Low-Loss Prism-Waveguide Optical Coupling for Ultrahigh-Q Low-Index Monolithic Resonators

Research Article Vol. 5, No. 2 / February 2018 / Optica 219

Low-loss prism-waveguide optical coupling for ultrahigh-Q low-index monolithic resonators

1,† 2,† 1 1 1 GUANGYAO LIU, VLADIMIR S. ILCHENKO, TIEHUI SU, YI-CHUN LING, SHAOQI FENG, 1 1 2 2 2 KUANPING SHANG, YU ZHANG, WEI LIANG, ANATOLIY A. SAVCHENKOV, ANDREY B. MATSKO, 2 1, LUTE MALEKI, AND S. J. BEN YOO * 1Department of Electrical Engineering and Computer Science, University of California-Davis, Davis, California 95616, USA 2OEwaves Inc., 465 North Halstead Street, Suite 140, Pasadena, California 91107, USA *Corresponding author: [email protected]

Received 31 October 2017; revised 29 December 2017; accepted 17 January 2018 (Doc. ID 310191); published 20 February 2018

While compact and low-loss optical coupling to ultrahigh-quality-factor (Q) crystalline resonators is important for a wide range of applications, the major challenge for achieving this coupling stems from the relatively low refractive index of the crystalline resonator host material compared to those of the standard waveguide coupling materials. We report the first demonstration of a single-mode waveguide structure (prism-waveguide coupler) integrated on a low- loss compact silicon nitride platform resulting in low-loss and efficient coupling to magnesium fluoride crystalline resonators by achieving the phase-matched and the mode-matched evanescent wave coupling. The coupling is characterized with 1 dB loss at 1550 nm wavelength. We further present a photonic integrated chip containing a pair of waveguides successfully coupling light into and out of the resonator, demonstrating a planar-waveguide-coupled crystalline resonator with a loaded Q of 1.9 × 109. We assemble this waveguide-coupled resonator and a distributed-feedback-laser chip into a butterfly package to realize a miniature Kerr optical frequency comb source using self-injection locking of the distributed feedback laser to the waveguide-coupled crystalline resonator. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

OCIS codes: (230.4555) Coupled resonators; (230.7380) Waveguides, channeled; (230.3120) Integrated optics devices.

https://doi.org/10.1364/OPTICA.5.000219

1. INTRODUCTION ultrahigh-Q crystalline resonators utilized either free-space Kerr optical frequency combs [1–5], based on high-Q whispering- prisms, angle-cut fibers, or tapered fibers, suffering from the com- gallery-mode (WGM) resonators [6–10], have significantly plexity of implementation onto a photonic platform that fre- quently calls for an active optical alignment [32–35]. Primary stimulated the applications in optical clocks, optical synthesis, op- challenges have been the material incompatibility between the tical communications, optical sensing, cavity quantum electrody- crystalline resonator and the commonly used photonic platforms namics, and precision navigation [11–19]. Dielectric resonators (e.g., silicon nitride, silica, silicon) [36,37], the low refractive in- on various platforms (e.g., silica, silicon nitride, lithium niobate) dex (e.g., 1.37) of the typical crystalline resonator [e.g., magne- have been monolithically integrated with on-chip waveguides, Q – sium fluoride (MgF2)] compared to that of the conventional achieving loaded as high as 100 million [20 25]. On the other waveguide materials at telecommunication bands (e.g., 1.98 hand, crystalline resonators [26,27], suffering less from intrinsic for silicon nitride, 3.48 for silicon), and the incompatible fabri- Q and surface Rayleigh scattering, reported with record high- of cation processes between traditional photonic platforms and the 300 billion [28], have not yet been implemented with low-loss mechanically polished crystalline resonators. on-chip optical couplers. Such optical coupling to crystalline res- Planar optical coupling is proved to be a robust and practical onators should be based on a phase- and mode-matched power approach to implement WGM resonators with on-chip wave- exchange between a resonator mode and a wave propagating in a guides [38–41]. Specifically, on-chip waveguides have been ver- specially engineered coupler (a waveguide or a prism). An ideal tically coupled to crystalline resonators providing loaded Q coupler should ensure phase synchronization, the optimal spatial exceeding 108 [42–44]. In this work, we exploit the principle overlap between the resonator and the coupler modes, mode se- of evanescent wave coupling under total internal reflection lectivity, and criticality of the coupling [29–31]. Mode selectivity (TIR) condition in traditional prism elements [45–48] and is important for multi-mode structures as it allows interaction demonstrate, for the first time to our knowledge, a compact with a particular mode without disturbing the other modes of prism-waveguide coupler on a silicon nitride platform. This the structure. Previous demonstrations of optical coupling to prism-waveguide coupler laterally couples light into and out of

Q an ultrahigh- crystalline MgF2 resonator with 1 dB loss and (a) 1.9 × 109 loaded Q, realizing a new method for integration between crystalline resonators and on-chip waveguides [49]. We further present a packaged module containing the prism- Q waveguide-coupled ultrahigh- MgF2 resonator to generate a stable and low-noise optical frequency comb at the 26 GHz repetition rate.

