https://doi.org/1.138/s41586-18-598-9 Bttery-operted integrted frequency com genertor Brin Stern 1,2, Xingchen Ji 1,2, Yoshitomo Okwchi 3, Alexnder L. Get 3 & Michl Lipson 2 * Opticl frequency coms re rodnd sources tht offer mutully coherent, equidistnt spectrl lines with unprecedented precision in frequency nd timing for n rry of pplictions 1. Frequency coms generted in microresontors through the Kerr nonlinerity require single-frequency pump lser nd hve the potentil to provide highly compct, sclle nd power-efficient devices 2,3. Here we demonstrte device lser-integrted Kerr frequency com genertor tht fulfils this potentil through use of extremely lowloss silicon nitride wveguides tht form oth the microresontor nd n integrted lser cvity. Our device genertes low-noise soliton-mode-locked coms with repetition rte of 194 gighertz t wvelengths ner 1,55 nnometres using only 98 milliwtts of electricl pump power. The dul-cvity configurtion tht we use comines the lser nd microresontor, demonstrting the flexiility fforded y close integrtion of these components, nd together with the ultr low power consumption should enle production of highly portle nd roust frequency nd timing references, sensors nd signl sources. This chip-sed integrtion of microresontors nd lsers should lso provide tools with which to investigte the dynmics of com nd soliton genertion. Frequency coms sed on chip-scle microresontors offer the potentil for high-precision photonic devices for time nd frequency pplictions using highly compct nd roust pltform. By pumping the microresontor with single-frequency pump lser, dditionl discrete, equidistnt frequencies re generted through prmetric four-wve mixing (FWM), resulting in Kerr frequency com 2,4 7. Under suitle conditions temporl cvity solitons cn e excited, which results in stle, low-noise coms with ultrprecise spcing 8 13. Mny pplictions require such tight frequency nd timing stility, including spectroscopy 14 16, low-noise microwve genertion 17, photonic frequency synthesis 18, opticl clocks 19, distnce rnging 2,21 nd telecommunictions 22. Although one of the most compelling dvntges for microresontor coms is the potentil for the pump source nd the microresontor to e fully integrted, previous demonstrtions using integrted resontors hve relied on externl pump lsers tht re typiclly lrge, expensive nd power-hungry, preventing pplictions where size, portility nd low power consumption re criticl. Power-efficient integrted lsers hve een developed using silicon lser cvities with onded or ttched III V mterils to provide opticl gin 23 26, ut losses in these silicon wveguides mke com genertion imprcticl t low power. On the other hnd, silicon nitride (Si 3 N 4 ) microresontors were recently demonstrted with record low prmetric oscilltion thresholds 27 due to the high qulity fctors (Q > 3 1 7 ), high nonliner refrctive index (n 2 2.4 1 19 m 2 W 1 ) nd smll mode volume (ring rdius pproximtely 1 µm). Additionlly, owing to the high index of refrction of Si 3 N 4 (n 2.) nd its low loss, compct, tunle Si 3 N 4 lser cvities with nrrow linewidth hve een demonstrted 28,29. Si 3 N 4 is common complementry metl oxide semiconductor (CMOS)- comptile deposited mteril tht cn e fricted t wfer scle, nd the comintion of efficient com genertion nd ville integrtion of ctive devices mke it n idel pltform for complete integrtion of opticl frequency coms. Here we demonstrte Kerr com source on n integrted hyrid III V/Si 3 N 4 pltform, using compct, low-power, electriclly pumped source. In our pproch (Fig. 1), gin section sed on III V reflective semiconductor opticl mplifier (RSOA) is coupled to Si 3 N 4 lser cvity, which consists of two Vernier microring filters for wvelength tunility nd high-q nonliner microresontor (Fig. 1). The nonliner microresontor serves two purposes. First, it genertes nrrownd ck-reflection