Spatial resolution enhancement in coherent optical frequency domain reflectometer by nonlinear frequency sweep suppression

Coherent Optical Frequency Domain Reflectometry (C-OFDR) is a promising technique for high-resolution metrology that can find applications also in DOFS (Distributed Optical Fibre Sensors). In the basic configuration, the coherent OFDR consists of a tunable laser source (TLS) whose frequency can be swept continuously in time without mode hops and an optical interferometer comprising a reference path and a measurement path. The device under test (DUT) is connected to the measurement path whereas the reference path is used as local oscillator. The interference signal between the reference signal from reference path and different reflections coming from the DUT are electrically detected and Fourier transformed allowing the visualization of beat frequencies. If the optical frequency of the TLS is modulated at a constant rate, beat frequencies are proportional to the optical path differences between the reflections in the DUT and the reference path. The critical point of OFDR implementations is the requirement for sophisticated optical sources providing a fast and linear frequency tuning over a broad frequency range. However, the available lasers exhibit in practice fluctuations in their optical frequency tuning rates. Due to these nonlinear tuning characteristics, sampling of the interference signal with a constant spacing in time gives rise to a non-uniform sampling in optical frequency which, in turn, degrades the spatial resolution of the OFDR measurement This problem can be avoided by sampling the interference signal at equidistant instantaneous optical frequency points rather than equally spaced time intervals. In this paper we analyze the tuning characteristics of a commercial external cavity laser (ECL). We use this analysis then to suppress nonlinear frequency sweep in an OFDR and demonstrated highly effective spatial resolution enhancement of more than 30 times.