Widely-Tunable Ring-Resonator Semiconductor Lasers

applied sciences

Review Widely-Tunable Ring-Resonator Semiconductor Lasers

Tin Komljenovic 1,*, Linjun Liang 2, Rui-Lin Chao 3, Jared Hulme 4, Sudharsanan Srinivasan 5, Michael Davenport 1 and John E. Bowers 1

1 Electrical & Computer Engineering, University of California Santa Barbara, Santa Barbara, CA 93106, USA; [email protected] (M.D.); [email protected] (J.E.B.) 2 Institute of Lightwave Technology, Beijing Jiaotong University, Beijing 100044, China; [email protected] 3 Department of Electrical Engineering, National Central University, Taoyuan 451700, Taiwan; [email protected] 4 Hewlett Packard Labs, 1501 Page Mill Rd, Palo Alto, CA 94304, USA; [email protected] 5 Juniper Networks, Inc., 1133 Innovation Way, Sunnyvale, CA 94089, USA; [email protected] * Correspondence: [email protected]; Tel.:+1-(805)-892-2149

Academic Editor: Ralf Hellmann Received: 27 June 2017; Accepted: 12 July 2017; Published: 17 July 2017

Abstract: Chip-scale widely-tunable lasers are important for both communication and sensing applications. They have a number of advantages, such as size, weight, and cost compared to mechanically tuned counterparts. Furthermore, they allow for integration in more complex integrated photonic chips to realize added functionality. Here we give an extensive overview of such lasers realized by utilizing ring resonators inside the laser cavity. Use of ring resonators for tuning allows for wide-tunability by exploiting the Vernier effect, and at the same time improves the laser linewidth, as effective cavity length is increased at ring resonance. In this review, we briefly introduce basic concepts of laser tuning using ring resonators. Then, we study a number of laser cavity configurations that utilize two ring resonators, and compare their tuning performance. We introduce a third ring resonator to the laser cavity, study three different cavity configurations utilizing three ring resonators, and select the optimal one, for which we show that laser tuning is straightforward, provided there are monitor photodetectors on-chip. Finally, we give a literature overview showing superior linewidth performance of ring-based widely-tunable lasers.

Keywords: semiconductor lasers; cavity resonators; laser tuning; photonic integrated circuits; ring resonators

1. Introduction Chip-scale widely-tunable lasers have been of interest for some time [1]. Applications range from communications, especially wavelength-division multiplexing systems (WDM), to a number of sensing applications, such as wavelength-steered light detection and ranging (LIDAR)for autonomous driving [2]. There are also numerous other sensor applications that rely upon the ability to sweep the laser frequency over a wide wavelength range, such as spectroscopy or backscatter reﬂectometers. A fully-integrated widely-tunable semiconductor laser has a number of advantages compared to the mechanically tuned counterparts—size, weight and cost are most obvious ones, but equally important, is the ability to integrate them with other components to provide added functionality on the same chip. Integration using monolithic or heterogeneous processes assembles many devices or optical functionalities on a single chip, so that all optical connections are on-chip and require no

Appl. Sci. 2017, 7, 732; doi:10.3390/app7070732 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 732 2 of 21 external alignment. The need for external alignment can become a bottleneck at large enough volumes, and result in increased cost. Photonic integrated circuits (PIC) can be realized in a number of platforms, but for efficient on-chip light generation, a direct bandgap semiconductor is needed. Historically, the first integrated photonic circuits with lasers were realized on gallium arsenide (GaAs) or indium phosphide (InP) substrates for the same reason [3]. The performance of lasers on native III-V substrate is considered state-of-the-art, but the cost due to the lower size of wafers, and higher cost of substrates, still presents a barrier to some markets. More recently, optical components are being integrated on silicon (Si) substrates for fabricating large-scale photonic integrated circuits that co-exist with micro-electronic chips. Additionally, the economy of scale in the microelectronics industry offers much a larger size (300 mm) and lower cost wafers, which consequently offers cheaper cost per unit die area, provided the volumes are high enough to offset higher initial investment [4]. Furthermore, the maturity of Si processing is vastly superior as shown with the latest generation of microprocessors having more than a few billion transistors densely packed in an area of ~450 mm2. This is many orders of magnitude more complex than the most advanced InP PICs. As the complexity of PICs increase, yield can be a limiting factor, and that is one more reason why the Si platform is actively being pursued for complex PICs. In the past 15 years, a whole range of photonic components, including filters, (de) multiplexers, splitters, modulators, and photodetectors, have been demonstrated, mostly in the silicon-on-insulator (SOI) platform [5]. The SOI platform is especially suited for standard communication bands at 1300 nm and 1550 nm, as silicon (n = 3.48) and its oxide SiO2 (n = 1.44) are both transparent, and form high-index contrast, high-confinement waveguides ideally suited for medium to high-integration PICs. In the shorter term, the rationale of silicon photonics is a reduction of the cost of photonic systems, most probably the ones targeted to the large volume data center market, but in the longer term, photonics should help solve the communications bottleneck, with the introduction of optics inside a high-performance electronic chip [6]. However, electrically pumped efficient sources on silicon remain a challenge due to the indirect bandgap of silicon. There have been some demonstrations with, e.g., strained Ge [7], but the performance of such sources is still very far from practical products. A large number of current Si photonic PICs use hybrid approaches in which Si PIC is coupled to III/V PIC that provides gain and lasing. Such an approach requires precise alignment, and arguably has scaling limitations. An alternative approach, most commonly called heterogeneous integration, bonds pieces of III/V materials on patterned Si wafer and then processes them using the same lithography tools. The advantage of this approach is that the alignment tolerances are significantly reduced, as devices are defined using lithography alignment marks after bonding of larger III/V pieces. Such an approach is currently widely used by both academia (e.g., University of California Santa Barbara (UCSB) [5,8], Ghent University [9,10]) and industry (Intel [11], Juniper Networks (formerly Aurrion) [12], Hewlett Packard Enterprise (HPE) [13]). This approach still has a theoretical disadvantage, as it uses expensive III/V wafers, and there is a size mismatch between typical Si wafers (12”), and typical III/V wafers (3”). Due to this reason, direct growth of III/V on silicon is currently being actively researched, with some very impressive demonstrations [14,15]. Direct growth has a potential to further reduce cost, but currently still has problems with lifetimes of the devices, despite using quantum dots, that are significantly more resilient to defects and dislocation impairments, than quantum wells. Direct growth, although a topic of great current interest, lies outside the scope of this manuscript. All the devices demonstrated in this manuscript use III/V quantum wells bonded to patterned SOI wafers. This approach gives the best of both worlds: high-gain from optimized III/V materials, and superior passive and waveguide technology of the SOI platform. The paper is organized as follows. In Section2, we introduce basic concepts of laser tuning utilizing Vernier effect and ring resonators, and provide theoretical considerations on the influence of ring resonators to laser linewidth and other key parameters. In Section3, we compare tuning and linewidth performance of a number of laser structures that utilize two different ring lengths Appl. Sci. 2017, 7, 732 3 of 21 for achieving wide tunability. In Section4, we introduce methods to further improve the linewidth: use of an on-chip external cavity, and use of an on-chip high-Q resonator. We study their placement, and performance influence, and select a high-Q ring internal to the laser cavity as the optimal design. We then demonstrate that tuning of lasers with three rings of different circumference is straightforward, provided the PIC has on-chip monitor photodetectors. Lastly, we give a brief overview of widely-tunable lasers in literature in Section5, and give conclusions in Section6.

2. Basic Principle of Wide-Tunability Utilizing Ring Resonators Wide-tunability in chip-scale semiconductor lasers is commonly achieved by utilizing the Vernier effect. The effect has been utilized both with sampled Bragg grating reflectors and ring resonators. Ring resonators, provided that the utilized waveguide platform offers sufficiently low propagation losses, have an advantage, as the effective cavity length at ring resonance is significantly enhanced, directly reducing linewidth [16,17]. A single ring resonator in add-drop configuration placed inside the laser Fabry-Perot cavity can be used for selecting an individual longitudinal mode. A typical transmission curve of a single ring resonator is shown in Figure1a. An ideal ring with ideal broadband couplers (no wavelength dependence on the coupling strength) has a periodic response separated by ring free-spectral range (FSR), corresponding to the inverse round-trip time inside the ring.

FSR [Hz] = c/(L·ng), (1) where c is the speed of light in a vacuum, L is the circumference of the ring, and ng is the group refractive index. To guarantee single longitudinal mode operation, the FSR of the ring should be larger than the gain bandwidth of the laser. With typical gain bandwidths in the 60 nm range, corresponding to ~7.5 THz around 1550 nm, ring circumferences, assuming ng = 3, are limited to very small values, around 10 µm. Such small circumferences might be impractical for at least two reasons, despite SOI technology providing very sharp bend radii with low loss. Tight bends, with losses lower than 0.09 dB per bend for bending radiuses of only 1 µm, were demonstrated more than ten years ago [18]. The first problem is designing such short couplers (two are needed for add-drop ring configuration), and the second problem is providing wide wavelength tuning. In Si photonics, two tuning mechanisms are typically used: carrier based (either injection or depletion), or thermal based. Carrier injection results in increased losses due to free carrier absorption, while carrier depletion generally has much lower efficiency, but allows for higher speed, and is the method of choice for high-speed modulators. Thermal tuning is often used for slower adjustments (10s of kHz, limited by thermal constants of the whole structure), especially as with proper design, there is no change in optical losses with tuning. This is the main reason we use thermal heaters for all of our devices, as low losses allow for narrower linewidth. In this work, the heaters are deposited above Si waveguides, with ~1 µm of SiO2 deposited in-between, to prevent the interaction between the metal and the optical mode, which would result in increased propagation loss. Devices based on silicon are readily thermally tunable due to the large thermo-optic coefficient (dn/dT) of silicon. A value of dn/dT of 1.87 × 10−4/K at room temperature and 1.5 µm is generally reported, and the coefficient increases at shorter wavelength, to 1.94 × 10−4/K at 1.3 µm [19]. This translates to tuning of ~0.08 nm/◦C at 1550 nm, so a temperature change for 60 nm of tuning can be as high as 700 ◦C. In practice, for passive devices, tuning ranges of up to 20 nm have been demonstrated [20], but still, such high temperatures close to the gain section would have negative influence on the laser performance. Much more efficient tuning is possible by using two or more rings with slightly different circumferences. Typical circumferences of the two rings are designed so that the FSR is between 200 GHz and 400 GHz, corresponding to ~1.6 nm or ~3.2 nm in wavelength. This allows for tuning the ring in full FSR by changing the temperature by only 20–40 ◦C, which is straightforward to achieve. Due to the slight difference in circumference of the two rings, their resonances are mostly not aligned, Appl. Sci. 2017, 7, 732 4 of 21 and allow for single-mode operation in a much wider bandwidth that the FSR of an individual ring, as shown in Figure1b. It is common to define tuning enhancement parameter M:

