OPTICAL COUPLER AND/OR MULTIPLEXER

Abstract: An optical fibre coupler has at least an m-core optical fibre (100) optically coupled to an n-core optical fibre (110), where m and n are positive integers and m is greater than 1. A channel add/drop multiplexer comprises such an optical fibre coupler and a Bragg grating (130) disposed on the m-core fibre outside the coupling region (120).

Description: OPTICAL COUPLER AND/OR MULTIPLEXER This invention relates to optical couplers and/or multiplexers. Fused optical fibre couplers, in which two optical fibres are fused together at a coupling region, are known. This invention provides an optical fibre coupler having at least an m-core optical fibre optically coupled to an n-core optical fibre, where m and n are positive integers and m is greater than 1. The invention provides a new type of coupler, where at least one of the fibres involved in the coupler has two or more light-transmitting cores. It can provide a way of selectively coupling light into or out of one core of the m-core fibre. Embodiments of the invention can use established simple fused coupler techniques, and can introduce very low (e.g. less than 1 dB) insertion losses for the coupled light. An exemplary embodiment of the invention comprises a dual core fibre coupled to a single core fibre. If one core of the dual core fibre is substantially identical (in terms of its optical propagation constants) to the core of the single core fibre, light can be made to couple between that core of the dual core fibre and the single core fibre, whereas light in the other core of the dual core fibre is essentially unaffected. In wavelength division multiplexed (WDM) optical transmission systems, multiple information channels are transmitted at different respective wavelengths so that the channels can all be carried on a single waveguide (e.g. an optical fibre). There is a need in such systems for so-called channel-add multiplexers and so- called channel-drop demultiplexers (these two functions can be combined in a single channel add/drop multiplexer). A channel add multiplexer is an optical device capable of receiving two optical signals - one generally being a multi-channel WDM signal and the other generally being a new wavelength channel to be added to the WDM signal. These two signals are received at respective input ports of the device, and a composite WDM signal comprising the original WDM signal and the new wavelength channel is supplied at an output port of the device. Similarly, in a channel drop demultiplexer, a multi-channel WDM signal is received at an input port of the device. One or more of the wavelength channels of the WDM signal is separated from the others and is supplied at a first output port, while the remainder of the WDM signal is supplied at another output port. Figure 1 of the accompanying drawings schematically illustrates a previously proposed optical fibre channel add/drop multiplexer having two input ports Al, A2 and two output ports A3, A4. The device comprises two optical circulators 10, 20 and a fibre Bragg grating 30. In Figure 1, a multi-channel WDM signal X^ is launched into port Al . A channel to be added, λ ADD , is launched into port A4 and a channel λ DR op to be separated from the WDM signal λ m is output at port A2. The WDM signal with the channel X D RO P dropped and the new channel X ADD added is output at port A3. The device works in a straightforward way. The Bragg grating is arranged to reflect light at the wavelength λ DRO p of the channel to be dropped and the channel to be added. So, the channel to be dropped passes from the circulator towards the grating, is reflected by the grating and is output by port A2 of the circulator. Similarly, the channel to be added enters at port A4, passes from the circulator to the Bragg grating where it is reflected, and is output at port A3 of the circulator. The remaining channels of the WDM signal, λ oτHERS , are unaffected by the Bragg grating and so emerge at port A3 of the circulator. The device of Figure 1 makes good use of the wavelength-selective properties of a fibre Bragg grating, but because the grating 30 is a two-port device the multiplexer needs the two circulators 10, 20. Circulators are expensive bulk optical devices, so it is undesirable to use them in an all- fibre system. Also, there are inevitable losses caused by the need to connect fibres to the bulk optical circulators. A simpler device could use two 50:50 fused fibre couplers, but at the expense of an increased insertion loss of 6 dB (decibels) for the add/drop multiplexer. Embodiments of the invention provide a channel drop demultiplexer comprising a coupler as defined above in which the fibres are coupled at a coupling region; and a grating is disposed on the m-core fibre away from the coupling region, in which a core of the m-core optical fibre to a first side of the coupling region provides an input port for a WDM signal; that core of the m-core fibre, to a second side of the coupling region, provides an output port for a WDM