Mode shaping of a graded-reflectivity-mirror unstable resonator with an intra-cavity phase element

A graded-reflectivity-mirror (GRM) unstable resonator with low output coupling is described, where a custom-made optical phase element is used inside the resonator to provide maximally flat output. The phase element removes the dip in the output beam by pre-compensating the internal Gaussian mode. An experiment is performed with a flashlamp-pumped Nd:YAG laser. The resonator's magnification (M) and the GRM's central reflectivity (R_o) are 2.3 and 0.7, respectively. The large dip in the center of the output is removed using the custom-made phase element. This resonator has the advantage over a conventional GRM unstable resonator of being suitable for lower-gain laser media. The gain required to overcome fundamental mode cavity losses for maximally flat output is decreased from 22.3 (for a conventional GRM resonator) to 4.3 (for the resonator containing the phase element). This reduction in required gain comes with essentially no loss to the resonator modal discrimination.