Variable Reflectivity Mirror, Graded Reflectivity Mirror

Variable‐Reflectivity Mirrors (VRM)

Vigitek, Inc. is one of a few companies that in collaboration with the manufacturers of a thin-film coated optics developed a technology of Variable-Reflectivity Coatings (aka Graded Reflectivity, or GRM) which delivers consistent and repeatable results. And while Nd:YAG mirrors with Ro ~ 30-40% can be obtained from several manufacturers, very few places can fabricate mirrors for low-gain lasers like Alexandrite , where Ro must be on the order of 75-80%, which requires a multi-layer coating that at the same time must withstand a high intracavity fluence of a few J/cm2. We can also design the unstable resonator in accordance with desired output characteristics.

Description

Description

Typically, the reflectivity of the mirror is greatest at the center and decreases towards the edge of the mirror. Vigitek offers GRMs with circularly symmetric profiles the reflectivity of which is described by formula: R(r) = Ro · exp [‐2(r/Wo)N], where Ro is the central reflectivity, r is the distance from the mirror’s centre, Wo is the radius of the coated area, where R≥Ro/e2, and n designates the order of the (super-)Gaussian profile (for n=1 the profile is simply Gaussian).

Example of the GRM coating design (output coupler for the Qswitched Alexandrite unstable resonator)

The graded reflectivity mirror (GRM) (also known as the variable reflectivity, Gaussian, or super-Gaussian mirror) is a special mirror whose reflectivity at a particular wavelength varies gradually as a function of the position on the mirror’s surface. GRM provides a better modal discrimination in an unstable resona-tor, i.e. it favors the oscillation of the fundamental mode against higher order modes. It also improves the optical quality of the output beam especially at low magnifications. The tapered reflectivity profile also leads to a decrease of ripples in the field profile and thus reduces the risk of optical damage when used with high intensity lasers.

Calculated GRM
Coating Function

Substrate Specification:
Material: BK7 glass, Fused Silica
Size: standard 25.4mm
Dia. x 9.5mm Thk., 12.7 mm Dia. x 6.35 mm Thk.
Meniscus Geometry (ROC subject to the unstable resonator design), Wedge 1 degree.

Coating Specification:
Wavelength: 755, 1064 nm (others on request)
WO typical diameter from 0.5 to 5.0 mm
Convex Side – Central Reflectivity RO = 5% to 80 % (+/-1 % to +/-5% depending on the reflectivity level)
Residual reflectivity RR = 5% of RO or 0.5%.
Concave Side – appropriate AR-coating
Damage threshold not less than 15J/cm2 @1064nm for 10 nsec pulsewidth

Green Line – measured reflectivity

Red/Magenta Line – super-Gaussian function with n=5, w=2.97 mm, Rmax=75 %

Teal Line – super-Gaussian function with n=5, w=2.97 mm, Rmax=70 % (normalized to fit reflectance with the same width and n as previous).

Purple Line – super-Gaussian function with n=4, w=3.66 mm, Rmax=70 % (normalized to fit reflectance, and shape parameters for measured curve).

Vigitek, Inc. fabricates OEM-type mounts & holders for mirrors, lenses, 25 and 35mm Dia. Pockels Cells, thin-film polarizers, retardation waveplates, etc. that we consider one of the best and most stable in the industry. The distinctive features include:

Mirror replacement without disturbing the alignment due to controlled surface pressing against three embedded steel balls;
100 TPI alignment screws with backlash-free alignment screws split locking mechanism;
Overall mount lock, Sapphire stops, etc.

Lasers containing these mounts were shipped worldwide and maintained their original alignment. This was the case even with Q-switched Nd:YAG lasers with GRM output coupler, designed for a low-magnification unstable resonator, which is sensitive to even slight misalignment.