due to coupling etween counterpropgting circulting ems resulting from Ryleigh scttering 3, effectively serving s n output mirror of the pump lser cvity, s we previously demonstrted 28. Second, the microresontor genertes frequency com through prmetric FWM. In this wy, the com genertion nd pump lser re inherently ligned, configurtion tht ws previously explored using resontors in fire lser cvities with fire mplifiers 31,32. Integrting the com source with the lser llows the flexiility to use such configurtion, voiding the typicl chin of discrete components found in ll previous Kerr com demonstrtions. Figure 1c shows the ssemled millimetre-sized com source, which hs only electricl inputs nd n opticl output (see Methods for friction detils). We designed the Si 3 N 4 lser cvity to ensure tunle, single-mode lsing nd provide sufficient pump output power for com genertion in the nonliner microresontor. The lsing wvelength is controlled y the lignment of the two microring Vernier filters 25, which re in turn ligned with one of the modes of the lrger microresontor shown in Fig. 1. The filters rdii re 2 µm nd 22 µm, corresponding to free spectrl rnges (FSRs) of 1.18 THz nd 1.7 THz, respectively, which result in trnsmission t only single frequency when the filters re ligned. Their resonnce positions cn e widely tuned using integrted resistive microheters, s shown in Fig. 2. The filters trnsmission ndwidth is designed to hve full-width t hlf-mximum of 15 GHz y ensuring strong coupling to the two djcent wveguides with 5 µm coupling length. The opticl gin in the lser cvity comes from electricl pumping of the III V wveguide on the RSOA, which is coupled to the Si 3 N 4 cvity t one end nd strongly reflects t the opposite end (see Methods). The output coupler of the lser cvity is 12-µm-rdius microresontor with mesured reflection of 4% on resonnce, s shown in Fig. 2. This level of reflection llows for high lser output power due to the high round-trip gin of the RSOA. The mesured trnsmission spectrum of the microresontor (Fig. 2) corresponds to n intrinsic Q of (8. ±.8) 1 6. Bsed on this Q nd the nomlous group-velocity dispersion for the 73 nm 1,8 nm wveguide, simultions indicte tht soliton-stte frequency com cn e generted with 7 µw of pump power in the us wveguide just efore the microresontor (Extended Dt Fig. 1). We find lsing with up to 9.5 mw output opticl power using the integrted Si 3 N 4 lser. In order to chrcterize the lser, we first operte the microresontor slightly detuned from resonnce to ensure tht only 1 School of Electricl nd Computer Engineering, Cornell University, Ithc, NY, USA. 2 Deprtment of Electricl Engineering, Columi University, New York, NY, USA. 3 Deprtment of Applied Physics nd Applied Mthemtics, Columi University, New York, NY, USA. *e-mil: ml3745@columi.edu
Driver circuit III V mplifier (RSOA) c 25 μm Amplifier Microresontor Com output 5 μm Microring filters Si 3 N 4 chip High-Q microresontor Bckreflection Four-wve mixing Com output Microheter wires RSOA Si 3 N 4 microresontor Filters Heter wires Output fire Fig. 1 Integrted frequency com source., The concept of n integrted Kerr com source with n on-chip mplifier nd microresontor., Microscope imge nd digrm of the integrted com source, including the lser cvity nd the high-q nonliner microresontor for com genertion. The reflective III V semiconductor opticl mplifier (RSOA) wveguide provides electriclly pumped opticl gin nd includes reflective fcet on one end (top), while the opposite side is coupled to lsing occurs nd frequency com is not generted. We oserve lsing with side-mode suppression rtio (SMSR) of more thn 6 db (Fig. 2c). As shown in Fig. 2d, the lsing threshold is 49 ma, with slope efficiency of 52 mw A 1. The mximum on-chip output power of 9.5 mw is otined t 277 mw (22 ma) electricl pump power consumption, P elec. This corresponds