2. DESIGN AND SIMULATIONS Q A. Ultrahigh- MgF2 Resonator

We used MgF2 resonators [shown in Fig. 1(a)] with an intrinsic finesse of one million following the fabrication technique in the Method section. MgF2 is characterized with excellent mechanical stability, relatively large hardness, as well as high optical transpar- (b) ency. The anomalous group velocity dispersion (GVD) at the communications band and a small thermo-refractive constant of this material are optimal for generation of optical mode-locked Kerr frequency combs. The anomalous GVD phase matches the nonlinear process. The small thermo-refractive constant increases the threshold of low-frequency thermo-optical instabilities, which often hinders mode locking of the comb harmonics. Figures 1(a) and 1(b) show the photograph and the design of the resonator fabricated for the Kerr frequency comb oscillator. The final diameter and thickness of the resonator were 2.66 mm and 150 μm, respectively. Thickness (20 μm) of the mode localization area was designed for a 10 μm × 15 μm wedge-shaped WGM profile [simulation shown in Fig. 2(a)] for an optimal mode matching Q Fig. 2. (a) Lowest-order WGM profile within the resonator with toward the prism-waveguide coupler mode. The loaded -factor μ μ x y was measured using the ring-down technique [28]. The fabricated approximate 10 m × 15 m ( axis × axis) mode size. (b) Ring-down measurement of the resonator indicating intrinsic Q-factor of 6 × 109. resonator was coupled nearly critically using a standard BK7 prism coupler to measure its Q-factor. The measured amplitude ring-down time was τ 10 μs [shown in Fig. 2(b)] at the 1.55 μm wave- ∕ length, which corresponds to 6 × 109 loaded Q-factor. the Si3N4 SiO2 waveguide platform suitable for the coupling to Q an ultrahigh- MgF2 resonator, we considered the waveguide B. Prism-Waveguide Couplers mode size and propagation loss as the most important parameters.

The Si3N4 material platform on silicon is attractive because of its The waveguide mode size at the interaction area [at the reflection relatively low loss and compact size, convenient for further inte- normal line shown in Fig. 3(a)] should match that of the WGM gration of the laser and photodiode chips [50–55]. In designing [shown in Fig. 2(a)] in the resonator to maximize the mode overlap integral and thus reduce the coupling loss resulting from mode mismatch. Si3N4 waveguide core thickness at 50 nm has the low- (a) est mode-confinement factor (below 5%) at the core region com- pare to those with thicker cores, providing the largest mode size to match the designed resonator mode (10 μm × 15 μm). Besides, ultralow propagation loss has been achieved with a Si3N4 waveguide platform with core thickness around 50 nm [56,57] utilizing wafer bonding and high-temperature annealing. We directly deposited a thick SiO2 over-cladding layer via the low-pressure chemical vapor deposition technique, which is free of the com- plexity arising from wafer bonding. (Fabrication details are shown in Section 3 (Methods). The mode shape of the single-mode 50-nm-thick Si N core (b) 3 4 waveguide (6 μm in width) is approximately 5 μm × 2 μm, much smaller and elongated compared to that of the WGM in the resonator. Thus, we utilized the 200-μm-long adiabatic taper structure [shown in green in Fig. 3(a)] to expand the mode to nearly a Q round shape with approximately 10 μm diameter where the wave- Fig. 1. (a) Camera photo of a transparent ultrahigh- MgF2 resonator on the holding pedestal. (b) Geometric dimension of the cross-section guide core width tapers down to 200 nm, which is close to the view of a half-resonator with 1330 μm radius, 150 μm height, and resolution of the lithography tool used in the fabrication process. 20-μm-high wedge region for the WGM shown in red. This achieves nearly ideal mode-matching to the fundamental Research Article Vol. 5, No. 2 / February 2018 / Optica 221

(a) (d)

(e) (b)

(c)

Fig. 3. (a) Geometric configuration of prism-waveguide couplers maintaining phase synchronized coupling toward the resonator (used in FDTD simulation): G denotes the gap distance between the photonic integrated chip (PIC) and resonator; D denotes the distance between the tip of prism-waveguide coupler and the reflection surface normal; d denotes the distance between the tip of the prism-waveguide coupler and the edge of PIC; tw denotes the tip core width; θ denotes the reflection angle; L denotes the length of the taper structure. (b) Testing PIC (10 mm × 4.5 mm) containing individual prism-waveguide couplers with variations in θ, d, and tw. (c) PIC (3.6 mm × 0.42 mm) containing a pair of prism-waveguide couplers (single prism-waveguide coupler with the best performance tested through mask shown in (b) and an end-to-end waveguide for alignment assistance). (d) Add–drop resonator system, analogy to our testing setup for single prism-waveguide coupler: bottom red dashed rectangular r k γ region represents prism-waveguide-to-resonator coupling area with self-coupling ratio 1, cross-coupling ratio 1, and additional loss ; top red dashed r k rectangular region represents free-space prism to resonator coupling area with self-coupling ratio 2, cross-coupling ratio 2, and no additional loss (ideal prism) (e) The semi-numerical simulation results of the drop port transmission power at different gap values [G in (a)] between the prism-waveguide chip and the resonator at 1550 nm.

