7B2 - Laser brightness gain and mode control by compensation for thermal distortion

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Abstract
In the limit of excellent optical quality of laser rods, the beam divergence will be determined by the optical distortion produced in the rod brought about by nonuniform absorption of pump light. For ruby lasers this has been shown to be approximately spherical and reproducible from firing-to-firing. Thus it should be possible to compensate for this by means of an external mirror having a curvature opposite to that induced in the rod, and by this technique reduce the beam divergence. The distortion produced in the rod is described in terms of a ray matrix of a thick lens whose focal length is a function of the input energy to the rod. A procedure is given for measuring this focal length to within ± 3 percent; it involves measuring the beat frequencies between transverse modes of a flat-mirror cavity containing the laser rod as an active element. After compensating an excellent optical quality rod with an external mirror, a brightness gain of about 100 was achieved simultaneously with single transverse mode operation (TEM00) at pump energies as high as 40 percent over threshold. This procedure for achieving single transverse mode operation is advantageous compared to other methods since 1) no extra elements are needed in the cavity, 2) the beam divergence is near the diffraction limit for the rod holder, and 3) the TEM00mode is not only selected but enhanced in volume, thus increasing the power output in this mode and the mode stability of the laser.

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