Chip-based Brillouin lasers as spectral purifiers for photonic systems Jiang Li, Hansuek Lee, Tong Chen, Oskar Painter and Kerry Vahala T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA (Dated: January 23, 2012) High coherence lasers are essential in a wide 1.4 range of applications, however, such performance is normally associated with large laser cavities, 1.2 because increasing energy storage reduces quan- 2 tum phase noise and also renders the laser fre- W) 1 m 0 quency less sensitive to cavity vibration. This r ( 1 2 basic scaling rule is at odds with an emerging we set of optical systems that place focus on com- o0.8 n p SBS pact (optimally integrable) sources of high coher- s a e 0 J ence light. These include phase-coherent opti- ok0.6 pump 0 cal communication using quadrature-amplitude- n St m)−20 2 modulation, and also record-low phase noise mi- oui0.4 T6h0rµeWshold m (dB−40 ] cnriqowueasv.e soIunrcetshisbasweodrku,potnheopfitricsat,l ccohmipb-btaescehd- Brill0.2 pectru−60 s t S c Brillouin laser is demonstrated. It features 1s 1,554 1554.3 1554.6 Wavelength (nm) i high-efficiency and single-line operation with the 0 t p smallest recorded Schawlow-Townes frequency 0 0.5 1 1.5 o Pump power (mW) noise for any chip-based laser. Because the fre- . s quency offset between the laser’s emission and c the input pump is relatively small, the device FIG.1: Data showing output SBL power versus input si provides a new function: spectral purification of pump power. Insets: (upper) Image of SBL ultra-high-Q y resonator with magnified edge region; (lower) optical spec- compact, low coherence sources such as semicon- h trumoftheSBLwiththepumpsuppressedonaccountofthe ductor lasers. p contra-directional emission of theSBL. [ Ultra-high coherence (low frequency noise) has emerged as a priority in a remarkably wide range 1 of applications including: High-performance microwave v 2 oscillators1, coherent fiber-optic communications2, re- form factor is decreased. In this work, we demonstrate 1 mote sensing3 and atomic physics4,5. In these appli- astimulatedBrillouinlaserthatattainsthe highestlevel 2 cations, the laser forms one element of an overall sys- ofcoherencereportedforachip-baseddevice. Schawlow- 4 tem that would benefit from miniaturization. For ex- Townes noise less than 60 milliHz2/Hz is measured at . 1 ample, in coherent communication systems a laser lo- an output power of approximately 400µW. Also signifi- 0 cal oscillator works together with taps, splitters and de- cant is that the low-frequency technical noise is compa- 2 tectors to demodulate information encoded in the field rable to commercial fiber lasers. The devices use a new 1 amplitude of another coherent laser source. A require- ultra-high-Q cavity9,10 that enables precise matching of : v mentinthesesystemsistocreatenarrow-linewidthlasers theBrillouingainshifttothefree-spectral-range,thereby i so that a large number of information channels can be guaranteeing reliable oscillation (see inset in Figure 1). X encoded as constellations in the complex plane of the The devices are efficient, featuring more than 90% con- r field amplitude2,6. In yet another example, the lowest version of lower-coherence pump to high-coherence out- a close-to-carrier phase-noise microwave signals are now put and threshold powers as low as 60 microWatts. derived through frequency division of a high-coherence Apre-requisiteforhighcoherenceisthattheoscillator laser source using an optical comb as the frequency mustoperatesinglelinewithhighsidemodesuppression. divider1. The miniaturization of these all-optical mi- Mutli-lineoperationnotonlylowerstheoverallcoherence crowave oscillators could provide a chip-based alterna- of source, but the presence of even relatively weak side- tive to electrical-based microwave oscillators, but with modes introduces low-frequency, mode partition noise13. unparalleled phase noise stability. Moreover, with the There are also significant collateral problems caused by advent of microcombs7,8, such an outcome seems likely multiline operation that are specific to certain applica- provided that similar strides are possible in miniaturiza- tions (e.g., dispersion in optical fiber communication). tion of high-coherence sources and reference cavities. Intracavity etalons or grating filters are normally used Typically, however, miniaturization comes at the ex- to induce high levels of mode selection in lasers. In the penseofcoherence,becausequantumandtechnicalnoise present work, the relatively narrow SBS gain spectrum contributions to laser coherence increase as laser-cavity provideshighmodeselectionwithouttheneedforetalons 2 or grating filters. diameter to obtain a match and more realistically it re- Single-line