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Reflecting Optics (mirrors, partial reflectors, beamsplitters, output couplers)
UV Mirror Substrates
The most important considerations in selecting a UV mirror substrate include the high absorption characteristics of UV light (see UV transmitting optics) and the ease of fabricating the optic. In the case of mirrors, one might assume that substrate selection is
irrelevant because it merely serves as a vessel for the coating. Yet, even with high quality, dielectric mirror coatings, there is always some “leakage” into the substrate. Having the leakage pass through (or transmit) with minimal absorption is the key to good UV mirrors. Therefore it is important to select a good UV transmitting substrate even for mirrors, especially in higher power applications (i.e. lasers). RMI recommends UV grade fused silica (UVFS) for most high damage threshold UV mirrors.
In cases where high power is not a critical issue (non-laser or low power laser applications), a cheaper substrate like BK7 or fused
silica may be an adequate solution.
UV light is also more susceptible to scattering than visible or IR light because of its shorter wavelength. Under UV light, the effects of surface imperfections (scratches and pits) are amplified. Therefore it is critical to have excellent surface quality to minimize energy loss within the system.
VIS/NIR Mirror Substrates
As in the case of UV mirrors, VIS/NIR mirror substrates must transmit any “leakage” that gets through the coating. In general, both BK7 and fused silica do this well for VIS/NIR light. If extreme thermal conditions and the need for mechanical strength are key considerations, then fused silica is the better choice because of its stability under thermal shock and high mechanical strength (see VIS/NIR transmitting substrates). In cases where these are not an issue, BK7 is the more economical choice.
IR Mirror Substrates
Since IR light is largely made up of heat, it is important to choose a substrate that has good heat dissipating properties. Silicon is widely used for IR mirrors, particularly in low power thermal imaging applications, due to its high durability, wide availability, light weight and low price. Additionally, silicon is a popular IR mirror substrate because of good IR transmission and heat dissipation in cases where heat absorption can lead to component damage.
For higher power laser mirrors (e.g. CO2 laser mirrors), RMI recommends copper mirror substrates. Copper is known for its tremendous heat dissipating properties that will minimize the damaging effects of absorption from high power IR lasers. RMI also produces Molybdenum (also referred to as “Moly”,) mirror substrates for more special/higher power CO2 lasers.
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Due to their potential strategic high value, export of RMI products and related technical data may be subject to U.S. or other export licensing requirements.
Please consult your export control administrator or contact RMI for more information. |
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