EFFECT OF DIFFUSION ON GAIN SATURATION IN CO2 LASERS
MASSACHUSETTS INST OF TECH LEXINGTON LINCOLN LAB
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The theory of gain saturation of a two-level system is briefly reviewed and then generalized to include an arbitrary number of upper and lower levels the upper levels are coupled to each other, as are the lower levels. The laser radiation is assumed to induce transitions between one upper and one lower level. In the limit of tight coupling among the upper and lower levels by themselves, the equations reduce to those of a two-level system, with effective relaxation rates that are weighted sums of the relaxation rates of the multilevel system. The relaxation rates occurring in a CO2 laser system are comparatively low so that one would expect spatial diffusion to play an important role in determining saturation. A theory of diffusion is carried through and it is shown that diffusion effects can indeed be important for optical beam diameters of a few millimeters. Finally, experiments on a sealed- off CO2 laser oscillator and amplifier system are reported. Amplifier gain is measured for four different beam radii. The equivalent saturation parameter derived from the measurements decreased monotonically from 97 to 25Wsquare centimeters as the average input beam radius increased from 0.9 to 2.5 mm in the 9-mm radius discharge tube of the amplifier.
- Lasers and Masers