Patent Description:
Optical communications, such as passive optical networks (PONs), are increasingly used to give network access (typically Internet access) to residential or office gateways, or data centers, in the scope of FTTH ("Fiber To The Home") technology deployment. Optical communications may also be used to ensure mobile infrastructure backhauling for instance in the scope of the deployment of <NUM> (<NUM>rd generation) or <NUM> (<NUM>th generation) mobile technologies using typically Point-to-Point arrangements.

With the emerging <NUM> (<NUM>th generation) mobile technology, fronthauling is about to appear and the needs in terms of data rate capabilities are significantly increased. One may refer to the International Mobile Telecommunications (IMT) recommendations <NPL>. In such a framework, fronthauling is achieved by moving upstream in the mobile infrastructure processing functions that were previously performed at base stations or nearby in the <NUM> or <NUM> mobile technologies. It is referred to as split options in the specifications <NPL> (see more particularly therein Section <NUM> and Table A-<NUM>).

Therefore, <NUM> mobile technology has a wider set of requirements as compared to FTTH requirements that have driven optical access technology evolution until now. <NUM> mobile technology translates thus into higher nominal data rates, reduced latency and denser deployment, among others. This translates to higher nominal data rates for optical access but also expected split ratio greater than <NUM>: <NUM> whereas FTTH split ratio is typically <NUM>:<NUM> with current PON systems.

To cope with this trend, coherent optical transmission is envisaged. Coherent optical transmission is a technique that uses modulation of amplitude and phase of light to enable transporting high volume of data over an optical fiber. Digital Signal Processing (DSP) is then typically used in order to achieve robust performance of coherent transmission over the optical fiber.

Coherent optical transmission is well-known since it has revolutionized the metro and core network segments: beyond its ability to capture the entire diversity of the channel and thus its ability to gain high spectral efficiency, one can cites good optical receiver sensitivity and good wavelength selectivity as well.

Fundamentals of coherent optical transmissions as used in metro and core network segments can be found in "<NPL>. This document depicts a coherent optical receiver employing phase and polarization diversities. Two phase-diversity homodyne receivers based on <NUM>° optical hybrid power splitters in order to retrieve phase and quadrature components of injected coherent optical signal are placed in a polarization-diversity configuration, with usage of a common Local Oscillator (see more particularly therein Fig. <NUM>). DSP arrangement has to be complementarily used in order to retrieve modulated symbols by digitally processing phase and polarization diversities output so as to restore complex signal amplitude in a stable manner despite fluctuations of carrier phase and signal state of polarization. To do so, the DSP arrangement comprises an anti-aliasing filter, a <NUM>-channel Analogue-to-Digital Converter (ADC), a Frequency Domain Equalizer (FDE) enabling symbol rate extraction for driving back the ADC, an adaptive Finite Impulse Response (FIR) equalizer enabling clock phase recovery and a carrier phase estimator (see more particularly therein Fig. <NUM>).

One main issue with coherent optical transmissions is that transmission over long-range optical fibers implies slow but large state-of-polarization changes large enough to be sensible at the coherent optical receiver. To ensure transmission continuity, two alternative approaches are used in order to avoid signal fading due to mismatch between the state of polarization of the signal received via the optical fiber and the state of polarization of the local oscillator. One approach is to compensate for change of state of polarization induced by the long-range optical fiber using complex Digital Signal Processing with full diversity polarization receivers. The other approach is to track the state of polarization using Optical Phase Lock Loop (OPLL), for instance in the case where full polarization diversity cannot be recovered. Both approaches lead to complex analog and/or digital arrangements with at least two analog branches. One may for example refer to "<NPL>. As disclosed therein, tracking state of polarization thus typically supposes endless polarization transformer coupled with an adaptive control loop, which is a rather complex and costly arrangement.

It appears from the foregoing explanation that intrinsic theoretical and hardware implementation applied to metro and core network segments are not compatible with low design complexity, low power consumption, low manufacturing cost and complexity, low deployment cost and complexity, and small size, which are key requirements in the network access segment. Indeed, the quantity of equipment to install in the network access segment is far higher than in the metro and core network segments, especially to meet geographical density expectations of base stations in <NUM> mobile technology, and furthermore, the places to install such equipment are often particularly limited in terms of space (home device, base stations installation sites. Furthermore, the optical transmissions in metro and core network segments suffer from noise presence due to signal amplification used to cover point-to-point long distances. On the contrary, the optical transmissions in network access segments operate on shorter distances but suffer from low signal strength (quantity of photons received by the optical receiver) due to split ratio considerations, and thus optical receiver sensitivity is a main concern. Therefore, the coherent optical transmissions technology as used in the metro or core network segments is not suitable for <NUM> mobile technology.

In addition to the scope of the <NUM> mobile technology, coherent optical transmissions might be suitable for FTTH evolution. Indeed, the increasing demand for high-speed Internet gives rise to increasing the split ratio of current PON systems in addition to higher data rates. However, as for the deployment of base stations in the <NUM> mobile technology, Optical Network Units (ONUs) as used in such PON systems require low design complexity, low power consumption, low manufacturing cost and complexity, low deployment cost and complexity, and small size, since they are deployed in users' premises or nearby. Furthermore, as for the <NUM> mobile technology context, the optical transmissions in metro and core network segments suffer from noise presence due to signal amplification used to cover point-to-point long distances. On the contrary, the optical transmissions in network access segments operate on shorter distances but suffer from low signal strength (quantity of photons received by the optical receiver) due to split ratio considerations, and thus optical receiver sensitivity is a main concern. Therefore, the coherent optical transmissions technology as used in the metro or core network segments is not suitable for FTTH evolution, either.

It is therefore desirable to provide a cost-effective solution of coherent optical receiver suitable for access network considerations. It is desirable to provide a solution that is as simple as possible.

It can be noted that it is known from the background art the document <CIT>, which discloses a heterodyne detector in which radiation of a local oscillator is modified to influence the state of polarization so that the ratio of the intensities of two orthogonally polarized components is equal to the corresponding ratio in an incoming signal beam. The radiation of the local oscillator and of the incoming signal beam is combined in a coupler, subsequently split into two orthogonally polarized components by a polarization splitter and then detected by two respective opto-electric converters.

It can be noted that it is known from the background art the document <CIT>, which discloses an optical heterodyne detection device using Faraday rotators or birefringent electro-optical crystals to influence the state of polarization of radiation originating from a local oscillator to correspond to a signal beam transmitted through a long-distance transmission fiber.

