The main drawback of fiber lasers is their high sensitivity to fluctuation in the properties of their surroundings (temperature, pressure, vibration, etc.). Even a minuscule fluctuation in the ambient parameters can destabilize them. Consequently, complex techniques are often utilized to isolate and stabilize fiber lasers.
A new, passive, feedback mechanism inherent to Brillouin fiber lasers (BFLs) was discovered and studied in our group. This mechanism, stemming from the interplay between thermal optical-length variations and the gain-line induced frequency dependent lasing power, triggers unexpected and counter-intuitive phenomena such as self-frequency-stabilization, multi-stability and memory effects. The direct benefit of this feedback mechanism is that it allows for passive self-stabilization of fiber lasers with minimal means of isolation. This nonlinear process can be controlled and modified by engineering the gain lineshape, rendering BFLs highly attractive as a platform for studying nonlinear dynamics and providing novel tools for various possible applications.
The following video (VID1) shows experimentally the unique properties of the feedback. The video presents the temporal evolution of the RF spectrum of the heterodyne beating signal between the pump and the Brillouin lasing. The inset shows the voltage being applied to the PZT at the same time, corresponding to the generated perturbations. The plotted span of the RF spectrum is approximately 30MHz which covers slightly more than a single FSR of the fiber cavity, and is centered at the gain line center. Note that the heterodyne beating between the higher (optical) frequency pump and the lower frequency BFL output results in an RF signal with a spectrum that is a mirror image of the BFL spectrum and hence higher RF frequencies are in practice lower optical frequencies and vice versa.