A demand-access optical CATV system using FM frequency multiplexing is developed. This system allows us to transmit 40 video signals including MUSE high vision signals over 40 km on a single-mode fiber trunkline and can transmit arbitrary four video signals to subscribers at 10 km on subscribers' single-mode optical fibers. The feature of the present system is the possibility to fabricate an inexpensive hub by using a newly developed channel selector which combines single heterodyne and RF switch and also to make use of a cheap conventional DBS tuner at the subscribers' receiving terminal. For a trunkline, an optical modulation depth has to be carefully determined by considering an intermodulation distortion. The present paper describes the degradation in effective CN ratio due to a white noise produced by the FM modulated third-order distortion and shows an optimum optical modulation depth for a given transmitting distance. Also, a system design is discussed for certain distances of a trunkline and a subscriber line.One can estimate using the present design the CN ratio at a subscriber terminal. The present system can provide the video quality of 4.5 (CN ratio 17.5 dB) for high vision.

A new bidirectional external optical modulator using Fresnel reflection light has been proposed and developed. Since weak light reflected from the end of a crystal can be modulated in this modulator, no laser source is required if the modulator is intended for home use. This modulator also has no significant effect on a video signal in the downstream. In the external modulator designed for trial use, insertion loss was less than 3 dB, the half‐wave voltage was 10 V, and the half‐wave voltage difference and phase difference between the two modes were 2 percent and 5 degrees. A small module of the present device was created for use in a home optical terminal station of a demand access‐type Hivision CATV system. In this bidirectional transmission experiment, stable channel selection and good Hivision receiving image with C/N > 20 dB were confirmed. For the uplink 64 kbit/s NRZ signal, bit error rate (BER) of less than 10−8 was obtained.

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