SNR analysis of conventional and optimal fiber-optic low-coherence interferometer topologies

The most straightforward (and currently the most common) interferometer for optical coherence tomography (OCT) is a simple Michelson interferometer. A low-coherence source illuminates the interferometer. The light is split by a 50/50 beamsplitter into a sample and a reference path. Light retroreflected from the reference and the sample is recombined at the beamsplitter and half is collected by a photodetector in the detection arm of the interferometer. Half of the light is returned towards the source, where it is lost. In addition, the reference arm light is typically attenuated by orders of magnitude in order to improve signal to noise ratio. Thus, using this typical configuration, virtually 75% of the optical power supplied by the source is not used for image formation. We present a family of novel interferometer designs incorporating optical circulators, unbalanced couplers, and/or balanced detection which are designed to optimize optical power efficiency and system signal to noise ratio. We evaluate the expected performance of the novel interferometers as compared with the standard Michelson interferometer. We review signal and noise sources important for the design of OCT interferometers, and specify the design equations for evaluation and optimization of signal to noise ratio. This analysis, based on sound, experimentally verified literature, predicts improved sensitivity for all of the new interferometer designs.