In direct-detection OFDM systems, the nonlinear effects caused by optical modulation and fiber transmission can degrade the system performance severely. In this study, we propose a new nonlinear companding transform to improve the performance of direct detection optical OFDM transmission systems. The demonstration is realized by Monte-Carlo simulation of the intensity modulation and direct detection DCOOFDM optical transmission system at 40 Gbps over a 80 km of standard single mode fiber link. The influence of the companding parameters on the performance of system in different nonlinear transmission conditions has been investigated via simulation.

An overview of the important techniques for detection of optical radiation from the ultraviolet, through visible to infrared spectral regions is presented. At the beginning single-point devices are considered. Next, di.erent application circuits used in direct detection systems together with elucidation of the design of front-end circuits and discussion of their performance are presented. Third part of the paper is devoted to advanced techniques including coherent detection. Finally, the updated information devoted to readout of signals from detector arrays and focal plane arrays is included. It is shown that detector focal plane technology has revolutionized many kinds of imaging in the past 25 years.