  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.
