GB2381683A

GB2381683A - A re-configurable wavelength add-drop multiplexer

Info

Publication number: GB2381683A
Application number: GB0125964A
Authority: GB
Inventors: Tony Kelly; Ivan Andonovic
Original assignee: Kamelian Ltd
Current assignee: Kamelian Ltd
Priority date: 2001-10-30
Filing date: 2001-10-30
Publication date: 2003-05-07
Also published as: GB0125964D0

Abstract

A re-configurable wavelength add-drop multiplexer comprising: a wavelength multiplexer/demultiplexer (1) for receiving a first multi-wavelength optical signal and separating this into second optical signals each of a different wavelength; a plurality of switching devices (Figure 4) each associated with one of the second optical signals for selectively transmitting the second signal back to the wavelength multiplexer/demultiplexer (1) or dropping the second signal and replacing it with another signal which is transmitted back to the wavelength multiplexer/demultiplexer (1), the wavelength multiplexer/demultiplexer (1) multiplexing the signals transmitted back to it from each of the switching devices (Figure 4) into a third (multi-wavelength) optical signal, each switching device (Figure 4) comprising a first channel (31) arranged to receive said second signal and transmit it back to the wavelength multiplexer/demultiplexer (1), a second channel (33) arranged to receive said second signal and to transmit it away from the wavelength multiplexer/demultiplexer (1) to drop the signal and a third channel (34) arranged to receive said another signal and transmit this back to the wavelength multiplexer/demultiplexer (1), each of the channels (31, 33, 34) comprising a semiconductor optical amplifier (13, 14, 18) which is selectively operable to determine whether an optical signal can pass through that channel. A non-reflective version with two separate AWG's is also disclosed.

Description

A RE-CONFIGURABLE WAVELENGTH ADD-DROP MULTIPLEXER This invention relates to a re-configurable wavelength add-drop multiplexer.

The functionality required of such a device is the ability to manipulate individual wavelengths such that they can be passed through the device as a multiplex of wavelengths, dropped out of the device as individual wavelengths and a new data stream input to replace the one which has been dropped. Such a device therefore has three'paths' : the through path, the dropped paths (there will be N of these where N is the number of wavelengths in the multiplex) and N add paths.

There are a variety of known arrangements for demultiplexing an optical signal, manipulating individual wavelengths and then multiplexing the signals again.

US5414548 describes an arrayed waveguide grating multi/demultiplexer with loop-back optical paths in which each signal goes through a processor before being multiplexed back into the output signal. US5748811 discloses a similar arrangement having add-drop circuits.

US5771112 discloses a reconfigurable device for insertion and extraction of wavelengths comprising add-drop multiplexers receiving signals from an optical switch and an optical coupler for re-combining the signals.

US6049640 discloses a wavelength division multiplexer (WDM) cross-connect using a pair of angular dispersive elements with phase shifters, the refractive index of which is controllable to alter the effective optical length thereof, connected between them. In another arrangement, a reflector unit replaces one half of the WDM cross-connect.

'40-Channel Add-Drop Filter by C. R. Doerr et al IEEE Photonics Technology Letters, Vol. 11, No. 11, November 1999 describes a device comprising a

demultiplexer in the form of an arrayed waveguide grating the outputs of which are directed to an array of Mach-Zehnder switches which are thermally operated by heating strips provided over the waveguides. When no power is applied to the switches, the signals are routed to a striped high reflection coating, which reflects the signal back through the switches and grating. When the switches are activated, two halves of the channel are directed to a 50/50 coupler, interfere out of phase and exit the chip between the high-reflection stripes. One output can be used as a drop port and the other as an add port.

The present invention aims to provide an arrangement which has fewer components than many of the known devices and which uses switching devices having advantages over those used in the known arrangements.

According to a first aspect of the invention, there is provided a re-configurable wavelength add-drop multiplexer comprising: a wavelength multiplexer/demultiplexer for receiving a first optical signal comprising a plurality of wavelengths and separating this into a plurality of second optical signals each of a different wavelength; a plurality of switching devices each associated with one of the second optical signals for selectively transmitting the second signal back to the wavelength multiplexer/demultiplexer or dropping the second signal and replacing it with another signal which is transmitted back to the wavelength multiplexer/demultiptexer, the wavelength multiplexer/demuttiplexer multiplexing the signals transmitted back to it from each of the switching devices into a third optical signal comprising a plurality of wavelengths, each switching device comprising a first channel arranged to receive said second signal and transmit it back to the wavelength multiplexer/demultiplexer, a second channel arranged to receive said second signal and to transmit it away from the wavelength multiplexer/demultiplexer to drop the signal and a third channel arranged to receive said another signal and transmit this back to the wavelength multiplexer/demultiplexer, each of the channels comprising a semiconductor optical amplifier which is selectively operable to determine whether an optical signal can pass through that channel.

