Abstract:
A molded glass lens is taught that includes a molded two-dimensional reference surface at a first end of the lens body, a first molded optical surface that is longitudinally displaced from the two-dimensional reference surface, and a molded second optical surface at a second end of the lens body. The first and second optical surfaces may be plano, convex or concave. The molded two-dimensional reference surface is planar and preferable annular. By physically locating the lens with the molded two-dimensional reference surface and one of the first or second optical surfaces, the lens can be held in a given orientation. Thus, the molded reference surface at the end of the cylindrical body allows for accurate and safe capture, positioning, handling, and placement for subsequent finishing operations, allowing for the creation of one or more additional lens datums.

Description:
FIELD OF THE INVENTION  
         [0001]    This present invention relates generally to glass optical elements, and more particularly to molded glass optical elements with datum(s) formed therein in the molding process that decrease the difficulty of subsequent manufacturing steps.  
         BACKGROUND OF THE INVENTION  
         [0002]    Various methods and apparatus for the compression molding of glass optical elements are known in the prior art. With these methods and apparatus, optical element preforms sometimes referred to as gobs are compression molded at high temperatures to form glass lens elements. The basic process and apparatus for molding glass elements is taught in a series of patents assigned to Eastman Kodak Company. Such patents are U.S. Pat. No. 3,833,347 to Engle et al., U.S. Pat. No. 4,139,677 to Blair et al., and U.S. Pat. No. 4,168,961 to Blair. These patents disclose a variety of suitable materials for construction of molds used to form the optical surfaces in the molded optical glass elements. Those suitable materials for the construction of the molds include glasslike or vitreous carbon, silicon carbide, silicon nitride, and a mixture of silicon carbide and carbon. In the practice of the process described in such patents, a glass preform or gob is inserted into a mold cavity with the mold being formed out of one of the above mentioned materials. The molds reside within a chamber in which a non-oxidizing atmosphere is maintained during the molding process. The preform is then heat softened by increasing the temperature of the mold to thereby bring the preform up to a viscosity ranging from 10 7 -10 9  poise for the particular type of glass from which the preform has been made. Pressure is then applied to force the preform to conform to the shape of the mold cavity. The mold and preform are then allowed to cool below the glass transition temperature of the glass. The pressure on the mold is then relieved and the temperature is lowered further so that the finished molded lens can be removed from the mold.  
           [0003]    Molded glass lenses may be manufactured with upper and lower molds residing in a cylindrical mold sleeve (U.S. Pat. No. 5,718,850 to Takano et al.). In such a process the final molded lens element is typically cylindrical (and circular in cross-section). The diameter and concentricity of the cylinder are critical to subsequent handling, positioning and mounting operations. Therefore, it has been necessary to control the diameter of the cylinder either during molding, or during a subsequent grinding operation. Controlling the diameter during molding is difficult. Although a cylindrical mold sleeve produces a lens with a well-constrained outer diameter, molding tool life can be decreased due to a variety of factors. One contributor to decreased molding tool life is variability in the volume of the preforms. The preforms are the glass material (usually in the shape of a sphere) from which the lenses are molded. If the preform volume is slightly larger than the mold cavity, the excess glass can exert excessive force on the cylindrical sleeve during molding. It can also become difficult to remove the lenses from the cylindrical sleeve after multiple molding cycles.  
           [0004]    Grinding a specified outer diameter on a lens after molding is often referred to as centering. As lens elements become smaller it becomes increasingly difficult to accurately center such lens elements as well as to position and align such elements in subsequent assembly operations.  
           [0005]    A lens geometry is needed which allows for accurate centering, handling, positioning, and mounting operations and that does not rely on the accuracy of the outside diameter of the cylindrical body of the lens as molded.  
         SUMMARY OF THE INVENTION  
         [0006]    It is therefore an object of the present invention to provide a molded lens having a geometry that allows for accurate centering, handling, positioning, and mounting operations after molding.  
           [0007]    It is a further object of the present invention to provide a molded lens having a geometry that does not rely on the accuracy of the outside diameter of the cylindrical body of the lens for post molding operations.  
           [0008]    Yet another object of the present invention is to provide a molded lens having a geometry that is not critically dependent upon optical element preform volume for the creation of a reference surface.  