To that end, it is proposed a coherent optical receiver intended to receive an amplitude-shift keying modulated optical signal from a coherent optical transmitter, comprising: a local oscillator; a polarization-diversity actuator configured for modifying an optical signal output by the local oscillator so as to form an optical signal with elliptical polarization with main axis orientation φl and ellipticity phase shift Ψl; a <NUM> x <NUM> coupler, having one input aiming at receiving the amplitude-shift keying modulated optical signal received from the coherent optical transmitter and the other input receiving another optical signal which is output by a set formed by a local oscillator and the polarization diversity actuator, so as to enable the local oscillator to provide a boosting effect to the amplitude-shift keying modulated optical signal received from the coherent optical transmitter, the <NUM> x <NUM> coupler further having one output connected to a set formed by a photodiode followed by a Direct Current filter removing a continuous component of an analog electrical signal output by the photodiode; a transimpedance amplifier converting current electrical signal output by, the Direct Current filter into voltage electrical signal; a controlling unit in form of electronic circuitry configured for performing a domain-switching procedure instructing a configuration change of the polarization diversity actuator by modifying the ellipticity main axis orientation φl and/or the ellipticity phase shift Ψl when the voltage electrical signal output by the transimpedance amplifier is below or equal to a predetermined first threshold THdsp corresponding to a predetermined percentage of theoretical maximum achievable by the boosting effect. Furthermore, the electronic circuitry is further configured for performing a phase-refining procedure inserting a controlled error signal in the phase of the optical signal output by the set formed by the local oscillator and the polarization diversity actuator, and adjusting a static component of the controlled error signal toward a maximum voltage electrical signal output by the transimpedance amplifier. Thus, by using such a single-branch coherent optical receiver, no complex Digital Signal Processing and no Optical Phase Lock Loop is used and therefore cost-effective solution is provided for access network considerations. Moreover, the phase-refining procedure enables optimizing coarse configuration provided by the domain-switching procedure.

In a particular embodiment, during domain-switching procedure, the modification of the ellipticity main axis orientation φl is performed by applying a predefined shift equal to <MAT> and the modification of the ellipticity phase shift ψl is performed by applying a predefined shift equal to <MAT>. Thus, recovering effective boosting effect (thanks to boosting terms provided by the local oscillator) is easily achieved.

In a particular embodiment, the polarization-diversity actuator comprises a variable waveplate for enabling modification of the ellipticity main axis orientation φl and for enabling modification of the ellipticity phase shift ψl. Thus, configuration modification is easily implemented, at low complexity and cost.

In a particular embodiment, the electronic circuitry is configured to implement an automatic configuration of a standby time period between successive executions of the domain-switching procedure, comprising: a monitoring of evolution of the electrical signal output by the transimpedance amplifier is first continuously performed in the scope of the domain-switching procedure; history of polarization state related variations is tracked by the monitoring and a time period between instructed successive changes in ellipticity parameter is monitored; once evolution of polarization state variations becomes stable, the stand-by time period between successive executions of the domain-switching procedure is defined so as to be lower than a stabilized time period between successive crossings of the predetermined first threshold THdsp. Thus, energy of the coherent optical receiver can be preserved during the standby time period between successive executions of the domain-switching procedure, and the standby time period is automatically defined.

In a particular embodiment, the electronic circuitry is configured to implement an automatic configuration of a standby time period between successive executions of the phase refining procedure, comprising: a monitoring of evolution of the electrical signal output by the transimpedance amplifier is first continuously performed in the scope of the phase-refining procedure; history of time periods between successive crossings of a predetermined second threshold THprp is monitored; the stand-by period between successive executions of the phase-refining procedure is defined so as to be lower than a stabilized time period between successive crossings of the predetermined second threshold THprp. Thus, energy of the coherent optical receiver can be preserved during the standby time period between successive executions of the phase-refining procedure, and the standby time period is automatically defined.

In a particular embodiment, the controlled error signal is a modulated signal. Thus, the controlled error signal can be easily tracked in the voltage electrical signal output by the transimpedance amplifier.

In a particular embodiment, the controlled error signal is a modulation on the phase of the optical signal output by the set formed by the local oscillator and the polarization diversity actuator, which can be expressed under the following form: <MAT> wherein ϕ<NUM> is said static component of the controlled error signal. Thus, the controlled error signal can be easily generated.

In a particular embodiment, during the phase-refining procedure, the electronic circuitry is configured for monitoring transimpedance amplifier output variations at a frequency defined by Ω and for adjusting ϕ<NUM> toward reaching the maximum voltage electrical signal output by the transimpedance amplifier where ϕ<NUM> equals to ϕmax.

In a particular embodiment, the electronic circuitry is configured for transferring mitigation of the static component ϕ<NUM> by modifying the ellipticity phase shift Ψl so as to compensate for ϕmax and further by resetting ϕ<NUM>. Thus, dynamics of a modulator used for generating the controlled error signal are preserved.

In a particular embodiment, the electronic circuitry is configured for transferring mitigation of the static component ϕ<NUM> in the case where the maximum voltage electrical signal at the output of the transimpedance amplifier does not vary beyond a predetermined third threshold THmax over a time period that corresponds to bandwidth of variable waveplate that is used in the polarization-diversity actuator for enabling modification of the ellipticity main axis orientation φl and for enabling modification of the ellipticity phase shift Ψl. Thus, transfer of the mitigation of the static component ϕ<NUM> is performed once stabilization is reached.

In a particular embodiment, the electronic circuitry is configured for monitoring evolution of phase detuning so as to detect a continuous and monotonic evolution of phase detuning and adjusts wavelength configuration of the local oscillator so as to compensate for the continuous and monotonic evolution of phase detuning detected. Thus, refinement of the phase of the optical signal output by the set formed by the local oscillator and the polarization-diversity actuator is not untimely triggered due to wavelength drift.

In a particular embodiment, once wavelength adjustment has been performed, the electronic circuitry is configured for reinitiating the phase-refining procedure. Thus, new optimization easily takes into account wavelength adjustment.

In a particular embodiment, the electronic circuitry is configured for reinitiating the phase-refining procedure at time intervals during reconfiguration transitory period of the local oscillator. Thus, phase refinement is smoothly performed during the reconfiguration transitory period of the local oscillator.

In a particular embodiment, the coherent optical receiver further comprises a temperature sensor capturing temperature of the local oscillator and a look-up table that establishes a relationship between temperature drift and wavelength drift. , and wherein the electronic circuitry is configured for monitoring evolution of temperature captured by the temperature sensor, for retrieving from the look-up table wavelength drift corresponding to temperature drift shown by the evolution of temperature and superimposing to the controlled error signal a contribution that is the opposite of phase evolution induced by the wavelength drift. Thus, wavelength drift effects on optical signal phase are easily managed and refinement of the phase of the optical signal output by the set formed by the local oscillator and the polarization-diversity actuator is not untimely triggered due to wavelength drift.

It is further proposed an Optical Network Unit intended to be used in a Passive Optical Network, wherein the Optical Network Unit includes the coherent optical receiver for receiving an amplitude-shift keying modulated optical signal transmitted by a coherent optical transmitter included in an Optical Line Terminal of the Passive Optical Network. Thus, low complexity and cost Optical Network Unit solution is provided to Passive Optical Network infrastructures.

The characteristics of the invention will emerge more clearly from a reading of the following description of at least one example of embodiment, said description being produced with reference to the accompanying drawings, among which:.

It has to be noticed that, since wavelength and frequency are tied together through a direct inverse relationship, these two terms are indifferently used by the one skilled in the art, as they refer to the same concept.

