oscillator circuits. For an inverter oscillator (Fig. 5), we coupled five inverters together, with the o u t~u t voltage of one inverter providing the input for the following. If the initial input to the first inverter is 0 V (-V,), then the output of the fifth will be -V, (0 V). Returning the output of the fifth inverter as the i n~u t for the first inverter causes all of the inverters to sequentially change logic state. Consequently, the circuit oscillates with a characteristic frequency f,,,. If all transistors are identical, fClSc is %o of the switching delay frequency of an individual inverter. In reality, fc,,, is limited by the slowest gate. For pentacene ring oscillators, f,,, is in the range of 100 to 500 Hz, and for PTV ring oscillators, fc,,, is in the range of 10 to 50 Hz. These values are dominayed by the RC time constant of the channel resistance R of the load transistor (the nonswitching transistor), which is giv-

When illuminated with a focused beam from He-Ne lasers, As2S3 films with thickness of ∼50 μm exhibit the thickness expansion reaching to 3 μm, which is approximately 10 times as great as that expected from the conventional photoexpansion phenomenon. The enhancement seems to be caused by photorelaxation of strain generated by photoexpansion.

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Bragg gratings have been fabricated in As(2)S(3) optical fibers through the photoinduced refractive-index change process. Bragg filters in the fiber show a more-ideal response than those fabricated in the corresponding bulk material. Some features characteristic of chalcogenide fibers and Ge-doped oxide fibers are discussed comparatively.