Guilong Yang

Theoretical calculation and experimental study of acousto-optically Q-switched CO2 laser.

Using resonator inserted with acousto-optically modulator, the experiments of the compacted CO(2) laser were performed with Q-switch. According to various factors that influenced the output of laser, the theoretical calculation of its main parameters was conducted by Q-switched pulsed laser rate equations. Based on the results, the technical route and approach were presented for optimization design of this laser. The measured peak power of this laser device was more than 4000W and pulsed width was 180ns which agreed well with the theoretical calculation. The range of repetition frequency could adjust from 1 Hz to 100 kHz. The theoretical analyzes and experimental results showed that the acoustic traveling time of ultrasonic field could not influence the pulse width of laser so that it did not require inserting optical lens in the cavity to reduce the diameter of beam. The acoustic traveling time only extended the establishingtime of laser pulse. The optimum working frequency of laser is about 1 kHz, which it matched with the radiation life time (1 ms) of CO(2) molecular upper energy level. When the frequency is above 1 kHz, the pulse width of laser increased with the frequency. The full band of wavelength tuning between 9.2 microm and 10.8 microm was obtained by grating selection one by one which the measured spectrum lines were over 30 in the condition of Q-switch.

High power 4.65 μm single-wavelength laser by second-harmonic generation of pulsed TEA CO2 laser in AgGaSe2 and ZnGeP2

A high power 4.65 μm single-wavelength laser by second-harmonic generation (SHG) of TEA CO2 laser pulses in silver gallium selenide (AgGaSe2) and zinc germanium phosphide (ZnGeP2) crystals is reported. Experimental results show that the average output power of SHG laser is not only restricted by the damage threshold of the nonlinear crystals, but also limited by the irradiated power of fundamental-wave laser depending on the operating repetition-rate. It is found that ZnGeP2 can withstand higher 9.3 μm laser irradiation intensity than AgGaSe2. As a result, using a parallel array stacked by seven ZnGeP2 crystals, an average power of 20.3 W 4.65 μm laser is obtained at 250 Hz. To the best of our knowledge, it is the highest output power for SHG of CO2 laser by far.

High power repetitive TEA CO2 pulsed laser

A high power repetitive spark-pin UV-preionized TEA CO2 laser system is presented. The discharge for generating laser pulses is controlled by a rotary spark switch and a high voltage pulsed trigger. Uniform glow discharge between two symmetrical Chang-electrodes is realized by using an auto-inversion circuit. A couple of high power axial-flow fans with the maximum wind speed of 80 m/s are used for gas exchange between the electrodes. At a repetitive operation, the maximum average output laser power of 10.4 kW 10.6 μm laser is obtained at 300 Hz, with an electro-optical conversion efficiency of 15.6%. At single pulsed operation, more pumping energy and higher gases pressures can be injected, and the maximum output laser energy of 53 J is achieved.

Computer modeling and experimental study of non-chain pulsed electric-discharge DF laser

Computer simulation and experimental study of a pulsed electrical-discharge DF laser pumped by the SF(6)-D(2) non-chain reaction are presented. The computer model encompassing 28 reactions is based on laser rate equations theory, and applied to approximately describe the chemical processes of non-chain DF laser. A comprehensive study of the dependence of number density on time for all particles in the gain area is conducted by numerical calculation adopting Runge-Kutta method. The output performance of non-chain pulsed DF laser as a function of the output mirror reflectivity and the mixture ratio are analyzed. The calculation results are compared with experimental data, showing good agreement with each other. Both the theoretical analysis and experimental results present that the laser output performance can be improved by optimizing the mixture ratio and output mirror reflectivity. The optimum values of mixture ratio and output mirror reflectivity are respectively 10:1 and 30%. The single pulse energy of 4.95J, pulse duration of 148.8ns and peak power of 33.27 MW are achieved under the optimum conditions.

