FR2614999A1 - Ferrimagnetic monomode two-dimensional optical waveguide, its method of manufacture and its use in an integrated-optic isolator - Google Patents

Abstract

The invention relates to a ferrimagnetic monomode two-dimensional waveguide as well as to its method of manufacture. The waveguide consists of a rectilinear bar 7 having a substantially square cross-section made with an yttrium-iron garnet, which may or may not be substituted with rare earths, and of a homogeneous medium composed of a substrate 9 and of a layer 10 deposited by epitaxy, and consisting of a gadolinium-gallium garnet. The invention is applicable, in particular, to the production of integrated-optic isolators.

Description

Two-dimensional ferrimagnetic optical waveguide,
its manufacturing process, and its use
in an integrated optical isolator.

 The invention relates to a two-dimensional ferrimagnetic optical waveguide, its manufacturing process, and its use in an isolator in integrated optics.

 Optical fibers are increasingly used not only for the transmission of information but also to "process" information in the broadest sense of the term. Thus by associating a single-mode optical fiber with integrated optical circuits, it is possible to carry out modulation, switching, multiplexing, demultiplexing, polarization, etc. functions.

 In fiber optic transmissions, it is necessary to prevent reflections which may occur from disturbing the transmission of information itself, either by action on the optical signal or by action on the light source. I1 must therefore somehow isolate the emission part from the use part. An optical isolator circuit is used for this purpose which is arranged for example close to the light source, in the path of the beam, so as to prevent any parasitic light return which would disturb the operation of the light source. It is understood that the device to be protected by an optical isolator circuit can be an optical modulator, an optical gyrometer, etc.

 One of the most used optical isolator circuits is of the volume component type implementing the Faraday effect, that is to say using the effect of rotation of the plane of polarization of a light ray passing through a transparent body. isotropic placed in a magnetic field.

More precisely, in the case of a light source such as a laser diode, the light ray first passes through a polarizer which therefore lets through only the light rays having a certain polarization. The polarized light ray then passes, for example, through a transparent ferrimagnetic medium placed in a magnetic field parallel to the axis of propagation of the light ray. The path in this ferrimagnetic medium is such that the deviation of the direction of polarization is 45 ". As a light ray moving in opposite direction undergoes a rotation of 45" in the same direction, the polarization of any reflected ray corresponding to the ray incident will be deviated by 90 "and, therefore, will not be able to pass through the polarizer. Consequently, the light source will not be disturbed by such a reflection. A unidirectional propagation line has thus been produced.

In integrated optical circuit technology, a ferrimagnetic guide can be produced using an ytrium and iron garnet often designated by the abbreviation YIG, deposited by epitaxy on a gadolinium and gallium garnet substrate, often designated by the abbreviation
CGC. Advantageously, the composition of YIG, of general formula Y3Fe5,012, is modified to introduce rare earths such as gadolinium and gallium. In fact, gadolinium is substituted for ytrium and gallium is substituted for iron so as to obtain the general formula Gdx Y3-x Gay Feg, y 012 in which x = 0.45 and 0.8 (y (1, 1.

 The layer of YIG, which has a thickness close to one micron, for wavelengths of one to five microns, and a width of a few millimeters, constitutes a waveguide for a light ray but, like its width , in the transverse direction compared to the ray path, is large compared to its thickness, this guide propagates several TE and TM propagation modes. However, the presence of several propagation modes considerably hinders the operation of the device because the rotation of the polarization plane by Faraday effect varies from one mode to another.

 In addition, the use of such a guide as an isolator requires that the speeds of the TE and TM modes be adapted. In the known art, these speeds are adapted by introducing crystal stresses during the epitaxy of the YIG guide on the substrate, but this method is not very reproducible.

 The object of the present invention is therefore to produce a ferrimagnetic waveguide, which does not have the aforementioned drawbacks, and which is in particular two-dimensional single mode and particularly well suited to the production of an integrated optical isolator having good performance. insulation.

 According to the invention a two-dimensional ferrimagnetic waveguide is characterized in that it consists of a straight bar with a substantially rectangular cross section which is produced with a garnet of ytrium and iron and a homogeneous medium with an index less than that of the bar which serves both as a support and an envelope for said bar.

 The homogeneous medium can be a gadolinium and gallium garnet or an ytrium and iron garnet which has a lower refractive index than that of the garnet constituting the straight bar.

 The straight bar which constitutes the propagation medium can be produced either in the form of a mesa or in the form of a trench.

 Garnet of ytrium and iron may contain rare earths partially replacing ytrium and iron, these rare earths being preferably gadolinium and gallium.

 In all cases, the substrate is preferably a gadolinium and gallium garnet.

