Visible light photonic integrated Brillouin laser

Narrow linewidth visible light lasers are critical for atomic, molecular and optical (AMO) physics including atomic clocks, quantum computing, atomic and molecular spectroscopy, and sensing. Stimulated Brillouin scattering (SBS) is a promising approach to realize highly coherent on-chip visible light laser emission. Here we report demonstration of a visible light photonic integrated Brillouin laser, with emission at 674 nm, a 14.7 mW optical threshold, corresponding to a threshold density of 4.92 mW μm−2, and a 269 Hz linewidth. Significant advances in visible light silicon nitride/silica all-waveguide resonators are achieved to overcome barriers to SBS in the visible, including 1 dB/meter waveguide losses, 55.4 million quality factor (Q), and measurement of the 25.110 GHz Stokes frequency shift and 290 MHz gain bandwidth. This advancement in integrated ultra-narrow linewidth visible wavelength SBS lasers opens the door to compact quantum and atomic systems and implementation of increasingly complex AMO based physics and experiments.


INTRODUCTION
Ultra-narrow linewidth (UNLW) visible light lasers provide the spectral purity required for precision atomic, molecular and optical (AMO) applications including atomic clocks 1,2 , atomic and molecular spectroscopy [3][4][5] , and quantum sensing 1,6,7 .Historically, it has been necessary to use macroscopic laser systems locked to large optical reference cavities to obtain the low phase noise and high frequency stability needed to address narrow optical clock transitions in atoms 8,9 .While providing state of the art performance 1,10,11 , these lab-scale systems pose challenges for atomic and molecular experiments of ever-growing complexity, and for a portable or even autonomous optical clock.There is a need for lasers that are smaller and more reliable so that experiments can scale up, and in general, photonic integration will lead to reduced size, weight, and power consumption, as well as reduced sensitivity to environmental disturbances.Photonic integration is a promising approach to miniaturize laser systems as well as improve their reliability [12][13][14] , thereby enabling systems with larger number of entangled atoms 15,16 , higher sensitivity quantum sensors 6,17 , higher precision positioning, timing and navigation 18 , and probing of complex molecules [19][20][21][22][23][24] .
Brillouin scattering (SBS) lasers, with their pump linewidth narrowing properties and ultra-low phase noise emission 25 are a promising candidate for AMO and quantum applications.Emission in visible has been achieved with fiber optic based resonators, exotic fiber, and bulk optic implementations [27][28][29][30][31][32][33] .Recently the coherence properties of an NIR fiber SBS laser were transferred to the visible to address the clock transition of strontium, however, this work required bulky, power inefficient, nonlinear frequency conversion 26 .To reduce system complexity and improve reliability, it is desirable to use a "direct-drive" approach, where the SBS laser directly emits at the desired visible wavelength, without intermediate conversion stages.Chip-scale SBS lasers operating in the NIR have exhibited impressive performance [34][35][36][37][38][39][40][41] , achieving sub-Hz fundamental linewidth 34 , 30 Hz integral linewidth over 100 ms, and 2×10 -13 fractional frequency stability 42 .To date, visible light emission in a photonic integrated SBS laser has remained out of reach.This lack of progress has been primarily due to barriers such as realizing ultra-low loss Brillouin-active planar waveguides in the visible.Overcoming these barriers, as well as realizing a visible light SBS laser in a wafer-scale integration platform, will reduce size and cost, as well as enable reliable scaling to more lasers for precision AMO science and applications.
