Replacing Germanium with Chalcogenide Glass in MWIR & LWIR Optical Design

Germanium has historically dominated infrared optical design due to its high refractive index and low dispersion. However, its high cost, strong thermal sensitivity, and incompatibility with scalable manufacturing increasingly limit its suitability for modern infrared systems.

This application note demonstrates that chalcogenide glasses are a fully feasible replacement from optical design, thermal performance, and manufacturing perspectives. With modern design techniques and precision glass molding, chalcogenide-based systems can achieve equivalent or superior performance while enabling cost-effective mass production.

1. Material and Optical Design Feasibility

Replacing Ge with chalcogenide glass does not require material equivalence. Instead, feasibility is achieved through optical design freedom and manufacturing enablement.

Table 1: Key material and optical design trade-offs when replacing Ge with chalcogenide glass.

Aspect	Germanium (Ge)	Chalcogenide Glass	Design Implication
Refractive Index	~4.0	2.2–2.8	Lower index compensated by element count and aspheres
Dispersion	Very low	Moderate–high	Managed via DOEs or multi-glass correction
Lens Count	Low	Moderate	Manufacturing scalability offsets complexity
Aspheres	High cost	Mold-friendly	Aspheres become economically feasible
Thermal dn/dT	High	Low	Passive athermalization achievable

2. Optical Performance Equivalence

Published MWIR and LWIR design studies demonstrate that chalcogenide-based optical systems can achieve MTF, distortion, and field performance comparable to Ge benchmarks. Performance equivalence is enabled through modest increases in optical complexity.

Table 2: Optical performance feasibility comparison between Ge and chalcogenide designs.

Design Type	Lens Count	Special Surfaces	MTF vs Ge	Feasibility
2× Ge	2	None	Baseline	Reference
2× ChG	2	None	Lower	Not sufficient
2× ChG + DOE	2	DOE	Equivalent	Feasible
3× ChG (2 glasses)	3	Optional asphere	Equivalent	Feasible
4× ChG (1 glass)	4	Asphere	Near-equivalent	Feasible

From Table 2, a 2-element Ge optical system can be replaced with equivalent MTF performance by the same number of chalcogenide lenses with one DOE surface, or with one aspherical chalcogenide lens plus two other IR optical materials such as ZnSe or ZnS. In some designs — especially where Ge is combined with other materials — Ge can be replaced by an equivalent number of chalcogenide elements with the same surface complexity, as the example below shows.

To illustrate, two designs with the same focal length and F/# — with and without Ge — are compared in Table 3, where IRG denotes chalcogenide glass from Schott:

Table 3: Parameters for two designs with 1Ge+3IRG and 4IRG.

Parameter	1Ge + 3IRG	4IRG
EFL	50 mm	50 mm
F/#	1.0	1.0
Detector	1280×1024, 12 µm	1280×1024, 12 µm
Wavelength	8–12 µm	8–12 µm
Distortion	≤1%	≤1%
Relative Illumination	≥96%	≥93%
TTL	63.76 mm	63.89 mm
Back Focal Length	18.03 mm	14.73 mm
Aspheric Surfaces	2	2
DOE Surfaces	1	1
First Lens Coating	DLC/HD/AR	HD/AR

The optical performance of the 4IRG design is equivalent to the 1Ge+3IRG counterpart, with only the relative illumination at the edges being slightly lower. A further limitation is that DLC coating is not currently available for chalcogenide — this is a coating process limitation rather than a design constraint.

Figure 1: MTF comparison across field — (a) 1Ge+3IRG vs (b) 4IRG.

Figure 2: Spot diagram comparison across field — (a) 1Ge+3IRG vs (b) 4IRG.

Figures 1 and 2 show the MTF and spot diagrams for the 1Ge+3IRG and 4IRG configurations. With the same number of lenses, the two systems exhibit similar performance, with the 4IRG design performing slightly better.

3. Thermal Feasibility and System Robustness

Thermal performance is a decisive advantage of chalcogenide glasses. Their low dn/dT enables passive athermal systems, reducing mechanical complexity and improving reliability.

Table 4: Thermal feasibility comparison between Ge and chalcogenide optical systems.

Metric	Ge System	ChG System	System Impact
dn/dT	High	Low	Reduced focus drift
Refocus Needed	Yes	No	Simpler mechanics
Thermal Darkening	Possible	None	Higher robustness

Due to their superior thermal properties, chalcogenide lenses can fulfil athermal requirements, maintaining near-constant performance across a wide temperature range without focal plane adjustment.

