Single-frequency fiber laser is a high-precision single-longitudinal mode ultra-narrow linewidth laser source based on semiconductor laser pumping, rare earth doped fiber as gain medium and relatively complex control technology, which can provide low phase noise and relative intensity noise. , extremely narrow spectral line width and long coherence length. At present, mainstream single-frequency fiber laser technology is mainly divided into four types:
1. Short straight cavity DFB type single frequency fiber laser: The advantage is that the resonant cavity has a simple structure and is easy to realize single longitudinal mode output. The disadvantage is that the 1/4 wavelength phase shift fiber grating requires high technical difficulty, and it is necessary to select a suitable rare earth doped fiber, and the phase shift grating package requires high requirements;
2, short straight cavity DBR type single frequency fiber laser: high doping concentration of phosphate glass fiber can achieve higher gain, short cavity design can achieve higher power output. However, it is difficult to achieve high-strength welding, the reliability of the cavity is poor, and the grating used for frequency selection is required to be high. In addition, short straight cavities are difficult to manufacture and difficult to adapt to harsh working environments;
3, composite cavity technology: also known as "virtual ring cavity technology", with the basic structure of the linear cavity, the fiberscope constitutes a straight cavity cavity mirror, essentially still a short straight cavity structure, the FSR determined by the cavity length is still The key to obtaining a single longitudinal mode requires a short cavity length. The virtual annular cavity technology combines the idea of short straight cavity and annular cavity, and realizes the traveling wave operation in the standing wave cavity. From the technical point of view, it belongs to a technology that is excellent at present, but the technical process is difficult to implement and lacks. Reproducible and costly;
4, ring cavity and composite ring cavity technology: Shanghai Haoyu launched the CoSF-R & CoSF-D series of products. Among them, the CoSF-R single-frequency fiber laser adopts the "optimized traveling wave cavity" design to eliminate the standing hole spatial hole burning effect, and the long cavity length design allows the longer gain fiber to fully absorb the pump light. High power output can be realized; the relative intensity noise and phase noise of the fiber laser can be greatly reduced when the traveling wave is running, and the ultra-narrow line width is <1 kHz; the all-fiber narrow-band filter is injected to select the single longitudinal mode output; the long saturated absorber is stable. Longitudinal mode operation; single polarization control eliminates polarization hole burning effect and achieves stable single polarization output with polarization extinction ratio of up to 25dB. CoSF-D type DFB narrow linewidth single-frequency fiber laser adopts short straight cavity distributed feedback (DFB) design, high quality active fiber phase-shifted fiber grating for stable single longitudinal mode operation, single frequency, single polarization, single vertical Mode output. And the phase noise and relative intensity noise is lower, the patented anti-environment packaging technology allows the product to be used in 1.0um, 1.5um, 1.9-2.1um bands.