WO2013110842A1

WO2013110842A1 - System, method, transmitter and receiver for optical communications

Info

Publication number: WO2013110842A1
Application number: PCT/ES2013/070027
Authority: WO
Inventors: Todor Kirilov Kalkandjiev; Jordi Mompart Penina; Alejandro Turpin Aviles; Yurii Loiko
Original assignee: Universitat Autonoma De Barcelona
Priority date: 2012-01-26
Filing date: 2013-01-23
Publication date: 2013-08-01
Also published as: ES2389434A1; ES2389434B1

Abstract

The system comprises: a transmitter (T) formed by a demultiplexer and a multiplexer including optical elements (C1, C2) which, together with a modulator (M1) disposed therebetween, allow the sub-beams of an input light beam (H1) to be used as optical channels for the transmission of data in the form of a multiplexed light beam (HM); and a receiver (R) which is disposed to receive the aforementioned beam after it has passed through the transmission means and which comprises a demultiplexer including an optical element (C3) which recovers the optical channels of the multiplexed light beam (HM). The method is adapted to be carried out using the system of the invention. The transmitter and the receiver are adapted to perform the functions of the transmitter and the receiver of the system of the invention respectively.

Description

System, method, transmitter and receiver of optical communications

Technical sector

The present invention concerns, in a first aspect, an optical communications system that allows the encoding and transmission of information in a light beam, and more particularly a system that includes a series of optical elements that allow the use of sub-beams. that make up the light beam as optical channels for data transport.

In a second aspect, the invention concerns a method adapted to be implemented using the system of the first aspect.

Third and fourth aspects of the invention concern a transmitter and a receiver adapted to perform the functions of, respectively, the transmitter and the receiver of the system of the first aspect of the invention. Prior art

Laser connections are currently used for FSOC ("Free-Space Optical Communications") point-to-point over long distances. Most advantages over other communication technologies come from the great coherence achieved in laser radiation and the low beam divergence. The technologies of FSOC using lasers are considered to be those with the highest efficiency and data rate (in bandwidth and capacity) for any optical channel in free space. These advantages are crucial for deep-space optical communications [1], not forgetting that laser connections can even be considered as an alternative (or an additional tool / channel) to radiofrequency and fiber optic communications for some applications terrestrial

Different properties of optical fields can be explored to efficiently encode and decode information. So far, efficient techniques for signal processing in high frequency modulation laser sources, ultrafast sensors and other ultrafast optical and electronic components have been developed. In addition, the capacity of a communication channel can be remarkably increased if one efficiently combines and decomposes (without loss of information) beams with slightly different characteristics such as, for example, does the wavelength division multiplexing (WDM) technique. ), which has been developed for fiber optic communications. In order to increase the capacity of the channel in FSOC, it has been considered to use the orbital angular momentum (OAM) of light to encode information in a monochromatic laser beam [2-5]. Thus, as the OAM light can be decomposed into an infinite number of orthogonal, ie states, widely known eigenmodes TEM _{p /} given by the functions of Laguerre-Gauss, the channel capacity can be arbitrarily increased, in beginning. However, there are practical drawbacks that make it difficult to encode information in the OAM of light [6-8] such as the fact that high orders of OAM modes are highly divergent and, consequently, cannot be used for FSOC over long distances That is why today there is no feasible communication system based on multiplexing using OAM.

On the other hand, the phenomenon of conical refraction, or CR (from English "Conical Refraction") is known by various references [9-17], although despite being a phenomenon discovered in 1832 [10] its use for implementation Optical systems with specific applications has so far been scarce.

One such implementation has been described in WO2010084317A1, where an optical system with an optical input source, such as a laser, is proposed to project an input light beam along an optical axis and an optical element that creates a conical refraction beam from the input beam, projecting a light ring on Lloyd's plane and subsequently reconstructs the input beam from the light ring. The optical element comprises one or more conical refractive elements which, in some of the embodiments, are oriented inversely.

The objective of the system proposed in WO2010084317A1 is to generate a light beam, such as a laser beam, which has high performance in terms of power and quality, as well as high efficiency.

It is not taught or suggested in WO2010084317A1 the use of the optical system proposed there for purposes other than those indicated and related to the improvement of the characteristics of the light beam supplied, not suggesting at all the use of such a system for the transmission of data.