           [0009]    Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by providing a molded lens that includes a molded two-dimensional reference surface at a first end of the lens body, a first molded optical surface that is longitudinally offset from the two-dimensional reference surface, and a molded second optical surface at a second end of the lens body. The first and second optical surfaces may be plano, convex or concave. The molded two-dimensional reference surface is an annular plano surface. The lens body (that portion of the lens that is between the second optical surface and the molded two-dimensional reference surface or the molded two-dimensional reference surface, and outside the diameters of the second optical surface and the molded two-dimensional reference surface) may be allowed to partially or fully free-form during molding or may be constrained during molding to provide a generally cylindrical shape to the lens body. If the lens body is allowed to free-form, it is subsequently subjected to a grinding operation to yield a generally cylindrical shape. Whether the generally cylindrical shape of the lens body is accomplished by molding or grinding, the generally cylindrical shape may include an additional datum surface(s) formed therein. Also, the molded lens of the present invention may include a molded three-dimensional reference surface at the second end of the lens body. If the molded lens includes a molded three-dimensional reference surface at the second end of the lens body, that reference surface will be interrupted by the second optical surface. The first and second optical surfaces are designed to image light from an object point to an image point. The molded two-dimensional reference surface is of a specified shape and location with respect to the first and second optical surfaces. By physically locating the lens with the molded two-dimensional reference surface and one of the first or second optical surfaces, the lens can be held in a given orientation. Thus, the molded reference surface(s) at the end(s) of the cylindrical body allow for accurate and safe capture, positioning, handling, and placement for subsequent finishing operations, allowing for the creation of one or more additional lens datums. These finishing operations can include, but are not limited to, grinding, polishing and cutting. These functions of capture, positioning, handling, and placement for subsequent operations can be performed using a centering cup that engages the molded reference surface(s) at the end(s) of the cylindrical body thereby allowing subsequent operations to be performed without reliance on the outside diameter of the lens body.  
           [0010]    As mentioned above, the first molded optical surface is longitudinally offset from the two-dimensional reference surface. That is, the first molded optical surface is positioned along the cylindrical or optical axis of the lens but the two-dimensional reference surface and the first molded optical surface reside at different distances from the second optical surface. The offset may be such that the first molded optical surface is closer to or further from the second optical surface as compared to the molded two-dimensional reference surface. In other words, the offset may take the form of an axial recess or an axial projection. In fact, the offset may be simultaneously a partial axial recess and a partial axial projection.  
           [0011]    The lens of the present invention can be made with an angled plano optical surface, a convex optical surface and a lens datum. This lens datum can then be used for subsequent processing operations (such as grinding) to add additional datums to the lens. These additional datums can be placed in a precise location with respect to the optical axis of the lens element. One of the additional lens datums can be a cylindrical surface that enables mounting of the lens either in a V-groove type structure or in a precise tube. In addition, the lens can be molded without the need for a precisely controlled cylindrical preform. The lens can be centered using existing centering equipment. Because the lens does not have to be held on the optical plano surface, it reduces the chances of scratching this surface. Scratches can lead to scatter and reduce the light transmitted by the lens. This is particularly important for situations where the beam diameter of the light directed onto the plano optical face may be only 50 microns such as those optical elements used in conjunction with optical fibers. In such an instance, a scratch of only a few microns in size could cause a measurable decrease in the amount of light transmitted by the lens to the receiving fiber.  
           [0012]    Either as a result of a post-molding grinding operation or as result of the molding operation itself, the lenses of the present invention typically will be “generally cylindrical.” Further, such lenses will typically be circular in cross-section. However, there may be lens applications where it is beneficial to form generally cylindrical lenses which, in cross-section perpendicular to lens axis, are not circular (e.g. elliptical). Thus, the term “generally cylindrical” as used herein is intended to include cylindrical lenses that may or may not be circular in cross-section. In addition, the term “generally cylindrical” as used herein is intended to include those lenses which have datum(s) formed in the cylindrical surface thereof such as, for example, flat datum(s) and recessed datum(s) as will be discussed in more detail hereafter. Therefore, the formation of such datum(s) in the cylindrical surface of a lens will not remove such lens from the definition of “generally cylindrical” as used herein. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a perspective view of an exemplary lens of the present invention including a two-dimensional reference surface;  
