IR Windows Innovations: Empower 2024 with Infrared Windows

Author: Pradyumn – R&D Intern

Editor: Qu Yingli – R&D Director

Editor: Bryan Ng – Marketing Manager

Optical windows are selectively transparent components designed to allow specific wavelengths of light to pass through, based on their material. These windows are carefully designed to maintain optical clarity, withstand environmental conditions, and minimize any distortion or alteration of the light passing through them. They are primarily used to protect delicate optical components, facilitate measurements, and enable observation or imaging in different applications.

1. Introduction to Optical Windows and Materials

Optical windows allow specific wavelengths to pass through while reflecting, absorbing, or blocking others. Optical windows can be made from an extended range of materials, such as fused silica, Silicon, Calcium Fluoride (CaF₂), Germanium (Ge), Potassium Chloride (KCl), Potassium Bromide (KBr), Sapphire (Al₂O₃), N-BK7, and Zinc Selenide (ZnSe).

Each of these materials has its own transmission profile, which is chosen based on the application. Material properties, including the transmission, refractive index, and hardness of the window substrate, can be critical for deciding which window is the best choice for the application. Before we delve into infrared windows (IR windows), let’s glance at other types of optical windows to gain a better understanding of the whole category.

1.1 A Brief Walkthrough of Other Windows

For transmission in the visible spectrum (350nm – 750nm), N-BK7 and fused silica are commonly used as substrates for optical windows. With fused silica having a high thermal shock resistance and high LIDT (laser-induced damage threshold), it is ideal for applications such as those involving high-power laser optics, visible range imaging systems, and spectroscopy.

Table 1: Wavelength Opto-Electronic Fused Silica (WFS series) and N-BK7 (WBK series) Windows

N-BK7, on the other hand, can be used for camera lenses, optical filters, and general-purpose visible light optics, due to its excellent optical transmission in the visible spectrum (approximately 350 – 2200nm). BK7 is also relatively hard and shows good scratch resistance. However, it is not recommended for temperature-sensitive applications such as precision mirrors.

2. IR Windows

For applications within the IR spectrum, materials such as ZnSe, Al₂O₃, Si, and Ge are used. An optimal IR window should allow all infrared radiation to pass through it with zero losses. Such windows are typically used for separating environments of varied pressures or temperatures while allowing light energy at a specified electromagnetic wavelength to pass between the two environments.

Table 2: Wavelength Opto-Electronic Al₂O₃ (WSP series) and ZnSe (WZ series) Windows

Additionally, they empower inspections of live, energized components and connections within electrical cabinets without requiring the removal of their covers. When using these windows for industrial purposes, it’s crucial to ensure they meet the requisite strength and environmental standards specific to the equipment they are installed in. These windows come in a variety of sizes and thicknesses to allow for proper installation.

2.1 Difference Between IR Windows and Other Windows

Infrared light can consist of near IR (NIR), short-wavelength (SWIR), mid-wavelength (MWIR), long-wavelength (LWIR), and far-infrared (FIR). For applications within the infrared region, Ge is often used as a substrate material for optical windows. Unlike other materials like fused silica and N-BK7, which allow the transmission of wavelengths of light from the visible and UV regions of the electromagnetic spectrum, Ge, and Si are opaque to UV and visible light, but have a wide transmission range in the infrared region.

Materials such as Al₂O₃, ZnSe, Zinc Sulphide (ZnS), and CaF₂ have a wide transmission band that ranges from UV to MWIR for CaF₂ and Al₂O₃ and from the visible spectrum to LWIR for ZnSe and ZnS. Hence, applications requiring the transmission of solely IR waves should use Ge or Si windows.

2.2 Ge IR Windows and Applications

As seen from the transmission profile, Ge serves as a long-pass filter for wavelengths greater than 2µm. Due to its high index of refraction (4.0 from 2µm to 14µm), it has minimal chromatic aberration, and anti-reflection coating is used on it. In addition, it demonstrates scratch resistance and inertness to air, water, alkalis, and a variety of acids. Its relatively high density (5.323 g/cm³), should be considered in applications where weight is a restriction.

Table 3: Wavelength Opto-Electronic Ge IR (WGE series) Windows

In addition, the transmission characteristics of Ge are significantly affected by temperature. As the temperature reaches 100°C, absorption increases to the extent that Ge becomes nearly opaque, and at 200°C, it loses all transmissive properties. Ge optical windows are extensively utilized in the defense and aerospace industries, life and medical sciences, industrial OEM, and a variety of other infrared applications. Refraction makes it suitable for wide-angle lenses and microscopes. In thermal imaging systems, Ge is commonly used for IR windows and lenses.

