Ultra-Low-Loss Silicon Waveguides for Heterogeneously Integrated Silicon/III-V Photonics

applied sciences

Article Ultra-Low-Loss Silicon Waveguides for Heterogeneously Integrated Silicon/III-V Photonics

Minh A. Tran *, Duanni Huang, Tin Komljenovic, Jonathan Peters, Aditya Malik and John E. Bowers ID

Department of Electrical and Computer Engineering, University of California at Santa Barbara, Santa Barbara, CA 93106, USA; [email protected] (D.H.); [email protected] (T.K.); [email protected] (J.P.); [email protected] (A.M.); [email protected] (J.E.B.) * Correspondence: [email protected]; Tel.: +1-805-893-2149

Received: 16 June 2018; Accepted: 9 July 2018; Published: 13 July 2018

Featured Application: Ultra-low-loss Si waveguide platform for heterogeneous integration with III/V.

Abstract: Integrated ultra-low-loss waveguides are highly desired for integrated photonics to enable applications that require long delay lines, high-Q resonators, narrow filters, etc. Here, we present an ultra-low-loss silicon waveguide on 500 nm thick Silicon-On-Insulator (SOI) platform. Meter-scale delay lines, million-Q resonators and tens of picometer bandwidth grating filters are experimentally demonstrated. We design a low-loss low-reflection taper to seamlessly integrate the ultra-low-loss waveguide with standard heterogeneous Si/III-V integrated photonics platform to allow realization of high-performance photonic devices such as ultra-low-noise lasers and optical gyroscopes.

Keywords: ultra-low-loss waveguide; silicon photonics; heterogeneous integration; narrow linewidth lasers; high Q resonators

1. Introduction Photonic integration promises significant improvement in performance, reliability and SWaP-C (size, weight, power and cost) due to integration of components, reduced coupling losses and reduced sensitivity to environmental perturbation. Photonic integrated circuits (PICs) have been demonstrated in various material systems such as GaAs, InP, Si, Si3N4, LiNbO3 with various levels of functionality [1–6]. An ideal platform would combine the efficiency of electrically pumped optical sources available in GaAs and InP systems, with high-contrast superior waveguide structures available in Si and ultra-low-loss waveguides available in e.g., Si3N4 platform. Heterogeneous Si/III-V photonics [5,7] is an attractive platform for photonic integration as it combines the best of what silicon and III-V platforms can offer and provide many kinds of high-performance passive and active optical devices on a single chip. High-capacity, large-scale PICs for communications and sensing applications have been successfully demonstrated [8–11]. The high index contrast waveguide formed between silicon and its native oxide (SiO2) allows for small waveguides and tight bends enabling very compact photonic devices with small footprint, which is desirable for electronics-photonics integration [12]. However, the high index contrast also results in more scattering loss, especially in waveguide with small cross-sectional mode area. For the best performances of optical amplifiers and lasers on the Si/III-V platform, the silicon thickness is optimally chosen in the range of 400–500 nm as discussed in detail in [13,14] and Section2 below. With SOI thickness in the sub-micron range, typical propagation loss of single-mode Si waveguides ranges from 0.5 dB/cm to 2 dB/cm [15–17], and the lower number requires best-in-class immersion

Appl. Sci. 2018, 8, 1139; doi:10.3390/app8071139 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 1139 2 of 13

193 nm lithography. This limits the performance of devices requiring on-chip delays larger than a few centimeters. When it comes to applications that demand very low propagation loss, such as optical buffer technology [18] or interferometric waveguide coil optical gyroscopes [19], multi-micron scale SOI [20] and dielectric platforms with lower index contrast [21–24] are superior. Davenport et al. has demonstrated the possibility of incorporating the silicon, III-V, and ultra-low loss Si3N4 waveguides on the same chip [25]. The downside of this approach, however, is the increased complexity of the fabrication, which involves an extra bonding step and planarization. It is advantageous to follow a simpler path to ultra-low-loss waveguides that can provide higher yield and lower cost, while maintaining compatibility with III-V heterogeneous integration. The most straightforward approach to achieve this goal is to make lower loss waveguides directly on silicon using the rib waveguide geometry as opposed to a deeply etched geometry. It has been shown in the literature that a propagation loss as low as 2.7 dB/m is achievable with a 200 nm rib on 1 µm thick silicon waveguide [26], and 15 dB/m loss can be obtained with half-etch rib waveguides on 3 µm thick SOI platform [20]. In this work, we apply the shallow rib waveguide idea to achieve ultra-low propagation loss on the 500 nm SOI waveguides of the Si/III-V heterogeneous platform. We report on a propagation loss as low as 4 dB/m that is seamlessly incorporable into the heterogeneous Si/III-V platform with only one additional lithography and etch step. We utilize these ultra-low loss silicon waveguides to successfully make long optical spiral delay lines, high quality-factor (Q) ring resonators and low kappa (κ), narrow bandwidth Bragg grating reﬂectors with record performance for sub-micron SOI waveguides. These passive components pave the way to realizing advanced photonic devices, e.g., optical gyroscopes and ultra-narrow linewidth lasers, fully integrated on a silicon chip.

2. Ultra-Low Loss Waveguides for Si/III-V Heterogeneous Integration To enable a full design space for lasers and amplifiers in Si/III-V heterogeneous platforms, control of the optical mode confinement in the gain (III-V) layer and passive (silicon) layer should be obtainable by engineering the waveguide widths of the III-V and silicon waveguides. In principle, as described in [27,28], the ratio between effective indices of the individual III-V guide and that of the silicon guide should be tailorable from above unity (regime a.1 in Figure1a) to below unity (regime a.3 in Figure1a). To satisfy that requirement, the thickness of the SOI waveguides need to be optimized in accordance to the III-V stacks. Since the current is injected vertically through the III-V active region, the III-V stack is typically about 2 µm thick to separate the contact metal sufficiently far from the optical mode. As shown in Figure1b, a thickness of 500 nm for the silicon layer results in a good index match with the III-V layer, allowing for a great flexibility in designing the mode hybridization. Other considered thicknesses, such as the widely utilized 220 nm SOI waveguides [2] or multi-micron thick SOI waveguides [6], do not suit well because their indices are fairly far off from that of the III-V. The heterogeneous Si/III-V photonic design kit (PDK) developed at University of California at Santa Barbara (UCSB) for 1550 nm wavelength in recent years has been built and optimized for 500 nm thick silicon on 1 µm buried oxide on silicon substrates. The ultra-low loss (ULL) waveguides in our work here, therefore, is also developed on the same SOI platform. Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 13 Appl. Sci. 2018, 8, 1139 3 of 13 Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 13

