Frequency-doubling PFM system to prolong the distance of optical fibre TV programme transmission

Abstract

In the square-wave PFM system for transmission of TV programmes through optical fibres, square-wave PFM pulses are generated at the transmitter with a relatively low centre frequency, whereas double-frequency equal-width narrow pulses are produced in the receiver before the demodulating low-pass filter, at both the rising and falling edges of the transmitted pulses. The use of the frequency-doubling technique in such a system results in narrow transmission bandwidth and high receiver sensitivity, thus prolonging the transmission distance significantly as compared with ordinary PFM. In the experiment, the system is operated at 1.3 μm with multimode fibre having bandwidth 700 MHz km, a laser diode as optical source and a PIN/FET as detector. It can effectively transmit the high-quality broadcast TV programme with accompanying sound channel to a distance of 50.7 km. Experimental results are: receiver sensitivity −40 dB m, weighted SNR for video 59.3 dB, differential gain and differential phase less than 1% and 1°, respectively.