Optical system for modifying the polarisation of light

Abstract

Disclosed is an optical system for selectively modifying the polarisation components of an incident light signal (Wi), comprising: o a signal splitter receiving as input the incident light signal (Wi) and generating as output two secondary signals (W1, W2), o a controllable phase shifter (7) for introducing a predefined phase shift between the two signals, o an interferometer (8) receiving as input the signals thus phase shifted and generating as output secondary signals (W'1, W'2) having interfered, a distribution of power between the secondary signals being a function of the phase shift, o at least one polarisation rotator (10) on the path of at least one of the signals coming from the interferometer, for introducing a corresponding difference between the polarisation directions of the secondary signals, o a signal combiner (5) for generating a combined signal, from the signals (W''1, W''2) coming from the interferometer and having passed through the at least one polarisation rotator.

Description

Description Title of the invention: Optical system for modifying the polarization of light technical field

[0001] The present invention relates to the field of optical systems for modifying the polarization of light, which can in particular be applied to optical communication systems. Previous technique

[0002] EP4407896A1 describes an optical communication system comprising a polarization modifier placed in the propagation path of light generated by a transmitter, configured to change the direction of the light's polarization and introduce a phase shift between two perpendicular components of the light's electric field. Several solutions for modifying the polarization are presented, including one that has the drawback of combining a polarization ellipticity modifier and a polarization direction modifier, making the circuit complex. Another proposed solution uses as many polarization rotators as there are possible rotation values, each rotator rotating the electric polarization of the incident wave by a fixed angle.This results not only in a high cost of manufacturing the device but also in the impossibility for the user to precisely adjust the angle of rotation, for example to adapt this angle of rotation to various configurations of a system.

[0003] The rotation of a linearly polarized electromagnetic wave can also be achieved using a Faraday rotator, in which a magnetic field in the same direction as the wave propagation direction is applied to the wave's propagation path. To make this principle electrically controllable, a solenoid surrounding the propagation path is required. This is difficult to implement on a flat waveguide, requires energy during operation, and cannot be used at high frequencies due to the time constant induced by the solenoid's inductance. Description of the invention

[0004] Therefore, there remains a need to further improve optical systems to modify the polarization of a signal and to remedy all or part of the disadvantages of the prior art described above. Summary of the invention The invention addresses this need through an optical system for selectively modifying the polarization components of an incident light signal, comprising at least: o a signal splitter receiving the incident light signal as input and generating two secondary signals as output, o a controllable phase shifter to introduce a predefined phase shift between the two signals, o an interferometer receiving at input the signals thus phase-shifted and generating at output secondary signals that have interfered, with a power distribution between the secondary signals that is a function of said phase shift, o at least one polarization rotator in the path of at least one of the signals from the interferometer, to introduce a corresponding difference between the polarization directions of the secondary signals, o a signal combiner to generate, from the signals coming from the interferometer and having passed through said at least one polarization rotator, a combined signal.

[0005] Thanks to the invention, the polarization components of the combined signal depend on the power distribution of the secondary signals, therefore on the phase shift introduced by the controllable phase shifter.

[0006] The invention makes it possible to easily produce a combined signal having desired polarization components, which can thus take on a very large number of values, if desired, and can be easily adjusted by the user.

[0007] Furthermore, the invention allows the easy use of fibers or other waveguides, including planar ones, which facilitates the creation of a reliable and compact system.

[0008] In examples of implementation of the invention, the incident light signal is linearly polarized.

[0009] Preferably, the polarization rotator is configured to introduce a 90° rotation of the polarization. Alternatively, the polarization rotator transforms a transelectric wave into a transmagnetic wave, or vice versa if the waves are carried by a planar waveguide.

[0010] The signal splitter may include an optical coupler. Alternatively, the signal splitter includes a semi-reflective blade.

[0011] The controllable phase shifter can be electrically controlled. Alternatively, the controllable phase shifter is optically controlled.

[0012] The controllable phase shifter may include an electro-optical element such as a Pockels cell.