signal; and the grating promotes coupling of light of a channel to be dropped between cores of the m-core fibre. Embodiments of the invention also provide a channel add multiplexer comprising a coupler as defined above in which the fibres are coupled at a coupling region; and a grating is disposed on the m-core fibre away from the coupling region, in which a core of the m-core optical fibre to a first side of the coupling region provides an input port for a WDM signal; that core of the m-core fibre, to a second side of the coupling region, provides an output port for a WDM signal; and the grating promotes coupling of light of a channel to be added between cores of the m- core fibre. In these embodiments a channel add multiplexer and a channel drop demultiplexer are provided which do not require expensive and lossy bulk optical devices such as optical circulators but which do not introduce the insertion losses of conventional fused taper couplers. The insertion loss can be very low (e.g. < ldB or even < 0.5 dB), which is much lower than the simple coupler arrangement described in a paper by Bilodeau et al: IEEE Photonics Technology Letters, 7, 1995, pp388-390 and a paper by Bakhti et al: Electronics Letters, 33, 1997, pp803-804. Compared to these prior art devices, embodiments of the invention are not interferometric devices and so there is no need to balance optical path lengths within the devices to a few wavelengths of the light. This makes embodiments of the invention much easier to manufacture. Again, compared to the paper by Bakhti et al and also WO97/08574, the grating can be written over a length of fibre instead of onto the waist of the coupler, this means that coupler manufacture and grating manufacture are different processes which can reliably be done separately without affecting the other process. In any event, it is difficult to produce a coupler waist of good uniformity over a long length, so any irregularities will affect the grating performance and restricts the length and number of gratings which can be used in the prior art devices. In contrast, in the present embodiments, multiple gratings can be used, of good quality because they are impressed onto fibre rather than onto a coupler waist. Tuning of the gratings e.g. by compression or stretching is also possible, helped by the non-interferometric nature of the embodiments and the high physical strength of fibre away from the coupler waist. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 schematically illustrates a previously proposed channel add/drop multiplexer; Figures 2a and 2b schematically illustrate cross sections through optical fibres; Figure 3 schematically illustrates a coupler formed of the fibres shown in Figures 2a and 2b; Figure 4 schematically illustrates a channel drop demultiplexer using a coupler as shown in Figure 3; Figure 5 schematically illustrates a channel add/drop multiplexer; and Figure 6 schematically illustrates an optical transmission system. Figures 2a and 2b schematically illustrate cross sections through two respective optical fibres used in the devices described below. Figure 2a illustrates a conventional single-mode optical fibre having a cladding region 40 surrounding a core 50. The core in this embodiment has a numerical aperture (NA) of 0.14, and is formed of boron and germanium doped silica glass. The fibre shown in Figure 2b has a cladding region 60 similar to the cladding 40 of the fibre of Figure 2a, surrounding two cores 70, 80 disposed about 14 μm apart. The core 80 is substantially central in the fibre and has an NA of 0.25 and is formed of phosphorus and germanium doped silica glass. The other core 70 is non- axial (off-centre), has an NA of 0.14 and is formed of boron and germanium doped silica glass (i.e. it is similar to the core 50). Accordingly, Figure 2a illustrates a single core (SC) fibre and Figure 2b illustrates a dual core (DC) fibre. The two cores of the DC fibre are mismatched with respect to the other, although one of them is matched to the core of SC fibre. One core of the DC fibre is also axial to facilitate easy connection to standard telecommunication fibre. Figure 3 schematically illustrates a fused taper coupler formed by fusing together a length of DC fibre 100 and a length of SC fibre 110 at a coupling region 120. This forms a six-port device having ports B1..B6. The transmission properties of this six-port device, when light is launched into any one of ports B1..B3, are shown in the following tables: Table 1 The device of Figure 3 is symmetrical so the following results also apply: Table 2 Thus, light launched into port B2 (the larger core of the DC fibre) emerges preferentially at port B6, whereas light launched into port Bl (the smaller core of the DC fibre) emerges preferentially at port B4. A number of uses can be envisaged for such a coupler. One such use. as part of a channel drop, a channel add or a channel add/drop multiplexer, will be described below. Figure 4 schematically illustrates a channel drop multiplexer comprising a coupler of