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Product Announcement

Sapphire Windows

01 Dec 2021

Sapphire (Al2O3) is the second hardest crystal available (after diamond), and this alone makes it an optimal substrate for optical windows. But on top of its extreme hardness, it also boasts numerous other benefits.

These include:

  • A high transmission from ultraviolet (UV) to infrared (IR), specifically 0.17µm to 5.5µm.
  • High durability, meaning it can withstand abrasives such as sand and other particulates
  • Chemical stability and resistance
  • High thermal shock resistance

As sapphire has a wide transmission band and is much stronger than other optical materials, it is often used as the front-facing optic to protect more delicate systems. It is one of the materials of choice for optical domes and windows used in deep-sea applications, where its high strength, high-pressure resistance and durability provide an effective barrier to external sources of damage.

Another property of sapphire is that it exhibits a phenomenon known as birefringence, meaning it has two different refractive indices when light enters the optic. Here at Knight Optical, we offer sapphire windows that are expressly cut along a particular plane to remove this birefringent property – these are referred to as our C-cut sapphire windows, which are available as both stock and custom options.

Alternatively, we also offer random-cut sapphire, which can have any orientation and, therefore, may display this birefringence. However, this is a more inexpensive option and can be used in applications where imaging is not necessarily a priority.

Listed below are our typical stock specifications. Knight Optical currently holds both C-cut and random-cut sapphire windows in 5mm up to 100mm diameters.

  • Diameter tolerance:  +0/-0.20 mm
  • Length/width:  ±0.1 mm  
  • Thickness:  ±0.1 mm 
  • Flatness:  < 5λ
  • Surface quality:   < 60-40 scratch/dig 
  • Parallelism:  < 5 arcmi

Alongside our extensive stock range, we also provide custom-made sapphire windows, manufactured to your requirements. Highlighted below are our custom sapphire capabilities.

  • Diameter:  < 5mm to 200mm+
  • Flatness:  < 1λ
  • Surface quality:  < 60-40 scratch/dig
  • Parallelism:  < 3 arcmin

What's more, we can also apply a coating to the surface, including antireflective and filter coatings, to optimise the performance of your application.

There are a few key factors to think about when enquiring about our sapphire windows.

These include:

Considering fluorescence. Sapphire can fluoresce slightly under UV light if impurities, such as iron or chromium, exist within the lattice. We can provide fluorescence-free sapphire, although this must be specifically requested when raising an enquiry as our random-cut and C-cut sapphire are not guaranteed to be fluorescence free.

Deeming whether the birefringence property of sapphire would be an issue within your application. If so, again, please raise this at the enquiry stage so that our technical team can ensure C-cut sapphire is provided.

We can also provide high accuracy sapphire windows with lambda/10 flatness, <10 arcsecond parallelism and 10/5 scratch-dig, perfect for high-precision and laser-based applications. Please note, for such accuracy, there needs to be a thickness-to-diameter ratio of at least 1:10 due to the hardness of sapphire.

Our sapphire windows are checked for quality in our state-of-the-art Metrology Laboratory using equipment from ZYGO, Agilent Technologies, and Starrett.

Contact our technical sales team who can help you with your enquiry and provide any further information you may require.

UK, Europe, Asia & RoW: E-Mail info@knightoptical.co.uk

Tel +44 (0)1622 859444

USA & Canada: E-Mail usasales@knightoptical.com

Tel +001 401-583-7846

Knight Optical Ltd
Roebuck Business Park
ME17 1AB
United Kingdom
Tel: 44 (0)1622 859444
Fax: 44 (0)1622 859 555
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