Selecting Infrared Optical Materials Jay Vizgaitis University of Arizona Optics 521

Selecting IR Materials +How do you select the proper materials for your infrared system? +Design with the materials that will best meet your needs +May result in trade-off between cost and performance -Performance: MTF, weight, transmission, operating environment -Cost: molding, diamond turning, bulk material +Best materials to design with, tend to be the most expensive!! Know your key system requirements!!

Atmosphere Transmittance LWIR covers a very wide spectral band ~6.5  m Broadband IR is considered 3.0 – 14.0  m

Transmission Range for Common IR Materials MgO CaF 2 BaF 2 GaAs AMTIR-4 AMTIR-3 Germanium ZnSe AMTIR-1 Sapphire Silicon ZnS (Cleartran) As 2 S3 GASIR1 GASIR Wavelength (  m) Amorphous Glasses Crystalline Materials Only a handful of usable materials, most don’t cover MWIR and LWIR

Material Refractive Index dn/dT (K -1 )Spectral Range =4  m =10  m Germanium – 17.0  m Gallium Arsenide – 16.0  m ZnSe – 20.0  m ZnS (cleartran) – 14.0  m AMTIR – 14.0  m AMTIR – 14.0  m AMTIR – 14.0  m GASIR – 14.0  m GASIR – 14.0  m Silicon3.4255N/A – 9.0  m Sapphire1.6753N/A – 5.5  m BaF – 12.5  m CaF – 10.0  m As 2 S – 8.0  m MgO1.6679N/A – 8.0  m Table of Infrared Optical Materials

-LWIR color correction is easily achieved with Germanium optics only -Dispersion of Germanium in LWIR is significantly different from that of MWIR -Color correction in MWIR can be quite challenging -Broadband correction is extremely challenging due to lack of materials and changing v-number of Germanium Comparison of MWIR and LWIR Dispersion

Reflective Optics Cassegrain Three Mirror Anastigmat -Reflective optics are very useful for broadband performance -Diamond turned mirrors are feasible due to long wavelengths -Obscurations can have significant performance impact -Off-axis designs are often used instead Obscuration degrades diffraction limited MTF Unobscured aperture results in ideal MTF

Special Material Considerations Germanium: -LWIR designs can best be achieved with solely germanium optics -Large index (n = 4.0) and high v-number (>1200) However… -Temperature > 57°C will cause germanium to start to go opaque -Large dn/dT values require athermalization after ~10°  T -Germanium can be very expensive

Special Material Considerations Chalcogenide Glasses: -Chalcogenide glasses can provide the most cost effective solution -Some chalcogenide glasses are moldable (AMTIR-4, 5, GASIR 1,2) However… -Inhomogeneity is greater than crystalline materials -Hardness is much less than crystalline materials -Lower melting points of some glasses make coating more difficult -Dispersion characteristics aren’t as good as other materials Molded GASIR lenses from Umicore

Conclusions +Lens material choices in the infrared is both easier and harder -Fewer materials, but higher indices -Aspheres and diffractives are easily fabricated +Every choice of material has its penalty and advantage +Spectral band and cost tend to dictate what materials are best