Self-pulsing instability of a CH2F2-laser

A single mode self-pulsing instability is reported for the 117 μm optically pumped far infrared CH₂F₂ laser     

IR-FIR Transferred lamb-dip spectroscopy in optically pumped molecular lasers

Pump saturation effects are reported for CH₃ and CH₃F optically pumped molecular lasers. The Lamb-dip of the pumping transition is observed directly in the optoacoustic absorption signal and as a transferred Lamb-dip in the emission of various FIR laser lines (CH₃F 496 μm, CH₃OH 119, 70.5 and 96.5 μm) . The offset of the absorbing transitions and their Stark effect are directly investigated. Results are also reported for the unassigned CH₃OH 164 μm line.     

Efficient doubling and CW difference frequency mixing in the infrared using the chalcopyrite CdGeAs2

We have obtained fifteen percent average internal power conversion efficiency for second harmonic generation in CdGeAs₂ of radiation from a TEA CO₂ laser operating at 10.6 μm. This is the highest reported doubling efficiency for a CO₂ laser. We have also observed cw difference frequency mixing in CdGeAs₂ at wavelengths between 11.4 and 16.8 μm using grating-tuned CO and CO₂ lasers. The CdGeAs₂ crystals are significantly improved in size and optical quality over those previously available.     

Frequency stabilization of laser diodes using 1.51-1.55 μm absorption lines of 12C2H2 and 13C2H2

Frequency stabilization of an 1.5 μm distributed feedback laser diode (DFB-LB) is demonstrated using a vibrational-rotational absorption of acetylene ¹²C₂H₂ and ¹³C₂H₂ molecules (VRAM). Frequency stability within 2 MHz peak-to-peak fluctuation can be achieved in the wavelength region of 1.51-1.55 μm. Frequency-stabilized distributed feedback (DFB) laser compact modules have been constructed. When the fiber-pigtail-type acoustic optical modulator was used for small laser frequency scanning, frequency stability within 3 MHz peak-to-peak fluctuation was achieved without any broadening of laser spectral linewidth. In addition, temperature and pressure dependencies of the acetylene absorption lines are characterized.     

Absorption coefficients of O3 at CO2 laser wavelengths

The O₃ absorption coefficients for the rotational lines P(12)–P(28) of the 9.4 μm emission band of the CO₂ laser are presented. Measurements were made in O₃–air dilute mixtures (20–600 ppm) at 25°C and a total pressure of 1013.25 h Pa using a frequency stabilized cw CO₂ laser and values have been determined with greater precision than in previously reported studies.     

Fine frequency tuning of a high power CO2 laser and its application to optically-pumped molecular lasers

We describe a novel technique for shifting the output frequency of a high power TEA CO₂ laser. The performance of a 16 μm laser is shown to be greatly improved when optically pumped by this tunable source.     

The refractivities of water vapour for CO2 laser lines

The refractivities of water vapour in the 10 μm region at temperatures near 20°C are measured with an accuracy of 1% using a 10.6 μm CO₂/0.63 μm He-Ne laser interferometer with a 2 m long vacuum cell. The measured refractivities are 30% less than the value calculated by extrapolating the dispersion equation for water vapour in the visible region. From this result, the CO₂ laser is found to be useful as a light source for a three-wavelength laser ranging system of correcting for the air refractivity in the path.     

A gigahertz tunable, high power, high repetition rate rf CO2 laser operating without He

This paper reports on a gigahertz mode-tunable waveguide CO₂ laser operating with high-pressure He-free mixtures (0.4 bar), with a long pulse (20 μs) high repetition rate (1 kHz) radio-frequency discharge. Single-mode tuning at 0.92 GHz with good mean laser linewidth (<40 MHz) and large peak power (150 W) have been obtained over 40 laser lines, with 0.3 W mean power. By using standard He reach mixtures both the laser line number and mean power are slightly increased.     

