Photorefractive Fiber Research Articles

Growth Modeling of Fiber Gratings: A Numerical Investigation

We have developed a simple method to analyze the growth process of fiber gratings (FGs) in the core of photorefractive optical fibers. It is based on the exponential saturation model and it takes into account the type of photosensitive fiber used, the exposure times, and the incident optical power. The photorefractive fiber has been characterized by two parameters: the saturation photoinduced refractive index change and the saturation energy, which can be obtained experimentally. We carry out a numerical investigation for the growth process of a uniform FG. The experimentally observed phenomena of shifting the Bragg optical frequency with exposure times and the appearance of higher-order Bragg resonances are predicted and discussed.

Photorefractive fibers for adaptive correlation filtering of a speckle-pattern displacement

We demonstrate recording of the gradient-type correlation filter for a speckle pattern in Bi 12SiO 20 (BSO) single-crystal fibers grown by the laser-heated pedestal-growth (LHPG) method. The filter linearly transforms small lateral displacements of the speckle pattern into the modulation of intensity transmitted through the photorefractive fiber. Despite the fibers possess high concentration of inhomogeneities, the intensity modulation depth reaches 25% that is comparable with values obtained with a high-optical-quality samples cut from a bulk crystal.

APPLICATION OF PHOTOREFRACTIVE FIBER AND WAVEGUIDE GRATINGS TO FAST SPEED NARROW BAND TUNABLE FILTERS

In this paper, the progress of the application of photorefractive fiber and waveguide crystal gratings to a narrow band, fast speed, tunable filter is presented. This novel filter is based on tunable Bragg grating. It uses the diffraction effect of Bragg grating to select the wavelength and the electro-optic effect of the grating to tune the effective grating period. Several approaches of fabricating this type of tunable Bragg grating based filter are discussed and some preliminary results are provided. It is shown that the tuning speed of this type of filter can be in the ns range and the bandwidth can be as narrow as 0.01 nm.

Optical Storage and Retrieval: Memory, Neural Networks, and Fractals

Optical disk storage holographic memory and some applications storage dynamics of photorefractive fibre holograms three-dimensional optical storage optical neural network architectures applications of photorefractive crystals for optical neural networks tools and neural architectures for direct image processing optical associative memory and adaptive learning.

Role of the filter spectral profile in the control of soliton transmissions

We analyse the propagation of optical solitons in a transmission system with lumped Butterworth or photorefractive fiber grating filters. The simulations show that fixed-frequency filters with a properly shaped frequency profile effectively suppress both Gordon-Haus time jitter and the soliton interactions, in analogy with sliding-frequency Fabry-Perot etalons.

Use of e-beam written, reactive ion etched, phase masks for the generation of novel photorefractive fibre gratings

E-beam field writing strategies have been developed to reduce field stitching errors to nigligible levels. Applying these strategies, 40mm × 4mm grating structures have been written onto phase masks which have in turn been used to print gratings into photo-sensitive fibres. Transmission measurements from constant pitch gratings with uniform phase and single phase shifts as well as chirped gratings are reported.

Nonlinear self-channeling of a laser beam at the surface of a photorefractive fiber

We experimentally observed the self-channeling of the light beam in photorefractive Bi12TiO20 fiberlike sample owing to the strong fanning effect. It results in significant speed up of the photorefractive response time in waveguides and in anomalous dependence of the response time on the applied ac electric field amplitude: the higher electric field, the faster response.

Moving fibre/phase mask-scanning beam techniquefor enhancedflexibility in producing fibre gratings with uniform phase mask

A new and flexible technique for producing photorefractive fibre gratings with a uniform phase mask is demonstrated. By slowly moving the fibre relative to the mask as the writing beam is scanned, wavelength shifts, pure apodisation and phase-shifted gratings can be achieved.

High accuracy dispersion measurements of chirpedfibre gratings

A wavelength scanning interferometric technique has been used to provide phase dispersion and time delay measurements of photorefractive fibre gratings with sub-picosecond time delay and 3 pm wavelength resolutions for the first time. Chirped fibre grating filters for dispersion compensation in long fibre telecommunications links have been fully characterised.

Experimental investigation of picosecond pulse reflection from fiber gratings

The dispersion of picosecond pulses on reflection from efficient photorefractive fiber gratings is explored experimentally. Unlike simple measurements of reflectivity, this approach allows both the amplitude and the phase of the grating response to be probed as a function of frequency.

Holographic memory using long photorefractive fiber array

Space division combined with angular or wavelength multiplexing volume holography using a two-dimensional array of long photorefractive fibers is investigated to realize RAM for images and a buffer memory for temporal signals. Focusing on the fiber length, characteristics of image holography and spectral volume holography have been thoroughly clarified. The optical implementation is proposed, and the preliminary experimental results show the feasibility.

