# High Dynamic Range Externally Time-gated Photon Counting Optical Time-domain Reflectometry

## TL;DR

This paper introduces an externally time-gated photon counting scheme using a high-speed optical switch to enhance the dynamic range of optical time-domain reflectometry, demonstrated through theoretical analysis, simulation, and experiments.

## Contribution

It proposes a novel externally time-gated approach with a Mach-Zehnder interferometer to improve PC-OTDR dynamic range, validated by experiments at 800 nm wavelength.

## Key findings

- Achieved 30.0 dB dynamic range over 70 meters with 50 ns gating.
- Improved dynamic range by 11.0 dB compared to no gating.
- Successfully detected a 0.37 dB loss event with gating.

## Abstract

Single photon detector (SPD) has a maximum count rate due to its dead time, which results in that the dynamic range of photon counting optical time-domain reflectometry (PC-OTDR) de-creases with the length of monitored fiber. To further improve the dynamic range of PC-OTDR, we propose and demonstrate an externally time-gated scheme. The externally time-gated scheme is realized by using a high-speed optical switch, i.e. a Mach-Zehnder interferometer, to modulate the back-propagation optical signal, and to allow that only a certain segment of the fiber is monitored by the SPD. The feasibility of proposed scheme is first examined with theoretical analysis and simulation; then we experimentally demonstrate it with our experimental PC-OTDR testbed operating at 800 nm wavelength band. In our studies, a dynamic range of 30.0 dB is achieved in a 70 meters long PC-OTDR system with 50 ns external gates, corresponding to an improvement of 11.0 dB in dynamic range comparing with no gating operation. Furthermore, with the improved dynamic range, a successful identification of a 0.37 dB loss event is detected with 30-seconds accumulation, which could not be identified without gating operation. Our scheme paves an avenue for developing PC-OTDR systems with high dynamic range.

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Source: https://tomesphere.com/paper/1907.05948