JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 16, AUGUST 15, 2012 2583 Photon-Counting Optical Time-Domain Reflectometry Using a Superconducting Nanowire Single-Photon Detector Junhui Hu, Qingyuan Zhao, Xuping Zhang, Labao Zhang, Xiaodong Zhao, Lin Kang, and Peiheng Wu Abstract—A novel photon-counting optical time-domain reflec- tometry ( -OTDR) based on superconducting nanowire single- photon detector (SNSPD) is proposed and demonstrated ex- perimentally. Benefiting from the low noise equivalent power (NEP), high repetition rate and low timing jitter of the SNSPD, our -OTDR system achieves a dynamic range of 22 dB after measurement time of 15 minutes. This obtainable dynamic range corresponds to a sensing length of 110 km. The system exhibits 6.0 cm spatial resolution at the end of 2 km and 1.1 m spatial reso- lution at the end of 26 km standard single-mode fiber. Considering the performance we obtained now and the increasing improve- ment of the fabrication technology, the SNSPD is promising in the field of fiber sensors. Index Terms—Optical fiber sensors, optical time-domain reflectometry, photon counting, superconducting nanowire single-photon detector. I. INTRODUCTION O PTICAL TIME-DOMAIN REFLECTOMETRY (OTDR) is one of the most successful diagnostic tools for nondestructively measuring the attenuation of a fiber link and locating the discrete defects on optical fiber systems [1]. Generally, the commercial OTDR systems adopt analogue detectors such as p-i-n or avalanche diodes (APDs) as detection unit. Their dynamic range and spatial resolution are limited by the sensitivity and response time of detectors. To enhance the performance, photon counting OTDR ( -OTDR) was proposed and demonstrated experimentally [2]–[7]. In these experiments, Ge or InGaAs/InP avalanche photodiode detectors (APDs) [2]–[4], [6] or silicon-based APDs [5], [7] were used and the results showed either centimeter spatial resolution or very large dynamic range at the telecommunication wavelengths. The advantages and limitations of InGaAs/InP APD-based Manuscript received February 03, 2012; revised April 15, 2012, June 04, 2012; accepted June 04, 2012. Date of publication June 08, 2012; date of current version July 04, 2012. This work is supported by the National Basic Research Program of China under grant No. 2010CB327803 and National Natural Sci- ence Foundation of China under Grant 61027017 and Grant 60644001. J. Hu now is with the Institute of Optical Communication Engineering, Nan- jing University, Nanjing, 210093, China and also with Guangxi Normal Univer- sity, Guilin, 541004, China. X. Zhang and X. Zhao are with Institute of Optical Communication Engi- neering, Nanjing University, Nanjing 210093, China (e-mail: xpzhang@nju. edu.cn). Q. Zhao, L. Zhang, L. Kang, and P. Wu are with Research Institute of Super- conductor Electronics (RISE), Nanjing University, Nanjing 210093, China. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2012.2203786 -OTDR have been systematically discussed [6]. The In- GaAs/InP APD-based -OTDR exhibited the dynamic range with 10 dB larger than that of conventional state-of-the-art OTDR, as well as the better two-point resolution with en- hancement factor of 20. However, Ge or InGaAs/InP-APDs have to operate under the Geiger-mode to reduce the afterpulse effect which significantly distorts the OTDR data. Therefore, it results in a complex control system and long measurement time [2]–[4]. Si-APD permits nongated mode operation with negligible afterpulse probability. Unfortunately, Si-APDs are not sensitive at the telecommunication wavelengths. One has to convert the photon near 1550 nm into visible light zone based on the principle of frequency up-conversion, and then detects this photon with Si-APDs [5], [7]. This method increases the instability and complexity of the system. Superconducting nanowire single photon detector (SNSPD) has been developed in the last decade. Due to the high sensitivity, high repeating rate, low dark count rate (DCR) and high system detection efficiency (SDE), SNSPD is a promising candidate in many applications such as quantum cryptography, high speed optical communications monitoring, biomolecule fluorescence detection and integrated circuit analysis. Recently, the -OTDR based on SNSPD was used to ascertain the photon leakage at the splicing points and the fiber joints for quantum key distribution (QKD) system [8]. The -OTDR illustrated there was used to confirm the bending positions and the leakage points, and thus to estimate the crosstalk between parallel fibers. More recently, a single-mode fiber-optic distributed Raman sensor based on SNSPD was successfully demonstrated for absolute tempera- ture measurement [9]. This experiment achieved the order of centimeter resolution at the sensing range of 2.8 m. In this paper, we experimentally demonstrate a high perfor- mance -OTDR based on SNSPD. We focus on the dynamic range and spatial resolution which determine how long and how precise the OTDR could test, respectively. The SNSPD-based -OTDR exhibits a dynamic range of 110 km under 100 m spa- tial resolution. By using the low timing jitter (TJ) SNSPD and short laser pulse (46.1 ps), we reach a spatial resolution of 6.0 cm at the end of 2 km sensing distance, which is much smaller than the resolution of most commercial OTDR instruments. II. CHARACTERISTICS OF SUPERCONDUCTING SINGLE PHOTON DETECTOR A. Noise Equivalent Power Both SDE and DCR are critical parameters of single-photon detectors for practical applications. They determine the noise 0733-8724/$31.00 © 2012 IEEE