Method and system for demultiplexing and demodulating signals multiplexed with variable orbital angular momentum - Patents.com

Abstract

A method is described for demultiplexing and demodulating (in particular "locally" demultiplexing and demodulating) amplitude-modulated signals grouped by orbital angular momentum multiplexing. The method comprises detecting a phase difference value ΔP detected by a beam detector arranged downstream of an interference structure 40 to which two portions of an electromagnetic beam carrying modulated channels, multiplexed with orbital angular momentum variables, are provided as input. ab and demultiplexing and demodulating the information a(t), b(t) modulated on the first modulated beam Fm1 and the at least one second modulated beam Fm2, respectively, based on ΔR. A corresponding system 100 for demultiplexing and demodulating amplitude-modulated signals capable of implementing the above-described method is also described.

Description

[Technical field]

[0001] The present invention relates generally to the technical field of transmitting and receiving electromagnetic beams, in particular optical / laser and microwave beams, detecting the orbital angular momentum of such beams, as well as to the field of telecommunications based on electromagnetic beams, in particular optical / laser and microwave beams, orbital angular momentum modulation and / or multiplexing.

[0002] The present invention particularly relates to demultiplexing and demodulation of signals multiplexed with orbital angular momentum variables (ie, orbital angular momentum multiplexed signals). [Background technology]

[0003] The theory of electromagnetic beam propagation, in particular laser and microwave beams, has relatively recently demonstrated the existence of orbital angular momentum (orbital angular momentum).

[0004] Orbital angular momentum, from a conventional point of view, is a concept related to the various transverse modes of beam propagation.

[0005] This can also be interpreted as an indication of the fact that the propagation front of the orbital angular momentum (OAM) wave is not simply flat, but has an evolution that is represented by a spiral surface.

[0006] In other words, the Poynting vector and the wave vector are not simply parallel to the direction of propagation, but are twisted relative to the direction of propagation.

[0007] Orbital angular momentum is, from a quantum point of view, treated as an additional quantum number distinct from spin.

[0008] Recently, beams whose orbital angular momentum can take on different values ​​other than 0 have also been experimentally demonstrated.

[0009] Due to the above characteristics, the "orbital angular momentum" variable is particularly difficult to detect and characterize when the detector is illuminated only by a limited portion of the radiation beam, even if it is very far from the singularity. In fact, there are no reliable systems and methods that allow to detect the orbital angular momentum of a received electromagnetic beam, e.g., a laser, by local measurement, utilizing only a limited portion of the beam incident on the detector, even if it is very far from the singularity.

[0010] On the other hand, a need is felt to detect the orbital angular momentum of a received beam for a variety of reasons including, for example, the characterization of the beam and the utilization of angular momentum variables for telecommunications purposes.

[0011] Such needs are currently not met by known technical solutions with local measurements.

[0012] Applicants have also identified the promising possibility of utilizing the orbital angular momentum variable as an additional degree of freedom, which can be advantageously used for both signal modulation and signal multiplexing.

[0013] However, the considered technical background does not provide a reliable telecommunications solution based on orbital angular momentum multiplexing and / or modulation, and therefore the need for such a solution is particularly felt.

[0014] Additionally, an important field of telecommunications applications is believed to require a method and system that can effectively demodulate and demultiplex multiple signals transmitted in two or more amplitude modulated and angular momentum multiplexed electromagnetic beams. Summary of the Invention

[0015] In light of the above, it is an object of the present invention to provide a method for demultiplexing and demodulating amplitude modulated signals grouped by multiplexing on orbital angular momentum variables, which makes it possible to at least partially overcome the drawbacks mentioned herein with reference to the prior art and to meet the above-mentioned needs, particularly felt in the considered technical field.

[0016] Such an object is achieved by the method according to claim 1.

[0017] Further embodiments of such a method are defined in claims 2-17.

[0018] The invention further relates to a system for demultiplexing and demodulating amplitude modulated signals grouped by multiplexing in orbital angular momentum variables, capable of implementing the above-mentioned method. Such a system is defined in claims 18 to 23. [Brief description of the drawings]

[0019] Further features and advantages of the above-mentioned method and system according to the invention will become apparent from the following description of preferred embodiments, given as illustrative and non-limiting examples, with reference to the attached drawings, in which:

[0020] [Figure 1] 1 shows a simplified diagram of the transmitting part of an embodiment of a system for transmitting and receiving an electromagnetic radiation beam according to the invention, such figure 1 simultaneously showing several steps of the corresponding method. [Diagram 2] 2 shows a simplified diagram of the receiving part of an embodiment of a system for transmitting and receiving an electromagnetic radiation beam according to the invention, which simultaneously illustrates some other steps of the corresponding method. [Diagram 3] Fig. 3 shows a simplified diagram of the transmission part of an embodiment of a telecommunication system according to the invention.Such figure 3 shows simultaneously several steps of the corresponding method. [Figure 4] 4 shows a simplified diagram of the receiving part of an embodiment of a telecommunication system according to the invention, which also illustrates some other steps of the corresponding method. [Diagram 5] 2 shows an embodiment of a system according to the invention comprising a correlator; [Figure 6] 1 shows some geometric quantities used in the system diagram. [Figure 7] 1 illustrates a system for demultiplexing and demodulating a signal according to an embodiment of the present invention. [Figure 8] 5 shows systems for demultiplexing and demodulating signals according to two respective further embodiments of the invention; [Figure 9] 5 shows systems for demultiplexing and demodulating signals according to two respective further embodiments of the invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] 1 to 6, a method for transmitting and receiving an electromagnetic radiation beam is described, adapted to determine the orbital angular momentum of a received electromagnetic radiation beam.

[0022] The method includes the steps of first generating at least one main electromagnetic radiation beam F1 characterized by a first orbital angular momentum L1, a first spectrum in a first frequency band, and a first beam radius of curvature, and generating a reference electromagnetic radiation beam F0 characterized by a second orbital angular momentum L0, a second spectrum in a second frequency band different from said first frequency band, and a second beam radius of curvature substantially matching said first beam radius of curvature.

[0023] It should be noted that the above characterization based on the first orbital angular momentum L1 and the second orbital angular momentum L0 can correspondingly be described in terms of the topological charge (l1,l0), since the angular momentum L and the topological charge l are linked by the following relation:

[0024] L = (l * h) / 2π, where h is Planck's constant.

[0025] The method therefore comprises generating a composite electromagnetic radiation beam Q1 consisting of a superposition of at least one main beam F1 and a reference beam F0 as mentioned above, and transmitting the composite electromagnetic radiation beam Q1 thus generated.

[0026] The method further comprises the steps of receiving said composite electromagnetic radiation beam Q1 by a first beam detector 1 positioned at a first position to generate a first composite beam electrical signal D1 representative of the electric field and / or magnetic field and / or intensity of the electromagnetic radiation of the composite beam at such first position, and receiving said composite electromagnetic radiation beam Q1 by a second beam detector 2 positioned at a different second position relative to said first position to generate a second composite beam electrical signal D2 representative of the electric field and / or magnetic field and / or intensity of the received electromagnetic radiation of the composite beam at such second position.

[0027] The method further includes the steps of performing frequency discrimination of the first composite beam electrical signal D1 to obtain a first main beam electrical signal P1 representative of the electric field and / or magnetic field and / or intensity due to the main beam at the first position and a first reference beam electrical signal R1 representative of the electric field and / or magnetic field and / or intensity due to a reference beam at the first position, and performing frequency discrimination of the second composite beam electrical signal D2 to obtain a second main beam electrical signal P2 representative of the electric field and / or magnetic field and / or intensity due to the main beam at a second position and a second reference beam electrical signal R2 representative of the electric field and / or magnetic field and / or intensity due to the reference beam at the second position.

[0028] The method ultimately includes determining an orbital angular momentum L1 of the main electromagnetic radiation beam and / or a spatial phase variation of the main electromagnetic radiation beam due to the main beam orbital angular momentum L1 based on said first main beam electrical signal P1, second main beam electrical signal P2, first reference beam electrical signal R1 and second reference beam electrical signal R2.

[0029] According to an embodiment of the method, the determining step includes determining a first phase difference value ΔP corresponding to the difference between the phase of the first main beam electrical signal P1 and the phase of the second main beam electrical signal P2. Furthermore, a second phase difference value ΔR corresponding to the difference between the phase of the first reference beam electrical signal R1 and the phase of the second reference beam electrical signal R2 is determined. The second phase difference value ΔR divided by the second wave number k' is then subtracted from the first phase difference value ΔP divided by the first wave number k to obtain a difference value (Q2=ΔP / k-ΔR / k'), which is independent of positional gradient conditions between the first and second detectors as mentioned above, originating from the relative positions of the two detectors with respect to the beam propagation, said difference value being independent of phase variations due to disturbances suffered by the transmitted composite beam before reception. The orbital angular momentum of the main electromagnetic radiation beam is then determined based on said obtained difference value (Q2=ΔP / k-ΔR / k').

[0030] The first wave number k corresponds to the main beam and is defined as k=2π / λ, where λ is the wavelength of said main beam in said first frequency band, and the second wave number k' corresponds to the reference beam and is defined as k'=2π / λ', where λ is the wavelength of said reference beam in said second frequency band.

[0031] The definition of "position inclination" (or "position tilt") is meant to indicate the angle formed between a line connecting two detectors and its (orthogonal) projection on a plane perpendicular to the beam propagation axis.

[0032] According to a particular embodiment, determining the orbital angular momentum of the main electromagnetic radiation beam comprises determining the orbital angular momentum of the main electromagnetic radiation beam based on the following equation:

number

[0033] According to an implementation option, the step of determining the first phase difference value ΔP comprises comparing the phase of the first main beam electrical signal P1 with the phase of the second main beam electrical signal P2 by a first phase comparator 3. The step of determining the second phase difference value ΔR comprises comparing the phase of the first reference beam electrical signal R1 with the phase of the second reference beam electrical signal R2 by a second phase comparator 4.

[0034] According to another implementation option, the step of determining the first phase difference value ΔP comprises performing a correlation operation between the first main beam electrical signal P1 and the second main beam electrical signal P2, and the step of determining the second phase difference value ΔR comprises performing a correlation operation between the first reference beam electrical signal R1 and the second reference beam electrical signal R2.

[0035] According to one embodiment of this method, the orbital angular momentum of the reference beam is always known.

[0036] Depending on the implementation option, the orbital angular momentum of the reference beam takes on a constant value L0=0.

[0037] According to one embodiment of the method, the first position of the first detector 1 and the second position of the second detector 2 are fixed and constant and different from the position of the beam singularity.

[0038] According to another embodiment of the method, the first position of the first detector 1 and / or the second position of the second detector 2 are movable and the interrelationship between said first and second positions is always known.

[0039] According to an implementation option, the second frequency band is substantially monochromatic.

[0040] According to a particular implementation option, the second frequency band is adjacent to the first frequency band.

[0041] According to a possible embodiment of the method, the step of performing frequency discrimination of the first or second composite beam electrical signal comprises performing frequency filtering or performing frequency separation by heterodyne techniques or other frequency separation methods.

[0042] According to one embodiment of the method, at least one main electromagnetic radiation beam is not modulated.

[0043] According to another embodiment of the method, the at least one main electromagnetic radiation beam is amplitude modulated, and / or phase modulated, and / or frequency modulated, and / or orbital angular momentum modulated.

[0044] According to one embodiment of the method, all of the aforementioned transmitted and received electromagnetic beams are optical and / or laser beams.

[0045] A detailed example of the implementation of the method is given below together with the relevant physical-mathematical analysis.

[0046] In the following description and in Figures 1 and 2, points where different signals are present (first composite beam electrical signal D1, second composite beam electrical signal D2, first main beam electrical signal P1, first reference beam electrical signal R1, second main beam electrical signal P2, second reference beam electrical signal) are shown with the same name as the respective signal for simplicity.

[0047] As already observed, the composite beam Q1 is generated by at least one beam having orbital angular momentum L=L1 (other than 0), defined herein as the main beam F1, superimposed on a beam having angular momentum L=L0, defined herein as the reference beam F0.

[0048] The main beam may be modulated or unmodulated. The reference beam has a frequency band that does not overlap with the frequency band of the main beam. The frequency band of the reference beam is preferably quasi-monochromatic and adjacent to the frequency band of the main beam. The reference beam has substantially the same curvature and the same propagation vector as the main beam. The reference beam preferably has a topological charge l0=0, which also means an orbital angular momentum L0=0.

[0049] Knowledge of the spatial phase difference generated by the main beam of orbital angular momentum L1 is obtained by using two detectors located at any position in space, except at the vortex singularity.

[0050] As is known, the expression "vortex singularity" means a point within a vortex at which the electromagnetic field consequently decreases to zero and the phase of the electromagnetic field cannot be determined.

