Background of the invention Acoustic waves can be used to create dynamic gratings in optical fibres. Typically, acoustic energy in the MHz region is launched into the fibre as a flexural wave. The resulting bending of the optical fibre causes periodic increases and decreases in the refractive index of the optical fibre, which in turn can act as a long period grating (LPG). The LPG can be utilised to couple light propagating in a core mode of the optical fibre to a cladding mode or other co- propagating mode of the optical fibre.

The inventor's have recognised that the acoustically created LPG is not radially uniform.

They have recognised that the nature of the refractive index change with bending is such that it is zero on a centre axis of the optical fibre, and increases with radial distance from the axis, at least in the direction of bending. The mode coupling can be described by the overlap of the two respective fields and the An distribution in the optical fibre. It has been recognised by the inventors that for a perfectly uniform, concentric fibre, the coupling between the fundamental core mode and some certain cladding modes is at a local minimum, and that the coupling between the fundamental core mode and all other modes is identically zero, due to an on-axis zero in the An radial distribution in the optical fibre, and the asymmetrical distribution of the An in any section of the fibre. Thus, in order to get sufficient coupling between modes, large fibre length and high driving power of the acoustic wave are required.

At least preferred embodiments of the present invention provide a fibre design with improved coupling efficiency for coupling between a core mode and a cladding mode in the presence of an acoustic grating. In preferred embodiments, the coupling efficiency is increased <BR> <BR> by optimising the fibre structure (and especially the location of the core (s) ) for use with an acoustically induced grating. This in turn achieves a reduction in the physical length of the fibre along which the acoustic grating is induced and the driving power required in order to achieve the same coupling efficiency.

Summary of the invention In accordance with a first aspect of the present invention there is provided an optical fibre comprising a core and a cladding formed around the core, wherein a position of the core with respect to a centre axis of the optical fibre is chosen such that a coupling efficiency of the optical fibre for coupling between a core mode and a co-propagating mode in the presence of an induced acoustic grating in the optical fibre is increased compared with an efficiency achievable with a concentric core for a given acoustic power.

The co-propagating mode may comprise a cladding mode of the optical fibre.

In one embodiment, the position of the core is chosen based on an absolute value of a refractive index change in the core as a result of the induced acoustic grating and/or an overlap of the core mode with the co-propagating mode and/or a polarisation state dependence of the coupling and/or any other factor on which the coupling efficiency is dependent.

In one embodiment, the optical fibre comprises two or more cores, wherein the position with respect to the centre axis of the optical fibre, the diameter, and the refractive index of each core are chosen to substantially maximise a coupling efficiency of the optical fibre for coupling between one core mode and a co-propagating mode in another core. In such an embodiment, the position, the diameter, and the refractive index of each core may be chosen to substantially maximise a coupling efficiency of the optical fibre for direct mode coupling between two of the cores in the presence of the acoustic grating. Alternatively or additionally, the position, the diameter, and the refractive index of each core may further be chosen to substantially maximise a coupling efficiency of the optical fibre for mode coupling between two of the cores via a coupling to and from an intermediate mode.

In accordance with a second aspect of the present invention, there is provided an optical device comprising an optical fibre of the first aspect. The optical device may comprise one or more of the group of a tunable notch filter, a modulator, an optical switch, a tunable attenuator, and an optical add/drop multiplexer (OADM).

In accordance with a third aspect of the present invention there is provided a method of mode coupling in an optical fibre, the method comprising the steps of launching acoustic energy into the optical fibre for inducing an acoustic grating in the optical fibre, whereby a core mode of the fibre is coupled to a co-propagating mode of the optical fibre, and selecting a position of the core with respect to a centre axis of the optical fibre such that a coupling efficiency between the core mode and the co-propagating mode in the presence of the acoustic grating in the optical fibre is increased compared with an efficiency achievable with a concentric core for a given acoustic power.

The method preferably further comprises the step of selecting a polarisation state of the core mode and/or the co-propagating mode to substantially maximise the coupling efficiency.

Brief description of the drawings Figure 1 is a schematic drawing showing a cross-sectional view of an optical fibre embodying the present invention.

Figure 2 is a schematic drawing showing a side view the optical fibre of Figure 1.

Figures 3 a) and b) show the calculated coupling coefficients between a LPoI core mode and HE21 and HEn cladding modes respectively in an offset core fibre embodying the present invention.

Figure 4 shows calculated plots illustrating the dependence of coupling coefficients between a LPoI core mode and the HE21 cladding mode on the polarisation of the core mode and the angular position of the core, for a given radial offset of the core, in offset core fibres embodying the present invention.

Figure 5 shows calculated plots illustrating the dependence of coupling coefficients between a LPoi core mode and the TEol cladding mode on the polarisation of the core mode and the angular position of the core, for a given radial offset of the core, in offset core fibres embodying the present invention.

Figure 6 shows calculated plots illustrating the dependence of coupling coefficients between a LPoI core mode and the TMol cladding mode on the polarisation of the core mode and the angular position of the core, for a given radial offset of the core, in offset core fibres embodying the present invention.

Figure 7 is a schematic drawing showing a side view of another optical fibre embodying the present invention.

