Angled viewports
Brewster Window or Angled Viewport?
Earlier this year we manufactured some viewports that from a first glance you might think are Brewster windows… but they actually aren’t!
While it might seem like just a technicality, the distinction between angled viewports and Brewster windows has huge implications for the applications and their functionality. Let’s explore what we manufactured and how it differs from a Brewster window!
What was the final product?
We manufactured two viewports with a specialised grade of fused silica renowned for its improved transmission within the DUV (deep-ultraviolet) spectrum. The customer had an antireflective coating added to target a specific wavelength in the ultraviolet range. These optics were polished to a high degree of flatness at lambda / 10, λ = 632 nm and the viewports were constructed from a non-magnetic DN40CF flange with the optic indium sealed into the assembly at a 45° angle.
How do Brewster windows work?
Brewster windows are used to linearly polarise light. Light is extremely complicated, capturing the attention of physicists to this day, so we will look at a simplistic way to understand it in the context of a Brewster window. When light interacts with a surface, which in our case is an optic, the area where the light beams are involved is called the plane of incidence.
Within this plane, you can consider light to have two components, one that oscillates in parallel to the surface and 90 degrees to the plane of incidence (s-polarised) and the other that is at a 90-degree angle to the surface and in parallel to the plane of incidence (p-polarised).
When light strikes the surface at a certain angle – called the Brewster angle, p-polarised light has its reflectance completely removed – which increases the transmission, whereas the s-polarised light is reflected normally.
The Brewster angle is calculated based on the refractive index of the material (how much the material bends light). However, it is also impacted by other things such as the refractive index of the air (or materials before the optic) which itself depends on the wavelength of the beam! When this angle is calculated correctly the refracted beam (partially p-polarised light) and the reflected beam (s-polarised light) are at a 90° angle to each other.
Why are these not Brewster Windows?
So far, from the description and the photos, these viewports seem like typical Brewster windows! However, there are a couple of giveaways, that they are not what they first appear. Optics for Brewster windows must have the angle they are set to precisely calculated so they can polarise incoming light. The angle for these optics was 45°, which is not something you would expect for a fused silica Brewster window. While it depends on the wavelength of light to be polarised and the refractive index of the specific grade of fused silica used, we would expect the Brewster angle for this optic to be closer to 56°.
Additionally, you might have also noticed that we mentioned these viewports are coated! This is another clue that they aren’t Brewster windows.
The Brewster angle is so powerful at polarising the beam, you would not require an anti-reflective coating, in fact it would hinder the process by changing the refractive index! Therefore, these are angled viewports. They would not linearly polarise the light, however they still could be used for beam splitting (non-polarising) which could be useful for interferometry!
If you would like to learn more about an actual Brewster window we manufactured, check out our post about a customer installing theirs here: https://torrscientific.co.uk/brewster-installation/