We report the first demonstration, to the best of our knowledge, of visible light SBS lasing and Brillouin gain in a photonic integrated waveguide platform.This platform is compatible with foundry-level wafer-scale integration and can incorporate other photonic elements to realize systems on-chip 19 .The Brillouin gain medium is a high aspect ratio (20 nm tall by 2.3 μm wide) silicon nitride waveguide core surrounded by a thermal oxide on silicon lower-and tetraethoxysilane pre-cursor plasma-enhanced chemical vapor deposition (TEOS-PECVD) uppersilica cladding.The ability to realize visible Brillouin lasing using this waveguide structure leverages continuous SBS generation from long lifetime photons in an ultra-low loss (~1 dB/m) optical waveguide, an ultra-high quality factor (Q) resonator (>50 Million), and a short phonon lifetime due to the absence of acoustic waveguiding 34 .These properties lead to a sub-Hz fundamental laser linewidth and other benefits at 1550 nm 34 .We measure spontaneous Brillouin scattering in a 2 meter spiral waveguide, resulting in a 25.110 GHz Stokes frequency shift and 290 MHz Brillouin gain bandwidth.The shift and gain bandwidth are accurately predicted using a fullvectorial numerical simulation and enables accurate detection of the weak back-reflected spontaneous-Stokes pump heterodyne beat note.By combining our multi-physics simulations with measured data (Brillouin gain in fiber + waveguide, Brillouin gain in fiber), we can estimate (see Supplementary Information) the SBS gain to be 2.73 (W m) -1 .Brillouin lasing is demonstrated in an 8.9 mm radius silicon nitride bus-coupled ring resonator, with intrinsic quality factor (Q) = 55. 4  Million and loaded Q = 27.7 Million 43 and a 3.587 GHz free spectral range (FSR) and a resonancelocked 674 nm semiconductor pump laser 31 .SBS lasing at 674 nm is verified by measuring a 14.7 mW on-chip pump threshold power and demonstrated linewidth reduction as the pump is increased from below to above threshold.The first order Stokes (S1) fundamental linewidth is determined by measuring the far-from-carrier frequency noise, with a resulting orders of magnitude linewidth decrease from the cold-resonator linewidth (i.e., cavity linewidth without light injected), confirming pump Brillouin phase noise suppression via long photon-and short phononlifetimes 25,34 .To highlight the versatility of this laser, we demonstrate 698 nm SBS lasing (the neutral strontium clock transition wavelength) using the same waveguide materials and design with mask-only changes to the ring diameter and change in the pump laser (698 nm).
To illustrate the utility of these integrated lasers, an integrated frequency stabilized SBS optical laser oscillator (OLO) can be used to "direct-drive" the strontium ion clock transition (see Fig. 1).A 674 nm pump laser is locked to the SBS resonator that in turn generates a backward propagation first order Stokes wave (S1) in the SBS resonator 14,44,45 .The Stokes wave is filtered by a three ring dual bus filter 46 , and routed to an acousto-optic modular (AOM) that generates a single sideband (SSB), that in turn is locked to an on-chip optical reference cavity 47,48 .The associated electronics for laser tuning, SBS resonator locking, reference cavity stabilization, and AOM drive are illustrated as well as resonator tuning elements (e.g., thermal tuning).The stabilized 674 nm OLO beam is coupled to a 88 Sr + ion in an electrostatic trap 49,50 using a waveguide to free-space grating coupler 51 .3-dimensional (3D) cooling and repump beams are provided via on-chip or off-chip lasers and waveguide to free-space grating couplers.This scalable architecture could readily be to address an ion array by integrating an array of OLOs and Gratings on PIC.A heterogeneously integrated 674 nm external cavity Si3N4 tunable laser serves as the SBS laser pump.A three ring optical filter isolates and routes the SBS first order Stokes (S1) to a power splitter that sends a portion of S1 to an onchip reference cavity.The reference cavity provides feedback to the pump laser and/or the SBS laser, to reduce the integral linewidth and stabilize the SBS laser for locking to the narrow atom transition.A single 88 Sr + ion trap is shown in a chip-integrated trap.The ion is addressed with the 674 nm light, as well as other wavelengths necessary for clock operation.Fiber-coupled 1092 nm and 1033 nm lasers are converted to free-space beams from large-area grating emitters, incident on the ion, for state re-pumping [51][52][53] .