WGM in the resonator. We determined the 200 μmlengthtobe The first step shown in Fig. 3(a) presents the finite-difference time- the optimized adiabatic length for the taper structure by simulating domain (FDTD) simulation setup at the coupling region from the the output power to preserve near 100% power transmission. The prism-waveguide toward the resonator. We used an estimated- adiabatic taper structure also ensures the single-mode power trans- channel MgF2 waveguide ring resonator to represent the ultra- mission to provide better mode coupling selectivity toward the low- high-Q wedge disk resonator used in the experiment with a similar est-order WGM within the resonator compared to traditional free- mode shape (10 μm × 15 μm) and kept the incident angle to be space prisms that excite multimodes at the reflection surface. 71.5°. The air-gap distance [value G in Fig. 3(a)] between the To satisfy the prism-like evanescent wave coupling toward the prism-waveguide chip and the resonator is the most critical param- r resonator under the TIR condition, we selected a specific incident eter to determine the self-coupling coefficient ( 1)andthecross- θ k angle [ , defined as an angle formed by the reflection surface nor- coupling coefficient ( 1) in the coupling region. The critical cou- mal and the center-line of the prism-waveguide coupler tip, pling condition for an add–drop resonator system [Fig. 3(d)](our r a r a shown in Fig. 3(a)] to fulfill the phase-matching condition at measurement setup analogy) requires 2 1,where denotes the chip–air interface. The effective index of the optical mode the single-trip amplitude transmission coefficient within the reso- a Q out of the prism-waveguide coupler tip is close to that of the nator. The value of our ultrahigh- MgF2 resonator is close to 1 n μ SiO2 cladding material ( waveguide 1.44) at 1.55 m wave- (ultralow scattering loss), which requires the gap value (G) to be k length, and the effective index of the fundamental WGM inside finely tuned until the 1 value is close to 0 to maintain critical n the resonator is approximately 1.37 ( resonator 1.37). The coupling. The simulated results from Fig. 3(a) are used in the sta- analytical phase-matching condition for evanescent coupling re- ble-state add–drop resonator model [58] shown in Fig. 3(d) to fur- quires that the propagation constant of the optical mode inside ther derive the transmitted power at the drop port, assuming no loss the resonator matches that of the prism-waveguide coupler tip at the second coupling region (free-space prism in our experiment). along the lateral direction at the reflection surface; this leads to Figure 3(e) shows the normalized transmission power simulated at θ n ∕n a relationship: arcsin resonator waveguide . Thus the angle the drop port of the system utilizing prism-waveguide couplers for Q value is calculated to be approximately 71.5°. The TIR condition the ultrahigh- MgF2 resonator. We could achieve the maximum is automatically satisfied at this reflection angle, since the TIR transmission at the center wavelength at the gap value around critical angle for the material is 44°. 1 μm, which confirmed our expectation and the principle for Figures 3(a) and 3(d) explain the model we used for numerical sim- prism-waveguide evanescent wave coupling to ultrahigh-Q crystal- ulation of our prism-waveguide coupling system (see Supplement 1). line resonators. Research Article Vol. 5, No. 2 / February 2018 / Optica 222

