Self-compensated polarization-related attenuation optical isolator

Abstract

The invention relates to an optical isolator (1) transmitting a passing beam (2), which is divided into a first beam (22) in a perpendicular polarization (s) and a second beam (24) in a parallel polarization (p), these two polarizations being orthogonal, the first, respectively second, beam propagates along a first, respectively second, optical path.According to the invention: the first beam passes through a first polarization rotator, a polarizing optical element, a second polarization rotator, and a second delay plate; and the second beam passes through the second polarization rotator, the polarizing optical element, the first polarization rotator, and a first delay plate. The two beams are in a mutually identical polarization state when they are incident on a plurality of reflective surfaces, which are placed on both the first and second optical paths. The first beam, now in parallel polarization, and the second beam, now in perpendicular polarization, are recombined to reform the passing beam. Figure 5 is shown in the abbreviated version.

Description

Title of the invention: Optical isolator with self-compensated polarization-related attenuation Technical field of the invention

[0001] The present invention relates generally to free space optical components.

[0002] It relates more particularly to an optical isolator with self-compensated polarization-related attenuation.

[0003] The invention finds a particularly advantageous application in optical systems and assemblies, such as for example optical amplification systems or telecommunications systems, where having the lowest possible polarization-related attenuation is critical. State of the art

[0004] An optical isolator is an optical device comprising an assembly of several components, and whose function is to allow the passage of light in a chosen direction, while blocking the passage in the opposite direction. Thus, an optical isolator can be compared by analogy to an electronic diode, allowing the passage of electric current in one direction only.

[0005] This type of optical device is particularly useful for eliminating parasitic reflections, which propagate back within an optical system and can potentially damage fragile optical components or generate unwanted parasitic signals.

[0006] An optical isolator generally comprises a polarization rotator, for example a Faraday rotator, inducing a rotation of the polarization state of an incident light beam by magneto-optical effect. This property, known as the Faraday effect, makes it possible to obtain a non-reciprocity in the polarization obtained at the output of such a polarization rotator, depending on the direction of passage of the light. This therefore results in a passing propagation direction and a blocking propagation direction.

[0007] Polarization-independent optical isolators are known in the prior art, i.e., devices that can be used for any input polarization. Such devices usually comprise a first element, such as a birefringent prism or a polarized beam splitter, which divides an incident light beam of any polarization into two light beams of orthogonal polarizations. A Faraday rotator, possibly associated with a half-wave plate, rotates the polarization of each of the two beams of orthogonal polarizations thus obtained propagating parallel and in the same direction in the Faraday rotator. A second birefringent prism, or even a polarized beam splitter, then recombines the two beams of orthogonal polarizations into a single beam.

[0008] Such optical isolators of the prior art nevertheless have a difference in the overall transmission rate between the two orthogonal polarizations, and therefore an optical power or intensity at the output of the optical isolator dependent on the polarization of the input signal. In other words, there is an attenuation of the light intensity linked to the state of polarization at the input. Such an optical isolator therefore has losses linked to the polarization or PDL (for Polarization Dependent Loss in English terminology).

[0009] This variation in the output optical power as a function of the input polarization of the light beam can be problematic in certain applications, since it causes fluctuations in the output optical power. Thus, for these applications, attenuation linked to the quasi-zero polarization is necessary.

[0010] The state of the art thus proposes to minimize the attenuation linked to the polarization below the threshold of 0.2 decibels offered by standard optical isolators, by using very high quality optical components, obtained according to strict specifications. These components then make it possible to reduce the attenuation differences between the orthogonal polarizations to 0.1 decibels on average, which remains prohibitive in the context of certain applications, such as for example in the context of optical power amplification. In addition, these solutions prove to be expensive and restrictive for industrial production. Presentation of the invention

[0011] In order to overcome the aforementioned drawback of the state of the art, the present invention proposes an optical isolator architecture with self-compensated polarization-related attenuation.

[0012] More particularly, according to the invention, there is provided an optical isolator configured to transmit a light beam called a passing beam, propagating in the optical isolator in a passing direction from an input to an output of the optical isolator, the optical isolator comprising an optical polarization splitter device, a plurality of reflective surfaces and an optical isolation block, the optical polarization splitter device being configured to divide the beam passing at the input of the isolator into a first beam in a perpendicular polarization state and a second beam in a parallel polarization state, the parallel polarization state being orthogonal to the perpendicular polarization state, the first, respectively second, beam propagating according to a first, respectively second, optical path, the first optical path and the second optical path being distinct from each other at the optical isolation block. It is provided in this optical isolator that: - each of the reflective surfaces of the plurality of reflective surfaces is placed both on the first optical path and on the second optical path, - the first beam and the second beam are counterpropagating, parallel and offset from each other by a distance at least equal to a diameter of the passing beam, during their propagation through the optical isolation block, - the optical isolation block comprises a plurality of polarization rotators, a polarizing optical element, and at least two delay plates, arranged so that the first beam successively passes through a first rotator of the plurality of polarization rotators, the polarizing optical element, a second rotator of the plurality of polarization rotators, and a second delay plate of the at least two delay plates and emerges from the optical isolation block in the parallel polarization state, while the second beam successively passes through the second polarization rotator, the polarizing optical element, the first polarization rotator, and a first delay plate of the at least two delay plates and emerges from the optical isolation block in the perpendicular polarization state, - the first beam and the second beam are in a mutually identical polarization state when the first beam and the second beam are incident on each of the reflective surfaces of the plurality of reflective surfaces, - the polarization splitter optical device being configured to recombine the first beam in the parallel polarization state with the second beam in the perpendicular polarization state and form the beam passing at the output of the optical isolator.

[0013] Thus, thanks to the invention, each of the beams propagating within the optical isolator, i.e. the first beam and the second beam, undergoes equivalent optical losses. Indeed, the first beam and the second beam are reflected by the same reflecting surfaces. Although these reflecting surfaces may have reflection rates dependent on an incident polarization state, the fact that the first and second beams are in the same polarization state, when they are incident on a given reflecting surface, makes it possible to cause systematically identical optical losses on the first beam and the second beam. Thus, in order to obtain an identical polarization state between the two beams for a given reflecting surface, the two beams are counterpropagating at the optical isolation block, and their polarization states are permuted when passing through this block.In this way, both beams experience optical paths si . miles, whether by the optical components encountered on this path, or by the polarization states along this path.

[0014] These systematically self-compensated optical losses between the two beams, associated with orthogonal polarization states at the input of the optical isolator, ensure that the attenuation of the optical power during propagation within the optical isolator is not polarization dependent. It is therefore by construction that the proposed optical isolator has a zero PDL.

