Managing Brillouin frequency spacing for temperature measurement with Brillouin fiber laser sensor

Managing Brillouin frequency spacing for temperature measurement with Brillouin fiber laser sensor is proposed and demonstrated. By managing Stokes light wave resonance of different orders, Brillouin lasers with different Brillouin frequency spacing are obtained. In the absence of EDFA, using an optical coupler with 5 km SMF, only the 1st-order (S1) with single frequency shift circulates in the ring. Using a three-port optical circulator with 5 km SMF and adding EDFA, only the 2nd-order (S2) with double frequency shift is in the ring. When the three-port circulator is replaced with a four-port, fiber under test 1 (FUT1) with 10 km SMF and FUT2 with 5 km SMF are used as Brillouin gain medium and generate the 3rd-order Stokes (S3) with triple frequency shift. Temperature is measured directly by beat frequencies between the pump and the 1st-order, 2nd-order, 3rd-order Stokes. About 1.036 MHz/degrees C, 2.006 MHz/degrees C and 3.104 MHz/degrees C sensitivity are consistent with the theoretical value and +/- 0.05 dB power fluctuation and 0.2 degrees C temperature stability keep the same with the previously reported results. It is expected to be widely used in the field of fiber optic sensing.