Modeling of the power oscillation observed in single mode intracavity absorption of CO2 laser lines

Single mode intracavity absorption is investigated from an analytical point of view in order to explain the periodical modulation of the power observed in intracavity absorption of CO₂ laser lines by CFC gases. Intracavity absorption is incorporated into the laser rate equations in form of an additional broadband loss. Detuning due to changes in the refractive index with increase in absorber pressure is accounted for by considering the impact of the frequency shift of the oscillating mode on the effective pump rate and the population densities of the laser upper and lower levels. The steady state solution of the rate equations leads to an analytical formula, which fits the measured intracavity absorption curves with only two fit parameters being adjusted to the actual values of the absorption cross section and the mean linear polarizability of the absorber molecule. The formula is applied to analyze the intracavity absorption of the CO₂ 10R lines by CFC-11. The obtained results are in reasonable agreement with the values available in the literature.     

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. PACS 33.20.Ea; 42.55.-f; 42.55.Lt     

Evaluation of ammonia absorption coefficients by photoacoustic spectroscopy for detection of ammonia levels in human breath

Photoacoustic spectroscopy represents a powerful technique for measuring extremely low absorptions independent of the path length and offers a degree of parameter control that cannot be attained by other methods. We report precise measurements of the ammonia absorption coefficients at the CO₂ laser wavelengths by using a photoacoustic (PA) cell in an extracavity configuration and we compare our results with other values reported in the literature. Ammonia presents a clear fingerprint spectrum and high absorption strengths in the CO₂ wavelengths region. Because more than 250 molecular gases of environmental concern for atmospheric, industrial, medical, military, and scientific spheres exhibit strong absorption bands in the region 9.2–10.8 μm, we have chosen a frequency tunable CO₂ laser. In the present work, ammonia absorption coefficients were measured at both branches of the CO₂ laser lines by using a calibrated mixture of 10 ppm NH₃ in N₂. We found the maximum absorption in the 9 μm region, at 9R(30) line of the CO₂ laser. One of the applications based on the ammonia absorption coefficients is used to measure the ammonia levels in exhaled human breath. This can be used to determine the exact time necessary at every session for an optimal degree of dialysis at patients with end-stage renal disease.     

Absolute frequency measurement of CO2 laser lines with a femtosecond laser comb and new determination of the CO2 molecular constants and frequency grid

Absolute frequency measurements of a CO₂ laser stabilized on saturated absorption resonances of CO₂ laser lines are reported. They were performed using a femtosecond-laser frequency comb generator and two laser diodes at 852 and 782 nm as intermediate oscillators, with their frequency difference phase-locked to the CO₂ laser. Twenty ¹²C¹⁶O₂ laser lines in the P and R bands at 9 μm were measured with a relative uncertainty of a few 10⁻¹² limited by the CO₂ frequency reproducibility. A new determination of the CO₂ molecular constants was obtained from these data and previous measurements in the 10 μm band. The CO₂ frequency grid was also calculated, with an improvement of two orders of magnitude compared to the previous grid of Maki et al. [J. Mol. Spectrosc. 167 (1994) 211].     

A compact, highly stable CW CO2 laser without brewster windows

We describe an open resonator, quasi sealed-off; 70cm cavity length CO₂ laser with very high stability of power and frequency, which lases on 96 lines from 9R42 (2W) to 10P52 (5W) with a power of >11W on 9R20, 9P20, 10R20 and 10P20, plus on 18 hotband lines with ≈2W. This laser was used successfully to pump an FIR ring laser [1] enabling an FIR power stability of ΔP/P ≈ 10⁻⁴ by use of a simple PI control loop, thus demonstrating its superb frequency stability.     

Measurements and assignments of new large offset CD3OH FIR laser lines

By using an acoustooptic modulator we extend the 300 MHz tunability of a waveguide CO₂ laser to 480 MHz. The CD₃OH was optically pumped by the 10R(32), 10R(34), and 10R(36) CO₂ laser lines, and 17 new FIR laser lines were discovered. The Stark effect on previously known FIR laser lines was investigated, and some tentative FIR laser lines assignments are suggested.     

HCOOH high-resolution spectroscopy in the 9.18 μm region

We report on highly accurate absolute frequency measurement against a femtosecond frequency comb of six saturated absorption lines of formic acid (HCOOH) with an accuracy of 1 kHz. We also report the frequency measurement of 17 other lines with an accuracy of 2 kHz. Those lines are in quasi coincidence with the 9R(36) to 9R(42) CO₂ laser emission lines and are probed either by a CO₂ or a widely tunable quantum cascade laser phase locked to a master CO₂ laser. The stability of HCOOH stabilized lasers is characterized by a fractional Allan deviation of 3.1 × 10⁻¹² τ^−1/2. They give suitable frequency references for Doppler-free two-photon spectroscopy.     

