OPTICAL DEVICE

Abstract: An optical device comprises a multicore optical fibre having m circularly symmetric cores where m>1, the multicore optical fibre having a region of steadily narrowing cross-sectional area. This invention also provides an optical coupler comprising a multicore optical fibre having m circularly symmetric cores optically coupled at a narrowed coupling region to another optical fibre having n cores, where m and n are positive integers and m is greater than 1, the multicore optical fibre having regions of steadily varying cross-sectional area linking the narrowed coupling region to non-narrowed regions of the multicore optical fibre.

Description: OPTICAL DEVICE This invention relates to optical devices. Currently, in order to implement optical devices such as multiplexers, demultiplexers and band pass filters in optical transmission systems, fibre Bragg gratings (FBGs) are used in conjunction with a circulator. Circulators are expensive non-fibre devices that show relatively high insertion losses and polarisation sensitivity. There is thus a need for all optical fibre systems that accomplish a similar function and have low insertion loss and negligible polarisation sensitivity. Previous attempts have been made to transform a reflective band stop device (the FBG) into a transmissive band pass device using just optical fibre technology. A four port configuration has been reported with a 50:50 coupler arranged to couple two identical fibres with identical FBGs written at both output ports. Light launched into one input fibre is equally split at the two output ports where it is subsequently reflected by the gratings and recombined through the coupler to re-appear on the fourth port. As this device is based on interferometric principles, however, it requires careful matching of the output ports. An all-fibre multi-core device has also been proposed that performs such transformation (GB 9708045.1). It comprises a mismatched dual-core fibre fused with a standard single-core, single-mode fibre over a particular coupling region. The single-core fibre is phase matched to, and therefore exchanges power with, only one of I the cores of the dual core fibre. The length of the coupling region is adjusted to give substantially complete cross-coupling. The grating is imprinted on the other (mismatched) core of the dual core fibre outside the coupling region and it is used to optically couple the two cores by a technique known as grating assisted backward coupling. Power launched into the mismatched core of the dual core fibre will travel unaffected through the coupling region and remain in the same core throughout the entire length of the device. This device, based on grating assisted backward coupling, is non interferometric and so does not require any adjustment of the fibre length for the output ports. Dual core fibres, however, are known to exhibit polarisation sensitivity, rendering the operation of the entire device polarisation dependent. This 2 means that the system designer must ensure the entering light has the appropriate polarisation - a factor over which there will typically be no control. This invention provides an optical device comprising a multicore optical fibre having m circularly symmetric cores where m>l, the multicore optical fibre having a region of steadily narrowing cross-sectional area. Using this basic configuration a number of different optical components such as transmissive, polarisation insensitive bandpass short and long period gratings, add/drop multiplexers, as well as acoustically activated band pass filters can be achieved. Compared to the prior art these devices can be made cheap, polarisation insensitive and have to have negligible insertion loss. There is no need to use expensive circulators, with their associated insertion losses. This invention also provides an optical coupler comprising a multicore optical fibre having m circularly symmetric cores optically coupled at a narrowed coupling region to another optical fibre having n cores, where m and n are positive integers and m is greater than 1, the multicore optical fibre having regions of steadily varying cross-sectional area linking the narrowed coupling region to non-narrowed regions of the multicore optical fibre. Embodiments of the invention will now be described with reference to the accompanying drawings, throughout which parts are referred to by like references and in which: Figure 1 shows schematically the refractive index profile of a. typical circularly symmetric m-core fibre; Figures 2a to 2f show schematically a mode transformation along a tapered region of a two core circularly symmetric fibre when light is initially launched into the inner core; Figures 3 a to 3f show schematically a mode transformation along a tapered region of a two core circularly symmetric fibre when light is initially launched into the outer