4. Manufacturing and Scalability Feasibility

Precision glass molding transforms optical feasibility into economic feasibility. Chalcogenide glasses enable scalable production of complex optical surfaces at low marginal cost.

Table 5: Manufacturing feasibility comparison.

Manufacturing Aspect	Ge	Chalcogenide Glass	Impact
Fabrication Method	Diamond turning	Precision molding	Cost scaling advantage
Asphere Cost	High	Low incremental	Design freedom
Volume Scaling	Poor	Excellent	Mass-market ready

5. Our Existing Germanium-Free Infrared Lenses

We have developed a range of Germanium-free infrared lenses in which Germanium has been replaced by chalcogenide glasses.

Fixed-Focus Lenses

Table 6: Germanium-free fixed-focus infrared lenses.

Part Number	Focal Length	Wavelength	Detector
IRG6.81.0-17A	6.8 mm	8–12 µm	640×480, 17 µm
IRG9.61.0-17A	9.6 mm	8–12 µm	640×480, 17 µm
IRG111.0-17A	11 mm	8–12 µm	640×480, 17 µm
IRG1001.0-12A	100 mm	8–12 µm	1024×768, 12 µm
INFRA-MW133.6-10	13 mm	3.6–4.2 µm	640×512, 10 µm
INFRA-MW253.6-10	25 mm	3.6–4.2 µm	640×512, 10 µm
INFRA-MW503.6-10	50 mm	3.6–4.2 µm	640×512, 10 µm
INFRA-MW13.72.0-15V2	13.7 mm	3–5 µm	640×512, 15 µm
INFRA-MW62.0-15V2	6 mm	3.7–4.8 µm	640×512, 15 µm

Figure 3: A Germanium-free infrared fixed-focus lens.

Zoom Lenses

Table 7: Germanium-free zoom infrared lens.

Property	Specification
Focal Length	18–180 mm
F/#	0.8 / 1.2
Wavelength	8–12 µm
Detector	640×512, 15 µm
Focus Range	3 m – ∞
Focus Type	Auto Focus

Figure 4: A Germanium-free infrared zoom lens.

Athermal LWIR Lenses

We have developed more than 100 types of athermal LWIR lenses with chalcogenide glass and other non-Ge materials, with focal lengths ranging from 0.9 mm to 110 mm.

Table 8: Athermal LWIR fixed-focus lenses without Germanium.

Focal Length	F/#	Wavelength	BWD	Sensor
0.9 mm	1.1	8–12 µm	2.4 mm	120×90, 12 µm
1.6 mm	1.0	8–14 µm	2.36 mm	120×90, 12 µm
3.2 mm	1.0	8–12 µm	2.53 mm	256×192, 12 µm
5 mm	1.0	8–12 µm	6.1 mm	384×288, 12 µm
10 mm	1.0	8–12 µm	8.1 mm	384×288, 17 µm
16.8 mm	1.0	8–12 µm	8 mm	640×512, 12 µm
25 mm	1.0	8–12 µm	10 mm	640×512, 12 µm
35 mm	1.0	8–12 µm	10 mm	1280×1024, 12 µm
50 mm	1.0	8–12 µm	11 mm	1280×1024, 12 µm
55 mm	1.0	8–12 µm	10 mm	640×512, 12 µm
75 mm	1.0	8–12 µm	8.075 mm	1280×1024, 12 µm
110 mm	1.3	8–12 µm	10.29 mm	640×512, 17 µm

6. Why Choose Wavelength Opto-Electronic

Wavelength Opto-Electronic (WOE) combines extensive IR optics expertise with end-to-end manufacturing capabilities, making it an ideal partner for Germanium-free solutions. The company’s integrated operations cover the entire value chain — from in-house chalcogenide glass development and purification to optical design, precision fabrication, coating, and assembly — enabling full production of IR lenses and systems without relying on Ge.

Wavelength’s specialized R&D in chalcogenide materials (including proprietary purification and growth processes) and significant production capacity (~10 tons of chalcogenide glass per year) ensure it can deliver high-performance MWIR/LWIR optics at scale. With decades of IR lens design experience — spanning athermal imaging to continuous zoom systems — and advanced technologies like durable DLC coatings tailored for chalcogenide optics, Wavelength is well positioned to support system designers and manufacturers in deploying scalable, high-performance Ge-free IR optical systems.

7. Conclusion

Replacing Germanium with chalcogenide glasses is not only feasible but strategically advantageous for modern MWIR and LWIR optical systems. Enabled by optical design freedom and precision molding, chalcogenide glasses deliver performance parity, thermal robustness, and scalable manufacturability.