We taped out the first type of single prism-waveguide mask ASML 5500/300 DUV lithography tool on 6 in. (150 mm) shown in Fig. 3(b) with variations in the incidence angle (θ), dis- p-type silicon wafers. We started with depositing 5-μm- thick tance (d) between the tip of the single prism-waveguide and the edge low-temperature-oxide (LTO) at 450°C by low-pressure chemical of the chip, and the prism-waveguide tip width (tw) to mitigate the vapor deposition (LPCVD) as the Si3N4 waveguide bottom clad- fabrication inaccuracy. Furthermore, to facilitate the power out- ding. We then deposited 50-nm-thick stoichiometric Si3N4 at coupling from the resonator, we designed the second type of the 800°C by LPCVD as the waveguide core [shown in step 1 in paired prism-waveguide coupler masks [shown in Fig. 3(c)]contain- Fig. 4(a)]. We defined the alignment marks for facet etching proc- ing mirror-positioned single prism-waveguide couplers (with the best ess on the Si3N4 layer followed by step 2 in Fig. 4(a) to pattern the configuration tested shown in the following Results section) with Si3N4 waveguide core by inductively coupled plasma (ICP) separation distance (2D) based on a trigonometric relationship be- etching. Another 5-μm-thick LTO oxide was deposited as the tween the incidence angle (θ)andthedistance(d). This separation waveguide overcladding in step 3 in Fig. 4(a). We deposited distance is set to achieve the best phase-matching condition between 500-nm-thick amorphous silicon (a-Si) by LPCVD as the hard μ both prism-waveguide coupler arms with the resonator. The combi- mask for facet etching through a 10- m-thick SiO2 layer together nation of both input and output prism-waveguide couplers should with a 150-μm-thick silicon substrate. We performed lithography function as a single free-space prism element for the specific crystal- on 5-μm-thick DUV photoresist for the assistance of facet deep line resonator. Besides, we particularly made the input/output beam etching. In step 4 in Fig. 4(a), we conducted a xenon difluoride μ angle of this paired prism-waveguide couplers PIC to be 20° to (XeF2) etching to remove an approximately 30- m-thick silicon suppress the surface reflection at the input edge for achieving self- substrate underneath the tip of the prism-waveguide couplers injection locking [59,60] toward a distributed feedback (DFB) laser [transparent regions in Fig. 4(b)]. Finally, dicing with 5 μm ac- chip using Rayleigh scattered light from the resonator. curacy using a diamond saw was performed on the wafer to shape PICs into the exact 0.5 mm × 4mmrectangular die [Fig. 4(b)]to 3. METHODS facilitate coupling to the resonator in a compact platform. A. Ultrahigh-Q Magnesium Fluoride Resonator Fabrication Technique 4. RESULTS We start with a high-quality crystalline MgF2 wafer and produce a thin cylindrical preform. The crystalline cylinder is then ground A. Demonstration of Single Prism-Waveguide Coupled to Ultrahigh-QMgF Resonator using a diamond slurry. The shape of the surface is set to be op- 2 timal for the waveguide platform (Si3N4) that the resonator is We finalized the PIC facet by deep etching through the cladding μ intended for. We have achieved ultrahigh finesse by careful selec- material (SiO2) and 150 m deep into the silicon substrate fol- tion of the parameters that control the mechanical shaping of the lowed by an isotropic XeF2 etching, which removes the silicon resonator utilizing the algorithm best for MgF2 at the specific substrate from the tip of the prism-waveguide coupler to avoid crystal cut. The control of the surface quality is achieved when resonator WGM coupling into the high-refractive-index silicon no Rayleigh scattering in the resonator is observed. Electron substrate. We measured the transmission spectrum of the resona- microscopy tests show that in this case, the roughness of the tor using a tunable narrow linewidth laser. The resonator was resonator surface is of the order of a crystalline lattice period. overloaded, and the measurement had shown that the loaded Q-factor was on the order of a billion [Fig. 5(b)]. The transmis- B. Prism-Waveguide Coupler Fabrication Technique sion loss through the resonator did not exceed 3 dB. The low Figure 4(a) illustrates the important fabrication steps for insertion loss suggests that the Q-factor of the resonator the prism-waveguide coupler PICs [Figs. 3(b) and 3(c)] using did not degrade noticeably during the packaging procedure.

Fig. 4. (a) Important fabrication steps: 1. Si3N4 LPCVD deposition; 2. Si3N4 waveguide core patterning; 3. SiO2 overcladding deposition; 4. PIC facet deep etching followed by XeF2 release of substrate silicon at PIC edge. (b) Fabricated PIC picture at left, center, and right position based on mask shown in Fig. 3(c). Research Article Vol. 5, No. 2 / February 2018 / Optica 223

(a) the fiber and the output beam of the waveguide at the level 3.8 dB; this value included residual reflections, modal mis- matches, and waveguide propagation losses. After that, the input launcher optics were aligned to send the light into the prism- waveguide coupler [green curved line shown in Fig. 5(a)]. The MgF2 resonator was positioned in the proximity of the waveguide chip edge with angled waveguide tips, and adjusted using Thorlabs NanoMax 302 three-axis translation stage with additional lead zirconate titanate (PZT) drives. An identical translation stage was used to bring the conventional glass coupling prism to couple the light out of the WGM of the resonator. The light exiting the prism was intercepted by a large area multimode fiber [“light pipe” in Fig. 5(a)] and sent into a photodetector for observation of the WGM, measurement of the loaded Q-factor using a tunable laser at 1550 nm (New Focus Velocity 6302), and evaluation of transmission losses. To validate the symmetry of in- (b) put and output coupling, an additional optical launcher [“WG Launcher 2” in Fig. 5(a)] was used to match the WGM field at the TIR surface of the prism. By varying and equalizing the two gaps, we maximized the optical throughput of the setup and estimated the transmission loss of the prism-waveguide-to- resonator-to-outcoupling prism as a function of loaded bandwidth. The latter has been measured using calibration of the laser sweep assisted by a fiber interferometer. We measured the minimum coupling loss of 1 dB at a heavily loaded bandwidth of 3.2 MHz via the best single prism-waveguide coupler with the following geometric specs: incident angle θ 71.5°, distance d 2 μm,andprism- waveguide tip width tw 200 nm. While the intrinsic unloaded bandwidth of resonator, measured shown in Fig. 5(b),was 100 KHz (Q near 1.9 billion at 1.55 μmwavelength). Fig. 5. (a) Measurement setup for single prism-waveguide coupler and straight calibration waveguides (shown in green lines). The insert is the B. Demonstration of Prism-Waveguide-Assisted microscope picture of the fabricated chip containing prism-waveguide cou- Optical Injection Locking of DFB Laser to Resonator plers aligned to a resonator where silicon substrate released region [step 4 in Fig. 4(a)] is shown in blue dotted rectangle. (b) Normalized transmitted As the next step in the experimental evaluation of our novel power received at the light pipe from (a) in weak loading regime, indicating prism-waveguide coupling system for MgF2 resonators, we per- the unloaded bandwidth of 100 kHz, Q-factor 1.9 × 109. formed and evaluated parameters of optical injection locking of a DFB laser. This was considered a crucial step to achieve waveguide-based optical frequency comb generator based on Q To evaluate the performance of the designed single prism- the ultrahigh- MgF2 resonator. To validate the quality of the waveguide couplers, we prepared a setup [shown in Fig. 5(a)] coupler, we created an external DFB semiconductor laser utilizing allowing us to (1) estimate the efficiency of waveguide insertion; the self-injection locking method for locking the laser to the (2) qualify the waveguide transmission loss; (3) couple light into WGM of the resonator from the prism-waveguide coupler; (4) couple light out of WGM using a conventional prism; (5) measure the full power exiting the WGM; and (6) deconstruct the overall transmission loss of the full optical train, and estimate the efficiency of coupling between the prism-waveguide coupler to the WGM in the resonator. The insert in Fig. 5(a) depicts the actual interface between the angled tip of the prism-waveguide coupler and the adjacent MgF2 resonator. Light delivered by a single-mode fiber (SMF-28) was collimated and focused using Thorlabs aspheric lenses C230 and C280 with focal distances approximate 9 mm and 18 mm, respectively; the resulting system formed the focal spot with a full width at 1∕e2 diameter of approximate 5 μm, well matched to the input mode of the silicon nitride waveguide. We used a straight waveguide [green straight line in Fig. 5(a)] to assess the insertion loss by comparing the Fig. 6. Photograph of the injection-locking setup illustrating two chips optical power output of the waveguide (with light collected using containing single prism-waveguide coupler implemented to a resonator. integrating sphere), to the optical power in the input fiber. This (1) is the resonator, (2) and (3) are the prism-waveguide coupler PIC, (4) comparison yielded the value of the overall insertion loss between and (5) are fiber launchers. Research Article Vol. 5, No. 2 / February 2018 / Optica 224

