Optimal Coherent Image Amplification By Two-Wave Coupling In Photorefractive BaTiO3

Fainman, Yeshaiahu; Klancnik, Edward G.; Lee, Sing H.

doi:10.1117/12.7973810

Cited by 168 publications

(24 citation statements)

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“…2(b)]. 4 The special crystal cut effectively gives a large dynamic range for the index variation An that we can write. The two writing beams were first turned on, and the readout-beam direction was adjusted for maximum diffraction efficiency.…”

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confidence: 99%

“…5 that the exposure schedule that must be followed in order to yield a set of holograms with equal amplitudes results in a strength parameter equal to qmax/M for each component. Under this constraint the maximum number of holograms is determined by the detectability of each component, so that 'kmax M= 0miny (4) which yields Mmax = R _ 103. This value is consistent with the discussions and experiments found in Refs.…”

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Diffraction efficiency of strong volume holograms

et al. 1990

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We investigate the diffraction efficiency of strong volume holograms in which the coupling parameter is several times that needed for maximum diffraction efficiency. We discuss the implications of our findings on photorefractive implementations of various neural network systems.The study of volume gratings has led to useful applications in many areas of optics, including integrated optics, acousto-optics, and holography. In most situations the coupled-mode analysis of volume holograms established by Kogelnik 1 accurately describes the diffraction behavior of a thick hologram and predicts a diffraction efficiency that is a periodic function of the index perturbation amplitude-thickness product.The diffraction efficiency X from a volume index grating with the direction of the readout beam tuned for the Bragg condition is given bywhere a is the absorption coefficient, d is the thickness of the hologram, An is the amplitude of the index perturbation, and 0 is the angle of incidence of the readout beam with respect to the normal to the surface of the hologram (we assume an unslanted grating); 0 and X are assumed to be measured inside the medium. Typically the amplitude of And realized in most holographic materials is such that only the increasing part of the first period of X is observed. As the grating amplitude and/or the thickness of the grating is increased beyond this regime, further coupling between the two waves results in a reversal of the energy-transfer direction to yield a drop in the diffraction efficiency as predicted by Eq. (1).We have been able to observe this effect in a photorefractive barium titanate (BaTiO 3 ) crystal. The use of photorefractive crystals for such a purpose is particularly appropriate since the dynamic nature of photorefraction allows us to record easily and accurately the temporal evolution of An, the index change. For example, by monitoring X during the holographic recording process, we are able to observe the functional dependence of q on And from And = 0 to saturation. Although X cannot exceed unity, a large saturation value of And is desirable because it corresponds to a large dynamic range for hologram recording, which in turn implies that a large number of holograms may be recorded. By monitoring the time dependence of 7, we are able to estimate the saturation value of And in our BaTiO 3 sample and calculate the storage capacity of the crystal for dense holographic interconnections.Such interconnections are useful in parallel information-processing applications such as artificial neural networks. ' 3In a photorefractive crystal the steady-state value of An is proportional to the modulation depth of the intensity interference pattern responsible for writing the hologram. Hence, by manipulating the intensities of the writing beams, one can monitor the functional relationship between An and the diffraction efficiency. For a fixed writing-beam modulation depth, the entire range of index perturbation amplitudes from zero to the maximum attainable value can be scanned by performing...

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Diffraction efficiency of strong volume holograms

et al. 1990

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“…Two-beam coupling via the photorefractive effect in media such as BaTiO 3 crystals has been studied by several researchers because of its wide-ranging applications in coherent image amplification [1][2][3][4], image processing [5,6] and optical logic operation [7], etc . This two-beam coupling forms the underlying process for both signal beam amplification in two-wave mixing and beam fanning [8][9][10][11] .…”

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Effect of Strong Pump Depletion on Two-beam Coupling in Photorefractive BaTiO₃at 442 nm

Joseph

Kamra

Singh

et al. 1992

Journal of Modern Optics

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Photorefractive BaTiO 3 crystal illuminated at 442 nm gives a large initial scattering level, which leads to strong pump depletion into beam fanning . Studies on the time development of beam fanning show a temporal shift of the fanned beam maximum towards the c-axis direction . The effect on the signal beam amplification of the strong pump depletion into beam fanning has been investigated, revealing different temporal development of the amplified signal for various cross-angles . Our observations on beam fanning in the presence and in the absence of a signal reveal that there is appreciable coupling between the fanned beam and the signal .