[0051] If the main beam is not modulated, the electric field E1 or related signal at the first detector 1 (shown as D1 in FIG. 2) can be expressed by the following analytical formula:

[0052]

number

[0053] where t is time, A1 and B1 are non-zero arbitrary amplitudes, l1 is the topological charge of the main beam, l0 is the topological charge of the reference beam, and θ1 is the angular position of the detector measured on a plane orthogonal to the composite beam propagation vector that contains the first detector 1.

number

number

number

number

[0054] Similarly, the electric field E2 or related signal on the second detector 2 (shown as D2 in FIG. 2) may be represented by the following analytical expression:

[0055]

number

[0056] where t is time, A2 and B2 are arbitrary non-zero amplitudes, l1 is the topological charge of the main beam, l0 is the topological charge of the reference beam, and θ2 is the angular position of detector 2 measured on a plane orthogonal to the composite beam propagation vector that contains detector 2.

number

number

number

number

[0057] As a further explanation of the geometrical quantities defined above, Fig. 6 shows by a dashed line the propagation axis z of the composite beam Q1 generated by the composite beam generating system already previously described (shown in Fig. 6 with reference to datum 30). Fig. 6 also shows the xy plane orthogonal to the propagation axis z, the position vectors of the two detectors 1 and 2, respectively

number

number

[0058] The fields or related signals are frequency separated by various possible techniques known per se, so that at R1 and R2 the fields or related signals are present only in the frequency band of the reference beam and at P1 and P2 the fields or related signals are present only in the frequency band of the main beam.

[0059] Therefore, the following analytical expression can be obtained:

[0060]

number

[0061] The second phase comparator 4 provides a quantity proportional to the phase difference of the fields and associated signals between R1 and R2.

[0062]

number

[0063] The first phase comparator 3 provides a quantity proportional to the phase difference of the fields and associated signals between P1 and P2.

[0064]

number

[0065] Since the main beam has a curvature substantially equal to that of the reference beam and a propagation direction substantially coinciding with the propagation direction of the reference beam, the phase difference associated with the tilt is, to a good approximation,

[0066]

number

[0067] Since the distortion phenomena due to propagation are very similar for the main beam and the reference beam (which are superimposed during transmission in the composite beam), the phase difference associated with the distortion is, to a good approximation,

[0068]

number

[0069] Furthermore, the proportionality constants of the two phase comparators may be selected so that they are matched.

[0070] Based on the above, signal Q2 will provide an amount proportional to the difference:

[0071]

number

[0072] Such quantities are, where appropriate, independent of position gradients and propagation disturbances.

[0073] Once the value of Q2 (i.e., ΔP / k-ΔR / k') is measured, θ1, θ2, k, k', and the value of l0 (the topological charge of the reference beam that can be set initially) are known. The topological charge value l1 of the main beam is easily obtained from the above equation. Therefore, the orbital angular momentum L1 of the main beam is also known. Note that:

[0074] L = (l * h) / 2π

[0075] If the main beam is phase modulated, the equations for points R1, R2, P1, and P2 are:

[0076]

number

[0077] Here, δ(t) is the time-varying phase term due to the phase modulation detected equally by the first and second detectors. Since the phase term δ(t) is compensated for by the output of the second phase comparator 2, the following is also obtained:

[0078]

number

[0079] If the main beam is frequency modulated, the equations for points R1, R2, P1, and P2 are:

[0080]

number

number

number

number

[0081] where m(τ) is the modulating signal over time, and k f is a constant.

number

[0082]

number

[0083] A method for carrying out electrical communication of signals modulated according to any known modulation technique and grouped by orbital angular momentum variable multiplexing is now described, which is also encompassed by the present invention.

[0084] Such a method includes generating a first electromagnetic radiation beam F1 characterized by a first orbital angular momentum L1, and generating at least one second electromagnetic radiation beam F2 characterized by at least one respective second orbital angular momentum L2, both of which have respective spectra in the same first frequency band and further have respective radii of curvature that substantially match the first beam radius of curvature value.

[0085] The method then comprises modulating, by any modulation technique, a first piece of information to be transmitted represented by a first modulation function a(t) onto the first electromagnetic radiation beam F1 to obtain a first modulated beam Fm1, and further modulating, by any modulation technique, at least one second piece of information to be transmitted represented by a second modulation function b(t) onto at least one second electromagnetic radiation beam F2 to obtain a second modulated beam Fm2. Then, the method comprises generating a reference electromagnetic radiation beam F0 characterized by a second orbital angular momentum L0, a second spectrum in a second frequency band different from said first frequency band, and a second beam radius of curvature having a value substantially corresponding to a value of said first beam radius of curvature.

[0086] Next, the method includes a step of superimposing and / or combining the aforementioned reference beam F0, the first modulated beam Fm1 and the second modulated beam Fm2 to generate a composite electromagnetic radiation beam Q1 consisting of a superposition of the reference beam F0 and a main beam consisting of a superposition of the aforementioned first modulated beam Fm1 and at least one second modulated beam Fm2.

[0087] Next, the method includes transmitting the generated composite beam of electromagnetic radiation Q1.

[0088] The method then includes receiving said composite electromagnetic radiation beam by a first beam detector 1 located at a first location to generate a first composite beam electrical signal D1 representative of the electric and / or magnetic field and / or intensity of the electromagnetic radiation of the composite beam at said first location, and receiving said composite electromagnetic radiation beam by a second beam detector 2 located at a different second location relative to the first location to generate a second composite beam electrical signal D2 representative of the electric and / or magnetic field and / or intensity of the received electromagnetic radiation of the composite beam at said second location.

[0089] The method further includes the steps of performing frequency discrimination of the first composite beam electrical signal D1 to obtain a first main beam electrical signal P1 representing the electric field and / or magnetic field and / or intensity due to the main beam at the first position and a first reference beam electrical signal R1 representing the electric field and / or magnetic field and / or intensity due to the reference beam at the first position, and performing frequency discrimination of the second composite beam electrical signal D2 to obtain a second main beam electrical signal P2 representing the electric field and / or magnetic field and / or intensity due to the main beam at the second position and a second reference beam electrical signal R2 representing the electric field and / or magnetic field and / or intensity due to the reference beam at the second position.

[0090] The method further includes determining a phase of the first main beam electrical signal P1 and a phase of the second main beam electrical signal P2, and further determining a phase of the first reference beam electrical signal R1 and a phase of the second reference beam electrical signal R2. Then, a first phase difference value ΔP corresponding to a difference between the phase of the first main beam electrical signal P1 and the phase of the second main beam electrical signal P2 is determined. ab In this case, such a first phase difference value ΔP ab depends on the values ​​taken by the first modulation function a(t) and the second modulation function b(t), and further includes determining a second phase difference value ΔR corresponding to the difference between the phase of the first reference beam electrical signal R1 and the phase of the second reference beam electrical signal R2.

[0091] The method then calculates a first phase difference value ΔP divided by the first wavenumber k. ab The second phase difference value ΔR divided by the second wave number k' is subtracted from the second phase difference value Q2 to obtain the difference value (ΔP ab The method includes obtaining a wave number k corresponding to a main beam, defined as k=2π / λ, where λ is the wavelength of said main beam in said first frequency band. The method includes obtaining a wave number k' corresponding to a reference beam, defined as k'=2π / λ', where λ is the wavelength of said reference beam in said second frequency band.

[0092] The aforementioned difference value Q2 is independent of the positional gradient conditions between the first detector 1 and the second detector 2, and also independent of phase variations due to disturbances suffered by the transmitted composite beam before reception, but represents a combination of values ​​taken by the first modulation function a(t) and the second modulation function b(t).

[0093] This method finally uses the previously determined difference value (Q2 = ΔP ab / k-ΔR / k′), demultiplexing and demodulating information modulated on each of the first modulated beam Fm1 and the at least one second modulated beam Fm2.

[0094] According to one embodiment of such a method, the number of modulated beams that are orbital angular momentum multiplexed is greater than two.

[0095] According to one embodiment of such a method, the first electromagnetic radiation beam F1 and the at least one second electromagnetic radiation beam F2 are digitally amplitude modulated according to the amplitudes of a first modulation function a(t) and at least one second modulation function b(t).

[0096] In this case, the difference value (Q2 = ΔP ab / K-ΔR / K′) may take on a number of expected values, representing respective combinations of digital amplitude values ​​taken by the first modulation function a(t) and the at least one second modulation function b(t).

[0097] According to an implementation option, the first electromagnetic radiation beam F1 and the at least one second electromagnetic radiation beam F2 are digitally amplitude modulated in a binary manner, the amplitudes of the first modulation function a(t) and the at least second modulation function b(t) may take on the logical values ​​0 or 1.

[0098] In this case, the method includes the further step of detecting (by detector 16) a received power or intensity Q3 corresponding to the first main beam electrical signal P1 or the second main beam electrical signal P2 and comparing the received power or intensity with a minimum threshold value.

[0099] The determined difference (Q2 = ΔP ab / K-ΔR / K') is the first expectation value (ΔP 10 / k-ΔR / k') or a second expectation value (ΔP 01 / k-ΔR / k'), or a third expectation value (ΔP 11 / k-ΔR / k').

[0100] The steps of demodulating, demultiplexing and demodulating the modulated information are carried out by determining the difference (ΔP ab / k-ΔR / k') is the first expected value (ΔP 10 / k-ΔR / k'), the first modulated beam Fm1 carries information corresponding to 1 and the second modulated beam Fm2 carries information corresponding to 0, and the determined difference (ΔP ab / k-ΔR / k') is the second expected value (ΔP 01 / k-ΔR / k'), the first modulated beam Fm1 carries information corresponding to 0 and the second modulated beam Fm2 carries information corresponding to 1, and the determined difference (ΔP ab / k-ΔR / k') is the third expected value (ΔP 11 / k-ΔR / k'), the first modulated beam Fm1 transmits information corresponding to 1 and the second modulated beam Fm2 transmits information corresponding to 1; and recognizing that, if the received power or intensity Q3 is lower than the aforementioned minimum threshold, the first modulated beam Fm1 transmits information corresponding to 0 and the second modulated beam Fm2 transmits information corresponding to 0.

[0101] According to another embodiment of such a method, the first electromagnetic radiation beam F1 and the at least one second electromagnetic radiation beam F2 are digitally modulated based on their angular momentum, where the angular momentum of the first beam F1 can take two different discrete values ​​based on a first modulation function a(t) and the angular momentum of the at least one second beam F2 can take two different discrete values ​​based on a respective at least second modulation function b(t).

[0102] Difference value (Q2=ΔP ab / k-ΔR / k′) may take on a number of expected values, each representing a respective combination of digital amplitude values ​​taken by the first modulation function a(t) and the at least one second modulation function b(t).

[0103] According to implementation options, the amplitudes of the first modulation function a(t) and of the at least one second modulation function b(t) may take on the logical values ​​0 or 1.

[0104] The determined difference (ΔP ab / K-ΔR / K') is the first expected value (ΔP 10 / K-ΔR / K'), or if the first modulation function (t) takes the value 0 and the second modulation function takes the value 1, the second expected value (ΔP 01 / K-ΔR / K'), or if the first modulation function a(t) takes the value 1 and the second modulation function takes the value 1, the third expectation value (ΔP 11 / K-ΔR / K'), or if the first modulation function (t) takes the value 0 and the second modulation function takes the value 0, the fourth expectation value (ΔP 00 / K-ΔR / K').

[0105] In this case, the steps of demodulating, demultiplexing and demodulating the modulated information are carried out by determining the difference (ΔP ab / k-ΔR / K') is the first expectation (ΔP 10 / k-ΔR / K′), the first modulated beam Fm1 carries information corresponding to 1 and the second modulated beam Fm2 carries information corresponding to 0, and the determined difference (ΔP ab / k-ΔR / K') is the second expected value (ΔP 01 / k-ΔR / k'), the first modulated beam Fm1 carries information corresponding to 0 and the second modulated beam Fm2 carries information corresponding to 1, and the determined difference (ΔP ab / k-ΔR / K') is the third expected value (ΔP 11 / k-ΔR / k'), the first modulated beam Fm1 carries information corresponding to 1 and the second modulated beam Fm2 carries information corresponding to 1, and the determined difference (ΔP ab / k-ΔR / K') is the fourth expectation (ΔP 00 and recognizing that when Fm1=Fm2 / k−ΔR / k′, the first modulated beam Fm1 carries information corresponding to 0 and the second modulated beam Fm2 carries information corresponding to 0.

[0106] According to an embodiment of such a method, the transmitted and received electromagnetic beams are light beams and / or laser beams.

[0107] A detailed embodiment of the above telecommunication method is given below together with the relevant physical and mathematical analysis.

[0108] In the following description and in Figures 3 to 5, points where different signals are present (first composite beam electrical signal D1, second composite beam electrical signal D2, first main beam electrical signal P1, first reference beam electrical signal R1, second main beam electrical signal P2f, second reference beam electrical signal R2) are shown with the same name as the respective signal for simplicity.

[0109] As shown in Fig. 3, a first beam (herein defined as the first main beam F1) of angular momentum L1 and a second beam (herein defined as the second main beam F2) of angular momentum L2 are superimposed on a reference beam of angular momentum L0. The two main beams have coincident and / or overlapping frequency bands and are digitally amplitude modulated (in the examples detailed herein). Furthermore, the two main beams have substantially coincident curvatures.

