A 565 Mbit/s FSK coherent system using commercial DFB lasers

Coherent systems are attracting considerable interest for a diversity of applications such as broadband distribution networks and long haul links. Such systems require optical source of high spectral purity. The current work is directed towards developing a coherent system using commercially available distributed feedback (DFB) laser sources. The system considered here operates at 565 Mbit/s utilising single filter asynchronous detection. The DFB lasers used are Toshiba TOLD385S devices operating at 1550 nm and 0 dBm output power. The lasers are butterfly packaged with an integral isolator and cooler. The measured 3 dB optical linewidths for the devices are approximately 40 to 50 MHz. The experimental arrangement features a balanced front end with wideband transimpedance amplifier, a novel demodulator circuit, automatic gain and frequency control loops (AGC and AFC respectively) and frequency compensation of the transmitter laser. A sensitivity of -41.8 dBm was achieved and no evidence of an error rate floor was observed