Shot noise limited optical measurements at baseband with noisy lasers

Abstract

This paper describes a simple, all electronic noise cancellation scheme which allows wideband, shot noise limited optical measurements at baseband, with noisy lasers, in many kinds of optical systems. With this system, it is usually possible to achieve the performance of a complex heterodyne system with a much simpler homodyne approach. Most optical systems and detectors have very wide temporal bandwidths and excellent linearity; thus at all frequencies of interest, the sample photocurrent has exactly the same instantaneous fractional excess noise fluctuations as the laser beam itself, with no differential gain or phase. The actual noise cancellation bandwidth is very wide, and does not depend on the feedback bandwidth, only on that of the differential bipolar transistor current divider.