Molded Glass Aspheric Optics Hit the Target for Precision and Cost

Molded glass aspheric optics hit the target for precision and cost

EDWARD PATTON, DAVID STITES, AND SCOTT CAHALL

FIGURE 1. Molded glass asphere lenses can be used to create cost-effective lens assemblies in such applications ntil the advent of precision glass niold- as scanning lenses or microscope objectives, (Courtesy of ing (PGM) nearly two decades ago, Rochester Precision Optics) the use of aspheric lenses was a sel- dom used and expensive optical solu- ciated with molding glass aspheres are generally more tion for imaging-op- than that of tradi- tics applications, especially where plastic Contrary to popular belief, tional grind and pol- aspheric lenses lacked the precision or ish (G&P) lenses, they refractive properties required by the precision glass molding is an are typically one-time lens design. Today, though, consumers expenses. Even at routinely use products enabled by glass economical and high-quality modest volumes, tool- molded aspheric lenses such as digital choice for aspheric lenses. ing cost can be rela- cameras, cell-phone cameras, and DVD tively insignificant players. Even with such widespread appli- Two case studies grind away when amortized over cation, many designers still regard glass a number of parts— aspheres as too expensive to use in all but at the assumptions of molded hundreds perhaps- large-scale imaging optics applications—a making their cost notion that stems from a long-held belief glass versus traditional lenses. comparable to that of that problems associated with aspheric G&P spherical lenses optics, such as reproducibility, accuracy, associated cost, (and considerably lower than that of G&P aspheres). As and risk, cannot be affordably overcome. such, it may even be economical to mold the spherical Better performance and lower cost are no longer mu- glass elements of an imaging system as well. With the tually exclusive, however. While the tooling costs asso- economic barrier for using quality glass aspheres effec- tively removed, lens designers should seriously consid- EDWARD PATTON is director of sales and marketing at Rochester er the additional degrees of freedom provided by PGM Precision Optics, 850 John St., West Henrietta, NY 14586; e-mail: [email protected]: www.rpoptics.com. DAVID STITES is an optical aspheres (see Eig. 1). consultant and designer at Total Optic Solutions, 8401 Ninth Street North, Tooling costs for a small (6 mm diameter) PGM lens— Suite 250, St. Petersburg FL 33702; dstites@totalopticsolutions. SCOTT CAHALL is an optical consultant and designer at Moondog Optics, 264 aspheric or spherical—can be less than $10,000. Once Village Landing, Fairport NY 14450; [email protected]. amortized, the customer benefits from lenses ranging

Laser Focus World www.laserfocusworld.com December 2007 71 MOLDED OPTICS, continued Time-Bandwidth^ Products Retractive index, (X. = 587.6 nm)

Ultrafast E-FDS1 * 1.90 • CDGM

Hoya /H-ZLftF55 H-ZLftF53 Ohara TAft H-ZLAF52A •S-LAH60 H-ZLAf^B • •H-2LAF52 • 1.80 K-VC89 H-ZLAF56 Sumita TAF'I . yNPHI * Schott ; -'fAC4 .N-LAF2 / -LAua» •H-LAKS4 / 1.70 .^.LAL13, D-LAK6 128 JU.121D-LAK70 •N-UAK22 BACD14 . K-VC79 1.60 • KCSK120 N-SK5* P-SK57 Picosecond pulses for 1.50 N-FJJ51 micro machining processes 1 1 1 1 90 80 70 60 50 40 30 20 Abbe number, V FIGURE 2. The Abbe diagram can be used to categorize the composition and position of moldable giass manufactured at Rochester Precision Optics. The Abbe number, K and index of refraction, n, are based at the Fraunhaufer d line of helium at >. = 587.6 nm.

from $2 to SIO per piece, depending on ultraviolet to near infrared (IR) wave- the material type and production vol- lengths (380 nm to 2.0 [im). Eastman umes. Tooling and parts cost for lenses Kodak and its successor Rochester Pre- at the upper limits (60 mm diameter) cision Optics (RPO) have developed of the process can be two to five times processes that can precisely mold glass- Femtosecond pulses higher, whereas tooling and parts cost es with a T up to 800°C. This results in for 2-Photon microscopy for G&P aspheric optics can be 10 to an even broader material offering for 40 times higher. Further savings from molding, giving the optical designer molding result from less waste, an im- considerable flexibilityan d ease when portant consideration when expensive converting traditional spherical sys- optical glass types are required. tems into higher-performing aspberic The glass molding process for aspher- designs. In doing so, however, the de- ic optics has significantly improved in signer must ensure the lens design uses -micro machining the molded glass refractive-index values -photocathode illumination recent years—largely due to the advances in computer-numerically-controlled that are slightly lower than that of the -seeding amplifiers (CNC) machinery used to manufac- raw glass in strip or block form. These "time-resolved fluorescence ture the tooling and measure the tools "index drop" values can be found in the -flow cytometry and optics. Surface profilometry and glass libraries of most lens design soft- interfcrometric testing of transmitted ware packages such as ZEMAX and -pump-probe spectroscopy Code V (or are readily provided by most -opto-electronic testing and retlected wavefront shapes help to achieve "surface figure" accuracies of PMG manufacturers). -UV generation 1/4 wave (0.16 [4m) or better (see table). The PGM process is not limited to -sync pump dye lasers The PGM process involves the forming small optics. For the right element ge- -OPO pumping of softened glass using heated molding ometry, it can enable economical pro- -THz generation tools of ground and polished silicon- duction of aspheric lenses up to 60 mm carbide and other materials, sometimes in diameter and more than 20 mm with specially coated tooling surfaces, thick. Additionally, IR glasses such as depending upon the softening point, T , chalcogenide that transmit at wave- and the chemical makeup of the glass or lengths as long as 14 [4m are routinely Time-Bandwidth Products AG glassy ceramic used. molded, replacing more expensive dia- Zurich, Switzerland A variety of glass types can be used in mond-turned IR materials such as ger- p: +41-44-445-3120 the glass molding process. Most optical manium, silicon, zinc sulphide, and [email protected] glass companies offer a dozen or more zinc selenide in thermal imaging and moldable glasses with low T (300°C other IR applications. Lens designers in the US/Canada: to 550°C) properties (see Fig. 2). These have provided the following case stud- Time-Bandwidth Products Inc. glasses offer the designer good flexibil- ies to share details of how they devel- p: (413)564-0610 ity and typically transmit in the long oped cost-effective design solutions. saies-usa{5)time-bandwidth .com