Stimulated rotational Raman scattering at multiwavelength under tea CO2 laser pumping with a multiple-pass cell

Stimulated rotational Raman scattering (SRRS) at multiwavelength pumped by TEA CO2 laser was demonstrated in this paper. Raman mediums were cooled by liquid-N2 and a multiple-pass cell (MPC) with 25 passes was designed and used. When the para-H2 was pumped by single-longitudinal-mode (SLM) circular polarized TEA CO2 laser on 10P(20), 9P(20), and 10R(20), 50 mJ 16.95 μm, 350 mJ 14.44 μm, and 536 mJ 16.9 μm radiations were obtained, corresponding to energy conversion efficiency of 1.2, 11.7, and 13.4%, respectively. When the ortho-D2 was pumped by CO2 laser on 10R(18), 108 mJ 12.57 μm Raman laser was obtained with energy conversion efficiency of 2.9%.

High-energy long duration frequency-doubled Nd:YAG laser and application to venous occlusion

Laser treatment represents an attractive option to other methods of vessel diseases especially varicose veins. A long pulse (30~50ms) 532nm laser (Fig.1) is used in our experiments with the pulse duration matching the thermal relaxation time of the vessels and the green laser matching the absorption spectrum peak of the blood. Laser irradiates nude vein vessels directly or exterior skin to finish operation faster and to acquire the practical data for upper enteron varicose vein treatment in several animal experiments performed in vivo. The 5J-energy pulse allows us to finely occlude rabbit or dog’s vein vessels up to 2 mm in diameter when irradiating them off external skin (Fig.2). Blood vessels are occluded at once and later biopsy specimens show the immediate and long-term lasting occlusion effect. While irradiating vessels directly (Fig.3), the vessels are usually irradiated to perforate, detailed causes are still under investigation. Animal experiments show long pulse green laser therapy is a safe and effective solution to the vein’s occlusion, which promises such laser with high energy of each pulse and 30~50 ms duration is an ideal candidate for vessel diseases treatment.

The mode-matching model of diode-end-pumped alkali vapor lasers

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.

Experimental investigation of a diode-pumped powerful continuous-wave dual-wavelength Nd:YAG laser at 946 and 938.6 nm

In this paper, a diode-pumped high-power continuous-wave (cw) dual-wavelength Nd:YAG laser at 946 and 938.6?nm is reported. By using an end-pumped structure, comparative experiments indicate that a 5?mm-length Nd:YAG crystal with a Nd3+-doping concentration of 0.3?at.% is favorable for high-power laser operation, and the optimal transmissivity of the output coupler is 9%. As a result, a maximum output power of 17.2?W for a dual-wavelength laser at 946 and 938.6?nm is obtained at an incident pump power of 75.9?W, corresponding to a slope efficiency of 26.5%. To the best of our knowledge, this is the highest output power of a quasi-three-level dual-wavelength laser using a conventional Nd:YAG crystal achieved to date. By using a traveling knife-edge method, the beam quality factor and far-field divergence angle at 17?W power level are estimated to be 4.0 and 6.13?mrad, respectively.

Influence of the helium-pressure on diode-pumped alkali-vapor laser

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.

Diode-pumped high-repetition-rate acousto-optically Q-switched Nd:YVO4 laser operating at 914 nm

A diode-pumped high-repetition-rate acousto-optically (A-O) Q-switched Nd:YVO4 laser operating at 914 nm was reported in this paper. Employing a compact linear laser cavity, at an operating repetition rate of 10 kHz, a maximum average output power of 2.2 W 914 nm laser was obtained at an incident pump power of 45.3 W, corresponding to an optical conversion efficiency of 4.9% and a slope efficiency of 8.8%. Minimum pulse width of 24 ns and maximum peak power of 8.0 kW of 914 nm laser was also achieved at an incident pump power of 40.8 W. To the best of our knowledge, this is the highest peak power of 914 nm laser at 10 kHz by far. Moreover, the highest operating repetition rate of pulsed 914 nm can even reach 100 kHz.