Mar 13, 2016 - The laser action was observed when the discharge-tube walls were cooled with .... or glass tube, connected to the discarge, was inserted into.
Home
Search
Collections
Journals
About
Contact us
My IOPscience
Carbon monoxide laser operating at room temperature
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1974 Sov. J. Quantum Electron. 3 484 (http://iopscience.iop.org/0049-1748/3/6/A08) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 152.23.71.245 This content was downloaded on 13/03/2016 at 20:53
Please note that terms and conditions apply.
Carbon monoxide laser operating at room temperature T. F. Kacheva, V. N. Ochkin, and N. N. Sobolev (Submitted March 20, 1973) Kvantovaya Elektron. (Moscow), No. 6(18), 58-63 (1973) The temperature dependence of the output power was determined for a CO laser emitting in the infrared range. The laser action was observed when the discharge-tube walls were cooled with running water. If the mixture contained xenon, the laser action was achieved under continuous-flow conditions and in a sealed laser. The output power of a sealed discharge tube 980 mm long was about 5 W and this value was obtained in tests lasting 80 h.
1. The carbon monoxide laser is attracting considerable interest. This is due to its high cw output power in the wavelength range 5-6 p. The walls of discharge tubes of such high-power CO lasers have to be cooled with liquid nitrogen, which is an inconvenient feature. Moreover, it is not normally possible to use a sealed construction (i.e., to avoid continuous flow) since some of the products of the dissociation are frozen out irreversibly on the walls. Continuous stimulated emission from the CO molecules without liquid-nitrogen cooling of the active medium was first achieved by mixing CO with active nitrogen.1 The output power was 370 mW and this system operated only under rapid flow conditions. The development of an electric-discharge CO laser with a sufficiently high power output but cooled simply with running water2 prepared the way for the construction of sealed systems. The present paper reports our attempts to build a sealed CO laser. In the course of our investigation we noted published reports3'4 that other workers had also achieved positive results. 2. The correct selection of the composition of the working gas mixture and of the discharge parameters is of primary importance in high-temperature laser action. In order to determine the right conditions we measured the dependences of the output power on the cooling temperature of the discharge-tube walls. We used a quartz discharge tube with a gap 980 mm long and with an internal diameter of 15 mm (the total length of the tube was 1140 mm). The tube had a double cooling jacket. The internal jacket (of 24 mm diameter) was filled with petroleum ether or isopentane, whereas the external jacket (of 45 mm diameter) was filled with liquid nitrogen. This construction enabled us to vary smoothly the temperature of the discharge-tube walls from 77 to 300°K (heating above 300°K was undesirable because it was accompanied by turbulent evaporation of the ether). The tube with the ether-filled internal jacket was first cooled to a temperature close to that of liquid nitrogen. The gradual heating resulted from the discharge itself. The temperature dependence of the output power was determined in the course of such heating. The tube ends were closed with CaF2 windows oriented at the Brewster angle. The resonator consisted of two gilt mirrors and one of them had an aperture for the extraction of the radiation. This radiation was measured with a liquid-nitrogen-cooled InSb detector. Our measurements indicated that for all the investigated discharge parameters (direct current 3-50 mA, gas pressure 3-20 mm Hg) the laser action in CO + N2 + He + O2 mixtures with different ratios of the components was observed only up to temperatures not exceeding— 40 to— 50°C. 484
Sov. J. Quant. Electron.. Vol. 3, No. 6, May-June 1974
