Fast axial flow CO2 laser excited by silent discharge

A fast axial flow CO₂ laser excited by silent discharge has been developed. With a new electrode system applied to the laser, homogeneous discharge of power density of up to 80 W/cm³ is obtained without applying any stabilization techniques on the discharge. An output laser power of 920 W in stable TEM₀₀ mode operation is attained with an efficiency of over 14%. Beam fluctuation common to fast axial flow lasers is suppressed by the combination of smooth gas flow and low CO₂ molar fraction     

Diagnostics of TM010-mode microwave cavity discharges inCO2-N2-He laser gas mixtures. I. Measurement ofdielectric constant

Plasma parameters in the microwave discharged CO₂ laser gas mixtures of CO₂-N₂-He=0.9/2.5/20 at 25 Torr obtained using the perturbation method for a TM₀₁₀-mode cylindrical resonant cavity were examined. From the changes in resonant frequencies and Q values of the cavity, dielectric constant of the discharged plasma under the condition of RF to laser output power conversion ratio of 14% at mass flow rate of 4.2 kg/h was determined to be nearly 0.96-0.01j. It was also obtained from the dielectric constants of the plasma that electron number density, the electron temperature, and discharge parameters (E/n) were (0.3-3)×10¹⁵m^-3, 0.5-2.7 eV, and (0.5-2)×10^-15 Vcm², respectively     

Plasma chemistry of the closed-cycle 1 kHz transversely excitedatmospheric CO2 laser with an efficient catalytic CO2 regenerator

A high-power closed-cycle 1 kHz transversely excited atmospheric (TEA) CO₂ laser with an efficient catalytic CO₂ regenerator was used to investigate the variations of the average laser output power and the concentrations of the CO₂, CO, and O₂ molecules in the laser gas mixture with the operational performance of the CO₂ regenerator. It was experimentally shown that for the laser gas mixture of CO₂-N₂-He=15-15-70% and the output coupler reflectivity of 70%, η of 0.1 was required to maintain the laser output power greater than 90% of the initial laser output power of 570 W at an input energy density and a clearing ratio of 150 J/L and 3.0, respectively     

Diagnostics of TM010-mode microwave cavity discharges inCO2-N2-He laser gas mixtures. II. Measurement ofvibration temperature

For pt. I see ibid., vol., 31, no. 8, p. 1525-32 (1995). The plasma temperatures in microwave discharged CO₂-N₂-He laser gas mixtures were examined using the spectroscopic and electrostatic probe methods. A vibration temperature of N₂ molecules, obtained spectroscopically, was determined to be nearly 7000 K without gas circulation and to be nearly 4000 K at the mass flow rate of 4.2 kg/h. It is found that an efficiency of laser output power exceed 14% (RF to laser output power conversion ratio) below the vibration temperature of 4000 K. The values of vibration temperature obtained were higher than those reported in DC discharges     

Theory and experiments on the power modulation of CO2lasers

The operation of a pulsed transverse flow CO₂ laser system with RF excitation is studied theoretically by computer simulation. The calculations are supported by experimental results which confirm that the pulse repetition frequency of the system is restricted by the time delay between the electric and the laser pulse and by relaxation oscillations. The time delay is roughly inversely proportional to the gas pressure and not very sensitive to variations of the CO₂ fraction and the output coupling of laser radiation. A pulse repetition frequency over some 30 kHz can be attained only if the excitation pulse period is equal to the delay time, but this operation condition is very sensitive to changes of the duty cycle. The calculations show that full modulation depth should be attainable up to a repetition frequency of 360 kHz     

Frequency chirp in a 10 ATM RF-excited CO2 waveguidelaser

Various mechanisms causing frequency chirping in multiatmospheric-pressure CO₂ lasers are discussed. The frequency chirp has been measured in a pulsed 10 atm CO₂ waveguide laser. It has been found that the dominating cause of the frequency chirp in this laser is heating of the gas by the RF discharge. The chirp increases with increasing RF input power. The chirp levels off at about 100-150 MHz/μs for 25 kW of input power. This leveling off the chirp is believed to be due to the negative lensing effect of the gas density perturbation. The effect of the anomalous dispersion on the chirp is observed when the laser is operated at the flanks of the CO₂ gain branches     

Maximizing the output power of a CO2 laser using theTaguchi technique

The results of an experiment aimed at optimizing the output beam power of an industrial fast-axial-flow CO₂ laser machine using the Taguchi method are reported. Four control factors (design variables) were explored: blower speed, laser gas pressure, cooling fans, and output mirror transmittance. The various parameters were assigned to a modified L18 orthogonal array. The experiment was conducted with two repetitions and complete randomization within block was practiced. It was found that higher laser power could be obtained with a blower speed of 2526 r.p.m., laser gas pressure of 62 torr, cooling fans, and output mirror transmittance of 40%. The laser gas composition was kept constant at 5% CO₂, 14% N₂, and 81% He. Twenty confirmatory tests were performed and the results fell within the predicted 95% confidence interval     