Figure 1. (a) Optical mode profiles of the heterogeneous (Si/III-V) waveguide with 500 nm thick > silicon in 3 different operating regimes: (a.1) nnmv− si with a slab III-V on 0.7 µm wide silicon FigureFigure 1. 1.(a )(a Optical) Optical mode mode profiles profiles of the of heterogeneous the heterogene (Si/III-V)ous (Si/III-V) waveguide waveguide with 500 with nm≈ 500 thick nm silicon thick waveguide, which results in higher confinement in the active III-V layer. (a.2) nnmv− si with a in 3 different operating regimes: (a.1) nm−v > nsi with> a slab III-V on 0.7 µm wide silicon waveguide, silicon in 3 different operating regimes: (a.1) nnmv− si with a slab III-V on 0.7 µm wide silicon slab III-V on 1.3 µm wide silicon waveguide, which results in equal confinement in the III-V and which results in higher confinement in the active III-V layer. (a.2) nm−v ≈ nsi with a slab III-V≈ on 1.3 µm waveguide, which< results in higher confinement in the active III-V layer. (a.2) nnmv− si with a silicon.wide silicon (a.3) nn waveguide,mv− si withwhich 0.5 results µm wide in equal III-V confinementon 1.3 µm wide inthe silicon III-V waveguide, and silicon. which (a.3) n resultsm−v < innsi slab III-V on 1.3 µm wide silicon waveguide, which results in equal confinement in the III-V and higherwith 0.5confinementµm wide III-Vin silicon. on 1.3 (bµ)m Effective wide silicon indices waveguide, of strip silicon which waveguides results in higher of three confinement thicknesses in < µ (220silicon.silicon. nm, (b500 ()a.3 Effective )nm nn mvand− indices 3 µm) si ofwith with strip 0.5 varying silicon µm wide waveguides width III-V is on plotted 1.3 of threeµm together wide thicknesses silicon with waveguide, (220 the nm,effective 500 which nm index and results 3of m)a in µ typicalwithhigher varying 2 µm confinement thick width III-V is plotted inwaveguide silicon. together (b (used) Effective with in [29]). the indices effective Noted of that indexstrip all ofsilicon simulations a typical waveguides 2 werem thick carried of III-Vthree out waveguide thicknesses at 1550 nm(used(220 wavelength. innm, [29 500]). Noted nm and that 3 all µm) simulations with varying were width carried is out plotted at 1550 together nm wavelength. with the effective index of a typical 2 µm thick III-V waveguide (used in [29]). Noted that all simulations were carried out at 1550 2.1.2.1. Waveguide Waveguidenm wavelength. Geometry Geometry TheThe ULL ULL waveguide waveguide cross-sectional cross-sectional geometry geometry is is shown shown in in Figure Figure 2a.2a. To To lower lower the the optical optical mode mode 2.1. Waveguide Geometry interactioninteraction with with the the rib rib sidewall, sidewall, the the waveguide waveguide rib rib he heightight is is targeted targeted to to be be about about 10% 10% of of the the total total Si Si layerlayer thickness. thickness.The ULL An waveguide An etch etch depth depth cross-sectional of of 56 56 nm nm was was geometry chosen chosen because is because shown it it incan can Figure be be accurately accurately 2a. To lower controlled controlled the optical using using mode a a laser-basedlaser-basedinteraction interferometricinterferometric with the rib sidewall, etch etch depth thedepth monitorwaveguide monitor (Intellemetrics rib (Intellemetricsheight is LEP500) targeted LEP500) system, to be about whichsystem, 10% is commercially ofwhich the total is Si commerciallyavailable.layer thickness. Thermal available. An oxide etch Thermal serves depth as ofoxide the 56 bottomnm serves was buried chosenas the oxide becausebottom (BOX) itburied can while be oxidethe accurately top (BOX) cladding controlled while is depositedthe using top a claddingoxide,laser-based either is deposited byinterferometric plasma-enhanced oxide, ei theretch chemical bydepth plasma-enhanced vapormonitor deposition (Intellemetrics chemical (PECVD) vapor orLEP500) sputtering.deposition system, Each(PECVD) claddingwhich or is sputtering.layercommercially is 1 µ Eachm thick claddingavailable. to minimize layer Thermal substrateis 1 µm oxide thick leakage serves to minimize or as loss the due substratebottom to the metalburied leakage heaters, oxide or loss which(BOX) due aretowhile the commonly metalthe top heaters,putcladding on thewhich topis depositedare of thecommonly waveguide oxide, put ei resonators.onther the by top plasma-enhanced of The the effective waveguide index chemical resonators. of the transversevapor The depositioneffective electric index (TE)(PECVD) of modes the or transverseversussputtering. the electric waveguide Each (TE) cladding widthmodes layer is ve plottedrsus is 1 theµm in Figurewaveguidethick to2b, minimize suggesting width issubstrate thatplotte thed leakagein waveguide Figure or 2b, loss guides suggesting due a to single the that metal TE themodeheaters, waveguide when which the guides widthare commonly a issingle 1.8 µ mTE put or mode lower. on thewhen top the of thewidth waveguide is 1.8 µm resonators. or lower. The effective index of the transverse electric (TE) modes versus the waveguide width is plotted in Figure 2b, suggesting that the waveguide guides a single TE mode when the width is 1.8 µm or lower.

FigureFigure 2. 2. (a)( aCross-sectional) Cross-sectional geometry geometry of the of theultra-low ultra-low loss loss(ULL) (ULL) Si waveguides Si waveguides in this in work. this work.The thicknessThe thickness of the ofburied the buried oxide (BOX) oxide (BOX)and the and Si devi thece Si layer device are layer 1 µm are and 1 µ500m andnm, respectively. 500 nm, respectively. The 56 nmTheFigure tall 56 Si nm 2.rib tall(a is) SiCross-sectional formed rib is formed by dry-etch bygeometry dry-etching,ing, leavingof the leaving ultra-low a 444 a nm 444 loss nmthick (ULL) thick Si slab. Si Si slab. waveguides (b) ( bEffective) Effective in index this index work. versus versus The waveguidewaveguidethickness width of width the inburied in the the 56oxide 56 nm nm (BOX)rib rib Si Si andwaveguide. waveguide. the Si devi The Thece layerwaveguide waveguide are 1 µm is is quasi-singleand quasi-single 500 nm, moderespectively. mode within within Thethe the 56 yellowyellownm tallcolored colored Si rib region. region.is formed Inset: Inset: by Electric Electric dry-etch field ﬁelding, profile proﬁle leaving of of the thea 444fundamentalfundamental nm thick mode modeSi slab. in in 1.8 ( 1.8b )µm µEffectivem wide wide waveguide. waveguide.index versus waveguide width in the 56 nm rib Si waveguide. The waveguide is quasi-single mode within the yellow colored region. Inset: Electric field profile of the fundamental mode in 1.8 µm wide waveguide. Appl.Appl. Sci. Sci. 20182018, 8,,8 x, 1139FOR PEER REVIEW 4 4of of 13 13

