GB2382244A

GB2382244A - Add-drop multiplexer with MEMs (Micro-Electro-Mechanical) tunable filters

Info

Publication number: GB2382244A
Application number: GB0127386A
Authority: GB
Inventors: Anthony Needham; John Garside
Original assignee: Marconi Optical Components Ltd
Current assignee: Marconi Optical Components Ltd
Priority date: 2001-11-15
Filing date: 2001-11-15
Publication date: 2003-05-21
Also published as: GB0127386D0

Abstract

An optical communications module for incorporation in an optical telecommunications network includes one or more MEMs (Micro-Electro-Mechanical) tunable filters 3, 28, 29, 30 and one or more receivers 5 and / or tunable transmitters 24 to enable one or more wavelength division multiplexed (WDM) channels to be added or dropped/extracted from an optical signal being transmitted through the network. Several filters may be arranged in cascaded fashion (figures 2, 3, 6 and 8). In one embodiment a beam splitter (27, fig. 7) is used to add/drop channels.

Description

Optical Communications Modules The present invention relates to optical communications modules including tunable filters combining add and/or drop multiplexing functions.

In telecommunications optical networks Wavelength Division Multiplexing (WDM) or Dense Wavelength Division Multiplexing (DWDM) is a common method for increasing the bandwidth of data that can be transmitted down a single fiber. In this technique the data to be transmitted is split between wavelength channels each of which is allocated a specific wavelength. Conventionally, in order to recover the signals from individual channels, fixed filters are used to select an individual channel, which is extracted (or"dropped") and interrogated using a receiver. To re-transmit on that channel, the signal from either a fixed wavelength transmitter is multiplexed in with the other wavelength channels ("added"), or the signal from a tunable transmitter is coupled into the fiber. There is now a move towards reconfigurable optical networks, where different wavelength channels may be dropped or added at different times at the same physical location in the network. To achieve this using fixed wavelength filters requires a high component count and causes relatively high losses.

Proposals have been made to use tunable optical filters to reduce the component count but such an arrangement still leads to bulky systems.

The present invention seeks to provide optical telecommunications modules incorporating receivers and/or transmitters together with tunable filters in a much more cost-effective and smaller size than previously known arrangements.

A ccord According to a first aspect of the present invention there is provided an optical telecommunications module incorporating a MEMS tunable filter adapted to receive a multiplexed optical signal comprising a plurality of discrete channels, and to filter out a predetermined channel of said optical signal, and receiver means to receive said predetermined channel, further output means being provided to receive the remainder of said optical signal for onward transmission.

According to an alternative aspect of the present invention there is provided an optical telecommunications module incorporating a MEMS tunable filter adapted to receive a

multiplexed optical signal comprising a plurality of discrete channels and transmitter means to add a predetermined channel to said optical signal to form a combined signal, and means to transmit the combined signal to an output for onward transmission.

According to another aspect of the invention there is provided an optical telecommunications module incorporating a MEMS tunable filter adapted to receive a multiplexed optical signal comprising a plurality of discrete channels, the tunable filter being adjustable to filter out a predetermined channel of said optical signal, and receiver means to interrogate said predetermined channel and transmitter means to add to said optical signal a further optical signal comprising a channel having the same wavelength as said predetermined channel to form a combined signal, and means to transmit the combined signal to a main output for onward transmission.

Preferably, the combined signal or said remainder of the optical signal are reflected from the tunable filter on to a further tunable filter which is adapted to add and/or extract a selected channel from the optical signal incident thereon. In a further development, the combined signal or said remainder of the optical signal are reflected sequentially on to a plurality of further tunable filters, each of which is adapted to add and/or extract a predetermined channel from the signal incident thereon.

In an alternative embodiment, the optical signal or said remainder of the optical signal is transmitted through a beam splitter, in which case, the module may include a tunable transmitter adapted to add a channel to the signal passing through the beam splitter.

I Preferably, the tunable filter comprises a small MEMS tunable filter. In preferred embodiments of the present invention, the tunable filter comprises a filter or filters of the type described and claimed in our earlier co-pending application entitled "Polysilicon Tuneable Optical Notch Filter", bearing Patent Application No.

0110728. 3, dated 2nd May 2001, the disclosure of which is incorporated herein.

The Polysilicon Tuneable Optical Notch Filter described in that application has a number of key features, namely, it is small, it is able to select an individual optical

channel and it has a low loss characteristic. Importantly for the applications to be discussed it reflects all channels that are not extracted and is able to hop from one channel to the next selected channel without scanning and hence without interfering with data carried on the intermediate channels.

Preferred embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which :Figure 1 shows a single channel tunable receiver, Figure 2 shows a multiple tunable drop receiver using additional mirrors, Figure 3 shows a multiple tunable drop receiver using direct reflection, Figure 4 shows a tunable add multiplexer, Figure 5 shows a tunable add-drop multiplexer, Figure 6 shows a tunable add-drop multiplexer using two tunable filters, Figure 7 shows a tunable add-drop multiplexer using a single filter and a beam splitter, and Figure 8 shows a multi-channel tunable add-drop multiplexer.