[0015] Other advantageous and non-limiting characteristics of the optical isolator according to the invention, taken individually or in all technically possible combinations, are the following: - the optical isolator comprises an optical component of refractive index n which is arranged to be placed both on the first optical path and on the second optical path, - the optical isolation block is arranged so that an optical path traveled by the first beam from the polarization splitter optical device to the optical isolation block is equal to an optical path traveled by the second beam from the polarization splitter optical device to the optical isolation block, - the polarization splitter optical device comprises a polarization splitter plate, - the polarization splitter optical device comprises a polarization splitter cube, - the polarization splitter optical device comprises an assembly of prisms, including a polarization splitter cube corner, the polarization splitter cube corner comprising a face oriented at 45 degrees to an optical axis of propagation of the passing beam and a rhombohedral prism comprising two faces oriented at 45 degrees to the optical axis of propagation of the passing beam, one of the two faces oriented at 45 degrees of the rhombohedral prism is juxtaposed with the face oriented at 45 degrees of the polarization splitter cube corner and in which the other of the two faces oriented at 45 degrees of the rhombohedral prism constitutes one of the surfaces of the plurality of reflective surfaces, - the polarization splitter optical device and the plurality of reflective surfaces which comprises a first reflective surface, a second reflective surface, and a third reflective surface, are arranged perpendicular to a plane and are arranged according to vertices of a rectangle contained in this plane, the rectangle having two long sides of length Ll, - the rhombohedral prism having a refractive index n, the other of the faces oriented at 45 degrees of the rhombohedral prism constituting the first reflecting surface, the first reflecting surface and the polarization splitting optical device being arranged along one long side of the rectangle, the second reflecting surface and the third reflecting surface being arranged at the ends of another long side of the rectangle, the optical isolation block is placed between the second reflecting surface and the third reflecting surface, at a distance equal to (Llx(nl)) / (2n) from the second reflecting surface, - the plurality of reflecting surfaces comprises at least one plane mirror of the metallic mirror type, or dielectric mirror, or total internal reflection mirror, - the polarization splitter device comprises at least one birefringent optical element, - the optical isolation block is configured so that: - the first delay plate is a half-wave plate placed only on the optical path of the second beam, and a neutral axis of the first delay plate forms an angle of 45 degrees in absolute value with the parallel polarization state and the perpendicular polarization state, - the first polarization rotator rotates the polarization by 45 degrees in absolute values ​​in one direction of rotation, - the polarizing optical element has a polarization axis oriented at 45 degrees in absolute value relative to the parallel polarization state and relative to the perpendicular polarization state, - the second polarization rotator rotates the polarization by 45 degrees in a direction of rotation opposite to the direction of rotation of the first polarization rotator, - and the second delay plate corresponds to a half-wave plate placed only on the optical path of the first beam, and a neutral axis of the second delay plate forms an angle of 45 degrees with the parallel polarization state and the perpendicular polarization state. - the optical isolation block further comprises a first auxiliary polarizing optical element and a second auxiliary polarizing optical element, the first auxiliary polarizing optical element being placed between the polarization splitter optical device and the first delay plate and the second auxiliary polarizing optical element being placed between the second delay plate and the polarization splitter optical device, the first auxiliary polarizing optical element having a polarization axis adapted to allow a perpendicular polarization state to pass and the second polarizing optical element having a polarization axis adapted to allow a parallel polarization state to pass.

[0016] Of course, the various features, variants and embodiments of the invention may be combined with each other in various combinations to the extent that they are not incompatible or mutually exclusive. Detailed description of the invention

[0017] The description which follows with reference to the appended drawings, given as non-limiting examples, will make it clear what the invention consists of and how it can be implemented.

[0018] In the attached drawings:

[0019] [Fig-1] is a top view of an optical isolator comprising in particular an optical isolation block, and where an optical path of a light beam in the passing direction is illustrated;

[0020] [Fig.2] is a top view of a second embodiment for an optical isolator, comprising a polarized beam splitter cube;

[0021] [Fig.3] is a top view of a third embodiment for an optical isolator comprising a trapezoidal prism;

[0022] [Fig.4] is a top view of a fourth embodiment for an optical isolator comprising a birefringent optical element;

[0023] [Fig.5] is a perspective view of an optical isolation block of an optical isolator according to a first embodiment, representing in particular the polarization states during the propagation of light through the succession of optical elements in the passing direction;

[0024] [Fig.6] is a representation seen from above of the optical isolator of [Fig.l], where an optical path traveled by a light beam propagating in the isolator in the blocking direction is shown;

[0025] [Fig.7] is a perspective view of the optical isolation block of an optical isolator according to the first embodiment, showing light propagating in the blocking direction as in [Fig.6];

[0026] [Fig.8] is a perspective view of an optical isolation block of an optical isolator according to a second embodiment, representing in particular the polarization states during the propagation of light through the succession of optical elements in the passing direction. General description of an optical isolator

[0027] In [Fig.l], an optical isolator 1 is shown. This is an optical component through which a light beam can propagate only in a single direction, defined as the passing direction. Such a light beam is defined as a passing light beam, or passing beam 2. This optical isolator 1 has an input IN and an output OUT; the passing beam 2 propagates from the input IN to the output OUT of the optical isolator 1 by following an optical path through the different optical elements that this isolator comprises.

[0028] The optical isolator 1 also has a blocking direction. This blocking direction corresponds to a direction of propagation from the output OUT of the optical isolator 1, partly following the same optical path as the passing beam 2, but in the opposite direction. A light beam circulating within the optical isolator 1 in the blocking direction, called blocked beam 4, is then, for example, filtered by polarization within the optical isolator 1 or else sees its optical path deflected relative to the optical path of the passing beam 2. In this way, a light beam circulating from the output OUT does not propagate to the input IN of the optical isolator 1.

[0029] It is thus considered that the optical isolator 1 allows the propagation of light in one direction, and blocks it in the opposite direction; it therefore presents an optical non-reciprocity, that is to say that the passage of light through the isolator in a given direction is not equivalent to the passage of light in the opposite direction.

[0030] Such an optical component advantageously makes it possible to avoid potential parasitic reflections within an optical system. These parasitic reflections backpropagate, i.e. propagate in the opposite direction relative to a light beam of interest, through the optical system, and are then likely to damage certain fragile optical components within this system and / or induce instabilities in their performance.

[0031] The optical isolator 1, illustrated in [Fig.l] is independent of polarization, that is to say that it performs its function of blocking or passing light independently of a polarization state of an incident beam in the passing direction. For this, the optical isolator 1 comprises different optical elements, in particular an optical polarization splitter device 12, a plurality of reflective surfaces 131, 132, 133, as well as an optical isolation block 17.

[0032] As shown in the top view of [Fig.l], the polarization splitter optical device 12 and the plurality of reflecting surfaces 131, 132, 133 coincide with a plane, corresponding for example to the plane of [Fig.l]. In this embodiment, the aforementioned optical elements are arranged at vertices of a rectangle 6.

[0033] Thus, the optical isolator 1 here comprises three reflective surfaces, corresponding to a first, respectively a second and a third reflective surface 131, 132, 133.

[0034] This rectangle 6, shown in dotted lines in [Fig.l], has two long sides of length L1 and two short sides.

[0035] Other arrangements of the polarization splitter optical device 12 and the plurality of reflective surfaces 131, 132, 133 are also conceivable, where the aforementioned optical elements are arranged at polygon vertices, here, of a six-sided polygon, as shown in [Fig. 2]. In this embodiment, the optical isolator 1 comprises five reflective surfaces.

[0036] In order to simplify the rest of the description, an arbitrary HV reference frame corresponding to the horizontal and vertical directions of [Fig.l], is used to describe the positioning of the different optical elements that comprise the optical isolator 1. In practice, the optical isolator 1 can be positioned in different orientations within an optical system, and the orientation of the optical elements as described here is given for information purposes only.

[0037] For example, it is chosen that the two long sides of the rectangle 6 are arranged in the vertical direction, while the two short sides of the rectangle 6 are arranged in the horizontal direction.

[0038] In the embodiment currently described, it is considered that the three reflecting surfaces 131, 132, 133 are oriented at 45 degrees relative to the sides of the rectangle 6. The optical elements thus arranged according to the vertices of the rectangle 6 guide the propagation of the different beams which can propagate within the optical isolator 1.

[0039] Firstly, a light beam propagating in the passing direction of the optical isolator 1 is described, thus corresponding to a passing beam 2. This beam therefore passes through the optical isolator 1 from its input IN to its output OUT. This passing beam 2 corresponds here to a light beam emitted for example by a laser type source, not shown here. For example, the laser source corresponds to a fiber laser source, the emitted beam of which is collimated using an optical collimation device.

[0040] This laser source emits radiation that is assumed to be monochromatic around a central wavelength. This central wavelength is located in particular in the C band of the infrared telecommunications electromagnetic spectrum, which covers wavelengths ranging from 1520 nanometers to 1570 nanometers, which are wavelengths typically used in the context of standard telecommunications applications. In particular, in the embodiment described, the passing beam 2 has a central wavelength of 1550 nanometers. Other central wavelengths chosen for example, in the visible spectrum, from 380 nanometers to 780 nanometers, or in the near infrared, from 780 nanometers to 2500 nanometers are also conceivable, using optical elements adapted in terms of bandwidth.

