Bistability and passive Q-switching of a CO2 laser with saturable absorber

The coexistence of bistability and repetitive passive Q-switching of a CO₂ laser with a gaseous saturable absorber has been investigated experimentally. With CH₃F as the saturable absorber bistability has been observed simultaneously with passive Q-switching, whereas with CH₃OH either one of these phenomena can only exist, as demonstrated for the first time. The measured features of both systems can be explained qualitatively by rate-equation theory.     

Undamped undulation superposed on the passive Q-switching pulse of a CO2 laser

An undamped undulation superposed on the pulse tail of the passive Q-switching is observed using HCOOH gas as a saturable absorber. The pulse shapes with the undulation are nicely reproduced through the rate-equation analysis in which the laser gain medium is described as a three-level system. Good agreements between the observation and the calculation are also obtained in the dependence of the period and the width of passive Q-switching pulse on laser parameters. The mechanism of the undulation is interpreted as the relaxation oscillation attributed to the relaxation from the lower laser level. The collisional rate constant of HCOOH molecule is also obtained.     

High pulse repetition frequency RF excited waveguide CO2 laser with mechanical Q-switching

In this paper, a mechanical Q-switching is used in radio frequency (RF) excited waveguide CO₂ laser to obtain high pulse repetition frequency (PRF) laser. The Q-switching system includes two confocal ZnSe lenses and a high speed mechanical chopper, which is inserted into the cavity. The peak power is up to 730 W and the pulse width 200 ns at the highest PRF 20 kHz. The laser also has the advantages of compact, small-volume, and low-cost.     

Short single-mode CO2 laser pulse generation by pulsed Q-switching

A simple method for the generation of short, single-mode CO₂ laser pulses produced by applying two voltage gates (of amplitude 3U_λ/4 and U_λ/4) to an electro-optic Q-switch placed in a three-mirror cavity is proposed. Single, single-mode, well-synchronizable pulses of 3 ns duration and of 3 mJ energy have been experimentally achieved from a TEA CO₂ laser with an intracavity Pockels cell with 3 ns switching time. Using a numerical simulation it is shown that with shorter switching time (≈1 ns) the method enables one to obtain, from such a laser, a single, megawatt pulse of 1 ns duration.     

Observation of bistability in a CO2 laser exhibiting passive Q-switching

Optical bistability has been investigated experimentally in a CO₂ laser containing CF₃Br as a saturable absorber. The frequency dependence of the hysteresis cycle of this laser containing a saturable absorber (LSA) has been studied. It has also been shown that the LSA may undergo simultaneously hysteresis cycles and Q-switching. The phase diagram of the LSA has been extended towards regions where this behavior has been observed.     

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 ??m 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.     

Efficient self-sustained pulsed CO laser

In this paper a simple sealed-off TEA CO laser is described with a self-sustained discharge without an external UV preionization source. At 77 K this system yields more than 600 mJ from a lasing volume of about 60 cm³ CO-N₂-He mixture (45 J/? atm. with 15.6% efficiency).     

Enhancement of passive Q-switching performance with mixed Nd:LuxGd1-xVO4 laser crystals

Passive Q-switching operation has been demonstrated with a class of mixed Nd:Lu(x)Gd(1-x)VO(4) laser crystals. With respect to that obtained with Nd:GdVO(4), the passive Q-switching performance, including threshold, pulse energy, and peak power, was found to be greatly enhanced with the mixed vanadate crystals. The shortest pulse width of 6.2 ns, largest pulse energy of 192.5 microJ, and highest peak power of 31.1 kW were obtained at the incident pump power of 13.75 W with the mixed crystal for x=0.5.     

A repetitively pulsed sealed-TE CO2 laser using an oxygen tolerant discharge scheme

A repetitively pulsed (40 Hz) TE CO₂ laser using an oxygen tolerant discharge scheme is described. Long lived (>10⁵ shots) stable discharges at high pump energy density (200 J.l^–1 atm^–1) have been achieved both with and without the use of additive gases.     

Laser induced medium perturbation in a pulsed CO2 laser

Laser induced medium perturbation (LIMP) has been identified by time-resolved interferometry observations of the lasing/non-lasing boundaries in a pulsed, kilojoule CO₂ electron beam laser, working at atmospheric pressure. This fundamental effect destroys the quality of the laser output beam and therefore has serious implications in laser design.     

Theoretical considerations regarding pulsed CO2 laser annealing of silicon

We present a calculation of the surface temperature and investigate the “thermal runaway” phenomenon during pulsed CO₂ laser (λ = 10.6 μm) annealing of silicon. In calculating the temperature variation of free carrier absorption in n-Si, we have taken into account acoustic deformation potential scattering, optical deformation potential scattering, and ionized impurity scattering. The deformation potentials are adjusted to fit the experimentally observed values at 300°K. Also, we discuss the contribution of free carrier absorption during annealing with a Nd:glass laser (λ = 1.06 μm).     

反馈调QCO_2激光器的光学多稳性与不稳性

本文发现在CO_2激光器的腔内插入一个电光调制器,利用正比于输出光的光电转换信号反馈调节激光器的Q值,可以实现光学双稳性与多稳性.文中建立了描述这一系统的数学模型,它由三个一阶非线性微分方程描述,其稳态模型显示了光学双稳性和多德性特点,理论分析和数值计算都证实了这一结果.     

Influence of scanning velocity on bovine shank bone ablation with pulsed CO2 laser

The influence of scanning speed on hard bone tissue ablation is studied with a 10.6-μm laser. The groove morphology and the thermal damage created in bovine shank bone by pulsed CO2 laser are examined as a function of incident fluence by optical microscope following standard histological processing. The results show that ablation groove width, depth and ablation volume, as well as the zone of thermal injury, increase gradually with incident fluence. As compared to the result for high scanning speed, the lower scanning speed always produces larger ablation volume but thicker zone of thermal injury. It is evident that scanning speed plays an important role in the ablation process. In clinical applications, it is important to select appropriate scanning speed to obtain both high ablation rates and minimal thermal injury.     

Repetitive passive Q-switching and bistability in lasers with saturable absorbers

We report detailed experimental data on the passive Q-switching operation in a CO₂ laser with CH₃I saturable absorber, and on the transient behaviour in the near-Q-switching situation. Under suitable operating conditions, we found bistability in the output power. In some cases, we observed the simultaneous presence of bistability and passive Q-switching. The theoretical part of the paper starts from the four-level model of laser with saturable absorber, as formulated by other authors. By adiabatically eliminating the variables of the resonant levels, we reduce the problem to a set of three differential equations, from which we derive explicit analytical conditions for the rise of passive Q-switching. These conditions turn out to be in good qualitative and partially quantitative agreement with our experimental findings as well as with other experimental data previously obtained by other authors. Finally we classify the possible combinations of passive Q-switching and bistability that one can find in this type of experiments.