BACKGROUND OF THE INVENTION

[0001] Broadband beam collimation and focusing is a long-standing challenge for spectroscopists. The dispersive nature of refractive optics introduces chromatic aberrations in lens-based collimators. Although a combination of lenses can reduce the problem, the most natural solution to get achromatic collimation is the use of reflecting optics such as an off-axis parabola (OAP). Off-axis parabolas have been widely used to collimate and focus beams in spectrographs such as in Czerny-Turner systems.

However, while these optics are well-adapted to a slowly diverging beam, their curvature is typically too low to be used for fast optical fiber collimation and focusing.

[0002] There is a high technical challenge involved in fabricating an OAP with a very small radius of curvature and a high surface quality. OAPs are normally fabricated using a single point diamond turning (SPDT) process on a metallic substrate, such as aluminum, to create a concave, parabolic reflective surface. This technique leaves small grooves that must be polished away in subsequent fabrication steps. Doing so, however, produces a surface with a roughness incompatible with the need to focus light into an optical fiber core having a diameter of a few micrometers or less.

SUMMARY OF THE INVENTION

[0003] In accordance with the present invention, an apparatus is provided for changing the divergence angle of a light beam. A solid optical medium is provided that has a relatively high index of refraction, and is positioned to receive the light beam. Because of the high refractive index of the medium, divergent light coupled into it has a degree of divergence lower than it would be in a conventional medium with a lower refractive index, such as air. The light coupled into the optical medium is then incident upon a curved reflective surface along a first principal direction and is reflected. Due to the curvature of the surface, the reflected light undergoes a change in its angle of divergence and exits the optical medium along a second principal direction different than the first. [0004] In an exemplary embodiment of the invention, the curved surface has the shape of a portion of a paraboloid. In one example, the light is divergent and is directed toward the optical medium from a source, such as the tip of an optical fiber, located at a focal point of the paraboloid. The divergent light passes through the optical medium, and the reflection of the light off the curved surface results in its collimation. The collimated light then exits the optical medium along the second principal direction. In another embodiment, collimated light is directed into the optical medium along the first principal direction. Reflection of the collimated light at the curved surface results in its being focused (i.e., having a negative divergence) to a focal point of the paraboloid. A light collector, such as an optical fiber, may be located at the focal point of the paraboloid to collect the reflected light. If the light passes a boundary of the optical medium into a medium with a different refractive index, the focal point of the paraboloid may be located at this boundary to avoid any chromatic dispersion due to refraction.

[0005] In one particular embodiment, the optical medium is a solid material such as glass or some other machinable, optically-transparent material. In such an

embodiment, the solid material provides the desired high refractive index, and also allows the formation of the reflective surface from a surface of the solid material. For example, a glass medium may be machined and polished so that a surface of the glass is in the desired reflective shape, e.g., an off-axis parabola, for light within the medium. A reflective coating may then be applied to the exterior of the solid medium at the location of the curved surface to enhance its reflective qualities. The particular function of the reflector will depend on the application, but a notable use is the focusing of collimated light or the collimating of rapidly diverging light. Because the reflection of the light occurs within the interior of the solid body, the formation of the curved surface may be done by shaping of the exterior of the solid body, greatly simplifying the necessary machining operation and allowing for a concave reflector with a much smaller radius of curvature. To avoid chromatic distortion, a focal point of the curved reflector may be located at a surface of the solid material. The tip of an optical fiber may also be located at this focal point for directing light toward the reflective surface, or for collecting light from the reflective surface. A coupler may be used to hold the fiber in place, and an index matching material may be provided between the fiber and the solid material to minimize losses.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 is a schematic view of a machined substrate from which is formed an off-axis parabolic reflector according to the present invention.

[0007] Figure 2 is a schematic view of the substrate material of Figure 1 after being cut along a longitudinal axis.

[0008] Figure 3 is a schematic view of an off-axis parabolic reflector formed after cutting, polishing and coating of the substrate material shown in Figure 2.

[0009] Figure 4 is a schematic, perspective view of the reflector of Figure 3 shown in three dimensions.