[0041] It is assumed in the remainder of this description that optical elements and / or devices adapted to the central wavelength of the passing beam 2 are chosen.

[0042] The passing beam 2 entering the optical isolator 1 is described by its central wavelength, but also by other parameters, including an optical power, an intensity, a beam diameter, and a polarization state also called polarization.

[0043] The beam diameter corresponds, for example, to the width at half height of a dis transverse intensity distribution, or to the width at 1 / e2 of it, as is customary. The half-width at 1 / e2 or radius at 1 / e2 corresponds to the waist or neck of a Gaussian light beam. For example, the neck of the passing beam 2 is 200 micrometers, corresponding to a beam diameter of 400 micrometers.

[0044] In the remainder of the description, the passing beam 2 is associated with a propagation axis, as well as with a propagation direction, corresponding here to the passing direction. The direction of propagation of the light is indicated in the figures using arrows on the propagation axis.

[0045] The passing beam 2 propagates in the direction passing between the input IN and the output OUT of the optical isolator 1 by following a path described below.

[0046] The input IN is materialized by a physical element, for example a ferrule to connect the end of an optical fiber, or simply corresponds to an entry point of the light beam within the optical isolator 1, as is the case here. The same is true for the output OUT.

[0047] The polarization state of the passing beam 2 at the input IN of the optical isolator 1 is chosen arbitrarily, in order to describe an embodiment. For example, the passing beam 2 considered in this first embodiment, and the following embodiments, is initially linearly polarized in an arbitrary direction.

[0048] Alternatively, a beam 2 passing according to an elliptical polarization, partially polarized, or even non-polarized, i.e. comprising different random polarization states, is also conceivable.

[0049] A polarization splitting optical device 12, which is arranged at one of the vertices of the rectangle 6 previously described, is arranged in such a way that a splitting surface 121 of this polarization splitting optical device 12 forms an angle of incidence of 45 degrees with the passing beam 2.

[0050] The separating surface 121 is configured to divide the passing beam 2 into two spatially distinct light sub-beams. These two sub-beams correspond to a first beam 22, which is polarized in a first polarization state, and a second beam 24, which is polarized in a second polarization state. The first polarization state and the second polarization state are here orthogonal to each other at the output of the separating surface 121.

[0051] Indeed, the separating surface 121 of the polarization separating optical device 12 is adapted to decompose the incident polarization state into two orthogonal polarization states, these two states forming an orthogonal base.

[0052] Any incident polarization state that the passing beam 2 presents can be decomposed on this orthogonal basis, the optical power of the passing beam 2 being distributed between these two orthogonal polarization states, apart from any losses. Similarly, a partially, or even non-polarized, passing beam 2 sees its power optical to be distributed between the two orthogonal polarization states of the orthogonal base.

[0053] These two orthogonal polarization states are defined with respect to a plane of incidence, which corresponds to the plane perpendicular to the separating surface 121 and coplanar with the passing beam 2. One of these two orthogonal polarization states corresponds to a polarization state perpendicular to the plane of incidence, called perpendicular polarization state, and usually noted s, from the German senkrecht.

[0054] The state of polarization orthogonal to this perpendicular polarization s, corresponding to the state of polarization included in the plane of incidence, is described as a state of parallel polarization and noted p, from the German parallel. Other names are also in use to describe the two orthogonal states of polarization.

[0055] Here, the first polarization state corresponds to the perpendicular polarization state s, while the second polarization state corresponds to the parallel polarization state p. These polarization states are illustrated in [Fig.l], using the usual notations.

[0056] The separating surface 121 of the polarization separating optical device 12 reflects the first beam 22, and transmits the second beam 24, according to their respective polarization states.

[0057] The first beam 22 is deflected by an angle equal to 90 degrees relative to the passing beam 2 while the second beam 24 is transmitted by the separating surface 121 without deviation relative to the passing beam 2. The first beam 22 and the second beam 24 are then spatially at 90 degrees from each other, and propagate in divergent directions.

[0058] Such a polarization splitter optical device 12 corresponds, for example, to an assembly of prisms, in particular a polarization splitter cube corner 120, one of the faces of this polarization splitter cube corner 120 corresponding to the splitter surface 121 as described previously. The prism assembly is completed by a rhombohedral prism 122. This rhombohedral prism 122 has six faces in pairs parallel, among which two faces oriented parallel to the splitter surface 121 and two faces extending parallel to the long sides of the rectangle 6. Then, a distance between the two faces parallel to the splitter surface 121 is equal to the length of a long side, that is to say to LL

[0059] This embodiment is shown in [Fig.l].

[0060] One of the two faces parallel to the separating surface 121 of the polarization splitter optical device 12 is brought into optical contact with the separating surface 121. In other words, one of the faces of the rhombohedral prism 122 is brought closer to the separating surface 121, until they adhere to each other by molecular bonding, provided that they are not very rough. Alternatively, the face of the rhombohedral prism and the separating surface 121 are bonded, for example using epoxy glue or optical cement.

[0061] Here, the prism assembly is for example at least partially made of optical glass. The prism assembly has a refractive index of between 1.4 and 1.8. For example, a glass is chosen whose refractive index is rounded to 1.5.

[0062] In this embodiment where the polarization splitter optical device 12 is an assembly of prisms, a second face of the two faces parallel to the splitter surface 121, located at a distance L1 from this splitter surface 121 constitutes a first reflecting surface 131 among the plurality of reflecting surfaces 131, 132, 133 mentioned previously.

[0063] According to other embodiments, several other polarization splitter optical devices 12 are possible, such as, for example, a polarization splitter plate, one of the two main faces of which corresponds to the splitter surface 121.

[0064] According to yet another embodiment, the polarization splitter optical device 12 comprises a polarization splitter cube formed by the assembly of two cube corners, thus forming a polarization splitter cube, also known as a polarizing beamsplitter cube in English. Such an embodiment is shown in [Fig.2] and [Fig.3].

[0065] According to a third other embodiment, the polarization splitting optical device 12 comprises at least one birefringent crystal, the polarization splitting optical device in this case possibly being monolithic. This embodiment is shown in [Fig.4].

[0066] The optical isolator 1 comprises, in this third embodiment, two reflecting surfaces, a first reflecting surface 134 and a second reflecting surface 135, as well as a birefringent crystal or birefringent optical element. Here, this birefringent crystal corresponds to a calcite crystal, which acts as a polarization splitter optical device 12. This birefringent crystal has two main faces, perpendicular to the passing beam 2. When these main faces act as interfaces from the air to the birefringent medium, they are then adapted to divide the passing beam 2 into two sub-beams, a first beam 22, and a second beam 24. Thus, these two main faces are considered equivalently as splitter surfaces 121, depending on the direction of incidence.In other words, the first main face of the polarization splitter device 12 crossed by a light beam acts as a splitter surface.

[0067] As previously, the two sub-beams have orthogonal polarization states, the separation of the passing beam 2 being due to a difference in refractive index perceived according to the polarization state of the passing beam 2, according to the birefringence properties well known in the state of the art.

[0068] Alternatively, the polarization splitter optical device 12 comprises an assembly of birefringent crystals, such as, for example, a Wollaston prism, or a Rochon prism, as known in the state of the art.

[0069] These examples of polarization splitting optical device 12 are cited as examples, and the chosen polarization splitting optical device 12 has no influence on the technical effect obtained.

[0070] Thus, the remainder of the description mainly refers to an arrangement of the optical isolator 1, as described in the embodiment of [Fig. 1]. Nevertheless, the extension of the implementation of the invention to the other embodiments as described previously is within the reach of those skilled in the art.

[0071] Referring again to [Fig.l], after passing through the polarization splitting optical device 12, the passing beam 2 is subdivided into two orthogonally polarized sub-beams, between which the optical power of the passing beam 2 is distributed.

