FR3050832A1 - FARADAY ISOLATOR OPTICAL COUPLING DEVICE - Google Patents

Abstract

The invention relates to an optical coupling device (1) between an optical chip (10) and an external waveguide (40), comprising an optical isolator comprising an intermediate component (20) adapted to transmit, towards the guide external wave, a polarized light rectilinearly coming from the optical chip (10), providing a Faraday rotation of the plane of polarization of said polarized light, characterized in that said intermediate component (20) is formed of a stack layers, among which: a first layer (22), said rotation, comprising a magnetooptical material, and - a second layer (23), called magnetic field generation, comprising an electrically conductive track (24) adapted to generate a magnetic field at said rotation layer.

Description

FARADAY ISOLATOR OPTICAL COUPLING DEVICE TECHNICAL FIELD

[001] The field of the invention is that of the optical coupling between an optical chip and a waveguide external to the optical chip, and more specifically relates to the optical coupling with an isolation function of the optical chip vis-à-vis -vis of light beams from the external waveguide, this optical isolation being achieved by rotation of the plane of polarization of the light beams by Faraday effect.

STATE OF THE PRIOR ART

[002] In the field of telecommunications, and particularly in the context of optical fiber transmission networks, it is necessary to perform optical coupling between optical chips and external waveguides, for example optical fibers. Optical chips are optical components that can have various optical functions such as the transmission and reception of an optical signal, as well as, among other things, the multiplexing, the modulation, the filtering and the division of the optical signal propagating within integrated waveguides.

[003] During optical coupling, it may be sought to isolate the optical chip vis-à-vis light beams reflected or backscattered by the external waveguide, thereby minimizing disturbances in the transmission and / or signal processing optical in the optical chip. For this, it is possible to provide an optical coupling device whose optical isolation function uses a Faraday rotator.

[004] Document EP0372448 describes an example of a coupling device adapted to provide optical coupling between a DFB laser and an optical fiber, comprising a Faraday effect optical isolator. As illustrated in Figure 1, the coupling device 1 comprises an intermediate component 20 disposed between the DFB laser 10 and the optical fiber 30. This intermediate component 20 comprises in particular a magneto-optical material, in the form here of a blade thin disposed in a tubular permanent magnet. A rectilinear polarizer 30, here in the form of a polarizing fiber, is disposed between the intermediate component 20 and the optical fiber 40 along the optical path.

[005] Thus, the optical signal emitted by the laser 10, then polarized in electric transverse mode, or TE mode, is transmitted by the Faraday rotator 22, 23 undergoing a rotation of its plane of polarization of 45 °, then is focused by a lens on the input face of the polarizing fiber 30 whose transmission axis is then coincident with the plane of polarization of the incident light. Conversely, the reflected light is transmitted by the polarizing fiber 30, then is transmitted by the Faraday rotator 22, 23 which induces a rotation of its polarization plane of 45 ° in the same direction as above, which leads to that the light passes in magnetic transverse mode, or TM mode, and then undergoes a significant optical attenuation at the laser.

[006] However, there is a need to have an optical isolation coupling device comprising an intermediate component, adapted to form a Faraday rotator, having a small footprint. It is also desirable that this intermediate component can be assembled in a simplified manner to the optical chip, and preferably more precisely, especially when the coupling device is combined with an optical system imaging the optical beam (such as a lens or a non-plane mirror) to optimize the coupling ratio between the beam emitted by the optical chip and the external waveguide. It is also desirable to have a method for producing such an intermediate component which allows a collective embodiment of a plurality of identical or similar intermediate components.

STATEMENT OF THE INVENTION

The invention aims to remedy at least in part the disadvantages of the prior art. For this purpose, the object of the invention is an optical coupling device between an optical chip and a waveguide external to said optical chip, comprising an optical isolator adapted to isolate the optical chip from light beams coming from the optical waveguide. external wave, said optical isolator comprising: an intermediate component, disposed between the optical chip and the external waveguide, adapted to transmit, in the direction of the external waveguide, a linearly polarized light from the optical chip, ensuring a Faraday rotation of the plane of polarization of said polarized light.

According to the invention, said intermediate component is formed of a stack of layers, among which: a first so-called rotation layer, comprising a magneto-optical material adapted to transmit the polarized light by providing said rotation of the polarization plane of that when a magnetic field is generated at said rotation layer, and a second magnetic field generation layer, comprising an electrically conductive track adapted to generate a magnetic field at said rotation layer.

[008] Some preferred, but not limiting, aspects of this optical coupling device are as follows.

[009] The conductive track may comprise a coil formed of turns wrapping around a central zone capable of transmitting said polarized light.

The conductive track may extend between two ends substantially opposite one another with respect to the central zone. The intermediate component may comprise electrical connectors, adapted to provide an electrical connection of the conductive track, which extend from said ends and open on a so-called upstream face of the intermediate component intended to be opposite a so-called upper face of the optical chip.

The first rotation layer may be formed of a stack of a first sublayer, said magneto-optical, formed of said magneto-optical material, and a second sub-layer, called plasmonic, capable of ensuring by plasmonic effect, an increase in the polarization plane rotation of the polarized light transmitted by the magneto-optical sublayer. By increasing the rotation is meant an increase in the value of the angle of rotation of the polarization plane.

The plasmonic sub-layer may comprise a plasmonic network of rectilinear metal lines covered with a dielectric material, the metal lines being oriented substantially orthogonal to a transmission axis of a rectilinear polarizer located between the optical chip and the rotation layer.

The intermediate component may further comprise a second rotation layer, comprising a magneto-optical material adapted to transmit the polarized light by providing a rotation of the polarization plane thereof when a magnetic field generated by a layer of field generation is applied to said second rotation layer.

The magnetic field generation layer may be located between the first and second rotation layers, and may be adapted to generate a magnetic field at each of said rotation layers.

The intermediate component may comprise electrical connection and assembly pads, made of an electrically conductive material and located at the upstream face so as to be in contact with the electrical connectors, and intended to be in contact with the electrical connection portions disposed on said upper face of the optical chip.

The intermediate component may comprise a so-called polarizer layer, comprising periodic metal structures adapted to form a rectilinear polarizer, disposed between the rotation layer and a so-called downstream face opposite to a so-called upstream face intended to face the chip. optical.

