Seismic wave attenuation and the spectral decay parameter ...
Attenuation of 33-µm laser radiation by atmospheric water vapor
2
Attenuation of 33-μm laser radiation by atmospheric water vapor Z. Kucerovsky and E. Brannen University of Western Ontario, Physics Department & Centre for Radio Science, London 72, Ontario. Received 7 May 1976. The extinction coefficient of the 33.029-μm (302.76 ± 0.15 cm" 1 ) 1 H 2 O 16 laser radiation by atmospheric water vapor under laboratory conditions has been measured. The laser line is a 5 14 100 → 4 41 020 H 2 O 16 transition, which is close to two water vapor absorption lines located at 301.87 ± 0.02 cm" 1 and 303.02 ± 0.002 cm" 1 as measured by Rao et al. 2 The separations between the laser line and the absorption lines are 0.89 cm" 1 and 0.26 cm" 1 , respectively. Since the absorption lines are of approximately equal strength, the absorption is due predominantly to the latter line (303.02 cm" 1 ). The in- teraction of the laser line with these absorption lines is strong resulting in a value of extinction coefficient of 1.548 ± 0.039 Torr -1 m -1 . The laser used in these measurements was a pulsed longi- tudinal water vapor laser with a semiconfocal 5-m long reso- nator in which one mirror was replaced by a 50-mm × 50-mm echelette grating 3,4 in Littrow configuration in first order, blazed for 30 μm (Bausch & Lomb, 898-17-5-1). Single wavelength, single mode operation was thus ensured. The energy for the laser was provided by a modified 30-MW pp pulse power supply modulator (Burmac M 1201) with a thy- ratron switch and 2-μsec pulse-forming network. The ra- diation reflected into the N = 0 order of the grating was cou- pled out and used in the experiments measuring absorption. The laser pulse was 1.5 μsec long with peak power of 500 W pp and repetition rate of 30 Hz. The absorption cell was a 2.29-m long 10-cm diam glass tube provided with a 2.5-cm diam KRS-5 entrance window (Har- shaw). A 2.5-cm aperture germanium lens with a focal length of 20 cm was used as the output window. The tube was heated to 60°C to prevent condensation of the water vapor on the walls. The cell was pumped by a two-stage oil pump to pro- vide reference value of absorption with zero concentration of water vapor. A membrane sampler and apparatus for con- trolling flow through the cell was also included. The detection of the laser radiation was accomplished by two pyroelectric detectors, one of which supplied information about the output power of the laser. The signal from each detector was passed through a 40-dB v preamplifier, a sampling circuit (Hewlett-Packard M1782A), and then processed by an analog integrator, recorded, and digitized. The mixtures of air and water vapor were prepared by using two independent techniques: a volume sample preparation and a flow-through method. When the volume sample preparation method was used, the cell was first pumped down to ~10 -3 Torr. A calculated volume of distilled water was then injected through the heated membrane sampler with a special sampling syringe (Hamilton, 50 μl). The cell was then filled with air dried by a two-stage dryer (silica gel, molecular sieve 5A) to achieve the pressure of 760 Torr inside. The value of transmission for the prepared concentration was then recorded. When the flow-through method was used, about 20 ml of dried air/min was passed through a water bath and the ab- sorption cell. The concentration of water vapor in the ab- sorption cell was calculated from the temperature of the bath. The results of the measurement of the transmission, nor- malized to one water absorption path, are shown in Fig. 1. A least squares fit to the experimental data results in the value September 1976 / Vol. 15, No. 9 / APPLIED OPTICS 2027
Transcript of Attenuation of 33-µm laser radiation by atmospheric water vapor
Attenuation of 33-μm laser radiation by atmospheric water vapor Z. Kucerovsky and E. Brannen
University of Western Ontario, Physics Department & Centre for Radio Science, London 72, Ontario. Received 7 May 1976. The extinction coefficient of the 33.029-μm (302.76 ± 0.15
cm"1)1 H2O16 laser radiation by atmospheric water vapor under laboratory conditions has been measured. The laser line is a 514 100 → 441 020 H2O16 transition, which is close to two water vapor absorption lines located at 301.87 ± 0.02 cm"1
and 303.02 ± 0.002 cm"1 as measured by Rao et al.2 The separations between the laser line and the absorption lines are 0.89 cm"1 and 0.26 cm"1, respectively. Since the absorption lines are of approximately equal strength, the absorption is due predominantly to the latter line (303.02 cm"1). The interaction of the laser line with these absorption lines is strong resulting in a value of extinction coefficient of 1.548 ± 0.039 Torr -1 m - 1 .
The laser used in these measurements was a pulsed longitudinal water vapor laser with a semiconfocal 5-m long resonator in which one mirror was replaced by a 50-mm × 50-mm echelette grating3,4 in Littrow configuration in first order, blazed for 30 μm (Bausch & Lomb, 898-17-5-1). Single wavelength, single mode operation was thus ensured. The energy for the laser was provided by a modified 30-MWpp pulse power supply modulator (Burmac M 1201) with a thy-ratron switch and 2-μsec pulse-forming network. The radiation reflected into the N = 0 order of the grating was coupled out and used in the experiments measuring absorption. The laser pulse was 1.5 μsec long with peak power of 500 Wpp and repetition rate of 30 Hz.