Figure 1 shows a first example of module intended for incorporation in an optical communications transmission system, the module incorporating a tunable single channel drop receiver. In this example all channels in a wavelength division multiplexing optical signal are directed, from an optical fibre input 1 secured to the module, via a collimating optical system 2 onto a first mirror of a tunable optical filter 3 which is at an angle to the direction of the signal. The filter 3 is tuned to allow transmission of a selected wavelength only, referred to as a dropped channel and the

output signal 4 of the dropped channel is directed on to a detector 5 and amplifier 6 . ( enabling the data in that channel to be extracted. The remaining channels that are not separated out are reflected or refocused and passed to an output optical fibre 7 via a fibre launch optical system 8. As discussed the filter 3 may be readily retuned to extract another channel without disturbing the intermediate channels. This has a major advantage as the integration of filter and receiver (or transmitter) within a single module reduces system loss and size whilst increasing ease of integration.

The tunable filter comprises two optically coupled Fabry-Perot optical cavities having a central fixed dielectric mirror located between a pair of independently adjustable

dielectric mirrors. Each cavity can be independently tuned for resonance with a particular channel by varying the distance between each outer mirror and the central mirror. The tunable filter has a silicon body formed using micro-electro-mechanical systems (MEMS) technology combining deep reactive ion etching of bonded siliconon-insulator materials and surface machining of the polysilicon.

In a further embodiment of the above invention illustrated in Figure 2, a plurality of filters and receivers, namely four, are arranged to drop four channels within the one module. Here as before the light from the collimating optical device 2 and the input optical fibre 1 is directed on to a first tunable filter 3 at an angle. The filter 3 is tuned to extract or drop a single channel, whilst all the other channels are reflected. In this case the reflected light signal is directed on to a second filter 9. The light is reflected on to a second filter 9 via an additional mirror 10. The second filter 9 again extracts a particular channel 11 and all the remaining channels are reflected on to a third filter 12 via a mirror 13 to enable a third channel 14 to be extracted from the signal. This sequence may be repeated a number of times. In this embodiment, a fourth filter 15 and mirror 16 is also provided to extract a further channel 17 and from where the remaining channels are either dispersed within the module or, as shown, focussed on to an output fibre 7 via an optical fibre launch system 8. The use of the reflecting mirrors enables all of the signals from the extracted or dropped channels to be output on one side of the module. The remaining channels being transmitted to the output optical fibre 7 leave the module on the same side as the input signal but it will be understood that by using an additional mirror, the output signal can leave the module

on the same side as the signals derived from the extracted or dropped channels.

I Figure 3 shows a further configuration of a module consisting of a multiple tunable drop receiver using direct reflection from the tunable filters. In this module, the multichannel signal from the input fibre 1 is incident on a first tunable filter 3 from which a single optical channel is dropped and passed through a detector and amplifier to a single channel output 4. The remainder of the input signal is reflected on to a second tunable filter which, in turn, extracts a further individual channel 19 which is again passed through a detector and amplifier to a single channel output. The remainder of the signal is reflected on to a third tunable filter 20 where, in turn, a further single channel is extracted or dropped to provide a further single channel

output 21. From the third tunable filter 20, the remaining signal is reflected on to a fourth tunable filter 22 where a fourth single channel signal is extracted to provide a fourth single channel output 23. The remaining part of the signal is reflected to a fibre launch optical system for transmission to an output fibre 7.

Referring now to figure 4, there is shown a tunable add multiplexer in which a single channel signal is added to an existing multichannel signal. In this embodiment, the main multichannel signal from an input fibre I is incident on the tunable filter 3 at an inclined angle and is reflected in its entirety to an output fibre. The device includes a tunable transmitter 24 which directs, onto the rear of the filter at an inclined angle, a single channel signal 25 which is tuned to a vacant channel in the main signal. The tunable filter has the advantage that it will transmit a selected wavelength in both directions whilst reflecting all others. Thus, the single channel signal passes through the filter 3 and is added to the main signal. This characteristic enables the filter 3 to be integrated with a tunable transmitter 24 to form a tunable add-multiplexer. In this embodiment the input signals are directed as before at an angle on the surface of the filter 3. This embodiment will also remove any residual input signals that happen to be present on the channel to be added to, hence also cleaning up the channel to reduce noise and cross talk.

In a similar manner to the multiple channel tunable drop receivers described above with reference to Figures 2 and 3, a plurality of tunable filters and a plurality of tunable transmitters can be integrated into a single module to produce multiple channel tunable add transmitters.

I In a similar manner to the multiple channel tunable add transmitters and receivers described above, multiple channel tunable filters may be produced where the transmitters or receivers are replaced by optical fiber ports or other optical connections allowing light to be coupled into or out of the module.

