An experimental setup of mid-infrared Fe:ZnSe laser operating at room temperature has been established, which was end-pumped by a non-chain pulsed HF laser. The temperature has significant influence on the level lifetime of Fe:ZnSe laser. As the crystal temperature changes from 85 to 295 K, the level lifetime of Fe ions changes from 57 to 0.35 μs, it is important for matching the pump pulse width and the level lifetime. Electronic excitation HF laser with short pulsed width is a good pump source for Fe<sup>2+</sup>:ZnSe laser at room temperature. For the Fe<sup>2+</sup>:ZnSe crystal with size of 20×20mm, when the pumping spot diameters is lower than 9.2mm, the phenomenon of transversal parasitic oscillation could been suppressed effectively. At room temperature, the output energy of Fe<sup>2+</sup>:ZnSe reaches 294mJ, the slope efficiency is about 36%, and the optical to optical efficiency respecting to the pump energy is 34%.
Crystal thermal characteristic is a key factor to affect output laser property. In some applications, the facets of crystal will be contaminated by dust in the air, which will enhance the heat absorption of laser and cause local thermal unbalance. Therefore a novel crystal heat dissipation method is proposed in this paper. Crystal is mounted in a specially designed heat sink, heat conducts between the contacting surfaces of crystal and heat sink. Pump incident laser irradiates from the end facet of crystal. The end facet of crystal is cooling by convection heat transfer with flowing protect gas. The experiment device is established, the pump laser is Hydrogen Fluoride laser with the wavelength of 2.8μm, pulse energy of 600mJ, and repetition rate of 50Hz. The crystal is Fe: ZnSe with the dimension of 20mm× 20mm× 6mm. The beam quality is measured in the condition with and without heat sink for comparison, the results indicate that the heat dissipation method proposed in this paper is benefit for improving the beam quality.
Diode-pumped alkali vapor lasers are famous in the field of laser for their significant advantages such as very high quantum efficiency (Cs 99.5%, Rb 98.1%, K 95.2%), good thermal management performance and excellent beam output quality etc. A rate equation model fully considering the spatial distributions of pumping light and oscillating light is established under the hypothesis of quasi-two-level energy system of DPALs in this paper. Meanwhile, expressions of threshold pumping power, mode-matching efficiency and output power and slop efficiency in low pumping and strong pumping, respectively, are obtained. Then, the influences of mode-matching efficiency on working performance of DPALs are discussed and analyzed. Results show that mode-matching efficiency mainly impacts on threshold pumping power, output power and slop efficiency in low pumping but that nearly has no effects in strong pumping. Therefore, this model benefits the further research of DPALs.
Kilowatts class diode-pumped Cs vapor laser (DPCL) has been realized and this kind of lasers have military applications potentially for its high output power with high efficiency. Pumped by a fiber coupled laser diode, the key operating parameters of a DPCL are studied, including the spot size of focused pumping light, pressure ratio of buffer gases, vapor cell length, temperature of Cs vapor and reflectivity of output coupler. The spot size is properly chosen in the consideration of both the intensity scalability and mode matching. Pressure ratio is optimized under a modest pressure of mixed gases of helium and ethane. Under the optimized pressure ratio, the Cs vapor can absorb the pumping energy and convert it into laser energy efficiently. Besides, the temperature and reflectivity are also optimized to operate the DPCL in optimum state. The results have significant instructions for the experimental design of DPCL.
Diode-pumped alkali-vapor laser (DPAL) is a kind of laser attracted much attention for its merits, such as high quantum efficiency, excellent beam quality, favorable thermal management, and potential scalability to high power and so on. Based on the rate-equation theory of end-pumped DPAL, the performances of DPAL using Cs-vapor collisionally broadened by helium are simulated and studied. With the increase of helium pressure, the numerical results show that: 1) the absorption line-width increases and the stimulated absorption cross-section decreases contrarily; 2) the threshold pumping power decreases to minimum and then rolls over to increase linearly; 3) the absorption efficiency rises to maximum initially due to enough large stimulated absorption cross-section in the far wings of collisionally broadened D2 transition (absorption transition), and then begins to reduce; 4) an optimal value of helium pressure exists to obtain the highest output power, leading to an optimal optical-optical efficiency. Furthermore, to generate the self-oscillation of laser, a critical value of helium pressure occurs when small-signal gain equals to the threshold gain.
The modified Bridgman method with heat field rotation was used to grow ε-polytype single crystals of pure and 1, 2 and 10 mass % S-doped GaSe or solid solution crystals GaSe<sub>1-x</sub>S<sub>x</sub>, x = 0.002, 0.091, 0.412. The interaction of ultrashort laser pulses of ∼ 100 fs duration at 800 nm and 2 μm with the grown crystals was studied at room temperature. Up to 3.4-fold advantage of S-doped crystals in limit pump intensity (no decrease in the transmission) was found under 800 nm pump at S-content increase up to 10 mass %. The advantage became a half less at 2 μm pump due to a decrease of two-photon absorption in pure GaSe crystals. The spectral dependence of transient absorption is recorded with 37 fs resolution and interpreted. It was ascertained that first observable damage of high quality crystals is caused by dissociation of submicrometer thick surface layer to initial elements and do not influence the frequency conversion efficiency until alloying of dissociated Ga. Local microdefects, multiphoton absorption and transient transmission processes are identified as key factors responsible for damage threshold.
The working mechanism of the non-chain pulsed DF laser is analyzed in this paper, then the glow discharge with the method of UV-preionized discharge is displayed, and finally the experimental investigation on the output characteristics of non-chain pulsed DF laser is done. With SF<sub>6</sub>, which is non-toxic and non-corrosive, and D<sub>2</sub> as the active medium, the dependence of the output characteristics of pulsed DF laser on the gas mixture ratio and the total pressure are investigated. In the experiment, the optimal gas mixture ratio of SF<sub>6</sub>-D<sub>2</sub> and total pressure are 10:1 and 10.5kPa, respectively. At the same time, by making a measurement on the output spectrum of DF laser with a DF laser spectrum analyzer, 17 P-branch transition lines are achieved and the majority of output energy is concentrated on several lines near the 3.876μm line. The laser beam divergence angles in horizontal and vertical directions are 1mrad by using the laser spot ablation method. At the charging voltage of 39kV, under the best working condition(SF<sub>6</sub>: D<sub>2</sub> = 10:1, P<sub>total</sub>= 10.5kPa), the maximum single pulse output energy of 3.58J, pulse duration of 215ns, peak power of 16.65 MW, and electro-optical conversion efficiency of 2.08%, are obtained.
Damage threshold of non-linear GaSe crystals under IR fs (Ti:Sapphiere 800 nm laser and 1.1-2.9μm OPG) and ns (2. 79
Er<sup>3+</sup>:YSGG and 10.6μm CO<sub>2</sub> laser) pulse pumping is studded in details. Local micro defects and field induced effects (GaSe dissociation, multiphoton absorptions and transient transparency origin effects) are identified as responsible for damage threshold in this case. Local (including nano scaled) defects and thermal effects are identified as reason of damage threshold under ns pulse pumping.