Minimum bit rate of DPSK transmission for semiconductor laser with a long external cavity and strong linewidth reduction

A numerical model for a laser diode, weakly coupled to an external cavity, is presented. Using this model, the actual frequency noise spectrum is considered rather than a white frequency noise for estimating the minimum bit rate in a differential-phase-shift-keying (DPSK)-system. Despite possible linewidth reductions by up to 200 with long external cavities, the minimum bit rate (BER = 10^-9) remains nearly unchanged. However, a longer laser cavity ( m) connected to an external cavity of moderate length ( ≈10 cm) yields a significant reduction of the minimum bit rate (BER = 10^-9) for DPSK-systems at low feedback levels. If more stringent criteria are applied (e.g., receiver penalty < 1 dB) the minimum bit rate remains high also for longer laser cavities.