Noise in optical low-coherence reflectometry

It is theoretically and experimentally shown that Fresnel end reflection at a waveguide under test can degrade the sensitivity of an optical low-coherence reflectometer (OLCR) with a balanced detection scheme. Optical mixing of the local oscillator (LO) light and the end reflection produces beat noise whose current noise spectral density is represented by 2(1+P²)⟨I₁⟩⟨I₂ ⟩/δν, where P and δν are the degree of polarization and the effective linewidth of the light, respectively, and ⟨I₂⟩ and ⟨I₂⟩ are the total mean photocurrents of the LO light and the end reflection at the balanced mixer, respectively. The balanced detection technique suppresses the intensity noise of the light and the beat noise becomes the dominant source of sensitivity degradation. The minimum detectable reflectivity is derived which includes the effect of sensitivity degradation caused by beat noise