72 December 2007 www.laserfocusworld.com Laser Focus World Night-vision goggles Manufacturing tolerances for tem optimization, the aspheric A customer asked an optical molded aspheric lenses elements were reduced to one. designer to optimize a six-ele- reducing the cost even further. ment (four spherical and two Feature Standard Precision Key to this further optimiza- G&P aspheric lenses) night- quality quality tion was an intimate under- vision-goggle objective design. Center thickness ± 0.025 mm ± 0.012 mm standing ofthe capabilities The leading drivers for Diameter and restrictions of the glass night-vision goggle designs .0/-0.030mm +0/-0.010 mm molding process, which are are typically weight and per Surface deviation 5-2 3-1/2 unique to each manufacturer formance. The incorporation Power irregularity and very different from those (fringes) ot the two aspheres allowed of more traditional optical the goggles to have relatively ETD (wedge) 0.05 mm fabrication methods. few elements—an equivalent Axis alignment 5min all-spherical solution would Scratch-dig 60^40 IR LEDs have required an addition- .\ customer hired a designer Index of refraction (nj) ± 0.001 al element to meet the same lo optimize the collimating performance specifications, Abbe number iV,) ± 0.8% optics for a broad-area IR- wbich would have driven the ± 0.015 mm light-emitting diode (IRED) system to an unacceptable with ± 0.5° divergence error, weight. Unfortunately, tbe two I'ortiinalely, the list ot moldabic glass dominated by tbe 0,2 mm size ofthe aspheres were prohibitively expensive materials is continually expanding and LED die. The overall length from front and unsuited for higher-volume pro- extensive. Working with this large se- vertex of lens to the IRED die had to be duction (an eventual goal of the cus- lection, the designer was able to find less tban 6 mm and the outer diameter tomer). The challenge was to dramati- moldable materials roughly equivalent less than 5 mm to fit available space. cally reduce the cost, while at the same to the G&P aspheres and incorporate The size ofthe IRED die and tbe al- time holding (or improving) the per- them into the goggle optics with no in- lowed divergence in tbe coUimated out- formance and preserving (or reducing) crease in system weight and with com- put dictated a lens with a focal length the system weight. parable performance. With further sys- of 12 mm. A simple collimator lens lit

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THE MOLDING PROCESS tion—which called for single-point dia- lens solution based on Hoya E-FDSl glass mond-turned (SPDT) manufacturing. with a refractive index of 1.913. The de- FOR ASPHERIC OPTICS The consultant considered an SPDT sign required modification ofthe sharp HAS IMPROVED DUE aspheric singlet of zinc selenide (ZnSe; and nearly hemispherical 0.61-mm-di- index of 2.52). However, ofthe six ven- ameter concave surface feature and also TO ADVANCES IN CMC dors solicited, only one vendor respond- required a tight tolerance on the center ed—with an extremely high cost and thickness. This option avoided the assem- MACHINERY USED TO painfully long delivery. The diamond- bly complications of an air-spaced dou- MANUFACTURE THE turning process would require a new blet, and eliminated more costly diamond single-point tool to machine the concave turning and expensive ZnSe altogether. TOOLING AND surface feature, which was a steep radius Precision glass molding offered low on aclear aperture just 0.5 mm in diam- tooling cost and rapid delivery compared MEASURE THE TOOLS eter. This justified some ofthe added cost, to ground-and polished or diamond- AND OPTICS. but introduced risk by pushing the SPDT turned approaches. The cost in small manufacturing process beyond its limits. prototype quantities of 100 pieces was within the 6 mm space, but introduced If the designer specified a ZnSe dia- competitive with ground-and-polished a manufacturing problem. The lens de- mond-turned positive-meniscus lens lenses (and far better in production quan- sign would need to be f/2.5 and free element, it would need to be paired tities). Furthermore, the design con- of spherical aberration to satisfy the with a G&P hiconcave spherical lens strained the rear surface to a flat shape, beam divergence rerquirement. The use of N-SF57 (high-index 1.845 optical which simplified the post-molding me- of high-index optical materials would glass). In all, this solution proved more trology while saving time and cost. Over- minimize surface curvature. costly than the original thick singlet all, the PGM process provided lower cost, The designer realized that it would be because of tight lens spacing tolerance, faster delivery, and lower-risk than the impossible to satisfy these IRED colli- added high-precision mechanical com- other strained optics solutions that were mation requirements using conventional ponents costs, and a more complicated on the verge of "unobtanium." Q G&P spherical surfaces at//2.5. At least doublet lens assembly. one aspheric surface would he required Precision glass molding methodology Tell us what you think about this article. Send an to sufficiently eliminate spherical aherra- allowed another look at a viable thick- e-mail to [email protected].

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74 December 2007 www.laserfocusworld.com Laser Focus World