(a) (b) (c) (d)

Fig. 7. (a) 3D schematic illustration of paired-prism-waveguide couplers coupled to the resonator for comb generation. (b) Illustration of the packaged Kerr frequency comb source. (c) Photo of packaged planar-waveguide coupled comb source. (d) Representative Kerr frequency comb output from the package.

WGM resonator [61,62]. The coupler loss impacts the back scat- with a WGM. Figures 7(b) and 7(c) show the schematic illustra- tering significantly because it limits the amount of light both enter- tion and the actual photograph of the packaged Kerr frequency ing the mode and exiting the mode in the direction of the laser. To comb unit. The frequency comb was observed when the detuning demonstrate the self-injection locking we used the setup depicted value was properly selected and phase adjusted for positive optical in Fig. 6,inwhichMgF2 resonator (1 in Fig. 6) was simultaneously feedback. The 26 GHz optical frequency comb was observed us- coupled to two single prism-waveguide coupler chips (2 and 3 in ing an optical spectrum analyzer [Fig. 7(d)]. After confirmation of Fig. 6). The best prism-waveguide coupler in PIC (2 in Fig. 6)was the optimal optical comb regime, the light was sent to a high- coupled to the powered DFB laser assembly (4 in Fig. 6) (Emcore speed photodetector (U2t V2140), where we observed generation 702) equipped with a two-lens beam transformer that provides of a spectrally pure RF signal confirming coherence of the optical mode matching between the laser beam and the waveguide mode. comb harmonics and utility of the device as a packaged photonic The focused DFB laser assembly was suspended on a mechanical source of low-phase-noise microwave signals. extender tip mounted on the micro-positioner for active alignment. The second, weakly coupled, prism-waveguide PIC (3 in Fig. 6) 5. CONCLUSION was used for tapping the light output of the resonator, and a light We have demonstrated the first on-chip prism-waveguide pipe (5 in Fig. 6) was used for collection and transferring the light couplers on a compact silicon nitride platform for phase- and to the photodetector. An additional light pipe was used to monitor mode-matched evanescent coupling to a low-index ultrahigh-Q the input coupling at the reflection port of the first prism- crystalline resonator. This prism-waveguide-coupled MgF2 reso- waveguide coupler. With proper alignment and optimally chosen nator achieves 1.9 billion loaded Q-factor with 1 dB coupling loss. gaps, optical injection locking was achieved with optical resonance We further realized a packaged planar-waveguide-coupled feedback from the resonator, and a locking range of 5 GHz was Kerr optical frequency comb source at 26 GHz repetition rate obtained with our prism-waveguide coupler. This observation where the laser chip was self-injection-locked to this prism- confirmed the high quality of the prism-waveguide coupler, as waveguide-coupled MgF resonator. the locking is usually not observed if the coupler loss exceeds 2 5 dB. These parameters are similar to those obtained in existing Funding. Defense Advanced Research Projects Agency WGM-resonator-stabilized injection-locked lasers that use free- (DARPA) (HR0011-15-C-0054); National Aeronautics and space optics and prism couplers [62]. Space Administration (NASA) Small Business Technology C. Demonstration of Packaged Kerr Frequency Comb Transfer (STTR) (NNX17CC66P). Source via Paired Prism-Waveguide Couplers Acknowledgment. We acknowledge fabrication support Based on the successful demonstration of laser injection locking from the Marvell Nanofabrication Laboratory (Berkeley, CA) with the MgF2 resonator via single prism-waveguide coupler and Center for Nano-MicroManufacturing (Davis, CA). PICs, we followed the design shown in Fig. 3(c) and the fabrication process in Section 3 (Methods) to deliver the paired † These authors contributed equally for this work. prism-waveguide couplers PIC using the best single prism- waveguide coupler configuration tested above. Figure 4(b) shows See Supplement 1 for supporting content. photographs taken with the aid of a microscope of the fabricated 0.5 mm × 3.8 mm paired prism-waveguide coupler PIC. The silicon substrate was removed (shown in the light-colored region REFERENCES around the PIC) to avoid unnecessary WGM power leakage. 1. W. Liang, D. Eliyahu, V. S. Ilchenko, A. A. Savchenkov, A. B. Matsko, D. Figure 7(a) depicts the configuration of the packaged comb unit Seidel, and L. Maleki, “High spectral purity Kerr frequency comb radio ” based on paired prism-waveguide coupler PIC coupling toward frequency photonic oscillator, Nat. Commun. 6, 7957 (2015). 2. T. J. Kippenberg, R. Holzwarth, and S. Diddams, “Microresonator-based the MgF2 resonator. The PIC was rotated 20° to eliminate the optical frequency combs,” Science 332, 555–559 (2011). input facet reflection for a stable laser injection-locking condition. 3. C. Y. Wang, T. Herr, P. Del’Haye, A. Schliesser, J. Hofer, R. Holzwarth, A Kerr optical frequency comb is generated in the resonator when T. W. Hansch, N. Picque, and T. J. Kippenberg, “Mid-infrared optical fre- the laser light is efficiently coupled to the WGM, the power of the quency combs at 2.5 μm based on crystalline microresonators,” Nat. Commun. 4, 1345 (2013). light exceeds the comb generation threshold, and the frequency of 4. P. Del’Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth, and T. the pump is locked to the mode of the resonator. We increased Kippenberg, “Optical frequency comb generation from a monolithic the laser current to approximately 80 mA for self-injection locking microresonator,” Nature 450, 1214–1217 (2007). Research Article Vol. 5, No. 2 / February 2018 / Optica 225