2.2. Waveguide Loss Consideration 2.2. Waveguide Loss Consideration Due to the line-edge roughness associated with optical lithography, which is further aggravated Due to the line-edge roughness associated with optical lithography, which is further aggravated in dry-etching processes, the propagation loss of the planar waveguides is typically dominated by in dry-etching processes, the propagation loss of the planar waveguides is typically dominated by the the radiative loss induced by the interaction between the optical mode and the roughness on the radiative loss induced by the interaction between the optical mode and the roughness on the waveguide waveguide sidewalls. Here, we estimate the waveguide radiative loss using the nw model sidewalls. Here, we estimate the waveguide radiative loss using the n model approximation [30]. approximation [30]. This approximation is particularly useful if one onlyw needs to quantify the This approximation is particularly useful if one only needs to quantify the relative comparisons of relative comparisons of the propagation losses among different waveguide geometries under the the propagation losses among different waveguide geometries under the same fabrication conditions. same fabrication conditions. According to this model, waveguide loss can be approximated as: According to this model, waveguide loss can be approximated as: ∂n ασ= ()eff rceffALn ,, ∂ne f f (1) α = A σ, L , n ∂w (1) r c e f f ∂w where A is the proportionality factor determined by the sidewall roughness rms (σ) and the where A is the proportionality factor determined by the sidewall roughness rms (σ) and the roughness roughness correlation length ( Lc ), neff is the effective index of the waveguide optical mode, and w is correlation length (Lc), neff is the effective index of the waveguide optical mode, and w is the width theof thewidth waveguide. of the waveguide. A full expression A full ofexpressionA can be foundof A can in [ 31be]. found Given ain waveguide [31]. Given geometry a waveguide that is geometryfar from that mode is cut-offfar from condition, mode cut-off the magnitudecondition, the of themagnitude factor A ofin the general factor does A in not general depend does on not the dependwaveguide on the width. waveguide width. FigureFigure 33aa showsshows the the measured measured propagation propagation loss ofloss the of two the “more two conventional” “more conventional” silicon waveguide silicon waveguidegeometries geometries for the heterogeneous for the heterogeneous silicon platform silico usedn platform in previous used worksin previous [10,32 ,works33]. One [10,32,33]. is a fully One500 is nm a fully etched 500 strip nm etched waveguide, strip waveguide, and the other and is the 231 other nm etchedis 231 nm rib etched waveguide. rib waveguide. The widths The of widthsboth waveguides of both waveguides are 1 µm. are Empirical 1 µm. Empirical values of va thelues roughness of the roughness parameters parameters were obtained were (lineobtained edge

(lineroughness edge roughness rms σ ~5 rms nm andσ ~5correlation nm and correlation length Lc length~60 nm) Lc and ~60 then nm) Equation and then (1) Equation is used (1) to generateis used totheoretical generate curvetheoretical corresponding curve corresponding to the radiative to losses the atradiative varying etchlosses depth. at varying The theoretical etch depth. waveguide The theoreticalloss curves waveguide with varying loss etching curves depthswith varying (500 nm, etch 231ing nm depths and 56(500 nm) nm, are 231 then nm calculated and 56 nm) and are plotted then calculatedin Figure3 andb, showing plotted ain reasonable Figure 3b, agreement showing a with reasonable experimental agreement data with for the experimental 500 nm and data 231 nmfor etchthe 500depths. nm and According 231 nm etch to these depths. model According results, to low these propagation model results, loss low in the propagation range of 3–10 loss dB/min the range can be ofexpected 3–10 dB/m for can the be 56 expected nm rib silicon for the waveguides 56 nm rib sili withcon larger waveguides than 1.5 withµmwidth. larger than 1.5 µm width.

FigureFigure 3. 3. ((aa)) ExperimentalExperimental data data (circular (circular markers) markers) and theoreticaland theoretical curve (solidcurve line) (solid for theline) propagation for the propagationloss of 1 µm loss wide of waveguide1 µm wide atwaveguide varying etchat varying depth. etch (b) Theoreticaldepth. (b) Theoretical calculations calculations (solid lines) (solid of the lines)waveguide of the waveguide loss at varying loss waveguide at varying widthwaveguide at three width different at three etch different depth (500 etch nm, depth 231 nm (500 and nm, 56 231 nm). nmThe and experimentally 56 nm). The experimentally measured results measured are plotted results in circular are plotted markers. in circular markers.

3.3. Passive Passive Device Device Demonstrations Demonstrations

3.1.3.1. Long Long Waveguide Waveguide Delay-Line Delay-Line WeWe designed designed 52.2 52.2 cm cm long long spiral spiral delays delays (illustrated (illustrated in in Figure Figure 4a)4a) to to characterize characterize the the propagation propagation loss.loss. Waveguide Waveguide widths widths were were 1.8, 1.8, 3.0 3.0 and and 8.0 8.0 µmµm and and the the minimum minimum bend bend radius radius used used in in all all spirals spirals waswas 800 800 µm.µm. We We use use the the coherent coherent optical optical frequency frequency domain domain reflectometry reflectometry (OFDR) (OFDR) technique technique with with a commercially available system (Luna Inc. OBR 4400, Roanoke, VA, USA) [34] to quantify the Appl. Sci. 2018, 8, 1139 5 of 13 Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 13 apropagation commercially loss available of the fabricated system Si (Luna rib waveguides Inc. OBR 4400,. The technique Roanoke, has VA, been USA) proven [34] to to quantifycalculate thethe propagationpropagation lossloss ofwith the high fabricated accuracy Si rib independent waveguides. of Thethe techniquefiber-waveguide has been coupling proven loss to calculate uncertainty the propagation[21]. Figure loss4b shows with high the accuracydata measured independent for the of the52.2 fiber-waveguide cm long spiral couplingof 1.8 µm loss wide uncertainty ULL silicon [21]. Figurewaveguide.4b shows The the peaks data on measured the left forhand the side 52.2 cm(near long 0 m spiral distance) of 1.8 andµm wideright ULLhand silicon side (near waveguide. 0.5 m Thedistance) peaks are on thethe leftreflections hand side at (nearthe waveguide 0 m distance) face andts. rightThe handlinearity side of (near the 0.5trace m distance)verifies that are thethe reflectionswaveguide at loss the is waveguide not bend-loss facets. limited. The linearityThe measured of the tracemean verifies and deviation that the of waveguide spectral dependence loss is not bend-lossof the propagation limited. The loss measured of the three mean waveguide and deviation widths of are spectral plotted dependence in Figure of4c. the A single propagation mode 1.8 loss µm of thewide three waveguide waveguide achieves widths an are average plotted of in 16 Figure dB/m4 propagationc. A single mode loss 1.8overµ Cm + wide L band. waveguide The propagation achieves anloss average in multimode of 16 dB/m ULL propagation waveguide losswith over 3.0 µm C + Land band. 8.0 µm The widths propagation are lower, loss in at multimode 10 dB/m and ULL 4 waveguidedB/m, respectively. with 3.0 µm and 8.0 µm widths are lower, at 10 dB/m and 4 dB/m, respectively. TheThe measuredmeasured lossloss ofof thethe ULLULL waveguideswaveguides showshow aa relativelyrelatively largelarge discrepancydiscrepancy toto thethe valuesvalues predicted by the nw approximation method described in Section 2.2 (Figure 2b). The model had predicted by the nw approximation method described in Section 2.2 (Figure2b). The model had several simplificationsseveral simplifications such as ignoringsuch as ignoring the surface the roughnesssurface roughness of the etched of the slabetched surfaces. slab surfaces. For fully For etched fully etched (500 nm) or halfway etched (231 nm) waveguides, the optical field that interacts with the (500 nm) or halfway etched (231 nm) waveguides, the optical field that interacts with the etched surface etched surface is small enough to be neglected in the total loss calculation. However, with the very is small enough to be neglected in the total loss calculation. However, with the very shallow rib (56 nm) shallow rib (56 nm) waveguides, the optical field expands laterally (similar to a slab mode) and the waveguides, the optical field expands laterally (similar to a slab mode) and the interaction between interaction between the mode field and the etched surface cannot be ignored. The roughness rms of the mode field and the etched surface cannot be ignored. The roughness rms of the etched surface the etched surface is typically in order of several nanometers, which is more than one order of is typically in order of several nanometers, which is more than one order of magnitude larger than magnitude larger than the unetched waveguide top surface roughness (~0.1 nm). The scattering due the unetched waveguide top surface roughness (~0.1 nm). The scattering due to the etched surface to the etched surface roughness is significant and results in the discrepancy between the roughness is significant and results in the discrepancy between the experimental and theoretical experimental and theoretical waveguide loss. waveguide loss.

