Germanium Windows

Germanium windows are optical windows that are completely impermeable to UV and VIS light giving them a dark, metallic appearance to the naked eye. However, when it comes to the IR range this is where germanium truly shines with an excellent, broad transmission range from 2-16μm making it an ideal candidate for Mid-Wave-IR (MWIR) and Long-Wave IR (LWIR) applications. The sharp transmission cut-off before 2µm also enables germanium to be used as a long pass filter, only transmitting wavelengths in excess of 2µm and fully blocking everything before it.

Germanium Material Properties

Germanium (Ge) is a shiny, hard element with a Knoop Hardness of 780kg/mm2 making it, along with its far stronger sister sapphire, the default candidate for applications where the environment is likely to put a beating on the material. For a full breakdown of how to wade through all of the major optical window choices check our optical window guide.

In fact, at an eye-popping Knoop Hardness of 2000kg/mm2, one might wonder, why wouldn’t I just use sapphire windows instead of germanium? While sapphire is the undisputed champion of robust optics in the UV/VIS and mid-IR with a wavelength range that dips into the far UV range at 150nm, it can only be used up to 4.5µm. This leaves germanium as the best and only choice for brutally tough IR applications. This is typically why you will see germanium windows and lenses serving in places as inhospitable as outer space, battlefields and in the middle of high-powered CO2 laser systems. More on this later.

Another consideration is germanium’s density. At 5.33g/cm3 it does not float like a butterfly and sting like a bee. It’s heavy. Quite heavy and this will need to be considered when designing weight-sensitive systems.

One cautionary detail to note is that your germanium window will not appreciate being exposed to high temperatures. In fact, there is an inverse relationship to temperature and transmission when it comes to this material. As your temperature goes up, the transmission properties will drop precipitously, a property known as thermal runaway. Anything over 100ºC is not recommended. By the time it reaches 200ºC it is nearly opaque at all wavelengths. If you need a high temperature window, you’re better suited to stick with a material like MgF2, YAG or our old friend sapphire.

Since germanium windows have a high index of refraction of approximately 4.0 in the range of 2-16μm, transmission with minimal refraction is guaranteed but without any additional coatings only around 50% of the beam is able to pass through. Those are rookie numbers and in most applications we’ll need to boost our window signals via various coatings. While this may be a disadvantage for optical windows this property comes in handy for novel lens designs and ATR optics where refraction is desired.

Optical Coating Options

An uncoated germanium window is a decent IR generalist but specific optical coatings can really kick things up a notch. Diamond-Like Carbon (DLC) coatings can make this already hard material even more resistance to severe abrasions, environmental damage, mechanical strikes, thermal shock and whole other host of adversities.

Frequently, DLC coatings are often paired with Broadband AR Coatings (BBAR) to not only up the mechanical performance but also the transmission properties. Typically, a window is coated on one side with DLC and the other side with BBAR (typically in the 7-14μm range to boost both of these properties in tandem. This combination can increase the transmission to well over 85% in the target wavelength range. Optical windows treated in this manner will pass a variety of tests including MIL-C-675-C, MIL-STD-810 and APCM-01071.00001 and can survive salt water, humidity, high and low temperatures and hard smacks.

What’s a pirate’s favorite optical coating? ARRRR

If high transmission is the only consideration, you’d be best served to double coat your windows with BBAR over particular wavelength ranges and you can see transmissions all the way up to 99%. Now we’re talking!

Various BBAR coatings typically requested are 3-5µm, 3-12µm, 2-14µm and 8-12µm though we can do even more customization. You can fill out our custom request form and see what Firebird Optics can do for your application.

Firebird Optics manufactures these types of windows in various geometries, coatings and can also produce lenses, prisms and various other optical components from germanium. Now back to your regularly scheduled programming…

Why Germanium and not Silicon?

We’ve already explored why you would pick germanium over sapphire but why would you pick germanium over silicon or vice versa?

Silicon (Si) is germanium’s next door neighbor who frequently borrows sugar. Both have an atomic number of 14, absorb visible light and have sharp cut-offs making them also function as long pass filters. In addition, there is similarity with their transmission ranges with silicon transmitting from 1.2-7µm. However, this is where they begin diverging. Silicon is far lighter with a density of 2.33g/cm3, making it ideal for weight-sensitive applications and less than half the density of germanium. Moreover, it is more thermally resistant than germanium and with a Knoop Hardness of 1150 it is also harder and less brittle. Silicon also costs less making it a better candidate in several applications.

