EP2697920A1

EP2697920A1 - Optical receiver device

Info

Publication number: EP2697920A1
Application number: EP12748251.1A
Authority: EP
Inventors: Luca POTI'; Marco Secondini
Original assignee: Consorzio Nazionale Interuniversitario per le Telecomunicazioni (CNIT)
Current assignee: Consorzio Nazionale Interuniversitario per le Telecomunicazioni (CNIT)
Priority date: 2011-04-15
Filing date: 2012-04-16
Publication date: 2014-02-19
Also published as: WO2012140633A1; ITPI20110043A1

Abstract

A wavelength division multiplexing optical receiver circuit (40) com¬ prising a means for receiving a PSK-type optical signal (Γ), in partic¬ ular a DQPSK optical receiver device, a wavelength demultiplexing means (53) for demultiplexing the optical signal, configured to provide a plurality of optical signals (7',7",7"',7""), each optical signal hav¬ ing a specific wavelength, an optical signal conversion means (35,36) for converting each optical signals (7',7",7"',7"") into respective elec¬ tric outlet signals (8',8",8"',8""), a control and decoding means for the electric outlet signals (8',8",8"',8""), wherein a delay means and an optical hybrid coupler (54"), in particular a 90° optical hybrid coupler, are provided upstream of the demultiplexing means (53), for carry¬ ing out a combination of signals (5',5") before the wavelength demul¬ tiplexing step. The feature of the device is that the wavelength de¬ multiplexing means (53) for demultiplexing the optical signal compris¬ es an array of waveguide bows (53'), each of them having a different length with respect to the others and having a common origin. The in¬ vention provides a receiver with a number of components lower than the prior art DQPSK receivers, which allows more widely using the PSK techniques, in particular the DQPSK technique, even if a large number of channels is received, which case can be dealt with only by a pro¬ hibitively complex construction.

Description

TITLE

OPTICAL RECEIVER DEVICE DESCRIPTION

Field of the invention

The present invention relates to a PSK optical receiver device, in particular a DQPSK optical receiver device, for wavelength division multiplexed signals (WDM, Wavelength Division Multiplexing).

Background of the invention

The evolution and the increasing spread of optical communications is requiring modulation techniques having an always higher spectral efficiency, with respect to the simple traditional binary amplitude modulation and intensity detection techniques.

PSK (Phase Shift Keying) is a differential digital phase-shift modulation technique. According to this technique, a carrier signal is modulated by shifting its phase accordingly to the bits that have been transmitted, and the bit are coded by the phase change of the signals that are received thereafter, instead of by the phase true values. In a particular configuration of said DQPSK (Differential quadrature Phase Shift Keying), four symmetrical phase values are used.

In particular, DQPSK modulation systems have been developed, such as the systems described in PJ.Winzer and R.J.Essiambre, Proc. IEEE, vol. 94, no. 5, pp. 952-985, May 2006, in which a specifically developed device allows transferring the electric modulating signal directly in the optical signal, without changing the phase relationship. In particular, receivers are provided that comprise an interferometer means followed by a couple of balanced photodiodes.

Another type of receiver, as described in F. Vacondio et al., J. Lightw. Technol,, vol. 27, no. 22, pp. 5106-5114, Nov. 2009, comprises a step of treating the in-phase and quadrature components of the modulated signal in respective narrow optical filters and in subsequent photodiodes.

In order to extract data from DQPSK optical signals, two steps must be sequentially carried out;

a wavelength demultiplexing, since a WDM signal (Wavelength Division Multiplexed signal) is considered;

— a differential phase reception of each wavelength.

In order to perform the differential phase reception, 90° hybrid couplers may be used in combination with photodetectors. Preliminarily, the signal is treated to obtain two signals, the first one of which is equal to original signal, and a second signal is the original signal delayed by a predetermined time, for instance by one symbol time or also by a shorter time, i.e. it is a delayed copy or repetition of the first signal. Then, the first and the second signal are coupled in four different ways, in order to provide four phase combinations, so that during subsequent processing they form a succession of 90° phase- displaced signals. Afterwards, optoelectronic converters are used to convert the optical signals into electric signals and to demodulate them. Finally, the electric signals are analogously or digitally processed.

A possible architecture of these DQPSK optical receivers is described, for example, in WO2011005596A2 and in WO2011005597A2, which describe the use of hybrid couplers compactly integrated on a same substrate, and the use of balanced photodiodes to convert the demodulated optical signals into electric signals.

Similarly to what is taught by WO2011005596A2 and WO2011005597A2, according to the prior art summarised by the diagram of Fig. 1 , a circuit receiver 10 comprises a substrate 11 , typically a board for optical devices of known type. Receiver 10 receives a DQPSK wavelength division multiplexed type signal 1 (DQPSK WDM signal), More in detail, the optical signal 1 is received by an optical amplifier 12 from which an amplified optical signal 1 ' is obtained, which is received by a delay circuit 13. The delay circuit 13 carries out a wavelength demultiplexing of the optical signal, without yet performing a conversion into an electric signal. The demultiplexing circuit 13 provides n mutually delayed optical signals 2 to an array of hybrid optical circuits, indicated as a whole by 14, each of which processes a signal that has a specific wavelength. The optical signals 3 obtained by respective hybrid optical circuits 14 are then received by respective photodiodes 15, which create respective electric signals 4 that are received by a processor circuit 17, which decodes the signal.

