Methods for manufacturing optical coupling elements

[00002] This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/358,052, entitled “Manufacturing Transceiver Optic Couplers,” filed Feb. 19, 2002, which is incorporated herein by reference.

[00003] 1. The Field of the Invention

[00004] The present invention relates to manufacturing optical transceivers and coupling elements. More particularly, the present invention relates to systems and methods for manufacturing optical coupling elements in transceivers with multiple ports.

[00005] 2. The Relevant Technology

[00006] Communication over optical fiber networks utilizes light signals that are effectively trapped inside of optical fibers. Because the light signals are trapped within a particular fiber, many fibers can be included in a single cable without concern about interference from the light signals carried by nearby fibers. Optical fibers also have the property of strongly rejecting interference that would otherwise be caused by radio frequencies and electromagnetic radiation. These characteristics make fiber optics ideally suited for many applications.

[00007] In order to transmit light signals using optical fibers, it is first necessary to introduce light into the fibers. Exemplary sources of light used in fiber optics are light emitting diodes and laser diodes. Coupling the light to a fiber, however, is not always efficient because the numerical aperture of the fiber is typically smaller than the common laser diode sources. Thus, coupling optics are needed for effective conversion of the laser diode mode to the transmitted optical mode inside the single mode optical fiber. Because coupling optics also have transmission losses and aberrations, only a portion of the light emitted by a light source that satisfies the physical characteristics of the transmitted fiber mode is effectively transmitted through the coupling optics and effectively coupled with the optical fiber. In addition, the power consumed by the light source is often increased in order to ensure that the light signal transmitted through the optical fiber has sufficient transmission power. To overcome these losses, the light sources are typically coupled or connected with optical fibers using high optical quality coupling elements with efficient anti-reflection coatings. This has the effect of reducing the power consumed by the light source while directing more light into the optical fiber. In addition to coupling a light source to an optic fiber, coupling elements are also used to couple or connect one optical fiber to another optical fiber and to couple or connect an optical fiber to a light detector.

[00008] Typical coupling elements or transceiver ports typically use ball lenses, aspheric lenses or graded index (GRIN) lenses. Ball lenses are significantly less expensive than aspheric lenses and GRIN lenses because they are simple to manufacture. However, the performance of GRIN lenses and aspheric lenses is significantly better than the performance of ball lenses.

[00009] A typical ball lens used to couple a laser diode output to a single mode fiber shows diffraction limited performance only in the 0.15 numerical aperture (A) region of the optical source. The same ball lens, when used to couple a single mode fiber to another single mode fiber can only couple light effectively up to 0.11 NA of the input fiber. Higher NA light output from the laser diode or the single mode fiber is highly aberrated and does not couple effectively when using coupling elements that include ball lenses.

[00010] GRIN lenses and aspheric lenses, on the other hand, perform significantly better than typical ball lenses. Laser diodes with beam divergence angles in the 0.5 NA region can effectively be coupled into single mode fibers using appropriate aspheric lenses. Unfortunately, the lowest cost aspheric glass lenses are significantly more expensive than ball lenses.

[00011] The advantage of using GRIN lenses or aspheric lenses over ball lenses is that a fiber optic system can generate sufficient transmission power with minimum laser output for any given link. In addition, low aberration imaging of the fiber output onto small detectors permits effective capture of the transmitted light when using GRIN lenses or aspheric lenses. The disadvantage of using GRIN lenses or aspheric lenses is their cost. Currently, low cost aspheric lenses are 10 to 20 times more expensive than ball lenses. What is needed are systems and methods of manufacturing low cost, high efficiency coupling elements or ports that provide improved optical performance.

[00012] These and other problems are overcome by the present invention which relates to manufacturing coupling elements. The present invention more particularly relates to a method of making or manufacturing low cost coupling elements that perform better than simple ball lenses. One example of the coupling elements described herein is a low cost glass lens solution that has performance that is comparable to the performance of aspheric glass molded lenses.