[0110] The electric field in the electrical signal D1 can be expressed analytically as follows:

[0111]

number

[0112] where t is time, A1(t) and C1(t) are the amplitudes of the main beams varying over time, B1 is an arbitrary non-zero amplitude of the reference beam, l1 is the topological charge of the first main beam, l2 is the topological charge of the second main beam, l0 is the topological charge of the reference beam, and θ1 is the angular position of the first detector measured on a plane orthogonal to the composite beam propagation vector that contains the first detector 1.

number

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[0113] The electric field in the electrical signal D2 can be expressed analytically as follows:

[0114]

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[0115] where t is time, A2(t) and C2(t) are the amplitudes of the main beam varying over time, B2 is an arbitrary non-zero amplitude of the reference beam, l1 is the topological charge of the first main beam, l2 is the topological charge of the second main beam, l0 is the topological charge of the reference beam, and θ2 is the angular position of the second detector measured on a plane orthogonal to the composite beam propagation vector that contains the second detector 2.

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[0116] As already observed above, the signals D1 and D2 are measured by two detectors, the reference beam is differentiated in frequency from the main beam, and the signals R1, R2, P1, P2 are obtained using the following equations:

[0117]

number

[0118] R1 and R2 have only the reference beam, while P1 and P2 have the superimposed main beam.

[0119] The second phase comparator 4 provides a quantity proportional to the phase difference.

[0120]

number

[0121] In digital modulation, the amplitude is A1=A 1max a(t), A2=A 2max a(t), C1=C 1max b(t), C2=C 2max b(t), where the functions a(t) and b(t) take on values ​​0 or 1 depending on the information digitally modulated by the first and second modulators, respectively.

[0122] A 1max , C 1max is the maximum amplitude of the field or signal representing the main beam (first and second, respectively) received by the first detector. A 2max , C 2max is the maximum amplitude of the field or signal representing the main beam (first and second, respectively) received by the second detector. At the transmitter, the amplitude of the main beam can be set to be equal to:

[0123] A 1max =C 1max ,A 2max =C 2max

[0124] The first phase comparator 3 provides a quantity proportional to the phase difference between the fields or signals of P1 and P2, which depends on the digital encoding of the modulation functions a(t), b(t).

[0125] This considers all possible combinations.

[0126] When a(t)=0 and b(t)=0, the amplitudes of the main beams cancel each other out, so the phase difference cannot be determined.

[0127] For a(t)=1 and b(t)=0, only the first main beam with angular momentum L1 is present, so similar relationships apply as already described for the single main beam case.

[0128]

number

[0129] For a(t)=0 and b(t)=1, there is only a second main beam with angular momentum L2, so similar relationships apply as already described for the single main beam case.

[0130]

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[0131] For a(t)=1 and b(t)=1, both main beams are present, so the following holds:

[0132]

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[0133] Based on the above relations, it is possible to calculate all possible combinations of the modulation signals in order to eliminate arbitrariness in phase due to propagation-related position tilt and wavefront distortions, similar to what has already been described for the case of a single main beam.

[0134] Therefore, the conclusion is that the following relationship is obtained:

[0135] When a(t)=0 and b(t)=0, the amplitudes of the main beams cancel each other out, so the phase difference cannot be determined.

[0136] For a(t)=1 and b(t)=0, we have:

[0137]

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[0138] For a(t)=0 and b(t)=1, we have:

[0139]

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[0140] For a(t)=1 and b(t)=1, considering that the main beam also has a substantially matching curvature, we have

[0141]

number

[0142] The aforementioned quantity (ΔP 10 / k-ΔR / k'), (ΔP 01 / k-ΔR / k'), (ΔP 11 / k-ΔR / k') can be easily distinguished, i.e., by appropriately selecting the topological charges l0, l1, l2 (i.e., the respective orbital angular momentum) of the reference beam and the two main beams, they are set to three different predefined and known values. Such quantities measured on reception are therefore recognizable and indicate the modulation value 0 or 1 applied to each of the two main beams. The information encoded therein can therefore be decoded, i.e., demodulated and recognized.

[0143] Moreover, such a quantity advantageously becomes independent of phase differences due to position tilt and (as already noticed above) of distortions of the propagating wavefront that may be cancelled by the presence of a reference beam.

[0144] Possible examples of the choice of the value of the topological charge are as follows:

[0145] l0=0, l1=0, l2=2

[0146] Other combinations are clearly detectable.

[0147] The status a(t)=0, b(t)=0 is not yet recognized and its phase is undetermined (as described above). This status is easily identifiable since it is the only combination where the amplitudes of the fields or signals received in both main beams are cancelled. Thus, the status a(t)=0, b(t)=0 is deterministically identified when the intensity or power of the signal detected at point Q3 (by detector 16 shown in FIG. 4) falls below a predefined threshold. Alternatively, both the first composite beam electrical signal D1 and the second composite beam electrical signal D2 may be monitored to recognize the situation where both signals are below their respective predefined thresholds.

[0148] Below, a detailed embodiment of the above telecommunication method is given based on orbital angular momentum modulation, together with the relevant physical and mathematical analysis.

[0149] Angular momentum modulation can be explained using arguments similar to those already developed for digital amplitude modulation.

[0150] The modulation functions a(t), b(t) take on values ​​0 or 1 depending on the information digitally modulated by the first and second modulators, respectively. Such modulation functions determine a discrete change in the angular momentum of the first electromagnetic radiation beam and the at least one second electromagnetic radiation beam, respectively, depending on the binary value taken, i.e. L1 and L2 are functions which depend on the values ​​taken by a(t) and b(t).

[0151] L1 = L1(a(t)), L2 = L2(b(t)). That is, we refer to the topological charges equivalently: l1 = l1(a(t)), l2 = l2(b(t)).

[0152] As a result, the signals present at R1, R2, P1, and P2 can be expressed as follows:

[0153]

number

[0154] R1 and R2 have only the reference beam. P1 and P2 have the superimposed main beam.

[0155] The second phase comparator 4 provides a quantity proportional to the phase difference (as is the general case).

[0156]

number

[0157] The first phase comparator 3 provides a quantity proportional to the phase difference between the fields or signals of P1 and P2, which depends on the digital encoding of the modulation functions a(t), b(t).

[0158] Considering all possible combinations, we get:

[0159]

number

[0160] Here, the combinations are determined by the indices a, b and the corresponding values ​​obtained by the functions a(t), b(t).

[0161] Then, to eliminate the arbitrariness of the phase due to position tilt and propagation-related wavefront distortion, we calculate the difference ΔP for all possible combinations of the modulated signals.ab / k-ΔR / k' is calculated by the following formula:

[0162]

number

[0163] The topological charge values ​​l0, l1(0), l1(1), l2(0), l2(1), or their corresponding orbital angular momentum, are expressed as the corresponding quantities (ΔP 00 / k-ΔR / k'), (ΔP 01 / k-ΔR / k'), (ΔP 10 / k-ΔR / k'), (ΔP 11 / k-ΔR / k') are different from one another and therefore recognizable, and can be chosen to make it possible to decode (demodulate) the encoded (modulated) information.

[0164] Furthermore, the aforementioned quantities are independent of phase differences due to position tilt and distortion of the propagating wavefront, which can be eliminated by the presence of a reference beam.

[0165] An example of the choice of topological charge values ​​is l0=0, l1(0)=0, l1(1)=1, l2(0)=0, l2(1)=2, which gives:

[0166]

number

[0167] Thus, the four aforementioned quantities are distinct and can be recognized.

[0168] As with the above example, other value assignments are obviously possible.

[0169] It should be noted that angular momentum modulation is similar in many ways to amplitude modulation, and the block diagrams shown in Figures 3 and 4 also apply to angular momentum modulation, with the only obvious difference being that the modulator modulates angular momentum rather than amplitude. Furthermore, in this case, the threshold detector of Figures 4 and 5 is not required.

[0170] 1 and 2, a system for transmitting and receiving an electromagnetic radiation beam is now described, adapted to determine the orbital angular momentum of a received electromagnetic radiation beam.

[0171] Such a system comprises means for generating a main electromagnetic radiation beam F1 5, means for generating a reference electromagnetic radiation beam F0 6, means for generating (7) and transmitting (14) a composite electromagnetic radiation beam Q1 (shown in Figure 1), means for receiving the composite electromagnetic radiation beam, first beam detection means 1, second beam detection means 2, first frequency discrimination means 8, second frequency discrimination means 9, and means for determining orbital angular momentum 10 (shown in Figure 2).

[0172] The means 5 for generating a beam of main electromagnetic radiation is configured to generate a main electromagnetic radiation beam F1 characterized by a first orbital angular momentum L1, a first spectrum in a first frequency band, and a first beam radius of curvature.

[0173] The means 6 for generating a reference electromagnetic radiation beam is configured to generate a reference electromagnetic radiation beam F0 characterized by a second orbital angular momentum L0, a second spectrum in a second frequency band different from said first frequency band, and a second beam radius of curvature substantially matching said first beam radius of curvature.

[0174] The means for generating (7) and transmitting (14) a composite electromagnetic radiation beam is configured to generate a composite electromagnetic radiation beam Q1 consisting of a superposition of the aforementioned main beam F1 and reference beam F0, and to transmit such generated composite electromagnetic radiation beam Q1.

[0175] The means for receiving a composite electromagnetic radiation beam comprises a first beam detection means 1 arranged at a first position and configured to generate a first composite beam electrical signal D1 representative of the electric field and / or magnetic field and / or intensity of the composite beam electromagnetic radiation at the first position, and a second beam detection means 2 arranged at a different second position with respect to said first position and configured to generate a second composite beam electrical signal D2 representative of the electric field and / or magnetic field and / or intensity of the composite beam electromagnetic radiation at the second position.

[0176] The first frequency discrimination means 8 is configured to perform frequency discrimination of the first composite beam electrical signal D1 to obtain a first main beam electrical signal P1 representing the electric field and / or magnetic field and / or intensity due to the main beam at the first position and a first reference beam electrical signal R1 representing the electric field and / or magnetic field and / or intensity due to the reference beam at the first position.

[0177] The second frequency discrimination means 9 is configured to perform frequency discrimination of the second composite beam electrical signal to obtain a second main beam electrical signal P2 representing the electric field and / or magnetic field and / or intensity due to the main beam at the second position and a second reference beam electrical signal R2 representing the electric field and / or magnetic field and / or intensity due to the reference beam at the second position.

[0178] The means 10 for determining the orbital angular momentum is configured to determine the orbital angular momentum L1 of the main electromagnetic radiation beam and / or the spatial phase variation of the main electromagnetic radiation beam due to the main beam orbital angular momentum L1 based on said main beam first electrical signal P1, second main beam electrical signal P2, first reference beam electrical signal R1 and second reference beam electrical signal R2.

[0179] According to different implementation options, the system is configured to perform the method for transmitting and receiving an electromagnetic radiation beam according to any of the above embodiments.

[0180] According to an embodiment of the system, the means for generating a main electromagnetic radiation beam 5 and the means for generating a reference electromagnetic radiation beam 6 comprise one or more sources or transmitters of an electromagnetic beam known per se (e.g., in an implementation option, a laser).

[0181] According to implementation options, the means 5 for generating a main electromagnetic radiation beam further comprises an amplitude and / or frequency and / or phase modulator 50 and / or one or more angular momentum modulators 50 (such an angular momentum modulator 50 may, for example, be a spatial light modulator).

[0182] According to an embodiment of the system, the means 7 for generating a combined electromagnetic radiation beam comprises an electromagnetic beam combiner (eg a beam combiner) with two or more inputs and outputs known per se.

[0183] According to an embodiment of the system, the first beam detection means 1 comprise one or more diaphragms (optical apertures), or an antenna or a group of antennas, or any other electromagnetic beam receiver known per se, adapted to operate at the frequencies of the first and second beams. The means for transmitting the electromagnetic beam 14 comprise, for example, one or more transmitting antennas.

[0184] According to an embodiment of the system, the second beam detection means 2 comprises one or more diaphragms (optical apertures), or an antenna or group of antennas, or any other electromagnetic beam receiver known per se, adapted to operate at the frequencies of the first and second beams.

[0185] According to different embodiments, the first frequency discrimination means 8 and the second frequency discrimination means 9 may comprise frequency filters known per se.

[0186] According to an embodiment of the system, the means 10 for determining the orbital angular momentum comprises at least two phase comparators 3, 4 and at least one processor 15 configured to derive the orbital angular momentum by processing (e.g. according to the equations shown previously) based on the output signals from the phase comparators.

[0187] According to an embodiment of the system, the means 10 for determining the orbital angular momentum comprises at least two correlators 11, 12 and at least one processor 15 configured to derive the orbital angular momentum by processing (e.g. according to the equations shown previously) based on the output signals from the correlators.

[0188] Further details are provided herein with respect to implementation options including the use of correlators, with reference to FIG.

[0189] In this case, instead of using a phase comparator that provides a value proportional to the phase difference ΔP or ΔR, a correlator is used that provides a value proportional to the cosine of the phase difference cos(ΔP) or cos(ΔR). The phase difference is then determined by the inverse function:

[0190] ΔP=arccos[cos(ΔP)]

[0191] ΔR=arccos[cos(ΔR)]

[0192] Correlation may be determined by the direct product of the fields or signals representing P1 and P2, or R1 and R2.

