Optical waveguides – With optical coupler – Permanently fixed coupler
Patent
1996-06-26
1998-05-12
Palmer, Phan T. H.
Optical waveguides
With optical coupler
Permanently fixed coupler
385 42, 385 43, 385 46, G02B 626
Patent
active
057518734
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to fibre optic couplers and to their manufacture, and in particular to 1.times.4 or 2.times.4 biconical taper fused single mode couplers. The description hereinafter will principally refer to 1.times.4 couplers but the principles involved are also applicable to 2.times.4 couplers.
As employed herein, the term 1.times.4 coupler refers to a fibre optic device which will distribute a signal input through a single input port substantially or nearly equally into four output ports. Coupling ratios which are at least comparable in magnitude, for example in the range 0.15 to 0.35 are presently considered by the fibre optic industry to be "substantially or nearly equal" but of course the achievement of equal ratios of 0.25 is the ultimate object. Such devices are especially useful in fibre optic tree networks.
BACKGROUND ART
Fibre optic couplers of 1.times.4 configuration for single mode applications are presently characterised by a significant degree of wavelength dependence in their principal operating parameters such as coupling ratios and insertion losses. Thus, coupler suppliers generally specify a particular wavelength characteristic for their 1.times.4 single mode couplers and even so-called "wavelength flattened" 1.times.4 couplers have approximately flat coupling and insertion loss characteristics only within a single operating band, e.g. 40 nm or 100 nm. It would be desirable to produce a 1.times.4 single mode coupler which is wideband for the wavelength range 1250-1600 nm, at least in the sense that the coupler is operable in both of the principal transmission wavelength bands for fibre optic communications networks, 1260-1360 and 1430-1580 nm, but more preferably over the whole range 1250-1600 nm.
A number of cross-sectional configurations for the cores of 1.times.4 biconical taper couplers have been proposed. A simple square array for (1989), Vol. 25, No. 10, 682!, in the context of a particular class of couplers in which the fibres are drawn and fused within a capillary tube which has a lower refractive index than the fibre cladding and which remains an integral component of the coupler. If this array is adapted to a 1.times.4 splitter coupler, for any given fibre segment which serves as the incident/through fibre, the coupling ratio and insertion loss characteristics with respect to wavelength are similar for the two adjacent fibre segments (which are substantially symmetrically disposed) but quite different for the diagonally opposite fibre segments. Moreover, the cycling of the characteristics with taper ratio and drawn length are quite out of phase, whereby it can be difficult even to control the fusion process to achieve a desired substantially equal power distribution across all four output ports. five fibre ring in which one fibre is a dedicated input fibre and the other four are output fibres. A proposal for a wholly symmetrical 1.times.4 coupler configuration is disclosed by Mortimore et al at Electronics Letters (1991), Vol. 27, No. 24, 2252. This configuration entails a hexagonal ring arrangement of fibre segments about a core incident fibre. All fibre segments are of equal diameter and in contact, but only every alternative fibre of the ring is a tap-off fibre: the others are dummy fibres. The result is reported to a quite good broadband response, but a coupler of this kind would be expensive and complex to manufacture.
Australian patent 618108 discloses polygonal section sheaths for a variety of coupler arrays. Sheaths proposed include square and triangular sections, and the patent illustrates or describes 2, 3 or 4 fibre couplers in various arrays.
U.S. Pat. No. 4,798,436 to Mortimore proposes a general technique for adjusting or controlling the characteristics of fibre couplers by varying the relative propagation constants of the fibres of the coupler. For example, one fibre may be differently tapered or different fibres may be selected. Attempts to apply this technique by modifying the fibre segment diagonally opposite the primary or in
REFERENCES:
patent: 4798436 (1989-01-01), Mortimore
patent: 5175782 (1992-12-01), Bowen et al.
patent: 5408554 (1995-04-01), Cryan et al.
patent: 5553179 (1996-09-01), Cryan et al.
Arkwright, J., "Novel Structure for Monolithic Fused-Fibre 1 .times. 4 Couplers," Electronics Letters, Sep. 12, 1991, vol. 27, No. 19, pp. 1767-1768.
Mortimore, D., "Monolithic 4 .times. 4 Single-Mode Fused Coupler," Electronics Letters, May 11, 1989, vol. 25, No. 10, pp. 682-683.
Mortimore, D., "Theory and Fabrication of 4 .times. 4 Single-Mode Fused Optical Fiber Couplers," Applied Optics, Jan. 20, 1990, vol. 29, No. 3, pp. 371-374.
Mortimore et al., "Monolithic Wavelength-Flattened 1 .times. 4 Singlemode Fused Fibre Coupler," Electronics Letters, Nov. 21, 1991, vol. 27, No. 24, pp. 2252-2253.
Elias Martin Charles
Hesling Mark Ronald
Johnston Scott Anthony
AOFR Pty. Limited
Palmer Phan T. H.
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