[00013] The present invention utilizes lens arrays made out of glass, each having a plurality of lenslets. The glass lens arrays can be press molded such that the lenslets have spherical or aspherical surfaces with circular or square apertures. Each glass lens array is fire polished and the quality of the resulting lens surfaces is similar to the surface quality of conventional ground and polished single glass lenses or injection molded glass aspheric lenses.

[00014] To create an individual coupling element, a molded glass lens array is attached with another lens array to form an element array such that the lenslets of the respective arrays are opposite each other. Spacers may be included between the lens arrays to maintain a predetermined distance between the two lens arrays. The spacers may be molded as an integral part of one of the lens arrays.

[00015] The element array is cut or otherwise separated into rows and the rows are further separated or cut into individual coupling elements. Each individual coupling element thus includes a single lenslet from each of the original lens arrays. One of the lenslets acts as a collimating lens and functions to collimate light from a source or a fiber, for example. The other lenslet acts as a focusing lens to focus the collimated light on, for instance, a fiber or a detector.

[00016] In another embodiment of the present invention, a third optical layer is inserted between the two lens arrays. This third layer may be an array of prisms or other optical elements. The prisms allow each coupling element to also function as a beam splitter or an optical filter with specific wavelength or polarization characteristics. After the element array is cut into rows, one side of the exposed optical layer is polished and a third row of lenslets is attached to the polished area. Then, the row is cut or separated into individual coupling elements that have three ports.

[00017] The individual coupling elements manufactured or created in this manner have a cost that is less than the price of the low cost ball lens while offering performance similar to GRIN lenses or aspheric lenses.

[00018] Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.

[00028] In fiber optics and optical communications, small lenses are often used to couple laser light into transmission fibers, connect one transmission fiber to another transmission fiber, and couple the output of the transmission fibers onto receiver detectors. The present invention relates to a method of manufacturing optical couplers or coupling elements for use in transceiver ports. The present invention utilizes molded lens arrays where the individual lenslets in each lens array can be used independently in separate coupling elements. The molded lens arrays have the advantage of being significantly less expensive than a single high quality aspheric lens or GRIN lens as previously described.

[00029] The higher quality lenslets or lenses from the lens arrays translate into improved coupling of a light source and a fiber from, for example, the basic ball lens coupling elements, because the molded elements of the lens array are molded in a general aspheric surface form. The present invention improves the coupling ratio and less output strength or power is required of the laser or light source to generate an adequate transmission signal in the optical fiber. In one embodiment, the lens arrays provide good thermal stability and optical transmission because the lens arrays are molded glass. It is understood that the present invention is not limited to glass molded lens arrays.

[00030] For illustrative purposes, FIGS. 1A-4B are not necessarily to scale and some features, such as lens surface curvature and the like are exaggerated for illustration purposes. FIG. 1A is a perspective illustration of a lens array 100 and FIG. 11B is a side view of the lens array illustrated in FIG. 1A. In this example, the lens array 100 is molded glass and the lens array 100 includes a plurality of lenslets which are exemplified by the lenslets 102. The lenslets of the lens array 100 can be press molded to have spherical or aspherical surfaces with either circular or square apertures, for example. The lens array 100 is often fire polished, which results in a surface quality and a finish that is comparable to conventional ground and polished single lenses.

[00031] In this example of the lens array 100, the lens aperture 106 is on the order of 2 millimeters and the pitch 104 is on the order of 3 millimeters. The lens array 100 has a thickness of approximately 2.5 millimeters to retain the stability of the molding process in arrays that have a width on the order of 50 to 75 millimeters. However, to accommodate a more compact transceiver port design, the molded lens array can be thinned to less than 1 mm in thickness by a simple polishing process performed on the flat side of the array before it is used. The number of lenslets 102 on a lens array 100 depends on the length and width of the lens array 100. An exemplary lens array 100 has a length and a width of 50 to 75 millimeters. It is understood that the parameters of pitch, lens aperture, length, and width are exemplary in nature and the present invention is not limited to these parameters and that these parameters can be adjusted according to specified optical requirements.

