GB2376531A

GB2376531A - Multichannel wavelength monitoring apparatus

Info

Publication number: GB2376531A
Application number: GB0114379A
Authority: GB
Inventors: Mehdi Asghari
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2001-06-13
Filing date: 2001-06-13
Publication date: 2002-12-18
Also published as: WO2002101964A1; GB0114379D0

Abstract

Multichannel wavelength monitoring apparatus 31 comprises inputs 33 for receiving multichannel optical signals, means 34 to apply identifying signals f<SB>1</SB> to f<SB>4</SB> to the multichannel optical signals, a demultiplexer 32 and a photodetector array 36 for detecting the output of the demultiplexer 32. A digital signal processor 35 may determine, on the basis of the identifying signals detected by the array 36, the components of the multichannel signals. The means 34 for applying identifying signals may be a variable optical attenuator, and the demultiplexer may be an optical channel modulator or array waveguide. The photodetector array may consist of photodiodes. In another embodiment, an input signal may be passed through a splitter into two optically conducting branches. Each branch may be selectively attenuated, the demultiplexer acting on the signal from the first branch when the second is attenuated and vice vera (see figure 1). The apparatus is used to determine parameters of each channel of the optical input, such as wavelength, relative power or amplitude, or optical signal-to-noise ratio.

Description

"Multichannel Wavelength Monitoring Apparatus" The present invention relates to multichannel wavelength monitoring apparatus, such as optical channel modulators (OCM) or optical spectrum analysers (OSA), also known as optical performance monitors (OPM).

Recently there has been much research into the development of small-sized spectrometers for use in substance analysis outside the laboratory. US 6002479 discloses an integrated spectrometric sensor/transducer (IISS/T) in which the spectrum of the light to be measured is dispersed by a simple dispersion element, such as a grating, and is then detected by photodetectors producing electrical signals representative of the spectrum. An analogue-to-digital converter serves to convert the detected analogue signals into digital signals, and the resolution of the detected spectral data is then enhanced by digital signal processing (DSP). Such DSP is capable of providing much better resolution than would otherwise be obtained by use of the low cost sensor and transducer components used.

Furthermore spectral analysers using waveguide-based optical technology have been developed for wavelength division multiplexing (WDM) in optical communication systems. In WDM different data channels within a light signal transmitted along a single optical fibre or waveguide are differentiated according to the wavelength band of the transmitted light corresponding to that channel. Signal processing in such WDM systems involves multiple combination and/or separation of the individual multiplex channels using multiplexer/demultiplexer devices. Similar technology can also be used to produce small-sized spectrometers for use in substance analysis outside the laboratory.

WO 99/57834 (University of Maryland) discloses a real time wavelength monitoring circuit for monitoring signals in a WDM optical communication system using a phased array waveguide grating (PAWG), such a circuit being used to monitor the wavelength distribution of signals at different nodes within the communication system. In order to increase the wavelength range of the circuit the PAWG has multiple

inputs, and the input signal to the circuit is switched so as to be supplied successively to a first centre wavelength offset input and a second centre wavelength offset input, the corresponding outputs from the PAWG being detected by detectors which in turn

supply signals uy way 01 ampmiers anu an /i converter to a signai processor,-me signal processor analyses the output signals produced in each measurement phase, and accurately monitors the wavelength of the signals using discrimination curves based on the ratios between pairs of output signals resulting from input signals supplied to different inputs of the PAWG. However such apparatus requires the use of input switches for switching between successive measurement phases, and this introduces both complexity and possible losses.

Each of the Applicants'co-pending British Patent Applications Nos. 0109656.9 (P097), 0107112.5 (P092) and 0106784.2 (P126) also discloses an AWG having multiple inputs to which signals are applied to produce respective demultiplexed signals in successive measurement phases.

It is known to modulate optical transmission signals with low-frequency tone signals. US 5485299 discloses a supervisory facility for an optical communications system which serves to detect a low-level intensity modulated tone which has previously been added to the transmitted signal and which can be recovered to provide direct measurement of received light level using a WDM/SKEW coupler in a copumping or by-pumping configuration.

Furthermore US 5654816 discloses monitoring apparatus in which the power of each channel is determined after each stage of optical amplification by monitoring the ratio between the power of the added tone and the total output power. Changes between the first and subsequent tone power/output power ratios reflect changes in network performance. The use of such superimposed low-frequency tone signals is also disclosed in US 5745270 and US 5969833.

It is an object of the invention to provide multichannel wavelength monitoring apparatus which is capable of monitoring a large number of channels at reasonable cost and with good performance.

