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Applied Optics

Fanning noise reduction in photorefractive amplifiers using incoherent erasures

Q. Byron He, Pochi Yeh

Published Jan 10, 1994• 46 citations
Publisher

Abstract

We propose and demonstrate a new scheme of beam fanning reduction in photorefractive amplifiers by using incoherent erasure. This simple and effective approach employs a pump beam with a mixed polarization state or introduces a third incoherent beam for selective erasure. The approach is analyzed, and the experimental results are presented and discussed.

DOI
10.1364/ao.33.000283

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We propose and demonstrate a new scheme of beam fanning reduction in photorefractive amplifiers by using incoherent erasure. This simple and effective approach employs a pump beam with a mixed polarization state or introduces a third incoherent beam for selective erasure. The approach is analyzed, and the experimental results are presented and discussed.
We propose and experimentally demonstrate two original methods for suppressing the beam fanning noise of photorefractive amplifiers. The first one involves two wave mixing in a slowly rotating crystal, while the second requires to tilt the crystal with respect to the recording beams. This method applies to materials which exhibit a sharp resonance of the gain versus the pump-probe beam angle. With both methods a large improvement of the S/N ratio is achieved and virtually noise free image amplifiers are demonstrated. In Bi12SiO20, subpicowatt optical signals are detected and amplified. By using combinations of LiNbO3/Bi12SiO20, as storage and amplification crystals respectively a long term readout (more than 20 hours) of a photorefractive memory is achieved.
We investigate the dynamics of multiple two-wave mixing in photorefractive materials. This is the first time to our knowledge that a study of the dynamics of many pairs of two-wave mixing interactions has taken place. A basic case with three waves undergoing multiple two-wave mixing is studied numerically. A numerical study is also performed in which many waves are used to model the time development of the fanning effect. Comparison of this numerical model with a basic experiment involving fanning in the presence of a two-wave mixing interaction is given. A qualitative agreement is exhibited. In addition, it is found that effectively reducing the coupling constant of a two-wave mixing interaction by the addition of an incoherent background beam can, under certain circumstances, significantly reduce the fanning without lowering the signal level.
The emergence of optical waves out of optical noise (fanning) in a photorefractive medium is considered in an unconventional manner, i.e., using the Lorentz lemma and ensemble averaging. The derivation of equations for four-wave interaction in the scheme of a double incoherent phase-conjugate mirror is described in detail. Numerical calculation based on the resulting system of equations provides insight into the effect of noise on the oscillation threshold and makes it possible to discover the “higher threshold” effect in saturation-free noisy materials.