For instance, Fig. 1 shows the case of seven wavelengths. The higher the wavelength number 2n, the higher would be the number of required hybrid optical circuits 14n. Therefore, since at least the same number of hybrid devices 14 is required as the number of the wavelengths of the WDM multiplexed signals, which in the commercial applications may be a somewhat high number, the receiving device would be remarkably complicated, and board 11a would have a relatively high size.

Another problem of the prior art architecture is how to arrange the waveguides on the boards like board 11 of Fig. 1 , which form and mutually connect the hybrid couplers, since the waveguides carrying optical signals 2,3 necessarily cross one another, as shown in WO2011005596A2 and WO2011005597 A2. Such cross passages cause interferences (cross-talk) that worsen the quality of the optical signal s.

Another problem of the prior art receivers is the sensitivity to the wavelength of the received signal(s). In other words, these receivers need a reconfiguration if this frequency even slightly changes, for example if it changes by a few GHz.

Still another problem of the prior art receivers is the sensitivity to construction tolerances, if two or more identical devices are placed beside one another, for instance in the case of WDM demultiplexers placed adjacent to one another: even very small length differences of the optical waveguides may cause relevant noises.

US 7,724,991 relates to an optical receiver apparatus for the coherent reception of a multi-wavelength optical signal comprising an AWG device (Arrayed Waveguide Gratings) that has at least two inputs, wherein at least one portion of the optical signal is applied to one of the inputs, wherein at least one portion of an optical local oscillator signal is applied to another input, and wherein the two inputs of the AWG are spaced apart by a distance substantially equal to one diffraction zone of the AWG.

Also WO 2010/080721 describes an apparatus for the coherent reception of a wavelength-multiplexed signal, comprising a polarization-diversity optical hybrid, at least 4 wavelength demultiplexing filters, 4M detectors and 4M analog-to-digital converters, wherein M is an integer number higher than 1. The hybrid has a first and a second input, respectively for receiving a wavelength-multiplexed signal with M sub-channels at different wavelengths, and a reference light source comprising M continuous-wave continuous references that approximates a center wavelength of the M sub-channels.

The devices described in US 7,724,991 and in WO 2010/080721 are coherent receivers, by which an outer signal is required, for example one produced by a local oscillator, to be combined with the received optical signal. Besides requiring a local oscillator, devices of this type remarkably complicates the signal decoding, which can be made only by complicated decoding devices.

US2010/0098252 discloses and describes an apparatus for a safe communication network that uses AlphaEta encryption, comprising a polarization insensitive optical receiver based on a 90° hybrid coupler, which is used for measuring and digitalizing an encrypted optical signal. Such a device is very difficult to be manufactured, in particular, as an integrated circuit.

Summary of the invention

It is therefore an object of the present invention to provide a PSK receiver device, in particular a DQPSK optical receiver device, for receiving wavelength division multiplexed optical signals, which includes less components than the prior art devices for receiving signals of the same type.

It is also an object of the present invention to provide such a receiver that can demodulate signals including a large number of wavelengths and which is not too difficult to manufacture.

It is also an object of the invention to provide a DQPSK optical receiver that is less sensitive to the wavelength of an incoming signal than the prior art receivers, in particular it is an object of the invention to provide a receiver in which the components can be selected and arranged regardless of the received signal wavelength.

It is a particular object of the invention to provide a structure of receiver that does not require reconfiguration if the frequency of the received signal(s) changes within a predetermined frequency range.

It is also an object of the present invention to provide such a receiver by which PSK techniques, in particular the DQPSK technique, can be more widely used than they are now, even if a large number of channels is received, in which case it is practically impossible or not advantageous to manufacture such devices by the prior art techniques.

It is still an object of the invention to provide such an optical receiver that is as far as possible insensitive to the construction tolerances of the waveguide.

These and other objects are achieved by a PSK-type, in particular a DQPSK-type, interferometric optical receiver circuit, comprising:

— a means for receiving a PSK-type optical signal, in particular a DQPSK- type wavelength division optical signal, (WDM PSK signal, or in particular

WDM DQPSK signal);

— a wavelength demultiplexing means of the optical signal, configured to provide a plurality of optical signals, each optical signal having a specific wavelength;

— an optical signal conversion means for converting each of said optical signals into respective electric signals;

— a control and decoding means of the electric signals;

wherein a delay and hybrid coupling block is provided upstream of the demultiplexing means, said coupling block comprising an optical hybrid coupler, wherein the block delay and matching is configured to produce, starting from the optical signal, a set of signals that are identical to one another apart from a predetermined phase shift between one another, so that the demultiplexing means is adapted to provide a plurality of optical signals starting from the set of signals produced by the optical hybrid coupler, wherein, in particular the optical hybrid coupler is a 90° optical hybrid coupler, and the set of signals comprises four signals that have a 90° phase shift with respect to one another.

The feature of the device is that the wavelength demultiplexing means of the optical signal comprise an array of waveguide bows, each having a specific length different form the others, said waveguide bows having a common origin.

Accordingly, the demultiplexing means of the optical signal can be integrated in a conventional optical circuit support, for example obtained by a technology based on glass or semiconductor or crystal supports.

