Enhanced Optical Time Domain Reflectometer Based On Sinusoidal Gating Avalanche Single Photon Detector

Optical time domain reflectometer (OTDR) could monitor the attenuation of entire sensing fiber by measuring the power of Rayleigh back scattered light. OTDR has many advantages such as distributed, anti-electronic interference, etc. With the development of society, typical application scenarios such as optical fiber anti eavesdropping, passive optical network monitoring requires longer sensing distance, lager dynamic range and higher spatial resolution. Compared with conventional OTDRs, photon-counting OTDRs (v-OTDRs) which employs a single photon detector as the detection apparatus, offer better sensitivity, superior spatial resolution and the absence of classic dead-zones. InGaAs/InP avalanche photodiodes (SAPDs) are most commonly used as single photon detectors in v-OTDRs due to their flexibility and robustness in room temperature. But it also has drawbacks in consideration of measurement time, spatial resolution and dynamic range. As in OTDR measurements, the power of Rayleigh back scattered signal decreases with the increasing length of fiber. In this condition, the optical power is strong at the beginning of sensing fiber which could saturate optical detector. At the end of sensing fiber, the optical power is weak which is difficult to detect. Present photon-counting OTDRs works in gated mode. In this mode, the performance of single photon detector is fixed. On the other hand, the gating signal is synchronized with optical sensing pulse in gated mode. Dead zones are used to suppress the affection of detector noise, but this decrease the performance of spatial resolution. Extra measurements could overcome the side effect, but results in longer measurement time.In the paper, we develop an enhanced optical time domain reflectometer based on avalanche single photon detector in order to solve the problem of dynamic range, spatial resolution and measurement time. Compared with former works, the detector in our technology works in a GHz sinusoidal gating mode. The dead time is short while the count rate is high. This improves the in spatial resolution in optical time domain measurements while decreases measurement time. Unlike gated mode, the detector works in a gated-free mode, in which the gating signal is asynchronous to optical sensing pulse. Extra measurements are not need to suppress the affection of dead zone.The enhanced optical time domain reflectometer consists of optical sensing parts and sinusoidal gating parts. The optical sensing parts are the same with traditional optical time domain reflectometer while the sinusoidal gating parts are customized. The sinusoidal gating parts consists of avalanche single photon detector, the driver of single photon detector and the reverse voltage module. The driver of single photon detector controls the temperature and supplied current of single photon detector. The reverse voltage module supplies the reverse voltage. In this paper, we get a sinusoidal gating frequency of 1 GHz with peak to peak power of 5.5 V in sinusoidal gating module. And the single photon detector could be cooled to -50 centigrade in room temperature with a fluctuation of 0.001 centigrade. Finally, bias voltage module offers a reverse voltage from 30-70V. The system works well in long time tests.We use an InGaAs/InP detector as single photon detector. Works in a sinusoid gating mode, our detector obtained a quantum efficiency of 2.5%, dark count efficiency of 1.8×10-6 and afterpulse probability of 0.047%. Our v-OTDR system obtained a dynamic range of 75km. Two reflection events with a distance of 30cm at the end of 50 km sensing fiber are also obtained. Our system exhibit the ability to get a spatial resolution of 15cm. Moreover, a loss event of 1% could be obtained by our system. To sum up, our system exhibits a new solution for dynamic range, spatial resolution and measurement time.