GB2170370A - Wavelength division multiplexed networks - Google Patents

Abstract

A wavelength division multiplexed network comprising a plurality of laser transmitters (1,1') for generating optical signals at different wavelengths, and a wavelength multiplexer (3) for wavelength division multiplexing the optical signals from the laser transmitters is described. A modulation signal from an oscillator (7) is imparted in turn via a switch (8) to each signal from the laser transmitters (1,1'). A monitor photodiode (10) and phase sensitive detector (11) sense the modulated signal from the multiplexed signals, and a microprocessor (9) adjusts the current applied to a corresponding Peltier device (6) in response to the output from the phase sensitive detector (11) to obtain substantially maximum power transmission through the wavelength multiplexer (3). <IMAGE>

Description

SPECIFICATION Wavelength division multiplexed networks The invention relates to a wavelength division multiplexed network and a method of controlling multiple signal sources in such a network.

Awavelength division multiplexed network is herein defined as comprising a plurality of optical signal sources for generating optical signals with different wavelengths, and wavelength multiplexing means for wavelength division multiplexing the optical signals from the optical sources.

Atypical wavelength division multiplexed (WDM) network as herein before defined is used in a telecommunications network in which at least each active subscriber is allocated an optical carrier signal having a wavelength different from other optical carrier signals in the network. A selected receiving subscriber has a receiver which is tuned to the transmitting subscribers wavelength. Other applications of WDM networks are known.

In order to achieve closely spaced wavelength multiplexing it is necessary to provide optical signal sources which generate optical signals having wavelengths which do not drift far enough in time for adjacent channels to interfere. In addition, the wavelength tolerance of the multiplexing means must be sufficiently small to prevent cross-talk between different channels. Both of these requirements mean that wavelength drift must be minimised.

One attempt at dealing with this problem is described in "Wavelength Sensed Mode Control of Semiconductor Lasers", Electron. Let., 1980,16(19), pp. 744-746.

In accordance with one aspect of the present invention, a method of controlling optical signal sources in a wavelength division multiplexed network as herein before defined comprises (1) imparting a characteristic modulation in turn to each signal from the optical signal sources; (2) sensing the modulated signal from the multiplexed signals; and (3) modifying the wavelength of the modulated signal to obtain substantially maximum power transmission through the multiplexing means.

We have recognised that the main purpose of maintaining tight wavelength control is to maximise power transmission through the multiplexing means. This can most simply be achieved by locking the wavelengths of the optical signals directly to the optical transmission properties of the multiplexing means. Furthermore, this can be achieved with a minimum of steps by poiling each of the optical signal sources in turn.

We believe that the invention will assist a closely spaced wavelength multiplexed system such as a large telecommunications network to be devised with wavelength separations between channels of nanometres or less. Channel spacings of this order would allow hundreds of channels to befitted into the 1.1.6 lim wavelength region.

In general, steps (2) and (3) will comprise a number of subsidiary sensing and modifying steps which are repeated until maximum power transmisson is obtained for each optical source in turn.

In a small network with only a small number of optical signal sources, all the signal sources in the network may be polled in turn. In a larger network, however, it may be more efficient to divide the optical signal sources into a number of groups which are individually polled.

Preferably, the sensing step comprises sampling the multiplexed signals from the multiplexing means and passing the sample to a sensor.

Conveniently, the characteristic modulation imparted to each signal is an amplitude modulation causing a variation in the output power of the optical signal source. Alternatively, wavelength modulation could be carried out provided the modulation is less than the channel spacing.

In some cases, for example where the optical signal sources comprise laser transmitters, particularly short external cavity controlled (SECC) laser transmitters, the wavelength of signals generated is responsive to the temperature of the sources. In these cases, the step of modifying the wavelength preferably comprises altering the temperature of the corresponding optical source.

In other cases the optical signal sources may comprise laser transmitters whose wavelength is controlled by a direct electrical input. In these cases the electrical input would be altered.

In accordance with a second aspect of the present invention, a wavelength division multiplexed network as herein before defined for generating wavelength multiplexed signals includes modulation means for imparting a characteristic modulation in turn to each signal from the optical signal sources; sensing means for sensing the modulated signal from the multiplexed signals, and modifying means for modifying the wavelength of the modulated signal to obtain substantially maximum power transmission through the multiplexing means.

This network may form part of a telecommunications network in which case the optical signal sources are associated with transmitting devices. The network may, however, be incorporated in other wideband networks, for example local area networks.

