A high efficiency Tm<sup>3+</sup>-doped fiber seed source system has demonstrated. The pumping laser and output laser in the optical fiber distribution were analyzed. High performance output laser would be obtained by reasonable design of laser parameters, selecting the optimum length of gain fiber, and increasing injection power. Tm<sup>3+</sup>-doping concentration of 7.5×10<sup>24</sup> m<sup>-3</sup> and the optimum length of gain fiber of 2 m were used in the experiments. The continuous wave laser with output power of 5.01 W and the beam quality of M<sup>2</sup><sub>x</sub> = 1.35, M<sup>2</sup><sub>y</sub> = 1.51 was obtained by fiber coil diameters of 10 cm and injection power of 17.61 W. A slope efficiency and conversion efficiency were up to 36.2% and 28.4%, respectively. The fiber length of 1 m, 1.5 m, 2 m, 2.5 m, 3 m were chosen in the experiments, and the effects of different fiber lengths on the output characteristic were researched. The laser loss was reduced, and the conversion efficiency and beam quality were improved effectively by selecting the length of gain fiber and Tm<sup>3+</sup>-doping concentration reasonably. Theoretical analysis and experimental results show that a higher output power can be achieved by increasing the injection power. It provides a strong experimental foundation for further researching on high power Tm<sup>3+</sup>-doped pulse fiber amplifier by using this Tm<sup>3+</sup>-doped fiber laser as a seed source system.
The affecting factors of 2 μm Tm<sup>3+</sup>-doped fiber laser output characteristics were theoretical analyzed. On the basis of the energy level structure and optical absorption properties of Tm<sup>3+</sup> ion, combining with the basic principle of Tm<sup>3+</sup>-doped fiber laser, and starting from the energy level structures and the cross relaxation processes of Tm<sup>3+</sup> ion, the three pumping methods of Tm doped fiber laser (TDF) were analyzed and discussed. The influences of output characteristics by different influence factors were simulated. Based on optimization of the equations, for different fiber lengths, doping concentrations and pumping absorption coefficients and other influence factors, the laser output characteristics under different conditions were obtained and analyzed. Combination the simulation analysis, through the reasonable design and the selection of the optimum parameters of the laser system, the high laser output performance scan be achieved by improving the injection power and controlling of fiber coil diameter. The influences of different factors on the output characteristics were analyzed in the issue. The high laser output performances can be obtained and the laser loss was reduced by selecting the parameters of the laser system properly.
266nm UV laser has a wide range of applications in many fields, such as laser medical treatment, laser processing,
precision measure and other applications for the reason of its advantages in wavelength, small diffraction effects, high single-photon energy, and high resolution and so on. BBO crystals absorb parts of the fundamental laser energy and second harmonic laser energy are unavoidable, and thus the temperature raise, so that the existing crystal phase matching conditions change, resulting in phase mismatching in the high-power frequency doubling, greatly influence the 266nm UV laser conversion efficiency. To further study the effect of phase mismatching to conversion efficiency, and improve the conversion efficiency, output power and other output characteristics of 266nm laser, the article mainly describe from the following three aspects. Firstly, took the use of three-dimensional nonlinear crystal temperature distribution which is obtained, the process of BBO crystal thermal-induced phase mismatching is analyzed. Secondly, based on frequency doubling theory, the effects of the thermal-induced phase mismatching affected of conversion in crystals are analyzed. Combining with the phase mismatching of the three-dimensional distributions, the fourth harmonic conversion efficiency with thermal phase mismatching changes of BBO 266nm UV laser are simulated for the first time. Thirdly, by using MATLAB software, the effects of phase mismatching to conversion efficiency in crystal for different waist radius, 532nm laser power and the fundamental beam quality are simulated. The results indicate a good physical interpretation of reasons of high power laser frequency doubling system. It shows that the model established explains the reason of the reduction of conversion efficiency, output power and the beam quality excellently. All results make a leading sense to the research on the compensating of the phase mismatching and on the improvement of conversion efficiency.
266nm UV laser has a wide range of applications in various fields by its advantages in high single photon energy and
high resolution, which also has a development gradually moving in the direction of high power and high conversion
efficiency. In the process of high-power laser frequency doubling, BBO crystal inevitably absorbs part of fundamental
light power and frequency doubled light power, it induced the temperature rise along the direction of radiation in crystal
and destroyed the phase-matching conditions of BBO crystal that lead to phase mismatching. In order to improve
harmonic conversion efficiency as well as reduce the influence of output power and beam quality caused by phase
mismatching, in this paper we analyzed the process of phase mismatching, established the thermal-induced phase
mismatching model by using analytical expression of the nonlinear crystal temperature field equation which has been
given, and the three-dimensional phase mismatching distribution were obtained. There are three major contributions in
the paper. Firstly, the working process of the nonlinear crystal was analyzed, and the physical and mathematical models
of temperature distributions were established, and the BBO crystal three-dimensional temperature distributions were also
obtained. Secondly, a variety of factors that affect the temperature distributions within the BBO crystal were summarized.
For different 532nm waist radius and 532nm input power, they were numerical simulated use of MATLAB. Finally,
combined with the above analysis, the physical and mathematical models of phase mismatching caused by energy
absorption of BBO in forth harmonics generation were established, the phase mismatching distributions in the crystal
were simulated as well, especially the changes to phase mismatching distributions with different parameter were
analyzed. Combination of the multiplier theory, the influence of phase mismatching on frequency doubling conversion
efficiency was analyzed. The results indicate that the physical model which established in this paper can explain the
physical reasons in high-power laser frequency doubling system very well, such as the reduce in conversion efficiency
and output power and beam quality. All research results play instructive effect at the improvement of conversion
efficiency and the compensation for the phase mismatching for further research.