By using split-step beam propagation method, self-pumped contra-directional two-beam coupling is simulated in a photorefractive medium for arbitrary shaped beams. The simulation shows a positive influence of the photovoltaic effect on the two-beam coupling efficiency, in agreement with published experimental observations.

We report ultraviolet-induced transient and stable absorption and two-color holographic recording at 514, 633, and 830 nm in thulium doped stoichiometric LiNbO3 crystal. The gating light at 355 nm increased the diffraction efficiency and sensitivity. We found the optimum gating-recording intensity ratio for maximum diffraction efficiency and weak nonvolatile holographic storage with gating.

Large built-in internal field is present in congruent LiNbO₃ and is due to bulk dipolar defect complexes of nonstoichiometric crystals. This field influence electric field switching of domains as well as other optical and electrical properties of crystals. Moreover it's time-temperature dependent and this feature can bias the stability of LiNbO₃ based devices. We investigate how the internal field could affect the electro-optic effect. To this aim an improved interferometric technique is used to obtain a spatially resolved measurement of the electro-optic coefficient of z-cut LiNbO₃ crystals. The samples are mounted in one arm of a reflective grating interferometer and resulting fringes patterns are visualized and stored by a CCD camera. Then this as-recorded data are processed by digital holographic technique in order to obtain 2D phase maps as function of the applied voltage across the crystal thickness. Hence spatial distribution of r₁₃ is achieved for crystal either in virgin state or in domain reversed one. Moreover, samples just after poling with two anti-parallel ferroelectric domains structure have been studied. Results show that the virgin area and the reversed one have quite different r₁₃ coefficient values. We suppose that this difference is due to the internal field, however further measurements are currently under investigation to confirm this hypothesis.

Ferroelectric Lithium Niobate (LN) possesses a combination of unique electrooptic, piezoelectric, pyroelectric, and photorefractive properties. These features make it suitable for applications in optical devices-as modulators, switches, and filters in communication systems and holographic recording medium, etc. Here, the growth of lithium Niobate doped with iron and doubly doped with iron and manganese ions will be described. The growth technique will be through Automatic Diameter Control Czochralski Design. From these grown crystals, critical electrooptical coefficients using null detection polarimetry are provided. The results of growth, electrooptic measurements, and some physical properties are compared and presented. Also, the use of doped LN crystals in devices is discussed.

Photopolymers have been applied widely in optical devices for communications, displays, bio-sensing, and data storage. The advantages of this material are easy to synthesize, high refractive index change, high sensitivity and good optical quality. Recently, we have investigated on the syntheses and analysis of 9,10-PhenanthreneQuinone doped Poly(MethylMethAcrylate) (PQ:PMMA) photopolymer. This material can be fabricated with large dimensions and with thickness in the range of several-centimeters. Experiments show that the material shrinkage is negligible after optical exposure such that our doped photopolymer is attractive for volume holographic applications. However, comparing with other photopolymers, our PQ:PMMA material has a drawback of lower sensitivity (~10 mW/cm²). In this presentation, we demonstrate that by adding organo-metallic compound (Zinc MethylAcrylate, ZnMA) into the PQ:PMMA photopolymer, we succeeded in reducing response time and increasing holographic diffraction efficiency of the material. We report the fabrication and experimental investigation of dye-doped photopolymer, and discuss their application for data storage.

Lithium niobate crystals used in quantum electronics and integrated optics are mainly doped by different impurities in order to strengthen or suppress one or another property. Here we discuss the influence of hafnium impurity on photorefractive and photoelectric properties of lithium niobate crystals. The investigations of photorefractive effect in hafnium doped lithium niobate crystals have shown the possibility of suppression of photorefraction in these crystals (up to 3 orders). The experimental results demonstrate that the suppression of photorefraction occurs as a result of correlated increase of crystal dark and photo conductivities by 5 orders which is explained by significant increase of polaron mobility in the conduction band. One of the possible causes of the polaron mobility increase is the modification of the phonon spectrum of the crystal and a reduction of polaron scattering on defects associated with the lithium deficiency.