Preferably, the modulation means comprises an oscillator for generating a modulation signal which is fed to each optical signal source in turn, and the sensing means comprises a phase sensitive detector which receives directly or indirectly the multiplexed signals and the modulating signal from the oscillator, the phase sensitive detector providing an output related to the power transmission of the modulated signal through the multiplexing means.

For example, the sensing means may comprise a photodetector which generates an electrical output signal which is fed to the phase sensitive detector.

Conveniently, the network further comprises switch means such as an electrical switch controlled bythe modifying means to pass the modulation signal from the oscillator to each of the optical signal sources in turn.

Preferably, the wavelength of signals generated by the optical sources is responsive to the temperature of the optical sources. This is a property of SECC laser transmitters which are also particularly advantageous in wavelength multiplexed systems in view of the degree of spectral control which can be achieved with these transmitters. In these cases, the modifying means may include temperature control means, such as Peltier effect thermoelectric cooler devices, associated with each optical source for altering the temperature of respective optical sources.

The multiplexing means may comprise any conventional wavelength division multiplexer such as a grating multiplexer using an optical fibre input array.

Although the various components of the network may be linked by a variety of optical waveguides, preferably monomode optical fibres are used.

An example of a method and network according to the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a block diagram of part of the network; and Figures 2a-2c illustrate mode selection in a SECC laser transmitter.

The network illustrated in Figure 1 comprises a number of SECC Iser transmitters, only two of which 1,1' are illustrated. Each laser transmitter 1 generates a carrier signal having a wavelength different from ail the other transmitters. This carrier signal is modulated by a data signal in a conventional manner. The modulated carrier signals are fed along monomode optical fibres 2 to a conventional wavelength multiplexer 3. A single monomode optical fibre 4 is connected to the output of the wavelength multiplexer 3 along which the wavelength multiplexed signals are fed to for example a cross-coupler or other distribution point (not shown).

A proportion of the multiplexed signals are tapped off from the optical fibre 4 along an auxiliary monomode optical fibre 5.

Each SECC laser transmitter 1,1' has been modified to include a Peltier effect thermoelectric cooler device 6. This allows a two-fold wavelength selection operation to be carried out. Initially, for example during manufacture, the external cavity of the lasertransmitter is adjusted so asto select the laser longitudinal mode closest to the desired operating wavelength. It is important that the laser transmitter should operate in a single longitudinal mode so as to avoid system penalties caused by fibre dispersion. Fine turning is then achieved by altering the temperature of the laser transmitter to pull the longitudinal mode onto the exact wavelength required.Since the laser transmitter operating wavelength would remain fixed, the external cavity tuning methods used previously will not be required, thus simplifying module construction.

It is desirable to operate the laser transmitter 1,1' at a temperature as close as possible to ambient since this both reduced the required Peltier device current, and also minimises the increase in laser threshold current and rate of laser degradation which would result from a high laser transmitter temperature. Condensation inside the transmitter module could also be a problem at low operating temperatures if hermeticaliy sealed transmitter passages were not used. Since the laser mode selected may have to be temperature tuned by up to haif of the mode separation in order to reach the required wavelength, it is desirable that the laser modes spacing should not be too great.For a laser mode spacing of 1 nm, and taking a laser mode temperature coefficient of 0.1 nm/"C, a temperature range of +5 C aboutthe nominal centre temperature would be required.

The number of laser modes which can be selected by the external cavity has an important effect in determining how many laser centre wavelengths will be required to cover the wavelength range of interest. Previous results have indicated that at least three modes are possible. This means that it is possible to select the central mode of the freerunning laser transmitter, or the mode on either side of it. Figures 2a-2C illustrate how mode selection by the external cavity and the tuning range which can be achieved by changing the laser transmitter temperature together determine the wavelength range over which a particular laser transmitter can be operated. The three modes are labelled a, b, and c.It will be seen that with a laser transmitter with a mode spacing of 1 nm, it should be able to operate at a wavelength within +1.5 nm of the laser centre wavelength.

It should be appreciated from the above description, that in orderto maximise power transmission through the multiplexer each laser transmitter 1,1' is selected in turn and its Peltier device current adjusted. It is important, however, that traffic should not be interrupted while laser transmitter tuning is taking place. Forthis reason, each laser transmitter is modulated in turn with a low amplitude, low frequency square wave signal from an oscillator 7. The electrical signal from the oscillator 7 is fed to an electronic switch 8 and from there to the transmitters. The electronic switch 8 is controlled by a microprocessor 9 to pass the square wave signal to each of the transmitters 1, 1' in turn.

The square wave signal modulates the bias current of each transmitter.