Visible wavelength SBS laser resonator:
The SBS laser resonator is based on an ultra-low loss single mode Si3N4 waveguide that is designed to operate at 674 nm.The waveguide consists of a 20 nm tall and 2.3 μm silicon nitride core deposited and etched on a lower thermally grown oxide cladding on a silicon substrate, with a TEOS-PECVD deposited upper cladding 54 (Fig. 2a), for further details, see Methods, below.The spontaneous Brillouin gain frequency shift and Brillouin gain spectrum are measured using heterodyne detection between the pump and the backscattered signal in a 2 meter long spiral waveguide (Fig. 2b).Multi-physics vectorial simulations that incorporate actual materials and device parameters are used to predict the frequency offset and the Brillouin gain shape (red curve in Fig. 2c).The weak, back-scattered signal is measured by heterodyne detection of the pump-Brillouin beat note (Fig. 2c) with an electrical spectrum analyser (ESA), for details see the Supplementary Information.We measure a 25.110 GHz peak frequency shift and 290 MHz gain bandwidth, which agrees with our numerical simulations.The broad gain bandwidth is due to the continuous generation of photons in the ultra-low loss optical waveguide without acoustic waveguiding, which permits coupling to a continuum of bulk acoustic phonon states within the waveguide oxide cladding 34,37 with a resulting Brillouin gain coefficient of 2.73 (W m) -1 .Brillouin scattering in the optical fiber used to deliver the 674 nm pump laser light is distinguished from the waveguide Brillouin scattering (Fig. 2c blue curve) by decoupling the fiber from the chip and making an independent measurement (blue curve of Fig. 2c), described further in the Supplementary Information).The SBS laser resonator is a 8950.9μm radius bus-coupled ring structure with a free-spectral range (FSR) designed to be 1/7 of the measured 25.110 GHz peak Stokes shift at 674 nm (Fig. 3b and  3c) and a bus-to-ring power coupling coefficient 34 к 2 of ~1.5%.We design the ring to have multiple FSRs per Brillouin Stokes frequency shift, in order to reduce the fundamental linewidth through increased cavity volume, as well as to provide robustness to manufacturing variations 34 .An intrinsic Q of 55.4 Million and loaded Q = 27.7 Million at 674 nm is measured (Fig. 3d) using an RF calibrated MZI [54][55][56] , yielding a propagation loss of 1.09 dB m -1 (see Methods section).Visible light 674 nm SBS lasing: The SBS laser resonator is pumped by a tapered amplifier (TA) that is seeded with 674 nm light from an external cavity diode laser (see experimental setup in Supplementary Information Fig. 3).The TA output is coupled to the waveguide SBS resonator through a high power fiber circulator.A maximum of 35 mW on-chip power is delivered to the waveguide bus, limited by the maximum 180 mW TA output power and ~4 dB fiber-to-facet coupling loss.The backward propagating SBS S1 signal is measured using a 3-port fiber optic recirculator located between the pump laser and the resonator input.The measured and simulated S1 powers are plotted versus the pump power in Fig. 4a.A clear S1 threshold is observed for an on-chip pump power of 14.7 mW where a 45% slope efficiency is measured, both in good agreement with our SBS model 55 (see Supplementary Information).Modelling of the S1 power (black dots) accurately predicts the measured S1 power as pump is increased from below to above threshold.The modelled S2 threshold is ~60 mW (yellow dots).b, Measurement of SBS first order Stokes (S1) emission linewidth, plotted on a logarithmic scale, below threshold (blue trace), just below threshold (red trace) and just after threshold (green trace).The Brillouin emission linewidth evolves from a spontaneous dominated linewidth of 16.5 MHz, which is approximately the SBS gain filtered by the cold-cavity resonator linewidth ~16 MHz, to the onset of stimulated Brillouin measuring a 12 MHz linewidth just below threshold, to a stimulated dominant 120 kHz linewidth just above threshold.c, Frequency noise measurements of S1 using a radio frequency (RF) calibrated fiber optic Mach-Zehnder interferometer (MZI) frequency discriminator.Fundamental linewidths are indicated by horizontal dotted lines that are tangent to the far-from-carrier frequency noise.Free-running pump frequency noise (purple trace) is for an unlocked pump (i.e., not locked to the stable cavity or SBS resonator).As the pump is increased, a decrease fundamental linewidth (curves ii to v) is measured.For this data, the back reflected pump is not optically filtered before frequency noise discrimination, leading to a contribution to the measured frequency noise of S1.Just above threshold, the conversion from pump to S1 is low, and the white noise floor at 16 mW pump (green) is as sum of pump and S1 and their beat note.As the on-chip pump, Pon-chip, is increased, the intra-cavity S1 photon number increases while the pump signal decreases to below 10dB of the Stokes for all other FN traces.d, Summary of beat note and fundamental linewidths from (b) and (c).