oscillators suffer from two general types of quires1:10,000controlfor consistentlow-thresholdturn- laserfrequency noise: one that is associatedwith techni- on power. cal noise such as from thermal drift and vibration; and In the recent demonstrations, an approach was imple- a second of fundamental origin, the Schawlow-Townes mented in which the high spectral density of transverse (ST)noisefromspontaneousemission. STnoiseisbroad- modes in spheres or CaF2 resonators offered a reason- band and has a double-sided frequency-fluctuation spec- ablelikelihoodoffindingawell-matchedpairofresonator tral density given by11,12: modes. Inthe presentwork,thisapproachisreplacedby precision control of the cavity free spectral range (FSR) using a new ultra-high-Q resonator geometry9,10. These µhν3 devices are the first chip-based Brillouin devices. Their SST(f)= (1) ν 2PQ Q coherence sets a record for frequency stability of any T ex chip-based device. The technical noise of the devices is where µ is the spontaneous emission factor, h is the alsoremarkablylowandcomparableto commercialfiber Planck constant, ν is the laser frequency and P is the lasers (currently a benchmark for good frequency sta- output power; Q is the total cavity quality factor and bility). Moreover, the same issue that has made these T relates to the external cavity quality factor Q and in- devices so difficult to fabricate in microcavity form(the ex trinsic cavity quality factor Q through 1 = 1 + 1 . relativelynarrowgainspectrum)becomesanassetincre- 0 QT Qex Q0 atinghighlystable,single-lineoscillation. Asnowshown, In an ideal case of no other contribution to frequency the relatively small offset frequency between the pump noise, the spectral lineshape of the laser is Lorentzian andthelaseroutputofthesedevicesmakesthesedevices withafull-width-half-maximumlinewidthgivenby∆ν = 2πSST(f). The ST noise scales inversely with laser out- well suited as compact spectral purifiers in any system ν requiring high coherence. put power and inverse quadratically with the laser Q factor11,12. This latter dependence is responsible for the difficulty in maintaining high coherence as device form factor is reduced; and nearly all laser cavities suf- fer Q factor degradation as the cavity size is reduced, either on account of increasing output coupling losses as in a Fabry-Perot laser or on account of the chal- lenge presented by fabrication of high-Q micro-cavities in a chip-based device. For example, state-of-the-art, high-coherence, monolithic semiconductor lasers feature an ST linewidth in the range of 3 kHz14 by using a long-cavity,corrugation-pitch-modulatedlaserstructure. While these levels are currently acceptable for coher- ent communications, further enhancements in coherence FIG. 2: Experimental setup. A tunable CW laser is am- would improve performance. Morever, other applica- plifiedthroughanEDFAandcoupled intothediskresonator tions, such as to microwave oscillators, require nearly 4 using thetaper-fiber technique. The SBL propagating in the orders of magnitude narrower linewidths. backward direction propagates through the fiber circulator, Among optically pumped, chip-based laser oscillators, and is monitored by photodetector (PD B) and optical spec- trum analyzer (OSA). The pump is monitored by a separate abeatnotelinewidthinasplit-mode,Erbium-dopedmi- crotoroid laser produced a value of 4 Hz20 and a micro- photodetector (PD A) and is also coupled to a balanced- homodyne detection setup (H¨ansch-Couillaud technique) to toroid Raman laser showed an ST linewidth of 3 Hz21. generate an error signal for locking the pump laser to the However, the Raman and Erbium gain spectra are very cavity resonance. broadsothatnofrequencyselectionmechanismexistsin thesedevices. Assuch,thedevicesarenotwellcontrolled spectrallyandfrequentlylaseonmultiplelinesorexhibit The resonators used in this work are new and attain mode hopping17. Recently, a related process, stimulated Q factors as high as 875 million9,10, even exceeding the Brillouin laser (SBL) action, has been demonstrated in performance of microtoroids26. Significantly, these de- discrete micosphere15 and CaF micocavities16. While vices are fabricated entirely from standard lithography 2 Brillouin scattering is well known in optical fiber com- and etching, avoiding any kind of silica reflow step26. munication,includingitsuseforgaininnarrow-linewidth Thecombinationofultra-high-Qandprecisioncontrolof fiber lasers22, generation of slow light23,24 and informa- FSR has not previously been possible, and is essential tionstorage25,realizationofmicrocavitybasedSBLshas forreliable fabricationoflow-thresholdSBL lasers. Very proven very challenging on account of the requirement briefly, devices use