In this paper, a unique all-fiber tunable filter based on the combination of single resonant band long period grating (LPG) and harsh environment electro-optic polymer second cladding layer is presented. The single resonant band LPG is used to select the resonant wavelength and the tuning of resonant wavelength is realized by changing the refractive index of electro-optic polymer cladding layer via external electric field. Although the basic operational principle and implementation of this unique tunable filter have been previously reported by authors, this paper is focused on athermal operation design and synthesis of harsh environment electro-optic polymer, which enhances the practicability of proposed tunable filter.

In this paper, an oscillatory characteristic of diffraction is observed in a weakly oxidized LiNbO₃:Ce:Cu crystal during the holographic recording. Experiment indicates that, after the first diffraction maximum, there exists an optimal switching time from the recording step to the fixing step which can produce a high diffraction of fixed hologram. High diffraction efficiency of 70% is observed in LiNbO₃:Ce:Cu crystal. For theoretical simulation, the time-space dynamic evolution of the nonvolatile holographic recording in doubly doped LiNbO₃ crystals by jointly solving the two-center material equations and the coupled-wave equations is performed. The results show that the optimal switching will lead theoretically the fixed diffraction up to 100 % with a suitable switching time. This method provides us a new idea to obtain high persistent diffraction efficiency. The relationship between the optimal switching and the crystal thickness is studied.

We propose, analyze, and demonstrate the use of a holographic method for cohering the output of a fiber tapped-delay-line (FTDL). We perform a theoretical examination of the phase-cohering process and show experimental results for an RF spectrum analyzer based on a phase-cohered FTDL that shows 50 MHz resolution and bandwidths in excess of 2 GHz. Phase-cohering holography can operate on thousands of fibers in parallel, enabling both fiber tapped-delay-lines and the coherent fiber remoting of optically-modulated RF signals from antenna arrays.

A waveguid-wavelength selector was fabricated using ion implanted SiC substrate and Ga/Ge thin film using laser ablation techniques. The device was used as a CO₂ laser lines selector. The theory of the operation is based on visibility of the CO₂ laser to produce a thermal grating which drives the optical selector with maximum efficiency of 46 MHz of laser offset between 10P20 and 10P18 CO₂ laser lines. The threshold of the thermal damage of the device was overcomed using a miniature heat exchanger, which is triggered by the excess of the thermal heat during the operation. The different potential use of the device will be presented as well.

A tunable demultiplexer based on the holographic grating (HG) using photopolymer is described. The system is based on the 1st order Born approximation of volume diffraction, optical aberration of the lens system, and mode correlation of fiber coupling. The merits of the holographic grating are that it has relatively easy fabrication process, non-uniform grating period and multiplexing are possible, and anisotropic diffraction can be exploited. And also the strain tuning method on the polymer grating for wavelength tuning is proposed. In the setup, the polymer grating is attached on the two XYZ linear stages. If one stage is fixed and the other stage moves away, the polymer grating is extended. Hence all the transmission wavelengths move to longer wavelength with grating period variation caused by the strain induced on the polymer. The objective of this method is to change the transmission wavelengths of the demultiplexer linearly or arbitrarily.

Dielectric properties of Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ single crystals grown by a modified Bridgman method are investigated under strong, high frequency (>100 kHz) AC field. It is found that there is a phase transition due to the applied AC field, which may be due to the following reasons: (1) strong AC field quickly switches the polarization directions of domains that heats up the crystal due to the friction of domain change; (2) phase transition happens because of the increase of the temperature. Comparing with conventional heating techniques, AC field induced phase transition is a quicker more effective way. Experimental results confirm the increase of d₃₃ and the change of transmittance under strong AC field.