One of the more common applications for Ge windows is in low-power CO₂ laser systems. With a LIDT (Laser-Induced Damage Threshold) of 10 J/cm², Ge windows are not suited for high-power or continuous wave (CW) lasers. Part of the reason for this is higher higher-powered lasers cause temperature increases, dramatically dropping transmission properties over 100ºC and eventually damaging the substrate itself once temperatures near 600ºC are reached. On the other hand, an AR-coated Ge is well-suited in a low-power pulsed laser setup. One particularly noteworthy application is in quantum cascade lasers (QC), which are used in high-end materials science. 

2.3 Si IR Windows and Applications

In addition to Ge, Si is also widely used for IR windows. Si is one of the hardest minerals and optical materials available for use in the NIR (1µm) to about 6µm. Optical quality Si is usually doped (5 to 40 ohm-cm) to prevent absorption bands within the transmission waveband. Si has a lower refractive index than Ge and has a lower density, which makes for less weighty optical designs.

Si is ideal for use as windows in the 3 to 5µm (MWIR) waveband and as a substrate for optical filters and Si’s low density (half that of Ge or ZnSe) makes it ideal for weight-sensitive applications, especially those applications between the 3 – 5µm range. It has a density of 2.329 g/cm³ and a Knoop hardness of 1150, so it is harder and less brittle than Ge.

With its high thermal conductivity, Si is better suited for high-power lasers compared to Ge. This is particularly important in fields such as industrial inspections and surveillance. However, as seen from its transmission profile, it has a strong absorption band at 9µm, which does not make it suitable for CO₂ laser applications.

Si windows are used in various applications. It is an integral component in thermal imaging devices, enabling the detection of temperature variations in objects and environments. It is also widely used in IR spectroscopy equipment to analyze the composition of different materials, as well as in the defense and security industry for target detection and night vision goggles.

2.4 Difference Between Ge and Si IR Windows

Si and Ge are semiconductor materials used in various applications, including window technologies. The main difference lies in their physical properties and optical characteristics. Si windows offer better transparency in the SWIR and MWIR but are less efficient in LWIR. On the other hand, Ge windows have superior infrared transparency for LWIR, making them ideal for thermal imaging and infrared spectroscopy applications.

However, Ge is generally more expensive and more fragile than Si. Si is a commonly found compound on the Earth’s surface. On the other hand, Ge is a rare material that is commonly found in lead, silver, and copper deposits. Additionally, the processing costs of Ge are also higher than those of Si, which makes Ge a more expensive compound. The choice between Si and Ge windows depends on the specific requirements of the application, such as wavelength range, cost, and mechanical durability. 

3. Windows Coating

Anti-reflection (AR) coatings are often put on optical windows to maximize transmission in the desired wavelength range. Most AR coatings are also very durable, providing resistance to both physical and environmental damage. For these reasons, the vast majority of transmissive optics include some form of anti-reflection coating.

When choosing an AR coating for a window, the full operating spectral range of the specific application must be thoroughly considered. While an AR coating can significantly improve the performance of an optical system, using the coating at wavelengths outside the design wavelength range could potentially decrease the performance of the system. It is recommended that an AR coating is used for Ge windows. 

4. Conclusion

Optical windows are crucial in the optics industry and are used for a range of applications with varying purposes. Various materials can be used as optical windows to filter out specific wavelengths of light, based on their transmission profiles. ZnSe, ZnS, Al₂O₃, and CaF₂ are a few compounds used for windows that allow for the transmission of light in the visible and IR spectrum. On the other hand, Ge and Si are useful in applications requiring only wavelengths from the IR spectrum to pass through.

Various factors can affect which material should be chosen for a specific application. In addition to transmission range, this includes factors such as density, hardness, operating temperature, nature of operation, and cost. Other considerations include the addition of an anti-reflective coating, which can alter the transmission profile to be within the desired range of application. In the ever-evolving field of optics, the significance of optical windows remains crucial, serving as a gateway to unlock the potential of optical technologies and applications.

Wavelength Opto-Electronic designs and manufactures optical windows of different materials from standard to high precision specifications. Our engineers are equipped with vast experience, and with our state-of-the-art facilities, you can be assured that our windows are high-quality, measured, and tested with our comprehensive metrology.