Nature of the temperature dependence of the absorption coefficient in the remote wing of the 4.3 μm CO2 band

The complex nature of the temperature dependence of the absorption coefficient beyond the edge of the CO₂ 4.3 m band — the change of the sign of the derivative of the absorption coefficient with respect to the temperature near the edge and in the remote wing of the band — is explained on the basis of the theory of spectrum line wings. The first change in the sign is due to the temperature dependence of the classical potential of intermolecular interaction; the second depends mainly on the difference in the quantum energies of molecule interaction. Therefore, a study of the dependence of the absorption coefficient on the temperature in the band wings provides information about the nature of the terms of the interacting molecules.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 42–45, December, 1987.     

Gain dynamics of the 4.3 μm CO2 laser

A detailed study of the gain dynamics of the pulsed, optically pumped 4.3 m CO₂ laser is described. Small-signal gain coefficients as high as 14%/cm are measured in a 4.3 m amplifier using low-power pulses from a 4.3 m probe laser. The measurements are compared with a rate-equation model and good quantitative agreement is obtained. The model, which uses no adjustable parameters, is described in detail. Gain is studied as a function of optical pumping power, gas mixture, gas pressure and discharge excitation of the 4.3 m amplifier. Optimization of the gain is discussed.     

Spectral line parameters including temperature dependences of self- and air-broadening in the 2←0 band of CO at 2.3 μm

Temperature dependences of pressure-broadened half-width and pressure-induced shift coefficients along with accurate positions and intensities have been determined for transitions in the 2←0 band of ¹²C¹⁶O from analyzing high-resolution and high signal-to-noise spectra recorded with two different Fourier transform spectrometers. A total of 28 spectra, 16 self-broadened and 12 air-broadened, recorded using high-purity (≥99.5% ¹²C-enriched) CO samples and CO diluted with dry air (research grade) at different temperatures and pressures, were analyzed simultaneously to maximize the accuracy of the retrieved parameters. The sample temperatures ranged from 150 to 298 K and the total pressures varied between 5 and 700 Torr. A multispectrum nonlinear least squares spectrum fitting technique was used to adjust the rovibrational constants (G, B, D, etc.) and intensity parameters (including Herman–Wallis coefficients), rather than determining individual line positions and intensities. Self- and air-broadened Lorentz half-width coefficients, their temperature dependence exponents, self- and air-pressure-induced shift coefficients, their temperature dependences, self- and air- line mixing coefficients, their temperature dependences and speed dependence have been retrieved from the analysis. Speed-dependent line shapes with line mixing employing off-diagonal relaxation matrix element formalism were needed to minimize the fit residuals. This study presents a precise and complete set of spectral line parameters that consistently reproduce the spectrum of carbon monoxide over terrestrial atmospheric conditions.     

Calculations and measurements of N2O absorption at high temperature in the 4.5 μm region

An approximate N₂O spectroscopic database suitable for high temperature and medium resolution applications has been created in the 4.5 μm region. Intensities of ¹⁴N₂ ¹⁶O hot bands have been extrapolated pragmatically from the v₃ band intensity and energies of vibrational levels have been computed by diagonalization of the effective Hamiltonian. The new parameters have been merged with the data available in the HITRAN database and in the recent experimental work of Toth. The entire list has then been used to generate individual line parameters. Pure N₂O spectra have been recorded with a Fourier Transform spectrometer up to 900 K and with 1 cm⁻¹ resolution. A good agreement between these spectra and line-by-line calculations using the new database is obtained while the use of HITRAN greatly underestimates absorption at high temperature.     

Decomposition of broad absorption band at about 3·2 μm in multicomponent aluminophosphate glasses

The decomposition of the broad absorption band at about 3·2 m in multicomponent aluminiumphosphate glasses was made using a computer Tesla 200. The results show that the band is composed of at least three bands. It is suggested that the two side bands are due to the stretching vibrations of the residual water molecules in the glass and only the middle band belongs to the protons.     

Short-term and long-term frequency characteristics of 2 μm single frequency Tm:YAG laser at room temperature

Placing one 0.1 mm YAG F-P etalon at nearly Brewster angle and combined use 1 mm fused silica in the cavity, a diode-pumped linear-polarized single-longitudinal-mode (SLM) Tm:YAG laser operating at 2 μm is achieved. This paper is focused on the stability of the linear-polarized SLM laser, including power stability, long-term frequency stability and short-term frequency stability. And the factors affecting the frequency characteristics of laser were also analyzed. The instability of the linear-polarization SLM laser is less than 1%. The long-term frequency stability is in the range from 1.16 × 10⁻⁷ to 1.75 × 10⁻⁷ monitored by the wave meter. And the short-term frequency stability is 97 Hz/μs measured with the self-beating heterodyne detection method.     