to 3.4% wll-plug efficiency (tht is, output opticl power divided y electricl power). Additionlly, we mesure nrrow lser linewidth of 4 khz using the delyed self-heterodyne method (see Methods). The reltively high output power nd nrrow linewidth re competitive with those of mny ulk pump lsers, yet the present lser is much more compct. the Si 3 N 4 portion of the lser cvity. The microring filters nd the lrger microresontor re tunle using integrted microheters. The ltter genertes prtilly reflected em to form second effective mirror of the lser cvity. This microresontor lso hs high Q to enle FWM nd com genertion. c, Photogrph of the integrted com source. The RSOA is edge-coupled to the Si 3 N 4 chip nd supplied with electric current vi wires, while the com output is mesured using n opticl fire. Using our cvity design, we generte Kerr com spnning more thn 8 THz nd chieve mode-locked, single-soliton stte with P elec less thn 1 mw, enling ttery-opertion pplictions. We oserve new opticl frequencies eginning to pper djcent to the 1,579 nm pump owing to FWM in the microresontor once the lser power mesured fter the ring (P opt ) reches threshold of 1.1 mw t P elec = 78 mw. We then increse P elec ove threshold to 13 mw nd monitor com formtion s the microresontor is tuned using its integrted microheter (see Methods for the set-up nd tuning procedure). When the microresontor is first detuned slightly, we mesure P opt = 2.5 mw for the single lsing mode (Fig. 3). As the microresontor is tuned into resonnce, greter circulting power leds to com Trnsmission (db) c 5 1 15 2 1,55 1,56 1,57 1,58 1,59 1,6 Wvelength (nm) 2 4 6 6 db SMSR 1,5 1,55 1,6 1,65 Wvelength (nm) Fig. 2 Chrcteriztion of the integrted III V/Si 3 N 4 lser., Mesured trnsmission spectr (normlized) for the Vernier filter microrings (filter 1 nd filter 2, digrm t right). By djusting the voltge pplied to the microheters, the filters reltive detuning is djusted nd single trnsmission wvelength is selected. Key t right shows voltge pplied in the formt filter 1, filter 2., Mesured opticl trnsmission nd reflection spectr (normlized) of the high-q microresontor. The 32-MHz resonnce ndwidth revels Q of 8 1 6. The nrrownd Filter tuning (V) 6.,. 5.5, 1.6 4.8, 2.3 4.2, 3. 4.9, 5.1 4.7, 5.9 Filter 1 Filter 2 d Output power (mw) 1 8 6 4 2 Normlized power 1.8.6.4.2 Lsing threshold Microresontor 5 5 Detuning (MHz) Trns. Refl. 52 mw A 1 5 1 15 2 Current (ma) reflection is generted y coupling vi Ryleigh scttering etween counter-propgting ems in the ring (rrows show em directions, colour code s spectr), which is pprent due to the resonnce splitting oserved from these degenerte ems. c, Lser output spectrum t 85 ma showing single-mode lsing with side-mode suppression rtio (SMSR) of more thn 6 db. d, Output opticl power of lser versus pump current t 1,58 nm with slope efficiency of 52 mw A 1.
2 4 Pump lser 4 5 6 7 8 6 9 2 4 6 Com formtion 4 5 6 7 8 9 c d 2 4 6 2 4 6 Single soliton sech 2 fit Bttery-operted SNR 2 4 6 1,58 1,584 1,52 1,56 1,6 1,64 Wvelength (nm) Fig. 3 Genertion of mode-locked soliton frequency coms. c, Left, spectr of output from the com source s mesured y n opticl spectrum nlyser t vrying stges of com genertion; right, corresponding RF spectr (resolution ndwidth 1 khz). A current supply provides electricl pump power of 13 mw., Spectrum of the lser output efore tuning fully into resonnce. The RF noise is low since there is only single-frequency lsing., Spectrum of the frequency com. Becuse the com is not yet mode-locked, eting etween different com lines produces high RF noise elow 2 MHz. c, Single-soliton frequency com spectrum with the chrcteristic sech profile (see Extended Dt Fig. 2). Inset, the signl-to-noise rtio (SNR, grey verticl rrow) is 5 7 9 2 4 6 4 5 6 7 8 9 2 4 6 8 RF frequency (MHz) Com source AAA ttery pproximtely 5 db; vriles plotted