According to the present invention there is provided a multichannel wavelength monitoring apparatus comprising first and second input means for receiving respectively a first multichannel optical signal and a second multichannel optical signal, signal identifying means for applying an identifying signal to at least one of the first and second signals, demultiplexing means for demultiplexing the first and second signals, photodetection means for detecting optical output signals from the demultiplexing means produced by demultiplexing of the first and second signals and for producing an electrical output signal, and digital processing means for differentiating, on the basis of the identifying signal detected by the photodetection means, first components of the electrical output signal derived from the first signal from second components of the electrical output signal derived from the second signal.

Such apparatus enables either the same measurement resolution or signal discrimination to be obtained using fewer photodetectors, or an increased measurement resolution to be obtained using the same number of photodetectors. As a further alternative the same measurement resolution may be obtained utilising less hardware complexity and/or relaxed component tolerances resulting in cost savings. It is contemplated that, in some embodiments of the invention, a substantially larger number of measurements than signals will be used so that increased measurement resolution is obtained by utilising a group of photodetectors to detect each part of each signal. In other embodiments of the invention a plurality of multichannel input signals from different sources may be simultaneously monitored by means of the apparatus.

The provision of digital processing enables the required differentiation between measurement points to be made at less cost than would be incurred if such differentiation were to be achieved by a purely optical monitoring system. In particular the application of the identifying signal, which may be a low-frequency tone or a digitally encoded signal, to at least one of the first and second signals and the

subsequent detection of the identifying signal enable those components of the output signals derived from the different input signals to be differentiated.

The first and second signals may be transmission signals received from different locations and input into the apparatus separately. Alternatively the first and second signals may be derived from a single multi-channel input signal and may each incorporate some or all of the spectral components of the input signal. In this case further input means may be provided for receiving the input signal, and splitter means may be provided for splitting the input signal so as to supply the first signal to the first input means and the second signal to the second input means.

The demultiplexing means is preferably a dispersive device, such as an arrayed waveguide (AWG), which provides an output in the form of optical signals which are spaced apart relative to a mean wavelength position with the spacing being dependent on the wavelength of the signal relative to the mean wavelength. Thus the output signals corresponding to the demultiplexed channels may be detected by an array of suitably spaced photodiodes.

Various embodiments are contemplated within the scope of the invention. In one of these embodiments the signals are simultaneously demultiplexed by the demultiplexing means, and the processing means incorporates transform means, utilising Fourier transformation for example, which serves to differentiate the components of the electrical output signal from the photodetection means. In this case Fourier transformation of the output signals may be used to yield the power or amplitude of each channel with the differentiation between the channels being made on the basis of the or each applied identifying signal or tone. Thus, for the case in which separate multiplexed signals from different locations, for example from Australia and Canada, are supplied to the apparatus, the electrical signals derived from Australia can be distinguished from the electrical signals derived from Canada during processing, and the applied identifying signals or tones allow the different multiplexed spectra of the two signals to be analysed simultaneously. In this case the identifying signal may be applied within the apparatus, that is on the chip, with the specific object of providing

such signal differentiation, or alternatively may be in the form of an identifying signal or tone applied to the signal at a remote location.

In another embodiment of the invention the apparatus includes attenuating means for selectively attenuating the first and second signals such that the demultiplexing means serves, in a first demultiplexing phase, to demultiplex the first signal whilst the second signal is attenuated by the attenuating means, and, in a second demultiplexing phase, to demultiplex the second signal whilst the first signal is attenuated by the attenuating means. In this case the attenuating means may serve either to completely block the second signal or to significantly reduce the magnitude of the second signal.

Such apparatus enables a large number of channels, for example 80 channels, to be monitored with an array of photodetectors having only a limited number of photodetectors, for example 40 photodetectors. This avoids the need to use large and costly arrays of photodetectors to monitor a large number of input channels, and enables existing, well-tried arrays to be used for this purpose. For instance, it is possible to use a 40-channel optical channel modulator (OCM) with 100 GHz channel spacing, which is a commercially available product, in place of a 80-channel OCM which would be more costly to produce because of the lower yield obtained in fabrication of such arrays.

The requirement to increase the number of input channels in the system also brings with it the requirement to decrease the channel spacing, and the invention enables the channel spacing to be decreased without either undesirably increasing the size of the apparatus or undesirably degrading the performance.

Preferably absorber means are provided for absorbing stray light outside the waveguides in order to reduce crosstalk between the channels.

Generally the wavelengths of the channels of the first component signal alternate with the wavelengths of the channels of the second component signal, and the channels of the first component signal and the channels of the second component signal are equal in number, although other arrangements are possible within the scope of the invention.

Preferably the attenuating means comprise variable optical attenuators (VOA) associated with each of the first and second input means for attenuating the different channels within the first and second component signals. Most preferably each of the variable optical attenuators is an electronic variable optical attenuator (EVOA).

However the attenuating means may alternately comprise digital optical switches (DOS) or Mach-Zehnder interferometers (MZI).

