A single laser ring BOTDA system based on Microwave external modulation is proposed. The light source is divided into two channels by coupler or two fiber pairs are used to realize single end incident. In this way, the equipment is simplified, the measurement time is reduced and the measurement accuracy is high.
The system uses a narrow linewidth laser with 1550nm working wavelength. The light source is divided into two channels through a 3 dB coupler. One of the optical signals is modulated by an electro-optic modulator (eom1) and amplified by an erbium-doped fiber amplifier (EDFA). The other optical signal of the coupler is modulated by an electro-optic modulator (eom2) to produce a frequency shift of about 11 GHz. When the frequency difference of the two paths of light in the fiber is consistent with the Brillouin frequency shift of the fiber, the stimulated Brillouin scattering is the strongest. The probe light enters the photodetector through the circulator, and then the superposition average and spectrum fitting are carried out by the high-speed data acquisition equipment (A / D) and the computer. The frequency difference corresponding to the maximum Brillouin gain of each section of the optical fiber can be determined. The frequency difference is equal to the Brillouin frequency shift on each section of the optical fiber, Therefore, it is possible to determine the temperature and strain linearly related to Brillouin frequency shift, so as to realize the distributed measurement of strain or temperature.