on the xes re the sme s in the min pnel. The com linewidth is seprtely mesured s 4 khz (see Methods). The RF spectrum confirms the trnsition to low-noise stte. d, Left pnel, frequency com spectrum mtching soliton sech profile generted with n AAA ttery supplying pump power of 98 mw. Left inset, digrm of ttery operted device, showing filters 1 nd 2 (see Fig. 2) nd the microresontor (lrge circle). Right inset, RF spectrum showing low-noise stte; y xis, power in dbm, x xis, RF frequency in MHz. Right pnel, photogrph of integrted com source (shown oxed in red) with printed circuit ord nd the ttery (rrowed) next to US qurter for scle. formtion, ccompnied y high rdio frequency (RF) noise (Fig. 3). Tuning the resonnce further results in stle coms with smooth spectrl envelopes chrcteristic of temporl cvity solitons 8. We mesure single-soliton stte with 8.6 THz (72 nm) 3-dB ndwidth ccompnied y drop in RF noise (Fig. 3c). Once generted, the soliton exhiits stle ehviour without feedck electronics or temperture control, with no visile chnges in the opticl spectrum or output power until the microresontor is intentionlly detuned. The power of the com lines totls.24 mw, indicting tht higher effective pump power my e resulting from our plcement of the microresontor in the lser cvity. Such efficient opertion llows us to lso show ttery-opertion of the com source y supplying the pump current using stndrd AAA ttery. At P elec = 98 mw from the ttery, we mesure P opt = 1.3 mw nd com mtching the single-soliton profile (Fig. 3d). These results represent unprecedented low-power consumption for generting Kerr frequency coms nd solitons with n integrted microresontor. In order to show the verstility of this pltform, we demonstrte more trditionl ut still lser-integrted configurtion in which the com is generted in microresontor tht is distinct from the pump lser. In this second design, shown in Fig. 4, the Vernier filters nd RSOA function in the sme wy s in the first design, ut Sgnc loop mirror is now included to serve s the output coupler with pproximtely 2% reflection. Becuse this mirror hs rodnd reflection, tunle lsing cn tke plce independent of the resonnce position of the com microresontor. With the microresontor fully off-resonnce, we mesure single-mode lsing t 1,582 nm with P opt = 4.9 mw nd over 6 db SMSR (Fig. 4) t P elec = 162 mw. By tuning the microresontor into resonnce with the lser wvelength, we cn generte frequency com (Fig. 4c). Through further tuning of the resonnce (see Methods), we oserve multiple-soliton-stte frequency com spnning 13.4 THz (15 nm) 3-dB ndwidth with the chrcteristic drop in RF noise (Fig. 4d). We model two-soliton-stte com nd otin profile closely mtching tht of the experimentl com (Fig. 4d). Single-soliton coms should lso e chievle with this configurtion, ut in this device we only oserved two or more solitons. Multiple-soliton coms in microresontors hve een used previously to demonstrte dul-com spectroscopy, for exmple 16. The mesured com power is 8 µw, corresponding to conversion efficiency of 1.6%. The com power scles with the numer of solitons, s does the numer
III V mplifier (RSOA) Si 3 N 4 chip Pump lser Filter 1 Pump output High-Q microresontor Four-wve mixing Com output Filter 2 Sgnc mirror 2 4 6 c 2 4 6 Pump lser Com formtion 8 9 1 11 12 13 8 9 1 11 12 13 d 2 4 6 8 Two-soliton stte Mesured 9 Simultion 1 11 12 13 2 4 6 8 1,55 1,6 1,65 RF frequency (MHz) Wvelength (nm) Fig. 4 Modulr configurtion of the integrted com source., Schemtic of the modulr com source configurtion. Here the integrted lser (turquoise dshed ox) is distinct from the nonliner microresontor, with Sgnc loop mirror serving s the lser output coupler. The rrows show the pth of light trvelling through the lser cvity nd reflecting ck t the reflective end (left) of the RSOA nd t the Sgnc mirror, with the lser output