Preferred and optional features of the invention will be apparent from the following description and from the subsidiary claims of the specification.

The invention will now be further described, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a generic form of an AWG-based reconfigurable add-drop multiplexer ; Figure 2 is a schematic diagram of a generic form of a reflective form of the multiplexer of Figure 1; Figure 3 is a schematic diagram of a known form of switching device used in each channel of an arrangement such as that shown in Figure 1; Figure 4 is a schematic diagram of a switching device used in an embodiment of the present invention; and Figure 5 is a more detailed diagram of an array of two such switching devices as used in a preferred embodiment of the invention.

Figure 1 shows a generic AWG based reconfigurable add-drop multiplexer comprising a first AWG 1 receiving an input signal comprising a plurality of wavelengths and separating this into a plurality of individual signals each of a different wavelength and a second AWG 2 for multiplexing a plurality of signals and outputting a multi-wavelength signal. A switching fabric or block 3 is connected between the two AWGs to determine which signals are transmitted through from AWG 1 to AWG 2, which signals are dropped and to add in other signals to replace those dropped.

The switching block 3 can be of any technology so long as it provides the required switching functionality. It may, for example, comprise thermo-optic switches or mechanical switches.

Figure 2 shows a generic reflective arrangement, In this case, the switching block 3'is configured such that the inputs and outputs use the same AWG 1 as the demultiplexer and multiplexer. A circulator 4 is provided in an optical fibre 5 leading to the AWG to selectively route input signals from an input fibre 5A to the AWG 1 or to route output signals from the AWG 1 to an output fibre 5B.

Such circulators are well known so will not be described further. This Figure is similar to the prior art described in the Doerr et al paper referred to above.

Figure 3 shows a switching device in which semiconductor optical amplifiers (SOAs) are used. The switching is performed by turning the SOA bias currents on (20dB signal gain) or off (50dB absorption of the signal). The device comprises a first silica chip 10 on which one of the outputs of the AWG 1 is received by a waveguide 11. This is divided into two by a Y-junction 12 (or other suitable component) the first part of the signal being directed to a first SOA 13 and the second part to a second SOA 14. The outputs of the SOAs 13 and 14 are coupled to further waveguides 15 and 16, respectively, provided on a second silica chip 17. A third SOA 18 is provided to receive an add signal via a waveguide 19 on the first chip and to transmit this to a waveguide 20 on the second chip 17. The waveguides 15 and 20 join at a Y-junction 21 and lead to an output waveguide 22 and the waveguide 16 leads off the chip 17. The switching device thus comprises three channel: a through channel (through components 11,12, 13,15, 21,22), a drop channel (though components 11, 12,14, 16) and an add channel (through components 19,18, 20,21, 22). Thus, by appropriate biasing of the three SOAs 13,14, 18, a signal may be transmitted though the device from AWG 1 to AWG 2 or a signal from AWG 1 may be dropped and replaced by another signal which is transmitted through to AWG 2. Each wavelength, i. e. each output from the AWG 1, has its own switching device of the type shown in the figure.

The switching block 3 shown in Figure 1 may comprise a plurality of such switching devices (one for each wavelength channel).

Figure 4 shows a reflective'broadcast and select'switching device which performs the same role as the device shown in Figure 3. An input waveguide 30 divides into two waveguides 31,32 the first of which leads to a first SOA 13 and the other of which divides further into waveguides 33,34 leading to second and third SOAs 14 and 18. The second SOA 14 conducts the signal off the chip so as to drop the signal and the third SOA 18 is connected to receive an add signal. The main difference is the operation of the through channel where the first SOA 13 is operated in reflective mode, by the provision of a high reflection coating 13A on one end thereof, whereby the same AWG 1 is used for both demultiplexing and multiplexing. A circulator (not shown) is provided to provide a similar role to that described in relation to Figure 2. It is known to provide a reflective coating on an SOA for use in other applications. A suitable form of SOA for this is described in"Fabrication of multiwavelength simultaneous monitoring device using arrayed-waveguide grating"by Okamoto et al, Electronic Letters, 14 March 1996, Vol. 32, pp569-570.

Figure 5 shows an array of switches for use with a multiplex of two wavelengths (a number chosen for simplicity) of the type shown in Figure 4. A greater number of wavelengths would generally be used (e. g. up to 160) and the array would thus be extended with the corresponding number of sets of SOAs.