The invention also provides multichannel wavelength monitoring apparatus comprising first and second input means for receiving respectively a first multichannel optical signal and a second multichannel optical signal, signal identifying means for applying a tone signal to at least one of the first and second signals, demultiplexing means having multiple inputs respectively connected to the first and second input means, the inputs being spaced apart by a predetermined distance related to the wavelength spacing of the channels of the first and second signals, and photodetection means for detecting optical signals from the demultiplexing means produced by demultiplexing of the first and second signals and for selectively supplying first output signals corresponding to the separated channels of the first signal and second output signals corresponding to the separated channels of the second signal.

In this embodiment an identifying signal or tone is preferably applied to at least one of the first and second signals to enable the corresponding output signals to be distinguished and separately analysed, for example by Fourier transformation. Typically the spacing between the inputs is a half or one and a half times the channel spacing where two inputs are provided, or a quarter or one and a quarter times the channel spacing where four inputs are provided for example. In this embodiment there are no switches provided to selectively block light transmitted along the first and second input means or only imperfect switches are provided, such as Mach-Zehnder switches operating to minus 20dB only.

The invention further provides multichannel wavelength monitoring apparatus comprising input means for receiving an input signal incorporating a plurality of

channels of different wavelengths, first and second optical conductor means, splitter means for splitting the input signal so as to supply a first signal to the first optical conductor means and a second signal to the second optical conductor means, attenuating means for selectively attenuating the first and second signals, channel demultiplexing means for demultiplexing the first signal whilst the second signal is attenuated by the attenuating means, and for demultiplexing the second signal whilst the first signal is attenuated by the attenuating means, and photodetection means for detecting first optical output signals produced by demultiplexing of the first signal and second optical output signals produced by demultiplexing of the second signal and for producing an electrical output signal.

The apparatus can be integrated in a waveguide-based optical chip, such as a silicon-on-insulator optical chip for example.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a schematic diagram of an exemplary embodiment of multichannel wavelength monitoring apparatus in accordance with the invention; and Figure 2 is a graph showing the spacing of the channels in operation of such apparatus; Figure 3 is a schematic diagram of a further embodiment of the invention; Figure 4 shows graphs of typical outputs of photodiodes 1 and 2 in the embodiment of Figure 3; Figure 5 shows graphs of the signal amplitude against frequency for the photodiodes 1 and 2;

Figure 6 is a schematic diagram of another embodiment of the invention; Figure 7 is a schematic diagram of a still further embodiment of the invention; Figure 8 shows a graph of a typical output of one of the photodiodes of the embodiment of Figure 7; Figure 9 shows a graph of the amplitude of the output signal of such a photodiode against frequency ; and Figure 10 is a block diagram of a control circuit used in one implementation of the invention.

The specific embodiment of the invention shown in Figure 1 is an optical channel monitor (OCM) having 80 channels and a 50GHz channel spacing. However it should be understood that the invention is also applicable to OCMs having different numbers of channels and different channel spacings, as well as to other types of multichannel wavelength monitoring apparatus for use either in WDM optical communication systems or in spectrometers for use in substance analysis.

The embodiment of Figure 1 is in the form of an integrated optical chip 10 incorporating a 40-channel OCM 11 of known design having a channel spacing of 100 GHz and a respective photodiode for providing an electrical output signal corresponding to the light detected in each channel. Such a design of 40-channel OCM can be fabricated using silicon-based technology at reasonable cost, and has been shown to provide good performance in operation. In addition the chip 10 incorporates an input waveguide 12 for the input signal incorporating 80 multiplexed channels of different mean wavelengths, a 50: 50 splitter in the form of a Y-coupler 14 for splitting the input signal into first and second multichannel signals, and first and second branch waveguides 16 and 18 extending from the two output arms of the Y-coupler 14 for conducting the first and second signals. Furthermore each of the first and second

branch waveguides 16 and 18 is provided with a respective attenuator in the form of a bank of variable optical attenuator (VOA) elements 20 or 22.

Each bank of VOA elements 20 or 22 includes a plurality of VOA elements for successively attenuating the signal conducted by the associated branch waveguide 16 or 18 so as to obtain a total attenuation of the signal of about 10dB. Additionally a bank 24 of toning VOA elements is associated with the first branch waveguide 16 only (and not with the second branch waveguide 18) for modulating the first signal with an identifying tone frequency (utilising about 3dB attenuation, for example) for enabling alternate channels to be more readily distinguished during subsequent processing as described more fully below. The first and second branch waveguides 16 and 18 are connected to two inputs 26 and 28 of the OCM 11, and the 40 outputs 30 of the OCM 11 provide 80 electrical output signals corresponding to the 80 channels for further processing. The identifying signal is preferably a low frequency tone signal, typically in the form of a sine wave, although other types of identifying signal, such as a digital identification code in an n-bit repeating pattern, may also be used.

