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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Reflections From A Dynamic Grating In A Dye Solution

Marc D. Davidson, Hugo J. van Staveren, C. Snoek, A. Dönszelmann

Published Mar 8, 1989• 0 citations
Publisher

Abstract

In addition to results of earlier experiments where Bragg scattering from a distributed feedback dye laser is observed, in the experiment described here the occurrence of a phase conjugated reflected beam is demonstrated. It is shown that a distributed feedback laser can be used as a frequency selective reflector for a seperate dye laser. Some time ago Golub and Shuker investigated the feasibility of optical switching by Bragg scattering of a light beam from a spatially modulated gain grating. This grating formed the basis of a distributed feedback dye laser (DFDL) and the reflection was only visible if the DFDL was pumped above threshold. In that case reflectivities of more than 100% for small input signals could be obtained. Golub and Shuker emphasized that in their experiment phase conjugation could not occur. Their main argument was, that the probe beam and the pump beams had completely different origins. Consequently these beams had no constant phase relation, and hence degenerated four-wave mixing leading to phase conjugation was excluded.

DOI
10.1117/12.949988

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In addition to results of earlier experiments where Bragg scattering from a distributed feedback dye laser is observed, in the experiment described here the occurrence of a phase conjugated reflected beam is demonstrated. It is shown that a distributed feedback laser can be used as a frequency selective reflector for a seperate dye laser. Some time ago Golub and Shuker investigated the feasibility of optical switching by Bragg scattering of a light beam from a spatially modulated gain grating. This grating formed the basis of a distributed feedback dye laser (DFDL) and the reflection was only visible if the DFDL was pumped above threshold. In that case reflectivities of more than 100% for small input signals could be obtained. Golub and Shuker emphasized that in their experiment phase conjugation could not occur. Their main argument was, that the probe beam and the pump beams had completely different origins. Consequently these beams had no constant phase relation, and hence degenerated four-wave mixing leading to phase conjugation was excluded.
Degenerate four-wave mixing (DFWM) is a simple method to achieve phase conjugation. It uses the third-order nonlinearity of the electric susceptibility to produce changes in the optical properties of materials. A dynamic grating is then induced. Such gratings might be used as fast optical memories in optical computing. Our aim is to find media with large nonlinear susceptibilities and fast response time. We report phase conjugation of 532-nm laser radiation by DFWM in dye solutions. The time dependence of the conjugated wave is investigated experimentally and compared to theoretical predictions. We have also studied the influence of the solvents, dye concentration, and pump intensity on the phase conjugated signal. The results are compared with those predicted by three nonlinear mechanisms. Thermally induced refractive-index changes, Kerr effect, and saturable absorption are considered.
The origin and mechanism of backward stimulated Rayleigh scattering in two-photon absorbing media are studied theoretically and experimentally. This type of stimulated scattering has the unusual features of no frequency shift and low pump threshold requirement compared to all other known stimulated scattering effects. This frequency-unshifted stimulated Rayleigh scattering effect can be well explained by a two-photon-excitation-enhanced Bragg grating reflection model. The reflection of the forward pump beam from this stationary Bragg grating may substantially enhance the backward Rayleigh scattering beam, providing a positive feedback mechanism without causing any frequency shift. A two-counterpropagating-beam-formed grating experiment in a two-photon absorbing dye solution is conducted. The measured dynamic behavior of Bragg grating formation and reflectivity properties are basically consistent with the predictions from the proposed model.
A frequency-unshifted and backward stimulated scattering can be efficiently generated in one-photon-absorption free but two-photon absorbing materials. Using a number of novel two-photon absorbing dye solutions as the scattering media and nanosecond pulsed laser as the pump beams, a highly directional backward stimulated scattering at the exact pump wavelength can be readily observed once the pump intensity is higher than a certain threshold level. The spectral and spatial structures as well as the temporal behavior and optical phase-conjugation property of this new type of backward stimulated scattering have been experimentally studied. This stimulated scattering phenomenon can be explained by using a model of two-photon-excitation enhanced standing-wave Bragg grating initially formed by the strong forward pump beam and much weaker backward Rayleigh scattering beam; the partial reflection of the pump beam from this grating provides an positive feedback to the initial backward Rayleigh scattering beam without suffering linear attenuation influence. Comparing to other known stimulated (Raman, Brillouin, Rayleigh-wing, and Kerr) scattering effects, the stimulated Rayleigh-Bragg scattering exhibits the advantages of no frequency-shift, low pump threshold, and low spectral linewidth requirement.
The exact theory of degenerate four-wave mixing in a Kerr-like medium in the Bragg scattering regime is used to study the reflectivity of phase-conjugate mirrors pumped by intense laser beams. The analysis takes into account pump depletion, nonlinear phase modulation and the relaxation time of the nonlinearity. The steady-state response exhibits multivalued reflectivity, hysteresis and potential bistability; however, the transient response shows that at most one value of the phase-conjugate reflectivity is stable. Periodic or chaotic pulsations are predicted beyond threshold values of pump intensity. The onset of a natural oscillation in the Kerr-like medium is also predicted in the absence of a continuous probe beam; the resulting field amplitude is stable and weakly sensitive upon the presence of external noise.
Phase conjugate reflectivity of the photorefractive four-wave mixing system is usually quite unstable and it is serious issue for practical use of these systems. The cause of this instability is mainly combination of the fluctuation of the phases of input signal and pump beams and the slow response time of the photorefractive crystal. When all input beams in the four-wave mixing system are mutually coherent and the phases of the input beams are not stable, four kinds of gratings are compete each other and the phase conjugate output becomes quite unstable. To avoid this problem, we can make one of the pump incoherent to the signal and another pump1. The phase conjugate output is, however, still unstable due to the rewriting of the grating caused by the fluctuation of the phase of the input beams. In this paper, we report our feedback system to stabilize the power of the phase conjugate output from the four-wave mixing system by controlling the phase of the signal beam. In this system, we detect the phase difference between signal and forward pump beam with lock-in amplifier and control the phase of the signal with a piezoelectric mirror to stabilize the phase difference.