5. V. Brasch, M. Geiselmann, T. Herr, G. Lihachev, M. H. P. Pfeiffer, M. L. 31. M. L. Gorodetsky and V. S. Ilchenko, “Optical microsphere resonators: Gorodetsky, and T. J. Kippenberg, “Photonic chip–based optical fre- optimal coupling to high-Q whispering-gallery modes,” J. Opt. Soc. Am. B quency comb using soliton Cherenkov radiation,” Science 351, 357– 16, 147–154 (1999). 360 (2016). 32. D. Rowland and J. Love, “Evanescent wave coupling of whispering gal- 6. V. S. Ilchenko, A. A. Savchenkov, A. B. Matsko, and L. Maleki, “Nonlinear lery modes of a dielectric cylinder,” IEE Proc. J. 140, 177–188 (1993). optics and crystalline whispering gallery mode cavities,” Phys. Rev. Lett. 33. M. Cai, O. Painter, and K. J. Vahala, “Observation of critical coupling in a 92, 043903 (2004). fiber taper to a silica-microsphere whispering-gallery mode system,” 7. K. J. Vahala, “Optical microcavities,” Nature 424, 839–846 (2003). Phys. Rev. Lett. 85,74–77 (2000). 8. A. B. Matsko and V. S. Ilchenko, “Optical resonators with whispering gal- 34. V. S. Ilchenko, X. S. Yao, and L. Maleki, “Pigtailing the high-Q micro- lery modes I: basics,” IEEE J. Sel. Top. Quantum Electron. 12,3–14 sphere cavity: a simple fiber coupler for optical whispering-gallery (2006). modes,” Opt. Lett. 24, 723–725 (1999). 9. V. S. Ilchenko and A. B. Matsko, “Optical resonators with whispering- 35. L. Maleki, “Sources: the optoelectronic oscillator,” Nat. Photonics 5, gallery modes-part II: applications,” IEEE J. Sel. Top. Quantum 728–730 (2011). Electron. 12,15–32 (2006). 36. V. R. Almeida, C. A. Barrios, R. R. Panepucci, and M. Lipson, “All-optical 10. D. Armani, T. Kippenberg, S. Spillane, and K. Vahala, “Ultra-high-Q control of light on silicon chip,” Nature 431, 1081–1084 (2004). toroid microcavity on a chip,” Nature 421, 925–928 (2003). 37. D. J. Moss, R. Morandotti, A. L. Gaeta, and M. Lipson, “New CMOS- 11. M. Aspelmeyer, T. J. Kippenberg, and F. Marquardt, “Cavity optome- compatible platforms based on silicon nitride and Hydex for nonlinear chanics,” Rev. Mod. Phys. 86, 1391–1452 (2014). optics,” Nat. Photonics 7, 597–607 (2013). 12. T. J. Kippenberg and K. J. Vahala, “Cavity optomechanics: back-action 38. B. Little, J.-P. Laine, D. Lim, H. Haus, L. Kimerling, and S. Chu, “Pedestal at the mesoscale,” Science 321, 1172–1176 (2008). antiresonant reflecting waveguides for robust coupling to microsphere 13. J. Pfeifle, V. Brasch, M. Lauermann, Y. Yu, D. Wegner, T. Herr, K. resonators and for microphotonic circuits,” Opt. Lett. 25,73–75 (2000). Hartinger, P. Schindler, J. Li, D. Hillerkuss, R. Schmogrow, C. 39. T. Le, A. A. Savchenkov, H. Tazawa, W. H. Steier, and L. Maleki, Weimann, R. Holzwarth, W. Freude, J. Leuthold, T. J. Kippenberg, “Polymer optical waveguide vertically coupled to high-Q whispering gal- and C. Koos, “Coherent terabit communications with microresonator lery resonators,” IEEE Photon. Technol. Lett. 18, 859–861 (2006). Kerr frequency combs,” Nat. Photonics 8, 375–380 (2014). 40. X. Zhang and A. M. Armani, “Silica microtoroid resonator sensor with 14. J. Zhu, S. K. Ozdemir, Y. Xiao, L. Li, L. He, D. Chen, and L. Yang, “On- monolithically integrated waveguides,” Opt. Express 21, 23592–23603 chip single nanoparticle detection and sizing by mode splitting in an ultra- (2013). high-Q microresonator,” Nat. Photonics 4,46–49 (2010). 41. F. Blom, D. Van Dijk, H. Hoekstra, A. Driessen, and T. J. Popma, 15. B. Peng, S. K. Ozdemir, F. Lei, F. Monifi, M. Gianfreda, G. L. Long, S. “Experimental study of integrated-optics microcavity resonators: toward Fan, F. Nori, C. M. Bender, and L. Yang, “Parity-time-symmetric an all-optical switching device,” Appl. Phys. Lett. 71, 747–749 (1997). whispering-gallery microcavities,” Nat. Phys. 10, 394–398 2014. 42. G. N. Conti, S. Berneschi, F. Cosi, S. Pelli, S. Soria, G. C. Righini, M. 16. M. D. Baaske, M. R. Foreman, and F. Vollmer, “Single-molecule nucleic Dispenza, and A. Secchi, “Planar coupling to high-Q lithium niobate disk acid interactions monitored on a label-free microcavity biosensor plat- resonators,” Opt. Express 19, 3651–3656 (2011). form,” Nat. Nanotechnol. 9, 933–939 (2014). 43. A. A. Savchenkov, H. Mahalingam, V. S. Ilchenko, S. Takahashi, A. B. 17. T. Herr, V. Brasch, J. D. Jost, C. Y. Wang, N. M. Kondratiev, M. L. Matsko, W. H. Steier, and L. Maleki, “Polymer waveguide couplers for fluo- Gorodetsky, and T. J. Kippenberg, “Temporal solitons in optical micro- rite microresonators,” IEEE Photon. Technol. Lett. 29,667–670 (2017). resonators,” Nat. Photonics 8, 145–152 (2014). 44. M. Ghulinyan, F. Ramiro-Manzano, N. Prtljaga, R. Guider, I. Carusotto, 18. S. Spillane, T. Kippenberg, and K. Vahala, “Ultralow-threshold Raman laser A. Pitanti, G. Pucker, and L. Pavesi, “Oscillatory vertical coupling be- using a spherical dielectric microcavity,” Nature 415, 621–623 (2002). tween a whispering-gallery resonator and a bus waveguide,” Phys. 19. F. Vollmer and S. Arnold, “Whispering-gallery-mode biosensing: label-free Rev. Lett. 110, 163901 (2013). detection down to single molecules,” Nat. Methods 5, 591–596 (2008). 45. P. Tien and R. Ulrich, “Theory of prism-film coupler and thin-film light 20. L. Razzari, D. Duchesne, M. Ferrera, R. Morandotti, S. Chu, B. E. Little, guides,” J. Opt. Soc. Am. 60, 1325–1337 (1970). and D. J. Moss, “CMOS-compatible integrated optical hyper-parametric 46. R. Ulrich, “Theory of the prism-film coupler by plane-wave analysis,” oscillator,” Nat. Photonics 4,41–45 (2010). J. Opt. Soc. Am. 60, 1337–1350 (1970). 21. J. S. Levy, A. Gondarenko, M. A. Foster, A. C. Turner-Foster, A. L. 47. M. Gorodetsky and V. Ilchenko, “High-Q optical whispering-gallery micro- Gaeta, and M. Lipson, “CMOS-compatible multiple-wavelength oscillator resonators: precession approach for spherical mode analysis and emission for on-chip optical interconnects,” Nat. Photonics 4,37–40 (2010). patterns with prism couplers,” Opt. Commun. 113,133–143 (1994). 22. D. T. Spencer, J. F. Bauters, M. J. Heck, and J. E. Bowers, “Integrated 48. R. Ulrich and R. Torge, “Measurement of thin film parameters with a