[00032] FIG. 2 is a block diagram illustrating a pair of lens arrays that have been attached to form an element array of coupling elements. In FIG. 2, a collimating lens array 202 is connected with a focusing lens array 204. These matched collimating and focusing lens arrays are designed and molded with the same lenslet pitch but not necessarily using the same aperture size or shape. Each lenslet (lenslets 208 and 212, for example) of the collimating lens array 202 functions to collimate the light from a source while each lenslet (lenslets 210 and 214, for example) of the focusing array 204 are used to focus the light onto a destination such as a fiber or detector. Because of the size and shape of the collimating lens array 202 and the focusing array 204, spacers 206 are used to maintain the distance between the collimating lens array 202 and the focusing array 204. The spacers 206 can be molded as part of one of the arrays during the manufacturing process. The spacers 206 can also be formed or molded to establish a predetermined distance between the focusing lens array 204 and the collimating lens array 202. The dotted lines 216 and 218 indicate where the element array of FIG. 2 is cut or separated into rows of coupling elements.

[00033] FIG. 3A is a perspective view of a single row of coupling elements that has been cut or separated from the coupling element array illustrated in FIG. 2. Dicing marks in convenient forms of lines, crosses, dots, or circles can be molded between the lenslets to ease the cutting process of individual ports. The row 300 includes a row 302 of the collimating lens array that is attached to a row 304 of the focusing array. Any number of attachment methods such as Ultra Violet curable epoxy or heat processed adhesives could be used to permanently attach the arrays. The row 300 of coupling elements further includes lenslets as described above where a collimating lenslet 308 is opposite a focusing lenslet 310. The dashed line 312 indicates where the row 300 is cut or separated in order to produce or separate the individual coupling elements from the row 300. The spacers 306, which connect the two rows of lenslets, are also illustrated.

[00034] FIG. 3B is a perspective view of an individual coupling element that has, in this example, two ports. The coupling element 320 includes a collimating lenslet 328 and a focusing lenslet 326 that are opposite each other and are, typically, aligned due to the initial alignment of the each lenslet pair within the lenslet array. The collimating lenslet 328 and the focusing lenslet 326 have thus been separated from an array of coupling elements as described above and the coupling element 320 may be used, for example, to couple a laser diode output into a fiber, to couple the output of a fiber onto a detector, to couple one fiber to another, or as a single optical coupling module at this point. Each coupling element has spacers 330 to provide support and rigidity to the coupling element 300.

[00035] FIG. 3C is a side view of the coupling element 300 of FIG. 3B and illustrates an example of how a coupling element may be utilized in optical communication. A laser diode 340 produces laser light than needs to be focused or introduced into a fiber 342. The laser diode 340 is typically placed a distance f₁ from the collimating lenslet 328, where f₁ is the focal length of the collimating lenslet 328. The purpose of the collimating lenslet 328 is to collimate the light output of the laser diode 340 or other light source in part because the collimated beam between the collimating lenslet 328 and focusing lenslet 326 can be used by other optical components to perform other functions on the beam. These functions can be performed using optical components that include, but are not limited to, wavelength filters, attenuators, wave plates, isolators, and the like or any combination thereof.

[00036] The collimated light 344 is incident upon the focusing lenslet 326, which focuses the collimated light 344 upon the fiber 342. The fiber 342 is located a distance f₂ from the focusing lenslet 326, where f₂ is the focal length of the focusing lenslet 326. The focusing lenslet 326 is typically designed to account for the numerical aperture of the fiber 342. The focal length f₂ of the focusing lenslet 326 over the focal length f₁ of the collimating lenslet 328 determines the magnifications of the coupling optics. The magnification of the coupling element 300 is designed as the ratio of the NA of the input to the coupling element 300, which may be a laser diode in this example, to the NA of the output of the coupling element 300, which is often a fiber, for efficient coupling.

