Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
1998-09-21
2001-04-24
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling function
C385S012000, C385S016000, C385S123000
Reexamination Certificate
active
06222960
ABSTRACT:
The invention relates generally to passive optical components.
Optical communication of signals through optical fibres and other waveguides is used more and more, both locally in individual devices and in larger systems such as various types of networks for communicating information over large distances. In these devices and systems a need exists for different kinds of optical elements for performing various kinds of operations on the optical signals. An example of such elements comprises couplers for transmitting an optical signal from a single waveguide to two different waveguides. Such couplers can be based on multimode interference in planar waveguides, see e.g. the summarizing article “Optical Multimode Interference Devices Based on Self-Imaging: Principles and Applications”, L. B. Soldano and E.C.M. Pennings, Journal of Lightwave Technology, vol. 13, No. 4, April 1995, pages 615-627. In these known devices a transversal multimode waveguide is excited in one point, whereby a multitude of optical or equivalently electromagnetic propagation modes are initiated. The modes propagate in the waveguide with different phase velocities and are at a cross-section of the waveguide, they are composed to an electromagnetic field distribution. Further, the field distributions differ much from each other in cross sections taken at different places along the longitudinal direction of the light waveguide. For example, one can obtain a rather exact copy of the exciting distribution, inverted distributions, multiple or multifold distributions, etc. For example, 1×N couplers have been disclosed made of GaAs and InP based waveguides for values of N between 2 and 20. A probable disadvantage of such devices is that the reflectance of the waveguide laterally can be very significant and that it can be difficult to avoid losses and/or undesired phase variations in the reflection.
In the Japanese patent application having the publication No. 60-225804 (application No. 59-083579) it is disclosed how a piece of an optical fibre
6
,
6
′ is coupled to another fibre
4
at either side thereof, for selecting the optical mode which propagates in the fibre
4
. The fibre
6
,
6
′ has an annular core and is single-mode type, whereas the fibre
4
is common multimode type.
In said patent application and the European patent application 0 387 740 optical fibres are disclosed having annular cores. These fibres are used for transmitting light existing in the shape of a single optical mode.
SUMMARY
It is an object of the invention to provide optical elements in which multimode interference is utilized and which have a simple construction and which can be manufactured in a relatively simple way.
For the multimode interference a piece of an optical fibre of multimode type is used, i.e. an optical fibre, in which, for the intended or considered wavelengths, a multitude of optical modes can propagate simultaneously. In principle, such a multimode fibre can allow multimode interference to take place like in a planar waveguide. However, all complications owing to lateral reflectance do not exist since there are no sides or surfaces by which the can be caused. In the preferred case the multimode fibre is an optical fibre having a core which has the shape of a ring or equivalently is a tubular structure. Such a fibre can be said to be similar to a planar waveguide which has been bent around a longitudinal axis in order to form a half of a circular tube and which is then completed with another similar half of a circular tube in order to obtain a complete circular tube. The thickness of the annular core is in principle of the same magnitude of order as the thickness of the core of the corresponding planar waveguide. When such an optical fibre is excited optically, having an exciting centre located in a point at or close to the annular core, different optical propagation modes are initiated, which substantially have the same ratios of their phase velocities in relation to each other as the different modes in a corresponding planar waveguide.
Generally, an optical element is provided, the main component of which is a piece of an optical fibre which in its general configuration can be a substantially conventional type. Thus, the fibre piece has a substantially cylindrical core together with a surrounding substantially cylindrical cladding. Further, a connection is provided to a light source at an end of the fibre piece, so that light emitted from the light source passes through the fibre piece. An output device receives light which has passed through the fibre piece, for providing this light to some other optical device, e.g. a receiving, detecting or evaluating device. In particular, the fibre piece should be multimode type and its connection be made in such a way that when coupled to a light source a multitude of optical modes of light are excited in the fibre piece.
Further, the fibre piece should be designed and in particular its length should be adapted or chosen in such a way, that in the cross section at the end surface of the fibre piece at the output device a mode picture exists, i.e. a light intensity distribution, corresponding to the different modes existing at this end surface, which is a substantially true image or reproduction of the mode picture which exists at the cross section of the input end, and this condition should be fulfilled for at least some frequencies/wavelengths for or for some wavelength interval of the injected light. For example, the mode picture at the cross section at the end surface can be substantially a multiple image of the mode picture existing at the cross section of the input end.
In the preferred case a ring shaped or tubular shaped core is provided in the fibre piece. The exterior and interior diameters of the core are then advantageously selected in such a way that the fibre piece for the considered wavelength range is multimode type in the circumferential direction of the fibre piece in relation to the longitudinal axis thereof. Further, the fibre piece should be type single-mode radially, i.e. in directions extending perpendicularly from the axis of the fibre piece.
Such an optical element can be used as for example a filter or as a sensor or a switch. In the latter case the output device can comprise an optical fibre of single-mode type. The connection and the output devices are designed and the length and shape of the fibre piece in an initial position are adapted in such a way that only light having a wavelength within a definite wavelength interval can be received by the output device. For a change of the length and/or the shape of the fibre piece from its initial state the mode pictures of light having wavelengths within the definite wavelength interval will be changed, so that the light within this interval is received with a modified intensity by the output device. The intensity at the cross section at the output device will in the preferred case be reduced significantly and can in certain cases almost vanish.
When using the optical element as a 1×N-coupler the connection can comprise an optical fibre of single mode type and the output device can comprise N optical fibres of singlemode type, where the cores of the latter ones connect to different positions on the core of the fibre piece in its end surface at the connection. When using it as a 1×2-coupler the output device comprises advantageously two optical D-fibres of type single-mode, the flat surfaces thereof being located facing and contacting each other.
REFERENCES:
patent: 3633035 (1972-01-01), Uchida et al.
patent: 4087159 (1978-05-01), Ulrich
patent: 4127320 (1978-11-01), Li
patent: 4205901 (1980-06-01), Ramsay et al.
patent: 4701011 (1987-10-01), Emkey et al.
patent: 4859014 (1989-08-01), Schmitt et al.
patent: 5031989 (1991-07-01), Morishita et al.
patent: 5796891 (1998-08-01), Poustie et al.
patent: 0 387 740 (1990-09-01), None
patent: 60-225804 (1985-11-01), None
patent: 95/12828 (1995-05-01), None
Soldano, Lucas B. et al., “Optical Multi-mode Interference Devices Based on S
Augustsson Torsten
Borak Georges
Granberg Mats
Stensland Leif
Burns Doane Swecker & Mathis L.L.P.
Lee John D.
Telefonaktiebolaget LM Ericsson (publ)
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