The present invention can be applied for implementing a low complexity coherent optical receiver intended to receive and detect optical signals transmitted by a coherent optical transmitter. More particularly, the coherent optical receiver is suitable for receiving amplitude-shift keying (ASK) modulated optical signals. For example, the amplitude-shift keying (ASK) modulated optical signals are Non-Return-to-Zero On-Off Keying (NRZ-OOK) modulated optical signals. <FIG> described hereafter introduce examples of contexts in which the present invention may be beneficial.

<FIG> schematically represents an arrangement of an optical communications system <NUM> of passive optical network (PON) type, in which the present invention may be implemented.

The optical communications system <NUM> in <FIG> comprises a master device <NUM>, typically an OLT (Optical Line Terminal) device, and a plurality of slave devices <NUM>, <NUM>, <NUM>, typically ONU (Optical Network Units) devices.

The optical communications system <NUM> in <FIG> may further comprise at least one spectral splitter device <NUM> and/or at least one power splitter.

Each spectral splitter device <NUM> comprises a pair of optical band-pass sets of filters for each PON, aiming at filtering respective wavelength bands, and thus enabling said spectral splitter device <NUM> to perform Wavelength Division Multiplexing (WDM). It can be noted that an equivalent arrangement can be obtained by sticking filtering films on reception diodes instead of using the spectral splitter device <NUM>.

Each power splitter enables increasing the quantity of slave devices that can be connected to the master device <NUM>, by dividing input signal power by the quantity of outputs towards the slave devices connected thereto. Each output of the power splitter device thus transmits the same optical information as received as input signal, the power splitter device having only impact on signals power.

The slave devices <NUM>, <NUM>, <NUM> are interconnected with the master device <NUM> via the at least one spectral splitter device <NUM> and/or the at least one power splitter using optical fibers <NUM>.

In the context of PON, ONUs are typically intended to be located at end-user households for FTTH services, and OLT enables ONUs to access a metropolitan or a core network (not shown). Such PON may also be used for mobile network infrastructures services.

<FIG> schematically represents an arrangement of an optical communications system of point-to-point type, in which the present invention may be implemented.

The optical communications system <NUM> in <FIG> comprises a master device <NUM> and a slave device <NUM>. The master device <NUM> and the slave device <NUM> are interconnected using optical fiber <NUM>.

In mobile network fronthauling infrastructure, the slave device <NUM> is typically located at a remote radio head location and the master device <NUM> is located at a baseband unit (BBU) location. Such arrangement may also be used for mobile network backhauling infrastructure between BBUs and core network access terminals.

In both arrangements of <FIG>, optical transmissions of information from the master device <NUM> to one or more slave devices are referred to as downlink transmissions and in the reverse direction to as uplink transmissions.

In both arrangements of <FIG>, it is desirable to install a low complex and cost-effective optical receiver, at least in the slave devices <NUM>, <NUM>, <NUM>, <NUM>, and potentially in the master device <NUM>. Moreover, it is desirable that the optical receiver provides high sensitivity so as to cope with potential low signal strength.

<FIG> schematically represents an arrangement of coherent optical receiver <NUM> which can be used in the optical communications system <NUM> of <FIG>. The coherent optical receiver <NUM> outputs signals <NUM> to be processed for demodulation.

According to the arrangement shown in <FIG>, the coherent optical receiver <NUM> comprises a 2x2 coupler <NUM>. For example, the 2x2 coupler <NUM> is a fiber fused coupler or a directional coupler.

On one input <NUM> of the 2x2 coupler <NUM> is injected an optical signal transmitted by a coherent optical transmitter. On the other input <NUM> of the 2x2 coupler <NUM> is injected another optical signal which is output by a set formed by a local oscillator LO <NUM> and a polarization diversity actuator PDA <NUM>.

The local oscillator LO <NUM> is a laser diode or an arrangement comprising a laser diode. It has to be noted that vertical-cavity surface-emitting laser (VCSEL) may be used as local oscillator.

To one output <NUM> of the 2x2 coupler <NUM> is connected a set formed by a photodiode <NUM> followed by a DC (Direct Current) filter <NUM> of the coherent optical receiver <NUM>. The DC filter <NUM> is an arrangement removing the continuous component of the analog electrical signal output by the photodiode <NUM>. The other output <NUM> of the 2x2 coupler <NUM> is left unused. The other output <NUM> of the 2x2 coupler <NUM> may be used for detection improvement or for other purposes. Doing so leads to a single analog branch coherent optical receiver.

Generally speaking, a polarized optical signal E(t) can be written in the Jones representation as follows: <MAT> which corresponds to a vector that describes an ellipse in a transverse plane with respect to the propagation direction of the optical signal. In the formula above, E<NUM> is the amplitude of the optical signal, ω is the wavelength of the optical signal and ϕ is the phase of the optical signal. Moreover, in the formula above, φ ∈ [<NUM>; π[ is the orientation of the main axis of the polarization ellipse and Ψ is the ellipticity phase shift. We can find the particular cases of Ψ = <NUM> for a linear polarization and of <MAT> , for a circular polarization.

Thus, on the output <NUM> of the 2x2 coupler <NUM>, the 2x2 coupler <NUM> provides the following combined signal C(t): <MAT> wherein the optical signal parameters with the index s concern the optical signal received from the coherent optical transmitter, with signal amplitude E<NUM>s modulated at modulation symbol duration rate, and the optical signal parameters with the index l concern the optical signal output by the set formed by local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM>, with non-modulated signal amplitude E<NUM>l, and finally the optical signal parameters with the index lo concern the optical signal output by the local oscillator LO <NUM> (here the phase ϕlo).

It has to be understood here that the optical signal output by the set formed by local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM> is not modulated with respect to transmission of information, but the amplitude E<NUM>l may be changed in order to help improving optical detection dynamics. It thus means that variations of the signal amplitude E<NUM>l contain no meaningful information as such.

It can be noted that, when E<NUM>l = E<NUM>s, ωl = ωs, φl = φs and <MAT> , then the combined signal C(t) vanishes to zero at any time. And it can be further noted that such a situation occurs only for two fields that differ only by a phase shift (same amplitude, same frequency, same state of polarization with fields shifted by π).

Thus, considering amplitude shift keying optical signals and further considering the impact of the DC filter <NUM> at the output of the photodiode <NUM>, the current i(t) output by the DC filter <NUM> can be expressed as follows: <MAT>.

In the equation of i(t) above, the following terms: <MAT> and <MAT> provide a boosting effect, thanks to the set formed by local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM>, and are referred to as boosting terms.

Control of the polarization diversity actuator PDA <NUM> thus enables adjusting the boosting effect. In order to control the polarization diversity actuator PDA <NUM>, the coherent optical receiver <NUM> further comprises a controlling unit <NUM> in the form of electronic circuitry, which is present at output of the DC filter <NUM> and loops back to the polarization diversity actuator PDA <NUM> via a line <NUM>. The controlling unit <NUM> may further also control the local oscillator LO <NUM> and therefore loop back to the local oscillator LO <NUM> via at least one line, for example a line <NUM> and a line <NUM>.

