Snapshot: Computers,

environmental regulations,

and new materials provide new

opportunities

and challenges for

the age-old art

B y A l i x C l a r e

A master optician us ing a mechanized pol ishing wheel .

and science of optical glass

fabrication.

Glass has always been a material of choice in the opt ics industry because it has many of the optical attributes of a l iq­uid, such as transparency, while exhibit­ing the physical properties of a solid. In glass, the local structure is s imi lar to that of its crystalline analogs. For exam­ple, crystalline and vitreous silica both contain rigidly adhered to S i O 4 tetrahe¬dral units, but, unl ike the crystal, the glass exhibits no long-range order or periodicity. Therefore, glasses are opti­ca l ly homogeneous wh i le r e ta i n i ng much of the basic electronic structure (and transparency) of the single crystal so l id . A network structure is usual ly

attributed to optical glasses. Most optical glasses are still based on oxides, of which most of those are based on silicates. In general, glasses have three types of elements associated with them:

• Network formers, e.g., silica, are units usually linked through bridging anions to other network formers. "An ion" and "cat ion" are used, although bonding is usually about 50% covalent.

• Network modif iers, e.g., sod ium, are cations that break up the network and result in non-br idg ing anions.

• Intermediates, e.g., lead, are cations that are ambiva­

lent and can act as either a network former or modi­fier depending on the circumstances. Optical glasses are made of all of these types of ions

and the ratio and identity of these ions determine much of the behavior in these materials.

Glass fabrication

Melting Reasonably pure glasses may be melted in platinum con­tainers in electric furnaces from pre-prepared cullet ( c rushed glass) o r batch c o m p o n e n t powders . Alternatively, mass-produced optical components can be made from a continuous melting system in which the glass is either drawn or cast into shapes prior to finish­ing. To avoid bubbles in the glass, additives in the melt ensure expulsion or re-dissolution of gases into the melt and mechanical mix ing or remelting ensure homoge¬nization (lack of density fluctuations). The elimination of bubbles and the homogenization together constitute " f i n i ng . " Glass can be poured into molds that may approximate the component shape or may produce a machinable preform. After cooling, annealing is neces­sary to alleviate any stresses built up due to differential cooling in the part which could lead to cracking.

Sol-gel method Two newer methods used to make optical glasses do not

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Pouring and forming g lass .

involve melting. The sol-gel method is a chemical means of creating glass, in which organic precursors such as tetraethylorfhosilicate (TEOS) form a glass network by a series of hydrolysis and condensation reactions. This method is fairly expensive because of the components used, but requires very small amounts of heat and pro­duces, chemically, very pure glasses. Organic molecules can be incorporated into these glasses because they are only heated to relatively low temperatures (a few hun­dred degrees to drive off water and aid consolidation). The flexibil ity provided by this treatment means that dyes, even laser dyes like rhodamine, can be incorporat­ed into these materials or polymers, and can be stabi­lized by the rigid glass network and aligned to give non­linear optical response in a part icular direct ion. The chemical processing of these glasses means f lexible geometries for optical parts.

Unfortunately, challenges st i l l exist in processing these materials. Large quantities of residual water may contaminate the final product and residual porosity can cause some l ight scattering. In add i t ion , the dry ing stages can result in considerable cracking. The shrinkage

in volume associated with starting from a l iquid and forming a solid is extensive, depending on the method of addit ives, but is generally in excess of 80%. However, several companies e.g., Geltech, have overcome many of these prob­lems and are now able to make large bulk pieces by these methods. However, bulk sol-gel optics have yet to enjoy wide market acceptance.

The use of the sol-gel method to coat opti­cal components is more widely accepted. These coatings may be colored or functional such as ant i - ref lect ion or electronical ly conduct ing. Many of the problems, such as shrinkage and drying, that have been associated with mono­lithic sol-gel pieces are not issues in the coating arena and sol-gel has certainly captured its niche in that particular market.

