## 1 Introduction  
Optical time domain reflectometry (OTDR) is a common technique for measuring the optical fiber length, attenuation and insertion loss of connectors, splices and breakpoints. It operates by injecting probe optical pulses into the fiber under test (FUT) and successively detecting the backscattered optical power with the round-trip time [1, 2]. OTDR is widely used in passive optical networks (PONs) for fault detection, localization and identification [3, 4]. With the rapid development of PONs, OTDR methods with high dynamic range and spatial resolution are crucial for their monitoring. There are three main ways of improving the OTDR dynamic range: (1) increasing the energy of the probe optical pulse, (2) improving the receiver sensitivity [5, 6, 7, 8], and (3) adopting pulse coding techniques [9, 10, 11]. The first one can be actuated in two manners. One involves using a high-power laser but incurs high costs, and the increased optical power injected into the FUT would cause nonlinear effects, such as stimulated Brillouin scattering [12] and modulation nonlinearities [13], affecting the detection of backscattered light. The other involves increasing the duration of the forward probe pulse, but this would deteriorate the spatial resolution, affecting the OTDR performance and fault localization accuracy. The second approach is mainly realized using cryogenic detectors, photon counters and coherent detection. Cryogenic detectors and photon countersare not suitable for practical applications due to their costs and cooling requirements [14]. Therefore, coherent detection seems the most practical method to achieve high receiver sensitivity. Unfortunately, its detection distance is limited by factors such as the coherence length of the light source, phase noise and polarization state of the backscattered light [15]. In the third approach, the optical pulse is encoded and successively injected into FUT as a probe signal. The backscattering curve is derived from the received backscattered signal via the correlation operation, which is equivalent to injecting a narrower probe pulse with the same energy. However, given the logarithmic relation between the encoding gain and length of the encoding OTDR, it is useless to keep increasing the encoding length above 256 bits to improve the encoding gain [16]. In the conventional OTDR, the spatial resolution is determined by the pulse width. The optical frequency domain reflectometry (OFDR) based on the Fourier transform has been proposed to solve this problem [17; 18]; a tunable light source is employed to generate a probe signal with a periodic optical linear frequency modulation (LFM), and the information on FUT can be obtained by analyzing the backscattered light in the frequency domain. The OFDR spatial resolution is determined by the sweep frequency range of the LFM signal rather than the pulse width. Therefore, OFDR can overcome the conflict between spatial resolution and pulse width of the conventional OTDR. However, there are also some problems with OFDR using coherent detection. Firstly, the OFDR detection distance is usually limited to only a few hundred meters because of the coherence length and phase noise of the laser source and the randomly changing polarization state of the backscattered light [19; 20]. Secondly, improving the OFDR performance usually requires the use of a laser with small phase noise and narrow linewidth, but such lasers are generally characterized by large volumes, low output optical powers and high costs. Recently, based on the concept of LFM pulse compression radar, an optical pulse compression reflectometry (OPCR) has been proposed [21]. The OPCR uses optical probe pulses with LFM and then uses a matched filter to realize pulse compression on the received backscatter trace. As with the OFDR, the spatial resolution of the OPCR is also only determined by the sweep frequency range of the LFM signal.