Phase conjugate mirrors on the base of Bi 12TiO 20 photorefractive fibre

The distribution of two-wave mixing coefficient as a function of pump beam incidence has been measured for the Bi 12TiO 20 photorefractive fibre. Double phase-conjugate mirror with its conversion efficiency higher than 5% has been recorded in a wide range of the pumps incidence angles at λ=632.8 nm for a 2×3 mW input light power. Self-pumped phase-conjugate mirror with efficiency of 3% was obtained by using internal reflections of fanning beam inside the fibre.

Application of a fiber-speckle hologram to fiber sensing

A sensitive fiber-speckle field sensor using a Ce:Fe-doped LiNbO(3) photorefractive fiber hologram is introduced. We have shown that the sensitivity of this photorefractive fiber specklegram sensor can be of the order of 0.05 µm. The proposed system would offer the widespread use of practical fiber-sensing applications.

Coupling of mutually incoherent beams in photorefractive Bi 12TiO 20 fiber

A novel geometry of double phase conjugate mirror in photorefractive single crystal Bi 12TiO 20 fiber is proposed, when pump beams are repeatedly internally reflected from the fiber faces. The phase conjugate wave reflectivities up to 18% and with response time of a few seconds were obtained at λ = 632.8 nm, supplying the fiber with an alternating electric field of square wave form. Two possible effective configurations of mutually incoherent beam coupling inside the photorefractive fiber were compared experimentally.

Growth of single-crystal photorefractive fibers of Bi 12SiO 20 and Bi 12TiO 20 by the laser-heated pedestal growth method

Photorefractive single-crystal fibers of Bi 12SiO 20 up to 90 mm in length and Bi 12TiO 20 up to 50 mm in length with diameters of 650–1200 μm were grown by the laser-heated pedestal growth (LHPG) method. Both Bi 12SiO 20 and Bi 12TiO 20 fibers were pulled from stoichiometric (6:1) melts at pulling rates of 0.3 and 0.2 mm/min, respectively, the feeding rate was 0.1 mm/min. Growth conditions, morphology and chemical composition of the grown fibers were studied. The possible mechanisms of crystallization of incongruently melting Bi 12TiO 20 fiber from the stoichiometric melt were discussed. The energy-exchange effect was observed in the BSO fibers at the wavelength λ = 0.5145 μm.

Dispersion compensation for high bit rate fiber-optic communication using a dynamically tunable optical filter

A detailed analysis of dispersion-compensating fiber-optic filter is provided. Design parameters of the filter that could provide dispersion compensation for wide spectral sources are discussed. The dispersion-compensating fiber filter designed here can be dynamically tuned to compensate for the drift in the laser center frequency caused by temperature change and other deteriorations. The filter can be fabricated by exposing the photorefractive fibers to ultraviolet radiation. A design methodology for fabricating the refractive-index variation along the longitudinal direction of the fiber with appropriate chirp and taper coefficients is also given here. Dispersion-compensation capability of the fiber filter is demonstrated by simulating pulse broadening after transmission in a long fiber and the regeneration of the pulse after dispersion compensation.

Cross-talk noise in a wavelength-multiplexed reflection-type photorefractive fiber hologram

We have analyzed the cross-talk noise as affected by the wavelength spread that is due to the spectral width of the light source in a wavelength-multiplexed reflection-type photorefractive fiber hologram. The spectral bandwidth of each multiplexed channel limited by the cross-talk noise is discussed. The signal-to-noise (cross-talk) ratio is analyzed, whereby we have shown that this ratio is sensitive to the separation between adjacent multiplexed channels and to the spectral width of the light source. The cross-talk-limited storage capacity for practical photorefractive fibers is evaluated.

Wavelength-multiplexed holographic construction using a Ce: Fe: doped PR fiber with a tunable visible-light laser diode

A wavelength-multiplexed reflection-type photorefractive (PR) fiber hologram using a tunable visible-light laser diode is presented. We have shown that the wavelength-multiplexed PR fiber hologram offers a higher and more uniform wavelength selectivity than that of the transmission-type. Experimental demonstrations of wavelength-multiplexed reflection-type PR fiber hologram are provided, which are consistent with the theoretical analysis.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Simultaneous resonance of an S-shaped two-coupler optical fibre ring resonator

Proposed is the application of an active scheme using degenerate two-wave mixing (DTWM) with a photorefractive fibre to an S-shaped two coupler fibre ring resonator. It is demonstrated theoretically that the DTWM can bring about the simultaneous occurence of the first and second resonances of the resonator. New amplitude resonance conditions are derived. In addition, the nonreciprocity property of the device is studied, and it is found that the DTWM increases the nonreciprocity.

New Applications for Photorefractive Fibers

New Applications for Photorefractive Fibers