The optical coupling device may further comprise a collimation element, disposed on a so-called upstream face of the intermediate component, and adapted to ensure the collimation of the polarized light extracted from the optical chip, and / or a focusing element. disposed on a so-called downstream face of the intermediate component, and adapted to focus the polarized light transmitted by the intermediate component on an input face of the outer waveguide.

The invention also relates to a method for producing an intermediate component of an optical coupling device according to any one of the preceding characteristics, comprising: forming a stack of layers, on a support layer, comprising: a step of forming said magnetic field generation layer; and o a step of forming said rotation layer; cutting the stack of layers to form said intermediate component.

The method may comprise a step of forming electrical connectors by etching the support layer so as to obtain openings opening on the one hand on the conductive track, and on the other hand on a so-called upstream face of the support layer. then filling the openings thus obtained with an electrically conductive material.

The invention also relates to a method of collectively producing a plurality of optical coupling device intermediate components according to any one of the preceding features, wherein the method of producing an intermediate component according to the preceding characteristics. is performed, said step of cutting said stack of layers for forming said intermediate components.

The invention also relates to a method for assembling and self-aligning an intermediate component of an optical coupling device according to any one of the preceding features on an upper face of an optical chip, comprising the following steps: - contacting the connection pads of the intermediate component with the connection portions of the optical chip, the connection pads being made of a fusible material, and being in contact with connection portions located on the upstream face intermediate component; - Performing the melting of the material of said pads, the melt material ensuring a self-alignment by surface tension effect between the connection portions of the intermediate component and the connection portions of the optical chip.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, objects, advantages and features of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made by reference. to the accompanying drawings, in addition to Figure 1 already described, in which: Figure 2 is a schematic sectional view of an optical chip coupled to an optical fiber by a coupling device according to a first embodiment; FIGS. 3A and 3B schematically illustrate the transmission of a light beam through the intermediate component shown in FIG. 2, from the optical chip to the optical fiber (FIG. 3A), and in the opposite direction (FIG. .3B); FIGS. 4A and 4B are schematic sectional views of a coupling device according to a second embodiment (FIG. 4A) in which the rotation layer comprises a plasmonic sub-layer, and according to a variant of the second embodiment (fig.4B) having two layers of rotation; FIGS. 5A to 5J illustrate steps of an exemplary method for producing an intermediate component represented in FIG. 4B, followed by the assembly of the coupling device comprising such an intermediate component to an optical chip and to an optical fiber. ; Figure 6 is a schematic sectional view of the optical coupling device according to another embodiment, assembled to an optical chip having a diffraction grating for extracting the light beam.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

In the figures and in the following description, the same references represent identical or similar elements. In addition, the various elements are not represented on the scale so as to favor the clarity of the figures. The various embodiments and variants are not exclusive of each other and can be combined with each other. Moreover, the terms "substantially", "about", "of the order of" mean within 10% or within 10 °.

The invention relates generally to the optical coupling between an optical chip, for example a waveguide integrated in the optical chip, and a waveguide external to the optical chip, for example a fiber optical. Optical coupling is achieved by an optical coupling device which further provides an optical isolation function. The optical coupling device comprises an intermediate component disposed between the optical chip and the optical fiber, this component forming a Faraday rotator of the optical isolator.

By optical isolator is meant here a non-reciprocal optical device, in the sense that it has an optical attenuation in a direction of propagation said passing relatively low or almost zero, and a greater optical attenuation in the direction opposite says blocking. In other words, it allows the transmission of light in one direction and at least partially blocks the light in the opposite direction. The optical isolator thus makes it possible to limit the propagation of light beams reflected or backscattered by the optical fiber in the direction of the optical chip.

In the context of the invention, the optical isolator is based on the Faraday effect. More specifically, it comprises a Faraday rotator formed of a so-called rotation layer comprising a magneto-optical material adapted to transmit the incident light polarized rectilinearly, while ensuring a rotation of its plane of polarization when this layer is subjected to a field static magnet. The magneto-optical layer is therefore associated with a means for generating a static magnetic field. The plane of polarization of the light beam is the plane formed by the vector of the electric field and the direction of propagation of the beam.

In order to attenuate or even block the reflected light, the Faraday rotator is usually associated with a downstream rectilinear polarizer disposed between the Faraday rotator and the external waveguide, whose transmission axis coincides with the orientation of the plane of polarization of the light at the output of the Faraday rotator. It is also associated with an upstream rectilinear polarizer disposed between the optical chip and the Faraday rotator, the transmission axis of which is substantially orthogonal to the orientation of the polarization plane of the light at the output of the Faraday rotator propagating in the blocking sense. The operation of such a Faraday effect optical isolator will be described in more detail later with reference to FIGS. 3A and 3B.

In the context of the invention, the intermediate component intended to form the Faraday rotator comprises a stack of layers, among which a layer made of a magneto-optical Faraday material and a layer intended to generate a magnetic field at a magnetic field. of the magneto-optical layer. This intermediate component is adapted to be assembled on a so-called upper face of the optical chip at a zone in which the light beam is extracted from the optical chip.

FIG. 2 illustrates, in cross-section, a device according to a first embodiment allowing optical coupling, with optical isolation function by Faraday effect, between a waveguide 11 integrated in an optical chip 10 and a external waveguide 40.

Here we define a three-dimensional direct reference (Χ, Υ, Ζ), where the plane (X, Y) is parallel to the plane of the upper face 14 of the optical chip 10 and where the Z axis is orientated so that substantially orthogonal to the upper face 14 of the optical chip 10. The Z axis coincides with the stacking axis of the layers of the intermediate component 20. Thus, the terms "vertical" and "vertically" extend as being relative to an orientation along the Z axis, and the terms "lower" and "higher" extend as relative to an increasing position when moving away from the optical chip 10 in the + Z direction.

The integrated waveguide 11 is formed in a support forming an optical chip 10, it may be an optoelectronic chip having a light source such as a laser. The dimensions of the integrated waveguide 11 may be such that the guide has a width greater than its thickness, which implies that the light beam propagating in the guide is polarized essentially in TE mode, ie that it has a plane of polarization substantially parallel to the plane of the guide. They may alternatively be such that the guide has a thickness greater than its width, which implies that the light beam is essentially polarized in TM mode. The width of the guide is its transverse dimension in the plane (X, Y) and its thickness is its dimension along the Z axis. Thus, the integrated waveguide 11 has a rectilinear polarizer function, whose transmission axis is defined here at 0 ° by convention. The integrated guide may be formed of a silicon strip surrounded by a silicon oxide. The optical chip can then be produced in the context of so-called silicon photonics technology, in which the integrated silicon waveguide rests on an insulating layer of silicon oxide. The dimensions of the silicon strip may, for example, be of the order of 0.2 × 0.5 μm 2.

