Tunable CO2 Research Articles

Far-field detection of the super-lensing effect in the mid-infrared: theory and experiment

Super-lensing is an electromagnetic phenomenon, based on the amplification of evanescent waves, capable of increasing the resolution of an optical imaging device beyond the diffraction limit. A multi-layer planar super-lens can be constructed from two materials: one with a negative dielectric permittivity ε < 0 inserted between two slabs of another material with a positive dielectric permittivity ε d = −ε > 0. We numerically modelled and experimentally implemented a super-lens in the mid-infrared spectral range (around 11 μm) by creating a three-layered structure of submicron thickness, SiO2/SiC/SiO2, in which the polaritonic material SiC has a negative dielectric permittivity in the restrahlen band between the frequencies of the transverse and longitudinal optical phonons. A far-field diagnostic of super-lensing based on measuring transmission and reflection coefficients through the metal-coated super-lens has been implemented using a tunable CO2 laser.

Generation of megawatt-power terahertz pulses by noncollinear difference-frequency mixing in GaAs

Terahertz pulses with a peak power of ∼2MW were generated in a noncollinear phase-matched GaAs crystal at room temperature. Efficient difference-frequency generation was achieved using 250-ps CO2 laser pulses for which the surface damage threshold increased in comparison with standard 200-ns pulses. Tunable CO2 lasers in combination with this technique should yield a source in the 0.1–3.0-THz range.

In situ measurement of the infrared absorption and extinction of chemical and biologically derived aerosols using flow-through photoacoustics

In an effort to establish a more reliable set of optical cross sections for a variety of chemical and biological aerosol simulants, we have developed a flow-through photoacoustic system that is capable of measuring absolute, mass-normalized extinction and absorption cross sections. By employing a flow-through design we avoid issues associated with closed aerosol photoacoustic systems and improve sensitivity. Although the results shown here were obtained for the tunable CO2 laser waveband region, i.e., 9.20-10.80 microm, application to other wavelengths is easily achievable. The aerosols considered are categorized as biological, chemical, and inorganic in origin, i.e., Bacillus atrophaeus endospores, dimethicone silicone oil (SF-96 grade 50), and kaolin clay powder (alumina and silicate), respectively. Results compare well with spectral extinction measured previously by Fourier-transform infrared spectroscopy. Comparisons with Mie theory calculations based on previously published complex indices of refraction and measured size distributions are also presented.

Measurement of the refractive index dispersion around an absorbing line

The “anomalous” dispersion of the refractive index has been determined around the 10P18 CO2 absorbing line by recording the spectrum of a mode tunable CO2 laser emission transmitted through a high quality Fabry–Perot resonator. The weak phase shift observed agrees well with the usual quantum mechanic treatment of a two level transition.

Tunable CO 2 transport through mixed polyether membranes

Gas-separation membranes composed of polyethers such as poly(ethylene glycol)diacrylate (PEGda) or poly(propylene glycol)diacrylate (PPGda) exhibit high CO 2 solubility selectivity, which makes them attractive for use in H 2 and air purification. In this work, we investigate the factors governing CO 2 and H 2 transport in mixed polyether matrices. Addition of semicrystalline poly(ethylene oxide)s to amorphous PEGda lowers the net CO 2 permeability and CO 2/H 2 selectivity due to crystal formation. Gas permeation through the amorphous fraction, however, remains unaffected, confirming the existence of a molecular weight limit below which the entire membrane participates in gas transport. The permeabilities of CO 2 and H 2, as well as their activation energy of permeation, in miscible PEGda/PPGda blends follow the linear rule of mixtures over the temperature range explored. Incorporation of amine moieties employed in liquid membranes into either the PEGda matrix during crosslinking or the PEG backbone generally reduces CO 2/H 2 selectivity but occasionally improves CO 2 permeability.

Accurate measurement of the refractive index of CO 2, N 2, He, O 2, and air at 10.57 μ and T = 23 °C

The refractive index of carbon-dioxide, nitrogen, helium, oxygen and air have been measured at 23 °C temperature by means of a tunable CO 2 laser at 10.57 μm and of a differential interferometric technique. As the gas refractive index depends on the pressure P as n = 1 + α P P, the pressure coefficients α P has been measured with 5e–3 accuracy.

A 3 kW average power tunable TEA CO 2 laser

A high average power line-tunable TEA CO 2 laser is described. Average output power up to 3 kW is achieved at repetition rate of 180 Hz. The maximum output power is almost equal among the four spectrum bands of CO 2 laser (the P and R branches of 00°1–10°0 and 00°1–02°0 transition bands). Several special technologies including rotating spark gap switching, PCB (printed circuit board) preionization and zeroth-order grating coupling are employed.