[0072] Each of these sub-beams undergoes a separate treatment before being recombined at the output of the isolator, which makes it possible to obtain an optical isolator 1 independent of the input polarization, thanks to the decomposition into two orthogonal polarization states by the optical polarization splitter device 12.

[0073] Furthermore, the other optical elements that the optical isolator 1 comprises are chosen and arranged so that by construction, the first beam 22 and the second beam 24 have identical overall transmission rates, that is to say that the attenuation of the optical power between the input IN and the output OUT of the optical isolator 1 is independent of the polarization of the passing beam 2.

[0074] This is equivalent to saying that by construction, the losses linked to polarization, corresponding to the PDL, for “polarization dependent loss” in English, are minimized, or even almost zero.

[0075] Advantageously, the optical isolator 1 which is the subject of the present disclosure retains its property of minimizing the PDL, independently of the wavelength of the passing beam 2. Nevertheless, other properties of the optical isolator 1 may be degraded by use at a wavelength different from the nominal wavelength. The performance in terms of optical losses or isolation of the optical isolator 1 is for example affected, in a similar manner to a standard optical isolator. Use of the optical isolator 1 at a wavelength different from the nominal wavelength, but within a range of a few tens of nanometers around the nominal wavelength, however retains the performance of the optical isolator 1 within reasonable proportions.

[0076] The PDL is usually determined by measuring the optical power at the output of an optical system as a function of the input polarization. The PDL then measures a deviation peak-to-peak difference between a minimum of optical power and a maximum of optical power. Its value is usually given in decibels (dB).

[0077] To achieve the minimization of the PDL, it is advantageously proposed in the present disclosure to maintain equivalent optical losses on each of the two sub-beams, by rigorously compensating for each of the sources of losses and by overcoming variations in reflectivity and / or attenuations of the optical elements due to the incident polarization states.

[0078] In the first embodiment described herein, the compensation of optical losses on each of the two sub-beams is done by means of a spatial arrangement of the different optical elements in a ring or loop, and by rigorously maintaining an identical polarization state between the two sub-beams at the level of each of the reflective optical elements encountered.

[0079] Thus, the first beam 22 and the second beam 24 travel a quasi-identical optical path, apart from a translation of the point of incidence on the different elements. In other words, the first beam 22 and the second beam 24 are transmitted or reflected by common optical elements, but following an inverted order of incidence, before being recombined at the end of the loop by the same optical polarization splitter device 12 described previously. The passing beam 2 recombined by the splitter plate 121 then emerges from the optical isolator 1 via its output OUT.

[0080] The geometric arrangement of the optical elements, i.e. of the plurality of reflective surfaces 131, 132, 133, of the optical isolation block 17 relative to the polarization splitter optical device 12 is described first, followed by the description of the path of the first beam 22 and of the second beam 24 in the passing direction, before continuing with the description of the passage of a light beam in the blocking direction. Geometric arrangement of optical elements

[0081] As detailed previously, after the separating surface 121 of the polarization separating optical device 12, the first beam 22 propagates along a first optical path, and the second beam 24 propagates along a second optical path. These two optical paths are spatially distinct from each other, between their separation by the separating surface 121 and their recombination by this same separating surface 121, or its equivalent in the case of a polarization separating optical device 12 comprising at least one birefringent optical element.

[0082] According to [Fig.l], the first optical path and the second optical path are segmented into as many portions as there are sides of the rectangle 6 at the top of which optical elements are arranged.

[0083] Here, as mentioned previously, the optical isolator 1 comprises a polarization splitter device 12, three reflecting surfaces, the first, the second and the third reflecting surface 131, 132, 133. The first optical path and the second optical path are each segmented into four portions between the different aforementioned optical elements.

[0084] In the first embodiment described, where the polarization splitter optical device 12 is an assembly of prisms, including a rhombohedral prism 122, the first reflecting surface 131 has already been described.

[0085] The second, respectively third, reflecting surface 132, 133 corresponds for example to a plane mirror, more particularly to a gold metal mirror. Alternatively in this first embodiment, the reflecting surfaces 132, 133 correspond, in a non-limiting manner, to a dielectric mirror, or to a surface reflecting a light beam by total reflection.

[0086] Alternatively, an embodiment where the polarization splitter optical device 12 is a polarization splitter cube, and where the first reflecting surface 131, and the second reflecting surface 132 constitute faces of a trapezoidal prism is shown in [Fig.3].

[0087] In the first embodiment described, by traversing the vertices of the rectangle 6 in a clockwise direction from the separating surface 121 of the polarization separating optical device 12, we successively find the first reflecting surface 131, the second reflecting surface 132, then the third reflecting surface 133, before falling back on the separating surface 121.

[0088] In the remainder of the description, a first segment is defined between the separating surface 121 and the first reflecting surface 131, a second segment between the first reflecting surface 131 and the second reflecting surface 132, then a third segment between the second reflecting surface 132 and the third reflecting surface 133, and a fourth segment is defined between the third reflecting surface 133 and the separating surface 121.

[0089] Due to the arrangement of the optical elements in a ring, and the 90 degree angle formed between the first beam 22 and the second beam 24 after the separating surface 121, they travel parallel optical paths on each of the segments listed in the previous paragraph, but in opposite directions of propagation. Thus, for each of the segments, the first beam 22 and the second beam 24 are counter-propagating and parallel. By counter-propagating, it is understood that the first beam 22 and the second beam propagate in mutually opposite directions.

[0090] The optical isolation block 17 is arranged along one of the sides of the rectangle 6. Here, in the first embodiment described, the optical isolation block 17 is more specifically arranged at one of the two long sides of the rectangle 6.

[0091] In particular, in the first embodiment described, where the optical isolator 1 comprises three reflective surfaces, and an optical polar splitter device rization 12 in the form of a prism assembly, it is understood that the optical isolation block 17 is arranged on the third segment.

[0092] In the first embodiment, it is advantageously provided to place the optical isolation block along the long side located opposite the prism assembly, at a distance d from the second reflecting surface 132. This distance d is calculated so that a part of a first optical path associated with the first beam 22 and a part of a second optical path associated with the second beam 24 are equal.

[0093] Here, the notion of optical path refers to a distance traveled by each of the light beams after the optical isolation block 17 and up to the separating surface 121, taking into account the refractive indices of the media crossed by these beams.

[0094] Indeed, in order to rigorously compensate for the optical losses between the two sub-beams, it is recommended to ensure the good spatial and / or angular recombination of the first beam 22 and the second beam 24, that is to say, to ensure a spatial and / or angular superposition between the sub-beams when they are recombined by the separating surface 121 in order to reform the passing beam 2.

[0095] At the output OUT of the optical isolator 1, the passing beam 2 is for example reinjected into an optical fiber, by means of an optical injection system, comprising for example a lens. The position of the optical fiber and of the optical injection system is usually maintained at a fixed position relative to the output OUT of the optical isolator 1. Thus, in the event that a spatial and / or angular deviation occurs between the first beam 22 and the second beam 24, they are then not recombined by the separating surface 121 into a single passing beam 2 whose beam diameter is identical to the beam diameter at the input of the optical isolator 1.

[0096] In particular, an angular shift may be induced by the optical isolation block 17, for example if it is prismatic. In order to maintain an angular shift which results in an identical spatial displacement between the first beam 22 and the second beam 24, it is preferable that the optical paths traveled after crossing the block are identical up to the separating surface 121.

[0097] This condition is here translated by a distance d equal to J — £ | where, we have 2xn recalls, L1 is equal to the length of the long side of the rectangle 6, n is equal to the refractive index of the assembly of prisms corresponding here to the optical polarization splitter device 12.

[0098] Since the refractive index of the prism assembly is equal to 1.5 then this distance d is approximated to one sixth of the length L1.

[0099] This advantageous position of the optical isolation block is illustrated schematically in [Fig.l].

