Citation: | Wang Shuai, Wang Bin, Liu Qingwen, et al. Advances of key technologies on optical reflectometry with ultra-high spatial resolution[J]. Opto-Electronic Engineering, 2018, 45(9): 170669. doi: 10.12086/oee.2018.170669 |
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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.
The schematic illustration of the spectrum for the frequency sweep with high-order sidebands of external modulation[17]
Experimental setup of the injection-locking scheme. FL: fiber laser; IM: intensity modulator; VOA: variable optical attenuator; PC: polarization controller; DFB: distributed feedback diode laser; Amp: RF amplifier; AWG: arbitrary waveform generator; FUT: fiber under test; BPD: balanced photodetector; A/D: analog-to-digital converter[17]
(a) Optical spectrum of the slave laser which is injection locked to the 8th-order sideband of the master laser, and the inset is spectrum of the generated optical comb after IM; (b) Relative optical frequency as a function of time after the injection-locking[17]
Experimental result of OFDR system. (a) Reflection trace; (b) Details of reflection peak around 10 km after using phase noise compensation (PNC) algorithm[17]
Schematic of FWM[17]
Relative optical frequency changes as a function of time[17]
Measured reflection trace, and the inset shows the details of reflection peak at the end of the fiber under test (FUT) [17]
Experimental illustration of linear optical sampling[18]
Spatial resolution at different distances[18]
(a) Reflection peak without chromatic dispersion (CD) compensation; (b) Time-frequency map of the reflection peak without CD compensation; (c) Reflection peak with CD compensation; (d) Time-frequency map of the reflection peak with CD; (e) Details of the reflection peak at 10 km with/without CD compensation[18]
Experimental results. (a) A temporal frame of 10 ns linearly chirped pulse; (b) Shot time Fourier transformation (STFT) analysis of the linearly chirped pulse; (c) Calculated autocorrelation of the linearly chirped pulse[19]
Experimental results. (a) 20 ns measurement of linearly chirped pulse with phase modulation; (b) STFT analysis of the linearly chirped pulse with phase modulation; (c) Calculated autocorrelation of the 20 ns linearly chirped pulse with phase modulation; (d) Autocorrelation of the 1544 ns time-aperture pulse[19]