The o ther f a b r i c a t i o n m e t h o d — v a p o r deposition-—involves the deposition of the glass from the gaseous state on to a substrate. A vari­ety of approaches to vapor deposi t ion exist. Depend ing on the poros i ty of the result ing deposit, consolidation steps may be required to obta in a dens i f ied glass. Un l i ke the sol -gel m e t h o d , the vapo r d e p o s i t i o n techn ique involves fairly high temperatures but, like the sol-gel method, results in extremely pure glass­es. The vapor deposit ion method revolut ion­ized fiber optic communications by providing ultrapure materials that could transmit signals over tens of kilometers, therefore making the medium commercially viable. The vapor depo­sition method is now being used where ultra­high purity is crucial to operation (e.g., applica­tions in the U V region of the spectrum, partic­u la r l y those associated w i t h exc imer laser optics, where currently expensive and tempera­mental halide crystals are generally used).

The main barriers to the use of vapor deposit ion techniques are largely economic. The equipment is expensive and because toxic, and sometimes corrosive components are used, the safety issues associated with operation of vapor deposition equipment make it pro­h i b i t i v e l y expens ive w h e n ext reme p u r i t y is no t absolutely required.

Optical properties P a r t i c u l a r opt ical prop­erties are key to the su i t ­ability of cer­t a i n glasses for spec i f ied app l ica t ions . The mos t i m p o r t a n t p r o p e r t y is transparency. The mater ia l An Opticam CNC machine.

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of choice usually has to be transparent in the frequency range o f interest. Genera l ly , the t ransparency o f a homogeneous, non-porous material is determined at the low energy end (infrared [IR]) by the fundamental frequencies of bond vibrations in the material. A per­manent dipole moment associated with the bond (i.e., less than 100% covalent bond) must be present. The extent of IR transparency depends on the mass of the vibrating ions and the force constant of the bond (heavy ions and loose bonds give better IR transparency). The U V t ransparency is de te rm ined by the e lec t ron ic bandgap of the material. The more electrically insulat­ing the material is, the better the U V transparency. In between, transparency is determined by impurities and defects that can cause absorptions and scattering associ­ated with the glass. These are predominantly composi­tion and/or processing dependent.

Refractive index is also an important property. The refractive index is essentially controlled by the polariz¬ability of the constituent cations and anions, with the anions usually playing a larger role due to their negative charge and abundance of polar izable electrons. The more polarizable the constituents, the more the light­wave is s lowed , g i v i ng a h igher re f ract ive index . Ref rac t ive index is also a f u n c t i o n o f f requency. Generally, in normal dispersion glasses, the refractive index is higher at higher frequency.

There are various empir ical equations that can be used to fit the dispersion curve, but optical component manufacturers primari ly look at a figure of merit called the Abbe number. The Abbe number is the ratio of refractive index in the midd le of the vis ible region minus one and the difference between the refractive indices at the two ends of the visible region. A dispersive glass (one that would widely spread out visible light into its constituent colors) would have a low Abbe number while a less-dispersive glass wou ld have a high Abbe number. Lenses wi th different and opposing powers, made from two glasses that have the same dispersion, can be used to make achromats (pairs of lenses that compensate chromatic dispersion at particular wave­lengths). To help the lens designer, the Abbe number is plotted against the refractive index on the "Glass Map." This is used to help determine a suitable glass for a given application.

Even the most perfect match between specified and exhibited optical properties is not enough to make a glass composi t ion a candidate for an optical compo­nent. There are the pract ical considerat ions such as whether the component can be formed in a sensible and economical fabrication process—grinding and polishing the component and in-service life of the component. For example, beryll ium fluoride is a very good glass-for­mer, has excellent U V transparency, can be produced extremely pure by a vapor phase deposition route (simi­lar to that used for silica optical fibers), and yet is so toxic that economical fabrication and use is prohibitive. Zinc chloride is also a good glass-former and has a wide transmission window, but practical ly disappears as a puddle in front of your eyes because of inferior chemi­

cal durability. Many of these secondary considerations are determined by the physical and thermal properties of the materials. For example, the toughness and hard­ness of the glass are fundamental determinants of the gr indabi l i ty of the part, and in general, low thermal expansion is required especially for use in extreme cl i­mates. As wi l l be discussed later, the toxicity of the batch components, resultant glass, and byproducts of fabrica­t ion, has become a very important issue and is one of the major drivers toward new compositions containing non-regulated components.

