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Photorefractive Effects, Materials, and Devices

Phase Conjugate Resonators and Bistable Oscillations in BaTiO3

Putcha Venkateswarlu, Mostafa Dokhanian, P. Chandra Sekhar, H. Jagannath, Merin George

Published Jan 1, 1990• 0 citations
Publisher

Abstract

Kwang and Yariv(1) used a single domain BaTiO3 crystal to form a ring passive phase conjugator (RPPC) and auxiliary resonators pumped by a multi-mode Ar+ laser at 4880Å. The auxiliary resonators were bistable. Jagannath et. Al(2) from this laboratory, reported the operation of a unidirectional ring resonator (UDRR) in addition to the RPPC and found bistability between the two. They also observed a linear passive phase conjugator (LPPC) which is bistable with the RPPC. It was found that auxiliary oxci nations between the crystal and a mirror are bistable separately with the UDRR and the LPPC.

DOI
10.1364/pmed.1990.ep1

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IEEE Journal of Quantum Electronics•Aug 1, 1986•61 citations
Kwang and Yariv(1) used a single domain BaTiO3 crystal to form a ring passive phase conjugator (RPPC) and auxiliary resonators pumped by a multi-mode Ar+ laser at 4880Å. The auxiliary resonators were bistable. Jagannath et. Al(2) from this laboratory, reported the operation of a unidirectional ring resonator (UDRR) in addition to the RPPC and found bistability between the two. They also observed a linear passive phase conjugator (LPPC) which is bistable with the RPPC. It was found that auxiliary oxci nations between the crystal and a mirror are bistable separately with the UDRR and the LPPC.
Bistable oscillations have been observed in BaTiO(3) pumped by a single-mode 30-mW (488-nm) argon-ion laser, which is used simultaneously as a ring passive phase conjugator (RPPC) and as a unidirectional ring resonator (UDRR). Strong oscillations in the UDRR increase the threshold coupling strength required for oscillation in the RPPC. The oscillations in the RPPC reduce the fanning in the crystal necessary for the startup of oscillation in the UDRR. Auxiliary resonators pumped by the RPPC are also bistable with the UDRR. Similar bistable oscillations have been observed when the crystal is used simultaneously as a RPPC and as a linear passive phase conjugator.
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.
Optical bistability and hysteresis in BaTiO3 have been observed in a self-pumped ring resonator (SPRR). The SPRR can be controlled with an additional He–Ne laser erasing beam that makes an angle with the ring pump beam. The erasing beam could be coherent or incoherent with the pump beam. By injecting the erasing beam into the crystal in such a way as to partially erase the index grating and/or take energy from the pump beam, the coupling strength is modified. In either case, the output of the ring shows bistability and hystersis in the complete cycle of the on-off-on operation by monitoring the intensity of the erasing beam. The intensity change and the time interval between each change step of the erasing beam have effects on the hysteresis behavior. The hysteresis observed when it is controlled by a coherent erasing beam is more than the one by an incoherent erasing beam. Some fluctuations exist in the hysteresis loop when the coherent erasing beam is used.
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.
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.