Optical code division multiple access (O-CDMA) technique, which is considered as one of the most robust multiple-access techniques for future all-optical network has been studied in many groups. To make O-CDMA system more applicable for networks and switching, rapidly-tunable O-CDMA codes are required. The conventional coders have been reported including fiber Bragg gratings, arrayed-waveguide grating (AWG) devices and optical delay lines. And different types of CDMA coders are being proposed in many research groups. But among them, there are few coders which are freely tunable in real communication. In this paper, we propose an application of the polymer holographic grating as a tunable encoder or label swapper in O-CDMA systems. The polymer holographic grating can be an attractive approach to the spread-space, spread-spectrum CDMA systems or other combined types. The polymer holographic grating can be used for spreading and dispreading the energy of the information signal over a larger and smaller spatial domain in holographic or spread-space CDMA systems, which can be useful wireless optical communication systems. And also, it can be used in other types of CDMA systems. An application of the cyclic property of the proposed device will also be discussed.

The main function of a Fresnel lens is to provide light guidance. In this paper, we present an optimal design of Fresnel lens for a reading light system with multiple light sources of white light LEDs. The groove angles of Fresnel lens are chosen as design parameters and the design goal is to maximize the illuminace in a specified read surface while maintaining the distribution uniformity of the light rays. We develop a series of genetic algorithms which are implemented with SCHEME language and macros supported by TRACEPRO package to search for a set of optimal groove angles for Fresnel lens. Simulation results show that the optimally designed Fresnel lens indeed offers better light-guiding performance than typical Fresnel lens for a multiple-LED reading light system.

In this paper, we present a low grating lobe optical beam steering technique using unequally spaced phased array, in which the required spacings among phase elements are quantitatively analyzed so that the grating lobe can be minimized by the destructive interference from these unequally spaced phase elements. The large grating lobe is one of the major drawbacks of optical phased array technology, which limits the light efficiency and quality of the light beam. Thus, the low grating lobe technique presented in this paper could substantially improve the light efficiency and the quality of light beam, which may play an important role in a variety of applications such as fast speed ladar beam steering, large size high resolution display, and wide bandwidth free space optics communications.

We demonstrate anisotropic self-diffraction in cerium doped barium titanate crystal using one incident beam. The second writing beam is derived from the internal reflection, at the incident surface, of the generated contra-propagating self-phase conjugate inside the crystal. Experimental measurements of the permissible angle(s) of incidence fit the theoretical and experimental results using two incident beams investigated by Kukhtarev et al [1]. The result also shows that anisotropic higher orders can be observed even when the ratio between both writing beams (induced and incident) is less than 10%. In the second part of the paper, we investigate the gratings created during self-diffraction originating from two writing beams in cerium doped barium titanate, using a reading beam of different wavelength. We have previously used anisotropic self-diffraction using two incident beams for image correlation [2]. The results can be separated to two different regimes, Bragg-like regime and Raman-Nath like regime. In the Raman-Nath regime, the polarizations of the generated higher order(s) are orthogonal to the polarization of the reading beam. In the Bragg like regime, the polarizations are the same. Finally the reading of self-diffraction gratings created by one writing beam will be investigated using one beam at a different wavelength.

Lithium niobate is a material of great interest for both fundamental science and applications because of the richness of its physical properties. On periodical structured lithium niobate is based quasi-phase-matching technique that allows efficient conversion in nonlinear optical processes. A critical step is the ability to engineering ferroelectric domains on micrometer-scales necessary for the desired interaction. Many efforts have been made to achieve a good control of domain reversal process and to this aim become fundamental to study all effects that influence ferroelectric domains inversion. Among these lithium niobate internal field earns great importance because on it depends observed difference between electric fields required for switching ferroelectric polarization in opposite directions. Moreover it's time-temperature dependent and this feature can bias the stability of LiNbO₃ based devices. We perform high spatial resolution interferometric measurement of internal field in lithium niobate crystals. In this way we can analyse influence of micrometer size not homogeneous area on internal field values. The samples are mounted in one arm of a Mach-Zehnder interferometer in microscopic configuration: resulting fringes patterns are visualized and stored by a CCD camera, then recorded data are processed by digital holographic technique in order to obtain 2D phase map of the sample with desired spatial resolution.

