Shot-noise limited optical hybrid based on fused fiber couplers

We describe a fiber-based coherent receiver topology which utilizes intrinsic phase shifts from fiber couplers to enable instantaneous quadrature projection with shot-noise limited signal-to-noise ratio (SNR). Fused 3x3 fiber couplers generate 3 phase-shifted signals simultaneously that can be combined with quadrature projection methods to detect magnitude and phase unambiguously. We present a novel differential detection topology which utilizes a combination of 3x3 and 2x2 couplers to enable quadrature projection with fully differential detection. We present a mathematical analysis of this 3x3 differential detection topology, extended methods for signal calibration, and SNR analysis. We characterize the SNR advantage of this approach and demonstrate a sample application illustrating simultaneous magnitude and phase imaging of a chrome-on-glass test chart.