         [0014]    [0014]FIG. 2 is a side elevational view of the exemplary lens depicted in FIG. 1;  
         [0015]    [0015]FIG. 3 is a cross-sectional schematic view of an apparatus with a preform positioned therein at the beginning of a molding sequence;  
         [0016]    [0016]FIG. 4 is a cross-sectional schematic view of the apparatus of FIG. 3 with the apparatus actuated to compress the preform to yield an exemplary molded lens of the present invention with a free-form lens body or perimeter;  
         [0017]    [0017]FIG. 5 a  is a cross-sectional schematic view of the apparatus of FIGS. 3 and 4 with the apparatus actuated to release the exemplary molded lens with a free-form lens body or perimeter formed thereby;  
         [0018]    [0018]FIG. 5 b  is a side elevational view of an exemplary lens as molded with the apparatus depicted in FIG. 5 a;    
         [0019]    [0019]FIG. 6 a  is a cross-sectional schematic view of an alternative apparatus to that shown in FIGS. 3 through 5 a  for molding a lens of the present invention;  
         [0020]    [0020]FIG. 6 b  is a side elevational view of an exemplary lens as molded with the apparatus depicted in FIG. 6 a;    
         [0021]    [0021]FIG. 7 is a side elevational view of another exemplary molded glass lens of the present invention which includes a two-dimensional reference surface at one end thereof and a three-dimensional reference surface at the opposite end thereof;  
         [0022]    [0022]FIG. 8 is a side elevational view of yet another exemplary molded glass lens of the present invention and including a longitudinal flat reference surface formed on the lens body;  
         [0023]    [0023]FIG. 9 is a plan view taken from the perspective of line  9 - 9  of FIG. 8;  
         [0024]    [0024]FIG. 10 is a side elevational view of yet another exemplary molded glass lens of the present invention including a two-dimensional (planar) reference surface, and a recessed reference surface formed on the lens body;  
         [0025]    [0025]FIG. 11 is a schematic depicting one potential use of two of the lenses of the present invention in an optical fiber communications component;  
         [0026]    [0026]FIG. 12 a  is a schematic showing typical center of curvature separation, d, of a prior art lens;  
         [0027]    [0027]FIG. 12 b  is a shematic illustrating that the center of curvature separation, d, of a plano-convex lens is infinite;  
         [0028]    [0028]FIG. 13 is a schematic showing how the lens of the present invention can be used to increase the center of curvature separation, d, of the lens;  
         [0029]    [0029]FIG. 14 is a schematic side elevational view of an exemplary lens of the present invention supported between two centering cups in a post molding grinding operation to achieve a generally cylindrical lens body; and  
         [0030]    [0030]FIGS. 15 a  through  15   f  show cross-sectional views of various exemplary lenses of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]    Turning first to FIGS. 1 and 2, there is depicted an exemplary molded glass lens  10  of the present invention. Lens  10  includes a lens body  12  that is cylindrical (and in this example, circular in cross-section). At a first end of lens body  12  is a molded two-dimensional reference surface  14 . The molded two-dimensional reference surface  14  is a plano surface that is preferably annular. There is a first molded optical surface  16  that is longitudinally displaced or offset from the molded two-dimensional reference surface  14 . That offset may take the form of a recess or a projection. An exemplary projection  15  is depicted in FIG. 1. As depicted, exemplary projection  15  includes an intermediate surface  17  residing between the molded two-dimensional reference surface  14  and the first molded optical surface  16 . The intermediate surface  17  is preferably tapered (such as, for example, to form a frusto-conical shape) to facilitate removal of the lens from the mold. There is a molded second optical surface  18  at a second end of the lens body  12 . The first and second molded optical surfaces  16 ,  18  may be plano, convex or concave.  
         [0032]    As described above, the molded two-dimensional reference surface  14  is a plano surface that is preferably annular. The actual shape of molded two-dimensional reference surface  14  will typically depend, however, on the cross-sectional shape of the lens body  12  and the shape of recess or projection which forms the base for the first molded optical surface  16 .  
         [0033]    As depicted in FIGS. 1 and 2, first molded optical surface  16  is plano and molded second optical surface  18  is convex. Note that although first molded optical surface  16  is plano, it does not have to be perpendicular to the cylindrical axis  20  of lens  10 . This geometry has particular advantage in some collimating lens applications, which will be discussed in more detail hereinafter.  