In addition to establishing the laser threshold, we demonstrate a decrease in the S1 emission linewidth as the pump power is increased from below threshold, through threshold, and above threshold 34,56 .Below threshold, the optical power spectrum is measured using a heterodyne beat note produced by mixing the backward propagating S1 with the pump on an electrical spectrum analyzer (ESA) (Supplementary Information).To minimize the contribution of the pump linewidth to the measured beat note we lock the pump laser to a commercial high finesse ultra-stable cavity (Stable Laser Systems).Well below threshold (i), the scattered light is produced by uncorrelated spontaneous scattering from thermal phonons, and is linearly filtered by the cavity resonance which is approximately 16.1 MHz.As threshold is approached, the spontaneous emission spectra, point (ii) in Fig. 4b, measures FWHM at 12.0 MHz, indicating the onset of stimulated emission, since the emission spectra is narrower than the cold-cavity resonance FWHM.
At just above threshold (iii), we see a dramatic narrowing of the linewidth to 120 kHz on the ESA as stimulated Brillouin scattering dominates the emission (Fig. 4b, trace (iii)), which is order 100x reduced from the passive cavity resonance linewidth.At all points above threshold, we measure the frequency noise of S1 using an optical frequency discriminator (OFD) (see Methods and Supplementary Information sections).The fundamental linewidth (∆) is defined 34,38 as the farfrom -carrier white frequency noise floor, in Hz 2 Hz -1 , multiplied by , where here the noise floor for each pump power input is indicated by horizontal dashed lines (iii) -(vi) in Fig. 4c.As the pump power increases beyond S1 threshold, the fundamental linewidth drops dramatically from 1.1 kHz (iv) to 269.7 Hz (vi).These linewidth results are summarized in Fig. 4d, indicating the integral linewidths for points (i) -(ii) below threshold, and the fundamental linewidths for the frequency noise curves in (iii) -(vi) in Fig. 4c.We were not able to provide the required on-chip pump power, 59.4 mW, to achieve lasing of the second order Stokes (S2).Future work will look further into noise properties measured using stabilized pump sources and exploring linewidth behavior as S1 approaches the S2 lasing threshold.

Demonstration of 698 nm SBS lasing:
We have described a visible light integrated SBS laser approach and here describe briefly, demonstration of lasing at a second visible wavelength.The silicon nitride bandgap will support low loss for wavelengths longer than ~405 nm, making this laser a powerful tool to realize a broad range of atomic transition wavelengths by mask-only changes (the waveguide width, the ring diameter, the ring bus coupling gap), and the pump laser wavelength.To demonstrate this principal, we design and fabricate a 698 nm SBS resonator, a wavelength chosen to match the neutral strontium atom clock transition (Fig. 5a).The waveguide design and geometry are the same as for 674 nm (as verified by optical mode simulations, see Supplementary Information).Our multi-physics simulation predicts a 24.243 GHz Stokes shift and 300 MHz Brillouin gain spectra at 698 nm (Fig. 5b).Based on this expected shift we set the FSR such that the Stokes shift is separated from the pump by 7 FSRs (to maximize cavity volume and reduce sensitivity of alignment between Stokes shift and resonance), leading to a 9.4 mm radius resonator.We design a 3.4 μm bus to ring gap to operate in the under-coupled regime with a power coupling coefficient of ~1%.We measure a 12.7 MHz cavity resonance width, a 60 Million intrinsic Q and 33.8 Million loaded Q, and a 3.421 GHz FSR (see Fig. 5c and inset).First order SBS lasing is observed as S1 at the expected pump-Stokes frequency offset measured with an optical spectrum analyzer (OSA) as shown in Fig. 5c (the measured pump is reflected from the SBS resonator far facet).The S1-pump laser (Ti:sapphire) 23.892 GHz beat note is shown in Fig. 5c inset (see Supplementary Information for more details), and is 351 MHz off the simulated shift which can be explained by a slight offset between the peak of the gain and the narrow cavity resonance.The pump laser is free-running (i.e.stabilized neither to the resonator nor a supplementary Fabry-Perot optical cavity), so the beat note drifts at the 100 kHz level in tens of milliseconds.The on-chip pump power is 108 mW and the on-chip pump threshold power (Pth) is approximately 75 mW.

