Many optical systems require transparent windows for various reasons. These windows can safeguard a system from external hazards and provide environmental and vacuum seal to the system. A designer must be aware of several factors when designing an optical window for a system such as strength, transmission, durability and available coatings.
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The first of all, it is important to determine the quantity for optical windows, so as to evenly distribute the funds. One should keep in mind, the larger the quantity of the pieces, the less expensive will be each part as cost can be divided over total number of parts.
Select the material
This is an important consideration. For this, one needs to consider the features of the materials as well as application needs. There are many different types of materials used for optical windows based on the purpose of any application. Magnesium fluoride window is an optical window manufactured from hard, durable optical material lends itself well to use under harsh conditions. Similarly, Sapphire optics are an ideal material for applications where high pressure, vacuum, or corrosive atmospheres.
There are many different types of materials used for designing optical windows such as N-BK7®, fused silica, quartz etc. One needs to consider the features and capabilities of the materials and application requirements to make the right choice.
Determine/ selecting the shapes
Optical windows can be made in different shapes such as round, rectangular, square etc. The round shape is considered as the best choice for obtaining flatness/accuracy. These windows can also be made in custom shapes to provide more challenges to maintaining surface flatness. Proper mounting of the window is necessary to ensure that window is sealed from outside environment.
Define your mechanical tolerance
Before choosing an optical window, one should define the acceptable limits of both size and thickness required for an application. For instance, if optical window is made in round shape, one should provide tolerance for diameter. If it is in rectangular/Square, one should provide tolerance for LxW.
Density
The density of the optical window is important due to its correlative effect on light refraction. Also, it correlated to weight/. The denser the material, the more will be the weight. When you are designing an optical window where weight sensitivity is critical, one needs to use less dense material.
Coatings
Coatings are important aspect of optical windows. These coatings are selected on the basis of the operating environment and the desired wavelength of the system. In optical systems that operate over a wide range of wavelength, a broadband coating is necessary.
Knoop Hardness (kg/mm2)
Knoop hardness of the material is an important consideration for manufacturing optical window. One should bear in mind that if a material has a high Knoop hardness, it is generally more robust and can withstand greater pressure shocks. The lower the number, the more brittle will be the material of the window.
VIS are a more cost-effective, specific option for use in the small range of visible light from 400-700nm. A UV window would work in similar applications but Optical Glass is a more specific, economical choice. N-BK7 is the material most commonly referred to by the name Optical Glass and features a transmission range of 350-2,000nm.
A VIS window is a common tool for use in imaging/display systems as well as a standard base substrate for use with mirror and filter coatings. Featuring a high index of refraction, high transmission and a high standard of material purity, these windows are often a crucial component in various optical systems. Additionally, N-BK7 has a high degree of stain resistance
The extended family of IR windows encompasses the largest and most frequently used assortment of optical windows. You can visit the full family of optical windows on the Firebird Optics website.
Each particular window has its own unique property and transmission profile, which are needed for specialized applications. We will break down why you might be interested in each of these types of windows. Each material features a link which delves into more of the material specifics:
Barium Fluoride (BaF2)- features transmission from deep in the UV from 200nm-12μm, BaF2 can be used in multiple setups in the UV, VIS and IR range. Its main properties include resistance to high-energy radiation and its low index of refraction. AR coatings are often not needed.
Calcium Fluoride (CaF2)- is very similar to BaF2 in terms of its high damage threshold, low index of refraction and low absorption coefficient. CaF2's main standout is its outstanding transmission range of 130nm-9.5μm, which dips even deeper into the UV range and farther out into the IR range than UV Fused Silica. Like BaF2 it is mostly used in laser and cryogenic applications.
Germanium (Ge)- Germanium's standout property is its low dispersion, making it top choice for low power CO2 laser applications where a focused beam with minimal scattering is a must. Additionally, with its 2-16μm range, no unwanted radiation from the UV, VIS or even most of the NIR range can interfere with measurements. Germanium also has remarkable chemical properties and is inert to air, water, alkalis and many acids.
Potassium Bromide (KBr)- a mainstay in FTIR spectroscopy, KBr is sought after for its gigantic transmission range of 250nm all the way out to 26μm. KBr will withstand high temperatures up to 300ºC and mechanical shocks but care must be taken to avoid moist environments, which degrade the material.