We demonstrated experimentally the formation of the photorefractive filament array in PDMs by a femtosecond pulse laser. Using an optical microscopy and a fast CCD camera, we recorded the formation processes. The results showed that, by special design of the femtosecond laser parameters and the optical configurations, filaments can be placed in an orderly 1D or 2D arrangement array, and be reflected. Based on these observations, we have established a theoretical model to simulate the filaments.

The demand for sensors for detecting chemical and biological agents is greater than ever before, including medical, environmental, food safety, military, and security applications. At present, most detection or sensing techniques tend to be either non-molecule specific, bulky, expensive, relatively inaccurate, or unable to provide real time data. Clearly, alternative sensing technologies are urgently needed. In this paper, we present a novel sensor with a nanoparticle surface enhanced Raman scattering (SERS) substrate coated on D-shaped or end-polished fibers for chemical, biological, and environmental detection. The sensor will be highly sensitive, molecular specific, reliable, label-free, non-invasive, inexpensive, easy to produce commercially using existing technologies, compatible with existing lasers and detectors, and applicable to a large number of molecules of interest. This is made possible by the unique sensor architecture based on a combination of optical fiber technology and novel SERS substrates, where SERS provides the high sensitivity (10⁶-10¹⁵ enhancement factor), molecular specificity, and applicability to a wide range of compounds, while the novel fiber configurations provide the flexibility, compactness, reliability, low cost, and ease of production.

We have synthesized and grown crystal of very unusual dielectric material thallium gallium selenide (TlGaSe2) stoichiometric composition. The crystals of the sizes of several cm lengths were grown by Bridgman method. Samples of rectangular shape were cut and polished for the dielectric measurements. Crystals showed dielectric constant of several thousand at low frequency region.

In this paper, we present a new class of broadband spectral filters based on aperiodic multilayer structures that reduce the required number of layers. Furthermore, the filter performance is optimized by Genetic algorithm, which is particular useful for optimizing arbitrary shape broadband filter. Quantitative computer simulation results are provided, which are consistent with theory. We believe that thee unique broadband filters may have a variety of applications in th area of broadband spectroscopy and hyperspectral imaging.

Optical waveguiding using nano-strip embedded photonic crystal (NEPC) is proposed and analyzed with theoretical modeling and numerical analysis. The underlying physical origin of the wave propagation is elucidated to be a photonic version of orbital hybridization, i.e., multiple period s- and p-state cavity mode hybridization. Theoretical description of the phenomena is made by using ab initio tight binding approach in comparison with plane wave expansion method (PWE). This extends understanding of the wave propagation in the photonic crystal (PC) waveguide made by one-dimensional defect. As a practical application of the NEPC and its waveguiding characteristics, low group velocity and low dispersion propagation in the NEPC is explained in relation to other waveguides and PC slab waveguide structures. The NEPC case of low group velocity and low dispersion can provide very broad bandwidth for the wave guiding compared to the other waveguides. By exploiting the hybridization in the NEPC, a useful understanding and modeling for such unique wave propagation can be made, which is expected to be applicable for the development of a new design theory and optical structure for novel dispersion devices.

Acousto-optic methods are widely used not only in devices and systems for radio and optical signal processing, but also for studying physical properties of various materials. The paper presents the results of the investigation of the "non-resonance" (due to native piezoelectric effect) generation of elastic waves at super high frequencies in the Bi₁₂GeO₂₀ crystal, using the acousto-optic method. Acoustic field patterns are visualized in various crystal cuts, including the "degenerated" acoustic mode in the [111] direction in the crystal. Conditions are discussed under which acoustic modes and directions exist with the "self-collimation" effect (abnormally low power spread of acoustic wave). Quantitative estimation of collimation quality is made for various conditions. Experimental characteristics of high-order harmonics generation processes appeared due to elastic non-linearity of the material are found. Basing on the experimental data, some parameters of the elastic non-linearity are evaluated.

