Mirrorless oscillation in polychromatic phase conjugation

Polychromatic phase conjugation (PPC), in which the pump and probe are separated substantially in wavelength, has recently been described. By means of a noncollinearly phase-matched difference frequency generation between two pump waves at w/sub 2/ and a probe at w/sub 1/, the phase conjugate of the probe is generated at the frequency 2w/sub 2/ - w/sub 1/. The phase matching is achieved by adjusting the angle between the direction of propagation for the pump and the symmetry axis. This is done by merely changing the input angle for the pump. The device used as a conjugator is a plane-parallel slab of heavy flint glass (SF-59), which is covered with delectric mirrors. The pump beam is introduced through a slit in one of the mirrors, and the probe enters at the short end. Mirrorless oscillation (of the same type as in phase conjugation with DFWM) has been theoretically discussed and is now verified experimentally. The major feature of this phenomenon is that a perfect wavelength tunable parametric oscillator is created. Compared with a second-order parametric oscillator, there are two definite advantages. First, the wavelength tuning is done by changing the input angle instead of a sensitive and difficult phase-matching method in which the temperature dependence of the birefringence is utilized. Second, all difficulties because of cavity conditions disappear because there is no cavity. The disadvantage is that the intensity requirements are sharper for PPC because it is a third-order phenomenon