Brillouin Laser Research Articles

Integrated photonic molecule Brillouin laser with a high-power sub-100-mHz fundamental linewidth.

Photonic integrated lasers with an ultra-low fundamental linewidth and a high output power are important for precision atomic and quantum applications, high-capacity communications, and fiber sensing, yet wafer-scale solutions have remained elusive. Here we report an integrated stimulated Brillouin laser (SBL), based on a photonic molecule coupled resonator design, that achieves a sub-100-mHz fundamental linewidth with greater than 10-mW output power in the C band, fabricated on a 200-mm silicon nitride (Si3N4) CMOS-foundry compatible wafer-scale platform. The photonic molecule design is used to suppress the second-order Stokes (S2) emission, allowing the primary lasing mode to increase with the pump power without phase noise feedback from higher Stokes orders. The nested waveguide resonators have a 184 million intrinsic and 92 million loaded Q, over an order of magnitude improvement over prior photonic molecules, enabling precision resonance splitting of 198 MHz at the S2 frequency. We demonstrate S2-suppressed single-mode SBL with a minimum fundamental linewidth of 71±18 mHz, corresponding to a 23±6-mHz2/Hz white-frequency-noise floor, over an order of magnitude lower than prior integrated SBLs, with an ∼11-mW output power and 2.3-mW threshold power. The frequency noise reaches the resonator-intrinsic thermo-refractive noise from 2-kHz to 1-MHz offset. The laser phase noise reaches -155 dBc/Hz at 10-MHz offset. The performance of this chip-scale SBL shows promise not only to improve the reliability and reduce size and cost but also to enable new precision experiments that require the high-speed manipulation, control, and interrogation of atoms and qubits. Realization in the silicon nitride ultra-low loss platform is adaptable to a wide range of wavelengths from the visible to infrared and enables integration with other components for systems-on-chip solutions for a wide range of precision scientific and engineering applications including quantum sensing, gravitometers, atom interferometers, precision metrology, optical atomic clocks, and ultra-low noise microwave generation.

Dispersion engineering in a Brillouin fiber laser cavity for Kerr frequency comb formation

We conduct numerical and experimental investigations on Kerr comb generation in a nonlinear and non-reciprocal fiber cavity by leveraging both stimulated Brillouin backscattering and cascaded four-wave mixing. By engineering the net cavity dispersion to be either normal or anomalous, we enable the formation of diverse patterns and localized structures in the cavity field. The comb’s properties depend crucially on the mismatch between the frequency spacing of the bichromatic pump and the free spectral range of the Brillouin laser cavity in both cases. Particularly, in the anomalous regime, adjusting this parameter yields coherent, stable frequency combs in the modulation instability regime. This allows control and expansion of the spectral bandwidth up to 2 THz in normal dispersion and to 6 THz under anomalous net dispersion. This versatile and easily reconfigurable method holds potential for applications in high-speed communications and microwave synthesis.

Neodymium laser-pumped ultra-low noise tunable Brillouin fiber laser around 920 nm

We have developed an ultra-low noise tunable Brillouin fiber laser exhibiting three orders of magnitude better frequency noise performance than the Neodymium-doped fiber laser pump and remarkable optical signal-to-noise ratio exceeding 80 dB suitable for immediate applications in coherent nonlinear conversion, quantum computing and underwater communications. In addition, we have implemented a custom optical phase-locked loop to ensure long-term stable operation and have investigated its impact on frequency noise. We demonstrate the power scalability of the single frequency (Hz-class) Brillouin laser, delivering over 500 mW with tunability across the 900 nm to 930 nm range in an all-fiber fully polarization-maintaining architecture.

