Diode laser measurements of line strengths and widths in the 4.5-μm bands of N2O

We have made line-strength measurements in the N₂O ν₃-fundamental region using a tunable diode-laser spectrometer. From these measurements and the Herman-Wallis factor determined by Boissy et al., we find the ν₃-fundamental band strength to be S_v = 1203 ± 22 cm⁻² atm⁻¹ at 297 K. Line-broadening parameters for two ν₃-fundamental lines were determined using nitrogen (N₂) as the broadening gas. Measured strengths and N₂ line-broadening parameters for several (ν¹₂ + ν₃ − ν¹₂) hot-band lines are also presented.     

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%.     

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.     

Diode laser measurements of line strengths and collisional half-widths in the ν1 band of OCS at 298 and 200K

Absolute line strengths and self-broadened half-widths have been measured at 298 and 200 K for spectral lines ranging from J = 1 to 55 in the ν₁ band (860 cm^-1) of ¹⁶O¹²C³²S, using a tunable diode laser spectrometer. The vibrational transition moment (6.412 ± 0.16 × 10^-2D) as well as the absolute intensity (29.63±1.48 cm^-2-atm^-1 at 298 K), of the ν₁ band are determined from these line-strenght measurements. By applying two semi-classical impact theories of collisional broadening, we have obtained results for half-widths at 298 and 200 K which are significantly larger than the experimental data for |m|<50. However, the variation of the linewidths with temperature is well reproduced theoretically.     

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.     

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.     

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.     

High power 4.65 μm single-wavelength laser by second-harmonic generation of pulsed TEA CO2 laser in AgGaSe2 and ZnGeP2

A high power 4.65 ??m single-wavelength laser by second-harmonic generation (SHG) of TEA CO₂ laser pulses in silver gallium selenide (AgGaSe₂) and zinc germanium phosphide (ZnGeP₂) crystals is reported. Experimental results show that the average output power of SHG laser is not only restricted by the damage threshold of the nonlinear crystals, but also limited by the irradiated power of fundamental-wave laser depending on the operating repetition-rate. It is found that ZnGeP₂ can withstand higher 9.3 ??m laser irradiation intensity than AgGaSe₂. As a result, using a parallel array stacked by seven ZnGeP₂ crystals, an average power of 20.3 W 4.65 ??m laser is obtained at 250 Hz. To the best of our knowledge, it is the highest output power for SHG of CO₂ laser by far.     

Laser diode photoacoustic and FTIR laser spectroscopy of formaldehyde in the 2.3 μm and 3.5 μm spectral range

The room temperature operating GaInAsSb/AlGaAsSb based diode laser and 66 K InAsSb/InAsSbP laser diode both operating in spectral range of formaldehyde absorption 4350-4361 cm⁻¹ and 2821-2823 cm⁻¹ have been characterized and compared. Very precise arrangement of laser absorption together with high resolution Fourier transform technique was tested. The photoacoustic technique was employed to determine the detection limit of formaldehyde (less than 1 ppmV) diluted by nitrogen for the strongest absorption line of the ν₃ + ν₅ band in the emission region of the GaInAsSb/AlGaAsSb diode laser. The detection limit (less than 10 ppbV) of formaldehyde was achieved in the 2820 cm⁻¹ spectral range in case of InAsSb/InAsSbP laser (fundamental bands of ν₁, ν₅)_.     

Line coupling in the temperature and frequency dependences of absorption in the microwindows of the 4.3 μm CO2 band

The shapes of the self- and N₂-broadened ν₃CO₂ fundamental vibration-rotation band in the microwindows (troughs between the lines) have been measured at various temperatures. Important deviations with respect to the superposition of Lorentzian profiles are observed. These deviations are interpreted in terms of line coupling, which redistributes the intensity in the whole band. In order to take into account this line coupling, two models are considered within the frame of the impact theory. The first model uses the strong-collision approximation to describe the rotational energy transferred by collisions. It leads to a simple analytical expression for the band profile. The second model is based on the exponential-gap law. These two models account well for the frequency dependence of the measured absorption in the microwindows and for the temperature dependence in the case of the N₂-broadened CO₂ band but not in the self-broadened case. The influence of the line-coupling rotational distribution, which differs significantly in the two models, is discussed. The possible role of the finite duration of collision in rotational energy transfer is examined.     

Tunable laser via F 2 + and F 2 − colour centers in the spectral region 0.88–1.25 μm

Frequency-tunable generation by means of F ₂ ⁺ and F ₂ ^? colour centers in a LiF crystal is reported. Colour centers were created by illuminating LiF crystals with electrons of 3 meV energy at the electron current density of 1 μA/cm². The pumping source was a ruby laser with a peak power of 20 MW, a pulse duration of 20 ns, and a repetition rate of 1 Hz. The frequency tuning is obtained in the range of 0.88–1.25 μm. Discussed are the ways of pumping of colour centers and the possibility of lasing in the spectral region of 0.85 to 2 μm in the type of colour centers under investigation.     

