AN OPTICAL FSK HETERODYNE DUAL FILTER DETECTION SYSTEM FOR TAKING ADVANTAGE OF DFB LD APPLICATIONS

An optimum system configuration for an optical frequency-shift keying (FSK) heterodyne dual filter detection system with distributed feedback laser diodes (DFB LD’s) is investigated taking into consideration LD phase noise influence. Experimental and theoretical examinations show that an IF filter bandwidth greater than 10 times the heat spectral linewidth is necessary to avoid LD phase noise influence. A 301-km long span transmission experiment has been successfully carried out with an optimum configuration for 34 Mbit/s. High receiver sensitivity, -61.8 dBm with more than 10 dB improvement over the direct detection system, has also been achieved. Experimental results at higher bit rates of 140, 200, and 280 Mbit/s indicate that a modulation index greater than 2 is desirable to avoid crosstalk between mark and space signals. With sufficient frequency deviations, high receiver sensitivities, -54.7 dBm (140 Mbit/s) and −52.5 dBm (200 Mbit/s), have been achieved. These represent 9.6- and 9-dB sensitivity improvement, respectively, over direct detection systems. A guide for FSK dual filter detection system design is derived from the experimental and theoretical results. Potential application regions for a dual filter detection system with DFB LD’s is determined by comparing the system performance with both a CPFSK delay demodulation system and a single filter detection system. © 1990, IEEE