Second-order coherence across the Brillouin lasing threshold

Brillouin–Mandelstam scattering is one of the most accessible nonlinear optical phenomena and has been widely studied since its theoretical discovery one hundred years ago. The scattering mechanism is a three-wave-mixing process between two optical fields and one acoustic field and has found a broad range of applications spanning microscopy to ultra-narrow-linewidth lasers. Building on the success of utilizing this nonlinearity at a classical level, a rich avenue is now being opened to explore Brillouin scattering within the paradigm of quantum optics. Here, we take a key step in this direction by employing quantum optical techniques yet to be utilized for Brillouin scattering to characterize the second-order coherence of Stokes scattering across the Brillouin lasing threshold. We use a silica microsphere resonator and single-photon counters to observe the expected transition from bunched statistics of thermal light below the lasing threshold to Poissonian statistics of coherent light above the threshold. Notably, at powers approaching the lasing threshold, we also observe super-thermal statistics, which arise due to instability and a “flickering” in and out of lasing as the pump field is transiently depleted. The statistics observed across the transition, including the “flickering,” are a result of the full nonlinear three-wave-mixing process and cannot be captured by a linearized model. These measurements are in good agreement with numerical solutions of the three-wave Langevin equations and are well demarcated by analytical expressions for the instability and the lasing thresholds. These results demonstrate that applying second-order-coherence and photon-counting measurements to Brillouin scattering provides new methods to advance our understanding of Brillouin scattering itself and progress toward quantum-state preparation and characterization of acoustic modes.

Cascaded stimulated Brillouin laser and Brillouin-Kerr optical frequency comb in high-Q MgF2 disk resonators.

Backward stimulated Brillouin scattering (SBS) in optical microcavities has been widely used in nonlinear optics and microwave photonics. Compared with glass material microcavity, magnesium fluoride crystal microcavity has the advantages of small absorption coefficient, fewer defects and larger nonlinear coefficient, moreover, it usually has a narrow gain bandwidth of tens of megahertz. Here, we design a high precision machining system to produce ultra-smooth surface magnesium fluoride crystal disk cavities with a diameter of about 5 mm, Q value exceeding 108, FSR matching material Brillouin gain. By simply modulating the pump wavelength and coupling power, we observe SBS phenomena with a 13.47 GHz Brillouin frequency shift near 1.55µm and cascaded stimulated Brillouin lasers (SBL) of up to 12 orders. In addition, the Brillouin-Kerr optical frequency comb in this device is demonstrated, observing nearly 300 comb lines spanning across a spectral window of 250 nm. Our research provides a way to fabricate high-Q crystal microcavities and demonstrates the potential of these devices in applications such as microwave sources and nonlinear optics.

Dynamic Interplay Between Kerr Combs and Brillouin Lasing in Fiber Cavities

AbstractHighly coherent frequency combs are of crucial importance for optical synthesis and metrology, spectroscopy, laser ranging, and optical communications. Kerr combs, generated via cascaded nonlinear frequency conversion in a passive optical cavity, typically offer high repetition rates, which is essential for some of these applications. Recently, new ways of generating Kerr combs combining Kerr and Brillouin effects have emerged with the aim of improving some performances, especially in the fiber cavity platform. Direct coherent pumping is replaced by lasing on specific cavity modes, offering easily adjustable repetition rates, and enhanced coherence by Brillouin purification. In this study, such a scheme is implemented and investigated in a non‐reciprocal cavity. Highly coherent combs are demonstrated by finely controlling bi‐chromatic Brillouin lasing and the Kerr comb parameters. A suitable numerical model is introduced to account for the interplay between Brillouin scattering, Kerr effect, and cavity resonant feedback. Quantitative agreements with experiments reveal the importance of the pump lasers detuning in setting the comb's properties, through the mode pulling effect. This phenomenon, along with multi‐mode lasing that impedes the coherence, is not studied in previous fiber‐based demonstrations. These limitations are discussed, and several scaling laws are devised.

Asymmetric Mach-Zehnder prism interferometer coupler-based combined fiber ring resonator system for a Brillouin gyroscope.

Brillouin fiber-optic gyroscope (BFOG) is a fiber ring resonator (FRR)-based ring laser gyroscope with a large dynamic range and high sensitivity. Previous BFOGs operated under simultaneous resonance of the pump and Brillouin laser light in the FRR, resulting in complex control systems. This article introduces an asymmetric prism Mach-Zehnder interferometer coupler (APMZIC)-based FRR system, in which the coupling ratio of pump light is large, while the Brillouin laser resonates in the cavity with a negligible coupling loss, resulting in a simple control system. We analyzed the characteristics of the APMZIC and its combined resonator system both theoretically and experimentally and verified the feasibility of connecting the APMZIC with the resonant cavity of a Brillouin laser. This coupler system is compact, easy to fabricate, highly stable, and exhibits good repeatability, which are necessary for miniaturizing lasers and gyroscopes.

