The linewidth of a single-frequency laser is a concept in the frequency domain, which can be understood as the linewidth of the spectrum. From the description of the phase noise of the single-frequency laser pointer, it can be concluded that the phase noise can directly reflect the linewidth of the single-frequency laser pointer. The linewidth test of a single-frequency laser is usually carried out by a delayed self-heterodyne unbalanced M-Z interferometer. The linewidth test is related to the integration time (or the length of the delayed fiber).
Wavelength stability and frequency drift are intuitive manifestations of single-frequency laser performance. Because the single-frequency fiber laser has a long resonant cavity length, it uses fiber grating, a key component sensitive to temperature, strain and vibration, and its wavelength stability or frequency drift is a challenge.
Comparison of the advantages and disadvantages of four mainstream single-frequency fiber lasers. The current mainstream single-frequency fiber lasers are mainly divided into two categories: short straight cavity type and ring cavity type. Among them, the single-frequency fiber laser with short straight cavity structure includes distributed feedback type and distributed Bragg reflection type; while the single-frequency fiber laser with ring cavity structure has many configurations, and currently more mature and commercially available technologies include "virtual ring cavity" technology and Two kinds of "optimized traveling wave cavity" technology (represented by Shanghai HannStar).
Distributed feedback (DFB) single-frequency fiber laser has the advantages of simple structure and easy realization of single longitudinal mode output; strong anti-interference ability; low cost, suitable for batch applications. Its core technology is to write phase-shift fiber gratings inside the active fiber core doped with rare earths. There is a certain distance between two segments of uniform-period fiber Bragg gratings to achieve a specific phase shift, such as 1/4 wavelength. Or 1/2 wavelength. Through the excitation of pump light, the output of single frequency laser is obtained. The phase-shifted active fiber grating is used as the gain medium of the resonant cavity, and at the same time as the ultra-narrowband filter with single longitudinal mode output.
The shortcomings of this scheme are also obvious, including 1/4 wavelength phase-shift fiber grating requires high technical difficulty, the need to select appropriate rare-earth-doped fiber, and the higher requirements for phase-shift grating packaging. Distributed Bragg reflection type single-frequency fiber laser pointer achieves higher gain through high doping concentration phosphate glass fiber, and short straight cavity design can achieve higher power output. However, it is difficult to achieve high-strength welding, the reliability of the resonant cavity is poor, and the requirements for the frequency-selective grating are high. In addition, the short straight cavity is difficult to manufacture and it is difficult to adapt to the harsh working environment.
The virtual ring cavity technology is also called "composite cavity technology". It has the basic structure of a linear cavity. The cavity mirror of a straight cavity formed by fiber gratings is essentially a short straight cavity structure. The FSR determined by the cavity length is still a single longitudinal mode. The key is that the cavity length is shorter. The virtual ring cavity technology combines the ideas of a short straight cavity and a ring cavity, and realizes traveling wave operation in a standing wave cavity. From a technical point of view, it is a relatively excellent technology at present, but the technical process is difficult to achieve, and it lacks Reproducible and expensive.
The characteristic of the optimized traveling wave cavity technology is that while eliminating the effect of standing wave space hole burning, the longer gain fiber can fully absorb the pump light through the design of long cavity length to achieve high power output. At the same time, the relative intensity noise and phase noise of the fiber green laser pointer can be greatly reduced during traveling wave operation, and an ultra-narrow linewidth of <1kHz can be achieved.