The electronic circuitry forming the controlling unit <NUM> may be a chip or chipset, such as an FPGA (Field-Programmable Gate Array) or an ASIC (Application-Specific Integrated Circuit). In a variant, the electronic circuitry forming the controlling unit <NUM> may be a processor (microcontroller, a DSP (Digital Signal Processor). ) accompanied by at least one memory. After being powered on, the processor is capable of reading instructions from the memory and executing these instructions for implementing functions of the controlling unit <NUM>. In this case, the functions of the controlling unit <NUM> are in software form, which may be stored on a non-transitory storage medium, such as an SD (Secure Digital) card.

A transimpedance amplifier TIA <NUM> is present between the DC filter <NUM> and the controlling unit <NUM>, so as to convert input current (as output by the DC filter <NUM>) to output voltage. Preferably, the controlling unit <NUM> loops back to the transimpedance amplifier TIA <NUM> for electrical gain control via a line <NUM>.

The controlling unit <NUM> does not perform tracking of the state of polarization. In the case of the present invention, there is no need to precisely know the state of polarization of the optical signal received from the coherent optical transmitter, neither the state of polarization of the optical signal output by the set formed by the local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM>. In the scope of the present invention, the controlling unit <NUM> determines whether or not the state of polarization of the set formed by the local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM> is suitable for obtaining operable boosting terms, and if not (without knowing which relative states of polarization led to such situation), the controlling unit <NUM> adjusts in response the configuration of the polarization diversity actuator PDA <NUM>. There is no need to determine the exact states of polarization of the optical signals and a single analog branch is sufficient to do so, which minimizes analog complexity. Moreover, no optical phase lock loop is needed, and no complex digital signal processing is needed, which further minimizes complexity. Operable boosting terms are boosting terms greater than, or equal to, a predefined threshold percentage (e.g., <NUM>%) of their theoretical maximum; which is the maximum of the strength of the optical signal output by the local oscillator LO <NUM>.

It can be noted from the formula above that i(t) equals to zero independently of φs and φl when E<NUM>s' = E<NUM>l' (which means that E<NUM>s = E<NUM>l) with further Δϕ = (<NUM>q + <NUM>)π, <MAT>, and <MAT>.

It can be noted from the formula above that i(t) also equals to zero independently of φs and φl when E<NUM>s' = E<NUM>l' (which means that E<NUM>s = E<NUM>l) with further Δϕ = (<NUM>q + <NUM>)π, <MAT>, and Δψ = (<NUM>k + <NUM>)π, <MAT>.

It thus means that, in these two cases, the optical signal received from the coherent optical transmitter and the optical signal output by the set formed by the local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM> are in the same state of polarization but with a phase shift of π. In view of the conditions to face these cases, they might be considered as marginally occurring, since they might be faced in one sample among successive samples in a modulation symbol, which can be easily recovered during demodulation operations. Nevertheless, providing a solution, as herein detailed in at least one particular embodiment, when these cases are faced improves coherent optical reception performance.

More importantly, the local oscillator LO <NUM> enables boosting the optical signal coming from the optical fiber, as far as the following situations are not met:.

Apart from these situations, the local oscillator LO <NUM> thus enables increasing sensitivity of the optical receiver, which, among others, enables improving split ratio over PON and enables increasing transmission data rate. It appears to be unnecessary to provide an exhaustive and systematic polarization tracking and polarization transformation (either in an analog or digital way) to get an effective boosted signal at symbol granularity.

In order to avoid the situations above, the polarization diversity actuator PDA <NUM> is controlled by the controlling unit <NUM> so as to modify on demand the optical signal as output by the local oscillator LO <NUM>. In other words, controlling the polarization diversity actuator PDA <NUM> enables escaping the situations above when encountered. Indeed, even a small change of state of polarization of the signal injected in the input <NUM> of the 2x2 coupler <NUM> creates enough difference so that at least one of the boosting terms does not equal zero. Indeed, a boosting effect is obtained once at least one of these boosting terms does not equal zero and further when these terms do not compensate for each other.

Thus, the main function of the controlling unit <NUM> is to ensure that the electrical signal i(t) does not vanish to zero while the optical signal received from the coherent optical transmitter contains useful information and thus to ensure that the boosting effect supposed to be provided by the local oscillator LO <NUM> is maintained as much as possible above the predefined threshold percentage mentioned above.

Once the controlling unit <NUM> detects that the electrical signal output by the transimpedance amplifier TIA <NUM> is below a predetermined threshold THdsp, the controlling unit <NUM> controls the polarization diversity actuator PDA <NUM> so as to change the state of polarization of the optical signal output by the set formed by the local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM>. The boosting effect is consequently obtained in the case where the optical signal received from the coherent optical transmitter contains useful information, since a sensitive change of the state of polarization would allow escaping the encountered situation where the electrical signal i(t) has vanished to zero although the optical signal received from the coherent optical transmitter contains useful information. It can be demonstrated that, in the worst case, the contribution related to the boosting terms is greater than the contribution of the optical signal received from the coherent optical transmitter as soon as the states of polarization of both optical signals injected in the 2x2 coupler <NUM> are shift by a value that is small percentage (e.g., <NUM>%) of the distance between adjacent optimal relative states of polarization.

After several kilometers in the optical fiber, the state of polarization of the optical signal originally transmitted by the coherent optical transmitter is randomly distributed over the Poincaré sphere. Thus, the state of polarization of the optical signal as received by the coherent optical receiver <NUM> is very likely to be elliptical. Thus, the optical signal output by the set formed by the local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM> is controlled to be elliptically polarized. Thus, in order to change the state of polarization of the optical signal output by the set formed by the local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM> when the electrical signal i(t) falls in a zero region although an optical signal containing useful information is received from the coherent optical transmitter, the controlling unit <NUM> instructs the polarization diversity actuator PDA <NUM> to change at least one ellipticity parameter among the angle φl of orientation of the main axis of the polarization ellipse and the ellipticity phase shift ψl of the optical signal output by the set formed by the local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM>. It enables making operable the boosting terms and recovering a sensitive photocurrent. In a particular embodiment, the controlling unit <NUM> instructs the polarization diversity actuator PDA <NUM> to change, by a predefined shift, the at least one ellipticity parameter among the angle φl of orientation of the main axis of the polarization ellipse and the ellipticity phase shift Ψl of the optical signal output by the set formed by the local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM>.

In a particular embodiment, the predefined shift targets a stationary extremum of the electrical signal i(t). In this particular embodiment, the predefined shift is <MAT> for the orientation axis φl (and consequently for Δφ and <MAT> for the ellipticity phase shift ψl (and consequently for Δψ).

Thus, it is avoided that the current i(t) output by the DC filter <NUM> vanishes to zero, in a simple and cost-effective way. The procedure disclosed above performs coarse control of changes of the state of polarization without tracking state of polarization, and is herein referred to as "domain-switching procedure". The domain-switching procedure can be continuously executed or performed at time intervals, e.g., regular time intervals. For example, the domain-switching procedure is regularly performed on a time cycle basis which duration is defined so as to be lower than the time constant of polarization state variations. The order of magnitude of the time constant of polarization state variations is typically about one or few milliseconds, but may differ according to optical fiber deployment on field. For instance, the polarization state variations depends on vibrations incurred by the optical transmissions, and differs according to whether optical fiber deployment on field is aerial, deep in the ground or at short distance from ground surface.