Tunable CO2 laser emitting sequential and hot transitions

The schematic model of a resonator with a reflection diffraction grating, providing a high frequency selectivivty, a wide spectral range, and a low loss level at a selected frequency, is studied theoretically and experimentally. The frequency-selective parameters of the resonator are calculated and analysed. It is shown that the dependence of the loss increment on the diaphragm diameter for a fixed frequency detuning Δν is nonmonotonic. Lasing at 100 lines of conventional bands and 36 lines of sequential and hot transitions is obtained. The minimum separation ΔνL between resolvable lines is found to be ∼0.2 cm-1.

Infrared multiphoton dissociation of SiF 4: gas phase reactions of SiF 3 with F and H 2

The infrared multiphoton dissociation (IRMPD) of pure SiF 4 and in mixtures with different gases was studied using a tunable CO 2 TEA laser. The initial dissociation step of the IRMPD of SiF 4 was found to be the decomposition into SiF 3 and F. The gas phase reactions of trifluorosilyl, SiF 3, with F and H 2 was investigated. A kinetic scheme was proposed to explain the experimental results. The set of coupled differential equations associated to this scheme was numerically solved. The rate constants of the SiF 3+F→SiF 4 and SiF 3+H 2→SiF 3H+H reactions were determined.

Infrared fibers based on Te–As–Se glass system with low optical losses

Unclad optical fibers based on high-purity Te–As–Se glasses prepared by chemical and physical methods of purification have been drawn. The optical, thermal and mechanical properties of glasses and fibers were investigated. The minimum optical losses were 0.07 dB/m at 7.3 μm for Te 25As 40Se 35 glass fiber and 0.04 dB/m at 6.7 μm for Te 20As 30Se 50 glass fiber. Sixty five percent of input power of a tunable CO 2 laser emitting at 9.3 μm was transmitted through a 1 m long fiber with diameter of 900 μm.

Absorption of CO 2 laser emission by freon-12

The infrared (IR) absorption of freon-12 (CF2Cl2) was studied in the emission range of a 3-W tunable CW CO2 laser by using a brass cell with KBr windows that was located outside the laser resonator. The results show that CF2Cl2 absorbs all CO2 laser emission lines in the ranges of 1073–1083 cm-1 and 937–943 cm-1. The most strongly absorbed laser line was 10P (28) ( 937.21 cm-1). Absorption coefficient values were obtained for all available wavelengths of the CO2 laser as the CF2Cl2 pressure was varied from 5 to 1000 mbar. By using the HITRAN database for freon-12, the absorption coefficients were calculated at the 10P (28) and 9R (28) lines as functions of the gas pressure and compared with the experimental values. The calculated results are in reasonable agreement with the experiment.

Photoacoustic spectrometry for trace gas analysis and leak detection using different cell geometries

A photoacoustic (PA) spectrometer with high selectivity and sensitivity has been developed for trace gas analysis and for the detection of gas leak at part per trillion by volume (pptV) level. This PA system comprises of a resonant photoacoustic cell, a pulsed line tunable CO2 laser as an excitation source and a sensitive electret microphone as a photoacoustic detector with an option to trigger the safety alarm system for early warning of gas leaks. In this work, three resonant PA cells with various geometries have been developed at our laboratory for the detection of photoacoustic signal using pulsed laser system and their comparative performance have been studied. As a special application of this PA system, the detection of sulfur hexa fluoride (SF6) gas using these three cells has been carried out for optimizing the sensitivity. Besides this, our PA system can very well be applied for pollution monitoring and detection of hazardous gases in a noisy environment.

Resonance absorption of coherent infrared radiation by thin polypropylene films and its technological applications

Tunable CO 2 and non-tunable CO 2 lasers were used for the irradiation of thin polypropylene films. The wavelength of the IR radiation was adjusted in such a way that it coincided exactly with the absorbance peak in the spectrum of the polypropylene pattern. In this way, conditions of resonance absorption of IR radiation by polymer films were produced. Strong thermal effects in PP were observed under irradiation at the resonance wavelength. The threshold value of IR power, which causes thermal phenomena in irradiated films was established. We pulled films exposed to established threshold values with a constant speed in the plane normal to the laser beam. Highly developed relief and thin traces were formed depending on the pulling speed. Traces with a width smaller than the wavelength of the incident laser beam were obtained. The IR spectra of irradiated PP films were studied with a FTIR spectrometer. Changes in the spectra of PP films were established using correlation analysis of spectral data. The phenomenon could be applied for obtaining highly developed surface relief, storage of information in polymer films and welding of polymer materials.