XPS investigation on Upilex-S polyimide ablated by pulse TEA CO2 laser

Laser ablation of Upilex-S polyimide films 80 μm thick was performed in air using a pulse TEA CO₂ laser with wavelength 9.3 μm. A halo surrounding the hole was observed, which is covered with sub-micro particles. Pieces of ablation products protrude from the ablated surface, leading to considerable roughness of the ablated area. Chemical and structural changes of Upilex-S polyimide film surface irradiated by the pulse TEA CO₂ laser in air were analyzed by X-ray photoelectron spectroscopy (XPS). Relative C content in the ablated area was found to be higher, whilst both O and N contents were lower than in the untreated area. This means that TEA CO₂ laser ablation released both the O and N atoms. Also, the peak areas corresponding with carbonyl group (C=O) in the imide system were reduced much more and a new component at 287.0 eV assigned to the amide structure (N---C=O) was detected after laser ablation. These suggest that the pyrolysis of the Upilex-S polyimide was the decomposition of the imide ring between the nitrogen/aromatic carbon atom and carbonyl carbon atom. In addition, another new component arising from >C=O groups was also detected for higher fluence (7.83 J/cm²), and its peak areas is very small. This result indicates that the slight oxidation may take place with higher fluence during laser ablation in air. Based on above-mentioned experimental results, a possible thermally-induced decomposition path of Upilex-S polyimide ablated by TEA CO₂ laser is presented.     

Photo-operated two-photon absorption effects in the GeSe2–Ga2S3–PbI2 glasses

The possibility to operate the two-photon absorption (TPA) of newly synthesized GeSe₂–Ga₂S₃–PbI₂ glasses using the CO laser beam (λ=5.5 μm) as a photoinducing one has been demonstrated. As the fundamental laser beam we have used the illumination of 10.6 μm passively modulated 0.5 ns CO₂ laser with a rate repetition of about 10 Hz. We have established that the maximal photoinduced TPA is observed for the 8% doped samples (up to 14 cm/GW), which is achieved at a pump CO laser pump power density equal to about 0.6 GW/cm². The undoped PbI₂ samples show the TPA maximum at a pump power density of about 0.2 cm/GW. The minimal TPA values were observed for the samples with 5% of PbI₂. The obtained results show that these materials can be used as effective optically operated optical limiters.     

Beam quality of a high-power CO2 laser MOPA system

In this paper, the effects of amplification of diffraction-limited pulsed CO₂ laser radiation over several meters of amplification length on beam quality and pointing stability are documented. Millijoule pulses are amplified up to 3 J. Generation and amplification of the 10 μm wavelength pulses were performed in the discharge volume of an e-beam sustained CO₂ laser. Beam quality is measured according to the ISO/DIS 11146 standard in terms of the beam quality factor M². Fluence distributions were recorded with a beam analysing system of 100 μm spatial resolution. M² parameter values ranged up to 1.55 for amplified pulse energies of 3 J. The necessity of beam-quality restoring techniques is inferred for the multijoule pulse energy regime.     

Vortex penetration depth of κ-(ET)2Cu[N(CN)2]Br

The magnetic field dependence of the in-plane penetration depth λ_|(H) for single crystal κ-(ET)₂Cu[N(CN)₂]Br has been measured at 3, 9.6, and 36 MHz. Over a limited range, λ_| scales with a characteristic field H^*(T) that coincides with a shoulder in the λ_| vs. H curves. Above that field, λ_| increases sharply toward a second inflection point at H^**(T) that coincides with is close to the irreversibility line measured by magnetization. For fields larger than H^** the penetration depth diverges, suggesting that the vortex lattice has melted. The field dependence at one frequency agrees qualitatively with a model of pinned vortices at low fields giving way to flux flow at higher fields. However, the observed frequency dependence deviates significantly from the predictions of this model, suggesting that collective effects play a major role. Our technique also yields a new measurement for the interplane penetration depth λ 300 μm, implying an anisotropy λ/λ_| > 200.     

