Vacuum Optics ❯ Anti-reflective coatings
Anti-reflective coatings
AR, BBAR, SPAR, VAR and WAR coatings of vacuum optics
What anti-reflective coating solutions does Torr Scientific offer?
Torr Scientific offers a wide range of Anti-Reflective (AR) coatings to improve the efficiency of laser transmission and to minimise the reflections from optics within vacuum viewports.
There are a variety of different wavelengths that can be targeted with AR coatings, from deep ultraviolet (UV) all the way through to Infrared (IR). With the ability for a single wavelength, multiple, or a range of wavelengths to be targeted by altering the type of coating.
Torr is able to provide theoretical “curves” (plotted graphs) to display the estimated transmission and reflection levels for each AR coating and optic.
Please enquire to the sales team to request a curve.
Typical curves for the various coatings:
Request a curve/More information
Why choose an anti‑reflective coating?
AR coatings work by creating a stepping stone in the difference between the refractive index of the optic and the external world.
Differences between the refractive indices of the air and the optic leads to reflections. These reflections occur on both the air‑side and the internal face of the optic. Applying an anti‑reflective coating to both faces creates destructive interference to reduce reflections.
Torr uses the customer-requested wavelengths to design and produce a custom coating .
If you are interested in the science behind AR coatings, please read our article on the topic.
Figure 2: The graphic on the left shows how opposing troughs and peaks cancel each other. The graphic on the right shows how two reflected waves from a coated face can be made to interfere destructively in this way. Because R2 has travelled further before reflection, its phase (peak/trough position) is changed relative to R1 such that the troughs and peaks are opposed.
Examples of
anti-reflective
coatings
AR
This is the standard single-layer coating, covering a wide range of wavelengths.
BBAR
BBAR (broadband) uses multiple layers to optimise for a continuous range of wavelengths.
This high-performance coating works best when the lower range is no less than half the upper range (e.g., 500nm – 1000nm).
VAR
VAR uses two layers of coating and is a high-performance approach for a singular wavelength.
Recognisable by the distinctive “V” shape it forms on reflectance curves.
WAR
WAR coating is optimal when two separate wavelengths are required at a high-performance transmission.
Recognisable for the “W” shape the coating forms within reflectance curves.
SPAR
In complex cases where a greater number, or a larger range of wavelengths are required, a SPAR (special) coating is possible.
Please contact the sales team for more information if this is a consideration for a project or purchase.
Available coatings
AR anti‑reflective coatings
Torr Scientific offers fused silica viewports with an AR (anti‑reflective) coating on each face comprising a single quarterwave layer of coating material optimised for a customer‑specified wavelength range. Please advise the important wavelengths or wavelength range with your enquiry or order. Wavelength ranges between 190 nm and 1550 nm are offered as standard, although coatings for other wavelength ranges can be quoted on request.
VAR anti‑reflective coatings
Torr Scientific laser viewports are offered with a two‑layer V‑anti‑reflective (VAR, named for the shape of the reflectance curve) coating on both sides of the window optimised to a customer specified laser wavelength. The coating reduces reflectance to below 1% per face, or 2% total, at the wavelength specified. Please advise the laser wavelength with your enquiry or order. Wavelengths between 240 nm and 1550 nm are offered as standard, although coatings for other wavelengths can be quoted on request. Viewports with coatings for a wavelength range can also be offered.
WAR anti‑reflective coatings
Torr Scientific fused silica viewports are offered with a four‑layer W‑anti‑reflective (WAR, named for the shape of the reflectance curve) coating on both sides of the window optimised to customer‑specified laser wavelengths. In many cases, the coating reduces reflectance to below 1% per face, or 2% total, at the key wavelengths specified. Please advise the important wavelengths or wavelength range with your enquiry or order. Wavelength ranges between 240 nm and 1550 nm are offered as standard, although coatings for other wavelength ranges can be quoted on request.