M = L1/(L2 − L1), (2) where L1 and L2 are the circumferences of Ring 1 and Ring 2, as shown in Figure1b. The FSR of the combinedAppl. two Sci. 2017 ring, 7, 732 structure utilizing the Vernier effect then equals FSR1·M, where FSR41 ofis 21 the FSR of Ring 1. We call the two ring structure that utilizes the Vernier effect to select single longitudinal where L1 and L2 are the circumferences of Ring 1 and Ring 2, as shown in Figure 1b. The FSR of the mode a single wavelength filter (SWF). A typical widely-tunable ring-resonator semiconductor laser combined two ring structure utilizing the Vernier effect then equals FSR1·M, where FSR1 is the FSR architectureof RingAppl. using Sci. 1. 2017We the, 7call, 732 SWF the two placed ring insidestructure the that Fabry–Perot utilizes the Vernier cavity, effect with to a gainselect element single longitudinal and4 of phase21 tuner to alignmode the longitudinal a single wavelength mode filter to the (SWF). resonance A typical of widely-tunable the SWF, is shown ring-resonator in Figure semiconductor2. Fabry–Perot laser mirrors where L1 and L2 are the circumferences of Ring 1 and Ring 2, as shown in Figure 1b. The FSR of the architecture using the SWF placed inside the Fabry–Perot cavity, with a gain element and phase tuner could be realizedcombined as two cleaved ring structure facets, utilizing but that the would Vernier limiteffect thethen integrationequals FSR1·M potential, where FSR of1 is such the FSR lasers, so we utilize Sagnacto alignof Ring the loop 1. longitudinal We mirrors, call the twomode in whichring to structurethe tworesonance same-sidethat utilizes of the SWF, the outputs Vernier is shown of effect a in coupler Figureto select 2. single (directional,Fabry–Perot longitudinal mirrors MMI or any could be realized as cleaved facets, but that would limit the integration potential of such lasers, so we other kind)mode are a connected single wavelength together filter to (SWF). form A a typical loop. widely-tunable Such a structure ring-resonator acts as asemiconductor mirror whose laser reflectivity utilizearchitecture Sagnac usingloop mirrors,the SWF placedin which inside two the same-side Fabry–Pe rotoutputs cavity, of with a coupler a gain element (directional, and phase MMI tuner or any can be controlled by the coupling strength; from theoretically 100% for a 50:50 coupler, all the way otherto alignkind) the are longitudinal connected modetogether to the to formresonance a loop. of thSuech SWF, a structure is shown acts in Figure as a mirror 2. Fabry–Perot whose reflectivity mirrors to verycan lowcould be values, controlled be realized which by as the cleave are coupling importantd facets, strength; but forthat higherfrom would theoretically limit output the integr powers. 100%ation for potential Thea 50:50 analysis coupler,of such oflasers, all the the so complexway we to laser cavity isvery simplifiedutilize low Sagnacvalues, by loopwhich using mirrors, are the important effective in which fortwo mirror higher same-side theory output outputsR powers.eff [ of21 a]. couplerThe In the analysis (directional, case of the the MMIcomplex laser or cavityany laser shown cavityother is kind)simplified are connected by using together the effective to form mirror a loop. theory Such a R structureeff [21]. In acts the as case a mirror of the whose laser cavityreflectivity shown in Figure2, Reff is a complex wavelength dependent mirror reflectivity, that includes the passive phase in Figurecan be 2,controlled Reff is a complex by the coupling wavelength strength; dependent from theoretically mirror reflectivity, 100% for athat 50:50 includes coupler, the all passive the way phase to section, both ring resonators, and one of the loop mirrors (R2 in the case of Figure2). section,very lowboth values, ring resonators, which are andimportant one of for the higher loop mirrorsoutput powers.(R2 in the The case analysis of Figure of the 2). complex laser cavity is simplified by using the effective mirror theory Reff [21]. In the case of the laser cavity shown in Figure 2, Reff is a complex wavelength dependent mirror reflectivity, that includes the passive phase section, both ring resonators, and one of the loop mirrors (R2 in the case of Figure 2).

Figure 1. (a) Add-drop response of single ring resonator (L1 = 200 µm, FSR = 3.31 nm); (b) Vernier of Figure 1. (a) Add-drop response of single ring resonator (L = 200 µm, FSR = 3.31 nm); ( b) Vernier of two rings showing 60 nm of free-spectral range (FSR) (L1 = 2001 µm, L2 = 211 µm, M = 18.1); Propagation Figure 1. (a) Add-drop response of single ring resonator (L1 = 200 µm, FSR = 3.31 nm); (b) Vernier of two ringsloss showing is assumed 60 to nm be of 1 dB/cm free-spectral and all power range coupling (FSR) (L coefficients1 = 200 µm, areL equal2 = 211 to µ0.15.m, M = 18.1); Propagation loss is assumedtwo rings to showing be 1 dB/cm 60 nm of and free-spectral all power range coupling (FSR) (L1 coefﬁcients= 200 µm, L2 = 211 are µm, equal M = to18.1); 0.15. Propagation loss is assumed to be 1 dB/cm and all power coupling coefficients are equal to 0.15.

Figure 2. Typical laser cavity comprising of two ring resonators with slightly different radii forming

a singleFigure frequency 2. Typical filter laser (SWF) cavity incomprising between twoof two loop ring mirrors resonators (R1 and with R slightly2) forming different a Fabry–Perot radii forming cavity. Figure 2. Typical laser cavity comprising of two ring resonators with slightly different radii forming a Wea define single frequencyeffective reflectivity filter (SWF) plane in between Reff to twosimplify loop mirrors the analysis. (R1 and R2) forming a Fabry–Perot cavity. single frequencyWe define filter effective (SWF) reflectivity in between plane R twoeff to simplify loop mirrors the analysis. (R1 and R2) forming a Fabry–Perot cavity. We defineThe effective complex reflectivity amplitude planereflectivityReff to formula simplify is given the analysis. by the following expressions: The complex amplitude reflectivity formula is given by the following expressions: 22 2 rS=⋅⋅⋅()()()22 S S 2 r (3) eff=⋅⋅⋅21, passive 21, ring 1 21, ring 2 2 , rSeff()()()21, passive S 21, ring 1 S 21, ring 2 r 2 , (3)

−−αβLjL = −−αβpp pp See21, passive = ppLjL pp , (4) See21, passive , (4) Appl. Sci. 2017, 7, 732 5 of 21

The complex amplitude reﬂectivity formula is given by the following expressions:

2 2 2 re f f = S21,passive · S21,ring1 · S21,ring2 · r2, (3)

−αp Lp −jβp Lp S21,passive = e e , (4)

|κ|2X S = X , (5) 21,ringX 2 2 1 − τ XX

lX lX −αp −jβp Xx = e 2 e 2 , (6) where αp and βp are the waveguide electric field propagation loss, and the effective propagation constant in waveguides, respectively, r2 is the amplitude reflectivity of the back-side mirror, Lx is the circumference of the ring, and the subscript X takes values 1 and 2 for the two rings in the SWF section. A number of useful laser parameters, including effective cavity length, mode spacing, and the adiabatic chirp reduction factor, can be determined using reff [22]. The chirp reduction is given by 2 factor F = 1 + A + B, and the Lorentzian linewidth (∆v0) is reduced by F , where A and B are defined as

∆ν ∆ν = 0 , (7) F2 1 d 1 dϕe f f (ω) A = Re i ln re f f (ω) = − , (8) τin dω τin dω αH d αH d B = Im i ln re f f (ω) = ln( re f f (ω) , (9) τin dω τin dω where αH is the linewidth enhancement factor, τin = 2neffLa/c is the roundtrip time in the active part of the cavity, where neff is the effective index of the gain section, and La is the length of the gain section. The A term, corresponding to the linewidth reduction from reduced longitudinal mode confinement, is often denoted as the ratio of the external (passive section) cavity path length to the gain section path length. As the effective length of a ring resonator is maximized at resonance, the A factor is maximized when the ring is placed exactly at resonance. The B term corresponds to the reduction in linewidth from the negative feedback effect, where a decrease in wavelength increases reflectivity (increasing photon density in the cavity), and hence decreases carrier density, which in turn causes the wavelength to increase, due to the carrier plasma effect. We plot some key parameters in Figure3, where we assume a 3 mm long Fabry–Perot cavity and use identical SWF, as plotted in Figure1b. The first takeaway from Figure3 is that the effective cavity length increases at resonance, and it is well known that longer cavities generally correlate with lower linewidth. A longer “effective” cavity, at the same time, reduces the longitudinal mode spacing, so SWF has to be designed properly to provide sufficient passive side-mode suppression ratio (SMSR) for the laser to be single mode. The effective cavity length is actually a derivative of phase with respect to frequency, and, as expected, the phase changes faster around the resonance. Finally, we plot the A, B, and F factors in Figure3d. As the B factor is antisymmetric around the resonance, and has a maximum when slightly detuned from the resonance, the result is that the total effect F is actually maximized off resonance. The phase section in the cavity can be used for a slight detuning of the laser oscillation with respect to the minimum cavity loss condition (resonator resonance) for best linewidth performance. Appl. Sci. 2017, 7, 732 6 of 21 Appl. Sci. 2017, 7, 732 6 of 21 Appl. Sci. 2017, 7, 732 6 of 21

FigureFigure 3. 3. (a(a) )Effective Effective cavity cavity length length calculated calculated for for a a3 3mm mm long long cavity cavity with with SWF SWF from from Figure Figure 1b;1b; Figure 3. (a) Effective cavity length calculated for a 3 mm long cavity with SWF from Figure 1b; ((bb)) Longitudinal Longitudinal mode mode spacing spacing for for the the same same cavity; cavity; ( (cc)) Phase Phase of of the the complex complex amplitude amplitude reflectivity reﬂectivity rreffeff (b) Longitudinal mode spacing for the same cavity; (c) Phase of the complex amplitude reflectivity reff forfor the the same cavity;cavity; ((dd)) Coefﬁcients CoefficientsA A, B, Band andF calculatedF calculated from from Equations Equations (8)and (8) (9)and for (9) the for same the cavity.same for the same cavity; (d) Coefficients A, B and F calculated from Equations (8) and (9) for the same cavity.In all cases, In all thecases, horizontal the horizontal axis corresponds axis corresponds to the detuningto the detuning from the from resonance. the resonance. cavity. In all cases, the horizontal axis corresponds to the detuning from the resonance. 3. Comparison of Classical Widely-Tunable Architectures 3.3. ComparisonComparison ofof ClassicalClassical Widely-TunableWidely-Tunable Architectures Architectures We compare four ring-based laser topologies, as shown in Figure 4, all of which have rings with WeWe compare compare four four ring-based ring-based laser laser topologies, topologies, as as shown shown in in Figure Figure4 ,4, all all of of which which have have rings rings with with two different circumferences to form the Vernier effect for tuning. twotwo different different circumferences circumferences to to form form the the Vernier Vernier effect effect for for tuning. tuning.

