Optimization of OCT signal-to-noise ratio, when excess photon noise increases due to mismatch of the balanced receiver

In optical coherence tomography (OCT), it is often assumed that the signal-to-noise ratio, at higher optical power regimes, is limited by beat noise. Nonetheless it was often noticed that the limiting value could not be experimentally achieved, despite minimizing the stray reflectance and increasing the optical source effective bandwidth. In this work we present a new model for the noise in a balanced OCT configuration, which accounts for the limited spectral response of fiber directional couplers, as well as imperfections in the photo-detectors. As a consequence, and due to different spectrum content between the two balanced-OCT outputs, excess photon noise is larger than previously considered. The model also allows for the determination of SNR optimized parameters for maximum performance of the balanced system, and can be applied to any coupler or broadband source. Results for optimized and non-optimized configurations, for different balanced detection configurations, implemented in an OCT system, are presented and discussed.