| Fiber lasers have excellent beam quality,low operating threshold and compact structure,show the characteristics of integration and precision,and have become an important branch of lasers.Among them,single longitudinal mode(SLM)narrow linewidth fiber lasers show excellent performances such as ultra-low noise interference,ultra-high monochromaticity and coherence in practical application scenarios,and have received extensive attention from researchers.The laser with ring cavity structure can eliminate standing waves and avoid the appearance of space hole burning effect;select a longer cavity to narrow the linewidth to a certain extent;use a longer gain fiber to obtain higher power output.However,lasers with ring cavity are prone to form multi-longitudinal mode oscillations,which greatly affect the performance of lasers.This paper focuses on the performance requirements of lasers in the two research fields of quantum secure communication and fiber-based time-frequency transfer systems.In view of the technical pain points of single-frequency stability and narrow linewidth that ring cavity lasers need to solve,two types of ring cavity lasers are designed.The main research contents and innovations are as follows:1.In order to meet the requirements of lasers for quantum secure communication,an S-band SLM narrow linewidth fiber laser based on passive multiple-subring resonator is proposed and experimentally verified.The design principle is:based on the traveling wave ring cavity,a stable single-frequency laser output with good performance is realized under the combined action of fiber Bragg grating,passive multi-ring auxiliary cavity and saturable absorber.The designed passive multiple-subring resonator structure constitutes a narrow-band filter with good mode selection performance,expands the equivalent free spectral range,and eliminates dense longitudinal modes.The output optical signal noise ratio(OSNR)of the proposed laser is about 70 dB,the center wavelength is 1499.77 nm,and the single-frequency operation of the S-band is obtained.In addition,the linewidth of Lorentz fitting is about 763 Hz,and the wavelength jitter is 0.04 nm,realizing narrow linewidth and wavelength-stable output.As a frequency-stabilized light source in the S-band,the laser has been tested in the quantum secure communication system,providing important support for the implementation of the quantum secure communication project.2.In order to meet the special requirements for narrow linewidth lasers in time-frequency synchronization systems based on fiber transmission,a sub-kHz fully polarization-maintaining erbium-doped fiber laser based on crossed-double-ring passive subring resonator is proposed and experimentally verified.The laser adopts full polarization-maintaining devices,and forms two double-coupled fiber loops through the crossed-double-ring passive subring resonator structure.This structure can function as a precision filter,improve the construction efficiency of the coupler,effectively eliminate dense longitudinal modes,and realize stable single frequency output.The output OSNR of this laser is about 70 dB,and the center wavelength is 1550.06 nm.After Lorentz fitting,its linewidth is about 278 Hz,which can meet the needs of fiber-based high-precision time and frequency transmission systems.In this paper,two novel fiber laser schemes are proposed for the requirements of quantum secure communication and fiber-based time-frequency transfer system respectively,and the feasibility and performance advantages of the proposed schemes are verified by experiments.The two fiber lasers developed in the paper have been used in corresponding scientific research projects,which are of great significance and value to the research on quantum communication and fiber-based time-frequency transfer. |
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