Measurement of Brillouin gain spectrum with dual-frequency laser for optical fibre sensor

In this Letter, the authors present a first measurement of Brillouin gain spectrum with dual-frequency solid-state laser. This allows a coherent detection of the Brillouin backscattered spectrum in a frequency range below the hundreds of MHz by fine-tuning of the difference between the two cross-polarised fields emitted by the laser. The photodetection at low-frequency benefits from the balanced detection technique where high dynamic range measurements are largely available. The narrow line-width of the cross-polarised field permits also the direct measurement of the Brillouin spectrum line-width. The system is demonstrated by measuring the Brillouin frequency shift with temperature when standard single-mode fibre-28 (SMF-28) and large effective area fibre (LEAF) are fully heated. This gives temperature coefficients close to theoretical expectations with values of 1.16 MHz/degrees C for SMF-28 and 1.15 MHz/degrees C for LEAF.