Assignments of optically pumped CD3OH laser lines

Six FIR laser lines from CD₃OH pumped by the 10R(36) and the 10R(18) CO₂ laser lines are assigned to specific rotational energy levels in the excited C–0 stretch state. It is found that their upper laser levels are shifted by a Fermi resonance between the C–0 stretch vibration and the third and forth harmonics of the torsional mode. The Fermi resonance shifts are +0.332 cm^–1 and +2.251 cm^–1 for the upper laser levels pumped by the 10R(36) and the 10R(18) CO₂ laser lines, respectively. Calculated frequencies of the pump and the laser transitions agree with those of the pump CO₂ laser lines and the observed FIR laser lines within estimated accuracy.     

The amplified spontaneous emission in TE CO2 laser with a half-cavity

The new structure with a half-cavity in TE CO₂ laser is described. In this device, a total of nearly 50 lines from transitions in P and R branches of the 9.6 and 10.6 μm bands is observed. The energy, the spot pattern and pulse width of those lines are tested.     

Tunable te-CO2 laser-pumped formaldehyde far-infrared lasers: Offsets, assignment, and superfluorescence

Sixty and three absorption transitions in D₂CO and H₂CO, respectively, have produced a number of far infrared laser lines when they are pumped by an etalon-tuned TE-CO₂ laser. Almost all the absorption transitions pumped previously by a free runing TE-CO₂ laser have been efficiently pumped by the etalon tuned CO₂ laser and found to have offset within ±500 MHz from the line-center of the relevant CO₂ pump lines. 22 (1) absorption and 63 (4) emission lines of D₂CO (H₂CO) are assigned. Some of these lines have generated superfluorescence. In paticular, the D₂CO 319-m line pumped by CO₂-9P(32) delivered an output energy of approximately one half that of the well-known D₂O 66-m. It is shown that a large electric dipole moment and an appreciable amount of fractional population in the lower level of the pump transition of this line are responsible to the superfluorescence.     

New far-infrared emissions and frequency measurements in hydrazine

By means of a new CO₂ laser we performed a new investigation of the far-infrared laser emission spectrum of hydrazine excited by the 10P(32) and 10R(8) CO₂ laser lines. We found seven new lines and measured the frequency of four of them; moreover we measured the frequencies of two more lines previously reported in the literature with only wavelength measurements. The frequencies of the far-infrared laser emissions have been measured by means of a frequency-synthesis chain based on new InP Schottky diodes. The detected signal was beat-note generated in a Schottky diode between the far-infrared radiation, the harmonics of a 72 GHz frequency reference and a rf signal. We also characterized all of the observed lines by their polarization relative to the pumping CO₂ laser, the optimum pressure and the offset relative to the CO₂ center frequency. PACS 42.55.Lt; 42.62.Fi     

室温下1.3μm附近CO2的高灵敏度半导体激光吸收光谱

用DFB（分布反馈）半导体激光器结合波长调制吸收光谱技术观测了CO2在1.31μm附近的泛频吸收光谱，并获得的相应的光谱参数（如：谱线位置，谱线强度以及自加宽系数），同时发现了15条弱的新谱线。在665 Pa压力下，本实验可探测的最弱谱线是2.25163′10-27 cm-1/(molecule×cm-2)，相应的吸收是3.88′10-8。     

Accuracy in determination of the temperature and partial pressure of CO<Subscript>2</Subscript> in CO<Subscript>2</Subscript>:N<Subscript>2</Subscript>:H<Subscript>2</Subscript>O:NO<Subscript>2</Subscript> mixtures by multiple-frequency laser probing

We present the results of analysis of the errors introduced by hot-band transitions 11¹0-01¹1, 03¹0-01¹1, 12⁰0-12⁰1 of the CO₂ molecule and the absorption lines of the H₂O and NO₂ molecules in determination of the temperature and partial pressure of CO₂, included in the gas mixture CO₂: N₂:H₂O: NO₂ at atmospheric pressure, by multiple-frequency laser probing using a CO₂ laser tunable over the lines of the 00⁰1-[10⁰0,02⁰0]_I,II ground-state laser transitions. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 6, pp. 810–815, November–December, 2007.     