prtilly trnsmitting through the ltter. d, Opticl output spectr t vrying stges of com genertion (left pnel) with corresponding RF spectr (right pnel; resolution ndwidth 1 khz)., Spectrum of lser output. The RF noise is low ecuse there is only single-frequency lsing. c, Spectrum of frequency com efore mode-locking with ssocited high RF noise. d, Spectrum of two-soliton frequency com. The RF spectrum confirms the low-noise stte. of pulses per round-trip. In Methods, we discuss the reltive dvntges of the two designs. This demonstrtion of lser-integrted Kerr com source presents opportunities in mny fields tht rely on the precision nd stility of frequency coms nd solitons, including sensing, metrology, communictions nd wveform genertion. The low power consumption of our pltform enles these pplictions in ttery-powered nd moile system without the need for externl lsers, movele optics, or lortory set-ups. Our pltform is CMOS-comptile for wfer-scle friction of roust integrted photonic chips, potentilly enling wide deployment of precision devices, such s portle spectrometers for moleculr sensing 14,15 or vehicle-mounted systems for distnce rnging 2,21. In future implementtions, the RSOA could e plced directly on the silicon sustrte, through pssively ligned mounting 25 or mteril onding 23, tking dvntge of the infrstructure for ssemly nd pckging of III V nd silicon chips tht is lredy scled to mss production for silicon photonic trnsceivers. Additionl photonic components such s filters for wvelength-division multiplexing 22 or wveguide couplers for mixing multiple coms 14,15,18 could lso e plced on-chip to comine frequency coms with more complex integrted photonic circuits. Online content Any methods, dditionl references, Nture Reserch reporting summries, source dt, sttements of dt vilility nd ssocited ccession codes re ville t https://doi.org/1.138/s41586-18-598-9. Received: 3 Mrch 218; Accepted: 8 August 218; Pulished online xx xx xxxx. 1. Newury, N. R. Serching for pplictions with fine-tooth com. Nt. Photon. 5, 186 188 (211). 2. Del Hye, P. et l. Opticl frequency com genertion from monolithic microresontor. Nture 45, 1214 1217 (27). 3. Psquzi, A. et l. Micro-coms: novel genertion of opticl sources. Phys. Rep. 729, 1 81 (218). 4. Jung, H., Xiong, C., Fong, K. Y., Zhng, X. & Tng, H. X. Opticl frequency com genertion from luminum nitride microring resontor. Opt. Lett. 38, 281 2813 (213). 5. Svchenkov, A. A. et l. Tunle opticl frequency com with crystlline whispering gllery mode resontor. Phys. Rev. Lett. 11, 9392 (28). 6. Levy, J. S. et l. CMOS-comptile multiple-wvelength oscilltor for on-chip opticl interconnects. Nt. Photon. 4, 37 4 (21). 7. Rzzri, L. et l. CMOS-comptile integrted opticl hyper-prmetric oscilltor. Nt. Photon. 4, 41 45 (21). 8. Herr, T. et l. Temporl solitons in opticl microresontors. Nt. Photon. 8, 145 152 (214). 9. Sh, K. et l. Modelocking nd femtosecond pulse genertion in chip-sed frequency coms. Opt. Express 21, 1335 1343 (213). 1. Yi, X., Yng, Q.-F., Yng, K. Y., Suh, M.-G. & Vhl, K. Soliton frequency com t microwve rtes in high-q silic microresontor. Optic 2, 178 185 (215). 11. Yu, M., Okwchi, Y., Griffith, A. G., Lipson, M. & Get, A. L. Mode-locked mid-infrred frequency coms in silicon microresontor. Optic 3, 854 86 (216). 12. Xue, X. et l. Mode-locked drk pulse Kerr coms in norml-dispersion microresontors. Nt. Photon. 9, 594 6 (215). 13. Volet, N. et l. Micro-resontor soliton generted directly with diode lser. Lser Photonics Rev. 12, 1737 (218).