The integrated waveguide 11 extends to an extraction zone 12 of the optical chip 10 at which the light beam is extracted from the chip 10. In this example, the area of extraction 12 is in the form of a notch 13 made from the upper face 14 of the chip 10, into which the waveguide 11 opens.

The external waveguide 40 may be that of an optical fiber 40. It is said external insofar as it is not integrated with the optical chip 10, that is to say that it is formed outside the chip 10. It is here placed above the upper face 14 and is held in position by a holding housing (not shown). The core of the external waveguide 40 may have, for example, a diameter between 3pm and ΙΟμιτι.

The optical coupling device 1 is disposed between the optical chip 10 and the external waveguide 40. It comprises an intermediate component 20 designed to form a Faraday rotator, which is assembled at the upper face 14 of the chip. 10 and is arranged facing the extraction zone 12. The coupling device 1 may comprise an upstream rectilinear polarizer and a downstream rectilinear polarizer, as well as an optical focusing of the light beam on the input face of the guide external wave. It can also include reflectors and a collimation optics. It is formed of a stack of layers which are arranged one on the other along a stack axis substantially orthogonal to the upper face 14 of the optical chip 10. Thus, the layers extend in a plane parallel to that of the upper face 14 of the optical chip 10.

The intermediate component 20 comprises a support layer 21 on which rests a rotation layer 22 by Faraday effect, and a layer 23 for generating a magnetic field. In this example, it also comprises a layer 30 forming a rectilinear polarizer.

The support layer 21 may be a thick layer of silicon, glass, sapphire or any other suitable material whose thickness along the Z axis may be of the order of a hundred microns, for example. example 100prn, and whose transverse dimensions, in the plane (X, Y), may be of the order of a few millimeters.

The layer 23 for generating a magnetic field here rests on the support layer 21. It is formed of a dielectric material, for example a nitride or a silicon oxide, here SiO 2, in which a track 24 is formed. electrically conductive, for example made of a metallic material such as copper or aluminum, which extends between two ends 26A, 26B so as to surround, in the form of a coil 25A, a central zone 27 of the intermediate component 20 for transmitting the light beam. The conductive coil 25A is adapted to be traversed by an electric current, and for this is connected to two electrical connectors 50 which extend vertically and open at the upstream face 28A of the intermediate component 20. These electrical connectors 50 thus cross the support layer 21 and open on the one hand at the ends 26A, 26B of the conductive track 24 and secondly on the upstream face 28A of the intermediate component 20, the latter facing the upper face 14 of the chip 10. In order to provide a connection with the electrical connectors 50, the conductive track 24 has a lower line 25B which connects the end 26B to the coil 25A, and which extends mainly under the conductive coil 25A along the Z axis. .

As an illustration, the field generation layer 23 is adapted to generate a static magnetic field of an intensity of the order of 0.05T at the level of the rotation layer 22. Thus, it may comprise a track conductor 24 forming a coil 25A of about 170 turns of a width each of 5pm and a thickness of 5pm of copper, surrounding a central zone 27 of 300pm in diameter, in which can circulate an electric current of about 6.5mA. The conductive track 24 may be spaced apart from the rotation layer 22 by a distance of lpm along the axis Z.

The rotation layer 22 covers the field generation layer 23, and extends in particular at the central zone 27 of optical transmission. It comprises a magneto-optical material capable of inducing, when subjected to a given value of a static magnetic field, a rotation of the plane of polarization of the incident light beam at an angle substantially equal to ar. This may be yttrium and iron garnet Y3FesOi2 (YIG) or any other suitable material, for example a terbium gallium garnet TbsGasO ^ (TGG) or a terbium and aluminum garnet TbsAIsO ^ (TAG ), or a garnet of bismuth and iron Bi3FesOi2 (BIG). The magneto-optical material may have doping, for example cerium ions Ce3 +. It presents a Verdet constant that reflects its ability to induce a rotation of the plane of polarization of light at a given angle. By way of example, the Verdet constant of the YIG for a wavelength of 1. 55 μm is of the order of 2700 ° / cm / T when deposited on a silicon oxide. Its thickness is predetermined to ensure, when subjected to the static magnetic field generated by the field generation layer 23, a rotation of the desired angle ar of the plane of polarization of the incident light beam. In this embodiment, the rotation layer 22 is formed essentially by the magneto-optical material, which extends over substantially the entire thickness of the rotation layer 22, and covers here substantially the entire upper surface of the layer 23 field generation.

In this example, optionally but advantageously, the intermediate component 20 further comprises a layer 30 forming a rectilinear downstream polarizer having a given transmission axis. This transmission axis is inclined vis-à-vis that of the upstream polarizer formed by the integrated guide 11 at an angle substantially equal to the angle ocr. This layer 30 covers the layer 22 of rotation. Thus, the intermediate component 20 is here formed of a stack of the field generation layer 23, the rotation layer 22 and the rectilinear polarizing layer.

The intermediate component 20 is mechanically assembled to the optical chip 10, at the upper face 14, and is positioned opposite the extraction zone 12. The assembly and the electrical connection are made jointly by studs. 51 connection and assembly, made of an electrically conductive material such as indium or aluminum. These conductive pads 51 are in contact with the upstream face 28A of the intermediate component 20 and the upper face 14 of the optical chip 10. They are more precisely in contact with conductive portions 15 disposed on the optical chip 10 and are in contact with the electrical connectors 50. The conductive portions 15 are connected to a source of electrical current (not shown), thus making it possible to circulate an electric current in the conductive track 24 thus making it possible to generate a static magnetic field at the level of the rotation layer 22 at a given intensity, for example 0.05T.