In operation of such an embodiment to produce 80 electrical output signals from an 80-channel multiplexed input signal, the input signal conducted by the input waveguide 12 is split by the Y-coupler 14 into the first and second signals of 80 channels and the VOA elements 20 are used to block the first signal in a first attenuating phase, whilst the VOA elements 22 are used to block the second signal in a second attenuating phase. Provided that the spacing between the inputs 26 and 28 is set to 50 GHz (that is half the channel spacing) the 40 odd-numbered channels supplied to the input 26 are demultiplexed and detected by the photodiodes in the first attenuating phase, whilst the 40 even-numbered channels fall in the gaps between the photodiodes, and the 40 even-numbered channels supplied to the input 28 are demultiplexed and detected by the photodiodes in the second attenuating phase, whilst the 40 oddnumbered channels fall in the gaps between the photodiodes.

Figure 2 diagrammatically shows the wavelengths XI, A3, 5, 7, 9, 11 ouf the odd-numbered channels of the first signal (solid lines) supplied to the input 26

alternating with the wavelengths 2, 4, 6, 8, IO, 12 of the even-numbered channels of the second signal (broken lines) supplied to the input 28 in the direction of increasing wavelength in the demultiplexed output (only some of the channels being shown to render the figure easter to read). More particularly the upper part of the figure shows the response spectra of the photodiodes to the demultiplexed output signal obtained from light supplied to the input 26, and the lower part of the figure shows the shifted response spectra of the photodiodes to the demultiplexed output signal obtained from light supplied to the input 28. It will be appreciated that the Y-coupler may be replaced by other forms of splitter, such as an evanescent coupler, a multimode interference (MMI) coupler, or a star coupler.

As a result, in the first attenuating phase, the 40 electrical output signals produced at the outputs 30 of the OCM 11 correspond to the 40 odd-numbered channels, and, in the second attenuating phase, the 40 electrical output signals produced at the outputs 30 of the OCM 11 correspond to the 40 even-numbered channels. The bank of VOA elements 20,22 are operated alternately to alternately provide signals at the outputs 30 corresponding to the even and odd-numbered channels.

By positioning the inputs 26 and 28 to the OCM 11 at half (or one and a half times) the channel spacing it is possible to ensure that, in each operating phase, the 40 demultiplexed channels are positioned correctly to be detected by the photodiodes. This ensures that, in each phase, 40 of the 80 channels will be measured by the photodiodes while the other 40 channels do not reach the photodiodes.

The electrical output signals from the OCM 11 are subsequently processed using a computer algorithm in order to provide data indicative of parameters, such as the power/amplitude and wavelength, of the different channels of the input signal. The electrical output signals are preferably processed in a digital signal processor (DSP), a digital-to-analogue converter being used to convert the analogue output signals from the photodiodes to digital signals which can be processed by such a processor. For example, such processing may be effected by one of the processing methods described in US 5991023 or US 6002479.

In this particular implementation the digital output signal from each photodiode is subjected to Fourier analysis to obtain the frequency peaks corresponding to the tone frequencies, and the analysis of the amplitudes of the tone frequencies obtained from each photodiode is used to discriminate between signals corresponding to the even and odd-numbered channels. Clearly such discrimination is assisted by the alternate blocking of the signals supplied to the two inputs 26 and 28 of the OCM 11. However, in practice, some light is still received by each input when the corresponding VOA elements are operated to block the signal supplied to that input, and thus the analysis of the tone frequencies in the output signals is preferably used to increase the sensitivity of measurement. However it should be understood that, in some applications, it may not be necessary to modulate either of the input signals with an identifying tone frequency.

Instead the necessary discrimination may be obtained by the alternate blocking or chopping of the signal supplied to the two inputs (coupled with the selectivity provided by the spacing of the inputs which ensures that each photodiode receives alternately even and odd-numbered channels) and preferably utilising synchronous detection in which the output from the detector array is chopped in synchronism with the chopping of the VOA elements.

Absorbers could be positioned between the channels to absorb crosstalk, although, in practice, the channels are physically narrow and there is little room for such absorbers. Accordingly, in a development of the invention, the outputs of the photodiodes are processed by means of a computer algorithm in order to substantially eliminate the crosstalk. This can be done because the level of crosstalk is predictable.

In a further, non-illustrated embodiment the toning VOA elements 24 are dispensed with, and only the blocking VOA elements 20,22 are used. However such an embodiment may not be suitable for some applications since the blocking VOA elements may not block the signals completely, and also the blocking VOA elements may not be acting in their linear regions so that distortion of the output signals may be produced. In the case where the blocking VOA elements do not block the signals completely, the toning VOA elements enable the output signals to be distinguished by

virtue of the toning frequencies. On the other hand, where the output signals are to be distinguished solely by means of blocking, it is necessary to ensure that sufficiently high power is applied to the blocking VOA elements to provide the required high degree of blocking. The provision of the additional toning VOA elements isolates the selection of the channels from the toning, thus enabling crosstalk effects to be removed from the output signals using DSP and providing better performance.

