Gaussian and exact analyses on the effect of local oscillator intensity noise in optical phase diversity FSK receiver

The impact of local oscillator intensity noise on the performance of an optical phase diversity FSK receiver using a delay-and-multiplying discriminator is analyzed for both Gaussian approximation and exact non-Gaussian statistics. The bit error rate (BER) is expressed in terms of Q-function using Gaussian approximation. Also, an exact equation for BER is derived by a non-Gaussian approach. Given data rate, frequency deviation, discriminator delay, composite laser linewidth, bandwidth ratio of pre- and post-detection lowpass filters, local oscillator relative intensity noise (RIN), and receiver thermal noise, the received signal power (sensitivity) can be calculated with respect to given local oscillator power for a specified BER. Both Gaussian and non-Gaussian numerical results show that there exists an optimal local oscillator power to achieve maximum sensitivity for RIN -150 dB/Hz, -135 dB/Hz, and -125 dB/Hz, respectively, at a BER of 10^-9 The deviation of sensitivity between Gaussian and non-Gaussian (exact) results is shown to be within 1 dB for the illustrated examples, when the product of frequency deviation and discriminator delay is equal to 0.25. If this product is far away from 0.25, the performance is greatly degraded as seen from the exact calculations and the deviation due to Gaussian approximation also becomes very significant