Patent Application
20120300312
The present invention relates generally to lenses, and, more specifically, to ball lenses used in an expanded beam optical connector.
Optical fiber connectors are a critical part of many different light transmission systems. For example, they are found in essentially all optical fiber communication systems and they are used in imaging light onto targets in military applications. Such connectors may be used to join segments of fiber into longer lengths, to connect fiber to active devices, such as radiation sources, detectors and repeaters, and to connect fiber to passive devices, such as switches, multiplexers, and attenuators. The principal function of an optical fiber connector is to hold the fiber end such that the fiber's core is axially aligned with an optical pathway of the mating structure. This way, light from the fiber is optically coupled to the optical pathway.
Of particular interest herein are “expanded beam” optical connectors. Such connectors are used traditionally in high vibration, high power and/or dirty environments, where “physical contact” between the fiber and the light path of a mating connector is problematic. To avoid problems of debris and vibration, expanded beam optical connectors have been developed which expand the optical beam and transmit it over an air gap between the connectors. By expanding the beam, its relative size increases with respect to the debris, making it less susceptible to interference. Further, transmitting the beam over an air gap eliminates component-to-component wear, thereby increasing the connector's endurance to vibration. Over the years, the expanded beam connector has evolved into a ruggedized multi-fiber connector comprising an outer housing which is configured to mate with the outer housing of a mating connector, typically through a screw connection. Contained within the outer housing are a number of inner assemblies or “inserts.” Each insert typically comprises an insert housing, a ferrule assembly contained within the insert housing and adapted to receive a fiber, and a ball lens at a mating end of the insert housing optically connected to the fiber.
The ball lens as an interface is particularly useful in military targeting. Such an application involves using a fiber to transmit light from a high power optical source (e.g., a laser) to an optical system which images the light onto a target. The ball lens interface is preferred because of the high power density of the light within the fiber. Expanding the beam reduces the power density at the separable interface, which reduces the probability of laser-induced damage caused by interaction of the laser with dirt and debris at the separable interface. In such an application, the laser wavelength is longer than the typical communication wavelength of approximately 1.31 μm to 1.55 μm.
One such ball lens 100 is shown in
Tyco Electronics Corporation (Harrisburg, Pa.) manufactures expanded beam (EB) connectors using the PRO BEAM® trademark. These expanded beam connectors are for use at typical communications wavelengths such as 0.85 μm, 1.3 μm, 1.31 μm and 1.55 μm, where the first two wavelengths are often used in multimode (MM) applications and the second two wavelengths are used for singlemode (SM) applications. These EB connectors use a 3 mm ball lens to collimate and then refocus the light. A ball lens with an index of refraction of approximately 2 focuses collimated light near the surface of the ball. Therefore in the SM design, the lens material is chosen such that the index of refraction is nearly equal to 2 at 1.31 μM and 1.55 μm. For MM applications, a lens material is used which has an index of refraction less than 2, such that the focal plane is outside of, i.e., external to, the lens. In the MM EB connectors, the mechanical housing (referred to as an “EB insert”) holds the fiber at a predetermined distance from the lens so that the fiber end is positioned at the external focal plane.
A ball lens can be easily manufactured with precise control of the diameter and the sphericity. Therefore the ball lens can be accurately located with respect to a fiber using passive alignment features that are built into the EB insert, using the mechanical characteristics of the ball/sphere shape, i.e., the geometry of a sphere provides a significant “registration surface” for use in positioning of a ball-shaped lens.
The above-described design functions quite well for typical communication wavelengths between 0.85 and 1.55 μm. However, situations arise where a particular application may require connectors that will function with higher wavelengths, such as in the military application described above, or in the near to mid-infrared wavelength range of 1.8 μm to 5 μm. This requires the use of a different glass than is used in the current EB connectors. One glass that is commonly used in infrared optics is zinc selenide (ZnSe), which can pass light in the infrared wavelengths. However, a problem exists when trying to use ZnSe for a ball lens: the index of refraction of ZnSe is greater than 2, and thus a collimated beam of light incident on a ball lens 300 made of ZnSe will focus the light to a point 312 inside of the ball, as illustrated in
The claimed invention is a truncated ball lens in which a ball lens, having a focal point internal to (i.e., inside of) the ball lens, is constructed by removing a surface of the ball lens so as to expose the focal point. A surface on the exposed side of the truncated ball lens faces the endface of a fiber directing light towards the lens. In a preferred embodiment, an optical axis of the truncated ball lens coincides with an optical axis of the fiber.