Brillouin lasers in a graphene microresonator for multispecies and individual gas molecule detection

Optical microcavities offer a promising platform for highly efficient light–matter interactions. Recently, the combination of microresonators and 2D materials in nanoscale has further enriched the optoelectronics of the microcavity geometries, spurring broad advances ranging from lasers, nonlinear converters, modulators to sensors. Here, we report the concept of a Brillouin laser sensor, by depositing graphene on an over-modal microsphere. Driven by a single continuous-wave pump at 1550 nm, multiple Brillouin lasers from distinct mode families are co-generated in a single device. The Brillouin lasers excited in the high Q cavity produce heterodyne beating notes with phase noise down to −161 dBc/Hz at 1 MHz offset, not only enabling label-freely identifiable detection of multispecies gas molecules adsorbed on the graphene in situ but also rendering it possible to trace individual molecules. Such a combination of graphene optoelectronics and Brillouin lasers in microcavities demonstrates a novel physical paradigm and offers insights into powerful tools for fast and precise optical sensing.

Polarization-maintaining photonic crystal fiber with high Brillouin gain coefficient for Brillouin lasers.

Stimulated Brillouin scattering (SBS) is effective for realizing a laser with an ultra-narrow linewidth. Although photonic crystal fiber (PCF) is considered an excellent medium to achieve SBS, it does not meet the requirements of low loss, large birefringence, and ease of fabrication. We propose a polarization-maintaining PCF (PM-PCF) structure and theoretically analyze the effects of the geometric structural parameters of the PM-PCF on various optical properties. Our theoretical analysis and experimental results contribute to the advancement of the field of ultra-narrow linewidth fiber lasers, distributed fiber sensing, and fiber-optic gyroscopes related to SBS.

Structure optimization and a detailed characteristic analysis for distributed Brillouin laser sensors

Distributed Brillouin laser sensor (DBLS) has shown its potential in terms of high signal-to-noise ratio (SNR) temperature/strain sensing without frequency sweeping. However, the discontinuous lasing in previous reports leads to limited resolution points and longer measurement time. In this letter, a structure optimization was proposed for DBLS to realize the continuous lasing and fully distributed sensing. Furthermore, a comprehensive experimental investigation on the sensing influence of key parameters, including net gain of lasing loop, peak power of pump pulse, and tolerance to the mismatch between pump pulse period and roundtrip time, has been explored detailly. The underlined physical mechanisms were also clarified experimentally. Compared with traditional Brillouin optical time-domain reflectometry (BOTDR), the substantially reduced standard deviation (STD) for distributed temperature sensing was demonstrated under the same averaging times. Related study can provide a new insight and design guideline for DBLS.

Intrinsic cascade-free intramode scattering Brillouin laser

The cascade operation of Brillouin lasers (BLs) is an identified obstacle to single-frequency power scaling and further compression of the fundamental linewidth. In this study, we reveal the relationship between the maximum cascade order and system parameters, starting from the phase-matching conditions of the Stokes cascade. The second Stokes is suppressed for modes that fall away the Brillouin gain linewidth (ΓB), which is heightened for Brillouin gain media with high sound velocity, large refractive index, and narrow linewidth. Diamond, with its extremely high product of speed of sound and refractive index, satisfies these requirements and is found to achieve cascade-free intramode scattering (TEM00) without manipulating cavity mode structures. This study elucidates a route to single-frequency, narrow-linewidth BLs via Brillouin material selection.

A narrow-linewidth high-power fused silica Brillouin laser

Free-space Brillouin lasers (BLs) generate high-power narrow-linewidth laser outputs at specific wavelengths. However, despite these impressive results, further compressing the Stokes linewidth while maintaining a high-power output remains a challenge. This study was conducted to investigate the design of BLs, starting from the phase-matching conditions and stability of a Brillouin cavity, for media with different phonon frequencies. A high-power and narrow-linewidth output can be realized using large-size, wide-gain-bandwidth Brillouin media. We experimentally demonstrated a BL using fused silica and diamond with the same cavity parameters. Stokes powers of 21.6 and 18.9 W were obtained with fused silica and diamond, respectively, at an available pump power of 58 W. Using fused silica, a Stokes linewidth of 1.2 kHz was obtained, which is two times narrower than that obtained using the diamond BL. Such BL design routes for various Brillouin media provide a path for achieving high-power, ultra-narrow-linewidth laser radiation at specific wavelengths.