FigureFigure 4.4. ((aa)) MaskMask layoutlayout ofof spiralspiral delaydelay lineslines withwith 52.252.2 cmcm totaltotal lengthlength andand 850850 µµmm minimumminimum bendbend radiusradius ((bb)) OpticalOptical BackscatterBackscatter ReflectometryReflectometry (OBR)(OBR) datadata fromfrom thethe spiralspiral withwith 1.81.8 µµmm width.width. AA linearlinear fitfit ofof thethe waveguidewaveguide backscatterbackscatter isis shownshown withwith thethe dasheddashed redred line.line. TheThe propagationpropagation lossloss ofof thethe waveguidewaveguide cancan be be approximated approximated as as 1/2 1/2 of of the the slope slope of of the the fitted fitted line. line. (c) ( Wavelengthc) Wavelength dependence dependence of the of propagationthe propagation loss loss (mean (mean and and standard standard deviation) deviation) of waveguides of waveguides with with different different widths widths (1.8 µ(1.8m, µm, 3.0 µ 3.0m andµm 8.0andµ 8.0m). µm). Appl. Sci. 2018, 8, 1139 6 of 13 Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 13

3.2.3.2. High High Quality Quality Factor Factor Ring Ring Resonators Resonators OneOne key key component component made made possible possible by bylow-loss low-loss waveguides waveguides is the is high- the high-Q ringQ resonator.ring resonator. The ringThe resonator ring resonator is an isessential an essential building building block blockto realize to realize compact compact optical optical filters filtersand delays and delayson chip. on Higherchip. Higher Q-factorsQ-factors enable enable sharper, sharper, narrowband narrowband filters filters and longer and longer delays, delays, which which are requirements are requirements for thefor aforementioned the aforementioned ultra-long ultra-long laser laser cavity. cavity. There There has has been been a plethora a plethora of ofrecent recent work work to to demonstrate demonstrate high-high-QQ integratedintegrated resonators resonators using using different different material materialss and and cavity cavity geometries. geometries. These These results results include include fullyfully integrated integrated resonators withwith an an intrinsic intrinsicQ Qof of 22 22 million million in siliconin silicon [26], [26], 81 million 81 million in a silicon in a silicon nitride nitrideresonator resonator [35] and [35] over and 200over million 200 million in a silica in a sili resonatorca resonator [36]. [36]. However, However, the resultsthe results of the of latterthe latter two twoare multimoded,are multimoded, which which can leadcan lead to multiple to multiple sets ofsets resonances of resonances or excess or excess loss at loss the couplingat the coupling regions regionsunless specialunless carespecial is taken care tois optimizetaken to optimize the coupler the [37 coupler]. These [37]. three These results three all haveresults bend all radiihave >2 bend mm, radiiwhich >2 limits mm, the which compactness limits the ofthe compactness device, and complicatesof the device, the designand complicates of a Vernier-effect the design filter dueof a to Vernier-effectthe small free filter spectral due rangeto the ofsmall each free ring. spectral Finally, rang it ise unlikelyof each ring. that Finally, these resonators it is unlikely were that designed these resonatorswith laser integrationwere designed in mind, with and laser cannot integratio be integratedn in mind, with and the heterogeneouscannot be integrated silicon/III-V with laserthe heterogeneouswithout significant silicon/III-V extra processing laser without [25,38 significant]. extra processing [25,38]. InIn this this work, work, we we demonstrate demonstrate ring ring resonators resonators readily readily integratable integratable with with the the heterogenous heterogenous silicon/III-Vsilicon/III-V platform platform with with a aloaded loaded QQ ofof 2.1 2.1 million, million, and and an an intrinsic intrinsic QQ ofof 4.1 4.1 million. million. The The ring ring is is in in anan all-pass all-pass configuration, configuration, and and is is 1.8 1.8 microns microns in in width, width, which which is is single-mode single-mode for for the the TE TE polarization. polarization. TheThe ring ring is is connected connected to to a abus bus waveguide waveguide of of the the same same dimensions dimensions using using a astraight-to-curved straight-to-curved coupler coupler withwith a a 1.2 1.2 micron micron coupling coupling gap. gap. The The transmission transmission spectra spectra of of the the ring ring is is measured measured by by sweeping sweeping a a high-resolutionhigh-resolution tunable tunable laser laser (0.1 (0.1 pm pm steps) steps) across across the the resonance, resonance, and and measuring measuring the the output output power. power. TheThe resulting resulting spectra spectra is depicted inin FigureFigure5 a,5a, and and fitted fitted with with a Lorentziana Lorentzian function. function. The The measured measured full fullwidth width half half max max (FWHM) (FWHM) is 91.3 is 91.3 MHz. MHz.

FigureFigure 5. ((aa)) Measured Measured spectral spectral responses responses of the of 750 theµm 750 radius µm Si radius ring resonator Si ring (all-pass resonator conﬁguration) (all-pass plotted with a Lorentzian ﬁt. The extracted intrinsic quality factor Q = 4.1 million. (b) Simulated configuration) plotted with a Lorentzian fit. The extracted intrinsic qualityint factor Qint = 4.1 million. (b) Simulatedbend-loss limitedbend-loss intrinsic limitedQ ofintrinsic the rings Q versusof the ringrings radius versus at threering etchradius depths. at three The etch experimental depths. The data experimentalof the targeted data 56 of nm the etch targeted depth 56 are nm depicted etch depth as blue are circular depicted markers. as blue circular markers.