In JP58202422A it is proposed to perform an optical multiplexing without using a wavelength selection element, thereby providing a small device that uses the conical refraction of a biaxial optical glass, and that allows multiplexing the light from a plurality of light sources to an optical fiber of exit arranged on an exit face of the glass, where the light of each of the light sources is transmitted through a respective fiber optic conduit with an end that strikes an input face of the glass opposite to said exit face crystal. The ends of the optical fibers that contact the glass, both the input and the output, must be fixed precisely at predetermined points so that, in theory, due to the conical refraction, the light of all the input beams converge at the exit point. But in practice this is not the case, due to the main drawback of the proposal made in JP58202422A, which is the following: Each light source mentioned in JP58202422A produces a conical refractive ring on the exit face of the glass. In the event that the input sources are displaced from each other, the CR rings on the output surface of the biaxial crystal will be displaced in the same amount. Therefore, for N light sources on the input surface, N CR rings will appear on the output surface. In addition, at the exit surface these CR rings intersect at some points, but do not overlap each other completely. This prevents them from being properly picked up by a fiber placed on the exit surface of the biaxial crystal.

Explanation of the invention.

It appears necessary to offer an alternative to the state of the art that covers the gaps found therein, in particular those existing in the communication systems based on the OAM multiplexing, and that allows its use for

FSOC over long distances.

To this end, the present invention concerns, in a first aspect, an optical communications system, comprising, in a manner known per se:

- a transmitter configured to transmit and encode information in a beam of light through a transmission medium; Y

- a receiver arranged to receive said beam, after having traveled through said transmission means, and configured to decode said encoded information included in the light beam.

Unlike the known proposals, in the system proposed by the first aspect of the present invention, in a characteristic manner:

- the transmitter comprises:

- a demultiplexer formed by at least a first optical element arranged and configured to receive, at a first end, a beam of light initial, demultiplexing it by changing the direction of the wave vectors of at least two of the sub-beams that make up said initial light beam, and supplying said sub-beams with wave vectors with directions changed by a second end opposite to said first end ;

- at least one modulator arranged and configured to perform said information coding by amplitude modulating said sub-beams with wave vectors with changed directions, each constituting an optical channel; Y

- a multiplexer formed by at least a second optical element arranged and configured to receive, at a first end, the light of said optical channels, and to supply said multiplexed optical channels towards said transmission means in the form of a multiplexed light beam, by a second end opposite said first end, and the receiver comprises a demultiplexer formed by at least a third optical element arranged and configured to receive, at a first end, said multiplexed light beam, and supply, by a second opposite end at said first end, said optical channels recovered after demultiplexing to the multiplexed light beam, changing the direction of the wave vectors of the sub-beams, which are at least two, that make up the multiplexed light beam.

What is described herein as attached sub-beams are beams that make up the initial light beam or the multiplexed light beam, the term "sub-beam" having been used simply for clarity in the description, avoiding thus confusions between the beams here referred to as sub-beams and the beams composed of such sub-beams.

For a preferred embodiment example:

- the first optical element is a first biaxial conical refraction crystal, arranged and configured to demultiplex said initial light beam by changing the direction of the wave vectors of the sub-beams that compose it, supplying said sub-beams with wave vectors with directions changed so that its projection in a plane takes the form of a light ring, where sub-beams with wave vectors in the same plane go to diametrically opposite sectors of the light ring;

- the modulator or modulators are arranged and configured to perform said information coding by modulating in amplitude, in said projection plane or in another or other planes, the sub-beams corresponding to different sectors of said light ring, or optical channels;

- and the second and third optical elements are respective second and third biaxial crystals of conical refraction.

The terms "ring of light" should not be understood herein and attached claims as restrictive in terms of radial uniformity, being able to cover both perfectly circular and oval rings.

The optical axes of the first and second optical elements are, in general, aligned with each other.

For an exemplary embodiment, the second biaxial conical refraction crystal is substantially the same size as the first.

The second biaxial conical refraction crystal is oriented inversely with respect to the first, that is to say at an angular distance of 180 ° around its optical axis, according to a preferred embodiment.

For another exemplary embodiment, the second biaxial conical refraction crystal is oriented with respect to the first an angular distance other than 180 °.

For another embodiment, the first, second and third optical elements are photonic crystals. It is known that the characteristics of photonic crystals can be modified by varying their structure and materials from which they are made. The photonic crystals used in the system proposed by the first aspect of the invention have such a structure that they allow a separation of light waves in a manner analogous to the way biaxial crystals do.

For another exemplary embodiment, the first, second and third optical elements are another class of optical elements designed to perform the explained functions.

According to an example of embodiment, the initial light beam is of the Gaussian type, although the system is also applicable, for other embodiments, to any beam that is part of the Laguerre-Gauss or Hermite-Gauss modes, depending on the level of divergence allowed by the application where the system is implemented and, above all, the distance between the transmitter and the receiver.

As regards the multiplexed beam of light, it is generally similar to a Gaussian beam, at least in terms of divergence characteristics, so that the distance it can travel to reach the receiver, without divergence between the different channels that conform it, it is considerable, of the same order as the distance traveled by a Gaussian beam.