In a further, non-illustrated embodiment, the Y-coupler and the attenuating VOA elements are replaced by a reversely directed interleaver which serves to selectively separate the odd-numbered channels from the even-numbered channels of the input signal before directing them to the respective inputs 26,28 of the OCM 11, preferably using the application of a tone frequency to alternate channels. Such an arrangement enhances crosstalk performance, but adds additional complications to the design and fabrication of the circuit.

In a still further, non-illustrated embodiment the VOA elements are replaced by a Mach-Zehnder interferometer (MZI) switch. However, in this case, it would be necessary to substantially increase the spacing between the inputs 26,28 of the OCM 11.

A further embodiment of the invention shown in Figure 3 enables four different multichannel signals from different sources (but having similar wavelength ranges) to be monitored simultaneously. This embodiment comprises an integrated optical chip 31 incorporating an OCM 32 having four inputs 33 with which four toning VOA banks 34 are associated, and a DSP 35 for processing electrical signals received from a photodiode array 36 of the OCM and having outputs 37.

In operation of such an embodiment the four multichannel input signals have respective identifying tone signals of different frequencies fl, f2, f3 and f4applied thereto by toning VOA banks 34, and are then supplied to the four inputs 33 of the OCM 32 which are spaced at a quarter (or one and a quarter times) the channel spacing in order to ensure that the output signals are correctly positioned to be detected by the

photodiodes. Within the OCM 32 the four signals are demultiplexed and the resulting optical spectra are detected by the photodiode array 36, with the resulting electrical output signals from the array 36 being analysed within the DSP 35 to enable data to be passed to the outputs 37 indicative of parameters of the channels of each input signal, such as the relative power/amplitude of each channel, the wavelength of each channel and the optical signal-to-noise ratio.

The electrical output signal from each diode of the array 36 typically varies with time in a manner as shown in Figures 4 (a) and 4 (b) for the first and second photodiodes of the array, and these signals are sampled over a sampling window extending from t = 0 to t = twindow. The sampled Fourier components of the signals are then analysed within the DSP 35 using a spectral analysis technique substantially as described in US 5991023 and US 6002479 using the identifying frequencies fl, f2, f3 and f4 (and the known amplitude of the tone frequency modulation relative to the amplitude of the input signals) to distinguish between the spectral components resulting from the four signals.

In this way a multichannel input signal received from, say, Canada can be distinguished within the chip from another multichannel signal received from, say, Australia.

With reference to Figures 4 (a) and 4 (b) the sampling window must be over a sufficient period of time to incorporate many oscillations of each identifying frequency.

Generally the longer the sampling window, the greater the discrimination between the different identifying frequencies. Typically the sampling window may be of the order of 10 seconds in which case, if the frequencies are of the order of 10 kHz, the number of oscillations of each frequency within the window will be of the order of 100, 000.

Figure 5 (a) shows the Fast Fourier Transform (FFT) function abs (fft (pl)) as a function of frequency, where pi denotes the output signal from the first photodiode of the array, and it will be seen that this incorporates peaks corresponding to the frequencies fi, f2, f3 and f4 (the further peaks-f4,-f3,-f2 and-fl being incidental artefacts of the Fourier transform analysis which can be ignored). The meaningful part of the function extends between a minimum frequency f = 1/twmdow and a maximum frequency f = twmdowl2 x sample rate. Figure 5 (b) shows a similar function abs (fft (p2)) for the output signal p2

from the second photodiode of the array, and in this case peaks of different magnitudes are obtained at the identifying frequencies f1, f2, f3 and f4

The output signal obtained from each photodiode due to the four input signal :) is then subjected to a sub-procedure as described in US 6002479 in which the amplitudes of the frequency peaks are measured, and the relative values of these amplitude measurements for successive photodiodes are used to determine features of each multichannel input signal, such as the presence or absence of a signal for each channel or the relative powers of the different channel signals. It is believed that the different processing procedures which can be used to obtain such data are readily apparent from the description of US 6002479, and these will therefore not be further described in this specification.

