Institut national d’optique - INO

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Graded Reflectivity Mirrors

Graded reflectivity mirrors (GRMs) are also employed in portable laser systems used for the restoration of valuable objects and artifacts

Q-switched Nd:YAG lasers with GRMs are widely used in dermatological and esthetic medicine

Optical thin film fabrication lab

Each of the GRMs fabricated at INO has special properties that meet specific client requirements

Client
Q-PEAK (formerly Schwartz Electro-Optics)

Background

INO has earned an international reputation for its graded reflectivity mirrors (GRMs), which are now used by major manufacturers of solid-state lasers and leading international R&D laboratories. In the case of Q-PEAK, our task was to push back the limits of existing technology to develop wideband GRMs with an 80 nm bandwidth and no major distortion of the reflectivity profile of Ti-Sapphire (Ti:Al₂O₃) tunable lasers.

Solution

The GRMs developed for our client required a new thin films design able to provide high reflectivity at the GRM center as well as efficient antireflection outside the GRM profile all along its operational bandwidth. The GRM offered performance that was far superior to what it would have been without GRM.

Plus…

GRMs 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.

The ABCs of GRMs

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.

GRMs at work

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.