A generation effect of higher harmonics for an externally applied signal in a photoconductive (PC) terahertz (THz)-wave emitter has been found. This effect is applicable to accurately measuring for frequencies of THz waves. This paper describes reasons why higher harmonics are generated in a PC device. The dependence of the photoconductance on the applied voltage in the PC device consists of a flat range and a negative slant range, and one sharply bending point is then formed at the boundary between the flat and slant ranges. When the PC device is irradiated by two laser beams with slightly different optical frequencies, the photoconductance is strongly modulated at the optical beat frequency in the THz region by photomixing the two laser beams. As a result, three bending points are formed in the average photoconductance (introduced as the average of the temporal photoconductance varying at the THz frequency). The slants comprised of the three bending points are different from each other. When the variation range of the applied voltage driven by the signal input on the biased voltage covers the voltage of one of the bending points, the photoconductance (or the average photoconductance in optical beating) varies along the different two slopes, the resultant temporal photocurrent is largely distorted, and then the harmonics of the signal input are generated in the photocurrent. The following features are clarified: (1) the harmonics of the signal input are generated by appropriately adjusting the bias voltage and the amplitude of the signal input, regardless of the presence/absence of optical beating; (2) the efficiency of the harmonic generation is about 10-4 -10-5; and (3) the harmonics over 35th order with almost flat amplitudes (-3.8 dB/octave) are generated.

A new fabrication method of a star coupler has been described. It is fabricated by stacking polymer films and then sticking them by a heat press. Using this method, the star coupler can be produced very easily and has square core cross sections. The 5-port multimode star coupler made on an experimental basis has an output power variation of 0.9 dB and an insertion loss of 3 dB.

The purpose of this research is to improve the process and the characteristics of computer generated hologram (CGH). In this paper we propose the method of the direct laser recording of CGH (DLR-CGH). At first we construct the direct laser recording system. Then we consider the most suitable hologram coding and scanning scheme for DLR-CGH. It becomes clear that the process of making CGH is considerably reduced by this recording system.

A multimode variable directional coupler has been fabricated by piling up films. It uses an electrostrictive element. The output powers can be continuously controlled over a wide range. The output powers from the two output ports of the coupler were equalized at a stress of 16.8 kg/cm2, and the loss due to the stress was small (0.19 dB).

Light intensity variation is performed in a multilayer optical circuit element consisting of stacked crossing thin film optical waveguides. The principle of operation is the interference of two guided light waves coupled with each other at the intersection of the waveguide of the optical circuit element. Measurements are carried out for each case of the coupling coefficient of the optical circuit element equal to 3 dB and not equal to 3 dB. The percentage variation obtained is 45%.

The propagation constant, the effective index, the effective guide thickness and the refractive index has been measured by using the method of a variable spacing of a high-index film and the waveguide surface. The spacing can be varied by the electrostrictive element.

Optical waveguide polarizers are realized by three dimensional polymer thin film waveguides with a-Si: H film clad. It has a low index buffer layer between the waveguide and a-Si: H film clad. It is experimentally found that a high extinction ratio for either TE or TM mode pass polarizer can be obtained by changing the hickness of the a-Si: H film clad.