14. Suh, M.-G., Yng, Q.-F., Yng, K. Y., Yi, X. & Vhl, K. J. Microresontor soliton dul-com spectroscopy. Science 354, 6 63 (216). 15. Dutt, A. et l. On-chip dul-com source for spectroscopy. Sci. Adv. 4, e171858 (218). 16. Yu, M. et l. Silicon-chip-sed mid-infrred dul-com spectroscopy. Nt. Commun. 9, 1869 (218). 17. Ling, W. et l. High spectrl purity Kerr frequency com rdio frequency photonic oscilltor. Nt. Commun. 6, 7957 (215). 18. Spencer, D. T. et l. An opticl-frequency synthesizer using integrted photonics. Nture 557, 81 85 (218). 19. Ppp, S. B. et l. Microresontor frequency com opticl clock. Optic 1, 1 14 (214). 2. Suh, M.-G. & Vhl, K. J. Soliton microcom rnge mesurement. Science 359, 884 887 (218). 21. Troch, P. et l. Ultrfst opticl rnging using microresontor soliton frequency coms. Science 359, 887 891 (218). 22. Mrin-Plomo, P. et l. Microresontor-sed solitons for mssively prllel coherent opticl communictions. Nture 546, 274 279 (217). 23. Fng, A. W. et l. Electriclly pumped hyrid AlGInAs-silicon evnescent lser. Opt. Express 14, 923 921 (26). 24. Vn Cmpenhout, J. et l. Electriclly pumped InP-sed microdisk lsers integrted with nnophotonic silicon-on-insultor wveguide circuit. Opt. Express 15, 6744 6749 (27). 25. Koyshi, N. et l. Silicon photonic hyrid ring-filter externl cvity wvelength tunle lsers. J. Lightwve Technol. 33, 1241 1246 (215). 26. Lee, J.-H. et l. Demonstrtion of 12.2% wll plug efficiency in uncooled single mode externl-cvity tunle Si/III-V hyrid lser. Opt. Express 23, 1279 1288 (215). 27. Ji, X. et l. Ultr-low-loss on-chip resontors with su-milliwtt prmetric oscilltion threshold. Optic 4, 619 624 (217). 28. Stern, B., Ji, X., Dutt, A. & Lipson, M. Compct nrrow-linewidth integrted lser sed on low-loss silicon nitride ring resontor. Opt. Lett. 42, 4541 4544 (217). 29. Oldeneuving, R. M. et l. 25 khz nrrow spectrl ndwidth of wvelength tunle diode lser with short wveguide-sed externl cvity. Lser Phys. Lett. 1, 1584 (213). 3. Ling, W. et l. Whispering-gllery-mode-resontor-sed ultrnrrow linewidth externl-cvity semiconductor lser. Opt. Lett. 35, 2822 2824 (21). 31. Psquzi, A. et l. Self-locked opticl prmetric oscilltion in CMOS comptile microring resontor: route to roust opticl frequency com genertion on chip. Opt. Express 21, 13333 13341 (213). 32. Johnson, A. R. et l. Microresontor-sed com genertion without n externl lser source. Opt. Express 22, 1394 141 (214). Acknowledgements We re grteful to S. Miller, C. Joshi, T. Lin, U. Dve nd J. Jng for discussions nd to M. Yu for help with soliton simultions. We lso thnk M. C. Shin nd O. Jimenez for pckging dvice. This work ws supported y AFRL progrmme wrd numer FA865-17-P-185; the ARPA-E ENLITENED progrmme (DE-AR843); the Defense Advnced Reserch Projects Agency (DARPA) under the Microsystems Technology Office Direct On-Chip Digitl Opticl Synthesizer (DODOS) progrm (N661-16-1-452) nd the Modulr Opticl Aperture Building Blocks (MOABB) progrmme (HR11-16-C-17); the STTR progrmme (N14-16-P-3); nd the Air Force Office of Scientific Reserch (AFOSR) (FA955-15-1-33). X.J. cknowledges the Chin Scholrship Council for finncil support. This work ws performed in prt t the Cornell NnoScle Fcility, n NNCI memer supported y NSF grnt ECCS-154281. Reviewer informtion Nture thnks W. Freude nd the other nonymous reviewer(s) for their contriution to the peer review of this work. Author contriutions B.S. conceived the work, designed nd ssemled the devices, performed the mesurements, nd prepred the mnuscript. X.J. fricted the devices. B.S. nd X.J. chrcterized the microring trnsmission. Y.O. simulted the soliton coms. M.L. nd A.L.G. supervised the project. All uthors discussed the results nd edited the mnuscript. Competing interests All uthors re listed s inventors in ptent ppliction relted to this work, filed y Columi University. Additionl informtion Extended dt is ville for this pper t https://doi.org/1.138/s41586-18-598-9. Reprints nd permissions informtion is ville t http://www.nture.com/ reprints. Correspondence nd requests for mterils should e ddressed to M.L. Pulisher s note: Springer Nture remins neutrl with regrd to jurisdictionl clims in pulished mps nd institutionl ffilitions.
