Patent Application
20050069256
The present invention relates to the optical coupling between a light source and an optical fiber. More particularly, the invention relates to an optical fiber having an integral microlens, and a method for forming microlenses of many different shapes on optical fibers of many diverse types.
Optical fiber technology is used in widely diverse applications. The use of optical fiber technology requires the optical fiber to gather light directed at the end of the fiber. The ability of the optical fiber to gather light is referred to as the coupling efficiency of the fiber. It is desired that as much light as possible be gathered by the optical fiber. For light to enter into an optical fiber from a light source, the light source and optical fiber are generally coupled by aligning the end of the optical fiber with the light source. However, due to divergence in the angle of emission of light from the light source, the coupling efficiency with optical fibers can be improved. Consequently, there is a need to improve the coupling efficiency between the light source and the optical fiber. It is known that the coupling efficiency can be improved dramatically by the use of a lens at the fiber end.
Numerous techniques are known for forming lenses at the ends of optical fibers. In some applications, discrete lenses are attached to the fiber end (for example, see U.S. Pat. Nos. 4,269,648; 4,380,365; 4,118,270 and 4,067,937). It is also known that a lens may be fabricated directly on the end of an optical fiber. This approach is generally preferable to attachment of a discrete lens because of its relative mechanical simplicity and freedom from complicated lens/fiber alignment procedures.
Direct lens fabrication techniques include cleaving the optical fiber to a square edge and then etching the end of the fiber (such as in an acidic solution) to form a rounded lens thereon (see U.S. Pat. No. 4,118,270 to Pan et al.). Another technique includes heating the optical fiber and pulling its ends so as to form a narrow waist, then cleaving the fiber at its waist to form a long substantially conically tapered lens (see U.S. Pat. No. 4,589,897 to Mathyssek et al.). Another technique for forming a lens on an optical fiber end is to heat the end of the fiber to its melting point to produce a rounded surface. Yet another technique includes abrasive lapping of the end of the optical fiber to achieve a conical lens (see U.S. Pat. No. 4,818,263 to Mitch) or wedge-shaped lens (see U.S. Pat. No. 5,845,024 to Tsushima et al.). In addition, these various techniques may be combined, for example, by abrasive lapping and then heating of the end of the optical fiber.
A variety of problems plague the known techniques for directly fabricating a lens on the end of an optical fiber. One problem is that many fabrication techniques are useful for forming only a limited range of lens shapes. Also, many prior art fabrication techniques are unable to form unusual lens shapes, or unable to form lenses on optical fibers having unusual geometries. Although most optical fibers have a circular core positioned in the center of the fiber cladding, other optical fiber geometries are also known. For example, some optical fibers have cores that are not circular in cross-section or that are not centered within the cladding. Prior art lensing techniques are typically unsuited for use with optical fibers having asymmetric geometries.
In addition to unusual fiber and lens geometries, some optical fibers, such as polarization maintaining (PM) fibers, include regions adjacent the fiber core that are highly doped with alumina or other material to induce a stress in the fiber that induces birefringence. These highly doped regions do not heat or etch at the same rate as the glass in other portions of the optical fiber. This difference interferes with the formation of lenses on these fibers by known techniques.
The present invention is an optical fiber having a lens integrally formed on an end of the optical fiber, and a method of fabricating a lens on the end of an optical fiber. The present invention is useful for forming diverse lens geometries and may be used with optical fibers having many different constructions and geometries.
In one aspect of the invention, the lens on the optical fiber has a finite radius of curvature in a first direction and a finite radius of curvature in a second direction orthogonal to the first direction. The radius of curvature in the first direction is different from the radius of curvature in the second direction, and at least one of the first and second directions is non-orthogonal to a longitudinal axis of the optical fiber. A transverse cross-section of the optical fiber has anisotropic physical properties according to one embodiment of the invention. According to another embodiment of the invention, the transverse cross-section of the optical fiber does not have anisotropic physical properties. According to another embodiment of the invention, the transverse cross-section of the optical fiber is non-circular.