3W average power 4.3 μm CO2 laser

A 3 W average power CO₂ laser oscillating in the range of 4.3m (10°1 to 10°O transition) is described. At the same time, the laser can emit 100 W in the sequence band 00°2 to 10°1 (10.6m). It is based on a commercial system with continuous-wave discharge of 12 m length and a slow gas flow. It operates in the Q-switched mode at pulse repetition rates up to 15 kHz. The pulse peak power is 1 kW and the pulse duration is 200 ns. The deviation from the theoretical efficiency limit has been decreased by a factor 2.5 in our device, due to saturation of the pumping (sequence band) radiation. We predict an improvement by another factor of 5 (possible average power of 10 to 20 W), if one avoids the absorption in the discharge-free zones.     

Sensitive detection of CO2 concentration and temperature for hot gases using quantum-cascade laser absorption spectroscopy near 4.2 μm

Mid-infrared quantum-cascade laser (QCL) absorption spectroscopy of CO₂ near 4.2 μm has been developed for measurement of temperature and concentration in hot gases. With stronger absorption line-strengths than transitions near 1.5, 2.0, and 2.7 μm used previously, the fundamental band (00⁰1–00⁰0) of CO₂ near 4.2 μm provides greatly enhanced sensitivity and accuracy to sense CO₂ in high-temperature gases. Line R(74) and line R(96) are chosen as optimum pair for sensitive temperature measurements due to their high-temperature sensitivity, equal signal-to-noise ratio (SNR), weak interference of H₂O transitions, as well as relatively strong line-strengths in high temperature and weak absorption in room temperature. The high-resolution absorption spectrum of the far wings of the R-branch (R56–R100) in the fundamental vibrational band of CO₂ is measured in a heated cell over the range 2,384–2,396 cm^?1 at different temperatures from 700 to 1,200 K. Taking three factors into consideration, including SNR, concentration detectability, and uncertainty sensitivity, the absorption line R(74) is selected to calculate CO₂ concentration. The tunable QCL absorption sensor is validated in mixtures of CO₂ and N₂ in a static cell for temperature range of 700–1,200 K, achieving an accuracy of ±6 K for temperature and ±5 % for concentration measurements.     

Line positions and intensities in the υ2 band of H216O

The constants involved in the rotational expansion of the transformed transition moment operator of the v ₂ band of H₂ ¹⁶O have been determined through a fit of about 110 measured line intensities. A comparison between theoretical and experimental values of these constants is given. The coefficient ²μ _x of the expansion of the dipole moment with respect to normal coordinates is deduced to be

Moreover, a knowledge of the transformed transition moment operator has been used to compute the whole spectrum of the v ₂ band.     

The refractivity of CO2 gas in the region of 10 μm

The refractivity of the CO₂ gas is measured with an experimental error of 2% in the 10-m region, using 10.4-m band CO₂ laser line. The frequency of the CO₂ laser is swept through the Doppler profile of the laser line. The experiment is achieved using a 0.63-m He–Ne/10.6-m CO₂-laser interferometer with a 2-m long vacuum cell. From the result, it is found that the Koch's formula also holds for the wavelengths in the 10-m region within an accuracy of 2%.     

CO concentration and temperature sensor for combustion gases using quantum-cascade laser absorption near 4.7 μm

A sensor for sensitive in situ measurements of carbon monoxide and temperature in combustion gases has been developed using absorption transitions in the (v′=1←v″=0) and (v′=2←v″=1) fundamental bands of CO. Recent availability of mid-infrared quantum-cascade (QC) lasers provides convenient access to the CO fundamental band near 4.7 μm, having approximately 10⁴ and 10² times stronger absorption line-strengths compared to the overtone bands near 1.55 μm and 2.3 μm used previously to sense CO in combustion gases. Spectroscopic parameters of the selected transitions were determined via laboratory measurements in a shock tube over the 1100–2000 K range and also at room temperature. A single-laser absorption sensor was developed for accurate CO measurements in shock-heated gases by scanning the line pair v″=0, R(12) and v″=1, R(21) at 2.5 kHz. To capture the rapidly varying CO time-histories in chemical reactions, two different QC lasers were then used to probe the line-center absorbance of transitions v″=0, P(20) and v″=1, R(21) with a bandwidth of 1 MHz using fixed-wavelength direct absorption. The sensor was applied in successful shock tube measurements of temperature and CO time-histories during the pyrolysis and oxidation of methyl formate, illustrating the capability of this sensor for chemical kinetic studies.     

