CaF2 Windows

A CaF2 window is an optical window made of crystalline calcium fluoride. These windows are versatile, environmentally stable and resistant to laser damage, and they exhibit a high, stable transmission from 200 nm to about 7 μm. Shanghai Optics produces high quality custom CaF2 windows optimized for UV and IR applications. We have a wide variety of windows in stock, and are able to manufacture windows to meet almost any custom specifications you might have.

Properties of CaF2 windows

The optical properties of CaF₂ are comparable to that of BaF₂, but with a Knoop hardness of 158.3 it is almost twice as hard. It is also significantly lighter: the density of CaF2 is 3.18 g/cm3, vs 4.89 g/cm3 for barium fluoride. CaF₂ optics tend to be more economical than their BaF₂ counterparts, as, unlike barium fluoride, the optical crystalline substrate can be found in nature.  Pure calcium fluoride is transparent, but since most CaF2 used for optical windows is naturally mined it may have absorption bands around 300 nm, due to impurities. Calcium fluoride and has a low refractive index (1.39908 at 5 μm).

Environmental Reactions

Calcium fluoride is resistant to water, HF, and many other acids, although it will dissolve slowly under contact with nitric acid.   At room temperature and 100% humidity, a CaF2 window won’t fog, and a polished surface should not degrade under normal working conditions.  At temperatures exceeding 600 C you should expect significant degradation in the presence of moisture or humidity, but in a dry environment these windows can be used up to 1000 C.

Temperature Dependence of CaF2 windows

In dry conditions CaF₂ does not experience significant degradation of form or optical properties until about 800 C.  At high temperatures it may be prone to thermal shock, but it is less sensitive to temperature changes than BaF₂. The coefficient of thermal expansion is 18.85 x 10-6/C, and melting point 1360°C.

Applications of CaF2

CaF2 is often a good choice for spectroscopic windows and cryogenically cooled thermal imaging systems; situations where broadband transmittance is essential.

The high damage threshold and low UV absorption of calcium fluoride make uncoated CaF2 windows a good choice for excimer laser optics. These specialty UV laser systems are used in micro-machining devices and in eye surgery, and we ensure that each of our CaF2 windows is precisely polished and features the high surface quality necessary for these precise laser applications. Our quality check is second to none, and we use state of the art metrology equipment to check that every window which levels our factory fully meets all applicable standards.

Orientation of Calcium Fluoride Windows

The cubic crystalline structure of CaF₂ means that a calcium fluoride window will have different properties depending on how it is cut. Our standard calcium fluoride windows are randomly cut. For broadband continuum generation (white light generation), we are also able to supply windows cut along the [001] axis. Please contact us to discuss the specific need of your application.

Factory Standard

Both square and circular calcium fluoride windows are available to order. Our factory standard CaF2 windows are produced with a clear aperture of greater than 85%. The diameter tolerance is +0.0, -0.05mm. This ensures a good, tight fit into any prepared mounting.

The parallelism is determined to be no more than 5 arc min, and the surface quality is 40-20 scratch dig per military standard MIL-PRF-13830B. This guarantees no dig with a diameter larger than 0.2mm, and no scratch brighter than 40. The sum of all scratch lengths with brightness 40 is not more than ¼ the diameter of the CaF2 window, and the total number of allowable digs is less than the diameter divided by 20.  As an example, a CaF2 window with a 200mm diameter will have no more than 10 digs, and the sum of all maximum brightness scratches will be less than 5 mm.

Factory Standard:

• Material: CaF2
• Irregularity (fringe): l /4
• Diameter Tolerance: +0.0, -0.05mm
• Thickness Tolerance: +/- 0.1mm
• Clear Aperture: >85%
• Parallelism: 5 arc min
• Surface Quality: 40-20
• Wavefront Distortion: l/4 per 25mm

Please contact us for manufacturing limit or custom specifications. Our design team would be happy to work with you as you determine the best optic for your specific application, and our broad range of precision equipment allows us to produce high quality windows to meet any requirements. We also carry CaF2 lenses and prisms.

Protective Bevel available