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

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