The absorption cell was a 2.29-m long 10-cm diam glass tube provided with a 2.5-cm diam KRS-5 entrance window (Har-shaw). A 2.5-cm aperture germanium lens with a focal length of 20 cm was used as the output window. The tube was heated to 60°C to prevent condensation of the water vapor on the walls. The cell was pumped by a two-stage oil pump to provide reference value of absorption with zero concentration of water vapor. A membrane sampler and apparatus for controlling flow through the cell was also included.
The detection of the laser radiation was accomplished by two pyroelectric detectors, one of which supplied information about the output power of the laser. The signal from each detector was passed through a 40-dBv preamplifier, a sampling circuit (Hewlett-Packard M1782A), and then processed by an analog integrator, recorded, and digitized.
The mixtures of air and water vapor were prepared by using two independent techniques: a volume sample preparation and a flow-through method. When the volume sample preparation method was used, the cell was first pumped down to ~10 - 3 Torr. A calculated volume of distilled water was then injected through the heated membrane sampler with a special sampling syringe (Hamilton, 50 μl). The cell was then filled with air dried by a two-stage dryer (silica gel, molecular sieve 5A) to achieve the pressure of 760 Torr inside. The value of transmission for the prepared concentration was then recorded.
When the flow-through method was used, about 20 ml of dried air/min was passed through a water bath and the absorption cell. The concentration of water vapor in the absorption cell was calculated from the temperature of the bath.
The results of the measurement of the transmission, normalized to one water absorption path, are shown in Fig. 1. A least squares fit to the experimental data results in the value
September 1976 / Vol. 15, No. 9 / APPLIED OPTICS 2027
Fig. 1. Transmission of 33.029-μm water vapor laser radiation through 1-m air containing a varying concentration of water vapor.
of the extinction coefficient for the atmospheric water vapor of 1.548 ± 0.039 Torr - 1 m - 1 . One can see that the 33-μm laser radiation is attenuated very strongly; only 6 cm of air under ordinary laboratory condition (20° C, 60% relative humidity) are necessary to decrease the radiation by the factor 1/e. The strong attenuation of the laser beam in air gives rise to an acoustic signal which at the laser output levels and repetition rate used in this experiment (about 500 Wp p and 30 Hz) is clearly audible. The signal can be detected by a microphone at much lower output power.
The extinction coefficient of water vapor for 33-μm water vapor laser radiation is of the same order as that of methane for the 3.392-μm laser radiation (KH2O = 2 p p m - 1 k m - 1 for water vapor laser compared with KCH4 = 0.8 p p m - 1 k m - 1 for 3.392-μm He-Ne laser radiation).5-6 The methane absorption is used to stabilize the frequency of the 3.392-μm He-Ne laser line. This strong absorption of a 33-μm laser line by water vapor in a ground state suggests that it may be possible to stabilize a water vapor laser in a manner similar to that of a He-Ne laser stabilized by the methane line.
This strong absorption from a ground level must also occur in the water vapor laser at 33 μm and may also help to explain why one does not find a more substantial increase in output power in a water vapor TEA laser operating on this line when higher pressures of water vapor-helium mixture are used. The absorption of the 33-μm radiation can be used as a sensitive method of measuring trace concentrations of water vapor in dry gases.
We thank D. G. Rumbold and K. C. Paulekat for their assistance in carrying out this experiment. This work was supported by grants from the National Research Council of Canada, Defence Research Board of Canada, and the Ontario Ministry of the Environment.
2028 APPLIED OPTICS / Vol. 15, No. 9 / September 1976
References 1. W. S. Benedict, M. A. Pollack, and W. J. Tomlinson III, IEEE J.
Quantum Electron. QE-5, 108 (1969). 2. K. N. Rao, W. W. Brim, V. L. Sinnett, and R. H. Wilson, J. Opt.
Soc. Am. 52, 862 (1962). 3. E. Brannen and W. J. Sarjeant, IEEE J. Quantum Electron. QE-6,
138 (1970). 4. W. Q. Jeffers, Appl. Phys. Lett. 11, 178 (1967). 5. B. N. Edwards and D. E. Burch, J. Opt. Soc. Am. 55, 174 (1965). 6. T. S. Chu and D. C. Hogg, Bell Syst. Tech. J., 301 (Feb. 1966)
![Preliminary estimation of kappa (κ) in Croatia · distance. The results are important for attenuation studies [4], re-creation, and re-calibration of attenuation of peak horizontal](https://static.fdocument.org/doc/165x107/604d24980407664546290426/preliminary-estimation-of-kappa-in-croatia-distance-the-results-are-important.jpg)
