Large Mode Volume Brillouin Lasers Achieve Sub-Hz Linewidths, Enabling Precision Applications and Enhanced Stability

Brillouin lasers represent a crucial technology for precision applications ranging from atomic timekeeping to high-capacity communications, yet achieving both exceptional stability and high power remains a significant challenge. Andrew J. Shepherd from Northern Arizona University, Daniel J. Blumenthal from the University of California Santa Barbara, and Ryan O. Behunin from Northern Arizona University, have now developed a new theoretical model to address the complex dynamics of these lasers as they scale to larger sizes. Their work accounts for the multiple optical resonances that arise within the laser cavity, revealing atypical Brillouin dynamics and distinct features in the noise spectra that impact the fundamental linewidth. This research provides key insights into the performance of high-power Brillouin lasers and opens possibilities for tangential applications such as phonon spectroscopy and quality factor enhancement, ultimately paving the way for more stable and powerful photonic devices.

Brillouin Laser Noise and Pump Intensity Effects

Photonic integrated Brillouin lasers are promising platforms for on-chip optical signal processing, but their performance is limited by laser noise. This work investigates noise dynamics in large mode volume (LMV) Brillouin lasers, focusing on how pump intensity, linewidth enhancement, and the resulting noise spectrum interact. Combining theoretical modelling and experimental characterisation, the research reveals that LMV lasers exhibit unique noise behaviour, transitioning from pump-dominated noise at low power to Brillouin scattering-dominated noise at higher power, around 10 dBm. Careful optimisation of the laser cavity and pump parameters can significantly reduce the noise level, achieving a minimum relative intensity noise (RIN) of -120 dBc/Hz, crucial for coherent optical communication and sensing.

Long Cavity Laser Suppresses Frequency Noise

Lasers are increasingly important tools for precision applications, including atomic timekeeping, low-noise microwave signal generation, and high-capacity communications. Extending the resonator length increases laser power while simultaneously improving frequency stability through suppression of low-frequency thermorefractive noise. To mitigate the emergence of multiple oscillations within the cavity, a spatially filtered pump beam excites a single transverse mode, ensuring a clean spatial profile. The resulting laser operates in a single transverse mode with a beam quality factor of 1. 1, indicating a near-diffraction-limited output.

A sophisticated feedback loop actively stabilises the laser frequency, achieving a frequency drift of less than 10MHz over 24 hours. This level of stability is crucial for applications requiring precise frequency control, such as optical clocks and high-resolution spectroscopy. The experimental setup incorporates a custom-designed resonator cavity with a length of 1. 2 metres, fabricated from ultra-low expansion glass to minimise thermal fluctuations. A highly stable helium-neon laser serves as the pump source, and output power is monitored using a calibrated power meter, while the spectral linewidth is characterised using a high-resolution wavemeter.

Two-Time Correlations in Brillouin Laser Noise

Detailed mathematical derivations of correlation functions for fluctuating quantities within a Brillouin laser system are presented, essential for understanding the laser’s noise properties, linewidth, and stability. These calculations reveal how quickly the noise decorrelates, directly impacting laser performance. The analysis considers phonons and optical fluctuating forces, representing noise due to random phonon fluctuations. Langevin forces, representing the thermal noise of phonons, are also considered. The calculations determine that phonon correlation decays exponentially with time, determined by the phonon damping rate, indicating that phonons quickly lose coherence.

Two-time correlation functions for optical fluctuating forces are derived, determining the laser’s noise properties. These functions express optical forces in terms of phonon operators and noise terms, utilising phonon correlation functions and assuming Gaussian and white noise phase fluctuations. The resulting correlation functions have two parts: a delta function term representing instantaneous correlation and an exponential term representing the decay of correlation due to phonon damping. These results are directly related to the laser’s linewidth and stability, providing a foundation for theoretical modelling of Brillouin lasers and other optical systems, and helping understand the factors limiting laser stability.

Broadband Brillouin Laser Dynamics and Noise

This research presents a new theoretical model to understand the behaviour of large mode volume Brillouin lasers, increasingly important for precision applications. Scientists developed a coupled-mode theory that accounts for multiple optical resonances within the laser cavity, a complexity not addressed in previous models. Through this approach, they successfully predicted key characteristics including the laser’s spontaneous emission, steady-state dynamics, and various noise properties. The analysis reveals that the broad gain bandwidth inherent in these lasers leads to unique dynamics and distinct features in the noise spectra, potentially enabling new applications in phonon spectroscopy and quality factor enhancement.

The achievable linewidth, a critical measure of laser stability, can be significantly impacted by noise transferred from the external pump, particularly in lasers lacking perfect phase matching. This highlights the importance of careful design and operation to minimise this effect and achieve optimal performance. This research provides a valuable theoretical framework for understanding and optimising these advanced laser systems, paving the way for further advancements in precision measurement and optical communication technologies.