Starting from a sealed-off CO laser with a long-life output of 29 W/m in multiline operation, we developed a tunable version of this device. The system described has a discharge length of 97 cm. It was made transition-selective by using a three-mirror configuration. In this way we were able to tune the laser to more than 70 oscillating vibrational-rotational transitions of the CO molecule

Krzyzanowski, Z.

Peters, P.J.M.

Witteman, W.J.

English

Starting from a sealed-off CO laser with a long-life output of 29 W/m in multiline operation, we developed a tunable version of this device. The system descried has a discharge length of 97 cm. It was made transition-selective by using a three-mirror configuration. In this way we were able to tune the laser to more than 70 oscillating v~rational-rotational transitions of the CO molecule. There is an increasing need for broadband tunable lasers, with a narrow line-width, in the near-infrared region. Especially spectroscopists are interested in the region around 5/am, but also for laser-induced chem-istry and air-pollution monitoring there are interest-ing applications for lasers in this wavelength region. A tunable cw CO laser will be a very attractive can-didate because of its high output power and the large number of potential laser lines. It is possible to cover with this laser the spectral range from about 5/am up to 8/am. To have a stable and high output power

P. J. M. Peters

W. J. Witteman

Z. Krzyzanowski

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TUNABLE SEALED-OFF CW CO LASER AT ROOM TEMPERATURE

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Tunable sealed-off CW CO laser at room temperature