In this paper, the applications of holographic devices for optical communications are reviewed and discussed. In dense wavelength division multiplexing (DWDM) optical communication systems, holographic devices have some potential applications to satisfy the complex requirements of the systems. We explain recent accomplishment in this field using holographic gratings (HGs) recorded in a photorefractive crystal or a photopolymer. General properties of the HG as a filter are reviewed. We note that the photopolymer HG can be used as a wavelength demultiplexer in DWDM system. It uses spatial dispersion properties of diffraction grating, which provides wavelength-dependent diffraction angle change to separate the multiplexed wavelength channels. The HG device has inherent advantages compared to the other device technologies. As specific applications of the device, we propose chromatic dispersion management of the multiple channel wavelength demultiplexer, channel wavelength tuning of the device output, and double band demultiplexer using superposed HGs. Also, we discuss optical signal routing using computer-generated holograms (CGHs) on a phase spatial light modulator. We expect that these HG devices and CGH system can exploit phase control, tunability, multiplexing and dynamic imaging control of holograms to cope with various DWDM system requirements.

Organic-inorganic nanocomposite films were prepared by dispersing an aromatic methacrylic monomer and a photo-initiator in organic-inorganic hybrid sol-gel matrices. The film properties could be controlled by optimizing the content of an organically modified silica precursor (TSPEG) in the sol-gel matrices. The photopolymer film modified with the organic chain (TSPEG) showed high diffraction efficiency (>90%) under an optimized condition. High diffraction efficiency could be ascribed to the fast diffusion and efficient polymerization of monomers under interference light to generate refractive index modulation. The TSPEG modified photopolymer film could be successfully used for holographic memory. Angular selectivity of the film were 0.46 ~ 0.16 depending on the film thickness in the incident angles between 20° ~ 70°. A digital holographic image and a real object were recorded successfully in the photopolymer film.

Random-phase-multiplexing storage using photorefractive crystals is one of the most important topics in the field of photorefractive optics. To achieve random phase recording, we can use a diffuser to encrypt the reference light in a holographic recording setup. To decrypt the recorded pattern, the same diffuser used in encryption must be used in the reconstruction light, and it must be set in the original orientation. In this way, a number of 2-D patterns can be stored in a single photorefractive crystal with a single diffuser set at different orientations for different patterns. A merit in this recording method is that the encryption is virtually not possible to be decrypted if the original diffuser for encrypting is not available. In this paper, we proposed a way to decrypt the encrypted information in a photorefractive lithium niobate crystal without the possession of the original diffuser. In this method, we suppose somehow we know one of the patterns stored in the crystal, and then we retrieve the original diffuser with this pattern. And ultimately all the other patterns stored in the crystal are decrypted and retrieved with this retrieved diffuser.

A novel integration technique for segmented 3D profiles measured by projected fringe profilometry is proposed. This method is able to match images successfully even in the presence of geometric deformations, illumination changes, and severe occlusions. Tests of the system performance have been carried out that accuracy of the registration scheme is one part in one hundred of one pixel. This technique is superior to the other methods because of its higher accuracy, improved robustness, and reduced computational cost.

In this paper, a brief review of recent advances on fabricating in-fiber gratings in single crystal sapphire fiber is presented, which includes (1) backgound on fiber gratings, (2) in-fiber grating fabrication methods in single crystal sapphire fiber, (3) the variation of spectral response of sapphire fiber grating induced by the change of ambient refractive index, and (4) reflection-type fiber specklegram generated in single crystal sapphire fiber. The application of this unique fiber grating to harsh environment, high temperature, distributed fiber optic sensor will also be discussed.

We propose a model of a holographic grating (HG) filter for WDM applications and provide a method to control the chromatic dispersion of WDM demultiplexer as one of its specific applications. The general filter characteristics can be described as a correlation between the scattered mode from the HG and the out-coupling optic mode. We note that the HG phase distribution and lens aberration of out-coupling optics provide effective phase modulation for the chromatic dispersion of the filtered output. Also the amplitude distributions of the two modes perform a kind of weighting function (i.e., apodization), which controls the overall filter spectrum. This filter scheme can exploit wavelength dependent diffraction angle transition (i.e., spatial dispersion) and can be used for multi-channel demultiplexer in WDM system. In this case, lens aberrations of the out-coupling optics cause chromatic dispersion problems in each separated channel and among the channels. The problems can be managed by controlling the HG phase distribution so that the overall chromatic dispersion of the demultiplexer can be reduced and managed over the demultiplexed channels. We characterize the chromatic dispersion properties according to the third order lens aberrations and propose methods to control them by using HG phase distribution.

