By H. Blauvelt, J. Paslaski, and J. Lipson, Ortel Corporation
Fiber optic links for transmitting AM video have typically operated with received optical power levels of about 1 mW. In the past, the most common practice was to split the laser output to serve multiple optical nodes. The higher the output power of the laser, the more optical splitting that could be done, but the received optical power was always approximately 1 mW. Recently there has been much interest in point to point links without any optical splitting. These links commonly have optical loss budgets in the 2-6 dB range. With conventional10 mW DFB lasers, the received optical power greatly exceeds 1 mW. To avoid overdriving the receiver, low power DFB lasers are used for these point to point links. Although point to point transmitters have lower output power requirements, the linearity demands on the laser are approximately the same as for high power DFB lasers. An alternative approach to point to point links is to use medium power (6-10 mW) lasers and higher received power. If the modulation depth is kept the same, this results in significantly higher link CIN. Alternatively, the modulation depth can be reduced to provide the same C/N, with substantially relaxed linearity requirements for the laser. In this paper, the factors influencing receiver linearity and designs capable of operating up to 5 mW received power will be described The impact of high optical power receivers on laser linearity requirements and on the channel capacity of AM fiber links will also be discussed.