It should be understood that, in some implementations, the number of photodiodes in the array will considerably exceed the number of channels, in which case output signals from a number of photodiodes may be used for monitoring each channel, and as a result greater detection resolution can be obtained. For example an array of 128 photodiodes and associated amplifiers may be used to monitor 40 channels. In this case the output signals from each group of photodiodes, for example 3 photodiodes, assigned to monitor each channel may be used to monitor the power of the channel, for example, by estimating the position of the peak frequencies within the group of output signals using a"centre of gravity"method. The peak frequency is then converted to a wavelength, and a calibration responsivity spectral table used to look up four responsivity values. The four output signals may then be converted into four optical power values which can be summed to obtain the estimated channel power. Such channel power estimates are generally higher than the actual channel power because crosstalk from other channels will be picked up by the photodiodes.

Generally the embodiment described with reference to Figure 3 can be adapted to have n inputs to the OCM for receiving n input signals, each input signal having m channels, and n different tone frequencies being applied to the input signals, and the resulting n spectra may be analysed simultaneously using the tone frequencies to

distinguish between the n different signals in order to provide the required data relating to the m channels of each signal.

The embodiment of Figure 6 is in the form of an integrated optical chip 41 incorporating an OCM 42 having four inputs 43 to which respective tone frequencies f1, f2, f3 and f4 are applied by VOA banks 44. However, in this case, the four input signals are identical and are derived from a single received signal which is supplied to a splitter 45 which splits the received signal into four signals of the same magnitude. Unlike the embodiment of Figure 1 no switches are provided to block the signal (or alternate channels of the signal) supplied to any of the inputs 43 or, if any such switches are provided, they are switches which operate only very imperfectly, such as Mach-Zehnder switches operating to minus 20dB only.

However, as in the embodiment of Figure 1, a predetermined spacing between the inputs 43 is required relating to the channel spacing in order to ensure that the corresponding optical outputs after demultiplexing within the OCM 42 are correctly positioned in relation to the photodiode array 46 of the OCM. Where there are four inputs 43, the input spacing should be a quarter of the channel spacing, or alternatively one and a quarter times the channel spacing. Alternatively, if there are only two inputs, the spacing between the inputs would be a half or one and a half times the channel spacing.

As in the other embodiments a DSP 47 is provided to analyse the electrical output signals from the photodiode array 46 and to supply data indicative of the relative parameters of the channels of the received signal. In this case the electrical output signal from each photodiode of the array 46 (with the exception of the endmost diodes of the array) incorporates components received from all four inputs 43, and analysis of these output signals using Fourier transformation and detection of the tone frequencies fl, f2, f3 and f4 enables the contributions from the different inputs 43 to be distinguished and separately analysed to obtain the required data in relation to the channels of the multichannel signal, such as relative channel powers, in a manner similar to that described in US 6002479. It will be appreciated that the fact that there are four input

signals supplied to the spaced inputs 43 of the OCM 42 means that, for a required resolution, fewer photodiodes can be used to monitor a particular number of channels than would otherwise be required, or alternatively greater resolution is obtained using

the same number of photodiodes. Furthermore the defined spacing of the four inputs 43 ensures that the relative positioning of the output spectra obtained for the four input signals to the OCM is known and can be made use of in the subsequent processing of the output signals from the photodiodes to obtain the data indicative of the relative parameters of the channels.

The embodiment of Figure 7 is in the form of an integrated optical chip 51 comprising an OCM 52 having four inputs 53 to which different identifying tone frequencies fi, f2, f3 and f4 are applied by four VOA banks 54. However, in this case, four different input signals are supplied to the inputs by an interleaving AWG 55 having four spaced inputs 56 to which further different tone frequencies fa, fb, fc and fd are applied by further VOA banks 57, and four spaced outputs for supplying the different input signals. The spaced inputs and outputs of the AWG 55 serve to separate the channels of the signals received at the inputs 56 into four separate interleaved wavelength groups (each of which has an identifying tone frequency fl, f2, f3 and f4 applied to it by respective one of the VOA banks 54. Thus each of the signals applied to the inputs 53 of the OCM 52 will incorporate channels from each of the four signals applied to the inputs 56 of the AWG 55, these channels being distinguished by the identifying tone frequencies fa, fb, fc and fd.

As in the preceding embodiment the spacing between the inputs 53 of the OCM 52 bears a predetermined relationship to the channel spacing, being, for example, one quarter of the channel spacing. Furthermore the demultiplexed optical signals are simultaneously detected by the photodiode array 58 of the OCM 52, and the electrical output signals from the array 58 are analysed in the DSP 59 to distinguish between the channels of the four signals received at the inputs 56 by means of the two sets of

identifying tone frequencies fa, fb, fc and fd and fi, f2, f3 and f4, in order to supply data to the outputs 60 indicative of the amplitude/power or wavelength of each channel and the signal-to-noise ratio for each signal.

