Germanium (Ge) is a robust, high-density optical material widely used in infrared (IR) systems. It effectively blocks ultraviolet (UV) and visible (VIS) light while transmitting infrared wavelengths from approximately 2 µm to 14 µm, making it a preferred material for MWIR and LWIR optical windows and lenses.
With the highest refractive index among common IR-transmitting materials and low optical dispersion, germanium enables compact, high-performance optical designs with strong focusing power. To reduce surface reflection losses caused by its high refractive index, anti-reflective (AR) coatings are strongly recommended.
Infrared Transmission and Thermal Performance
Uncoated germanium optical windows typically transmit ~45–50% of infrared light per element across the 2–14 µm range. With properly designed AR coatings, transmission can exceed 95% within the target IR band.
Germanium performs well at moderate operating temperatures, but its infrared transmission is temperature dependent:
- Up to ~50°C: Stable IR transmission
- ~100°C: Gradual increase in absorption
- Above ~200°C: Rapid degradation of IR transmission due to free-carrier absorption
Because of this behavior, germanium windows are not recommended for sustained high-temperature environments unless cooling or system-level thermal management is implemented.
Additional material considerations:
- High density (~5.32 g/cm³) may be a factor in weight-sensitive systems
- High hardness (~HK 780) provides good durability, slightly higher than GaAs, with similar mechanical characteristics
Applications of Germanium Windows
Thanks to its optical and mechanical properties, germanium is widely used in:
- Thermal imaging systems (front windows and lenses)
- Wide-angle IR lenses and microscope optics
- Forward-Looking Infrared (FLIR) systems
- Fourier Transform Infrared (FTIR) spectroscopy
- Scientific, analytical, and industrial IR instrumentation
Fabrication Process of Germanium Optical Windows
Producing a high-quality germanium window requires precise control at every manufacturing stage to ensure optical accuracy and long-term reliability.
1. Material Selection & Preparation
- High-purity germanium is selected for optimal IR transmission
- Intrinsic, N-type, or P-type material may be chosen based on application requirements
- Raw crystals are cut or cast into rough blanks
2. Shaping & Machining
- Diamond sawing to achieve near-net geometry
- Precision grinding to control thickness and diameter
3. Fine Machining & Polishing
- Lapping removes subsurface damage and improves flatness
- Optical polishing achieves low surface roughness and required scratch-dig quality
4. Edge & Surface Finishing
- Beveling prevents chipping and improves handling safety
- Thorough inspection and cleaning to remove residues
5. Optical Coating
- AR coatings applied using PVD or CVD to maximize IR transmission
- Optional protective coatings (e.g., DLC) for harsh environments
6. Quality Control & Testing
- Optical testing for transmission, reflectance, and surface quality
- Environmental testing for temperature stability and durability
- Final inspection to ensure compliance with specifications
7. Packaging & Shipping
- Clean, protective packaging to prevent contamination and damage
- Shipped with handling and installation guidance