         [0034]    Looking next in FIGS. 3 through 5 b , there is shown a cross-sectional view of an apparatus  22  for producing the molded glass lens  10  of the present invention depicting a molding sequence. The apparatus  22  of the present invention includes an upper mold  24  and a lower mold  26 . Upper mold  24  resides in an upper mold support  28  and lower mold  26  resides in a lower mold support  30 . The upper mold  24  includes a first optical mold surface  32 . First optical mold surface  32  is depicted as being concave but may include other optical geometries such as convex or plano features. The lower mold  26  includes a two-dimensional reference mold surface  34  and a second optical mold surface  36 . Surrounding upper and lower molds  24  and  26  is an induction heating coil  40 . In operation, a glass preform  38  (depicted as being spherical) with an optical quality surface is inserted into the depression in the lower mold  26  defined by the two-dimensional reference mold surface  34  and the second optical mold surface  36 . Through actuation of the induction heating coil  40 , the temperature of the upper and lower molds  24 ,  26  and preform  38  is raised to at least the glass transition temperature of the preform  38 . Then the preform  38  is pressed between the upper and lower molds  24 ,  26  causing the preform  38  to deform as depicted in FIG. 4, thereby imparting to the preform  38  first and second optical mold surfaces  32 ,  36  and molded two-dimensional reference surface  14 . Compression is performed (by means not shown) to a positive stop at which point the molds  24 ,  26  and the preform  38  are allowed to cool to below the glass transition temperature and preferably to below the annealing point of the glass. The volume of the cavity  42  defined by molds  24 ,  26  and mold support  28 ,  30  within the mold position as depicted in FIG. 4 is significantly greater than the volume of the preform  38 . Once the molds  24 ,  26  and the preform  38  cool, molds  24 ,  26  are parted as depicted in FIG. 5 a . In such manner, a molded glass lens  44  (See FIG. 5 b ) is molded which includes a lens body  46  with a free-formed perimeter  48 , a molded two-dimensional reference surface  50 , a first molded optical surface  52  displaced longitudinally from the molded two-dimensional reference surface  50 , and a molded second optical surface  54 . The molded two-dimensional reference surface  50  is an annular plano surface. The conical surface  51  is tapered to facilitate easy removal of the lens  44  from the mold. The free formed perimeter  48  is then preferably subjected to a grinding operation to produce a cylindrical lens body. In this manner, a finished lens  10  such as depicted in FIG. 1 is produced. The grinding operation can be efficiently performed as centering of the lens  44  is accomplished using the two-dimensional reference surface  50 . Molds  24 ,  26  may be made of a machinable material (such as electroless nickel) thereby allowing both molds  24 ,  26  to be machined. Alternatively, molds  24 ,  26  may also be made from a material that cannot be easily machined, such as glass or ceramic, by forming molds  24 ,  26  by machining mold tools which have surfaces that are negatives of the desired surfaces for molds  24 ,  26 . Then molds  24 ,  26  can be molded using such negative or inverse mold tools.  
         [0035]    It should be understood that upper and lower molds  24 ,  26  are not necessarily directly heated by induction. Rather, upper and lower molds  24 ,  26  preferably reside in mold supports  28 ,  30  fabricated from a conductive material such as graphite or molybdenum. The mold supports  28 ,  30  are heated by the induction heating coil  40  and the upper and lower molds  24 ,  26  are heated indirectly by conduction and radiant heat transfer.  
         [0036]    It should be understood that two-dimensional reference mold surface  34  does not have to be of the highest optical quality, since two-dimensional reference surface  14  will not be used to transmit light. However, the quality of two-dimensional reference mold surface  34  will affect subsequent centering operations.  
         [0037]    Looking next at FIGS. 6 a  and  6   b , there is shown an alternative apparatus  60  for molding an exemplary molded glass lens  61  of the present invention. Apparatus  60  includes an upper mold  62  and a lower mold  64 . Upper mold  62  resides in an upper mold sleeve  66  and lower mold  64  resides in a lower mold sleeve  68 . The upper mold  62  includes a first optical mold surface  70 . First optical mold surface  70  is depicted as being concave, but may include other optical geometries such as convex or plano features. The lower mold  64  includes a two-dimensional reference mold surface  72  and a second optical mold surface  74 . Operation of apparatus  60  is similar to operation of apparatus  22 . A glass preform  73  with an optical quality surface is inserted into the lower mold sleeve  68  and on top of lower mold  64 . The glass preform  73  used in the apparatus  60  as depicted is preferably cylindrical with spherical ends. The spherical ends would have optical quality surfaces. Through actuation of an induction heating coil, or other heating means, the temperature of the upper and lower molds  62 ,  64  and preform  73  is raised to at least the glass transition temperature of the preform  73 . Then the preform  73  is pressed between the upper and lower mold  62 ,  64  and confined by the upper and lower mold sleeves  66 ,  68  causing the preform  73  to deform to the shape of the mold cavity  75  defined thereby. In this manner, the first and second optical mold surfaces  76 ,  78  and the molded two-dimensional reference surface  80  are imparted to the preform  73  yielding lens  61  as depicted in FIG. 6 b . The conical surface  77  is tapered to facilitate easy removal of lens  61  from the mold. Compression is performed (by means not shown) to a positive stop at which point the molds  62 ,  64  and the lens  61  are allowed to cool to below the glass transition temperature and preferably to below the annealing point of the glass. At that point, upper and lower molds  62 ,  64  and upper and lower mold sleeves  66 ,  68  can be separated and lens  61  can be removed. Preferably, upper and lower mold sleeve  66 ,  68  join one another in an interlocking arrangement as shown in FIG. 6 a . Preferably, mold cavity  75  includes an annular channel  82  projecting into upper and lower mold sleeves  66 ,  68  proximate to where upper and lower mold sleeves  66 ,  68  are above one another when in molding position. In the embodiment depicted in FIG. 6 a , one-half of annular channel  82  is formed in upper mold sleeve  66 , and one-half of annular channel  82  is formed in lower mold sleeve  68 . Annular channel  82  allows for the volume of the preform  73  which is somewhat larger than the volume of the main portion of mold cavity  75 . In this manner, lens  61  can be formed with a generally cylindrical shape while avoiding putting too much pressure on upper and lower sleeves  66 ,  68  during molding operation. In other words, annular channel  82  provides a reservoir into which excess glass can flow. The excess glass that flows into annular channel  82  can be subjected to a subsequent grinding operation and removed thereafter.  