Dopants in doubly doped lithium niobate crystals are crucial for properties of nonvolatile holographic recording. In our experiments, a series of possible doubly doped congruent LiNbO3:Fe:X (X=Mn, Cu, Rh, Ru, Ni) are proposed and investigated for nonvolatile holographic recording. The experimental results demonstrate that the dopants with different distances of energy band to conduction band have different recording efficiency. Further analysis approve that the dopants which have moderate energy-band distance to conduction band can perform recording with both high diffraction efficiency and long lifetime. The compared results show that LiNbO3:Fe:Ni is promising for nonvolatile holographic recording, and its recording conditions are optimized.

A novel integration approach using a supercontinuum light illumination generated by launching femto second laser pulses into a highly nonlinear photonic crystal fiber is presented. The main advantages of using the presented technique for accurate 3D data fusion are: (1) a large depth of field; (2) very low aberration (even for a large field of view); and (3) no coherent noise (a major problem for the laser system). Matched points can be located in the fused partial topological maps at an accuracy of one hundredth of the pixel size.

In this research Electronic Fiber Speckle Pattern Interferometry (EFSPI) is proposed to investigate the area of directional sensing. Modal phasing caused speckle field phase variation was discussed and the sensor architecture based on electronic fiber speckle pattern interferometry (EFSPI) is proposed. The concept of vector sensing with EFSPI was analyzed and experimentally verified with fiber speckle interferogram results, signaling a great potential for directional sensing.

A high-quality single-layer panchromatic dichromated gelatin material is achieved successfully by employing new types of multi-color photosensitizers and photochemical promoters to conventional photo-crosslinking gelatin system. Its holographic recording characteristics such as spectral response, the photosensitivity of three primary colors, spectral selectivity of volume reflection hologram, angular and wavelength selectivity of volume transmission hologram, are studied in detail. Using red, green and blue lasers, namely three primary colors, the bright volume transmission and reflection holograms can be recorded on the panchromatic material at the exposure level of 30 mJ/cm². Some preliminary results of space, angle and wavelength multiplexing holographic storage for storing multiple binary and grey-tone optical images, are also reported in this paper.

A technique using diffractive elements for finding the absolute shape of a large-scale object is proposed. It is found that an accurate projected fringe profilometer can be built by applying the holographic technique in the system. The advantages of using the presented technique for projected fringe profilometry are: (1) a large depth of field; (2) very fringe distortion (even for a large field of view); and (3) a very compact design for the measurement system.

In this paper we propose a novel method to perform multiplexing. In our setup, we use only one beam of light to illuminate and pass through the object pattern and to image it into a photorefractive lithium niobate crystal. Image information is recorded in the crystal in the form of gratings due to fanning effect. We can read the image information using another collimated or white-light reading beam. To record multi-images in the same crystal, the object beam of each recording should be tilted to the last object beam for avoiding crosstalk. As a result, we can record multi-images in a lithium niobate crystal, and read them separately without crosstalk.

We here present a new method for synthesizing the fiber Bragg grating. This method, which we name as reborn Born method, is originated from the first order Born approximation. It provides more accurate results for synthesizing strong fiber Bragg grating. It is more accurate than layer-peeling algorithm with synthesizing uniform grating. The comparison shows that the reborn Born method enables to synthesize the gratings that layer-peeling algorithm fails to calculate.

Composite Over-Wrap Vessels are widely used in the aerospace community. They made of thin-walled bottles that are over wrapped with high strength fibers embedded in a matrix material. There is a strong drive to reduce the weight of space borne vehicles and thus pushes designers to adopt COPVs that are over wrapped with graphite fibers embedded in its epoxy matrix. Unfortunately, this same fiber/matrix configuration is more susceptible to impact damage than others and to make matters worse; there is a regime where impacts that damage the over wrap leave no visible scar on the COPV surface. In this paper FBG sensors are presented as a means of monitoring and detecting these types of damage. The FBG sensors are surface mounted to the COPVs and optically interrogated to explore the structural properties of these composite pressure vessels. These gratings optically inscribed into the core of a single mode fiber are used as a tool to monitor the stress strain relation in the composite matrix. The response of these fiber-optic sensors is investigated by pressurizing the cylinder up to its burst pressure of around 4500 psi. A Fiber Optic Demodulation System built by Blue Road Research, is used for interrogation of the Bragg gratings.