[0010] Figure 5 is a schematic view depicting a possible application of the reflector shown in Figure 3.

DETAILED DESCRIPTION

[0011] The present invention is directed to an optical reflector that changes the angle of divergence of the light it reflects, such as by focusing a collimated beam of light or by collimating a divergent beam. The reflector also includes an optical medium adjacent to its reflective surface that has a relatively high index of refraction. This higher refractive index results in light from a light source having a lower divergence angle than with a medium of lower refractive index, such as air. In one particular embodiment, the optical medium adjacent to the reflector is a solid material such as glass, and the reflector has a reflective surface in the shape of an off-axis parabola that is formed directly from a surface of the solid material (those skilled in the art will understand that "off-axis parabola" is a term of art used to describe a reflector of this nature, and that the actual three-dimensional shape of the reflector is that of a portion of a paraboloid). This allows the parabolic reflecting surface to be fabricated as a convex surface on the exterior of the material, rather than a concave surface, which is much more difficult to fabricate with low radius. Light directed toward the reflector thus passes through a relatively high index medium (e.g., glass), after which it is reflected by the reflector surface. [0012] Shown schematically in Figure 1 is a glass rod of material that is used to form the desired reflector for a first embodiment of the invention. A main portion 10 of the rod has a shape, such as a cylinder, that is appropriate for a standard machining apparatus. As shown, an end portion 12 of the glass rod has been machined to form a small radius paraboloid. Because of tool size limitations, the machining of a concave surface having such a small radius would be difficult or impossible. For example, it is generally accepted that tool size limitations prevent the machining of a concave surface having a radius of curvature less than a few tens of millimetres. However, such a size limitation does not exist for concave surfaces, and the parabolic shape on the surface of portion 12 is formed using standard machining tools.

[0013] The glass rod is subsequently cut to produce the desired reflector form. The particular shape of the reflector structure may be chosen to best suit the intended application. For example, a full parabolic reflector may be made by cutting the paraboloid shape along a plane perpendicular to the longitudinal axis, such as at the boundary between the cylindrical portion and the paraboloid portion. Regardless of the shape, it is important to take into account the location of the focal point of the

paraboloid. This is particularly true if the application requires access to that point, such as in the case of a reflector that focuses a collimated beam into an optical fiber, in which case the end of the fiber would be located at the focal point. In such a case, the material would have to be cut so that the fiber could be positioned appropriately.

[0014] An example of a reflector according to the present invention is shown schematically in Figure 2. After the glass substrate of Figure 1 has been machined and polished, the substrate is cut to form an off-axis portion, in this case the structure being cut along the longitudinal axis. It is not necessary to make this cut in a direction parallel to the longitudinal axis, but doing so somewhat simplifies the fabrication process. Since only part of the paraboloid surface will be used in a subsequent optical application, the substrate may be cut again so that only part of the machined portion 12 remains. Thus, as remaining portion may be such as that shown in Figure 3. This portion 16 is a glass structure with an off-axis parabolic surface and, with the application of a reflective coating 18 to the exterior of the parabolic region, it functions as a reflector for light within the interior of the material. That is, the portion 16 functions as an off-axis parabolic reflector that may be used in a variety of different optical applications. Figure 4 is a schematic view of the reflector from a three-dimensional perspective.

[0015] Shown in Figure 5 is a schematic diagram indicative of how the parabolic reflector of the present embodiment might function. In this example, light exits an optical fiber 20, the tip of which is located at the focal point of the parabolic reflecting surface (as known the art, the focal point for an off-axis parabola is the same as the focal point of the entire parabola from which the off-axis parabola is derived). If the light is to pass through a refractive index interface, it may be beneficial to locate the focal point at the interface to minimize any chromatic distortion of the light due to refraction. In the present embodiment, a tip of the fiber 20 is placed in contact with a surface of the reflector 16 such that any refractive index transition will be located at the focal point of the light. Since the light is coupled directly from the reflector medium to the fiber medium, it will not pass through an ambient environment (e.g., air). The example of Figure 5 makes use of a coupler 24 to hold the fiber in position, and index matching material, such as a fluid or adhesive, may be used between the fiber and the reflector surface to prevent any air gap between the two media. The coupler 24 may also be made adjustable so that its position along the surface of the reflector 16 may be changed to accommodate slight variations in the trajectory of the light.