[0013] The signal combiner may include a semi-reflective mirror. Alternatively, the signal combiner may include a birefringent crystal.

[0014] Signal propagation can occur in optical fibers. Alternatively, signal propagation from the device input to the polarization rotator can occur within at least one planar waveguide.

[0015] The invention also relates to the use of the system according to the invention to generate a set of discrete polarization values ​​in order to encode information on a light signal, in particular to generate a set of discrete polarization values ​​in order to encode information on a photon.

[0016] The information may include a plurality of bits encoded on a single transmission of light.

[0017] In this case, the controllable phase shifter is, for example, controlled to take on a succession of predefined values. The invention also relates to the use of an optical system for selectively modifying the polarization components of an incident light signal, comprising at least: o an input stage receiving the incident light signal as input and generating two secondary signals at output with a power distribution between the secondary signals that is a function of a control signal, o at least one polarization rotator in the path of at least one of the signals from the input stage, to introduce a corresponding difference between the polarization directions of the secondary signals, o a signal combiner to generate, from the signals coming from the input stage and having passed through said at least one biasing rotator, a combined signal, the input stage being controlled to generate a set of discrete bias values ​​in order to encode information on a light signal.

[0018] This aspect of the invention can be combined with all or part of the following features: - the information comprises a plurality of bits encoded in a single transmission of light, - the incident light signal is linearly polarized. - the polarization rotator introducing a 90° rotation of the polarization, - The entrance floor comprises: a signal splitter receiving the incident light signal as input and generating two signals as output, a controllable phase shifter allowing a predefined phase shift to be introduced between the two signals depending on the control signal, an interferometer receiving at input the signals thus out of phase and generating at output signals which have interfered, secondary, with a distribution of power between the secondary signals which is a function of the phase shift. - The entrance floor includes: a signal splitter receiving the incident light signal as input and generating two output signals, an optical coupler receiving the signals from the splitter and comprising between two waveguides separated by a material with a controllable refractive index, allowing the power distribution between the output signals to be varied according to the control signal, - The signal splitter includes an optical coupler. - The signal splitter includes a semi-reflective blade. - The controllable phase shifter is electrically controlled. - The controllable phase shifter is optically controlled. - the controllable phase shifter includes a Pockels cell. - The signal combiner includes a semi-reflective mirror - The signal combiner incorporates a birefringent crystal. - Signal propagation occurs through optical fibers. Signal propagation from the optical system input to the polarization rotator occurs within at least one planar waveguide. - the system is used to generate a set of discrete polarization values ​​in order to encode information on a photon.

[0019] Brief description of the drawings

[0020] The invention will be better understood upon reading the detailed description that follows, the non-limiting examples of its implementation, and upon examination of the attached drawing, on which:

[0021] [Fig. 1] Figure 1 schematically represents an optical system for modifying the polarization of a light wave according to an example of an implementation of the invention,

[0022] [Fig 2] Figure 2 represents a variant of the combiner,

[0023] [Fig 3] Figure 3 illustrates a variant of the interferometer,

[0024] [Fig 4] Figure 4 schematically represents a variant of the first subset, and

[0025] [Fig 5] Figure 5 schematically represents another variant of the first subset. Detailed description

[0026] Figure 1 shows an example of an optical system 90 according to the invention, applied to modifying the polarization of an incident light wave Wi, which is preferably linearly polarized along a given axis.

[0027] The system 100 is configured to generate from the incident signal Wi an optical output signal Wf, having polarization components whose values ​​are determined by a control unit 110, for example a microcomputer equipped with a suitable interface for controlling electro-optical elements.

[0028] The incident wave Wi propagates in a first optical propagation channel 2, preferably an optical fiber or a planar optical guide.

[0029] System O can include, as illustrated:

[0030] - a first optical sub-assembly 1 configured to provide, from the incident coherent light wave Wi propagating in the first optical propagation channel 2, a first secondary coherent light wave W'1 propagating in the first optical propagation channel 2 and a second secondary coherent light wave W'2 propagating in a second optical propagation channel 3. The percentage distribution of the intensity of the first and second secondary light waves W'1, W'2 is adjustable in a controlled manner under the effect of a control signal D1 from the control unit 110. The incident and secondary coherent waves are, for example, linearly polarized and both in the vertical direction.

