Relationship between LIDT and the Angle of Incidence
A 99.7% reflectivity mirror and a 99.3% reflectivity mirror can have the exact same coating design.
The difference is the angle of incidence for the light hitting the mirror. This issue catches out more optical setups than people admit. A coating optimized and quoted for normal incidence will not deliver the same performance, or survive the same fluence, once you fold it into a 45° beam path. Polarization sensitivity changes. Mechanical stress in the stack changes. Even the substrate needs to be thicker to stay rigid under oblique loading.
The scrutiny is needed for LIDT numbers on a datasheet. “35 J/cm²” means nothing on its own. Was that measured at 1064 nm, at 10 ns pulses, using ISO 21254 S-on-1 testing? A damage threshold quoted under one pulse regime will not necessarily transfer to your femtosecond amplifier or your 10 Hz industrial system.
This is why Manx Precision Optics (MPO) specifies laser line mirrors separately for 0° and 45° incidence, with coatings designed for each geometry, rather than adapted from one to the other. Also, the LIDT data are really tied to explicit test conditions. In other words, the LIDT should not be considered just a bare number.
Before every coating run, the staff at MPO asks one question: At what angle and in what polarization state was this reflectivity and LIDT figure actually measured? If you are specifying mirrors for a high-energy or ultrafast system, it is worth asking your supplier the same thing.
About Manx Precision Optics
Manx Precision Optics (MPO) manufactures optical components and precision optics. The company offers customized products as well as a wide range of off-the-shelf components at highly competitive prices. The main categories of MPO’s standard products are:
- Laser line mirrors
- Ultrafast laser mirrors
- Roof mirrors
- Polarization optics
- Large optics
- Lenses
- Etalons
- AR-coated windows
- Uncoated optical substrates
MPO works with customers to determine the most suitable optical coatings for their specific applications. With years of experience and an intimate knowledge of thin film coatings design, MPO can produce customized, quality coatings according to unique specifications.
To request more information or a quotation for Manx Precision Optics products, contact IL Photonics.