Jointed optical fibers

Details Jointed optical fibers Jointed optical fibers

1999-11-03

2002-01-08

Ullah, Akm E. (Department: 2874)

Optical waveguides

With splice

Fusion splicing

C385S099000

Reexamination Certificate

active

06336749

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to the field of optical fiber systems involving connections between dissimilar optical fibers.
BACKGROUND
In addition to the standard communications fiber, a number of new types of optical fiber have been introduced into optical fiber based communication systems in recent years. Two groups of new fibres can be identified in terms of their refractive index distributions. The first group comprises dispersion controlling fibres with multiple layered refractive index profiles, while the second group comprises fibers that have a small core diameter but a high numerical aperture (NA) as compared with the standard communications fiber. The NA is defined as the square root of the difference between the squares of the refractive indices of the core and the cladding.
The standard communications single mode fiber is exemplified by the Corning product SMF 28 step index single mode fibre which consists of a circular core region of raised, approximately uniform, refractive index and a surrounding cladding region of uniform refractive index. The fiber consists of silica glass with the core doped with germania to give a raised refractive index and the cladding is typically undoped pure silica. The core diameter is about 9.0 &mgr;m and the mode field diameter (MFD) is about 10 &mgr;m at the wavelength of 1.55 &mgr;m and the NA is about 0.1.
The dispersion controlling fiber (DCF) is exemplified by the Lucent Technologies DCF fiber which is used for dispersion compensation. This fiber has a multiple layered refractive index profile consisting of a raised refractive index core (doped with germania), surrounded by a ring layer of lowered refractive index (doped with fluorine), surrounded, in turn, by a slightly raised ring layer (doped with germania). The DCF fiber has a MFD of about 5.0 &mgr;m at the wavelength of 1.55 &mgr;m. The raising and lowering of the refractive index is with reference to the uniform silica cladding which surrounds the whole.
The high NA fibres are exemplified by the erbium doped fibre (e.g. Fibercore DF 1500F), the photosensitive fibre for Bragg grating writing (e.g. Fibercore PS1500) and the small core fibres used for pigtailing optoelectronic components (e.g. Fibercore SM1500).
In many potential applications, it is envisaged that these special optical fibers will be spliced permanently to standard communication fibers.
A standard method for connecting two lengths of standard communication fiber, referred to as fusion splicing, involves butting together the prepared ends of two fibers in the presence of a heat source e.g., a flame or electric arc such that the fiber ends melt and coalesce. Fusion splices are subject to optical losses, referred to collectively as “splice loss.” Various factors have been identified as contributing to splice loss, including lateral offset of the cores, differences in the optical characteristics of the mating fibers, and changes in the refractive index profile that take place during fusion.
When fibers having widely dissimilar mode field diameters (MFDs) and mode field shapes (MFSs) are spliced to one another, the mismatch of the mode fields at the location of the splice can result in high splice loss.
One technique for mitigating this contribution to the splice loss is described, for example, by D. B. Mortimore and J. V. Wright, “Low-Loss Joints between Dissimilar Fibers by Tapering Fusion Splices,” Electronics Letters, 22 (Mar. 13, 1986), pp. 318-319. This tapering technique involves first making a standard fusion splice and then drawing the softened glass in the vicinity of the splice such that the glass becomes constricted, decreasing the diameter of both the cladding and the core in the vicinity of the splice. This tapered region is said to function as a mode transformer that transforms the optical mode field of one fiber to that of the other with low optical loss. A standard communication fiber has reportedly been joined, with a total splice loss of 0.56 dB, to a fiber having a core diameter of 3.8 &mgr;m and an MFD of 4.34 &mgr;m. This tapering by drawing approach has never been practically demonstrated outside the laboratory.
An alternative approach to fusion splicing of fibers, based on the diffusion of dopants in the refractive index profile, was reported by, for example, W. Zell, et al., “Low-Loss Fusion Splicing of PCVD-DFSM Fibers,” Journal of Lightwave Technology, LT-5, (September 1987), pp. 1192-1195. The approach of Zell, et al. involves spreading the smaller of the cores of the (not very dissimilar) mating fibers by diffusing the index-raising dopant during an annealing step after the splice is formed. (The index-lowering dopant of the cladding was also found to diffuse during heating.). Zell, et al. reported that diffusion tapering was effective in reducing the optical loss in a fusion splice between a depressed cladding, single-mode (DCSM) fiber and a dispersion flattened, single-mode (DFSM) fiber having a smaller MFD than the DCSM fiber.
Significantly, the heat treatment, reported in that work, caused the concentrations of germanium and fluorine dopants, respectively, to exhibit a diffusion profile extending axially from the joint. At a wavelength of 1.3 &mgr;m, a splice loss of 0.30 dB was achieved by Zell, et al. This splice loss was smaller than the theoretical loss in a step joint between the two fibers, and the difference was attributed to diffusion tapering. However, at a wavelength of 1.55 &mgr;m, a somewhat greater loss, 0.35 dB, was observed, and no reduction of loss attributable to diffusion tapering was observed.
In a practical communication system, it is desirable for splices between different fibers to exhibit still smaller losses, e.g., losses much smaller than 0.3 dB. The Zell, et al. reference does not disclose a technique that can produce low-loss splices between fibers having drastically different core sizes, refractive index profiles, MFDs and MFSs. Indeed, at 1.55 &mgr;m, which corresponds approximately to the operating wavelength of erbium amplifiers, Zell, et al. has failed to show any improvement in splice loss by diffusion tapering. Moreover, the improved splice reported there involved a pair of only moderately dissimilar fibers both with relatively large cores, i.e., fibers with respective MFDs of 10.1 &mgr;m and 7.6 &mgr;m at a wavelength of 1.55 &mgr;m.
The work of Zell et el. has been extended by Cohen et al. (‘Optical communications system comprising a fiber amplifier’ U.S. Pat. No. 5,074,633 Dec. 24, 1991). Cohen et al. describes a splice joint with a loss of less than 0.15 dB at 1.55 &mgr;m between an erbium doped fiber with a MFD of less than 4 &mgr;m and a communications fiber with a MFD of about 10 &mgr;m. This result was also achieved with an annealing step after the splice was formed where the heat source was an oxy-hydrogen flame of about 0.6 mm in length.
Zell and Cohen each use diffusion tapering to a limited extent by preferentially diffusing the smaller of the cores at the annealing stage and keeping the diffusion of the larger of the cores to a minimum in an effort to equalise the core diameters. Thus, the diffusion takes place primarily in one fiber only. Cohen uses a maximum diffusion time of only 200 seconds at 2000° C.
The difference in MFD and MFS between DCF and standard communications fiber is very large compared to the difference in fibers which have been spliced using the prior art. Thus, practitioners in the art have until now failed to provide a fusion splice that is capable, for operation at about 1.55 &mgr;m, of joining a multiple layer fiber to a communications fiber having radically different MFDs and MFSs, with a total splice loss less than 0.2 dB over a wide wavelength range. Similarly, splice losses between the standard communications fibre and high NA fibres have not been demonstrated below about 0.13 dB.
Another problem arises in attempting to produce a low loss joint between fibers of different diameters such as a standard telecommunications fiber and a high NA fibers with small diameters as compared with the telec

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