Potassium Chloride (KCl)- often used interchangeably with KBr due to its similar transmission properties (210-20μm), KCl might be chosen over KBr due to its high damage thresholds and low index of refraction. Similar to Germanium, KCl is ideal for low-power CO2 laser applications but unlike Germanium, it can be used in the UV, VIS and NIR range.
Sapphire (Al2O3)- with a large transmission range of 150nm-4.5µm sapphire is a good generalist but where sapphire truly shines is its material robustness. You can use sapphire in almost any harsh environment and it will take the punishment. From extreme resistance to thermal conductivity, a high dielectric constant and chemical resistance, sapphire will take almost anything thrown at it and ask for seconds. Only behind diamond in terms of material hardness but unlike diamond can be made extremely thin, which further improves transmission.
Sodium Chloride (NaCl)- NaCl is the closest to a disposable option you'll find the IR window family. Since the window is essentially table salt, as you may imagine, it is sensitive to water and thermal shocks. With a wavelength range of 250-20μm, its main feature is that it is a cost-effective, wide range FTIR generalist.
Zinc Selenide (ZnSe)- The main reason to use a ZnSe window is in a high power Co2 laser system. High resistance to thermal shock, low absorption coefficient and low dispersion properties make it so you can concentrate high energy radiation and bring it to a focused, minimally scattered point through this window. Care must be taken as the material is soft and susceptible to scratches. ZnSe is not recommended for use in harsh environments. Get yourself a sapphire window instead!
Probably the first consideration you'll have when picking a window is what your wavelength operational range will be. Is this for a far UV application? FTIR? Visible light only? Typically, we say there is actual transmission when the value of radiation that reaches the detector from the monochromator is at least 80%.
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Wavelength range is a function of transmission, which refers to the light that is coming from the light source, passing through the optical window, the sample and finally arriving at the detector.
Therefore, if you have a material that is giving you a 50% transmission value at 18μm, this would not be included as part of the wavelength range. A material such as CaF2 whose range is from 130nm-9.5μm would be inappropriate. You would need a material, for example like KBr that has a range of 250nm-26μm, meaning that within this window (pun intended) there is over 80% transmission of light.
Example: let's say you are using a Krypton laser (presumably to hunt Superman), which operates from 416-799nm. You would need to choose an optical window that does not block light in this wavelength range and allows for proper transmission above 80%. Several candidates jump to the forefront such as Fused Silica (180-2,500nm), CaF2 (130nm-9.5μm) and NaCl (250-20μm) just on wavelength alone. However, this is only one consideration in your choice and we'll discuss other considerations in more depth below.
Lots can happen on the way to the detector that can slow down the light, bounce it around and absorb it, which leads to our next topic.
The refractive index refers to how much a beam of light changes direction or bends when traveling from one medium to another. Relevant to us, this will mostly refer to light transitioning from the air through an optical window.
While wavelength range is the main property that sets apart a UV window from an IR window, refractive index comes in a close second. An IR window material will have a higher index of refraction of around 1.4-4 while a UV/VIS material such as BK-7 or Fused Silica will be around 1-2.
The nd is closely correlated to the material's density, which makes sense as if you have a material that is denser such as sapphire or Germanium, the material will slow down and bend more intensely than a low index crown glass.
Example: if you were to need an optical window that required an intensely focused beam of light such as a laser to hit its target with minimal bending of light in the IR range, a low index of refraction, low density material like KBr would be an ideal candidate.
Since we're big on section transitions here, this leads neatly into our next section on material density and why it matters in your choice.
We've already covered most of density in our last section but more depth is required. The density not only matters because of its correlative effect on light refraction but also because the density of a material correlates to its weight. The denser a material, the greater the weight will be.
If you are designing an optical assembly and weight-sensitivity is critical, you would need to use a less dense material.
Example: you need a high transmitting IR optical window around 6µm and normally you'd consider Germanium but at 5.33g/cm3 it's denser than a lead zeppelin and is too heavy for your weight-sensitive application. In this case, a lighter, less dense material like Silicon with its density of 2.33g/cm3 is the right call.
Okay, fair enough. We couldn't make a good transition happen here so let's just discuss the Abbe number. The vd (get your mind out of the gutter), refers to how much light the material disperses and is closely associated to the index of refraction, which we discussed above. More specifically, the Abbe number is a ratio, which tracks the change in refractive index versus wavelength. Here is the formula:
In an Abbe calculation, you need the refractive index at three given wavelengths to solve for the vd. In the VIS range, NF, nD and nC are the refractive indices at 486.1nm, 589.3nm and 656.3nm. These particular three wavelengths are standard reference points known as the Fraunhofer lines but that's not important right now. The Abbe number is really just the refractive index of a material at these three wavelength points plugged into the equation.
