GB2378594A

GB2378594A - Polarisation splitting wavelength router

Info

Publication number: GB2378594A
Application number: GB0119495A
Authority: GB
Inventors: Peter Duthie
Original assignee: Marconi Caswell Ltd; Marconi Optical Components Ltd; Bookham Technology PLC
Current assignee: Marconi Caswell Ltd; Marconi Optical Components Ltd; Lumentum Technology UK Ltd
Priority date: 2001-08-10
Filing date: 2001-08-10
Publication date: 2003-02-12
Also published as: GB0119495D0; WO2003015454A3; WO2003015454A2

Abstract

A wavelength router comprising a plurality of input ports, a plurality of output ports and a routing means for routing optical signals between an input port and an output port, at least one input and output port being polarisation selective and the routing means being polarisation maintaining. In use, the router is adapted to polarise a light signal into two polarisations at the polarisation selective input ports. The router also comprises information encoding means adapted to write a data signal onto a respective polarisation of each wavelength of the light signal, thereby increasing capacity.

Description

Wavelength Router The invention relates to a wavelength router for use in an optical communications system.

The rapid increase in importance of data communication has led to a corresponding increase in demand for bandwidth in communication systems. Dense wavelength division multiplexing (DWDM) has been developed to expand the capacity of new and existing optical fibre systems, in which multiple wavelengths of light simultaneously transport information through a single optical fibre. Each wavelength operates as a single channel carrying a stream of data and accordingly the capacity of the fibre is multiplied by the number of DWDM channels available.

It has been proposed to use polarisation techniques to increase the capacity of WDM systems but in general this approach is not favoured as polarisation maintaining fibre is too expensive and suffers high losses compared to single mode fibre. Polarisation maintaining fibre also suffers from the production difficulty that it is difficult to join fibres so as to maintain polarisation. Additionally, any system relying on polarisation in this way is incompatible with existing single mode fibre systems and would therefore be unlikely to find general market acceptance.

In all communication networks, there is a need to connect individual channels to individual destination points such as another network or end customer. In traditional telecommunication systems, these are referred to as cross-connects and are implemented using electronics. However, due to the inherent advantages of all optical systems, there is great interest in developing cross-connects which operate at a wavelength level using photonic network elements.

Most conventional optical cross-connects are essentially physically reconfigurable switches. In known optical cross-connects, the incoming light stream must generally first be demultiplexed into its constituent wavelengths, each on an individual fibre. Each individual wavelength is then directed to its destination using an optical space switch, generally composed of a large number of switch elements. The wavelengths must then be remultiplexed or recombine before continuing onto the destination fibre. For full

connectivity, wavelength conversion may be required. A typical 40 channel DWDM system will require thousands of switch elements to cross connect all the wavelengths.

The complexity and loss of optical cross-connects is further increased by the need to provide access to parallel redundant switching planes to provide reliable operation.

Consequently, such optical cross-connects are highly complex and expensive systems and there exists a need to maximise the number of available channels and the bandwidth of each channel as cost-effectively and reliably as possible.

An alternative approach is the wavelength router. Instead of a space switch which is optically transparent to all wavelengths, a wavelength-selective optical interconnect provides alternative routes between input ports and output ports according to the wavelength of the optical signal. A wavelength-tunable optical transmitter or wavelength

converter at each input port can thereby selectively access each output port, providing a y e cross-connect function. A suitable wavelength-selective optical interconnect can be constructed by configuring an optical combiner with a wavelength demultiplexer. The optical loss is then large for significant channel numbers, dominated by the loss of the combiner. A better wavelength-selective optical interconnect can be constructed by combining the functions of a wavelength multiplexer with a wavelength demultiplexer in a single component. The loss for the overall NxN function is then reduced. Suitable technologies for constructing an NxN single stage wavelength-selective optical interconnect include the array waveguide grating (AWG) (fig la) and the free-space grating (fig lb). Components currently available commercially with NxN function are limited to 40x40.

The present invention seeks to provide a wavelength router with an increased capacity over known wavelength routers.