Emission spectroscopy of ν10 band of vinyl bromide

By pumping with a pulsed CO₂ laser we have observed many FIR emissions in vinyl bromide that we have assigned to the ν₁₀ band at 942 cm⁻¹. We have been able also to assign many other FIR lines observed in previous works, so that we have derived new spectroscopic constants for the vibrational excited level.     

Detection of small N2O concentrations using a frequency doubled CO2 laser

A reliable procedure for remote high-accuracy laser detection of N₂O as one of the principal destroyers of the protective ozone layer of the Earth has been developed. The procedure is based on using a CO₂-laser system emitting efficiently in the 4.5 μm range. In this case lasing from isotopic modification ¹²C¹⁸O₂ of carbon dioxide with its subsequent frequency doubling by a nonlinear crystal is used. With the object of reducing the price the composition of the active medium (both for TEA laser and low-pressure longitudinal-discharge-excitation laser) has been optimized. New high-efficiency intracavity frequency doubling schemes based on nonlinear AgGaSe₂ crystals have been developed for CO₂-lasers of both types. Low concentrations of N₂O and concentrations of the principal background gases CO and H₂O have been measured under real atmosphere conditions with the aid of the lidar complex built around these lasers.     

Reproducibility of optically stimulated luminescence measurements from single grains of Al2O3:C and annealed quartz

We have measured some aspects concerning the performance of a new instrument for the measurement of optically stimulated luminescence (OSL) from single sand-sized grains. The instrument uses a solid-state laser to stimulate OSL from each grain individually. These grains are placed in a regular grid of nine-by-nine holes, which are drilled in the sample disc. We report on tests carried out to determine the precision with which the laser beam can be directed at individual grains in these holes. Single grains of aluminium oxide (Al₂O₃:C) (90–180 μm) and annealed quartz (90–120 μm) were used to test the reproducibility with which the OSL signal can be measured. These experiments suggest that the laser beam can be positioned to within 30 μm and that the reproducibility of OSL measurement is 3.5% on an average.     

Observations of ω0/2 harmonic emission from 0.35μm laser-irradiated plasmas

Observations of ω₀/2 harmonic emission from both spherical and plane targets irradiated by 0.35 μm, 800 ps laser pulses have been obtained with simultaneous high spectral and temporal resolutions of 16 Å and 20 ps respectively. The ω₀/2 harmonic emission spectrum is interpreted as providing a direct measurement of the frequency of the ω₀/2 plasma waves and hence can be used to estimate the electron temperature.     

High resolution submillimetre spectroscopy at 890 GHz

A molecular absorption line of CF₂=CH₂ is in close coincidence with the 337 μm transition of the HCN maser. The absorption coefficient and the separation of this line from maser centre frequence have been measured.     

A continuously variable attenuator for the CO2 laser

A continuously variable attenuator for the CO₂ laser, causing minimal degradation of beam quality, is implemented using rotating ZnSe wedges, and is capable of transmissivities between 0.3% and 18%, and between 27% and 89%. The transmissivity at any particular setting was measured to be essentially constant for wavelengths from 9.27 μm (9R20) to 10.77 μm (10P36).     

High-resolution measurements of molecular absorption line profiles at 10 micrometers

Absorption line profiles of CO₂, C₂H₄ and NH₃ have been measured around a wavelength of 10 μm and under various conditions using a passive heterodyne detection technique with a resolution of 6 × 10^-4 cm^-1.     

Temperature dependence of intensities and widths of N2-broadened lines in the 15 μm CO2 band from tunable laser measurements

Intensities and nitrogen-broadened widths of several low-J lines in the Q-branch of the 15 μm band of CO₂ have been determined over the temperature range 200–300 K. Measurements were made with a tunable infra-red diode laser spectrometer having a spectral resolution 10^-4 cm^-1. Measured intensities are uniformly about 7% lower than available calculations which were based on previous measurements of band intensity. Measured line widths are higher than available calculations and generally followed the relation

b_L⁰(T)=b_L⁰(T₀)(T₀|T)ⁿ

with n = 0.74 (standard deviation 0.08).