[0100] Similarly, in the case where the optical isolator 1 comprises any optical element, of length L2, and of refractive index n, this optical element being arranged so as to be both on the first optical path and on the second optical path, on one of the sides of the rectangle 6, then the preceding formula remains applicable in order to determine an optimal position for the optical isolation block, provided that L1 is replaced by L2.

[0101] The optical isolation block 17 is adapted to allow light to pass in the forward direction, and to block it if it passes through the block in the opposite direction. Such an optical isolation block 17 is shown in detail in [Fig.5]. Such an optical isolation block 17 is also compatible with all the other embodiments described previously.

[0102] The arbitrary reference VH used in [Fig.l] is repeated in [Fig.5], in order to orient it in relation to [Fig.l].

[0103] To ensure the principle of non-reciprocity, the optical isolation block 17 comprises optical elements adapted to modify a polarization state, i.e. polarization modifying optical elements, including a plurality of polarization rotators 172, 176, at least one polarizing optical element 174 and at least two delay plates 171, 173.

[0104] These polarization modifying optical elements are arranged at normal incidence relative to the first beam 22 and the second beam 24.

[0105] An arbitrary reference frame Oxyz, orthogonal and direct, is defined on the third segment, in order to describe the orientations of the axes of interest of the polarization modifying optical elements. An axis Oz of this arbitrary reference frame Oxyz is parallel to the first beam 22 and to the second beam 24, and is oriented from the second reflecting surface 132 towards the third reflecting surface 133.

[0106] Furthermore, the orientation of the axes of interest of the polarizing elements is described in an xy plane, orthogonal to the Oz axis. The angles are given relative to an Ox axis, in the direct direction, which corresponds to a clockwise direction equivalent to a positive direction, which is opposed to an anti-clockwise direction which is equivalent to a negative direction.

[0107] In the first embodiment, the plurality of polarization rotators corresponds to two polarization rotators, a first polarization rotator 172 and a second polarization rotator 176.

[0108] The first polarization rotator 172 is configured to rotate an incident polarization 45 degrees clockwise, while the second polarization rotator 176 is configured to rotate an incident polarization 45 degrees counterclockwise.

[0109] Here, the first polarization rotator 172 and the second polarization rotator 176 are, for example, Faraday rotators.

[0110] In the first embodiment illustrated in Figures 5 and 7, the insulation block optical element 17 comprises a central polarizing optical element 174, which is more commonly referred to as a polarizer. By central polarizing optical element 174, it is meant here that this polarizing optical element 174 is arranged between the first polarization rotator 172 and the second polarization rotator 176. Such a polarizer is adapted to select a polarization direction, here, for linearly polarized light, and is similar to a polarization filtering device. This selected polarization direction corresponds to an axis of the polarizer.

[0111] Finally, the optical isolation block 17 comprises in the first embodiment exactly two delay plates, a first delay plate 171 and a second delay plate 173. These two delay plates 171, 173 correspond to two half-wave plates, which are described by two neutral axes, a slow axis and a fast axis, between which the phase delay takes place.

[0112] The arrangement of the polarizing elements in the first embodiment is as follows, starting from the second reflecting surface 132, along the third segment, to the third reflecting surface 133: - firstly, the first delay plate 171, which is placed only on the optical path of the second beam 24. More precisely, the first delay plate 171 is a half-wave delay plate. This first delay plate 171 is placed so as not to intercept the path of the first beam 22. For this, a half-wave plate with suitable dimensions is used, or a half-wave plate machined in the shape of a D. The fast axis of the first half-wave delay plate 171 is oriented at +45 degrees in the xy plane, relative to the polarization direction s; - the first half-wave delay plate 171 is followed by the first polarization rotator 172, placed on both the first optical path and the second optical path, which rotates the polarization states of the first and second beams by 45 degrees clockwise, as a reminder, the clockwise direction being defined relative to the arbitrary reference frame Oxyz; - then comes the polarizing optical element 174, also placed on both the first and second optical paths, at a central position of the optical isolation block 17, between the first polarization rotator 172 and the second polarization rotator 176. The axis of the polarizing optical element 174 is oriented at 45 degrees in the xy plane relative to the polarization direction p of the first beam 22; - the polarizer 174 is followed by the second polarization rotator 176, which rotates the polarization states of the first and second beams by -45 degrees, i.e. 45 degrees counterclockwise, - finally, the second half-wave plate 173, placed only on the first optical path associated with the first beam 22. Like the first half-wave plate, its dimensions and / or its shape is adapted to be crossed by the first beam 22 and not the second beam 24. Its fast axis is oriented at 45 degrees in the xy plane relative to the polarization direction s.

[0113] As a reminder, the angular orientations of the different axes describing the arrangement of the polarization modifying optical elements are described by taking the Ox axis as the origin in the xy plane and following the direct direction.

[0114] Now that the geometric arrangement of the different elements comprising the optical isolator 1 has been described, we are interested in the detailed description of the first optical path traveled by the first beam 22 and then in the detailed description of the second optical path traveled by the second beam 24.

[0115] During their propagation, the two sub-beams undergo optical losses. Here, it is reasoned that these optical losses are mainly induced by reflective surfaces and by the polarization splitter optical device 12. In particular, for reflective surfaces, the optical losses depend on the type of surface, for example, whether it is a reflective surface of the metal mirror type, or a surface reflecting light by total internal reflection (total internal reflection in English).

[0116] In the case of a reflective surface of the metal mirror type, the optical losses depend in particular on the polarization state of the incident beam, the angle of incidence, as well as the wavelength of the incident beam. Hence the importance, in the context of the present disclosure, of rigorously maintaining an identical polarization state on the two sub-beams, for each of the reflective surfaces.

[0117] In practice, in the context of the embodiment described, given an angle of incidence and reflection of the sub-beams on the reflecting surfaces 132, 133 equal to 45 degrees, and reflecting surfaces corresponding to gold mirrors, a reflection rate of the reflecting surface is equal to 97.5%, the symbol % corresponding to a percentage, for a p-polarized beam, and equal to 98.5% for an s-polarized beam. The two sub-beams here have a wavelength of 1550 nanometers, like the passing beam 2. In other words, the optical losses for such reflecting surfaces 132, 133 are equal to 2.5% for a p-polarized incident beam, and equal to 1.3% for an s-polarized incident beam.

[0118] In the case of a surface reflecting light by total reflection, such as for example here, the reflecting surface 131, the optical losses are in practice zero, regardless of the polarization state of the incident beam. Path of the first beam and second beam

[0119] As a reminder, after the separating surface 121, the first beam 22 is initially in the perpendicular polarization state s. Being reflected by the separating surface 121, the first beam 22 propagates on the first segment up to the first reflective surface 131.

[0120] In particular, in the first embodiment described, the first beam 22 propagates within the rhombohedral prism 122 of refractive index n, over a distance L1.

[0121] The first beam 22 is then reflected by the first reflecting surface 131. This surface is arranged so that an angle of incidence of the first beam 22 on the first reflecting surface is equal to 45 degrees.

[0122] When reflected by the first reflecting surface 131, the first beam 22 possibly undergoes optical losses. Here, given that the first beam 22 is reflected by total reflection on the reflecting surface 131, the optical losses are zero.

[0123] The first beam 22 then propagates on the second segment between the first reflecting surface 131 and the second reflecting surface 132.

[0124] The second reflecting surface 132 is positioned so that the second beam 24 has an angle of incidence of 45 degrees. In the same manner as before, the first beam 22 experiences optical losses when reflected by the second reflecting surface 132. Given the type of reflecting surface, and the characteristics exhibited by the first beam 22 at the second reflecting surface 132, the first beam loses 1.3% of its initial optical power.

[0125] The first beam 22 then propagates along the third segment, and passes through the optical isolation block 17. Upstream of the optical isolation block 17, the first beam 22 is, as a reminder, polarized in the perpendicular polarization state s, which corresponds to a linear polarization, in the direction of the axis Ox of the arbitrary reference frame Oxyz.