METHODS Friction. The Si 3 N 4 devices re fricted 27 y first growing 4 µm of SiO 2 on crystlline silicon wfer using therml oxidtion to form the ottom cldding of the wveguides. Then 73 nm of Si 3 N 4 is deposited using low pressure chemicl vpour deposition (LPCVD). The wfer is nneled in two stges to remove hydrogen impurities. The wveguides re then ptterned using electron em lithogrphy nd etched using CHF 3 plsm etching. The wveguides re cld with 2 µm SiO 2. The microheters re plced over the wveguides using 1 nm of sputtered pltinum (with titnium dhesion lyer) nd lift-off ptterning. RSOA/Si 3 N 4 coupling nd electricl connection. The III V RSOA gin chip used here is commercilly ville from Thorls (SAF 1126) nd provides rod gin ner 1,55 nm. One side hs 93% reflection nd the other side is nti-reflection coted. This second side is coupled to the Si 3 N 4 chip with the wveguides ngled reltive to the fcets to further prevent reflections 28. The Si 3 N 4 chip is polished up to the end of tpered 28-nm-wide wveguide which is simulted to hve less thn 1 db coupling loss to the mode of the RSOA wveguide. The two chips re ttched nd ligned using three-xis stges with micrometers. We mesure n experimentl 2 db coupling loss. The RSOA is wireonded to n electricl printed circuit ord (PCB) for supplying the pump current from either Keithley 24 SourceMeter or n AAA ttery with tunle potentiometer. The microheters of the Si 3 N 4 chip re connected to pds nd interfced with DC wedge proe (GGB Industries) nd controlled y DAC (Mesurement Computing) supplying out 3 mw to ech heter. The Si 3 N 4 wveguide output is formed s n inverse-tper to edge-couple to lensed single-mode fire. Lser set-up nd com genertion procedure. In order to rech mode-locked soliton coms in the first configurtion, which uses the dul-cvity design, we first clirte the lser y ligning the resonnces of the two Vernier microring filters using the integrted heters. This my e done y monitoring the trnsmitted mplified spontneous emission (ASE) noise through the filters from the RSOA or y using seprte lser to clirte the wvelength tuning. Next, the nonliner microresontor is tuned using its heter to lign to the filters resonnces. Once the three re ligned with the pump current ove threshold, the device egins to lse. The cvity phse shifter heter, which is positioned over section of wveguide etween the filters nd the RSOA, is then tuned to mximize the output power, nd the filters my gin e djusted slightly to mximize the output. After this initiliztion procedure, the resonnce of the nonliner microresontor is tuned to longer wvelength such tht the originl lsing mode is lue-detuned nd lsing ceses ecuse the microresontor is no longer on resonnce to provide the ck-reflection s the lser s output mirror. From this point, the heter is tuned ck in the opposite direction to lue-shift the resonnce nd go through the stges of Fig. 3 c: first lsing, then chotic com genertion, nd finlly soliton sttes 33. The resonnce producing soliton sttes corresponds to n effectively red-detuned lser 8, where the detuning results in typicl pump-to-com conversion efficiency of severl per cent 34. With further tuning of the resonnce, output power egins to drop nd eventully lsing ceses once the microresontor is fully detuned from the filters nd the cvity mode. This procedure llows repetle genertion of soliton sttes y tuning t rtes up to out 1 khz using function genertor pplying tringle wve voltge to the heter, s shown previously y Joshi et l. 33 ; however, we re lso often le to rech the soliton sttes y mnul tuning of the heter voltge without function genertor. This reltive ese of mode-locking is likely to e feture of the self-ligning dul-cvity configurtion. In the second, modulr configurtion, the soliton genertion procedure is identicl to the first, with the exception tht lsing my tke plce with the nonliner microresontor off-resonnce, llowing simpler clirtion set-up ut without the inherent lignment of the microresontor. In oth configurtions, we were lterntively le to tune the lser from shorter to longer wvelengths cross the microresontor resonnce using the cvity phse shifter nd lso chieve soliton mode-locked coms. Owing to the low pump power needed to generte frequency coms in the microresontor, we did not oserve significnt therml shifts in the resonnce. If scled to higher powers where such shifts ecome stronger, the speed of the resonnce tuning cn e djusted to mtch the power dissiption in the soliton stte 8,33. While we did not require ctive feedck to mintin the soliton stte during our experiments (timescles up to n hour), future systems could ccount for environmentl fluctutions using ctive feedck 35 to stilize the soliton sttes indefinitely. This