Figure 8 shows a graph of a typical electrical output signal from one of the photodiodes p7 of the array 58 in a sampling window extending from t = 0 to t = twmdow, and Figure 9 is a graph of the amplitude against frequency of such a corresponding function abs (fft (p7) ) showing frequency peaks corresponding to two different input signals received from Australia and Canada, by way of example. In this case the effect of the modulation provided by the VOA banks 55 is to place sidebands on the peak frequencies obtained from the modulation applied by the VOA banks 57. Thus, in the case of an input signal received from Australia, for example, a series of frequency peaks fAustraha + fi, australia + 2, fAustraha + f3 and fAustralia + 4 are obtained as shown in Figure 9.

Although this embodiment is described as being provided to receive multichannel signals from different sources, it should be appreciated that a similar embodiment could be used for analysing a single multichannel signal applied simultaneously to all four of the inputs 56 of the AWG 55. This would enable either additional saving in the number of photodiodes required to monitor a particular number of channels or alternatively a still further increase in the resolution or signal discrimination obtained using the same number of photodiodes.

In each of the above described embodiments, the OCM is described as having two or four inputs. However it should be appreciated that the OCM may have any required number of inputs within the scope of the invention in order to suit the particular application.

In a practical implementation of any of the above described embodiments a closed loop feedback control circuit may be used to improve the measurement accuracy, and Figure 10 is a block diagram of a control circuit for controlling one input of a multiinput system for determining the relative power levels of the channels in a multichannel wavelength monitoring apparatus. One optical path through the system is shown in order to simplify the diagram, although it will be understood that in practice other optical paths and their associated control circuits will be provided.

Referring to Figure 10, an optical signal is applied to an input 101 (which may be the 1 waveguide of an AWG and passes through an electrically controlled variable optical attenuator (VOA) 102 supplied with electrical power having an AC component at a particular frequency i, and a DC component. The frequency f, corresponds to the identifying tone frequency referred to above. The DC component sets the average attenuation of the optical signal, whereas the AC component is used for subsequent control and signal processing as already discussed. The optical signal continues through an AWG 103 which may cause some undesirable attenuation. At or after the AWG 103, an optical waveguide tap 104 diverts a small proportion of the light out of the main waveguide to one or more monitor photodiodes 105. The tap may be an integral part of the AWG. The resulting detection currents outputted by the photodiodes 105 are converted to voltage by amplifiers 107, and demodulators 108 separate the DC average photocurrent from the AC photocurrent. The AC signal from one of the demodulators 108 incorporating the applied tone frequency is supplied to the feedback control circuitry.

The optical signal leaves the system through the output waveguide 106. The purpose of the feedback control circuitry is to regulate the power of this optical signal k, to a desired set point average power as it leaves the system by way of the output waveguide 106. The DC signals, DCA and DCB, from each of the demodulators 108 may be used as follows. DCA may be used for a total power measurement, of use to amplifier controllers beyond this system, and both DCA and DCB may be supplied to a temperature stabilisation circuit which balances their powers to centre the tuning of the AWG 103. The AC signal ACB from the second demodulator 108 should be identical to AC signal ACA from the first demodulator 108 and this will not be referred to further.

The AC signal ACA is passed through a further amplifier 110 and then digitised by an analogue-to-digital converter 111 to supply a digital byte output signal proportional to the analogue signal outputted by the amplifier 110 at regular intervals, typically 100k Samples/s, to a central control processor 112. The central control processor 112 receives a waveform of samples and operates on this waveform to measure the amplitude of the frequency component at the identifying tone frequency f,.

This can be extracted by Fast Fourier Transform (FFT) or other methods from the waveform, even in the presence of noise and other tones present in the common output of an AWG. The processor 112 compares the measured amplitude of the tone frequency f, to the required amplitude to obtain the correct set power of At. This necessarily uses a calibration table 113 of the derivatives dA/dI of the VOA 102 which usually depend on the DC setting of the VOA 102. The DC level of the VOA 102 is adjusted by an adjustment circuit 115 and the AC tone at frequency f, produced by a modulating circuit 114 is added in an adder 116.

In this manner, closed loop feedback between one or more monitor photodiodes after an AWG and one or more VOA elements before it are achieved. The AWG may have other inputs, each identified by sinusoidal tones of a different frequency, and extracted from the AC component of the monitor photodiode signal by digital signal processing.

