Impact of spontaneous emission noise on the sensitivity of direct-detection lightwave receivers using optical amplifiers

A simple technique for analysing the sensitivity and bit-error rate performance of direct-detection lightwave receivers using optical amplifiers is presented. The analysis provides closed-form expressions for the system performance. For zero (negligible) linewidth, the theory predicts an average signal energy of 42.3 photon/bit at BER=10^-9. In comparison, more accurate techniques predict an average signal energy of 38 photon/bit under the same conditions.