Only the addition of xenon in amounts comparable with CO enabled us to shift the maximum temperature of the laser action to room temperature. A typical dependence of the output power on the temperature of the wall sofa discharge tube containing a mixture of CO + N2 + He + Xe + O2 is plotted in Fig. 1. This dependence is not monotonic. It shows a very strong dip at ~ 180°K. The laser action disappears at this temperature and then reappears again and is observed up to room temperature. This type of dependence can be explained as follows. The fall of the output power during the first stage of heating is due to the weakening of the population inversion mechanism because of a nonresonant vibration— vibrational exchange.5 The reappearance of the laser action at temperatures above 180°K is due to the release of xenon, which remains frozen on the tube walls at lower temperatures. (The dependence shown in Fig. 1 should be regarded as approximate because measurements of the temperature of the tube walls with thermocouples located at different points indicated that the transverse temperature field was not homogeneous. The lower part of the tube closer to the evaporating nitrogen was cooled more strongly than the upper part. This inhomogeneity was particularly strong at low temperatures when it amounted to several tens of degrees. At temperatures above 190-200°K the cooling of the tube was practically uniform.) Thus, the use of a gas mixture containing xenon enabled us to achieve the laser action throughout the investigated range from liquid—nitrogen to room temperature. The positive role of xenon in the population inversion in the CO laser was discussed in ref. 6. Xenon increased the electron density and reduced the average energy of electrons. It would seem to us unlikely that the addition of xenon to a discharge could give rise to any other mechanism than the nonresonant vibration—vibrational exchange mechanism. In discussing the influence of temperature on the operation of the CO laser it is useful to note the following point. We calculated the radial distributions of the gas rel. units Fig. 1. Dependence of the output power W on the temperature of the tube walls Tw deduced from several independent experiments using a CO + N2 + He + Xe + O2 (1: 6.7 : 28: 0.67: 0.04) mixture with a total gas pressure p = 16.4 mm Hg and a discharge current I = 20 mA. 100
ISO
200
TV,°K
Copyright © 1974 American Institute of Physics
484
100
T w = 280"K
KO -
Fig. 3. Dependences of the output power on the discharge current for a CO + N2 + He + Xe + Oj (1: 6.7 : 28 : 0.67 : 0.04) mixture.
ISO
-4
a
Fig 2. Radial distributions of the gas temperature !„ for different discharge currents in a CO + N2 + He + Xe + O2 (1: 6.7:28: 0.67: 0.04) mixture at p = 16.4 mm Hg.
temperature in the discharge by solving numerically the heat conduction equation on a computer. These calculations were carried out for different wall temperatures and various discharge conditions. Some of the dependences obtained are plotted in Fig. 2. The results indicated that the gas temperature varied much less than the wall temperature. For example, for a current 30 mA and an increase in the wall temperature from 77 to 280°K (i.e., by 203°) the gas temperature on the discharge axis increased from 220 to 330°K (by 110°). This was due to an increase in the thermal conductivity of the gas with rising temperature and with decreasing intensity of the longitudinal electric field, i.e., with decreasing power supplied to the discharge. 3. Figure 3 shows the dependence of the output power on the discharge current obtained for different pressures in a CO + N2 + He + Xe + O2 mixture when the walls of the discharge tube were cooled with running water (Tw ~ 280°K). These results were obtained employing the same tube as in the study of the temperature dependences of the output power. The rate of flow of the gas was 0.5 m/sec. A characteristic feature of these dependences was, inmost cases, the rise of the output power with increasing current in the low-current range followed by a maximum and a fall at high currents. When the pressure was reduced the maximum output power shifted toward higher currents. At a pressure of 32 mm Hg we observed only the falling part of the dependence because in this case the discharge conditions in the low-current range were unstable. The initial rise of the output power with the discharge current was due to an increase in the electron density and the corresponding increase in the rate of excitation of the molecular vibrations. At higher currents the gas temperature rose sufficiently to suppress considerably the mechanism of transfer of energy to the laser levels by the nonresonant vibration—vibrational exchange. At higher pressures the temperature rose more rapidly than at lower pressures. The heating of the gas explained also the dependence of the output power on the gas pressure for fixed values of the discharge current (Fig. 4). In this case the maximum output power was reached at lower pressures when the discharge current was increased. It should be noted that all these results were obtained at considerably lower gas pressures than those reported in ref. 2.