feedck nd the initiliztion procedure could potentilly e controlled using low-power microcontroller integrted longside the photonic chip (Fig. 1) implementing pulse width modultion to efficiently tune the heters 36. Comprison of the designs. The two designs demonstrted here, consisting of dul-cvity com source nd trditionl modulr configurtion, enle new flexiility in designing the pump lser for generting the frequency com. The dul-cvity configurtion ensures tht the microresontor is inherently ligned with the lser ecuse the feedck reflection completes the lser cvity 37. Detuning the microresontor is still possile, ut we oserved lower sensitivity to the exct heter settings thn found in the modulr design, llowing for esier tuning into soliton mode-locked coms through mnul tuning (lthough the utomted tuning procedure ove ws successfully pplied to oth designs). Additionlly, this first design showed strong output com power reltive to the pump output. Becuse the microresontor is prt of the lser cvity, we cnnot directly mesure the pump power input to the microresontor, ut the theoreticl conversion efficiency for solitons 34 suggests tht the effective pump input my e notly stronger thn the pump output fter the microresontor. The com genertion process in the second, modulr design is directly nlogous to most previous Kerr com experiments 8,1,33,38,39. Despite the potentil dvntges of the first design, the trditionl pproch my e desirle if the pump lser nd microresontor need to e discretely controlled rther thn tuned together. For exmple, the lser my e locked to stle reference t fixed wvelength, simplifying the tuning controls only the microresontor need e tuned. Lser nd com linewidth mesurement. The lser linewidth is mesured using the delyed self-heterodyne method 28. The lser output t 8 ma pump current is sent to n interferometer with one pth delyed y 12 km of fire (corresponding to dely of 58 µs). The other pth is phse modulted t 3 MHz. The resulting et signl is mesured on n electricl spectrum nlyser (Agilent E447B) nd 4 khz Lorentzin linewidth is determined. The com linewidth is mesured y eting single com line with 1,56 nm 2.4 khz-linewidth reference lser (Redfern Integrted Optics). With pump current of 12 ma, single soliton com is generted (s in Fig. 3c), nd the output is sent to 5:5 coupler, with the other input coming from the reference lser followed y polriztion controller. The heterodyne output is sent to photodiode nd the RF et note corresponds to the com linewidth, which we mesure to e pproximtely 4 khz, mtching tht of the pump lser. Dt vilility The dt tht support the findings of this study re ville from the corresponding uthors on resonle request. 33. Joshi, C. et l. Thermlly controlled com genertion nd soliton modelocking in microresontors. Opt. Lett. 41, 2565 2568 (216). 34. Bo, C. et l. Nonliner conversion efficiency in Kerr frequency com genertion. Opt. Lett. 39, 6126 6129 (214). 35. Yi, X., Yng, Q.-F., Yng, K. Y. & Vhl, K. Active cpture nd stiliztion of temporl solitons in microresontors. Opt. Lett. 41, 237 24 (216). 36. Cong, G. W. et l. Power-efficient gry-scle control of silicon thermo-optic phse shifters y pulse width modultion using monolithiclly integrted MOSFET. In Opticl Fier Communiction Conference (215) M2B.7 (Opticl Society of Americ, 215). 37. Peccinti, M. et l. Demonstrtion of stle ultrfst lser sed on nonliner microcvity. Nt. Commun. 3, 765 (212). 38. Husmnn, B. J. M., Bulu, I., Venktrmn, V., Deotre, P. & Lončr, M. Dimond nonliner photonics. Nt. Photon. 8, 369 374 (214). 39. We, K. E., Erkintlo, M., Coen, S. & Murdoch, S. G. Experimentl oservtion of coherent cvity soliton frequency coms in silic microspheres. Opt. Lett. 41, 4613 4616 (216).
-2-4 -6 Simulted soliton P in = 7 µw 154 156 158 16 162 Wvelength (nm) Extended Dt Fig. 1 Com genertion simultion t low opticl power. Shown is the simulted opticl spectrum of soliton com generted with 7 µw opticl pump power (P in ) in the us wveguide efore the microresontor. The microresontor dimensions used in the model re 73 nm 1,8 nm with rdius of 12 µm, corresponding to 194 GHz FSR.
Simulted soliton -2-4 -6-2 -4 Mesured soliton -6 152 156 16 164 Extended Dt Fig. 2 Comprison of simulted nd mesured solitons., Simultion of single-soliton com generted with 2 mw opticl pump power in the us wveguide efore the microresontor (1.66 mw fter the microresontor). The microresontor dimensions used in the model re 73 nm 1,8 nm with rdius of 12 µm, corresponding to 194 GHz Wvelength (nm) FSR., Opticl spectrum of mesured single-soliton com (from Fig. 3c) with 1.66 mw pump power in the us wveguide fter the microresontor. The sech profile nd com ndwidth qulittively mtch those of the simulted com.