Dynamic Holograms Research Articles

Self-adjusting dynamic binary phase holograms

An optical system constructed around a dynamic diffractive optic element, a ferroelectric liquid-crystal spatial light modulator in binary phase-only modulation mode, is investigated. The spatial light modulator is successively adjusted according to the direct binary search technique to diffract an incoming laser light beam into a predecided intensity distribution by use of feed back from the diffracted light. It was found that the feedback signal was noisy and that vibrations and limited bistability in the spatial light modulator's pixels were the main noise sources. The final diffraction efficiency depends on the degree of noise in the feedback signal, but even under fairly noisy conditions the iterations were found to converge properly.

Influence of the purity of the starting materials on the recording of dynamic holograms in Bi12TiO20 crystals

The parameters of Bi12TiO20 crystals grown with bismuth oxide of different degrees of purity were measured by a holographic recording technique in an external ac electric field and were compared with a crystal specially doped with chromium. Various crystal parameters such as the diffusion length of the photoexcited carriers and the Debye screening length were determined. It was found that the absorption spectrum of the material and the holographic recording efficiency were strongly influenced by light chromium doping and by insufficient purity of the starting mixture.

Nonlinear formation of dynamic holograms in a dye solution

A scheme for the formation and reconstruction, at the doubled frequency, of volume dynamic holograms is considered in the case of the fifth-order nonlinearity. The results are given of an experimental investigation of the diffraction of laser radiation by a transmission dynamic grating formed in a solution of polymethine dye No.3274U. A theoretical model is proposed for describing quadratic hologram formation and suitable for calculation of the energy efficiency of conversion of the radiation frequency.

Correction of segmented mirror aberrations by phase conjugation and dynamic holography

A novel approach is proposed for using the phase conjugation phenomenon for dynamic correction of distortions in beam forming laser systems. The method is based on the well-known application of a special diffraction structure on the surface of corrected optical elements ensuring nonreciprocal beam propagation in a system including a phase conjugate mirror. The main feature of the proposed approach is that it uses the diffraction structure which is not static (as usually) but is written dynamically by a small auxiliary laser source irradiating the corrected element surface simultaneously with the working beam. This makes it possible to extend the capabilities of the known method with respect to scaling-up the size of compensated mirrors and allows replacement of a large solid mirror by a segmented one. The novel method of correction has been tested experimentally by the use of 1.06 μm radiation for recording the dynamic hologram and a 10.6 μm laser beam as the working beam, distorted by a two-segment mirror with a dynamic hologram on its surface and corrected by a phase conjugate mirror. The means of dynamic hologram recording in different spectral ranges are discussed.

Correction of segmented mirror aberrations by phase conjugation and dynamic holography

A novel approach is proposed for using the phase conjugation phenomenon for dynamic correction of distortions in beam forming laser systems. The method is based on the well-known application of a special diffraction structure on the surface of corrected optical elements ensuring nonreciprocal beam propagation in a system including a phase conjugate mirror. The main feature of the proposed approach is that it uses the diffraction structure which is not static (as usually) but is written dynamically by a small auxiliary laser source irradiating the corrected element surface simultaneously with the working beam. This makes it possible to extend the capabilities of the known method with respect to scaling-up the size of compensated mirrors and allows replacement of a large solid mirror by a segmented one. The novel method of correction has been tested experimentally by the use of 1.06 μm radiation for recording the dynamic hologram and a 10.6 μm laser beam as the working beam, distorted by a two-segment mirror with a dynamic hologram on its surface and corrected by a phase conjugate mirror. The means of dynamic hologram recording in different spectral ranges are discussed.

Dynamic holograms and phase conjugation in multilevel resonant media

The generation of dynamic holograms and four-wave phase conjugation in resonant media has been investigated under conditions of interaction between radiation and excited singlet and triplet states. Two mechanisms of optical control of resonant media non-linearities using independent pump-up to increase the diffraction efficiency of dynamic holograms have been considered. The peculiarities of non-linear recording of holograms, and a variant of quadratic recording, have been investigated. The dependence of the efficiency of diffraction by dynamic holograms on the intensity of interacting waves and the spectroscopic characteristics of the medium has been analysed.

