Optical interconnection network

Abstract: An optical interconnection network comprises an optical bus (60) formed by a D-fibre (62) embedded in a thermoplastic substrate (64) with the flat of the D-fibre (62) flush with the top surface of the substrate. A module 70 similarly constructed is dimensioned to be a push fit in a wall structure formed on the substrate 64 with the fibres 72 and 62 in a position to evanescently couple optical signals from one fibre to the other.

Description: This invention relates to optical interconnection networks (for optically interconnecting one or more transmitter stations with one or more receiver stations). Photonics technology could play a significant role in future telecommunications and computer switching systems within which areas there will then be a requirement for large non-blocking interconnect systems, for example, which each of say 10,000 transmitters will need to be independently connectable to any one of 10,000 receivers. Such large systems are not presently commercially achievable by known optical switching systems. Further, small systems which can be achieved at the present time are not in general susceptible to ready expansion should new transmitters or receivers need to be added to the network. In WO85/00484 there is disclosed an optical interconnection network comprising an optical signal bus consisting of an optical waveguide, and at least one connector module for connection to the bus, the module having an optical waveguide forming an evanescent coupling with the waveguide of the signal bus when the module is connected to the bus. According to one aspect of the present invention such a network is characterised in that the signal bus comprises a first substrate supporting a plurality of the optical waveguides, the waveguides each having a flat surface substantially co-planar with a surface of the substrate; the connector module comprises a second substrate supporting at least one optical waveguide, the waveguide having a flat surface substantially co-planar with a surface of the second substrate; and means are provided for releasably coupling the module to the signal bus in a face-to-face relationship to form the evanescent coupling between the two waveguides. The connector module provides optical coupling to or from the optical bus waveguide as required. The network can be readily extended by coupling additional modules to the optical bus. The releasable coupling means preferably includes upstanding guide walls on one of the substrates adapted to locate respective side edges of the other substrate such that the respective waveguides of the two substates are aligned relative to one another over the length of the module. The guide walls preferably define a socket, and the connector module is preferably a push-fit in the socket. According to a second aspect of the invention, there is provided a method of optically interconnecting a transmitter station with a receiver station in an optical communication system having an optical signal bus between the two stations, characterised in that the method comprises forming a plurality of spaced-apart optical waveguides in the signal bus to define a first plurality of spatially-multiplexed optical signal channels, releasably coupling a connector module to the signal bus at the transmitter and/or receiver stations to form an evanescent face-to-face coupling between at least one of the waveguides in the bus and respective waveguides in the connector module, and providing a further plurality of non-spatially multiplexed signal channels by independently multiplexing the signals transmitted along each of the spatially-multiplexed channels. Spatial multiplexing is included by providing spaced-apart waveguides on the signal optical bus, the receiver station being provided with means for selecting a signal from the appropriate waveguide. The independent multiplexing can be, for example, wavelength and/or time division multiplexing. The allocation of the channels of the various multiplexes used in the hierarchy may be dedicated, i.e. fixed, for each module, or assignable on a demand basis. Preferably, the interconnection network will employ wavelength multiplexing as one of the hierarchy of multiplexes. This can be achieved by providing each transmitter station with a fixed or tunable source of optical radiation, for example a laser, or by providing a reference optical bus of at least two reference optical waveguides each couplable to an optical source of a distinct wavelength, each transmitter waveguide being evanescently couplable to at least one of the waveguides of the reference optical bus. This hierarchy of wavelength, spatial and other multiplexing systems allows a larger interconnect network to be formed than presently possible using any one of such multiplexing systems individually and the use of modules optically interfacing to the busses provides, in conjunction with the hierarchical multiplexing; a readily extendable network. Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a first embodiment of the present invention; Figure 2 is a schematic diagram of a second embodiment of the present invention in which the receiver modules employ coherent homodyne optical demodulator; Figure 3 is a schematic diagram of a third embodiment of the present invention in which the receiver modules employ coherent heterodyne optical demodulators; and Figure 4 is a perspective schematic view of an optical bus and a module according to the present invention. Referring to Figure 1 an interconnect network 2 has a reference bus 4 of m optical waveguides R i to R m each coupled to a respective optical source of distinct wavelength λ i to λ m shown collectively as a reference generator 8, and a signal bus 10 of N optical waveguides S l to S N . For clarity there is shown only one transmitter module T i of upto NxM possible transmitter modules and one receiver module 14 of up to NxM possible receiver modules connectable to the network of this particular embodiment. The transmitter module 12 in this example when fixed in position relative to the busses 4 and 10 has a first and second group of waveguides 16 and 18 evanescently coupled to the waveguides of the busses 4 and 10 respectively. Each waveguide of the group of waveguides 16 is coupled to one of the waveguides R i and in turn is optically coupled to a reference selector switch 20 which is arranged to couple one of the waveguides R i to an optical modulator 22. The modulator 22 modulates the carrier signal λ i coupled from waveguide S i with the information signal to be transmitted. The output of the modulator 22 is couplable via a connector switch 24 to a selected one waveguide of the group of waveguides 18 and hence to a selected one of the waveguides S i . The receiver module 14 includes a group of waveguides 26 and an N to 1 selector switch 28 which in the same manner as the selector switch 16 and waveguides 16 of the transmitter module 12 can selectively couple one of the signal waveguides S i to a demodulator 29 via a frequency selective optical filter 30. An example of a means for coupling the waveguides of the busses 4 and 10 the module waveguides of groups 16, 18 and 26 will be described in detail below. The principle behind the bus architecture of this embodiment present invention is that the signal bus forms a message bearer bus carrying optical signals from the transmitter modules in a manner that is accessible to the receiver modules and that the reference bus provides a range of optical frequencies of carrier signal which can be selected by a given transmitter module 12. An interconnection path is established by selecting one wavelength of carrier signal by means of the selector switch 20 modulating the carrier so obtained by the message information and then connecting the modulated carrier via the connector switch 24 to the selected signal waveguide S i . Each waveguide S i can therefore carry a wavelength multiplex of M wavelengths, each transmitter being associated with a unique combination of wavelength λi and waveguide S j . A receiver can select the information signal to be received by connecting the appropriate waveguide S j via the signal selector switch 28 and passing only the required wavelength λ i to the optical demodulator by means of the frequency selective filter 30. The principle of using a hierarchy of multiplexing techniques can be extended by, for example, using time coding of transmission, coding of transmissions and so. Thus, for example a three layer hierarchy of space, frequency (as exemplified by the Figure 1 arrangement) together with time domain multiplexing of the information signals to the modulators 22, each with a dimension of 100 channels would provide an interconnect power of 10⁶. This multiplication of the multiplexing power of each multiplexing technique therefore allows a much greater interconnect power than available from any one technique alone. The ability to removably couple transmitter and receiver modules onto the busses 4 and 10 permits ready growth of the network by the additional of modules until all the multiplex combinations are used when whereupon one additional multiplex may be added to the hierarchy to further increase the interconnect power of the network and for transmitter and receiver modules to be unplugged and moved to different locations on the optical bus as necessary. Thus, for example, once NxM transmitters and receivers had been connected to the network, the introduction of P time slot channels would allow expansion of the network to NxMxP transmitters and receivers. The transmitter modules 12 and 14 can independantly, be installed with dedicated channels, ie. the selection and connector means coupling the modulator/demodulators to a single predetermined waveguide of each bus, in which case the groups of waveguides 16, 18 and 26 need comprise only one waveguide without a requirement for selector or connector switches, or they may be as shown in Figure 1 which allows the transmitters and receivers to select the wavelength and space channel combination for transmission and reception. Alternatively, the transmitter modules may be provided with fixed or tunable optical sources, for example lasers, which eliminates the need for the reference bus 4. If two or more non-spatial hierarchies are employed then the signal bus need have only one waveguide, each transmitter and receiver module being evanescently coupled to it exclusively. In the case of spatial multiplexing, i.e. two or more waveguides in the signal optical bus, the modules can, analogously to the wavelength multiplexing example above, be dedicated or assignable. If the modules are dedicated then the signal connector or signal selector switch may be