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“…It is important to note that when in TWM the pump-to-signal ratio is high (a ratio of 440 was used in [7]) the suppression of the fanning of the pump beam (only) is enough to guarantee low overall fanning. So, the above technique is perfectly suited to applications where the range of very high pump-to-signal ratios is of interest, for example coherent image ampli®cation [8]. There are, however, applications where a low pump-to-signal ratio is used.…”

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Optimum signal polarization in two-wave mixing with incoherent erasure of the fanning process

Danailov¹,

Laurito²,

Pecchia³

1998

Journal of Modern Optics

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We present a study of the optimum polarization states of the pump and signal beams in two-wave mixing ampli®cation using BaTiO3 where the main objective is to improve the performance by reduction of the beam fanning. We demonstrate numerically and experimentally that at low (less than 50) pump-to-signal ratios it is advantageous to use both pump and signal in mixed polarization states with respect to the principal plane of the crystal.Beam fanning is known to be a serious limiting factor in photorefractive twomove mixing (TWM) ampli®ers. In BaTiO 3 , by far the most popular photorefractive medium, the fanning causes the beam quality to deteriorate strongly and dramatically decreases the gain. The photorefractive beam fanning originates from the energy exchange between the incident beam and the portion of this beam scattered by the crystal defects (for example [1, chapter 4]). It involves the creation of a number of noise gratings corresponding to various angular components of the scattered light. Several techniques for reduction of the beam fanning in BaTiO 3 have been proposed and studied [2±7]. In general, they are based on erasure of the noise gratings or avoiding their e ective interaction with the pump beam. The technique of incoherent erasure using pump beam with a mixed-polarization state, proposed by He and Yeh [7], is very simple and seems to be the most universal. In this scheme, the pump beam has an ordinary polarized component, which works as incoherent erasure for the noise gratings and suppresses the pump beam fanning. It is important to note that when in TWM the pump-to-signal ratio is high (a ratio of 440 was used in [7]) the suppression of the fanning of the pump beam (only) is enough to guarantee low overall fanning. So, the above technique is perfectly suited to applications where the range of very high pump-to-signal ratios is of interest, for example coherent image ampli®cation [8]. There are, however, applications where a low pump-to-signal ratio is used. For example, when TWM is applied to a high-power laser beam clean-up, the pump-to-signal ratio is usually less than 50 [9,10]. Then, the contribution of the signal to the fanning wave also becomes signi®cant and a ects the performance of the ampli®er. Taking into account that for the TWM beam clean-up the power extraction e ciency is crucial for its applicability to practice, it appears very important to analyse the fanning suppression technique of He and Yeh [7] in the case of a strong signal and to consider how its performance can be improved in this regime.

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Optimal Coherent Image Amplification By Two-Wave Coupling In Photorefractive BaTiO3

Cited by 168 publications

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Diffraction efficiency of strong volume holograms

Diffraction efficiency of strong volume holograms

Effect of Strong Pump Depletion on Two-beam Coupling in Photorefractive BaTiO₃at 442 nm

Optimum signal polarization in two-wave mixing with incoherent erasure of the fanning process

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Optimal Coherent Image Amplification By Two-Wave Coupling In Photorefractive BaTiO3

Cited by 168 publications

References 0 publications

Diffraction efficiency of strong volume holograms

Diffraction efficiency of strong volume holograms

Effect of Strong Pump Depletion on Two-beam Coupling in Photorefractive BaTiO3at 442 nm

Optimum signal polarization in two-wave mixing with incoherent erasure of the fanning process

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Effect of Strong Pump Depletion on Two-beam Coupling in Photorefractive BaTiO₃at 442 nm