Mirror Resources: Limitations of Replication

The coatings used on thin film epoxy replicas are suitable for most applications from x-ray to ultraviolet, from visible thru the far infrared. The substrate that the mirror surface is replicated onto determines the thermal stability characteristics of the replica. It is the epoxy itself that limits the replica in the following applications:

  • Very high storage or operating temperatures
  • Special coatings requiring high temperature deposition
  • High power CW laser operation
  • Very high power short pulse laser operation
  • Thermal distortion of ultra-light-weighted optics
  • Limited spectral transmission range

Typical standard production epoxies are suitable for storage and use at maximum temperatures ranging from 80° to 125°C. Special high temperature epoxies are available for use up to 300°C, but since they must be cured at elevated temperatures, they often do not yield the same precision as the standard production epoxies. At the low temperature range, both epoxies have been used down to liquid nitrogen temperatures and can potentially be used at liquid helium temperatures.

Special coatings that require high temperature deposition can be used for flat replicas because the master is typically made of glass. The master with its release layer can be heated to 300°C without damage. For aspheres, the master used in the final replication process to produce the replica is typically a replica containing a layer of cured epoxy itself. It therefore cannot be heated to high temperatures as is often required for many special thin film coatings.

A glass aspheric master can sometimes be used directly instead of a replicated sub-master. However the risk to the master is not insignificant and the cost of making multiple masters for a reasonable production run typically make it impractical.

High Power Laser Operation

Replicas are used successfully with a variety of lasers including CO2, excimer, laser diode, helium-neon, Nd:YAG, etc. Success or failure depends on the amount of radiation that will be transferred as heat through the epoxy (which has a thermal conductivity that is lower than glass) to the substrate. For pulsed lasers, it depends on the penetration of the laser pulse into the coating and into the epoxy. A good rule of thumb is that if a glass mirror cannot survive the laser power, a replica won't either. Typical acceptable power levels for pulsed lasers are 80 MW/cm2 at 1065 nm (for dielectrically over-coated aluminum coatings).

Thermal Distortion

The distortion of a replica due to ambient temperature changes can be calculated. Except for highly accurate surfaces (i.e. 1/20 wave), or ultra-light weight substrates, this effect is insignificant. It is much less than for an electroless nickel plated aluminum mirror that has been polished on one surface only.