In another aspect of the invention, the lens is formed on the optical fiber by drawing the tip of the optical fiber over an abrasive media in a spiral curvilinear pattern. The curvilinear pattern is shaped to abrade the tip of the optical fiber such that the result is the desired lens shape. In one embodiment according to the invention, the curvilinear pattern is shaped to compensate for asymmetric physical properties in the transverse cross-section of the optical fiber.
In another aspect of the invention, the lens is formed on the optical fiber by drawing the tip of the optical fiber over an abrasive media in a curvilinear pattern that is selected from the group consisting of substantially oval patterns, substantially elliptical patterns, substantially egg-shaped patterns, substantially pill-shaped patterns, and substantially iron-shaped patterns.
One purpose of a lens on an optical fiber is to route light from a light source into the core of the optical fiber as efficiently as possible. Typically, the light produced from the light source diverges. The divergent light pattern may take nearly any shape. The generated light pattern may be a generally circular shape, but more often takes a generally elliptical shape. In the case of a generally elliptically shaped light pattern, cylindrical lenses are usually employed because cylindrical lenses couple the light more efficiently than conical or spherical lenses. However, cylindrical lenses are not completely efficient, because light at the extreme ends of the light source ellipse is not coupled into the optical fiber core, but rather coupled into the cladding of the optical fiber. The light directed into cladding of the optical fiber is therefore lost. To increase the lens coupling efficiency, a biconic lens shape is required to capture this extra light and focus it into the core. As used herein, a biconic lens shape is a lens having a finite maximum radius of curvature along a first axis, and a finite radius of curvature along a second axis orthogonal to the first axis, where the radii of curvature are different from each other. As used herein, spherical lenses, conical lenses and cylindrical lenses are excluded from biconic lens shapes, as they either have at least one radius of curvature which is not finite (e.g., a cylindrical lens), or have radii of curvature that are not different from each other (e.g., conical and spherical lenses).
In other instances, a spherical, conical or cylindrical lens is desired, but the lens is required on an optical fiber having a transverse cross-section with anisotropic physical properties. The anisotropic physical properties in the transverse cross-section of the optical fiber make it very difficult or impractical to fabricate a spherical or conical lens shape on the optical fiber using prior mechanical fabrication techniques. As used herein, “anisotropic physical properties” refer to those properties of an optical fiber that differ depending upon the direction measured in the transverse cross-section of the optical fiber. For example, the bending stiffness or abrasion resistance of an optical fiber may vary in different directions across the transverse cross-section of the fiber. As an example, using abrasion techniques as shown in U.S. Pat. No. 4,818,263 to Mitch, it is difficult to produce a truly conical lens on a polarization maintaining optical fiber because the bending stiffness of the fiber varies with rotation about the fiber axis. As a result, attempts to produce a conical lens on polarization maintaining fibers often produce elliptical lens patterns with variations in the major and minor radius by as much as 40%. One aspect of the present invention allows the fabrication of a wide variety of lens shapes on many diverse types of optical fibers.
Specific types of optical fibers with which the present invention may be successfully employed include polarizing maintaining (PM) optical fibers and polarizing (PZ) optical fibers. Polarization maintaining (PM) optical fiber is a single mode optical fiber that is designed to have a large internal birefringence caused by geometric and stress effects in the fiber. The polarization state of linearly polarized light that is launched on a birefringent axis is maintained as is propagates along the fiber. Polarizing (PZ) optical fiber is a highly birefringent, single mode optical fiber that is designed so that one polarization state has much higher loss than another. Unpolarized light that is launched in to the PZ fiber will emerge as polarized light.
In one embodiment according to the invention, as also seen in
The lensed optical fibers described herein may be fabricated across a wide range of lens shapes and on a wide range of optical fiber constructions using the fabrication technique according to the invention, in which a tip of the optical fiber is drawn over a flat abrasive media in a curvilinear pattern, as further described below.
