# Distributed vibration sensing based on forward transmission and coherent detection

## TL;DR

This paper introduces a new ultra-long distributed vibration sensing system using forward transmission and coherent detection, achieving high SNR and long-range localization of vibrations over 1008 km of fiber.

## Contribution

The novel sensing scheme combines forward transmission with coherent detection for ultra-long range vibration detection and localization, surpassing traditional backward scattering methods.

## Key findings

- Achieved over 50dB sensing SNR after long-haul transmission.
- Demonstrated localization of vibrations at 400 Hz, 1 kHz, and 10 kHz.
- Sensed vibrations with less than 50 m spatial resolution over 1008 km.

## Abstract

A novel ultra-long distributed vibration sensing (DVS) system using forward transmission and coherent detection is proposed and experimentally demonstrated. In the proposed scheme, a pair of multi-span optical fibers are deployed for sensing, and a loop-back configuration is used by connecting the two fibers at the far end. The homodyne coherent detection is used to retrieve the phase and state-of-polarization (SOP) fluctuations caused by a vibration while the localization of the vibration is realized by tracking the phase changes along the two fibers. The proposed scheme has the advantage of high signal-to-noise ratio (SNR) and ultra-long sensing range due to the nature of forward transmission and coherent detection. In addition, using forward rather than backward scattering allows detection of high frequency vibration signal over a long sensing range. More than 50dB sensing SNR can be obtained after long-haul transmission. Meanwhile, localization of 400 Hz, 1 kHz and 10 kHz vibrations has been experimentally demonstrated with a spatial resolution of less than 50 m over a total of 1008 km sensing fiber. The sensing length can be further extended to even trans-oceanic distances using more fiber spans and erbium-doped fiber amplifiers (EDFAs), making it a promising candidate for proactive fault detection and localization in long-haul and ultra-long-haul fiber links.

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Source: https://tomesphere.com/paper/1907.07675