Fig. 1: Concept of time-expanded ΦOTDR.

a An example of a traditional coherent detection-based ΦOTDR scheme. A train of optical probe pulses (with a comb-like spectrum composed of in-phase lines) is launched into the fibre under test. The backscattered light is beaten with a continuous-wave local oscillator (LO) and photo-detected. Both the amplitude and phase of the electromagnetic field are acquired over a bandwidth BW identical to that of the launched probe. b Proposed time-expanded ΦOTDR scheme. A periodic probe signal is launched into the fibre under test. Its spectrum is a random phase-modulated optical comb. The backscattered light is beaten with an LO that is a comb with the same number of lines and identical phase modulation as those of the probe comb but with a line spacing difference δf. After photo-detection, a low-pass filter passes the comb generated by the interference of the lines of the probe with the neighbouring lines of the LO. Both the amplitude and phase of the electromagnetic field are acquired over a compressed bandwidth BW/CF, where CF is the ratio between the probe line spacing and δf. This produces a trace temporally expanded by a factor CF. In the detection stage of both figures (central dashed boxes), the signals that are involved in the product (denoted by the symbol ʘ) are electromagnetic fields