TheThe ring ring in in Figure Figure 5a5a has a 750 µmµm bend bend radius, radius, for for which which we we did did not not observe observe any any bend-loss bend-loss inducedinduced radiation radiation in in the resonator.resonator. ToTo confirm confirm this, this, we we investigated investigated rings rings with with varying varying radii, radii, and and plot plottheir their intrinsic intrinsicQ factor Q factor alongside alongside the theoretical the theoretical bend-loss bend-loss limit for Qlimitin Figure for Q5 b.in The Figure experimental 5b. The experimentalresults are extracted results are from extracted the measured from the FWHM, meas whileured FWHM, the dashed while lines the are dashed simulated lines using are simulated Lumerical usingMODE Lumerical solutions. MODE The simulations solutions. areThe shown simulations for three are different shown for etch three depths, different as that etch is the depths, most sensitive as that isparameter the most sensitive to the bend-loss. parameter We to find the that bend-loss. our ring We in thisfind experimentthat our ring is actuallyin this experiment overetched is compared actually overetchedto the target compared 56 nm, judging to the target by the 56Q nm,of the judging smaller by rings. the Q We of dothe not smaller observe rings. significant We do not bend observe loss in significantthe rings untilbend the loss radius in the is rings less thanuntil 550the µradiusm. For is practical less than reasons, 550 µm. the For ring practical radius reasons, should bethelarger ring radiusthan 700 shouldµm to be avoid larger bend-loss than 700 possibly µm to causedavoid bend-loss by an underetch possibly of caused the waveguide. by an underetch of the waveguide. Appl.Appl. Sci. Sci. 20182018, ,88, ,x 1139 FOR PEER REVIEW 77 of of 13 13

3.3.3.3. Narrow Narrow Bandwidth Bandwidth Bragg Bragg Gratings Gratings AnotherAnother useful componentcomponent thatthat can can be be utilized utilized with with low-loss low-loss waveguides waveguides is a low-kappais a low-kappa (κ) Bragg (κ) Bragggrating. grating. These gratingsThese gratings are widely are usedwidely in filters,used in sensors, filters, andsensors, semiconductor and semiconductor lasers as a wavelengthlasers as a wavelengthselective element, selective much element, like the much ring like resonator. the ring The resonator. bandwidth The ofbandwidth the grating of is the directly grating proportional is directly proportionalto power coupling to power coefficient couplingκ, and coefficient therefore κ can, and be minimizedtherefore can by reducing be minimized the perturbation by reducing strength the perturbationof the grating strength [39]. At of the the same grating time, [39]. the lengthAt the (Lsame) of thetime, grating the length should (L be) of long the enough grating (κshouldL ~1)and be longthe loss enough of the (κ waveguideL ~ 1) and must the loss be low of inthe order waveguide to obtain must sufficient be low reflection. in order Care to obtain must besufficient taken in reflection.the design Care of the must grating be taken in order in the to design achieve of weak the gratingκ as conventional in order to achieve sidewall weak or top κ as surface conventional gratings sidewalltend to haveor top stronger surfaceκ gratings. Instead, tend we to utilize have “post”stronger (or κ inversely,. Instead, “hole”)we utilize gratings “post” in (or which inversely, small “hole”)areas of gratings the waveguiding in which small material areas are of placed the waveguiding (removed) periodically material are onplaced either (removed) side of the periodically waveguide. onThe either dimensions side of the of thewaveguide. posts (holes) The anddimensions proximity of the to the posts waveguide (holes) and can proximity be lithographically to the waveguide defined, canand be tailored lithographically to achieve thedefined, desired andκ. Thistailored approach to achieve has been the successfuldesired κ. forThis low approach loss silicon has nitride been successfulgratings [40 for]. Forlow ourloss waveguides, silicon nitride it isgratings simpler [40]. to etch For holes our waveguides, on either side it ofis thesimpler waveguide, to etch asholes shown on eitherin Figure side6a. of The the resulting waveguide,κ of as the shown grating in is Figure directly 6a. controlled The resulting by the κ hole-to-waveguide of the grating is distance,directly controlledwhile the periodby the ofhole-to-waveguide the grating is set distance, to be 240 while nm, corresponding the period of the to a grating stopband is set at 1563to be nm 240 for nm, an unperturbed waveguide n = 3.2560. corresponding to a stopbandeff at 1563 nm for an unperturbed waveguide neff = 3.2560. µ WeWe fabricate fabricate uniform uniform gratings gratings with with lengths lengths of of 0.5 0.5 and and 1.0 1.0 cm. cm. The The waveguide waveguide width width is is 1.8 1.8 µm,m, andand the the length length is is 2.2 2.2 cm. cm. We We used used an an e-beam e-beam lithography lithography to to write write the the grating grating teeth, teeth, although although DUV DUV immersionimmersion lithography lithography can can also also resolve resolve 120 120 nm nm fe features.atures. Transmission Transmission and and reflection reflection spectra spectra are are measured,measured, and and fitted fitted using using a a transfer transfer matrix matrix model model in in Lumerical Interconnect. This This approach approach also also accountsaccounts for for Fabry-Perot Fabry-Perot cavities cavities formed formed by by the the waveguide waveguide facets, facets, which which affects affects the the grating grating shape. shape. FromFrom this, this, we we extract extract the the bandwidth bandwidth and and peak peak reflecti reflectivityvity of ofthe the grating, grating, which which can can then then be used be used to estimateto estimate κ. Aκ. few A few selected selected gratings gratings are are depicted depicted in inFigure Figure 6b6b along along with with the predicted gratinggrating bandwidth.bandwidth. Using 11 cmcm long long gratings, gratings, we we were were able able to achieve to achieve a 50 pma 50 FWHM pm FWHM bandwidth bandwidth with roughly with roughly70% reflectivity, 70% reflectivity, with potential with topotential further reduceto furthe ther bandwidthreduce the withbandwidth a longer with grating. a longer Figure grating.6c and Figured show 6c two and examples d show two of theexamples measured of the reflection measured and reflection transmission and transmission spectra of the spectra 1 cm long of the grating 1 cm −1 −1 longwaveguides grating waveguides with κ = 1.25 with cm κ and= 1.25κ =cm 4.0−1 cmand κ, respectively.= 4.0 cm−1, respectively. Finally, Figure Finally,6e shows Figure the 6e relation shows thebetween relation the between distance the between distance the between holes to thethe coreholes waveguide’s to the core edgewaveguide’s and κ strength. edge and The κ measuredstrength. Thedata measured was fitted data well was with fitted an exponentialwell with an curve.exponent Theial right curve. y-axis The right of Figure y-axis6e of shows Figure the 6e calculatedshows the calculatedeffective index effective difference index difference between the between waveguide the waveguide with and without with and the without hole gratings. the hole gratings.

FigureFigure 6. 6. ((aa)) An An SEM SEM image image of of the the fabricated fabricated low low κκ BraggBragg grating grating waveguide waveguide with with a a close-up close-up of of the the holesholes on on both both sides sides of of the the waveguide. waveguide. (b (b) )Theoretical Theoretical calculations calculations of of gratings’ gratings’ full full width width half half max max (FWHM)(FWHM) versus versus varying varying κκLL forfor 0.5, 0.5, 1 1 and and 2 2 cm cm long long grating grating wa waveguides.veguides. Circular Circular blue blue markers markers show show thethe measured measured data. data. ( (cc,d,d)) Spectra Spectra of of reflection reflection and and transmission transmission of of 1 1 cm cm long long grating grating waveguides waveguides with with −1 −1 κκ == 1.25 1.25 cm cm−1 andand κκ == 4.0 4.0 cm cm−1, respectively., respectively. The The ripples ripples are are caused caused by by reflections reflections off off the the waveguide waveguide facets.facets. ( (ee)) The The extracted extracted κκ,, Δ∆nn, ,and and exponential exponential fit fit as as a a function function of of waveguide waveguide to to hole hole distance. distance.

