Patent Application
20060056762
The invention pertains to optical couplers and particularly to couplers used for conveying laser light from a source into an optical fiber.
Several patent documents may be related to optical coupling between optoelectronic elements and optical media. They include U.S. Pat. No. 6,086,263 by Selli et al., issued Jul. 11, 2000, entitled “Active Device Receptacle” and owned by the assignee of the present application; U.S. Pat. No. 6,302,596 B1 by Cohen et al., issued Oct. 16, 2001, and entitled “Small Form Factor Optoelectronic Receivers”; U.S. Pat. No. 5,692,083 by Bennet, issued Nov. 25, 1997, and entitled “In-Line Unitary Optical Device Mount and Package therefore”; and U.S. Pat. 6,536,959 B2, by Kuhn et al., issued Mar. 25, 2003, and entitled “Coupling Configuration for Connecting an Optical Fiber to an Optoelectronic Component”; which are herein incorporated by reference.
Coupling efficiency between light sources and optical media is an important factor in various communications and other applications. Coupling efficiency, for instance, from a laser source to a single mode fiber not only is affected by a mismatch between the laser field/fiber-mode but also by aberrations in the coupling optics. A single ball lens may be used for single mode fiber coupling, but because of the spherical aberration from the ball lens, the coupling efficiency may be only about fifty percent. However, many communications applications need higher coupling efficiencies because of distance, weak light sources and high data rates. An aspherical glass lens is able to achieve high fiber coupling but its cost may be too high for practical use.
The present invention is a low cost, highly efficient system for coupling light from a light source into optical fiber. Among other features, it may have a spherical lens and an aspherical lens situated on the same optical path.
Source 32 of
VCSEL 12 may be a single mode source. Light 11 may propagate through a protective window 14 of a (hermetically) sealed package containing the VCSEL onto a ball lens 15. The distance between VCSEL 12 and the surface of window 14 closer to VCSEL 12 may be about 380 microns (15 mils). Window 14 may be about 203 microns (8 mils) thick and consist of BK7™, Corning #7052, or any suitable transmissive material. The distance between the surface of the window 14 (closer to lens 15) and lens 15 may be about 280 microns (11 mils) along the optical axis. Spherical lens 15 may be about 1.5 millimeters (59 mils) in diameter. Lens 25 may be a glass ball lens. It may be composed of BK7™, LaSFN9, or any suitable material. Light 11 may move through lens 15 and out of it into an aspherical lens 16. The distance between lens 15 and lens 16 may be about 561 microns (22.1 mils). Light 11 may propagate through lens 16 into optical fiber 13. The end of fiber 13 may be in physical contact with lens 16 but not required to be so. The length of lens 16 may be about 209 microns (82.3 mils). The above-noted length measurements are along the optical axis. Lens 16 may be a convex lens made from Zeonex™ E48R available from Zeon Chemicals L.P., 4111 Bells Lane, Louisville, Ky. 40211. The lens may also be made from GE ULTEM. A 1.5 mm ball lens 15 of BK7™ material may be available from Edmund Industrial Optics, 101 East Gloucester Pike, Barrington, N.J. 08007-1380. Optical fiber 13 may be an SMF-28™ single mode optical fiber available from Corning Incorporated, One Riverfront Plaza, Corning, N.Y. 14831. One may note that the dimensions illustrated above are typical and other geometries may be functional as well.
The present optical coupler may have both high coupling efficiency and low cost. The coupling optics may use a glass ball lens and a molded aspherical lens. The aberration of the ball lens may degrade the efficiency of the coupling system. However, the ball lens' spherical aberration may be compensated by the light ray directing properties of the aspherical plastic lens. Since the ball lens may have significantly more optical power than the plastic lens in the coupling system, the plastic lens' poor thermal properties may be compensated for and minimized. Therefore, an appropriately designed combination of a glass ball lens and plastic molded aspherical lens may provide a thermally stable and highly efficient optical coupling system.