The invention also relates to a method for manufacturing this two-dimensional monomode waveguide, characterized in that it comprises the following operations:
- the deposition of a layer of ytrium garnet and iron on the gadolinium and gallium garnet substrate,
- removal of a substantial part of said layer to leave only a straight bar, and
- depositing a layer of gadolinium and gallium garnet on the substrate and the bar.

In the case of a homogeneous medium made of ytrium and iron garnet, the process is characterized by the following operations:
- depositing a first layer of ytrium and iron garnet on a gadolinium and gallium garnet substrate,
the deposition on the first layer of a second layer of ytrium garnet and of iron having a refractive index higher than that of the first layer,
- the removal of a substantial part of the second layer to leave only a straight bar, and
- depositing a third layer of ytrium garnet and iron on the first layer and the bar, said third layer having a refractive index which is equal to that of the first layer.

 In the case where the straight bar is made in a trench, the first deposition operation is preceded by the digging of said trench.

 The various deposits are preferably carried out by epitaxy in the liquid phase.

 The operation of removing a substantial part of a layer is carried out by chemical or ionic machining of the reactive type or not.

 The invention finally relates to the use of such a two-dimensional single-mode guide in an integrated optical isolator.

Other characteristics and advantages of the present invention will appear on reading the following description of particular embodiments, said description being made in relation to the appended drawings:
- Figure 1 is a perspective view of an effect waveguide
Faraday according to the prior art
- Figure 2 is a cross section of a first example of
realization of a two-dimensional single-mode effect waveguide
Faraday according to the present invention
- Figure 3 is a cross section of a second example of
realization of a two-dimensional single-mode effect waveguide
Faraday according to the present invention and
- Figure 4 is a cross section of a third example of
realization of a two-dimensional single-mode effect waveguide
Faraday according to the present invention.

 As mentioned in the preamble to produce for example a unidirectional propagation line of a light ray, it is necessary to associate, depending on the direction of propagation sought, a polarizer and a ferrimagnetic waveguide, with Faraday effect, that the 'is also called one-way section. This Faraday effect waveguide can be constituted as described in the known art (Figure 1) for example by a layer 1 of ytrium garnet and iron (YIG) having a thickness of one micron which has been deposited by liquid phase epitaxy on a gadolinium and gallium garnet substrate 2 having a thickness of approximately 500 microns.

dans laquelle
ITM est l'intensité du mode TM à la sortie 6 (1 = L) ITE est l'intensité du mode TE à l'entrée 5 (1 = 0)
K est la constante de Faradav

ou est la longueur au rayonnement lummeux,

étant les constantes de propagation des modes respectivement TEo et TMo
Cette formule montre que ce rapport et donc l'isolation obtenue est maximum lorsque a ss est égal à zéro, c'est-à-dire lorsqu'il y a l'égalité des indices nTE et nTM . Or, dans la couche 1, il ne peut y avoir cette égalité car les con0ditions aux limites sont très différentes selon le mode envisagé.En outre, dans une telle couche, il y a dispersion de l'énergie lumineuse entre l'entrée 5 et la sortie 6 car la couche I a une largeur importante par rapport à la longueur d'onde.A light ray 3 propagates from the left (entry 5) to the right (exit 6) of figure 1 in the layer 1 and is subjected on the length L to a magnetic field M having the direction of the arrow, it is -that of the propagation of the light ray 3. Being a waveguide, only electric Transverse (TE) and Magnetic Transverse (TM) modes can propagate inside layer 1 and there As a result, the Faraday effect in this type of propagation is characterized by the proportion of TU mode which appears at output 6 when the light ray is in TE mode at input 5. This proportion is given by the formula

in which
ITM is the intensity of the TM mode at output 6 (1 = L) ITE is the intensity of the TE mode at input 5 (1 = 0)
K is the Faradav constant

where is the length of the light radiation,

being the constants of propagation of the modes TEo and TMo respectively
This formula shows that this ratio and therefore the insulation obtained is maximum when a ss is equal to zero, that is to say when there is equality of the indices nTE and nTM. However, in layer 1, there cannot be this equality because the boundary conditions are very different depending on the mode envisaged. In addition, in such a layer, there is dispersion of light energy between input 5 and the output 6 because the layer I has a large width relative to the wavelength.

 The two above observations lead to producing, according to the present invention, a ferrimagnetic waveguide whose dimensions are limited in the two directions so as to have a propagation of the single-mode type in the two dimensions. Three embodiments of a ferrimagnetic waveguide are described below which have characteristics of the present invention.

 In FIG. 2, the ferrimagnetic waveguide consists of a guide produced in the form of a mesa 7 in ytrium garnet and iron resting on a part of the upper face 8 of a garnet substrate 9 gadolinium and gallium. A layer 10 of gadolinium and gallium garnet covers the mesa 7 and the part of the upper face 8 of the substrate 9 not covered by the mesa 7.