         [0038]    Those skilled in the art will recognize that the lens of the present invention could be formed with a mold apparatus similar to that depicted in FIG. 6 a  but having only a single sleeve rather than a split sleeve. However, a single sleeve would prevent the inclusion of annular channel  82  in the mold cavity. Such a design would have problems associated therewith. These problems are particularly true when molding glass optical elements that are only about 2 mm or less in diameter. Maintaining control of the inner diameter of a bore that is only about 2 mm in diameter is difficult. Furthermore, repeated glass pressing operations tend to degrade the surface quality inside the bore, leading to increased probability of the lens sticking in the mold. In this type of molding operation the variability of preform volume must be controlled very precisely to reduce potential stresses that might damage the sleeve.  
         [0039]    The lens of the present invention can be molded to include more than one datum or reference surface. Looking at FIG. 7, there is depicted another exemplary molded glass lens  100  of the present invention which includes two reference surfaces. Lens  100  includes a lens body  102  that is cylindrical (and in this example, circular in cross-section). At a first end of lens body  102  is a molded two-dimensional reference surface  104 . There is a first molded optical surface  106  that is displaced longitudinally from the molded two-dimensional reference surface  104 . The molded two-dimensional reference surface  104  is an annular plano surface. At a second end of lens body  102  is a molded three-dimensional reference surface  107 . The molded three-dimensional reference surface  107  is curvilinear and may be a spherical, aspherical or conical segment. As such, the second molded optical surface  108  may be thought of as interrupting the three-dimensional reference surface  107 . The second molded optical surface  108  abutts and is formed integrally with molded three-dimensional reference surface  107 . The first and second optical surfaces  106 ,  108  may be plano, convex or concave. As depicted in FIG. 7, first molded optical surface  106  is plano and second molded optical surface  108  is convex. Note that although first molded optical surface  106  is plano, it does not have to be perpendicular to the cylindrical axis  109  of lens  100 . This geometry has particular advantage in some collimating lens applications which will be discussed in more detail hereinafter. However, for other lens applications which include a plano optical surface it may be preferred to have the plano surface perpendicular to the cylinder and/or optical axis of the lens. This embodiment of the invention allows independent location of the center of curvatures of the two reference surfaces  104 ,  107  that are held in the cups during a centering operation. That is, the location of the lens for grinding does not depend on the molded optical surfaces  106 ,  108  that are used to implement the optical function of the lens  100 .  
         [0040]    [0040]FIGS. 8 and 9 show yet another exemplary molded glass lens  110  of the present invention. Exemplary lens  110  is similar to lens  10 . Lens  110  includes a lens body  112  that is cylindrical (and in this example, circular in cross-section). At a first end of lens body  112  is a molded two-dimensional reference surface  114 . There is a first molded optical surface  116  longitudinally displaced from the molded two-dimensional reference surface  114 . The molded two-dimensional reference surface  114  is an annular plano surface. There is a molded second optical surface  118  at a second end of the lens body  112 . The first and second optical surfaces  116 ,  118  may be plano, convex or concave. As depicted in FIGS. 8 and 9, first molded optical surface  116  is plano and second molded optical surface  118  is convex. As shown, first molded optical surface  116  is plano but is not perpendicular to the cylindrical axis  120  of lens  110 . However, first molded optical surface  116  may be formed to be perpendicular to the cylindrical axis  120  of lens  110  depending on the particular lens application.  