Advantageously, these pads 51 also provide self-alignment of the intermediate component 20 on the optical chip 10. For this, each pad 51 is in contact with the connecting portion 15 disposed on the upper face 14 of the chip 10, as well as a connecting portion 52 disposed on the upstream face 28A of the component 20, these connecting portions 15, 52 being opposite one another. The stud 51 is made of a fusible material, such as indium, and a pad melting step is performed during the assembly of the intermediate component 20 on the optical chip 10. The pad 51 melt has a shape of ball under the action of surface tension forces, these same forces causing the self-alignment, or passive alignment, connection pads 15, 52 relative to each other. To do this, the lateral dimensions of the connection portions 15, 52 are adapted to the volume of the ball 51 in fusion, as described in document WO98 / 26318, or the Bernabé et al. Document, Highly integrated VCSEL-based. lOGb / s miniature optical sub-assembly, Electronic Components and Technology Conference, 2005, Proceedings, 55th, 1333 - 1338 Vol. 2. After solidification of the balls 51, the intermediate component 20 is assembled and aligned with the optical chip 10.

In order to orient the light beam extracted from the optical chip 10 towards the intermediate component 20, the coupling device here comprises a first reflector 2 disposed on the upstream face 28A of the intermediate component 20 and arranged so that the the light beam extracted from the integrated guide 11 propagates substantially parallel to the stacking axis of the intermediate component 20. It thus extends from the intermediate component 20 and partially houses in the notch 13 of the zone of 12. Alternatively, the first reflector 2 can be assembled to the optical chip 10 and not to the intermediate component 20, being disposed in the notch 13 of the extraction zone 12.

[0044] Preferably, it is structured so that the reflected light beam is collimated. In this case, an antireflection layer (not shown), formed for example of a stack of dielectric layers, is disposed between the reflector 2 and the support layer 21, for example at the upstream face 28A.

The coupling device also comprises a second reflector 3 disposed on the downstream face 28B of the intermediate component 20 and arranged so that the light beam transmitted by the intermediate component 20 is reflected towards the optical fiber 40. Preferably it is structured so that the reflected light beam is focused on the input face of the waveguide of the optical fiber 40, for example by giving the mirror a curvature such that the focal point of the mirror corresponds to the position of the guide external wave. As a variant, the second reflector 3 can be assembled to the holding casing 4, and not to the intermediate component 20.

The operation of the optical coupling device 1 is as follows. With reference to FIG. 3A, a light beam propagates in the waveguide 11 of the optical chip 10 with a rectilinear polarization, here in TE mode (here we note oc = 0 ° the angle of inclination of the plane of polarization). The integrated waveguide 11 is shown schematically in FIG. 3a as an upstream rectilinear polarizer whose transmission axis forms the angle α = 0 °. The light beam is extracted from the waveguide 11 at the level of the extraction zone 12, then is reflected, and preferably collimated, by the first reflector 2 towards the intermediate component 20. It is transmitted by the rotator of FIG. Faraday of the intermediate component 20 while undergoing a rotation of its plane of polarization by a predetermined angle ar, for example + 45 °, by the action of the rotation layer 22 undergoing the static magnetic field generated by the generation layer 23. The beam is then transmitted by the polarizing layer 30 without substantial optical attenuation, insofar as the transmission axis here coincides with the inclined polarization plane of the light beam. It is then reflected by the second reflector 3 in the direction of the optical fiber 40, while being focused on the input face of the external waveguide 40. Thus, there has been optical coupling between the optical chip 10 and the optical fiber 40, by transmitting the incident beam from the integrated waveguide 11 into the external waveguide 40 without any significant optical attenuation. The optical intensity of the beam transmitted to the optical fiber 40 has a value lt substantially equal to that 1, of the beam extracted from the optical chip 10.

The optical coupling device 1 furthermore makes it possible to isolate the optical chip 10 from the reflected or backscattered light beams coming from the optical fiber 40. In fact, with reference to FIG. 3B, such a beam reflected by the optical fiber 40 is transmitted by the downstream polarizer 30 and thus has a plane of polarization oriented along the transmission axis, that is to say here according to the angle ar, for example 45 °. It is then transmitted by the Faraday rotator 22, 23 which provides a rotation of the polarization plane of an additional ar angle so that the plane of polarization is then inclined by 2ar, here 90 °, that is to say that it is now polarized a TM mode. It is then reflected in the direction of the integrated waveguide 11 and then substantially attenuated at the input of the integrated waveguide 11 since this guide only allows the optical propagation of the light beams in TE mode. The intensity of the light beam at the level of the integrated waveguide 11 has a value lt much lower than that I, of the beam reflected by the optical fiber 40, which can be estimated at li.cos2 (2ar) according to the law of Malus .

It is possible to increase the optical insulation rate by adjusting the properties of the magneto-optical layer in terms of Verdet constant and / or layer thickness, as well as by the use of a static magnetic field of greater or lesser intensity.

The advantages of such an optical coupling device 1 are as follows. The optical coupling device 1 has a small footprint insofar as the polarization rotator is made in the intermediate component 20 in the form of a stack of layers whose dimensions are much smaller than those of the elements forming the Faraday rotator. example of the prior art mentioned above. In addition, it presents an assembly to the optical chip 10 simplified when the connection pads 51 provide both the assembly and the electrical connection. The alignment of the intermediate component 20 is further improved here when the connection pads 51 are made of a fusible material and a melting step ensuring the self-alignment is performed.

FIG. 4A illustrates an intermediate component 20 of an optical coupling device 1 according to a second embodiment.

The intermediate component 20 is distinguished here from that described with reference to Figure 2 essentially in that the rotation layer 22 is formed of a stack of two elementary sub-layers, a first sub-layer 22-1 called magneto-optical, and a second sub-layer 22-2 called plasmonic.

An underlayer 22-1 here covers the field generation layer 23. It is made of a magneto-optical material having a real Verdet constant. Preferably, it is formed of a garnet of bismuth and iron BisFesO ^ (BIG). Alternatively, it may be formed of a yttrium garnet and FeSiFeO4 (YIG), a terbium and gallium garnet TbsGasO ^ (TGG), a terbium garnet and TbsAlSO 4 aluminum ( TAG), or any other suitable magneto-optical material. The magneto-optical material may have doping, for example cerium ions Ce3 +.

Its thickness is predetermined to ensure, when subjected to the static magnetic field generated by the field generation layer 23, a rotation of the desired ocr angle of the plane of polarization of the incident light beam. By way of example, the sub-layer 22-1 may be formed of a thickness of 22 μm of BIG material, whose Verdet constant is real, that is to say the Verdet constant in the absence of an amplification by plasmonic effect of the magneto-optical rotation, is of the order of 4.30 ° / μιτι / Τ. The BIG material may be deposited on a thin nucleation layer made of YIG, for example of the order of 10 nm. Prior to the deposition of the BIG material on the nucleation layer, an annealing thereof may be carried out, for example at 1000 ° C.

A second plasmonic sub-layer 22-2 here covers the magneto-optical sub-layer 22-1. It comprises a structuring adapted to ensure by plasmonic effect an increase in the rotation of the plane of polarization of the incident beam. In other words, by the plasmonic sub-layer 22-2, the rotation layer 22 provides a rotation of the polarization plane of the incident beam by an angle greater than it would be in the absence of this sub-layer. plasmonic layer 22-2. At the rotational layer 22 is then associated a so-called Verdet constant effective.

The plasmonic sub-layer comprises a periodic network of rectilinear metal lines covered with a dielectric material. The metal lines are oriented longitudinally so that the plane of polarization of the incident beam, in particular when it comes from the optical chip 10, is substantially orthogonal to the longitudinal axis of the metal lines. Moreover, the dimensions of the metal lines, in terms of width and thickness, as well as the pitch of the grating, are smaller than the wavelength of the incident beam.

By way of illustration, in the case of an underlayer 22-1 of BIG material, the underlayer 22-2 may thus comprise a periodic network of lines, or son, of gold, each resting on the underlayer 22-1 and extending along a rectilinear longitudinal axis. The period may be of the order of a few hundred nanometers to a few microns, for example 495 nm, and the wires may have a thickness of the order of a few tens to hundreds of nanometers, for example 65 nm, and a width of a few tens to a few hundred nanometers, for example 120nm. These values are purely illustrative and can be adjusted in particular according to the wavelength of the incident beam so as to obtain a more or less significant increase in the magnetooptical rotation by plasmonic effect. The article by Chin et al. entitled Nonreciprocal plasmonics enables giont enhancement of thin-film Faraday rotation, Nat. Common. 4: 1599 doi: 10.1038 / ncomms2609 (2013) thus gives an example of a rotational layer formed of a sub-layer of BIG material associated with a plasmonic sub-layer making it possible to obtain an increase of 8.9 times of magneto-optical rotation.

The polarizing layer 30 covers the rotational layer 22 and the structures are oriented in an inclined manner to the longitudinal axis of the metal lines of the plasmonic network.

In operation, the coupling device 1 provides greater magneto-optical rotation than in the case of the first embodiment, by coupling the magneto-optical effect provided by the magneto-optical sublayer 22-1. and a resonance of localized surface plasmons generated at the periodic grating of the sublayer 22-2 by the incident light beam.

By way of illustration, for a wavelength of the incident beam of 1. 55 μm, the plasmonic sub-layer 22-2 may comprise a periodic network of gold metal lines of about 75 nm width and about 80 nm height, distributed with a period of about 2.5pm. The magneto-optical sublayer 22-1 may be made of 22 μm thick BIG material placed in a static magnetic field of 0.05T of intensity. This results in a magneto-optical rotation of about 42 ° and an optical transmission of the rotation layer 22 of the order of 75%.

Note that the rotation layer 22 has a maximum of magneto-optical rotation at least optical transmission. In the preceding example, the optical transmission minimum is 75% for a wavelength of 1. 55 μm of the incident beam, which leads to a maximum of magneto-optical rotation. It is therefore possible to size the periodic plasmonic network to increase the value of the transmission of the rotation layer 22, which leads to a decrease in the plasmonic effect, resulting in a decrease in the value of the effective Verdet constant. which can then be compensated by an increase in the thickness of the magneto-optical layer 22-1.

FIG. 4B illustrates an intermediate component 20 of an optical coupling device 1 according to a variant of the second embodiment.

The intermediate component 20 differs here from that described with reference to FIG. 4A essentially in that it comprises a second rotation layer 22i arranged in the stack of layers so as to be placed in the field generated by the magnetic field generation layer 23. This rotation layer 22i is here identical or similar to the rotation layer 22, that is to say that it also comprises a plasmonic sub-layer 22I-2 associated with a magneto-optical sublayer 22i-1. Alternatively, it may be similar or identical to the rotation layer illustrated in FIG. 2 and thus not comprise a plasmonic sub-layer.

It advantageously has the shape of a layer portion, in the sense that it does not extend over the entire surface of the upper face of the support layer 21. It is disposed between the upstream face 28A and the layer 23 of field generation, and has a lateral dimension, in the plane (X, Y), such that the connectors 50 do not cross.

It can thus be performed in a notch located in the upper face of the support layer 21, this notch being formed by dry etching, for example RIE type (Reactive-lon Etching, in English). Alternatively, it may be formed in a notch made in a dielectric layer, for example SiO 2, deposited on the support layer 21.

The two rotation layers 22, 22i have such properties, in terms of the Verdet constant and the thickness of the layers, that a rotation of the polarization plane of the incident light beam is ensured, when these layers are subjected to static magnetic field generated by the generation layer 23. Each layer 22, 22i can thus induce a rotation of the polarization plane of the order of ar / 2 of the polarization plane, for example about 22.5 °. Thus, during the transmission of a polarized beam by the lower rotation layer 22i, the beam undergoes a first rotation of its plane of polarization by an angle of 22.5 °, then a rotation of an angle of 22 , 5 ° additional during transmission by the upper rotation layer 22, thus bringing to 45 ° the inclination of the polarization plane after transmission by the Faraday rotator.

Thus, by way of illustration, for an intermediate component 20 comprising: - two layers of rotation 22, 22i of BIG material 11 μm thick, deposited on a SiO 2 layer, and having an effective Verdet constant of 38 ° / pm / T, the layer 23 for generating a static magnetic field generating a field of an intensity of the order of 0.050T at the level of said rotation layers 22, 22i, a rotation of the polarization plane of the beam is obtained light of the order of ar / 2 = 21 ° during transmission by the two layers of rotation 22, 22i, which results in an optical isolation rate of cos2 (2ocr) = about 99%. The optical coupling device thus makes it possible to reduce the transmission, to the optical chip 10, of reflected or backscattered light beams coming from the external waveguide 40.