Lens 16 may be composed of glass or be a single aspherical glass lens. Glass aspherical lenses may have good thermal properties and less aberration than a ball lens. They may be somewhat expensive and difficult to produce. Plastic aspherical lenses may be easily and inexpensively producible; however, they do not have thermal properties as good as the glass lenses. Yet the plastic aspherical lenses have much less aberration than the ball lenses. For instance, light rays coming from a spherical lens periphery may form an image before the ideal focal point. For this reason, the spherical aberration (a blurred image) may occur at the center portion of the image formed. Or if the focus is readjusted for the center portion of the image, then the spherical aberration (again, a blurred image) may occur at the periphery of the image. In other words, it may not be possible for all of the parallel rays going through a spherical lens to converge at one point. An awkward and cumbersome multitude of spherical lenses might be designed to partially correct this aberration problem. However, one aspherical lens may be designed to gather or converge all of the parallel rays of light to one focal point. The aspherical lens may have surface with a specially designed curvature to achieve this convergence of the light rays. The aspherical lens surface does not completely conform to the shape of a sphere like that of a spherical lens. Mass production technologies including plastic mold technology may be used to mold aspherical lenses by pouring or injecting plastic material into a rather precise aspherical mold. Further, the aspherical lens may achieve a coupling efficiency into a single mode fiber above ninety percent for coupling systems 10, 20 and 40. This is a desired performance feature for VCSEL communication applications since VCSEL optical power is relatively low compared to other laser sources. Significant power is better conveyed with a glass aspherical lens; however, the cost of a glass aspherical lens is high (i.e., greater than eight dollars per lens in year 2000 with high volume pricing). The inexpensive (i.e., less than a dollar with high volume pricing) aspherical lens may be the poured or injection molded plastic lens. The aspherical lens may be made of another material similar to plastic. The plastic lens may have poor thermal characteristics but a glass ball lens may compensate for those characteristics in a coupling system with the plastic lens. The ball lens may be made of another material similar to glass.
A design for the aspherical convex lens 16 may be indicated by the following equation and parameter values.
z={cr2/[1+(1−(1+k)c2r2)1/2]}+A6r6+A8r8
Surface 1
Illustrative examples of the invention have an optical design which may possess both high coupling efficiency and low cost. The spherical aberration of ball lens 15 may be compensated for by aspherical plastic lens 16. Because ball lens 15 may convey the most optical power in system 10, the combination of a glass ball lens and plastic molded optics may provide thermal stability and high coupling efficiency for optoelectronic element and single mode optical fiber coupling applications.
Lens 25 may be a 1.5 mm ball lens made of LaSFN9™ material available from Edmund Industrial Optics. Lens 26 may be made of Zeonex™ E48R material available from Zeon Chemicals L.P. Fiber 23 may be an SMF-28™ single mode optical fiber available from Corning Incorporated. Window 24 may be made from BK7™ material available from various vendors. Window 24 may be a hermetically sealed window of a TO-56 can or other package incorporating light source 22 such as a VCSEL.
Like system 10, coupler system 20 may have thermal stability and high coupling efficiency for coupling light into SM (single mode) optical fiber 23. In the above-described systems 10, 20 and 40, end faces of optical fibers 13, 23 and 43, respectively, may be situated so as to be in contact with aspherical lenses 16, 26 and 46, as shown in the respective
In systems 10, 20 and 40, light sources 12, 22 and 42 may be single mode VCSELs or other sources of that mode. However, they may be multimode VCSELs or other sources of that mode. The optical fibers 13, 23 and 43 of these systems may be single mode or multimode, as applicable.
A design for aspherical concave lens 26 may be indicated by the following equation and parameter values.
z={cr2/[1+(1−(1+k)c2r2)1/2]}+A2r2+A4r4
Surface 1
The following chart shows the coupling efficiency of system 20 versus deviation of the alignment of the output of the system with optical fiber 23. This chart appears to reveal system 20 as having a good tolerance to some misalignment of its output with optical fiber 23 to which system 20 is coupling light from light source 22.
Coupler systems 10, 20, 30 and 40 may be a part of an array of light sources such as VCSELs and an array of fibers to which that the light is coupled. On the other hand, components 12, 22, 32 and 42 may be detectors receiving light from their respective coupling systems that are receiving light from an optical fiber or fibers. The coupled light may include light signals such as communications signals.
Although the invention has been described with respect to at least one illustrative embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.