[0126] The crossing of the optical isolation block 17 by the first beam 22, as well as the successive polarizations of this beam during this crossing, are shown in detail in [Fig.5]. The polarization states are illustrated here in the xy plane.

[0127] It is validly assumed that the optical losses induced by the polarization modifying optical elements are zero. Thus, during its passage through the optical isolation block 17, it is here considered that the first beam 22 does not undergo optical losses.

[0128] The first beam 22 first passes through the first polarization rotator 172. This rotates the first polarization state 45 degrees clockwise. As illustrated in [Fig.5], the first beam 22 after passing through the first polarization rotator 172 is linearly polarized and oriented 45 degrees in the xy plane.

[0129] The polarizing optical element 174, the axis of which is oriented at 45 degrees in the xy plane relative to the Ox axis, therefore transmits the first beam 22 given that the latter is polarized in the same orientation as the polarizer axis.

[0130] The first beam 22 then passes through the second polarization rotator 176. The second polarization rotator 176 rotates the polarization of the first beam 45 degrees counterclockwise. Thus, after the second polarization rotator 176, the first beam 22 is linearly polarized at 0 degrees in the xy plane, i.e., along the Ox axis.

[0131] Finally, the first beam 22 passes through the second half-wave delay plate 173, whose fast axis is oriented at 45 degrees in the xy plane. The first delay plate 173 therefore induces a phase shift in the polarization of the first beam 22. After passing through the second half-wave delay plate 173, the first beam 22 is now linearly polarized along the Oy axis. This polarization corresponds to the parallel polarization state p.

[0132] The first beam 22, now polarized in a state orthogonal to its polarization upstream of the optical isolation block 17, propagates to the third reflecting surface 133, which reflects the first beam 22.

[0133] Like the previous reflective surfaces, this third reflective surface 133 is oriented at 45 degrees relative to the first beam 22. During its reflection, the first beam 22 again undergoes possible optical losses. Since it is a metal mirror, similar to the second reflective surface, these optical losses are worth, for example, 2.5%.

[0134] Finally, the first beam 22 travels through the rest of the ring, on the fourth segment, before returning to the level of the polarization splitter optical device 12, and in particular its splitter surface 121.

[0135] The second optical path, traveled by the second beam 24, is then described. This is polarized in the parallel polarization state, after the separating surface 121.

[0136] The separating surface 121 transmits the second beam 24, which propagates first along the fourth segment of the rectangle 6, between the separating surface 121 and the third reflecting surface 133.

[0137] This third reflecting surface 133 is adapted to reflect the second beam 24. During its reflection by the third reflecting surface 133, the second beam 24 may undergo optical losses. Given the type of the third reflecting surface 133, here, a metal mirror, and the characteristics presented by the second beam 22, the optical losses are worth 2.5%.

[0138] After reflection on the third reflecting surface 133, the second beam 24 then propagates on the second segment, between the third reflecting surface 133 and the second reflecting surface 132.

[0139] The second beam 24 then propagates along the second segment, along an optical path parallel and counterpropagating relative to the optical path of the first beam. 22. Nevertheless, the two sub-beams are offset by a distance E equal to at least a diameter of the passing beam 2. This offset value makes it possible to ensure selective passage of the beams through the first delay plate 171 and the second delay plate 173 of the optical isolation block 17. For example, the distance E is at least greater than or equal to four times the neck of the beam, the neck (waist in English) having been defined previously as the radius of the beam at a radial intensity profile equal to 1 / e2, for a beam having a Gaussian profile. In order to limit a spatial footprint of the optical isolator 1, it is nevertheless suggested to maintain the distance E at a reasonable scale relative to a scale of the optical system within which the optical isolator 1 is inserted.

[0140] In practice, in order to obtain a distance E between the first beam 22 and the second beam 24, the optical axis of the passing beam 2 is positioned parallel to the fourth segment of the rectangle 6, at a distance equal to E / 2 relative to the fourth segment.

[0141] Thus, on each of the segments of the rectangle 6, the first beam 22 and the second beam 24 are spaced by the distance E.

[0142] The crossing of the optical isolation block 17 by the second beam 24 is also illustrated in detail in [Fig.5], where the polarization of the second beam 24 in the xy plane is also shown.

[0143] Upstream of the optical isolation block, the second beam 24 is polarized in the parallel polarization state p, corresponding to a linear polarization state, oriented along the Oy axis.

[0144] The second beam 24 first passes through the second polarization rotator 176 which rotates the polarization of the second beam 24 by 45 degrees counterclockwise relative to the arbitrary reference frame Oxyz. Thus, after the second polarization rotator 176, the second beam 24 is linearly polarized at an orientation of 45 degrees relative to the Ox axis.

[0145] With this polarization, the second beam 24 is therefore adapted to pass through the polarizing optical element 174, since its state of polarization is aligned with the axis of the polarizer.

[0146] The second beam 24 then passes through the first polarization rotator 172 which rotates the polarization state of the second beam 24 by 45 degrees clockwise, relative to the arbitrary reference frame Oxyz.

[0147] After passing through the first polarization rotator 172, the second beam 24 is again polarized in the parallel polarization state p.

[0148] The first delay plate 171, placed exclusively on the optical path of the second beam 24, then induces a phase shift of the polarization of this second beam 24 along the neutral axes of the plate. The second beam 24 emerges from the first delay plate delay 171 polarized in the perpendicular polarization state s, corresponding to a linear polarization oriented along the Ox axis.

[0149] Thus, downstream of the optical isolation block 17, the second beam 24 has a polarization state orthogonal to its polarization upstream of the optical isolation block 17.

[0150] In the same way as the first beam 22, it is reasonably considered that the second beam 24 does not undergo optical losses within the optical isolation block 17.

[0151] The second beam 24 continues its optical path, being reflected by the second reflecting surface 132, here, a gold metal mirror. Thus, this surface is associated with optical losses then equal to 1.3%, given the angle of incidence, equal to 45 degrees and the state of polarization of the incident light beam, here the state of parallel polarization p.

[0152] The second beam 24 then propagates over the second segment of the rectangle, before finally being reflected by the first reflecting surface 131. This surface is associated with optical losses which are zero here, given that the second beam 24 is reflected by total internal reflection.

[0153] Thus, during their propagation on their respective optical path, the first beam 22 and the second beam 24 have undergone optical losses of strictly equal values ​​by construction of the optical isolator 1 described, and this, independently of the variation of the transmission rates of the reflecting surfaces as a function of the incident polarization.

[0154] In the first embodiment described, where the polarization splitter optical device 12 is an assembly of prisms, the second beam 24 propagates over a distance equal to L1, i.e. the length of a long side of the rectangle 6, within a rhombohedral prism 122 of refractive index n, up to the splitter surface 121.

[0155] After having traveled their respective optical path, the first beam 22 and the second beam 24 are recombined by the same separating surface 121 as previously, in order to reform the passing beam 2 at the output OUT of the optical isolator 1.

[0156] A difference in optical losses between the first beam 22 and the second beam 24 possibly exists due to the fact that one of the beams was transmitted by the separating surface 121, while the other of the beams was reflected. To compensate for this difference, it is advantageous to recombine the two sub-beams by the polarization separating optical device 12 in the following manner: - the first beam 22 is now transmitted by the separating surface 121, since polarized in the parallel polarization state p, after having been reflected during the division of the passing beam 2, given that the first beam 22 was then in the perpendicular polarization state s; - similarly, the second beam 24 had been transmitted by the separating surface 121 during of the division of the passing beam 2, since it was in the parallel polarization state p. Now, during the recombination, the second beam 24 being in the perpendicular polarization state s, it is reflected by the separating surface 121.

[0157] This recombination is enabled by the two delay plates 171 and 173 introduced into the optical isolation block 17, which permute the polarization states of the sub-beams at the input and output of the optical isolation block 17.

