Overview: As the core technology of distributed fiber-optic sensing, optical reflectometry may realize the non-destructive measurement at a remote position. It can be used to retrieve the distributed information such as reflectance, refractive index, polarization state along the optical fiber, and to diagnose the irregular "event" on fiber-optic links. In this paper, we summarized the research status on state-of-art optical reflectometry technologies, and reviewed the advances of key technologies on optical reflectometry with ultra-high spatial resolution and long measurement range. We proposed three different methods to improve the performance, and tried to promote their applications on distributed fiber-optic sensing systems.
Firstly, we propose and demonstrate a millimeter-resolution long-range optical frequency domain reflectometry (OFDR) using an ultra-linearly 100 GHz swept optical source realized by injection-locking technique and cascaded four-wave-mixing (FWM) process. The ultra-linear swept source is realized using an external modulation method with a linearly swept radio frequency (RF) signal. By using the injection-locked frequency swept laser as the optical source of OFDR, we obtain a spatial resolution of 4.2 mm over 10 km measurement range. To further improve the spatial resolution, FWM process is used to broaden the frequency sweeping span. A frequency sweeping span of ~100 GHz is achieved with cascaded FWM. We demonstrate a 1.1 mm spatial resolution over 2 km measurement range with the proposed ultra-linearly swept optical source.
Then, we demonstrate an ultra-high-resolution optical time domain reflectometry (OTDR) system by using a mode-locked laser as the pulse source and a linear optical sampling technique to detect the reflected signals. Taking advantage of the ultrashort input pulse, the large detection-bandwidth, as well as the low timing jitter of linear optical sampling system, a sub-mm spatial resolution is achieved. As the pulse-width is broadened with the increase of distance due to the chromatic dispersion and the large bandwidth of the ultrashort pulse, by adopting digital chromatic dispersion compensation, we achieved a spatial resolution of 340 mm when measuring the reflector at 10 km.
The final method is based on linear optical sampling and pulse compression method. We propose an all-optic sub-THz-range linearly chirped optical source and a high-bandwidth detection system to characterize it. Taking advantage of the chromatic dispersion effect, ultrashort optical pulses are stretched to be ~10-ns linearly chirped pulses with sub-THz range, which yields a large time-bandwidth product of 4500, a high compression ratio of 4167 and a chirp rate of 45 GHz/ns. A ultra-high spatial resolution of 120 μm with 150 m measurement range and 20.4 dB extinction ration is finally demonstrated.