Moreover, the device is based on a model of an optical receiver for PSK- modulated signals, in particular DQPSK-modulated signals, which is substantially independent from the wavelength, and can be easily adapted to the case of a receiver of a wavelength division multiplexing (WDM) transmission system. In fact, if the signals combination made by the 90° hybrid coupler is fed upstream of the wavelength demultiplexing, a WDM optical receiver has the advantage of requiring only one hybrid coupler for all the wavelength-multiplexed signal. Therefore, four wavelength division multiplexed (WDM) signals are obtained, in particular, at the output side of the hybrid coupler, wherein the components of said signals for each wavelength have already been treated in a delay and hybrid coupling step, in particular a 90^e-delay step, in the case of DQPSK signals, said step required in a DQPSK decoding procedure. Similar solutions can be provided for other PSK coding procedures.

Furthermore, electric signals can be accordingly decoded by simple techniques and means, for example by a conventional computer device or processor means, configured to receive and to process the electric wavelength-demultiplexed signals and configured to reconstruct the information contained therein, i.e. to reconstruct the information carried by the WDM signal that is received by the receiver.

In an interferometric optical receiver, i.e. in a non-coherent receiver, the incoming optical signal is not combined with an external signal, for example with a signal produced by a local oscillator, but is combined with a copy of itself, produced starting from the received signal that is suitably treated in a delay circuit.

In particular, the demultiplexing means comprises couples of AWG devices (Arrayed Waveguide Gratings), normally used when multiplexing and demultiplexing according to WDM technique.

In an exemplary embodiment, a delay circuit is provided upstream of the hybrid coupler. In an advantageous exemplary embodiment, the delay circuit comprises two waveguide branches that have different lengths with respect to each other.

In particular, the waveguide branches have opposite curvatures. This way, cross passages can be avoided between the waveguide branches, which often cause cross-talk noise events. ln particular, two matching waveguides of equal length are provided for each wavelength, said waveguides configured to receive from said hybrid coupler distinct optical signals that are produced by the signals combination that takes place in the hybrid coupler. The matching waveguides can be waveguides of AWG filters.

Preferably, the optical signal conversion means comprises two coupled photodiodes for each wavelength. This way, the photodiodes are automatically balanced, in particular, the photodiodes are made as coupled photodiodes, each couple receiving the optical signal leaving a waveguide of the array of waveguide bows of the demultiplexing means,

In an advantageous exemplary embodiment, the waveguide bows are configured for receiving at respective ends distinct optical signals produced by the combination that takes place in the hybrid coupler, such that each waveguide bow is engaged in both its opposite senses. This simplifies the structure of the circuit receiver, since a limited number of waveguides is required.

Furthermore, for each couple of distinct signals coming from the hybrid coupler, equal length must not be provided, for example waveguides of respective AWG filters, which have narrow tolerance as required by filtering the distinct signals of respective frequencies that engage the waveguide. In fact, since a same multiple waveguide is engaged in the two opposite directions, dimensional differences may not be present.

Said optical receiver device may consist of optical circuits that are unaffected by the polarization.

If the optical receiver is made of optical circuits that depends or are sensitive to polarization, and if the input polarization of the signals is unknown and/or variable, a polarization diversity scheme Is provided comprising a polarization separator upstream of the hybrid optical circuit, said polarization separator configured to provide two orthogonal components of the PSK or DQPSK WDM signal, which can be processed in a device having the structure described above, according to the invention.

In particular, a delay circuit and an optical hybrid coupler are provided for each component of the PSK WDM signal, and downstream of each optical hybrid coupler one respective WDM demultiplexer is provided for each combination of signals supplied by the coupler.

Similarly, it is possible the case of polarization PSK WDM signals and, also in this case, a polarization separator is provided upstream of the hybrid optical circuit, said separator configured to provide two orthogonal components of the optical signal, one for each polarization, wherein the above described architecture is provided also in this case for each of said polarizations, i.e. a delay circuit and an optical hybrid coupler are provided for each component of the DQPSK WDM signal, and one respective WDM demultiplexer is provided for each combination of signals supplied by the coupler, downstream of each optical hybrid coupler.

If the output signals cannot be directly used, but they contain phase or polarization rotations that must be eliminated or corrected, the control and decoding means is advantageously configured for carrying out a linear combination by multiplying each signal by respective suitable coefficients and by subsequently summing the products thereof, such that clean signals are obtained as desired, this step of combination can be performed on signals that are already in the digital form, and/or on analog signals, by means of high frequency electronic circuits of sum and amplification. Even in this case, the steps of linear combination depend upon the signal, and can be obtained by algorithm well known and usual for a skilled person,

In particular, in a possible embodiment, the control and decoding means is configured to receive electric output signals from the optical signal conversion means and to process them to obtain the desired signal, and to forward the signal as an output to a data and clock signals recovery circuit, as well as to possible output to an error decoding and/or correction circuit (FEC). This processing and these circuits, both the analog and the digital part, are well known and usual for a skilled person, and then are not described more in detail.

In particular, the scope of the invention includes an integrated optical receiver that has the above described features.

