FM‐FDM optical CATV system employing demand access technology

Abstract

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.