In a particular embodiment, an automatic configuration of a standby period between successive executions of the domain-switching procedure is implemented. At first, a monitoring of evolution of the electrical signal output by the transimpedance amplifier TIA <NUM> is continuously performed in the scope of the domain-switching procedure. History of polarization state related variations is thus tracked and the time period between successive changes in ellipticity parameter ordered by the controlling unit <NUM> is monitored. Once the evolution of polarization state variations becomes stable, the controlling unit <NUM> defines a stand-by period between successive executions of the domain-switching procedure so as to be lower (e.g., half or with a predefined time margin) than a stabilized time period between successive crossings of the predetermined threshold THdsp (i.e., triggering changes in at least one ellipticity parameter ordered by the controlling unit <NUM>). Thus, the time intervals between successive executions of the domain-switching procedure do not need to be specifically configured by an operator with respect to the effective deployment of the optical fiber on field.

Further details about the domain-switching procedure are disclosed hereafter with respect to <FIG>.

Refined control of changes of the state of polarization is performed by adjustment of the phase ϕl of the optical signal output by the set formed by the local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM>. To do so, a modulated signal is introduced by the polarization diversity actuator PDA <NUM> under control of the controlling unit <NUM> in a "phase-refining procedure".

The phase-refining procedure is preferably performed consequently to a change of at least one ellipticity parameter during execution of the domain-switching procedure. The phase-refining procedure may be performed independently of the domain-switching procedure. The phase-refining procedure can be executed when detecting that evolution of the electrical signal output by the transimpedance amplifier TIA <NUM> is above a predetermined threshold THprp. In a variant, the phase-refining procedure can be executed at time intervals, for example at regular time intervals.

As for the domain-switching procedure, a standby time period between successive executions of the phase-refining procedure may be automatically defined. At first, a monitoring of evolution of the electrical signal output by the transimpedance amplifier TIA <NUM> is continuously performed in the scope of the phase-refining procedure. The controlling unit <NUM> tracks history of time periods between successive crossings of the predetermined threshold THprp. Once stabilized, the controlling unit <NUM> defines a stand-by period between successive executions of the phase-refining procedure so as to be lower (e.g., half or with a predefined time margin) than a stabilized time period between successive crossings of the predetermined threshold THprp.

Determining how the phase ϕl has to be adjusted can be achieved thanks to a controlled error signal introduced by a phase modulator PM <NUM> in the polarization diversity actuator PDA <NUM>, as detailed hereafter, more particularly with respect to <FIG> and <FIG>. Indeed, introducing an error signal as a stimulus on signal phase implies a noticeable effect on the current i(t) output by the DC filter <NUM>, which enables finding the value of the phase ϕl which provides maximum signal strength.

In a particular embodiment, the controlled error signal is a modulation on the phase of the optical signal output by the set formed by the local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM>, which can be expressed as: <MAT>.

It leads to the following expression of the current i(t) output by the DC filter <NUM>, when considering the case of the local oscillator with a circular polarization with φl = π/<NUM> and ψl = π/<NUM>: <MAT>.

Therefore, ϕ<NUM> enables adjusting a constant component of the phase ϕl and Δϕ<NUM> enables oscillating around this constant component, which creates noticeable variations of the current i(t) output by the DC filter <NUM>. It can be noted that φ fixes a temporal origin of the controlled error signal, which only matters for synchronous detection, as disclosed hereinafter.

Back to <FIG>, the controlling unit <NUM> comprises, in a particular embodiment, a value decision block VDB <NUM>, a polarization diversity manager PDM <NUM> and a detuning manager DM <NUM>.

The value decision block VDB <NUM> analyzes the electrical signal output by the transimpedance amplifier TIA <NUM>. The value decision block VDB <NUM> thus analyzes the voltage output by the transimpedance amplifier TIA <NUM>. The value decision block VDB <NUM> compares the value of the voltage output by the transimpedance amplifier TIA <NUM> with the predefined threshold percentage (e.g., <NUM>%) of the theoretical maximum achievable by the boosting effect, i.e., theoretical maximum achievable by the boosting terms. The theoretical maximum is defined according to the gain of the local oscillator LO <NUM>, to the conversion rate R by the photodiode <NUM> of incident photons into electrons and to the gain of the transimpedance amplifier TIA <NUM>. When the voltage output by the transimpedance amplifier TIA <NUM> is below the predefined threshold percentage of the theoretical maximum achievable thanks to the boosting terms, the value decision block VDB <NUM> acts to change configuration of the polarization diversity actuator PDA <NUM> so as to modify at least one ellipticity parameter among the angle φl of orientation of the main axis of the polarization ellipse and the ellipticity phase shift ψl of the optical signal output by the set formed by the local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM>.

The polarization diversity manager PDM <NUM> controls the polarization diversity actuator PDA <NUM> via the line <NUM>. The polarization diversity manager PDM <NUM> converts instructions, which are received from the value decision block VDB <NUM> via a line <NUM> for modifying at least one ellipticity parameter among the angle φl of orientation of the main axis of the polarization ellipse and the ellipticity phase shift Ψl, of the optical signal output by the set formed by the local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM>, into adequate commands suitable to the polarization diversity actuator PDA <NUM> (e.g., drive voltage change). The polarization diversity manager PDM <NUM> consequently controls the polarization diversity actuator PDA <NUM> via the line <NUM>.

The detuning manager DM <NUM> controls the local oscillator LO <NUM> via a line <NUM> and the polarization diversity actuator PDA <NUM> via a line <NUM>. The detuning manager DM <NUM> converts instructions, which are received from the value decision block VDB <NUM> via a line <NUM> for modifying the wavelength of the optical signal output by the local oscillator LO <NUM>, into adequate commands suitable to the local oscillator LO <NUM>. The detuning manager DM <NUM> consequently controls the local oscillator LO <NUM> via the line <NUM>. For instance, the detuning manager DM <NUM> adjusts the Peltier current when the local oscillator LO <NUM> is equipped with a Peltier module, or drive voltage when the local oscillator LO <NUM> is equipped with an optical ring resonator. The detuning manager DM <NUM> converts instructions, which are received from the value decision block VDB <NUM> via the line <NUM> (or another line dedicated thereto) for modifying parameters (ϕ<NUM> and Δϕ<NUM> in the formula above) of the controlled error signal introduced by the phase modulator PM <NUM>, into adequate commands suitable to the phase modulator PM <NUM>.

Furthermore, the value decision block VDB <NUM> may provide instructions to the transimpedance amplifier TIA <NUM> via the line <NUM> for modifying electrical gain of the transimpedance amplifier TIA <NUM>.

<FIG> schematically represents a particular embodiment of the polarization diversity actuator PDA <NUM>. The particular embodiment of <FIG> is suitable for using a local oscillator providing a linearly-polarized optical signal, as commonly found off-the-shelf; however, the particular embodiment of <FIG> is also suitable for using a local oscillator providing a circular or elliptical polarization.

