Resonant Pumped 2μm Holmium - Filled Solid - State Lasers

Recently,2μm coherent light sources have become one of the important research direction in the field of laser technology due to their important applications in the military, industrial and medical. Ho ion doped laser gain medium can be used to directly obtain2.1μn laser with achieving very high optical-optical conversion efficiency which benefits from the in-band pumping technology. In addition, Ho ion doped crystal materials can be used to obtain high energy pulse laser output for their energy storage advantages. In this paper, several new Ho ion doped garnet laser materials are explored to obtain high efficient continuous wave and passively Q-switched operations, including new type of optical functional materials Ho:YAG, Ho:LuAG transparent ceramics, mixed crystal Ho:LuYAG and single crystal Ho:LuAG.In this paper, a technology route of in-band pumped Ho lasers by Tm fiber laser are confirmed after the laser transition level of Ho doped materials are discussed. And a Tm fiber laser was built with a volume Bragg grating (VBG) as wavelength selection and narrowing device. When the LD pumped power is164W (wavelength is792nm), the output power is44W with a very narrow linewidth at1907nm wavelength.For Ho:YAG transparent ceramic laser, we study the transmission spectrum, absorption and emission spectra of the gain medium varying with doping concentration. Using a plane-concave cavity with low loss,21W high efficient laser output power were realized with pumped power of35W, by optimizing the doping concentration and transmittivities of the output mirror. The influence of Ho:YAG transparent ceramics doping concentration on the laser operation were studied comparatively and the sample with doping concentration as high as4at.%could still be launched laser while the Ho:YAG crystal of the same doping concentration was not reported. In addition, Ho:YAG transparent ceramics laser were realized passively Q-switched operation with a graphene as a saturable absorber device. This could verify the Ho:YAG transparent ceramics could be used to obtain pulse laser and the graphene has a broadband saturable absorption properties (at2097nm).For Ho:LuAG transparent ceramics, the spectral characteristics were explorded and continuous wave operation was achieved. Due to the gain medium is not with antireflective coating, the efficiency of the laser is lower. However, three laser transition processes (corresponding to three laser wavelengths) of Ho:LuAG transmission ceramics and their competition phenomenon were observed when transmissivities of output mirrors were changed. Then its causes were discussed.For mixed crystal Ho:LuYAG and single crystal Ho:LuAG, the spectral characteristics, passively Q-switched operation and continuous wavelength operation were studied. The spectra is the line broadening of Ho3+in LuYAG than in LuAG. This inhomogeneous broadening is probably due to the presence of a mixed occupancy of Lu and Y ions on dodecahedral sites, resulting in a variation in the crystal field at different Ho3+sites. In this research work, the highest continuous wave output power have been achieved with both of crystals. When using a saturable absorption mirrors (SESAM) as a passively Q-swithed components, relatively stable laser pulses have been obtained with both kinds of crystals. In general, Ho:LuYAG is more suitable for laser operation than Ho:LuAGSeveral important results have been obtained in this paper on in-band Ho doped solid state laser pumped by Tm fiber laser, including:1. We report on the high-power and high-efficiency operation of a polycrystalline Ho:YAG ceramic laser and a single crystal Ho:LuAG laser in-band pumped by a Tm fiber laser at1907nm. Over21.4W at2097nm and18W at2124.5nm of cw output power have been generated under35W of incident pump power, corresponding to average slope efficiencies with respect to the incident pump power of63.6%and53.4%. To date, these are most high powers corresponding to both of Ho:YAG ceramic and Ho:LuAG single crystal.2. With the same pump condition, the high-power and high-efficiency operation of the mixed crystal Ho:LuYAG was achieved of21W output power at2124nm, corresponding to a sloped efficiency of61.5%. To our knowledge, this was reported for the first time.3. Ho:LuAG ceramic (without antireflective coating) was realized to generate output power for the first time. The competition among three wavelengths was observed and explained.4. Q-switched operation of a polycrystalline Ho:YAG ceramic laser was achieved with a graphene as the saturable absorber for the first time.0.26W at2097nm pulse output power has been generated under a max. incident pump power of3.27W, corresponding to repetition frequency of64kHz at the max. and pulse width of2.6μs at the min.5. For the first time, the mixed crystal Ho:LuYAG and single crystal Ho:LuAG lasers have been used to obtain passively Q-switched pulse with a SESAM as a saturable absorber.