Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
  • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
  • G02B6/126—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind using polarisation effects
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals

Definitions

• the invention relates to an optical waveguide structure in which birefringence is controlled.
• the structure comprises an optical waveguide supported by a substrate, the optical waveguide having at least one micronanostructured guide layer.
• Optical waveguides are mediums for the transport of optical signals that are widely used in optoelectronics. However, they have the disadvantage of being sensitive to the polarization of light: it is said that the optical waveguides are birefringent. Birefringence is conventionally defined as being the difference between the effective indices assigned to the TE (for "Transverse Electric” in English) and TM (for "Transverse Magnetic”) polarizations:
• the polarization states TE and TM are defined such that: TE corresponds to the electric field parallel to the plane of the circuit and to the magnetic field perpendicular to the plane of the circuit,
• the shape birefringence is due to the asymmetrical shape ratio of the section of the waveguide. Indeed, when an optical waveguide has an arbitrary shape ratio, it does not propagate the two polarization states TE and TM of an optical signal at the same speed, unless the section of the guide is strictly square. An optical polarization dispersion of the optical mode is then observed within the optical waveguide.
• the birefringence of material, noted B matr is due to the material in which is inscribed the waveguide.
• the birefringence of material may be intrinsic, as in the case of anisotropic materials, or be induced by waveguide manufacturing processes.
• the known waveguides are mainly structures consisting of vertical stacks of thin layers.
• Document [1] (referenced at the end of this description) describes a "double-core" waveguide consisting of a core layer and a thin layer.
• the addition of the thin layer just below the main core layer compensates for the polarization dispersion of the core core layer.
• the thin layer is chosen to have a thickness of a few tens of nanometers and a high refractive index, and a birefringence of opposite sign to that of the main core layer.
• the thin layer is a Si 3 N 4 layer having a refractive index n -2 and the main core layer is SiON.
• the double - core waveguide only compensates for weak birefringences (of the order of 10 ⁇ 3 , which corresponds to material birefringence values). It is therefore necessary for the waveguide to have a roughly square section in order to avoid the appearance of a form birefringence.
• the thickness of the thin layer is chosen so that the sum of the thicknesses of the main layer of heart and the thin layer is approximately equal to the width of the waveguide.
• the height H and the width L of the waveguide are chosen so that the section of the waveguide has a shape ratio very close to the unit. , that is to say, a substantially square shape, because this multilayer waveguide only compensates for weak birefringence (some 10 ⁇ 4 ). It is therefore important to avoid creating a shape birefringence in the waveguide.
• the waveguides of the prior art must have a substantially square cross section in order to limit the shape birefringence.
• the heights of these waveguides, having values close to the widths of these guides, are of a few micrometers.
• the object of the invention is therefore to propose an optical waveguide which makes it possible to control the birefringence, and this, whatever the aspect ratio of the section of the waveguide.
• an optical waveguide structure comprising: - a support substrate, - an optical waveguide disposed on one side of the support substrate, the optical waveguide comprising at least one guiding layer having a refractive index neither intended to guide an optical wave along a direction of propagation parallel to the surface of the guide layer, said structure being characterized in that said at least one guide layer comprises at least one zone having a valuable birefringence B, said zone comprising recesses which are made in the thickness of the guiding layer, the recesses being filled with a fluid or a material having a refractive index n 2 different from ni and being organized in at least two parallel rows, each row lying in a plane substantially perpendicular to the surface of the guide layer and substantially parallel to the direction of propagation of the optical wave, and c each row extending over a distance greater than or equal to the wavelength of the optical wave intended to be propagated in the guiding layer, the width of the recesses being less than or equal to one-tenth of the wavelength of the optical
• ni and n 2 being the filling factor of the refractive index material ni and being a value between 0 and 1, the values 0 and 1 being excluded;
• n H , ⁇ E and n H , ⁇ i4 being respectively the effective refractive indices of the TE mode and the TM mode of the upper refractive index ("high") guidance layer
• n B , ⁇ E and n B , ⁇ M being respectively the effective refractive indexes of the TE mode and the TM mode of the fluid or filling material of the lower refractive index ("low”) recesses.
• ni and n 2 are different, there is necessarily one which is superior and the other which is inferior.