eliminated, the optical waveguide of the modules being located so that it couples to the required waveguide of the signal optical bus when co-located with the bus. Referring now to Figure 2, an interconnection network 32 is shown which is as the network 2 except that the wavelength selection in the receiver module 14 is not by way of a tunable filter 30 but by means of coherent, homodyne detection. This is achieved by means of an additional selector switch 34 which selectively couples one of the reference waveguides R i top the coherent demodulator 36 corresponding to the wavelength channel to be demultiplexed. Heterodyne detection can be achieved in a similar manner as by the incorporation of frequency shifters 38 between the reference generator and the receiver modules 14 of the network of Figure 2 as shown in Figure 3. A possible structure of the busses 4, 10, of the groups of waveguides 16, 18, 26 and a the method of inter-coupling them will now be described with reference to Figures 4 and 5. The busses were fabricated using optical fibres having a D-shaped cross-section, "D-fibres". This fibre is formed by cutting away one side of a preform of a normal optical fibre so that one side is close to the core. When drawn this produces a long, continuous length of fibre with a lateral flat surface 46 close to the core 44 (as shown on Figure 4 with respect to only one of the figures 44). In this particular embodiment the core was about 0.5µm from the flat surface. The three D-fibres 40, 42 and 44 when placed with their centres 250µm apart and press-moulded into a polymer substrate 50 30mm. The moulding was performed against a heated optical flat which ensure that the flat surfaces of the D-fibres align accurately in a common plane. The fibres 40, 42 and 44 where then cleaved and fusion spliced to standard single mode optical fibre in turn splices to the tails of semiconductor lasers of wavelength 1.3µm (not shown). A second substrate was fabricated (not shown), the D-fibres being spliced at one end of single mode tails to couple them to optical power meters. The two of the waveguide substrate 50 coupled to the lasers (not shown) performed the function of the optical bus 4 carrying the power. The third acts as a single signal bus waveguide but was coupled to a laser also to measure the coupling performance. The second substrate, acting as one of the module couplers, was positioned so it overlapped the first by 0.5mm. The close proximity of the cores provided evanescent coupling between them allowing a small fraction of the optical power to be tapped out of the fibre bus into the corresponding waveguide of the second substrate. A photomicrograph of the substrate 50 showed that the D-fibres appear as ridges 0.5µm high and 85µm wide, the remainder of the fibre diameter being concealed by polymer 'ears' which grip the fibres and slope away to the general substrate level. The top points of the flat surfaces of the fibre occupy a plane to well within an accuracy of 100nm with a spacing of 250µm ± 20µm. The power splitting ratios between fibres are displayed in the table below, where the primed numerals indicate the equivalent fibres on the second substrate. The cross talk between unwanted pairs of fibres was below the measurement limit of -78dB. The ratio of wanted to unwanted signal was therefore at least 43dB at each tapping point. A detailed treatment of the tolerancing of the bus and connector substrates shows that with the present design, a lateral and angular misalignment 250µm, and 1° respectively, a fibre height variation of 0.75µm, or a wavelength change from 1.3 to 1.5µm, each give smaller variation in output power than 3dB. All these parameters are well within achievable engineering limits and have been achieved here. A more critical parameter is the "rocking" of the connector in the two available dimensions. However, by taking advantage of the close tolerance of the planes formed by the D-fibre flats (<100nm), the natural contact of these planes as the connector contacts the bus is well within the tolerance needed. It is obvious that the position of contact along the bus will be immaterial. Many connectors could therefore be connected to the bus along its length. Each connector substrate in principle only takes up a few mm of space on the D-fibre bus. The optical loss of D-fibre is below 1dB/m. It can therefore be envisaged that by expanding the present geometries and maintaining the physical tolerances already achieved an optical distribution bus capable of interconnecting large numbers of communicating terminals could be built using present optical technologies. Referring now to Figure 5 an optical interconnection network shows a portion an optical fibre bus 60 having one signal optical D-fibre 62 embedded in a thermoplastic substrate 64. A portion of the fibre 62 lies within a wall structure 65 having a slot 66 and a channel 68 to avoid contact with the flat of the D-fibre 62. A module 70 comprising an optical D-fibre 72 embedded in a thermoplastic substrate 74 is dimensioned to be a push fit in the wall structure 68. When pushed into structure 68 the fibres 72 and 62 are held in position to permit evanescent coupling. The fibre 72 extends through the slot 66 to a receiver or transmitter as required. The module 70 may be provided with a handle to ease removal of the module 70 from the wall structure 64.

