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A novel scheme using multi-wavelength coherent detection for SNR enhancement in a Brillouin optical time domain reflectometer (BOTDR) is presented. The probe pulse of the BOTDR contains multiple wavelengths with small equal-wavelength spacing, and the Brillouin scattering light waves of the multiple wavelengths beat with the corresponding local reference light waves with the same equal spacing to generate overlapped multiple Brillouin scattering spectra. The overlapped scattering spectra can be detected by a conventional photodetector. A three-wavelength heterodyne detection BOTDR is demonstrated and it provides 4.2 dB SNR enhancement compared to a conventional single wavelength heterodyne detection system.Introduction: The Brillouin optical time domain reflectometer (BOTDR) can be used to simultaneously measure distributed temperature and strain by accessing only one end of the fibre under test. The signal-noise-ratio (SNR) of a BOTDR, which determines the dynamic range and measurement accuracy of the BOTDR [1], is limited by the intrinsic attenuation and unexpected nonlinear effects of optical fibre. To improve the SNR of a BOTDR, Er-doped fibre amplifier (EDFA), Raman amplification, coding probe pulse and coherent detection were used alone or in combination to amplify the weak Brillouin backscattered light [1 -4]. In a existing single-wavelength coherent detection BOTDR, the SNR can be improved by increasing the power of the local oscillator (LO). However, when the LO is strong enough, the SNR no longer depends on the LO power, but is linearly proportional to the Brillouin signal power [5]. Therefore, it is the enhancement of Brillouin signal power that plays a more important role in the improvement of the SNR of the BOTDR.In this Letter, a new scheme using multi-wavelength heterodyne detection is proposed to improve the SNR of the BOTDR. The scheme combines the advantages of high probe power and coherent detection, and can achieve high sensing efficiency in the same measurement time compared with the conventional single-wavelength BOTDR.

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