waveguide coupled Si3N4 resonators in the ultrahigh-Q regime,” Optica 1, prism coupler,” Appl. Opt. 12, 2901–2908 (1973). 153–157 (2014). 49. G. Liu, K. Shang, S. Li, T. Su, Y. Zhang, S. Feng, R. Proietti, S. J. B. Yoo, 23. M. Zhang, C. Wang, R. Cheng, A. Shams-Ansari, and M. Lončar, V. S. Ilchenko, W. Liang, A. A. Savchenkov, A. B. Matsko, and L. Maleki, “Monolithic ultra-high-Q lithium niobate microring resonator,” Optica 4, “Low-loss on-chip prism-waveguide coupler to high-Q micro- 1536–1537 (2017). resonator and optical frequency comb generation,” in Optical Fiber 24. P. Del’Haye, T. Herr, E. Gavartin, M. Gorodetsky, R. Holzwarth, and T. J. Communications Conference and Exhibition (OFC) (IEEE, 2017), pp. 1–3. Kippenberg, “Octave spanning tunable frequency comb from a microre- 50. K. Okamoto, Fundamentals of Optical Waveguides (Academic, 2010). sonator,” Phys. Rev. Lett. 107, 063901 (2011). 51. Y. A. Vlasov and S. J. McNab, “Losses in single-mode silicon-on- 25. A. Gondarenko, J. S. Levy, and M. Lipson, “High confinement micron- insulator strip waveguides and bends,” Opt. Express 12, 1622–1631 (2004). scale silicon nitride high Q ring resonator,” Opt. Express 17, 11366– 52. J. Cardenas, C. B. Poitras, J. T. Robinson, K. Preston, L. Chen, and M. 11370 (2009). Lipson, “Low loss etchless silicon photonic waveguides,” Opt. Express 26. I. S. Grudinin, A. B. Matsko, A. A. Savchenkov, D. Strekalov, V. S. 17, 4752–4757 (2009). Ilchenko, and L. Maleki, “Ultra high Q crystalline microcavities,” Opt. 53. K. Shang, S. Pathak, B. Guan, G. Liu, and S. Yoo, “Low-loss compact Commun. 265,33–38 (2006). multilayer silicon nitride platform for 3D photonic integrated circuits,” Opt. 27. I. S. Grudinin, V. S. Ilchenko, and L. Maleki, “Ultrahigh optical Q factors of Express 23, 21334–21342 (2015). crystalline resonators in the linear regime,” Phys.Rev.A74, 063806 (2006). 54. A. Himeno, K. Kato, and T. Miya, “Silica-based planar lightwave circuits,” 28. A. A. Savchenkov, A. B. Matsko, V. S. Ilchenko, and L. Maleki, “Optical IEEE J. Sel. Top. Quantum Electron. 4, 913–924 (1998). resonators with ten million finesse,” Opt. Express 15, 6768–6773 (2007). 55. K. Shang, S. Pathak, G. Liu, S. Feng, S. Li, W. Lai, and S. J. B. Yoo, 29. D. Farnesi, G. C. Righini, A. Barucci, S. Berneschi, F. Chiavaioli, F. Cosi, “Silicon nitride tri-layer vertical Y-junction and 3D couplers with arbitrary S. Pelli, S. Soria, C. Trono, D. Ristic, M. Ferrari, and G. Nunzi Conti, splitting ratio for photonic integrated circuits,” Opt. Express 25, 10474– “Coupling light to whispering gallery mode resonators,” Proc. SPIE 10483 (2017). 9133, 913314 (2014). 56. J. F. Bauters, M. J. R. Heck, D. D. John, J. S. Barton, C. M. Bruinink, A. 30. S. Spillane, T. Kippenberg, O. Painter, and K. Vahala, “Ideality in a fiber- Leinse, R. G. Heideman, D. J. Blumenthal, and J. E. Bowers, “Planar taper-coupled microresonator system for application to cavity quantum waveguides with less than 0.1 dB/m propagation loss fabricated with wa- electrodynamics,” Phys. Rev. Lett. 91, 043902 (2003). fer bonding,” Opt. Express 19, 24090–24101 (2011). Research Article Vol. 5, No. 2 / February 2018 / Optica 226

