A calculation is reported of the reflection coefficients for degenerate four-wave mixing in thermal gratings that form in the active media of solid-state lasers. It is shown that these reflection coefficients may be close to the reflection coefficients of amplitude gratings in the active media of ruby crystal and neodymium glass lasers.
The experimental relaxation time of degenerate four-wave mixing based on the nonlinearity of saturation of the active media of TEA CO 2 lasers (~0.3 μs) is shown to be an order of magnitude less than the time calculated on the basis of the existing theories. A new mechanism is proposed to account for the relaxation of such mixing in the active media of TEA CO 2 lasers. This mechanism postulates erasure of gratings, formed in the course of the fourwave mixing, of the population inversion of the active levels of a laser due to a high rate of filling of the upper level by resonant energy transfer from nitrogen molecules.
A system of equations is derived to describe a transient opposed degenerate four-wave interaction (FWI) in fields of arbitrary intensity in gaseous media exhibiting resonant and thermal nonlinearity mechanisms. A system of closed equations describing an opposed degenerate FWI in fields of arbitrary intensity is obtained for the steady-state case. It is found that, unlike linearly absorbing media, the optimal reflection coefficients of a signal wave in nonlinearly absorbing media are only achieved at reference wave intensities comparable with the saturation intensity of the resonant medium. Numerical calculations illustrate a fall in the reflection coefficient as the amplitude of the signal wave is increased.
A theoretical analysis is made of the efficiency of typical ring systems of phase-conjugating mirrors utilizing a degenerate four-wave interaction in media with a thermal nonlinearity. The analysis is made for steady-state conditions taking into account wave saturation effects in an amplifier situated in a positive feedback circuit. It is shown that the reflection coefficient in these systems is extremely sensitive to the level of the phase-conjugate signal (as the ratio of the signal intensity to the saturation intensity of the amplifier active medium is increased, the reflection coefficient of the ring phase-conjugating mirror decreases sharply). It is also found that at a high level of the nonlinearity and for an appreciable gain per pass, these systems exhibit multistability (i.e., several different steady states exist).
An experimental investigation was made of the reflection by a nonlinearity from gain saturation in the active medium of a TEA CO 2 laser under degenerate four-wave mixing conditions. A new mechanism of relaxation of the response to such a nonlinearity was identified and processes with submicrosecond and subnanosecond relaxation times were found in CO 2 . The saturation intensities for these processes and the conditions for achieving high reflection intensities as a result of such a mixing were determined.