[0031] - downstream of the first optical sub-assembly 1, a second optical sub-assembly 4 configured to modify the first and second secondary light waves W'1, W'2 linearly polarized into first and second light waves W”1 and W”2 whose first and second polarization directions make a fixed non-zero angle between them, preferably + / -90 degrees, these waves being further phase-shifted from each other by a given phase shift, preferably zero but which can be controlled by a signal D2 from the control unit 110.

[0032] - a light wave combiner 5 located downstream of the second optical sub-assembly 4, configured to mix the first and second modulated light waves W”1, W”2 on the same final optical channel 13, to generate the wave Wf.

[0033] Signals D1 and D2 can be electrical and / or optical.

[0034] The system 100 according to the invention allows, by receiving at input an incident light wave Wi linearly polarized, to provide at output a final light wave Wf whose polarization components can be controlled in a controlled and continuous manner by the control unit 110.

[0035] More specifically, the final light wave Wf is the sum of the first and second modulated light waves W”1, W”2 coming out of the second optical subset 4 which are linearly polarized, preferably at 90 degrees to each other, whose amplitudes and phase shift are controllable by the control unit 110.

[0036] This solution has the advantage of being usable at high frequencies. The invention also has the advantage of being applicable both to free-field optical paths and to guided media, such as optical fibers or planar waveguides, or a combination of these.

[0037] First optical sub-assembly

[0038] The first optical subset 1 (also called the input stage) allows the linearly polarized incident light wave Wi to be separated into two with a predefined distribution of intensities, which are preferably equal, to generate the waves W1 and W2.

[0039] The first subset 1 allows the waves W1 and W2 to be phased out from each other, with a phase shift controllable by the signal D1 from the control unit 110.

[0040] The first subset 1 also allows these waves, thus out of phase, to interfere in order to bring them out in the form of the two waves W'1 and W'2 on two optical channels, with respective intensities depending on said phase shift.

[0041] Preferably, the first subset 1 includes at least one first splitter 6 configured to divide the incident light wave Wi into the first light wave W1 and the second light wave W2.

[0042] This splitter 6 is for example an optical coupler comprising two optical paths brought close together over a given length, but still remaining separated from each other in such a way that the incident wave entering one of these two optical paths emerges at substantially equal intensity from the output of each of these two optical paths.

[0043] Alternatively, the wave divider comprises a semi-reflective blade through which half of the incident wave Wi passes, while the other half is reflected at 45 degrees.

[0044] Preferably, the first subset 1 comprises at least one first phase shifter 7, controllable by the signal D1, associated with the first optical propagation channel 2, to induce a first phase shift of the first light wave W1 relative to the second light wave W2 in a controlled manner. The phase shifter 7 is preferably composed of at least one Pockels cell. The Pockels cell may be longitudinal or transverse. Its optical axis is preferably aligned with the direction of the coherent light. If the coherent light is not linearly polarized along a fixed axis and the Pockels cell used is transverse, then the phase shifter is preferably composed of two successive Pockels cells oriented at 90° to each other such that the phase shift induced on the light is the same along two perpendicular components of its electric field.

[0045] Preferably, the first subset 1 includes at least one optical interferometer 8 configured to induce a distribution of the intensities of the first and second light waves W1, W2, dependent on the phase shift introduced by the phase shifter 7.

[0046] This interferometer 8 is preferably an optical coupler, consisting of two optical paths brought close together over a given length, but still separated from each other over another given length, such that the incident waves entering these two optical paths emerge from the two optical paths with respective intensities depending on the phase difference of the waves entering said interferometer. Alternatively, the interferometer 8 comprises a semi-reflective plate through which the two incident waves propagating along paths 2 and 3 pass, each at 45 degrees to said semi-reflective plate and at 90 degrees to each other, as illustrated in Figure 3.