Figure 5 shows two input waveguides 30A and 30B each of which divides in the manner described in relation to Figure 4, leading, respectively, to SOAs 13A, 14A and 18A and SOAs 13B, 14B and 18B. Preferably, the reflective SOAs 13A and 13B are formed in a first array and transmissive SOAs 14A, 14B, 18A and 18B are formed in a second array.

This allows the reflective and transmissive devices to be fabricated separately, which improves device yield and also means that there are no compromises made related to fabricating different devices on the same wafer. The AWG and splitter/recombiner waveguides may be fabricated using silica or silicon waveguides. The SOAs are made of InP with integrated mode expanders, e. g. as disclosed in GB0027344.1 (Publication No. GB......), or GB0100975. 2 (Publication No. GB............) comprising a narrow waveguide mounted on a wider waveguide. The optical mode may be transferred from the narrow waveguide to the wider waveguide by tapering the end (s) of the narrow waveguide or by altering the refractive index thereof, e. g. by varying the band gap in a GalnNAs system.

Instead of using two AWGs, one to demultiplex and one to multiplex, the arrangement shown in Figure 5 operates in reflective mode and uses the same AWG to perform both tasks. Furthermore, the arrangement has fewer discrete components and therefore waveguide alignments compared to a conventional switch matrix (of the type shown in Figure 1). One less AWG and waveguide splitting network is required at the expense of a circulator at the input/output. In addition, the size of the device can be reduced, e. g. from 50-60 cm long as in the prior art to about 50-100 mm long.

One of the most important aspects of AWG performance is matching the passband wavelengths to that of the ITU grid (a precisely defined set of absolute wavelengths and wavelength spacing of the wavelength multiplex used in intemational telecommunication systems). Currently AWGs can reliably achieve the correct spacing but, due to manufacturing variability, the absolute values of the wavelength passbands are variable and have to be matched by using the temperature dependence of the passband wavelengths. The AWG chip is therefore either heated or cooled to achieve the correct wavelength.

Using silica based devices, this means that the package temperature has to be controlled to within 1 degree. For silicon devices the temperature has to be

controlled to within 0.1 degrees. A problem arises when two AWGs are used in the same package where the wavelengths have to be matched, as would be required in the arrangement shown in Figure 1. To achieve the same wavelength of operation, it is likely that the AWGs have to be controlled to different temperatures. This is not desirable as it leads to thermal gradients in the package and hence mechanical stress on the components.

Having the same fibre as the input and output also reduces the number of fibre alignments and allows the drop/add fibres to be provided at one end of the package rather than at the sides thereof. More importantly, it reduces the number of parts in the whole subsystem that have to be aligned. The arrangement shown in figure 1 has five devices that have to be aligned (the demux AWG 1, the switching fabric 3, the add wavelength source, the dropped wavelength receiver and the mux AWG 2) whereas in the reflective topology only three devices have to be aligned (the demux/mux AWG, the splitter/recombination network, and the SOA arrays).

In a further arrangement (not shown), the silica chip could be replaced by a network of optical fibres.

For maintenance of system performance, the equalisation of all wavelength channels in the multiplex is desirable. Normally this is executed by a'channel monitoring'overhead, most often realised by additional hardware that samples the input multiplex, separates and detects each individual channel and produces a control (electronic) signal.

In a preferred form of the arrangement described herein, the output optical gain and power of each channel can be monitored and controlled on each SOA by providing three contacts on the SOA, all electrically isolated from each other, together with appropriate circuitry connected thereto. Then, in addition to loss compensation and switching, the SOA can provide simultaneous detection of an optical signal. Appropriate interconnection of the end sections of the SOA with

the middle (bias) section thereof produces an electronic signal that is related to the optical gain of the SOA. Further details of an appropriate form of SOA to provide this function are given in US5309469 the disclosure of which is incorporated herein. This feature can be applied to each of the three SOAs in each switching device.

For optical power control, means are provided for applying an additional amplitude modulation on the input signal, e. g. a pilot tone, to induce an electrical signal proportional to the optical output power which is sensed by control means which can then control the output power by varying the bias current applied to the relevant SOA. Further details of such an arrangement are given in US4918396 the disclosure of which is incorporated herein.

In the arrangements described above, closed loop feedback control of the electrical power supplied to the SOA can be used to reduce the polarisation sensitivity of the device. SOAs are sensitive to different polarisations and the polarisation of the signals received by the SOAs is random. This could give rise to random variations in the output power of the SOAs. However, by providing a closed loop feedback which adjusts the output of each individual SOA if its output power goes above or below set limits, these variations can be kept within acceptable limits without the need to use more sophisticated (and expensive) methods of reducing the polarisation sensitivity of the SOAs.