Figure 4. Four considered widely-tunable ring-based laser designs. Thermal heaters are drawn in FigureFigure 4.4.Four Fourconsidered consideredwidely-tunable widely-tunable ring-basedring-based laserlaserdesigns. designs. ThermalThermal heatersheaters areare drawndrawn inin yellow: (a) Ring-bus-ring laser (RBR); (b) Single Coupled-Ring-Resonator laser (CRR 1×); (c) Vernier yellow:yellow: (a(a)) Ring-bus-ring Ring-bus-ring laser laser (RBR); (RBR); ( b(b)) Single Single Coupled-Ring-Resonator Coupled-Ring-Resonator laser laser (CRR (CRR 1 ×1×);); ((cc)) VernierVernier racetrack resonator and (d) Double Coupled-Ring-Resonator laser (CRR 2×). racetrackracetrack resonator resonator and and ( d(d)) Double Double Coupled-Ring-Resonator Coupled-Ring-Resonator laser laser (CRR (CRR 2 ×2×).). The first is a ring-bus-ring (RBR) laser, which utilizes two ring filters in series in add-drop The first is a ring-bus-ring (RBR) laser, which utilizes two ring filters in series in add-drop configuration,The first and is a ring-bus-ringtwo Sagnac loop (RBR) mirrors. laser, The which second utilizes is a two single-sided ring filters coupled-ring in series in resonator add-drop configuration, and two Sagnac loop mirrors. The second is a single-sided coupled-ring resonator (CRRconfiguration, 1×) laser which and two utilizes Sagnac a CRR loop mirror mirrors. on one The side second as both is areflector single-sided and filter, coupled-ring and a Sagnac resonator loop (CRR 1×) laser which utilizes a CRR mirror on one side as both reflector and filter, and a Sagnac loop mirror(CRR 1on×) the laser other which side. utilizes The third a CRR is mirrora Vernier on oneracetrack side as laser, both which reflector utilizes and filter, two andbus awaveguides, Sagnac loop mirror on the other side. The third is a Vernier racetrack laser, which utilizes two bus waveguides, onemirror of which on the has other a gain side. region The thirdand phase is a Vernier tuner, racetrackand two ring laser, resonators which utilizes that are two coupled bus waveguides, to the bus one of which has a gain region and phase tuner, and two ring resonators that are coupled to the bus waveguides.one of which The has afourth gain regiondesign and is a phase double-sided tuner, and coupled-ring two ring resonators resonator that (CRR are coupled 2×) laser, to thewhich bus waveguides. The fourth design is a double-sided coupled-ring resonator (CRR 2×) laser, which utilizeswaveguides. a CRR The mirror fourth on design both issides a double-sided as both reflec coupled-ringtor and filter. resonator All lasers (CRR 2have×) laser, resistive which heaters utilizes utilizes a CRR mirror on both sides as both reflector and filter. All lasers have resistive heaters overlayinga CRR mirror the onrings both to sidesprovide as bothactive reflector tuning andof th filter.e wavelength. All lasers Ring have circumferences resistive heaters of overlayingthe RBR laser the overlaying the rings to provide active tuning of the wavelength. Ring circumferences of the RBR laser arerings 256 to µm provide and active271 µm, tuning giving of the an wavelength. expected tuni Ringng circumferencesrange of 42.1 nm, of the with RBR the laser power are 256 couplingµm and are 256 µm and 271 µm, giving an expected tuning range of 42.1 nm, with the power coupling coefficient271 µm, giving same anfor expected all couplers, tuning and range equal of to 42.1 20% nm,. For with the the CRR power 1× laser, coupling the selected coefficient circumference same for all coefficient same for all couplers, and equal to 20%. For the CRR 1× laser, the selected circumference valuescouplers, for Ring and equal1 and toRing 20%. 2 were For the337 CRR µm and 1× laser,368 µm, the respectively, selected circumference and the selected values power for Ring coupling 1 and values for Ring 1 and Ring 2 were 337 µm and 368 µm, respectively, and the selected power coupling coefficientRing 2 were values 337 µ form and Κ1, Κ 3682 andµm, Κ respectively,3 are 2.25%, 2.25% and the and selected 36%, respectively. power coupling This coefficient results in valuesan expected for K1 , coefficient values for Κ1, Κ2 and Κ3 are 2.25%, 2.25% and 36%, respectively. This results in an expected tuningK and rangeK are of 2.25%, 20.4 2.25%nm. A and more 36%, detailed respectively. description This results of CRR in anmirror expected properties tuning is range found of 20.4in [23]. nm. tuning2 range3 of 20.4 nm. A more detailed description of CRR mirror properties is found in [23]. VernierA more racetrack detailed description laser has two of CRR rings, mirror with properties circumferences is found equal in [ 23to]. 400 Vernier µm and racetrack 420 µm, laser with has twothe Vernier racetrack laser has two rings, with circumferences equal to 400 µm and 420 µm, with the powerrings, withcoupling circumferences coefficient equalsame for to 400all couplers,µm and 420 andµ m,equal with to the20%. power The desig couplingn tuning coefficient range sameis equal for power coupling coefficient same for all couplers, and equal to 20%. The design tuning range is equal to 31.6 nm. Finally, the CRR 2× laser has two pairs of rings, each pair having nominally the same to 31.6 nm. Finally, the CRR 2× laser has two pairs of rings, each pair having nominally the same radius of 368 µm and 402 µm. The coupling coefficient values for both pairs are equal and the same radius of 368 µm and 402 µm. The coupling coefficient values for both pairs are equal and the same Appl. Sci. 2017, 7, 732 7 of 21

all couplers, and equal to 20%. The design tuning range is equal to 31.6 nm. Finally, the CRR 2× laser Appl. Sci. 2017, 7, 732 7 of 21 µ µ Appl.has twoSci. 2017 pairs, 7,of 732 rings, each pair having nominally the same radius of 368 m and 402 m. The coupling7 of 21 tocoefﬁcient those on valuesCRR 1× for laser. both The pairs design are equal tuning and range thesame is 18.6 to nm. those The on calculated CRR 1× laser. function The of design the passive tuning ringtorange those filters is 18.6on is CRR shown nm. 1× The laser.in calculated Figure The 5.design function tuning of therange passive is 18.6 ring nm. ﬁlters The calculated is shown in function Figure5 .of the passive ring filters is shown in Figure 5.

Figure 5. Synthesized spectra of all the considered ring structures: (a) Ring-bus-ring (RBR) resonator Figure 5. Synthesized spectra of all the considered ring structures: (a) Ring-bus-ring (RBR) resonator structure;Figure 5. Synthesized(b) Single Coupled-Ring-Resonator spectra of all the considered (CRR ring 1×) structures: structure; (a ()c Ring-bus-ring) Vernier racetrack (RBR) resonatorresonator structure; (b) Single Coupled-Ring-Resonator (CRR 1×) structure; (c) Vernier racetrack resonator structurestructure; and (b) (dSingle) Double Coupled-Ring-Resonator Coupled-Ring-Resonator (CRR (CRR 1×) 2×) structure; structure. (c ) Vernier racetrack resonator structure and (d) Double Coupled-Ring-Resonator (CRR 2×) structure. structure and (d) Double Coupled-Ring-Resonator (CRR 2×) structure. We have fabricated the lasers using heterogeneous III/V silicon process. Optical photographs of lasersWe are have shown fabricated in Figure thethe 6. laserslasers Wavelength usingusing heterogeneousheterogeneou tuning for laserss III/VIII/V was silicon mapped process. out Optical vs. ring photographs heater voltage of squaredlasers are (proportional shown in Figure to power),6 6.. WavelengthWavelength as shown tuningtuning in Figu forforre lasers7.lasers The was wasRBR mapped mappedlaser had outout a measured vs.vs. ringring heaterheater tuning voltagevoltage range ofsquared 42 nm, (proportional the CRR 1× laser to power), had a measuredas shown tuningin Figu Figure rerange 77.. TheThe of RBR21RBR nm, laserlaser while hadhad the aa measured measuredVernier racetrack tuningtuning rangerange laser hadof 42 a nm,measured the CRR tuning 11×× laserlaser range had had of a a47 measured measured nm, and tuning tuningthe theo range rangeretical of of tuning21 21 nm, nm, range while while ofthe the CRR Vernier Vernier 2× laser racetrack is 18.6 laser nm. Wehad do a measurednot show tuningthe 2D range tuning of map 47 nm,for CRR and the2× lase theoreticaltheor reticaldue to tuningunpredictable range of tuning CRR 22× performance,× laserlaser is is 18.6 18.6 thatnm. nm. weWe addressed dodo notnot showshow in Section thethe 2D2D tuningtuning3.1. The mapmap tooth-shaped forfor CRRCRR 22× ×tuning laselaserr duemap due toof unpredictable the Vernier racetrack tuning performance, laser is due to that the wavelengthwe addressed hopping in in Section Section between 3.1. 3.1 .The The two tooth-shaped tooth-shaped wavelengths tuning tuningsepa maprated map of by ofthe the the Vernier design Vernier racetrack tuning racetrack rangelaser laser is of due is~31 due to nm, the to whenwavelengththe wavelength the laser hopping work hopping at between the between states two where two wavelengths wavelengths net gain between sepa separatedrated these by by twothe the modesdesign design aretuning tuning similar range range in magnitude.of of ~31 ~31 nm, nm, Thewhen maximum the laser measuredwork at the double-sided states where output net gain power between between was these these7.7 mW two two for modes modes the RBR are are laser,similar similar 10.5 in in mWmagnitude. magnitude. for the CRRThe maximum 1× laser laser, measured measured 12.7 mWdouble-sided double-sided for the Vernier output output powerracetr powerack was was laser, 7.7 7.7 mW and mW for 14.2 for the themW RBR RBR forlaser, laser,the 10.5 CRR 10.5 mW 2× mW for laser. the for TheCRRthe CRRoutput 1× laser 1× powerslaser laser, laser, were 12.7 12.7 measuredmW mW for forthe using the Vernier Vernieran integrating racetr racetrackack sphere.laser, laser, and and 14.2 14.2 mW mW for for the the CRR CRR 2× 2× laser.laser. The output powers were measured usingusing anan integratingintegrating sphere.sphere.