core; Figure 4 shows schematically one embodiment of the invention as a transmissive short-period grating device; Figure 5 shows schematically an add/drop multiplexer-demultiplexer; and 3 Figure 6 shows schematically a transmissive grating device with a long period. A cross section of the refractive index of a typical m-core optical fibre used in this embodiment is shown schematically in Figure 1. An innermost core 10 has a standard top hat refractive index profile 50 while all outer cores 30, 40 have a ring like concentric geometry. The cores are separated by a lower refractive index cladding material 20. These circularly symmetric geometries facilitate the splicing of the device into standard telecom fibres and show no polarisation sensitivity. As with all multicore waveguide structures, the fibre shown in Figure 1 supports a number of guided eigenmodes. When such a multimode guiding structure is adiabatically tapered (as illustrated schematically in Figure 2a), a mode transformation takes place along a tapered region 70. Examples of such mode transformations for a two core fibre similar to the one shown in Figure 1 are presented in Figures 2 and 3. Figure 2b shows the normalised radial power distribution of a fibre mode initially launched into the inner fibre core 10 in an untapered region 60. As the mode enters the tapered region 70, it gradually transforms into the corresponding mode of the composite tapered structure. Thus for a Gaussian-like initial mode shown in Figure 2b a continuous transformation into an oscillatory spatial mode, shown in Figure 2f takes place, intermediate stages being shown in Figures 2c, 2d, and 2e. Once again, Figure 3a illustrates the tapered fibre structure, and Figure 3b shows the normalised power radial distribution of a fibre mode initially launched into an outer ring fibre core 40 in the untapered region 60. As the mode enters the tapered region 70, it gradually transforms into the Gaussian mode, shown in Figure 3f, intermediate stages being shown in Figures 3c, 3d and 3e. These different stages correspond to different taper ratios - defined as the ratio of the taper outer diameter over the untapered initial diameter. The intermediate stage shown in Figure 3c corresponds to a taper ratio of 0.8, for example. Such mode transformation is achieved here with a refractive index profile 50 where the refractive index of the outer ring cores is higher than or equal to the central fibre cores. This taper effect may be used to achieve a wide variety of all-fibre optical devices. In a first embodiment, illustrated in Figure 4, a transmissive band pass filter 4 is shown. The device comprises a circularly symmetric multicore fibre 200 optically coupled to a standard single core, single mode fibre 210. The optical coupling is achieved by tapering 180 and fusing 190 together the two fibres 200, 210. The device has four ports: 220, 230, 240 and 250. A grating 260 is incorporated in the port 240 of the multicore fibre, which is not the one into which light is launched. To avoid back reflections the grating 260 is preferably formed in the outer ring core only. Light is initially launched into the innermost top-hat core of the multicore fibre 200 in the port 220 which does not contain the grating 260. As it enters a tapered region 180 it gradually transforms into an oscillatory mode pattern that is spatially incompatible and phase mis-matched to the Gaussian-like mode of the other tapered fibre 210. It therefore passes the fused region 190 without exchanging any power and appears in the inner core of the outlet port 240. This change of mode in the broadening taper is due to symmetry and reciprocity. The grating 260 is now used to couple light from the forward propagating inner core mode into a backward propagating outer ring core mode. This is described as a grating assisted backward coupling process and is known to be wavelength dependent. Light in the outer core mode is gradually transformed into a Gaussian mode as it re-enters the tapered region 180, which is designed to be phase matched to the Gaussian-like mode of the fibre 210 in the fused region 190. The fused length is adjusted to achieve substantially full power exchange between the two fibres. As a result light initially launched into the port 220 of the multicore fibre without a grating will appear at the port 230 of the single core fibre on the same side of the fused region as the initial launch port 220 with negligible insertion losses and no polarisation dependence, providing its wavelength lies within the range of wavelengths reflected by the grating. Light with a wavelength such that it is not affected by the grating will enter at port 220 and leave at port 240. In another embodiment two devices similar to the one shown in Figure 4 may be combined to make an add/drop multiplexer-demultiplexer, as shown in Figure 5. Multiple channels are launched