In another aspect of the invention, the above indicated objects are achieved by an Integrated optical circuit for differential optical receivers, for executing a delay, a combination and a demultiplexing of a WDM optical signal comprising: — a delay circuit configured to produce two signals, a first signal of which is coincident with the original signal, and a second signal is the WDM original signal delayed by a predetermined time;

— a hybrid coupler, in particular a 90° hybrid coupler configured to combine the two signals and to create a further delay, in order to provide four signals obtained by coupling the two signals and by introducing the further delay;

— at least one couple of AWG filters configured to demultiplex four signals obtained by coupling the two signals according to a plurality of signals that have different wavelengths with respect to one another;

— a couple of balanced optoelectronic converters for each wavelength which are located at one end portion of respective AWG.

Brief description of the drawings

The invention will be now shown with the description of an exemplary embodiment thereof, exemplifying but not limitative, with reference to the attached drawings, in which like reference characters designate the same or similar parts, throughout the figures of which:

— Fig. 1 is a block diagram of a DQPSK receiver according to the prior art;

— Fig, 2 is a block diagram of a receiver according to an exemplary embodiment of the invention;

Fig. 3 shows a delay and hybrid coupling block, more in detail;

Fig. 4 is a block diagram of an optical receiver according to a particular exemplary embodiment of the invention;

Fig. 5 diagrammatically shows an optical receiver according to a particular exemplary embodiment of the invention, which is provided with a demultiplexing means each formed by waveguide bows that have different lengths form one another, each length intended for a respective multiplexed wavelength;

Fig. 6 diagrammatically shows an optical receiver according to an exemplary embodiment of the invention, wherein each waveguide is engaged by signals of the same wavelength, in both its opposite senses;

Fig. 7 is a block diagram of an optical receiver according to a further exemplary embodiment of the invention, which is well-suited to case in which the input polarization of the signals is unknown or variable, and in the receiver optical circuits are used dependent or sensitive to the polarization, i.e. if the polarization of the incoming signal is unknown or variable.

Description of a preferred exemplary embodiment

With reference to Fig. 2, a PSK receiver 20 is described, in particular a

DPQSK receiver, according to the invention, for a wavelength division multiplexed optical signal 1. Receiver 20 may advantageously comprise a conventional optical signal amplifier 22, associated with a means for receiving an optical signal, which also is conventional and is not shown, to obtain an amplified wavelength division multiplexed optical signal 1 '.

Downstream of amplifier 22 is provided a delay and hybrid coupling block 24, a possible exemplary embodiment of which is described herein, with reference to Fig. 3. Delay and hybrid coupling block 24 is configured to receive amplified optical signal 1' and to responsively provide wavelength-multiplexed signals, indicated as a whole by 6, which are shifted by a predetermined angle with respect to each other, in particular, in the case of DPQSK signals, four DQPSK WDM signals 6 shifted by 90" with respect to one another, to which the phases at 0°, 90°, 180°, 270^e are conventionally assigned, they also shown more in detail in Fig. 3. The receiver further comprises an optical signals wavelength demultiplexing means 23, which is configured to receive WDM DQPSK signals 6 and to responsively provide, for each multipiated wavelength, a plurality of DQPSK wavelength-demultiplexed optical signals 7.

For each signal 7, i.e. for each wavelength, a optoelectronic converter device 25 is provided that is configured to receive a respective optical signal 7 of this wavelength, and is configured to responsively provide an electric signal 8 associated with this wavelength,

Moreover, a conventional control and decoding means 27 is provided, preferably in the form of a processor, which is configured to reconstruct the information conveyed by each demultiplexed electric signal 8 and then to completely reconstruct the information conveyed by DQPSK WDM signal 1 as received by receiver 20. The components 22,23,24,25, along with an optical connection means between these components, such as waveguides, is arranged on a conventional optical circuit board 21. The delay depends only upon the symbol rate, which must obviously be the same for all the transmitted channels.

With reference to Fig. 3, a delay and hybrid coupling block 24 is shown that comprises a delay circuit 24' configured to receive an optical signal, for example wavelength-multiplexed optical signal 1' of Fig, 2, and to responsively provide a first wavelength-multiplexed optical signal 5' that is identical to incoming optical signal 1', and a second wavelength-multiplexed optical signal 5" that is identical to first optical signal 5' apart from being delayed by a predetermined delay, i.e. it is a delayed repetition or copy of incoming optical signal 1 '.

Preferably, the delay time is shorter than or equal to a symbol time, in particular it is equal to a symbol time.

The delay circuit can be a conventional one; a delay circuit according to an advantageous exemplary embodiment is described herein, with reference to Figs. 5 and 6.

Still with reference to Fig. 3, delay and coupling block 24 also comprises a hybrid coupler 24", in particular a 90° hybrid coupler, which is a wide band device and is configured for contemporaneously analysing the inlet signals. Hybrid coupler 24" is configured to receive mutually delayed first optical signal 5' and second optical signal 5", and to combine these signals, in order to obtain a plurality of WDM signals, i.e. signals that are still wavelength- multiplexed signals, and are shifted by a predetermined angle with respect to each other. In particular, in the case of DPQSK coding, hybrid coupler 24" is adapted to combine the first and the second signal 5' and 5" in four different ways, thus creating four WDM signals 6 that during further processing can be treated to obtain signals shifted by 90° with respect to one another, which correspond to the phases at 0°, 90', 180°, 270°.

