Variable Radius Mirrors

The II-VI Variable Radius Mirror (VRM) allows users to dynamically change their beam characteristics on the fly. By controlling the VRM’s radius of curvature with water pressure, users can adjust the laser beam divergence.

VRMs allow focus depth adjustment during material piercing; this produces optimum cutting speeds. It also allows flying optics systems manufacturers to compensate for focal length variations across the working table. This is especially important with large working tables, where laser beam divergence changes at the lens as the optical path moves across the work area.

The VRM is designed for use at near-normal angle of incidence. Many laser cutting systems use two mirrors as telescope optics. The telescope is made of one convex and one concave mirror. Replacing one of these mirrors with a VRM allows all of the benefits listed above.

Pressure Control
There are at least two ways to control the pressure in the VRM and, as a result, control the radius of the mirror surface. The key component is either a variable-speed pump or a proportional control valve. These items are driven by an amplifier. Input to the amplifier is typically a 0 to 10 volt signal. The amplifier is run open-ended or in a closed-loop system.

Custom Designs
II-VI has the engineering capability to design adaptive mirrors for any beam delivery system. Using proprietary design techniques, II-VI can accurately model the VRM shape and predict how it will deform under pressure. The mirror shape is optimized to match the pressure-radius curve defined by the customer.

Water Pressure System Example
The drawing below shows the closed-loop system that uses a pressure transducer to measure the pressure in the mirror cavity. This signal is fed back to the CNC controller.


Substrate: Copper
Standard Mirror Diameter: 57.1 mm, 79.0 mm
Usable Clear Aperture: 35 mm, 50 mm
Radius Range*: 6 MCC - 6 MCX
3 MCC - PO
PO - 3 MCX
1.2 MCX - 1.6 MCX
Pressure Range: 3 to 11 bar
Water Flow Rate: ~1 liter/minute
Angle of Incidence: Near normal
Reflectivity with MMR-A Type Coating: > 99.8%
Pointing Stability: <= 30 arc seconds
*Customized radius range available.
M is meter, CC is concave, CX is convex , PO is plano



Materials Processing, CO2 Laser Microprocessing, High Power CO2 Laser Processing, Mirrors, INFRARED

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