4. Photonic Integrated Circuits with Ultralow Loss Silicon Waveguides The ability to integrate ultralow loss waveguides inside the laser cavity or form fully integrated delayAppl. lines Sci. 2018 with, 8, x activeFOR PEER devices REVIEW promises vastly superior performance at much-reduced footprint8 of 13 compared to e.g., ﬁber-based delays. Furthermore, full integration improves the robustness of devices compared to e.g., fiber-based delays. Furthermore, full integration improves the robustness of and reduces cost of packaging. The key for integration is a low loss taper to transition from ULL devices and reduces cost of packaging. The key for integration is a low loss taper to transition from waveguidesULL waveguides to a waveguide to a waveguide geometry geometry with with smaller smaller bend bend radius, radius, which which wewe describe in in detail detail in in followingfollowing section. section. This This allows allows usus toto expandexpand thethe versatility of of the the heterogeneous heterogeneous silicon silicon platform. platform. Finally,Finally, we we give give two applicationstwo applications that beneﬁtthat bene greatlyfit greatly from havingfrom having ULL waveguides: ULL waveguides: narrow linewidthnarrow laserslinewidth and interferometric lasers and interferometric optical gyroscopes. optical gyroscopes.

4.1.4.1. Transition Transition Taper Taper to to Standard Standard Silicon Silicon Waveguides Waveguides FigureFigure7 shows 7 shows the the schematic schematic of of an an adiabatic adiabatic taper taper forfor aa lowlow loss, low low reflection reflection transition transition of of the the opticaloptical mode mode from from the the 56 56 nm nm etched etched rib rib ULLULL waveguidewaveguide to a a deeper deeper 231 231 nm nm etched etched rib rib waveguide, waveguide, whichwhich has has a smallera smaller bend bend radius radius (50 (50µ µm).m). The The waveguidewaveguide width is is set set at at 1.8 1.8 µm.µm. The The mode mode transition transition happenshappens along along the the linear linear taper taper formed formed byby etchingetching onon thethe Si slab. Figure Figure 7a7a–c–c shows shows the the simulated simulated profilesprofiles of theof the waveguide’s waveguide’s fundamental fundamental mode mode at at pointspoints a,a, bb and c denoted on on the the schematic schematic figure. figure.

Figure 7. Schematic of the taper converting the optical mode from an ULL waveguide to standard rib Figure 7. Schematic of the taper converting the optical mode from an ULL waveguide to standard rib (231 nm etch depth) waveguide. Figures (a–c) show the waveguide mode profiles at locations labeled (231 nm etch depth) waveguide. Figures (a–c) show the waveguide mode profiles at locations labeled a—ULL silicon waveguide, b—central part of the taper, and c—standard silicon waveguide along the a—ULL silicon waveguide, b—central part of the taper, and c—standard silicon waveguide along the taper structure. taper structure. In order to ensure an adiabatic transition from fundamental mode of the 56 nm etched waveguideIn order toto ensurethe 231 an nm adiabatic etched waveguide, transition from the length fundamental of the linear mode taper of the must 56 nmbe sufficiently etched waveguide long. to theAs shown 231 nm in etched Figure waveguide,8a, at 1550 nm the wavelength, length of the high linear order taper modes must in bethe sufficiently deeper rib waveguide long. As shown are in Figureexcited8 a,if atthe 1550 taper nm length wavelength, is shorter high than order 25 µm. modes With in a the taper deeper longer rib than waveguide 50 µm, areboth excited mode if theexpansion taper length and is finite-difference shorter than 25 time-domainµm. With a taper(FDTD) longer simulations than 50 µshowm, both that mode essenstially expansion 100% and finite-differencetransmission in time-domain the fundamental (FDTD) mode simulations is achieved show. The thatreflection essenstially power 100%is also transmission simulated and in is the fundamentalshown to be mode lower isthan achieved. −60 dB level. The To reflection be safe, powerthe taper is length also simulated of 200 µm andis chosen is shown in practice. to be lowerThe thanwavelength−60 dB level.dependence To be of safe, the thetransmission taper length effici ofency 200 andµm reflection is chosen power in practice. are again The simulated wavelength in dependenceFDTD and ofshown the transmission in Figure 8b. efficiencyLarger than and 99.8% reflection transmission power areand againlower simulatedthan −55 dB in reflection FDTD and shownshould in be Figure achievable8b. Larger over the than whole 99.8% C + transmission L band. and lower than −55 dB reflection should be achievable over the whole C + L band. Appl. Sci. 2018, 8, 1139 9 of 13 Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 13

Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 13

FigureFigure 8. 8. (a()a )Simulation Simulation results results at at 1550 1550 nm nm wavelength wavelength of of the the optical optical transmissions transmissions and and reflections reflections at at thethe deep-shallow deep-shallow taper. taper. The The fundamental fundamental mode mode was was launched launched from from the the ULL ULL silicon silicon waveguide waveguide side. side. TransmittedTransmitted and and reflected reflected powers powers to to the the fundamen fundamentaltal mode mode and and high high order order modes modes were were simulated simulated usingusing two two methods. methods. The The solid solid lines lines show show the the re resultssults using using mode mode expansion expansion on on the the PhotoDesign PhotoDesign FimmProp,FimmProp, while while the the markers markers are are the the data data points points simulated simulated with with Lumerical Lumerical FDTD FDTD (b (b) )Wavelength Wavelength dependencedependence of of the the transmission transmission and and reflection reflection fr fromom fundamental fundamental input input mode mode to to fundamental fundamental output output