Annealing: A post-melting heat treatment to alleviate stresses built up by differ­ential cooling.

Transparency As mentioned earlier, transparency is ultimately deter­mined at the high frequency end by the bandgap of the material and at the low frequency end by the fundamen­tal vibrations of the material. For most applications in the visible region, oxide glasses suffice, but sometimes applications require extended capabilities outside the v i s i b l e r e g i o n . The i n f r a red t ranspa rency can be improved by choosing a material either without a per­manent dipole moment, or, choosing one with heavier ions and looser bonds. Silicate glasses have an infrared cut-off at approximately 5 urn, but heavy metal oxides can t ransmi t l igh t out to app rox ima te l y 8 -9 µm. Fluor ide glasses can transmit to approximately 7 µm (fluoroaluminates) and to 8 µm (fluorozirconates) with the weak ionic bonding and heavy cations contributing. Chalcogenide glasses (based on arsenic antimony and germanium slenides, sulfides and tellurides) transmit anywhere from 6 µm (sulfides) to 10-20 µm (selenides and tellurides). If halide and chalcogenides are mixed, transmissions up to 15 µm are routine. Increased IR transparency is of interest in military applications par­ticularly for components that operate in the 3-5 µm and 8-12 µm atmospher ic w indows. A l so , materials for night vision and optical sensing are required and many of these operate quite far into the IR. The search for new compositions with desirable properties for IR transmis­sion is ongoing, and improvements in existing composi­tions such as new purification techniques and forming processes are continually advancing these fields. One of the biggest challenges that exists in the field of IR mate­rials is controll ing the temperature dependence of the refractive index. This is significant because absorption and subsequent self-focusing has impl icat ions in IR

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Glossary

Fining: A combination of gas expulsion or re-dissolution in the melt and homoge¬nization to ensure a minimum of light scattering.

Lapping: Final polishing process to obtain an optical finish on the product.

Preform: Solid shaped, somewhat larger than the final product that will go through a further formatting process to reach the final product.

optics, not to mention applications in high power laser (CO 2 ) delivery.

U V transparency basical ly depends on the band structure of the glass. The better electrical insulator a composit ion is, the more l ikely it is to transmit high energy U V l ight. U l t r a h igh pur i ty s i l ica and s i l ica doped with halide have very high U V transparency, but silica is not easy to fabricate having high viscosity at extremely h igh temperatures. Some glasses recently developed at Schott have very good U V transmission. These are typ ica l ly f luorophosphate ( f luorocrown) compositions. At Alfred we have found that the more ionic a composi t ion, the more U V transmitt ing it is. Therefore, the family of glasses based on aluminum flu­oride are highly U V transparent. Ultimately, U V trans­parency wi l l be determined by the material's purity. Any defect or impurity severely compromises the U V trans­parency of the material. U V transparency has become increasingly important for the development of optical components to assist in appl icat ions invo lv ing high power U V lasers (e.g., photol i thographic techniques). Interestingly, increased U V transparency can reduce dis­persion because the phase change associated wi th the response of the material to electromagnetic oscillation causes the dispersion. The farther the bandgap transi­t ion is f rom the region of appl icat ion, the lower the overall chromatic dispersion. The biggest challenges at this end of the spectrum focus on purity. Many of the desirable glasses are part icularly diff icult to produce using high purity techniques such as vapor deposition. In addition, multi-component glasses are very expensive to produce with the requisite purity.

