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.
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 xcexcm. The optical surfaces are specified as plano/convex asphere. The surface figure requirement is better than xcex/8 at where the wavelength (xcex) is 632 nm, and the center thickness tolerance of the array must not exceed 2 xcexcm.
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 1xc3x978, 1xc3x9712, 10xc3x9710, 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 106 and 109 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.
The method described above works perfectly well when molding a discrete lens from a single cavity mold from one preform or gob. Press molding an array of microlenses using one or more preforms is subject to many difficulties which include alignment of mechanical and optical axes of each lens element with respect to a common axis, and location of each lens element with respect to a reference point in the array. Furthermore, it is extremely difficult to machine convex aspheric mold cavities using conventional techniques if the microlens diameter is less than 1 mm. U.S. Pat. No. 6,305,194 B1 to Budinski et al. teaches a method and apparatus for compression molding a glass microlens array from a single gob. Careful control is required to make the gob flow uniformly over the entire mold surface without trapping any air or gas molecules in the individual mold cavities. If there is variation of temperature across the mold surface, the glass flow may be non-uniformn and leave flow marks on the lens elements. More importantly, this invention relies on making negative mold surfaces using conventional machining techniques, which is practical if the lens diameter is greater than 1 mm.
Consequently, there is a need for an improved method of forming microlens arrays which may not involve the conventional MEMS or RIE techniques, but a novel compression molding process.
It is therefore an object of the present invention to provide a method for compression molding integrated microlens arrays of precisely aligned glass optical elements and the microlens arrays molded therewith.
It is a further object of the present invention to provide a method for compression molding linear or two-dimensional arrays of microlenses.
It is another object of the present invention to provide a method for compression molding linear or two-dimensional arrays of microlenses wherein the individual lens elements are in registration with respect to a reference point or each other.
Still another object of the present invention is to provide a method for compression molding linear or two-dimensional arrays of microlenses which utilizes a mounting structure in the molding operation that forms a permanent part of the integrated molded microlens arrays.
Yet another object of the present invention is to provide a method for compression molding linear or two-dimensional arrays of microlenses which uses discrete glass preforms for each individual lens element in the array.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a reading of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by providing a method wherein an array of optical elements is formed by forming a mount, including a plurality of element receiving bores therethrough arranged in a predetermined array; placing the mount on a lower mold surface of a lower mold; inserting a glass preform in each of the plurality of element receiving bores, each glass preform extending through a respective element receiving bore, each glass preform extending beyond a top surface of the mount; heating at least the glass preforms to at least a glass transition temperature thereof, aligning an upper mold having an upper mold surface with a plurality of desired optical features formed therein with the predetermined array; and pressing the glass preforms between the upper mold surface and the lower mold surface to mold the glass preforms into the array of optical elements permanently retained in the mount, the pressing of the glass preforms causing glass from each glass preform to flow generally radially outward therefrom across the top surface of the mount thereby creating an upper flange on each element that aids in retaining each optical element in the mount. The lower mold surface is also configured to impart to the discrete glass preforms the desired optical features to the bottom surfaces thereof. Each preform includes optical quality top and bottom surfaces prior to compression molding. Typically, the optical quality surfaces are provided by well-known polishing operations. The mount is preferably made from a ceramic material selected to have a coefficient of thermal expansion that approximates the coefficient of thermal expansion of the glass preforms in order to prevent any distortion of the final product.
The ceramic mount and the lower mold are preferably configured to create a chamber or plurality of chambers between a bottom surface of the mount and a lower mold surface with each glass preform extending through the chamber to rest on the lower mold surface. When this chamber is present, the method also includes the step of causing glass from each glass preform to flow generally radially outward therefrom in the chamber(s) thereby creating a lower flange on each optical element that aids in retaining each optical element in the mount.
After the array of lens elements has been molded into the mount, it is then cooled to below the glass transition temperature. Then the array of lens elements, now permanently retained in the mount, is removed from between the upper mold and the lower mold. In the practice of the method of the present invention, the apparatus chamber in which molding is preformed is environmentally controlled. A vacuum may be drawn or the chamber may be purged with a non-reactive gas-like nitrogen or argon to promote an oxygen-free environment.
Through the practice of the method of the present invention, microlens arrays can be produced wherein the arrays include discrete lens elements having diameters less than 1 mm. Further, imperfections or artifacts that can be created by glass flow when an entire array is formed from a single preform are eliminated because the discrete glass preforms used in the practice of the present invention are not forced into a semi-viscous flow situation.