an 8-10 micron thick thermal oxide to precisely match the Brillouin shift to a pair of cavity layer that is lithographically patterned and etched using modes. Specifically,thenarrowlinewidthoftheBrillouin buffered HF into disks. The etched, oxide disks then act gain requires better than 1:1000 control of the resonator as a mask for selective dry etching of the silicon. This 3 dry etch process creates a whispering gallery resonator were tested at a series of pump wavelengths in the 1500 through undercut of the silicon. By proper control of nm band. In each device, the pump wavelength was se- both the wet and dry etching processes, the Q of the re- quentially tuned alongresonancesbelonging to the same sulting resonator can be nearly 1 billion. Moreover, the azimuthalmodefamily. Theminimumthresholdforeach lithographyandetchingprocessprovidediametercontrol device corresponds to excitation at the Brillouin gain of 1:20,000, more than sufficient to place the microcav- maximum(i.e., g (∆ω−Ω =0)). The rise in threshold b B ity FSR within the Brillouin frequency shift. The upper awayfromtheminimum(foragivenresonatordiameter) left inset in Figure 1 shows a top view of a resonator reflectstuningoftheBrillouinshiftfrequencywithpump fabricated using this procedure. Additional details on wavelength noted above. The variation of this peak fre- fabrication and characterization of these resonators are quency shift versus the corresponding pump wavelength given elsewhere9,10. is plotted in Figure 3c. There is good agreement with Figure 2showstheexperimentalsetupusedtotestthe the expected theoretical dependence. SBLs. Pump poweris coupledinto the resonatorby way ofafibertapercoupler18,19. SBLemissioniscoupledinto a ~6mm disks, Threshold tuning the opposing direction and routed via a circulator into 0.5 a photodiode, optical spectrum analyzer or an interfer- 0.45 ometer (not shown) for measurement of frequency noise. The transmitted pump wave is monitored using a bal- 0.4 FSR=10.793GHz anced homodyne receiver so as to implement a Ha¨nsch- 0.35 FSR=10.823GHz Couillaud locking of the pump to the resonator27,28. W) FSR=10.861GHz By proper control of taper loading, the SBL can be d (m 0.3 FSR=10.898GHz operated in two distinct ways: cascade or single-line. In ol0.25 h cascade, the waveguide loading is kept low so that once s e 0.2 oscillation on the first Stokes line occurs, it can function hr t asapumpwaveforasecondBrillouinwaveandsoon. In S 0.15 B contrast,single-lineoperationcanbeobtainedbysetting S 0.1 waveguide loading to a value that critically couples the 0.05 pump wave at the target laser output power. This op- erational mode suppresses oscillation on all modes lead- 0 1535 1540 1545 1550 1555 1560 1565 1570 ing to high side-mode suppression. Figure 1 shows the Pump wavelength(nm) output power versus coupled input power for single line b c operation. The lowerrightinsetto the figure is the opti- 10.95 cal spectrum showing both the Stokes line and the sup- Hz)10.9 F−i7t .l1in6eM sHlozp/nem= pressed pump wave. The threshold for the device shown G is 60microWatts,andthe differentialpumping efficiency hift (10.85 s is95%. Sidemodesuppressionoftheneighboringcavity S B modesexceeds70dBataround1milliWattoutputpower S10.8 (measured by observing the beatnote of the main mode 10.75 andweaksidemodesonanelectricalspectrumanalyzer). 15 40 1545 1550 1555 1560 1565 Pump wavelength(nm) The thresholdfor SBL actionis givenby the following expression. FIG. 3: Illustration of tuning control of the SBL de- vices. a, SBL threshold is measured as a function of pump 2π2n2V P = eff (2) wavelength using four slightly different resonator diameters. th gbQpQbλpλb FSRs are indicated in the legend. b, Illustration of the con- troloftheSBSgainwiththechangeofFSRandpumpwave- Beyond the importance of high cavity Q factor evident length. c, Measured Brillouin-shift frequency (circles) versus in this expression,it is essentialto maintaina large SBL pumpwavelength. gain parameter, g (∆ω −Ω ) (where gain = g P , b B b pump ∆ω is the free-spectral range, and Ω is the Brillouin B shift). Because the gain spectrum is relatively narrow To characterize the SBL frequency noise, a Mach- (typical full-width half maximum is 20-60 MHz22,29), Zehnder interferometer having a free spectral range of this requires a precise match of the free-spectral-range 6.72MHz is usedas a discriminator andthe transmitted to the Brillouin shift. Ω depends on the pump wave- optical power is detected and measured using an electri- B length λ and phonon velocity V through the relation cal spectrum analyzer (ESA). To suppress the intensity p a Ω /2π = 2nV /λ . An illustration of the control pos- noise, the complementary outputs of the interferometer B a p