Abstract

To use coherent heterodyne system configurations effectively and to take full advantage of the potential sensitivity improvement over direct detection systems, it is essential to develop low-noise heterodyne-receiver frontends. So far, such development has proved difficult (especially for gigabit-per-second applications) because of the parasitics of typical photodiodes. The best results so far with bandwidths of ≥2 GHz have used inductive tuning¹ to obtain typical noise spectral densities of $~10\text{pA}/\sqrt{\text{Hz}}$^2,3 for frontends with one photodiode. In many systems, the influence of relative intensity noise (RIN) on the local oscillator laser can be important.⁴ The requirements to the RIN level can be significantly eased using a balanced- (two-photodiode) receiver frontend, but this frontend tends to increase the influence of photodiode parasitics and requires careful coupling of the photodiodes. We have developed a design, usable for gigabit-per-second applications, which simultaneously gives a high common-mode rejection owing to the balancing and the lowest level of the noise spectral density reported so far.

© 1991 Optical Society of America