RF excitation of a flowing gas CO2 waveguide laser

Investigation of an RF excited CO₂ waveguide laser in flowing gas operation is reported. Power extraction of 0.8 W/cm with an efficiency of 10.3% has been achieved. Using W.W. Rigrod's analysis (1965), values of the small-signal gain α₀ and saturation parameter I_s have been determined for different excitation levels and for different pressures of the amplifying medium. The parameters α₀ and I_s, have been determined as 0.6%/cm and 10.4 kW/cm ², respectively, at 125 torr and 100 W/cm³ RF loading power. These values are close to those reported for sealed-off RF CO₂ waveguide lasers with xenon added to the gas mixture     

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     

A new helical coupling microwave antenna excited high-powerCO2 laser using a cylindrical resonant cavity

A new helical coupling antenna which was installed into a cylindrical microwave cavity (2.45 GHz) has been investigated to obtain high-power CO₂ laser operation. Laser experiments using a system of swirling fast axial gas flow parallel to the beam have been performed. The geometric properties of the helical antenna and the gas flow structures were optimized by using numerical codes analyzing the electric fields and gas flow dynamics. The swirling flow of the laser gas mixtures of CO₂-N₂-He=0.7-5-19 was caused in the discharge tube by an 8-slit nozzle. It is shown that the electric field obtained by the proper design of the helical coupling coil yields homogeneous discharges. The maximum output power and the efficiency (RF to laser power conversion ratio) was obtained as 1010 W and 15%, respectively, at a gas pressure of 45 torr and a gas flow rate of 57 kg/h     

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     

Hooting modes in a CO2 waveguide laser

CO₂ waveguide lasers `hoot' when, instead of the solitary resonator mode expected from a homogeneously broadened gas laser, two or more resonator transverse modes laser simultaneously on a common laser line. The unwanted mode or modes, even if very faint and present only occasionally, can impede heterodyne measurements where a clean frequency spectrum is required. The authors report measurements of transverse-mode beat frequencies from a small RF-excited CO₂ laser with a hybrid Al/Al₂O₃ square-bore waveguide and plane mirrors; these frequencies alter with mirror tilt by up to 70 MHz mrad^-1. A minimum beat frequency is seen when the laser is well assigned. The authors suggest that a model of the misaligned resonator may help to identify the faint modes responsible for hooting     

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     

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     

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     

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     

Discharge-induced frequency modulation of RF excited CO2waveguide lasers

The mechanisms causing a shift in the oscillation frequency of an RF-excited CO₂ waveguide laser through changes in the discharge excitation power are investigated. Frequency shifts in the range of ±0.5 to 1 MHz per watt of RF input power were measured. These are shown to be consistent with the effects of thermal expansion of the laser gas caused by an increase of the gas temperature due to RF power absorption. It is also shown that the effects of gas dissociation are small but significant, whereas, contrary to earlier suggestions, the effects of electron density fluctuations are negligible. The discharge-induced frequency shift may be used as a simple frequency modulator with a frequency deviation of ±30 MHz, although the usefulness of this effect is limited to a bandwidth of about 500 Hz, due to the acoustic resonance of the waveguide channel     

New FIR laser lines from fully deuterated dichloromethane:completion of perpendicular/parallel emission pairs

We report 16 new far-infrared (FIR) laser emissions of CD₂ Cl₂. These lines are obtained by optically pumping the active molecule by means of a large-tunability waveguide CO₂ laser. Our tunability of 290 MHz around each CO₂ laser emission allows for the excitation of large-offset absorption transitions of CD₂Cl₂, not reachable by conventional CO₂ Lasers. While the overwhelming majority of the known FIR laser emissions of CD₂Cl₂ are polarized parallel to the polarization of the pump CO₂ line, 13 of the 16 new lines reported in this paper are polarized perpendicularly to the corresponding CO₂ pump line. The presence of both perpendicular and parallel lines is extremely important for the assignment of laser transitions     

射频激励金属板条波导CO2激光器的功率输出特性

为了获得较高的激光输出功率,介绍了射频激励扩散冷却全金属板条波导CO2激光器,放电区域由左右两个铝合金壁和上下两个铝合金电极构成,放电区域高2 mm,宽20 mm,长386 mm,工作气体混合比CO2:N2:He:Xe=1:1:3:0.26,该技术代替了过去的金属陶瓷结构,结合纵向电压均匀分布技术和面增比技术,获得了127 W的激光输出功率,光电转换效率高达14%。实验中,照相机放在激光器的纵向轴心上,清楚地记录了CO2激光器的放电现象。实验结果表明,该激光器小信号增益1%/cm,饱和光强1 200 W/cm2,促进了CO2激光器的发展。     

Optimization of CO2 laser frequency stabilization viaexternal Doppler dither modulation