According to the invention there is provided a wavelength router comprising a plurality of input ports, a plurality of output ports and a routing means for routing optical signals between the input ports and the output ports, at least one input and output port being polarisation selective and the routing means being polarisation maintaining, wherein the router is adapted to separate a light signal into two polarisations at the polarisation

selective input ports and the router comprises information encoding means adapted to write a data signal onto a respective polarisation of each wavelength of the light signal In a first preferred embodiment, each input port has two transmitters connected thereto and each output port has two receivers connected thereto, the signal from each transmitter being combined using a polarisation maintaining optical coupler and the signal from the router being polarisation resolved by a polarisation beam splitter. Full connectivity is therefore provided between transmitters and receivers. Preferably, contention resolution is provided for the case when more than one transmitter requires access to the same receiver at the same time. For M channels, M tuneable lasers, M data modulators, M receivers, M/2 combiners, M/2 polarisation beam splitters and 1 M/2xM/2 polarisation maintaining wavelength router are required The tuning requirement of a laser diode is a function of channel width and the bandwidth of each channel. However, the tuning range of any given laser diode is limited for physical reasons. The invention advantageously permits the effective number of channels to be increased by using the polarisation of light to encode different data signals on different polarisations of the same wavelength without any commensurate increase in wavelengths. By making the routing means polarisation maintaining, any polarisation control at the receiver can be avoided, which, as such controls are typically slow, will facilitate the routers use in high capacity communication systems. The polarisation maintaining routing means or interconnect thus has a different routing characteristic for each polarisation and wavelength combination, thereby simplifying the design and manufacturing costs for the device.

In a second preferred embodiment, each input port has a transmitter connected therewith, which transmitter is adapted to produce two signals having the same wavelength but transversely differing polarisation. Preferably, the transmitter includes first and second modulators, each adapted to write a data signal at different polarisations. Preferably the modulators are in series with one another and a polarisation rotator is located between the first and second modulators. Alternatively, the modulators may be parallel to one another with a polarisation beam splitter located in front of the modulators and a polarisation beam combiner located after the modulators. In this latter case, a polarisation beam

rotator may be included between the splitter and one modulator. Although reduced connectivity is thereby provided between transmitters and receivers, (also preferably subject to contention resolution when more than one transmitter requires access to the same receiver at the same time), a reduced number of tuneable lasers is required to obtain the capacity increase. For M channels, M/2 tuneable lasers, M data modulators, M receivers, M/2 combiners, M/2 polarisation beam splitters and I M/2xM/2 polarisation maintaining wavelength router are required.

Exemplary embodiments of the invention will now be described in greater detail with reference to the drawings in which: Fig. la schematically shows a known wavelength interconnect ;

Fig. lb schematically shows an alternative known wavelength interconnect ; c, TV. an JLLerJ [WL Fig. 2 shows a schematic diagram of a wavelength router ; Fig. 3 shows a schematic diagram of an alternative router; Fig. 4 shows a schematic diagram of the polarised signal generator; Fig. 5 shows an alternative polarised signal generator to Fig. 4 Fig. 6 shows an alternative arrangement for the modulator component.

Figs. la and 1 b show known wavelength selective interconnects. The function of the wavelength selective interconnect is that for any input port, a choice of output ports can be made by selecting appropriate optical wavelengths. Each of N discrete wavelengths will select one of N ports. There is no intrinsic splitting loss between input and output ports.

In the arrayed waveguide grating (AWG) implementation of Fig. la, N input ports are combined at a mixer 100, split between an array of waveguides whose length incrementally increases before recombination and splitting at a second mixer 101 between output ports. The length differences in the waveguide between the mixers 100 and 101 correspond to phase changes for different wavelengths, resulting in different output routing.

The equivalent function is available in bulk form (see Fig. Ib) where a diffraction grating provides wavelength selectivity in the routing. The routing means comprises incident lens 111, a diffraction grating 110 and output lens 112.

Figures 2 and 3 show an NxN wavelength router according to the invention having a plurality of input ports 1-8-N, a plurality of output ports 11-18-N and routing means 20.

The routing means comprises a known arrayed waveguide grating (AWG) as described above, which is passive and also adapted to maintain the linear and orthogonal polarisation of the light signal passing through the waveguide. Again, it would be possible to use alternative passive polarisation maintaining structures such as a bulk grating in appropriate circumstances. In Figure 2, a system with full connectivity is shown in which each channel has an associated tuneable laser to generate a signal. Figure 3 shows an alternative embodiment in which a single tuneable laser is provided for each pair of channels. This has reduced connectivity but also requires only half the number of tuneable lasers.

In use, two effective optical transmitters Tx are connected to each input port and two effective optical receivers Rx to each output port. Typically the transmitters and receivers will comprise optical transceivers or wavelength translators connected to an optical communications network based on single mode fibre.

At each of the transmitters, for example TxlA, a data signal is encoded on to the light signal at a particular polarisation and the polarised signal is passed down a polarisation maintaining fibre 21 to a combiner to effectively combine the signal with that from transmitter Tria, which combiner is connected to an input port 1-8. The signal is then routed by the routing means 20 to the appropriate output port 11-18 for that wavelength, for example A. At the outp. ort, the signal is passed to a polarisation beam splitter, which routes the to Receiver Rxl or Rx2 according to its polarisation. Similarly for a wavelength A. 2 with polarisation P I I or P21 due to its different wavelength will be routed from input port 1 to a different output port 14, where it will pass through a polarisation beam splitter to be passed to Receiver R. 14 or Rx24 according to its polarisation.

