- The paper demonstrates the first observation of stimulated Brillouin scattering in ultra-high Q CaF₂ resonators with thresholds around 3 μW.
- Experimental results reveal multiple Stokes components and ultranarrow linewidths down to 30 μHz, showcasing potential for stable microwave generation and sensing.
- The study utilizes advanced techniques like the Pound-Drever-Hall method to enhance resonator stability, underpinning its practical applications in integrated photonics.
Brillouin Lasing with a CaF<sub\>2</sub> Whispering Gallery Mode Resonator
In their paper, Grudinin, Matsko, and Maleki have explored the phenomenon of stimulated Brillouin scattering (SBS) within whispering gallery mode (WGM) resonators constructed from CaF<sub\>2</sub> materials. Their study demonstrates, for the first time, the occurrence of SBS in an ultra-high Q centimeter-scale optical resonator. This research highlights unique opportunities for using these resonators in applications such as optical generation of microwaves and precision sensing gyroscopes.
Theoretical Underpinnings and Experimental Setup
The study elaborates on how SBS occurs due to the interaction between light and acoustic waves within the resonator, resulting in the gain of laser-like properties. Brillouin lasers, in contrast to traditional lasers, are characterized by their ability to generate multiple Stokes components in a cascading manner, with very low power thresholds, typically below 1 mW. Such characteristics enable SBS lasers to emit microwaves within the 10–100 GHz range and exhibit a significant linewidth narrowing effect, reducing the relative intensity noise.
The authors utilized ultra-high Q WGM resonators due to their efficacious ability to enhance nonlinear optical processes. This enhancement is facilitated by the spatial confinement and extended effective optical paths within the resonators, contributing to reduced lasing thresholds for phenomena such as stimulated Raman scattering (SRS) and SBS itself.
They adapted a two-resonator setup with Nd:YAG laser sources operating at 1064 nm to induce WGMs, achieving coupling efficiencies up to 80%. The study also incorporates advanced techniques such as the Pound-Drever-Hall method, further refining the stability of the laser cavity system.
Experimental Results
The authors fabricated two high-quality CaF<sub\>2</sub> resonators with different radii of curvature to investigate SBS thresholds and characteristics. It was found that SBS threshold in these resonators is around 3 μW, which accords closely with theoretical predictions. Experimental outcomes revealed Stokes lines at offsets of 17.7 GHz attributed to Brillouin scattering from the longitudinal phonon branch in the material. This observation was theorized based on the calculated Brillouin frequency offset for a CaF<sub\>2</sub> resonator and supported by measured spectral asymmetries between forward and backward beams.
Interestingly, the data indicated that SBS laser linewidth could be exceedingly narrow, potentially down to 30 μHz. This narrowing is potentially due to high optical and acoustic quality factors of the resonators. The observation of Raman lasing at higher pump power levels provides additional evidence of the resonator's capability to support multi-line lasing processes.
Implications and Future Directions
The realization of such laser systems holds promise in numerous domains, specifically in applications demanding stable and precise frequency standards. Particularly in sensitive rotation measurements and high-stability microwave generation, the observed linewidth narrowing effect from SBS in WGM resonators is highly beneficial. Furthermore, the study’s findings suggest the possibility of exploiting such resonators in compact and efficient integrated photonics devices, shedding light on the instrumental role of phononic and photonic interactions within crystallographic materials.
To expand the utility of CaF<sub\>2</sub> WGM resonators, future research could consider addressing potential limitations, such as temperature-related noise affecting Stokes linewidth in room-temperature environments. Also, the development of strategies for SBS suppression can facilitate the application of these devices in scenarios where Brillouin lasing may prove counterproductive.
In conclusion, the authors have established a significant groundwork for realizing SBS in high-Q microresonators, charting a path for advancements in optical and microwave photonic applications. The remarkable optical properties exhibited by CaF<sub\>2</sub> WGM resonators open a multitude of avenues for further exploration in both fundamental research and practical implementations in the future.