Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2002-04-05
2004-07-27
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S015000, C385S115000
Reexamination Certificate
active
06768837
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for the collective production of microlenses at the end of a set of optical fibres, of the ribbon of fibres type.
The present invention applies to optical and optoelectronic modules amongst other things for optical telecommunications. It applies more particularly to the production of microlenses on optical fibres in order to improve the coupling between optical and optoelectronic components. These microlenses are particularly well adapted to collective coupling with active components in arrays, such as lasers, semiconductor amplifiers, VCSELs or photodetectors for example.
2. Discussion of the Related Art
In the literature a large number of articles are found having methods for the individual manufacture of microlenses at the end of fibres which improve the coupling between active components and monomode fibres. The history of these microlenses is presented in the collection of publications “Microlenses Coupling Light to Optical Fibers”, Huey-Daw Wu, Frank S. Barnes, 1991, pp. 149-213: “Microlenses Coupling Light to Optical Fibers” IEEE Lasers and Electro-optics Society 1991 [
1
].
On the other hand, very few articles are found concerning collective coupling lenses.
The most recent articles report on combinations of lengths of fibres of different natures and the fashioning of a lens at the end of fibres, but always in order to produce individual microlenses.
In fact, individual coupling lenses are known. Reference can be made to the article by K. Shiraishi et al. (University of Utsunomiya, Japan) “A fiber with a long working distance for integrated coupling between laser diodes and single-mode fibers”, Journal of Lightwave Technology, Vol. 13 No 8, pp. 1736-1744, August 1995 [
2
], which presents a lens whose working distance is 160 &mgr;m for laser-fibre coupling losses of 4.2 dB and lateral and angular axial positioning tolerances respectively of 35 &mgr;m, 2.6 &mgr;m and 0.8° for an additional loss of 1 dB. The results were obtained for a laser emitting at a wavelength of 1.49 &mgr;m with a mean total half-height divergence of 20.5° (that, is to say 34° at 1/e
2
). This is a length of fibre
1
without a core and with a hemispherical end, welded to a monomode fibre
1
whose core has been locally enlarged by heat treatment, as shown by FIG.
1
.
In a more recent article, Shiraishi and Hiraguri “A lensed fiber with cascaded Gi-fiber configuration for efficient coupling between LDs to single-mode fibers” ECOC '98, 20-24 September, Madrid Spain, pp. 355-356 [
5
], propose a new lens consisting of two lengths of monomode fibres, of different natures, whose focusing parameters are different, welded together and to a monomode fibre by electric arc. A hemispherical profile is conferred on the end multimode fibre by means of an electric arc welder. Losses of 2 dB are obtained in front of a laser diode emitting at 1.3 &mgr;m, whose total divergence in far field at half-height of the maximum is 24.90×19.50 (that is to say 42.2°×33.1° at 1/e
2
). The working distance is 50 &mgr;m.
If the publications concerning individual fibre laser coupling lenses are numerous, those dealing with collective lenses intended for multichannel optical modules are more rare.
A method is known which consists in interposing an array of microlenses (not fixed to the fibre ribbons). By way of example, the coupling lens shown in FIG. 2 of G. Nakagawa and al. (Fujitsu Laboratories, Japan) “Highly efficient coupling between LD array and optical fiber array using Si microlens array” IEEE Photonics Technology Letters, Vol. 5, No 9, pp. 1056-1058, September 1993 [
4
], makes it possible to obtain 4.8±0.3 dB by dynamic coupling between the array
4
of four lasers with a total half-height divergence of 30° (that is to say 44° at 1/e
2
) and 4 monomode fibres
2
1
,
2
n
by means of a matrix of silicon lenses. This type of coupling complicates the assembly steps, since it adds an additional component to be positioned very precisely.
In 1996, J. Le Bris “High performance semiconductor array module using tilted ribbon lensed fibre and dynamical alignment” ECOC '96 Oslo THc.2.3, p. 4.93, from the company Alcatel (AAR, France) proposes a lensing method on a fibre ribbon which consists of chemically etching a ribbon of monomode fibres and reworking the end of each fibre of the ribbon by electric arc. With this method 3.6 dB of loss is obtained in front of an array of semiconductor amplifiers with ribbons misaligned by 20×25° of total half-height divergence (that is to say 34×42.5° at 1/e
2
). The wavelength is 1.55 &mgr;m.
The recommended solutions for the “lensing” of the fibres (the fitting of lenses at the end of fibres) which make it possible to obtain good coupling levels are not collective methods in the case of references [
1
] to [
3
].
In addition, the outside diameter of the 125 &mgr;m fibre is not maintained all along the microlens, which poses a problem for the hybridisation on a silicon platform in precise positioning Vs and for precision ferrule fitting.
For the collective methods known at the present time, the coupling losses are still too high. In addition, the use of discrete lenses described in reference [
4
] requires several successive alignments, which increases the number of assembly steps compared with microlenses attached at the end of the fibre. The method described in reference [
5
] also imposes a very short working distance of less than 15 &mgr;m in addition to the fact that it is complex.
SUMMARY OF THE INVENTION
The purpose of the present invention is to improve the coupling between an array of active elements and a set of aligned fibres of the fibre ribbon type.
To this end, the invention relates to a method for the collective production of microlenses at the end of a set of aligned optical fibres, principally characterised in that it comprises a step of heating the end of all the fibres by means of an electric arc in order to form the microlenses, the plane in which the ends of the fibres are situated being distant from the line of hottest points of the electric arc in order to round their end evenly.
The method according to the invention also has the advantage of being collective and therefore compatible with mass production, and with a very high performance.
According to another characteristic of the invention, the distance between the optical fibre ends and the line of hottest points is between 850 micrometres and 950 micrometres.
Advantageously, the set of optical fibres consists of a ribbon.
According to a preferred embodiment of the invention, the ribbon comprises monomode fibres whose terminations comprise a length of silica welded to a length of fibre with an index gradient, the microlenses being produced at the end of the lengths of fibres with an index gradient.
REFERENCES:
patent: 4804395 (1989-02-01), Clark et al.
patent: 5563969 (1996-10-01), Honmou
patent: 5595669 (1997-01-01), Le Bris
patent: 5598493 (1997-01-01), Bonham et al.
patent: 5930430 (1999-07-01), Pan et al.
patent: 0 825 464 (1998-02-01), None
Shiraishi et al.;A Fiber Lens with a Long Working Distance for Integrated Coupling Between Laser Diodes and Single-Mode FibersJournal of Lightwave Technology, US, IEEE, vol. 13, No. 8, Aug. 1995.
Chanclou Philippe
Lostec Jean
Thual Monique
France Telecom (SA)
Lee John D.
Lin Tina M
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