Gel-Silica Optics

8 Gel-Silica Optics

Silica optics are preferred for many optical systems because of the characteristics listed in Table 8-1. The sol-gel-derived process for making silica optics described in previous sections achieves all the features listed in this table. In addition, ultrastructure processing results in an optical component with a final net shape and, with appropriate molds, a surface with unique optical features.

Table 8-1. Features of Silica Optics

.

3. 4. 5. 6. 7. 8.

9. 10.

Excellent optical transmission from the ultraviolet (170 nm) to near infrared wavelengths (3400 nm). Excellent refractive index homogeneity. lsotropic optical properties. Small strain birefringence. Very low coefficient of thermal expansion of about 0.55 x 106/~ Very high thermal stability. Very high chemical and environmental durability. Small number of bubbles or inclusions. Ability to be polished to high standards. Index of refraction match with silica core fiber optics.

80

Gel-Silica Optics

81

There are four primary methods of manufacturing silica optics, Table 8-2, based upon Bruckner. [128] The first two processes involve melting naturally occurring quartz crystals at high temperatures. The resulting materials are termedfused quartz. Deficiencies of fused quartz optics can include substantial amounts of cation impurities (Type I), hydroxyl impurities (Type II), inhomogeneities, seeds, bubbles, inclusions, and microcrystallites. [128] The relative extent of these defects depends on the grade of fused quartz. The higher the grade, the fewer the defects, but the higher the cost and the more difficult the production of large optics. Table 8-2. Methods of Silica Optics Manufacture Fused Quartz

Type I

Electric melting of natural quartz crystals heat

Si02(quartz) Type II

vacuum

Si02(glass)

Flame fusion of natural quartz crystals heat

Si02(quartz)

oxyhydroge~-

Si02(glass)

Fused Silica

Type III

Vapor-phase hydrolysis of pure silicon tetrachloride carried out in a flame SiCI4 + 02

Type IV

~ SiO2(glass) + 4HCI

Oxidation of pure silicon tetrachloride, which is fused electrically or by means of a plasma SiCI4 + 02

~ SiO2(glass) + 2C12 Gel-Silica

Type V

Gelation ofalkali silicate colloidal solutions with full densification (1500 to 1720~ or Hydrolysis and condensation of an alkoxide precursor with full densification of(p = 2.2 g/cc) at 1150 to 1350~

Si(OR)4 + 4H20 Si(OH)4 Type VI

~ Si(OH)2 + 4(ROH)

~ SiO2 (dense gel-silica) + 2(H20)

Hydrolysis and condensation of an alkoxide precursor with partial densification (p= 1.6 to 2.0 g/cc) at 800~ to 1200~ Si(OR)4 + 4H20 Si(OH)4

~ Si(OH)2 + 4(ROH)

~ SiO2(porous gel-silica) + 2(H20)