Simultaneous emission on 10P and 10R branch lines from a hybrid CO2 laser

The use of cavity length control to obtain multiline emission from a hybrid CO₂ laser has been investigated. We have obtained simultaneous laser oscillation on up to four lines in the P and R branches of the 10 m band by operating the c.w. section above the lasing threshold. For sub-threshold operation of the c.w. section the hybrid laser emission is restricted to the stronger 10P lines.     

A 3 kW average power tunable TEA CO2 laser

A high average power line-tunable TEA CO₂ 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 CO₂ 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.     

FIR Stark spectroscopy on optically pumped CH3OH laser

An extensive theoretical and experimental analysis of the absorption and emission spectrum of a CH₃OH FIR-laser excited by a conventional CO₂ laser is presented. Particular interest is devoted to the Stark shifts of the pump and lasing lines and to the electric field dependence of the Fir-laser output of the various lines. The offsets with respect to the exciting radiation and the Stark shifts of the IR absorption (pump) lines are measured by means of the transferred Lamb dip technique. The theoretical behaviours of the Stark patterns are calculated for several choices of the quantum numbers and selection rules involved in the transitions. A large variety of experimental results are reported and compared to theory. Non-linear Stark shifts have been observed for the 37.5m FIR laser line and for the IR-pump transitions excited by the 9-P(38) and 10-R(38) CO₂ laser Lines. Line assignments are proposed and new FIR laser lines are reported.     

Diode laser and Fourier transform spectroscopy of the13CH3OH C-O stretching mode

In this work we present high resolution Doppler limited absorption spectra measurements of the C-O stretching mode of¹³CH₃OH, obtained from diode laser spectroscopy, and the Fourier Transform spectrum obtained at 0.12 cm^–1 resolution. By using these data and previously known spectroscopic information, we determined the frequency and the J quantum number for the multiplets of the P and R(J) branches of the C-O stretching fundamental band. Infrared transitions in coincidence with emission lines of the regular CO₂ laser and some of its isotope parents are pointed out.     

The rotationally-resolved absorption spectrum of formaldehyde from 6547 to 6804 cm

The room temperature absorption spectrum of formaldehyde, H₂CO, from 6547 to 6804 cm⁻¹ (1527-1470 nm) is reported with a spectral resolution of 0.001 cm⁻¹. The spectrum was measured using cavity-enhanced absorption spectroscopy (CEAS) and absorption cross-sections were calculated after calibrating the system using known absorption lines of H₂O and CO₂. Several vibrational combination bands occur in this region and give rise to a congested spectrum with over 8000 lines observed. Pressure broadening coefficients in N₂, O₂, and H₂CO are reported for an absorption line at 6780.871 cm⁻¹, and in N₂ for an absorption line at 6684.053 cm⁻¹.     

N2 and O2 pressure broadening and pressure shift in the 4ν2 band of OCS

To measure accurately OCS concentrations in planetary atmospheres, it is important to know precisely nitrogen and oxygen pressure broadening and pressure-induced shift coefficients for the lines used in the retrievals. We present in this study the corresponding coefficients for lines of the P and R branches of the 4ν₂ band of the primary isotopologue of carbonyl sulfide (¹⁶O¹²C³²S).For this purpose, infrared absorption spectra of a natural carbonyl sulfide (OCS) gas sample were recorded at an unapodized resolution of 0.004 cm⁻¹, at room temperature for different pressures of N₂ and O₂, using a Bruker IFS125HR spectrometer at the LISA Laboratory in France. The line parameters were derived using the multispectrum fitting method applied to the measured shapes of the lines, including the interference effects caused by the line overlaps.The results are compared with earlier measurements and with values calculated using a semi-classical model based upon the Robert and Bonamy formalism that reproduces rather well the experimental m (m=−J for P(J) lines and m=J+1 for R(J) lines) quantum number dependence of the N₂ and O₂ broadening coefficients. On the other hand most of the lines studied here have positive shift coefficients, which do not show any systematic dependence on m. However, in previous studies of the ν₃, 2ν₃ and ν₂ bands, these coefficients were negative for all lines.     

N2F4 as saturable absorber of CO2 laser radiation

Tetrafluorohydrazine induces passive Q switching in a continuous-wave CO₂ laser in both the P and R branches of the 10.4 μ system, showing comparable effectiveness on all the accessible lines. At higher pressures, this gas forces the laser to oscillate in the 9.3 μ system in non frequency-selective cavities.