DISCUSSION
We have demonstrated the first, to the best of our knowledge, visible light SBS laser in a waveguide photonic integrated circuit.The 674 nm laser, based on an ultra-low loss silicon nitride bus-coupled ring resonator, is designed to serve as a "direct-drive", spectrally pure, chip-scale source that can couple to the 88 Sr + clock transition, without the need for intermediate frequency translation.To guide our design, we use a combination of multi-physics simulation and Brillouin spectroscopy to identify the first order Stokes (S1) frequency shift gain bandwidth, at 674 nm.Lasing is demonstrated with an on-chip 14.7 mW threshold, a 45% slope efficiency and Brillouin emission linewidth narrowing as the pump is increased from below threshold to above threshold, achieving 269.7 Hz fundamental linewidth at an on-chip pump power of 36 mW.Assuming the slope efficiency is constant and the calculated S2 threshold is 59 mW (Fig. 4a), at the onset of S2 lasing (threshold), we estimate it is possible to increase the S1 cavity photon number by a factor of 2x, resulting in an S1 fundamental linewidth reduction to ~153 Hz 55 .The measured frequency noise components result from various sources, including the SBS fundamental noise 38 , intrinsic noise of the SBS cavity, pump amplitude noise that converts to SBS noise from locking the pump to the SBS resonator, and the technical noise sources in the pump laser and SBS resonator as well as pump amplitude to phase noise conversion from locking the laser to the cavity.
The ability to achieve SBS lasing in the visible comes about from the combination of long photon lifetime and ultra-high cavity Q using ultra-low loss optical waveguiding, and short phonon lifetime.This design enables lasing at other visible SBS emission wavelengths with mask-only changes and a change in the pump source.To illustrate this versatility, we demonstrate lasing at 698 nm, a wavelength suitable to probe long-lived transitions in neutral strontium.Looking forward, we will be characterizing the full frequency noise, coherence, and linewidths at other visible laser emission wavelengths.
In order to improve SBS laser efficiency and further reduce linewidth, our simulations show that with closer FSR matching to the Brillouin gain peak shift we can achieve a 7 mW threshold with the current design and generate higher S1 optical power.This can be achieved by adjustment of the SBS cavity using waveguide tuning technique including thermal 57 and piezoelectric 58 .Our simulations show that continued increase in S1 photon number up to S2 threshold will lead to a fundamental linewidth reduction to ~ 2 Hz.Other possible improvements to further reduce the linewidth include modulating the laser resonator with a grating, to split the second order Stokes (S2) resonance and prevent S2 emission and further increase in S1 optical power 55 .Given the transparency and bandgap of silicon nitride, and the low loss achievable down to ~405 nm, this platform will support a wide range of SBS photon-phonon interactions and as such, wavelengths for a variety atomic and molecular transitions.Future work will involve demonstrating this design across the broad range of silicon nitride waveguide transparency (e.g., Yb @ 578 nm, Ca + @ 729 nm and waveguides with higher bandgap that can support the UV (e.g., Al+ 267.4 nm).

