Resonant Acoustic Modes in DFB Fiber Lasers Characterized via Frequency Noise Measurements

This paper investigates the impact of resonant acoustic waves on the frequency noise spectrum of Distributed Feedback fiber lasers. A phase-shifted Fiber Bragg Grating is used to probe acoustic waves traveling along an optical fiber and resonating within the mechanical cavity formed by the fiber's contact points with the substrate. These acoustic waves are fully characterized through careful Fourier-analysis, with vibrational simulations providing additional insights into the wave's mode shape within the fiber. We identify two distinct acoustic modes, predominantly traveling on the surface of the fiber, with measured velocities of 3273 m/s and 1619 m/s and causing a calculated oscillation of the silica fiber's diameter of approximately 10 fm. To mitigate the detrimental impact of these oscillations on the laser's frequency noise, we propose dampening the acoustic waves by either using softer fiber coating materials or surrounding the fiber with viscous liquids. These findings offer practical insights for enhancing the performance of low-noise Distributed Feedback fiber lasers.