[0047] A device (not shown) can be inserted downstream of at least one of the outputs of subassembly 1 to ensure that the two waves exiting it are in phase. This device is, for example, an additional optical path inserted at the end of one of the two outputs of said subassembly 1.

[0048] The first subset 1 can be similar to a Mach Zehnder interferometer used to measure the phase shift introduced by the phase shifter 7.

[0049] Thus, in one variant, schematically illustrated in Figure 4, subassembly 1 includes a Mach-Zehnder interferometer, preferably of the optical fiber type, incorporating an electro-optical material that allows the refractive index to be varied according to the applied control signal, and thus the phase shift. An acousto-optic modulator can also be used to modify the refractive index, as can p-n junction phase shifters (made from p- and n-doped semiconductors) or temperature-dependent phase shifters. In another variant, illustrated schematically in Figure 5, subassembly 1 includes an input splitter 6, for example with a 50 / 50 split between the outputs, and then an optical coupler 130 having two waveguides separated by a material 131 whose refractive index is controllable by the signal D1, for example an electro-optical material. The material 131 allows the distribution of output power between the signals W'1 and W'2 to be varied according to the control signal D1.

[0050] Second optical subset

[0051] The second optical subset 4 allows, on the one hand, the transformation of the initially linearly polarized waves W'1 and W'2 along the same direction into two linearly polarized waves in two polarization directions, preferably perpendicular, and advantageously, the introduction of a phase shift between the two waves W1', W2', this phase shift being controllable by the signal D2.

[0052] Preferably, the second optical subset 4 includes, as illustrated, at least one second phase shifter 9 controllable by the signal D2, associated with the first optical propagation channel 2, configured to induce in a controlled manner a second phase shift of the first light wave W'1 with respect to the second light wave W'2.

[0053] The phase shifter 9 is preferably composed of at least one Pockels cell. The Pockels cell can be longitudinal or transverse. Its optical axis is preferably aligned with the direction of the coherent light. If the coherent light is not linearly polarized along a fixed axis and the Pockels cell used is transverse, then the phase shifter 9 is preferably composed of two successive Pockels cells oriented at 90° to each other such that the phase shift induced on the light is the same along two perpendicular components of its electric field.

[0054] Preferably, the second optical subassembly 4 includes at least one polarization rotator 10 associated with the second propagation channel 3 configured to rotate the second polarization direction of the second light wave W'2 by a predefined angle. This rotator includes, for example, a solenoid around a gallium arsenide (GaAs) crystal through which the light wave propagates, creating a magnetic field parallel to the propagation path of the electromagnetic wave, or it may include a half-wave plate oriented at 45° to the electric field of the light wave if the latter is linearly polarized, or a circuit transforming a trans-electric wave into a trans-magnetic wave or vice versa within a planar waveguide.

[0055] Advantageously, this configuration allows the direction of the waves initially linearly polarized in the same direction to be transformed into linearly polarized waves in two directions, preferably perpendicular to each other in the case where the rotator induces a 90-degree rotation.

[0056] The first Pockels 7 cell and / or the second Pockels 9 cell preferably comprise a nonlinear crystal such as beta barium oxide (BBO) or cadmium telluride (CdTe).

[0057] An element 11 can be placed on the path of one of the signals of the second optical subset to introduce a phase shift allowing the optical path to be adjusted.

[0058] Signal combiner

[0059] Preferably, the light wave combiner 5 includes at least one birefringent crystal 11.

[0060] Preferably, this birefringent crystal 11 is coated with at least one anti-reflective surface treatment. For example, the birefringent crystal 11 is a prism that includes one or more anti-reflective layers at the entrance and / or exit.

[0061] Alternatively, as illustrated in Figure 2, the combiner 5 is a semi-reflective blade.

[0062] Propagation channels

[0063] Preferably, the first propagation channel 2 and the second propagation channel 3 each comprise a planar waveguide. Alternatively, they comprise, for example, an optical fiber.

[0064] Preferably, couplers 6 and 8 are made of waveguides, planar or optical fibers allowing a closer proximity of the first propagation channel 2 and the second 3.