A wavelength demultiplexing is carried out at the outlet of hybrid coupler 24". To this purpose, a waveguide is provided for each signal 6, which is configured to transfer each signal 6 to a wavelength demultiplexing or demux device 23, i.e. globally to wavelength demultiplexing means 23 such as the demultiplexing means of receiver 20 of Fig. 2, which provide wavelength- demultiplexed signals (7',7"J'",7""). As described more in detail hereinafter, for step of wavelength demultiplexing identical filters can be used for all four signals 6 produced by hybrid coupler 24", including for instance waveguide arrays, as in the case of the AWG devices or filters (Figs. 5 and 6).

Fig. 4 shows a block diagram of a DQPSK optical receiver 40 according to an exemplary embodiment of the invention, which has a structure similar to the one of the device to which the block diagram of Fig. 2 relates. In particular, optical receiver 40 comprises a delay circuit 44' that is configured to produce, starting from incoming optical signal 1', a first DQPSK WDM optical signal 5' that is identical to incoming signal 1 , and a second DQPSK WDM optical signal 5" that is a copy of signal 5 delayed by one symbol time or bit time, a 90° hybrid coupler 24" is arranged downstream of delay circuit 44', for example of the above mentioned type or in any case of a type described hereinafter, that is configured to receive WDM DQPSK signals 5,5' and to produce, starting from these, four WDM DQPSK signals 6. Signals 6\6",6",6^m, which form four signals 6, are equal to signals 5,5' apart from a 90° phase shift with respect to one another, respectively. Optical receiver 40 also comprises a filter AWG 43 for each DQPSK WDM signal 6, which is configured, along with respective balanced photodiodes 35',35" and 36', 36" of respective optoelectronic converters 45, to produce wavelength-demultiplexed electric signals 8. A control means 27 is also provided, in the form of a conventional computer device or processor, which is configured to receive and analyse wavelength- demultiplexed electric signals 8 and to reconstruct the information conveyed by electric signals 8.

Figs. 5 and 6 diagrammatically show two optical receivers 50 and 60 according to respective exemplary embodiments of the invention. Each receiver 50 and 60 has a means 48 for receiving an optical signal 1 ', which is in communication, through a waveguide portion, with a first common end 49 of two waveguide branches 51 and 52, which form a delay circuit of the receiver such as the circuit 44' of Fig. 4. Waveguides 51 and 52 have different lengths with respect to each other, as measured between the first common end 49 and the respective second ends 55' and 55", in particular waveguide 51 is longer than waveguide 52. This way, signal 5" that reaches the second end 55" of branch 52 is in delay with respect to signal 51' that reaches second end 55' of branch 51, in a time depending upon the length difference of the branches of wavelength 51 and 52. Such length difference can be chosen in such a way that the delay is lower than or the same as a symbol time, in particular it is the same as a symbol time.

Another feature of devices 50 and 60 is that they comprise a hybrid coupler 54" consisting of four waveguide branches 56,57,58,59, wherein branches 56,57,58 have the same length, while branch 59 is longer or shorter than this same length. The length difference between branch 59 and branches 56,57,58 can create a further 90° delay. Therefore, signal 6' collected downstream of branch 56 is equal to signal 5', which is in turn equal to incoming signal 1 ', and signals 6",6"' respectively collected downstream of waveguide branches 57,58 are equal to signal 5", which is delayed, for instance, by a symbol time with respect to incoming signal 1 ', while the signal 6"" collected downstream of branch 59 differs from signal 5' by a 90° delay.

A further feature of devices 50 and 60 is that they comprise respective pluralities of AWG-type multiplexing devices 53,63, one for each signal 6',6",6"\6"" at the outlet of hybrid coupler 54". In particular, receiver 50 of Fig. 5 comprises four AWG devices 53, each including, in turn, a plurality of optical guide bows 53'. Optical guide bows 53' of each device AWG 53 are in optical communication with optical guide branches 56,57,58,59, respectively, of hybrid mixer 54". More in detail, waveguide bows 53' of a same demultiplexing device 53 have a common origin on one of four waveguide branches 56,57,58,59 of hybrid coupler 54". Furthermore, optical guide bows 53' of each AWG multiplexing device 53 have different length with respect to one another.

The length of each arch 53' is chosen such that the optical signals reach with different delays an outlet portion of respective device 53, i.e. in a further common origin of waveguide bows 53' of a same demultiplexing device 53, where such signals are recombined and then split again, in a known way, into respective outlet optical guides of device 53 as a plurality of wavelength- demultiplexed optical signals 7\7^,,,7^,"₎7"", which include the previously multiplexed wavelength in a multiplexing device, not shown, which produce signal 1' incoming into receiver 50.

As shown in the details of Figs. 5' and 5", each optical guide bow 53' has an end portion that is in optical communication with a respective photodiode 35', 35" or 36', 36". More in detail, each guide bow that is in optical communication with branch 56, which carries signal 6', is in optical communication with a respective photodiode 35' and each guide bow that is in optical communication with branch 57, which carries signal 6", delayed by a symbol time with respect to signal 5', is in optical communication with a respective photodiode 35". Similarly, each guide bow that is in optical communication with branch 58, which carries signal 6"', is in optical communication with a respective photodiode 36" and each guide bow that is in optical communication with branch 59, which carries signal 6"", 90° delayed with respect to signal 5", is in optical communication with a respective photodiode 36'. In other words, an optoelectronic conversion system 35,36, shown by balanced photodiodes 35', 35" and 36', 36" is provided that is repeated for each output couple of AWG filters 53.