Brillouin laser spectrometer based on spectral compression

We introduce a spectrometer design that uses Brillouin lasing to perform broadband spectral compression. This approach enables the entire C-band (optical frequencies covering 4 THz, or 25 nm) to be compressed into a 230 MHz wide RF spectrum—allowing a 4 THz wide optical spectrum to be monitored continuously using a standard analog-to-digital converter. This technique is based on the linear dependence of the Brillouin frequency shift on optical wavelength. To use this dependence for spectral analysis, we couple the input optical spectrum into a fiber ring cavity where it acts as an optical pump, exciting a frequency-shifted Brillouin lasing spectrum. The Brillouin lasing spectrum is then referenced to a copy of the original optical spectrum using heterodyne detection, converting the Brillouin frequency shift associated with each lasing mode to the RF domain. The narrow linewidth of the lasing modes enables a precise measurement of individual lines with an uncertainty of 28 MHz (0.2 pm) across a 4 THz band at an update rate of 200 Hz. This simple approach, constructed using standard fiber-coupled telecom components, provides an efficient method for high-resolution, broadband spectral analysis.

Ultra-Narrow Bandwidth Microwave Photonic Filter Implemented by Single Longitudinal Mode Parity Time Symmetry Brillouin Fiber Laser.

In this paper, a novel microwave photonic filter (MPF) based on a single longitudinal mode Brillouin laser achieved by parity time (PT) symmetry mode selection is proposed, and its unparalleled ultra-narrow bandwidth as low as to sub-kHz together with simple and agile tuning performance is experimentally verified. The Brillouin fiber laser ring resonator is cascaded with a PT symmetric system to achieve this MPF. Wherein, the Brillouin laser resonator is excited by a 5 km single mode fiber to generate Brillouin gain, and the PT symmetric system is configured with Polarization Beam Splitter (PBS) and polarization controller (PC) to achieve PT symmetry. Thanks to the significant enhancement of the gain difference between the main mode and the edge mode when the polarization state PT symmetry system breaks, a single mode oscillating Brillouin laser is generated. Through the selective amplification of sideband modulated signals by ultra-narrow linewidth Brillouin single mode laser gain, the MPF with ultra-narrow single passband performance is obtained. By simply tuning the central wavelength of the stimulated Brillouin scattering (SBS) pumped laser to adjust the Brillouin oscillation frequency, the gain position of the Brillouin laser can be shifted, thereby achieving flexible tunability. The experimental results indicate that the MPF proposed in this paper achieves a single pass band narrow to 72 Hz and the side mode rejection ratio of more than 18 dB, with a center frequency tuning range of 0-20 GHz in the testing range of vector network analysis, which means that the MPF possesses ultra high spectral resolution and enormous potential application value in the domain of ultra fine microwave spectrum filtering such as radar imaging and electronic countermeasures.

Detecting strain with a fiber optic cable on the seafloor offshore Mount Etna, Southern Italy

Oceans cover more than 70 percent of the Earth's surface making it difficult and costly to deploy modern seismological instruments here. The rapidly expanding global network of submarine telecom cables offers tremendous possibilities for seismological monitoring using laser light. Recent pioneer studies have demonstrated earthquake detection using lasers in onland and submarine fiber optic cables. However, permanent strain at the seafloor has never before been measured directly as it happens. With this aim, we deployed a dedicated 6-km-long fiber optic strain cable, offshore Catania Sicily, in 2000 m water depth, and connected it to a 29-km long electro-optical cable for science use. We report here that deformation of the cable equivalent to a total elongation of 2.5 cm was observed over a 21-month period (from Oct. 2020 to Jul. 2022). Brillouin laser reflectometry observations over the first 10 months indicate significant strain (+25 to +40 microstrain) at two locations where the cable crosses an active strike-slip fault on the seafloor, with most of the change occurring between 19 and 21 Nov. 2020. The cause of the strain could be fault slip or seabottom currents. During the following 11 months, the strain amplitude increased to +45 to +55 microstrain, affecting a longer portion of the cable up to 500 m to either side of the first fault crossing. A sandbag experiment performed on the distal portion of the cable (3.2–6.0 km) starting Sept. 2021 demonstrates how the fiber optic cable deforms in response to an applied load and how the deformation signal partially dissipates over time due to the elastic properties of the cable. These preliminary results are highly encouraging for the use of BOTDR (Brillouin Optical Time Domain Reflectometry) laser reflectometry as a technique to detect strain at the seafloor in near real time and to monitor the structural health of submarine cables.

