[OPTICAL REVIEW Vol. 2, No. 5 (1995) 366-370]

Improvements in Carrier-to-Noise-Ratio (CNR) of Lightwave Amplitude Modulated Vestigial-Sideband (AM-VSB) Transmission Systems by the Use of Optical Isolators

Takashi TAKEO,1 Tomohiko KANIE,2 Katsuhiro SATOH,3 Shigeyuki AOYAMA,3 Haruhiko ITOH1 and Hajime HATTORI1

1Department of Electronics, Nagoya Municipal Industrial Research Institute, 3-4-41, Rokuban, Atsuta-ku, Nagoya, 456 Japan, 2Faculty of Materials Science, Japan Advanced Institute of Science and Technology, Hokuriku, 15, Asahidai, Tatsunokuchi, Nomi-gun, Ishikawa, 932-12 Japan, 31st Technical Services Department, Aichi Electronics Co., Ltd., 1-15, Himegaoka, Shimogiri, Kani, 509-02 Japan

(Received April 10, 1995; Accepted June 26, 1995)

In lightwave community antena television (CATV) systems as well as other optical fiber communication networks, a transmitted optical signal is known to be degraded by an intensity noise produced within the fiber due to the interference between the signal and doubly reflected light. We report on the improvements to the signal degradation due to the double Rayleigh backscattering by inserting optical isolators in the trunk lines of the systems. A carrier-to-noise ratio (CNR) has been calculated as a function of the number and the insertion loss of the isolators. 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 results, we conducted experiments for the specific case of employing one isolator, and obtained good agreements between the two.

Key words : AM-VSB transmission system, CATV, Rayleigh backscattering, relative intensity noise (RIN), CNR, optical isolator