REQUIREMENTS FOR LD FM CHARACTERISTICS IN AN OPTICAL CPFSK SYSTEM

The required LD FM characteristics for a coherent CPFSK heterodyne delay demodulation system are derived through theoretical and experimental investigations. A novel procedure for evaluating the influence of a nonideal FM response on the bit error rate characteristics is proposed which explains experimental results fairly well. Using the procedure, criteria for the LD FM response are obtained in order to realize low penalty operation with a long bit pattern. It turns out that the position and the shape of the dip (typical for a conventional LD FM response), especially the slope of the higher frequency part of the dip, are the dominant factors which determines the nonideal FM response influence of transmitter devices. The criteria obtained give guidelines for the LD FM response equalization, as well as the device selection. For devices to be applied in practical systems with an FM equalizer, a dip frequency, sufficiently below 4 x 10(-4) of the bit rate, should be selected in order to realize less than 0.5-dB sensitivity penalty operation, since the passive electrical equalization is only effective for the frequency region higher than the dip frequency.