Coherent Phase-OTDR Operating at 850 nm Wavelength for Enhancing SNR

We propose a distributed vibration sensor operating at the wavelength of 850-nm. Because of the λ−4 dependence of Rayleigh scattering, operation at 850-nm provides a signal-to-noise ratio (SNR) enhancement of the backscattered signal, being maximum at the pulse input section (about 7 dB), while decreasing along the fiber in virtue of the higher loss at 850 nm. As dynamic range, spatial resolution, and SNR are closely related parameters in phase-OTDR operation, this wavelength-driven enhancement may be traded-off to increase the spatial resolution, and/or reduce the measurement time. In our tests, carried out using a heterodyne coherent detection scheme, we demonstrate the successful detection of vibrations up to 5 kHz.