As for the transmission medium, this is, for an exemplary embodiment, an isotropic medium, such as free space, thus the present system being suitable for carrying out the known FSOC communications, of the English "Free Space Optical Communication".

For another example of embodiment, the transmission medium is a waveguide, constituted for example by optical fiber.

As for the type of modulator used in the system of the first aspect of the invention, depending on the exemplary embodiment, it comprises at least one mask of angular amplitude arranged in said plane where the light ring is formed or in another plane or a modulator known as SLM, the acronym "Spatial Light Modulator", ie space light modulator.

As regards the initial light beam, this is a monochromatic light beam or a polychromatic light beam depending on the exemplary embodiment.

According to an embodiment, the modulator is configured to independently modulate part or all of the optical channels, by spatial modulation or spatial and temporal modulation.

For another embodiment, the modulator is configured to modulate in a dependent manner at least part of the optical channels, by spatial modulation or spatial and temporal modulation.

According to an exemplary embodiment, the transmitter also comprises at least a first lens disposed in front of the first end of the first optical element to focus the initial beam of light on it, and at least a second lens disposed behind the second end of the second optical element. to collide said multiplexed beam of light.

The receiver also comprises, for one embodiment, at least one lens arranged in front of the first end of the third optical element, so that it receives the multiplexed beam of light after passing through the lens, and at least one photodetector device (such as a CCD device) arranged in front of the second end of the third optical element to receive the light from the optical channels and process the information contained therein. The system comprises, for some embodiments, cascading two or more groups such as the one described here, aligned with each other or with their respective Optical axes forming an angle, in the latter case the beam of output of an optical group is directed towards the entrance of the consecutive optical group by means of an optical element that deflects the direction of the beam (such as a reflector or a prism).

That is to say that the system comprises, for said embodiments, after the third optical element, at least one optical group that includes:

- at least a second modulator arranged and configured to encode information by amplitude modulating the optical channels recovered by the third optical element,

- at least a second multiplexer formed by at least a fourth optical element arranged and configured to mutiplex the optical channels encoded by the second modulator, and

- at least a second demultiplexer formed by at least a fifth optical element arranged and configured to demultiplex the light beam multiplexed by the second multiplexer.

After said second demultiplexer, or after the last demultiplexer for an exemplary embodiment for which more than two optical groups such as those described are arranged, the system comprises a photodetector device to receive the light from the optical channels and process the information contained in the themselves, as part of a last receiver arranged at the end of the line of optical groups.

According to another embodiment, the system combines sections that include one or more of the described optical groups with sections formed by another class of communication, optical or other devices (in the latter case implementing the necessary conversion of optical signals to the magnitude used by communication devices of another class).

A second aspect of the invention concerns a method of optical communications, comprising:

a) transmit and encode information in a beam of light through a transmission medium; Y

b) receiving said beam of light, after having traveled through said transmission means, and decoding said encoded information included in the beam of light.

Unlike the known methods, in the one proposed by the second aspect of the invention, step a) comprises:

- emit an initial beam of light towards a first optical element; - receiving, by said first optical element, said initial beam of light, demultiplexing it by changing the direction of the wave vectors of at least two of the sub-beams that make up said initial beam of light, and supplying said sub-beams with vectors of wave with changed directions;

- carrying out said information coding by modulating in amplitude said sub-beams with wave vectors with changed directions, each constituting an optical channel; and send the light of said optical channels to a second optical element; Y

- receiving and multiplexing, by means of said second optical element, the light of said optical channels, supplying said multiplexed optical channels towards said transmission means in the form of a multiplexed light beam,

and step b) comprises receiving and demultiplexing, by means of a third optical element, said multiplexed light beam, supplying said optical channels recovered after demultiplexing to the multiplexed light beam, changing the direction of the sub-beam wave vectors, which They are at least two, which make up the multiplexed beam of light.

For a preferred embodiment, the first, second and third optical elements are respective first, second and third conical refractive biaxial crystals, where each of said demultiplexings of steps a) and b) comprises changing the direction of the vectors of wave of the sub-beams that make up the initial light beam, as regards stage a), and the multiplexed beam, as regards stage b), supplying said sub-beams with wave vectors with directions changed so that its projection in a plane takes the form of a light ring, where sub-beams with wave vectors in the same plane go to diametrically opposite sectors of the light ring.

The method of the second aspect of the invention is intended to be implemented using the system of the first aspect.

Depending on the exemplary embodiment, said initial light beam is linearly or circularly polarized, or depolarized. A third aspect of the invention concerns an optical communications transmitter, which is configured to transmit and encode information in a beam of light to through a transmission medium, where, unlike known transmitters, it comprises:

- a demultiplexer formed by at least a first optical element arranged and configured to receive, at a first end, an initial beam of light, demultiplexing it by changing the direction of the wave vectors of at least two of the sub-beams that make up said beam of initial light, and supplying said sub-beams with wave vectors with directions changed by a second end opposite to said first end;

- a modulator arranged and configured to perform said information coding by amplitude modulating said sub-beams with wave vectors with changed directions, each constituting an optical channel; Y

- a multiplexer formed by at least a second optical element arranged and configured to receive, at a first end, the light of said optical channels, and to supply said multiplexed optical channels towards said transmission means in the form of a multiplexed light beam, by a second end opposite said first end. The transmitter of the third aspect of the invention is configured, according to different embodiments, to perform the functions of the transmitter of the system of the first aspect, that is to say that the transmitter described as included in the system of the first aspect of the invention is, for some realization examples, the one proposed by the third aspect.

A fourth aspect of the invention concerns an optical communications receiver that is arranged to receive a light beam that includes encoded information, having traveled through a transmission medium, and configured to decode said encoded information included in the light beam, the receiver comprising, in a characteristic way, a demultiplexer formed by at least one optical element arranged and configured to receive, at a first end, a light beam with information encoded in optical channels corresponding to multiplexed light sectors in said beam of light, and supplying, by a second end opposite said first end, said optical channels recovered after demultiplexing the multiplexed light beam by changing the direction of the wave vectors of the sub-beams that make up the multiplexed light beam. The receiver of the fourth aspect of the invention is configured, according to different embodiments, to perform the functions of the receiver of the system of the first aspect, that is to say that the receiver described as included in the system of the first aspect of the invention is, for some realization examples, the one proposed by the fourth aspect.

Brief description of the drawings

The foregoing and other advantages and features will be more fully understood from the following detailed description of some embodiments with reference to the attached drawings, which should be taken by way of illustration and not limitation, in which:

Fig. 1 is a schematic perspective view of the system proposed by the first aspect of the invention, for an exemplary embodiment for which the optical elements are biaxial crystals of conical refraction;

Fig. 2 shows experimental images obtained for the system of Fig. 1 using masks for multiplexing with 2 (a), 4 (b), 6 (c), 8 (d), 10 (e), 12 (f ) and 36 (g) angular openings, illustrated in the first row; the second row shows the multiplexed beams produced by the transmitter, while in the third row the patterns demultiplexed by the receiver can be seen;

Fig. 3 shows, schematically, the system corresponding to a degenerate case of CR in 2-cascade, on which the system proposed by the present invention is based;

Fig. 4 shows, in relation to the system of Fig. 3, different captures made with a CCD device for a Gaussian input beam (a-c), strongly elliptical (d-f) and obtained with an 8-star amplitude mask; the first column represents the images of the input beams, the second column the intensity pattern in the focal plane and the last column shows the output beams; Y

Figs. 5a and 5b show, schematically, the system proposed by the invention, for an example of an embodiment different from that of Fig. 1, for which the system is bidirectional, illustrating in Fig. 5a the path of a beam entering from the left, and in Fig. 5b the path of a beam that enters from the right, according to the illustrated position of the system. Detailed description of some embodiments In this section, some embodiments will be described with reference to the accompanying Figures representative of the preferred case described above, that is to say, which uses as optical elements biaxial crystals of conical refraction.

First, an explanation is made of the central idea on which the system proposed by the present invention is based, as regards its preferred embodiment for which the transmitter of the latter comprises two biaxial crystals of refraction in one provision that is called degenerate case of CR in 2-cascade.

The degenerated case of CR in 2-cascade (that is, with two crystals) with oppositely oriented CREs is the central idea of the preferred embodiment of the present invention, for coding the information of a Gaussian monochromatic beam. It consists of a cascade of two CREs with the same length (preferably <100 nm difference) with two opposite-oriented vectors, placed between a pair of LF / LC focusing / collimating lenses, as illustrated in Fig. 3. The first CRE1 crystal transforms the initial Gaussian beam Hi in the CR ring, projected in the focal plane PF, and the second CRE2 crystal transforms said ring towards the initial Gaussian beam, in the form of an output beam Ho. In other words, the degenerated 2-cascade CR can be considered as a direct-inverse 2D transformation of a monochromatic beam, analogously to the two-dimensional Fourier transform.

