Graded reflectivity mirrors (GRM)

Graded reflectivity mirrors (GRM) are used as output couplers for unstable laser resonators, such as solid-state Nd:YAG Q-switched and TEA-CO2 gas lasers.



GRMs can be used to change the shape of a beam (e.g., 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.

Fabrication process

  • Our 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 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 µsec. pulses at 1.064 µm.

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