Saturable absorbers for passive Q-switching of erbium lasers emitting in the region of 3 μm

The phototropic properties of Fe:ZnSe, Co:ZnSe, and Co:ZnS single crystals have been investigated. It is shown that these crystals can be used to advantage as the saturable absorbers in solid-state erbium lasers emitting in the region of the 3-μm range. The absorption cross sections of the ground states of the Co²⁺ ion in the ZnSe (σ_GSA = 11·10⁻²⁰ cm²) and ZnS (σ_GSA = 5.6·10⁻²⁰ cm²) crystals and of the Fe²⁺ ion in the ZnSe (σ_GSA = 50·10⁻²⁰ cm²) crystal at λ = 2.79 μm were determined experimentally. It has been established that the above-indicated crystals in the excited state absorb light weakly. The use of these crystals as passive Q switches made it possible to realize, for the first time, the regime of Q-switching of a Cr,Er:YSGG laser emitting at a wavelength of 2.79 μm. Single pulses with an energy of 60 mJ and a duration of 170 nsec were obtained in the regime of passive Q-switching. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 747–751, November–December, 2005.     

Development of 385-μm D2O laser for plasma diagnostics

An optically-pumped 385-μm D₂O laser has been constructed for Thomson scattering ion temperature measurements in tokamak plasma. Stable single mode and stable tunable (over ±1 GHz) operation of the pump 9R (22) TEA CO₂ laser is performed by using an intracavity ZnSe etalon which is temperature-controlled within ±0.01°C. The saturation broadening with D₂O absorption line is observed for the first time using the tunable 9R (22) CO₂ laser. The pressure broadening coefficient of the N₂O absorption line is measured to be 7.6 MHz/torr using the ±1 GHz tunable 385-μm D₂O Raman laser. At 385 μm, an output quantum efficiency as high as 21% is obtained.     

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.     

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.     

Frequency stabilization of diode laser to 1.637 μm based on the methane absorption line

The frequency of an external-cavity diode laser has been stabilized to 1.637 μm by using the reference of absorption lines of methane. The method can be applied to wavelength division multiplexed optical communication, fiber-optic sensing systems, as well as the high-sensitivity detection of methane. The derivative-like error signal yielded by frequency modulation and phase sensitivity detection technology is inputted into the PI feedback loop circuit in order to stabilize the frequency to the line center. After stabilization, the frequency fluctuation of diode laser is held within 5.6 MHz, and the root of Allan variance of error signal reaches a minimum of 1.66×10-10 for an average time of 10 s.     

Wavelength-modulation-spectroscopy for real-time, in situ NO detection in combustion gases with a 5.2 μm quantum-cascade laser

A mid-infrared absorption strategy with calibration-free wavelength-modulation-spectroscopy (WMS) has been developed and demonstrated for real-time, in situ detection of nitric oxide in particulate-laden combustion-exhaust gases up to temperatures of 700 K. An external-cavity quantum-cascade laser (ECQCL) near 5.2 μm accessed the fundamental absorption band of NO, and a wavelength-scanned, 1f-normalized WMS with second-harmonic detection (WMS-2f/1f) strategy was developed. Due to the external-cavity laser architecture, large nonlinear intensity modulation (IM) was observed when the wavelength was modulated by injection-current modulation, and the IM indices were also found to be strongly wavelength-dependent as the center wavelength was scanned with piezoelectric tuning of the cavity. A quantitative model of the 1f-normalized WMS-2f signal was developed and validated under laboratory conditions. A sensor was subsequently designed, built and demonstrated for real-time, in situ measurements of NO across a 3 m path in the particulate-laden exhaust of a pulverized-coal-fired power plant boiler. The 1f-normalized WMS-2f method proved to have better noise immunity for non-absorption transmission, than wavelength-scanned direct absorption. A 0.3 ppm-m detection limit was estimated using the R15.5 transition near 1927 cm⁻¹ with 1 s averaging. Mid-infrared QCL-based NO absorption with 1f-normalized WMS-2f detection shows excellent promise for practical sensing in the combustion exhaust.     

Absolute line intensities for oxirane in the 11.4 μm spectral region

Absolute individual line intensities of numerous transitions of the fundamental ν₁₅, ν₁₂ and ν₅ bands of oxirane (ethylene oxide, cyc-C₂H₄O) have been measured in the 750–950 cm^?1 region using eight high-resolution Fourier transform spectra recorded at 0.002 cm^?1 resolution and various pressures. These line intensities were least-squares fit using a theoretical model which takes into account the vibration–rotation interactions linking the upper state rotational levels and, therefore, accurate rotational expansions of the transition moments of the ν₁₅, ν₁₂ and ν₅ vibrational bands were derived. Using the coefficients obtained in the fitting, a line list has been generated and used to perform comparisons with the present measurements. Also, comparisons with measurements taken at medium–low resolution of the ν₁₅/ν₁₂/ν₅ system show excellent agreement.