In this paper we report the design and implementation of a multimode fiber sensing technique for displacement sensing. To exploit the spatial information contents for sensing, a multimode fiber specklegram sensor with a hetero-core fiber structure is used. The sensor utilizes the inner product of multimode fiber speckle fields, which is highly sensitive on the geometrical shape change of the sensing section. The sensitivity and the dynamic range of the displacement sensing are investigated for hetero-core structure fiber specklegram sensor and straight multimode fiber specklegram sensor. It’s found that the sensitivity of the hetero-core FSS offers sensitivity as high as 0.1 μm, with a dynamic range of about 3 μm, which is superior to straight structure multimode fiber FSS. Dynamic response of the hetero-core FSS for displacement sensing was also studied.

Optical switches with wavelength selectivity are important and useful in dense wavelength division multiplexing (DWDM) systems, especially in network reconfiguration. One example of such devices is a switchable add/drop filter which is capable of switching between all-through state and wavelength adding (or dropping) state. The building block of such switches is a 2x2 wavelength switch. Most of the existing 2x2 switches, such as those involving two prisms operating in total internal reflection mode or total transmission mode, require light being coupled in and out of the fiber resulting in large device size and high insertion loss. In this paper, we design, analyze, and demonstrate a novel wavelength-selective 2x2 switch by recording electrically switchable holographic gratings in a layer of holographic polymer dispersed liquid crystal (H-PDLC) sandwiched between two side-polished fibers. We first demonstrate our novel idea experimentally, then analyze our result theoretically, and finally design optimized switches for both 100GHz and 50GHz DWDM systems. This device provides in-line operation capability and is particularly suitable for DWDM network reconfiguration.

We report on the birefringence of photonic crystal fibers as functions of fiber structure parameters and incident wavelength. It is found that the sign of the fiber birefringence can be changed by controlling the incident wavelength. The guided-mode coupling properties of the dual-core photonic crystal fibers are highly engineerable by tailoring the birefringence. New types of polarization splitters based on the photonic crystal fibers are presented.

Fabrication of various digital fringe patterns, such as digital sinusoidal gratings, two-dimensional fringe patterns, and two-frequency fringe patterns is presented. The advantage of using various digital patterns instead of other traditional patterns for projected fringe profilometry are (1) high geometrical accuracy (<0.5μm); (2) high contrast ratio; and (3) very low high order harmonic distortions.

Enhanced spectra of an object in high-frequency components by a photorefractive BaTiO₃ two-wave mixing are imaged on a phase-only filter of a reference object displayed by a liquid-crystal spatial phase modulator. Beam propagation inside a crystal includes to show an actual process in two-wave mixing. The experiments with the correlation performance are shown.

The influence of elastic anisotropy on effects of power spread in crystals from the langasite family is under study. Conditions for self-collimation directions for which power spread is much less than that for the isotropic variant, are formulated. Directions with the quality of collimation close to the maximum possible value are found in langasite and langatate (LGT) crystals. Presented are computed spatial distributions of acoustic wave power at different distances from the source. Those distributions illustrate the advantages of self-collimation directions. Computed are the angular distributions of collimation coefficients for several planes and the optimal shape for the top electrode is found for resonator structures that uses self-collimation directions in the crystals from the langasite family.

ZnO thin film is the perspective material for using as active layer in solid-state UV photodetectors. Here we present our investigations of photoelectric properties of the developed photosensitive field-effect transistor. Pure and lithium doped ZnO films were produced by vacuum electron-beam deposition method. Field effect was studied in Li doped ZnO films having high resistivity and in heterostructures consisting of three ZnO layers doped by 1, 5 and 10 at% of Li impurity accordingly. The photoelectric characteristics were measured (currents ratio, charge carriers mobility, ampere-watt sensitivity in UV diapason, NEP sensitivity, and photocurrent kinetics). The open and close current ratio was 106 and the field-effect mobility was ~10 cm²/Vsec. We have also studied the low-frequency noises (0.001÷100 kHz) of UV photodetector and suggested the methods of noise suppression. It was found that the dark current noises and photocurrent noises have different mechanisms.