82

Sol-Gel Silica

Types III and IV are termed syntheticfusedsilica. The cation impurity content of fused silica optics is substantially lower than fused quartz optics due to the higher purity ofthe raw materials. [8~ However, the chemical reactions indicated in Table 8-2 for these types of silica may not go to completion. Consequently, water contents up to a few thousand parts per million can be present in Type III silica. Also, C1 ion contents of a few hundred parts per million can be retained as an unreacted residue in both types of fused silica and, in particular, Type IV. Other defects are somewhat lower for fused silicas than for fused quartz. The chemical processes involved in production of Types I, II, III and IV silicas makes the manufacture of near net shape or net surface optics impossible. Price increases considerably as the quality of fused silica and the size of the optics increase.[ 129] The price of precision optics also depends greatly on the extent of grinding and polishing required. As much as 80% ofthe final cost can be due to finishing because ofthe intensive hand labor involved. [129] The economic consequences of high finishing costs has been a shift of supply to third world sources where labor costs are low. [129] However, as the photonic field moves towards smaller optical devices, traditional grinding and polishing methods become less and less viable. Plastic components which can be injection molded or formed by pressing to net shape have become attractive even though they have poorer properties. For example, the change in refractive index with temperature (An/AT)of optical plastics is 6 to 50 times greater than glass. [13~ The coefficient of thermal expansion of optical plastics is also 10 times higher than optical glass and 60 times higher than optical silica. [13~ Thus, net-shape, net surface processing of optical silicas is a major industrial need. Chemically based sol-gel processes are now used to manufacture Types V and VI silicas as described in Table 8-2. [119] These processes offer the potential for improving many features of silica optics listed in Table 8-1. Additional advantages over Type I to IV silicas are also possible, as described in Table 8-3. Successful processing controls have been achieved for alkoxidederived gel-silica optics for dimensions from 1 mm optics to 100 mm (Fig. 81). Results from optical property measurements of Type V gel-silica show no evidence of bubbles, no striae, a superior index ofrefraction, homogeneity of about 1-6 x 10"6, and very low strain birefringence 0f4-6 nm/cm (Table 8-4). [80][82][129][131][132] These characteristics of alkoxide-derived sol-gel silica (Gelsil | are equal or superior to the common Types I to IV optical silicas.j80][132]

Gel-Silica Optics Table 8-3. Advantages of Gel-Silica Optics

Net ShapWSurface Casting Complex geometries Lightweight optics Aspheric optics Surface replication (e.g., Fresnel lenses) Binary/diffractive optics Internal structures Reduced or no grinding Reduced or no polishing Net shape optics Net surface optics

Improved Physical Properties (Type V) Lower coefficient of thermal expansion Lower vacuum ultraviolet cutoff wavelength Higher optical transmission No absorption due to H~O or OH bands Lower solarization Higher homogeneity Fewer defects

Transparent Porous Structures (Type VI) Impregnation with organic polymers Graded refractive index lenses Laser-enhanced densification Controlled chemical doping Control of variable oxidation states of dopants

83

84

Sol-Gel Silica

Figure 8-1. Examples of alkoxide-derived gel-silica (Gelsil| optical components made commercially. (Photo courtesy of Geltech, Inc.) T a b l e 8-4. Specifications o f Gelsil | High Purity Silica Wavelength Range CTE Density Abbe Constant (ve) Knoop Hardness

0.17 to 3.4 ~tm 0.52 • 10-6 crn/cm~ 2.2 g/cm3 67.6 300g load, kg/mm2= 520 100g load, kg/mm2= 545

Bubbles & Inclusions Class 0 as low as 3 x 10.6 Homogeneity as low as 0.1 nm/cm Strain Infractive Index at Various Wavelengths (~tm) 1.464 @ 0.4800 (Blue Cd) 1.461 @ 0.5461 (Hg) 1.459 @ 0.5896 (Na) 1.457 @ 0.6438 (Red Cd) Gelsil| registered U.S. Trademark of Geltech, Inc., Orlando, FL 32826

Gel-Silica Optics

85

An incentive for low-temperature, chemically based processing is achieving a higher level of purity than traditional glass and ceramicprocessing methods. Alkoxide gel-silica glasses have very few cation and hydroxyl impurities. An important consequence of the elimination of impurities is the improvement of transmission throughout the optical spectrum. Figure 8-2 compares the ultraviolet optical transmission ofcommercial ultraviolet (UV)-grade optical silica (Type III) with a typical spectrum from an alkoxide-derived gel-silica (Type V). [82] The vacuum UV cutoffwavelength is substantially improved for the gel-silica material for reasons discussed in Ch. 5.

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Optical

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silicas.