Subnanosecond multi-gigawatt CO2 laser

A semiconductor switching technique has been utilized to produce 30-300 ps variable duration CO₂ laser pulses of 0.5-MW peak power. Eight passes through a 1.2-m long, UV-preionized, 3-atm TE CO₂ amplifier raise the output laser peak power to the 10¹⁰ W level. Sampling the amplifier gain in linear and saturated regimes using CO₂ laser radiation ranging from CW to 30 ps pulse length permits comparison with computer modeling of picosecond CO₂ pulse amplification. The potential for further peak power scaling of picosecond molecular lasers is discussed     

High peak power and high repetition rate characteristics in acurrent-pulsed Q-switched CO2 laser with a mechanical shutter

An electro-mechanical Q-switched (EMQ) CO₂ laser is Q-switched by a mechanical beam chopper in combination with a pulsed discharge current. Such a system can produce pulses with high peak powers (>10 kW) and high repetition rates (>1 kpps). In order to analyze the output characteristics, the peak power and the duration of the output pulses have been measured experimentally in detail over a wide range of Q-switching times up to 250 ns. For a low-pressure (<4 kPa) CO₂ gas system, the standard rate equations adequately explain the experimental results by introducing a new switching function for the form of the cavity loss for the mechanical chopper. In an EMQ-laser with a high initial inversion density (4.5·10¹⁵ /cm³ at 150 mA peak current), multiple peak pulses or pulse distortion have been observed. This is due to the plasma screening effect induced by the burning of the metal shutter blades placed inside the cavity. It is found that tungsten metal shutter blades can be used up to a power density of 259 MW/cm² for a focused beam without this effect occurring, The solutions of the rate equations show that optimum coupling can prevent the plasma screening effect even for a Q-switching time longer than the pulse buildup time. The EMQ-laser configured for optimum coupling has produced a peak output power of 30 kW for the 9P20 transition branch in the CO₂ spectrum without any pulse distortion. This value has been obtained even though the discharge length was only 1.3 meters     

光纤气体激光光源研究进展及展望(Ⅱ):基于粒子数反转

粒子数反转和受激拉曼散射是实现光纤气体激光器输出的最常见的两种基本原理。与光纤气体拉曼激光光源不同,基于粒子数反转原理的光纤气体激光器是通过气体分子振转能级的本征吸收跃迁实现激光输出。由于绝大多数气体分子的振转能级对应的激射跃迁谱线都在中红外波段,这种激光器的输出波长基本都在中红外波段。简要分析了基于粒子数反转原理的光纤气体激光器在产生中红外波段激光方面的优势,重点回顾了其发展历史与研究现状,并对下一步的发展趋势进行了展望。     

基于可调谐CO2激光器的SF6差分光声检测研究

为了对电力场所SF₆气体浓度进行有效监测,采用光声光谱气体检测技术,基于波长可调谐CO₂激光器,设计了一套大气环境下的SF₆痕量气体检测系统,并提出一种差分光声光谱技术以提升光声系统的检测灵敏度。结果表明,所设计的SF₆气体检测光声系统的共振中心频率为1066Hz,品质因数为32.04,光声池常数为89.74Pa·m·W-1;利用单谱线光声法,在激光谱线10P12处检测SF₆气体的灵敏度为0.06×10-6（体积分数）;采用差分光声光谱气体技术后,在激光谱线10P12和10P16处3W强度调制光的照射下,光声系统的灵敏度提升到0.02×10-6（体积分数）。差分光声光谱技术能有效降低噪声影响,提升光声检测系统的灵敏度,具有一定的实用价值。     

2-kHz repetition rate XeF laser

High-repetition-rate laser action, up to 2 kHz, has been demonstrated in XeF molecules at 351 and 353 nm by using a blowdown fast transverse-flow system and a four-circuit, thyratron-switched, low inductance pulse generator. For a typical run, the transverse flow was uniform, and the average flow velocity was 25 m/s across a discharge region of1.4 times 0.4 times 30cm³. The gas mixture used was He:Xe:NF3= 100:1.5:0.5, and the total pressure was varied from 600-1200 torr. For single-pulse operation, the maximum laser output energy was 22 mJ/pulse, and the electric efficiency was 0.4 percent. For a 2-kHz repetition rate, the average laser output energy was approximately 12 mJ/pulse with 50 percent variations. Hence, an average output power of 24 W was obtained.     