Polarization-maintaining photonic crystal fiber with high Brillouin gain coefficient for Brillouin lasers

Polarization-maintaining photonic crystal fiber with high Brillouin gain coefficient for Brillouin lasers

140-Hz Narrow-Linewidth Brillouin/Erbium- Ytterbium Fiber Laser

We demonstrate an ultra-narrow-linewidth Brillouin/Erbium-Ytterbium co-doped fiber ring laser, which uses a length of 80-cm Erbium-Ytterbium co-doped fiber as both the Brillouin gain and linear gain media. The 976-nm pump threshold is only 80 mW when the Brillouin pump power is 20 mW. The output power of this laser reaches about 200 mW. The laser linewidth is as narrow as 140 Hz. The relative intensity noise of the fiber laser is -145 dB/Hz. The phase noise at 1 kHz is as low as to -107 dB re rad/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> (1-m optical path difference). Moreover, the 140-Hz-linewidth Brillouin laser light is transferred from a 2-MHz-linewidth Brillouin pump light. This fiber laser presents outstanding linewidth-narrowing characteristic. It presents many potential applications in coherent communications, optical fiber sensors, and etc.

Soliton Microcombs Multiplexing Using Intracavity-Stimulated Brillouin Lasers.

Solitons in microresonators have spurred intriguing nonlinear optical physics and photonic applications. Here, by combining Kerr and Brillouin nonlinearities in an over-modal microcavity, we demonstrate spatial multiplexing of soliton microcombs under a single external laser pumping operation. This demonstration offers an ideal scheme to realize highly coherent dual-comb sources in a compact, low-cost and energy-efficient manner, with uniquely low beating noise. Moreover, by selecting the dual-comb modes, the repetition rate difference of a dual-comb pair could be flexibly switched, ranging from 8.5 to 212MHz. Beyond dual-comb, the high-density mode geometry allows the cascaded Brillouin lasers, driving the co-generation of up to 5 space-multiplexing frequency combs in distinct mode families. This Letter offers a novel physics paradigm for comb interferometry and provides a widely appropriate tool for versatile applications such as comb metrology, spectroscopy, and ranging.

Brillouin scattering of zero-valent Au-, Cu-, Ag-intercalated hexagonal boron nitride

Brillouin laser light scattering spectra of hexagonal boron nitride were measured in its pure form as well as with zero-valent copper, gold, and silver intercalants including one sample co-intercalated with copper and silver. Up to four of the five independent elastic stiffness constants were determined along with the associated sound velocities for the five distinct materials. Intercalation radically changes the optical color of the boron nitride. Brillouin scattering measurements show almost no change in the ${c}_{33}$ out-of-plane elastic stiffness and the ${c}_{66}$ in-plane shear modulus, while the ${c}_{11}$ in-plane stiffness increases by roughly 100 GPa with copper-silver co-intercalation and decreases by roughly 30 GPa with silver intercalation. The results lie within the broad range of literature values for ${c}_{11},$ ${c}_{33},$ and ${c}_{44}$ but differ substantially for ${c}_{66}$.

Analytical investigation of GeO2-doped optical fibers to build Brillouin lasers at 1300 nm

The 1.5 µm spectral region concentrates the most substantial attention on building fiber laser sources for communications applications. However, data transmission in this spectral region is quite congested, requiring exploring new neighboring regions. In particular, 1.3 µm, usually defined as the zero-dispersion band, currently constitutes an active field of research, and the development of fiber laser sources in this wavelength band is of substantial interest to expand the capabilities of existing photonic operations. Previous experimental investigations have already shown that the peak stimulated Brillouin scattering efficiency is strongly affected by the concentration of GeO2 doping in single-mode fibers. In this context, this work presents an analytical investigation of the performance of a Brillouin fiber laser at 1.3 µm using optical fibers doped with different GeO2 concentrations as a Brillouin gain medium. The results obtained suggest that optical fibers doped with high GeO2 concentrations can be a simple and effective solution to build Brillouin lasers with better efficiency in the 1.3 µm band employing shorter fiber lengths with low pump power values.