[0158] The optical losses generated by the polarization separating surface 121 of the polarization separating optical device 12 are therefore compensated for on each of the two sub-beams. In fact, the first beam 22 and the second beam 24 each undergo reflection and transmission by this separating surface 121.

[0159] Each sub-beam undergoes both the optical losses linked to reflection by the separating surface 121 and the optical losses linked to transmission by the separating surface 121.

[0160] In summary, given the optical isolation block 17 described, the two optical paths of the first and second beams each correspond to two portions, a portion upstream and a portion downstream of the optical isolation block 17, the upstream and downstream being described with respect to their respective direction of propagation.

[0161] The polarization of the first beam 22 and the second beam 24 is identical on the downstream portion of the first beam 22 and the upstream portion of the second beam 24, and vice versa. Thus, by considering a surface among the plurality of reflecting surfaces 131, 132, 133, the parameters influencing the optical losses, i.e., the state of polarization, the wavelength, and the angle of incidence are kept identical for the first beam 22 and the second beam 24.

[0162] In addition, the optical elements of the optical isolator 1, with the exception of the first delay plate 171 and the second delay plate 173, are all arranged on the optical path of both the first beam 22 and the second beam 24.

[0163] Thus, by construction, all sources of optical losses induced on one of the sub-beams are reflected on the other sub-beams. The sources of optical losses are therefore said to be self-compensated, because no element is specifically added for this purpose. This results in an optical isolator configuration 1 where the attenuation linked to the polarization is minimized, and is less than a few tenths of decibels, or even less than a few hundredths of decibels.

[0164] Advantageously, these very low polarization-related attenuation rate performances are accessible using standard quality, and therefore inexpensive, optical components. Passage in the blocking direction

[0165] After the description of the propagation of a passing beam 2, propagating from the input to the output of the optical isolator 1, a beam is now described light propagating within the optical isolator 1 in the blocking direction.

[0166] Such a light beam, hereinafter referred to as “blocked beam 4”, propagates from the output OUT of the optical isolator 1 without however reaching the input IN of the optical isolator 1.

[0167] In the first embodiment, the blocked beam 4 travels along an optical path described in [Fig. 6]. The direction of propagation of the light is, as usual, indicated by arrows on the propagation axis.

[0168] This blocked beam 4 corresponds for example to a reflection at normal incidence of the passing beam 2 on a surface of an optical element within an optical system comprising the optical isolator 1.

[0169] The optical path of the blocked beam 4 is partly identical to the optical path of the passing beam 2, but in an opposite direction of propagation.

[0170] Starting from the output OUT of the optical isolator 1, following the optical path of the passing beam 2, the blocked beam 4 propagates to the optical polarization splitter device 12, more specifically to its splitter surface 121.

[0171] The blocked beam 4 is also separated by the separating surface 121 into two distinct sub-beams of orthogonal polarizations. These two sub-beams correspond to a first beam 42 and a second beam 44, the first beam 42 being polarized in the perpendicular polarization state s, while the second beam is polarized in the parallel polarization state p.

[0172] The separating surface 121 reflects the first beam 42 of the blocked beam 4. This first beam 42 propagates along a part of the second optical path as defined previously, in the opposite direction.

[0173] In particular, the first beam 42 of the blocked beam 4 is guided by the first reflecting surface 131 and the second reflecting surface 132, along the first segment and the second segment of the rectangle 6, to the optical isolation block 17.

[0174] The passage of the first beam 42 of the blocked beam 4 in the optical isolation block is described in more detail in [Fig.7]. The polarization states of the first beam 42 are represented in the xy plane.

[0175] Thus, the first beam 42 following the second optical path in the opposite direction first passes through the first half-wave delay plate 171. This first delay plate 171 induces a phase shift in the polarization of the first beam 42.

[0176] The first delay plate 171 transforms the perpendicular polarization state s of the first beam 42 into a parallel polarization state p, oriented along the Oy axis.

[0177] Then, the first polarization rotator 172 rotates the polarization of the first beam 42 by 45 degrees clockwise. The first beam 42 is then polarized at +135 degrees relative to the Ox axis in the xy plane.

[0178] The polarizing optical element 174 blocks the propagation of the first beam 42 in the blocking direction, because the axis of the polarizer 174 is in a crossed direction relative to the polarization state of the first beam 42. In other words, the orientations of the axis of the polarizer and the polarization state of the first beam 42 form an angle of 90 degrees relative to each other.

[0179] As for the second beam 44 in the blocking direction, this is initially transmitted by the separating surface 121 in the p polarization state, before being guided by the third reflecting surface 133 to the optical isolation block 17. Here, the second beam 44 travels part of the first optical path described previously, but in the opposite direction.

[0180] Within the optical isolation block, the second beam 44 in the blocking direction, in the parallel polarization state p, first encounters the second delay plate 173. This transforms the parallel polarization state p of the second beam 24, oriented along the Oy axis, into a linear polarization oriented along the Ox axis. This polarization corresponds to the perpendicular polarization state s.

[0181] The second polarization rotator 176 then rotates the polarization of the second beam 44 45 degrees counterclockwise.

[0182] The second beam 44 in the blocking direction is then also linearly polarized with an orientation of +135 degrees relative to the Ox axis.

[0183] The polarizing optical element 174 therefore also blocks the propagation of the second beam 44, since the orientation of the axis of the polarizer 174 is orthogonal to the orientation of the polarization of the second beam 24.

[0184] Thus, a light beam propagating in the blocking direction is extinguished at the optical isolation block 17, in particular by the polarizing optical element 174.

[0185] The optical isolator 1 therefore clearly has a direction of propagation of the passing light, as opposed to a direction of propagation of the blocking light.

[0186] The performance of such an optical isolator 1 is characterized for example by an isolation rate. This isolation rate is measured in decibels, and is proportional to the logarithm of the ratio between an optical power measured at the input after propagation of the blocked beam 4, and an optical power of the blocked beam, measured before passing through the optical isolator 1.

[0187] More specifically, here, the insulation rate is given by the following formula

[0188] Isolation = 10 los—^— ' corresponds to the optical power of the beam °?WT blocked 4 measured at the IN input of optical isolator 1, P^ovt corresponds to the optical power of the blocked beam 4 measured at the OUT output of optical isolator 1, and the log operator corresponds to the logarithm function in base 10.

[0189] This isolation rate, sometimes simply called isolation, quantifies the extent to which the optical isolator 1 effectively prevents the passage of light into the blocked direction.

[0190] Typically, the isolation rate measured on the optical isolator 1 as described according to the first embodiment is measured around 20 dB.

[0191] In the context of certain applications, in particular those using optical elements that are particularly fragile to retroreflected radiation, it is advantageous to increase this insulation rate. Second embodiment, double layer

[0192] Thus, a second embodiment of the optical isolator 1 described previously is proposed, making it possible to improve the isolation rate.

[0193] The isolation block of such an optical isolator 1 is shown in [Fig. 8]. In particular, the optical isolation block 17 is shown there. This has an architecture similar to that described previously, which included a single isolation layer. Now, the optical isolation block 18, as illustrated in [Fig. 8], includes a second isolation layer.

[0194] For this, two polarizing optical elements 181, 182, called auxiliary polarizing optical elements, are added to the optical isolation block 18, and placed on either side of the single-layer optical isolation block 17. More precisely, the first auxiliary polarizing optical element 181 is placed between the polarization splitter optical device 12 and the first delay plate 171, for example between the second reflecting surface 132 and the first delay plate 171. Similarly, the second auxiliary polarizing optical element 182 is placed between the second delay plate 173 and the polarization splitter optical device 12, for example between the second delay plate 173 and the third reflecting surface 133.

[0195] In other words, the single-layer optical isolation block 17 is nested within the second layer, this second layer being constituted by two auxiliary polarizing optical elements 181, 182.

[0196] The first auxiliary polarizing optical element 181 has a polarization axis oriented so as to allow a polarization corresponding to the perpendicular polarization state s to pass.

