Generation of far-infrared pulses by use of a passively mode-locked high-pressure CO2 laser

Summary Our present study demonstrates for the first time generation of frequency-tunable intense infrared radiation pulses in the nanosecond time regime by mode-locking a high pressure CO₂ laser using p-doped germanium as a saturable absorber. These pulses were transferred into the FIR region via stimulated Raman scattering of the CO₂ laser radiation in CH₃F resulting in subnanosecond FIR laser pulses.     

Generation and detection of tunable ultrashort infrared and far-infrared radiation pulses of high intensity

We report on generation and detection of intense pulsed radiation with frequency tunability in the infrared and far-infrared spectral regions. Infrared radiation is generated with a transversally electrically excited high pressure CO₂ laser. A laser pulse of a total duration of about 300 ns consisted, due to self mode locking, of a series of single pulses, some with pulse durations of less than 450 ps and peak powers larger than 20 MW. Using these pulses for optical pumping of a Raman D₂O laser, trains of short far-infrared pulses with durations less than 400 ps were obtained. For detection a new ultrafast superconducting detector was used.     

Single mode CO2-laser pulses of high power

Single longitudinal mode CO₂-laser pulses of 200 MW power have been produced by seeding an unstable resonator cavity with single mode radiation from a hybrid low pressure — high pressure CO₂ laser system. The results indicate that a ratio of dominant over residual mode intensities of better than 10⁵ were achieved.     

High precision materials processing using a novel Q-switched CO2 laser

Holes with diameters of about 400 µm have been laser trepanned in Ti6Al4V and carbon fibre reinforced polymer (CFRP) thin sheets with a thickness of 0.5 mm. A commercial CO₂ laser (SM1500E, FEHA LaserTec, Germany) and a novel Q-switched CO₂ laser (µ-storm, IAI, Netherlands) were used as radiation sources. Optical microscopy, scanning electron microscopy and replicas of the processed holes were used to investigate the influence of the CO₂ laser pulse parameters (e.g. pulse energy, duration and peak power) on the processing quality. It was shown that melt formation and high temperature oxidation reactions of Ti6Al4V during thermal laser processing were reduced significantly by using short and high intense Q-switched CO₂ laser pulses. During trepanning of CFRP heat affected zones resulting from the extremely different thermal properties (melting and vaporisation temperature, heat conduction) of the reinforcing carbon fibres and the polymer matrix were reduced significantly by using the Q-switched CO₂ laser. The results demonstrate that Ti6Al4V and CFRP can be processed very precisely with CO₂ laser radiation and air as processing gas without melt formation and thermal damage.     

Nonlinear absorption at 10.6 μm in Hg1−xCdxTe

Near band-edge illumination of Hg_1?xCd_xTe (x ≈ 0.2) by short bursts of CO₂ laser radiation has been shown to lead to highly nonlinear absorption and saturation. For sufficiently high infrared intensities the transmission through an absorber is enhanced more than three orders of magnitude. The effect should be useful in temporal shaping of high intensity CO₂ laser pulses.     

Mixing of 30 THz laser radiation with nanometer thin-film Ni-NiO-Ni diodes and integrated bow-tie antennas

Mixing experiments with 30 THz CO₂-laser radiation as well as the detection of 35 ps 30 THz pulses of an optical-free-induction-decay CO₂-laser system have been performed with the first nanometer thin-film Ni-NiO-Ni diodes with a minimum contact area of 0.012 µm². Difference frequencies up to 85 MHz were detected by mixing two different CO₂-laser beams coupled to the diode with an integrated bow-tie antenna. The dependence of the beat signal on bias voltage, laser power and polarization of the infrared laser radiation was determined.     

Generation and detection of tunable ultrashort infrared and far-infrared radiation pulses of high intensity

We report on generation and detection of intense pulsed radiation with frequency tunability in the infrared and far-infrared spectral regions. Infrared radiation is generated with a transversally electrically excited high pressure CO₂ laser. A laser pulse of a total duration of about 300 ns consisted, due to self mode locking, of a series of single pulses, some with pulse durations of less than 450 ps and peak powers larger than 20 MW. Using these pulses for optical with durations less than 400 ps were obtained. For detection a new ultrafast superconducting detector was used.     