Dynamic holography in alkali metal vapour

Problems of dynamic hologram recording on resonant absorption lines of alkali metal vapour are discussed. Light beams and media parameters are determined that are necessary for hologram recording with usable efficiency. The effects of thermal atomic motion and the radiative transport of excitation influence on the spatial frequency transfer function of two-level media are investigated. Results of hologram recording in vapour of Rb (780.0 nm) and Cs (852.1 nm) using low-power CW-lasers are presented. Transmission holograms in the volume media and reflection holograms near the boundary of the resonant vapour and the dielectric are compared. For transmission holograms the advantage of collinear read-out when using pure vapour in comparison with a counterpropagating read-out is shown, and this makes it possible to achieve higher values of hologram efficiency over a wide range of atomic density. It is shown that reflection hologram recording is characterized by higher resolution, as compared with hologram recordings in the volume of extended media. Problems of metal vapour hologram usage for laser parameter control are discussed. The feasibility of holograms in Cs-vapour usage as a spectral-selective real-time feedback element in a semiconductor laser external cavity is shown.

Nd:YAG amplifier with a semiconductor phase-conjugate mirror

We investigate an all-solid-state Nd:YAG multipass amplifier with a phase-conjugate mirror operating in the nanosecond pulsed regime. The phase conjugator employed a dynamic phase hologram written within a semiconductor (Si, CdTe:V, GaAs, GaAs:Cr) by free carrier generation. The amplification factor of such architecture is calculated to be as high as 100 with a four-pass amplifier. Net amplifications of 3.5 are obtained experimentally with a two-pass flash-lamp-pumped Nd:YAG amplifier with a semiconductor phase-conjugate mirror. The ability of this device to compensate for severe phase distortions is demonstrated.

Self-adaptive solid-state laser oscillator formed by dynamic gain-grating holograms

We describe the operation of a self-starting Nd:YAG laser oscillator incorporating a design in which laser oscillation occurs by means of diffraction from spontaneously generated three-dimensional gain gratings produced by spatial hole burning in the Nd:YAG amplifier. The transient onset and spectral selectivity of the gain gratings produce an output with energy of 600 mJ in a 10-ns single-longitudinal-mode pulse at 10 Hz. The self-adaptation of the gain gratings produces compensation of intracavity phase distortion. A transient numerical modeling of the nonlinear resonator gives good agreement with the experimental system and also provides insight into the temporal dynamics of the gain grating.

Photorefractive coupler for fault-tolerant coupling

We report the first experimental demonstration of a fault-tolerant laser-to-fiber coupler that is insensitive to both angular and lateral misalignments. Fault-tolerance is achieved by sending a laser beam into a photorefractive crystal that automatically senses and tracks the location of the fiber facet, and diffracts the input beam (via self-generated adaptive dynamic holograms) into the fiber. Using an argon laser (514.5 nm), a barium titanate crystal, and a multimode fiber (core diameter=400 μm, NA=0.16), we have achieved a coupling efficiency of 58% and a tolerance of more than 0.5 mm of lateral misalignment. Techniques to improve the coupling efficiency and the degree of fault tolerance, and to extend this method for the coupling of a laser diode output into a single-mode fiber are discussed.

Polarisation insensitive operation of ferroelectricliquidcrystal devices

The accepted mode of operation of ferroelectric liquid crystal devices is to place them between crossed polarisers. The authors demonstrate that such devices may be used as diffractive modulators without polarisers. A one-to-many polarisation independent switch is presented for singlemode optical fibre networks, based on dynamic liquid crystal holograms.

Positive and negative feedback effects of four-wave mixing in photorefractive crystals with two active centres

Based on a two-level model of optical transitions, the problem of positive and negative feedback effects during simultaneous reading and writing of dynamic holograms in photorefractive crystals has been solved analytically. The diffraction efficiency and the gain coefficient are shown to be nonlinear functions of the readout beam intensity and are nonreciprocal with respect to the readout from the two input ports. The readout beam intensity value that affects a previously recorded grating is shown to depend on microscopic parameters such as the cross-section of photoionization, the recombination coefficient and the concentration of photorefractive centres. An analysis of the amplitude of the overall shared grating has shown that with increasing readout beam intensity the holographic grating thickness decreases. In this case, under negative feedback the grating amplitude decreases, while with positive feedback it increases. The possibility is shown of achieving (under positive feedback) a maximum value of diffraction efficiency and complete energy transfer in a finite thickness of the photorefractive crystal, in contrast to the infinite thickness required in the standard two-beam coupling case.