We present calculations of the temporal evolution of beam fanning in doubly-doped photorefractive crystals that is initiated by scattering from noise based on jointly solving the two-center material equations and the coupled-wave equations. The optimal conditions for material prescriptions and oxidation-reduction processing et al are discussed in detail. The theoretical results can confirm and predict experimental results.

A new type of volume holographic grating lenses for converting a plane wave to a lateral-spread spherical wave and vice versa, has been designed. The lenses have the advantages of small volume, light weight, high wavelength selectivity and simplicity of fabrication. Grating pitch and grating vector of the grating lenses which derived from intensity distribution of wavefront interfering field of a plane wave and a spherical wave are discussed. And basis is provided to design and fabricate volume holographic grating lenses devices. On the basis of two beams coupled-wave theory, coupled wave differential equations and diffraction efficiency are derived. The practical applications of the lenses are demonstrated.

Multilayer storage in a shift-multiplexed holographic disk is proposed. An optical head comprising a fiber array is used for multiplayer storage in a LiNbO₃ holographic disk. High longitudinal selectivity of spherical reference waves is obtained and permits the implementation of multiplayer storage in the hologram. The technique yields the efficient usage of dynamic range and promotes the storage capacity potentially.

Two schemes for ultrabroad-band wavelength converters based on four-wave mixing in a semiconductor optical amplifier are proposed, both utilizing a conventional semiconductor optical amplifier (SOA) and two orthogonal-pump waves. In scheme I, the efficiency of wavelength conversion is measured for the wavelength shifts from 1500 nm to 1640 nm. The variation of conversion efficiency is < 0.9 dB over the wavelength range from 1530 nm to 1560 nm (C-band), and is < 4.5 dB over the wavelength range from 1560 nm to 1610 nm (L-band). The maximum conversion efficiency is about -8.7 dB. In scheme II, wavelength conversion with the polarization sensitivity less than 1.3 dB is obtained over a range from 1510 nm to 1620 nm.

Optical windows have been widely used in optical spectrographic processing system. In this paper, various window profiles, such as rectangular, triangular, Hamming, Hanning, and Blackman etc., have been investigated in detail, regarding their effect on the generated spectrograms, such as joint time-frequency resolution ΔtΔw, the sidelobe amplitude attenuation etc.. All of these windows can be synthesized in a photorefractive crystal by angular multiplexing holographic technique, which renders the system more adaptive. Experimental results are provided.

Hetero-core fiber structure consists of a sandwiched structure of fibers with different core diameters. Hetero-core fiber structure has found its application in both intensity based and phase change based sensors. In this paper, an analysis of hetero-core fiber structure in photonic crystal fibers (PCF) and waveguides is presented. With the finite-difference time-domain (FDTD) method, the performance of such a structure in a fiber specklegram sensor and the refractive index modulation on PCF section are investigated. It is found that the unique photonic nanostructures can substantially enhance the sensitivity of the specklegram sensor with added dynamic range tunability, which can lead to many practical applications in optical sensors.

Optical spectrographic processing systems have been shown to be the most widely used techniques for time-varying signals that usually contain very distinct characteristics in spectral distributions and are difficult to categorize in time (spatial) domain. In this paper, an optical architecture for spectrograph generation, by which time-varying signal processing can be performed, is proposed and its implementation is described in detail. Some potential applications using optical spectrographic processing system in phonetics, linguistics, speech identification are investigated. Simulation results are also presented to demonstrate the effectiveness of the optical spectrographic processing system in those application areas. Other applications of the optical spectrographic analysis system in identifying individuals through "voice prints" or in detecting mechanical fatigue through abnormal noise are also indicated.