Figure 6. Optical images of fabricated lasers: (a) Ring-bus-ring laser (RBR); (b) Single Coupled-Ring- ResonatorFigure 6. Optical laser (CRR images 1×); of ( cfabricated ) Vernier lasers:racetrack ( (aa)) Ring-bus-ringresonator Ring-bus-ring and laser laser(d) Double (RBR); (RBR); Coupled-Ring-Resonator( (bb)) Single Single Coupled-Ring- Coupled-Ring- laserResonator (CRR laser 2×). (CRR 11×);×); ( (cc)) Vernier Vernier racetrack resonator and (d) DoubleDouble Coupled-Ring-ResonatorCoupled-Ring-Resonator laser (CRR 2×). 2×). The laser linewidths were measured using the delayed self-heterodyne method, with 10 km of fiber Thein one laser arm, linewidths and an acousto-optic were measured modulator using the wi delth 100ayed MHz self-heterodyne of frequency method, shift in thewith other 10 km arm. of Wefiber quote in one the arm, 3 dB and Lorentzian an acousto-optic linewidth modulator calculated wi fromth 100 −20 MHz dB points. of frequency We show shift the in measurements the other arm. inWe Figure quote 8. the All 3 thedB Lorentzianmeasured configurations linewidth calculated have linewidths from −20 dBin thepoints. 200 kHzWe show range, the with measurements best results atin 150Figure kHz. 8. ForAll the casemeasured of CRR configurations 2× laser, in some have certain linewidths lasing in wavelengths, the 200 kHz itsrange, measured with best linewidth results at 150 kHz. For the case of CRR 2× laser, in some certain lasing wavelengths, its measured linewidth Appl. Sci. 2017, 7, 732 8 of 21

Appl.can be Sci. as2017 narrow, 7, 732 as 196 kHz, however, the stable single-mode lasing performance cannot be sustained8 of 21 across the whole theoretical tuning range.

Appl. Sci. 2017, 7, 732 8 of 21 can be as narrow as 196 kHz, however, the stable single-mode lasing performance cannot be sustained across the whole theoretical tuning range.

Figure 7.7. PlotPlot ofof peakpeak wavelength wavelength vs. vs. ring ring tuning tuning power power for for (a )(a RBR) RBR laser; laser; (b )( CRRb) CRR 1× 1×laser; laser; and and (c) Vernier(c) Vernier racetrack racetrack laser. laser. We We do do not not show show the the CRR CRR 2× 2×laser laser tuning tuning performanceperformance duedue to unpredictability.

The laser linewidths were measured using the delayed self-heterodyne method, with 10 km of ﬁber in one arm, and an acousto-optic modulator with 100 MHz of frequency shift in the other arm. We quote the 3 dB Lorentzian linewidth calculated from −20 dB points. We show the measurements in Figure8. All the measured conﬁgurations have linewidths in the 200 kHz range, with best results at 150 kHz. For the case of CRR 2× laser, in some certain lasing wavelengths, its measured linewidth can be asFigure narrow 7. asPlot 196 of kHz,peak however,wavelength the vs. stable ring tuning single-mode power for lasing (a) RBR performance laser; (b) CRR cannot 1× laser; be sustained and across(c the) Vernier whole racetrack theoretical laser. tuningWe do not range. show the CRR 2× laser tuning performance due to unpredictability.

Figure 8. Measured linewidths for (a) RBR laser; (b) CRR 1× laser; and (c) Vernier racetrack laser. The linewidth of the CRR 2× laser was 196 kHz at the one wavelength where it was measured.

3.1. Discussion and Conclusions Having analyzed, fabricated, and measured four different laser architectures, we conclude that the best design to realize a widely-tunable ring-resonator based laser is the RBR. The RBR laser has very predictable tuning, stable lasing due to the standing-wave nature of the resonator, and high passive side-mode suppression ratio (SMSR), resulting in high lasing SMSR. Added benefits are Figure 8. Measured linewidths for (a) RBR laser; (b) CRR 1× laser; and (c) Vernier racetrack laser. Figure 8. Measured linewidths for (a) RBR laser; (b) CRR 1× laser; and (c) Vernier racetrack laser. largelyThe fixed linewidth mirror of the reflectivities CRR 2× laser due was to 196 the kHz br atoadband the one wavelength Sagnac loop where mirrors it was measured. used. The biggest disadvantageThe linewidth of this of thearchitecture CRR 2× laser is a was somewhat 196 kHz atlo werthe one output wavelength power, where although it was that measured. can be corrected 3.1.by placing Discussion a booster and Conclusions semiconductor optical amplifier (BSOA) after the laser. In a heterogeneously integrated3.1. Discussion design, and integrationConclusions of a BSOA is straightforward. The reason for lower output power is Having analyzed, fabricated, and measured four different laser architectures, we conclude that the tuningHaving section, analyzed, where fabricated, light go andes through measured each four ring di fferenttwice, laserincreasing architectures, the insertion we conclude loss of thatthe the best design to realize a widely-tunable ring-resonator based laser is the RBR. The RBR laser has structure,the best design and consequently, to realize a widely-tunable the cavity loss, ring-res but at theonator same based time, laser this isresults the RBR. in improved The RBR filtering,laser has very predictable tuning, stable lasing due to the standing-wave nature of the resonator, and high andvery higher predictable passive tuning, SMSR. stable lasing due to the standing-wave nature of the resonator, and high passive side-mode suppression ratio (SMSR), resulting in high lasing SMSR. Added benefits are largely passiveWe showside-mode tuning suppression of the RBR ralasertio (SMSR),in more detailresulting in Figure in high 9a, lasing where SMSR. directions Added of fine benefits (dashed are fixed mirror reflectivities due to the broadband Sagnac loop mirrors used. The biggest disadvantage redlargely line) fixed and coarsemirror (dotted reflectivities blue line) due tuningto the arbreoadband drawn. TheSagnac black loop line mirrorsshows continuousused. The tuningbiggest of this architecture is a somewhat lower output power, although that can be corrected by placing withoutdisadvantage mode ofhops, this architecturewhere phase is sectiona somewhat corrects lower for output the cavity power, length, although as wavelength that can be iscorrected tuned. a booster semiconductor optical amplifier (BSOA) after the laser. In a heterogeneously integrated Toby guaranteeplacing a sufficientbooster semiconductor phase shift, the optical phase tuningamplifie serction (BSOA) has toafter be optimized,the laser. Inand a inheterogeneously general, up to design, integration of a BSOA is straightforward. The reason for lower output power is the tuning aintegrated few nanometers design, of integration continuous of tuning a BSOA is possible, is straightforward. in practice, beforeThe reason temperatures for lower at output phase andpower ring is section, where light goes through each ring twice, increasing the insertion loss of the structure, tunersthe tuning become section, excessive, where and light added goes heat through starts toeach influence ring twice, the laser increasing performance the insertion significantly. loss of the and consequently, the cavity loss, but at the same time, this results in improved filtering, and higher structure,The CRR and 1× consequently, laser has higher the cavityoutput loss, power but than at the RBR, same primarily time, this due results to lower in improved losses at thefiltering, CRR passive SMSR. reflector,and higher compared passive SMSR.to RBR filter realization. The biggest problem with CRR is the lower passive We show tuning of the RBR laser in more detail in Figure9a, where directions of fine (dashed red SMSR,We translating show tuning to smaller of the tuningRBR laser ranges, in more and/or detail lower in Figurelaser SMSR. 9a, where directions of fine (dashed line) and coarse (dotted blue line) tuning are drawn. The black line shows continuous tuning without red line) and coarse (dotted blue line) tuning are drawn. The black line shows continuous tuning mode hops, where phase section corrects for the cavity length, as wavelength is tuned. To guarantee without mode hops, where phase section corrects for the cavity length, as wavelength is tuned. sufficient phase shift, the phase tuning section has to be optimized, and in general, up to a few To guarantee sufficient phase shift, the phase tuning section has to be optimized, and in general, up to nanometers of continuous tuning is possible, in practice, before temperatures at phase and ring tuners a few nanometers of continuous tuning is possible, in practice, before temperatures at phase and ring become excessive, and added heat starts to influence the laser performance significantly. tuners become excessive, and added heat starts to influence the laser performance significantly. The CRR 1× laser has higher output power than RBR, primarily due to lower losses at the CRR reflector, compared to RBR filter realization. The biggest problem with CRR is the lower passive SMSR, translating to smaller tuning ranges, and/or lower laser SMSR. Appl. Sci. 2017, 7, 732 9 of 21 Appl. Sci. 2017, 7, 732 9 of 21

Figure 9. ((aa)) Tuning Tuning map map for for the RBR laser showing direct directionsions of of fine ﬁne (dashed red line) and coarse (dotted(dotted blue blue line) tuning. The The bl blackack line shows continuous tuning without without mode mode hops, hops, where where phase sectionsection corrects corrects for the the cavity cavity length length as as wavelength wavelength is is tuned; tuned; ( (bb)) The The bidirectional bidirectional lasing lasing nature nature of of the the Vernier laser where direction of lasing changes as laser bias is tuned. The clockwise (CW) direction is plotted in blue, and thethe counterclockwisecounterclockwise (CCW) (CCW) direction direction in in red red (similar (similar effect effect happens happens with with tuning tuning or orchange change of laserof laser operating operating temperature); temperature); (c) Mask(c) Mask layout layout of CRR of CRR structure structure showing showing Si waveguides Si waveguides and andthermal thermal heaters heaters (green); (green); (d) Reﬂection(d) Reflection from from CRR CRR structure structure when when both both rings rings are are aligned; aligned; (e) ( Splittinge) Splitting of ◦ ofCRR CRR resonance resonance when when two two rings rings have have a difference a difference in temperaturein temperature of 2of 2C. °C.