Detailed description of the embodiments The preferred embodiment described provides an optical fibre design with improved coupling efficiency for a coupling between a core mode and a co-propagating mode in the presence of an acoustic grating.

In Figure 1, an optical fibre 10 embodying the present invention comprises a core 12 located off-set from a centre axis 14 of the optical fibre 10. The off-set between a centre axis 16 of the core 12 and the centre axis 14 of the fibre 10 is Ar. The position of the core 12 is further defined by an angle 9, as indicated in Figure 1.

It has been recognised by the inventors that a refractive index change An in the presence of an acoustic grating induced in the optical fibre 10 depends on Ar and 9. More particularly, An increases with radial distance from the centre axis 14 of the optical fibre 10, at least in the direction of bending, in the example shown in Figure 1 assumed to be in the direction corresponding to 8=90' (arrow 17). The inventors have found that coupling between the fundamental core mode and some certain cladding modes is at a local minimum in a conventional concentric optical fibre, and is identically zero for all the other cladding modes, due to the on-axis zero in the An radial distribution, and the asymmetrical distribution of the An in any section of the fibre.

In addition to the consideration of the refractive index change An dependence on Ar and 9, the overlap in the mode field intensity distribution between a fundamental mode 20 and the cladding mode 22 needs to be considered, as illustrated in Figure 2. It will be appreciated by a person skilled in the art, that an optimal position for the core 12 can be chosen in the example embodiment, to increase the coupling efficiency between the core mode 20 and the cladding mode 22 in the optical fibre 10, in the presence of an acoustic grating for a given acoustic power.

Figures 3 a) and b) show the calculated coupling coefficients between the LPoI core mode and HE21 and HEn cladding modes respectively in an offset core fibre embodying the present invention. The propagation constants and field distribution of the modes were calculated based on the nominal parameters and measured core index of SMW3 fibre, a single mode fibre equivalent to standard SMF-28 fibre, an acoustic amplitude of 3nm and an acoustic pitch of 703pm. The trend is that the coupling coefficient (curves 50,52) reaches its maximum (54,56) when the core offset, Ar (compare Figure 1), is tens of microns away from the axis; and for coupling to different modes the offset value where the maximum is reached is different (compare radial offsets corresponding to maxima 54,56 respectively).

The calculations shown in Figures 3 (a) and (b) for the HE21 and HEn cladding modes respectively assume that the core is located in the position S=90° (see Figure 1), which is parallel to the vibration direction 17 (see Figure 1), and the core mode is also polarized on this direction. Another assumption in these calculations is that for the core mode the cladding is infinite and the cladding-air boundary has no effect on the core mode. If the offset Ar is too large, the core will approach the cladding-air boundary and this assumption will not be true.

Additionally or alternatively, the dependence of the coupling efficiency on any other factors including polarization orientations of any mode in coupling can be considered in optimising or tailoring the coupling efficiency through positioning of the core.

Figures 4,5, and 6 show the dependence of coupling coefficients between the LPol core mode and the HE21, Tex,, and TMol cladding modes respectively on the polarisation of the LPol core mode and the angular position, X, of the core. The vertical coordinates in Figures 4,5, and 6 are coupling coefficients (in arbitrary units), and the horizontal coordinates are the core mode polarization. For each mode coupling pair, the coupling coefficient has been calculated for angular positions of the core at S = 0°, S = 30°, S = 60°, and s = 90° (see Figure 1) respectively, while the radial offset, Ar = 40 microns, and the direction of bending 17 (9 = 90°, see Figure 1) remain constant. A linearly polarised core mode was considered, that is, the core mode has only one polarisation direction. Another assumption is that the cladding mode is assumed to not be affected by the refractive index of the core (due to the small difference in cladding to neore)- It can be seen from the Figures 4,5, and 6, that for coupling to different modes, the optimum core positions are different, and in some cases depend on the core polarisation.

Turning now to Figure 7, in another optical fibre 30 embodying the present invention, two non-concentric cores 32,34 are formed within a cladding 36 of the fibre 30.

It will be appreciated by the person skilled in the art that in the optical fibre 30, the position and optical parameters of the cores 32,34 can be chosen such that mode coupling between a fundamental mode 38 of the one core 32 to a fundamental mode 40 of the other core 34 can occur via intermediate coupling to and from a cladding mode 42, in the presence of an acoustic grating.

In an alternative design embodying the present invention, the positions of two (or more) cores in a multi-core fibre, and their respective diameters and refractive indices, may be chosen such that direct coupling between two of the cores is achievable in the presence of an acoustic grating.

It will be appreciated by the person skilled in the art that the designs mentioned above may be optimised such that the dependence of coupling in these designs on any other factors can be changed to meet the various needs of the devices.

It will be appreciated by the person skilled in the art that numerous modifications and/or variations may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

It will further be appreciated by the person skilled in the art, that the present invention does have broad applications in optical devices, such as e. g. in OADM devices.

In the claims that follow and in the summary of the invention, except where the context requires otherwise due to express language or necessary implication the word"comprising"is used in the sense of"including", i. e. the features specified may be associated with further features in various embodiments of the invention.