Referring now to
Prior to forming a lens 18 on the end of the optical fiber 10, 10′, the outer coating 32 of the protruding fiber 10, 10′ is stripped off of the optical fiber via either mechanical or chemical means. A short length of the outer coating 32 is optionally left protruding from the end 34 of the holding fixture 30. The short length of outer coating 32 acts as a protective sleeve and also as a strain relief mechanism for the protruding length of bare glass fiber 36 during the remainder of the lensing process.
The protruding bare glass fiber 36 is next cleaved to leave a desired free length L using a fiber optic cleaver as is commonly available. A free length L remains protruding from the holding fixture 30. A preferred free length L of glass fiber 36 will depend upon the fiber properties, such as diameter and bending stiffness. The minimum free length L of glass fiber 36 is dictated by the width of the cleaver blade, and may be as small as 1 mm.
The holding fixture 30 with its secured optical fiber 10, 10′ is then mounted to a movable stage (not shown). The movable stage moves the holding fixture 30 and fiber tip 16 relative to an abrasive media 40 along a programmed path within a three dimensional space bounded by the travel limits of the movable stage. When working with an optical fiber 10′ having a transverse cross-section with anisotropic physical properties, the orientation of the optical fiber 10′ relative to the movable stage depends upon the lens design (i.e., the orientation of the anisotropic physical properties relative to the direction of the stage movement should be known).
After holding fixture 30 and optical fiber 10, 10′ are secured to the moveable stage, the tip 16 of optical fiber 10, 10′ is dragged across abrasive surface 40 in a predetermined curvilinear pattern to remove material from the fiber tip. As the optical fiber 10, 10′ is dragged across the abrasive media 40, the optical fiber 10, 10′ bends and the tip 16 of the fiber 10, 10′ becomes oriented at a precise contact angle β with respect to the abrasive surface 40 (
Thus, according to an exemplary embodiment, the contact pressure exerted on the tip of the fiber and the contact angle can be controlled by one or more of the following parameters: the free-fiber length (L) of unsupported fiber, the distance between the end of the holding fixture and the abrasive surface, and the physical properties of the fiber (e.g., bending stiffness, diameter, composition).
The fiber tip 16 rotates with vertical position of the holding fixture 30. Thus, lowering the fiber tip 16 into position at the beginning of the fiber abrasion process and pulling the tip 16 up when finished without creating undesirable artifacts on the lens surface should be addressed. Potential problems can be averted by providing a smooth (non-abrasive) film 42 over the ends of the abrasive media 40 to allow correct positioning of the optical fiber tip 16 as it contacts the abrasive media 40.
As best illustrated by the arrows 46 in
After the optical fiber is properly positioned on the abrasive media, the tip 16 of the optical fiber is drawn over a flat abrasive media 40 in curvilinear patterns. By carefully manipulating the curvilinear pattern traced by the fiber tip 16, lenses of many shapes may be created, and any anisotropic physical properties of the optical fiber may be accommodated. For example, when a conical lens is desired on an optical fiber 10′ having a transverse cross-section with anisotropic physical properties (such as a polarization maintaining fiber), the fiber tip 16 may be moved in a curvilinear pattern that offsets the anisotropic properties (e.g., bending stiffness and abrasion resistance) to produce a truly circular conical shaped lens, and thus produce a more efficient lens on the fiber to couple, for example, with a light source.
Examples of fiber abrasion patterns are illustrated in
In alternate fiber abrasion patterns according to the invention, the radius on one end of the generally elliptical pattern may be different from the radius at the opposite end of the pattern to create an “egg” shaped pattern or generally “iron” shaped pattern as shown in
The curvilinear abrasion patterns according to the invention are not limited to the exemplary abrasion patterns of
It should be noted that terms used herein describing shapes, such as “ellipse”, “elliptical”, “toric”, “circle”, “circular”, “spiral”, etc., are not intended to be limited by their mathematical definitions, and are rather understood to generally or substantially resemble such shapes.
