Extended Range UHV Viewports

EXTENDED RANGE UHV: Sealed Optics offers metal sealed Conflat (CF Flange), extended-range optics in 0.50-inch, 1-inch, and 2-inch view diameters with transmission covering the extended range from 121nm to 20 microns. Before extended range viewports became available, fused silica and sapphire were the most common windows for construction of vacuum viewports. Ultra-high Vacuum (UHV) viewports use materials for windows such as Zinc Selenide, Magnesium Fluoride, and Calcium Fluoride. These optical materials are useful in that the extend the useful transmission range for UHV viewports.

Calcium Fluoride (CaF2) is used optically as windows, prisms and lenses transmitting from the vacuum ultraviolet (VUV) into the infrared.  It is a physically and chemically stable optical material with good water and chemical resistance. Two levels of quality are manufactured, the first for visible and infrared applications and the second especially for deep ultraviolet applications. 

Magnesium Fluoride (MgF2) is slightly birefringent and is normally supplied with the optical axis cut perpendicular to the window face.  Magnesium Fluoride transmits well into the VUV region and is an excellent choice for excimer laser applications.  Magnesium fluoride is one of two materials that will transmit in the VUV 121nm range. The other, lithium fluoride, is too fragile to manufacture viewports of this type.

Zinc Selenide (ZnSe) is used for Infrared windows and lenses and transmits well between 0.5 microns and 22 microns. These viewports are available uncoated and BBAR coated from 8 to 12 microns. Zinc selenide is attractive for wavelengths far into the infrared having very low absorption coefficient at 10.6 microns. Because ZnSe has a high refractive index the reflective losses are considerable and therefore ZnSe is often anti-reflective coated.

The table below illustrates the relative strengths of these materials compared to fused silica (glass). Only zinc selenide is comparable to glass in strength. It can be understood from this table that protecting the viewport window from impact is vital.

 Modulus of Rupture (MOR)        MPa      PSI

Fused Silica (glass)                       55.0      8,000

Zinc Selenide                                 55.1      8,000

Magnesium Fluoride                      49.6      7,200

Calcium Fluoride                            36.5      5,300 

Other alternative window materials are similar in strength and suitable for UHV viewport construction.