Reduced Parasitic Lasing in Ti:Sapphire Lasers. Removing a Bottleneck to New Ways of Acceleration

Powerful solid state lasers can be efficient sources of high energy electrons in a new concept of particle acceleration. One limitation to the power of these lasers is the phenomenon of parasitic lasing, in which oscillations transverse to the intended lasing direction can drain the stored energy and thus clamp the gain at some maximum value. This problem becomes more severe the larger the laser crystal becomes. The source of the problem is the internal reflections in the larger sized gain material. This program was aimed at eliminating this bottleneck to more powerful lasers, to allow more intense high energy beams to be generated by this technique. The basic approach was to eliminate the internal reflections by making an absorbing layer on the outside of the crystal that had the same refractive index as the crystal. Thus, any transverse light would be absorbed before it reached the edge of the crystal and had a chance to reflect back into the interior. During the first year, furnaces were renovated that and used to produce evaluation crystals. The first approach was to oxidize the outside surface of the titanium doped sapphire laser gain crystal. This oxidation is known to make the crystal be absorbing in the lasing wavelengths. A number of tests showed some modest absorption. After the first year, 2 parallel paths were pursued to improve on this initial results – overgrowth and oxidation and grooving with index matched thermoplastic. Experiments on one of these techniques progressed to near-operational status as a commercial coating process. A test Ti:sapphire rod with a commercial coating technique was also supplied to the subcontractor for baseline testing. The final product of the Phase II was a process that was reduced to a commercialized operation involving the complex series of steps of fabricating the crystal, polishing, grooving, providing an AR coating, then producing the absorbing layer and packaging the crystal in a manner that would allow easy handling for installation and deinstallation.