Modeling of laser mode self-adapted filtering by photorefractive Fabry–Perot interferometers

Abstract : The reduction of the number of oscillating modes by a self-adapted Fabry–Perot (FP) filter was first demonstrated in a dye laser in 1987 [Opt. Lett. 12 (1987) 117]. A renewal of interest in this technique arose recently with the observation of a dramatic reduction of the number of modes in a pulsed Sapphire:Ti laser [Appl. Phys B 69 (1999) 155]. A self-adapted Fabry–Perot filter is made of a photorefractive crystal inserted inside a linear laser cavity between the amplifier and the output coupler. This coupler acts as one mirror of this Fabry–Perot while the Bragg reflection grating recorded by the standing waves inside the crystal plays the role of the second mirror. The dynamic properties of the photorefractive effect make this Fabry–Perot continuously adapted to the oscillating modes. Its spectral reflectivity reinforces the competition between these modes which, in some cases, leads to a single-mode operation. We analyze the laser mode competition taking into account the laser gain properties and the wavelength dependence of the cavity losses induced by the self-adapted Fabry–Perot.