mode,mode, simulated simulated with with Lumerical Lumerical FDTD. FDTD. Figure 8. (a) Simulation results at 1550 nm wavelength of the optical transmissions and reflections at the deep-shallow taper. The fundamental mode was launched from the ULL silicon waveguide side. 4.2.4.2. Applications Applications Transmitted and reflected powers to the fundamental mode and high order modes were simulated using two methods. The solid lines show the results using mode expansion on the PhotoDesign 4.2.1. ExtendedFimmProp, Cavity Lasers while the markers are the data points simulated with Lumerical FDTD (b) Wavelength 4.2.1. Extended Cavitydependence Lasers of the transmission and reflection from fundamental input mode to fundamental output TheThe heterogeneous heterogeneousmode, simulated silicon silicon with Lumericalplatform platform FDTD. has has demonstrated demonstrated superior superior laser laser linewidth linewidth performance performance comparedcompared to to 4.2.typical typical Applications linewidths linewidths of of III/V III/V lasers lasers that that are are in in a afew few MHz MHz range range [41] [41.]. There There are are at at least least two two reasonsreasons for for superior superior linewidth.linewidth. TheThe platformplatform allows allows for for the the control control of confinementof confinement in the in quantumthe quantum wells 4.2.1. Extended Cavity Lasers wellsand providesand provides a low-loss a low-loss waveguide waveguide platform, platfo openingrm, opening a newpossibility a new possibility in improving in improving the coherence the coherenceby providing by aproviding mechanismThe heterogeneous a to mechanism separate silicon platform the photonto has separa demonstrated resonatorte the superior (inphoton silicon) laser resonatorlinewidth and highly-absorbing performance (in silicon) activeand compared to typical linewidths of III/V lasers that are in a few MHz range [41]. There are at least two highly-absorbingmedium [42reasons]. The active secondfor superior medium improvement linewidth. [42]. The The comesplatform second with allows to improvementfor the the use control of ring of confinementcomes resonators with in the insideto quantum the the use laser of cavityring resonatorsto create a inside widely-tunablewells andthe provideslaser cavity laser. a low-loss Ringto create waveguide resonators, a widely-tunable platfo providedrm, opening that laser. a thenew Ring utilizedpossibility resonators, waveguide in improving provided platform the that offers the utilizedsufficiently waveguide lowcoherence propagation platform by providing losses,offers a mechanism havesufficiently an advantage to lowsepara propagationte as the the photon effective losses, resonator cavity have (in length ansilicon) advantage at ringand resonance as the highly-absorbing active medium [42]. The second improvement comes with to the use of ring effectiveis significantly cavityresonators length enhanced, inside at ring the directly laser resonance cavity reducing to createis signif linewidth. a widely-tunableicantly Bothenhanced, laser. reasons Ring directly resonators, benefit reducing provided from loss thatlinewidth. reduction, the Both and reasonsit was predictedbenefitutilized from that waveguide loss sub-kHz reduction, platform Lorentzian offersand itsufficiently was linewidths predicted low propagation should that sub-kHz be losses, attainable have Lorentzian an with advantage waveguide linewidths as the platform should beproviding attainable sub-0.5 effectivewith waveguide dB/cmcavity length propagation platformat ring resonance providing loss [is43 signif]. icantlysub-0.5 enhanced, dB/cm directly propagation reducing losslinewidth. [43]. Both We extendreasons the benefit analysis from loss to platformreduction, and providing it was predicted 0.1 dB/cm that sub-kHz propagation Lorentzian linewidths and keep should the same basic We extendbe attainable the analysis with waveguide to platform platform providing providing sub-0.50.1 dB/cm dB/cm propagationpropagation loss and [43]. keep the same basic laserlaser structure structure asWe as in extend in[43] [43 comprising the] comprising analysis to ofplatform three of three providingring ring resonators 0.1 resonators dB/cm insidepropagation inside the andFabry-Perot the keep Fabry-Perot the same laser basic cavity. laser cavity. The estimatedThe estimated Lorentzianlaser Lorentzian structure linewidth as in linewidth [43] comprising for optimized for optimizedof three ringlaser resonators laser fabricated fabricated inside thewith withFabry-Perot such such low-loss laser low-loss cavity. waveguides waveguidesThe is is sub-100sub-100 Hz Hz asestimated as shown shown Lorentzian in in Figure Figure linewidth 9.9. for optimized laser fabricated with such low-loss waveguides is sub-100 Hz as shown in Figure 9.

Figure 9. Simulated Lorentzian linewidth of widely-tunable laser comprising of three rings as a Figure 9. Simulatedfunction of Lorentzian coupling strength linewidth of the highof widely-tunableQ ring for four waveguide laser comprising propagation loss of three scenarios. rings The as a function of couplinganalysis strength ignores of the non-linear high- Qeffects.ring for four waveguide propagation loss scenarios. The analysis ignores non-linear effects. Figure 9. Simulated Lorentzian linewidth of widely-tunable laser comprising of three rings as a function of coupling strength of the high-Q ring for four waveguide propagation loss scenarios. The analysis ignores non-linear effects. Appl. Sci. 2018, 8, 1139x FOR PEER REVIEW 10 of 13

4.2.2. Optical Gyroscopes 4.2.2. Optical Gyroscopes Low SWAP and low cost gyroscopes are of great improtance in the fields of robotics, unmanned aerialLow vehicle SWAP and and automotives low cost gyroscopes and recently are has of greatrapidly improtance become even in the more fields so ofwith robotics, the emergence unmanned of augmentedaerial vehicle reality and automotivestechnology and and drones. recently In has many rapidly applications become that even operate more so in with environments the emergence with highof augmented shock and realityvibrations, technology optical andgyroscopes drones. are In more many robust applications and reliable that compared operate in to environments their MEMS withcounterparts high shock because and vibrations,they do not optical have gyroscopes any moving are parts. more robustHowever, and the reliable large compared size and tocost their of traditionalMEMS counterparts optical gyroscopes, because they which do are not based have on any di movingscrete components, parts. However, make the them large difficult size and to be cost as widelyof traditional usable opticalas MEMS gyroscopes, gyros. In which a previous are based work on [10], discrete we used components, integrated make silicon them photonics difficult to miniaturizebe as widely the usable optical as MEMSdriver (OD) gyros. part In for a previous interferometric work [10 fiber], we gyroscopes used integrated (IFOGs) silicon to a small photonics chip. Theto miniaturize integrated OD the opticalchip contains driver every (OD) optical part for component interferometric needed fiber for gyroscopesthe gyroscope, (IFOGs) except to the a small fiber sensingchip. The coil, integrated which needs OD chip to containsbe sufficiently every opticallong to component achieve high needed sensitivity. for the gyroscope,For applications except that the requirefiber sensing lower coil, sensivity, which however, needs to be an sufficiently integrated longwaveguide-based to achieve high delayline sensitivity. might For deliver applications adequate that performance.require lower In sensivity, the following, however, we anshow integrated the architec waveguide-basedture of an interferometri delaylinec might optical deliver gyroscope adequate fully performance.integrated on Insilicon, the following, making use we of show the U theLL architecture Si waveguide of anas its interferometric Sagnac sensing optical coil. gyroscope fully integratedThe schematic on silicon, mask making layout use ofof thethe ULL integrated Si waveguide gyroscope as its is Sagnac shown sensing in Figure coil. 10. The general architectureThe schematic of the circuit mask is layout very similar of the integratedto [44]: The gyroscope light from is the shown sources in Figuregoes through 10. The two general 3-dB broadbandarchitecture couplers of the circuit to get is verysplitted similar into to two [44]: waves The light and from then the goes sources through goes throughpush-pull two phase 3-dB modulators.broadband couplers The two to waves get splitted enter intothe ULL two wavessilicon andwaveguide then goes sensing through coil push-pull (with 4 dB/m phase propagation modulators. Theloss) twoand wavespropagate enter in the counter-directions ULL silicon waveguide before comi sensingng back coil (withto beat 4 on dB/m a photodiode propagation on loss)a port and of propagatethe first 3-dB in counter-directionscoupler. The Sagnac before phase coming shift can back beto detected beat on and a photodiode used to measure on a port the of rotation the first of 3-dB the system.coupler. The Sagnac phase shift can be detected and used to measure the rotation of the system.