So, why does this matter and what does this have to do with picking an optical window? A low Abbe number means high dispersion, specifically of colors. While you might want high dispersion in another application such as a prism, this quality might be something you seek to minimize in an optical window.
Example: you are considering using a high Abbe Silicon (Si) window (vd of 3.422) in the VIS range. Keep in mind this is already a bad idea as Si transmits from 1.2-8.0μm, completely bypassing the VIS range. Naturally, you are experiencing chromatic aberrations or optical color defects where varying wavelengths of colored light are not converging at the same point after passing through your optical window. Instead of getting a focused, concentrated beam at your focal point it's looking more like the dark side of the moon. Bummer. You'd be better served switching to a higher Abbe number material such as CaF2 with its low Abbe number of 1.434. On top of this, the wavelength range is right at 130nm-9.5μm.
Now we get into the more physical properties, such as how much punishment the material can take. If you're exposing your optical window to extreme temperatures and fast temperature differentials, you'd better consider your coefficient of thermal expansion. This represents how the size of the glass will expand and contract relative to changes in temperature. For instance, an optical window that is 75mm at room temperature might be slightly larger at 400ºC. As you'd imagine, if your material is doing this your measurements will be thrown off and there is a high likelihood that a crack could occur rendering your expensive window as useful as a concrete parachute.
Example: if you've read this far you can probably guess what material will be used as an example here. If you are building an optical system for a deep sea exploration vessel where the pressure will be extreme and you can expect the system to be bounced around, sapphire is the obvious choice. With a Knoop hardness of , the sapphire will easily absorb the punishment from the extreme pressures of the deep sea and also withstand Kraken attacks with nary a scuff (okay, you get the idea).
Meanwhile, if you had used a more lightweight material such as NaCl with a Knoop hardness of 18.2 it would be sleeping with the fishes. Not to mention, the NaCl would not hold up well in such a moist environment.
We saved some of the best for last. What good is your optical window going to be if it is covered with scratches and scuffs? Each one of these imperfections gets in the way of the incoming light and causes serious disruptions to your measurements. Scattered light may not cause huge disruptions in many optical systems but in certain applications it is a necessity that a high level of surface quality or what's referred to as "scratch-dig" is achieved.
The scratch portion of scratch-dig is determined by comparing the scratches along a given optical surface to a set of standard scratches under specific lighting conditions set forth in the MIL-PRF-B. The "dig" simply refers to the size of a divet or dig into the material. This is calculated by dividing the diameter in microns of the dig by 10. This gives you a scratch-dig number. The most commonly seen ones are 80-50, 60-40, 40-20, 20-10 and 10-5. The lower this number, the higher the surface quality.
Unlike with other material qualities, scratch-dig is not specific to the material itself but rather a function of how much polishing is applied to the window during production.
Example: you are looking for an optical component for a low power Co2 laser system. You have already narrowed down all of the optical properties and have landed on KCl. Since this is for use with a laser application, high precision is needed and the beam will be so focused that any surface imperfections will cause problems in your system. Garden variety 60-40 scratch dig is not precise enough. For this application you would need something far more demanding such as 20-10.
Perfection may be the goal but all of us ultimately fall short. Even our optical windows. Surface flatness is a measure of the deviation away from the perfection of a perfectly flat surface. This is measured with an interferometer relative to what's called an optical flat, which is as close to perfect that exists for an optical window.
When the optical window is placed against the optical flat, deviation can be measured in visible fringes. If the fringes are evenly spaced and perfectly straight then the optical window is as flat as the optical flat. However, if the fringes are curved, these fringes can be calculated in terms of wave dimensions (λ).
Where surface flatness is not critical such as in commercial applications, it's common to see values of 1λ or above. Meanwhile, when surface flatness is critical, you are looking for a fraction with a large denominator such as λ/10. In laser applications, this is often where surface flatness is a necessity. If there is any waviness on the surface, a focused laser beam would distort on the surface and cause all sorts of headaches for the experiment.
Example: you are devising a high power laser system and have settled on a ZnSe window with a scratch-dig of 20-10. You are all covered in terms of surface imperfections, material quality and minimal dispersion. All you have left to deal with is the surface flatness. Playing it smart, you decide on a λ/20 ZnSe window.
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