AsThe it CRRis very 1× challenginglaser has higher to design output a single power CRR than structure RBR, primarily with high due SMSR, to lower a logical losses idea at the would CRR bereflector, to replace compared the Sagnac to RBR loop filter mirrors realization. on both The sides biggest of problemthe cavity with with CRR CRR is thestructures. lower passive This further SMSR, reducestranslating the to cavity smaller losses, tuning as ranges,shown, and/orwith the lower highest laser output SMSR. powers obtained with CRR 2× laser. The laserAs it isalso very features challenging high topassive design SMSR, a single but CRR the structure tuning with is extremely high SMSR, challenging, a logical idea for would several be reasons.to replace First the Sagnacis that the loop laser mirrors now on has both four sides contro of thels cavityfor tuning, with CRRas each structures. ring inside This the further CRR reduces has its ownthe cavity tuner, losses,and the as fifth shown, control with would the highest be the phase output to powersalign the obtained longitudinal with mode. CRR 2 A× waylaser. to The simplify laser thealso control features would high passivebe using SMSR, a single but heater the tuning shared is per extremely CRR structure. challenging, A problem for several with reasons. this approach First is isthat that the the laser rings now have has to four be exactly controls the for same tuning, circumference as each ring to inside act as a the reflector, CRR has and its any own non-symmetry tuner, and the orfifth thermal control crosstalk would be will the lead phase to resonance to align the splitting longitudinal, as shown mode. in A Figure way to 9d,e. simplify The problem the control is further would complicatedbe using a single due heaterto a need shared to have per distinctly CRR structure. different A problem coupling with values this between approach the is thattwo therings rings and have the busto be waveguide exactly the same[23], which circumference makes tofull act symmetry as a reflector, impossible. and any non-symmetryFor our laser, orwe thermal have designed crosstalk independentwill lead to resonance heaters for splitting, each ring, as shown and have in Figure also pr9d,e.ovided The the problem ability is to further use the complicated two heaters due as toone a bigneed heater to have covering distinctly both differentrings for couplingeach CRR values structure, between but in the all cases, two rings the tuning and the was bus very waveguide challenging. [23], Duewhich to makessuch complicated full symmetry controls, impossible. it is hard For our to recommend laser, we have the designed CRR 2× independent structure, despite heaters it forhaving each thering, highest and have output also power. provided the ability to use the two heaters as one big heater covering both rings for eachThe CRRlast considered structure, butstructure, in all cases,Vernier the racetrack tuning waslaser, very has challenging.high output po Duewer, to is such straightforward complicated tocontrols, control, it and is hard has to decent recommend SMSR,the but CRR as it 2 ×hasstructure, a ring type despite of cavity, it having it inherently the highest has output bidirectional power. lasing,The where last consideredtwo travelling structure, waves compete Vernier racetrackfor gain. We laser, show has the high clockwise output power,(CW) and is straightforward counterclockwise to (CCW)control, output and has powers decent in SMSR, Figure but 9b, as and it has as a the ring bias type current of cavity, is changed, it inherently the hasoutput bidirectional powers in lasing, two directionswhere two also travelling change. The waves same compete can happen for gain. as the We laser show is tuned, the clockwise or laser operating (CW) and temperature counterclockwise changes. (CCW) output powers in Figure 9b, and as the bias current is changed, the output powers in Appl. Sci. 2017, 7, 732 10 of 21 two directions also change. The same can happen as the laser is tuned, or laser operating temperature changes.

4. Advanced Designs for Linewidth Reduction Linewidths of Si ring based semiconductor lasers are typically in sub-MHz range (for comparison, see Section5), compared with typical linewidths of III/V lasers that are in a few MHz range. Here we describe methods for improving linewidth even further. It should be noted that methods described here generally improve Lorentzian, or white-noise limited, linewidth. The 1/f contribution of the phase noise to laser linewidth depends on many parameters, including sources of technical noise, such as current supplies, etc. To improve long-term stability of a semiconductor laser, locking to stable high-Q resonators is typically employed. A demonstration of locking a ring based widely-tunable semiconductor laser to a high-Q resonator made in SiN has shown signiﬁcant reduction of low-frequency noise [24], and such resonators can be integrated on the same chip [25]. Laser locking to the external resonator lies outside the scope of the paper, but shows potential for extremely stable chip-scale laser, especially as SiN has lower thermo-optic coefﬁcient than Si.

4.1. Utilizing Delayed On-Chip Optical Feedback A technique that has often been employed for improving the laser linewidth uses delayed optical feedback. Semiconductor lasers, as is well known, can significantly be affected by external optical feedback. For that reason, in the majority of applications, great care is taken to coat the laser facets with anti-reflection coating, and to place optical isolators as close to the cavity as possible; but as controlled feedback can improve certain laser parameters, it has been extensively studied. Depending on specific conditions, optical feedback has been shown to make the laser multi-stable, have hysteresis phenomena, enhance, prolong or suppress the relaxation oscillation in the transient output, improve the laser’s performance through noise suppression, reduce nonlinear distortion and modulation bandwidth enhancement, and, what is most relevant for our application, improve the laser linewidth [26–31]. The effects of feedback are typically divided into five distinct regimes with well-defined transitions [32]. The classic paper from 1980 has been revisited and expanded [33]. It is shown that the change of amplitude signal (∆E) always accompanies that of frequency change (∆v), due to feedback. The dependence is sinusoidal for weak feedback (∆E ≈ E0·cos(2kL), 2kL being the optical phase associated with distance to external reflector), and becomes more complicated, leading to switching, hysteresis, and chaos, for higher levels of feedback. The full dynamics are complex and depend on many laser parameters, such as gain, loss, photon and carrier lifetime, and above all, the linewidth enhancement factor αH. The feedback behavior can further be separated into four distinct regions: short vs. long cavities, and coherent vs. incoherent feedback. The latter condition relates the external cavity length L to the coherence length Lcoh of the unperturbed source. The distinction between short and long cavities is made based on the relation between the relaxation resonance frequency (fR), and the external cavity frequency. For short cavities, the length L < c/(2fR) and the oscillation regime is dependent upon the phase of the external path length. For long cavities L > c/(2fR), no dependence of the oscillation regime on phase should be observed. A schematic diagram of the integrated widely-tunable laser with on-chip external cavity is shown in Figure 10. The gain section is inside a 2 mm long cavity formed by loop-mirrors. The lasing wavelength is determined by the SWF formed from two ring resonators, and a cavity phase section, all of which are controlled by thermal phase tuners. The front loop mirror (at the output of the laser) has a 10% power reflection, and the output of the laser is terminated at the facet at an angle of 7◦, to minimize reflections. The back loop mirror, after the wavelength tuning section, has a power reflection of 60%, which couples part of the light to the external cavity. In order to allow for a long external cavity, a low-loss waveguide platform is needed. Here, we utilize optimized silicon waveguides with a loss of 0.67 dB/cm. The external cavity is ~4 cm long and has its own phase adjustment section and gain section. The gain section in the external cavity is used to control the level Appl. Sci. 2017, 7, 732 11 of 21 of feedback. The external cavity is coupled on the side with the tuning section, so that the tuning rings filter out any spontaneously emitted light from the external semiconductor optical amplifier (SOA), Appl.improving Sci. 2017, the7, 732 phase noise, and consequently, the linewidth performance. In the case of sufficient11 of 21 forward bias in the external SOA, the extended cavity can lase by itself. At the laser output, there is an thereon-chip is an monitor on-chip photodetector monitor photodetector (MPD) that (MPD) uses the that same uses epitaxial the same material epitaxial as the material laser and as the the laser SOA. andWe havethe SOA. demonstrated We have demonstrated 54+ nm of tuning 54+ nm range of tuning with highrange SMSR with high (>45 SMSR dB). Output (>45 dB). power Output is around power is10 around mW, which 10 mW, can which easily can be increased easily be byincreased the use by of athe booster use ofSOA. a booster SOA.

FigureFigure 10. A schematicschematic view view of of a tunablea tunable laser laser design design with with integrated integrated external external cavity. cavity. Tuners Tuners are yellow are yellow(two phase (two phase sections sections and two and ringstwo rings for widefor wide tuning), tuning), and and SOAs SOAs are are dark dark orange orange (gain (gain sectionsection inside thethe laser laser cavity, cavity, and external SOA th thatat is used to control the level of feedback). The output monitor monitor photodiode (MPD) is used to measure the laser ou outputtput power for adjustment of laser parameters.

TheThe on-chip on-chip MPD MPD is is a a very useful tool to moni monitortor the on-chip performance of of a a laser laser with with an an externalexternal cavity. ItIt hashas been been shown shown that that lasers lasers can can be be influenced influenced by feedbackby feedback levels levels as low as low as − 90as dB−90 [ 34dB], [34],and and a straightforward a straightforward way way to measureto measure the the feedback feedback is is to to change change the the phase phase ofof the returned signal, signal, while monitoring the the laser laser output output power. power. We We sh showow such such a a measurement in in Figure Figure 11a,11a, where where the externalexternal phase phase heater heater is is tuned tuned for for various various gain gain se sectionction biases, biases, and and output output power power is is recorded recorded by by the on-chipon-chip MPD MPD placed placed at at the laser output. The The resulting resulting effect effect is an almost sinusoidal modulation of outputoutput power, power, with with the the phase phase of of the the returning returning sign signalal as as predicted predicted for for lower lower levels levels of of feedback feedback [33]. [33]. This signal signal can can be be used used to to phase phase align align the the two two ca cavities,vities, and and improve improve the the laser laser linewidth. linewidth. We We measured measured thethe linewidth linewidth of of the the laser laser with with optimized optimized feedback feedback fr fromom the the external external cavity. cavity. Th Thee feedback feedback was was manually manually tunedtuned for for best best performance, performance, and and results results are are shown shown in in Figure Figure 11b.11b. The The linewidths linewidths were were below below 100 100 kHz kHz acrossacross the entire tuning range, and the best single-modesingle-mode linewidth is 50 kHz.kHz. The The quoted quoted 3 3 dB linewidthslinewidths are are calculated calculated from from −2020 dB dB points points by by assuming assuming a a Lorentzian Lorentzian lineshape lineshape using using a a delayed self-heterodyneself-heterodyne method. method. Without Without the the external external cavity, cavity, the the linewidths linewidths were were in in the the 300 300 kHz kHz range. range. ItIt should should be be noted noted that that the the lasers lasers were were not not packag packaged,ed, so so there there were were a a number of of sources sources of of technical noisenoise present. present. WeWe were able to measure even lower linewidths (below 20 kHz) at higher currents supplied to thethe external external SOA, SOA, but but the the laser laser was was multimode, multimode, wi withth mode mode spacing spacing determined determined by by the the external external cavity cavity length.length. In In this this case, case, both both the the relative relative intensity intensity noise noise (RIN) (RIN) and and frequency frequency noise noise spectrums spectrums have have peaks peaks atat this mode separation. This This shows shows that that the the extern externalal cavity cavity can can provide provide ev evenen better performance if if thethe filtering filtering section section is is optimized. optimized. We We also also believe believe that that the the external external SOA SOA increased increased the the noise noise in in the the feedbackfeedback signal, signal, due due to to random random sp spontaneousontaneous emission emission events, events, thus thus limiting limiting the the linewidth linewidth improvement improvement performance overover thethe theoretically theoretically predicted predicted one, one, as foras for the the measurements measurements shown shown in Figure in Figure 11b, it 11b, was itoperated was operated below transparency.below transparency. Driving Driving the external the SOAexternal more SOA strongly more would strongly result would in multimode result in multimodebehavior, as behavior, the SWF cannotas the SWF filter cannot out a single filter longitudinalout a single longitudinal mode of the external mode of cavity the external in such cavity a design. in suchThe Q a ofdesign. tuning The rings Q of is tuning in the 10,000rings is range. in the 10,000 range. AsAs an an improvement improvement of this this work, work, we we suggest suggest use use of of a a variable variable optical optical attenuator attenuator (VOA) (VOA) in in place place ofof an SOA. VOAs could provide similar functiona functionalitylity in in controlling the feedback level, with some differences. They only attenuate the signal, and so, depending on the configuration, potentially higher levels of feedback (regime V) would not be achieved, and the extinction ratio can be limited, at least for single stage structures. By cascading the VOAs, higher extinction ratios can be achieved, but with an increase in the insertion loss. Appl. Sci. 2017, 7, 732 12 of 21 differences. They only attenuate the signal, and so, depending on the configuration, potentially higher levels of feedback (regime V) would not be achieved, and the extinction ratio can be limited, at least for single stage structures. By cascading the VOAs, higher extinction ratios can be achieved, but with Appl. Sci. 2017, 7, 732 12 of 21 an increase in the insertion loss.