[0193] Alternatively, the correlation can be expressed by interference with intensity I P1 and I P2 P1 and P2, or respectively, with intensity I R1 and I R2 The average strength of the interference between the fields at R1 and R2 with By measuring the following can be knowingly determined:

[0194] cos(ΔP)=( -I P1 -I P2 ) / (2(I P1 I P2 ) 1 / 2 )

[0195] cos(ΔR)=( -I R1 -I R2 ) / (2(I R1 I R2 ) 1 / 2 )

[0196] According to an embodiment of the aforementioned system, the transmitted and received electromagnetic beams are light beams and / or laser beams.

[0197] 3 and 4, a system for effecting electrical communication of modulated signals according to any known modulation technique and grouping by multiplexing with orbital angular momentum variables is described.

[0198] Such a system comprises means for generating electromagnetic beams 5, 6, modulation means 50, beam combining and / or superposition means 7, transmission means 14, beam receiving means 1, 2, 8, 9, phase determining means 20 and processing means 15.

[0199] The means 5, 6 for generating electromagnetic beams are configured to generate a first electromagnetic radiation characterized by a first orbital angular momentum L1, and to generate at least one second beam of electromagnetic radiation F2 characterized by at least one respective second orbital angular momentum L2.

[0200] Both the first electromagnetic radiation beam F1 and the at least one second electromagnetic radiation beam F2 have respective spectra in the same first frequency band and further have respective radii of curvature that substantially match a radius of curvature value of the first beam.

[0201] The means 5, 6 for generating an electromagnetic beam are further configured to generate a reference electromagnetic radiation beam F0 characterized by a second orbital angular momentum L0, a second spectrum in a second frequency band different from said first frequency band, and a second beam radius of curvature having a value substantially matching said first beam radius of curvature value.

[0202] The modulation means 50 is configured to modulate a first information to be transmitted, represented by a first modulation function a(t), onto a first electromagnetic radiation beam F1 by any amplitude and / or phase and / or frequency modulation technique to obtain a first modulated beam Fm1, and to modulate at least one second information to be transmitted, represented by a second modulation function b(t), onto at least one second electromagnetic radiation beam F2 by any amplitude and / or phase and / or frequency modulation technique to obtain a second modulated beam Fm2.

[0203] The beam combining and / or superposition means 7 is configured to superpose and / or combine the aforementioned reference beam F0, the first modulated beam Fm1 and the second modulated beam Fm2 to generate a composite electromagnetic radiation beam Q1 consisting of a superposition of a reference beam and a main beam consisting of a superposition of the aforementioned first modulated beam Fm1 and at least one second modulated beam Fm2.

[0204] The transmitting means 14 is configured to transmit said generated composite electromagnetic radiation beam.

[0205] The means for receiving a composite electromagnetic radiation beam comprises first beam detection means 1, second beam detection means 2, first frequency discrimination means 8 and second frequency discrimination means 9.

[0206] The first beam detection means 1 is arranged at a first location and configured to generate a first composite beam electrical signal D1 representative of the intensity of the electric and / or magnetic field and / or electromagnetic radiation of the composite beam at the first location.

[0207] The second beam detection means 2 is arranged at a different second position with respect to the first position and is configured to generate a second composite beam electrical signal D2 representative of the electric and / or magnetic fields and / or intensity of the electromagnetic radiation of the composite beam at the second position.

[0208] The first frequency discrimination means 8 is configured to perform frequency discrimination of the first composite beam electrical signal D1 to obtain a first main beam electrical signal P1 representing the electric field and / or magnetic field and / or intensity due to the main beam at the first position, and to obtain a first reference beam electrical signal R1 representing the electric field and / or magnetic field and / or intensity due to the reference beam at the first position.

[0209] The second frequency discrimination means 9 is configured to perform frequency discrimination of the second composite beam electrical signal to obtain a second main beam electrical signal P2 representative of the electric field and / or magnetic field and / or intensity due to the main beam at the second position and to obtain a second reference beam electrical signal R2 representative of the electric field and / or magnetic field and / or intensity due to the reference beam at the second position.

[0210] The phase determining means 20 is configured to determine the phase of the first main beam electrical signal P1 and the phase of the second main beam electrical signal P2 and to determine the phase of the first reference beam electrical signal R1 and the phase of the second reference beam electrical signal R2.

[0211] The phase determining means 20 determines a first phase difference value ΔP corresponding to the difference between the phase of the first main beam electrical signal P1 and the phase of the second main beam electrical signal P2. ab wherein such first phase difference value ΔP ab is dependent on the values ​​taken by the first modulation function a(t) and the second modulation function b(t). Furthermore, a second phase difference value ΔR is determined, which corresponds to the difference between the phase of the first reference beam electrical signal R1 and the phase of the second reference beam electrical signal R2, and the first phase difference value ΔP divided by the wave number k is determined. ab Then, subtract the second phase difference value ΔR divided by the wave number k' to get the difference value Q2 = ΔP ab / K-ΔR / k', where the first wave number k is the wave number corresponding to the main beam and is defined as k=2π / λ, where λ is the wavelength of said main beam belonging to said first frequency band, and the second wave number k' is the wave number corresponding to the reference beam and is defined as k'=2π / λ', where λ' is the wavelength of said reference beam belonging to said second frequency band.

[0212] The difference value Q2 above = ΔP ab / K-ΔR / k' represents a combination of values ​​taken by the first modulation function a(t) and the second modulation function b(t), and is independent of the positional tilt condition between the first detector 1 and the second detector 2, and is also independent of phase fluctuations due to disturbances experienced by the transmitted composite beam before reception.

[0213] The processing means 15 calculates the difference value Q2=ΔP ab / k-ΔR / k′, and configured to demultiplex and demodulate modulation information of each of the first modulated beam Fm1 and the at least one second modulated beam Fm2.

[0214] According to different embodiments, a system is configured to perform an electromagnetic radiation beam telecommunication method according to any of the above embodiments.

[0215] According to an embodiment of the system, the means for generating an electromagnetic beam 5, 6 comprise one or more sources or transmitters of an electromagnetic beam known per se (eg, in an implementation option, a laser).

[0216] According to an embodiment of the system, the modulation means 50 comprise an amplitude and / or frequency and / or phase and / or angular momentum modulator known per se.

[0217] According to an embodiment of the system, the first and second beam detection means 1, 2 comprise one or more diaphragms (optical apertures), or an antenna or group of antennas, or any other electromagnetic beam receiver known per se, adapted to operate at the frequencies of the first and second beams, respectively.

[0218] According to an embodiment of the system, the first and second frequency discrimination means 8, 9 comprise frequency filters known per se.

[0219] According to an embodiment of the system, the phase determining means 20 comprise at least two phase comparators 3, 4 known per se.

[0220] According to an embodiment of the system, the first and second frequency discrimination means comprise correlators 11, 12 known per se. With regard to such correlators the same considerations apply as given above with regard to the system for transmitting and receiving electromagnetic beams.

[0221] According to an embodiment of the system, the processing means 15 comprises one or more processors known per se, and associated software.

[0222] According to an embodiment of the aforementioned system, the transmitted and received electromagnetic beams are light beams and / or laser beams.

[0223] With reference to FIGS. 7-9, a method for demultiplexing and demodulating amplitude modulated signals grouped by multiplexing in orbital angular momentum variables is described below.

[0224] The method is applied when the signals to be demultiplexed and demodulated constitute a composite electromagnetic radiation beam Q1 consisting of a superposition of a reference beam F0 and a main beam consisting of a superposition of a first modulated beam Fm1 and at least one second modulated beam Fm2.

[0225] The first modulated beam Fm1 is obtained by modulating a first information, represented by a first modulation function a(t), onto the first electromagnetic radiation beam F1 by any amplitude modulation technique.

[0226] The at least one second modulated beam Fm2 is obtained by modulating at least one second information represented by a second modulation function b(t) on at least one respective second electromagnetic radiation beam (F2) by any amplitude modulation technique.

[0227] The first electromagnetic radiation beam F1 is characterized by a first orbital angular momentum L1, and the at least one second electromagnetic radiation beam F2 is characterized by a respective at least one second orbital angular momentum L2, where both the first electromagnetic radiation beam F1 and the at least one second electromagnetic radiation beam F2 have respective spectra in the same first frequency band and further have respective radii of curvature that substantially match the first beam radius of curvature value.

[0228] The aforementioned reference electromagnetic radiation beam F0 is characterized by a second orbital angular momentum L0, a second spectrum in a second frequency band different from the first frequency band, and a second beam radius of curvature having a value substantially corresponding to the first beam radius of curvature value.

[0229] The method includes first passing the aforementioned composite electromagnetic radiation beam Q1 through two apertures Z1, Z2 to obtain a first composite beam portion Q1(x1) downstream of the first aperture Z1 and a second composite beam portion Q1(x2) downstream of the second aperture Z2.

[0230] The method further includes transmitting a first composite beam portion Q1(x1) along a first branch A of the interferometer 40 and transmitting a second composite beam portion Q1(x2) along a second branch B of the interferometer 40.

[0231] Next, the method involves superimposing the first composite beam portion Q1(x1) and the second composite beam portion Q1(x2) by the beam splitter 41 of the interferometer along a third branch D of the interferometer to obtain a respective third electromagnetic beam D, and obtaining a respective fourth electromagnetic beam C along a fourth branch C of the interferometer.

[0232] The method further includes a step of performing frequency discrimination of the third electromagnetic beam D around the first frequency band to obtain a third filtered electromagnetic beam F in which contributions originating from the reference beams of the first composite beam portion Q1(x1) and the second composite beam portion Q1(x2) are cancelled and components originating from the respective first modulated beams Fm1(x1), Fm1(x2) and at least one second modulated beam Fm2(x1), Fm2(x2) remain.

[0233] Next, a frequency discrimination of the fourth electromagnetic beam C is performed around the second frequency band of the reference beams to obtain a fourth filtered electromagnetic beam E consisting of a superposition of two reference beams F0(x1), F0(x2) belonging to the first composite beam portion Q1(x1) and the second composite beam portion Q1(x2), respectively.

[0234] The method includes detecting the third filtered electromagnetic beam F by a first beam detector 1 to generate a first electrical signal V1 representative of the intensity of the electromagnetic radiation of the third filtered electromagnetic beam F, and detecting the first phase difference value ΔP ab in the first beam detector 1, which depends on the values ​​taken by the first modulation function a(t) and the second modulation function b(t), a first phase difference value ΔP between the components of the third filtered electromagnetic beam originating from the first composite beam filtering portion Fm1(x1), Fm2(x1) and the components of the third electromagnetic beam originating from the second composite beam filtering portion Fm1(x2), Fm2(x2) is calculated based on the first electrical signal V1. ab and determining:

[0235] The method further includes detecting the fourth filtered electromagnetic beam E by the second beam detector 2 to generate a second electrical signal V2 representing the intensity of the electromagnetic radiation of the fourth filtered electromagnetic beam E, and determining in the second beam detector 2 a second phase difference value ΔR between two reference beams F0(x1), F0(x2) belonging to the first composite beam portion Q1(x1) and the second composite beam portion Q1(x2) based on the second electrical signal V2.

[0236] The method finally includes: ab and a step of demultiplexing and demodulating the information a(t), b(t) modulated by the first modulated beam Fm1 and the at least one second modulated beam Fm2, respectively, based on the second phase difference value ΔR.

[0237] It should be noted that the demodulation and modulation performed in this manner is advantageously "localized" modulation and demultiplexing, in the sense that it is based on detection of only a limited, or "localized," portion of the electromagnetic beam, without requiring detection of the entire wavefront of the beam.

[0238] According to an embodiment of the method, the aforementioned steps of demultiplexing and demodulation are performed by generating a first phase difference value ΔP ab The difference value Q2 = ΔP is obtained by dividing the second phase difference value ΔR by the second wave number k' from the product obtained by dividing the first wave number k. ab / k-ΔR / k', and then such a determined difference value Q2=ΔP ab / k-ΔR / k′, and demultiplexing and demodulating the modulated information for each of the first modulated beam Fm1 and the at least one second modulated beam Fm2.

[0239] The first wavenumber k is the wavenumber corresponding to the main beam defined as k=2π / λ, where λ is the wavelength of the main beam, and the second wavenumber k' is the wavenumber corresponding to the reference beam defined as k'=2π / λ', where λ' is the wavelength of the reference beam.

[0240] The difference value Q2 above = ΔP ab / k-ΔR / k' represents a combination of values ​​taken by the first modulation function a(t) and the second modulation function b(t), and is independent of the positional tilt condition between the first and second detectors, and is also independent of phase variations due to disturbances experienced by the transmitted composite beam before reception.

[0241] According to an implementation option of the method, the first electromagnetic radiation beam F1 and the at least one second electromagnetic radiation beam F2 are digitally amplitude modulated according to the amplitudes of a first modulation function a(t) and at least one second modulation function b(t).

[0242] Difference value Q2=ΔP ab / k-ΔR / k′ may take on a number of expected values, representing respective combinations of digital amplitude values ​​taken by the first modulation function a(t) and the at least one second modulation function b(t).