485
Sov. J. Quant. Electron., Vol. 3, No. 6, May-June 1974
20
30
I, mA
4. The observation of the laser action in the CO laser with water-cooled discharge-tube walls suggested that it should be possible to construct a sealed laser. We found that the laser action could be obtained in such a sealed laser for a fairly long period and this was true of quartz and glass discharge tubes. A tube made of molybdenum glass had a discharge gap of 980 mm and an internal diameter of 15 mm. The water-cooling jacket was 980 and 1280 mm, the cooled sections were 975 and 1260 mm long, and the internal diameters were 18 and 15 mm. In all cases the electrodes were located in side tubes at a distance of 50 mm from the discharge axis. We used nickel and tantalum cylindrical electrodes with closed bottoms. An additional quartz or glass tube, connected to the discarge, was inserted into the open end of the cylinder. This construction reduced considerably the sputtering of the metal and ensured more stable discharge conditions. Before the tubes were assembled, their internal surfaces were treated in hydrofluoric acid. After the assembly, the tube was evacuated to a pressure of 10~4 mm Hg and conditioned by discharges in air, inert gases, andworking mixtures. The electrodes were heated with a high-frequency inductor when connected to a high-vacuum outlet. Before sealing, a continuous-flow discharge was allowed to take place for 20-30 min. Then a helical trap, placed in the gas-admission system, was either cooled with dry ice or with liquid-nitrogen vapor. The latter procedure resulted in a somewhat higher subsequent power output which was likely to be due to the removal of the traces of water vapor present in gases of technical purity. The laser was sealed by a simultaneous closing of the entry W, rel. units
Fig. 4. Dependence of the output power on the pressure in a CO + N2 + He + Xe + O2 (1: 6.7 : 28 : 0.67 : 0.04) mixture obtained for different discharge currents.
4-0 -
20 •
20 p, mm Hg
Kacheva et al.
485
v, watts »
n,watts
20 a
zu
30 t,h
°°
to t,i
Fig. 5. Dependences of the output power on the duration of operation of a laser sealed in a quartz tube (a) and in a glass tube with a bypass channel (b). The arrows indicate the moments of switching off of the discharge. The interval between switching off and on was 12 h. The mixture was CO + N2 + He + Xe + O2 (1: 6.7; 28: 0.67: 0.04). The pressure was 16.4 mm Hg.
and exit vacuum valves. The pressure, discharge current, and gas mixture composition were made the same as those observed at the optimal continuous-flow conditions in a water-cooled tube. Figure 5a shows the time dependences of the output power obtained during 36-h operation of a sealed quartz tube of 980 mm length. The power was measured with a KIM-1 calorimeter. The arrows in Fig. 5 identify the moments when the laser was switched off. After each such occasion the laser was switched on again after 12 h. We could see that the power fell after each such operation. Similar observations were reported in ref. 7 for a sealed CO2 laser and attributed to the adsorption of the individual components of the gas mixture by the electrode metals (regeneration of the composition was incomplete).
tions were smoothed out which was probably due to additional conditioning of the tube under working conditions. It should be possible to increase further the service life of such a laser by a careful conditioning of the tube and suitable preparation of the construction elements. Further improvements may be expected from the use of high-purity gases.4 It is also obvious that the service life of a laser can be extended considerably by providing an additional ballast enclosure for the working gas mixture. 1
Figure 5b shows the dependence of the output power on the duration of operation of a laser in a molybdenum glass tube with a bypass channel, which was a tube of 12 mm diameter connecting the cathode to the anode. In this case the service life of the laser was considerably longer and the output power level was much more stable. During the first few days of tests we observed behavior similar to that shown in Fig. 5a. However, eventually these varia-
N. Legay-Sommaire, L. Henry, and F. Legay, C. R. Acad. Sci. (Paris), 26013, 3339 (1965). Z M. L. Bhaumik. Appl. Phys. Lett., 17. 188 (1970). 3 C. Freed, Appl. Phys. Lett., 18^, 458 (1971). *H. J. Seguin, J. Tulip, and B. White, Can. I. Phys., 49, 2731 (1971). 5 N. N. Sobolev, V. V. Sokovikov, V. N. Strelets, Kratk. Soobshch. Fiz., No. 9, 13 (1971). 6 M. Z. Novgorodov, A. G. Sviridov, N. N. Sobolev, and P. Shvarts, Kratk. Soobshch. Fiz., No. 5, 20 (1972). 7 E. N. Lotkova, V. I. Makarov, V. N. Ochkin, T. P. Pyataeva, and N. N. Sobolev, Preprint No. 10 [in Russian], Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow (1972). 8 Yu. A. Pekar, Z,h. Tekh. Fiz., 3T_. 1112 (1967) [Sov. Phys. -Tech. Phys., 12, 800 (1967)].
486
Kacheva et al.
Sov. J. Quant. Electron.. Vol. 3. No. 6. May-June 1974
486