Broadband anti-reflective coatings
Torr Scientific fused silica viewports are offered with a four‑layer broadband anti‑reflective (BBAR) coating on both sides of the window optimised to a customer-specified laser wavelength range. In many cases, the coating reduces reflection to below 1% per face or 2% total at the key wavelengths specified. Please advise the important wavelengths or wavelength range with your enquiry or order. Wavelength ranges between 240 nm and 1550 nm are offered as standard, although coatings for other wavelength ranges can be quoted on request.
Theoretical reflectance curves for a single BBAR-coated face of fused silica. As the wavelength range increases the reflectance gets poorer.
Other coatings or other materials are available on request.
Example of sapphire:
Vacuum optics available for AR coatings
AR coatings are typically manufactured for vacuum optics viewports, Torr offers a wide range of optic materials. A full list of coated and non-coated available optics can be found on the Torr Scientific Vacuum Viewports webpage.
Fused silica
- This is the most popular option when working with lasers, for its optical qualities and due to the high-performance grades available. Coatings then improve transmission even further.
Sapphire
- Sapphire naturally has a broad transmission range from UV to near IR, which coatings can complement. Moreover, sapphire can be utilised at high temperatures which can offer more options for bakeouts.
Zinc selenide
- This optical material occurs naturally with a high transmission rate within IR, of which coatings can improve upon further. The specificity of these optics’ transmission within IR allows for the use of CO2
Kodial
- Kodial is typically used in viewports for visibility purposes, of which coatings can help to improve by minimising reflections.
Z-cut quartz
- This optic has unique qualities, by being birefringent with transmissions ranging from 250nm to 3µm and 50µm-1000µm. This quality means the refractive index depends on the polarisation of the light.
- Coatings can improve this already naturally ideal THz material.
Calcium fluoride
- An optic used primarily for excimer laser applications, which has high transmission rates across UV into near IR. This can then be increased with the use of AR coatings.
Other optics options for coating include lead glass, barium fluoride and magnesium fluoride. Find out more on the Torr Scientific Vacuum Viewports webpage.
How to choose an AR coating?
It can be complex to determine which AR coating is best for each project, it is a combination of factors surrounding the desired application and required wavelengths.
Additionally, there are a wide range of optical materials available for AR coatings, all of which have their own unique benefits.
The starting point of this is determining the purpose of the viewport:
- Viewports aimed at assisting with vision in the vacuum chamber may not need to focus on very specific wavelengths and solely want to reduce the reflections across a wide range.
- Whereas viewports allowing for lasers to enter the chamber, will have a specific wavelength or multiple wavelengths that are necessary.
Vacuum viewports, AR coatings and lasers
The standard laser damage threshold for our vacuum viewports:
- Pulsed lasers = 2 J/cm2 for wavelength 530 nm and pulse width 10 ns
- Continuous lasers = 500 W/cm2 at wavelength 530 nm.
These figures are based around a conservative estimate what our AR coatings can withstand.
How is AR coating developed and tested?
Coating recipes
Torr formulates their own recipes in-house for custom coatings. These recipes are produced using specialist software. Test pieces are coated and then tested for compliance using our UV-vis-IR and FTIR spectrophotometers.
Manufacturing & measuring
AR coatings at Torr Scientific are deposited using an e-beam evaporation system.
To coat the optics, they are placed within a vacuum chamber in which a high energy electron beam heats the coating material to over 1000°C. As the material is rapidly heated, it transforms into a vapour cloud, coating the optic.
Coating thickness are controlled by a thin-film thickness monitor.
To determine the success of the AR coatings, the transmission and reflection rates are measured using our spectrophotometers. These fire beams of light moving sequentially from UV to IR at the coated optic. The detector then measures the coating’s transmission and reflectance across the specified range, in comparison to the theoretical curve.
This process helps to ensure that the optic and it’s coating are targeting the correct wavelengths and achieving optimal transmission levels.
Standard vs custom AR coatings
Below you can find a selection of popular and commonly requested AR curves.
If these are not suitable, please contact our sales team and they will happily discuss your options. They can advise about coatings regarding viewport applications and provide theoretical curves for your convenience.
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