Component production Glass has been used to make many different types of t rad i t iona l opt ica l components . D i f f rac t ing pr isms made out of glass disperse light into its component col­ors of the visible spectrum, lenses focus light or diverge light using the refractive index and workabi l i ty, and mirrors consisting of silvered glass reflect light using the thermomechanical properties. Throughout the years, many improvements have been made in the opt ical qua l i ty of these components . M o r e pure mater ials (reduced extraneous absorption) and better fining pro­duce opt ical ly-clear bu lk glasses on a regular basis. Techniques for fabricating high tolerance geometric shapes from melted glass boules have been slower to develop.

Machining Since the dawn of optical science, master opticians have worked in optical fabrication shops producing intricate shapes from a variety of glass compositions. Glass is not a trivial material to machine. Anyone who has dropped a glass bottle (usually full of something that badly stains your clothing) wi l l attest to the brittle nature of glass. Cutting usually proceeds by diamond tool, grinding by successively smaller grit sizes followed by lapping with a polishing agent on a soft cloth. The role of master opti­cians has gradually changed with mechanization of cer­

tain processes. F rom the first mechanized pol ish ing wheels came machines that would, with extremely high tolerances in their mot ion, be used to grind precision optical parts at a reasonably high rate. These machines had to be carefully set up and aligned by the master opticians. Set up would often constitute about 10 times the amount of time it would take to actually grind the piece, the advantage being that multiple pieces could be ground at once.

Wi th the advent of many machines using computer numer i c con t ro l ( C N C ) , there is a fur ther change occur r ing in the role of the master opt ic ian. These machines require that a set of parameters be p ro ­grammed into the computer which, given the correct parameters, could in principle, repeat the same process and grind many such parts. The main obstacle to mech­anization is the inability to emulate the "feel" that the optician has when minute changes in position or pres­sure are needed, especially when cutting and grinding asymmetrical parts. Therefore, the initial determination of computer parameters for grinding a new part from new materials can be somewhat "hit or miss." Steps are being taken to address and overcome these shortfalls in computer-aided fabrication wi th instruments like the "Opt icam" series for deterministic microgrinding. This was developed and tested at the Center for Opt ics M a n u f a c t u r i n g ( C O M ) , U n i v e r s i t y o f Rochester . Successful cutting of spherical lenses has led researchers at C O M to investigate modifications to make aspheres and conformal (unusual) optics. Even more exciting are developments from the same group in magnetorheo¬logical f inishing (MRF) in which a magnetic slurry is induced to perform the final pol ishing steps for glass parts making the final polishing steps more intune with C N C techniques.

Gradient index materials In some cases there are alternative approaches to fabri­cation of optical components. For example, i f the pur­pose of a lens is to bend light in a controlled way (e.g., to a focus), the light is focused by a radial change in op t i ca l pa th leng th . T h i s , as stated p rev ious ly , is ach ieved by ca re fu l m a c h i n i n g and p o l i s h i n g . Alternatively, one could envisage changing the refractive index in a controlled way and leaving the length of pas­sage constant to achieve the same effect. A gradient index (GRIN) lens is born and light can be focused with equal precis ion. This can be achieved by ensuring a composit ional gradient in the glass itself. There are a number of ways of achieving this. One way is to use one of the deposi t ion techniques descr ibed above w i th gradual compositional changes giving a gradient index to the outside of the glass. As stated before, this can be expensive. Sol-gel can also be used in this manner by building up successive layers of different index.

