A comparison of sensitivity degradations for optical homodyne versus direct detection of on-off keyed signals

The sensitivity of an optical homodyne receiver with practical degradations is analytically evaluated using a semiclassical photodetection model which is valid for arbitrarily low levels of local oscillator (LO) power. This model provides several important advantages over the usual treatment in which large LO is assumed. In addition to giving an intuitive picture of the evolution of the detection sensitivity from the direct detection case (LO=0) to the shot noise limited coherent detection case (large LO), it allows for a quantitative comparison between degradations caused by some important receiver nonidealities for the two limiting cases. Sensitivity loss due to dark current, thermal noise, and electronic distortion is evaluated and the LO power required to reach shot noise limited operation in the presence of these degradations is determined