[0065] The dimensions of the approaches, such as the gap and / or the length of the approach, depend on the refractive indices of the waveguides and / or fibers, the refractive index of the medium separating them, and the profile of the waveguides and / or fibers.

[0066] If the first propagation channel 2 and the second propagation channel 3 are waveguides then they can be brought close together over a short distance.

[0067] Any source capable of generating the coherent incident light wave Wi can be used as the system input, this incident light being in the form of any light or photon of a given wavelength. The generated light can be linearly polarized. The source can be a laser.

[0068] Waveguides and / or optical fibers are preferably selected to preserve polarization.

[0069] Waveguides can be flat.

[0070] Information transmission The optical system according to the invention can be used to transmit information that is coded on a set of discrete polarization values ​​of the light signal. Examples of information transmission are disclosed in application WO2024 / 156821 by the same inventor, with, for example, a receiver enabling the measurement of the ellipticity and polarization of light.

[0071]

[0072] Of course, the invention is not limited to the examples of embodiment described. Various modifications can be made without departing from the scope of the invention. For example, Pockels cells can be replaced by Kerr cells.

[0073] The first light wave splitter 6 may include a separating or semi-reflective blade dividing the incident wave into two waves of equal intensity.

Claims

Demands 1. Use of an optical system enabling the selective modification of the polarization components of an incident light signal (Wi), comprising at least: o an input stage (1) receiving the incident light signal (Wi) as input and generating two secondary signals at output with a power distribution between the secondary signals which is a function of a control signal (D1), o at least one polarization rotator (10) in the path of at least one of the signals from the input stage, to introduce a corresponding difference between the polarization directions of the secondary signals, o a signal combiner (5) to generate, from the signals (W”1 , W”2) coming from the input stage (1) and having passed through said at least one biasing rotator (10), a combined signal, the input stage (1) being controlled in order to generate a set of discrete bias values ​​in order to encode information on a light signal.

2. Use according to claim 1, the information comprising a plurality of bits encoded on a single transmission of light.

3. Use according to any one of claims 1 and 2, the incident light signal (Wi) being linearly polarized.

4. Use according to any one of the preceding claims, the polarization rotator (10) introducing a 90° rotation of the polarization.

5. Use according to any one of the preceding claims, the input stage (1) comprising: a signal splitter (6) receiving the incident light signal (Wi) as input and generating two output signals (W1, W2), a controllable phase shifter (7) allowing a predefined phase shift to be introduced between the two signals (W1, W2) as a function of the control signal, an interferometer (8) receiving at input the signals thus out of phase and generating at output signals (W'1 , W'2) which have interfered, secondary, with a distribution of power between the secondary signals which is a function of the phase shift.

6. Use according to claim 5, the signal splitter comprising an optical coupler (6).

7. Using claim 5, the signal splitter comprising a semi-reflective blade.

8. Use according to any one of claims 1 to 4, the input stage (1) comprising: a signal splitter (6) receiving the incident light signal as input and generating two output signals (W1, W2), an optical coupler (130) receiving the signals (W1, W2) from the distributor and comprising two waveguides separated by a material (131) with controllable refractive index, allowing the power distribution between the output signals (W'1, W'2) to be varied according to the control signal.

9. Use according to any one of claims 5 to 7, the controllable phase shifter (7) being electrically controlled.

10. Use according to any one of claims 5 to 7, the controllable phase shifter being optically controlled.

11. Use according to any one of claims 5 to 7, the controllable phase shifter (7) comprising a Pockels cell.

12. Use according to any one of the preceding claims, the signal combiner comprising a semi-reflective mirror 13. Use according to any one of claims 1 to 12, the signal combiner comprising a birefringent crystal.

14. Use according to any one of the preceding claims, signal propagation taking place in optical fibers 15. Use according to any one of the preceding claims, the propagation of signals from the input of the optical system to the polarization rotator taking place within at least one planar waveguide.

16. Use of the system according to any one of the preceding claims, to generate a set of discrete polarization values ​​for the purpose of encoding information on a photon.

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