What Do We Need To Get Great Link Performance?

Abstract

It is well known that the RF performance of optical fiber links falls short of what is desired, and often required. These seem to be the facts of life: links using colmmercially available components have at least 20 dB of RF loss, their noise figures are even greater than their loss, and their dynamic ranges are marginal to acceptable [ 11. Further, all three of these performance parameters degrade significantly with increasing frequency. This sitate of affairs is in stark contrast with the fundamental limits of link performance [2], which have shown that low loss, low noise figure, high linearity links should be possible, even up to fairly high frequencies. So why have we not been able to realize links with such performance? We will pursue the answer to this question by hypothesizing a set of link requirements and seeing what level of device performance would be required to meet it. Assume that we desire a link with ,an RF-to-RF gain (GI) of -3 dB over a bandwidth of less than one octave, a noise figure (NF) of 6 dB, and an intermodulation-free dynamic range (IMFDR) of 145 dB in a 1 Hz bandwidth. For the purposes of this discussion we will also limit consideration to amplifierless links using intensity modulation with direct detection (IMDD) and with passive impedance matching at the input and output ends of the link.