[00037] In this manner, individual coupling elements are produced by attaching a pair of lens arrays and then separating or cutting the attached arrays into individual coupling elements. One advantage of maintaining a space between the lenslet arrays is that the collimated light can be used for other optical elements without change in the optical performance of the coupling element. The collimation space between the collimating lenslet and the focusing lenslet, occupied by the spacer elements or by other extra optical elements can be different in each case without a need to change the collimator and focusing lens design. Thus optical components can be inserted in the design between the two lenses and the spacer length can be varied without affecting the coupling performance of the coupling optics.

[00038] Another advantage is the automatic pre-alignment of the collimating and focusing lenses in the array form. Once the alignment of the two lens arrays is done, there is no need for further adjustment of the elements within the port.

[00039] The alignment between the opposing lenslets (the collimating lenslet 328 and the focusing lenslet 326) of the coupling element 320 is also not critical because the light between the opposing lenslets is collimated. Extreme misalignments, however, may cause the collimated light to be vignetted by the lens apertures. Arranging two opposing lens arrays as described and cutting the lens arrays into individual coupling elements generate two-port optical devices that can be used for a variety of purposes including, but not limited to, coupling laser light into a fiber, coupling the output of one optical fiber to another optical fiber, and coupling the output of an optical fiber to a detector.

[00040] As previously mentioned, having a space between the opposing arrays or lenslets in the coupling element permits other optical elements to be inserted between the collimating lens array and the focusing lens array. These optical elements can perform, for example, isolation, attenuation, and the like. FIGS. 4A and 4B illustrate an example where an array of prisms is inserted between the collimating lens array and the focusing lens array.

[00041] FIG. 4A is a side view of an element array that includes an optical element between the lens arrays. In this example, a pair of prism arrays (prism array 410 and prism array 411) have been inserted between the collimating lens array 402 and the focusing lens array 404. The prism arrays function as a beam splitter for the collimated light that passes through the coupling elements. The interface 412 between the prism array 410 and the prism array 411 can be appropriately coated as known in the art before the prism arrays are attached or connected and inserted between the collimating lens array 402 and the focusing lens array 404. The interface 412 is positioned, for example, at 45 degrees.

[00042] The element array of FIG. 4A is then separated or cut into rows as previously described. When the rows are separated, each side of the optical element is exposed. In the example where the optical element is a prism array, then a side of the prism is exposed. In other words, a prism surface is exposed. The prism side is polished. After the prism side is polished, another lenslet row 430, as illustrated in FIG. 4B, is attached to the polished surface 431 of the prism 410. The pitch of the lenslet row 430 is made out to be the same as the collimator and focusing lenslet arrays. The row of coupling elements 400 is now prepared to be cut or separated into individual three port coupling elements. Each three port coupling element is pre-aligned in the array form as previously described and the collimated beam advantages in alignment tolerance are also true for the third port of the coupling elements manufactured in this manner.

[00043] The interface 412 shown in FIG. 4B acts as a beam splitter for the light collimated by the collimating lenslet 406. In this example, a source 422 emits light that is collimated by the collimating lenslet 406. The focusing lenslet 408 receives some of the collimated light and focuses the light on a fiber 424 or a detector as previously described. The detecting lenslet 428 which receives light that is redirected or split at the interface 412 focuses the light on a destination such as a detector 426, for example. Alternatively the beam splitter can detect light that is received from another fiber instead of a light source. The beam splitter can also direct an optical signal to another fiber.

[00044] The beam splitter or prism arrays described above are usually created from molded glass. In another example, the beam splitter that is inserted between the collimating lens array and the focusing lens array can be created using plastic molded parts that are similar to the glass prism arrays. Exemplary plastics have good transmission characteristics for certain wavelengths. The plastic array is not required to have high surface quality as the beam splitter surface. Instead, a glass bar that has been coated to enable beam splitting is positioned on the interface surfaces of the plastic prism arrays. After the glass bars have been properly positioned, the plastic arrays are filled and attached using optical epoxy. The optical epoxy ensures, for example, that the glass bars are securely fixed within the element array of coupling elements and that the optical element within each coupling element will function properly. For example, the interface 412 may instead represent a glass bar that is securely fixed using optical epoxy.

[00045] The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.