New azo-dye-doped polymer systems as dynamic holographic recording media

Polymer materials show their impact on optical storage technology for developing high information density and fast access-type memories with a high read-out efficiency. New azo-dye-doped polymer materials have been developed and used for recording dynamic holograms. Dynamic holograms with reasonably high diffraction efficiency have been recorded at 468 nm, and the efficiency of the Write, Read and Erase (WRE) cycle in these azo-dye-doped polymer materials was determined. Significant observations in these materials are: (i) there is no need to change the polarization of the writing beam to erase the recorded holograms, (ii) the whole WRE cycle is very fast (3–6 s) and (iii) recording of more than 250 WRE cycles without any fatigue of the recording materials is possible.

Optical restoration of photorefractive holograms through self-enhanced diffraction

For the first time to our knowledge, we demonstrate the use of self-enhanced diffraction to optically restore weak dynamic volume holograms. This restoration process is achieved through the use of a novel beam-toggling scheme that significantly enhances the steady-state diffraction efficiency of volume index gratings stored in photorefractive media. Experiments demonstrating and confirming theory are presented and discussed.

Dynamic white light holography using an optically addressed multiple quantum well spatial light modulator

Using achromatic gratings techniques we have written dynamic holograms on an optically addressed spatial light modulator with a temporally and spatially incoherent white light source. The resolution and depth-of-field of these holograms are theoretically derived and experimentally verified. We demonstrate writing speeds over an order of magnitude faster than video framing rates.

Optically transparent digitally tunable wavelengthfilter

A new digital architecture combining a spatial light modulator written dynamic hologram and a fixed diffractive grating has been demonstrated to produce a tunable wavelength filter with a tuning resolution of 2.5 nm over a 160 nm range. Three modes of a broadband 1.45 mu m laser diode have been isolated using the filter and selectively coupled into singlemode communications fibre.

Dynamic holographic interconnects that use ferroelectric liquid-crystal spatial light modulators

<p>Dynamic interconnect holograms are designed by the use of a simulated annealing algorithm and written to a 128 × 128 pixel ferroelectric spatial light modulator that is used in a binary-phase mode. Dynamic holograms are used to implement a 2 × 2 crossbar with single-mode fiber inputs and outputs, which function with as high as 27 dB of isolation between output ports. The principle is extended to two-dimensional interconnection holograms, and arbitrary fan-out to as high as 64 points is demonstrated with good performance.</p><p>Images of interconnection holograms are transferred from the spatial light modulator to an optically addressed spatial light modulator that is used in a binary-phase mode. The addition of a fixed array generator computer-generated hologram permits replication of the hologram image, thus creating a larger hologram with a high space-bandwidth product on the optically addressed spatial light modulator.</p><p>Results of a preliminary experiment are presented.</p>

Mutated bacteriorhodopsins-versatile media in optical image processing

Mutated bacteriorhodopsins are considered as versatile media in optical image processing. The following topics are discussed: the biological function of bacteriorhodopsin (BR); the photocycle of BR; light-controlled absorption of BR films; BR films in transmission- and reflection-type dynamic holograms; holographic interferometry (image processing in the time domain); holographic pattern recognition (image processing in the space domain). >

Pulse shaping in volume reflection holograms

Ultrashort pulses are shaped by reflection from dynamic volume holograms recorded in azo-dye-doped epoxies. The spectral resolution and controllability of the reflection geometry are optimal for this application.

Optical competitive learning with VLSI/liquid-crystal winner-take-all modulators

A new approach to self-aligning unsupervised optical learning based on the competitive learning algorithm and adaptive holographic interconnectións is introduced. A volume hologram is used to diffract the light from an input spatial light modulator so that it focuses upon a custom winner-take-all very-large-scale-integrated circuit liquid-crystal spatial light modulator. The units that receive the most light switch to a reflective state, and the light reflected from the winning pixels interferes in the volume of the dynamic hologram with the phase conjugate of the input pattern in order to add an outer-product perturbation to the current holographic interconnection. As a set of input patterns is cycled through many times, the system learns to diffract the light from a particular input class upon a self-organized set of detectors that recognize similar input patterns without the aid of a teacher or any required alignment. Beam-propagation simulations are used to show that the holographic optical learning network faithfully reproduces the behavior of the ideal competitive learning algorithm. A winner-takeall detector/modulator device containing a total of 576 optical neurons grouped into 31 separate competitive patches in the required sparse-grid topology has been successfully fabricated, and experimental results are presented.