METHODS
Fabrication process.The substrate and lower cladding consist of a 15-µm-thick thermal oxide grown on a 100-mm diameter silicon wafer.The main waveguide layer is a 20-nm-thick stoichiometric Si3N4 film deposited on the lower cladding thermal oxide using low-pressure chemical vapor deposition (LPCVD).A standard deep ultraviolet (DUV) photoresist layer was spun and then patterned using a DUV stepper.The high-aspect-ratio waveguide core is formed by anisotropically dry etching the Si3N4 film in an inductively coupled plasma etcher using a CHF3/CF4/O2 chemistry.After the etch, the wafer is cleaned using a standard Radio Corporation of America (RCA) cleaning process 59 .A 6-µm-thick silicon dioxide upper cladding layer was deposited in two 3-µm steps using plasma-enhanced chemical vapour deposition (PECVD) with tetraethoxysilane (TEOS) as a precursor, followed by a final two-step anneal at 1050 °C for 7 hours and 1150 °C for 2 hours.
Resonator linewidth measurements.674 nm resonator: We used the same cateye diode tunable pump laser (from mogLabs) for these measurements.A ~50 m unbalanced fibre-based radiofrequency (RF) calibrated Mach-Zehnder interferometer (MZI) was used to measure the Q 60 .To calibrate the MZI free spectral range (FSR), an RF electro-optic phase modulator (EOM) was used to create two sidebands.While scanning across a resonance, the two sidebands are used to calibrate the MZI FSR.The MZI FSR is measured to be 3.99±0.02MHz.The MZI is acoustically isolated to minimise noise in the fringes.We simultaneously scan the laser through both the MZI and the resonator and the MZI FSR provides a RF calibrated frequency reference for calibrating the resonance linewidth.698 nm resonator: We use a Ti:Sapphire laser at 698 nm to measure the Q.To calibrate the frequency, we use two different phase modulators to add sidebands at 1.4645 GHz for FSR measurements and at 60 MHz for Q measurements.

Frequency noise measurements.
We measured the frequency noise and fundamental linewidth of our laser with an optical frequency discriminator (OFD) consisting of a fiber based unbalanced MZI (UMZI) and a balanced photodetector.The frequency noise of the laser,  !() in (Hz 2 Hz -1 ) is related to the power spectral density of the detector output  "#$ () in (V 2 Hz -1 ) as:  !() =  "#$ () (  ( % ) && .
' where  % is the optical delay of the UMZI,  is the frequency offset,  && is the peak-to-peak voltage of the detector output.The fundamental linewidth Δ =  ( is determined by the value  ( , that is tangent to the lowest point of the  !() far-from-carrier noise measurements at frequencies typically above 1 MHz.The S1 power from the reflection port of the circulator is sent into the acoustically isolated, 50m fiber-delay UMZI with a FSR of 3.99 MHz.Both UMZI outputs are connected to the balanced photodetector (Thorlabs PDB450A) with a bandwidth of 150 MHz in order to reduce the impact of intensity variations in the detector output.The power spectral density  "#$ () of the detector difference (RF) output was measured using a digital sampling oscilloscope (Keysight DSOX1204G with 200 MHz bandwidth).The RF output triggers the scope at the quadrature operating point of the UMZI and the power spectral density data is averaged over 16 traces with a Hann window is applied.The frequency noise is calculated using equation (1).

Fig 1 .
Fig 1. Integrated optical clock with a stabilized SBS laser for addressing a trapped Sr ion.Illustration of an integrated visible wavelength silicon nitride (Si3N4) waveguide stimulated Brillouin scattering (SBS) laser as a strontium ion ( 88 Sr + ) clock transition optical laser oscillator (OLO) and cooling beam delivery interface.A heterogeneously integrated 674 nm external cavity Si3N4 tunable laser serves as the SBS laser pump.A three ring optical filter isolates and routes the SBS first order Stokes (S1) to a power splitter that sends a portion of S1 to an onchip reference cavity.The reference cavity provides feedback to the pump laser and/or the SBS laser, to reduce the integral linewidth and stabilize the SBS laser for locking to the narrow atom transition.A single 88 Sr + ion trap is shown in a chip-integrated trap.The ion is addressed with the 674 nm light, as well as other wavelengths necessary for clock

Fig. 2 .