57. M. J. Heck, J. F. Bauters, M. L. Davenport, D. T. Spencer, and J. E. 60. M. L. Gorodetsky, A. D. Pryamikov, and V. S. Ilchenko, “Rayleigh scatter- Bowers, “Ultra-low loss waveguide platform and its integration with sil- ing in high-Q microspheres,” J. Opt. Soc. Am. B 17, 1051–1057 (2000). icon photonics,” Laser Photon. Rev. 8, 667–686 (2014). 61. B. Dahmani, L. Hollberg, and R. Drullinger, “Frequency stabilization of 58. W. Bogaerts, P. De Heyn, T. Van Vaerenbergh, K. De Vos, S. Kumar semiconductor lasers by resonant optical feedback,” Opt. Lett. 12, Selvaraja, T. Claes, P. Dumon, P. Bienstman, D. Van Thourhout, and 876–878 (1987). R. Baets, “Silicon microring resonators,” Laser Photon. Rev. 6,47–73 62. W. Liang, V. Ilchenko, A. Savchenkov, A. Matsko, D. Seidel, and L. (2012). Maleki, “Whispering-gallery-mode-resonator-based ultranarrow line- 59. R. Lang, “Injection locking properties of a semiconductor laser,” IEEE J. width external-cavity semiconductor laser,” Opt. Lett. 35, 2822–2824 Quantum Electron. 18, 976–983 (1982). (2010).