Page 36 - High Power Laser Handbook

8 G a s , C h e m i c a l , a n d F r e e - E l e c t r o n L a s e r s Carbon Dioxide Lasers 9

avoided by using nonoxidizing materials, such as quartz or ceramic,
or by passivating the metallic materials used. Passivation of alumi-
num electrode surfaces, for example, can be achieved by chemical
reactions between aluminum and a strong oxidizer, such as nitric
acid. Other methods include anodizing and applying conversion
8
coatings. The CO partial pressure can also be conserved by using a
2
catalyst to accelerate the back reaction of CO and oxygen to CO .
2
The most commonly used method for stabilizing the CO partial
2
pressure is the use of pre-dissociated gas mixes, in which CO and
sometimes oxygen are added to the gas mix. This approach avoids
not only CO dissociation but also the creation of oxygen. Avoiding
2
the creation of oxygen is of interest because oxygen quenches both
14
the upper laser level and the exited N molecule. 15
2
1.4 CO Laser Types
2
Like any other laser, the CO laser has a limited efficiency. Efficiently
2
removing the waste heat from the laser gas and keeping its tempera-
ture below 600 K is key to laser performance. Two types of CO laser
2
designs on the market efficiently remove the heat from the active
medium: fast-flow and diffusion-cooled lasers. In fast-flow designs,
the gas is circulated with speeds of up to half the speed of sound
through the discharge area. The gas is then cooled in heat exchangers
before returning to the discharge area. In diffusion-cooled designs, the
laser gas is in contact with cooled surfaces, and the heat is removed by
diffusion of the hot gas molecules to the water-cooled electrodes. These
two categories of lasers are described in later sections of this chapter.
The different types of CO lasers can be further categorized accord-
2
ing to their excitation method, their design, and their operating param-
eters. The gas discharge for laser excitation can be direct current,
medium frequency (0.3 to 3 MHz), radio frequency (3 to 300 MHz), or
microwave (0.3 to 3 GHz) powered discharge. Further categories are
sealed-off lasers, waveguide lasers, transversely excited atmospheric
pressure (TEA) lasers, and gas dynamic lasers. The next sections focus
on designs that are relevant for today’s industrial applications.

1.4.1 Diffusion-Cooled CO Lasers
2
The laser power (P ) of diffusion-cooled lasers scales with the surface
L
area A, which removes the heat from the gas, and the distance between
the water-cooled electrode surfaces (the interelectrode gap) d:
P ∝ A/d
L
The available power levels of diffusion-cooled lasers range from a
few milliwatts up to 10 kW. In this section, we distinguish between
high- (>1 kW) and low-power (<1 kW) lasers; we also describe the most
common electrode geometries for diffusion-cooled high-power lasers.