Technical Information

TN2015-01: Calcium Fluoride CaF2

Calcium Fluoride (CaF2) is a crystalline material that occurs naturally as fluorspar. When occurring naturally, CaF2 is often strongly coloured due to the presence of colour centres. In pure form, CaF2 is a very useful material for a wide range of optical applications due to its wide transmission range from 130nm to 10µm for the purest material.

CaF2 also offers low axial and radial stress birefringence and high refractive index homogeneity. All these characteristics make it the material of choice for many UV applications.

It is also a very good material for applications in the visible and IR and especially where the application involves a spectrally broad wavelength range, its low refractive index (approx. 1.43-1.44 in the visible) and resulting surface reflectivity often removes the need for an anti-reflection coating.

Apart from being used as a material for optical substrates, CaF2 is also commonly used as an optical coating material.

When compared to BK7 or Fused Silica, CaF2 is much softer with a Mohs hardness of 21 (SiO2: 100, Al2O3: 400). This makes it a more difficult material to polish to a high surface quality as the material and while ’10-5′ is a standard scratch-dig specification for materials like BK7 or Fused Silica, CaF2 is generally only specified as ’20-10′ or even worse.

To achieve high optical coating quality combined with a high Laser-induced damage threshold (LIDT), it is important to have a very good scratch-dig specification as well as a very good RMS roughness.

Manx Precision Optics Ltd. (MPO) is a company well known for its manufacture of high LIDT components. To achieve the best results, MPO undertakes the full manufacturing process of optical components – shaping/grinding/ polishing/coating/assembly in-house, giving the company full control over the entire process.

With its highly experienced workforce the company therefore developed a polishing process to achieve the highest surface quality on CaF2 substrates. This opens up new applications for CaF2 in combination with high LIDT applications.

This is best demonstrated by a typical example – a 50.8mm diameter substrate.

The flatness over >90% clear aperture is λ/20 as the interferogram illustrates. Two measurements undertaken with a profilometer (50x magnification, checked area approx. 170um 170um) between the centre and the edge of the clear aperture demonstrate the excellent RMS roughness of <0.2nm (2 Angstrom) that MPO can achieve. Flatness and surface quality standards of this type make the substrate usable for very high end applications, especially where low scatter losses and a high LIDT are essential. TN-2015-01-image 1

Interferogram demonstrating the excellent surface flatness achievable in a 2″ (50.8mm) diameter CaF2 substrate

For further information, please do not hesitate to contact us.

Manx Precision Optics Ltd.
4 Albert Street
Douglas
Isle of Man IM1 2QA
British Isles

Tel: +44- (0) 1624-620 800
email: sales@mpo.im
web: www.mpo.im