Adaptive interferometry based on dynamic reflective holograms in cubic photorefractive crystals

Contribution of the inverse flexoelectric effect to counterpropagating two-wave mixing of light beams in photorefractive crystals

“…1) in a photorefractive crystal causes a dynamic hologram to form in it, on which they are simultaneously selfdiffracted. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]19 This involves the interference of each of the transmitted beams with one of the diffracted beams-i.e., reference beam I P interferes with I S1 , and I S interferes with I P1 . The output intensity of the signal beam transmitted through the crystal is expressed as…”

“…[8][9][10][11][12][13][14][15] One possible application of volume holograms in photorefractive crystals is a laser interferometer. [1][2][3][4][5][6][7]11,[14][15][16][17] Laser interferometers convert phase modulation of light into intensity modulation, and this allows the mechanical vibrations of surfaces that reflect or scatter the light beams to be measured with high accuracy. The maximum sensitivity of a classical interferometer to the amplitude of the vibrations being measured is limited by the shot noise of the photodetector and is estimated in Ref.…”

“…The dynamic character of the holograms in photorefractive crystals, which have a finite response time when they are overwritten, makes it possible to carry out adaptive processing of nonsteady-state light-field patterns in laser interferometers, ensuring both effective phase demodulation and the compensation of the low-frequency modulation caused by the slow drift of the external conditions. Counterpropagating wave mixing, used in such crystals to form effective reflective holograms by a diffusion mechanism in the absence of applied external electric fields, [8][9][10][11][12][13][14][15][16][17] is attractive for adaptive laser interferometry.…”

“…[1][2][3] An important feature of reflective volume holograms recorded with wave mixing in the visible region is the small spatial period, which is about 100 nm. 1,3,[15][16][17] Such reflective holograms with high spatial frequency are characterized by electric-field gradients that can reach gigantic values, measured in TV∕m 2 . Because of the inverse flexoelectric effect, which causes the medium to exhibit elastic strains (elastic stresses) proportional to the gradient of the electric polarization (electric stress), [20][21][22] appreciable elastic strains can appear in the crystal that introduce an additional contribution to the photoelectric response of the medium because of the photoelectric effect.…”

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

“…In the former case, the light intensity changes detected by a photodetector (photodiode) have only a constant component, but, in the latter case, oscillations at the phase modulation frequency (sometimes, at the dou bled frequency) are observed. Because the sinusoidal phase modulation can be performed due to the reflec tion from an oscillating object, the schemes of adap tive interferometers were proposed [6,7] and also those allowing the measurement of the velocity [8].…”