Fig. 2. Waveguide design and 674 nm spontaneous Brillouin measurement: a, Depiction of the waveguide cross section.b, Photograph of 2 meter single mode waveguide spiral used for the spontaneous Brillouin gain measurement, shown while illuminated with 674 nm light.5 mm scale is shown for reference.c, Measured and simulated spontaneous Brillouin gain with 25.110 GHz first order Stokes (S1) frequency shift, 2.73 (W m) -1 gain peak, and 290 MHz bandwidth.The measured blue curve shows the Brillouin contribution from both the fiber and silicon nitride waveguide, while the grey trace shows contribution from only the fiber, which confirms that the peak at 25.110 GHz is due to waveguide spontaneous Brillouin scattering.

Fig. 4 .
Fig. 4. Stimulated Brillouin scattering (SBS) Stokes threshold, power and linewidth measurements.a, Pump power on-chip (Pon-chip) vs. Stokes power.Measured first order Stokes (S1) laser threshold of 14.7 mW and 45% slope efficiency.Modelling of the S1 power (black dots) accurately predicts the measured S1 power as pump is increased from below to above threshold.The modelled S2 threshold is ~60 mW (yellow dots).b, Measurement of SBS first order Stokes (S1) emission linewidth, plotted on a logarithmic scale, below threshold (blue trace), just below threshold (red trace) and just after threshold (green trace).The Brillouin emission linewidth evolves from a spontaneous dominated linewidth of 16.5 MHz, which is approximately the SBS gain filtered by the cold-cavity resonator linewidth ~16 MHz, to the onset of stimulated Brillouin measuring a 12 MHz linewidth just below threshold, to a stimulated dominant 120 kHz linewidth just above threshold.c, Frequency noise measurements of S1 using a radio frequency (RF) calibrated fiber optic Mach-Zehnder interferometer (MZI) frequency discriminator.Fundamental linewidths are indicated by horizontal dotted lines that are tangent to the far-from-carrier frequency noise.Free-running pump frequency noise (purple trace) is for an unlocked pump (i.e., not locked to the stable cavity or SBS resonator).As the pump is increased, a decrease fundamental linewidth (curves ii to v) is measured.For this data, the back reflected pump is not optically filtered before frequency noise discrimination, leading to a contribution to the measured frequency noise of S1.Just above threshold, the conversion from pump to S1 is low, and the white noise floor at 16 mW pump (green) is as sum of pump and S1 and their beat note.As the on-chip pump, Pon-chip, is increased, the intra-cavity S1 photon number increases while the pump signal decreases to below 10dB of the Stokes for all other FN traces.d, Summary of beat note and fundamental linewidths from (b) and (c).

Fig. 5 .
Fig. 5. SBS lasing at 698 nm.a, Neutral Sr transitions with 698 nm clock transition.b, Multi-physics simulation of waveguide spontaneous Brillouin gain spectra for pump at 698 nm.Brillouin gain spectrum width of ~300 MHz and frequency offset of 24.243 GHz.c, The measured full width half maximum (FWHM) of the resonator linewidth at 698 nm is 12.7 MHz and intrinsic Q of 60 Million and loaded Q of 33.8 Million.Inset shows the 3.421 GHz measured free spectral range (FSR) of the resonator for the transverse electric field (TE).d, First order Stokes (S1) and the pump measured on an optical spectrum analyzer (OSA), with inset showing the pump-S1 beat note measured at 23.892 GHz on an electric spectrum analyzer (ESA).Inset on left shows the 9.4 mm radius resonator fiber-coupled with 698 nm pump laser light during measurements.