Custom Germanium

Additional Information

• Spherical germanium lenses have a low optical dispersion, due to its high refractive index, and therefore display minimal chromatic aberration.
• Aspheric germanium lenses offer diffraction-limited performance, their design eliminating spherical aberration and can therefore greatly simplify an optical system by removing the need for multiple elements.
• Germanium is often used as a substrate for infrared longpass filters as it naturally blocks the UV and visible wavelengths. It is a semiconducting material and therefore has an abrupt cut-on (intrinsic edge) at 1.7µm.
• Due to germanium’s naturally high reflectivity, AR coatings are usually applied to increase transmission, reaching over 85%. A DLC coating can be applied to an exposed surface, increasing durability and transmission.

Considerations

• Germanium has a high density which needs to be taken into account if the optical system is weight sensitive, silicon has almost half the density of germanium so could present a better option if weight is an issue.
• It is also subject to thermal runaway – as the temperature increases, germanium’s absorption also increases meaning its transmission is reduced, at 100°C it is almost opaque and by 200°C it does not transmit at all. Therefore it should be used in systems working below 100°C to reduce the risk of optical failure
• Germanium dust is hazardous if inhaled, so care should be taken when handling

Typical Applications

FLIR and Thermal Imaging

• Germanium transmits in both the MWIR and LWIR wavebands making it ideal for thermal imaging systems both as protective windows and/or lenses within the systems.

Low powered CO₂ Lasers

• Germanium transmits at 10.6µm and therefore works in a CO₂ laser systems, however it cannot withstand the higher powered lasers, zinc selenide is more commonly used here.

Fourier Transform Infrared (FTIR) Spectroscopy

• Germanium is often the material of choice for attenuated total reflection (ATR) prisms used in FTIR spectroscopy. It is especially used when the sample being measured has a high refractive index.

Pyrometers

• Due to germaniums comparative robustness, especially when a DLC coating has been applied to the exposed surface, it is commonly used as a protective window in pyrometers.