Optical Review Vol. 6(1999) No. 6

Liquid Crystal (LC) lens cells with variable focusing properties are fabricated using nematic LC materials and applicable to optical devices, and our recent work on these cells is described. First, the LC lens cells are prepared using lens-shaped substrates coated with transparent electrodes. Their focal length can be continuously varied between the values for an ordinary ray and an extraordinary ray by changing the voltage passing across them. Methods of improving properties and some applications of the lens-shaped LC lens are briefly described. The lens properties of these cells with plane-paralleled structure are then demonstrated, where the refractive index is graded to a quadratic distribution resulting from an axially symmetric non-uniform electric field. LC cells with axially distributed tilt angles are constructed using a pair of circular hole-patterned electrode substrates and very small LC lens (LC microlens) with variable focusing can be fabricated. Optimizing the electrode structure, device parameters, and material parameters of the LCs, excellent focusing properties can be obtained. The properties of the LC microlens are improved by using the polymer stabilization technique. The LC microlens with a divided electrode structure shows three-dimensional beam steering and focusing properties, and the astigmatic aberration caused by the molecular orientation effect can be compensated. Applications of the LC microlens to optical devices and systems are introduced.

We present the design of a novel metal micro-aperture surface emitting laser for tera byte optical data storage. The field distribution near micro-aperture is calculated by 2-dimensional finite element method, and it is shown that a spot size as narrow as 100 nm beyond a diffraction limit is obtainable.

This paper describes the fabrication method and the measured spectral behavior of a 1.55 μm transmission glass-echelon grating filter. This echelon grating consists of a pile of identical plane-parallel glass plates constructed using a special assembling tool. The experimental result confirmed the accuracy of the Fourier transform which indicated that the spectral passband width was inversely proportional to the beam size. This feature makes it possible to use the filter as a bandpass filter or a comb filter when constructing photonic networks. The echelon filter has certain intrinsic advantages including a low insertion loss and polarization independence. Moreover, it offers good cost performance and excellent design flexibility.

It is found numerically that the time-bandwidth product (TBP) of optical pulse from a gain-switched semiconductor laser operating under a sufficient gain saturation condition increases monotonously with increasing modulation amplitude. This enhancement of the TBP becomes remarkable for longer-wavelength semiconductor lasers (1.3, 1.55 μm) having large value of the gain compression factor ε (~10^{−23} m³), making the pulse chirp estimation using the TBP difficult.

If illuminating light with expected spectral distribution can be synthesized, it will be a great help in many applications, such as computer vision, pattern recognition, industrial quality control, and remote sensing. A light source system containing a liquid crystal spatial light modultor (LCSLM) is proposed for this purpose. A supervised neural network with a single layer was used to control the system. After training the neural nrtwork with data acquired from the system, it gives the data which should be written to LCSLM in order to synthesize illuminating light having certain spectral distribution. Eight spectral distributions were synthesized to test the performance of the system, and the average synthesizing error was 12%.

The finite-difference time-domain (FDTD) method is applied to the numerical evaluation of diffraction from a binary relief grating with subwavelength structures. It is demonstrated numerically that the binary relief grating made of isotropic materials works as a polarization selective optical device. It is further shown that the polarization effects depend on the arrangements of subwavelength structures.

We demonstrate a new tool called optical spectrogram scope for visualization of a spectrogram or a scalogram of optical ultrafast phenomena. The optical spectrogram scope is constructed on the basis of the time-to-two-dimensional-space conversion technique capable of converting a set of time-varying frequency distributions into two-dimensional spatial ones.

A white light system based on a 65 cm×35 cm diffractive screen is demonstrated to be capable of displaying three-dimensional figures with continuous horizontal parallax. Three computer-controlled mirrors and a diffractive-refractive optical system are employed for positioning each element of the figure. No visual accessories are necessary and more than one observer can watch it simultaneously.

Two fabrication methods of surface relief electro-optic (EO) gratings in azo-polymer films are described: surface deformation by a continuous wave laser and laser ablation by a pulse laser. In the fabrication process, the periodic intensity modulation of laser beams is formed by an interference fringe or a phase mask. The surface relief EO grating is fabricated by poling the surface relief grating (SRG) after the SRG fabrication process. The linear dependence of an applied voltage in the modulation efficiency of the first-order diffraction beam is observed. The proposed methods are effective for highly-efficient surface relief EO grating fabrication.

The effects of the direction of transition dipole moment on polarization-dependent phase-conjugate (PC) reflectivity by degenerate four-wave mixing in a saturable-dye-doped film are theoretically investigated. On the basis of a four-energy-level model for the saturable dyes, the pump-intensity dependence of the PC reflectivity is calculated for the two cases of a probe wave being polarized either parallel or perpendicular to pump waves, when the transition dipole moment associated with excited-state absorption tilts from that associated with ground-state one. For the parallel polarization, in the case of two transition dipole moments having different directions, a dip appears in the PC reflectivity curve near a resonant frequency on account of the characteristics of nonlinear absorption, whereas it disappears when the frequency of light is off resonance due to the influence of nonlinear refractive index.

In phase-shifting Fizeau interferometers, nonlinear motion of the phase shifter and multiple-beam interference are the most common sources of systematic errors affecting high-precision phase measurement. A new class of algorithms with extended compensating capability for these errors is proposed. Measurement errors for the new algorithms and two groups of conventional algorithms: discrete Fourier algorithms and the Schwider-Larkin-Hibino algorithms are estimated as a function of the number of sampled images when these systematic error sources are equally dominant. It is shown that the conventional phase-measuring algorithms produce significant errors when the reflectivity of the testing surface exceeds ten percent. Also, these algorithms have an optimum number of samples at around seven with which the residual errors become minimum. The new class of algorithms shows a substantial reduction of the residual errors when the number of samples exceeds ten. There is no optimum number of samples for the new algorithms. For fewer than six samples, discrete Fourier algorithms which have no error-compensating capability for the nonlinearity of phase modulation give a minimum error.

A new lidar method for measuring water cloud particle size is proposed, and the feasibility of the measurement is discussed. The method utilizes the phenomenon known as the glory which is observed in open air. The proposed lidar consists of a multicolor laser transmitter and two receiver systems looking at the scattering from the target cloud with different scattering angles. Results of the theoretical study show that a system with five laser wavelengths (355, 532, 750, 1064 and 1500 nm) and two receivers located at scattering angles of 180 and 177.5-179 deg is useful for measuring particle size (mode radius of the size distribution) in a range of 4 to 12 μm.