Tunable Sealed - off Cw Co Laser at Room Temperature

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 chemistry and air-pollution monitoring there are interesting applications for lasers in this wavelength region. A tunable cw CO laser will be a very attractive candidate 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 cont 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 discharge tube that was provided with a small cell containing zeolite [1 ]. The zeolite is used as an absorb'er of any water that is either released from the construct ion materials or produced in the discharge from released 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 experiments for developing a tunable laser starting from the sealed-off CO laser mentioned above. Several techniques 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 FabryPerot etalon has one broad-band reflector and a rotatable wavelength-selective grating. Using this three-mirror configuration, we developed in the past a cw sealed-off CO 2 laser, with a discharge 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-adjustments 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 expectations.