基于特种玻璃光纤的中红外拉曼激光器研究进展

Significance: High power mid-infrared (MIR) fiber lasers have important applications in fundamental research, atmospheric communications, environmental monitoring, national defense & security, and so on. Raman fiber lasers and rare earth ions doped fiber lasers are two key technologies for MIR laser generation. Now, 34 μm fiber lasers have been demonstrated based on rare earth ions doped fluoride glass fibers. Whereas, due to the narrow and isolated emission bands of rare earth ions, the operational wavelengths of rare-earth-doped fiber lasers cannot cover all wavelengths in the MIR region. Moreover, the obtained output power decreases with the increase of operational wavelength due to the low emission efficiency and large quantum defects in the MIR region for rare earth ions. Raman fiber lasers are realized based on the stimulated Raman scattering (SRS) effects in optical fibers. SRS is an important nonlinear optical process in optical fibers. When the pump laser is launched into the fiber core, it would interact with the medium, and be scattered to Stokes light with a longer operational wavelength. The frequency difference between the pump and Stokes light is named as Raman shift. The generated Stokes light can serve as the pump light and cause the generation of next order Stokes light with a longer operational wavelength, which means that cascaded operation can be achieved for Raman fiber lasers. That is to say, if there is pumping light with a suitable wavelength and an infrared glass fiber with a low loss and high laser damage threshold, fiber lasers operating at any wavelength within the transmission window of the glass matrix can be achieved based on the Raman shift and cascaded Raman operation in principle, which is inaccessible for rare earth ion doped fiber lasers. Since then, to explore glass fibers with wide MIR transmission windows, good stabilities, high laser damage thresholds, big Raman shifts, and large Raman gain coefficients has been of great significant for the development of high power MIR fiber lasers. Progress: By using silica fibers as Raman gain media, the output powers of ~1 μm Raman fiber lasers have exceeded 1 kW, and Raman fiber lasers with a tunable wavelength ranging from 1 μm to 2 μm have also been demonstrated. Whereas, the longest operational wavelength of the Raman fiber lasers based on traditional silica fibers is limited within 2.5 μm due to the high background loss in the MIR region. Currently, researches on MIR Raman fiber lasers are based on tellurite, fluoride or chalcogenide glass fibers which have relative low transmission losses in the MIR region. Chalcogenide glass has a relative large Raman gain coefficient. By using As₂S₃ glass fibers as Raman gain media, researchers demonstrated second-order cascaded Raman lasing at 3.77 μm with an average output power of 9 mW (Fig. 9). Compared with chalcogenide glasses, fluoride glasses have a high laser damage threshold. By using fluoride glass fibers as Raman gain media, researchers reported a 3.7 W Raman fiber laser at 2231 nm (Fig. 13). Compared with chalcogenide and fluoride glasses, tellurite glasses have a large Raman shift. By using tellurite glass fibers as Raman gain media, 35 μm MIR Raman fiber lasers with a tens of watts output power could be achieved in principle (Fig.16). Recently, as the development of fiber fabrication technologies, hollow core fibers with low MIR transmission losses can be obtained. By using CO₂ filled hollow core fibers as Raman gain media, researchers demonstrated a 4.3 μm Raman laser with an output power of 82 mW. Besides, a widely tunable Raman soliton pulse could be obtained by employing the soliton self-frequency shift (based on intrapulse Raman scattering effect) in optical fibers. Researchers demonstrated 24.3 μm tunable Raman soliton generation in fluoride fibers (Fig. 14). Very recently, the authors have developed fluorotellurite glass fibers with a wide transmission window (0.46 μm), good stability, high laser damage threshold, big Raman shift (~780 cm^-1), and large Raman gain coefficient (1.28×10^-12 m/W@2 μm), and based on them, the generation of 1.962.82 μm tunable Raman solitons (Fig. 21) and "a shower of Raman solitons" at a preset wavelength of ~3 μm are obtained (Fig. 24), which preliminarily verifies the potential of fluorotellurite glass fibers for constructing mid-infrared Raman fiber lasers. Conclusion and Prospect: Raman fiber lasing is an efficient approach for the generation of high power MIR fiber lasers. Currently, by using tellurite, fluoride or chalcogenide glass fibers as the Raman gain media, the 3.77 μm second order cascaded Raman fiber laser and 24.3 μm tunable Raman soliton fiber laser have been developed. However, limited by the characteristics of infrared glass and high quality fiber & device fabrication technologies, the research on MIR Raman fiber lasers is still in the development stage, and the output power is quite low in the order of watts. Very recently, the authors have developed fluorotellurite glass fibers with good stabilities and high laser damage thresholds, and preliminarily verified their potential for constructing high power MIR Raman fiber lasers. We believe that tunable MIR Raman soliton lasers covering the whole 25 μm region could be obtained by optimizing the parameters of fluorotellurite glass fibers and pump lasers, and hundred-watt-level high power MIR Raman fiber lasers would be achieved by improving the quality of fluorotellurite fibers and with the development of high quality MIR fiber devices (e.g. fiber gratings) in the future.