[0243] According to an implementation option of the method, the first electromagnetic radiation beam F1 and the at least one second electromagnetic radiation beam F2 are digitally amplitude modulated in a binary manner, the first modulation function a(t) and the at least second modulation function b(t) being capable of taking the logical values ​​0 or 1.

[0244] In this case, the method includes the further step of detecting the received power or intensity of the third filtered electromagnetic beam F, which corresponds to the first electrical signal V1 detected by the first beam detector 1, and comparing the received power or intensity with a minimum threshold value.

[0245] The determined difference (Q2 = ΔP ab / k-ΔR / k') is the first expectation value (ΔP 10 / k-ΔR / k'), or the second expectation value (ΔP 01 / k-ΔR / k'), or a third expectation value (ΔP 11 / k-ΔR / k').

[0246] In this case, the step of demultiplexing and demodulating the modulated information comprises: -Determined difference (ΔP ab / k-ΔR / k') is the first expected value (ΔP 10 / k-ΔR / k′), the first modulated beam Fm1 carries information corresponding to 1 and the second modulated beam Fm2 carries information corresponding to 0; -Determined difference (ΔP ab / k-ΔR / k') is the second expected value (ΔP 01 / k-ΔR / k′), the first modulated beam Fm1 carries information corresponding to 0 and the second modulated beam Fm2 carries information corresponding to 1; -Determined difference (ΔP ab / k-ΔR / k') is the third expected value (ΔP 11 / k-ΔR / k′), the first modulated beam Fm1 carries information corresponding to 1 and the second modulated beam Fm2 carries information corresponding to 1; - recognizing that the first modulated beam Fm1 transmits information corresponding to 0 and the second modulated beam Fm2 transmits information corresponding to 0 when the received power or intensity of the third filtered electromagnetic beam F is lower than the aforementioned minimum threshold.

[0247] According to another embodiment of the method, the first phase difference value ΔP ab is dependent on an average intensity Im1 of the third filtered electromagnetic beam F associated with the respective interference pattern, and said first electrical signal V1 represents such average intensity of the third filtered electromagnetic beam F detected by the first beam detector 1. The second phase difference value ΔR is dependent on an average intensity Im2 of the fourth filtered electromagnetic beam E associated with the respective interference pattern, and said second electrical signal V2 represents the average intensity Im2 of the fourth filtered electromagnetic beam E detected by the second beam detector 2.

[0248] In this case, the method further comprises the step of keeping the average intensity Im2 of the fourth filtered electromagnetic beam E constant at a predetermined intensity value and of the second phase difference value ΔR being set to a predetermined value and kept constant by a feedback control loop controlled by the second electrical signal V2 and acting on the first composite beam portion Q1(x1).

[0249] According to an implementation option, the first electromagnetic radiation beam F1 and the at least one second electromagnetic radiation beam F2 are digitally amplitude modulated according to the amplitudes of a first modulation function a(t) and at least one second modulation function b(t).

[0250] In this case, the first phase difference value = ΔP ab may take on a number of expected values, representing respective combinations of digital amplitude values ​​taken by the first modulation function a(t) and the at least one second modulation function b(t).

[0251] According to an implementation option, the first electromagnetic radiation beam F1 and the at least one second electromagnetic radiation beam F2 are digitally amplitude modulated in a binary manner (e.g. by On-Off Keying -OOK-modulation or other modulation methods known per se), the amplitudes of the first modulation function a(t) and of the at least one second modulation function b(t) may take on the logical values ​​0 or 1.

[0252] In this case, the method includes the further step of detecting the received power or intensity of the third filtered electromagnetic beam F, which corresponds to the first electrical signal V1 detected by the first beam detector 1, and comparing the received power or intensity with a minimum threshold value.

[0253] The determined first phase difference value ΔP ab is the first expectation value ΔP that depends on the first orbital angular momentum L1 10 , or the second expectation value ΔP depending on the second orbital angular momentum L2 01 , or a third expectation value ΔP that depends on the combination of the first and second orbital angular momentum 11 It is possible to take.

[0254] The step of demultiplexing and demodulating the modulated information includes: The determined first phase difference value ΔP ab is the first expected value ΔP 10 recognize that, if Fm1=Fm2, the first modulated beam Fm1 transmits information corresponding to 1 and the second modulated beam Fm2 transmits information corresponding to 0; The determined first phase difference value ΔP ab is the second expected value ΔP 01 recognize that, when Fm1=Fm1+Fm2, the first modulated beam Fm1 transmits information corresponding to 0 and the second modulated beam Fm2 transmits information corresponding to 1; The determined first phase difference value ΔP ab is the third expected value ΔP 11 recognize that, when Fm1=Fm2, the first modulated beam Fm1 transmits information corresponding to 1 and the second modulated beam Fm2 transmits information corresponding to 1; - recognizing that when the received power or intensity of the third filtered electromagnetic beam F is lower than said minimum threshold, the first modulated beam Fm1 transmits information corresponding to 0 and the second modulated beam Fm2 transmits information corresponding to 0.

[0255] According to one embodiment of the method, the number of orbital angular momentum multiplexed modulated beams is equal to N, which is greater than two.

[0256] Each modulated beam is characterized by a respective orbital angular momentum Li and is amplitude modulated by a respective modulation function, and each possible combination of values ​​of the modulation functions for the N modulated beams corresponds to a first phase difference ΔP a1...aN and / or the difference between them (Q2 = ΔP a1...aNb / k-ΔR / k').

[0257] According to the embodiment of the method shown in FIG. 8, the method further comprises the steps of arranging in the interferometer, prior to superposition of the first and second composite beam portions, a first additional optical path (along the branches shown as A and G in FIG. 8) for the first composite beam portion Q1(x1) and arranging a second additional optical path (along the branches shown as B, B', G in FIG. 8, where B' makes two passes) for the second composite beam portion Q1(x2).

[0258] The aforementioned first and second additional optical paths are each provided with a phase difference between the second composite beam portion Q1(x2) and the first composite beam portion Q1(x1) due to the total optical path difference between the respective entrance apertures and the first beam detector 1.

number

number

[0259] According to another embodiment of the method, the step of keeping the average intensity Im2 of the fourth filtered electromagnetic beam E constant comprises controlling, by a control unit 61 receiving as input the second electrical signal V2 and the setting signal SP, the first mirror 42 on which the first composite beam portion Q1(x1) falls in order to vary in a controlled manner the length of the optical path A traversed by the first composite beam portion (Q1(x1)), thereby varying the phase shifts of said first and second electromagnetic beam portions, Q1(x1) and Q2(x2), respectively, and modifying in a controlled manner dependent on the second electrical signal V2 and the setting signal SP, the average intensity (Im2) detected by the second detector 2, in order to keep the second phase difference value ΔR, and the average intensity Im2 of the fourth filtered electromagnetic beam E constant, and setting them to respective predefined values ​​dependent on the setting signal SP.

[0260] According to an implementation option, the variation of the optical path A is obtained by translating the first mirror 42 in a controlled manner by an actuator 43 controlled by a control unit 61 (this can be configured, for example, by performing a "PID control", proportional-integral-derivative).

[0261] According to a further embodiment of the method shown in FIG. 9, the step of keeping the average intensity Im2 of the fourth filtered electromagnetic beam E constant comprises the step of detecting at the second beam detector 2 the intensity I R1 and at the second beam detector 2, the intensity I of the reference beam associated with the second composite beam portion is compensated for fluctuations in the intensity I R2 and compensating for variations in

[0262] The aforementioned step of compensating includes: - deflecting a part of the beam corresponding to a first composite beam portion Q1(x1) in a first branch A of the interferometer; a third beam detector 63 detects the intensity of the deflected portion of the beam corresponding to the first composite beam portion Q1(x1) and generates a first compensated electrical signal GI R1 and - deflecting a part of the beam corresponding to the second composite beam portion Q1(x2) in a second branch B of the interferometer; a fourth beam detector 64 detects the intensity of the second beam portion and generates a second compensated electrical signal GI representative of the intensity of the second beam portion; R2 and a first compensation electrical signal GI R1 and a second compensation electrical signal GI R2 and the processing unit 65 modifies the setting signal SP based on the

[0263] According to implementation options of the method, all said electromagnetic beams are light beams and / or laser beams.

[0264] According to an embodiment of this method, the orbital angular momentum of the reference beam is always known.

[0265] According to a particular implementation option, the orbital angular momentum of the reference beam takes a constant value L0=0.

[0266] According to an embodiment of the method, the second frequency band is substantially monochromatic.

[0267] According to a particular implementation option, the second frequency band is adjacent to the first frequency band.

[0268] According to an implementation option, the step of performing frequency discrimination of the first or second electrical signal of the composite beam comprises performing frequency filtering.

[0269] With reference to FIGS. 7-9, a system 100 for demultiplexing and demodulating amplitude modulated signals that have been grouped by multiplexing on orbital angular momentum variables is described below.

[0270] The type of signal to be demultiplexed and demodulated comprises a composite electromagnetic radiation beam Q1 consisting of a superposition of a reference beam F0 and a main beam consisting of a superposition of a first modulated beam Fm1 and at least one second modulated beam Fm2, in accordance with what has already been described previously with reference to the method according to the invention.

[0271] The system 100 comprises a screen with two apertures Z1, Z2 and is configured to pass a composite electromagnetic radiation beam Q1 through the two apertures Z1, Z2 such that a first composite beam portion Q1(x1) is obtained downstream of the first aperture Z1 and a second composite beam portion Q1(x2) is obtained downstream of the second aperture Z2.

[0272] The system 100 further comprises an interferometer 40 arranged downstream with respect to the two apertures Z1, Z2, comprising a first interferometer branch A configured to be traversed by the first composite beam portion Q1(x1), a second interferometer branch B configured to be traversed by the second composite beam portion Q1(x2), and a beam splitter 41 configured to superimpose the first composite beam portion Q1(x1) and the second composite beam portion Q1(x2) along a third branch D of the interferometer to obtain a respective third electromagnetic beam D, and to obtain a respective fourth electromagnetic beam C along a fourth branch C of the interferometer.

[0273] The interferometer 40 further comprises a first frequency discrimination means 45 and a second frequency discrimination means 44 .

[0274] The first frequency discrimination means 45 is configured to frequency discriminate the third electromagnetic beam D around the first frequency band to obtain a third filtered electromagnetic beam F in which components originating from the reference beams of the first composite beam portion Q1(x1) and the second composite beam portion Q1(x2) are cancelled and components originating from the respective first modulated beams Fm1(x1), Fm1(x2) and at least one second modulated beam Fm2(x1), Fm2(x2) remain.

[0275] The second frequency discrimination means 44 is configured to discriminate in frequency the fourth electromagnetic beam C around a second frequency band of the reference beams to obtain a fourth filtered electromagnetic beam E consisting of a superposition of two reference beams F0(x1), F0(x2) belonging to the first composite beam portion Q1(x1) and the second composite beam portion Q1(x2), respectively.

[0276] The system 100 further comprises a first beam detector 1 configured to detect the aforementioned third filtered electromagnetic beam F and generate a first electrical signal V1 representative of the intensity of the electromagnetic radiation of the third filtered electromagnetic beam F.

[0277] The system 100 further comprises a second beam detector 2 configured to detect the aforementioned fourth filtered electromagnetic beam E and generate a second electrical signal V2 representative of the intensity of the electromagnetic radiation of the fourth filtered electromagnetic beam E.

[0278] The system 100 finally comprises a processing means 65, which is - based on the first electrical signal V1, in the first beam detector 1, a first phase difference value ΔP between the components of the third filtered electromagnetic beam originating from the one composite beam filtering portion Fm1(x1), Fm2(x1) and the components of the third electromagnetic beam originating from the second composite beam filtering portion Fm1(x2), Fm2(x2) ab Such a first phase difference value ΔP ab depends on the values ​​taken by the first modulation function a(t) and the second modulation function b(t); - determining, in a second beam detector 2, a second phase difference value ΔR between two reference beams F0(x1), F0(x2) belonging to the first composite beam portion Q1(x1) and to the second composite beam portion Q1(x2) based on said second electrical signal V2; the first phase difference value ΔP ab and configured to demultiplex and demodulate information a(t), b(t) modulated by the first modulated beam Fm1 and the at least one second modulated beam Fm2, respectively, based on the second phase difference value ΔR.

[0279] According to an implementation option of the system, the first frequency discriminator means 45 comprises an optical bandpass filter whose passband corresponds to said first band frequencies.

[0280] According to an implementation option of the system, the second frequency discrimination means 44 comprises a narrowband optical bandpass filter (laser line) whose passband corresponds to said second band frequency of the reference beam.

[0281] According to an implementation option of the system, the first and second beam detectors 1, 2 comprise a first photodiode and a second photodiode, respectively.

[0282] According to an implementation option of the system, the processing means 65 comprises one or more electronic processors 65 .

[0283] According to one embodiment, the system 100 further comprises a control unit 61 adapted to receive as input the second electrical signal V2 and the setting signal SP, an actuator 43 controlled by the control unit 61, and a first mirror 42 controlled by the actuator 43 and positioned such that the first composite beam portion Q1(x1) falls on it.