FigureFigure 11. 11. (a(a) )The The influence inﬂuence of of feedback feedback from from external external cavity cavity when when SOA SOA is biased is biased at 0 at V. 0 V.The The x-axisx-axis is 2 theis the squared squared current current (I2~P) (I ~P) applied applied to external to external cavity cavity phase phase tuner, tuner, the they-axisy-axis is the is thecurrent current collected collected by monitorby monitor photodetector photodetector (MPD) (MPD) at the at laser the laser output output and every and every curve curve corresponds corresponds to different to different laser laserbias betweenbias between 30 and 30 and80 mA 80 mAin steps in steps of 1 of mA; 1 mA; (b) ( bMeasured) Measured linewidth linewidth across across the the full full tuning tuning range. range. TheThe horizontal horizontal line line shows shows the the linewidth linewidth measured on similar laser structure with no external cavity.

4.2.4.2. Utilizing Utilizing On-Chip On-Chip High-Q High-Q Resonator Resonator High-High-QQ resonatorsresonators are are another another way way ofof realizing realizing narrow-linewidth narrow-linewidth lasers. lasers. We Wehave have explored explored the possibilitythe possibility of improving of improving the thelinewidth linewidth of ofwidely-tunable widely-tunable se semiconductormiconductor lasers lasers by by utilizing utilizing an an integratedintegrated high- high-QQ ringring cavity-on-chip cavity-on-chip [35]. [35]. Three Three potential potential strategies strategies were were analyzed: analyzed: with with the the high- high-QQ ringring used asas anan externalexternal cavity cavity with with optical optical feedback feedbac ink in either either all-pass all-pass or dropor drop configuration, configuration, and withand withthe high- the high-Q ringQ beingring being an integral an integral part ofpart the of laser the laser cavity. cavity. TheThe laser laser performance performance can can be be influenced influenced by by couplin couplingg the the laser laser to toa distanced a distanced single single mirror, mirror, or to or ato high- a high-Q cavity.Q cavity. For For a laser a laser coupled coupled to toa single a single mirror, mirror, two two modes modes of of operation operation can can be be identified, identified, dependingdepending onon ifif the the external external cavity cavity free-spectral free-spectral range ra isnge larger, is larger, or smaller, or smaller, than the relaxationthan the relaxation oscillation oscillationfrequency (frequencyωR) of the ( standaloneωR) of the standalone laser [26]. For laser short [26]. cavities For short (external cavities cavity (external FSR > ωcavityR), the FSR linewidth > ωR), thereduction linewidth is modest, reduction but is themodest, feedback but the system feedback has a system very broad has a effectivevery broad bandwidth effective ablebandwidth to suppress able tonoise suppress to frequencies noise to frequencies much larger much than largerωR. Inthan the ω caseR. In the of a case long of cavity a long (externalcavity (external cavity cavity FSR < FSRωR),

Figure 12. (((aa)) Schematic Schematic of of external external cavity cavity laser laser with with high- high-Q ringring in in all-pass all-pass configuration; conﬁguration;configuration; ( (b) Schematic of external cavity laser with high-high-Q ringring inin add-dropadd-dropadd-drop conﬁguration.configuration.configuration.

A thirdthird approachapproach forfor using using the the high- high-QQresonator resonator to to improve improve laser laser linewidth linewidth isthe is the inclusion inclusion of the of high-the high-Q resonatorQ resonator in ain drop a drop conﬁguration configuration as as an an integral integral part part of of the the laser laser cavity,cavity, asas shownshown inin Figure 13a.13a. The inclusioninclusion ofof aa thirdthird ring ring resonator resonator inside inside the the cavity cavity increases increases the the cavity cavity loss, loss, and and there there is ais trade-off a trade- inoff the in the loaded loadedQ and Q and insertion insertion loss. loss. Simulations Simulations show show that that inclusion inclusion of aof ring a ring with withQ inQ thein the range range of ~160of ~160 k, resultsk, results in ~1.9in ~1.9 dB dB of of double-pass double-pass loss, loss, which which can can easily easily be be accounted accounted for for with with proper proper design design of theof the gain gain section. section.

Figure 13. ((a(a) )Schematic Schematic of of laser laser with with high- high-Q ringringQ ring insideinside inside thethe cavity; thecavity; cavity; ((b) Ring (b) Ring 1 and 1 2 and provide 2 provide for widefor wide-tunabilitytunability due to due Vernier to Vernier effect, effect, and filter-out and ﬁlter-out a single a single resonance resonance of ring of 3. ring 3.

The third ringring hashas aa substantiallysubstantially differentdifferent circumferencecircumference (>10(>10×× largerlarger than than the the Vernier rings used to realize the SWF SWF in in our our study), study), as as shown shown in in Figure Figure 13b. 13b. Rings Rings 1 1and and 2 2are are used used to to filter filter out out a asingle single resonance resonance of of ring ring 3 3whose whose spacing spacing is is equal equal to to 25 25 GHz. GHz. Ring Ring 3 3 is is used used to to filter filter out a single longitudinal modemode ofof thethe cavity, whosewhose spacingspacing isis aa functionfunction ofof thethe ringring Q and the detuning from ring resonances. Simulations show that single-wavelength operation is straightforward to achieve, as the cavity length enhancement that incr increaseseases with ring quality factor is compensated by the reduction of 3 dBdB linewidthlinewidth ofof the the resonator. resonator. Furthermore, Furthermore, the the phase phase of onlyof only one one cavity cavity has has to be to optimized be optimized for best for performance,best performance, which which simplifies simplifies laser operation.laser operation. There There are two are disadvantages two disadvantages of this of approach: this approach: first is thatfirst theis that theoretical the theoretical linewidth linewidth improvement improvemen is the smallestt is the between smallest the threebetween considered the three configurations, considered owingconfigurations, to the fact owing that theto the loaded fact Qthatis thethe lowest,loaded andQ is thethe secondlowest, is and a potential the second limitation is a potential due to non-linearlimitation due effects to non-linear at high intra-cavity effects at intensities.high intra-cavity The latter intensities. can be suppressedThe latter can to abe certain suppressed degree to by a utilizingcertain degree rib type by waveguides,utilizing rib type or using waveguides, SiN for waveguides. or using SiN for waveguides.

4.3. Control of Widely-Tunable Laser with High-Q Resonator A three ring resonator widely-tunable laser is a more complicated device to control than a standard widely-tunable laser, like the one shown in Figure2. For a standard design using the Vernier effect, either with Bragg gratings or ring resonators, generation of a look-up table (LUT), which will map the laser temperature and grating/ring resonator controls to the output wavelength, Appl. Sci. 2017, 7, 732 14 of 21 is required. The procedure usually involves an optical spectrum analyzer or a wavemeter to monitor wavelength,the output wavelength,as a function as of a Vernier function element of Vernier control element signals. control This signals. process Thisinevitably process takes inevitably time, astakes the number time, as of the points number for measur of pointsements for measurements tends to be relatively tends tolarg bee. relatively For tuning large. just the For two tuning Vernier just rings/gratings,the two Vernier the rings/gratings, result is a 2D the tuning result map is a 2Dthat tuning can also map have that canlaser also temperature have laser temperaturecorrections. Tocorrections. optimize Tothe optimize side-mode the suppression-ratio side-mode suppression-ratio (SMSR), a third (SMSR), variable, a third control variable, of controlthe phase of the section, phase shouldsection, be should introduced. be introduced. This results This results in 3D intuning 3D tuning maps maps and and significantly signiﬁcantly slower slower LUT LUT generation. generation. TheThe high- high-QQ laserlaser increases increases the the complexity complexity by by one one more more dimension, dimension, resulting resulting in in a a 4D 4D tuning tuning map map if if brute-forcebrute-force generation generation is is attempted. attempted. The The problem problem is further is further complicated complicated by narrow by narrow resonances resonances of the of high-the high-Q resonator,Q resonator, resulting resulting in an in anincrease increase in ingranularity granularity of ofthe the control control signals. signals. While While all all this this is is theoreticallytheoretically possible, possible, a a simpler simpler and and faster faster laser laser control control mechanism mechanism is is possible, possible, provided provided that that the the laser laser includesincludes on-chip on-chip monitor monitor photodetec photodetectors.tors. In In the the case case of of a a fully-integrated fully-integrated laser laser design, design, the the monitor monitor photodetectorsphotodetectors are are usually usually straightforward straightforward to to realize, realize, with with no no extra extra processing processing steps, steps, which which highlights highlights thethe benefits beneﬁts of of integrated integrated designs. designs. WeWe have have designed designed and and fabricated fabricated a a laser laser using using th thee architecture architecture shown shown in in Figure Figure 13a,13a, but but have have includedincluded two two monitor monitor photodetectors photodetectors to to control control the the laser laser operation, operation, as as shown shown in in Figure Figure 14a 14a [39]. [39]. AA photograph photograph of of the the fabricated fabricated chip chip on on a a temper temperatureature controlled controlled stage is shown in Figure 14b.14b.

Figure 14. (a) Schematic of laser with high-Q ring inside the cavity that includes two monitor Figure 14. (a) Schematic of laser with high-Q ring inside the cavity that includes two monitor photodetectorsphotodetectors and and a a booster booster SOA SOA (BSOA); (BSOA); ( (bb)) A A probed probed laser laser bar bar on on a a stage stage comprising comprising of of a a number number 2 ofof laser laser structures. structures. A A single single device device with with high- high-QQ ringring inside inside the the cavity cavity occupies occupies an an area area of of ~4 ~4 mm mm2. .