A n alternative is to start w i th a regular homoge­neous composition and use ion exchange to achieve the gradient effect. Ion exchange involves mobile ions in the structure (usually modif iers) which, in contact wi th chemically similar ions and heat, wi l l effect an exchange

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that alters the polarizability and, hence, the refractive index. Ion exchange is a process that is also used for strengthening due to the fact that exchanging smaller ions for larger ions will generally compress a surface, closing the cracks and flaws that make it weak. The depth of this exchange is usually quite small compared to those required for gradient index optics so the latter are usually electric field assisted. GRIN materials are used in large lens arrays, for example for optical com­puting and image reproduction, endoscopy, and in small lenses for photonic applications. Currently, the exchangeable materials available, the speed of the ion exchange process, the geometric limitations, and the size of viable components are the main barriers for wide­spread application. The possibilities for new ion exchange compositions are always increasing and new methods such as ultrasonically enhanced ion exchange might speed up the process. However, the creation of conformal optics by these methods will remain a chal­lenge at least in the near future.

Glass ceramics The stability of mirrors has been an issue, with respect to their environment, part icularly thermally. Developments in these areas have involved the applica­tion of athermal glasses and glass-ceramics. Athermal glass compositions have competing processes in their thermal expansion that ensure that the density does not change significantly as a function of temperature. This is achieved by only a small increase in the average anion-cation bond length as a function of temperature and/or flexibility in the connectivity of the basic struc­tural units. Free volume in the glass takes up the slack instead of expansion occurring. These tend to be opti­cally homogenous and can be used for athermal lenses and as mirror substrates. Glass ceramics are essentially glasses that contain small crystallites which contribute to the properties of the base glass. Glass ceramics may be transparent or opaque depending on crystallite size, shape, and composition. In general, glass ceramics are mechanically tougher than glasses, but also by careful compositional control, can be made to have very small or zero thermal expansion. In this case, the crystals can compensate for the thermal expansion of the glass with a negative thermal expansion coefficient. This is very useful for mirrors in precise optical equipment that is likely to experience extremes in temperature. The main challenges associated with glass ceramics involve adapt­ing the grinding and polishing techniques developed for glass to these materials. Ceramics tend to be far less pre­dictable and more sensitive to processing variations than glasses, so a slightly modified set of rules for deter­ministic grinding may be necessary.

Environmental issues In many cases the search for new materials is driven by an increased awareness of the environment. The fabrica­tion of components, transportation of raw materials components and waste, and the subsequent disposal of waste have led many glass manufacturers to try to devel­

op compositions that are free of heavily regulated mate­rials. Lead is one such material and it is ubiquitous in many optical glasses. This element's high polarizability results in high refractive indices and indeed many of the commercial flint glasses are on the order of 60% by weight lead oxide. Lead glasses tend to be "sweet" glasses which means that they are easy to work. On a smaller scale, arsenic and antimony oxides are excellent fining agents and yet are regulated materials. This trend has led companies such as Schott, Corning, and Hoya to search for alternatives to lead. Corning has favored barium and strontium in decorative lead crystal and Schott has worked on both barium and lanthanum containing alternatives. Hoya has developed a series of lead-free and arsenic-free glasses. Meanwhile, lead still gives the most desirable combination of properties. In addition to the refractive index, it acts as a flux for most glass melts allowing lower melting temperatures and, of course, the glass is less expensive to produce than most of the alternatives. The search for alternatives will prob­ably increase in the near future especially with the envi­ronmental responsibility falling more heavily on the manufacturer. Legislation now makes the manufacturer responsible for the product and by-products from "the cradle to the grave" unless these constituents become a raw material in another process. As a result, once the expense of disposal is added to the cost of manufacture, the cost margin between the product cost of lead glasses and the alternatives will rapidly decrease.

What next? Work will continue toward automation of grinding and polishing processes with more emphasis on complex parts. The new work ethic has all but eliminated the apprenticed trades, and the role of the master optician is quickly changing in response to this and new develop­ments in CNC. To better define parameters for CNC, work is in progress to try to quantify some of the empir­ical data on deterministic grindings in terms of figures of merit based on simple property measurements. There will always be a search for new and better materials, pushing the frontiers of transparency and decreasing losses. There will also be other determining factors for compositional development, which are not directly related to optical properties, but which influence the economics of manufacture.

Alix Clare is associate professor of glass science at the New York State College of Ceramics at Alfred University.

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