Theoretical And Experimental Study On Large Mode Area Tm-doped Fiber Laser In MOPA Regime

In recent year, with the study on rare ion-doped fiber lasers deepen increasingly and the needs for application fields, the development of high power rare ion-doped fiber lasers turns to the exploration of the new wavelength band. As 2μm laser works at eye-safe wavelengths, atmospheric transmission window, and the absorption peak of plastic and water molecules, so 2μm laser is widely used in laser radar, medicine, laser processing,optical parametric oscillation and so on. Latest research for 2μm fiber laser at home and abroad shows that thulium-doped, holmium-doped fiber laser are the best way to achieve laser output for the wavelength, over 1 kW output power has already been achieved with MOPA technical proposal. However, compared to the conventional 1μm Yb-doped fiber laser, power scaling of 2μm laser has no related reports since 2010. On the based of these advantages, a large mode area thulium-doped fiber laser in MOPA regime, for example, was theoretically and experimentally studied.At first, this paper reviews the current development on CW Tm-doped fiber laser in an all-round way, introduces the technologies of MOPA regime, after the summarization of the implementation plan for 2μm laser, it indicates the majority of a large mode area Tm-doped laser research effort is being invested into scaling average output powers.Then the physical limits relevant to power scaling of 2μm laser and efforts to the methods of power scaling were showed analytically. Based on this, a physical limit is analyzed for the maximum extractable power of 2μm laser were analyzed and calculated with the consideration of thermal fracture, thermal lens, melting of the core, optical damage, finite pump brightness and the nonlinear effects. As a result, a maximum extractable power of 56.6 kW and 70.2 kW could be achieved by theTm-doped fiber sources and Ho-doped sources in resonantly-pumped configuration, respectively. Our analysis shows that in all cases we considered, 2μm laser sources espescially in resonantly-pumped regime have a powerful advantage to obtain high output power.Based on the theoretical studies of the space-propagation mode for TDFA, this paper has performed a great deal of experiments on power scaling of TDFA in MOPA regime. However, such output powers are being scaled with on degree of control on laser system temporal behavior. Because of the generation of parasitic lasing and unstable temporal behavior with the increasing ASE, we have found that the effect of ASE should be taken into consideration. In the same time, a medical experiment with a 2μm CW laser of ten-watt was carried out, which offers advantage support for medical applications of 2μm laser.In this paper, ASE experimental investigation of the impact of time-domain features are carried out, from which we found the ASE of reverse injection obtained by self feeback plays an important role in time-domain features of the system.In addition, an examination of operation in nanosecond passively mode-locked Tm-doped fiber laser with output pulse energy up to 366 nJ was obtained at the repetition rate of 17.75 MHz.