Energy scaling of 4.3 μm room temperature Fe:ZnSe laser

We demonstrate a fourfold increase of the output energy of the gain-switched mid-IR Fe:ZnSe laser. Iron doping of the ZnSe polycrystalline samples was realized using a postgrowth thermal-diffusion method from the metal film. Gain-switched Er:Cr:YSGG (2.8 μm) laser pumped Fe:ZnSe lasing was studied in a Fabry-Perot cavity over a 236-300 K temperature range. The maximum output energy reached 4.7 mJ at 4.3 μm and 3.6 mJ at 4.37 μm at 236 K and 300 K and was limited only by available pump energy. The laser threshold was about 8 mJ and was practically unchanged over the studied temperature range. The laser slope efficiencies, measured with respect to the input pump energy, decreased from 19% to 16% with an increase of temperature from 236 to 300 K. The output radiation featured a Gaussian spatial profile with M(2) = 2.6.     

Saturation spectroscopy of CO2 and frequency stabilization of an optical parametric oscillator at 2.77 μm

We report the frequency stabilization of a CW single-frequency, singly resonant optical parametric oscillator (OPO) to the saturation absorption center of the (12)C(16)O2[10°1,02°1](II)>←00°0 P(14) line at 2.77 μm. The CO2 molecules were excited by the OPO idler wave, and the absorption signal was monitored through the fluorescence at 4.3 μm using a gold-coated longitudinal cell. The idler frequency was stabilized onto the line center by wavelength modulation method. The linewidth of the saturation dip was estimated to be 4.7 MHz, and the achieved frequency stability was 3.9 kHz (3.6×10(-11)).     

Laser absorption spectrometer using frequency chirped intensity modulation at 1.57 μm wavelength for CO2 measurement

We have demonstrated the laser-absorption spectrometer system using frequency chirped intensity modulation at 1.57 μm wavelength for measurement of CO(2) concentration. Using this technique, backscattered laser radiation from different ranges can be discriminated in the frequency domain of the electrical signal. We have reported the discrimination of two signals from the targets with different ranges. It is shown that stable measurements with short time fluctuation corresponding to 4 ppm (rms) were obtained with 32 s measurement intervals. Furthermore, there is qualitative good agreement on, at least, the diurnal changes between the results of the laser absorption spectrometer system and the in-situCO(2) sensor.     

Analysis of the D2O absorption spectrum near 2.5 μm

The D₂O absorption spectrum recorded with a selective modulation Girard spectrometer in the region of 3690–4190 cm⁻¹ (resolution ∼0.07 cm⁻¹) has been analyzed. Based on the fitting of experimental data the spectroscopic parameters of the vibrational state (011) have been determined and the parameters of the vibrational states (110) and (030) have been estimated.     

Line Intensities of the Phosphine Dyad at 10 μm

Over 1000 measured line intensities of phosphine (PH₃) are reported for the region 830 to 1310 cm⁻¹, which contains the two lowest fundamentals in Coriolis interaction. These measurements are fitted to 1.5% for ν₂ at 992.13 cm⁻¹ and 2.1% for ν₄ at 1118.31 cm⁻¹, respectively, using five intensity parameters that include three Herman-Wallis type terms. In addition, some 60 intensities of the 2ν₂-ν₂ hot band are modeled. The corresponding assignments and line positions of the dyad from previous work [L. Fusina and G. Di Lonardo, J. Mol. Struct.517-518, 67-78 (2000)] are combined with the present intensity study to provide an improved PH₃ database for planetary studies. The total integrated intensity for the dyad is 156.(4) cm⁻² atm⁻¹ at 296 K.     

Cascaded carbon monoxide laser frequency conversion into the 4.3-4.9 μm range in a single ZnGeP2 crystal

Collinear cascaded mid-IR frequency conversion in a single nonlinear optical crystal was accomplished. Concurrent collinear generation of the sum frequency of multiline fundamental band carbon monoxide laser radiation as a first frequency conversion cascade resulted in collinear difference frequency generation within the 4.3 to 4.9 μm spectral range when mixing this sum frequency radiation with the fundamental one as the second cascade in the same ZnGeP(2) nonlinear optical crystal.     

Low temperature laser absorption spectra of methane in the near-infrared at 1.65 μm for lower state energy determination

Direct absorption spectra of the 2ν 3 band of methane （CH₄） from 6038 to 6050 cm～（-1） were studied at different low temperatures using a newly developed cryogenic cell in combination with a distributed feedback （DFB） diode laser.The cryogenic cell can operate at any stabilized temperature ranging from room temperature down to 100 K with temperature fluctuation less than ±1 K within 1 hour.In the present work,the CH 4 spectra in the range of 6038-6050 cm～（-1） were recorded at 296,266,248,223,198,and 176 K.The lower state energy E and the rotational assignment of the angular momentum J were determined by a "2-low-temperature spectra method" using the spectra recorded at 198 and 176 K.The results were compared with the data from the GOSAT and the recently reported results from Campargue and co-workers using two spectra measured at room temperature and 81 K.We demonstrated that the use of a 2-low-temperature spectra method permits one to complete the E and J values missed in the previous studies.