Elimination of OH radicals from gel-silica optics also results in elimination of absorption bands in the near infrared, as shown in Fig. 8-2. This figure shows the difference between Type V gel-silica and Type III silica which exhibits absorption bands at 1400 and 2200 nm and a very broad absorption band at 2730 nm. A very low coefficient of thermal expansion (CTE) is an especially important physical characteristic of optical silica. The alkoxide process leads to production of a Type V silica having a lower CTE than other types of silica. [8~ Figure 8-3 compares the CTE values and temperature dependence of Type V silica (Gelsil | optics with the NIST silica reference and Types III and IV commercial silicas over the temperature range of 25 to

86

Sol-Gel Silica

700~ The alkoxide sol-gel optical silica has a very low value of CTE throughout most of the temperature range. ,

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Figure 8-3. Comparison of coefficient of thermal expansion of sol-gel derived silica (Type V Gelsil| with other commercial silicas.

One of the most important uses of the alkoxide sol-gel process is to produce surface feature optics such as binary optics, sinusoidal gratings, and high-fill factor micro lens a r r a y s . [132][133] Table 5 lists some of the optical devices made using the gel-silica process. The concept of casting to final shape and surface presents a great challenge due to the high level of shrinkage which occurs in sol-gel processing, as shown earlier in Chs. 3 and 4. Mold design is based on a multitude of data collected on the shrinkage of the gel during the process as a function of its shape and its dimensions (diameter, thickness, radius of curvature, etc.). Shrinkage data is used to build a model which reduces the number of iterations needed to achieve the production of parts with the targeted dimensions and tolerances. Every optical component requires the design of a custom mold and adoption of the sol-gel process to meet the requirements of that design, but these operations are performed only once and their costs are amortized over a large volume of optical parts. A schematic ofthe replication process developed byNogues et al. [133] which can be used for large volumes ofproduction is shown in Fig. 8-4. First, a micro-optic design (A) is used to design and manufacture a master mold (B).

Gel-Silica Optics

87

The design of the master mold compensates for shrinkage and geometry changes that occur during the sol-gel process. The master can be fabricated using single point diamond turning, as discussed below. T a b l e 5. Gel-Silica Optical Product Development Programs and Applications* Laser Densified Porous Glass for Light Waveguide Applications Micro-Lens Arrays for Laser Diode Systems Fresnel Lenses for Space Systems Radiation-Hard 100% Silica Optical Fiber Waveguides Sol-Gel Glass Prismatic Cover for Photovoltaic Cell for Space Applications Binary Elements for Diffractive Optics Applications Pure Silica Gratings for High-Powered Laser Applications Optical Components with Internal Cavities for Flow Cell Use GRIN Lenses for Laser Diode, Fiber Optic, and Camera Applications Side-Fire Laser Scalpel for Fiber-Optic Surgery Hybrid Optics for Two Color Correction Complex Aspheric Lens Porous Gel-Silica Matrices for Toxic Gas Detection *Courtesy of Geltech, Inc., Orlando, FL 32826

In a second step, the active surface (C) to be incorporated into the complete mold system (D) is manufactured by using plastic forming techniques such as compression molding or low pressure injection molding. The completed mold (D) is filled by casting the alkoxide-base sol, described in Ch. 2, which leads to the production of the optical component (E) after gelation, drying and stabilization, and densification. Accuracy of mold replication is in the submicron range, as described below. A recent study by Zhu etal. demonstrated that replication of diffractive and Fresnel optics produces full density Type V silica lenses as good as their parent plastic Fresnel lenses from which they are molded. [134] The 38 mm gelsilica Fresnel lenses were made by the sol-gel replication process shown in Fig. 8-4. Surface profiles of the lenses were obtained with a threedimensional optical profilometer/interferometer (Zygo Corporation).

88

Sol-Gel Silica

A) Micro-opticDesign B) Micro-opticMaster

[

C) PlasticRepLicaof Master L ......... ....i............................ ........7._..........