into the inner core of a port 290 of the multicore fibre at the left of the Figure. One channel, at a wavelength, λ D , which is to be demultiplexed, is reflected back at the grating 330 and dropped, by the mechanism of 5 Figure 4 detailed above, at a port 300 of a single core fibre 360 at the far left of the Figure. The rest of the channels are at wavelengths lying outside the grating backward assisted coupling band and continue along the multicore fibre. A channel to be added, at wavelength, λ Λ , is launched into the inner core of the fibre at a port 310 beyond a further fused section 280. It is subjected to grating assisted backward coupling at a grating 330 and where it joins the rest of the channels in the outer core of the fibre. These transfer to the inner core as they pass through the taper. The channels are thus coupled to single mode, single core fibre 370 at fused section 280 and exit the device at the outlet port 320 of this fibre. In a different embodiment a long period band pass transmissive device can be made as illustrated in Figure 7. A long period grating 350, which may be generated by a travelling acoustic wave, is used to couple the Gaussian-like mode of the inner core into the oscillatory mode of the outer ring core before the light enters the tapered region 70. Along the tapered region 70 the ring mode is transformed back into a Gaussian-like mode of the tapered fibre and is launched efficiently into a single-mode fibre 340 that is spliced into it. This is a wavelength-dependent, grating-assisted forward-coupling process. Light of wavelength outside the bandwidth affected by the grating remains in the Gaussian-like mode and eventually transforms into an oscillatory mode upon entering the tapered region 70. This mode is coupled to high order cladding modes in the spliced fibre 340 and finally lost. The grating 350 is formed in the outer ring cladding of the fibre 330 and can either be a permanent, optically written grating or a transient grating.

Claims: CLAIMS 1. An optical device comprising a multicore optical fibre having m circularly symmetric cores where m>l, the multicore optical fibre having a region of steadily narrowing cross-sectional area. 2. A device according to claim 1, in which one of the m cores of the multicore optical fibre is an axial core. 3. A device according to claim 1 or claim 2, comprising an optical grating formed in one or more cores of the multicore optical fibre. 4. A device according to claim 3, in which the grating is a transiently-formed grating. 5. A device according to claim 3, in which the grating is an optically- written grating. 6. A device according to any one of claims 3 to 5, in which the grating is formed in a non-axial core of the multicore optical fibre. 7. A device according to any one of claims 3 to 6, in which the grating is formed at a non-narrowed region of the multicore optical fibre. 8. A device according to any one of the preceding claims, in which the refractive indices of the m cores increase in a sequence from the innermost core to the outermost core. 9. An optical filter comprising: an optical device according to any one of claims 3 to 8; and 7 a launching optical arrangement for launching an input optical signal into an axial core of the multicore optical fibre in a direction of narrowing of the multicore optical fibre. 10. An optical coupler comprising: an optical device according to any one of the preceding claims, coupled at the relatively narrowed region to another optical fibre having n cores, where n is a positive integer, the optical device being arranged so that regions of steadily varying cross-sectional area linking the narrowed coupling region to non-narrowed regions of the multicore optical fibre. 11. An optical coupler comprising a multicore optical fibre having m circularly symmetric cores optically coupled at a narrowed coupling region to another optical fibre having n cores, where m and n are positive integers and m is greater than 1, the multicore optical fibre having regions of steadily varying cross-sectional area linking the narrowed coupling region to non-narrowed regions of the multicore optical fibre. 12. An optical coupler according to claim 10 or claim 11, in which a grating is impressed onto at least one of the cores of the multicore optical fibre at a position away from the coupling region. i 13. An optical add/drop multiplexer/demultiplexer comprising two or more optical 1 couplers according to any one of claims 9 to 12 connected in series, the couplers being connected at a region having a grating.

Inventors: LAMING RICHARD IAN, ZERVAS MICHAEL NICKOLAOS, LAMING, RICHARD, IAN, ZERVAS, MICHAEL, NICKOLAOS
Applicants: LAMING RICHARD IAN, UNIV SOUTHAMPTON, ZERVAS MICHAEL NICKOLAOS, LAMING, RICHARD, IAN, UNIVERSITY OF SOUTHAMPTON, ZERVAS, MICHAEL, NICKOLAOS
Application year: 1999
Main classification: G02B6
IPC codes: G02B6