A unique all-fiber tunable filter is based on the combination of a single resonant band long period grating (LPG) and an electro-optic polymer second cladding layer. The single resonant band LPG is fabricated by etching the cladding of a standard 125 μm thick fiber and using either ultraviolet (UV) illumination or electric arc discharge to write the grating. Once a single resonant band has been achieved, a polymer second cladding layer is applied to the LPG. The refractive index of the polymer cladding determines the resonant wavelength of the filter and is tuned by applying an external electric field. The grating fabrication method and type of polymer used for the second cladding affect filter performance, and both must be considered when designing an application specific all-fiber filter.

We propose a new mechanism for fiber optic high temperature sensing based on whispering gallery mode resonance in bent optical fibers. Due to the strong whispering gallery mode resonance in cladding-thinned bent optical fibers, long-period grating-like deep spectral peaks are formed in the wavelength domain. The wavelength shift of the resonance peaks as a function of temperature is investigated. It is demonstrated that the single loop of bend ring in the fibers can be used as a transducer for high temperature sensing without any internal structure.

High precision control is highly desirable when using the selective chemical etching technique to fabricate tapered fibers for many practical applications. So far, various methods have been proposed on this topic. In this paper, we proposed a novel and effective method to make tapered fibers in different shapes and sizes based on automatic control of the immersion depth in chemical etching. We adopted the diluted Hydrofluoric acid as etching solution in our preliminary experiment, and common selective chemical etching scheme was also implemented in our experiment in which the buffered hydrofluoric acid solution is used. We found out in our study that the etching process can be further controlled by controlling the evaporation of the etching solution. Under near-saturation condition, the ammonium fluoride (NH₄F) in the etching solution tends to crystallize as the water evaporates. The evaporation of the water and the crystallization of the ammonium fluoride cause the immersion depth of the etched fiber to decrease in certain rate, which leads to different etching time on different parts of the etched fiber. This fact enables the etched fiber to have a very smooth tapered part. By controlling the changing rate of immersion depth and other etching conditions, we can finely control the shape and size of etched fibers.

In electro-optic (EO) modulator devices ferroelectric crystals of strontium barium niobate (SBN) are attractive due to exceptional high EO coefficients and low half wave voltage. SBN single crystals grown by laser heated pedestal growth are investigated to explore frequency dependent EO property at low frequency and near resonant frequency range. The mechanism of its frequency dependence is discussed briefly.

A new method of phase-coded multiplexing is proposed and tested. The construction of this multiplexing scheme combines a rotating cylindrical-collimating lens system (RCCLS) with a random phase transparent mask. It is verified that such a system is capable of storing over 1000 images in a doped LiNbO3 crystal. Experimental results and theoretical analyses presented in this paper demonstrate that a compact, all optical, secure and high capacity volume holographic memory system can be implemented with further exploitation of the method.

In this paper, we have developed the synthesized method for volume Hologram grating by layer-peeling algorithm. Layer-peeling algorithm is known as an efficient tool for synthesizing fiber Bragg grating which is one-dimension grating without absorption. By taking account for attenuation and angles of incident wave, we have modified the layer-peeling algorithm that can synthesize volume Hologram grating in lithium niobate crystal.

We here present an efficient analysis method for fiber Bragg grating, called inverse layer-peeling algorithm, which is originated from the layer-peeling algorithm. The layer-peeling algorithm is known as an efficient method for synthesizing fiber Bragg grating. The profile of grating can be reconstructed accurately from its strongly complex reflection spectrum with this algorithm. We found that this method also has the ability for analyzing fiber Bragg grating by inversing its synthesis procedures. With comparing the current analysis method, the inverse layer-peeling algorithm provides accuracy results and is faster by two orders of magnitude than the transfer matrix method.

For optical iris recognition, the eigen-images correlation recognition method is modified. The 2-D separable approximations of wavelet packet bases are constructed with the help of the cascade algorithm. Expanding the scale of basis selection, mutli-mother multi-vanishing moment joint best bases are chosen from the basis set of 25 mother wavelets including the mothers constructed by the lifting scheme. Using the corresponding eigen-images generated and the post-processing method based on statistic features, optical experiment is implemented. The experimental result agrees with the simulation result.