With the increasing of the number of users in optical fiber communications, fiber-to-home project has a larger market value. Then the need of dynamic optical couplers, especially of N broad-band couplers, becomes greater. Though some advanced fiber fusion techniques have been developed, they still have many shortcomings. In this paper we propose a dynamic optical coupled system employing even-numbered Dammann gratings, which have the characteristic that the phase distribution in the first half-period accurately equals to that in the second-period with π phase inversion. In our experiment, we divide a conventional even-numbered Dammann grating into two identical gratings. The system can achieve the beam splitter and combiner as the switch between them according to the relative shift between two complementary gratings. When there is no shift between the gratings, the demonstrated 1×8 dynamic optical coupler achieves good uniformity of 0.06 and insertion loss of around 10.8 dB for each channel as a splitter. When the two gratings have an accurate shift of a half-period between them, our system has a low insertion loss of 0.46 dB as a combiner at a wavelength of 1550 nm.

Recently the experiments demonstrating the advantages of direct optical control of ferroelectric domain profiles have been carried out. Here we present our findings and detailed analysis on time dependence of the ferroelectric coercive field after domain inversion of the LiTaO₃ and LiNbO₃ crystals. We have investigated the mechanisms of light influence and possibilities of optical control of domain structures in ferroelectrics, in particularly the roles of the various internal field components, their origins and dynamic behavior following domain reversal. Our experiments have shown the possibility of direct optical control of domain patterning in ferroelectrics.

A new structure of polarization-selective elements consisting of two holographic gratings and a dove prism coupler is proposed. The absence of a multi-stage wave-guide, compact size, and lightweight volume are the outstanding features of the new structure. Based on the coupled-wave theory, the analysis and design of the structure are discussed in detail to calculate the required index modulation. Several parameters such as the recording intensity, the exposure time, and the recording angles for the fabrication of the proposed element are determined. Under the conditions the element is fabricated in Dupont photopolymer HRF-150-38 material and with the operating wavelength of 532nm. A simplified pick-up head is constructed to evaluate the performance of the fabricated element.

Optical transparency and high diffraction efficiency are two essential factors for high performance of the photopolymer. Optical transparency mainly depends on the miscibility between polymer binder and photopolymerized polymer, while diffraction efficiency depends on the refractive index modulation between polymer binder and photopolymerized polymer. For most of organic materials, the large refractive index difference between two polymers accompanies large structural difference that leads to the poor miscibility and thus poor optical quality via light scattering. Therefore, it is difficult to design a high-performance photopolymer satisfying both requirements. In this work, we prepared a new phase-stable photopolymer with large refractive index modulation and investigated the optical properties. Our photopolymer is based on modified poly (methyl methacrylate) as a polymer binder, acryl amide as a photopolymerizable monomer, triethanolamine as initiator, and yellow eosin as a photosensitizer at 532nm. Diffraction efficiency over 85% and optical transmittance over 90% were obtained for the photopolymer.

Thermal electron dynamics at the interfaces of thin gold film/ferroelectric lithium niobate (LiNbO₃) is studied by using step-function type laser irradiation. Our measurements reveal an anomalous transient optical transmission fluctuation caused by thermal electron dynamics at the interfaces. To our knowledge, such phenomenon and its theoretical explanation haven't been reported.

A ferroelectric single crystal fiber placed inside a microwave cavity is designed to perform pulse uptuning, downtuning, or reshaping by utilizing chirping effects on optical pulses traversing through it. For electrooptic modulator devices ferroelectric crystal fibers are of significant interest due to their high electrooptic coefficients and near-circular cross-sectional waveguide configuration. Single crystal fibers of strontium barium niobate grown by the laser heated pedestal growth method are shown to have high sensitivity to microwave electromagnetic field. Quantitative exploration for the influence of chirping on the performance of this modulator in a microwave cavity waveguide is carried out and reported.

External electric field has great effect on grating formation via additional carrier drift during nonvolatile holographic recording. The photorefractive properties of doubly doped LiNbO₃:Fe:Mn crystals are theoretically investigated by jointly solving the two-center material equations with nonzero external electric field and the coupled-wave equations. The external electric field dependence of the refractive-index changes n₁, the diffraction efficiency η as well as the photorefractive sensitivity S are studied for oxidized LiNbO₃:Fe:Mn crystals. Significant improvement of the photorefractive performance has been revealed by applying external electric fields to oxidized LiNbO₃:Fe:Mn. The enhanced material performance under external electric field improves the applicability of doubly doped LN crystals.

