Optical waveguides – With disengagable mechanical connector – Optical fiber/optical fiber cable termination structure
Utility Patent
1999-08-05
2001-01-02
Font, Frank G. (Department: 2877)
Optical waveguides
With disengagable mechanical connector
Optical fiber/optical fiber cable termination structure
C385S061000, C385S078000, C385S076000, C385S060000, C385S057000, C385S055000
Utility Patent
active
06168319
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to optical components employing optical fibers, and in particular to fiber collimators.
2. Technical Background
Optical fiber has become increasingly important in many applications involving the transmission of light. Over long distances, single-mode silica optical fiber is used in optical communications networks to transmit large amounts of data with low loss and immunity to interference. Telecommunication fibers are usually designed to transmit only a fundamental optical mode. That is, they are single-mode at the preferred telecommunication bands near 1310 and 1550 nm, which are in the infrared portion of the spectrum.
The light transmitted through the fiber can also be subjected to different types of optical interactions to filter, modulate, split, combine, or otherwise act on the light. In most cases two or more fibers are led into an enclosure operating as an optical system. The input light entering the enclosure, usually but not always on one fiber, interacts with some optical device within the enclosure, and the resulting light exits the enclosure via one or more fibers. One example of a two-port system is an optical isolator in which two polarizers sandwiching a Faraday rotator are positioned between the two fibers which have collimating lenses adjacent their free ends. The polarization angles are set such that light can propagate in one direction through the isolator but is prevented from propagating in the opposite direction. Another example is a dielectric interference filter which transmits or reflects selected wavelengths.
In practice, the fibers used in such optical systems are typically held in collimator assemblies which are easily aligned to the enclosure of the optical system. A commercial collimator includes as its most fundamental components the fiber, a small glass tube (sometimes referred to as a capillary) which holds the exposed fiber end, and a graded-index lens (GRIN) lens. A GRIN-type lens used with optical fiber is a generally cylindrically shaped piece of optical glass with a length longer than its diameter. It is fabricated to have a radially varying index of refraction that is greater towards the center, with the result being that it produces a focusing effect similar to a convex lens. GRIN lenses are commercially available under the trade name Selfoc.® Both the fiber and the GRIN lens are inserted and held in the tube s;o that the GRIN lens collimates the light diverging from (or focuses the light to) the smaller core of the optical fiber.
Conveniently, the fiber is held in the collimator assembly by a cylindrical ferrule which closely fits inside the small glass tube. The distance between the fiber end (as determined by the ferrule position) and the GRIN lens is crucial for collimation. Once the distance has been set, the fiber and GRIN lens are fixed within the tube by epoxy, for example, so as to maintain the desired collimation. The collimator can then be inserted into the optical system with alignment provided by sleeves, for example, into which the glass tube and other optical components fit snugly. The fixed displacement between the fiber and the GRIN lens in the collimator should provide an optically well-characterized beam, and result in minimum insertion loss between the fiber and the optical system. Although for some applications the beam should preferably be collimated in the far field, in other applications there are alternate considerations. For example, the beam may be focused to a minimum spot size at a predetermined distance, or the desired beam may be characterized as having some combination of beam size and beam divergence at some predetermined distance. Such aligned collimators are often commercially available or fabricated independently of the optical system in which they are to be used.
Conventionally, the assembly and alignment of collimators have involved a long, tedious, and labor-intensive operation. In many cases, the collimators which are so aligned must be paired together for use within a dual- or multi-port optical system. The pairing imparts additional complexity to the distribution, sale, and use of collimators.
In the typical process of assembling a collimator, the GRIN lens is bonded into the glass tube with its angled (or faceted) side inside the tube and it plano side facing outwardly toward the intermediate optical component. The fiber is received and held within the bore of the ferrule along its central axis, and the ferrule-fiber assembly is inserted into the tube (or sleeve). Thereafter, the ferrule's position (and hence the fiber's position) are varied or adjusted along the length of the tube until some optimum condition is achieved. At that point, the ferrule is bonded to the tube.
The fabrication of an input matched-pair collimator has typically been a two-step process involving the alignment of two collimators. For example, in the first step the assembly of the tube and GRIN lens bonded to one end of the tube is held in a fixture. The ferrule-fiber assembly is fit into the other end of the tube, and a visible laser is coupled to the opposing free end of the fiber or “pigtail.” The position of the ferrule-fiber assembly is varied axially along the z-direction in the tube, and the spot size for the beam exiting the GRIN lens is visually observed striking a surface of an infrared-sensitive phosphorescent card placed from 0.5 to 6 feet (15 to 100 cm) from the plano face of the GRIN lens. At the ferrule position at which the spot size is minimized, the ferrule-fiber assembly is permanently fixed within the tube. At these distances, a minimally-sized beam is focused and essentially collimated with no divergence or convergence. However, a problem with aligning at these distances is that azimuthal misalignment between the faceted faces of the ferrule and GRIN lens introduces an angular offset that results in a beam displacement at these large distances that is too large for automated equipment to easily accommodate. As a result, the alignment of the input collimator has been accomplished visually by the operator with little opportunity for simplified automation without complex measurement equipment and software routines.
It is generally felt that two collimators independently aligned according to this method are not adequately aligned to each other to allow their use in one system. As a result, it is conventional to align a second collimator using the first aligned collimator as a source. In one conventional process, an already-aligned input collimator is used to align a second matched collimator, and the two are thereafter maintained as a pair. In this process, a laser is attached to the pigtail of one collimator, and in optical intensity detector is attached to the pigtail of the second (or output) collimator. The already-aligned input collimator is mounted onto a fixture allowing tip-tilt adjustment (that is, two orthogonal angular adjustments). The bonded assembly (including the GRIN lens and the glass tube for the collimator to be matched) is held in a fixture providing two degrees of adjustment in tilt-tip angle, and an additional two degrees of freedom in the x- and y-directions perpendicular to the tube axis. A first stage holding the tube-GRIN lens assembly provides adjustability for the two tip-tilt angles and the x- and y-directions. A second stage holding the fiber-ferrule provides the z-direction adjustment. The various stages are manually adjusted to minimize insertion loss, which is measured by observing the magnitude of the optical signal on a detector attached to one of the pigtails.
Typically, the ferrule is adjusted for maximum signal quality, and then the other five dimensions are adjusted to further increase signal quality. Whenever the z-axis adjustment is made, any angular offset from the collimator affects the spatial offset and significant adjustment of the remaining controls in the x- and y-directions is typically required. Once the performance of the matched collimator has been op
Alden Philip G.
Corning Incorporated
Font Frank G.
Ratliff Reginald A.
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