Accurate gas diagnostics for sealed-off CO2 lasers usingnear-infrared DFB semiconductor lasers

The selective spectroscopic detection of CO₂ and H₂O in the gas mixture of a 400-W sealed-off CO₂ slab laser is reported. The detection is based on the acquisition of absorption Voigt profiles of two vibrational overtone transitions of CO ₂ and H₂O, obtained using two InGaAsP distributed feedback (DFB) semiconductor lasers. This spectroscopic method enables a direct absolute measurement of the temporal evolution for the concentrations inside the laser active volume. Monitoring takes place during laser operation without gas extraction. The experiments carried out on CO₂ and H₂O abundances confirm the paramount importance of wall desorbance phenomena in all-metal radio frequency (RF) excited lasers     

The near field of omnidirectional helical antennas

The close near field of helical antennas, radiators widely used in connection with two-way portable communication equipment, is investigated theoretically and experimentally. The investigation has been motivated by safety related considerations. A simplified mathematical model for the radiation from helical antennas with a large number of turns is derived. The near E-field intensity obtained from the theoretical model is compared to the values measured using an accurate E-field probe. The agreement between experimental and theoretical values is excellent. The results show that there is a substantial buildup of static-type electric energy in the close vicinity of helical antennas. The intensity of these electric fields in the vicinity of a helical radiator depends essentially on the Q factor of the antenna. For one experimental helix the far-field power density equivalent (|E|²/377) of the electric field at 1-cm distance from the radiator exceeds some proposed safety standards for less than 250-µW radiated power. These values are in complete agreement with the results of previous studies which showed that helical radiators are very ineffective in depositing electromagnetic energy into simulated muscle tissue located in the close vicinity of the antenna. If safety standards of independent or government agencies do not take into account the peculiar nature of the electromagnetic energy in the close vicinity of some radiating devices, it is conceivable that the power of portable two-way communication equipment might be forced down to useless levels.     

Optical gain and saturation intensity in a transverse-flow CW CO2 laser

Simple expressions for the small-signal gain and the saturation intensity in a transverse-flow CW CO₂ laser have been derived considering the effect of gas flow and the finite time required for N ₂ molecule to transfer its energy to CO₂ molecules. These expressions provide estimates of these parameters fairly close to the experimentally measured values in high-power transverse-flow CW CO₂ lasers     

CO2 laser performance with a distributed gold catalyst

The performance of a CO₂ laser was improved by coating the inside wall of the discharge tube with a discontinuous gold film. In the presence of the discharge, the gold acts as an ambient temperature catalyst to reform decomposed CO₂. The gold coated laser delivered 122 W/m for both sealed-off and flowing operation. CO₂ decomposition levels are presented as a function of input power, gas pressure, and temperature. Atomic oxygen, generated by the discharge, appears to be required for gold to exhibit any detectable catalytic activity     

An interferometric study of CO2-1aser-produced sparks

The expansion of sparks produced in various gas mixtures by a helical TEA CO2laser of modest power (∼ 0.5 MW) has been examined using time-resolved photography and two-wavelength interferometry. The latter was accomplished with the aid of a synchronized ruby laser, the fundamental and second-harmonic frequencies of which illuminated a Mach-Zehnder interferometer. The interferograms have been analyzed using Abel inversion techniques and two-dimensional profiles of the electron density within the sparks obtained.     

FM mode-locked high-pressure CW RF-excited CO2 waveguidelaser

The authors report the results from a study of a FM mode-locked continuous-wave (CW) RF-excited CO₂ waveguide laser operated at 0.25-2 atm gas pressures. It is shown that electrooptic FM modulations can be efficiently used to mode lock a CW CO₂ laser. The combination of a high gas pressure and a high modulation frequency makes it possible to generate pulses which are substantially shorter than those previously reported for CW mode-locked CO₂ lasers. A theoretical approach is used for simulation of the FM mode-locked laser. The experimental pulses of a few hundred picoseconds FWHM are considerably shorter than previously reported for CW mode-locked CO₂ lasers. The experimental results are compared with the results of numerical calculations using a frequency domain simulation model     

Preionizer-induced pulsed-mode operation of a transverse-flowtransversely excited CW CO2 laser

A pulse-preionized, dc-excited, transverse-flow, CW CO₂ laser has been operated in pulsed mode without using a discharge power-switching scheme. The pulsation in laser power is produced by a pulsed current that flows from the dc power supply due to the collapse in the discharge impedance whenever a preionizer pulse appears. By properly choosing the preionizer-pulse repetition frequency, dc power-supply filter-capacitor value, and gas composition, the laser pulsation has been enhanced. This laser has been operated at 1-kW average laser power level in pulsed mode at 1.3-kHz pulse repetition frequency with a peak-to-average power ratio of 3.35 and laser-power modulation depth of almost 100%     

Beam deflection in a transverse-flow gas laser

The beam deflection phenomenon of a transverse-flow discharge-excited CO₂ gas laser is studied by measuring the beam pattern with the rotating-wire method at gas velocities of 35-70 m/s, gas pressures of 5.3-10.6 kPa (10³ N/m²), and laser powers of 50-700 W. The beam deflection distance increased with increasing laser power and gas pressure and with decreasing gas velocity. All the data points of beam deflection distances observed experimentally fell on one line when normalized by the gas pressure and plotted against the measured temperature gradient in the resonator. It is shown that the most important factor affecting the beam deflection is the refractive index variation of the gas produced by the gas temperature gradient in the resonator. Methods to reduce the beam-deflection distances are suggested