Nobukazu Yoshikawa

Phase optimization of a kinoform by simulated annealing

The design of a kinoform by the use of simulated annealing is discussed. The simulated annealing process is applied to decrease the reconstruction noise and to adjust the phase distribution of the kinoform to the configuration of the recording device. A liquid-crystal spatial light modulator is used to display the kinoform. The reconstructed image of the optimized kinoform is found to be in good agreement with the computed image. Some experimental results obtained with a liquid-crystal spatial light modulator are presented. The phase quantization effect of the kinoform is discussed.

BINARY COMPUTER-GENERATED HOLOGRAMS FOR SECURITY APPLICATIONS FROM A SYNTHETIC DOUBLE-EXPOSURE METHOD BY ELECTRON-BEAM LITHOGRAPHY

We have developed a new multiplexing method for producing binary computer-generated holograms (CGHs) for security applications. This method is based on double recording of two types of coding method upon binary CGHs. The CGH synthesized by the proposed method can have multiple image planes from the region close to the hologram (image region) to infinity (Fraunhofer region) without severe degradation of reconstructed images in the image region. A CGH containing simultaneous image- and Fourier-type holograms is fabricated by electron-beam lithography. Some experimental results are presented.

Quantized phase optimization of two-dimensional Fourier kinoforms by a genetic algorithm

We have developed a phase optimization method of a quantized kinoform by a genetic algorithm. Because the genetic algorithm inherently deals with discrete values, the quantized phase of the kinoform can be easily estimated. The two-dimensional Fourier kinoform can utilize effectively the periodicity of the discrete Fourier transform in the genetic algorithm. This condition enables us to perform the crossover process that is one of the processes in genetic algorithm without a spatial bandwidth of the kinoform. The optimization has been performed successfully in computer simulation. The optically reconstructed image agrees well with the theoretical one.

Fringe pattern correlator for three-dimensional object recognition

We propose a novel three-dimensional (3-D) object-recognition method based on a Fourier-transform profilometry technique and a two-dimensional (2-D) correlation technique. Height information on 3-D objects is transformed to phase information on 2-D complex amplitude by use of the Fourier-transform profilometry technique. 3-D objects are recognized using correlation by use of the transformed complex amplitude.

Mutually pumped phase conjugator with a rainbow configuration in BaTiO 3 :Ce crystal using nanosecond pulses

A new type of mutually pumped phase conjugator with a rainbow configuration has been demonstrated experimentally in cerium-doped barium titanate crystal that uses pulsed beams with a pulse width of 3.0 ns and a repetition rate of 40 Hz at a wavelength of 532 nm. The highest phase-conjugate reflectivity was ~24%. The dependence of the phase-conjugate reflectivity on the incident angle and distance, as well as the grating formation time for achieving 1 - e(-1) of equilibrium reflectivity, has been measured.

Poled Polymer Etalon Light Modulator on Integrated Circuits

Electrically addressed reflective spatial light modulators with polymeric thin films are fabricated on the surface of an n-type metal-oxide-semiconductor (NMOS) chip. The use of a resonator structure is proposed to minimize the driving voltage. The developed device is composed of 1×5 NMOS transistors and nonlinear polymeric materials sandwiched between aluminum electrodes. The electrooptical polymer consists of disperse red 1 (DR1)-doped poly-methyl-methacrylate (PMMA). Poling was demonstrated on this chip and the light modulation was observed using a lock-in amplifier. A modulation efficiency of 2.0 × 10-5 was obtained.

Interpolation of reconstructed image in Fourier transform computer-generated hologram

Abstract An interpolation method of image points reconstructed by a Fourier transform computer-generated hologram is described, so that a sufficient large size hologram can be synthesized by a small size hologram. We have made a mosaic hologram which consists of K × K subholograms with N × N sampling points multiplied by an appropriate weighting factor. It is shown that the mosaic hologram can reconstruct an image with NK × NK image points. The main advantage of the present algorithm is that a sufficiently large size hologram of NK × NK sample points is synthesized by K × K subholograms which are successively calculated from the data N × N sample points and also successively plotted.

Time-space conversion of femtosecond light pulse by spatio-temporal joint transform correlator

Abstract A new method to obtain the auto-correlation of an ultrafast light pulse spatially and parallel is proposed. The method is based on a conventional optical computing technique, joint transform correlator, and a spectral filtering method to control ultrafast light pulses temporally. A simple experimental determination of the pulse-separation of twin pulses, is demonstrated. In the experiment, the optically addressable spatial light modulator is used as a spectrum-power spectrum conversion device, and the temporal accuracy of the system is 12.4 fs.

Spatial filtering using poled polymer etalon light modulators

Spatial filtering using poled polymer light modulators is studied. The use of a resonator structure is proposed to minimize the driving voltage. Side-chain polymer poly-orange tom-1 isophoronedisocyanate is used as a material. The measured electro-optic coefficient r33 is 23 pm/V at the wavelength of 633 nm. Spatial filtering of the input images are realized with 30 V applied voltage.

Use of generic algorithm for computer-generated holograms

The phase optimization of a quantized kinoform by the genetic algorithm is discussed. Because the genetic algorithm inherently deals with discrete values, the quantized phase of the kinoform can be easily estimated. The periodicity of the discrete Fourier transform for the 2D Fourier kinoform enables us to perform the crossover process, which is one of the processes in genetic algorithms, without concern of a spatial bandwidth of the kinoform. We introduce a step-quantization method for the multilevel kinoform. Improvements are shown to be significant with this method in computer simulations. The optically reconstructed image agrees well with the calculated one.