D),,Sol-Gel,MoldAssembly,,

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E) DenseSol-GelMicro-optic Figure 8-4. Replication process for mold fabrication. Figure 8-5a shows the central portion ofthe active surface ofthe plastic lens having a positive aspheric curvature and a facet width of approximately 0.25 mm. A statistical analysis indicates that the average facet width is 0.251 + 0.004 mm. This statistical analysis was made on a total of 28 facet width readings from two cross sectional profiles of the active surface tested. The facet angles for the first four facets counting from the center of the active surface are given in Table 8-6. For comparison, the same type of surface profiles are plotted in Fig. 85b for the glass Fresnel replica having a negative aspheric curvature. Note that the test surface of the glass lens has well-defined profiles with a facet width of approximately 0.10 mm. A statistical analysis based on 36 measurements indicates that the facet width ofthe glass Fresnel lens is 0.101 + 0.002 mm, and that the standard deviation is only half of that for the mold active surface. This reduction in the facet width tolerance is due to the shrinkage occurring in the processing and is one of the advantages of this sol-gel process.

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Gel-Silica Optics

91

Table 8-6. Comparison of Facet Angles for Active Surface and Silica

Fresnel Lens. Active Surface

Facet Number

4

Silica Replica

Facet Angle (degree)

Std. Dev. (degree)

Facet Angle (degree)

Std. Dev.

Variance

(degree)

(degree)

0.263

0.007

0.264

0.010

0.001

0.446

0.003

0.476

0.027

0.030

0.672

0.013

0.677

0.021

0.005

0.904

0.018

0.890

0.031

-0.014

Table 8-6 shows that the first four facet angles of the glass Fresnel replica are within 0.033 ~ofthe facet angle ofthe active surface, which is well within the tolerance of 0.25 ~given by Fresnel Optics, Inc., the supplier ofthe active surface. The greatest significance of these results is the fact that the facet angle in the replica is essentially the same as that of the parent active surface. This proves that the shrinkage of the small surface features which occurs during processing is very uniform in all three dimensions, making accurate replicas of these types of features possible. The 0.5 micron curvature at the bottom of the plastic active surface (Fig. 8-5a) was accurately replicated in the glass Fresnel lens (Fig. 8-5b). Also, note the high fidelity in the Y-direction with the height of the surface profile of the replicated glass Fresnel lens in the micron range, as shown in Fig. 8-5b. This replication capability of the surface features at micrometer levels of fidelity is one of the major advantages of the sol-gel molding process which can be used to produce micro-optic elements such as microlens arrays, gratings, binary optical elements, and total internal reflection optics. Table 8-7 compares the optical performance ofboth the active surface and the replica of the Fresnel lenses. As expected, the silica Fresnel lens replica is an inverted (negative) lens with smaller dimensions in comparison to the original active surface. These data, together with modulation transfer functions of the lenses, show that the quality of the silica Fresnel lenses produced by replication via sol-gel processing of an active surface is very high. Consequently, the gelsilica process can be used for low-cost manufacture of glass with surface feature optics, such as Fresnel lenses, and other diffractive optical elements, gratings, microlens arrays, and total internal reflection components.

92

Sol-Gel Silica

Table 8-7. Focal Length and f-Number of the Active Surface and Silica

Fresnel Replica Focal Length ( m m )

Diameter( m m )

./=Number

Active Surface

104

101.6

1.02

Silica Replica

52

38.1

1.36

Another recent study by Bemacki et al. {135] has demonstrated that single point diamond turning (SPDT) is an excellent means for producing the master molds (A in Fig. 8-4) required for mass production of gel-silica hybrid optics. This is an important development because: 1. Recent combinations of diffractive and refractive functions in the same optical component (hybrid optics) allow designers additional opportunities to make systems more compact and enhance performance. 2. The complementary dispersive nature of refractive and diffractive optics can be used to render two-color correction in a single hybrid optical element. 3 Techniques previously suitable only in the infrared can now be applied to components used at visible wavelengths. 4. Combining diamond turning and sol-gel processing offers a cost-effective method for producing highly customized and specialized optical components in high quality silica glass. With the sol-gel casting method of replication, diamond-turned mold costs can be shared over many pieces. 5. Diamond turning takes advantage of all of the available degrees of freedom in a single hybrid optical element: aspheric surface to eliminate spherical aberration, kinoform surface for control of primary chromatic aberration, and the flexibility to place the kinoform on nonplanar surfaces for maximum design flexibility.

