Measurement of the DPSK Detection Sensitivity at 1.4 Gbit/s as a Function of Local Oscillator Power

So far the best receiver sensitivities in optical fiber transmission systems have been achieved with coherent systems, with the most impressive transmission distances obtained with differential phase shift keying[1] (DPSK) and continuous phase frequency shift keying[2] (CPFSK) performed at modulations speeds in the Gbit/s range. Although DPSK and CPFSK modulation schemes put stringent requirements on the laser linewidth, (s few tenth of % of the bit rate), they have been successfully implemented using monolithic laser source at both high[2-3] and low bit rates,[4] in the presence of a finite level of phase noise without substantial degradation of the system performance. However, studies of the constraints on the system performance for high capacity coherent systems have led to a prediction of increasing ISI with increasing bit rate.[1,5-6]