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Volume 45, number 3 OPTICS COMMUNICATIONS 1 April 1983 TUNABLE SEALED-OFF CW CO LASER AT ROOM TEMPERATURE P.J.M. PETERS, W.J. WITTEMAN and Z. KRZYZANOWSKI Department of  Applied Physics, Twente University of  Technology, Enschede, The Netherlands Received 2 November 1982 Revised manuscript received 29 December 1982 Starting from a sealed-off CO laser with a long-life output of 29 W/m in multiline operation, we developed a tunable version of this device. The system descried has a discharge length of 97 cm. It was made transition-selective by using a three-mirror configuration. In this way we were able to tune the laser to more than 70 oscillating v~rational-rotational transitions of the CO molecule. There is an increasing need for broadband tunable lasers, with a narrow line-width, in the near-infrared region. Especially spectroscopists are interested in the region around 5/am, but  also for laser-induced chem- istry and air-pollution monitoring there are interest- ing applications for lasers in this wavelength region. A tunable cw CO laser will be a very attractive can- didate because of  its high output  power and the large number of  potential  laser lines. It is possible to cover with this laser the spectral range from about 5/am up to 8/am. To have a stable and high output  power, many investigators use cw CO lasers that are cooled down to liquid-nitrogen temperature and with con- t inuously flowing gas mixtures. It will be clear that a sealed-off CO laser, working at room temperature,  would be much easier to work with and to transport  from one place to another. Many at tempts have been made to develop such a sealed-off laser. Now it is generally believed that for long-life operat ion the elimination of  hydrogen or waver vapour is essential. In 1980 we reported on a solution for this problem by using gold electrodes and a quartz dis- charge tube that was provided with a small cell con- taining zeolite [1 ]. The zeolite is used as an absorb'er of  any water that is either released from the construc- t ion materials or produced in the discharge from re- leased hydrogen. With this sealed-off tube working at room temperature we got a multiline output  power of  29 W/m. After one year of  regular use the output  power o f  this tube was still 22.5 W/m. In this paper we will report  on our recent experi- ments for developing a tunable laser starting from the sealed-off CO laser mentioned above. Several tech- niques can be used to make a cw laser wavelength-selec tive. We chose a three-mirror configuration, which can yield single-mode emission. The principles of  this three-mirror device have been described for cw CO 2 lasers by Ernst and Witteman in 1971 [2]. It consists of  an output  coupling window with a suitable chosen radius o f  curvature on one side and on the other side of  the laser tube a Fabry-Perot etalon. The Fabry- Perot etalon has one broad-band reflector and a rotat- able wavelength-selective grating. Using this three-mirror configuration, we develop- ed in the past a cw sealed-off CO 2 laser, with a dis- charge length of  about 90 cm. This system met some special requirements. The first one was that the laser should give a very stable output  power for each of  the selected laser lines. The second requirement was to develop a system that was easily transportable from one place to another without  any need for re-adjust- ments of  either the mirror or the grating. The system designed appeared to be tunable over more than 80 transitions in both  the P and R branches of  the 0 0 0 1 -  0 0 10°0 and 00 1 - 0 2  0 vibrational bands of  the CO 2 molecule. From experiments with this CO 2 laser we could conclude that  the design came up to our ex- pectations. 0 0 3 0 4 0 1 8 / 8 3 / 0 0 0 0 - 0 0 0 0 / $  03.00 © 1983 North-Holland 193 Volume 45, number 3 OPTICS COMMUNICATIONS 1 April 1983 piezoelectric central mounting-block concave [R_- 5ml / translator / outcoup! !ng \ pyrex support-tube rnlrror : M 1 \ r e f l e c t i n g  1 70°~ I qua r t z  discharge tube g~ld-coated grating plane mirror:M 2 w i t h  coolant jacket Fig. 1. Cross-section of the tunable CO laser. The same system we now investigated with a sealed-off CO laser. The experimental set-up is shown in fig. 1. Inside a pyrex support tube the quartz discharge tube has been mounted. On-one side the tube has been glued in the central mounting block. In this solid block also the grating has been mounted, with a possi- bility to turn the grating very precisely around a vertical axis perpendicular to the optical axis. In our measurements we used a 3600 ~/inch gold-coated grating, blazed at 5.4/am, from Photo Technical Re- search Inc. The grating was mounted on a piezo-elec- tric translator, in order to tune the length of  the Fabry-Perot on the oscillating wave-length. The quartz discharge tube is on the other side, on which the outcoupling mirror has been glued, connected to the pyrex support tube, by means of  a slightly ad- justable support structure. It should be noticed that the pyrex support tube has been fixed only on one side to the central mounting block. This block, on its turn, has been mounted to a thick, rigid, iron pro- file. The laser tube had a discharge length of  about 90 cm and an internal diameter of  6 mm. As electrode material we used gold. For the spectral analysis of  the laser output radia- tion, the beam was chopped and then passed through a microprocessor driven Hilger & Watts monochroma- tor, which was equipped with a 150 ~/mm grating blazed at 5.4/am. The output radiation from the