Low costs laser used nowadays in network access segment are typically semiconductor lasers that are linearly polarized. Such lasers can be used as the local oscillator LO <NUM>. A way to transform a linear polarization into an elliptical polarization is to place a waveplate (also referred to as wavelength plate) at the output of the laser. Waveplates are based on birefringent materials with two orthogonal axes of different velocity, which induce a phase shift between the projections of a given field on the two axes of the birefringent materials. Injecting therein a linearly-polarized optical signal outputs an elliptically-polarized optical signal. It however may be taken into account that a waveplate couples both polarization ellipticity and phase shift, which means that phase shift between the optical signal output by the local oscillator LO <NUM> and the optical signal received from the coherent optical transmitter may also be impacted by the waveplate. Thus, in case the ellipticity has to be independently controlled with a waveplate, a compensating for phase shift may be applied on both axes of the optical signal output by the local oscillator LO <NUM> to counter the waveplate phase shift thus introduced.

In order to control ellipticity of the optical signal output by the set formed by the local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM>, a variable waveplate can be used. Variable waveplates are particular waveplates the phase shift of which between their slow (extraordinary) and fast (ordinary) axes can be controlled. Liquid crystal variable retarders and pockets cells are examples of variable waveplates. Tunable highly birefringent photonic Liquid Crystal Fibers can alternatively be used. Retardance is then tuned over a specified range by changing drive voltage.

According to the embodiment shown in <FIG>, the optical signal issued from the local oscillator LO <NUM> is injected in the aforementioned phase modulator PM <NUM> that introduces the controlled error signal. The phase modulator PM <NUM> can be an electrically driven optical modulator based on Pockels cells, or a liquid crystal modulator, or a variable length waveguide, or a waveguide with variable optical index.

At the output of the phase modulator PM <NUM>, the polarization diversity actuator PDA <NUM> further comprises a variable waveplate VW <NUM>. The variable waveplate VW <NUM> is here used to adjust ψl and thus to consequently mitigate the difference between ψl and ψs.

The polarization diversity actuator PDA <NUM> may further comprise a controller CTRL <NUM> configured for interpreting instructions received from the controlling unit <NUM>, and more particularly from the polarization diversity manager PDM <NUM> or the detuning manager DM <NUM>, and for converting these instructions into commands (such as appropriate drive voltage) to the variable waveplate VW <NUM> and to the phase modulator PM <NUM> respectively.

In a variant, the controlling unit <NUM>, and more particularly the polarization diversity manager PDM <NUM> and the detuning manager DM <NUM>, directly transmits commands (such as appropriate drive voltage) to the variable waveplate VW <NUM> and to the phase modulator PM <NUM>. In this case, the controlling unit <NUM>, and more particularly the polarization diversity manager PDM <NUM>, may comprise a look-up table LUT that links, on one hand, actuation of changes in φl (and consequently in Δφ) and/or in ψl (and consequently in Δψ) and, on the other hand, commands to be applied to the polarization diversity actuator PDA <NUM>, more particularly to the variable waveplate VW <NUM>; moreover, the controlling unit <NUM>, and more particularly the polarization diversity manager PDM <NUM>, may comprise a look-up table LUT that links, on one hand, phase modulation parameters and, on the other hand, commands to be applied to the polarization diversity actuator PDA <NUM>, more particularly to the phase modulator PM <NUM>.

<FIG> schematically represents a particular embodiment of the value decision block VDB <NUM>.

The value decision block VDB <NUM> comprises a level detector LD <NUM>. The level detector LD <NUM> detects voltage output by the transimpedance amplifier TIA <NUM> and performs conversion into digital data more easily handled for further analysis.

The value decision block VDB <NUM> further comprises an analyzer AN <NUM>. The analyzer AN <NUM> receives digital data from the level detector LD <NUM>. The analyzer AN <NUM> compares the value of the voltage output by the transimpedance amplifier TIA <NUM> with the predefined threshold percentage (e.g., <NUM>%) of the theoretical maximum achievable thanks to the boosting terms. The analyzer AN <NUM> then makes a decision regarding whether or not at least one action has to be taken in order to ensure that operable boosting terms are met.

The value decision block VDB <NUM> further comprises a manager MGR <NUM>. The manager MGR <NUM> receives from the analyzer AN <NUM> instructions with respect to action to be taken. The manager MGR <NUM> consequently provides polarization diversity-related instructions to the polarization diversity manager PDM <NUM> via the line <NUM> and/or electrical gain control instructions to the transimpedance amplifier TIA <NUM> via the line <NUM> and/or phase tuning-related instructions to the detuning manager DM <NUM> via the line <NUM> and/or wavelength tuning-related instructions to the detuning manager DM <NUM> via the line <NUM>.

In a preferred embodiment, the value decision block VDB <NUM> further comprises a squaring module SQ <NUM>. The squaring module SQ <NUM> squares the electrical signal received from the transimpedance amplifier TIA <NUM> (i.e., the squaring module SQ <NUM> multiplies the input electrical signal by itself). It simplifies analysis performed by the analyzer AN <NUM>, by ensuring that only positive values are handled (which is easier for comparing them in absolute value with a threshold). To do so, the squaring module SQ <NUM> is for example an analog amplifier with both inputs receiving the electrical signal received in input of the value decision block VDB <NUM>. Usage of the squaring module SQ <NUM> further advantageously increases contrast for the boosting terms with respect to the optical signal received from the coherent optical transmitter.

On the contrary, operating without the squaring module SQ <NUM> enables implementing NRZ-OOK-alike demodulation based on negative minimum and positive maximum levels.

The value decision block VDB <NUM> outputs the signals <NUM> to be processed for demodulation. These signals may be the output of the squaring module SQ <NUM>. These signals may simply be the output of the transimpedance amplifier TIA <NUM>.

<FIG> schematically represents an algorithm performed by the controlling unit <NUM> for configuring the coherent optical receiver <NUM> in the scope of the domain-switching procedure. The controlling unit <NUM> continuously receives analog electrical signal from the transimpedance amplifier TIA <NUM>.

In a step S501, the controlling unit <NUM> performs signal level detection with respect to the electrical signal output by the transimpedance amplifier TIA <NUM>, as already explained above. The step S501 outputs digital representation, according to a sampling rate, of the analog electrical signal (voltage) output by the transimpedance amplifier TIA <NUM>. Level detection preferably includes integration or averaging over a predetermined quantity of samples.