[0284] The control unit 61 is configured to vary in a controlled manner the length of the optical path A traversed by the first composite beam portion Q1(x1), thereby varying the phase shift of said first Q1(x1) and second Q2(x2) electromagnetic beam portions and modifying the average intensity Im2 detected by the second detector 2 in a controlled manner dependent on the second electrical signal V2 and the setting signal SP, and is configured to set and keep constant the second phase difference value ΔR and the average intensity Im2 of the fourth filtered electromagnetic beam E to respective predefined values ​​dependent on the setting signal SP.

[0285] According to one embodiment, the system 100 further comprises means for balancing the optical paths 46, 47, 48, 49 and is configured to arrange a first additional optical path (A, G) for the first composite beam portion Q1(x1) and a second additional optical path (B, B', B', G) for the second composite beam portion Q1(x2) before the superposition of the first and second composite beam portions in the interferometer.

[0286] Such a step of arrangement results in a phase difference between the phase of the second composite beam portion Q1(x2) and the phase of the first composite beam portion Q1(x1) due to the total optical path difference between the respective entrance apertures and the first beam detector 1.

number

number

[0287] According to an implementation option, the aforementioned means for balancing the optical paths comprises a second mirror 48, a compensator 47, a semi-reflector 46 and a third mirror 49 arranged along the path of the second composite beam portion Q1(x2) along the second branch B of the interferometer so as to form the aforementioned second additional optical path (B, B', B', G).

[0288] The aforementioned semi-reflector 46 is further disposed along the path of the first composite beam portion Q1(x2) and configured to transmit the first composite beam portion Q1(x1) and the second composite beam portion Q1(x2) to the beam splitter 41.

[0289] According to one embodiment, the system 100 measures the intensity of a reference beam associated with the first composite beam portion at the second beam detector 2 (I R1 ) and the intensity of the reference beam associated with the second composite beam portion at the second beam detector 2 (I R2 and beam intensity compensation means configured to compensate for variations in

[0290] The beam intensity compensation means comprises a first semi-reflector 66, a first narrowband optical bandpass filter 68, a third beam detector 63, a second semi-reflector 67, a second optical bandpass filter 69, a fourth beam detector 64 and a processing unit 65.

[0291] The first semi-reflector 66 is configured to deflect a portion of the beam corresponding to the first composite beam portion Q1(x1) in the first branch A of the interferometer.

[0292] The first narrowband optical bandpass filter 68 is configured to filter the deflected portion of the beam corresponding to the first composite beam portion Q1(x1) around a second frequency band of the reference beam.

[0293] A third beam detector 63 detects the intensity of the deflected portion of the beam corresponding to the first composite beam portion Q1(x1) and generates a first compensated electrical signal GI R1 The method is configured to generate

[0294] The second semi-reflector 67 is configured to deflect a portion of the beam corresponding to the second composite beam portion Q1(x2) in the second branch B of the interferometer.

[0295] The second optical bandpass filter 69 is configured to filter the polarized portion of the beam corresponding to the second composite beam portion Q1(x2) around a second frequency band of the reference beam.

[0296] A fourth beam detector 64 detects the intensity of the deflected portion of the beam corresponding to the second composite beam portion Q1(x2) and generates a second compensated electrical signal GI R2 The method is configured to generate

[0297] The processing unit 65 outputs a first compensated electrical signal GI R1 and a second compensation electrical signal GI R2 The setting signal SP is changed based on the

[0298] According to implementation options of the system, the aforementioned electrical signals generated by the different beam detectors are suitably amplified in a self-evident and known manner by respective electronic amplifiers (indicated in Figures 7 to 9 by reference numbers 71, 72, 73, 74).

[0299] According to a possible embodiment, the aforementioned system 100 is configured to carry out a method according to any of the method embodiments described above.

[0300] By way of non-limiting example, a detailed description of embodiments of methods and systems according to the present invention is provided below, in which interferometric techniques and some variants thereof enable communication of OOK (On-Off Keying) amplitude modulated and orbital angular momentum multiplexed signals using composite beams.

[0301] In this example, a light beam is considered and the beam detector is a photodiode.

[0302] As shown in FIG. 7, the composite beam Q1 generated, modulated and multiplexed as described above (see, for example, FIG. 3) enters two branches A and B of the interferometer through two apertures Z1 and Z2.

[0303] The beam in branch A is split into two beams along branches C and D by a beam splitter 41 .

[0304] The beam in branch B is also split into two beams along branches C and D by a beam splitter 41 .

[0305] The beam propagating along branch C is filtered by a laser line bandpass filter 44 that allows only the reference beam to be transmitted (frequency discrimination).

[0306] The center band wavelength of the laser line filter 44 must be equal to the wavelength of the laser used as the reference beam in transmission.

[0307] In section E, on the plane containing photodiode 2, along the optical axis, the fields of the reference beam alone originating from branches A and B, respectively, can be expressed as:

[0308]

number

number

[0309] Where:

number

number

[0310] Phase term [Number] [Number] It should be noted that [it] is due to the optical paths of the two arms of the interferometer. In an unbalanced configuration as shown in Figure 7, there is no limitation as long as ΔL < c / Δυ. Here, ΔL is the difference in the optical paths of the interferometer arms, Δυ is the information bandwidth, and c is the speed of light in a vacuum.

[0311] In the case of limitations arising from this perspective, for example, in the case of a high information bandwidth (GHz or dozens of GHz), or when the distance between the two apertures Z1 and Z2 is large, an embodiment using a balanced interferometer described later with reference to Figure 8 can be used.

[0312] Anyway, since the unbalanced interferometer has a very simple structure, it still attracts great attention and can be used practically if the relationship of ΔL < c / Δυ is sufficiently verified.

[0313] field [Number] [Number] is defined as follows.

[0314] [Number]

[0315] Here, the various quantities have the meanings already explained above.

[0316] place

number

number

[0317]

number

[0318] Here, IR1 and IR2 are the

number

number

[0319] Interference pattern intensity is converted into an electrical signal by the photodiode 2. The electrical signal is suitably amplified, conditioned and applied at the input of the control unit 61.

[0320] According to implementation options, such a control unit 61 performs a control of the PID type (Proportional-Integral-Derivative) known per se. The PID control is based on the intensity measured and amplified by the photodiode 2. is substantially equal to the value of the set signal SP.

[0321] According to various possible embodiments, said actuators of the first mirror 43 are piezoelectric, magnetic or capacitive or of any other type known per se.

[0322] Even if the position and tilt of the wavefront illuminating the interferometer changes, the intensity is further kept approximately constant by PID control. The tilt can be caused by the relative displacement between the wavefront and the interferometer, or by vibrations or perturbations of the propagating wavefront.

[0323] Strength I R1 and I R2 is substantially constant during the measurement, the quantity ΔR is also substantially constant, as shown in relationship [1].

[0324] Strength I R1 and I R2 If is not constant, an embodiment with a compensated set signal, described below with reference to FIG. 9, is used.

[0325] It should be noted that this feedback solution of the interferometer (also called "locking interferometer") is rejected here in a new and unique way, since the "locking" of the interferometer is performed by a reference beam of the composite beam (by frequency discrimination), and the main beam of the composite beam is discriminated in frequency along the other branch of the interferometer.

[0326] The beam along branch D is filtered by a bandpass filter 45 that allows only the main beam to be transmitted.

[0327] The center band wavelength of the bandpass filter 45 must be equal to the wavelength of the laser used to generate the main beam during transmission, while the filter band must be equal to or greater than the information band to be transferred.

[0328] In section F, on the plane containing photodiode 1, along the optical axis, the fields of the main beams emanating from branches A and B, respectively, can be expressed as:

[0329]

number

number

[0330] Where:

number

number

number

number

[0331] place

number

number

number

[0332] place

number

number

[0333]

number

[0334] Here, I P1 and I P2 are the locations

number

number

[0335] The phase difference ΔR in the field of the section E of the interferometer of the photodiode 2 is given by:

number

[0336] The phase difference ΔP of the field of the section F of the interferometer of the photodiode 1 for various combinations of the digital modulation functions a(t)b(t) is:

[0337] For a(t)=0, b(t)=0: Since the amplitudes of the main beams are cancelled, the phase difference cannot be determined.

[0338] For a(t)=1, b(t)=0: There is only the first main beam with angular momentum L1.

number

[0339] For a(t)=0, b(t)=1: There is only a second main beam with angular momentum L2.

number

[0340] For a(t)=1, b(t)=1: With both main beams, we get:

number

[0341] By calculating the phase difference ΔP / k−ΔR / k′ for all possible combinations of modulated signals, we obtain:

[0342] For a(t)=0, b(t)=0: Since the amplitudes of the main beams are cancelled, the phase difference cannot be determined.

[0343] For a(t)=1, b(t)=0: The following equation [3] holds true.

number

[0344] For a(t)=0, b(t)=1: The following equation [4] holds true.

number

[0345] For a(t)=1, b(t)=1: The following equation [5] holds true.

number

[0346] As already indicated above, advantageously the relationship is independent of phase differences due to position tilt and distortion of the propagating wavefront, which can be eliminated by the presence of a reference beam.

[0347] Here, from equations [3], [4] and [5], we obtain the value of the phase difference ΔP for various combinations of the modulation functions a(t) and b(t).

[0348] For a(t)=0, b(t)=0: Since the amplitudes of the main beams are cancelled, the phase difference cannot be determined.

[0349] For a(t)=1, b(t)=0: The following equation holds:

number

[0350] For a(t)=0, b(t)=1: The following equation holds:

number

[0351] For a(t)=1, b(t)=1: The following equation holds:

number

number

[0352] The quantity ΔR is a fixed quantity that can be selected in various combinations by changing the value of the set point signal SP of the PID control.

[0353] Such a quantity ΔR also remains constant even as the position tilt, the phase difference due to the position inclination, and the distortion of the propagating wavefront are changed by the negative feedback of the PID control.

[0354] Furthermore, the quantity (k / k')

number

number

[0355] As a result, the quantity ΔP 10 , ΔP 01 , ΔP 11 can be set to three different values ​​simply by appropriately choosing the topological charges l0, l1, l2 (i.e., the respective orbital angular momenta L0, L1, L2) of the reference beam and the two main beams.

[0356] Value ΔP 10 , ΔP 01 , ΔP 11 is also chosen to be as many different known predefined nonzero values ​​as possible, as shown in equation [2]. 10 、 01 、 11 must be chosen to produce an average intensity detected at photodiode 1 having

[0357] As a result, the respective average intensities measured on reception are recognizable and indicate the modulation value applied to each of the two main beams, 0 or 1. The information encoded therein can therefore be decoded, i.e. demodulated and recognised.

[0358] In fact, by measuring the intensity of the photodiode 1 following a pre-calibration operation, it is possible to demodulate the transmitted information.

[0359] Calibration, necessary to associate to each combination a(t), b(t) the corresponding average intensity value detected by photodiode 1, consists of transmitting a known sequence of all combinations and recording by the processing unit the corresponding average intensities measured by photodiode 1.

[0360] The demodulation consists in associating each combination, as defined in the previous calibration, with each intensity measured by the photodiode 1. Such association can be performed by the same processing unit used during the calibration step.

[0361] Obviously, the system is P1 , I P2 , and ΔP are designed to produce a one-to-one dependence between the combination of modulation functions a(t)b(t) and the average intensity measured by photodiode 1 under various conditions of use.

[0362] For stationary applications, calibration may be performed once or periodically to compensate for drift. For non-stationary applications, calibration must be performed periodically using a specific communication protocol.

[0363] The status a(t)=0, b(t)=0 is not yet known and its phase is undetermined (as above). This combination is identified deterministically when the intensity of photodiode 1 is cancelled, 00 =0.

[0364] Note that in this example illustration, two main beams with two modulation functions are used.

[0365] However, in other possible implementations there may be more main beams and respective modulation functions.

[0366] Indeed, by appropriately choosing the orbital angular momentum of the main beam, it can be ensured that the phase difference ΔP for various combinations of modulation functions is different.

[0367] For interferometers, the relative average intensity is calculated by substituting each ΔP in equation [2] They are different from each other, and thus it is also necessary to confirm that they are recognizable.

[0368] According to another embodiment shown in FIG. 8, a balanced interferometer is used.

[0369] The function of the balanced interferometer corresponds to the function of the unbalanced interferometer with respect to what occurs after the beam splitter (FIG. 7).

[0370] In this configuration, the beam of branch A is reflected by the half-reflecting plate 46 and moves along section G before being split into two beams along branches C and D (as in the configuration using an unbalanced interferometer), while the beam B hits the second mirror 48, crosses the compensating plate 47, is reflected by the half-reflecting plate 46 towards the third mirror 49, is reflected by the mirror 49 towards the half-reflecting plate 46, and moves along section G before being split into two beams along branches C and D (as in the configuration using an unbalanced interferometer).

[0371] The advantage of this configuration is that by appropriately arranging the mirrors 42, 48, 49,

Number

Number

[0372] to G and twice the optical path of section B'.

Number

Number

[0373] Compensation plate 47 is used to compensate the light path within semi-reflector 46 (eg, as is done in other types of interferometers, such as a Michelson interferometer).

[0374] According to another embodiment, shown in FIG. 9, a compensation of the setting signal SP is used.

[0375] According to the relationship [1], the constant average strength corresponds to a constant phase ΔR, which is the sum of the two quantities I R1 and I R2 This only occurs if is essentially constant.

[0376] However, in non-stationary applications, such quantities can vary significantly and need to be compensated for in order to keep the phase difference ΔR of the reference beam constant.

[0377] To do this, it is possible to act on the setpoint signal SP of the PID control.

[0378] As shown in FIG. 9, it is sufficient to use two semi-reflectors 66, 67, two laser line filters 68, 69 with a wavelength matching that of the reference beam used for transmission, two photodiodes 63, 64 and two amplifiers 73, 74 to take the portion of the beam that hits the apertures Z1 and Z2.

[0379] The voltage value of the set signal SP applied to the PID controller for compensation is determined according to the following relationship:

number

[0380] G.I. R1 and G.I. R2 are the intensities I R1 and I R2 is the voltage value of the signal at the output of the amplifier proportional to, where G is the proportionality constant.

[0381] G is the first electrical signal V1 and its intensity is a voltage value measured at the output of amplifier 71 (see FIG. 7) that is proportional to (or corresponds to), G is a proportionality constant, and ΔR is the value of the phase difference that should be kept constant. The calculations may be performed by processing unit 65 (which may correspond to a processing unit configured to obtain the demodulated functions a(t) and b(t) or may be performed using an additional processor).

[0382] As may be noticed, the objectives of the present invention are fully achieved by the above-presented system and method thanks to its functional and structural features.

[0383] Indeed, the systems and methods for transmitting and receiving electromagnetic beams presented above can accurately and reliably detect the orbital angular momentum of a received beam in a manner that is independent of the receiver's position tilt and independent of the distortions experienced by the beam during propagation.

[0384] This is achieved by dual spatial detection at two different points of a composite beam which, in addition to the beam under investigation, also contains an additional reference beam.

[0385] The ability to accurately and reliably detect the orbital angular momentum of a received beam can then be advantageously applied to a number of different applications, including, for example, exploiting angular momentum variables for beam characterization and telecommunications purposes.

[0386] With respect to telecommunications applications, the method and system of the present invention allows for the exploitation of orbital angular momentum variables as an additional degree of freedom, and can be advantageously used for both signal modulation and signal multiplexing.

[0387] In particular, orbital angular momentum provides an additional level of multiplexing (with consequential obvious advantages), allowing signals that are identical in terms of other multiplexing variables (e.g., time or frequency) and that are identifiable based on different orbital angular momentum to be grouped together.

[0388] Furthermore, the present invention provides an efficient method for locally demultiplexing and demodulating amplitude modulated signals grouped by multiplexing in orbital angular momentum variables.

[0389] Such demultiplexing and demodulation is advantageously local, since it can be performed based on local detection and by detecting even a small portion of the wavefront of the beam.

[0390] Thanks to such a demultiplexing and demodulation method and associated system, it is possible to realize the aforementioned telecommunication method and system based on orbital angular momentum multiplexing and to manage the simultaneous transfer of multiple pieces of information on a single transmission channel (using orbital angular momentum multiplexing).

[0391] Those skilled in the art can modify and adapt the above system and method embodiments and substitute other functionally equivalent elements to meet their foreseeing needs without departing from the scope of the following claims. Each of the features described as belonging to possible embodiments can be achieved without regard to the other embodiments described.

Claims

1. 1. A method for demultiplexing and demodulating amplitude modulated signals grouped by multiplexing with orbital angular momentum variables, comprising: The demultiplexed and demodulated signal comprises a composite electromagnetic radiation beam (Q1) consisting of a superposition of a reference beam (F0) and a main beam consisting of a superposition of a first modulated beam (Fm1) and at least one second modulated beam (Fm2), said first modulated beam (Fm1) is obtained by modulating, by any amplitude modulation technique, a first information represented by a first modulation function a(t) on a first electromagnetic radiation beam (F1), and said at least one second modulated beam (Fm2) is obtained by modulating, by any amplitude modulation technique, at least one second information represented by a second modulation function b(t) on at least one respective second electromagnetic radiation beam (F2), The first electromagnetic radiation beam (F1) has a first orbital angular momentum (L １ ), wherein the at least one second electromagnetic radiation beam (F2) has at least one respective second orbital angular momentum (L ２ ), both the first electromagnetic radiation beam (F1) and the at least one second electromagnetic radiation beam (F2) having respective spectra in the same first frequency band and further having respective radii of curvature substantially matching the first beam radius of curvature value; The reference beam (F0) has a second orbital angular momentum (L ０ ), a second spectrum in a second frequency band different from the first frequency band, and a second beam radius of curvature having a value substantially corresponding to the first beam radius of curvature value; The method comprises: - passing said composite electromagnetic radiation beam (Q1) through two apertures (Z1, Z2) to obtain a first composite beam portion (Q1(x1)) downstream of the first aperture (Z1) and a second composite beam portion (Q1(x2)) downstream of the second aperture (Z2); - transmitting said first composite beam portion (Q1(x1)) along a first branch (A) of an interferometer (40) and transmitting said second composite beam portion (Q1(x2)) along a second branch (B) of the interferometer; - superimposing the first composite beam portion (Q1(x1)) and the second composite beam portion (Q1(x2)) along a third branch (D) of the interferometer using a beam splitter (41) of the interferometer to obtain a respective third electromagnetic beam (D) and along a fourth branch (C) of the interferometer to obtain a respective fourth electromagnetic beam (C); performing a frequency discrimination of the third electromagnetic beam (D) around said first frequency band in order to obtain a third filtered electromagnetic beam (F) in which the contributions originating from the reference beams of the first composite beam portion (Q1(x1)) and the second composite beam portion (Q1(x2)) have been cancelled and components originating from the respective first modulated beam (Fm1(x1), Fm1(x2)) and at least one second modulated beam (Fm2(x1), Fm2(x2)) remain; performing a frequency discrimination of a fourth electromagnetic beam (C) around said second frequency band of the reference beams in order to obtain a fourth filtered electromagnetic beam (E) consisting of a superposition of two reference beams (F0(x1), F0(x2)) belonging respectively to the first composite beam portion (Q1(x1)) and to the second composite beam portion (Q1(x2)); - detecting said third filtered electromagnetic beam (F) by a first beam detector (1) to generate a first electrical signal (V1) representative of the intensity of the electromagnetic radiation of the third filtered electromagnetic beam (F); - based on said first electrical signal (V1), in the first beam detector (1) a first phase difference value (ΔP ａｂ ), wherein the first phase difference value (ΔP ａｂ ) depends on the values ​​taken by the first modulation function a(t) and the second modulation function b(t); - detecting said fourth filtered electromagnetic beam (E) by a second beam detector (2) to generate a second electrical signal (V2) representative of the intensity of the electromagnetic radiation of the fourth filtered electromagnetic beam (E); - determining, on the basis of said second electrical signal (V2), in a second beam detector (2), a second phase difference value (ΔR) between two reference beams (F0(x1), F0(x2)) belonging to the first composite beam portion (Q1(x1)) and to the second composite beam portion (Q1(x2)); - the first phase difference value (ΔP ａｂ demultiplexing and demodulating information (a(t), b(t)) modulated on the first modulated beam (Fm1) and the at least one second modulated beam (Fm2), respectively, based on the first modulated beam (Fm1) and the second phase difference value (ΔR); A method comprising:

2. Said steps of demultiplexing and demodulation include: - the first phase difference value (ΔP ａｂ ) to subtract the second phase difference value (ΔR) divided by the second wave number k′, and obtain the difference value (Q2=ΔP ａｂ / k-ΔR / k′), where a first wavenumber (k) corresponds to the main beam and is defined as k=2π / λ, where λ is the wavelength of the main beam; a second wavenumber (k') corresponds to the reference beam and is defined as k'=2π / λ', where λ' is the wavelength of the reference beam; The difference value (Q2 = ΔP ａｂ / k-ΔR / k′) represents a combination of values ​​taken by a first modulation function a(t) and a second modulation function b(t), which is independent of positional tilt conditions between the first and second beam detectors and independent of phase variations due to disturbances suffered by the transmitted composite beam before reception; - the determined difference value (Q2 = ΔP ａｂ demultiplexing and demodulating the modulation information for each of the first modulated beam (Fm1) and the at least one second modulated beam (Fm2) based on the ΔR / k-ΔR / k'; The method of claim 1 , comprising:

3. the first electromagnetic radiation beam (F1) and the at least one second electromagnetic radiation beam (F2) are digitally amplitude modulated according to the amplitudes of a first modulation function a(t) and at least one second modulation function b(t), Difference value (Q2=ΔP ａｂ 3. The method of claim 2, wherein the expectation value a(t) represents a combination of digital amplitude values ​​taken by the first modulation function a(t) and the at least one second modulation function b(t).

4. the first electromagnetic radiation beam (F1) and the at least one second electromagnetic radiation beam (F2) are digitally amplitude modulated in a binary manner, the amplitudes of the first modulation function a(t) and of the at least one second modulation function b(t) being capable of taking the logical values ​​0 or 1; the method comprises the further step of detecting a received power or intensity of a third filtered electromagnetic beam (F), which corresponds to the first electrical signal (V1) detected by the first beam detector (1), and comparing said received power or intensity with a minimum threshold value, - the determined difference (Q2 = ΔP ａｂ / k-ΔR / k′) is the first orbital angular momentum (L １ ) the first expectation value (ΔP １０ / k-ΔR / k′), or the second orbital angular momentum (L ２ ) The second expectation value (ΔP ０１ / k-ΔR / k′), or a third expectation value (ΔP １１ / k-ΔR / k′), The step of demultiplexing and demodulating the modulated information comprises: - the determined difference (ΔP ａｂ / k-ΔR / k′) is the first expected value (ΔP １０ / k-ΔR / k′), the first modulated beam (Fm1) transmits information corresponding to 1 and the second modulated beam (Fm2) transmits information corresponding to 0; - the determined difference (ΔP ａｂ / k-ΔR / k′) is the second expected value (ΔP ０１ / k-ΔR / k′), the first modulated beam (Fm1) transmits information corresponding to 0 and the second modulated beam (Fm2) transmits information corresponding to 1; - the determined difference (ΔP ａｂ / k-ΔR / k') is the third expected value (ΔP １１ / k-ΔR / k′), the first modulated beam (Fm1) transmits information corresponding to 1 and the second modulated beam (Fm2) transmits information corresponding to 1; - recognizing that the first modulated beam (Fm1) transmits an information corresponding to 0 and that the second modulated beam (Fm2) transmits an information corresponding to 0, if the received power or intensity of the third filtered electromagnetic beam F is lower than said minimum threshold value; Including, The method according to claim 3.

5. - the first phase difference value (ΔP ａｂ ) is dependent on an average intensity (Im1) of a third filtered electromagnetic beam (F) associated with the respective interference pattern, said first electrical signal (V1) representing said average intensity of the third filtered electromagnetic beam (F) detected by the first beam detector (1); a second phase difference value (ΔR) dependent on an average intensity (Im2) of a fourth filtered electromagnetic beam (E) associated with the respective interference pattern, said second electrical signal (V2) representing said average intensity (Im2) of the fourth filtered electromagnetic beam (E) detected by a second beam detector (2); The method comprises: controlling the length of the optical path traversed by the first composite beam portion (Q1(x1)) by means of a feedback control loop controlled by a second electrical signal (V2) acting on said first composite beam portion (Q1(x1)) such that a second phase difference value (ΔR) is set to a predefined value and kept constant, thereby keeping said average intensity (Im2) of the fourth filtered electromagnetic beam (E) constant at a predefined intensity value, The method of claim 1 , further comprising:

6. the first electromagnetic radiation beam (F1) and the at least one second electromagnetic radiation beam (F2) are digitally amplitude modulated according to the amplitudes of a first modulation function a(t) and at least one second modulation function b(t), First phase difference value=ΔP ａｂ 6. The method of claim 5, wherein x(t) can take on a number of expected values, representing respective combinations of digital amplitude values ​​taken by the first modulation function a(t) and the at least one second modulation function b(t).

