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Use of two output mirrors ,a sapphire or quartz etalon and a resonant OC, maintained single axial mode at all power levels and temperatures. However, having two independently mounted OC mirrors which have to work as one large composite etalon in fact increases the chance for a mis-aligned resonator which  does favor higher order transverse modes or poor TEM00 mode quality due to multiple reflections. Consideration must be given to passive resonator stability and accurate alignment and keeping a very low Fresnel number  resonator geometry.  resalign.htm for more information. You can get good tem00 mode with the above arrangement as was demonstrated inr7beam.htm   and  r7osc.htm.  

The decision to try this new arrangement was strictly experimental just to gather data. New oscillator is in a Plano Plano resonator configuration. This design replaces the dielectric spherical concave mirror used as an HR for a resonant reflector. 

noc.jpg Image of new resonator.

nhr.jpg View of 4 plate HR housing as installed in resonator kinematic mount.

3eta.jpg  Picture of an etalon of the type being used for this setup.

   HR mirror is a resonant reflector made of a  2.5mm quartz plate uncoated etalons separated by 25mm spacers. A third spacer of 25mm and a 3.175mm sapphire etalon is used as a fourth plate. All plates are aligned by lambda/10 and the three 2.5mm quartz plates are flat to 1/20 wave and are of equal thickness to lambda/8 and spacers are equal to same. This four plate resonant reflector should be at least 86 percent reflectivity. The OC also is a  resonant reflector of  two quartz plates 7.9mm thick and spacer of 7.9mm at 39 percent reflectivity.

This new  resonator was shortened to 54cm to increase axial mode spacing. Dye cell is used as a mode selector.

Aperture was  placed near OC since both ends are plano and the aperture can filter stray offaxis reflections before exiting on the OC side.

The laser operated in single axial mode at all times. TEM00 mode quality generally was not good due to multiple reflections. Due to use of a 86 percent reflector for the HR and a 39 percent reflector for the OC, increased the threshold for the laser and also increased the loss associated with the resonator. The effect was to reduce the output to 6 to 10 millijoule in single mode and to increase the pulse width to 20 nanosecs.

Due to poor TEM00 mode quality, the double resonant reflector design was abandoned for the proven plano-concave resonator used before.   So the 4 plate resonant reflector was removed. The sapphire etalon used as the 4th plate was removed and a 3mm quartz etalon was installed instead as the 4th plate inorder to achieve 81 percent reflectivity and the 4 plate resonant reflector was instead used as an OC replacing the 2 plate OC. The 7 meter spherical concave mirror was reinstalled as the HR. This  plano-concave resonator arrangement worked fine in single axial mode due to use of the 4 plate resonant reflector and the dye q switch as the mode selector. The aperture was left on the OC side and TEM01 mode generally was the mode at 1 mm aperture size. Only when 0.7mm aperture was used did TEM00 mode emerge but the power loss due to diffraction allowed only 3 millijoule output. When the aperture was moved back over to the HR side, at 1.2 mm good TEM00 mode was achieved and 14 mj output in single axial mode at 12 nanosec pulse length.

The conclusion: 

An aperture controlled the fresnel number of the resonator and was the TEM mode selector used in the trials. A dye q switch assisted in achieving consistent single axial mode  and no attempt to control the resonator by temperature means was practical in the air cooled laser. 

Of the various designed used for version 7, the top two designs that gave the best results were:

Composite OC  resonator provided single axial mode and marginal to fair TEM00 Mode.

Plano-concave with  4 plate OC  gave the best performance of TEM00 and single axial mode.

Of the designs that did poorly:

Plano-concave with 2 plate OC which gave good TEM00 mode but rarely was single axial mode.

Double resonant reflector which was single axial mode but poor TEM quality and too sensitive to rod pumping geometry.