The embodiments described above use a multiplexer/demultiplexer in the form of an AWG. However, any other form of multiplexer/demultiplexer may be used.

The arrangements described are preferably formed on a totally integrated hybrid circuit comprising the AWG, the splitter waveguides and the SOA arrays.

However, other arrangements may be used, e. g. discrete devices may be used for one or more of the components (multiplexer/demultiplexer, splitters, SOAs) joined, as appropriate by optical fibres. For instance, in the arrangement shown in Figure 5, the SOA arrays are integrated with passive waveguide splitters on a

silica chip and this may be connected to a multiplexer/demultiplexer via optical fibre ribbons.

The SOAs described above may be used to switch single wavelengths or wavelength bands, e. g. comprising 4 or 8 different wavelength signals.

Claims

CLAIMS 1. A re-configurable wavelength add-drop multiplexer comprising: a wavelength multiplexer/demultiplexer for receiving a first optical signal comprising a plurality of wavelengths and separating this into a plurality of second optical signals each of a different wavelength; a plurality of switching devices each associated with one of the second optical signals for selectively transmitting the second signal back to the wavelength multiplexer/demultiplexer or dropping the second signal and replacing it with another signal which is transmitted back to the wavelength multiplexer/demultiplexer, the wavelength multiplexer/demultiplexer multiplexing the signals transmitted back to it from each of the switching devices into a third optical signal comprising a plurality of wavelengths, each switching device comprising a first channel arranged to receive said second signal and transmit it back to the wavelength multiplexer/demultiplexer, a second channel arranged to receive said second signal and to transmit it away from the wavelength multiplexer/demultiplexer to drop the signal and a third channel arranged to receive said another signal and transmit this back to the wavelength multiplexer/demultiplexer, each of the channels comprising a semiconductor optical amplifier which is selectively operable to determine whether an optical signal can pass through that channel.
2. An add-drop multiplexer as claimed in claim 1 in which the wavelength multiplexer/demultiplexer comprises an arrayed waveguide grating.
3. An add-drop multiplexer as claimed in claim 1 or 2 in which the wavelength multiplexer/demultiplexer is connected to said switching devices by an arrangement of integrated waveguides provided on a substrate.

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4. An add-drop multiplexer as claimed in claim 3 in which the arrangement comprises a waveguide for receiving each of said second optical signals which then divides by Y-junctions into three waveguides forming said first, second and third channels.
5. An add-drop multiplexer as claimed in claim 3 or 4 in which said arrangement of integrated waveguides is formed on a silica substrate.
6. An add-drop multiplexer as claimed in any preceding claim in which the semiconductor optical amplifier on the first channel is provided with a high reflection coating on one end thereof so as to operate in a reflective mode.
7. An add-drop multiplexer as claimed in claim 6 in which each of the semiconductor optical amplifiers on the first channel for each of the second optical signals is provided on a first common array.
8. An add-drop multiplexer as claimed in any preceding claim in which each of the semiconductor optical amplifiers on the second and third channels for each of the second optical signals is provided on a second common array.
9. An add-drop multiplexer as claimed in claim 3,7 and 8 in which the first and second arrays are mounted to a first edge of said substrate.
10. An add-drop multiplexer as claimed in claim 9 in which said wavelength multiplexer/demultiplexer is mounted to a second edge of said substrate opposite said first edge.
11. An add-drop multiplexer as claimed in any preceding claim in which the semiconductor optical amplifiers are formed of InP.

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12. An add-drop multiplexer as claimed in any preceding claim in which each of the semiconductor optical amplifiers comprises at least one integrated mode expander.
13. An add-drop multiplexer as claimed in any preceding claim in which each of the semiconductor amplifiers is provided with three, electronically isolated electrical contacts by means of which the optical power therein can be monitored.
14. An add-drop multiplexer as claimed in any of claims 1 to 12 comprising means for applying additional amplitude modulation on each of the second optical signals to provide a pilot tone thereon, and control means for sensing the output power of the pilot tone and controlling the respective semiconductor optical amplifier to adjust the optical gain thereof.
15. An add-drop multiplexer as claimed in any preceding claim comprising a closed loop feedback control of the electrical power applied to each SOA arranged to maintain variations in the output power thereof due to polarisation sensitivity within predetermined limits.
16. An add-drop mulitplexer as claimed in any preceding claim in which each of said second optical signals comprises a plurality of signals of different, discrete wavelengths each being of a different wavelength to those of the other second optical signals.
17. A reconfigurable wavelength add-drop multiplexer substantially as herein before described with reference to and/or as shown in Figures 4 and 5 of the accompanying drawings.