An external Doppler dither modulation scheme for locking the CO ₂ laser frequency to the center of the 4.2-μm fluorescence Lamb dip of CO₂ is discussed. The optimum values for the frequency dither, fluorescence cell temperature and CO₂ absorber gas pressure depending on the experimental conditions are determined. The design of two different dither mirror arrangements is described. With optimum parameters and with τ=40 ms integration time, it is possible to lock the CO₂ laser to within ±10 kHz to line center over several minutes. Measurements of ultrahigh-resolution Lamb dip spectra of the υ₂ as R (3,3) transition of NH₃ show the superiority of external dither modulation over the conventional internal modulation scheme with respect to the spectral purity of the laser radiation     

Continuously tunable high-pressure CO2 laser forspectroscopic studies on trace gases

A high-pressure CO₂ laser with unique characteristics in terms of continuous tunability and emission bandwidth is presented. It is operated at a pressure of 11.5 bar and transversely excited by short, high-voltage pulses generated by a double LC inversion circuit. Auxiliary discharges parallel to the electrodes provide a sufficient free-electron density through UV ionization of the laser gas mixture. The laser resonator consists of a near-grazing-incidence grating setup in which the grating is positioned at a large incidence angle of 77°. A theoretical model for the calculation of the emission bandwidth is presented and its predictions are compared to direct measurements and show excellent agreement. The achieved very narrow bandwidth of 0.018 cm^-1 constitutes the ultimate wavelength resolution of any detection system using this laser as radiation source. It allows the resolution of any fine structure in the spectra of absorbing gases at atmospheric pressure. Continuous tunability has been achieved over 76 cm^-1 between 932 cm^-1 and 1088 cm ^-1 with minimum pulse energies in excess of 10 mJ. The narrow bandwidth precludes the occurrence of mode-pulling effects so that the laser exhibits a linear wavelength tuning behavior throughout the entire emission range. The calibration of the laser wavelength is performed by photoacoustic measurements on low pressure CO₂ gas. An absolute accuracy of ±10^-2 cm^-1 is achieved. A great potential improvement in detection selectivity can thus be expected from a scheme with the high-pressure CO₂ laser as radiation source     

Laser action in hydrazine: observation and characterization of newlarge offset FIR laser lines

Using for the first time a waveguide CO₂ laser with a tunability of 300 MHz, in order to pump large offset infrared absorptions, we obtained 34 new far-infrared laser lines from the hydrazine molecule; their wavelengths range from 155.2 to 669.3 μm. All of the new lines and some other already known laser emissions are characterized in the wavelength, the polarization relative to that of the pumping CO₂ laser, the optimum pressure of operation, relative intensity, and the offset relative to the CO₂ center frequency     

Observation and characterization of new FIR laser lines from formicacid

We report the observation and characterization of seven new far-infrared (FIR) laser lines from H¹²COOH and H¹³COOH in the range 232.2 to 746.6 μm optically pumped by a wide-tunability waveguide CO₂ laser. Their absorption frequency is outside of the tuning range of a conventional CO₂ pump laser     

Observation of a large number of new laser lines from12CD3F gas optically pumped with a continuouslytunable high-pressure pulsed CO2 laser

An observation is reported of a large number of new laser lines from ¹²CD₃F gas optically pumped with a continuously tunable high-pressure pulsed CO₂ laser. Making use of the coincidence of the 10 μm P and R branches of CO₂ with the v₃ and v₆ vibrational-rotational absorption bands of ¹²CD₃F, 180 laser lines were found in the wavenumber range between 8 and 55 cm^-1, all of them yet unknown; these lines are studied for characteristic properties of laser action. All laser lines are assigned as pure rotational transitions in the vibrational excited or ground states     

Laser-induced ionization of air in the presence of preionization

Ionization of air at different pressures is studied for the following situations: (1) a spark gap acting as a preionizer alone, (2) a preionizer with a pulsed CO₂ laser, (3) a preionizer with a CW CO₂ laser, (4) a preionizer with a Q-switched Nd:YAG laser, and (5) and a Nd:YAG laser with a CO₂ laser. Results are quantified and analyzed with respect to some available data     

Characterization of new FIR laser lines from CD3-rockingand asymmetric CD3 deformation modes of 13CD3OD

In this paper, we report new optically pumped far-infrared (FIR) laser lines from the in-plane CD₃-rocking and asymmetric CD ₃ deformation absorption vibrational bands of ¹³CD ₃OD. A waveguide CO₂ laser of wide tunability (290 MHz) was used as the pump source, and a Fabry-Perot open cavity as the FIR laser resonator. Optoacoustic absorption spectrum was used as a guide to search for new FIR laser lines. We could observe 13 new laser lines in the range 103-491 μm. The lines were characterized according to wavelength, relative polarization, relative intensity, and optimum working pressure. The transferred lamb-dip technique was used to measure the frequency absorption transition for both new and previously reported laser lines