On the capacity of intensity modulated systems using optical amplifiers

We study the information capacity and the information density of communication systems using optical amplifiers, in the direct detection intensity modulation case. Our results are compared with Shannon's formula for coherent detection, where we find that in the relevant range of parameters the information density of intensity modulated systems is equal to one half of the density of coherent systems minus 1 bit/s/Hz. We show that with typical values of optical signal-to-noise ratio of the order of 20 dB, multilevel signaling can increase the information density by no more than a factor of 3.5 relative to binary modulated systems. The highest information density is obtained with a symmetric optical spectrum and it cannot be improved by removing one sideband as in the single sideband modulation case.