[0016] The fiber of the present embodiment has a relatively large numerical aperture, and light exiting the fiber has a fast divergence. However, because the reflector medium, in this case glass, has a relatively high index of refraction, the degree of divergence of the beam will be less than if the light from the fiber exited into a medium with a lower index of refraction (e.g., air), as is the case with conventional reflectors. As the reflector 16 of Figure 5 is selected for the particular fiber and particular layout of the system shown, the divergent beam from fiber 20 is collimated by the reflected surface, and exits the reflector body as a collimated beam, as shown at 22. Those skilled in the art will understand that, because the collimated beam exits the reflector medium in a direction perpendicular to the reflector surface, there will be no refraction or

corresponding chromatic dispersion.

[0017] The embodiment shown in Figure 5 is described above as a collimation of light diverging from a fiber, but it represents equally the capacity of the reflector to focus a collimated beam of light, either into a fiber or to some other destination where focused light is desired. In such a case, collimated light entering the reflector medium

perpendicular to a flat surface will remain collimated. After focusing by the reflective surface, however, the light will be convergent (i.e., will have a "negative divergence"), and will exit from the reflector medium to the appropriate collection optics. If the surface of the reflector medium also represents a refractive index interface, the light will be focused at this interface, as is shown in Figure 5. In this way, chromatic distortion due to refraction at the interface will be avoided. In the present embodiment, the surface of the refractive medium 16 represents an interface between the glass material of the reflector and the glass of the fiber core. Thus, if a beam of collimated light 22 was introduced to the reflector medium as shown, the off-axis parabolic surface of the reflector would focus the light at the interface between the reflector medium and the fiber. Of course, if the collecting optics were to have an index of refraction identical to that of the reflector medium, there would be no chromatic distortion at the boundary between the media. However, for the present embodiment, the focus will be at the core of the fiber nonetheless in order to ensure proper collection of the light.

[0018] The small radius off-axis parabolic reflector of the present invention provides a means for providing collimation of a fast diverging beam, or focusing of a collimated beam to a fast converging beam. Moreover, a reflector according to the present invention that has a given parabolic shape will be effective for light having a higher degree of divergence or convergence than would be possible for a conventional reflector having the same parabolic shape, due to the substrate material having a higher index of refraction than air.

[0019] In the foregoing embodiment of the invention, the reflector is formed using single-point diamond turning (SPDT). SPDT is a machining process that uses single crystal cutting tools and specialized machinery to produce a precision surface on selected materials by accurately cutting away a thin portion of the surface. An example of this fabrication process uses the following steps:

1 ) preform or conventionally machine the part to rough shape with excess material left on all the surfaces to be processed;

2) heat treat the part to relieve stress; 3) mount the part with minimal induced stress in an appropriate fixture on the SPDT machine;

4) align the selected diamond tool;

5) finish machining the part to the desired final shape and surface quality with multiple light cuts under computer control; and

6) if necessary, polish the optical surface to smooth it before applying an appropriate optical coating.

[0020] The polishing step may or may not be necessary, depending on the application and the preparation techniques used. Typically present in SPDT is the production of a periodic grooved surface that scatters and absorbs incident radiation. The polishing step may be used to remove these grooves and eliminate the resulting effect.

[0021] Those skilled in the art will understand that there are a number of different curved internal surfaces that may be desired, and that may be formed in accordance with the present invention. In addition, the particular material being used may be selected for its specific characteristics, such as machinability and/or index of refraction. It should also be noted that the formation of a reflector as shown in Figures 1 -3 involves a symmetrical cutting of the substrate in two after forming the parabolic surface. As mentioned previously, such a symmetrical cut is not necessary for creating a desired reflector. However, in this embodiment, doing so would allow a substrate such as that shown in Figure 1 to be formed into two identical reflectors.

What is claimed is: