Coherent Optical Receiver

Abstract

To Improve the sensitivity of coherent optical receiver.SOLUTION: In an optical communication system, a coherent optical receiver 200 includes a local oscillator 210, a polarization diversity actuator 211 that modifies an optical signal output by the local oscillator, a 2×2 coupler 220 for combining the optical signal output by the polarization diversity actuator and an optical signal which has been subjected to amplitude shift keying modulation from an optical transmitter, and two photodiodes 230a and 230b each connected to the output of the 2×2 coupler. The local oscillator provides a boost effect to the optical signal which has been subjected to the amplitude shift keying modulation. The coherent optical receiver additively combines contribution amounts from both the photodiodes and generates an electrical signal representing the combined contribution amounts. If a control unit 280 detects that the level of the electrical signal is insufficient, the control unit 280 commands the polarization diversity actuator to execute the change of elliptical principal axis orientation and / or the change of elliptical phase shift.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] This invention relates generally to optical communications, and more particularly to coherent optical receivers for optical communications. [Background technology]

[0002] Optical communications, such as passive optical networks (PONs), are increasingly being used to provide network access (typically Internet access) to home or office gateways or data centers with the widespread adoption of Fiber To The Home (FTTH) technologies. Optical communications are also used to ensure mobile infrastructure backhaul, typically using a point-to-point mechanism, for example, within the scope of the deployment of 3G (third generation) or 4G (fourth generation) mobile technologies.

[0003] With the rise of 5G (5th generation) mobile technology, fronthaul is emerging and the need in terms of data rate capabilities is increasing significantly. Reference can be made to Non-Patent Document 1. In such a framework, fronthaul is achieved by moving processing functions that are traditionally performed at or near the base station in 3G or 4G mobile technologies to the upstream of the mobile infrastructure. This is referred to as the split option in Non-Patent Document 2 (for more details, see Section 11 and Table A-1 of Non-Patent Document 2).

[0004] 5G mobile technology therefore has broader requirements than the FTTH requirements that have driven the deployment of optical access technology to date. 5G mobile technology will therefore lead to, among other things, higher nominal data rates, reduced latency, and higher-density deployment. While this will result in higher nominal data rates for optical access, current PON systems are expected to achieve split ratios of over 1:128, compared to the typical FTTH split ratio of 1:64.

[0005] To address this trend, coherent optical communication is envisioned. Coherent optical communication is a technique that uses modulation of the amplitude and phase of light to enable high-capacity data transmission over optical fibers. Digital signal processing (DSP) is typically used to achieve the robustness of coherent communication over optical fibers.

[0006] Coherent optics is well known for its revolutionary role in metro and core networks. Beyond the ability to capture the full diversity of channels and therefore obtain high spectral efficiency, coherent optics also offers good optical receiver sensitivity and wavelength selectivity.

[0007] The principles of coherent optical communications used in metro and core network fields are described in Non-Patent Document 3. This document describes a coherent optical receiver that utilizes phase diversity and polarization diversity. Two phase diversity homodyne receivers based on a 90-degree optical hybrid power splitter are arranged in a polarization diversity configuration, using a common local oscillator, to extract the phase and quadrature components of an input coherent optical signal (see Figure 9 in Non-Patent Document 3 for more details). A DSP mechanism must be additionally used to extract the modulated symbols by digitally processing the outputs of the phase and polarization diversity receivers to stably recover the complex signal amplitude despite fluctuations in the carrier phase and polarization signal conditions. To this end, the DSP mechanism comprises an anti-aliasing filter, a four-channel analog-to-digital converter (ADC), a frequency domain equalizer (FDE) that enables symbol rate extraction to backdrive the ADC, an adaptive finite impulse response (FIR) equalizer that enables clock phase recovery, and a carrier phase estimator (see Figure 18 of non-patent document 3 for more details).

[0008] One major problem with coherent optical communications is that communication over long-distance optical fibers involves slow, yet large enough, polarization state changes that can be detected by coherent optical receivers. To ensure communication continuity, two alternative approaches have been used to avoid signal fading due to mismatch between the polarization state of the signal received over the optical fiber and the polarization state of the local oscillator. One approach is to use a full-diversity polarization receiver and perform complex digital signal processing to compensate for the polarization state changes caused by the long-distance optical fiber. The other approach is to use an optical phase-locked loop (OPLL) to track the polarization state, for example, when full polarization diversity cannot be restored. Both of these approaches lead to complex analog and / or digital mechanisms with at least two analog branches. For example, see "Polarization State Tracking," by "Polarity-Dependent Transformers," in "Polarization State Tracking ...

[0009] From the above description, it is clear that the inherent theoretical hardware implementations applied in the metro and core network field do not meet the essential requirements in the network access field, such as low design complexity, low power consumption, low manufacturing cost and complexity, low deployment cost and complexity, and small size. In fact, the number of devices to be installed in the network access field is much higher than that in the metro and core network field, especially in order to meet the expected geographical density of base stations in 5G mobile technology, and moreover, the locations where such devices are installed are often particularly limited in terms of space (home devices, base station installation locations, etc.).

[0010] Furthermore, optical communications in metro and core network fields suffer from the presence of noise due to signal amplification used to cover long point-to-point distances. In contrast, optical communications in network access fields operate over relatively short distances but suffer from low signal strength (the amount of photons received by an optical receiver) considering the splitting ratio, and therefore optical receiver sensitivity is a major concern. Therefore, coherent optical communications technology such as that used in metro and core network fields is not suitable for 5G mobile technology.

[0011] In addition to the scope of 5G mobile technology, coherent optical communications may be suitable for FTTH deployment. Indeed, the growing demand for high-speed Internet has led to higher data rates as well as higher split ratios of current PON systems. However, for base station deployment in 5G mobile technology, the optical network units (ONUs) used in such PON systems are deployed at or near user premises, requiring low design complexity, low power consumption, low manufacturing cost and complexity, low deployment cost and complexity, and compact size. Furthermore, in the context of 5G mobile technology, optical communications in metro and core network fields suffer from the presence of noise due to signal amplification used to cover long point-to-point distances. Conversely, optical communications in network access fields operate over relatively short distances but suffer from low signal strength (the amount of photons received by an optical receiver) considering the split ratio, and therefore optical receiver sensitivity is a major concern. Therefore, coherent optical communications technology as used in metro and core network fields is not suitable for FTTH deployment either. [Prior art documents] [Non-patent literature]

[0012] [Non-Patent Document 1] the International Mobile Telecommunications (IMT) recommendations ITU-R M.2083 “IMT Vision: Framework and overall objectives of the future development of IMT for 2020 and beyond”, released in September 2015. [Non-patent document 2] the specifications 3GPP TR 38.801 V 14.0.0 “Study on new radio access technology: Radio access architecture and interfaces”, released in March 2017. [Non-patent document 3] “Fundamentals of Coherent Optical Fiber Communications”, K. Kikuchi, Journal of Lightwave Technology, vol. 34, n°1, released in January 2016. [Non-patent document 4] “Endless Polarization Control Systems for Coherent Optics”, Noe' et al., IEEE Journal of Lightwave Technology, vol. 6, n°7, pp. 1199-1207, released in July 1988. Summary of the Invention [Problem to be solved by the invention]

[0013] It is therefore desirable to provide a cost-effective solution for coherent optical receivers that is suited to access network considerations, and it is also desirable to provide a solution that is as simple as possible. [Means for solving the problem]

[0014] To this end, a coherent optical receiver for receiving an amplitude shift keying modulated optical signal from a coherent optical transmitter includes a local oscillator and an ellipse having a major axis orientation φ l and the elliptical phase shift ψ l a polarization diversity actuator configured to change an optical signal output by a local oscillator to form an optical signal having an elliptical polarization of φ; a 2×2 coupler that receives at one input an amplitude shift keying modulated optical signal received from a coherent optical transmitter and at the other input another optical signal output by the set of local oscillator and polarization diversity actuator so as to enable the local oscillator to provide a boost effect to the amplitude shift keying modulated optical signal received from the coherent optical transmitter; and two photodiodes, one of which is connected to one output of the 2×2 coupler and the other of which is connected to the other output of the 2×2 coupler. The coherent optical receiver is configured to additively combine contributions from both photodiodes and generate an electrical signal representing the combined contribution. The coherent optical receiver monitors the electrical signal and, if the electrical signal is below a predetermined threshold corresponding to a predetermined percentage of the theoretical maximum achievable by the boost effect, determines that the elliptical major axis orientation φ l and / or elliptical phase shift ψ land a control unit in the form of electronic circuitry configured to command a configuration change of the polarization diversity actuator by changing the local oscillator and polarization diversity actuator. Hence, no complex digital signal processing or optical phase-locked loop is used, providing a cost-effective solution for access network considerations. Furthermore, using both output branches of the 2x2 coupler as described above makes it possible to limit situations in which the photocurrent processed by the electronic circuitry vanishes towards zero, thereby further reducing the need to change at least one elliptical parameter of the optical signal output by the set of local oscillators and polarization diversity actuators.

[0015] In certain embodiments, the ellipse major axis orientation φ l The modification of is performed by applying a predefined shift equal to π / 4, resulting in an elliptical phase shift ψ l The modification of is performed by applying a predefined shift equal to π / 2, so that restoring the effective boost effect is easily achieved.

[0016] In certain embodiments, the polarization diversity actuator has an elliptical major axis orientation φ l and allows for the modification of the elliptical phase shift ψ l The present invention is provided with a variable wave plate that allows for the change of the wave length. Therefore, configuration changes are easily implemented with low complexity and cost.

[0017] In a particular embodiment, the two photodiodes are configured as balanced photodiodes between a positive voltage source and a negative voltage source, and the coherent optical receiver is configured such that the difference in photocurrents generated by the two photodiodes is sent to a transimpedance amplifier that generates an output voltage proportional to the difference, forming an electrical signal that is monitored by the control unit. Therefore, in this embodiment, inherent photodiode noise and dark current are easily removed.

[0018] In another specific embodiment, each of the two photodiodes is followed by a DC current filter and then a transimpedance amplifier. The transimpedance amplifier is followed by a squaring module that squares the voltage electrical signal output by the transimpedance amplifier. The coherent optical receiver further includes an adder that sums the voltage electrical signals output by the transimpedance amplifier to form an electrical signal that is monitored by the control unit. Therefore, a low-complexity design is achieved (e.g., matched photodiodes are not required).

[0019] In yet another specific embodiment, each of the two photodiodes is followed by a DC current filter and then a squaring module that squares the current electrical signal output by the DC current filter. The coherent optical receiver further includes an adder that sums the current electrical signals output by the squaring modules. The adder is followed by a transimpedance amplifier (e.g., a general transimpedance amplifier) ​​to form an electrical signal that is monitored by the control unit. Therefore, a low-complexity design is achieved.

[0020] In certain embodiments, the control unit controls the ellipse major axis orientation φ l and / or elliptical phase shift ψ l The coherent optical receiver may be further configured to change the gain of the local oscillator and / or change the gain of the transimpedance amplifier before applying the configuration change by selecting at least one configuration change from the changes in the first and second configuration changes. Therefore, the responsiveness of the coherent optical receiver is improved.

[0021] In a particular embodiment, the control unit checks whether the average level of the voltage electrical signal output by the transimpedance amplifier over the integration or averaging time is suitable, i.e., above a predefined threshold percentage and below an upper predefined threshold percentage so as to avoid saturation; if the average level is not suitable in that a signal average level reduction is required, then as a first priority, the signal average level is reduced by adjusting the gain of the transimpedance amplifier and as a second priority, the signal average level is reduced by adjusting the gain of the local oscillator; if the average level is not suitable in that a signal average level increase is required, then checks whether there is still a margin to increase the gain of the transimpedance amplifier or to increase the gain of the local oscillator; if such a margin exists, then as a first priority, the signal average level is increased by adjusting the gain of the transimpedance amplifier and as a second priority, the signal average level is increased by adjusting the gain of the local oscillator; if not, then the control unit checks whether there is still a margin to increase the signal average level by adjusting the gain of the transimpedance amplifier and as a second priority, the signal average level is reduced by adjusting the gain of the local oscillator; l and / or elliptical phase shift ψ l and instructing the configuration change by selecting at least one configuration change from among the changes in the first and second inputs. Therefore, the responsiveness of the coherent optical receiver is further improved.

[0022] In certain embodiments, the control unit controls the ellipse major axis orientation φ l and / or elliptical phase shift ψ l Therefore, the ellipse major axis orientation φ is provided with a look-up table linking the actuation of the change in l and / or elliptical phase shift ψ l Changes are easily implemented.

[0023] In certain embodiments, various configurations of polarization diversity actuators are used to determine the elliptical major axis orientation φ l and / or elliptical phase shift ψ land the look-up table further comprises reactivity information for switching from any current configuration of the polarization diversity actuator to any other target configuration of the polarization diversity actuator, and the control unit is configured to select a configuration of the polarization diversity actuator that implies the shortest adjustment time in view of the current configuration of the polarization diversity actuator when changing the configuration of the polarization diversity actuator, thus improving the reactivity of the polarization diversity actuator.

[0024] It is further proposed an optical network unit for use in a passive optical network, the optical network unit comprising the coherent optical receiver disclosed above in any one of its embodiments, 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, a low complexity and cost optical network unit solution is provided for a passive optical network infrastructure.

[0025] The characteristics of the invention will emerge more clearly from a reading of the following description of at least one example of embodiment, the said description being made with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0026] [Figure 1A] 1 is a schematic diagram of the architecture of an optical communication system of the passive optical network type in which the present invention can be implemented; [Figure 1B] 1 is a schematic diagram of the architecture of an optical communication system of point-to-point type in which the present invention can be implemented; [Figure 2] 1C is a schematic diagram of a coherent optical receiver arrangement that can be used in the optical communication system of FIG. 1A or FIG. 1B. [Figure 3] FIG. 3 is a schematic diagram of a mechanism for a polarization diversity actuator of the coherent optical receiver of FIG. 2. [Figure 4]3 is a schematic diagram of the organization of a value decision block suitable for the coherent optical receiver of FIG. 2; [Figure 5] 3 is a schematic diagram of an algorithm executed by the control unit of the coherent optical receiver of FIG. 2 to adjust the configuration of the coherent optical receiver; [Figure 6] 4 is a schematic diagram of an algorithm executed by a control unit to adjust the configuration of a coherent optical receiver in a particular embodiment; [Figure 7] 1C is a schematic diagram of another coherent optical receiver arrangement that can be used in the optical communication system of FIG. 1A or FIG. 1B. [Figure 8] 8 is a schematic diagram of another arrangement of a value decision block suitable for the coherent optical receiver arrangement of FIG. 7. [Figure 9] 1C is a schematic diagram of yet another coherent optical receiver arrangement that can be used in the optical communication system of FIG. 1A or FIG. 1B. DETAILED DESCRIPTION OF THE INVENTION

[0027] It should be noted that wavelength and frequency are linked to each other through a direct inverse relationship, and the two terms are used interchangeably by those skilled in the art as they refer to the same concept.

[0028] The present invention relates to a local oscillator and a method for modulating an optical signal output by the local oscillator to obtain an elliptical major axis orientation φ. l and the elliptical phase shift ψ land a polarization diversity actuator configured to form an optical signal having an elliptical polarization of φ. The coherent optical receiver further includes a 2x2 coupler and two photodiodes connected to an output of the 2x2 coupler, one of the photodiodes connected to one output of the 2x2 coupler and the other of the photodiodes connected to the other output of the 2x2 coupler. The coherent optical receiver is configured to additively combine contributions from both photodiodes and generate an electrical signal representative of the combined contribution. The coherent optical receiver monitors the electrical signal and, if the electrical signal is equal to or less than a predetermined threshold corresponding to a predetermined percentage of the theoretical maximum achievable by the boost effect, indicates that the elliptical major axis orientation φ l and / or elliptical phase shift ψ l and a control unit in the form of electronic circuitry configured to command a configuration change of the polarization diversity actuator by modifying . Several variants of mechanisms are disclosed hereinafter for additively combining the contributions from both photodiodes to generate an electrical signal that is monitored by the control unit.

[0029] The present invention can be applied to implement a low-complexity coherent optical receiver intended to receive and detect an optical signal transmitted by a coherent optical transmitter. More specifically, the coherent optical receiver is suitable for receiving an amplitude shift keying (ASK) modulated optical signal. For example, the amplitude shift keying (ASK) modulated optical signal is a non-return-to-zero on-off keying (NRZ-OOK) modulated optical signal. Figures 1A and 1B, described below, provide an example of a context in which the present invention can be useful.

[0030] FIG. 1A shows a schematic representation of the architecture of an optical communication system 100 of the passive optical network type in which the present invention can be implemented.

[0031] The optical communication system 100 in FIG. 1A includes a master device 110, typically an OLT (Optical Line Terminal) device, and a plurality of slave devices 141, 142, 143, typically ONU (Optical Network Unit) devices.

[0032] The optical communication system 100 in FIG. 1A may further include at least one spectral splitter device 130 and / or at least one power splitter.

[0033] Each spectrum splitter device 130 includes a pair of optical bandpass filter sets per PON, intended to filter a respective wavelength band, thus enabling the spectrum splitter device 130 to perform wavelength division multiplexing (WDM). Note that instead of using a spectrum splitter device 130, an equivalent mechanism can be obtained by applying a filtering film to the receiving diode.

[0034] Each power splitter allows for an increase in the number of slave devices that can be connected to the master device 110 by dividing the input signal power by the number of outputs going to the slave devices connected to the master device 110. Therefore, each output of the power splitter device transmits the same optical information that it received as the input signal. The power splitter device only affects the signal power.

[0035] The slave devices 141, 142, 143 are interconnected to the master device 110 using optical fibers 120 via at least one spectral splitter device 130 and / or at least one power splitter.

[0036] In the context of a PON, the ONUs are typically intended to be located at end-user premises for FTTH services, and the OLT enables the ONUs to access a metropolitan or core network (not shown). Such PONs may also be used for mobile network infrastructure services.

[0037] FIG. 1B shows a schematic representation of the architecture of an optical communication system of the point-to-point type in which the present invention can be implemented.

[0038] 1B includes a master device 110 and a slave device 140. The master device 110 and the slave device 140 are interconnected using an optical fiber 120.

[0039] In a mobile network fronthauling infrastructure, the slave device 140 is typically located at a remote radio head location and the master device 110 is located at a baseband unit (BBU) location. Such a mechanism may also be used in a mobile network backhauling infrastructure between a BBU and a core network access terminal.

[0040] In both the arrangements of Figures 1A and 1B, optical transmission of information from master device 110 to one or more slave devices is referred to as downlink transmission, and transmission in the reverse direction is referred to as uplink transmission.

[0041] 1A and 1B, it is desirable to install low-complexity, cost-effective optical receivers in at least the slave devices 140, 141, 142, 143, and potentially in the master device 110. Furthermore, to cope with potentially low signal strengths, it is desirable for the optical receivers to be highly sensitive devices.

[0042] Figure 2 shows a schematic representation of a coherent optical receiver arrangement that can be used in the optical communication system 100 of Figure 1A or 1B. The coherent optical receiver 200 outputs a signal 202 that is to be processed for demodulation.

[0043] 2, the coherent optical receiver 200 includes a 2x2 coupler 220. For example, the 2x2 coupler 220 is a fiber fused coupler or a directional coupler.

[0044] An optical signal transmitted by a coherent optical transmitter is input to one input 221 of the 2x2 coupler 220. Another optical signal output by a set of a local oscillator LO 210 and a polarization diversity actuator PDA 211 is input to the other input 222 of the 2x2 coupler 220.

[0045] The local oscillator LO 210 is a laser diode or an arrangement comprising a laser diode. A vertical cavity surface emitting laser (VCSEL) may also be used as the local oscillator.

[0046] The optical signal output by the 2×2 coupler 220 as output 223 is captured by the photodiode 230 a of the coherent optical receiver 200 .

[0047] The optical signal output by the 2x2 coupler 220 as output 224 is captured by a photodiode 230b of the coherent optical receiver 200, which is identical to the photodiode 230a. The photodiodes 230a and 230b are matched photodiodes (the same semiconductor chip fabricated according to the same process).

[0048] Photodiodes 230a and 230b are arranged as balanced photodiodes, i.e., for balanced photodetection (also known as differential photodetection) between a positive voltage source V+ and a negative voltage source V−.

[0049] At point P in FIG. 2, the difference in the photocurrents generated by photodiodes 230a and 230b is captured by transimpedance amplifier TIA 245, which generates an output voltage proportional to the difference.

[0050] Any asymmetry in the electronic components and optical path must be avoided. Since high frequency detection is the goal, matched optical fiber lengths must be used between the outputs 223 and 224 of the 2x2 coupler 220, as this would otherwise introduce an unexpected phase shift and affect the sensitivity. Also, the 2x2 coupler 220

number

[0051] In general, a polarized optical signal E(t) can be written in the Jones representation as follows:

number

number

[0052] Therefore, considering an amplitude shift keying optical signal, photodiode 230a generates a current i, which can be expressed as: a Output (t).

number

[0053] Similarly, photodiode 230a generates a current i which can be expressed as: b Output (t).

number

[0054] Therefore, the photocurrent signal output by the photodiode 230b (second branch) is a so-called mixing detuning term (i a (t) and i b (t)) differs from the photocurrent signal output by the photodiode 230a (first branch) by a phase shift of π in the bottom two rows of each of the above expressions.

[0055] Here, the optical signal output by the set of local oscillator LO 210 and polarization diversity actuator PDA 211 is not modulated for information transmission, but rather has an amplitude E 212 to help improve optical detection dynamics in some embodiments detailed below. 0l It should be understood that the signal amplitude E 0l This means that the variability does not contain significant information itself.

[0056] At point P, the balanced photodetection jointly performed by photodiodes 230a and 230b generates a current i P (t). This current is the photocurrent i output by the photodiode 230b. b The photocurrent i output by the photodiode 230a from (t) a (t) minus the value, and can therefore be expressed as:

number

[0057] Therefore, common-mode rejection is performed by the balanced photodetection, which removes the direct current (DC) component of the photocurrents output by photodiode 230a and photodiode 230b, as well as the inherent photodiode noise and dark current.

[0058] E 0l =E 0s , ω l =ω s , φ l =φ s , and

number

[0059] As a result, the local oscillator LO 210 can boost the optical signal coming from the optical fiber, unless the following conditions are met:

number

[0060] Therefore, except for these situations, the local oscillator LO 210 makes it possible to increase the sensitivity of the coherent optical receiver, and in particular to improve the splitting ratio across the PON and increase the transmission data rate.

[0061] To avoid the above situation, the polarization diversity actuator PDA 211 is controlled to modify the optical signal as required when output by the local oscillator LO 210. In other words, by controlling the polarization diversity actuator PDA 211, it is possible to avoid the above situation when it is encountered. Even a small change in the polarization state of the signal input to the input 222 of the 2x2 coupler 220 will create a sufficient difference such that at least one of the following terms, called boosting terms, is not equal to zero:

number

number

[0062] To control the polarization diversity actuator PDA 211, the coherent optical receiver 200 further comprises a control unit 280 in the form of electronic circuitry. The control unit 280 is present at the output of the transimpedance amplifier TIA 245 and is looped back to the polarization diversity actuator PDA 211 via line 261. The control unit 280 also controls the local oscillator LO 210 and may therefore be looped back to the local oscillator LO 210 via at least one line, for example line 271 and line 291. The control unit 280 is preferably looped back to the transimpedance amplifier TIA 245 for electrical gain control via line 251.

[0063] The electronic circuitry forming the control unit 280 may be a chip or a chipset, such as an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit). In a variant, the electronic circuitry forming the control unit 280 may be a processor (microcontroller, DSP (Digital Signal Processor)...) associated with at least one memory. After power-on, the processor is able to read instructions from the memory and execute these instructions to implement the functions of the control unit 280. In this case, the functions of the control unit 280 are in software form, which may be stored on a non-transitory storage medium, such as an SD (Secure Digital) card.

[0064] It should be understood that the control unit 280 does not perform polarization state tracking. In the present case, it is not necessary to accurately know the polarization state of the optical signal received from the coherent optical transmitter, nor the polarization state of the optical signal output by the set of local oscillator LO 210 and polarization diversity actuator PDA 211. Within the scope of the present invention, the control unit 280 determines whether the polarization state of the set of local oscillator LO 210 and polarization diversity actuator PDA 211 is suitable for obtaining an operable boost term. If not suitable (without knowing which relevant state of polarization caused such a situation), the control unit 280 changes the polarization state of the optical signal output by the set of local oscillator LO 210 and polarization diversity actuator PDA 211. Therefore, no optical phase-locked loop is required, and no complex digital signal processing is required, thereby minimizing design complexity. Actionable boost terms are boost terms that are greater than or equal to a predefined threshold percentage (e.g., 20%) of their theoretical maximum value, which is the maximum value of the intensity of the optical signal output by the local oscillator LO 210.

[0065] 6, the control unit 280 may alternatively, as a temporary or supplemental action, vary the intensity (optical gain) of the optical signal output by the local oscillator LO 210. Under some circumstances, as disclosed hereinafter with respect to FIG. 6, the control unit 280 may alternatively, as a temporary or supplemental action, vary the electrical gain of the transimpedance amplifier TIA 245.

[0066] Therefore, the function of the control unit 280 is to control the electrical signal i while the optical signal received from the coherent optical transmitter contains useful information. P(t) does not disappear and become zero, and therefore the boost effect that is supposed to be provided by the local oscillator LO 210 is maintained as much as possible above the predefined threshold percentage mentioned above. This means that as long as the condition for the predefined threshold percentage mentioned above is met, even if a degradation of the boost effect is detected by the control unit 280, there may be no need to adjust the boost effect.

[0067] Therefore, the control unit 280 does not have to run continuously, among other things, but operates at intervals, for example periodically. As will be explained below, the control unit 280 adjusts the optical signals output by the set of local oscillator LO 210 and polarization diversity actuator PDA 211 so that the electrical signal i P Some time will pass before a new situation arises where there is a risk that (t) will disappear and become zero, since transmission over long-distance optical fiber implies slow changes in polarization state. The control unit 280 can take into account the evolution history of the electrical signal output by the transimpedance amplifier TIA 245 to determine when to activate next. The control unit 280 can calculate the evolution history of the electrical signal i relative to the predetermined threshold percentage mentioned above. P In other words, the control unit 280 controls the standby time to be proportional to the distance (t). P If the risk of the evolution of (t) disappearing and becoming zero is far from zero, the standby time can be increased, and the electrical signal i P The standby time can be reduced if the deployment of (t) is in the reverse situation.

[0068] When the control unit 280 detects that the electrical signal output by the transimpedance amplifier TIA 245 is below a predetermined threshold, the control unit 280 controls the polarization diversity actuator PDA 211 to change the polarization state of the optical signal output by the set of the local oscillator LO 210 and the polarization diversity actuator PDA 211. As a result, a boost effect is obtained in cases where the optical signal received from the coherent optical transmitter contains useful information, because the sensitive change of the polarization state can prevent the electrical signal i from being output even if the optical signal received from the coherent optical transmitter contains useful information. P This allows us to avoid encountering a situation where (t) vanishes to zero. In the worst case, it can be demonstrated that as soon as the polarization states of both optical signals entering the 2×2 coupler 220 shift by an amount that is a small fraction (e.g., 5%) of the distance between adjacent optically associated states of polarization, the contribution associated with the boost term becomes larger than the contribution of the optical signal received from the coherent optical transmitter.

[0069] After traveling several kilometers in an optical fiber, the polarization state of the optical signal initially transmitted by the coherent optical transmitter is randomly distributed on the Poincaré sphere. Therefore, the polarization state of the optical signal when received by the coherent optical receiver 200 is very likely to be elliptically shaped. Therefore, the optical signal output by the set of the local oscillator LO 210 and the polarization diversity actuator PDA 211 is controlled to be elliptically polarized.

[0070] Even if an optical signal containing useful information is received from a coherent optical transmitter, the electrical signal i P When (t) falls into the zero region, the control unit 280 controls the polarization diversity actuator PDA 211 to change the polarization state of the optical signal output by the set of the local oscillator LO 210 and the polarization diversity actuator PDA 211 by adjusting the angle φ of the orientation of the major axis of the polarization ellipse of the optical signal output by the set of the local oscillator LO 210 and the polarization diversity actuator PDA 211.l and the elliptical phase shift ψ l , which allows the boost term to be activated and the sensitive photocurrent to be recovered. In a particular embodiment, the control unit 280 instructs the polarization diversity actuator PDA 211 to change the angle φ of the orientation of the major axis of the polarization ellipse of the optical signal output by the set of the local oscillator LO 210 and the polarization diversity actuator PDA 211. l and the elliptical phase shift ψ l to change at least one ellipse parameter from among the ellipse parameters by a predefined shift.

[0071] In a preferred embodiment, the predefined shift targets a stationary extremum of the electrical signal i(t). In this preferred embodiment, the predefined shift is l (and therefore Δφ) for π / 4, and the elliptical phase shift ψ l (and therefore Δψ) when π / 2.

[0072] In a particular embodiment, the control unit 280 comprises a value decision block VDB 250 and a polarization diversity manager PDM 260. In a more particular embodiment, the control unit 280 further comprises a signal strength manager SSM 290. In an even more particular embodiment, the control unit 280 further comprises a detuning manager DM 270.

[0073] The value decision block VDB 250 analyzes the electrical signal output by the transimpedance amplifier TIA 245. Therefore, the value decision block VDB 250 analyzes the voltage output by the transimpedance amplifier TIA 245. To do this, the value decision block VDB 250 compares the value of the voltage output by the transimpedance amplifier TIA 245 with the theoretical maximum value achievable by the boost effect, i.e., a predefined threshold percentage (e.g., 20%) of the theoretical maximum value achievable by the boost term. The theoretical maximum value is defined according to the gain of the local oscillator LO 210, the conversion rate R of incident photons to electrons by the photodiodes 230a and 230b, and the gain of the transimpedance amplifier TIA 245. If the voltage output by the transimpedance amplifier TIA 245 falls below a predefined threshold percentage of the theoretical maximum achievable by the boost term, the value decision block VDB 250 functions to change the configuration of the polarization diversity actuator PDA 211 and / or the local oscillator LO 210. According to the present invention, the value decision block VDB 250 determines the angle φ of the orientation of the major axis of the polarization ellipse of the optical signal output by the set of local oscillator LO 210 and polarization diversity actuator PDA 211. l and the elliptical phase shift ψ l , operative to change the configuration of the polarization diversity actuator PDA 211 to change at least one ellipse parameter from among

[0074] Under some circumstances, the value decision block VDB 250 may alternatively or supplementarily modify the intensity of the optical signal output by the local oscillator LO 210 (i.e., modify the gain of the local oscillator LO 210) and / or modify the gain of the transimpedance amplifier TIA 245. Thus, the value decision block VDB 250 improves the signal-to-noise ratio of the electrical signal to be used to perform demodulation.

[0075] The value decision block VDB 250 can take into account the evolution history of the electrical signal output by the transimpedance amplifier TIA 245, as already explained.

[0076] The value decision block VDB 250 can further perform a detuning analysis function by detecting a beat signal over a predefined number of modulation symbols at a frequency equal to the wavelength detuning, i.e., the deviation between the wavelength of the optical signal received from the coherent optical transmitter and the wavelength of the optical signal output by the set of local oscillator LO 210 and polarization diversity actuator PDA 211. Beat signal detection can be performed by fast Fourier transform and spectral analysis to identify a peak at the beat frequency corresponding to the wavelength detuning Δν, as known to those skilled in the art. Another approach is to perform beat signal detection by analog convolution with a cosine signal, for example, as known to those skilled in the art. In the case of detuning, the value decision block VDB 250 then functions to adjust the wavelength of the optical signal output by the local oscillator LO 210 according to the detected beat frequency. The value decision block VDB 250 then instructs the detuning manager DM 270 to change the wavelength configuration of the local oscillator LO 210 accordingly.

[0077] The polarization diversity manager PDM 260 controls the polarization diversity actuator PDA 211 via line 261. The polarization diversity manager PDM 260 receives from the value decision block VDB 250 via line 252 the angle φ of the orientation of the major axis of the polarization ellipse of the optical signal output by the set of local oscillator LO 210 and polarization diversity actuator PDA 211. l and the elliptical phase shift ψ linto appropriate commands (e.g., drive voltage changes) suitable for the polarization diversity actuator PDA 211. As a result, the polarization diversity manager PDM 260 controls the polarization diversity actuator PDA 211 via line 261.

[0078] Signal strength manager SSM 290 controls local oscillator LO 210 via line 291. Signal strength manager SSM 290 converts instructions to change the strength of the signal output by local oscillator LO 210, received via line 253 from value decision block VDB 250, into appropriate commands suitable for local oscillator LO 210. As a result, signal strength manager SSM 290 controls local oscillator LO 210 via line 291. For example, signal strength manager SSM 290 adjusts the input current incident on local oscillator LO 210 if it is a Directly Modulated Laser (DML) or adjusts the drive voltage signature if oscillator LO 210 is an Electro-absorption Modulated Laser (EML).

[0079] The detuning manager DM 270 controls the local oscillator LO 210 via line 271. The detuning manager DM 270 converts instructions to change the wavelength of the optical signal output by the local oscillator LO 210, received via line 254 from the value decision block VDB 250, into appropriate commands suitable for the local oscillator LO 210. As a result, the detuning manager DM 270 controls the local oscillator LO 210 via line 271. For example, the detuning manager DM 270 adjusts the Peltier current if the local oscillator LO 210 comprises a Peltier module, or adjusts the drive voltage if the local oscillator LO 210 comprises an optical ring resonator.

[0080] Additionally, the value decision block VDB 250 can provide instructions to the transimpedance amplifier 245 via line 251 to change the electrical gain of the transimpedance amplifier TIA 245 .

[0081] Figure 3 schematically illustrates a particular embodiment of the polarization diversity actuator PDA 211. The particular embodiment of Figure 3 is suitable for use with a local oscillator that provides a linearly polarized optical signal, as commonly found off-the-shelf. However, the particular embodiment of Figure 3 is also suitable for use with a local oscillator that provides a circularly or elliptically polarized signal.

[0082] Low-cost lasers currently used in network access applications are typically linearly polarized semiconductor lasers. Such lasers can be used as the local oscillator LO 210. A method for converting linear polarization to elliptical polarization is to place a waveplate (also called a wavelength plate) at the output of the laser. A waveplate is based on a birefringent material with two orthogonal axes of different velocities, which induces a phase shift between the projection of a given field on the two axes of the birefringent material. Inputting a linearly polarized optical signal into the waveplate results in an elliptically polarized optical signal being output. However, it can be considered that the waveplate couples both the polarization ellipse and the phase shift. This means that the phase shift between the optical signal output by the local oscillator LO 210 and the optical signal received from the coherent optical transmitter can be similarly affected by the waveplate. Therefore, in cases where the ellipse must be independently controlled using a waveplate, a compensating phase shift can be applied to both axes of the optical signal output by the local oscillator LO 210 to counteract the waveplate phase shift thus introduced.

[0083] A variable waveplate can be used to control the ellipse of the optical signal output by the set of local oscillator LO 210 and polarization diversity actuator PDA 211. A variable waveplate is a specific waveplate that can control its phase shift between the slow (exceptional) axis and the fast (normal) axis. Liquid crystal variable retarders and pocket cells are examples of variable waveplates. Alternatively, a tunable highly birefringent photonic liquid crystal fiber can be used. The retardance is then adjusted over a specified range by changing the driving voltage.

[0084] According to the embodiment shown in Figure 3, the optical signal output from the local oscillator LO 210 is preferably input to a variable delay line VDL 301. The variable delay line VDL 301 may be constructed from an isotropic material. The variable delay line VDL 301 introduces an adjustable delay to the overall optical signal, which affects the phase of the optical signal. Here, the variable delay line VDL 301 controls the phase Φ of the optical signal output by the local oscillator LO 210. l For example, the variable delay line VDL 301 is a liquid crystal variable retarder whose delay parameter ΔL is in the working range [0;2π] and is aligned in the factory with the polarization of the linearly polarized laser that forms the local oscillator LO 210.

[0085] At the output of the variable delay line VDL 301 (if present), the polarization diversity actuator PDA 211 further comprises a variable wave plate VW 302. The variable wave plate VW 302 here has a phase difference ψ l As a result, ψ l and ψ s As already mentioned, the variable wave plate VW 302 can further affect the phase shift. In this case, the variable delay line VDL 301 and the variable wave plate VW 302 collectively reduce the difference between Φ l As a result, Φ l and Φs can be used to reduce the difference between

[0086] The polarization diversity actuator PDA 211 may further comprise a controller CTRL 303 configured to interpret instructions received from the control unit 280, more particularly from the polarization diversity manager PDM 260, and to convert these instructions into commands (such as appropriate drive voltages) for the variable delay line VDL 301 and the variable wave plate VW 302.

[0087] In one variant, the control unit 280, more particularly the polarization diversity manager PDM 260, sends commands (such as appropriate drive voltages) directly to the variable delay line VDL 301 and the variable wave plate VW 302. In this case, the control unit 280, more particularly the polarization diversity manager PDM 260, on the one hand controls φ l (and therefore Δφ) and / or ψ l 3. The polarization diversity actuator PDA 211 may be provided with a look-up table LUT linking the actuation of the change in φ (and therefore Δψ) on the one hand and the commands to be applied to the polarization diversity actuator PDA 211, more specifically to the variable delay line VDL 301 and the variable wave plate VW 302 on the other hand. The various configurations of the polarization diversity actuator PDA 211, i.e. the variable wave plate VW 302 in the mechanism of FIG. 3, are l and / or ψ lcan bring about the same change. Thus, in certain embodiments, the look-up table LUT further includes reactivity information for switching from any current configuration of the polarization diversity actuator PDA 211 to any other target configuration of the polarization diversity actuator PDA 211. More specifically, the look-up table LUT further includes reactivity information for switching from any current configuration of the variable wave plate VW 302 to any other target configuration of the variable wave plate VW 302. The look-up table LUT may further include reactivity information for switching from any current configuration of the variable delay line VDL 301 to any other target configuration of the variable delay line VDL 301. As a result, the control unit 280, more specifically the polarization diversity manager PDM 260, φ l and / or ψ l When a change in must be applied, the polarization diversity actuator PDA 211, i.e., the polarization diversity actuator PDA 211 that implies the shortest adjustment time in view of the current configuration of the variable wave plate VW 302, i.e., the configuration of the variable wave plate VW 302, is selected. Moreover, this knowledge of the adjustment time of the variable wave plate VW 302 and optionally the variable delay line VDL 301 can be used by the control unit 280 to determine at any point in time whether the adjustment is effectively valid.

[0088] FIG. 4 schematically represents a particular embodiment of the value determination block VDB 250 suitable for the mechanism of the coherent optical receiver 200 shown in FIG. 2.

[0089] The value determination block VDB 250 comprises a level detector LD 402. The level detector LD 402 detects the voltage output by the transimpedance amplifier TIA 245 and performs a conversion to digital data that is more easily handled for further analysis.

[0090] The value decision block VDB 250 further comprises an analyzer AN 403. The analyzer AN 403 receives digital data from the level detector LD 402. The analyzer AN 403 compares the value of the voltage output by the transimpedance amplifier TIA 245 with a predefined threshold percentage (e.g., 20%) of the theoretical maximum value achievable by the boost term. The analyzer AN 403 then makes a decision as to whether at least one action must be selected to ensure that the actionable boost term is met.

[0091] The value decision block VDB 250 further comprises a manager MGR 404 which receives instructions regarding the action to be taken from the analyzer AN 403. The manager MGR 404 consequently provides polarization diversity related instructions to the polarization diversity manager PDM 260 via line 252, and / or signal strength instructions to the signal strength manager SSM 290 via line 253, and / or electrical gain control instructions to the transimpedance amplifier 245 via line 251, and / or detuning related instructions to the detuning manager DM 270 via line 254.

[0092] The value decision block VDB 250 further comprises a squaring module SQ 401. The squaring module SQ 401 squares the electrical signal received from the transimpedance amplifier TIA 245 (i.e., the squaring module SQ 401 multiplies the input electrical signal by itself). The squaring module SQ 401 simplifies the analysis performed by the analyzer AN 403 by ensuring that only positive values ​​are considered (which is easier to compare the absolute electrical signal to a threshold). To do this, the squaring module SQ 401 is, for example, an analog amplifier whose both inputs receive the electrical signal received at the input of the value decision block VDB 250. The use of the squaring module SQ 401 further advantageously enhances the contrast of the boost term for the optical signal received from the coherent optical transmitter.

[0093] The value decision block VDB 250 outputs signals 202 to be processed for demodulation. These signals may be the outputs of the squaring module SQ 401. These signals may simply be the outputs of the transimpedance amplifier TIA 245.

[0094] 5 shows a schematic representation of the algorithm executed by the control unit 280 to configure the coherent optical receiver 200. The control unit 280 continuously receives an analog electrical signal from the transimpedance amplifier TIA 245.

[0095] In step S501, the control unit 280 performs signal level detection on the electrical signal output by the transimpedance amplifier TIA 245, as already explained above. Therefore, the control unit 280 monitors the electrical signal output by the transimpedance amplifier TIA 245. Step S501 outputs a digital representation of the analog electrical signal (voltage) output by the transimpedance amplifier TIA 245 according to a sampling rate. The level detection preferably includes integration or averaging over a predetermined number of samples.

[0096] In step S502, the control unit 280 performs a comparison of the level detected in step S501 with a predefined threshold percentage (e.g., 20%) of the theoretical maximum signal level achievable by the boost term. As already mentioned, the theoretical maximum is defined according to the gain of the local oscillator LO 210, the conversion rate R of incident photons to electrons by the photodiode 230, and the gain of the transimpedance amplifier TIA 245. The conversion rate R of incident photons to electrons by the photodiode 230 is known to the control unit 280 by pre-configuration (e.g., at the factory). If the gain of the local oscillator LO 210 is not defined (i.e., controlled) by the control unit 280, the gain of the local oscillator LO 210 is known to the control unit 280 by pre-configuration (e.g., at the factory). If the gain of the transimpedance amplifier TIA 245 is not defined (i.e., controlled) by the control unit 280, the gain of the transimpedance amplifier TIA 245 is known to the control unit 280 by pre-configuration (e.g., at the factory). Alternatively, the theoretical maximum signal level achievable by the boost term is known to the control unit 280 by pre-configuration (e.g., at the factory). Still alternatively, the applicable threshold is known to the control unit 280 by pre-configuration (e.g., at the factory). In this latter case, the pre-configured applicable threshold maintains a pre-defined threshold percentage (e.g., 20%) of the theoretical maximum signal level achievable by the boost term, except that the effective value of the pre-defined threshold percentage (e.g., 20%) is not dynamically determined by the control unit 280. The pre-defined threshold percentage may change over time. For example, the control unit 280 is instructed by the application layer of the optical coherent receiver 200 or the demodulator of the optical coherent receiver 200 that this change over time means that the signal-to-noise ratio of the electrical signal should be enhanced or otherwise degraded.

[0097] In step S503, the control unit 280 checks whether the comparison indicates that the level detected in step S501 is greater than or equal to the threshold. If the level detected in step S501 is lower than the threshold, step S504 is executed. If the level detected in step S501 is greater than or equal to the threshold, step S501 is repeated. A waiting or standby time can be inserted before executing step S501 again. The duration of the waiting or standby time is shorter than the time required for the polarization of the carried optical signal to change by a predefined percentage in polarization space, as observed for the optical fiber. For example, the duration of the waiting or standby time is 100 milliseconds.

[0098] In step S504, the control unit 280 commands a reconfiguration of the set of local oscillators LO 210 and polarization diversity actuators PDA 211. The reconfiguration is performed by the following set Li: ellipse major axis orientations φ l and / or elliptical phase shift ψ l The configuration change is at least one selected from the changes described above.

[0099] Step S501 is then repeated, with preferably no waiting time applied at this point for the control unit 280 to determine whether another phase shift modification and / or ellipse modification of the optical signal output by the set of local oscillator LO 210 and polarization diversity actuator PDA 211 has to be performed.

[0100] 6 schematically illustrates an algorithm executed by the control unit 280 to adjust the configuration of the coherent optical receiver 200 in a particular embodiment. In this particular embodiment, if the control unit 280 determines that level detection for the electrical signal output by the transimpedance amplifier TIA 245 is insufficient, the control unit 280 adjusts the configuration of the coherent optical receiver 200 by changing the power of the optical signal output by the local oscillator LO 210 and / or changing the gain of the transimpedance amplifier TIA 245 before deciding to apply the configuration change by selecting at least one configuration change from the set Li. Indeed, in most schemes using off-the-shelf components, changes in the elliptic parameters are less responsive (i.e., take longer) than changes in the oscillator signal power, which in turn are even less responsive than changes in the transimpedance amplifier gain (EGC, electrical gain control).

[0101] In step S601, the control unit 280 performs signal level detection and integration or averaging over a predefined number of samples, as previously described. Over time, the optical signal received from the coherent optical transmitter is substantially distributed over a dynamic range, so the signal average level represents the dynamic range.

[0102] In step S602, the control unit 280 checks whether a suitable average level is met. In other words, the control unit 280 checks whether the average level of the signal over the integration or averaging time is above a predefined threshold percentage (e.g., 20%). Furthermore, the control unit 280 checks whether the average level of the signal over the integration or averaging time is below an upper predefined threshold percentage (e.g., 80%) to avoid saturation. If the average level is suitable, step S601 is repeated, preferably along with applying a standby time period; if not, step S603 is executed.

[0103] In step S603, the control unit 280 checks whether a signal average level increase is required. If a signal average level increase is required, step S605 is executed; if not, a signal average level decrease is required, and step S604 is executed.

[0104] In step S604, the control unit 280 reduces the signal average level by adjusting the gain of the transimpedance amplifier TIA 245 as a first priority, and by adjusting the gain of the local oscillator LO 210 (the intensity of the optical signal output by the local oscillator LO 210) as a second priority. In other words, if there is still a margin to reduce the gain of the transimpedance amplifier TIA 245, the control unit 280 does so; otherwise, the control unit 280 reduces the gain of the local oscillator LO 210. By doing this, the transimpedance amplifier TIA 245 has a higher chance of operating later, which allows it to react more quickly in cases where an increase in the signal average level is required, because the response of adjusting the gain of the transimpedance amplifier TIA 245 is better than the response of adjusting the gain of the local oscillator LO 210. Then, step S601 is repeated, preferably with a waiting or standby time period applied.

[0105] In step S605, the control unit 280 checks whether there is still a margin to increase the gain of the transimpedance amplifier TIA 245 or increase the gain of the local oscillator LO 210. If there is still a margin to increase the gain of the transimpedance amplifier TIA 245 or increase the gain of the local oscillator LO 210, step S607 is executed; otherwise, step S606 is executed.

[0106] In step S606, the control unit 280 commands a configuration change by selecting at least one configuration change from the set Li mentioned above. Since the effective achievement of the configuration change may require some non-negligible time, the control unit 280 may gradually reduce the signal average level by adjusting the gain of the transimpedance amplifier TIA 245 as a first priority, and by adjusting the gain of the local oscillator LO 210 as a second priority, while the configuration of the polarization diversity actuator PDA 211 is gradually changed. Therefore, if the configuration change of the polarization diversity actuator PDA 211 is effectively effective, saturation is avoided. Then, step S601 is repeated, preferably with the application of a waiting or standby time period.

[0107] In step S607, the control unit 280 increases the signal average level by adjusting the gain of the local oscillator LO 210 (the intensity of the optical signal output by the local oscillator LO 210) as a first priority, and increases the signal average level by adjusting the gain of the transimpedance amplifier TIA 245 as a second priority. In other words, if there is still margin to increase the gain of the local oscillator LO 210, the control unit 280 does so; otherwise, the control unit 280 increases the gain of the transimpedance amplifier TIA 245. However, in certain embodiments where the evolution of the signal average level indicates the need for faster adjustment (in terms of responsiveness of changes in the gain of the local oscillator LO 210), the control unit 280 increases the gain of the transimpedance amplifier TIA 245 as a first priority to benefit from better responsiveness. This can also be completed by increasing the gain of the local oscillator LO 210. Step S601 is then repeated, preferably with a waiting or standby time period applied.

[0108] FIG. 7 illustrates schematically another coherent optical receiver arrangement 700 that can be used in the optical communication system of FIG. 1A or FIG. 1B.

[0109] The arrangement in Figure 7 also uses two photodiodes. However, the arrangement in Figure 7 differs from the arrangement in Figure 2 in that the photocurrents provided by each of the photodiodes 230a and 230b are processed independently and then summed for further analysis by the control unit 280. The photodiodes 230a and 230b do not need to be matched photodiodes in this arrangement.

[0110] More precisely, the output of the photodiode 230a is sent to a DC (direct current) filter 240a. The DC filter 240a is a mechanism that removes the continuous component of the analog electrical signal output by the photodiode 230a. The output of the DC filter 240a is sent to a transimpedance amplifier TIA 245a. The output of the transimpedance amplifier TIA 245a is sent to a squaring module SQ 702a. The squaring module SQ 702a squares the electrical signal received from the transimpedance amplifier TIA 245a (i.e., the squaring module SQ 702a multiplies the input electrical signal by itself). To do this, the squaring module SQ 702a is, for example, an analog amplifier whose both inputs receive the electrical signal provided by the transimpedance amplifier TIA 245a.

[0111] Similarly, on the other branch, the output of photodiode 230b is sent to DC filter 240b, which removes the continuous component of the analog electrical signal output by photodiode 230b. The output of DC filter 240b is sent to transimpedance amplifier TIA 245b, which in turn is sent to squaring module SQ 702b, which squares the electrical signal received from transimpedance amplifier TIA 245b in the same manner as squaring module SQ 702a did for the electrical signal received from transimpedance amplifier TIA 245a.

