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

Absolute Phase Shift of Phase Conjugators

Ian C. McMichael, Pochi Yeh, M. Khoshnevisan

Published Jun 16, 1986• 0 citations
Publisher

Abstract

We present theoretical expressions and experimental measurements of the absolute phase shifts of phase conjugators. Photorefractive media, transparent media, saturable absorbers and saturable amplifiers are considered in the analysis. Experimental measurements of the absolute phase shifts are presented for barium titanate, strontium barium niobate and ruby.

DOI
10.1117/12.960377

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We present theoretical expressions and experimental measurements of the absolute phase shifts of phase conjugators. Photorefractive media, transparent media, saturable absorbers and saturable amplifiers are considered in the analysis. Experimental measurements of the absolute phase shifts are presented for barium titanate, strontium barium niobate and ruby.
We present measurements of the two-wave mixing gain as a function of frequency detuning in photorefractive crystals. In many cases, this function is asymmetric, indicating that the phase shift of the photorefractive inde grating with respect to the light interference pattern is not exactly 90 degrees , as is often assumed. In four-wave mixing, the phase of the phase-conjugate wave contains the phase of the pumping and probe waves and a phase shift determined by the interaction taking place in the nonlinear medium. This second term, the phase shift of the phase conjugator, is a function of the type of grating and the phase shift of this grating. The phase shift of the grating obtained from the two-wave mixing measurements compares well with that obtained from four-wave mixing measurements of the phase of the phase conjugator.
When the phase-conjugate replica of an optical beam is generated by traditional four-wave mixing with external pumping beams, the frequency of the phase-conjugate beam is fixed by the frequencies of the three input beams. In self-pumped phase conjugation, there is only one input beam, and the frequency of the output beam is free to roam. In photorefractive materials, this frequency difference between the incident and output beams is typically a few hertz. We present a method for controlling this frequency shift in the self-pumped phase-conjugate mirror that uses a ring configuration1 in photorefractive barium titanate. We show that two-beam coupling of an additional control beam with either of the optical beams already present in the ring cavity can alter the frequency of the phase-conjugate beam. We show that the control beam can alter the nonreciprocal phase shift2 of light propagating around the ring and thereby alter the frequency of the phase-conjugate beam. We also calculate the magnitude and the sign of the expected frequency shift as a function of the intensity, direction, and frequency of this control beam.
A photorefractive system based on the combined operation of a strontium barium niobate input amplifier and a self-pumped barium titanate phase-conjugator crystal is presented and analyzed. Amplified real-time phase-conjugate wave-front generation in this system is demonstrated. The system's ability to all-optically, temporally modulate a weak probe signal and to transmit it back to its source by means of amplified dynamic phase conjugation demonstrates its potential in adaptive optical transmission schemes.