Method of forming an optical fibre coupler and a coupler so form

Details Method of forming an optical fibre coupler and a coupler so form Method of forming an optical fibre coupler and a coupler so form

1991-08-20

1992-12-29

Lee, John D.

Optical waveguides

With optical coupler

Particular coupling structure

G02B 626

Patent

active

051757796

DESCRIPTION:

BRIEF SUMMARY
This invention relates to methods of forming optical fibre couplers and couplers so formed.
In this specification, the term "optical" is intended to refer to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infrared and ultraviolet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.
1.times.N single-mode star couplers are attractive components for use in future optical networks which rely on passive splitting of optical signals from a central station to many outstations. These networks will, in all probability, be wavelength multiplexed with wavelengths covering a broad wavelength range so it is important that star couplers are not only available but are also wavelength insensitive to ensure equal signal distribution to the outstations. Although wavelength-flattened couplers may be fabricated by knitting together many 2.times.2 wavelength flattened couplers of the kind disclosed in the applicant's patent U.S. Pat. No. 4,798,436, the resulting devices tend to be large and relatively complex while 1.times.N devices made by the method of the above patent requires the use of fibres having dissimilar propagation constants and consequently the need for controlled pre-tapering etching or use of different optical fibres.
Japanese patent application 03-2173141(A) describes the production of an optical branching filter in which a bundle of optical fibres is inserted into a glass tube having an inside wall layer of lower refractive index and melting point. This assembly is then heated and drawn until the central fibre and one of the surrounding fibres propagate the same optical power from an optical source coupled to the central fibre on the other side of the filter. This, however, provides an equal 1:n split at one wavelength only.
According to the present invention a method of forming an optical fibre coupler includes the steps of locating two or more secondary optical fibres about a central optical fibre to form a fibre bundle such that all the secondary fibres are similarly disposed with respect to every one of the other fibres; and forming a fused, tapered coupler from the fibre bundle the tapering being stopped when the proportions of light coupled out of the central fibre at two distinct predetermined frequencies are first equal.
The fibres may be located within a capillary tube to ensure the required geometry is maintained during the usual heating and pulling of the fibre bundle to form the tapered structure. Alternatively, they can be located by holes in a support member such as a glass cylinder.
To ensure equal coupling of light into each of the secondary fibres, each secondary fibre must be in the same position relative to all the other fibres (where mirror symmetric disposition are to be regarded as the same). This is readily achieved by arranging the secondary fibres equidistantly spaced in a ring about the central fibre. As will be discussed in more detail below, it may be advantageous to reduce the coupling into the secondary fibres. It has been found that this can be obtained, in the case where there are an even number of secondary fibres, by grouping the secondary fibres in pairs. Each of the paired fibres will be in mirror symmetric dispositions relative to all the other fibres and so will all experience the same coupling from the central fibre, but the asymmetric disposition round the fibre reduces the amount of coupling.
The point at which to stop tapering may be determined by injecting into the central fibre a light signal at one or both of the distinct frequencies and monitoring the light exiting the central fibre or one or more secondary fibres. If the coupler manufacturing process is sufficiently consistant it may be possible to monitor only one frequency (which may or may not be one of the distinct frequencies) and stop tapering when the monitored output reaches a level which it is known from previous coupler manufacture corresponds to

REFERENCES:
patent: 4392712 (1983-07-01), Ozeki
Patent Abstracts of Japan, vol. 12, No. 363, 29 Sep. 1988, & JP A 63-115112 (Sumitomo Electric Ind Ltd) 19 May 1988.
Patent Abstracts of Japan, vol. 12, No. 383, 13 Oct. 1988, & JP A 63-129307 (Sumitomo Electric Ind Ltd) 1 Jun. 1988.
Patent Abstracts of Japan, vol. 13, No. 9, 11 Jan. 1989, & JP A 63-217314 (Nippon Telegr & Teleph Corp) 9 Sep. 1988.
Journal of the Optical Society of America, vol. 62, No. 11, Nov. 1972 "Coupled-Mode Theory for Optical Fibers", Snyder, pp. 1267-1277.

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