【摘要】 提出并验证了一种基于三环复合子腔滤波器的单纵模（SLM）掺铥光纤激光器（TDFL）。TDFL内部包含一段未泵浦的掺铥光纤（TDF），将其作为饱和吸收体（SA）与六耦合器三环（SCTR）子腔滤波器共同作用，实现谐振腔内纵模选择功能。仿真分析并比较了复合子环在三级级联和四级级联时的滤波性能，分析了2μm波段饱和吸收体的工作原理并给出了相应的参数公式，定量仿真了SA长度对带宽的影响。实验结果表明，所提复合长腔TDFL可在1942.34 nm处实现测试时长为60 min的SLM稳定激射，输出激光的光信噪比（OSNR）大于30 dB。在持续时间为30 min、光谱数据记录次数为7的稳定性测试中，输出激光的中心波长无漂移，峰值功率波动小于0.24 dB。更多还原

【Abstract】 Objective Thulium-doped fiber lasers(TDFLs) serve as promising seed sources for a variety of applications, including masteroscillator power-amplifier(MOPA) systems, free-space optical communication, and ultra-long-range integrated sensing, where stability in both central wavelength and output power is critically important. The incorporation of composite sub-ring structures and saturable absorbers(SAs) has been identified as a key strategy for achieving stable single-longitudinal-mode(SLM) operation in ringcavity TDFLs, a topic that has attracted considerable research attention. In this study, we propose and experimentally validate a longcavity ring TDFL with ultra-stable wavelength characteristics, which integrates a composite six-coupler triple-ring(SCTR) sub-cavity filter and a saturable absorber. The proposed laser exhibits outstanding output power stability and robust SLM operation. Our findings not only expand the range of feasible cavity configurations for SLM operation in TDFLs but also provide important insights into the operating mechanisms of composite sub-rings and SAs, as well as the influence of cascade stages on composite filter performance. This work offers valuable guidance for researchers in optical communications who are designing high-performance SLM rare-earthdoped fiber laser cavities, thereby contributing to the further development and application of TDFLs.Methods This research adopts an integrated theoretical and experimental approach to develop and realize a SLM thulium-doped fiber laser based on a composite sub-cavity filter and SA. The theoretical analysis employs the transfer matrix method to establish the structural configurations of both the composite SCTR and extra-cavity quadruple ring(ECQR) filters, with explicit derivation of the transmission expression for the SCTR filter. Numerical simulations are performed to characterize the transmission spectra of the SCTR and ECQR filters. We systematically investigate the operational mechanisms of composite sub-cavity structures and saturable absorbers, with particular emphasis on the longitudinal mode selection principles enabled by composite sub-cavity configurations within ring resonators. Parametric models for transient fiber gratings are developed, and the Kramers-Kronig relations are applied to construct refractive index variation models. MATLAB-based simulations computationally analyze the relationship between dynamic grating bandwidth and SA length. For experimental validation, an optical spectrum analyzer is used to measure the spectral characteristics of ultra-fine fiber Bragg gratings(UFBGs), laser center wavelength drift, and power fluctuations. A radio frequency spectrum analyzer confirms SLM operation through spectral linewidth characterization. The proposed configuration effectively suppresses mode competition while maintaining wavelength stability in high-power fiber laser systems.Results and Discussions Fig. 1 illustrates the experimental setup of the thulium-doped fiber laser incorporating a multi-ring composite sub-cavity filter. The structural schematic of the SCTR is presented in Fig. 2. Comparative analysis between Fig. 3 and Fig. 4 indicates that under similar ring parameters in cascaded composite sub-ring configurations, the SCTR demonstrates superior transmittance performance compared to the ECQR. Fig. 5 displays the reflection spectrum of the fiber Bragg grating(FBG) within the TDFL resonator cavity, showing a measured full width at half maximum of 0.269 nm. Fig. 6 presents the numerically simulated relationship between dynamic grating bandwidth and saturable absorber length. At a pump power of 5.91 W(λ=793 nm), the TDFL achieves stable single-wavelength lasing operation. Fig. 7 provides: 1) a typical single-wavelength spectrum recorded over a 30-minute period; 2) the corresponding fluctuations in central wavelength and maximum output power. As shown in Fig. 7(a), the central wavelength is stabilized at 1942.34 nm with an optical signal-to-noise ratio exceeding 30 dB. The data in Fig. 7(b), collected at five-minute intervals over 30 minutes(seven datasets total), demonstrate negligible wavelength drift and peak-power fluctuations below 0.24 dB. Fig. 8 shows the single-wavelength output spectrum across a 0-1 GHz span, revealing no observable longitudinalmode components. Throughout a 60-minute stability test with twelve measurements taken at five-minute intervals(inset of Fig. 8), no longitudinal-mode components were detected, confirming stable single-longitudinal-mode operation. Table 1 compares performance metrics of SLM thulium-doped fiber lasers based on various techniques, highlighting the exceptional wavelength and output-power stability of the proposed laser.Conclusions We have proposed and experimentally demonstrated a SLM ring-cavity TDFL operating in the 2 μm wavelength band. The laser architecture incorporates a composite ring SCTR filter, a SA, and a FBG. Comprehensive characterization of the TDFL’s output performance and operational stability confirms excellent laser characteristics. Analysis of the SCTR filter’s filtering properties combined with numerical simulations evaluating the impact of additional sub-ring stages on filter performance demonstrate that the SCTR filter exhibits superior filtering capabilities compared to the ECQR filter. The laser achieves stable single-wavelength output at 1942.34 nm, with no observable wavelength drift over a 30-minute observation period and power fluctuations maintained below 0.24 dB. The optical signal-to-noise ratio(OSNR) reaches 33.73 dB. During extended 60-minute monitoring, no significant longitudinal mode frequency components are detected within the 0-1 GHz range, verifying stable SLM operation. Future work will focus on further optimization of the experimental setup to enhance output performance and explore potential applications in coherent beam combining, wavelength-division multiplexing, and next-generation lidar systems.更多还原