Abstract

Many optical as well as fiber-optical sensors require the precise measurement of the range of a distinct optical reflection. Whereas the spatial resolution of two adjacent reflection events along the light path is determined by the spectral or modulation bandwidth of an emitted light pulse, the precision of range measurements of a single reflector can be much lower than this two-point resolution if a good signal-to-noise ratio (SNR) after optical detection is achieved. In this paper, theoretical limits defined by the Cramer-Rao Lower Bound due to thermal and shot noise in the optical detector are compared for different common modulation techniques like harmonic intensity modulation as well as optical time- and frequency domain reflectometers. These theoretical results help to decide, whether efforts for improvement of a real system should focus on signal-to-noise ratio after photo detection, or on phase noise of oscillators as well as timing jitter of data acquisition for demodulation and signal processing.

© 2018 The Author(s)