R.P.M. Green

Self-adaptive solid-state laser oscillator formed by dynamic gain-grating holograms

We describe the operation of a self-starting Nd:YAG laser oscillator incorporating a design in which laser oscillation occurs by means of diffraction from spontaneously generated three-dimensional gain gratings produced by spatial hole burning in the Nd:YAG amplifier. The transient onset and spectral selectivity of the gain gratings produce an output with energy of 600 mJ in a 10-ns single-longitudinal-mode pulse at 10 Hz. The self-adaptation of the gain gratings produces compensation of intracavity phase distortion. A transient numerical modeling of the nonlinear resonator gives good agreement with the experimental system and also provides insight into the temporal dynamics of the gain grating.

Investigation of multipass geometries for efficient degenerate four-wave mixing in Nd:YAG

Degenerate four-wave mixing in the saturable gain of a flash-lamp-pumped Nd:YAG amplifier has been investigated. Three different geometries are examined in which the probe beam experiences (a) one pass, (b) two passes, and (c) four passes of the four-wave interaction region. It is found that multipassing the gain medium has a dramatic effect on the efficiency of the process, with a phase-conjugate reflectivity of 2500 and a conjugate energy extraction efficiency of greater than 200% demonstrated.

Phase conjugate reflectivity and diffraction efficiency of gain gratings in Nd:YAG

Abstract It is shown that gain gratings can be used as efficient phase conjugate and diffractive optical elements. Experimental results are presented of saturable gain four-wave mixing in a Nd:YAG amplifier using orthogonally polarised pump beams of pulse duration 16 ns. Phase conjugate reflectivity and diffraction efficiency are measured for both the reflection and transmission grating cases. Results show a higher efficiency for the transmission geometry. Transient (experimental results are comjpaed to a steady state theory.

Holographic laser resonators in Nd:YAG

We report the operation of Nd:YAG ring laser resonators formed by diffractive coupling from a gain volume hologram written in a Nd:YAG amplifier. Stable diffraction-limited output in a TEM(00) mode is demonstrated in a Nd:YAG system with thermal lensing and without the requirement of spatial mode control.

High-reflectivity four-wave mixing by gain saturation of nanosecond and microsecond radiation in Nd:YAG

High-reflectivity four-wave mixing by gain saturation in a Nd:YAG laser amplifier is demonstrated. With a four-pass geometry, the phase-conjugate reflectivity of a Q-switched pulse (18-ns duration) is ∼2500, and for a long pulse (relaxation oscillation duration ∼100 μs) it is ∼30. Temporal and energy characteristics of the conjugate beam are presented. A phase-conjugate Nd:YAG TEM00 laser resonator is also produced in which the Nd:YAG rod is simultaneously the laser gain medium and the phase-conjugating device.

Transient modeling of pulsed phase conjugation experiments in a saturable Nd:YAG amplifier

Abstract We model the transient dynamics of degenerate four-wave mixing in a saturable gain medium. We confirm that the transmission gain grating case has a higher reflectivity than the reflection grating case in correspondence to experiments. A transient model of self-pumped phase conjugation in a saturable amplifier in a loop arrangement is compared to a pulsed experiment in a Nd:YAG amplifier. Results from the model show agreement in the behavioural characteristics of the experimental system. Dynamical evolution of the system is described with reference to temporal build-up of the gain gratings during the writing pulse and the subsequent erasure during the lasing output pulse.

Spatial characteristics of a laser oscillator formed by optically-written holographic gain-grating

Abstract We investigate the spatial characteristics of a Nd:YAG unidirectional ring resonator formed by an adaptive holographic gain-grating. Diffraction-limited spatial output is demonstrated from the resonator even with very strong intracavity phase distortion. We also show that, despite good intracavity phase compensation and good spatial output quality, the output mode size is not generally the same as the self-intersecting input beam that forms the gain hologram. A simplified theory, to account approximately for a spatially-Gaussian input beam, gives qualitative agreement with the key features observed in the experimental resonator.

Reflectivity and oscillation conditions of a gain medium in a self-conjugating loop geometry

The intensity-dependent reflectivity of a gain medium in a self-phase-conjugating loop geometry is numerically modeled by use of exact integration equations without restriction on the degree of gain saturation. High conjugate reflectivity and high energy extraction efficiency of the gain medium are predicted. A weak saturation analysis is also presented to derive useful expressions for the threshold input intensity for oscillation of the backward conjugate mode and the resonant oscillation frequencies.

Bragg-selectivity of a volume gain grating

Abstract We adapt the theory of volume grating holography to model analytically the diffraction efficiency of a volume gain grating and its variation due to deviation from Bragg-matching. We present modelling of the case when the gain grating is optically-written in a saturable laser amplifier by spatial hole burning, under both steady-state and transient conditions. We perform an experimental measurement of the angular-sensitivity of a volume transmission gain grating optically-written in a Nd:YAG amplifier rod. It is shown that the amplification of the writing beams needs to be considered since this leads to a spatial variation of the grating modulation depth and thereby to the reduction of its angular sensitivity compared to the uniform grating case.

Intensity-induced birefringence in Cr4+: YAG

Abstract We studied both experimentally and theoretically the transmission and polarization characteristics of a Cr4+ : YAG saturable absorber crystal as a function of the intensity and polarization state of an incident laser beam. We found that a birefringent absorption behaviour is induced and we show that the experimental results are well described by a full numerical model which includes excited-state absorption as well as a ground-state cross-absorption term. By using pump-probe measurements we show that a finite cross-saturation of the absorption is achieved along the crystal axes orthogonal to the polarization of the optical radiation. No induced refractive index birefringence is observed.