Relaxation dynamics and photoconductivity inp-type germanium

The relaxation of holes inp-Ge excited by CO₂ laser radiation is analyzed in a detailed cascade model. The scattering times are taken in part from theory and from recent saturation-spectroscopic measurements. From this model we calculate the photoconductive response quantitatively. Experimentally we observe the conductivity change induced by single ns CO₂ laser pulses and find good agreement with the predictions.     

Key methods for intensifying soft X-ray emission from a laser plasma for lithography

A two-pulse two-wave (Nd and CO₂ lasers) scheme is proposed for irradiating a laser target, which ensures the highest factor of laser radiation conversion to the X-ray range (13.5 nm ± 1%). Analytic estimates are obtained for parameters of pulses and of the target made of Xe or Li. Numerical optimization is performed for X-ray emission from a spherical Xe target exposed to a CO₂ laser pulse. The maximal factor of conversion of laser radiation to X rays is ～1%. Angular and spectral characteristics of X-ray radiation are obtained. The flux of fast Xe ions ejected from the target and damaging the Mo/Si coating of X-ray mirrors is estimated.     

Efficient generation of FIR radiation by optical pumping of D2 18O

In this paper we show that D₂ ¹⁸O vapour, optically pumped with a continuously tunable high pressure CO₂ laser, is an excellent source for far infrared radiation. Both high photon conversion coefficients and broad Raman gain regions were found for a large number of new laser transitions spread over the frequency range from 25 cm^–1 to 240 cm^–1. We demonstrate that these Raman gain regions can be used to generate far infrared laser pulses with high intensity and durations of about 100 ps.     

Short delay time UV preionized TEA CO2 laser operating with binary CO2/H2 and CO2/He gas mixtures

In order to obtain short tail-free output laser pulses from a TEA CO₂ laser, parametric study of the laser operation with CO₂/H₂ and CO₂/He binary gas mixtures containing high CO₂ concentrations was carried out. A small scale UV preionized short delay time TEA CO₂ laser was employed. In terms of the maximum extractable output pulse energy and power, the more conventional CO₂/He gas mixture was found to be inferior in comparison with the CO₂/H₂ mixture proposed here.     

Tunable TEA CO2 laser for long-range DIAL lidar

The high-power pulse-periodic TEA CO₂ laser intended for use in the structure of mobile ground-based long-range IR DIAL lidar is developed. For achievement of the highest peak power of radiation, the system of laser excitation with a voltage of ±40 kV and effective preionization allowing one to work at high pressure of various gaseous mixtures is created. A study of energetic, temporal and spatial parameters of laser radiation for used gaseous mixtures are executed at a pressure of 0.5–2.4 atm. The pulses of laser emission with an energy more than 10 J and duration on FWHM of ∼30 ns are obtained for the case of heliumless mixtures. The maximal obtained peak power of the laser radiation is 100 MW. The maximal efficiency of the laser including the energy deposited in the glow discharge is 12.6%. At the use of the selective cavity, the laser operation at 85 emission lines of the CO₂ molecule is obtained and at 60 lines, the energy of radiation exceeded 4 J.     

10 kW cavity operation of a submillimetre CH3F laser

Operation of a CH₃F laser at 496μ has been obtained in a hemispherical optical cavity pumped by up to 42 J of 9.55μ CO₂ TEA laser radiation. Highly modulated 1μ pulses delivering 6 mJ at peak powers greater than 10 kW have been recorded.     

