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Annual Meeting Optical Society of America

Bistable phase-conjugate resonator using an intracavity saturable absorber

M. B. Klein, Gilmore J. Dunning, Stephen W. McCahon, D. M. Pepper

Published Jan 1, 1991• 0 citations
Publisher

Abstract

Phase-conjugate resonators have several attractive properties for image processing and storage. In the work described here, we demonstrate improved thresholding and storage properties of a phase-conjugate resonator (PCR) by the addition of an intracavity saturable absorber. It can be shown that the resonator will be bistable if (1) the small-signal absorption of the saturable absorber exceeds the small-signal gain of the phase-conjugate mirror, and (2) the absorption saturates more readily than the gain (i.e., reflectivity) of the PCM. Recently, a saturable absorber was used inside a PCR to control the transverse modes,1 but no evidence of bistability was obtained. In our experiment, we used a PCR bounded at one end by a conventional mirror and at the other end by an externally pumped BaTiO3 PCM. The saturable absorber was a solid film of fluorescein- doped boric acid glass.2 By adjustment of the total intensity and beam ratio at the PCM, we were able to demonstrate bistability. The input in our experiment was an incoherent control beam incident noncollinearly onto the intracavity saturable absorber. In the absence of a control beam, no oscillation was observed. When the control beam was turned on, the resonator switched to the on state in ~5 sec. The control beam could then be removed, and the cavity oscillation continued. To return to the off state, the resonator path was blocked.

DOI
10.1364/oam.1991.thee1

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Phase-conjugate resonators have several attractive properties for image processing and storage. In the work described here, we demonstrate improved thresholding and storage properties of a phase-conjugate resonator (PCR) by the addition of an intracavity saturable absorber. It can be shown that the resonator will be bistable if (1) the small-signal absorption of the saturable absorber exceeds the small-signal gain of the phase-conjugate mirror, and (2) the absorption saturates more readily than the gain (i.e., reflectivity) of the PCM. Recently, a saturable absorber was used inside a PCR to control the transverse modes,1 but no evidence of bistability was obtained. In our experiment, we used a PCR bounded at one end by a conventional mirror and at the other end by an externally pumped BaTiO3 PCM. The saturable absorber was a solid film of fluorescein- doped boric acid glass.2 By adjustment of the total intensity and beam ratio at the PCM, we were able to demonstrate bistability. The input in our experiment was an incoherent control beam incident noncollinearly onto the intracavity saturable absorber. In the absence of a control beam, no oscillation was observed. When the control beam was turned on, the resonator switched to the on state in ~5 sec. The control beam could then be removed, and the cavity oscillation continued. To return to the off state, the resonator path was blocked.
We report our observations of optical bistabilities and hysteresis1 in photorefractive passive phase conjugate resonators with BaTiO3, caused by grating erasure effects between two resonators in three different configurations. Bistable oscillations have been observed between two semilinear oscillators, two ring resonators, and also between a ring and a semilinear resonator. In the experimental arrangement used for bistability studies, a single-mode argon ion laser beam (4880 Å) entered the crystal in a face appropriate for self-pumped phase conjugation. However, the orientation is kept in such a way that no self-pumped phase conjugation is observable if none of the resonators is in operation. In this geometry, bistability and hysteresis were observed in the signal intensity of one resonator as the reflectivity of the variable beam reflector in the second resonator was increased and then decreased in a cycle. Bistability was also seen in the signal intensity of a single resonator as a function of the reflectivity of the external mirror.
This paper is the second in a series describing propagation of electromagnetic radiation in phase-conjugate oscillators. We apply the formulation developed in the first paper to study the phase-conjugate resonator (PCR) and the Fabry–Perot cavity with an intracavity phase-conjugate mirror (PCM). Our results show that nondegenerate oscillation occurs in the PCR for large parametric gains and fixed separation between the conventional reflector and the PCM. The bandwidth is considerably reduced from that of a PCM alone. In the phase-conjugate oscillator, oscillation will occur at the pump frequency only if there is no separation between the conventional mirrors and the phase-conjugate element and if the nonlinear medium fills the entire Fabry–Perot cavity.
When a passive phase conjugate mirror (PPCM) is conjugating an incident beam, a phase conjugate oscillation can be obtained by placing a mirror near to the PPCM. By placing an additional mirror adjacent to the first mirror, two oscillations can be observed. If we consider these two oscillations as two states of the system, the system exhibits interesting bistable behavior: the presence of one oscillation will prevent the other oscillation building up and vice versa. The system will remain in that state unless it is being disturbed, for example, blocking the oscillation by a piece of paper, then the system switches to the other state and remains in that state even after that piece of paper is removed. The switching between states can also be controlled by an external control beam. A theoretical model based on the threshold of oscillation is proposed. This device has potential application in bistable switches and associative memories.
We have developed a theory for nondegenerate oscillations in optical resonators containing an intracavity phase-conjugate element. The phase-conjugate element consists of a nonlinear transparent medium that is pumped externally by a pair of counterpropagating laser beams of the same frequency and intensity. Phase conjugation of an input beam of slightly different frequency occurs because of nondegenerate four-wave mixing. The theory takes into account linear absorption (or gain) in the medium and is applied to study the threshold behavior of phase-conjugate oscillators. For the special case of no conventional mirrors, the phase-conjugate oscillator reduces to an ordinary phase-conjugate mirror, and our general formulation yields the results of previous studies. Our analysis shows that the parametric gain required for oscillation increases (or decreases) as a result of linear absorption (or gain) in the medium, and oscillation can occur at a frequency different from that of the pump beams in the presence of large linear gain (or loss). The effects of linear absorption (or gain) on the filter operation are also examined.