The creation of an angled toric lens with a large torus radius is illustrated in
The creation of an angled toric lens with a small torus radius is illustrated in
The creation of a conic lens on a fiber with anisotropic physical properties is illustrated in FIGS. 10A-B and 11. In this example, a PM optical fiber (Tiger fiber Type 7129 available from 3M Company of Saint Paul, Minn., U.S.A.) with anisotropic physical properties was loaded into a collet so that the fiber protruded from the bottom face of the collet by 6.25 mm (0.246 inch), with the major axis of the fiber stress ellipse loaded consistently in one direction. The fiber was subjected to a series of nine process stages consisting of drawing the fiber tip across flat abrasive lapping films in a series of true elliptical spiral patterns similar to those shown in
The spiral paths used in each process stage are described by a set of X-Y coordinates referenced to an X-Y Cartesian coordinate system lying on the abrasive film. The Y-axis is defined as parallel to the major axis of the fiber stress ellipse as loaded into the collet. The set of X-Y coordinates describing the true elliptical spirals can be described mathematically as follows:
X=x cos(φ)+y sin(φ)
Y=−x sin(φ)+y cos(φ)
where x and y represent the set of Cartesian coordinates describing the spiral in a second x-y Cartesian coordinate system co-located at the same origin as the X-Y coordinate system but whose x-axis is rotated by an angle φ with respect to the X-axis.
The coordinates x and y can be calculated as follows:
x=D cos(Θ) y=D sin(Θ)
where D and Θ are a set of polar coordinates describing the path in a polar coordinate system co-located at the origins of the XY and xy Cartesian coordinate systems, and where Θ=0 represents a direction parallel to the x-axis with increasing Θ moving in a counter clockwise direction towards the positive y-axis. The coordinates D and Θ are related as follows:
where, a0 and Θ0 are the radial and angular polar coordinates representing the starting point of the path, aN represents the final radius of the spiral (measured at Θ0), N is the number of cycles required to achieve the final radius, K is the aspect ratio of the ellipse defined as the ratio of the radius of a true ellipse measured at Θ=½π radians divided by the radius of the ellipse measured at Θ=0 radians.
During stage 1, the fiber was drawn across a 0.5 μm diamond grade flat lapping film in a clockwise elliptical spiral pattern defined by the following parameters: φ==0, Θ0=0, K=1.05, a0=0.165 inches, af=0.125 inches, N=200. The spiral pattern was followed by drawing the fiber in 10 cycles around an elliptical pattern defined by the aspect ratio and final radius of the spiral pattern. During stage 2, the fiber was drawn across the same lapping film as stage 1 and in the same pattern as stage 1 but in a counter-clockwise direction with φ=¼π. During stage 3, the fiber was drawn across a 0.1 μm diamond grade film in a clockwise pattern identical to the pattern in stage 1. During stage 4, the fiber was drawn across the same 0.1 μm diamond grade film in a counter-clockwise pattern identical to stage 2. In stages 5, 7 and 9, the fiber was drawn across a cerium-oxide lapping film on a “flocked” backing in a pattern identical to stage 1. In stages 6 and 8, the fiber was drawn across the same flocked cerium-oxide lapping film in a pattern identical to stage 2.
During stages 1-4 the collet to film distance was fixed at 5.3 mm (0.21 inch). During stage 5-8 the collet to film distance was set at 5.8 mm (0.23 inch) and in stage 9 the distance was set at 6.1 mm (0.24 inch). The transitions down to and up from the collet to film distance used in the spirals were handled by drawing the fiber in a helical pattern defined by the aspect ratio and initial radius of the spiral until the collet was moved into the proper vertical position. During the helical transitions a cover film was placed over the abrasive lapping film to avoid the creation of non-uniformities in the fiber lens.
The resulting lensed fiber had a wedge angle of 98.5°, a maximum lens radius of 6.81 μm and a minimum radius of 6.48 μm measured orthogonally to the maximum radius. The farfield pattern of the lens is shown in
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.