Photodiodes 35', 35", as well as photodiodes 36', 36", are vertically coupled, in order to provide couples of balanced photodiodes 35', 35" and 36',36". This way, each couple of photodiodes can combine a respective wavelength-demultiplexed signal, that has a wavelength corresponding to a couple of optical guides 53' coming from two distinct branches 56,57 or 58,59 of hybrid coupler 54".

A step of sum and difference of signals 6' and 6" is carried out in each couple of balanced photodiodes 35',35", while a step of sum and difference of signals 6"' and 6"". is carried out in each couple of photodiodes 36' and 36".

Each couple of balanced photodiodes 35', 35" and 36', 36" has an electric outlet, not shown for the sake of clearness, at which a couple of electric signals is available, such as electric signals 8\8",8"',8"" of Figs. 2 and 4, which are the result of the above steps of sum and of difference, such that two electric signals 8', 8" (Fig. 3) are obtained, for each frequency, from the respective couple of balanced photodiodes 35', 35" and two electric signals 8"', 8"" (Fig. 3) are obtained from the respective couple of balanced photodiodes 36\36".

Signals 8',8",8"\8"" form, for each multiplexing frequency, a set 8 of four signals that are shifted by 90° with respect to one another, to which the phases at 0°, 90°, 180°, 270° are conventionally assigned.

As shown, receiver 50, according to the invention, comprises a single hybrid coupler 54" upstream of the wavelength demultiplexing means 53, such that incoming optical signal 1 ' is delayed at first, and then prepared for subsequent wavelength demultiplexing, thus obtaining four signals 6',6",6"' and 6"" that are transferred to the AWG devices consisting of waveguide bows 53', respectively, and then are transferred to the couples of balanced photodiodes 35\35" and 36\36", respectively.

As shown in Fig. 5, this can be made without forming waveguides cross passages between hybrid coupler 54" and demultiplexing means 53, which simplifies the construction of receiver 50, as well as the interference events which would occur in the case of receivers made according to prior art architectures, in which these cross passages cannot be avoided.

Therefore, the receiver according to the invention can be advantageously, used for contemporaneously receiving a plurality of wavelength-multiplexed channels, whatever the channel number may be, even in case of a very high channel number.

With reference to Fig. 6, receiver 60 comprises two AWG devices 63 each in turn including a plurality of optical guide bows 63". More in detail, waveguide bows 53' of a same demultiplexing device 53 have a common origin on one of four waveguide branches 56,57,58,59 of hybrid coupler 54". Furthermore, optical guide bows 63' of each device AWG 63 are in optical communication, at their own opposite end portions, with optical guide branches 56 and 57, or with optical guide branches 58,59 of a hybrid mixer 54", Optical guide bows 63' of each device AWG 63 have a different length with respect to one another.

The length of each arch 63' is chosen such that the optical signals reach with different delays an outlet portion of respective device 63, i.e. in a further common origin of the waveguide bows 63' of a same demultiplexing device 63, where such signals are recombmed and then split again, in a known way, into respective outlet optical guides of device 63 as a plurality of wavelength- demultiplexed optical signals 7',7",7'",7"", which include the previously multiplexed wavelength in a multiplexing device, not shown, which produce signal V incoming into receiver 60.

As shown in the details of Figs. 6' and 6", each optical guide bow 63' has a first end portion that is in optical communication with a respective photodiode 35' or 36" and a second end portion that is in optical communication with a respective photodiode 35" or 36", respectively. More in detail, each guide bow that is in optical communication, at its own opposite ends, with branches 56 and 57 is also in optical communication, at such respective opposite ends, with respective photodiodes 35", 35', while each guide bow that is in optical communication, at its own opposite ends, with branches 58 and 59 is also in optical communication, in such respective opposite ends, with respective photodiodes 36", 36'. This way, each optical guide bow 63' that is in communication with branches 56,57 of optical coupler 54", is engaged by signal 6' in a first direction and by signal 6" in the opposite direction, of which signals only respective components are allowed that have a multiplexed wavelength, while each optical guide bow 63' that is in communication with branches 57,59 of optical coupler 54", is engaged by signal 6"' in a first direction and by signal 6"" in the opposite direction, of which signals only respective components are allowed that have a multiplexed wavelength,

Photodiodes 35' and 35", as well as photodiodes 36' and 36" are vertically coupled, in order to provide couples of balanced photodiodes 35\35" and 36',36". This way, as in receiver 50 of Fig. 5, each couple of photodiodes can combine a respective wavelength-demultiplexed signal, that has a wavelength corresponding to a couple of optical guides 53' coming from two distinct branches 56,57 or 58,59 of hybrid coupler 54", A step of sum and difference of signals 6' and 6" is carried out at each couple of balanced photodiodes 35',35", while a step of sum and difference of signals 6"' and 6"" is carried out at each photodiodes couple 36' and 36". Each couple of balanced photodiodes 35', 35" and 36', 36" has an electric outlet, not shown for the sake of clearness, at a couple of electric signals is available like electric signals 8',8",8"\8"" of Figs. 2 and 4, which are the result of the above steps of sum and difference, such that two electric signals 8', 8" (Fig, 3) are obtained, for each multiplexing frequency, from the respective couple of balanced photodiodes 35', 35", and two electric signals 8"',8"" are obtained from the respective couple of balanced photodiodes 36',36". Signals 8',8",8"',8"" form, for each multiplexing frequency, a set 8 of four signals that are shifted by 90" with respect to one another, the phases at 0°, 90^c, 180°, 270° are conventionally assigned,