Building further on the fact that the tunable filter will transmit a selected wavelength in both directions whilst reflecting all others enables the realization of modules that can both drop and add selected channels. An example of such a module is schematically described with reference to Figure 5. In this embodiment, all channels

are directed to a tunable filter 3 at a slight angle and the filter is tuned to extract an individual wavelength channel, the transmitted light of the dropped channel falling on to a detector 5. In addition a tunable transmitter within the module has its output directed at the rear mirror of the filter 3 at the same angle as the original signal The output of the transmitter 24, when tuned to the same channel as that signal extracted, is allowed to pass through the filter 3 to replace the dropped channel.

Further potential alternative arrangements are disclosed in Figures 6 and 7. Referring now to Figure 6, there is shown a module incorporating a first filter 3 adapted to extract a first channel and this dropped channel is passed through a detector 5 and amplifier to an output 4. The remainder of the input signal is reflected from the first filter 3 to a second filter 26, the input signal again being incident on the second filter 26 at an angle. The module includes a tunable transmitter 24 which adds a channel by transmitting the added channel signal to the rear of the second filter 26 through which it passes to be added to the main signal. The main signal is then passed in the usual way to an output optical fibre 7. The channel signal added by the tunable transmitter may be the same wavelength as the channel extracted in the first filter stage, but this embodiment is primarily intended to enable the addition of a channel of a different wavelength from that extracted.

Figure 7 shows a further embodiment in which the input signal from the input fibre 1 is transmitted to a first filter 3 where a single channel is extracted and passed through a detector 5 and amplifier to an output 4. The remainder of the input signal is reflected on to a beam splitter 27. The module incorporates a tunable transmitter 24

which transmits to the beam splitter 27 a signal for a channel to be added to the main f signal. The main signal with the added channel, which again, may be, but is usually not, the same wavelength as the dropped channel, is transmitted to an output optical fibre 7 in the usual way.

Figure 8 describes a further module incorporating a cascaded add-drop transmitter arrangement enabling multiple channels to be extracted and replenished. In this module, the signal from the input optical fibre I is transmitted to a first tunable filter 3 in the usual way where one channel is extracted and passed to an output 4 through a detector 5 and amplifier and, at the same time, the module includes a tunable

transmitter 24 which adds a channel to the signal incident on the first filter in a similar way to the embodiment described with reference to Figure 5. The resultant signal is passed from the first filter 3 to a second tunable filter 28 where, again, one channel is dropped from the signal and a further channel added. The resultant signal is transmitted from the second filter 28 to a third tunable filter 29 where, again, a channel is dropped and a further channel added. The resultant signal is transmitted to a fourth tunable filter 30 where, once again, one channel is dropped and a further channel added. The resultant signal is transmitted to an output optical fibre 7.

In all the above embodiments of the present invention additional optical components may be incorporated to fold or deviate the optical path in order to reduce module size or produce alternative layouts. In an alternative embodiment of the tunable add/drop multiplexer, a single optical element may be used to couple both light into the detector and light out from the transmitter for a given channel. In addition further electronic components may be included on the output of the receive amplifiers or detectors or transmitter inputs within the module to further multiplex, demultiplex, modify or monitor the signals.

Claims

CLAIMS t. An optical telecommunications module incorporating a MEMS tunable filter adapted to receive a multiplexed optical signal comprising a plurality of discrete channels, and to filter out a predetermined channel of said optical signal and receiver means to receive said predetermined channel for interrogation, further output means being provided to receive the remainder of said optical signal for onward transmission.
2. An optical telecommunications module incorporating a MEMS tunable filter adapted to receive a multiplexed optical signal comprising a plurality of discrete channels and transmitter means to add a predetermined channel to said optical signal to form a combined signal, and means to transmit the combined signal to an output for onward transmission.
3. An optical telecommunications module incorporating a MEMS tunable filter adapted to receive a multiplexed optical signal comprising a plurality of discrete channels, the tunable filter being adjustable to filter out a predetermined channel of said optical signal, and receiver means to interrogate said predetermined channel, and transmitter means to add to said optical signal a further optical signal comprising a channel having the same wavelength as said predetermined channel to form a combined signal, and means to transmit the combined signal to a main output for onward transmission.
4. A module has claimed in any one of claims 1,2 or 3 wherein the combined signal or said remainder of the optical signal are reflected from the tunable filter on to a further tunable filter which is adapted to add and/or extract a selected channel from the optical signal incident thereon.
5. A module according to claim 4, wherein the combined signal or said remainder of the optical signal are reflected sequentially on to a plurality of further tunable filters, each of which is adapted to add and/or extract a predetermined channel from the signal Incident thereon.

<Desc/Clms Page number 9>
6. A module according to any one of the preceding claims, wherein the optical signal or said remainder of the optical signal is transmitted through a beam splitter.
7. A module according to claim 6, further including a tunable transmitter adapted to add a channel to the signal passing through the beam splitter.
8. A module according to any one of the preceding claims in which the or each filter comprises a filter as claimed in any one of the claims of our earlier co-pending Patent Application No. 0110728. 3.
9. An optical telecommunications module substantially as described herein with reference to, and as illustrated in, the accompanying drawings.