Analogously, a signal arriving at input port 8 with wavelength s with polarisations PI or P2 passed to the appropriate output port 11.

In the most general case, the transmitter comprises an information encoder or modulator such as an electro-optic modulator to encode a data signal at a particular wavelength. The data signal will be derived from an external device. The wavelength is determined by system characteristics and will typically be generated using a tuneable laser. For each wavelength, the light signal can have two orthogonal polarisations. In common usage, these are designated the (transverse electric) TE and (transverse magnetic) TM modes (neglecting the forward component of the vector in each case). Each of these modes can be used to encode separate data signals at the same wavelength.

The routing means will in general function in the known manner but maintaining the polarisation increases capacity up to double that of a known router within the constraints of this. Generally, a transmitter TxlA having generated a wavelength I with polarisation

Pa with corresponding data signa ! D ; A, passes The signal to input port 1, which combines . L a VV JLLJU unig uaL ig the signal with that from TxiB with Xj and polarisation Pb and data signal DlB, maintaining the polarisations, which input port then routes the signals to the appropriate output ports 11 and xx Fig. 4 shows a schematic diagram of the means for generating the polarised light signal.

In use, the tuneable laser 200 will generate a linearly polarized light signal. A'JJ4 waveplate 201 converts this to an equal quantity of TE and TM light. The TE mode polarisation is modulated with a first data signal, the light is then passed through a'JJ2 waveplate 203 and the unmodulated polarisation component, previously TM, now TE, is modulated with a second data signal In the most general case the transmitter comprises an information encoder, also known as

a modulator, to encode a data signal at a particular wavelength. Suitable ^adulation means may be either direct modulation of the laser, or by means ofan externalli lulator.

In a first further embodiment, the signal is passed to a first modulator 202 adapted to encode a data signal at the wavelength of the light signal, which modulator encodes the signal in the TM polarisation mode with no substantial effect on the TE mode. The signal is then passed to a second modulator 204, which encodes a second data signal at the wavelength of the light signal in the TE polarisation mode with no effect on the TM

mode. In a case such as this, a material with linear electro-optic tensor coefficients with appropriate zero terms, such as gallium arsenide must be used Crystals of cubic Zinc-blende structure, such as m-V semiconductors are optically isotropic by default and owe their electro-optic coefficients to their non-centrosymmetric nature. The electro-optic effect is described by a 6x3 tensor whose elements 41,52, and 63 alone are non-zero. The zero-populated upper-half of this tensor implies that none of the primary crystallographic axes provide electro-optic change for applied E-fields in their own direction for light polarised in the same direction. The lower-half tensor diagonal non-zero elements provide an electro-optic effect for in-plane, 45 -polarised light to an E-field applied perpendicular to the plane, where the planes are defined by major crystal axes.

Thus Ey produces a maximum effect for light polarised in the x-z plane at 45 to x and z axes, where x, y, z are crystallographic axes. These 450 directions also define cleavageplanes in GaAs, thereby defining preferred propagation directions and polarisation states also. TE-polarised light reacts optimally to normal (into-the-plane) fields. TM polarised light sees no effect since (as noted above) the polarisation direction is that of both the field and a major crystal axis.

As the TE and TM modes are orthogonal, a polarisation rotator 203 can be included between the first and second modulators, so that these are co-planar, which simplifies the manufacture of the modulators. Although it would be possible to dispense with the polarisation rotator by aligning the modulators orthogonally to one another, this will typically prove more complex and expensive than including an intermediate polarisation rotator.

Fig. 5 shows an alternative embodiment to Fig. 3. It is possible to have the two modulators 202 & 204 in parallel to each other by incorporating a polarisation beam splitter 210 in the optical path prior to the modulators and a polarisation beam combiner 212 after the modulators 202,204. In this case it would also be advantageous to include a polarisation beam rotator between the splitter and one modulator and another between the

modulator and the combiner so that the modulators can again be co-planar. Alternatively, a simple power splitter could be used, followed by a polarisation rotator in one arm This arrangement would be suitable for modulator materials such as lithium niobate, which do not have the aforementioned particular properties of gallium arsenide, that one polarisation can be modulated without significant effects on the other polarisation.