In a preferred embodiment the ball lens is formed of ZnSe; however, it is understood that the ball lens can be formed of any material suitable for use in the infrared wavelengths, and having an index of refraction greater than 2. A surface of the ball lens is removed (e.g., by grinding, drilling, laser processing, and/or polishing) to “expose” the focal point. Such a truncated ball lens offers some of the advantages of a ball lens, such as the ability to position the lens accurately with respect to a fiber using the surface of the ball lens and passive features in an EB insert.
In a preferred embodiment, the fiber is in contact with the truncated ball lens, using the truncated ball lens as a mechanical stop for the fiber, i.e., the generally flat exposed surface of the ball lens coincides with the focal point of the truncated ball lens. However, other variations are possible; for example, in an alternative embodiment, additional material may be removed from the ball lens such that the focal point is beyond the exposed surface, i.e., outside of the truncated ball lens but within the area defined by the ball lens prior to the removal of the material. The fiber and truncated ball lens can then be held at a predetermined distance (e.g., using features in an EB insert), in a manner similar to that described above with respect to the standard multimode EB connector. Such a design allows for active alignment of the fiber-lens distance in the axial direction in order to optimize performance. The choice of which design to use is a tradeoff between the cost of active alignment and the accuracy and cost of the manufacture of the lens.
In the ball lens of
In one embodiment, a hole large enough to accommodate fiber 410 is drilled into the ball lens 400, so that an opening is formed deep enough to enable the endface 412 of fiber 410 to be inserted up to focal point 414. This configuration would look essentially like the configuration of
In
Modeling confirms the excellent performance of the claimed invention. Specifically, modeling was performed in which it was assumed that the fiber is a step-index fiber with a numerical aperture (NA) of 0.3, and a core diameter of 100 μm. A 3 mm ZnSe ball lens was modeled. The distance from the fiber to the ball lens' curved surface and the distance between the two ball lenses were varied to optimize the performance at 1.8 μm and 5 μm. The result was that (ignoring reflection losses) the loss was 0.034 dB at both 1.8 μm and 5 μm.
It is noted that the only parts of the lens which are needed to collimate and transmit the light are those through which light passes. Therefore the ball lens can be modified in any way that may be useful or necessary, so long as the optical path shown above is not affected. For example, one may choose to polish/grind/cut portions to the point where the cone shape is more pronounced as shown in
In the embodiments disclosed above, the fiber is cleaved or polished such that the endface of the fiber is perpendicular to the optical axis and the endface is parallel to the generally flat face of the truncated ball lens. However, it is understood that in another embodiment, the fiber can be cleaved or polished at an angle, such as 8°, in order to reduce back-reflections, and the ball can be oriented such that the flat face of the truncated ball is parallel to the angled endface of the fiber. Further, as shown in
Further, although the portion where the focal point is exposed is referred to herein as comprising a “flat face,” it is understood that this surface could instead be curved (convex or concave) and still fall within the scope of the claimed invention. In preferred embodiments, the general shape of the truncated ball lens is greater than one-half of a sphere, so as to take advantage of the mechanical properties of a sphere-shaped lens, particularly with regard to alignment of the lens. A fundamental element of the truncated ball lens of the present invention is that the area in which the light travels through the lens remains essentially intact. Thus, for example, the truncated ball lens shown in
It is also understood that the terms “planar” and “parallel” as used herein, including in the claims, include situations where the described element is substantially planar or substantially parallel, respectively. For example, when a fiber endface is cleaved or when a portion of the rear area is removed, it is understood that certain imperfections may cause a portion of the endface of the cleaved fiber, or the surface of the rear area after material is removed, to have curved edges, excess material or other imperfections. Such a surface is considered to be planar for the purpose of the claimed invention. Similarly, due to slight imperfections in alignment, elements are considered to be parallel for the purpose of the claimed invention even where the elements are slightly out of a purely parallel alignment, i.e., when they are substantially parallel.
In any of these designs, it is also understood that an anti-reflection (AR) coating may be added to the lens at the lens/air interfaces. For those designs where the fiber is in contact with the lens, an AR coating on the lens and/or fiber will be optimized for the fiber/lens interface. For those designs where the fiber is not in contact with the lens, the surface of the lens nearest the fiber can be AR coated for the lens/air interface, and the fiber can be coated for the fiber/air interface. In any of these designs, it may be desirable (due to cost, for example) to not AR coat one or more of the surfaces. However, optimal performance will be achieved when the surfaces are AR coated.
While there has been described herein the principles of the invention, it is to be understood by those skilled in the art that this description is made only by way of example and not as a limitation to the scope of the invention. Accordingly, it is intended by the appended claims, to cover all modifications of the invention which fall within the true spirit and scope of the invention.
