Sensitivity of an optical heterodyne receiver in presence of an optical preamplifier

The effect of an optical preamplifier on the sensitivity of a heterodyne receiver near the shot noise limit is measured. Without the preamplifier, the sensitivity of the receiver is 76 photons/bit for an FSK modulated signal at 45 Mbit/s received with a BER of 10−9(2.8 dB from the shot noise limit). Optical preamplification degrades this sensitivity by 9.6 dB. The degradation expected from the amplifier spontaneous emission noise is shown to be very close to the measured value.