For both the embodiments of figures 4 and 5, it would be possible in a further embodiment to include a fast active polarisation changer in the optical path, so that once a data signal has been written in the TE mode, the TE and TM modes are swapped and the second modulator can also write a data signal in the TE mode. In general, signal modulation is easier to achieve with the TE mode than with the TM mode. This arrangement also benefits from the production advantage that the modulators will be co- planar.

In general changing polarisation state cannot be achieved instantaneously and therefore polarisation state changes will typically only be made between system data blocks. This applies to all embodiments of the invention.

Figure 6 shows an alternative arrangement for the modulator component. In this case, the modulators based on gallium arsenide have additional control circuitry mounted on the gallium arsenide comprising a simple FET or HEMT type GaAs switch 220 adapted to switch the data path between the first and second modulators. In this case, a data signal can be routed to either modulator and be written at either polarisation depending on its destination using a conventional semiconductor switching arrangement. This arrangement provides a simple cost-effective control over the routing of the signal.

As an alternative to the basic structure having a single routing means which is adapted to maintain both TE mode and TM mode polarisation, it would be possible to adapt the router to comprise two parallel routing means, each adapted to maintain a single polarisation mode. Although this inevitably increases the number components and hence complexity of the router design, due to the significant cost component of the polarisation maintaining routing means, it may prove to have lower production costs.

Although the invention has been particularly described using a single laser to generate a single signal having two polarisations, it would be possible to use two lasers to generate the different polarisations. If separate transmitters are used for each respective polarisation, then either fast polarisation switching in the optical domain is required or the ability to choose which polarisation is modulated. Although the light signals have been described as being generated by a tuneable laser, it would also be possible to use other known wavelength-selectable sources.

Notwithstanding the above description, the use of AWG technology is not meant to be to the exclusion of using the combiner: demux approach albeit, the architecture of the latter approach may rely upon combining input wavelengths down to a plurality of intermediate fibre stages. The polarisation state wavelength routing means, with increased data capacity, is equally applicable to either wavelength routing solution and therefore the invention is intended to cover the gamut of routing architectures betwixt the solution being entirely an AWG wavelength selection device, through to the solution being a multi-input combiner combining all wavelengths onto one fibre and demultiplexing said combined signal to their individual wavelengths thereafter. Intermediate solutions involve combining a plurality of wavelength inputs to multiplicity of intermediate fibres each of which is then demultiplexed using an asymmetric wavelength interconnect.

Claims

1. A wavelength router comprising a plurality of input ports, a plurality of output ports and a routing means for routing optical signals between the input ports and the output ports, at least one input and output port being polarisation selective and the routing means being polarisation maintaining, wherein the router is adapted to separate a light signal into two polarisations at the polarisation selective input ports and the router comprises information encoding means adapted to write a data signal onto a respective polarisation of each wavelength of the light signal.

2. A wavelength router according to Claim 1, wherein, in use, each input port has two transmitters connected thereto and each output port has two receivers connected thereto, the signal from each transmitter being combined using a polarisation maintaining optical coupler and the signal from the router being polarisation resolved by a polarisation beam splitter.

3. A wavelength router according to Claim 1, in which each input port has a transmitter connected therewith, which transmitter is adapted to produce two signals having the same wavelength but transversely differing polarisations.

4. A wavelength router according to Claim 2 or Claim 3, wherein the information encoding means includes first and second modulators, each adapted to write a data signal at a single polarisation.

5. A wavelength router according to Claim 4, wherein the modulators are in series with one another and a polarisation rotator is located between the first and second modulators.

6. A wavelength router according to Claim 4, wherein the modulators are parallel to each another with a polarisation beam splitter located in front of the modulators and a polarisation beam combiner located after the modulators.

7. A wavelength router according to Claim 6, wherein a polarisation beam rotator is included between the polarisation beam splitter and one modulator.

8. A wavelength router according to any one of Claims 4 to 7, wherein the data path to each respective modulator is controllable via a semiconductor switch.

9. A wavelength router according to any one of Claims 1 to 8, wherein routing means comprises a first wavelength selective interconnect adapted to maintain a first polarisation and a second wavelength selective interconnect adapted to maintain a second polarisation.

10. A wavelength router according to any one of Claims 1 to 9, wherein the information encoding means comprises an external modulator.

11. A wavelength router according to any one of Claims 2 to 9, wherein the transmitter comprises the information encoding means.

12. A wavelength router according to Claim 11, wherein the information encoding means is adapted to write the data signal by direct modulation of the transmitter laser.

13. A wavelength router according to any one of Claims 2 to 12, wherein contention resolution is provided for the case when more than one transmitter requires access to the same receiver at the same time.

14. A wavelength router substantially as described herein, with reference to and as illustrated in the accompanying drawings.