This invention provides a device for compensating for the phase distortion in higher order laser modes so that these modes can be focused to diffraction-limited spot sizes. Also, there is provided the means for forcing a laser to oscillate in a particular high-order laser mode....http://www.google.com/patents/US4219254?utm_source=gb-gplus-sharePatent US4219254 - Corrective optics for higher order mode lasers
