Analog Lightwave Links For CATV Applications

Analog lightwave links for CATV application continue to penetrate the CATV industry, especially as cable operators position themselves in the race to provide broadband information services to the mass market. The analog lightwave systems are remarkably simple. The RF spectrum of the CATV signal, without format change, modulates the transmitted light intensity, and an optical receiver reverts the light intensity back to RF. This simplicity comes at the price of very demanding requirements on the noise and distortion performance of the transmitter and receiver. The AM-VSB formatted signal currently used for television requires a signal-to-noise ratio (SNR) of about 50 dB. With an additional distortion requirement of -60 dB with respect to the signal, and the CATV spectrum of 40-60 subcarriers spread over a multi-octave spectrum (55 - 500 MHz), the source must be extremely linear [l]. A workhorse of the CATV lightwave industry is the directly modulated 1.3 Fm distributed feedback (DFB) laser. Recently introduced to the market is an externally modulated YAG laser at 1.3 pm. Erbium-doped fiber amplifiers (EDFAs) have not found wide application because the amplifier noise impairs the system noise performance [2], and suitable source lasers are not widely available. We present the system design issues for the various transmitter types.