         [0041]    In this alternate lens embodiment, the molded datum (molded two-dimensional reference surface  114 ) allows the addition of two other datums. The first added datum is a cylindrical surface  122 , the axis of which is coincident with the optical axis of the lens  110 . The second added datum is a flat reference surface or datum  126 . The flat reference surface  126  is parallel to the axis  120  of the cylindrical datum surface  122 . The flat surface  126  can be used during placement of the lens into an assembly to constrain the rotational orientation of the lens about the aspheric axis (which is assumed to be coincident with the axis of the created cylindrical datum). Preferably, datums  122 ,  126  are formed during the molding process. However, datums  122 ,  126  may also be formed in subsequent grinding operation(s) after the molding process is completed. When the lens body  112  is allowed to free-form in the molding operation, then it is necessary to form datums  122 ,  126  in subsequent grinding operations.  
         [0042]    [0042]FIG. 10 shows yet another exemplary molded glass lens  130  of the present invention. Exemplary lens  130  is also similar to lens  10 . Lens  130  includes a lens body  132  that is cylindrical (and in this example, circular in cross-section). At a first end of lens body  132  is a molded two-dimensional reference surface  134 . There is a first molded optical surface  136  displaced longitudinally from the molded two-dimensional reference surface  134 . The molded two-dimensional reference surface  134  is an annular plano surface. There is a molded second optical surface  138  at a second end of the lens body  132 . The first and second optical surfaces  136 ,  138  may be plano, convex or concave. As depicted in FIG. 10, first molded optical surface  136  is plano and second molded optical surface  138  is convex. As shown, first molded optical surface  136  is plano but is not perpendicular to the cylindrical axis  140  of lens  130 . However, first molded optical surface  136  may be formed to be perpendicular to the cylindrical axis  140  of lens  130  depending on the particular lens application. Lens  130  also includes a recess  142  formed therein. Recess  142  would be formed in a subsequent grinding operation after molding. The recess  142  allows relatively precise axial location of the lens  130 . Such a recess  142  could make subsequent placement, inspection and alignment of the lens  130  in an optical assembly easier. Those skilled in the art will recognize that although recess  142  is preferably annular, recess  142  may comprise one or more non-contiguous recess segments.  
         [0043]    As mentioned above, the geometry of lens  10  as depicted in FIG. 1, where the first molded optical surface  16  is plano but does not have to be perpendicular to the cylindrical axis  20  of lens  10  has particular advantage in some collimating lens applications. Looking at FIG. 11, when light is transferred from a first optical fiber  150  to a second optical fiber  152 , it is often accomplished with a pair of lenses  154 ,  156 . The first lens  154  collimates the output of the emitting fiber  150 , and the second lens  156  focuses that collimated beam  158  into the receiving fiber  152 . Other optical components (not shown) may be placed between these two lenses  154 ,  156  in the collimated beam  158  of light such as dichroic filters, beam splitters or birefringent materials that separate the beams. For these systems, it is desirable that the collimating optics be small. This minimizes the size of any supplemental optics, and decreases the overall package size. The optics must also work over a wide wavelength and temperature range. Glass optical elements are desired over plastic due to lower thermal and environmental sensitivity. In the manufacture of assemblies that use optical components such as lenses, it is desirable that the lenses have datums that can be used for accurately locating the lenses in the assembly. When using a lens  154  to collimate light from an optic fiber  150 , it is not desirable to have an optical surface  162  that is nearly perpendicular to the beams. A perpendicular optical surface may reflect light back along the same path, and back into the fiber. This reflected light could affect the laser source used in telecommunication systems. One possible design for a collimating lens element  154  which would overcome this reflection problem is to have plano optical surface  162  angled to the optical axis, and the second optical surface  164  be a convex asphere.  
         [0044]    Thus, the lens of the present invention describes a way of making lenses with an angled rear facet surface and producing a known datum on the lens. The inclusion of the known datum reduces difficulties in the subsequent centering operations.  
         [0045]    As mentioned above, the lens of the present invention allows more efficient centering operations. When centering lenses, it is desirable that the separation between the center of curvatures of the optical surfaces be a large as possible. FIG. 12 a  shows a molded glass lens  170  with two convex optical surfaces  172  and  174  and having corresponding radii of curvature, R 1  and R 2 , respectively. In this case, the center of curvature separation d is quite small making it difficult to grind the outside diameter of the lens precisely with respect to the optical axis.  
         [0046]    [0046]FIG. 12 b  shows a lens  175  with a plano surface  176  and a convex surface  178 . In this case the radius of curvature of the plano surface is infinite which causes the center of curvature separation d to also be infinite. This will greatly faciliate the centering of this plano-convex lens. The plano surface  176  minimizes the tilt of the centered lens  175 , and the convex surface  178  minimizes the decentration of the centered lens  175 . FIG. 12 b  shows a plano-convex lens, but the present invention can also be used on a plano-concave lens.  
