Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
2000-11-17
2003-07-29
Hoffmann, John (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S030100, C065S030130, C065S038000, C065S056000, C065S037000
Reexamination Certificate
active
06598429
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods for fabricating gradient-index rods and rod arrays, particularly gradient-index glass lenses with a radial index profile.
BACKGROUND OF THE INVENTION
The radial gradient refractive index (“GRIN”) glass rod is one of the fundamental optical communication system components. It is widely employed, for example, as an optical beam collimator in optical communication devices to couple signals and pump diode lasers into single mode optic fiber, to convert a diverging beam from a fiber into a collimated beam, and to refocus a collimated optical beam into an optical fiber. I. Kitano, H. Ueno, M. Toyama, “Gradient-index lens for low-loss coupling of a laser diode to single-mode fiber,” Applied Optics, Vol. 25(19), pp.3336, 1986. Most fiber optic devices, from isolators to n×n switches and dense wavelength division multiplexing devices, depend on a collimated optical beam to achieve minimum insertion loss and maximum efficiency. T. Towe, S. Cai, “OEM optical components: Gradient-index lenses make light work for beam directing,” Laser Focus World, Vol. 35, No. 10, 1999. Most necessary processing for the optical signals are carried out in the physical space between the collimators.
Many techniques have been investigated to fabricate radial GRIN materials including ion-exchange, I. Kitano, K. Koizume, H. Matsumura, T. Uchida, M. Furukawa, “A light-focusing fiber guide prepared by ion-exchanged techniques,” J. of Japan Society of Applied Physics, Vol. 39, pp.63, 1970, H. Kita, I. Kitano, T. Uchida, M. Furukawa, “Light-focusing glass fibers and rods,” J. Am. Ceram. Soc., Vol. 54, pp.321, 1971, S. Houde-Walter, “Recent progress in gradient-index optics,” SPIE Vol. 935, 1988; sol-gel, M. Yamane, J. B. Caldwell, D. T. Moore, “Preparation of gradient-index glass rods by the Sol-Gel process,” J. Non-Cryst. Sol., V.85, pp.244 (1986), K. Shingyouchi, S. Konishi, K. Susa, I. Matsuyama, “Radial gradient refractive index glass rods prepared by a sol-gel method,” Elec. Lett., V.22 pp.99, (1986); molecular stuffing, J. H. Simmons, R. K. Mohr, D. C. Tran, P. B. Macedo and J. A. Litovitz, “Optical properties of waveguide made by a porous glass process,” Applied Optics, 18, pp.2732, (1979); diffusion in plastics, D. Hamblen, Kodak and U.S. Pat. No. 4,022,855; chemical vapor deposition, M. A. Pickering, R. L. Taylor, and D. T. Moore, “Gradient infrared optical material prepared by a chemical vapor deposition process,” Appl. Optics, 25, pp.3364, 1986; photochemical, N. F. Borrelli and D. L. Morse, “Planar gradient-index structures,” in Technical Digest, Topical Meetings on Gradient-index Optical Imaging Systems, Kobe, Japan, D1, 1983; as well as neutron irradiation, P. Sinai, Applied Optics, 10, 99, 1971. Commercial radial GRIN rods are mostly fabricated by ion-exchange and sol-gel.
Ion-exchanged cesium or thallium glasses have been widely used for fabricating radial GRIN rods. Cesium (Cs
+
) or thallium (Tl
+
) glasses have many advantages, including excellent optical transparency, but they suffer from many serious drawbacks. The first is the very high toxicity of Cs
+
or Tl
+
ions in the mother glass rod. This is particularly dangerous in the glass melting stage where the temperature is high and the vapor pressures are high. The second is that the selection of the refractive index of the mother glasses is restricted. The refractive indices of the mother glasses are high. A high refractive index will produce a high back reflection. The third is the slow ionic diffusion process. Typically, the glass rod has to be dipped in the molten salt at a temperature around 500° C. for several hundred hours. The maximum diameter of the radial GRIN rod is limited to 3 mm due to the low ionic diffusion process.
