Teruo Shimomura

Fiber Bragg grating cryogenic temperature sensors

Temperature sensing to as low as 80 K was demonstrated with 1.55-μm fiber Bragg gratings. The gratings were bonded on substrates to increase sensitivity, and a shift of the reflection wavelength was measured. The temperature sensitivity was 0.02 nm/K at 100 K when an aluminum substrate was used and 0.04 nm/K at 100 K when a poly(methyl methacrylate) substrate was used. These values are smaller than those at room temperature because of the nonlinearity of both the thermal expansion and the thermo-optic effect. Extension to the liquid helium temperature is also discussed.

Computer control of fiber Bragg grating spectral characteristics using a thermal head

We present a new technique for arbitrarily controlling the temperature distribution in fiber Bragg gratings (FBGs) to control their spectral characteristics. A thermal head consisting of a microheater array, driven with computer control signals is used as a controlling device. Various spectral characteristics including a chirped grating and a transmission filter are demonstrated using a single unchirped grating. An effective improvement in response time constant of thermal control was observed on bonding of the fiber grating to thermal head (18 ms), in comparison to that without bonding (40 ms). The measured results did not show any hysterisis. Reliability studies of this technique are also reported. The same technique is further used for the dual wavelength operation of an Er-doped fiber laser.

Iterative stepwise binarization of digital amplitude holograms with added energy to the signal window

We propose a method for optimizing the process of iteratively binarizing digital amplitude holograms by utilizing the effect of adding energy to the reconstructed signal window from the hologram. For this purpose, we introduce a parameter called the energy factor and show that the performance of the iterative binarization process can be greatly enhanced if the value of the energy factor is properly chosen. Finally, we show that the optimal values of this energy factor can be estimated with an analytical function for different computational costs.

Stochastic approach in the efficient design of the direct-binary-search algorithm for hologram synthesis.

An algorithm incorporating a stochastic approach is proposed for reducing the computation time of the direct-binary-search algorithm for hologram synthesis. Two variants of this new algorithm are considered for a number of hologram-generation problems. Both variants can reduce the computation time significantly with a very small increase in the reconstruction error on average. The effectiveness of the proposed algorithm is found to improve with the increasing computational complexity of the design problem. Also, the algorithm is able to generate the hologram in a time that is relatively independent of the initial conditions.

Error reduction of quantized kinoforms by means of increasing the kinoform size

There are two kinds of method that utilize the redundancy in kinoform domains for reducing the reconstruction errors of quantized kinoforms. One is the iterative-dummy area (IDA) method, which increases the kinoform size indirectly by the addition of a dummy area to the desired image. The other is the interlacing technique (IT), which increases the kinoform size directly by the interlacing of a number of subkinoforms whose sizes are the same as the desired image. We compare the error reduction of quantized kinoforms between these two methods. Simulation results show that reconstruction errors from the IT method can be reduced further and faster than those from the IDA method when the kinoform size is increased to larger than 4 x 4 times the size of the desired image.

Interlacing technique approach for the synthesis of kinoforms

An interlacing technique algorithm is proposed for the synthesis of kinoforms. The conventional iterative methods are quite powerful for optimizing kinoforms, but there is still a large reconstruction error for a quantized kinoform. We suggest the use of a number of subkinoforms interlaced together to synthesize a multikinoform for reconstructing the desired image. The idea of our interlacing technique is to increase the size of a kinoform to reduce the reconstruction error. The first subkinoform is generated from the desired image. Other subkinoforms are generated from the error images between the desired image and the image reconstructed from the previous subkinoforms. A theoretical analysis shows that the reconstruction error will be reduced as the number of subkinoforms is increased. Simulation results show that our interlacing method can reduce the reconstruction error more than do the conventional iterative methods and that the reconstructed image can be improved.

Color Gamut of a Nematic Liquid Crystal Display

The theoretical color gamut of a nematic liquid crystal display is described. The color gamut of a tunable birefringence mode and a guest host mode are revealed with the CIE chromaticity diagram and color solid. In order to account for the quantitative color gamut, color matching between the given chromaticity coordinates and those calculated is investigated. Color matching is performed by a combination of three liquid crystal subcells (A, B, C), where each subcell receives the voltage VA, VB, VC or contains the dye amount a, b, c. Calculation of the value of voltage or dye amount was executed by the matrix representation method. The calculated data are in good agreement with the given data within 0.50 CIE-UNIT color difference in the 1964 CIE (U*, V*, W*) color scale.

Binary phase difference diffuser: Design and application in digital holography

Design of a simple diffuser, in which the absolute value of the phase difference between the diffusers adjacent samples is constant, is considered. A condition is imposed in the design so that the diffuser is suitable for use with the iterative method based on the iterative Fourier transform algorithm for calculating a two-dimensional complex amplitude function that possesses a bandlimited and smooth spectrum. A method for effectively designing this diffuser is proposed, and its application in the field of digital holography is presented.

Interferometric measurement of KrF-laser-induced refractive index changes in germanosilicate fibers

Abstract Photosensitivity in germanosilicate optical fibers is studied with a fiber Michelson interferometer. Saturation characteristics for multiple KrF-laser exposure are measured, and the origin of the index change is discussed. The total index change of a high-Ge (35 mol%) fiber is 6 times that of a low-Ge (3 mol%) fiber. In both fibers, the fast and the slow saturation components are found. An analysis using the Frantz-Nodvik equation shows that they are attributed to GeSi and Ge 2+ defects, resectively. The saturated index changes in highly reflective fiber gratings are due to Ge 2+ defects.

KrF-Laser-Induced Fiber Bragg Gratings: Formation Characteristics and Cutback Measurement

The formation characteristics of fiber Bragg gratings were studied by using the side-writing holographic method with a 248 nm injection-locked narrow-band KrF excimer laser. Type I gratings with reflectivity as high as 99% were formed in hydrogen-loaded fibers by multiple-pulse exposure. Type II gratings with the maximum reflectivity of 90% were also fabricated. The characterization of fiber gratings by the cutback method is demonstrated. Using this method, we determined the effective grating length and calculated the magnitude of photoinduced index change. Chirp characteristics of the gratings were also studied.