         [0047]    In the special case of a sphere, the center of curvature separation is zero and the ability to precisely center the lens become very difficult. A molded glass lens  180  of the present invention is depicted in FIG. 13 which includes convex optical surfaces  182 ,  184 . Convex optical surfaces  182 ,  184  may both be spherical and of the same radius. By adding a two-dimensional reference surface  186  to the lens  180 , the center of curvature separation d as defined by the reference surface  186  and convex optical surface  184  becomes infinite, making the centering operation much easier.  
         [0048]    As mentioned above, the molded lens of the present invention allows for ease of centering using standard optical centering equipment. In normal use, this equipment is used to center optical lenses with two spherical surfaces. It is understood that the equipment can also be used to center lenses with aspheric surfaces. Referring to FIG. 14, there is shown an exemplary lens  10  of the present invention (FIG. 1) supported between two centering cups  190 ,  192 . The first centering cup  190  engages two-dimensional reference surface  14 . The second centering cup engages the second molded optical surface  18 . As previously noted, the two-dimensional reference surface  14  is an annular plano surface. By way of example, FIG. 14 depicts a lens  10  positioned for removal of excess material using a grinding wheel  194 .  
       EXAMPLE  
       [0049]    An exemplary lens of the present invention similar to that depicted in FIGS. 1 and 2 was successfully molded from Schott SF-57 glass. The lens produced was a plano-convex collimator lens intended to be used in an arrangement similar to that shown in FIG. 11. The plano optical surface  16  was inclined by approximately 8° from being perpendicular to the cylindrical axis  20  to reduce back reflections into the transmitting optical fiber. An annular plano two-dimensional reference surface  14  was molded into the end of lens  10  that contained the plano optical surface  16 . The plano optical surface  16  was displaced longitudinally from the two-dimensional reference surface  14 . An aspheric optical surface  18  was integrally molded at the opposite end of lens  10 . The purpose of the aspheric optical surface  18  is to collimate the optical beam. The apparatus used to form the lens  10  was similar to that depicted in FIGS. 3 through 5. A spherical preform  38  was placed into the mold and heated to approximately 500° C. The spherical preform  38  was then compressed between upper and lower molds  24 ,  26  for approximately  30  seconds and then cooled. Once the upper and lower molds  24 ,  26  were separated, the molded lens  44  was removed and placed in a plastic tray. Subsequent to molding, the lens  44  was coated with an anti-reflection coating optimized at 1550 nm. Following this, the excess glass was removed using a conventional optical lens centering machine to yield a final lens geometry similar to that shown in FIGS. 1 and 2. Two centering cups  190 ,  192  were used to align the optical axis of the lens to the mechanical axis of the centering machine similar to the arrangement shown in FIG. 14. One cup  190  contacted the annular plano two-dimensional reference surface  14  and the other cup  192  contacted the aspheric optical surface  18 .  
         [0050]    As mentioned above, the first molded optical surface  16  is longitudinally offset from the two-dimensional reference surface  14 . The offset may be such that the first molded optical surface  16  is closer to or further from the second molded optical surface  18  as compared to the molded two-dimensional reference surface  14 . In other words, the offset may take the form of an axial or longitudinal recess or an axial or longitudinal projection. In fact, the offset may be simultaneously a partial axial recess and a partial axial projection. FIGS. 15 a  through  15   f  show cross-sectional views of various exemplary lenses of the present invention. FIGS. 15 a  and  15   b  depict offsets that are exemplary of axial recesses. FIGS. 15 c  and  15   d  depict offsets that are exemplary of axial projections. FIGS. 15 e  and  15   f  depict exemplary offsets that are simultaneously partially axially recessed and partially axially projecting.  
         [0051]    Those skilled in the art will recognize that although the lenses of the present invention are discussed herein as being individually molded, small versions (having diameters of 2 mm or less) of such lenses can be molded in arrays. The upper and lower mold would include cavities for molding multiple lenses as part of a single integrally formed sheet. The individual lenses could then be singulated in a subsequent cutting operation.  
         [0052]    From the foregoing, it will be seen that this invention is well adapted to attain all of the ends and objects hereinabove set forth together with other advantages which are apparent and which are inherent to the apparatus.  
         [0053]    It will be understood that certain features and subcombinations are of utility and may be employed with reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.  
         [0054]    As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth and shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.  
