1. Field of the Invention:
This invention relates generally to laser devices, and more particularly to laser devices in which a gap-space is present between the gain medium and optical mirrors.
2. Description of the Prior Art:
It is common practice in the design of laser devices to position a gain medium between a pair of opposing optical mirrors which form the resonator. The gain medium used to sustain oscillation may extend from mirror to mirror as in some helium-neon and carbon dioxide laser devices, or the extension may be short of the mirrors with a window provided at each of the terminal ends of the gain medium. In the latter case, commonly found in solid state and ion lasers and in some types of helium-neon lasers, there remains a physical gap between the gain medium and the laser mirrors. In operation, the beam resonates between the mirrors and passes through the gain medium as well as through the intervening gap space. Although this design offers many advantages and has been widely used, the occurrence of optical losses within the resonator cavity has been a consistent and continuing problem.
Since 1961, when such lasers were placed in operation, efforts have been made to overcome or at least minimize this problem. However, because there has not been a clear understanding as to what caused the increased losses in the resonator cavity, a solution to the problem has not emerged from these efforts.
It is, of course, obvious that the cavity not properly shielded would collect dust and other foreign matter and that these materials would fall on the optical surfaces to produce scatter and absorption. Yet, the provision of a dust cover for the cavity did not eliminate the problem in that optical losses continued to be experienced and the laser components degraded with time.
Although a clear understanding of the mechanism responsible for these laser losses and deterioration has eluded investigators for many years, the basic causes of the problem can now be explained as a result of recent research efforts.
It has been found that water vapor is absorbed into the materials comprising the laser mirrors and dust seals and is somehow expelled into the optical cavity where it causes losses. This is the most common and pervasive cause of the problem. In the case of gas lasers, the presence of some UV radiation also contributes to the problem. This can be explained by the fact that energy at the UV frequency converts oxygen to ozone in the optical cavity. The resulting ozone attacks cavity materials and seals to produce contaminants which deposit on optical surfaces and become suspended in the optical pathway to cause losses and deterioration of cavity components.
In addition, organic vapors emanating from solvents used to clean the optical surfaces at times diffuse into the optical cavity as a deleterious contaminant.
Thus, it is apparent that in order to assure a long-term stable performance of these laser devices, water vapor in particular must be continuously purged from the optical cavity, and in some instances there is likewise a need to remove any accummulation of ozone and organic vapors.