The widely known TEMp modes / formed by Laguerre-Gauss functions constitute a complete basis to represent any light field whose transverse amplitude and polarization have cylindrical symmetry. The intensity distribution of a CR beam produced by a circularly polarized Gaussian beam has cylindrical symmetry. However, CR beams cannot be represented in terms of the TEMp / modes. In fact, the latter have a symmetry such that the electric field vector for each two points opposite each other relative to the axis of symmetry are either collinear or anti-collinear. In contrast, the electric field vectors of two opposite points of the CR ring are always perpendicular. Consequently, TEMp / modes cannot be used as a basis to represent the conically refracted beam. In the same way, it can be concluded that the Hermite-Gauss modes, as well as other more exotic modes such as Ince-Gauss or the metric rivers, cannot be used either. as a representation base of the CR. For this reason, we must focus the decomposition of the CR beam from an alternative approach.

The investigations carried out by the present inventors have already shown that there are beams, known as CR filtered beams [17], that do not produce a CR ring when they propagate along the optical axis of a biaxial crystal. These beams are divided, in general, into two beams diagonally positioned at opposite points of the CR ring which is to be expected in any other case. The intensities of these divided beams change under the rotation of the CRE, similar to what happens in double refraction in calcite. For some particular orientations of the CRE, one of the divided beams has zero intensity and, using an analogy with the widely known phenomenon of double refraction, these filtered beams are identified as their own light modes of the effect of the CR. These proper modes are very useful for describing the propagation of any initial beam with an arbitrary intensity pattern along the degenerated 2-cascade CR configuration.

Fig. 4 shows the results of said investigations in the propagation of three input beams with different transverse patterns along a degenerated 2-cascade CR configuration. Clearly, only the Gaussian beam (see Fig. 4 (a)) produces the CR ring (see Fig. 4 (b)) after passing along the first CRE, restoring the ring to the initial beam (see Fig. 4 (c)) after going through the second CRE.

To generate the strongly elliptical shaped beam of Fig. 4 (d), a 25μηι opening slit was used to select only the central part of a Gaussian beam. In this case, similar to the case of filtered CR beams, only two refracted beams were obtained. If, in addition, this incident beam is linearly polarized, then there is a specific orientation of the first CRE for which only one beam is observed in the focal plane. It is in this sense that linearly polarized beams with a strongly elliptical profile are identified as their own modes of light for the phenomenon of CR.

A Gaussian beam can be represented by the Fourier transform as a superposition of plane waves with different k wave vectors. The strongly elliptical beam (see Fig. 4 (d)) can be seen as composed of plane waves whose vectors - / (are confined in a plane, known as the K plane, so that the incident Gaussian beam can be considered as a infinitesimal sum of own modes, that is to say strongly elliptical beams with different K planes. After the first CRE, each beam characterized by a K plane is projected at two opposite points of what a CR ring should be (see Fig. 4 (e)) and, after the second CRE, both points are restored to the initial beam while retaining the K plane, see Fig. 4 (f) .

Alternatively, the operability of the degenerated 2-cascade can be expressed considering that a Gaussian beam is spatially demultiplexed in a CR ring such that each pair of opposite points (with orthogonal polarizations) represents a plane K of the incident Gaussian beam . For the case with n planes K (n = 4 in Fig. 4 (g)), the CR ring will have 2n opposite points (8 in Fig. 4 (h)). The members of each pair of beams of opposite points are joined again (multiplexed) after the second CRE in a single beam, causing the initial beam to be reproduced after the second CRE; compare Figs. 4 (i) and (g). The second CRE2 crystal, which performs the inverse transform, can be seen as a multiplexer. It should be noted that the intensity of each point of the light ring CR before the multiplexing element (second CRE) can be modulated independently, allowing an increase in the capacity of the channel carried by the Gaussian beam.

After explaining the study on the degenerate case of CR in 2-cascade, which is the basis of the preferred embodiment of the system proposed by the first aspect of the invention, this is described below, which is designed to encode and decode information by CR, explaining in the following in terms of the three fundamental parts of an optical communication system in FSOC, ie, the transmitter T, the free space propagation itself and the receiver R, with reference to Fig. 1. The transmitter T comprises two biaxial crystals C1 and C2, which provide the direct-inverse transformation, a mask of angular amplitude M1 that is used as a light modulator and a lens L1 to focus the input beam H1 and another lens L2 to collimating the output beam Ho.

It should be noted that although the crystals C1 and C2 with different sizes have been illustrated in Fig. 1, the results illustrated in Fig. 2 correspond to an implementation of the system of Fig. 1 where C1 and C2 have the same size , which will be indicated below, and are inversely oriented.

