Graded Reflectivity Mirrors (GRM) manufactured by INO are mainly used in solid-state Nd:YAG Q-switched lasers and TEA-CO2 gas lasers, where they serve as output couplers for these laser unstable resonators. GRMs can be used to change the shape of a beam (i.e., from gaussian to super-gaussian or inversely) during reflection or transmission. A GRM with maximum on-axis transmittivity used in a laser resonator acts as a soft aperture to provide instant fundamental mode oscillation, but without any hard-edge-induced rippling of the field transverse profile.
INO GRMs are fabricated on anti-reflection-coated transparent substrates on which profiled dielectric layers have been deposited using two vacuum evaporation techniques. The first patented technique, exclusive to INO, uses a rotating mask of specific geometry to deposit one or more thin films of variable thickness. The mask rotates rapidly in front of the substrate, producing circular symmetrical reflectivity profiles. The second technique uses a fixed mask to produce non-symmetrical reflectivity profiles well suited to gain media with rectangular or square cross-sections.
Our fabrication techniques allow us to fabricate circular and non-circular reflectivity profiles on 10 mm to 150 mm diameter SiO2, Sapphire and ZnSe substrates. Reflectivity profiles can be gaussian, super-gaussian, or any other mathematical function.
The beam waist (Wn) can be as small as 0.7 mm. A GRM can withstand energy densities greater than 12 J/cm2 for 14 ns pulses at 1,064 nm.
G. Duplain, P. G. Verly, J. A. Dobrowolski, A. Waldorf, and S. Bussiere, “Graded-reflectance mirrors for beam quality control in laser resonators”, Applied Optics, Vol. 32, No 7, 1993.