the type shown in Figure 3 with a Bragg grating 130 impressed on one of the fibre cores using standard grating writing techniques. In the example of Figure 4. the grating is impressed on the larger core of the DC fibre between the coupling region 120 and the port B5. A WDM input signal having a range of wavelength channels (λ m ) is launched into port Bl . For those wavelengths unaffected by the Bragg grating (λ 0THERS ), the light emerges at port B4 according to the results shown in Table 1. However, the grating 130 has the effect of coupling light, at the wavelength Λ DROP ° a channel to be dropped, from the core leading to port B4 (the central, narrower core of the DC fibre) into the core leading to port B5 (the wider, off-centre core of the DC fibre) but in a reverse propagation direction. Accordingly, it is as though the light was entering from port B5, and so according to Table 2 above, the dropped channel λ DRO p emerges from port B3. The coupling condition for the grating 130 to have this effect is as follows: β.(λ) + β 2 (λ) = ^ Λ where β l and β 2 are the propagation constants of the central core and the non-central core respectively. The grating 130 can in fact be fabricated as a series arrangement of two or more gratings having different periods or the same periods, so that two or more channels can be dropped by the same device and/or the dropped channel can be "cleaned up" (attenuated) by a second grating at the same pitch. The grating 130 is disposed in the off-centre core of the DC fibre, so that for wavelengths at which the grating does not cause coupling from one fibre to another, the grating has little or no effect on the forward propagation of the non-dropped channels λ 0THERS . A particular resonance is at λ when This will cause a reflection at λ back down the central core, therefore causing a loss for the channel at λ. By writing the grating into the off-centre core, this reflection is reduced. Figure 5 schematically illustrates a channel add/drop multiplexer formed, in effect, by a back-to-back arrangement of two couplers of the type shown in Figure 3 with a grating disposed on one of the cores between the two couplers. The device of Figure 5 is an eight-port device having ports numbered Cl ..C8. The initial WDM signal is launched into port Cl, representing the central, narrower core of the DC fibre (equivalent to port Bl of the device of Figure 3). A dropped channel λ DRO p emerges from port C3. A channel to be added, is launched into port C8 and the non-dropped channels of the original WDM signal (X Q XHERS) along with the added channel λ ADD emerge from port C6. The channel dropping arrangement is identical to that shown in Figure 4. A grating 130' in the non-central core of the DC fibre causes coupling from the central core into the non-central core, in a reverse direction at the wavelength λ DRO p- This light is then coupled back to the port C3 as described above. Similarly, the channel to be added, λ ADD , is launched into port C8. This is equivalent to light being launched into port B6 of Figure 3, and from Table 2 it can be seen that the light emerges at port B2 of Figure 3, i.e. in the off-centre core of the DC fibre. Light propagating (from right to left as shown) in this core impinges on the grating 130', which couples the light into a reverse-propagating (with respect to the original direction of the added channel) signal in the central core. This reverse- propagating light (in fact, now propagating from left to right in Figure 5) emerges from the port C6 of the device. The channels unaffected by the grating, X 0T HERS > P ass through both couplers and emerge from the port C6. In other embodiments, the grating could be (or include) a chirped grating, such as a linearly chirped grating, to give a similar response over a range of channels or to provide dispersion compensation. Figure 6 schematically illustrates an optical transmission system using channel add/drop multiplexers of the type shown in Figures 4 or 5. In Figure 6, two optical transmitters 200, 210 at wavelengths λ, and λ 2 respectively are combined at a channel add multiplexer 220. A further channel from as transmitter 230 (wavelength λ 3 ) is added at a channel add multiplexer 240. The combined WDM signal then propagates through a length of fibre 250 before light at λ, is removed by a channel drop demultiplexer 260, to be received by an optical receiver 270. The remaining light continues to propagate to a channel drop demultiplexer 270 which separates the wavelengths λ 2 and λ 3 for reception by respective receivers 280, 290. While the embodiments described above have related to a 2 core fibre coupled to a 1 core fibre, in general other numbers of cores can be used, so long as at least one fibre has more than one core. So, for example, a 3 core fibre could be coupled to a 2 core fibre, and the grating could be impressed on a subset (possibly greater than 1) of the cores of one fibre. Of course, if multi-core fibres are used some of the cores could be arranged as unconnected "dummy" cores.