Gel-Silica Optics

93

In this experiment, performed at the Oak Ridge National Laboratory and Geltech, Inc., the mold master for a hybrid lens that offers two-color correction was made in the following manner. A layer ofelectroless nickel, approximately 4-6 mm, was deposited onto a stainless steel mold pin. A master profile, scaled up to account for the sol-gel shrinkage, was cut into its surface with SPDT. A polycarbonate negative of the profile was made by injection molding and the alkoxide-based sol was cast into the negative mold, as depicted in Figs. 2-3 and 8-4. Comparison ofthe results from each stage offabrication of a gel-silica (Gelsil | lens from a SPDT master is shown in Fig. 8-6. A region near the edge ofthe mold pin, injection mold, and Gelsil | hybrid lens, was probed with an optical surface profiler in each case. There is some loss of fidelity in the injection mold fabricated from the SPDT mold pin, but the sol-gel cast and densified lens is quite faithful and scaled to 40% of the size of the injection mold. The large isotropic shrinkage that occurs during casting also lessens the effects of SPDT tool marks, extending the use of SPDT to the fabrication of optics for visible bandwidth applications. The third example of use ofultrastructure sol-gel processing to make net-shape-net-surface optics is a side-fire laser tip, illustrated in Figs. 8-7 and 8-8, for endoscopic surgery. [136] The gel-silica laser tip is a compound lens/ mirror, 1.8 to 2.8 mm diameter, which aims and focuses the laser beam. One end ofthe gel-silica cylinder features an as-cast molded-in convex lens. The opposite end of the component is an as-cast beveled mirror, silvered on the outside. Laser light exits the optical fiber, passes through the lens, traverses the cylinder, strikes the mirror, and, depending on the design, is reflected at angles ranging from 3 to 167 degrees out of the cylinder (Fig. 8-7). A second lens can be formed into the side ofthe cylinder at the beam's exit point. Shapes can be molded that are impossible to grind and the sol-gel method needs little post-processing. Surgeons vary the focus by adjusting the distance from the tip to the target. At an interval of 1.0 to 1.5 mm the fiber optic tip provides a focused beam 1.5 times the fiber core diameter. Outside these dimensions, itproduces a defocused beam for coagulation or ablation.

94

Sol-Gel Silica 1.5 1 r

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100

150

200

Radial Coordinate (microns) Figure 8-6. Cross sectional traces with an optical surface profiler showing kinoform near the edge of the mold pin (top), polycarbonate injection mold (center), and Gelsil | hybrid singlet (bottom). The sag of the base surface has been removed to highlight the kinoform facet details. Lateral resolution for the mold pin and injection mold was 4.4 ~tm/sample, while that for the Gelsil | lens was 2.2 ~tm/sample.

Gel-Silica Optics

,,

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95

,

SIDE-FIRING RBER OPTIC TIP SLEEVE INTERNAL THREAD

BUFFER LAYER \ SLE~IE

ROD-MIRROR-LENS

LASER ENERGY INPUT

/OPTICAL FIBER LINK

t BLC~O VESSEL

OPTICAL RBER

ADJUSTABLE DISTANCE FOR BEAM FOCUSING

Figure 8-7. Endoscopic fiber-optic end-effector, the RTI Laser Sweep Tip, provides a variable-focused beam at oblique angles. Threaded attachment method could find use as a connector for fiber-optic communication lines.

96

Sol-Gel Silica

(a)

(b) Figure 8-8. (a) Gelsil | net shape side fire laser tip. (b) Laser light being focused by side fire lens made by net shape lasting of gel-silica (Gelsil|