It is possible to adjust the optical isolation ratio by choosing the properties of the rotation layer in terms of real and effective Verdet constant, and / or the thickness of the magneto-optical sublayers 22. 1, 22i-1 and the dimensions of the plasmonic gratings of the sub-layers 22-2, 22i-2, as well as by the use of a static magnetic field of greater or lesser intensity.

With reference to FIGS. 5A to 5J, an exemplary method for producing the intermediate component 20 of the optical coupling device 1 as shown in FIG. 4B, as well as the assembly of the intermediate component 20 on the chip, is now described. optical 10 and the positioning of the external waveguide 40.

The production method is of the "wafer-level" type, that is to say that the intermediate component 20 is made by stacking layers deposited on a substrate (or wofer in English), before being cut then assembled to the optical chip 10.

With reference to FIG. 5A, the upstream rotation layer is produced on a support layer 21. The support layer 21 may be a wafer-type substrate made of an optically transparent material at the wavelengths of the light beam intended to be propagate between the optical chip 10 and the optical fiber 40. It can be a substrate, with a thickness of about 700μιτι, made of silicon, glass, sapphire or any other suitable material.

In a first step, the magneto-optical sublayer 22i-1 is produced by depositing a magneto-optical material on the support layer 21 at least at the level of the central optical transmission zone 27. It may have a thickness of the order of a few microns to a few hundred microns, for example here llpm of a yttrium and bismuth garnet (BIG) deposited on a nucleation layer of an yttrium garnet and of iron (YIG) a hundred nanometers thick. Other materials may be suitable depending on the value of the Verdet constant. This material may be deposited by cathodic sputtering in a notch located at the upper face of the support layer 21, this notch being previously formed by dry etching. The deposition of the magneto-optical material may be performed so as to cover the entire upper surface of the support layer 21. An etching is then performed so as to keep only the portion of the magneto-optical material located in the notch. Crystallization annealing can be carried out under vacuum, for example at a temperature of between 600 ° C. and 900 ° C., followed by cooling at room temperature following a ramp of about ten degrees per minute. The thickness of the magneto-optical sub-layer is chosen so as to ensure a rotation of the polarization plane of the incident beam at a predetermined angle of rotation ar / 2. In addition, this thickness and its spacing vis-à-vis the conductive track 24 are chosen so that, in operation, it is located in an area in which the static magnetic field has a substantially uniform intensity, for example 10% near. The magneto-optic sublayer 22i-1 does not extend here over the entire upper surface of the support layer 21 but covers at least the surface located in the central transmission zone 27. Alternatively, it could completely cover the upper surface of the support layer 21.

In a second step, the plasmonic sub-layer 22i-2 is made in such a way that it covers the magneto-optical sub-layer 22i-1 at the level of the central zone 27 of optical transmission. The periodic network of rectilinear metal lines 22i-3 is produced from conventional steps of photolithography, etching and deposition. Preferably, the material of the metallic lines is in contact with the magneto-optical material, which makes it possible to obtain a good coupling between the plasmonic effect and the magneto-optical effect. The longitudinal axis of the metal lines is chosen so that the electric field of the incident beam is substantially orthogonal to this longitudinal axis. The width of the metal lines 22 and 3 and the period of the grating are less than the wavelength of the incident light beam. The metal lines are made of a metallic material, preferably gold. In this example, the metal lines are in contact with the BIG material. The plasmonic network is then covered with a dielectric encapsulation layer, for example SiO 2, with a thickness of a few hundred nanometers, then flattened by chemical mechanical polishing.

Referring to Figures 4B and 4C, there is provided, on the rotation layer 22i, the magnetic field generation layer 23. The magnetic field generation layer 23 is formed of a dielectric material, for example a nitride or a silicon oxide, here SiO 2, which comprises a so-called upper metallization level 25A, spiraling around the zone central 27 optical transmission. This level of metallization forms a conductive coil 25A comprising a plurality of turns extending from a first end 26A and surrounding the central zone 27. The layer 23 comprises a second metallization level 25B, said lower, forming a conductive line 25B extending from the end of the turn nearest the central zone 27 to the second end 26B of the conductive track 24. The conductive track 24, formed of the coil 25A and the line 25B, is covered with the dielectric material which encapsulates the track 24.

The metallization levels of the conductive track 24 can be achieved by conventional steps of photolithography, etching, deposition, and chemical mechanical planarization. The layer 23 may have a thickness of the order of a few microns, for example 10 μm, and the conductive coil 25A may comprise one to several turns, preferably several tens to more than one hundred turns, for example of the order 150 turns of thickness and width of the order of a few microns, for example 5pm. The conductive line 25B may have a thickness and a width of the order of a few microns. The conductive material forming the conductive track 24 may be, for illustrative purposes, copper, aluminum, or even doped silicon.

With reference to FIG. 5D, the upper rotation layer 22 is produced on the field generation layer 23. This rotation layer 22 is here similar to the lower rotation layer 22i, and differs essentially in that it completely covers the upper surface of the field generation layer 23. The realization of this layer of rotation is not described again. The plasmonic sub-layer 22-2 also comprises a dielectric encapsulation material, for example SiO 2, which covers the network of metal lines 22-3, and has a thickness of a few hundred nanometers. It is then flattened, for example, by chemical mechanical polishing.

With reference to FIG. 5E, a stack of layers forming a rectilinear polarizer 30 is produced. On a support layer 31 is formed a layer 32 comprising periodic rectilinear structures 33 made of metallic material. The support layer 31 may be a wafer-type substrate made of an optically transparent material at the wavelengths of the light beam intended to propagate between the optical chip 10 and the optical fiber 40. It may thus be a silicon substrate, made of glass, sapphire or any other suitable material.

Periodic rectilinear structures 33 can be made from conventional steps of photolithography, etching and deposition. The axis along which these structures extend 33 makes it possible to define a transmission axis inclined with respect to that of the upstream rectilinear polarizer 11. They form a plurality of periodic rectilinear lines between them, the width and the period of which are less than the wavelength of the incident light beam. They extend longitudinally so as to cover at least the transmission zone 27. By way of example, for an incident wavelength of 1.5 pm, the structures 33 may have a width of about 150 nm, a height of 400 nm. approximately, and are spaced apart at a time of about 400 nm. These structures 33 are then covered with a dielectric material for encapsulation, for example SiO 2, of a thickness of a few hundred nanometers, then flattened by chemical mechanical polishing.

