Oscillating holograms recorded in photorefractive crystals by a frequency detuned feedback loop

Abstract

We report an optoelectronic feedback loop suitable for generating noise-free interference patterns oscillating at arbitrary waveforms. The technique allows controlling the frequency detuning between the interfering beams through a phase modulator in a closed-loop interferometer. We use the dither signal method and propose a quasisynchronous demodulation scheme to create a phase modulated error signal for driving the loop. The dynamics of the interference fringes is easily controlled by a voltage waveform from a function generator, which is used in association with a time delay circuit for shifting the frequency of the reference signal used for lock-in demodulation. The technique is specially suited for applications involving low-frequency phase oscillations, such as those frequently encountered in the generation of space-charge waves in highly resistive photorefractive materials. The processing scheme allows real time monitoring of the hologram strength, and absolute values for the diffraction efficiency and the holographic phase shift can be obtained. Photorefractive wave oscillations ranging from approximately 100 mHz to 10 Hz were produced in a nominally undoped Bi 12 TiO 20 sample. The technique can be readily applied to other fields of optical interferometry, such as for testing optical surfaces, optimizing adaptive holographic devices, measuring physical quantities, among other applications.

Sign of the dominant charge carriers in photorefractive crystals determined by a phase-locked holographic technique

“…Several feedback techniques for stabilizing holographic setups have been reported in the literature; a general overview can be found in chapter 6 of Ref. [25], and more recent developments in [26][27][28]. In principle, the main goal of a feedback system is to generate an error signal suitable for the particularities of the experiment (desired phase shift, frequency bandwidth, etc.).…”

Near-infrared holographic photorefractive recording under applied electric field in undoped Bi$_{12}$TiO$_{20}$ sillenite crystal

“…However, the action of the E 0 perturbs the crystal index ellipsoid due to the electro-optic effect, inducing a birefringence in the medium making the diffracted beams elliptically polarized; this phenomena produces additional difficulties to separate the diffracted beams from the signal and pump ones and consequently making difficult to determine the diffraction efficiency and also to improve the signal-to-noise ratio (SNR) by simply doing the use of a polarization analyzer [3,5,7]. In order to overcome this difficult some phase modulation techniques that are applied to one of the recording beams have been proposed [8,9,10,11]. In this work we take advantage of phase-sensitive detection and measure diffraction efficiency by direct chopping of elliptically polarized diffracted beams in photorefractive hologram recorded in undoped BTO crystal at 1064 nm under the action of a dc E 0 .…”

Self-diffraction of frequency-modulated light in the Bi12TiO20 crystal

“…The two wave mixing is observed as the redistribution of the power or the phase between two light waves propagating in a photorefrac tive crystal. The optimization of the self diffraction regime and the study of characteristics of the photore fractive response are often realized using the frequency shift or the phase modulation of light beams [3][4][5]. As a result, the self diffraction effect is observed on a hologram that runs at a constant velocity or oscillates.…”