Germanium holds the high ground when it comes to transmission range getting that additional coverage from 7-16µm, which silicon lacks. On top of this, germanium has higher electrical conductivity making it a more suitable candidate as an optical component in laser systems.

Summary of Advantages and Disadvantages of Germanium

Advantages

No water solubility- advantage over salty windows such as KBr and NaCl that are extremely hygroscopic.
Good spectral properties in the IR range.
Sharp transmission cut-off at 2µm enables use as optical filter.
Excellent mechanical and chemical resistance.
High refractive index enables excellent performance for lenses, ATR optics or beamsplitters. For optical windows, AR coatings can be added to boost performance.
Relatively low cost compared to similar materials.

Disadvantages

Dense, heavy material not ideal for light-weight applications.
Low resistance to high temperatures.
High index of refraction makes an uncoated window less than ideal for transmission.

windex. works great for windows. for optical windows…not so much.

Cleaning Germanium Windows

You definitely don’t want to breathe in germanium dust so care must be taken while handling these optics. Using gloves whenever possible and washing hands should be part of any standard operating procedure.

The best way to clean the windows is to use either ethanol, isopropyl alcohol, methanol, reagent-grade acetone or lint-free lens cloths. You can also use nitric acid but this material, while okay for germanium can corrode other optics or mounts that may be nearby in your system.

Alternatively, you can use liquid CO2 from a specialized nozzle for cleaning though this requires additional cost in terms of equipment. While you will get a better, more controlled cleaning and can be used for stubborn stains, care must be taken to work in a controlled moisture-free environment and we do not recommend this for typical end-users.

When not in use, we recommend storing your germanium windows wrapped in lens tissue with humidity below 30% and between 15 and 25ºC. While germanium is pretty tough stuff, these are the ideal conditions that will prolong the life of your window.

Germanium Window Applications

Now for some fun stuff. We hear from plenty of our customers where their germanium windows are seeing action. Here are some highlights:

Aerospace

Germanium ends up in some interesting places including defense aircraft navigation, reconnaissance and surveillance systems soaring in the stratosphere on an airplane or even as part of a satellite.

One very common scenario we see germanium utilized is inside high and low speed wind tunnels for jet propulsion studies. Typically, these windows are much larger than stock configurations reaching sizes in excess of 190-200mm.

Germanium makes a good electromagnetic interference (EMI) shielding material and can shield IR systems on planes from other nearby signals that would render the system ineffective. This effectively creates an IR Faraday cage or as we like to refer to it, an aerospace tin foil hat. Typical resistance for EMI-grade germanium is approximately 4 Ohm per cm but this depends on the required level of spurious signal suppression. A germanium window made to these specs can effectively short out any errant signals and keep the IR system running well.

aerial thermography brought to you by your old pal, germanium

CO2 lasers

get wrecked! damage done to germanium windows by high power lasers. source: http://www.mdpi.com/2076-3417/10/10/3578/htm

One of the more common applications we see at Firebird Optics for germanium windows is in low power CO2 laser systems. With a Laser Induced Damage Threshold (LIDT) of 10 J/cm2, germanium windows are not suited for high power or continuous wave (CW) lasers. Part of the reason for this is 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 germanium will feel right at home in a low power pulsed laser setup. One particularly noteworthy application is in quantum cascade lasers (QC), which is used in high-end materials science. Quantum cascades are used by such institutions as the Max Planck Institute of Quantum Optics in Garching to produce ribbons and strip structures as well as to produce new materials for use in medical applications. Pretty cool stuff!

No we’re not building a death star. or are we?

Military and Night Vision applications

One common application for germanium is in night vision goggles. For these types of applications, both the inside and outside surfaces of the germanium is polished to a mirror finish and coated with multi-layered thin film filters to reduce any reflection of IR light. This enables optimal IR transmission essential to night vision/thermal imaging. As these windows will have excellent sensitivity at long range, this enables warfighters to retain long stand-off distances increasing safety and mission-effectiveness. Germanium is a literal life-saver.

As you’d imagine, since germanium tags along with warfighters, it needs to be durable in all adverse conditions from desert, sea, high altitude, etc. The DLC coatings can take a serious beating with temperatures ranging from -80ºF to 160ºF, 24 hours of continuous sea spray, ocean immersion for over 24 hours and being immersed in a sandstorm. Additionally, the coatings can withstand heavy mechanical damage and chemical attacks.

Happy trails and stay insanium in the germanium!

Firebird Optics

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Firebird Optics Information Station

The Essential Guide to Germanium Windows and Lenses