For the initial beam H1, for the experimental test whose results are illustrated in Fig. 2, a circularly polarized Gaussian beam and collimated with an unfocused waist radius of w ₀ = 1.5 mm obtained from a 640 nm laser diode coupled to a single mode fiber. The degenerated 2-cascade of CR is prepared with two identical biaxial crystals of KGd (W0 ₄ ) ₂ such that they offer a ring after the first C1 crystal of 1.5 mm radius. To focus and collimate the beam, L1, L2 lenses with a focal length of 200 mm have been used. The demultiplexing protocol can be divided into the following steps:

(i) The first CRE, or conical refractive element, C1 demultiplexes the Gaussian beam H1 into a ring whose points are independent communication lines or channels. Each point of the ring can be separated correctly and modulated. (ii) As a modulation element M1 amplitude masks have been used with n (up to 36) open circular sectors, each of them placed, alternatively, in the plane where the light ring is projected, to select some parts of the CR ring

(iii) The second CRE C2, identical and inversely oriented with respect to the first (for the experimental test described here), through whose optical axis pass the parts of the light ring selected by the mask used as modulator M1, and is responsible for of doing the inverse transformation of the beam, that is to say multiplexing the sectors modulated in an output beam HM with a divergence comparable to that of the incident beam H1. The experimental images obtained using masks for multiplexing with 2 (a), 4 (b), 6 (c), 8 (d), 10 (e), 12 (f) and 36 (g) are illustrated in Fig. angular openings The second row shows the multiplexed beams produced by the T transmitter, while in the third row the patterns can be seen demultiplexed by the R receiver. In the experimental setup described here, the free space propagation was about 1.5 m. It is important to note that the mask used has an influence on the profile but not on the divergence of the HM multiplexed beam, see second row in Fig. 2, which allows the system proposed by the invention to be used over long propagation distances. As for the R receiver, this consists of an objective L3 with focal length of

50 mm, a third CRE C3 12 mm long, which demultiplexes the HM multiplexed beam, and a CCD camera, as an FD photodetection device, that captures the resulting standard beam. The CRE C3 demultiplexes the beam propagated in free space HM, by the phenomenon of CR and recover the sectors modulated by the transmitter T. As can be seen in the third row of images in Fig. 2, it is possible to modulate clearly up to 12 sectors without interference with each other, that is to say with divergence characteristics comparable to those of a Gaussian beam. In this section the results obtained from a detailed investigation made by the present inventors on the property of the direct and inverse transform of an incoming beam of light propagated along the optical axis of a 2-cascade CR assembly have been shown. degenerated where two oppositely oriented biaxial crystals are used, which has proven to be a configuration to be used as a novel technique for multiplexing and demultiplexing monochromatic optical beams, according to the present invention. This technique can be used to increase the capacity of the channel in FSOC.

In fact, by means of the experimental implementation described with reference to the attached figures, an increase in an order of magnitude of the capacity of the channel has been shown in a propagation distance of 1.5 m, such that the divergence of the resulting beam does not exceed that of the incident. This technique can be implemented with both coherent and incoherent light and can be extended to any wavelength for which the biaxial crystal is transparent. It should be added that, if a polychromatic beam is sent, each wavelength can be independently modulated and, consequently, the proposed system and method can be used for multiplexing and demultiplexing of a particular spectral range (around ± 100 nm for the biaxial crystals of KGd (W0 ₄ ) ₂ used in the experiments performed by the present inventors). This CR technique can also be combined with other standard multiplexing / demultiplexing techniques such as WDM.

Making small changes or adjustments in the implementation of the system that has been used for the experimental test described, without departing from the scope of the invention as defined in the attached claims, it is expected to be able to further increase the capacity of the channel and extend the results to even greater distances. Likewise, for the mentioned experimental test, classic light sources have been considered and used, but new applications in secure communications can arise from the extension of multiplexing / demultiplexing with CR towards quantum light sources. Finally, in Figs. 5a and 5b illustrates an embodiment of the system proposed by the first aspect of the invention, more developed than that illustrated by Fig. 1, allowing bidirectional use thereof, especially thanks to the inclusion of two semi-transparent optical systems E1 and E2, such as two mirrors, that allow the passage of light in one direction and that reflect it when it affects them in the opposite direction.

In Fig. 5a the system is illustrated when a beam of light has entered from the left of it, according to the position illustrated, that is the beam H1 a, focused by the lens L1, this being demultiplexed by the first optical element C1 in corresponding sub-beams, or optical channels, changing the direction of their wave vectors, which are modulated by the modulator M1, all in a manner analogous to that described for the embodiment example of Fig. 1, and after that they pass through the semi-transparent mirror E1 and reach the second optical element C2.

Following the description of Fig. 5a, it can be seen how the second optical element C2 multiplexes said optical channels, as already explained with reference to Fig. 1, and the HMa multiplexed beam, once collimated by the lens L2, passes through the transmission medium and behind it the lens or lens L3 and the third optical element C3, where the optical channels that compose it are demultiplexed, after which the light of these optical channels is reflected in the semi-transparent mirror E2 directed towards the FD2 photodetector.