monochromator was detected by an InSb detector and this signal was fed into a lock-in amplifier. The output of  the lock-in amplifier can be recorded on an Y-T recorder. The resolving power of  our arrange- ment was about 8 A. We investigated two different optical configura- tions. In both cases we used on the outcoupling side a germanium window with a reflectivity of  88%. The transmission of  this window was about 10% and the radius of  curvature was 5 m. On the other side of  the tube we used two different germanium windows, one with a reflectivity of  90% and the other with a reflectivity of  70%. When the grating was blocked, both configurations nevertheless emitted laser radiation. According to gain measurements carried out by Bhaumik and col- leagues [3] this should be impossible. They estimated that the gain did not exceed 0.2% cm -1 in a similar CO laser. In our tube with the highest losses the maxi- mum gain clearly exceeded their measured values. We concluded this from the fact that the laser thresh- old value was 0.27% cm-1 in our laser and still there were at least five oscillating transitions. By compar- ing all oscillating rotational-vibrational transitions in multiline, operation, we see a remarkable differ- ence for the two optical configurations described. Looking at the lines with the highest intensity, we ob- serve a shift to lower vibrational bands and higher rotational numbers when more transitions oscillate at the same time. We have seen this effect also in a pulsed transversely excited CO laser working at cryo- genic temperatures [4]. It is due to an intrinsic fea- ture of  the CO laser: the cascade effect. With the grating not blocked we found much more oscillating transitions in both configurations. With the window of  90% reflectivity (at the Fabry- Perot side) we found only 10 to 15 new lines. This is due to the fact that most of  the vibrational energy, which is available in potentially lasing levels, is actual- ly emitted as multiline stimulated radiation because of  the low laser-threshold values. So the tuning effect 194 Volume 45, number 3 OPTICS COMMUNICATIONS 1 April 1983 Table 1 Measured v~rational-rotational transitions in a wavelength-selective, sealed-off, cw CO laser (T = 300 K). Note: lines with the same subscript coincide within 0.5 cm -1 Vibrational band P-branch transition 8--, 7 17 18 19 20 21 22 23 A 9~ 8 17 A 18 19 20 21 22 B 23 C 10--~ 9 16 B 17 C 18 19 20 21 22 D 11--,10 14 15 16 D 17 E 18 19 20 12--,11 14 16 F 17 G 18 19 20 13--,12 13 14 15 17 18 19 14-.13 14 15 16 17 18 19 15--,14 14 15 16 18 19 20 16--,15 14 16 17 18 17--,16 13 14 16 17 18 18~17 15 16 23 E 21 22 F 23 G 21 20 20 21 22 of  the grating is almost restricted to these transitions, which are either adjacent to the oscillating lines or the next step in the cascade chain. This has been con- firmed in our experiments. If the threshold is much higher, like in our second configuration, the situation is completely different, since there are only a few remaining oscillating transi- tions. It should be noted that,  in both  configurations, if  the system is tuned at a particular transition with the grating, we always observe at the same time some ad- ditional oscillating transitions. These transitions are generated in the cavity formed by  the mirrors M 1 and M 2 and it will be clear that in our second configura- t ion the gain of  these transitions will exceed 0.27% cm -1  . With M 2 = 70% we mostly observe the P9~8 (19), P10~9(17), Pll__,10(16), P12__,11(17), and P14~ 13(15) transitions. So in this case, first, there are a lot of  potential ly lasing levels for which the gain is not too far below the threshold value and, second, less vibrational ener- gy present in potential ly lasing levels is emitted as stimulated radiation. It was with this configuration that up to now we found more than 70 vibrational-rotational transitions by tuning the grating at the proper  angle and distance from the 70% reflection window. In table 1 all these measured transitions are shown ranging from 5.2/am up to about 6.1/am (which is from 1900 cm -1  down to 1640 cm-1) .  The missing transitions are probably a result of  atmospheric ab- sorptions. The atmospheric path length from the out- coupling mirror to the detector  was about 4 m. The output  power of  the different lines varied between 25 mW and 2 W. This can probably be more for some strong lines if  the losses in the cavity are made still higher, so that no transit ion can oscillate without  the grating. Conclusion. Although the wavelength-selective cavity we investigated was not ideal as the outcoupling losses were not high enough, we yet obtained valuable information about the possibilities o f  our three mir- ror wavelength-selective system for the CO laser. We suggest that  by decreasing the reflectivity o f  mirror M 2 the laser will oscillate on only one line and that even more transitions should be possible. The stabili ty of  the laser is good. If  there are fluctuations, they are mostly very slow. However, it might be possible to  avoid them by using a feed-back controlled piezo- electric scanner. References [ 1] P.J.M. Peters, W.J. Witteman and R.J. Zuidema, Appl. Phys. Lett. 37 (1980) 119. [2] G.J. Ernst and W.J. Witteman, IEEE J. Quant. Electron. QE-7 (1971) 484. [3] M.L. Bhaumik, W.B. Lacina and M.M. Mann, IEEE J. Quantum. Electron. QE-8 (1972) 150. [4] P.J.M. Peters, Ph.D. Thesis, Twente University of Tech- nology (1981). 195 

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Tunable sealed-off CW CO laser at room temperature

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