Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
2001-07-03
2004-07-20
Fortuna, José A. (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S408000, C065S381000, C065S406000
Reexamination Certificate
active
06763685
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to optical fiber couplers made by fusing and tapering the optical fibers and fabrication of such couplers so as to provide multiplexing and demultiplexing optical functions with minimal polarization effect. This invention also relates to the design of such couplers having predetermined wavelength periods.
2. Description of the Prior Art
Fused tapered couplers are made by laterally fusing and tapering two or more optical fibers. The technique enables an exchange of light power between two or more optical fibers and can be used to fabricate power splitters. One advantage of this method is that light never leaves the glass of the optical fiber and never encounters an interface, making the coupling process inherently reflection free.
Initially, this technique was discovered at the Canadian government communications research centre in Ottawa with reference to multimode optical fiber distribution systems, which lead to several patents, such as U.S. Pat. Nos. 4,291,940; 4,330,170; 4,439,221; 4,449,781; 4,586,784 and 4,763,977. It was soon realized that it also worked with monomode or single-mode fibers as disclosed, for example, in U.S. Pat. No. 5,054,874, but with a somewhat different behaviour.
With single-mode fibers, the coupling of light between the fibers was oscillatory, as a function of elongation, and thus the coupling ratio could be controlled. Furthermore, it was also observed that this behaviour was oscillatory in wavelength and thus the couplers could be used as wavelength multiplexers and demultiplexers as disclosed, for example, by Bures et al., Applied Optics, 1983, 22(12). In the telecommunications domain, the realization of multiplexers was published, for instance, by Lawson et al., 1984, Electronics Leters, 20(23). It was then determined that the period could be controlled by the number of coupling cycles which are observed during the elongation process. In the early 1980's, the only commercially available multiplexing fused couplers were of large periods (1300 nm-1550 nm) corresponding to 1.5 or 2 elongation cycles. However, it was later shown by Bilodeau et al., Optics Letters, 1987, 12(8), that couplers with large number of cycles had much smaller periods than those with a small number of cycles. The experimental wavelength response of long couplers shows a beating phenomenon, where the sinusoidal spectal response is modulated. This is explained by the modulation attibued to the slight difference in modal propagation constants, as disclosed, for example, by Love et al., Electronics Letters, 1985, 21(12). It became obvious then that to make a good multiplexer with a small wavelength spacing, one had to make a long coupler with many cycles and that for the multiplexed wavelengths, the two polarization states should be in phase, as shown, for instance, by McLandrich et al., Journal of Lightwave Technology, 1991, 9(4).
There are also patents that describe this principle. For example, U.S. Pat. No. 5,491,764 discloses a narrow band twisted optical fiber wavelength division multiplexer/demultiplexer (WDM) where a pair of fibers is first twisted to reduce polarization dependence and then fused to form a coupler. It is stated in this patent that although there exist fiber optic WDMs that use optical fibers which are aligned in parallel with one another and fused to form a fiber optic coupler, they are only capable of MUXing and DEMUXing two preselected wavelength lights, operating at wavelengths of 1310 nm and 1550 nm.
In U.S. Pat. No. 5,809,190 there is disclosed a multi-window wavelength-division multiplexer (MWDM) in which two fibers are crossed and fused together to form a multiplexer coupler. It is stated in this patent that it uses a crossed pair of fibers, instead of a prior art twisted pair of fibers, to improve the polarization dependent loss. By reducing the polarization sensitivity, U.S. Pat. No. 5,809,190 indicates that more than two wavelengths can be multiplexed which is obvious for sinusoidal wavelength response because such response is periodic. This principle was disclosed by Symon et al. in a paper entitled “Dense all fiber WDM by means of Mach-Zehnder interferometer” presented at SPIE Photonics West '96 conference on Functional Photonic and Fiber Devices, held in San Jose, Calif. on Jan. 28-Feb. 2, 1996 and published in the SPIE Proceedings Vol. 2695 pp. 114-122.
Neither of the above patents describes ways to achieve the correct spacing and to match the polarization phase siunultaneoasly, for any given channel spacing. Therefore, there is a need for multiplexing and demultiplexing couplers with narrow channel spacing, wherein one would simultaneously obtain a predetermined wavelength spacing and the required polarization phase match.
SUMMARY OF THE INVENTION
The present invention provides a method of fabrication of multiplexing and demultiplexing couplers with narrow channel spacing of 0.4 nm or larger by controlling the degree of fusion and the shape of the longitudinal profile of the fused fibers. This can be done without either twisting or crossing the single-mode fibers from which the couplers are made by fusion and elongation. This allows a more precise control of the response of the coupler and makes it possible to achieve a match between spacing and polarization for any given channel spacing, that will be reproducible in fabrication. This is possible because the control can be used to reduce or increase polarization dependence and wavelength dependence so that the match can be made for any desired condition. The invention also includes the novel couplers produced pursuant to the new fabrication process.
The principle of operation of single-mode fused fiber couplers is now well known. For simplicity, we will only describe the operation of a 2×2 coupler, i.e., a coupler composed of 2 fused single-mode identical fibers. Although the basic principle presented here is applicable to other fused structures, using more than 2 fibers or dissimilar fibers, most of the discussion herein is oriented towards making a 4 port-device, i.e., 2 input ports and 2 output ports, that can multiplex or demultiplex two series of wavelengths.
In making a 2×2 single-mode fused fiber coupler, two optical fibers are placed side-by-side after stripping of the protective polymer jacket, so that the optical claddings of the fibers are longitudinally in contact over a predetermined length. Such contact can be mechanically maintained or, as indicated in some prior art references mentioned above, the fibers can be crossed or twisted together. The exposed section is placed between two holding clamps that suspend it so that a heat source can be approached to fuse and soften the glass, and to create taper by pulling on the clamps. This creates a bi-taper structure, made of two fibers that share a single optical cladding because they are fused together. If the taper transverse dimensions are small enough, the fiber cores are reduced to a point where they do not guide the light anymore. This power is then guided by the optical cladding and the surrounding medium, which is usually air, thus forming a highly multimode waveguide.
Because of the transverse symmetry of the structure, composed of two fused identical fibers, the single-mode fiber core mode excites, in the down-taper region, a superposition of two optical modes of the fused and tapered region. These modes, hereafter called supermodes, are the fundamental mode, labelled LP
01
and the first asymmetric mode, labelled LP
11
. If the transition in the down taper region is adiabatic. i.e. the taper slope is not too abrupt, the two supermodes are exited equally and no power is lost to higher power modes. The two supermodes then propagate along the fused section, accumulating a phase difference &phgr;. In such adiabatic up-taper region, the supermodes interfere and the power goes back into the fiber cores. Depending on the phase however, the interference will be either constructive in the initial fiber core or if the modes are out of
Fortuna Jos'e A.
ITF Optical Technologies Inc.
Primak George J.
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