The 90-degree recording geometry in photorefractive often suffers low diffraction efficiency and small sensitivity. For the low diffraction efficiency, previous researches considered it mainly come from larger K-vector. But in experiments the diffraction efficiency in 90-degree geometry is so low that the large K-vector can't explain the origin of low diffraction efficiency. We think the main reason of low diffraction efficiency in 90-degree geometry comes from the coupling between the incident light and diffraction light. The volume grating formatted in 90-degree geometry must be volume grating with finite size and follow the diffraction characteristic of crossed-beam volume gratings. When the refractive index change Δn>1×10^-4, the diffractive efficiency in 90-degree recording geometry would be comparable with that in transmission geometry. However narrow grating space in 90-degree recording geometry makes it is more sensitive to ambient disturbs and leads to low effective modulation depth. That is why we can't obtain high diffraction efficiency for 90-degree geometry in experiments. For obtaining high diffraction efficiency in 90-degree geometry experimentally an active stabilization system is necessary.

For an ultrashort pulsed beam (UPB) with different polarization states diffracted by a volume holographic grating (VHG), the effects of the bandwidth to its diffraction properties are studied. By developing the coupled wave theory of Kogelnik, the formulas of the spectral and temporal distributions of the transmission and diffraction pulse beams are analytically given, with considering the effect of the dispersion of the grating media further. Then the bandwidths of the VHG for the UPB with different polarization states are investigated. We show the changes of the spectral and temporal distributions of the transmission and diffraction pulsed beams with different ratios of the bandwidth of the UPB to the bandwidth of the VHG. Finally, the varieties of the diffraction efficiency of the VHG for the UPB with different polarization states are studied.

Holographic recording experiments of doubly-doped LiNbO₃:Fe:Ni crystals were conducted by three kinds of different two-color recording schemes. The results show that the saturation diffraction efficiency, the fixing diffraction efficiency, and the recording sensitivity of oxidized LiNbO₃:Fe:Ni crystal are higher than those of other reported doubly-doped LiNbO₃ crystals. Based on the doped energy-band diagram, the effect of microcosmic optical parameter of the deep trap center on holographic recording properties of doubly-doped LiNbO₃ is analyzed theoretically. LiNbO₃:Fe:Ni has the potential of being a new highly efficient nonvolatile holographic recording material.

In this paper two-beam interference experiments are performed to investigate the diffraction characteristics of volume gratings in Fe:LiNbO₃ crystals (0.15wt% Fe₂O₃), the results show that the dependence of the diffraction characteristics on the oxidization-reduction state and the oscillatory diffraction can be found in the reduced crystals. The theoretical analysis shows that the strongest space charge field can be formed in the reduced crystals, which induce the strong refractive index change, and because of the sin² relation between refractive index change and diffraction efficiency, the oscillatory diffraction can be found in the reduced crystals. In application for high diffraction efficiency the oxidized treatment should be performed for Fe:LiNbO₃ crystals so that the depth integral of saturated refractive index change is equal to π/2.

Significant improvements of photorefractive properties such as diffraction efficiency with 90-degree holographic recording geometry have been obtained by the application of external electric fields. Experimental results show that the highest diffraction efficiency can be obtained at least 3 times than without external electric field in Fe-doped LiNbO₃ crystals. The vectorial Kukhtarev equations are modified in single center model taking into account the directions of optical axis and external electric field. The photorefractive properties of single doped LiNbO₃:Fe crystal using 90-degree storage geometry are theoretically investigated by jointly solving the vectorial material equations and the two dimensional coupled wave equations. The variation trends of the theoretical results are similar to the experimental results.

In this paper, effect of polarization of light on photorefractive grating formulation with any light modulation in birefringent photorefractive material is studied in detail. The diffraction efficiency with various modulations and different polarization mode is calculated. Theoretical analytic results are presented. It is shown that both the ratio of the initial intensity and the incident angle of the writing waves are selective when the polarization of mode of the readout wave and that of diffracted wave are different. But these two recording conditions are not selective for the isotropic diffraction. On the other hand, it is also found that the length of the grating is selective for anisotropic diffraction.