The sampled multiplexed signals are passed along the optical fibre 5 to a monitor photodiode 10 whose output is proportional to the intensity of the incoming signals. This output is fed to a phase sensitive detector 11. In addition, a low frequency square wave is also fed to the phase sensitive detector 11. This enables the phase sensitive detector 11 to provide an output related solely to the intensity of the optical signal modulated by the square wave signal. The output from the phase sensitive detector 11 provides a measure of the amplitude of the low frequency square wave and this is fed to the microprocessor 9. Maximum transmission through the wavelength multiplexer 3 will occur when the transmitter wavelength exactly coincides with the centre of the multiplexer response for that wavelength channel.

The microprocessor 9 responds to the output of the phase sensitive detector 11 and adjusts the control currentfed to the corresponding Peltier device 6 in an attempt to increase the signal from the detector 11.

Once the power has been maximised, the microprocessor 9 causes the switch 8 to pass the square wave signal to the next transmitter.

Two features of the system will ease the implementation of this control method. The first is that the transmission vs wavelength response for each channel of the multiplexer should be reasonably smooth, without local peaks and troughs which could fool the power maximising algorithm.

The second is that changes in transmitter wavelengths and multiplexer response should take place only slowly, allowing a single set of control electronics to be time multiplexed between all ofthe transmitters in the way described. If, due to rapid drifts orto large numbers of transmitters, this proved notto bethecase, it would be possibleto add further control systems, and to share out the transmitters between them. Only a single monitor photodiode would be required, and the control systems could operate simultaneously without interference as long as different and unrelated oscillatorfrequencies were chosen for each control system.

Ashared controller9 of this type might also be abide to take over other control functions economically.

The main candidate would be the control of laser bias current and pulse current for each of the laser transmitters. This would normally be done buy a monitor photodiode and circuitry associated with each transmitter package. It would require little extra circuitry forthe wavelength controllerto perform this control function in addition to its wavelength control function, and individually monitor photodiodes and circuitry for each of the transmitter packages would not then be required.

The control system might also be able to collect diagnostic and operating information. For example, it might monitor trends in Peltier device current and laser bias currentfor each transmitter, and raise a warning if these parameters started to change atan unusual rate,sincethiscould be an early warning of impending laserfailure. If the rest of the system were suitably designed it could even initiate the automatic transferoftrafficfrom a suspecttransmitterto a spare wavelength channel.

Claims (11)

1. A method of controlling optical signal sources in awavelength division multiplexed network as hereinbefore defined, the method comprising 1) imparting a characteristic modulation in turn to each signal from the optical signal sources; 2) sensing the modulated signal from the multiplexed signals; and, 3) modifying the wavelength of the modulated signal to obtain substantially maximum power transmission through the multiplexing means.

2. A method according to claim 1, wherein the wavelength of signals generated by the optical signal sources is responsive to the temperature ofthe sources, the step of modifying the wavelength comprising altering the temperature of the corresponding optical source.

3. A method of controlling optical signal sources in a wavelength division multiplexed network substantially as hereinbefore described with reference to the accompanyng drawings.

4. A wavelength division multiplexed network as hereinbefore defined for generating wavelength multiplexed signals, the apparatus including modulation means for imparting a characteristic modulation in turn to each signal from the optical signal sources; sensing means for sensing the modulated signal from the multiplexed signals; and modifying means for modifying the wavelength of the modulated signal to obtain substantially maximum powertransmission through the multiplexing means.

5. A network according to claim 4, wherein the modulation means comprises an oscillatorfor generating a modulation signal which is fed to each optical signal source in turn, and the sensing means comprises a phase sensitive detector which receives directly or indirectly the multiplexed signals and the modulating signal from the oscillator, the phase sensitive detector providing an output related to the powertransmission of the modulated signal through the multiplexing means.

6.Anetwork according to claim 5, further comprising switch means controlled by the modifying meansto passthemodulationsignalfrom the oscillatorto each of the optical signal sources in turn.

7. Anetworkaccording to anyof claims4to 6, wherein the wavelength of signals generated by the optical sources is responsivetothetemperature of the optical sources, the modifying means including temperature control means associated with each optical source for altering the temperature of respective optical sources.

8. A network according to claim 7, wherein the temperature control means comprise Peltier devices.

9. A network according to any of claims 4to 8, wherein the optical signal sources comprise SECC lasertransmitters.

10. Awavelength division multiplexed network substantially as hereinbefore described with reference to the accompanying drawings.

11. Atelephone network according to any of claims 4to 10.