In a step S502, the controlling unit <NUM> performs comparison of the level detected in the step S501 with the predefined threshold percentage (e.g., <NUM>%) of the theoretical maximum signal level which is achievable thanks to the boosting terms. As already mentioned, the theoretical maximum is defined according to the gain of the local oscillator LO <NUM>, to the conversion rate R by the photodiode <NUM> of incident photons into electrons and to the gain of the transimpedance amplifier TIA <NUM>. The conversion rate R by the photodiode <NUM> of incident photons into electrons is known to the controlling unit <NUM> by pre-configuration (e.g., in factory). When the gain of the local oscillator LO <NUM> is not defined (i.e., controlled) by the controlling unit <NUM>, the gain of the local oscillator LO <NUM> is known to the controlling unit <NUM> by pre-configuration (e.g., in factory). When the gain of the transimpedance amplifier TIA <NUM> is not defined (i.e., controlled) by the controlling unit <NUM>, the gain of the transimpedance amplifier TIA <NUM> is known to the controlling unit <NUM> by pre-configuration (e.g., in factory). Alternatively, the theoretical maximum signal level which is achievable thanks to the boosting terms is known to the controlling unit <NUM> by pre-configuration (e.g., in factory). Further alternatively, the applicable threshold THprp is known to the controlling unit <NUM> by pre-configuration (e.g., in factory). In this latter case, the preconfigured applicable threshold THprp remains a predefined threshold percentage (e.g., <NUM>%) of the theoretical maximum signal level that is achievable thanks to the boosting terms, except that the effective value of the predefined threshold percentage (e.g., <NUM>%) is not dynamically determined by the controlling unit <NUM>. The predefined threshold percentage may change over time: for example, the controlling unit <NUM> is instructed by an application layer of the optical coherent receiver <NUM>, or by a demodulator of the optical coherent receiver <NUM>, that means signal-to-noise ratio of the electrical signal should be strengthened or, on the contrary, loosened.

In a step S503, the controlling unit <NUM> checks whether or not the comparison shows that the level detected in the step S501 is greater than, or equal to, the threshold THprp in question. When the level detected in the step S501 is lower than the threshold THprp in question, a step S504 is performed. When the level detected in the step S501 is greater than, or equal to, the threshold THprp in question, the step S501 is repeated. A waiting or standby time may be inserted before executing again the step S501. The duration of the waiting or standby time is less than the time observed with optical fibers to change polarization of transported optical signals in a predefined proportion in the polarization space. For instance, the waiting or standby time duration is <NUM> milliseconds.

In the step S504, the controlling unit <NUM> instructs a configuration change of the set formed by the local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM>, wherein the configuration change is modification of the ellipticity main axis orientation φl and/or modification of the ellipticity phase shift ψl.

Then, the step S501 is repeated. No waiting time is preferably applied at that time, in order for the controlling unit <NUM> to determine whether another ellipticity modification of the optical signal output by the set formed by the local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM> has to be performed.

As already mentioned, the domain-switching procedure enables coarse control of changes of the state of polarization without effectively tracking state of polarization. Fine control is provided by the phase-refining procedure disclosed hereafter.

<FIG> schematically represents an algorithm performed by the controlling unit <NUM> for adjusting configuration of the coherent optical receiver <NUM> in the scope of the phase-refining procedure.

In a step S601, the controlling unit <NUM> configures the polarization diversity actuator PDA <NUM> so as to set the modulation parameters defining the controlled error signal. More particularly, the controlling unit <NUM> configures the phase modulator PM <NUM> with respect to the modulation parameters ϕ<NUM> and Δϕ<NUM>. Regarding the parameter Ω, it is set so as to correspond at a cycling period that is higher than polarization time constant (in the order of milliseconds) but lower than typical wavelength chirp or fast phase noise timescale (in the order of milliseconds microseconds). Modulation at hundreds of kHz can then be used.

At first, ϕ<NUM> is set to <NUM> and Δϕ<NUM> is set to a variation of Δϕ that corresponds to a predefined fraction of the threshold THprp that triggers the change of ellipticity parameter in the domain-switching procedure (e.g., half or a third of the threshold value) in order to limit probability that the photocurrent output of the DC filter <NUM> goes down to the threshold value in question due to the phase-refining procedure.

In a step S602, the controlling unit <NUM> tracks photocurrent variations at the frequency Ω/2π. More precisely, the controlling unit <NUM> tracks voltage electrical signal variations at the output of the transimpedance amplifier TIA <NUM>. The controlling unit <NUM> preferably performs integration over a predefined quantity of datacom samples (e.g., few hundreds of samples, such as <NUM> or <NUM>, depending on coding and interleaving robustness), which means that a few tracking samples (e.g., <NUM> or <NUM> samples, or few tens, or even <NUM> or <NUM> samples, depending on incurred noise) are acquired within the 2π/Ω time period. Datacom samples have to be understood as samples issued from the modulated optical signal transmitted by the coherent optical transmitter and tracking samples have to be understood as samples issued from the modulated optical signal generated by the phase modulator PM <NUM>.

According to a particular embodiment relying on synchronous detection, the controlling unit <NUM> monitors evolution of the acquired tracking samples during the 2π/Ω time period in view of the shape of the controlled error signal during said 2π/Ω time period. In view of the definition of Ω, propagation times of signals in the coherent optical receiver <NUM> are negligible and it is considered that variations of the controlled error signal and implied variations at the output of the transimpedance amplifier TIA <NUM> occur with a same time reference. Thus, the controlling unit <NUM> analyses the output of the transimpedance amplifier TIA <NUM> in parallel to the shape of the cos(Ωt + φ) contribution in the optical signal output by the set formed by the local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM>.

According to the starting position of the phase modulation introduced by the phase modulator PM <NUM> and its amplitude, the photocurrent output by the DC filter <NUM>, and consequently the analog electrical signal from the transimpedance amplifier TIA <NUM>, synchronously increase or decrease, or pass a maximum value (vertex), with respect to shape of the controlled error signal amplitude. In other words, the controlling unit <NUM> analyses whether, in view that the controlled error signal amplitude varies as a cosine wave between ϕ<NUM> - Δϕ<NUM> and ϕ<NUM> + Δϕ<NUM>, the analog electrical signal from the transimpedance amplifier TIA <NUM> synchronously increases or decreases, or passes the maximum value (vertex).

In a step S603, the controlling unit <NUM> checks whether the analog electrical signal from the transimpedance amplifier TIA <NUM> passes the maximum value (vertex). If such is the case, a step S605 is performed; otherwise, a step S604 is performed.

In the step S604, the controlling unit <NUM> modifies at least the modulation parameter ϕ<NUM> in order to start the modulation closer to the aforementioned maximum value and attempt finding the value φmax which provides said aforementioned maximum value. The controlling unit <NUM> increases or decreases the modulation parameter ϕ<NUM> by a predetermined step value, for example Δϕ<NUM>, so as to move the modulated signal toward the aforementioned maximum value (vertex). Typically, in synchronous detection, if the analog electrical signal from the transimpedance amplifier TIA <NUM> decreases while at the same time the shape of the cosine wave of the controlled error signal amplitude increases, or vice versa, the modulation parameter ϕ<NUM> has to be decreased; and if the analog electrical signal from the transimpedance amplifier TIA <NUM> increases (or decreases) at the same time as the shape of the cosine wave of the controlled error signal amplitude, the modulation parameter ϕ<NUM> has to be increased. Then the step S603 is repeated for further monitoring variations at the frequency Ω/2π.