The same system as in Fig. 5a is illustrated in Fig. 5b, but when the input beam is the one indicated as H1 b, that is, the one that enters from the right of the system, being used as a lens input indicated as L4 and as a demultiplexer the fourth optical element C4, which performs the same functions as the first optical element C1.

After the demultiplexion of Hb and modulation of their respective optical channels by means of the M2 modulator, the same process explained in relation to Fig. 5a is followed, but in this case it is the third optical element C3 that multiplexes the optical channels of the H1 beam b, having passed through the semi-transparent mirror E2, and the multiplexed beam HMb, once collimated by the lens L3, passes through the transmission medium and behind it the lens or lens L2 and reaches the second optical element C2, where it is demultiplexed recovering the optical channels that compose it, after which the light of these optical channels is reflected in the semi-transparent mirror E1 going towards the photodetector FD1.

That is, for the exemplary embodiment illustrated by Figs. 5a and 5b, the elements that a sense act as transmitters, T1 or T2, act in a sense opposite as receivers, R1 or R2, provided the said bidirectionality for data transmission.

The first C1, second C2, third C3 and fourth C4 optical elements are, depending on the exemplary embodiment, biaxial conical refraction crystals, photonic crystals or any optical element designed to perform the explained function.

A person skilled in the art could introduce changes and modifications in the described embodiments without departing from the scope of the invention as defined in the appended claims, being able, for example, to modify the bidirectional system of Figs. 5a and 5b, both in the number of elements that form it and in its arrangement within the system, maintaining the aforementioned bidirectional characteristics.

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Claims

1.- Optical communications system, of the type comprising:

- a transmitter (T) configured to transmit and encode information in a beam of light through a transmission medium; Y

- a receiver (R) arranged to receive said beam, after having traveled through said transmission means, and configured to decode said encoded information included in the light beam;

the system being characterized because:

said transmitter (T) comprises:

- a demultiplexer formed by at least a first optical element (C1) arranged and configured to receive, at a first end, an initial light beam (H1), demultiplexing it by changing the direction of the wave vectors of at least two of the sub - Beams that make up said initial light beam (H1), and supplying said sub-beams with wave vectors with directions changed by a second end opposite to said first end;

- at least one modulator (M1) arranged and configured to perform said information coding by amplitude modulating said sub-beams with wave vectors with changed directions, each constituting an optical channel; Y

- a multiplexer formed by at least a second optical element (C2) arranged and configured to receive, at a first end, the light of said optical channels, and to supply said multiplexed optical channels towards said transmission means in the form of a beam of multiplexed light (HM), by a second end opposite said first end,

and because said receiver (R) comprises a demultiplexer formed by at least a third optical element (C3) arranged and configured to receive, at a first end, said multiplexed light beam (HM), and supply, by a second opposite end at said first end, said optical channels recovered after demultiplexing to the multiplexed light beam (HM), changing the direction of the wave vectors of the sub-beams, which are at least two, that make up the multiplexed light beam (HM) .

2. System according to claim 1, characterized in that: - said first optical element (C1) is a first biaxial conical refraction crystal, arranged and configured to demultiplex said initial light beam by changing the direction of the wave vectors of the sub-beams that compose it, supplying said sub-beams with Wave vectors with changed directions so that their projection in a plane takes the form of a light ring, where sub-beams with wave vectors in the same plane go to diametrically opposite sectors of the light ring;

- said modulator (M1), which is at least one, is arranged and configured to perform said information coding by modulating in amplitude, in said projection plane or in another or other planes, the sub-beams corresponding to different sectors of said ring of light, or optical channels;

- and because said second (C2) and third (C3) optical elements are respective second and third biaxial crystals of conical refraction.

3. - System according to claim 1 or 2, characterized in that said initial light beam (H1) is of the Gaussian type.

4. - System according to claim 1, 2 or 3, characterized in that said multiplexed light beam (HM) is similar to a Gaussian beam, at least as regards divergence characteristics.

5. - System according to any one of the preceding claims, characterized in that said transmission medium is an isotropic medium.

6. - System according to claim 5, characterized in that said isotropic medium is the free space.

7. - System according to any one of claims 1 to 4, characterized in that said transmission means is a waveguide.

8. System according to any one of the preceding claims, characterized in that said transmitter (T) also comprises at least a first lens (L1) arranged in front of the first end of the first optical element (C1) to focus the initial beam of light (H1 ) thereon, and at least one second lens (L2) arranged behind the second end of the second optical element (C2) to collide said multiplexed light beam (HM).