Figure 10.10.Schematic Schematic mask mask layout layout of an integratedof an integrated interferometric interferometric optical gyroscope optical on gyroscope heterogeneous on heterogeneoussilicon photonics. silicon The sensingphotonics. part The of thesensing gyroscope part of is the four-metergyroscope longis the ULL four-meter silicon waveguidelong ULL siliconspiral with waveguide propagation spiral loss with ~4 dB/m.propagation The sensing loss coil~4 dB/m. waveguides The sensing were connected coil waveguides with a compact were connectedintegrated opticalwith a drivercompact ([10 ])integrated via tapers optical to standard driver and ([10]) compact via waveguides.tapers to standard and compact waveguides. The sensitivity of the proposed gyroscope as a function of the length of the Sagnac waveguide coil isThe estimated sensitivity by takingof the proposed into account gyroscope four sources as a fu ofnction noise, of i.e., the thermal length noise,of the shotSagnac noise, waveguide relative coilintensity is estimated noise and by thermo-refractivetaking into account noise. four sources The anaslysis of noise, is performedi.e., thermal in noise, the same shot mannernoise, relative in our previousintensity worknoise [and44] andthermo-refractive the contribution noise. from The each anaslysis noise term is performed to rotation in nosie the same spectral manner density in our are previousdepicted inwork Figure [44] 11 and. A the 50 mWcontribution output power from fromeach noise the light term sources, to rotation 4 dB/m nosie loss spectral in the ULLdensity silicon are depicted in Figure 11. A 50 mW output power from the light sources, 4 dB/m loss in the ULL silicon waveguide coil, and 0.02 dB/crossing (simulated value)√ were assumed. The minimum√ detection limit waveguideof the gyroscope coil, and is approximated 0.02 dB/crossing to be (simulated 113 ◦/hr/ value)Hz or were equivalently assumed. 1.88The ◦minimum/ hr. detection limit of the gyroscope is approximated to be 113 ° //hr Hz or equivalently 1.88 ° / hr . Appl.Appl. Sci. Sci.2018 2018, 8, 1139, 8, x FOR PEER REVIEW 11 of11 13 of 13

Figure 11. Contributions from various noise sources to the rotation noise spectral density as a Figure 11. Contributions from various noise sources to the rotation noise spectral density as a function function of Sagnac loop length for optical output power P0 = 100 mW and waveguide propagation of Sagnac loop length for optical output power P0 = 100 mW and waveguide propagation loss = 4 dB/m. Calculationsloss = 4 dB/m. were Calculations based on the were analysis base presentedd on the analysis in [44]. presented in [44].

5. Conclusions 5. Conclusions In conclusion, we have demonstrated ULL silicon waveguides using 500 nm thick SOI that can In conclusion, we have demonstrated ULL silicon waveguides using 500 nm thick SOI that can be be seamlessly integrated with heterogeneous silicon/III-V active components such as lasers, seamlessly integrated with heterogeneous silicon/III-V active components such as lasers, modulators, modulators, and photodetectors. Waveguide loss is as low as 4 dB/m for a multimode waveguide, and photodetectors. Waveguide loss is as low as 4 dB/m for a multimode waveguide, and 16 dB/m and 16 dB/m for a quasi-single mode waveguide. The ultralow loss directly benefits applications for a quasi-single mode waveguide. The ultralow loss directly beneﬁts applications such as optical such as optical gyroscopes in which a long delay line is needed. It can also be utilized in high-Q ring gyroscopes in which a long delay line is needed. It can also be utilized in high-Q ring resonators resonators and narrowband grating-based filters. We demonstrate rings with 4.1 million intrinsic Q, and narrowband grating-based ﬁlters. We demonstrate rings with 4.1 million intrinsic Q, which are which are promising to realize narrow linewidth lasers on silicon. Calculations show that linewidths promising to realize narrow linewidth lasers on silicon. Calculations show that linewidths on the order on the order of 100 Hz should be achievable using these waveguides. of 100 Hz should be achievable using these waveguides.

AuthorAuthor Contributions: Contributions:M.A.T. M.T. designed designed the the devices; devices; M.A.T., M.T., D.H. D.H. performed performed the the experiments; M.T., M.A.T., D.H., D.H., T.K. T.K.simulated simulated and and analyzed analyzed the the data; J.P.,J.P., D.H.,D.H., A.M. A.M. fabricated fabricated the the devices; devices; M.A.T., M.T., D.H., D.H., T.K., T.K., J.E.B. J.E.B. wrote wrote the the manuscript;manuscript; J.E.B. J.E.B. supervised supervised the the project. project.

Funding:Funding:DARPA DARPA MTO MTO Contract Contract N66001-16-C-4017. N66001-16-C-4017. Acknowledgments: This work was supported by DARPA MTO. The views and conclusions contained in this Acknowledgments: This work was supported by DARPA MTO. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing official policies of DARPA ordocument the U.S. Government. are those of the The authors authors and would should like not to be thank interpreted Paolo Pintusas represen for helpting official with simulation, policies of MichaelDARPA or Davenportthe U.S. for Government. processing advice, The authors Andreas wo Boesuld for like help to with thank testing, Paolo Warren Pintus Jin, for Chao help Xiang with and simulation, Paul Morton Michael for fruitfulDavenport discussions. for processing advice, Andreas Boes for help with testing, Warren Jin, Chao Xiang and Paul Morton Conflictsfor fruitful of Interest: discussions.The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References References 1. Coldren, L.A.; Corzine, S.W.; Masanovic, M.L. Diode Lasers and Photonic Integrated Circuits, 2nd ed.; 1. Wiley:Coldren, Hoboken, L.A.; NJ,Corzine, USA, 2012.S.W.; Masanovic, M.L. Diode Lasers and Photonic Integrated Circuits, 2nd ed.; Wiley: 2. Okamoto,Hoboken, K. ProgressNJ, USA, and2012. technical challenge for planar waveguide devices: Silica and silicon waveguides. 2. LaserOkamoto, Photonics K. Rev. Progress2012, 6, 14–23.and technical [CrossRef ]challenge for planar waveguide devices: Silica and silicon 3. Bauters,waveguides. J.F.; Heck, Laser M.J.R.;Photonics Demis, Rev. 2012 J.;, Dai,6, 14–23. D.; Tien, M.; Barton, J.S.; Leinse, A.; Heideman, R.G.; 3. Blumenthal,Bauters, J.F.; D.J.; Heck, Bowers, M.J.R.; J.E. Demis, Ultra-low-loss J.; Dai, D.; Tien, high-aspect-ratio M.; Barton, J.S.; Si3 NLeinse,4 waveguides. A.; Heideman,Opt. R.G.; Express Blumenthal,2011, 19,D.J.; 3163–3174. Bowers, [CrossRef J.E. Ultra-low-loss][PubMed ]high-aspect-ratio Si3N4 waveguides. Opt. Express 2011, 19, 3163–3174. 4. 4. Thomson,Thomson, D.; D.; Zilkie, Zilkie, A.; A.; Bowers, Bowers, J.E.; J.E.; Komljenovic, Komljenovic, T.; T.; Reed, Reed, G.T.; G.T.; Vivien, Vivien, L.; L.; Marris-Morini, Marris-Morini, D.; D.; Cassan, Cassan, E.; E.; Virot,Virot, L.; L.; Fé dFédéli,éli, J.M.; J.M.; et al.et al. Roadmap Roadmap on on silicon silicon photonics. photonics.J. Opt.J. Opt. (United (United Kingdom) Kingdom)2016 2016, 18, ,18 073003., 073003, [CrossRefdoi:10.1088/2040-8978/18/7/073003.] Appl. Sci. 2018, 8, 1139 12 of 13

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