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Amplified spontaneous emission is a source of broadband noise thatparasitically limits the achievable gain in laser amplifiers. While opticalbandpass filtering elements can suppress these broadband noise contributions,such filters are typically designed around specific frequencies or requiremanual tuning, rendering them less compatible with tunable laser systems. Here,we introduce a nonlinear self-adaptive filter and demonstrate the suppressionof amplified spontaneous emission surrounding the lasing mode of a tunable 780nm external cavity diode laser, using the two-beam coupling interaction inphotorefractive BaTiO$_3$. A peak suppression of $-$10 dB is observed $\pm$2.5nm from the lasing mode, with an overall 50% filter power throughput. Thedynamic photorefractive filter is automatically centered on the peak frequencydue to the continuous writing and readout of the volume holographic grating andcan thereby also automatically adapt to frequency tuning, drift, or modehopping with an estimated auto-tuning rate of 100 GHz/s under typicalconditions. Additionally, we present opportunities for enhancing filtersuppression characteristics via the input intensity ratio and tuning thebandwidth via the coupling angle, toward versatile, self-adaptive opticalfiltering.

Self-Adaptive Amplified Spontaneous Emission Suppression with a Photorefractive Two-Beam Coupling Filter