Optical waveguides – With optical coupler – Particular coupling function
Patent
1996-05-30
1998-08-18
Lee, John D.
Optical waveguides
With optical coupler
Particular coupling function
385 50, 372 6, G02B 626
Patent
active
057968918
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to an optical filter suitable for use, for example, in an optical telecommunications system. It also encompasses optical sources using such a filter.
The need to filter a signal to change its intensity as a function of wavelength arises in a number of different situation. For example, some optical sources, such as a rare-earth doped fixture amplifiers have highly uneven gain characteristics. It is desirable therefore to provide an appropriately tailored filter having a generally complementary characteristic to filter the output from such a source - a process known as "gain flattening". As another example, in optical communications systems using wavelength division multiplexing (WDM), precisely tuned filters are required to select a channel or channels from those present in the system.
In designing filters for these purposes there is a need to minimise the costs of fabrication, to ensure that the filter is readily integrable with the other circuits and devices in an optical system, to provide a filter which can be tuned to give a desired response characteristic, and which can maintain that characteristic stably.
A paper by Paloma R. Horche et al published at pp 104-107 of IEEE Photonics Technology Letters vol 1, no. 7, Jul. , 1989: "Spectral Behaviour of a Low-Cost All-Fiber Component Based on Untapered Multifibre Unions" describes the spectral characteristics of a system comprising a fibre waveguide of 50 microns diameter supporting multiple spatial modes coupled between input and output single-mode fibre waveguides of 9 microns diameter. The system is found to have an oscillatory power transmission characteristic.
A paper by Uri Ghera et al published at pp 1159-1161 IEEE Photonics Technology Letters 5 (1993 Oct., no. 10) "A Fiber Laser with a Comb-Like Spectrum" discloses a laser intra-cavity filter formed by joining an Nd doped fibre to a HiBi fibre. Both fibres are single mode fibres in terms of their spatial modes. The system relies upon the interaction of polarisation modes to produce a comb-filter characteristic.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, an optical filter comprising a multi-mode waveguide having a first number n modes and a second waveguide having a different number m modes coupled in series, where n and m are integers and n>m, the interaction of different modes at the n-mode/m-mode interface being effective to filter an optical signal propagating through the waveguides to provide a desired response characteristic, is characterised by means for changing a lateral offset between the central axes of the waveguides at their interface thereby controlling the response of the filter.
By a lateral offset is meant an offset in the direction perpendicular to the longitudinal central axis of the waveguides.
Preferably the second waveguide is a single-mode waveguide.
The present invention uses spatial-mode beating between a multiple-mode waveguide and, e.g., a single-mode waveguide to provide a filter which can readily be configured to give a desired response characteristic. Waveguides, be they fibre waveguides, planar devices or semiconductor devices, form the basic building blocks of most optical communications systems or optical signal processing devices. The filter of the present invention can therefore very readily be incorporated in such systems and can be manufactured using the conventional technologies developed for such systems. A further important advantage of the present invention is that the filter response is variable in response to a change in the offset between the waveguides.
Changing the offset between the waveguides at their interface has the effect of changing the extent to which the different modes in the multiple-mode waveguide are excited. For example, by adjusting the offset it is possible to change the filter from having a transmission peak at a particular characteristic wavelength to having a transmission minimum at that wavelength.
The filter may comprise a plurality o
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Finlayson Neil
Poustie Alistair J.
British Telecommunications PLC
Kang Ellen E.
Lee John D.
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