sible using the new resonator geometry is provided in weredetected using a balancedreceiver. This ESAspec- Figure 3a where four devices having diameters of 6020, trumisrelatedtothefrequency-fluctuationspectralden- 6044, 6062 and 6080 microns (lithography mask size) sity, S (f), through the following relation: ν 4 The 1/f noise that appears at lower carrier offset fre- quencies is given in Figure 4d and approximately tracks V22π2τ2sinc2(τ f)S (f) W (f)= pp d d ν (3) a similar-shaped noise spectrum in the laser pump over ESA R this frequency range. However, the absolute level of 1/f L noiseoftheSBLatagivenoffsetfrequencyisreducedby whereτ is the Mach-ZehnderdelayandV is the peak- d pp about30dBrelativetothe1/fnoiseinthepump. Indeed, to-peak voltage of the detected MZI output over one the level of technical noise in this band is comparable to fringe. Using this formula, the frequency-fluctuation several commercial fiber lasers that were characterized spectral density is plotted in Figure 4a. The singularity as part of this study. As such, the performance of the intheplotsat6.72MHzisanartifactofthedataconver- SBL, both in the quantum limited ST regime and the sion near the zero of the sinc2 function. The frequency- technical-noise limited 1/f regime, is excellent. fluctuationspectrahavearelativelyflat(whitenoise)re- The ability to obtain frequency noise levels compara- gion for carrier offset frequencies above 2 MHz and then ble to fiber lasers, but for on-chip operation opens new a 1/f-like region at frequencies below 2 MHz. The value possibilitiesforminiaturizationandintegrationincoher- of the white noise region is plotted both as a function ent communication, high-stability microwave oscillators, of SBL power and external cavity Q factor in Figures andpotentiallyinremotesensing. Ineachofthesecases, 4b and 4c, respectively. Also plotted are fits to inverse the SBL device provides a new role as a spectral puri- powerand inverseQ2 curves. These dependences as well fier, boosting the coherence of the pump wave by sev- ascalculationconfirmthatthemeasuredfrequencynoise eral orders of magnitude. The relatively small frequency component is the ST quantum noise of the laser. The shift created in this process (about 11 GHz) is easily minimum value of 60 milliHz2/Hz is to our knowledge compensated in the pump. For example, low coherence the smallest recorded ST noise for any chip-based laser. DFB lasers are manufactured with wavelengths set on It is also interesting to note that, to the authors knowl- the ITU grid by control of an integrated grating pitch. edge, the ST noise dependence on loading has not pre- However, final control is provided by temperature tun- viously been recorded. This, normally difficult measure- ing of the fully packaged device. A DFB laser could be ment, was possible here onaccountofthe ability to vary tuned through this same process to function as an SBL the taper loading of the resonator19. pump sothat the emitted SBL wavelengthresides atthe desired ITU channel. In this way, the existing WDM in- a b 104 frastructure could be adapted for high coherence opera- 0.6 tionin opticalQAM systems. The frequencynoise levels 2(f) (Hz/Hz)ν110002 2 (Hz/Hz)ν00..24 dlpitehhmaiscoensnsetomriasiceteo,dnitdhuiescrteeosrteixlmacsaeeetreddbeytvhe4an0tdsSBtqa.utUeasroienfg1t0ht2he4e-aQmrtAeaMmsuofrnoeord-- S S mats could be implemented using an SBL generated op- −2 tical carrier at 40GB/s. 10 0 0 5 10 0.1 0.2 0.3 0.4 offset freq (MHz) SBS Power (mW) While the current devices use a taper coupling for c d launch of the pump and collection of laser signal, the 1 1e8 SBL ability to precisely control the resonator boundary en- 0.8 Hz)1e6 Pump laser ables use of microfabricated waveguides for this process. Hz)0.6 2Hz/1e4 Severaldesigns are under investigation,the implementa- 2S (Hz/ν00..24 S(f) (ν1e21 tSiBonLodfewvihciecsh. wFoilrleexxtaemndpleth,ethreanSgBeLosfdaepmploicnasttiroantsedofhtehree 050 100 150 200 250 1e−120 0 1k 10k 100k 1M acrreeactaenHdeidrtazteosrlfoowrelroclokningg-tetormalrinefeewreidntches.caSvuictyhasosoausrtcoe Qex (million) Offset freq (Hz) on a chip might one day be combined with microcomb technology to realize a compact and high-performance FIG. 4: Measurements of the SBL frequency noise microwaveoscillator. Atthetable-topscale,thesecomb- characteristics. a, Laser frequency noise spectrum at dif- ferent output power levels from 0.047mW to 0.375mW (in- basedsystemshaverecentlyexceededtheperformanceof dicated by color). b, ST noise plotted versus output power. cryogenic electronic oscillators1. The dashed line is an inverse power fit to the data. c, ST noise plotted versus the external cavity quality factor Qex. 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