New phenomena related to pulsed far-infrared superradiant and Raman emissions

We report on new effects in relation to optical pumping of far-infrared (FIR) superradiance and Raman emissions in CH₃F, CH₃CN, D₂O, NH₃ by rapidly truncated 10 μm CO₂ laser pulses and optical-free-induction decay (OFID) 30 ps-10μm-CO₂ laser pulses. Thus, we have found a drastic reduction of the FIR-pulse duration which is closely related to the fast truncation of about 10 ps of the plasma shutter used in our OFID 10 μm-CO₂ laser system. The forward emissions exhibit the on-set of swept-gain superradiance and the appearance of Raman emission with increasing pressure in the FIR cell. This implies line competition between the superradiant and Raman emissions and thus results in different emission regimes which we have investigated systematically. The backward emissions are superradiant over the whole range of our investigations and show high quality with respect to stability and reproducibility which is important for applications.Furthermore, we have found that the CO₂-pump radiation and the generated FIR Raman emission interact mutually which results in anticorrelated fluctuations of the two fields. We have been able to interpret this effect as a result of periodic back-and-forth fluctuations of the Λ-like three-level molecular systems.Finally, we have observed the development of OFID of the truncated CO₂-pump pulses in the FIR-laser gases. This effect has been thoroughly investigated and as a result we have generated for the first time ps-10 μm-CO₂-OFID pulses with FIR-laser gases as spectral filters instead of the usual hot CO₂ gas. New phenomena and advantages of our new OFID system based on FIR laser gases are discussed.     

Simulation of parametric oscillation in the submillimeter range at pumping of the ZnGeP2 crystal by a train of 100-ps high-power pulses

Analysis is given for a possibility of singly resonant parametric oscillation in the submillimeter range at synchronous pumping of the ZnGeP₂ crystal by a train of 100-ps second-harmonic pulses from the CO₂ laser with the radiation energy 1.0 J. The calculation shows that using the ZnGeP₂ crystal and the second harmonic of the CO₂ laser with the energy density 1.8 J cm⁻², one can get the peak submillimeter radiation power from 3.6 to 12 MW in the range from 95 to 300 μm (1.0–3.3 THz). The expected peak power values are larger than the experimental ones obtained by other nonlinear optics methods.     

Injection locked single mode operation of a tea CO2 laser with high energy extraction

Generation of reproducible smooth pulses from longitudinal mode control of a TEA CO₂ laser by signal injection from a cw CO₂ laser is reported and the operating conditions investigated for several new geometries. Applied to unstable resonators, the efficient and spectrally pure, high power output obtains has important application in the area of optical pumping.     

Efficiency of optical-to-acoustic energy conversion upon the interaction of a pulsed laser radiation with a liquid: I. Calculation of the efficiency upon acoustooptic interaction

Relationships for the efficiency of conversion of laser radiation energy to acoustic energy for the acoustooptic (thermal) mechanism of interaction are derived. The cases of short and long laser pulses interacting with the rigid and free boundaries of a heavily absorbing liquid are considered. The efficiency is numerically calculated for the situation when the radiation of a transverse-excitation atmospheric (TEA) CO₂ laser interacts with water with regard for the temperature variation of the volume thermal expansion coefficient of the latter.     

Intracavity Raman conversion for 16-μm Stokes pulse generation in para-hydrogen

16-μm Stokes pulses were directly generated for the first time to our knowledge by an intracavity configuration for the para-hydrogen Raman laser. We have analyzed Stokes field growth using a focused gain model and designed a pump/Stokes cavity to satisfy CO₂ pump power and pulse duration requirements for Raman oscillation. The CO₂ laser oscillation with circular polarization was realized by seeding externally circularly polarized CO₂ radiation. An output energy of 2.4 mJ was obtained with the output coupler of 0.5% transmittance, which indicated that 420 mJ of Stokes pulse energy was stored inside the cavity. This suggests that a much higher energy can be extracted by the optimization of cavity parameters. Received: 18 November 1998 / Published online: 26 May 1999     

Design and operation characteristics of a high power transverse flow pulser sustained cw CO2 laser

A transverse flow, transverse discharge cw CO₂ laser in which de discharge is sustained by employing high repetition rate high voltage pulses has been developed. Pulser sustained discharge through electrodes of innovative design provided uniform excitation at electrical input power densities more than 10 W/cc. Laser output power more than 2.5 kW was obtained in a laser gas mixture consisting of 0.5 mbar of CO₂, 16 mbar of N₂ and 38.5 mbar of He. Design details and operational characteristics of this laser are presented.     

Mathematical simulation of high-power laser systems

The energetic, spectral, and time characteristics of a high pressure CO₂ laser at high pumping level are studied. It is shown that in the free generation regime high power radiation pulses R~ 10^–8 sec in duration with a multifrequency spectrum can be obtained. Laser output characteristics are presented as functions of gas pressure, pumping energy, and resonator parameters.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 15–20, June, 1981.