 In FIG. 3, the ferrimagnetic waveguide consists of a guide produced in the form of a valley or trench 11 dug in a substrate 12 on the upper face 13 and filled with ytrium garnet and iron 14. A layer 15 of gadolinium and gallium garnet covers the entire upper face 13 of the substrate 12, including the yeast and iron garnet 14 filling the trench 11.

 In the two embodiments of Figures 2 and 3 the substrate has a thickness of approximately 500 microns and a refractive index of 1.945; mesa 7 or trench 1 1 has dimensions of the order of a micron for wavelengths of one to five microns and a refractive index of 2.15; the upper layer 10 or 15 has a minimum thickness of approximately 5 microns but can reach 50 microns.

 In many applications, it is advantageous to increase the transverse dimensions of the guide, that is to say of the mesa 7 or of the trench 11, in order to facilitate the connections, the adjustments and the couplings with the adjacent optical circuits. In order to obtain an increase in the transverse dimensions of the guide, it is known to make the difference in the values of the indices of the wave guide and of the underlying material to be of the order of five thousandths. waveguides of the type of FIG. 1 are produced in which, on a gadolinium and gallium garnet substrate with an index of 1.945, is deposited, not a single layer of ytrium and iron garnet but two layers, the first of a thickness of three to five microns having a refractive index of 2.155 for a wavelength of one to five microns and the second of a thickness of six to seven microns having a refractive index of 2.160.

 The invention can also use such an arrangement and the structure shown in FIG. 4 is obtained. This structure comprises a substrate 16 made of gadolinium and gallium garnet having a thickness of approximately 500 microns and a refractive index equal to 1.945, a first layer 17 of ytrium and iron garnet having a thickness of approximately three to five microns and a refractive index equal to 2.155, a YIG guide 18 in the form of a mesa disposed on the first layer 17 having sides of six to seven microns and a refractive index equal to 2.160, and a second layer 19 of ytrium garnet and iron of composition identical to the first layer 17 but having a thickness at least equal to five microns and up to fifty microns.

 This third exemplary embodiment also lends itself to a trench or valley structure similar to that described in relation to FIG. 3 provided that a layer 17 is thick enough to dig a trench therein while having sufficient thickness on the bottom of the trench.

 In the particular examples of embodiment of the two-dimensional Faraday effect single-mode guide which have just been described, the dimensions of the guide, whether transverse or longitudinal, may vary with the material used and the wavelength λ of the light ray which crosses it, and the values that have been mentioned are only indicative. Of course, the material used can be ytrium garnet and pure iron but also ytrium garnet and iron substituted with rare earths such as gadolinium and gallium as indicated in the preamble. The length of the guide and therefore of the substrate depends on the amplitude of the Faraday effect in the magnetic ferri material of which the guide is made; as for the width of the substrate, it is not critical because it does not intervene in the Faraday effect.

 From the description of the specific examples of embodiment of the two-dimensional ferrimagnetic mode guide, it appears that this guide consists of a straight bar with a substantially rectangular cross section close to the square made of an yarn and iron garnet, whether or not substituted for rare earths, this bar being enveloped with a medium of homogeneous composition which is either a gadolinium and gallium garnet or an ytrium and iron garnet, whether or not substituted with rare earths, which has a refractive index slightly lower than that of the garnet of the guide bar.

 To manufacture the two-dimensional ferrimagnetic waveguides according to the invention, the method is as follows. In the first particular embodiment which has been described (FIG. 2), the first phase of the process consists in depositing, by liquid phase epitaxy on the upper surface 13 suitably prepared of the substrate 9 in gadolium and gallium garnet, a layer of garnet ytrium and iron substituted or not of rare earths such as gadolinium and gallium.

The epitaxial growth is stopped as soon as the thickness of the layer reaches about one micron. A large part of this layer is removed by chemical or ionic machining of the reactive type or not so as to keep only a central part 7 in the form of a mesa which constitutes the two-dimensional monomode guide. The layer 10 of gadolinium and gallium garnet is then deposited by liquid phase epitaxy, for example by immersing the substrate 9 surmounted by the guide 7 in a bath containing the oxides necessary for the synthesis of the material and brought to a temperature of 980 "C. The duration of the immersion depends on the growth rate chosen and on the thickness of the layer 10 which it is desired to obtain; this duration can be approximately two minutes. To manufacture the second embodiment which has been described in relation to FIG. 3, the upper face 13 of the substrate 12 is first hollowed out by chemical or ionic machining of the reactive type or not to obtain the trench 11. A layer of ytrium and iron garnet, substituted or not of rare earths, is then deposited by liquid phase epitaxy on the entire upper surface 13 and in the trench 11 so as to fill the said trench. The layer which has just been deposited is then machined to obtain an upper face pla presenting only yeast and iron garnet at the location of trench 11. After this operation, the layer 15 of gadolinium and gallium garnet is deposited, as for layer 10 of the example of figure 2.

