Optical: systems and elements – Single channel simultaneously to or from plural channels – By surface composed of lenticular elements
Reexamination Certificate
2001-12-21
2003-09-23
Mack, Ricky (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By surface composed of lenticular elements
C216S026000
Reexamination Certificate
active
06624948
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods for molding precision optical glass elements having an optical surface, and more particularly, to methods for molding arrays of precisely aligned glass optical elements at high temperatures with aspheric optical surfaces.
BACKGROUND OF THE INVENTION
Microlens arrays provide optical versatility in a miniature package for communications, display and image applications. Traditionally, a microlens is defined as a lens with a diameter less than one millimeter. From the practical point of view, lenses having a diameter as large as five millimeters are also considered microlenses. For many applications, microlenses formed on the ends of optical fibers are employed to couple light from sources such as laser diodes to the fiber.
The use of microlenses in the form of arrays stems from the demand from the end users to work with information in parallel. The technologies of the semiconductor industry, including MEMS (micro electromechanical systems), lend themselves to the formation of arrays.
Microlenses can include diffractive or refractive functions, or the combinations thereof for athermal or achromatic elements. In fact, the benefits of refractive lenses, including achromatism efficiency and high numerical apertures, make them the most attractive for communication applications.
As individual elements, microlenses can have a wide range of parameters. Diameters can range from a few microns to a few millimeters. Their focal ratios, that is, the ratio of focal length to lens diameter, can range from f/0.8 to infinity. The optical surface can be either spherical or aspherical. Microlenses can be made from a variety of materials such as plastics, glasses and exotic materials like gallium phosphide.
Design issues for discrete microlenses and microlens arrays are very similar to those of conventional large lenses, so the rules of optics still apply. Since the apertures of microlenses are so small, diffraction effects are more dominant than refraction effects. The most common fabrication techniques for microlens arrays include direct etching of the lens profile using photolithographic masks or contact masks, diffusing materials with different refractive indices into a substrate, swelling defined areas of a substrate, and forming and solidifying drops of liquid having desirable refractive index on a surface.
Manufacturing specifications and tolerances for micro optical arrays are governed by the specific application and defined by the end user accordingly. For example, the typical focal length variation across an array comprising microlenses having diameters in the range of from 0.1 mm to 1.0 mm for communication use is 1 to 2 percent. The cumulative pitch tolerance from one microlens to another must be less than 1 &mgr;m. The optical surfaces are specified as plano/convex asphere. The surface figure requirement is better than &lgr;/8 at where the wavelength (&lgr;) is 632 nm, and the center thickness tolerance of the array must not exceed 2 &mgr;m.
The stringent specification for the microlens arrays for telecommunication application makes it necessary to synchronize a web of manufacturing technologies to attain the final goals. The manufacturing process can be broadly divided into three basic steps. These steps are originating the shape of the lens, creating a master mold, and finally forming the lens profile on the surface of the selected optical material. The origination of the shape generally comprises photolithography technology to create a mask for reactive ion etching.
Another commonly used manufacturing technique is to reflow a photoresist. This method comprises coating a substrate with a selected photoresist, exposing it to LN radiation through a mask, or alternatively, subjecting the photoresist to gray scale laser exposure. Upon heating the substrate, the exposed photoresist melts and surface tension pulls the material in the form of convex lenses. The depth of the photoresist determines the focal length of the lens.
Ion exchange is another method, which has been used for some time to manufacture microlenses. Ions are diffused into a glass rod to give a radial refractive index distribution which guides the light and that forms a focus. The index of refraction is highest in the center of the lens and decreases quadratically as a function of radial distance from the central axis. Microlenses made using this ion exchange technology are widely used to collimate light from fibers as, for example, in telecommunications. As applications warrant larger and larger arrays of channels, users are moving away from discrete microlenses towards microlens arrays.
Depending on the application, microlens materials may vary. For high volume applications using visible light, it is desirable to mass produce plastic optics using an injection molding process. One advantage of injection molding is that high-resolution molding technique can mold the optical element as part of the system casing. This method is very cost-effective because the labor associated with alignment and assembly is eliminated.
The optimal transmission wavelength for telecommunication is in the far infra-red wavelength, which is around between 1300 and 1550 nm. Therefore, the materials that work in this wavelength region are becoming more important. The two most common materials are fused silica and silicon, both of which have advantages and disadvantages as well for this application. Other optical quality materials are being tested and considered.
As mentioned earlier, the applications for microlenses are very broad. The primary use of microlenses in telecommunication is to match light from free space into fibers and to collimate light coming out of fibers. The microlens will require a numerical aperture that matches the fiber and a diameter of about 1 mm so that the diameter matches the free space beam. The microlenses are used in individual channels, although they are normally arranged in arrays of channels in 1×8, 1×12, 10×10, or even higher configurations. Some of the larger free-space devices are now using more than 1000 channels.
The manufacturing process for the production of glass microlens arrays generally involves reactive ion etching (RIE) of fused silica. RIE of fused silica is a relatively standard technology but, fabrication of microlens arrays having the stringent specifications dictated by the telecommunication industry is by no means an easy or routine task. It is very difficult to meet all the requirements of microlens arrays using this technology. This technology also involves many steps before the final product is produced and consequently the yield is very poor and the products are not cost competitive.
Compression molding of optical quality glass to form microlenses is also well known. This method comprises compressing optical element preforms, generally known as gobs in the art, at high temperatures to form a glass lens element. U.S. Pat. No. 3,833,347 to Angle et al, U.S. Pat. Nos. 4,139,677 and 4,168,961 to Blair et al, U.S. Pat. No. 4,797,144 to DeMeritt et al, and U.S. Pat. Nos. 4,883,528 and 4,897,101 and U.S. Pat. No. 4,929,265 to Carpenter et al described the basic process and apparatus for precision glass molding of optical elements. These patents disclose a variety of suitable materials for construction of mold surfaces used to form the optical surfaces in the molded glass optical elements. In the compression molding process described in the above patents, a gob is inserted into a mold cavity. The molds reside within an oxygen-free chamber during the molding process. The gob is generally placed on the lower mold and heated above the glass transition temperature (Tg) and near the glass softening point so that the viscosity of the glass is within 10
6
and 10
9
poise. The upper mold is then brought in contact with the gob and pressure is applied to conform to the shape of the mold cavity. The molds and the molded lens are then allowed to cool well below Tg and the pressure on the molds are relieved and the lens is removed.
Carlton Donn B.
Ghosh Syamal K.
Miller Darryl E.
Bocchetti Mark G.
Eastman Kodak Company
Mack Ricky
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