A further advantage of receiver 60, in addition to those already highlighted for receiver 50, is that AWG filters 63 do not have distinct optical guides, such as the optical guides 53' of AWG filters 53 of receiver 50, for filtering components of signals 6',6" as well as of signals 6"' and 6"", which have the same wavelength. In fact, as already described, signals 6", 6" as well as signals 6"' and 6"", which have the same wavelength when leaving respective AWG filters, engage in opposite directions the same waveguide 63'. Therefore, waveguides of exactly the same wavelength, with very narrow tolerances, must not necessarily be provided in the case of receiver 60 to obtain couples of signals 6',6" as well as 6"' and 6"" of the same frequency, which is the case of receiver 50, on the contrary. In AWG devices 53 of receiver 50, these tolerances must be at most the same order of magnitude as the wavelength of the signal being carried, otherwise relevant noise events may take place.

With reference to the block diagram of Fig. 7, an optical receiver 70 is described according to a further exemplary embodiment of the invention. Optical receiver 70 is made according to a polarization diversity scheme, in which a polarization separator or polarization discriminator 71 is provided, upstream of hybrid optical circuit 24", that is configured to provide two orthogonal components 1" and 1"' of incoming WDM signal PSK V. Optical receiver 70 comprises respective optical circuits 72,73, according to the invention, for treating each orthogonal component 1" and V.

Each optical circuit 72,73 comprises in turn a delay circuit 44' configured to make signals 5' and 5" respectively identical to respective component 1" and 1"', and delayed with respect to the same by a predetermined delay time, for example by a symbol time. Each optical circuit 72,73 also comprises a respective hybrid coupler downstream of the delay circuit 44", in this case still a 90°-hybrid coupler 24", which provides, by combining respective signals 5' and 5" with one another, respective pluralities of signals 6\6"₁6"\6"", like the signals indicated by the same reference number in Fig, 5 and 6, in this case sets of four signals that are shifted by 90° with respect to one another, and correspond to the phases at 0°, 90°, 180°, 270°.

For each combination of signals that is provided by a respective hybrid coupler 24", a respective plurality of multiplexing devices is provided downstream of said coupler which, in the present exemplary embodiment, comprises AWG filters 43 for performing the wavelength demultiplexing of signals 6',6",6"·,6"", thus obtaining wavelength-demultiplexed optical signals 7. Downstream of the AWG filters 43 photoelectronic converter devices 45 are provided that comprise couples of balanced photodiodes 35', 35" and 36', 36" to obtain wavelength-demultiplexed electric signals 8 from optical signals 7. Downstream respective conventional processors 27 are provided as a control and decoding means of these electric signals, for reconstructing the information conveyed by each plurality of electric signals 8 produced by the optoelectronic circuits 72 and 73 starting from the two orthogonal components 1 ", 1 '" of incoming optical signal 1 '.

Incoming optical signal V may be a polarization multiplexed signal. In alternative, incoming optical signal V may be a signal of unknown or variable polarization, while optical circuits 72,73 comprise components depending upon the polarization or sensitive to polarization.

Optical circuits 72,73 for treating each orthogonal component 1" and 1"^* can be optical circuits of the above described type. Even if in Fig. 7 these respective circuits are substantially equal to circuits 40 as shown in Fig. 4, they may be, for instance, circuits of the type shown in Figs. 2, 5 and 6, according to the possible exemplary embodiments that are mentioned in the description.

In a particular exemplary embodiment of the present invention, an optical receiver circuit 20,40,50,60,70 (Figs. 2-7) is provided in which control and decoding means 27 is adapted to carry out a linear combination by multiplying each electric signal 8, more in detail, each of outlet signals 8' ,8", 8"', 8"" (Fig. 4) by respective suitable coefficients and to carry out a subsequent sum of the products obtained this way, such that clean signals are obtained.

In a particular exemplary embodiment of the invention, an optical receiver circuit 20,40,50,60,70 (Figs. 2-7) is provided in which, the control and decoding means 27 is configured to receive electric output signals 8, more in detail, electric signals 8', 8", 8"', 8"" (Fig. 4), from optical signal conversion means 25,45,35,36 and to treat them to obtain the desired signal, and is configured to provide the signal as an output to a data and clock signals recovery circuit, not shown, as well as to possible FEC decoding and correction circuits, that are well known and therefore are not described more in detail.

The foregoing description exemplary specific embodiments will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt for various applications such embodiment without further research and without parting from the invention, and, then it is therefore to be understood that such adaptations and modifications will have to be considered as equivalent to the specific embodiments. The means and the materials to realise the different functions described herein could have a different nature without, for this reason, departing from the Field of the invention. It is to be understood that the phraseology or terminology that is employed herein is for the purpose of description and not of limitation.

