Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
2000-01-19
2001-12-11
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling function
C385S027000, C385S031000, C385S042000, C385S043000, C385S123000, C385S126000
Reexamination Certificate
active
06330382
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to couplings between laser sources and multimode optical fibers, particularly for purposes of supporting consistent bandwidths over short distances.
2. Technical Background
Local area optical networks within and between buildings often employ multimode fibers for conveying high speed signals. Cores of the multimode fibers are larger in diameter than cores of single mode fibers and can carry more power. The larger core diameters also simplify splicing and alignment of the fibers with each other or with other network connections.
Most multimode fibers are tuned for use with conventional light emitting diode (LED) sources. These sources have limited modulation capability and, therefore, can only support data rates up to approximately 500 MB/s. In order to support higher speeds, such as 1.25 GB/s (as in the Gigabit Ethernet standard), systems require laser sources. However, lasers such as Fabry Perot (FP) lasers and vertical cavity surface emitting lasers (VCSELs), which are preferred for their higher power, narrower spectral width, higher modulation rates, and ability to couple to multiple arrays, generally couple less reliably to multimode fibers.
Compared with conventional LED sources, the lasers (FP lasers and VCSELs) produce smaller spot sizes that underfill the multimode fibers (i.e., not all of the modes are equally excited). Since each of the modes conveys light a little differently, the bandwidth varies between the different fill conditions. The smaller spot sizes also concentrate energy near the centers of the multimode fibers where refractive index profiles are particularly difficult to control. Small changes in the alignment of the lasers with the multimode fibers can produce further variations in the bandwidth.
Although the multimode fibers can be adjusted with respect to the laser sources to optimize bandwidth, the adjustments require in situ testing or other special attentions that can complicate and add to the costs of network installations. For example, repeated trials can be required to identify positions of alignment that support adequate bandwidth and special devices must be added to the link to ensure alignment is maintained.
SUMMARY OF THE INVENTION
Our invention provides more reliable bandwidth for multimode fiber networks. Multimode couplers arranged in accordance with our invention expand the light output beam of laser sources to more evenly fill the modes of the multimode fibers. The resulting bandwidths supported by the multimode fibers are less sensitive to alignment variations with the laser sources so that upgraded network installations can be made with FP lasers and VCSEL sources without trial and error adjustments of alignment positions.
A mode conditioned multimode fiber system according to one embodiment of our invention includes a multimode coupler having a first end connected to a laser source, a second end connected to a multimode optical fiber, and an intermediate section joining the two ends. The intermediate section of the coupler is tapered between the first and second ends to progressively increase a diameter of a single mode or other less than overfilled light beam entering the first end of the coupler from the laser source. The beam diameter is increased to a size that fills at least one-half and preferably all of the modes of the multimode optical fiber.
Within the coupler is a core surrounded by inner and outer layers of cladding. The core and at least the inner layer of the cladding are drawn down along the coupler length to smaller cross-sectional dimensions by amounts that force propagation of the beam beyond the core into the inner cladding layer. Enough inner cladding remains at the second end of the coupler to guide the expanded beam at the desired beam diameter.
The combined diameter of the core and inner cladding layer at the first end of the coupler is sized to form a single mode or slightly larger connection with the laser source. The numerical aperture and the beam diameter at the first end preferably match corresponding characteristics of the laser source. The combined diameter of the core and inner cladding layer at the second end of the coupler is sized to form a multimode connection with the multimode fiber. The numerical aperture and the beam diameter at the second end sufficiently match corresponding characteristics of the multimode fiber to excite the majority if not all of the modes of the multimode fiber. The beam diameter at the second end of the coupler is preferably equal to at least one-half of the core diameter of the multimode fiber.
A laser source, such as a FP laser or VCSEL source, can be coupled to a multimode fiber in accordance with our invention by connecting the first end of the multimode coupler to the laser source, connecting the second end of the multimode coupler to the multimode fiber, and expanding a diameter of a light beam along the length of the multimode coupler so that the modes of the multimode fiber are more evenly filled by the light beam. At least one-half to substantially all of the modes of the multimode fiber are preferably filled. The beam expansion is made by tapering a core of the multimode coupler to force more of the light beam into a surrounding inner cladding layer. An outer cladding layer confines the expanded light beam within the inner cladding layer.
The multimode coupler, which is positioned between the laser source and the multimode fiber, forms a single mode or slightly larger connection to the laser source and a multimode connection to the multimode fiber. The resulting light beam expansion reduces bandwidth sensitivity to radial alignment variations between the multimode coupler and the multimode fiber. Accordingly, the multimode couplings can be aligned to accuracies similar to couplings between multimode fibers.
Additional features and advantages of the invention will be set forth in the detailed description which follows and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operation of the invention.
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Harshbarger Douglas E.
Nolan Daniel A.
Thomas Leo C.
Truesdale Carlton M.
Alden Philip G.
Corning Incorporated
Healy Brian
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