       PARTS LIST  
       [0055]    [0055] 10  molded glass lens  
         [0056]    [0056] 12  lens body  
         [0057]    [0057] 14  two-dimensional reference surface  
         [0058]    [0058] 15  exemplary projection  
         [0059]    [0059] 16  first molded optical surface  
         [0060]    [0060] 17  intermediate surface of exemplary projection  15   
         [0061]    [0061] 18  second molded optical surface  
         [0062]    [0062] 20  cylindrical axis  
         [0063]    [0063] 22  apparatus  
         [0064]    [0064] 24  upper mold  
         [0065]    [0065] 26  lower mold  
         [0066]    [0066] 28  upper mold support  
         [0067]    [0067] 30  lower mold support  
         [0068]    [0068] 32  first optical mold surface  
         [0069]    [0069] 34  two-dimensional reference mold surface  
         [0070]    [0070] 36  second optical mold surface  
         [0071]    [0071] 38  glass preform  
         [0072]    [0072] 40  induction heating coil  
         [0073]    [0073] 42  cavity  
         [0074]    [0074] 44  molded glass lens  
         [0075]    [0075] 46  lens body  
         [0076]    [0076] 48  free-formed perimeter  
         [0077]    [0077] 50  molded two-dimensional reference surface  
         [0078]    [0078] 51  conical surface  
         [0079]    [0079] 52  first molded optical surface  
         [0080]    [0080] 54  second molded optical surface  
         [0081]    [0081] 60  alternative apparatus  
         [0082]    [0082] 61  molded glass lens  
         [0083]    [0083] 62  upper mold  
         [0084]    [0084] 64  lower mold  
         [0085]    [0085] 66  upper mold sleeve  
         [0086]    [0086] 68  lower mold sleeve  
         [0087]    [0087] 70  first optical mold surface  
         [0088]    [0088] 71  induction heating coil  
         [0089]    [0089] 72  two-dimensional reference mold surface  
         [0090]    [0090] 73  glass preform  
         [0091]    [0091] 74  second optical mold surface  
         [0092]    [0092] 75  mold cavity  
         [0093]    [0093] 76  first optical mold surface  
         [0094]    [0094] 77  conical surface  
         [0095]    [0095] 78  second optical mold surface  
         [0096]    [0096] 80  molded two-dimensional reference surface  
         [0097]    [0097] 82  annular channel  
         [0098]    [0098] 100  molded glass lens  
         [0099]    [0099] 102  lens body  
         [0100]    [0100] 104  molded two-dimensional reference surface  
         [0101]    [0101] 106  first molded optical surface  
         [0102]    [0102] 107  molded three-dimensional reference surface  
         [0103]    [0103] 108  second molded optical surface  
         [0104]    [0104] 109  cylindrical axis  
         [0105]    [0105] 110  molded glass lens  
         [0106]    [0106] 112  lens body  
         [0107]    [0107] 114  molded two-dimensional reference surface  
         [0108]    [0108] 116  first molded optical surface  
         [0109]    [0109] 118  second molded optical surface  
         [0110]    [0110] 120  cylindrical axis  
         [0111]    [0111] 122  cylindrical surface or datum  
         [0112]    [0112] 126  flat reference surface or datum  
         [0113]    [0113] 130  molded glass lens  
         [0114]    [0114] 132  lens body  
         [0115]    [0115] 134  molded two-dimensional reference surface  
         [0116]    [0116] 136  first molded optical surface  
         [0117]    [0117] 138  second molded optical surface  
         [0118]    [0118] 140  cylindrical axis  
         [0119]    [0119] 142  recess  
         [0120]    [0120] 150  first optical fiber  
         [0121]    [0121] 152  second optical fiber  
         [0122]    [0122] 154  first lens  
         [0123]    [0123] 156  second lens  
         [0124]    [0124] 158  collimated beam  
         [0125]    [0125] 162  first optical surface  
         [0126]    [0126] 164  second optical surface  
         [0127]    [0127] 170  molded glass lens  
         [0128]    [0128] 172  convex optical surface  
         [0129]    [0129] 174  convex optical surface  
         [0130]    [0130] 175  molded glass lens  
         [0131]    [0131] 176  plano surface  
         [0132]    [0132] 178  convex optical surface  
         [0133]    [0133] 180  molded glass lens  
         [0134]    [0134] 182  convex optical surface  
         [0135]    [0135] 184  convex optical surface  
         [0136]    [0136] 186  two-dimensional reference surface  
         [0137]    [0137] 190  first centering cup  
         [0138]    [0138] 192  second centering cup  
         [0139]    [0139] 194  grinding wheel  
         [0140]    d center of curvature separation  
         [0141]    R 1  radius of curvature  
         [0142]    R 2  radius of curvature