Volume hologram formation in photorefractive materials is a dynamic process which is generally analyzed by using material equations by Kukhtarev et al. and coupled-wave equations. Usual numerical solutions to material equations are based on layered photorefractive materials always using the Runge-Kutta method. We present a new way based on finite element method for photorefractive effect simulation with any light modulation. In this paper material equations are partially decoupled to two equations between the electron density and the electrostatic field (and hence modulate the refractive index via the electro-optic effect). Then these two equations are numerically solved by finite element method for a sinusoidal intensity pattern with an arbitrary modulation depth from the interference of two mutually coherent beams. The numerical solutions allow us to examine the validity of analytical theories for photorefractive effect. We can obtain time-space distribution of photorefractive grating, and the space-charge field buildup. We present results of a number of parameters. These solutions help to understand the dynamics of volume hologram formation in photorefractive materials.

A Bragg grating focusing device composed of electro-optically induced refractive index grating in periodically poled lithium niobate was designed for integrated optics systems. The device are induced and controlled by the application of an external electric field. The theoretical analysis of the proposed optical device is carried out using two-dimensional coupled wave theory. The focusing properties and the diffraction intensity are investigated at the focal plane. It is shown that the designed device has good performance of focusing when a plane wave incidents on it at a certain angle. This type of Bragg diffraction devices can be act as a focusing lens and will have many potential applications such as in integrated optics and opto-electronics.

Based on jointly solving the two-center material equations with nonzero external electric field and the coupled-wave equations, we have numerically calculated the variations of the depth of refractive-index change and the spatial phase shift (between the grating and the light interference pattern) in the steady state versus various external electric fields. Different effects are found in the recording and the fixing phases of the nonvolatile holographic recording, and consequently, external electric fields applied in the positive direction along c axis (or large one in the negative direction of c axis) in the recording phase and that applied in the negative direction of c axis in the fixing phase are proved to benefit strong photorefractive performance. Experimental verifications are given with small external electric field.

Diffraction gratings, which do not rely on the electro-optic effect, are recorded in copper-doped lithium niobate crystals: At elevated temperatures (approx. 180 °C), the crystals are illuminated by an interference pattern. The light pattern excites electrons from filled Cu⁺ traps into the conduction band. They are captured by empty Cu²⁺ traps elsewhere. The resulting electric fields are compensated by ions, such as H⁺, that are thermally mobilized. Therefore the redistribution of electrons can continue which finally results in a high modulation of Cu⁺ and Cu2+ traps. This process is called "thermal fixing" and is known since decades. However, here we report about a novel and relevant effect resulting from the electron concentration grating. The Cu⁺ traps absorb mainly in the near UV and visible spectral range, Cu²⁺ traps have their absorption peak in the near infrared. A modulation of Cu⁺ and Cu²⁺ trap concentrations therefore results in a pronounced absorption modulation. This immediately implies an absorption grating, but another consequence is even more dramatic: The Kramers-Kronig relations link changes of the absorption to changes of the refractive index. Thus the narrow absorption bands of copper in lithium niobate lead to refractive index changes over a vast wavelength range. Maximum index changes of the order of 10⁻⁴ are achieved. The resulting gratings can be recorded in any crystal orientation and are of interest for applications in non-linear optics.

An efficient and secure algorithm for random phase mask generation used in optical data encryption and transmission system is proposed, based on Diffie-Hellman public key distribution. Thus-generated random mask has higher security due to the fact that it is never exposed to the vulnerable transmitting channels. The effectiveness to retrieve the original image and its robustness against blind manipulation have been demonstrated by our numerical results. In addition, this algorithm can be easily extended to multicast networking system and refresh of this shared random key is also very simple to implement.

In this paper we analyzed and calculated the deformation based on noon symmetrical temperature distributions in the cross section of optical fiber. Deformation distortion causes the micro vibrations of the optical fibers under periodical thermal excitation applied to one side of cylindrical surface. Calculations were made to optimize the exposure and to minimize energy, needed for realization of this class of sensors based on vibrations. The same effect can play negative role for the fiber-optic communication's systems. The main feature in presented work from the similar theoretical considerations is the analysis of the non-symmetrical heating azimuthally with appropriate deformations.