At present, the most urgent need for single-frequency narrow linewidth lasers is the development of optical fiber sensor technology.With the increase of social needs and the development of optical fiber sensor technology itself, a large number of optical fiber sensors have gradually become practical.Many of these fiber optic sensors require a single-frequency narrow linewidth laser source.Using single-frequency narrow linewidth fiber lasers for optical fiber sensors can meet the needs of optical coherent detection. On the one hand, it is a basic method to realize the demodulation of phase or frequency modulation physical quantities, such as Brillouin scattered light time domain reflectometer BOTDR and interferometer Fiber optic sensors; on the other hand, can improve the signal-to-noise ratio of the sensing system, improve the dynamic range and spatial resolution of the sensing system, such as coherent optical time domain reflectometry COTDR, optical frequency domain reflectometry OFDR and Brillouin scattering Optical time domain reflectometer BOTDR and other application requirements.

   Currently, the most typical application examples of single-frequency narrow linewidth fiber lasers are OFDR technology using optical frequency linear modulation and Brillouin scattered light time domain reflectometry (BOTDR) technology.Figure 1 shows the typical composition structure of OFDR based on optical frequency linear modulation.The measurement system uses a narrow linewidth laser to output optical frequency linearly modulated light. The beam is divided into two paths. Part of the light is coupled into the reference arm and reflected back to the fiber coupler to form the local oscillation (LO) reference light in heterodyne coherent detection.Another part of the light is coupled into the sensing fiber, the measuring arm.The backscattered light and reference light generated in the sensing fiber are mixed in the fiber coupler to generate a beat signal, which is photoelectrically converted. The beat frequency is proportional to the sensing distance, and the measured value can be obtained from the beat frequency. distance information, while the beat frequency signal strength reflects the size of the sensing physical quantity.The sensing system uses optical coherent detection. On the one hand, the local reference beam can be used to amplify the backscattered signal, which can greatly improve the detection sensitivity. Currently, it can reach a high receiving sensitivity of -90dBm; on the other hand, any form added to the sensitive optical fiber Disturbances, such as pressure, temperature, sound, and vibration, may cause changes in the polarization state, frequency, or phase of the backscattered light, which will cause the beat frequency signal to change, so the sensing of these physical quantities can be realized.For example, a distributed optical fiber vibration sensor system can be built using a linearly swept narrow linewidth fiber laser to detect, locate and resolve vibration disturbance signals over a distance of 10 kilometers. This sensing system can be used for nuclear power plants, oil and gas pipelines, military bases and Provide effective security guarantees in border areas.

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