The silver ion is sometimes used as an alternative to the thallium ion because it has a higher mobility, S. Houde-Walter, D. T. Moore, “Delta-n control on GRIN glass by additives in AgCl diffusion baths,” Applied Optics, Vol. 25(19), pp.3373, 1986. However, it is difficult to fabricate silver glasses because the silver tends to precipitate out of the glass or form a fine metallic colloid in the glass, R. H. Doremus, “Optical properties of small silver particles,” J. Chem. Phys., V.41, pp.414, 1965. A technique called “ion-stuffing” avoids the difficulty of making silver glass from a batch melt, S. Ohmi, et al, “Gradient-index lenses made by double ion exchange process,” Appl. Opt., V. 27, pp.496, 1988. The technique uses a sodium containing glass and exchanges the sodium for silver in a silver nitrate molten salt. The second step is to immerse the glass rod in a sodium nitrate molten salt and exchange some of the silver ions out near the surface of the glass. So the glass rod has a higher silver concentration at the center than the surface. The major problem for radial GRIN rods fabricated by this process is the photostability. The glass will be colored due to the metal colloid.
The sol-gel method involves the synthesis of a multi-components alkoxide gel, which is shaped by a mold. Since the gel is porous, dopants can be removed or introduced rapidly by immersing the gel in acids for leaching or in a solution containing dopants for introducing. After the diffusion, the gel is dried and sintered to yield a GRIN glass. The sol-gel method has two major advantages over the ion-exchange technique. The first is the rapid transport process. Since the diffusion of leaching is conducted in a porous gel, the transport process is much more rapid than the ionic diffusion process in ion-exchange technique. It means that a larger diameter radial GRIN rod can be fabricated. The second is that a variety of ions can migrate through the gel pores. Bivalent ions can be easily transported through the gel pores. Here, major drawbacks for sol-gel method are that it is difficult to control the index profile and fracture during sintering. Since ions will continue to migrate during the drying and sintering processes, the index profile is difficult to control. During the drying and sintering processes, the outside part consolidates more quickly than the insider part. Thus, gaseous by-products can be trapped in the middle of the material. The thermal expansion coefficient of the inside part will then be higher than the outside part, which causes fracture during sintering process.
There is another type of GRIN material, the axial GRIN material, in which the refractive index varies along optical axis, D. T. Moore, “Design of singlets with continuously varying indices of refraction,” Journal of the Optical Society of America, Vol. 46(7), pp.998, 1971. In one known technique for fabricating axial GRIN glass, disclosed in U.S. Pat. Nos. 5,630,857 and 5,689,374, glasses having different refractive indices are stacked together. These glasses are then thermally diffused at a high temperature to form a monolithic glass blank with a smooth refractive index profile. However, the end surfaces of an axial GRIN must be individually polished to spherical shape in order to focus the optical beam, M. Murty, “Laminated lens,” Journal of the Optical Society of America, Vol. 61(7), pp. 886, 1971, which limits their application. By comparison, radial GRIN material has inherent focusing power since the refractive index varies perpendicularly to optical axis. The end surfaces of radial GRIN rods are flat, which simplifies the fabrication process and increases the compatibility significantly.
Therefore, there is a need for an improved method for forming GRIN optical elements that is safe and efficient, produces clear glass, and produces radial GRIN rods directly.
SUMMARY OF THE INVENTION
The present invention solves the aforementioned problems and meets the aforementioned need by providing a novel method of fabricating a GRIN optical element. In accordance with the invention, a central rod of optical glass having predetermined properties and predetermined outside dimensions is placed inside a tube of optical glass having predetermined properties and predetermined outside dimensions
Jiang Shibin
Lam Philip
BeamTek, Inc.
Birdwell Janke & Durando, PLC
Hoffmann John
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