IMPROVEMENTS IN CARRIER-TO-NOISE-RATIO (CNR) OF LIGHTWAVE AMPLITUDE-MODULATED VESTIGIAL-SIDE-BAND (AM-VSB) TRANSMISSION-SYSTEMS BY THE USE OF OPTICAL ISOLATORS

In lightwave community antena television (CATV) systems as well as oth er optical fiber communication networks, a transmitted optical signal is known to be degraded by an intensity noise produced within the fibe r due to the interference between the signal and doubly reflected ligh t. We report on the improvements to the signal degradation due to the double Rayleigh backscattering by inserting optical isolators in the t runk lines of the systems. A carrier-to-noise ratio (CNR) has been cal culated as a function of the number and the insertion loss of the isol ators. The calculated results indicate that there is an optimum number of isolators, and if the insertion loss is less than 0.3 dB, the CNR degradation can be restored by more than 60%. To test the calculated r esults, we conducted experiments for the specific case of employing on e isolator, and obtained good agreements between the two.