Optics, Materials & Coatings: UV Optics

Paul Hrich, Bicron

Ten years ago, the ultraviolet- transmitting optic was considered a mature, developed product. Today market forces are challenging suppliers to dramatically improve UV optical performance. Suppliers who can meet the challenge will participate in at least a decade of double-digit growth. It should come as no surprise, therefore, that many materials suppliers are working furiously to increase capacity, especially for calcium fluoride and magnesium fluoride.

Modern ultraviolet optical technology is the result of continuous product development stretching back more than six decades. To support the early, low-power systems, materials needed to transmit only UV photons. Optics with parts-per-million impurity levels provided adequate transmission, with typical bulk absorption in the range of 10^-3/cm.

The modern excimer laser has an enormous photon flux density, and the probability of multiple photon events in an excimer laser optic has become statistically significant. An optic with parts-per-million impurity levels can undergo damage site cascades that quickly lead to catastrophic failure. Laser manufacturers are challenging the market to supply optics that can withstand 100 billion laser pulses or more.

Equally formidable are the challenges for the optics fabricators. After the new "super" materials are in hand, they must be ground and figured to surfaces consistent with their superior bulk properties. Where once figures of λ/10 were discussed, now designers speak of λ/100 to λ/5000. As the industry moves to shorter wavelengths, subsurface flaws become a more serious concern. Shorter-wavelength photons scatter more easily. With each advance into the UV, the size of the maximum allowable surface or subsurface defect decreases.

After fabrication, the challenges continue. A new generation of coatings must survive the high fluence and corrosive fluorine environment of the excimer laser, and output couplers must withstand the etching effects of created ozone. Holders must cradle the new optics so gently that they do not induce stress birefringence.