With reference to FIG. 5F, the rectilinear polarizer 30 is assembled on the Faraday rotator, by direct bonding, also known as molecular bonding, of the layer 30 forming the rectilinear polarizer on the material of FIG. encapsulation of the underlayer 22-2. The direct bonding is carried out in a manner known per se, by bringing the surfaces of the polarizing layer into contact with that of the underlayer 22-2, preferably followed by an annealing of increase of the bonding energy, by example at a temperature of the order of 400 ° C. Preferably, prior to bringing the polarizing layer into contact with the encapsulation material of the underlayer 22-2, a thermal oxidation of the lower face of the support layer is carried out, or a thin film is deposited. a silicon oxide on this face. The surfaces then have hydroxyl groups (-OH) which give the bonding surfaces the ability to bind with water molecules through an adsorption mechanism. Direct bonding is then hydrophilic type and involves hydrogen bonding forces whose interaction intensity is particularly high.

Referring to FIG. 5G, thinning of the support layer 21 is then carried out by an abrasion-type technique (grinding, in English), so as to reduce the thickness of the support layer 21 to a value of about 100 microns. Then through openings are made from the upstream face 28A of the intermediate component 20, which open at the two ends 26A, 26B of the conductive track 24. These openings, of the type TSV (for Through-Silicon Via, in English) ), extend along the thickness of the support layer 21, and are filled with an electrically conductive material, such as copper or aluminum, so as to obtain electrical connectors 50. These may have a diameter of about ten microns.

With reference to FIG. 5H, a first reflector 2 is assembled here on the upstream face 28A of the intermediate component 20, the reflector 2 being arranged to orient the light beam extracted from the optical chip 10 along a substantially parallel axis of propagation. to the stacking axis of the layers of the intermediate component 20. It can also be structured to ensure the collimation of the reflected light beam. In the latter case, an antireflection layer (not shown), composed in known manner of a plurality of dielectric sublayers, is disposed between the first reflector 2 and the upstream face 28A of the intermediate component 20.

Furthermore, a second reflector 3 is assembled on the downstream face 28B of the intermediate component 20, the reflector 3 being arranged to orient the light beam transmitted by the intermediate component 20 towards the optical fiber 40. It can be structured for focusing the light beam on the input face of the optical fiber 40.

Finally, conductive portions 52 may be deposited on the upstream face 28A of the intermediate component 20 so as to contact the electrical connectors 50. The lateral dimensions of these conductive portions 52 are preferably defined according to the volume of the pads 51 of connection in fuse material, so as to ensure a self-alignment function in a subsequent step of melting said pads 51.

The stack of layers is then cut into appropriate lateral dimensions so as to obtain the intermediate component 20.

An intermediate component 20 of an optical coupling device 1 has thus been produced by a micro-electronic technique of the wafer-level type, that is to say on the substrate scale and by conventional steps of FIG. microelectronics deposition, photolithography and etching. It is then possible to collectively produce a plurality of intermediate components from the same stack of layers, said components being separated from each other when cutting the stack of layers to the desired format.

In this example, the intermediate component 20 comprises the first and second reflectors 2, 3 assembled on its upstream faces 28A and downstream 28B. Alternatively, it may not include these reflectors, which are then placed on the optical chip 10 and on the housing 4 for holding the optical fiber 40.

FIG. 51 illustrates a step of assembling the intermediate component 20 on the optical chip 10. This comprises an integrated waveguide 11 adapted to ensure the propagation of a light beam coming from a light source for example a laser source, the light beam being polarized here in TE mode.

The optical chip 10 comprises an extraction zone 12 of the light beam out of the chip, formed here of a notch 13 made from the upper face 14 of the chip and in which opens the integrated waveguide 11.

The optical chip 10 further comprises conductive portions 15 disposed on its upper face 14 on either side of the extraction zone 12, which are intended for the electrical connection of the conductive track 24. They are connected to a source of electrical power (not shown). The conductive portions 15 are intended to face the portions 52 of the intermediate component 20 and their lateral dimensions are chosen as a function of the volume of the connection pads 51 of fusible material.

Connection pads 51 are arranged on the conductive portions 15. The connection pads 51 are made of an electrically conductive material, for example indium or aluminum. They are intended to ensure the assembly of the intermediate component 20 on the optical chip 10 on the one hand, and the electrical connection of the conductive track 24 on the other. These pads 51 may be made of a fusible material, their volume then being adapted to the lateral dimensions of the conductive portions. Thus, during a step of melting the material of the pads 51, these become balls which ensure a self-alignment of the conductive portions of the intermediate component 20 vis-à-vis the conductive portions of the optical chip 10, by surface tension forces. This results in a self-alignment of the intermediate component 20 on the optical chip 10.

FIG. 5J illustrates a step of assembling the optical fiber 40 to the optical chip 10 by means of a holding casing 4 which further ensures the alignment of the fiber 40 with respect to the intermediate component 20.

Specific embodiments have just been described. Various variations and modifications will occur to those skilled in the art.

Thus, the extraction zone 12 of the optical chip 10 can be defined, not by a recess as described with reference to FIGS. 2 and 5, but by a diffraction grating 16 present at the level of the guide. integrated wave 11. As illustrated in FIG. 6, the optical chip 10 comprises an integrated waveguide 11 which extends under the upper face 14 of the chip 10. A diffraction grating 16 is formed in the guide of FIG. 11 in an extraction zone 12 opposite which is located the intermediate component 20. It is structured to ensure the extraction of the polarized beam propagating in the integrated waveguide 11. The extraction angle Θ of the beam can be defined from the known relation: neff = sin (0) + ιτι. (Λ / Λ), where neff is the effective index of the waveguide seen by the optical mode propagating in the guide , m is the diffraction order, λ is the wavelength of the light beam and Λ is the pitch of the grating. The diffraction grating can thus be structured in such a way that the angle of extraction of the light beam is substantially equal to 90 °.

Furthermore, as also shown in Figure 6, the assembly and the electrical connection of the intermediate component 20 on the upper face 14 of the optical chip 10 can be provided by the engagement of male studs 51 in housing females 15. Here, the male pads, made of an electrically conductive material, are fixed on the upstream face 28A of the intermediate component 20 and are in contact with the connectors. They are engaged in female housings 15 disposed on the upper face 14 of the optical chip 10, made of an electrically conductive material, and connected to a current source (not shown).