9. System according to any one of the preceding claims, characterized in that said receiver (R) comprises at least one objective (L3) arranged in front of the first end of the third optical element (C3), so that it receives the multiplexed light beam ( HM) after crossing said objective (L3), and at least one photodetector device (FD) arranged in front of the second end of the third optical element (C3) to receive the light from the optical channels and process the information contained therein.

10. - System according to any one of claims 1 to 8, characterized in that it comprises, after said third optical element (C3), at least one optical group that includes:

- at least a second modulator arranged and configured to encode information by modulating in amplitude said recovered optical channels,

11. - System according to claim 2 or any one of claims 3 to 10 when they depend on 2, characterized in that said modulator (M1), which is at least one, comprises at least one mask of angular amplitude disposed in said plane where the ring of light is formed or in another plane.

12. - System according to any one of the preceding claims, characterized in that said modulator (M1) comprises at least one spatial light modulator.

13. - System according to any one of the preceding claims, characterized in that said initial light beam (H1) is a monochromatic light beam.

14. - System according to any one of the preceding claims, characterized in that said modulator (M1) is configured to independently modulate at least part of said optical channels, by spatial modulation or spatial and temporal modulation.

15. - System according to any one of the preceding claims, characterized in that said modulator (M1) is configured to modulate in a dependent manner at least part of said optical channels, by spatial modulation or spatial and temporal modulation.

16. - System according to any one of the preceding claims, characterized in that the optical axes of said first (C1) and second (C2) optical elements are aligned with each other.

17. - System according to claim 16 when it depends on the 2, characterized in that said second conical refractive biaxial crystal is substantially of the same dimensions as the first.

18. - System according to claim 2 or any one of claims 3 to 17 when they depend on 2, characterized in that said second conical refractive biaxial crystal is oriented inversely with respect to the first.

19. - Method of optical communications, of the type comprising:

b) receiving said beam of light, after having traveled through said transmission means, and decoding said encoded information included in the beam of light;

the method being characterized because:

said stage a) comprises:

- emit an initial beam of light towards a first optical element;

- receiving, by said first optical element, said initial beam of light, demultiplexing it by changing the direction of the wave vectors of at least two of the sub-beams that make up said initial beam of light, and supplying said sub-beams with vectors of wave with changed directions;

and because said step b) comprises receiving and demultiplexing, by means of a third optical element, said multiplexed light beam, supplying said optical channels recovered after demultiplexing the multiplexed light beam, changing the direction of the sub-beam wave vectors, which are at least two, which make up the multiplexed beam of light.

20. - Method according to claim 19, characterized in that said first, second and third optical elements are respective first, second and third conical refractive biaxial crystals, wherein each of said demultiplexings of steps a) and b) comprises changing the direction of the wave vectors of the sub-beams that make up the initial light beam, as regards stage a), and the multiplexed beam, as regards stage b), supplying said sub-beams with wave vectors with directions changed so that their projection in a plane takes the form of a ring of light, where sub-beams with wave vectors in the same plane go to diametrically opposite sectors of the ring of light.

21. - Method according to claim 19 or 20, characterized in that it is intended to be implemented using the system according to any one of claims 1 to 18.

22. - Method according to claim 19, 20 or 21, characterized in that said initial light beam is linearly or circularly polarized.

23. - Method according to claim 19, 20 or 21, characterized in that said initial light beam is depolarized.

24.- Optical communications transmitter, of the type that is configured to transmit and encode information in a light beam through a transmission medium, the transmitter being characterized in that it comprises:

- a multiplexer formed by at least a second optical element (C2) arranged and configured to receive, at a first end, the light of said optical channels, and to supply said multiplexed optical channels towards said transmission means in the form of a beam of multiplexed light (HM), by a second end opposite said first end.

25. Transmitter according to claim 24, characterized in that it is configured to perform the functions of the transmitter (T) of the system according to any one of claims 1 to 18.

26.- Optical communications receiver, of the type that is arranged to receive a light beam that includes encoded information, after having traveled through a transmission medium, and configured to decode said encoded information included in the light beam, the receiver being characterized in that it comprises a demultiplexer formed by at least one optical element (C3) arranged and configured to receive, at a first end, a beam of light (HM) with information encoded in optical channels corresponding to sectors of light multiplexed in said beam of light (HM ), and supply, by a second end opposite said first end, said optical channels recovered after demultiplexing the multiplexed light beam (HM) by changing the direction of the wave vectors of the sub-beams, which are at least two, which make up the multiplexed light beam (HM).

27.- Receiver according to claim 26, characterized in that it is configured to perform the functions of the receiver (R) of the system according to any one of claims 1 to 18.