Assignment of the Ammonia ν2(2a->2s) Rotational MIR Laser Transitions Pumped by a Tunable CO2 Laser

In this work is reported the assignment of the NH3 MIR rotational laser lines previously observed inside the 2ν2 vibrational level. Some of them are due to the one photon pump of the hot band 1s->2a, the most part is assigned to off resonance two photon pumping (0a->1s->2a) induced at intermediate detunings as large as 600GHz

Photoacoustic gas sensing employing fundamental and frequency-doubled radiation of a continuously tunable high-pressure CO 2 laser

We present the first photoacoustic spectrometer for gas sensing employing both the fundamental and the frequency-doubled radiation of a continuously tunable high-pressure CO2 laser with room temperature operation. A quasi-phase-matched diffusion-bonded GaAs crystal is used in the system for second-harmonic generation. A pulsed photoacoustic detection scheme with a non-resonant cell, equipped with an 80-microphone array, is employed. The wide continuous tuning range in the fundamental (9.2–10.7 μm) and the frequency-doubled (4.6–5.35 μm) regimes, together with the narrow linewidth of 540 MHz (0.018 cm-1) for the 10-μm region and of 1050 MHz (0.0315 cm-1) for the 5-μm region, allow the measurement of gas mixtures, individual species and isotope discrimination. This is illustrated with measurements on NO and CO2. The measured isotope ratio 15NO/14NO=(3.58±0.55)×10-3 agrees well with the literature (3.700×10-3) and demonstrates the good selectivity of the system.

Line Narrowing of a Tunable High Pressure CO 2 Laser by Means of an Improved Non-Littrow Cavity

A model is reported to foresee ,for different mirror choices, the spectral properties of a high pressure, frequency tunable, CO2 laser, operating with a non-Littrow grating mounting. The calculations show a larger frequency selectivity for not collimated plane cavities. The model is experimentally verified in a suitable high pressure laser. In particular narrowed linewidths less than 300MHz ( FWHM) are obtained.

High-resolution spectra of bromo methane and methyl bromide obtained by a continuously tunable 10-bar CO2 laser based photoacoustic spectrometer.

High resolution spectral data of 100 ppmV (10(-6) per volume) concentrations of the trace gases bromo methane (BM, CH3Br), and methyl bromide (DME, (CH3)2O), buffered in synthetic air (80% N2, 20% O2) at atmospheric pressure and room temperature are reported. The spectra are recorded with a continuously tunable 10-bar CO2 laser based photoacoustic (PA) spectrometer. The tuning range covers 76 cm(-1) between 9.2 microm (1087 cm(-1)) and 10.7 microm (935 cm(-1)) at a constant narrow line-width of 0.018 cm(-1) (540 MHz). The non-resonant PA measuring cell employs an in-line 10-microphone array. The estimated detection limits for BM and DME are approximately 2 ppmV for a signal-to-noise ratio (SNR) of 3. This corresponds to a calculated detection limit of approximately 76 ppbV for ethylene.

Remote mapping of vegetation and geological features by lidar in the 9–11-µm region

We report examples of the use of a scanning tunable CO(2) laser lidar system in the 9-11-mum region to construct images of vegetation and rocks at ranges as far as 5 km from the instrument. Range information is combined with horizontal and vertical distances to yield an image with three spatial dimensions simultaneous with the classification of target type. Object classification is based on reflectance spectra, which are sufficiently distinct to allow discrimination between several tree species, between trees and scrub vegetation, and between natural and artificial targets. Limitations imposed by laser speckle noise are discussed.

IR excitation of ethylene molecules and clusters embedded in He4 droplets

Ethylene and ethylene clusters embedded in superfluid He4 droplets have been spectroscopically investigated with continuous wave line tunable CO2 and N2O lasers, utilizing the strong absorption of the ν7 mode of C2H4. The monomer and the different clusters of ethylene absorb in the same spectral region and show significant overlap of their spectra. By measuring the dependence of the signal on the pick-up pressure and fitting the resulting curves with a Poisson distribution and by further studying the spectral dependence on the droplet size, it was possible to assign two peaks. The origin of the 10 cm−1 broad gas phase dimer spectrum has been the subject of much discussion; we show that the broad peak is not due to homogeneous broadening, because the low temperature of the He droplet leads to a dimer full width at half maximum width of about 1 cm−1.

Multifrequency Diagnostics of a Vibrationally Equilibrium CO2-Containing Gas Mixture

We present a technique which makes it possible to simultaneously determine the temperature T and the partial pressure of carbon dioxide in a vibrationally equilibrium gas mixture at atmospheric pressure by using the experimentally measured spectral distribution of the absorption factor at the oscillation lines of a tunable CO2 laser. The technique developed can be employed for monitoring both the energy efficiency and the ecological purity of the processes of combustion of large amounts of hydrocarbon fuels accompanied by release to the atmosphere of combustion products containing carbon dioxide.