Claims: An optical interconnection network comprising an optical signal bus (4, 10, 50, 60) having at least one optical waveguide (S, 42, 62) and at least one connector module (70) for connection to the bus, the module having at least one optical waveguide (72) forming an evanescent coupling with a respective waveguide of the signal bus when the module is connected to the bus, characterised in that the signal bus (4, 10, 50, 60) comprises a first substrate (64) supporting a plurality of the optical waveguides (S, 42, 62), the waveguides each having a flat surface substantially co-planar with a surface of the substrate (64); the connector module (70) comprises a second substrate (74) supporting said at least one optical waveguide (72), the waveguide (72) having a flat surface substantially co-planar with a surface of the second substrate (74); and means (65) are provided for releasably coupling the module (70) to the signal bus (10, 50, 60) in a face-to-face relationship to form the said evanescent coupling between the two waveguides. A network according to claim 1 in which the releasable coupling means includes upstanding guide walls (65) on one of the substrates (64, 74) adapted to locate respective side edges of the other substrate such that the respective waveguides of the two substrates are aligned relative to one another over the length of the module. A network according to claim 2 in which the guide walls (65) include a first pair of opposed walls extending generally parallel to the direction of the waveguides in the respective substrates, and a second pair of opposed walls transverse to the first pair. A network according to claim 2 or 3 in which the guide walls (65) include aligned openings (66, 68) forming a passage for the respective optical waveguides in the two substrates when the module is coupled to the bus. A network according to any one of the claims 2-4 in which the guide walls define a socket and in which the connector module is a push-fit in the socket. A network according to claim 1 in which the releasable coupling is a plug-in coupling. A network according to any one of the preceding claims in which the waveguides comprise D-fibres. A network according to any one of the preceding claims in which the module (70) has a plurality of waveguides, one positioned to couple to each of the waveguides in the signal bus. A network according to claim 8 further comprising: an optical signal transmitting means (12); and first transmitter selection means (24) whereby the transmitting means is couplable to a selected one of the signal waveguides. A network as in claim 9 in which the optical signal transmitting means comprises a tunable laser. A network as in claim 8 further comprising a receiving means (14) provided with a first receiver selection means (28) whereby the receiving means is couplable to a selected one of the signal waveguides. A network as in claim 11 in which the receiving means (14) comprises a tunable optical filter. A network as in claim 9 further comprising a reference optical bus (4) which includes two or more reference optical waveguides each coupled to an optical source (8) of a distinct wavelength, and in which the transmitting means (12) has a waveguide positioned to couple to a predetermined one of the reference waveguides and means (22) for modulating an optical signal coupled from said reference waveguides. A network as in claim 13 in which the transmitting means (12) has a plurality of waveguides (16) positioned to couple to each of the reference waveguides, and second transmitter selection means (20) whereby the transmitting means is couplable to a selected one of the reference waveguides. A network as in claim 11 further comprising a reference optical bus (14) which includes two or more reference optical waveguides each coupled to an optical source (8) of a distinct wavelength, and in which the receiving means (14) comprises a coherent demodulator (36), and the receiving means has a waveguide positioned to couple to a predetermined one of the reference waveguides whereby the coherent demodulator is couplable to one of the optical sources. A network as in claim 15 in which the receiving means (14) has a plurality of waveguides positioned to couple to each of the reference waveguides and a second receiver selection means (34) whereby the receiving means is couplable to a selected one of the reference waveguides. A network according to any one of the preceding claims wherein the optical signal bus (4, 10, 50, 60) has plural optical waveguides (S, 42, 62) defining a first plurality of spatially-multiplexed optical signal channels; and the connector module (70) couples an optical signal processor (12, 14) to at least one optical waveguide of the bus, the processor being capable of processing optical signals on at least one of a second plurality of non-spatially multiplexed optical signal channels on said at least one optical waveguide. A network according to claim 17 wherein additional signal channels are provided by independently multiplexing the signals transmitted along each of the spatially-multiplexed channels. A network according to claim 18 in which the independent multiplexing comprises wavelength and/or time division multiplexing. A method of optically interconnecting a transmitter station (12) with a receiver station (14) in an optical communication system having an optical signal bus (4, 10, 50, 60) between the two stations, characterised in that the method comprises forming a plurality of spaced-apart optical waveguides in the signal bus to define a first plurality of spatially-multiplexed optical signal channels releasably coupling a connector module (70) to the signal bus at the transmitter and/or receiver stations to form an evanescent face-to-face coupling between at least one of the waveguides in the bus and respective waveguides in the connector module, and providing a further plurality of non-spatially multiplexed signal channels by independently multiplexing the signals transmitted along each of the spatially-multiplexed channels.

Inventors: CASSIDY STEPHEN ANTHONY, HEALEY PETER, SMITH DAVID WILLIAM, CASSIDY, STEPHEN ANTHONY, HEALEY, PETER, SMITH, DAVID WILLIAM
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1990
Main classification: H04B10
IPC codes: G02B6, H04B10, H04Q3, H04J14, H04Q11