 To manufacture the third exemplary embodiment which has been described in relation to FIG. 4, the first phase of the method consists in depositing, by epitaxy in liquid phase, on the substrate 16 the first layer 17 of ytrium garnet and of iron, substituted or not of rare earths, of index 2.155. The epitaxial growth is stopped as soon as the thickness of the layer 17 reaches three to five microns. To carry out mesa 18, a layer of yeast and garnet iron, substituted or not substituted with rare earths, of index 2.16 and of thickness equal to six or seven microns, is deposited by liquid phase epitaxy. This layer is then configured by reactive or non-reactive ionic chemical machining to obtain the mesa 18 guide only.

Finally, layer 19, of composition identical to layer 17, is deposited by liquid phase epitaxy on layer 17 and the mesa guide 18. The epitaxial growth of this layer 19 can be stopped as soon as the thickness reaches approximately five microns .

 In all cases, after leaving the bath used for epitaxial growth, the wafer is cleaned according to all known methods, for example by ultrasound.

 The use of such a two-dimensional ferrimagnetic waveguide in an integrated optical isolator is particularly advantageous because the fact of surrounding the guide with a material identical to that composing the layer of the substrate on which the guide is formed allows to obtain a completely symmetrical configuration. Thus the two polarizations of the modes propagating in the guide will propagate at the same speed which notably improves the conversion of one polarization into the other and increases the quality of the insulation.

Claims (15)

 1. Two-dimensional ferrimagnetic waveguide, characterized in that it consists of a rectilinear bar (7, 14 or 18) with a substantially rectangular cross section of ytrium garnet and iron and a homogeneous medium ( 9. 10 or 12, 14 or 17, 19) serve as a support and an envelope for said bar, the index of the homogeneous medium being lower than that of ytrium and iron garnet.  2. waveguide according to claim 1, characterized in that the homogeneous medium (9, 10 or 12, 15) is a garnet of gadolinium and gallium, part of which is a substrate (9, 12) which serves as a support and the other part a layer (10, 15) obtained by deposition.  3. Waveguide according to claim 2, characterized in that the bar is in the form of a mesa (7) on the upper layer of the substrate (9), the layer (10) being obtained by deposition enveloping the three other faces of bar.  4. Waveguide according to claim 2, characterized in that the bar is formed in a valley hollowed out in the substrate (12), the layer (10) covering the upper face of the bar and the substrate.  5. waveguide according to claim 1, characterized in that the homogeneous medium (17, 11) is a garnet of ytrium and iron which has a refractive index lower than that of the garnet constituting the bar (14), said homogeneous medium (17, 19) being obtained by successive deposits on a substrate (16) which serves as a support.  6. Waveguide according to any one of the preceding claims, characterized in that the yeast and iron garnet contains rare earths partially replacing ytrium and iron.  7. Wave guide according to claim 6, characterized in that the substitution rare earths are gadolinium and gallium.  8. Wave guide according to any one of claims 2 to 7, characterized in that the substrate is a garnet of gadolinium and gallium.  9. Method for manufacturing a waveguide according to claim 2, characterized in that it comprises the following operations:  - depositing a layer of ytrium and iron garnet on the gadolinium and gallium garnet substrate,  - removal in a substantial part of said layer to leave only a straight bar, and  - depositing a layer of gadolinium and gallium garnet on the substrate and the bar.  10. The method of claim 9, characterized in that the deposition of the layer of yeast and iron garnet is preceded by the digging of a straight trench in the substrate.  - depositing a third layer of ytrium garnet and iron on the first layer and the bar, said third layer having a refractive index which is equal to that of the first layer.  - the removal of a substantial part of this second layer to leave only a straight bar, and  the deposition on this first layer of a second layer of ytrium garnet and of iron having a refractive index greater than that of the first layer,  - depositing a first layer of ytrium garnet and iron on a gadolinium and gallium gear substrate,  11. Method for manufacturing a waveguide according to claim 5, characterized in that it comprises the following operations:  12. Method according to claim 11, characterized in that the deposition of the second layer of garnet-ytrium and iron is preceded by the digging of a rectilinear trench in the first layer.  13. Method according to any one of the preceding claims 9 to 12, characterized in that the successive deposits are made by epitaxy in the liquid phase.  14. Method according to any one of the preceding claims 9 to 13, characterized in that the removal of a substantial part of the layer of garnet dytrium and of iron and the digging of the trench are carried out by chemical or ionic machining of the reactive type or not.  15. Use of a two-dimensional ferrimagnetic guide according to any one of claims 1 to 6, in an integrated optical isolator, further comprising a polarizer and means for applying a magnetic field to the guide in the direction of propagation. .