[0197] The second auxiliary polarizing optical element 182 has a polarization axis oriented this time so as to allow the parallel polarization state P- to pass through

[0198] The use of a two-layer optical isolation block 18 makes it possible to increase the isolation rate. In particular, it makes it possible here to increase this isolation rate, for example, to double it in terms of decibels.

[0199] The addition of these auxiliary polarizing optical elements 181, 182 corrects, for example, possible imperfections in the optical elements of the optical isolation block, in particular the rotators, whose nominal rotation angle is sometimes subject to fluctuation.

[0200] It is also possible, in order to increase the isolation rate, to increase the number of layers. This consists of interposing a pair of auxiliary polarizing optical elements on either side of an optical isolation block with a lower isolation rate. Thus, it is possible to produce optical isolators with one, two, three, four, etc. layers. However, the improvement in the isolation rate is made at the expense of optical losses, of an equal quantity on the two sub-beams, but then degrading an overall transmission rate of the optical isolator 1. This overall transmission rate is defined as the ratio of the input light power to the output light power of the isolator. Variants

[0201] The present invention is in no way limited to the embodiments described and shown, but those skilled in the art will be able to provide any variant in accordance with the invention.

Claims

Claims

1. Optical isolator (1) configured to transmit a light beam called a passing beam (2), propagating in the optical isolator (1) in a passing direction from an input (IN) to an output (OUT) of the optical isolator (1), the optical isolator (1) comprising an optical polarization splitter device (12), a plurality of reflective surfaces (131, 132, 133) and an optical isolation block (17, 18), the optical polarization splitter device (12) being configured to split the passing beam (2) at the input (IN) of the isolator into a first beam (22) in a perpendicular polarization state (s) and a second beam (24) in a parallel polarization state (p), the parallel polarization state (p) being orthogonal to the perpendicular polarization state (s), the first, respectively second, beam (22, 24) propagating along a first, respectively second, optical path,the first optical path and the second optical path being distinct from each other at the optical isolation block (17), the optical isolator (1) being characterized in that:, - each of the reflective surfaces of the plurality of reflective surfaces (131, 132, 133, 135, 136) is placed both on the first optical path and on the second optical path, - the first beam (22) and the second beam (24) are counter-propagating, parallel and offset from each other by a distance (E) at least equal to a diameter of the passing beam (2), during their propagation through the optical isolation block (17, 18), - the optical isolation block (17, 18) comprises a plurality of polarization rotators (172, 176), a polarizing optical element (174) and at least two delay plates (171, 173), arranged so that the first beam (22) successively passes through a first polarization rotator (172) among the plurality of polarization rotators (172, 176), the polarizing optical element (174), a second polarization rotator (176) of the plurality of polarization rotators (172, 176) and a second delay plate (173) of the at least two delay plates (171, 173), delay (171, 173) and emerges from the optical isolation block (17, 18) in the parallel polarization state (p),while the second beam (24) successively passes through the second polarization rotator (176), the polarizing optical element (174), the first polarization rotator (172), and a first delay plate (171) of the at least two delay plates (171, 173) and emerges from the, optical isolation block (17) in the perpendicular polarization state (s), - the first beam (22) and the second beam (24) are in a mutually identical polarization state when the first beam (22) and the second beam (24) are incident on each of the reflective surfaces of the plurality of reflective surfaces (131, 132, 133, 134, 135), - the optical polarization splitter device (12) being configured to recombine the first beam (22) in the parallel polarization state (p) with the second beam (24) in the perpendicular polarization state (s) and form the passing beam (2) at the output (OUT) of the optical isolator (1).

2. An optical isolator according to claim 1, wherein the optical isolator 1 comprises an optical component of refractive index n which is arranged to be placed on both the first optical path and the second optical path.

3. An optical isolator according to any one of claims 1 or 2 wherein the optical isolation block (17, 18) is arranged such that an optical path traveled by the first beam (22) from the polarization splitter optical device (12) to the optical isolation block (17, 18) is equal to an optical path traveled by the second beam (24) from the polarization splitter optical device (12) to the optical isolation block (17, 18).

4. An optical isolator according to any one of claims 1 to 3, wherein the polarization splitter optical device (12) comprises a polarization splitter plate or a polarization splitter cube.

5. An optical isolator according to any one of claims 1 to 3, wherein the polarization splitter optical device (12) comprises a prism assembly, including a polarization splitter cube corner (120), the polarization splitter cube corner (120) having a face oriented at 45 degrees to an optical axis of propagation of the passing beam (2) and a rhombohedral prism (122) comprising two faces oriented at 45 degrees to the optical axis of propagation of the passing beam (2), one of the two faces oriented at 45 degrees of the rhombohedral prism (122) is juxtaposed with the face oriented at 45 degrees of the polarization splitter cube corner (120) and wherein the other of the two faces oriented at 45 degrees of the rhombohedral prism (122) constitutes one of the surfaces of the plurality of surfaces re- flexors (131, 132, 133).

6. An optical isolator according to any one of claims 4 or 5, wherein the polarization splitter optical device (12) and the plurality of reflective surfaces which comprises a first reflective surface (131), a second reflective surface (132), and a third reflective surface (133) are arranged perpendicular to a plane and are arranged along vertices of a rectangle (6) contained in this plane, the rectangle (6) having two long sides of length T 1

7. L / 1. Optical isolator according to claims 5 and 6, wherein the rhombohedral prism (122) is of refractive index n, the other of the faces oriented at 45 degrees of the rhombohedral prism constituting the first reflecting surface (131), the first reflecting surface (131) and the polarization splitting optical device (12) being arranged along one long side of the rectangle (6), the second reflecting surface (132) and the third reflecting surface (133) being arranged at the ends of another long side of the rectangle (6), the optical isolation block (17, 18) is placed between the second reflecting surface (132) and the third reflecting surface (133), at a distance equal to £] Xy from the second reflecting surface (161).

8. An optical isolator according to any one of claims 1 to 3, wherein the polarization splitter optical device (12) comprises at least one birefringent optical element.

9. Optical isolator according to any one of claims 1 to 8, wherein the plurality of reflective surfaces (131, 132, 133, 134, 135) comprises at least one plane mirror of the metallic mirror type, or dielectric mirror, or total internal reflection mirror.

10. An optical isolator according to any one of claims 1 to 9, wherein the optical isolation block (17) is configured such that: - the first delay plate (171) is a half-wave plate placed only on the optical path of the second beam (24), and a neutral axis of the first delay plate (171) forms an angle of 45 degrees in absolute value with the parallel polarization state (p) and the perpendicular polarization state (s), - the first polarization rotator (172) rotates the polarization by 45 degrees in absolute value in a rotation direction, - the polarizing optical element (174) has a polarization axis oriented at 45 degrees in absolute value with respect to the parallel polarization state (p) and with respect to the perpendicular polarization state (s), - the second polarization rotator (176) rotates the polarization by 45 degrees in a direction of rotation opposite to the direction of rotation of the first polarization rotator (172), - and the second delay plate (173) corresponds to a half-wave plate placed only on the optical path of the first beam (22), and a neutral axis of the second delay plate (173) forms an angle of 45 degrees with the parallel polarization state (p) and the perpendicular polarization state (s).

11. Optical isolator (1) according to one of claims 1 to 10, wherein the optical isolation block (18) further comprises a first auxiliary polarizing optical element (181) and a second auxiliary polarizing optical element (182), the first auxiliary polarizing optical element (181) being placed between the polarization splitter optical device (12) and the first delay plate (171) and the second auxiliary polarizing optical element (182) being placed between the second delay plate (173) and the polarization splitter optical device (12), the first auxiliary polarizing optical element (181) having a polarization axis adapted to allow a perpendicular polarization state (s) to pass and the second polarizing optical element (182) having a polarization axis adapted to allow a parallel polarization state (p) to pass.