Passive transverse-mode organization in a photorefractive oscillator with saturable absorber

We present and demonstrate a self-transverse-mode organization effect (passive transverse mode locking?) in a cavity that contains a saturable absorber. It gives narrowing and filamentation of the oscillating beam in the region of the absorber. In the demonstration, we have used a resonator formed by two photorefractive phase conjugate mirrors. The saturable absorber was bacteriorhodopsin in a polymer film. Light powers of 1–100 mW were used.