Claims: CLAIMS 1. An optical fibre coupler having at least an m-core optical fibre optically coupled to an n-core optical fibre, where m and n are positive integers and m is greater than 1. 2. A coupler according to claim 1 , in which the cores of the m-core fibre have different propagation constants. 3. A coupler according to claim 1 or claim 2, in which m=2 and n= l. 4. A coupler according to any one of claim 1 to 3, in which one of the cores of the m-core fibre has a propagation constant substantially identical to the propagation constant of a core of the n-core fibre. 5. A coupler according to any one of claims 1 to 4, in which a grating is impressed on at least one core of the m-core fibre. 6. A coupler according to claim 5, in which the grating comprises a periodic refractive index variation. 7. A coupler according to claim 6, in which the grating comprises a chirped grating. 8. A coupler according to claim 7, in which the grating comprises a linearly chirped grating. 9. A coupler according to claim any one of claims 5 to 8, in which one of the cores of the m-core optical fibre is disposed substantially axially within the m-core fibre, the grating being impressed on another of the cores of the m-core fibre. 10. A coupler according to any one of claims 5 to 9, in which: the fibres are coupled at a coupling region; and the grating is disposed on the m-core fibre away from the coupling region. 11. A coupler according to claim 10, in which the fibres are fused together at the coupling region. 12. A coupler according to claim 10 or claim 11 as dependent on claim 4, in which the grating is impressed on the core of the m-core fibre having a propagation constant substantially identical to the propagation constant of a core of the n-core fibre. 13. A channel drop demultiplexer comprising a coupler according to any one of claims 10 to 12; in which a core of the m-core optical fibre to a first side of the coupling region provides an input port for a WDM signal; that core of the m-core fibre, to a second side of the coupling region, provides an output port for a WDM signal; and the grating promotes coupling of light of a channel to be dropped between cores of the m-core fibre. 14. A channel drop demultiplexer according to claim 13, in which the grating is disposed in the m-core fibre to the second side of the coupling region, the grating promoting reverse direction coupling of light of a channel to be dropped between cores of the m-core fibre; and a core of the n-core fibre, to the first side of the coupling region provides an output port for the dropped channel. 15. A channel add multiplexer comprising a coupler according to any one of claims 10 to 12, in which a core of the m-core optical fibre to a first side of the coupling region provides an input port for a WDM signal; that core of the m-core fibre, to a second side of the coupling region, provides an output port for a WDM signal; and the grating promotes coupling of light of a channel to be added between cores of the m-core fibre. 16. A channel add multiplexer according to claim 15, in which the grating is disposed in the m-core fibre to the first side of the coupling region, the grating promoting reverse direction coupling of light of a channel to be added between cores of the m-core fibre; and a core of the n-core fibre, to the second side of the coupling region provides an input port for the channel to be added. 17. A channel add/drop multiplexer comprising: a channel drop demultiplexer according to claim 13 or claim 14; series-connected with: a channel add multiplexer according to claim 15 or claim 16. 18. A channel add/drop multiplexer according to claim 17, in which a common grating disposed between the channel add multiplexer and the channel drop demultiplexer performs the function of the grating of the channel add multiplexer and the grating of the channel drop demultiplexer. 19. A multiplexer or demultiplexer according to any one of claims 13 to 18, in which the grating comprises two or more grating sections having different grating periods to promote optical coupling at different respective wavelengths. 20. A multiplexer or demultiplexer according to any one of claims 13 to 19, in which the grating comprises two or more grating sections having substantially identical grating periods. 21. An optical transmission system comprising a multiplexer or demultiplexer according to any one of claims 13 to 20. 22. A wavelength-division-multiplexed optical transmission system comprising two or more series-connected multiplexers according to any one of claims 15 to 20, each multiplexer being arranged to add one or more respective wavelength channels to a wavelength-division-multiplexed optical signal. 23. A wavelength-division-multiplexed optical transmission system comprising two or more series-connected demultiplexers according to claims 13, 14 and 17 to 20, each demultiplexer being arranged to remove one or more respective wavelength channels from a wavelength-division-multiplexed optical signal.

Inventors: DONG LIANG, LAMING RICHARD IAN, DONG, LIANG, LAMING, RICHARD, IAN
Applicants: DONG LIANG, LAMING RICHARD IAN, UNIV SOUTHAMPTON, DONG, LIANG, LAMING, RICHARD, IAN, UNIVERSITY OF SOUTHAMPTON
Application year: 1998
Main classification: G02B6
IPC codes: G02B6