Claims

CLAIMS: 1. Multichannel wavelength monitoring apparatus comprising first and second

input means ior receiving respectively a first multichannel optical signal and a second multichannel optical signal, signal identifying means for applying an identifying signal to at least one of the first and second signals, demultiplexing means for demultiplexing the first and second signals, photodetection means for detecting optical output signals from the demultiplexing means produced by demultiplexing of the first and second signals and for producing an electrical output signal, and digital processing means for differentiating, on the basis of the identifying signal detected by the photodetection means, first components of the electrical output signal derived from the first signal from second components of the electrical output signal derived from the second signal.
2. Apparatus according to claim 1, comprising further input means for receiving a multichannel optical input signal, and splitter means for splitting the input signal so as to supply the first signal to the first input means and the second signal to the second input means.
3. Apparatus according to claim 2, wherein the splitter means is a coupler, which splits the input signal into two substantially identical signals.
4. Apparatus according to claim 2, wherein the splitter means comprises an interleaving device which separates the input signal into first and second component signals in dependence on wavelength such that the first signal incorporates some of the channels and the second signal incorporates the remainder of the channels.
5. Apparatus according to any preceding claim, wherein the demultiplexing means is a dispersive device, such as an array waveguide (AWG).
6. Apparatus according to any preceding claim, wherein the signal identifying means comprises attenuating means for attenuating at least one of the first and second signals so as to apply said identifying signal to said at least one signal.

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7. Apparatus according to any one of claims 1 to 6, further comprising attenuating means for selectively attenuating the first and second signals such that the demultiplexing means serves, in a first demultiplexing phase, to demultiplex the first signal whilst the second signal is attenuated by the attenuating means, and, in a second demultiplexing phase, to demultiplex the second signal whilst the first signal is attenuated by the attenuating means.
8. Apparatus according to claim 6 or 7, wherein the attenuating means comprises a variable optical attenuator (VOA) associated with each of the first and second input means.
9. Apparatus according to claim 6 or 7, wherein the attenuating means comprises a digital optical switch (DOS) associated with each of the first and second input means.
10. Apparatus according to claim 6 or 7, wherein the attenuating means comprises a Mach-Zehnder interferometer (MZI) associated with each of the first and second input means.
11. Apparatus according to any one of claims 1 to 5, wherein the processing means incorporates transform means which serves to differentiate the first and second components of the electrical output signal from the photodetection means during detection of the first and second signals simultaneously demultiplexed by the demultiplexing means.
12. Apparatus according to any preceding claim, wherein the demultiplexing means has multiple inputs to which the first and second signals are separately supplied, the inputs being spaced apart by a predetermined distance related to the wavelength spacing of the channels of the first and second signals.
13. Apparatus according to any preceding claim, further comprising interleaving means for receiving a plurality of multichannel optical input signals and for separating

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each input signal into first and second component signals in dependence on wavelength so that the first signal incorporates channels in one wavelength range derived from the input signals and the second signal incorporates channels in another wavelength range derived from the input signals.
14. Apparatus according to claim 13, wherein further signal identifying means is provided for applying a further identifying signal to at least one of the input signals to the interleaving means, and the processing means is adapted to monitor signal channel parameters on the basis of both the first-mentioned identifying signal and the further identifying signal.
15. Apparatus according to any preceding claim, wherein absorber means are provided for absorbing wavelengths between the wavelengths of the channels in order to reduce crosstalk between the channels.
16. Apparatus according to any preceding claim, wherein the processing means is adapted to monitor the amplitude or power of each channel.
17. Apparatus according to any preceding claim, wherein the processing means is adapted to monitor the wavelength of each channel.
18. Apparatus according to any preceding claim, wherein the processing means is adapted to monitor the signal-to-noise ratio.
19. Apparatus according to any preceding claim, wherein the processing means utilises a digital signal processor (DSP).
20. Apparatus according to any preceding claim, which is formed by silicon-oninsulator (SOI) optical chip technology.
21. Apparatus according to any preceding claim, which is incorporated in a wavelength division multiplexing (WDM) system.

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22. Multichannel wavelength monitoring apparatus substantially as hereinbefore described with reference to the accompanying drawings.
23. Multichannel wavelength monitoring apparatus comprising first and second input means for receiving respectively a first multichannel optical signal and a second multichannel optical signal, signal identifying means for applying a tone signal to at least one of the first and second signals, demultiplexing means having multiple inputs respectively connected to the first and second input means, the inputs being spaced apart by a predetermined distance related to the wavelength spacing of the channels of the first and second signals, and photodetection means for detecting optical signals from the demultiplexing means produced by demultiplexing of the first and second signals and for selectively supplying first output signals corresponding to the separated channels of the first signal and second output signals corresponding to the separated channels of the second signal.
24. Multichannel wavelength monitoring apparatus comprising input means for receiving an input signal incorporating a plurality of channels of different wavelengths, first and second optical conductor means, splitter means for splitting the input signal so as to supply a first signal to the first optical conductor means and a second signal to the second optical conductor means, attenuating means for selectively attenuating the first and second signals, channel demultiplexing means for demultiplexing the first signal whilst the second signal is attenuated by the attenuating means, and for demultiplexing the second signal whilst the first signal is attenuated by the attenuating means, and photodetection means for detecting first optical output signals produced by demultiplexing of the first signal and second optical output signals produced by demultiplexing of the second signal and for producing an electrical output signal.