By use of on-chip monitor photodetectors, it is straightforward to control both the high-Q By use of on-chip monitor photodetectors, it is straightforward to control both the high-Q resonator, as well as cavity phase. In Figure 15, we plot the currents from two monitor photodetectors resonator, as well as cavity phase. In Figure 15, we plot the currents from two monitor photodetectors when the laser is biased. From the periodicity, we can easily determine the tuning efficiencies; in this when the laser is biased. From the periodicity, we can easily determine the tuning efﬁciencies; in this case ~14 mW/π for phase section, and ~26 mW/π for the high-Q ring. Also, it is straightforward to case ~14 mW/π for phase section, and ~26 mW/π for the high-Q ring. Also, it is straightforward to position both elements at a particular position, without use of any external elements. An illustrative position both elements at a particular position, without use of any external elements. An illustrative alignment strategy would be to position the high-Q ring and phase to maximize the output power alignment strategy would be to position the high-Q ring and phase to maximize the output power (output monitor photocurrent), and then use the high-Q monitor to control and detune the high-Q (output monitor photocurrent), and then use the high-Q monitor to control and detune the high-Q ring ring for maximum linewidth reduction factor, as illustrated in Section 2. It is interesting to point out for maximum linewidth reduction factor, as illustrated in Section2. It is interesting to point out that that the position of maximum output power in Figure 15a corresponds to a local minimum of the the position of maximum output power in Figure 15a corresponds to a local minimum of the high-Q high-Q ring monitor photodetector shown in Figure 15b. In other words, maximum output power is ring monitor photodetector shown in Figure 15b. In other words, maximum output power is obtained obtained when cavity losses are minimized, which is when all three rings are aligned. The power at when cavity losses are minimized, which is when all three rings are aligned. The power at the high-Q the high-Q ring monitor photodetector is locally minimized due to the extinction ratio of the high-Q ring monitor photodetector is locally minimized due to the extinction ratio of the high-Q ring at the ring at the pass port. Ideally, if the high-Q ring was critically coupled, that power would be zero, but pass port. Ideally, if the high-Q ring was critically coupled, that power would be zero, but the ring is the ring is intentionally over-coupled to reduce the insertion loss and consequently cavity loss. intentionally over-coupled to reduce the insertion loss and consequently cavity loss. Appl. Sci. 2017, 7, 732 15 of 21

Appl. Sci. 2017, 7, 732 15 of 21 Appl. Sci. 2017, 7, 732 15 of 21

Figure 15. Monitor photodetector current for the (a) output MPD; and (b) high-Q ring MPD as a function of heater power delivered to phase section and high-Q ring. The blue rings show the region of maximum output power in (a) and identical position for the high-Q monitor that is in the local minimum (b). FigureFigure 15.15. MonitorMonitor photodetectorphotodetector currentcurrent forfor thethe ((aa)) outputoutput MPD;MPD; andand ((bb)) high-high-QQ ringring MPDMPD asas aa function of heater power delivered to phase section and high-Q ring. The blue rings show the region Byfunction using of on-chip heater powerphotodetectors, delivered to fast phase alignment section and of the high- high-Q ring.Q ring The and blue phase rings show section the is region possible, of maximum output power in (a) and identical position for the high-Q monitor that is in the local makingof maximum control of output this laser power very in ( asimilar) and identical in complexity position forto typical the high- laserQ monitor designs that utilizing is in the only local two minimumminimum ( b(b).). Vernier rings. The laser bias tuning and wavelength tuning are both very predictable. Figure 16a showsBy the using output on-chip spectra photodetectors, as we tune one fast of alignmentthe rings, while of the optimizinghigh-Q ring the and phase phase and section high- isQ possible, heaters By using on-chip photodetectors, fast alignment of the high-Q ring and phase section is possible, formaking maximum control current of this at thelaser output very monitorsimilar in photo complexitydetector. to The typical side-mode laser designssuppression-ratio utilizing only (SMSR) two making control of this laser very similar in complexity to typical laser designs utilizing only two acrossVernier 39 rings.nm of Thetuning laser range bias in tuning steps correspondingand waveleng thto tuningthe FSR are of oneboth of very the Vernierpredictable. rings Figure is greater 16a Vernier rings. The laser bias tuning and wavelength tuning are both very predictable. Figure 16a thanshows 40 dB,the outputwith best spectra values as exceeding we tune one 48 dB. of th Bye rings,tuning while both rings,optimizing it is possible the phase to hit and any high- wavelengthQ heaters shows the output spectra as we tune one of the rings, while optimizing the phase and high-Q heaters insidefor maximum the tuning current range at of the the output laser. The monitor same photo optimization,detector. andThe maximizationside-mode suppression-ratio of the output monitor (SMSR) for maximum current at the output monitor photodetector. The side-mode suppression-ratio (SMSR) photodetectoracross 39 nm ofcurrent, tuning results range in stepsoptimal corresponding laser relative to intensity the FSR noiseof one (RIN) of the performance, Vernier rings as is showngreater across 39 nm of tuning range in steps corresponding to the FSR of one of the Vernier rings is greater inthan Figure 40 dB, 16b. with Here best we values plot exceedinglaser RIN 48as dB. we By sweep tuning the both high- rings,Q ring it is heaterpossible and to hitthe any measurement wavelength than 40 dB, with best values exceeding 48 dB. By tuning both rings, it is possible to hit any wavelength clearlyinside theshows tuning direct range correlation of the laser. between The same laser optimization, RIN and outputand maximization power current. of the When output latter monitor is inside the tuning range of the laser. The same optimization, and maximization of the output monitor maximized,photodetector RIN current, is optimized. results This in optimal once again laser shows relative the intensity benefits noiseof using (RIN) on-chip performance, detectors asfor shown laser photodetector current, results in optimal laser relative intensity noise (RIN) performance, as shown in control.in Figure For 16b. this Herereason, we we plot expect laser that RIN as as the we comp sweeplexity the of high- photonicQ ring integrated heater and chips the increases, measurement they Figure 16b. Here we plot laser RIN as we sweep the high-Q ring heater and the measurement clearly willclearly include shows a larger direct number correlation of mo nitorbetween photodetectors laser RIN forand control. output power current. When latter is shows direct correlation between laser RIN and output power current. When latter is maximized, maximized,For proper RIN linewidth is optimized. measurements, This once againlaser packagingshows the isbenefits required of usingto suppress on-chip sources detectors of technical for laser RIN is optimized. This once again shows the beneﬁts of using on-chip detectors for laser control. noise,control. such For asthis air reason, drift overwe expect the laser that as(typically the comp wavelengthlexity of photonic changes integrated in 10 MHz/mK) chips increases, as well they as For this reason, we expect that as the complexity of photonic integrated chips increases, they will electricalwill include noise a largersources number coming of from monitor long photodetectors unshielded wires for control.of the DC probe card. The packaging is include a larger number of monitor photodetectors for control. currentlyFor properin development, linewidth andmeasurements, linewidth measuremen laser packagingts will is requiredbe presented to suppress in a future sources publication. of technical noise, such as air drift over the laser (typically wavelength changes in 10 MHz/mK) as well as electrical noise sources coming from long unshielded wires of the DC probe card. The packaging is currently in development, and linewidth measurements will be presented in a future publication.

Figure 16. (a) Optical spectra as one of the Vernier rings is tuned and high-Q ring and phase are Figure 16. (a) Optical spectra as one of the Vernier rings is tuned and high-Q ring and phase are adjusted for maximum current at the output monitor photodetector; (b) Correlation between laser adjusted for maximum current at the output monitor photodetector; (b) Correlation between laser relative intensity noise (RIN) measured at 0.55, 2.25 and 11.25 GHz, and monitor photodetector relative intensity noise (RIN) measured at 0.55, 2.25 and 11.25 GHz, and monitor photodetector currents, currents, as high-Q ring is swept. RIN is optimized when output power is maximized. asFigure high- Q16.ring (a) isOptical swept. spectra RIN is optimizedas one of the when Vernier output rings power is istuned maximized. and high-Q ring and phase are adjusted for maximum current at the output monitor photodetector; (b) Correlation between laser relative intensity noise (RIN) measured at 0.55, 2.25 and 11.25 GHz, and monitor photodetector currents, as high-Q ring is swept. RIN is optimized when output power is maximized. Appl. Sci. 2017, 7, 732 16 of 21

For proper linewidth measurements, laser packaging is required to suppress sources of technical noise, such as air drift over the laser (typically wavelength changes in 10 MHz/mK) as well as electrical noise sources coming from long unshielded wires of the DC probe card. The packaging is currently in development, and linewidth measurements will be presented in a future publication.

5. A Performance Review of Widely-Tunable Semiconductor Lasers Chip-scale widely-tunable semiconductor lasers have been developed on monolithic III/V, hybrid (where III/V gain region is butt-coupled to Si or SiN chip providing single mode filtering and tuning), and heterogeneous (where III/V material is bonded to Si) platforms. To cover a broad tuning range and provide single longitudinal mode lasing, a Vernier effect is typically used, either by using sampled grating distributed Bragg reflectors (SG-DBR), or micro-ring resonator (MRR) structures. An alternative approach for wide tuning utilizes an array of distributed-feedback (DFB) lasers with different design central wavelengths, where each laser covers a few nm of tuning. The output of this array is typically coupled to a single waveguide and amplified by an SOA, to compensate coupling losses and increase output power. Most tunable lasers generally utilize a booster SOA, which serves at least two functions. First is the amplification and control of output power without the need to control the laser bias current that could influence tuning. Second is the ability to quickly switch-off the laser output when the wavelength is tuned, which can be very important in, e.g., live WDM networks, to prevent crosstalk or disruption of neighboring channels. An additional benefit of using a booster SOA is the lower power inside the laser cavity, which reduces the nonlinear effects that can negatively impact laser performance, such as for example, two-photon absorption. The tuning range basically depends on the net gain bandwidth of gain material, and the design of the single wavelength filter (SWF). For designs having arrays of lasers, the tuning range is determined by the net gain bandwidth, number of parallel laser diodes, and the tuning range of each diode, which is usually designed in such way that there are no blind spots inside the tuning range. Our original idea was to compare tunable laser performance in terms of output power, tuning range, and linewidth, across years, platforms, and tuning mechanisms, but as tuning range and output powers did not show any clear trends, we decided to plot only the linewidth performance. This actually makes sense, as tuning range is mostly determined by the gain medium, if tuning filters are properly designed, while output power can easily be increased by optimizing booster SOA. Laser linewidth is extremely important for a number of applications, such as coherent communications, LIDAR, other types of optical sensing, etc. It has been extensively researched, and there has been a growing need for chip-scale lasers with narrow linewidth. In Figure 17, we compare the linewidth of widely-tunable lasers [23,40–58], and a few clear trends can be noted. First is that the linewidth has steadily been improving from MHz range to kHz range in the past 20 years, and the second is that ring based tunable lasers (MRR) offer superior linewidth performance, as shown with circles. This is to be expected, keeping in mind the effective cavity length enhancement and negative optical feedback effects introduced in Section2. We should point out that some numbers in the plot correspond to integrated linewidth as measured with the delayed self-heterodyne technique, while some correspond to the white noise limit or so-called instantaneous linewidth determined from phase noise measurements, which are not directly comparable. Nevertheless, the trends are clear. Next, we give descriptions of some representative laser designs. SG-DBR type designs are widely studied and used in monolithic III/V tunable lasers, where the wider tuning range filter is provided by the product of the two differently spaced and independently tuned reflection combs of the SG-DBRs at each end of the cavity. Wide tuning can be realized by carrier injection or thermal effect, both of which change the effective index of refraction, but do so at different speeds and different excess losses. The early SG-DBR lasers had linewidths in the MHz range, as the injection of carriers resulted in increased cavity loss, and consequently wider linewidth. For different lasing wavelengths, due to different carrier concentrations, the linewidth varies between Appl. Sci. 2017, 7, 732 17 of 21 several 100s kHz to several MHz, as showed by the error bar in Figure 17[ 40,41]. It should be noted that such lasers can be tuned extremely rapidly which can be beneficial for some application. Thermal tuning does not suffer from tuning induced loss, and has previously enabled linewidths as low as 400 kHz in a super-structure grating (SSG) DBR laser using on-chip microheaters [42]. Thermal tuning is generally much slower, in the 10s of kHz range. The SG-DBR lasers’ performance can further be Appl. Sci. 2017, 7, 732 17 of 21 optimized by thermal engineering and careful design of sampled gratings, so (instantaneous) laser linewidths down to 50–70 kHz were demonstrated, with +17 dBm of fiber fiber coupled power, and SMSR >50 dB over 41 nm of tuning range in C-band [[43].43].

