Advances of key technologies on optical reflectometry with ultra-high spatial resolution
Wang Shuai, Wang Bin, Liu Qingwen, Du Jiangbing, Fan Xinyu, He Zuyuan     
State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong University, Shanghai 200240, China

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.

Supported by National Key R & D Program of China (2017YFB0405500)