Claims

1. An interferometric optical receiver circuit (20,40,50,60,70) comprising:

— a means for receiving a PSK-type wavelength division optical signal

(1 ,1 ');

— a wavelength demultiplexing means (23,43,53,63) of said optical signal, configured to provide a plurality of optical signals (7), each optical signal having a specific wavelength;

— an optical signal conversion means (25,45,35,36) for converting each of said optical signals (7) into respective electric outlet signals (8);

— a control and decoding means (27) of said electric outlet signals (8); wherein a delay and hybrid coupling block (24) is provided upstream of said demultiplexing means (23,43,53,63), said coupling block comprising an optical hybrid coupler (24",54"),

characterised in that said delay and hybrid coupling block (24) is configured to produce, starting from said optical signal (1 ,1'), a set (6) of signals (6^,,6",6",6"') that are identical to one another apart from a predetermined phase shift between one another, so that said demultiplexing means (23,43,53,63) is adapted to provide a plurality (7) of optical signals starting from said set (6) of signals (6\6",6"_Ι6"') produced by said optical hybrid coupler (24", 54"),

and that said wavelength demultiplexing means (23,43,53,63) comprises a plurality of waveguide bows (53', 63'), that have lengths different form one another and have a common origin.

2. An optical receiver circuit (20,40,50,60,70) according to claim 1 , wherein said means for receiving a PSK-type optical signal is configured to receive a DQPSK-type optical signal.

3. An optical receiver circuit (20,40,50,60,70) according to claim 1 , wherein said optical hybrid coupler (24", 54") is a 90° optical hybrid coupler, and said set (6) of signals comprises four signals (6',6",6",6"') that have a 90° phase shift with respect to one another.

4. An optical receiver circuit (50,60) according to claim 1 , wherein a delay circuit (24') is provided upstream of said hybrid coupler (54"), wherein said delay circuit (24') comprises two waveguide branches (51 ,52) having different lengths from each other.

5. An optical receiver circuit (50,60) according to claim 4, wherein said waveguide branches (51 ,52) have curvatures opposite to each other, so that no cross passages are formed between said waveguide branches (51 ,52).

6. An optical receiver circuit (50,60) according to claim 1 , wherein said demultiplexing means comprises AWG devices (53,63).

7. An optical receiver circuit (50) according to claim 1 , wherein two equal-length waveguides (53') are provided for each wavelength, said waveguides configured for receiving from said hybrid coupler (54") distinct optical signals (6',6",6"',6"") that are produced by said combination.

8. An optical receiver circuit (40,50,60) according to claim 1 , wherein said optical signal conversion means (45) comprises two coupled photodiodes (35735",36736") for each wavelength.

9. An optical receiver circuit (60) according to claim 1 , wherein said waveguide bows (63') are configured for receiving, at respective ends, distinct optical signals (6',6",6"',6"") that are produced by said combination in said hybrid coupler (54"), such that each waveguide bow (63') is engaged by a couple (676",6"'/6"") of said distinct optical signals in both its opposite senses.

10. An optical receiver circuit (70) according to claim 1 , comprising a polarization separator (71) upstream of said hybrid optical circuit (24"), said polarization separator configured to produce two orthogonal components (Γ,Γ') of said optical signal (1 '), wherein a delay circuit (44') and an optical hybrid coupler (24") are provided for each of said orthogonal components (Γ,Γ'), and wherein a respective WDM demultiplexer (43) is provided for each combination of signals supplied by said hybrid coupler (24") downstream of each optical hybrid coupler (24").

11. An optical receiver circuit (20,40,50,60,70) according to claim 1 , wherein said control and decoding means (27) is adapted to perform a linear combination by means by multiplying each of said electric outlet signals (8) by respective suitable coefficients and by subsequently summing the products thereof such that clean signals are obtained.

12. An optical receiver circuit (20,40,50,60,70) according to claim 1 , wherein said control and decoding means (27) is configured to receive said electric outlet signals (8) from said optical signal conversion means (25,45,35,36) and to process said electric outlet signals (8) to obtain a target signal, and is configured to provide said signal as an output to a data and clock signals recovery circuit.

13. An optical receiver circuit (20,40,50,60,70) according to claim 12, wherein said control and decoding means (27) is configured to provide said signal as an output to an error decoding and/or correction circuit, in particular to a FEC circuit.

14. An integrated optical receiver circuit according to any of claims 1 to 13,

15. A integrated optical circuit for differential optical receivers, for carrying out a delay, a combination and a demultiplexing of a WDM optical signal comprising:

— a delay circuit configured to produce two signals, a first signal of which is coincident with said WDM optical signal, and a second signal is the original signal delayed by a predetermined time;

— a hybrid coupler;

— at least one couple of AWG filters configured to demultiplex four signals obtained by coupling said two signals according to a plurality of signals that have different wavelengths with respect to one another;

— a couple of balanced optoelectronic converters for each wavelength, which are located at one end portion of respective AWG filters.

16. A integrated optical circuit according to claim 15 wherein said hybrid coupler is a 90" hybrid coupler configured to combine said two signals and to create a further delay, in order to provide four signals obtained by coupling said two signals and by introducing said further delay.