[0112] The electrical signals output by the squaring modules SQ 702a and 702b are then summed by summer 701, with both branches thus providing additive contributions. Therefore, squaring the electrical signals output by the transimpedance amplifiers TIA 245a and 245b prevents the branches from canceling each other. The electrical signals output by summer 701 are then monitored and processed by control unit 280, as already described for the coherent optical receiver arrangement in FIG. 2. However, the value decision block VDB of control unit 280 does not include the squaring module SQ 401 shown in FIG. 4. An arrangement of a value decision block VDB suitable for the coherent optical receiver arrangement of FIG. 7 is shown in FIG. 8 (which is self-explanatory in light of the above disclosure related to FIG. 4).

[0113] With respect to coherent optical receiver 700, control unit 280 can control the electrical gain of transimpedance amplifier TIA 245a and transimpedance amplifier TIA 245b via respective lines 251a and 251b, and similar electrical gain control adjustments for both transimpedance amplifier TIA 245a and TIA 245b are then performed by control unit 280.

[0114] FIG. 9 schematically illustrates yet another coherent optical receiver arrangement 900 that can be used in the optical communication system of FIG. 1A or 1B. The coherent optical receiver arrangement 900 is similar to the coherent optical receiver arrangement 700 shown in FIG. 7. Here, the output of DC filter 240a is sent directly to squaring module SQ 702a, and the output of DC filter 240b is sent directly to squaring module SQ 702b. The electrical signals output by squaring modules SQ 702a and 702b are then summed by summer 701, and the output of summer 701 is sent to a transimpedance amplifier, such as transimpedance amplifier TIA 245. Whereas the summation in the coherent optical receiver arrangement 700 in FIG. 7 is performed in the voltage domain, here the summation is performed in the current domain. Therefore, the electrical signal output by the transimpedance amplifier TIA 245 is then monitored and processed by the control unit 280 as already described for the coherent optical receiver arrangement in FIG.

[0115] As previously mentioned, the control unit 280 can control the electrical gain of the transimpedance amplifier TIA 245 via line 251 .

[0116] 9 arrangement therefore allows for a reduction in bill of materials (BOM) and design complexity by using a single transimpedance amplifier, which is suitable for optical transmission systems where a high signal-to-noise ratio is expected to be achieved.

Claims

1. 1. A coherent optical receiver for receiving an amplitude shift keying modulated optical signal from a coherent optical transmitter, comprising: a local oscillator; Ellipse principal axis orientation φ ｌ and the elliptical phase shift ψ ｌ a polarization diversity actuator configured to modify the optical signal output by the local oscillator to form an optical signal having an elliptical polarization of a 2x2 coupler that receives at one input the amplitude shift keying modulated optical signal received from the coherent optical transmitter and receives at another input another optical signal output by the set of the local oscillator and the polarization diversity actuator, so as to enable the local oscillator to provide a boost effect to the amplitude shift keying modulated optical signal received from the coherent optical transmitter; two photodiodes, one of which is connected to one output of the 2x2 coupler and the other of which is connected to the other output of the 2x2 coupler; Equipped with the coherent optical receiver is configured to additively combine contributions from both photodiodes and generate an electrical signal representative of the combined contribution; The coherent optical receiver monitors the electrical signal and, when the electrical signal is below a predetermined threshold corresponding to a predetermined percentage of the theoretical maximum achievable by the boost effect, adjusts the ellipse major axis orientation φ ｌ and / or the elliptical phase shift ψ ｌ a control unit in the form of electronic circuitry configured to command a configuration change of the polarization diversity actuator by changing

2. The ellipse principal axis orientation φ ｌ The modification of ψ is performed by applying a predefined shift equal to π / 4, ｌ 2. The coherent optical receiver of claim 1, wherein the modification of {overscore (x)} is performed by applying a predefined shift equal to π / 2.

3. The polarization diversity actuator has the ellipse major axis orientation φ ｌ and allows the elliptical phase shift ψ ｌ 3. A coherent optical receiver according to claim 1, comprising a variable waveplate that allows for the change of

4. 4. A coherent optical receiver according to claim 1, wherein the two photodiodes are configured as balanced photodiodes between a positive voltage source and a negative voltage source, and the coherent optical receiver is configured such that a difference in photocurrents generated by the two photodiodes is sent to a transimpedance amplifier that generates an output voltage proportional to the difference forming an electrical signal that is monitored by the control unit.

5. 4. The coherent optical receiver according to claim 1, wherein each of the two photodiodes is followed by a DC current filter and then a transimpedance amplifier, the transimpedance amplifier being followed by a squaring module that squares the voltage electrical signals output by the transimpedance amplifiers, and the coherent optical receiver further comprises an adder that sums the voltage electrical signals output by the transimpedance amplifiers to form an electrical signal that is monitored by the control unit.

6. 4. The coherent optical receiver according to claim 1, wherein each of the two photodiodes is followed by a DC current filter and then a squaring module that squares the current electrical signal output by the DC current filter, and the coherent optical receiver further comprises an adder that sums the current electrical signals output by the squaring modules, and the adder is followed by a transimpedance amplifier to form an electrical signal that is monitored by the control unit.

7. The control unit controls the ellipse major axis orientation φ ｌ and / or the elliptical phase shift ψ ｌ 7. The coherent optical receiver according to claim 4, further configured to change a gain of the local oscillator and / or change a gain of the transimpedance amplifier before applying the configuration change by selecting at least one configuration change from among the changes

8. The control unit checking whether the average level of the voltage electrical signal output by said transimpedance amplifier over an integration or averaging time is adequate, i.e., above a predefined threshold percentage and below an upper predefined threshold percentage so as to avoid saturation; If the average level is not suitable in that a signal average level reduction is required, then reducing the signal average level by adjusting the gain of the transimpedance amplifier as a first priority, and reducing the signal average level by adjusting the gain of the local oscillator as a second priority; If the average level is not suitable in that a signal average level increase is required, check whether there is still a margin to increase the gain of the transimpedance amplifier or increase the gain of the local oscillator, and if such a margin exists, increase the signal average level by adjusting the gain of the transimpedance amplifier as a first priority, and increase the signal average level by adjusting the gain of the local oscillator as a second priority, otherwise, adjust the ellipse major axis orientation φ ｌ and / or the elliptical phase shift ψ ｌ commanding a configuration change by selecting at least one configuration change from among the changes in 8. The coherent optical receiver of claim 7, configured to:

9. The control unit controls the ellipse major axis orientation φ ｌ and / or the elliptical phase shift ψ ｌ 9. A coherent optical receiver according to claim 1, comprising a look-up table linking the actuation of the change of polarization diversity with the commands to be applied to the polarization diversity actuator.

10. The various configurations of the polarization diversity actuators are ｌ and / or the elliptical phase shift ψ ｌ 10. The coherent optical receiver of claim 9, wherein the look-up table further comprises reactivity information for switching from any current configuration of the polarization diversity actuator to any other target configuration of the polarization diversity actuator, and the control unit is configured to, when changing the configuration of the polarization diversity actuator, select the configuration of the polarization diversity actuator that implies the shortest adjustment time in view of the current configuration of the polarization diversity actuator.

11. An optical network unit for use in a passive optical network, the optical network unit comprising a coherent optical receiver according to any one of claims 1 to 10, which receives an amplitude shift keying modulated optical signal transmitted by a coherent optical transmitter included in an optical line terminal of the passive optical network.

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