In the step S605, the controlling unit <NUM> fixes the modulation parameter ϕ<NUM> to ϕmax. Phase is thus adjusted so that signal strength at the output of the transimpedance amplifier TIA <NUM> is optimized. The phase-refining procedure then ends, and the modulation parameter ϕ<NUM> remains unchanged, and Δϕ<NUM> is set to <NUM>, until the phase-refining procedure is launched again. When the phase-refining procedure is reinitiated, the controlling unit <NUM> preferably keeps the modulation parameter ϕ<NUM> to φmax as a starting point of the new phase-refining procedure.

Once φmax is obtained and the polarization diversity actuator PDA <NUM> is configured in accordance, any fluctuation of the maximum of the photocurrent output by the DC filter <NUM>, and consequently of the analog electrical signal from the transimpedance amplifier TIA <NUM>, comes from a change in the axes of the state of polarization of the optical signals. So, with this phase-refining procedure, any phase detuning is mitigated, independently of its nature (whether: wavelength detuning, polarization change or phase shift), and if several phase detuning phenomena simultaneously occur, they are jointly mitigated. Optimum signal strength is thus obtained at the output of the transimpedance amplifier TIA <NUM> in view of the orientations of the axes of the state of polarization of the optical signals.

In a particular embodiment of the step S605, the controlling unit <NUM> modifies the ellipticity phase shift Ψl, in order to compensate for ϕmax and resets the modulation parameter ϕ<NUM> (fixing ϕ<NUM> to <NUM>). Indeed, ellipticity phase detuning Δψ between the optical signal output by the set formed by the local oscillator LO <NUM> and the polarization diversity actuator PDA <NUM> and the optical signal received from the coherent optical transmitter acts similarly as wavelength detuning Δυ and phase detuning Δϕ on both polarizations induced contributions at the output of the DC filter <NUM>. Any one among said detuning phenomena can thus be compensated for, by adjustment of at least one of the others. In the arrangement of <FIG>, mitigation of the static component of the phase detuning is thus transferred from the phase modulator PM <NUM> to the variable waveplate VW <NUM>. It enables preserving dynamics of the phase modulator PM <NUM>. In a more particular embodiment, transfer of the mitigation of the static component of the phase detuning is initiated by the controlling unit <NUM> in the case where the maximum value of the electrical voltage at the output of the transimpedance amplifier TIA <NUM> does not vary beyond a predetermined threshold THmax over a time period that corresponds to the bandwidth of the variable waveplate VW <NUM>.

In a particular embodiment, the controlling unit <NUM> monitors evolution of the phase detuning so as to detect wavelength detuning and adjusts wavelength configuration of the local oscillator LO <NUM> in accordance. Processing resources and complexity needed to perform such monitoring are limited compared with processing resources tracking state of polarization. This aspect is presented hereafter with respect to <FIG>.

<FIG> schematically represents an algorithm performed by the controlling unit <NUM> for adjusting wavelength of the local oscillator LO <NUM>.

In a step S701, the controlling unit <NUM> monitors evolution of the phase detuning so as to detect wavelength detuning. Thus, in a step <NUM>, the controlling unit <NUM> checks whether or not a continuous and monotonic evolution of phase detuning is present at the output of the transimpedance amplifier TIA <NUM>. Indeed, wavelength drift of a laser is induced by adiabatic evolution of laser cavity. This can be caused by auto-modulation gain, thermal effects. Adiabatic evolution often refers to phenomena with time constants of the order of few milliseconds to several seconds or even more. Phase noise and frequency or wavelength detuning are distinguished the one from the other as reference to the time scale, i.e., symbol duration. Hence, any instabilities longer than the symbol duration is viewed as a wavelength drift, while any instabilities with a time constant shorter or about symbol duration is viewed as a phase noise.

If the controlling unit <NUM> detects a continuous and monotonic evolution of phase detuning, a step <NUM> is performed; otherwise, the step S701 is repeated. A waiting period may be applied before repeating the step S701.

In the step S703, the controlling unit <NUM> adjusts wavelength of the local oscillator LO <NUM> so as to compensate for the continuous and monotonic evolution of phase detuning detected. Then the step S701 is repeated. A waiting period may be applied before repeating the step S701.

Once wavelength adjustment has been performed, the controlling unit <NUM> preferably reinitiates the phase-refining procedure. In a more particular embodiment, the controlling unit <NUM> preferably reinitiates the phase-refining procedure at time intervals (e.g., regular time intervals) during reconfiguration transitory period (slow reactivity) of the local oscillator LO <NUM>. Thus, during the time needed by the local oscillator LO <NUM> to effectively adapt the wavelength, the controlling unit <NUM> compensates for transitory wavelength evolution with overmodulation of the controlled error signal.

In a particular embodiment, when the domain-switching procedure is initiated, and when a phase-refining procedure is on-the-way, the controlling unit <NUM> stops and resets the phase-refining procedure. The phase-refining procedure is reinitiated once the change of configuration of the polarization diversity actuator is achieved with respect to the ellipticity main axis orientation φl and/or the ellipticity phase shift ψl. The domain-switching procedure has thus higher priority than the phase-refining procedure, especially during initialization of the coherent optical receiver <NUM>.

<FIG> schematically represents a temperature sensor-based particular arrangement of the coherent optical receiver <NUM>.

Claim 1:
A coherent optical receiver (<NUM>) adapted to receive an amplitude-shift keying modulated optical signal from a coherent optical transmitter, comprising:
- a local oscillator (<NUM>);
- a polarization-diversity actuator (<NUM>) configured for modifying an optical signal output by the local oscillator (<NUM>), so as to form an optical signal with elliptical polarization with main axis orientation φl and ellipticity phase shift ψl;
- a <NUM> x <NUM> coupler (<NUM>), having one input (<NUM>) aiming at receiving the amplitude-shift keying modulated optical signal received from the coherent optical transmitter and the other input (<NUM>) receiving another optical signal which is output by a set formed by the local oscillator (<NUM>) and the polarization diversity actuator (<NUM>), so as to enable the local oscillator (<NUM>) to provide a boosting effect to the amplitude-shift keying modulated optical signal received from the coherent optical transmitter, the <NUM> x <NUM> coupler (<NUM>) further having one output connected to a set formed by a photodiode (<NUM>) followed by a Direct Current filter (<NUM>) removing a continuous component of an analog electrical signal output by the photodiode (<NUM>);
- a transimpedance amplifier (<NUM>) converting current electrical signal output by the Direct Current filter (<NUM>) into voltage electrical signal;
- a controlling unit (<NUM>) in form of electronic circuitry configured for performing a domain-switching procedure instructing a configuration change of the polarization diversity actuator (<NUM>) by modifying the ellipticity main axis orientation φl and/or the ellipticity phase shift ψl when the voltage electrical signal output by the transimpedance amplifier (<NUM>) is below or equal to a predetermined first threshold THdsp corresponding to a predetermined percentage of theoretical maximum achievable by the boosting effect,
and wherein the electronic circuitry is further configured for performing a phase-refining procedure inserting a controlled error signal in the phase of the optical signal output by the set formed by the local oscillator (<NUM>) and the polarization diversity actuator (<NUM>), and adjusting a static component of the controlled error signal toward a maximum voltage electrical signal output by the transimpedance amplifier (<NUM>).