Furthermore, the intermediate component 20 may comprise collimating and / or focusing lenses, located on its upstream 28A or 28B downstream faces, these lenses being produced by conventional resin creep or nano- or microimpression techniques. .

Furthermore, the downstream polarizer may have other shapes, for example a blade disposed against the input face of the external waveguide, or even a fiber providing the linear polarization function, assembled on the face of the waveguide. input waveguide as described in EP0372448 already cited. In the latter case, the second reflector 3 focuses the light beam on the input face of the polarizing fiber.

The coupling device may further comprise a specific upstream polarizer arranged between the optical chip 10 and the Faraday rotator, for example a metal structured layer similar to the polarizing layer 30, disposed on the support layer 21.

In these embodiments, the optical fiber 40 is disposed substantially orthogonal to the stacking axis Z of the layers of the intermediate component 20. It can of course be inclined vis-à-vis this axis so to form an angle strictly less than 90 °, or even be aligned with the extraction zone 12 and the central transmission zone 27.

Claims (14)

An optical coupling device (1) between an optical chip (10) and an external waveguide (40) to said optical chip, comprising: - an optical isolator adapted to isolate the optical chip (10) from light beams from external waveguide (40), said optical isolator comprising: an intermediate component (20) disposed between the optical chip (10) and the external waveguide (40) adapted to transmit towards the guide external wave, a polarized light rectilinearly coming from the optical chip (10), providing a Faraday rotation of the plane of polarization of said polarized light, characterized in that said intermediate component (20) is formed of a stack layers, among which: a first layer (22), called a rotation layer, comprising a magneto-optical material adapted to transmit the polarized light by ensuring said rotation of the polarization plane thereof when a magnetic field is generated at nive at said rotation layer (22), and o a second magnetic field generation layer (23) having an electrically conductive track (24) adapted to generate a magnetic field at said rotation layer (22). ). 2. Optical coupling device (1) according to claim 1, wherein the conductive track (24) comprises a coil (25A) formed of turns wrapped around a central zone (27) capable of transmitting said polarized light. Optical coupling device (1) according to claim 1 or 2, wherein the conductive track (24) extends between two ends (26A, 26B) substantially opposite one another with respect to the central zone. (27), and wherein the intermediate component (20) has electrical connectors (50), adapted to provide electrical connection of the conductive track (24), which extend from said ends (26A, 26B) and open on a so-called upstream face (28A) of the intermediate component (20) intended to face a so-called upper face (14) of the optical chip (10). 4. Optical coupling device (1) according to any one of claims 1 to 3, wherein the first rotation layer (22) is formed of a stack of a first sub-layer (22-1), called magneto-optical, formed of said magneto-optical material, and a second sub-layer (22-2), called plasmonic, capable of ensuring, by plasmonic effect, an increase in the polarization plane rotation of the transmitted polarized light by the magneto-optical sublayer (22-1). 5. The optical coupling device (1) according to claim 4, wherein the plasmonic sub-layer (22-2) comprises a plasmonic network of straight metal lines (22-3) covered with a dielectric material, the metal lines being oriented substantially orthogonal to a transmission axis of a rectilinear polarizer located between the optical chip (10) and the rotational layer (22). 6. Optical coupling device (1) according to any one of claims 1 to 5, wherein the intermediate component (20) further comprises a second rotational layer (22i), comprising a magneto-optical material adapted to transmit the polarized light by rotating the plane of polarization thereof when a magnetic field generated by a field generation layer (23) is applied to said second rotation layer (22i). An optical coupling device (1) according to claim 6, wherein the magnetic field generation layer (23) is located between the first and second rotation layers (22, 22i), and is adapted to generate a field magnetic at each of said rotation layers. 8. optical coupling device (1) according to claim 3, wherein the intermediate component (20) comprises pads (51) for electrical connection and assembly, made of an electrically conductive material and located at the upstream face. (28A) so as to be in contact with the electrical connectors (50), and intended to be in contact with electrical connection portions (15) disposed on said upper face (14) of the optical chip (10). 9. optical coupling device (1) according to any one of claims 1 to 8, wherein the intermediate component (20) comprises a so-called polarizer layer (30), comprising periodic metal structures (33) adapted to form a polarizer rectilinear, disposed between the rotating layer (22) and a so-called downstream face (28B) opposite an upstream face (28A) intended to face the optical chip. 10. Optical coupling device (1) according to any one of claims 1 to 9, further comprising a collimating element, disposed on a so-called upstream face (28A) of the intermediate component (20), and adapted to ensure collimation polarized light extracted from the optical chip (10), and / or a focusing element, disposed on a so-called downstream face (28B) of the intermediate component (20), and adapted to focus the polarized light transmitted by the intermediate component (20) on an input face of the outer waveguide (40). 11. A method of producing an intermediate component (20) of an optical coupling device (1) according to any one of claims 1 to 10, comprising: - the formation of a stack of layers, on a support layer (21), comprising: a step of forming said magnetic field generation layer (23); and o a step of forming said rotation layer (22); - Cutting the stack of layers to form said intermediate component (20). 12. The method of claim 11, including a step of forming electrical connectors (50) by etching the support layer (21) so as to obtain openings opening on the one hand on the conductive track (24), and on the other hand on a so-called upstream face (28A) of the support layer (21), then by filling the openings thus obtained with an electrically conductive material. A method of collectively producing a plurality of optical coupling device intermediate components according to any one of claims 1 to 10, wherein the method according to claim 11 or 12 is performed, said step of cutting said stack of layers. for forming said intermediate components. 14. A method for assembling and self-aligning an intermediate component (20) of an optical coupling device according to claim 8 on an upper face (14) of an optical chip (10), comprising the steps following: - contacting the connection pads (51) of the intermediate component (20) with the connection portions (15) of the optical chip (10), the connection pads (51) being made of a fusible material, and being in contact with connection portions (52,15) located on the upstream face (28A) of the intermediate component (20); - Performing the melting of the material of said pads (51), the melt material providing self-alignment by surface tension effect between the connection portions (52) of the intermediate component (20) and the connection portions (15) of the optical chip (10).