Variable Radius Mirrors

Variable Radius Mirrors

A Variable Radius Mirror (VRM) is a mirror that can dynamically change its radius of curvature when pressurized with air or water.  It is useful in laser cutting applications for controlling beam divergence and axial focus position. 

II-VI specializes in custom VRM solutions that mount into laser-specific beam delivery systems.  Our designs are based on many parameters which include radius range, pressure range, and angle of incidence.   
 

There are some limitations to the radius and pressure ranges to consider.  The best way to visualize the relationship between radius and pressure in a VRM is to plot curvature (1/radius) as a function of pressure.  Several design examples are listed in a table and plotted on a chart shown below.  A steep slope represents a large curvature change over a small pressure change (see Example A).  This results in a thin VRM design which can be difficult to produce.  A gradual slope represents a small curvature change over a large pressure change (see Example B).  This results in a thicker VRM which is easier to produce.  Steep curvatures can also be produced using a diamond turning process, provided the overall range of curvature is small (see Example C).  Most pressure ranges fall within 0 to 6 bar, but we produce mirrors with design pressures as high as 11 bar (see Example D). 

 

Design Example

Pressure (bar)

Radius (m)

Curvature (1/m)

A

0

3

0.333

5

-3

-0.333

B

0.3

22

0.045

3.5

-22

-0.045

C

3

-1.55

-0.645

6

-1.25

-0.800

D

3

6

0.167

11

-6

-0.167

 

 

VRM’s are designed to operate at a specific angle of incidence (AOI).  The most common AOI is 45°, but we have also produced versions for use at near-normal, 15° and 22.5°.  For any VRM design with an AOI greater than 0°, the radius becomes an effective radius, Reff.  For these designs there is a longer radius in the plane of reflection equal to Reff / cos(AOI) and a shorter radius in the other direction equal to Reff * cos(AOI).  A collimated beam reflecting from any of our VRM designs at the intended AOI will come to a focus at a distance of 0.5 * Reff.

The usable clear aperture for VRM designs is typically between 35 mm and 50 mm.  During the design process, the behavior of a VRM design is simulated over the pressure range and analyzed over the clear aperture using finite element analysis (FEA).  The contours of the mirror membrane are optimized to reduce irregularity and match the specified pressure-curvature range.  We also check to ensure the stresses are safely below the yield strength of the copper alloy so that the VRM does not plastically deform under normal operating conditions. 

Our VRM designs can be pressurized with air or water, although the base plate and fittings should be optimized for the specific type of pressurization that is implemented.   Water pressurized versions have the advantage of water-cooling directly behind the mirror surface, but the control system is more difficult to implement.  Air pressurized control systems are easier to implement, but then the water cooling is limited to the base plate instead of directly behind the mirror surface.