7. the first electromagnetic radiation beam (F1) and the at least one second electromagnetic radiation beam (F2) are digitally amplitude modulated in a binary manner, the amplitudes of the first modulation function a(t) and of the at least one second modulation function b(t) being capable of taking the logical values ​​0 or 1; the method comprises the further step of detecting a received power or intensity of a third filtered electromagnetic beam (F), which corresponds to the first electrical signal (V1) detected by the first beam detector (1), and comparing said received power or intensity with a minimum threshold value, The determined first phase difference value (ΔP ａｂ ) is the first orbital angular momentum (L １ ) the first expectation value (ΔP １０ ), or the second orbital angular momentum (L ２ ) The second expectation value (ΔP ０１ ), or a third expectation value (ΔP １１ ) can be obtained, The step of demultiplexing and demodulating the modulated information comprises: The determined first phase difference value (ΔP ａｂ ) is the first expected value (ΔP １０ ), the first modulated beam (Fm1) transmits information corresponding to 1 and the second modulated beam (Fm2) transmits information corresponding to 0; The determined first phase difference value (ΔP ａｂ ) is the second expected value (ΔP ０１ ), the first modulated beam (Fm1) transmits information corresponding to 0 and the second modulated beam (Fm2) transmits information corresponding to 1; The determined first phase difference value (ΔP ａｂ ) is the third expected value (ΔP １１ ), the first modulated beam (Fm1) transmits information corresponding to 1 and the second modulated beam (Fm2) transmits information corresponding to 1; - recognizing that the first modulated beam (Fm1) transmits an information corresponding to 0 and that the second modulated beam (Fm2) transmits an information corresponding to 0, if the received power or intensity of the third filtered electromagnetic beam F is lower than said minimum threshold value; The method of claim 6, comprising:

8. the number of orbital angular momentum multiplexed modulated beams is equal to N, which is greater than 2; Each modulated beam is characterized by a respective orbital angular momentum (Li) and is amplitude modulated by a respective modulation function, Each possible combination of values ​​of the modulation function for the N modulated beams corresponds to a respective value of the first phase difference (ΔP ａ１．．．ａＮ ) and / or the respective difference values ​​(Q2 = ΔP ａ１．．．ａＮ / k-ΔR / k'), 5. The method according to any one of claims 2 to 4.

9. The method comprises: - in the interferometer, prior to the superposition of the first and second composite beam portions, a first additional optical path (A, G) for the first composite beam portion (Q1(x1)) and a second additional optical path (B, B', B', G) for the second composite beam portion (Q1(x2)), Further comprising: The first additional optical path (A, G) and the second additional optical path (B, B', B', G) are caused by a total optical path difference between the respective entrance apertures and the first beam detector (1) to produce a phase difference between the phase of the second composite beam portion (Q1(x2)) and the phase of the first composite beam portion (Q1(x1)). [0010] To cancel the and the phase difference between the phase of the second composite beam portion (Q1(x2)) and the phase of the first composite beam portion (Q1(x1)) due to the total optical path difference between the respective entrance apertures and the second beam detector (2). [0025] It is predefined and sized to cancel 9. The method according to any one of claims 1 to 8.

10. The step of keeping the average intensity (Im2) of the fourth filtered electromagnetic beam (E) constant comprises: - controlling, by a control unit (61) receiving as input the second electrical signal (V2) and the setting signal (SP), the first mirror (42) on which the first composite beam portion (Q1(x1)) falls in order to vary in a controlled manner the length of the optical path A traversed by the first composite beam portion (Q1(x1)), thereby varying the phase shift of said first (Q1(x1)) and second (Q1(x2)) electromagnetic beam portions and modifying in a controlled manner the average intensity (Im2) detected by the second beam detector (2), which is dependent on the second electrical signal (V2) and the setting signal (SP), and setting and keeping constant the second phase difference value (ΔR) and the average intensity (Im2) of the fourth filtered electromagnetic beam (E), to respective predefined values ​​dependent on the setting signal (SP); The method of claim 5 , comprising:

11. The step of keeping constant the average intensity (Im2) of the fourth filtered electromagnetic beam (E) comprises detecting at the second beam detector (2) the intensity (I Ｒ１ ) and at the second beam detector (2), the intensity of the reference beam associated with the second composite beam portion (I Ｒ２ ) and compensating for variations in The step of compensating comprises: - deflecting a part of the beam corresponding to a first composite beam portion (Q1(x1)) in a first branch (A) of the interferometer; - detecting the intensity of the deflected part of the beam corresponding to the first composite beam portion (Q1(x1)) by a third beam detector (63) and obtaining a first compensation electrical signal (GI Ｒ１ ) - deflecting a part of the beam corresponding to a second composite beam portion (Q1(x2)) in a second branch (B) of the interferometer; - detecting the intensity of the second beam portion by a fourth beam detector (64) and producing a second compensated electrical signal (GI Ｒ２ ) - a first compensation electrical signal (GI Ｒ１ ) and a second compensation electrical signal (GI Ｒ２ modifying the setting signal (SP) by a processing unit (65) based on The method of claim 10, comprising:

12. 12. The method according to any one of claims 1 to 11, wherein all said electromagnetic beams are light beams and / or laser beams.

13. 13. The method according to any one of claims 1 to 12, wherein the orbital angular momentum of the reference beam is always known.

14. The orbital angular momentum of the reference beam is a constant value L ０ The method of claim 13, wherein:

15. 15. The method of claim 1, wherein the second frequency band is substantially monochromatic.

16. The method of claim 15 , wherein the second frequency band is adjacent to the first frequency band.

17. 17. The method of claim 1, wherein the step of performing frequency discrimination of the first or second electrical signal of the combined beam comprises the step of performing frequency filtering.

18. A system (100) for demultiplexing and demodulating amplitude modulated signals grouped by multiplexing with orbital angular momentum variables, comprising: The demultiplexed and demodulated signal comprises a composite electromagnetic radiation beam (Q1) consisting of a superposition of a reference beam (F0) and a main beam consisting of a superposition of a first modulated beam (Fm1) and at least one second modulated beam (Fm2), said first modulated beam (Fm1) is obtained by modulating, by any amplitude modulation technique, a first information represented by a first modulation function a(t) on a first electromagnetic radiation beam (F1), and said at least one second modulated beam (Fm2) is obtained by modulating, by any amplitude modulation technique, at least one second information represented by a second modulation function b(t) on at least one respective second electromagnetic radiation beam (F2), The first electromagnetic radiation beam (F1) has a first orbital angular momentum (L １ ), wherein the at least one second electromagnetic radiation beam (F2) has at least one respective second orbital angular momentum (L ２ ), both the first electromagnetic radiation beam (F1) and the at least one second electromagnetic radiation beam (F2) having respective spectra in the same first frequency band and further having respective radii of curvature substantially matching the first beam radius of curvature value; The reference beam (F0) has a second orbital angular momentum (L ０ ), a second spectrum in a second frequency band different from the first frequency band, and a second beam radius of curvature having a value substantially corresponding to the first beam radius of curvature value; The system (100) comprises: a screen comprising two apertures (Z1, Z2) and configured to pass said composite electromagnetic radiation beam (Q1) through said two apertures (Z1, Z2) so as to obtain, downstream of the first aperture (Z1), a first composite beam portion (Q1(x1)) and, downstream of the second aperture (Z2), a second composite beam portion (Q1(x2)); an interferometer (40) arranged downstream with respect to the two apertures (Z1, Z2), The interferometer (40) a first interferometer branch (A) arranged to be traversed by said first composite beam portion (Q1(x1)), a second interferometer branch (B) arranged to be traversed by said second composite beam portion (Q1(x2)), a beam splitter (41) configured to superimpose the first composite beam portion (Q1(x1)) and the second composite beam portion (Q1(x2)) along a third branch (D) of the interferometer to obtain a respective third electromagnetic beam (D) and along a fourth branch (C) of the interferometer to obtain a respective fourth electromagnetic beam (C); - first frequency discrimination means (45) configured to discriminate in frequency the third electromagnetic beam (D) around said first frequency band in order to obtain a third filtered electromagnetic beam (F) in which the contributions originating from the reference beams of the first composite beam portion (Q1(x1)) and the second composite beam portion (Q1(x2)) are cancelled and components originating from the respective first modulated beam (Fm1(x1), Fm1(x2)) and at least one second modulated beam (Fm2(x1), Fm2(x2)) remain; - second frequency discrimination means (44) configured to discriminate in frequency the fourth electromagnetic beam (C) around said second frequency band of the reference beams in order to obtain a fourth filtered electromagnetic beam (E) consisting of a superposition of two reference beams (F0(x1), F0(x2)) belonging respectively to the first composite beam portion (Q1(x1)) and to the second composite beam portion (Q1(x2)), The system (100) comprises: a first beam detector (1) configured to detect said third filtered electromagnetic beam (F) and to generate a first electrical signal (V1) representative of the intensity of the electromagnetic radiation of the third filtered electromagnetic beam (F); a second beam detector (2) configured to detect the fourth filtered electromagnetic beam (E) and to generate a second electrical signal (V2) representative of the intensity of the electromagnetic radiation of the fourth filtered electromagnetic beam (E); - processing means (65), The processing means (65) - based on said first electrical signal (V1), in the first beam detector (1) a first phase difference value (ΔP ａｂ ), wherein the first phase difference value (ΔP ａｂ ) depends on the values ​​taken by the first modulation function a(t) and the second modulation function b(t); - determining, based on said second electrical signal (V2), in a second beam detector (2), a second phase difference value (ΔR) between two reference beams (F0(x1), F0(x2)) belonging to the first composite beam portion (Q1(x1)) and to the second composite beam portion (Q1(x2)); - the first phase difference value (ΔP ａｂ demultiplexing and demodulating information (a(t), b(t)) modulated on the first modulated beam (Fm1) and the at least one second modulated beam (Fm2), respectively, based on the first modulated beam (Fm1) and the second phase difference value (ΔR); The system (100) is configured to:

19. The system (100) comprises: a control unit (61) adapted to receive as input a second electrical signal (V2) and a setting signal (SP); an actuator (43) controlled by a control unit (61); a first mirror (42) controlled by an actuator (43) and positioned such that the first composite beam portion (Q1(x1)) impinges on it; said control unit (61) is configured to vary in a controlled manner the length of the optical path (A) traversed by the first composite beam portion (Q1(x1)), thereby varying the phase shift of said first (Q1(x1)) and second (Q1(x2)) electromagnetic beam portions, modifying in a controlled manner the average intensity (Im2) detected by the second beam detector (2) in dependence on the second electrical signal (V2) and on a setting signal (SP), and to set and keep constant the second phase difference value (ΔR) and the average intensity (Im2) of the fourth filtered electromagnetic beam (E) to respective predefined values ​​in dependence on the setting signal (SP), 20. The system (100) of claim 18.

20. The system (100) comprises: The method further comprises: means for balancing the optical paths (46, 47, 48, 49), the means for balancing the optical paths (46, 47, 48, 49) comprising: in the interferometer, before the superposition of the first and second composite beam portions, a first additional optical path (A, G) for the first composite beam portion (Q1(x1)) and a second additional optical path (B, B', B', G) for the second composite beam portion (Q1(x2)), It is configured as follows: The arrangement of the first additional optical path (A, G) and the second additional optical path (B, B', B', G) ensures that the phase difference between the second composite beam portion (Q1(x2)) and the first composite beam portion (Q1(x1)) due to the total optical path difference between the respective entrance apertures and the first beam detector (1) is [0030] To cancel the and the phase difference between the phase of the second composite beam portion (Q1(x2)) and the phase of the first composite beam portion (Q1(x1)) due to the total optical path difference between the respective entrance apertures and the second beam detector (2). [0045] and arranging the first additional optical path (A, G) and the second additional optical path (B, B', B', G) so as to cancel 20. The system (100) of claim 19.

21. The means for balancing the optical path comprises: a second mirror (48), a compensator (47), a semi-reflector (46) and a third mirror (49) arranged along the path of the second composite beam portion (Q1(x2)) along the second branch (B) of the interferometer so as to form said second additional optical path (B, B', B', G), the semi-reflector (46) is further disposed along the path of the first composite beam portion (Q1(x1)) and configured to transmit the first composite beam portion (Q1(x1)) and the second composite beam portion (Q1(x2)) to a beam splitter (41); 21. The system (100) of claim 20.

22. The system (100) comprises: At the second beam detector (2), the intensity of the reference beam associated with the first composite beam portion (I Ｒ１ ) and at the second beam detector (2), the intensity of the reference beam associated with the second composite beam portion (I Ｒ２ and a beam intensity compensation means configured to compensate for variations in The beam intensity compensation means includes: a first semi-reflector (66) configured to deflect a part of the beam corresponding to the first composite beam portion (Q1(x1)) in the first branch (A) of the interferometer; a first narrowband optical bandpass filter (68) configured to filter the deflected portion of the beam corresponding to the first composite beam portion (Q1(x1)) around said second frequency band of the reference beam; - detecting the intensity of the deflected part of the beam corresponding to the first composite beam part (Q1(x1)) and obtaining a first compensation electrical signal (GI Ｒ１ a third beam detector (63) configured to generate a a second semi-reflector (67) configured to deflect a part of the beam corresponding to the second composite beam portion (Q1(x2)) in the second branch (B) of the interferometer; a second optical bandpass filter (69) configured to filter the deflected portion of the beam corresponding to the second composite beam portion (Q1(x2)) around said second frequency band of the reference beam; - detecting the intensity of the deflected portion of the beam corresponding to the second composite beam portion (Q1(x2)) and obtaining a second compensation electrical signal (GI Ｒ２ a fourth beam detector (64) configured to generate a - a first compensation electrical signal (GI Ｒ１ ) and a second compensation electrical signal (GI Ｒ２ a processing unit (65) configured to modify the setting signal (SP) based on Equipped with 22. A system according to any one of claims 18 to 21.

23. The system (100) according to any one of claims 18 to 22, wherein the system (100) is configured to perform a method according to any one of claims 1 to 17.

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