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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Gain media phase conjugators and self-adaptive solid-state lasers

Michael J. Damzen

Published Jan 1, 1994• 0 citations
Publisher

Abstract

A presentation is made of recent work in 3-D saturable gain-gratings, produced by spatial hole-burning, as efficient, externally programmable and/or self-adaptive diffractive optical elements for application in gain-media phase conjugators and as a technology for versatile adaptive spatial, spectral, and Q-switching control of high-gain solid-state lasers. Description is to be mainly made of gain-gratings written by spatial hole-burning when coherent beams, at the lasing wavelength, interfere in a gain medium. Such 3-D gain gratings, which have a persistence for the upper-state lifetime of the gain medium, can be considered as volume gain holograms and the application of these adaptive grating devices can be considered as a type of dynamic "gain holography."

DOI
10.1364/cleo_europe.1994.cmh5

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A presentation is made of recent work in 3-D saturable gain-gratings, produced by spatial hole-burning, as efficient, externally programmable and/or self-adaptive diffractive optical elements for application in gain-media phase conjugators and as a technology for versatile adaptive spatial, spectral, and Q-switching control of high-gain solid-state lasers. Description is to be mainly made of gain-gratings written by spatial hole-burning when coherent beams, at the lasing wavelength, interfere in a gain medium. Such 3-D gain gratings, which have a persistence for the upper-state lifetime of the gain medium, can be considered as volume gain holograms and the application of these adaptive grating devices can be considered as a type of dynamic "gain holography."
Phase conjugal ion by degenerate four-wave mixing (DTVVM) in saturable gain media is a nonlinear process of major interest for applications to dynamic holography and phase-distortion correction of an optical wave propagating in laser media [1,2,3]. The use of laser amplifiers for such operations presents very attractive features including the automatic matching of the nonlinearity with the laser wavelength, a fast response time and a high efficiency of the nonlinear process due to the laser amplification of all interacting beams. This process is based on the possibility of writing dynamic gain gratings when interfering beams are propagating in a laser medium by gain saturation.
Nonlinear optics normally involves the change in the susceptibility of a material due to the presence of optical radiation. In many cases, the light-matter interaction produces a change in the refractive index of the medium, as occurs in photorefractive media, as well as in Kerr-like media, Brillouin-active media and absorbing media with thermallyinduced refractive index changes. Such nonlinear refractive index variation corresponds to a change in the real part of the optical susceptibility %. In resonant materials, the optical radiation induces stimulated emission (or absorption) causing transfer of population between different energy states of the medium. This can lead to saturation of the amplification (or absorption) corresponding to changes in the imaginary part of the susceptibility x= i2odk where the amplitude gain coefficient a(I) is a saturable function of the intensity /, and k is the magnitude of the wavevector of the optical radiation. The interference of two (or more) coherent beams I(x) in the laser amplifier can cause a modulation of the population inversion N(x). This modulation of the inversion is commonly known as spatial hole burning in lasers and leads to formation of a gain-grating a(x) = 1/2GN(X) where G is the stimulated emission cross-section. The gain grating can act as a very efficient diffractive optical element exhibiting high diffraction efficiency that may be even greater than unity [16] and also very high phase conjugate reflectivity using four-wave mixing (FWM) geometries [5-7].
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Summary form only given. Gain gratings in laser media have been extensively studied to understand gain four-wave mixing (FWM) as a means of achieving phase conjugation, particularly in the steady-state and highly-transient regimes. We present an analysis of the generalized time regime (including pumping and spontaneous emission) and model the time-dependent evolution of the gain grating from the highly transient to the steady-state for arbitrary beam strength. For the first time to our knowledge, this analysis considers the formation of a combined transmission and reflection gain grating.
The first paper in this series presented a detailed analysis of the diffraction efficiency of a single holographic grating obtained by spectral hole burning. This paper covers the influence of phase changes which occur during the burning of a single hologram, and the interaction of spectrally adjacent two laser induced gratings. A theoretical description of the holographic efficiencies which takes into account the influence of the relative phases between the two gratings. Experimental results are presented and discussed in terms of the theory.
A new hologram type in spectral hole-burning systems is presented. During exposure, the frequency of narrow-band laser light is swept over a spectral range that corresponds to a few homogeneous linewidths of the spectrally selective recording material. Simultaneously the phase of the hologram is adjusted as a function of frequency-the phase sweep function. Because of the phase-reconstructing properties of holography, this recording technique programs the sample as a spectral amplitude and phase filter. We call this hologram type frequency and phase swept (FPS) holograms. Their properties and applications are summarized, and a straightforward theory is presented that describes all the diffraction phenomena observed to date. Thin FPS holograms show strongly asymmetric diffraction into conjugated diffraction orders, which is an unusual behavior for thin transmission holograms. Investigations demonstrate the advantages of FPS holograms with respect to conventional cw recording techniques in freq ncymultiplexed data storage. By choosing appropriate phase sweep functions, various features of holographic data storage can be optimized. Examples for cross-talk reduction, highest diffraction efficiency, and maximal readout stability are demonstrated. The properties of these FPS hologram types are deduced from theoretical considerations and confirmed by experiments.