可调谐单频光纤激光器具有调谐范围宽、光信噪比高、线宽窄、噪声低和兼容性好等特点，在光谱学、光学探测、光学传感、光纤通信等领域有着重要的应用价值，引起了国内外研究者的广泛关注。简单介绍了可调谐单频光纤激光的调谐和选模关键技术，对1.0、1.5、2.0 μm和中红外等不同波段的可调谐单频光纤激光器进行了总结与归纳，综述了其国内外研究现状，并展示了其在调谐范围、激光线宽、光信噪比、输出功率、输出功率平坦度等性能指标方面取得的成果。此外，结合笔者课题组近年来在可调谐单频光纤激光器方面的研究工作，介绍了基于复合腔结构实现可调谐单频光纤激光的最新进展，并展望了可调谐单频光纤激光器的未来发展趋势。

In this paper, a technique combining cascaded energy-transfer pumping (CEP) method and master-oscillator power-amplifier (MOPA) configuration is proposed for power scaling of 1.6-μm-band single-frequency fiber lasers (SFFLs), where the Er3+ ion has a limited gain. The CEP technique is fulfilled by coupling a primary signal light at 1.6 μm and a C-band auxiliary laser. The numerical model of the fiber amplifier with the CEP technique reveals that the energy transfer process involves the pump competition and the in-band particle transition between the signal and auxiliary lights. Moreover, for the signal emission, the population density in the upper level is enhanced, and the effective population inversion is achieved thanks to the CEP. A single-frequency MOPA laser at 1603 nm with an output power of 52.6 W and an improved slope efficiency of 30.4% is experimentally obtained through the CEP technique. Besides, a laser linewidth of 5.2 kHz and a signal-to-auxiliary laser ratio of 60.7 dB are obtained at the maximum output power. The proposed technique is anticipated to be promising for increasing the slope efficiency and power scaling for fiber lasers operating at L band.

Nd³⁺-doped fiber lasers at around 900 nm based on the ⁴F_3/2 → ⁴I_9/2 transition have obtained much research attention since they can be used as the laser sources for generating pure blue fiber lasers through the frequency doubling. Here, an all-fiber laser at 915 nm was realized by polarization-maintaining Nd³⁺-doped silica fiber. A net gain per unit length of up to 1.0 dB/cm at 915 nm was obtained from a 4.5 cm fiber, which to our best knowledge is the highest gain coefficient reported in this kind of silica fiber. The optical-to-optical conversion efficiency varies with the active fiber length and the reflectivity of the output fiber Bragg grating (FBG), presenting an optimal value of 5.3% at 5.1 cm fiber length and 70% reflectivity of the low reflection FBG. Additionally, the linear distributed Bragg reflector short cavity was constructed to explore its potential in realizing single-frequency 915 nm fiber laser. The measurement result of longitudinal-mode properties shows it is still multi-longitudinal mode laser operation with 40 mm laser cavity. These results indicate that the Nd³⁺-doped silica fiber could be used to realize all-fiber laser at 915 nm, which presents potential to be the seed source of high-power fiber laser.

Bismuth (Bi)-doped photonic materials, which exhibit broadband near-infrared (NIR) luminescence (1000–1600 nm), are evolving into interesting gain media. However, the traditional methods have shown their limitations in enhancing Bi NIR emission, especially in the microregion. Consequently, the typical NIR emission has seldom been achieved in Bi-doped waveguides, which highly restricts the application of Bi-activated materials. Here, superbroadband Bi NIR emission is induced in situ instantly in the grating region by a femtosecond (fs) laser inside borosilicate glasses. A series of structural and spectroscopic characterizations are summoned to probe the generation mechanism. And we show how this novel NIR emission in the grating region can be enhanced significantly and erased reversibly. Furthermore, we successfully demonstrate Bi-activated optical waveguides. These results present new insights into Bi-doped materials and push the development of broadband waveguide amplification.

A noise-sidebands-free and ultra-low relative intensity noise (RIN) 1.5 μm single-frequency fiber laser is demonstrated for the first time to our best knowledge. Utilizing a self-injection locking framework and a booster optical amplifier, the noise sidebands with relative amplitudes as high as 20 dB are completely suppressed. The RIN is remarkably reduced by more than 64 dB at the relaxation oscillation peak to retain below 150 dB/Hz in a frequency range from 75 kHz to 50 MHz, while the quantum noise limit is 152.9 dB/Hz. Furthermore, a laser linewidth narrower than 600 Hz, a polarization-extinction ratio of more than 23 dB, and an optical signal-to-noise ratio of more than 73 dB are acquired simultaneously. This noise-sidebands-free and ultra-low-RIN single-frequency fiber laser is highly competitive in advanced coherent light detection fields including coherent Doppler wind lidar, high-speed coherent optical communication, and precise absolute distance coherent measurement.

Transverse mode instability (TMI) has become the major limitation for power scaling of fiber lasers with nearly diffraction-limited beam quality. Compared with a co-pumped fiber laser, a counter-pumped fiber laser reveals TMI threshold enhancement through a semi-analytical model calculation. We demonstrated a 2 kW high-power counter-pumped all-fiberized laser without observation of TMI. Compared with the co-pumped scheme, the TMI threshold is enhanced at least 50% in counter-pumped scheme, moreover, stimulated Raman scattering and four-wave mixing are suppressed simultaneously.

Bismuth (Bi)-doped laser glasses and fiber devices have aroused wide attentions due to their unique potential to work in the new spectral range of 1 to 1.8 μm traditional laser ions, such as rare earth, cannot reach. Current Bi-doped silica glass fibers have to be made by modified chemical vapor deposition at a temperature higher than 2000°C. This unavoidably leads to the tremendous loss of Bi by evaporation, since the temperature is several hundred degrees Celsius higher than the Bi boiling temperature, and, therefore, trace Bi (～50 ppm) resides within the final product of silica fiber. So, the gain of such fiber is usually extremely low. One of the solutions is to make the fibers at a temperature much lower than the boiling temperature of Bi. The challenge for this is to find a lower melting point glass, which can stabilize Bi in the near infrared emission center and, meanwhile, does not lose glass transparency during fiber fabrication. None of previously reported Bi-doped multicomponent glasses can meet the prerequisite. Here, we, after hundreds of trials on optimization over glass components, activator content, melting temperature, etc., find a novel Bi-doped gallogermanate glass, which shows good tolerance to thermal impact and can accommodate a higher content of Bi. Consequently, we successfully manufacture the germanate fiber by a rod-in-tube technique at 850°C. The fiber exhibits similar luminescence to the bulk glass, and it shows saturated absorption at 808 nm rather than 980 nm as the incident power becomes higher than 4 W. Amplified spontaneous emissions are observed upon the pumps of either 980 or 1064 nm from germanate fiber.