High resolution optical reflectometry
Optical fiber sensors have become a very important intelligent monitoring method because of their low cost, small size, light weight, corrosion resistance and electromagnetic interference resistance, compared with traditional media. And they have found its prospect in many strategic fields. Due to the ease of deployment and special suitability for distributed and multi-point networked applications, distributed fiber optic sensors have been used for continuous monitoring of large critical infrastructure such as bridges, aircraft and oil pipelines. With 40 years of development, distributed optical fiber sensing technology has been made great progress in performance indicators such as detection distance, sensitivity and spatial resolution. As one of the most important techniques of optical fiber sensing, optical reflectometry is capable of fiber non-destructive detection. Usually, Raleigh scattering, Raman scattering and Brillouin scattering signals are used to capture the distribution reflectivity, refractive index and polarization state information, from which abnormal events such as fusion points, bending, cracking and corrosion can be extracted. However, because of the common restrain of receiving bandwidth and laser phase noise, the measurement distance and spatial resolution are greatly restricted.
Professor He Zuyuan’s group has been devoted in ultra-high resolution optical reflectometry to solve this problem. They have found that the measurement distance and spatial resolution are restricted by the phase noise of laser and sensing fiber, fiber chromatic dispersion and the bandwidth of receiving system. Optical frequency domain reflectometry is suitable for high resolution detection with linearly swept laser source, which is also capable of detection bandwidth compression. However, the accumulation of phase noise as relative long seeping time can not be ignored, and wideband high linearly swept laser source is also not accessible. To solve this problem, they develop a wideband seeping laser source and phase compensation technique. Firstly, they achieved a frequency multiplication of RF sweeping signal to 100 GHz with 32 times, which is realized with the higher-order sideband of the electro-optic modulation and four wave mixing, the linearity of the achieved swept sources is 300 kHz. Together with a laser phase noise compensation method, they achieved a spatial resolution of 1 mm. On the other hand, they found that the phase noise could be greatly reduced by decreasing the frequency tuning time at the cost of larger detection bandwidth. To break the bottleneck of electrical system, they combined the optical reflectometry with coherent optical sampling technique. The spatial resolution of optical time domain reflectometry and pulse compression reflectometry are respectively improved to 340 μm and 120 μm.
Fig.1 Optical multiplication technique with high-order sideband of electro-optic modulation
Fig.2 Ultra-high resolution reflectometry achieved based on coherent optical sampling
Center for Intelligent Photonics (CIP) of Shanghai Jiao Tong University currently has six teachers and more than forty master’s and doctoral students, the six teachers are respectively Prof. He Zu Yuan, Fan Xinyu, Liu Qingwen, Ma Lin, Du Jiangbing, and Zhang Wenjia. In recent years, they have carried out a series of cutting-edge problem related to reliability and consistency when putting in real applications in optical sensing and optical interconnection. These techniques have been widely adopted in communication, sensing, military, aerospace, oil and interconnection areas, which proving powerful support for scientific research and industrial production. At present, CIP has presided over the national R&D plan, the National Natural Science Foundation's Instrument Special Project, the Key International Cooperation Project, the Shanghai Science and Technology Commission Science and Technology Innovation Action Plan Project.
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.