System Tradeoffs in Optical PPM Communications with Diode Combining

Optical power combining with laser diodes is receiving considerable attention. The objective is to noncoherently combine the diffraction limited beams of N diodes into a single beam, thereby increasing the field intensity by N. To accomplish this it is necessary to operate the diodes at different wavelengths in order to achieve the required combining. A question then arises as to the most efficient way to encode the diodes for maximum digital performance. In this paper, three different system architectures are considered, with combining achieved by dichroic mirrors operating in conjunction with a pulse position modulated (PPM) format. The basic criterion is maximization of data rate with increasing number of diodes, while maintaining a fixed decoding bit error probability. The three systems are: 1) power combining into a single pulse, followed by PPM encoding; 2) parallel channels, in which each diode is separately PPM encoded; and 3) color-coding, in which diodes are encoded over a common wavelength-time slot alphabet. All systems use beam combining, but the latter two require wavelength splitting as well. Data rate performance is evaluated as a function of the number of diodes, mirror combining losses, PPM alphabet size, and the operating optical SNR.