Figure 17. Widely-tunable integrated lasers linewidth versus time. We make a distinction between Figure 17. Widely-tunable integrated lasers linewidth versus time. We make a distinction between III/V based monolithic lasers, hybrid lasers where III/V gain chips are butt coupled to Si or SiN chips, III/V based monolithic lasers, hybrid lasers where III/V gain chips are butt coupled to Si or SiN chips, and heterogeneous integrated lasers where the III/V gain material is bonded to silicon. (sampled and heterogeneous integrated lasers where the III/V gain material is bonded to silicon. (sampled grating distributed bragg reflector (SG-DBR) lasers, distributed feedback (DFB) lasers and micro-ring grating distributed bragg reflector (SG-DBR) lasers, distributed feedback (DFB) lasers and micro-ring resonator (MRR) lasers). resonator (MRR) lasers). A DFB laser array is another technique for widely tunable lasers commonly realized in monolithicA DFB III/V laser arrayplatform. is another DFB lasers technique are simpler for widely than tunable SG-DBR lasers type commonly lasers, and realized narrow in monolithiclinewidth III/V platform. DFB lasers are simpler than SG-DBR type lasers, and narrow linewidth performance performance can be achieved by increasing cavity length LDFB while maintaining the optimum κ·LDFB canproduct, be achieved where κ by is the increasing grating cavitycoupler length coefficientLDFB [44].while A maintainingnarrow (integrated) the optimum linewidthκ·LDFB in theproduct, range wherefrom 88κ kHzis the to grating160 kHz coupler was achieved coefficient by a [tunable44]. A narrowDFB laser (integrated) array with linewidtha long cavity in the(1500 range µm) fromDFB 88structure kHz to [45]. 160 kHzBy integrating was achieved a booster by a tunable SOA, waveguide DFB laser arraycoupler, with and a long12 DFB cavity lasers (1500 whichµm) were DFB structuretemperature [45 ].controlled By integrating in the range a booster from 15 SOA, to 50 waveguide °C, laser tuning coupler, range and cover 12 DFBed 40 lasers nm in which the L-band, were ◦ temperaturewith a fiber output controlled power in of the 13 dBm, range and from a high 15 toSMSR 50 ofC, >50 laser dB. tuning The heterogeneous range covered silicon 40 nmphotonics in the L-band,platform with opens a fiberup a outputnew possibility power of in 13 improving dBm, and the a high coherence SMSR of of se >50miconductor dB. The heterogeneous lasers, by providing silicon photonicsa mechanism platform to separate opens the up photon a new possibilityresonator and in improving highly-absorbing the coherence active medium of semiconductor [59]. Individual lasers, byDFB providing lasers with a mechanism sub-kHz toinstantaneous separate the photon linewidt resonatorhs were and demonstrated highly-absorbing [60],active and mediumit would [ 59be]. Individualrelatively straightforward DFB lasers with to sub-kHz integrate instantaneous them in a linewidthsDFB array wereto form demonstrated a widely-tunable [60], and chip-scale it would besemiconductor relatively straightforward laser. to integrate them in a DFB array to form a widely-tunable chip-scale semiconductorExtremely laser.impressive results were achieved by MRR based hybrid lasers, where reflective semiconductorExtremely optical impressive amplifiers results (RSOAs) were achieved are butt-co byupled MRR to based external hybrid waveguide lasers, circuits where reflectivewith ring semiconductorresonators forming optical the amplifiersSWF. The (RSOAs)SWF can arebe realized butt-coupled in various to external passive waveguide waveguide circuits platforms, with such ring resonatorsas Si and formingSiN. The the RSOA SWF. and The SWFthe SWG can be can realized be fabricated in various separately, passive waveguide and hand-picked platforms, for such best as Siperformance. and SiN. The With RSOA Si and based the SWGSWF, can over be fabricated20 dBm separately,fiber coupled and hand-pickedoutput power, for bestand performance.(integrated) Withlinewidth Si based narrower SWF, overthan 2015 dBmkHz along fiber coupled the whole output C–band, power, was and achieved (integrated) [50]. The linewidth authors narrower point at thanleast 15two kHz reasons along for the such whole an impressive C–band, was performance: achieved [50 the]. Theoptimized authors Si-wire point waveguides at least two reasonswith losses for suchlower an than impressive 0.5 dB/cm performance: in C–band, the and optimized the passive Si-wire alignment waveguides technology with losses for lower precisely than 0.5mounting dB/cm ina Si-waveguide C–band, and the tunable passive filter, alignment gain technologychip, and forbooster precisely SOA mounting on a common a Si-waveguide platform tunable on silicon, filter, with minimum excess loss. A similar structure, but using the SWF made in SiN, has shown a record low (instantaneous) linewidth of 290 Hz, thanks to the 0.1 dB/cm low-loss Si3N4 waveguide, and the use of a three ring structure inside the cavity, as analyzed in Section 4.2 [51]. The effective on-chip cavity length approaches 0.5 m, resulting in extremely narrow instantaneous linewidth. Tuning range is equal to about 81 nm, with maximum fiber-coupled output power of 11 dBm. Heterogeneous approaches, where III/V and Si are bonded prior to processing of III/V, offer an advantage of manufacturability. Such approaches are naturally more scalable if modified complementary metal oxide semiconductor (CMOS) processes are used, and such lasers can be integrated with many Appl. Sci. 2017, 7, 732 18 of 21 gain chip, and booster SOA on a common platform on silicon, with minimum excess loss. A similar structure, but using the SWF made in SiN, has shown a record low (instantaneous) linewidth of 290 Hz, thanks to the 0.1 dB/cm low-loss Si3N4 waveguide, and the use of a three ring structure inside the cavity, as analyzed in Section 4.2[ 51]. The effective on-chip cavity length approaches 0.5 m, resulting in extremely narrow instantaneous linewidth. Tuning range is equal to about 81 nm, with maximum fiber-coupled output power of 11 dBm. Heterogeneous approaches, where III/V and Si are bonded prior to processing of III/V, offer an advantage of manufacturability. Such approaches are naturally more scalable if modified complementary metal oxide semiconductor (CMOS) processes are used, and such lasers can be integrated with many other components on a single chip to realize advanced photonic integrated circuits, e.g., fully-integrated beam steerers [2], microwave signal generators, and tracking generators [47]. A laser providing 57 nm of tuning, >45 dB SMSR, and a (integrated) linewidth below 100 kHz in full tuning range, with best results down to 50 kHz and 10 dBm output power (without booster SOA), has been demonstrated [49].

6. Conclusions We have presented an extensive overview of chip-scale widely-tunable lasers that use ring resonators for tuning and filtering out a single longitudinal mode. Wide-tunability is commonly achieved by utilizing the Vernier effect, where two or more rings have slightly different radii. The use of the Vernier effect results in increased tuning efficiency. We analyze and measure four different laser cavity architectures having rings with two different radii, and point out the advantages and disadvantages of each approach. The use of ring resonators, provided that the utilized waveguide platform offers sufficiently low propagation losses (which both Si and SiN waveguides do, with losses in dB/cm or sub-dB/cm range), has an advantage, as the effective laser cavity length at ring resonance is significantly enhanced, directly influencing linewidth. We show approaches for further reducing the linewidth, either by using external on-chip delay, or by the introduction of a third ring resonator with high-Q factor. Introduction of the third ring increases the number of controls in the laser, but we show that by using on-chip monitor photodetectors, such lasers are straightforward to control and tune. Lastly, we have compared the linewidth performance of all chip-scale widely-tunable lasers, and have shown that the ring based tunable lasers offer superior linewidth performance with integrated linewidths approaching few kHz and instantaneous linewidths in the sub-kHz range, while at the same time providing wide tuning (40+ nm), and high output power (up to 20 dBm). A fully-integrated widely-tunable ring-resonator based laser realized in heterogeneous silicon process, is a perfect candidate to be used in future complex photonic integrated circuits for communications and sensing.

Acknowledgments: This work was supported by a DARPA EPHI and DODOS contracts. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing official policies of DARPA or the U.S. Government. Tin Komljenovic was in part supported by NEWFELPRO Grant 25. Linjun Liang acknowledges the support from the Fundamental Research Funds for the Central Universities (No. 2016YJS030) and China Scholarship Council. The authors would like to thank Daryl T. Spencer, Erik Norberg and Gregory A. Fish for helpful discussions. Author Contributions: Tin Komljenovic, Linjun Liang and Rui-Lin Chao performed the experiments; Tin Komljenovic, Linjun Liang and Sudharsanan Srinivasan analyzed the data; Tin Komljenovic, Jared Hulme, Sudharsanan Srinivasan and Michael Davenport designed the devices, Jared Hulme and Michael Davenport fabricated the devices, Linjun Liang analyzed the state of the art; John E. Bowers conceived the original idea; Tin Komljenovic and Linjun Liang wrote the manuscript. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results. Appl. Sci. 2017, 7, 732 19 of 21

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