Near-stoichiometric LiNbO₃ crystals slightly doped with Mn from different Li concentrations in the melt were grown by the top seeded solution growth method. The light-induced (small polarons) absorption under UV illumination was probed using near-IR of 780 nm. The dark decay process shows a stretched-exponential behavior. The dependences of the light-induced absorption, the decay time constant, and the stretching factor on the pumping intensity were presented. The photovoltaic current intensity and the photoconductivity under UV illumination were measured by applying a DC field along the z-axis of the sample. The photoconductivity intensity showed superlinear dependence of intensity. The experimental results can be explained quantitatively by developing the two-center charge transport model with direct excitation and recombination between Mn and small polarons.

In this paper the erasure characteristics with different readout conditions, especially off-Bragg readout, during nonvolatile holographic storage in doubly doped LiNbO₃ crystals are investigated. The results show that off-Bragg readout by red beam can achieve modest diffraction efficiency while self-enhancement readout gets highest and self-depletion readout has lest diffraction efficiency. During erasing phase, the fast erasure case is that off-Bragg red beam and ultraviolet light are simultaneously used for illumination. It is indicated that the greatest effect on the erasure of the former recorded gratings during multiplexing holograms is the recording of the others gratings, and new recording schedule during multiplexing technique should be considered according to off-Bragg erasing. The experimental results are well verified by theoretical simulation basing on two-center material equations and two beam coupling equations.

Vectorial Kukhtarev equations modified for nonvolatile holographic recording in doubly doped crystals are analyzed, in which the bulk photovoltaic effect and external electrical field are both considered. On the basis of the small modulation approximation, both the analytic solution to space-charge field with time in recording phase and the steady-state solution in readout phase are deduced. Because bulk photovoltaic current is determined by polarized state of incident light, and refractive index change depends on not only the modulus of space-charge field but also its direction, the optimum design parameters for maximizing space-charge field are different from those for maximizing refractive index change. Therefore a trade-off exists between them. Based on the vectorial analyses of band transport model for nonvolatile holographic recording in doubly doped crystals, an optimal recording direction is given for maximizing refractive index change in doubly doped LiNbO₃ :Fe:Mn crystals.

Depolarized Interferometric Fiber Optic Gyroscopes (D-IFOGs) that are constructed with inexpensive single mode (SM) fiber have provided an opportunity for developers to meet Army emerging missions goals for affordable, small volume, reliable inertial guidance systems for use in small missiles, munitions, and future micro-unmanned autonomous vehicles. However, there remain several vital issues associated with substantially reducing the diameter of the sensor coil. Optical fiber that is precision-wound onto a micro coil experiences increased stress due to small radius bending, fiber distortions at crossover sites, and increased interlayer pressures as a result of multiple layers of fiber wound under tension. Tension and small radius bending stresses can have a detrimental effect on the performance of D-IFOGs. Therefore, other scenarios for the application of SM fiber to a micro-sensor coil must be considered. One scheme involves taking advantage of the bending-induced birefringence and employing the low cost SM fiber as a polarization-maintaining (PM) fiber. The mechanics of how a substantial reduction in the coil radius produces PM fiber properties in SM fiber is investigated under this research effort. Conventional and specialty SM fibers are characterized to identify optimal fibers for the development of micro-sensor coils. The results from extinction ratio measurements on the SM fibers and micro-sensor coils are presented in this paper. The significant cross coupling suggests that scattering centers are present in very small radius bending. Also, measurements show that optical loss is significant in micro IFOG coils.

This paper presents the design of a reading light system consisted of multiple R, G, and B white light LEDs with Fresnel lens located before the light source. First, we use a general Fresnel lens and a Genetic Algorithm (phaseΙ) to search the best arrangement of multiple LEDs, then use another Genetic Algorithm (phaseΠ) to the result obtained before to get a better Fresnel lens of width-varied grooves. Experimental results show that the special arrangement of LEDs and the special structure of Fresnel lens make the reading light system more efficient in illuminance than the reading light system of no Fresnel lens.