• n 2 is strictly equal to the surrounding index n in v (in the case where the structured guide layer is placed in a liquid or in the most general case, in the air or in any gas) , then n B , ⁇ E and n B , ⁇ M will be taken equal to n in v
• the parameters of the above formula are modified so as to obtain, at the output of the waveguide, a compensated birefringence and in such a way that the two modes of propagation TE and TM are detected at the same time, even if they do not have the same speed of propagation. For example, if the TE mode propagates in the guiding structure with a certain delay compared to the TM mode, it is then desired to delay the TM mode so that at the output of the guiding layer, we can see the two modes TE and TM.
• the parameters of the formula are locally modified so that the two modes TE and TM propagate at the same speed within the guiding layer. For this, the effective indices will be exactly the same.
• the period between the rows is between 50 nm and 1 ⁇ m; the spacing between the recesses is between 50 nm and 1 ⁇ m; the depth of the recesses is between 10 nm and 1 ⁇ m.
• the guide layer is said to have lateral micronanostructuring.
• the recesses are present in at least one zone of the guiding layer where it is desired to modify the birefringence of the guiding layer.
• the recesses can thus be located in one or more precise zones of the guide layer or be present on the entire guide layer.
• the recesses can be present over the entire width of the guiding and / or over the entire length of the guiding layer.
• the refractive index n 2 is different from the refractive index n i.
• a refractive index neither greater than the index n 2 , so that the optical wave remains confined in the index material ni.
• the index material ni is silicon
• the material of index n 2 can be chosen from oxides (for example SiO 2 , TiO 2 ...) or nitrides (for example Si 3 N 4) . ).
• the light wave intended to be propagated in the guiding layer has a wavelength of between 400 nm and 10 ⁇ m.
• the adjacent recesses of adjacent rows are separated by a distance less than or equal to one-tenth of the wavelength of the optical wave intended to be propagated in the guiding layer.
• the distance between the adjacent recesses of the same row is constant.
• the distance between the rows is constant.
• each guideline passing through the center of the recesses of each row it is possible for example to obtain a waveguide structure in which the guiding layer comprises a set of recesses arranged periodically.
• the recesses is a slot extending in the direction of propagation of the optical wave.
• at least one row may advantageously comprise a single recess, this recess being a slot which extends in the direction of propagation of the optical wave.
• At least one of the recesses is a hole having a circular or square section.
• the width of the recesses varies within the same row of recesses.
• the variation of the width of the recesses within the same row is constant.
• the variation of the width of the recesses in the same row is periodic; for example, the variation may be a sinusoidal variation with an increase followed by a decrease, etc. of the width.
• the structuring parameters of the guide layer may vary over the width or length of the guide layer.
• the index profile is then locally modified. This modification makes it possible, for example, to perform advanced passive / active optical functions in the waveguide (focuser, mode converter, switch, etc.).
• the guiding layer has a thickness less than or equal to 10 microns. More particularly, the guiding layer may advantageously have a thickness of between 1 micrometer and 250 nanometers. Advantageously, the recesses have the same depth.
• the recesses are through recesses.
• the recesses are said to be through as they pass through the entire thickness of the guide layer.
• the structure further comprises two conductive electrodes, a first electrode being placed under the coating layer and the second electrode being placed above the coating layer.
• the two electrodes are placed near the region of the guide layer having the recesses.
• the electrodes may be electrically conductive or thermally conductive.
• the invention also relates to a method for producing such an optical waveguide structure comprising a support substrate and an optical waveguide, the waveguide comprising at least one guiding layer which comprises at least one zone having a birefringence value B.
• the method of realization comprises the following steps: providing a stack comprising a support substrate and at least a first material guiding layer on one face of the support substrate,
• each row being in a plane substantially perpendicular to the surface of the guide layer and substantially parallel to the direction of propagation of the optical wave in the guide layer, and each row being extending a distance greater than or equal to the wavelength of the optical wave to be propagated in the guide layer, the width of the recesses being less than or equal to one-tenth of the wavelength of the optical wave intended to be propagated in the guiding layer, the width of a recess being the maximum distance between two opposite edges of the same recess present in a plane perpendicular to the direction of propagation of the optical wave, each recess within a same row being remote from an adjacent recess by a distance less than or equal to one-tenth of the wavelength of the optical wave intended for propagated in the guide layer, the width
• ff being the filling factor of the index material ni and being a value between 0 and 1, 0 and 1 being excluded
• n H , ⁇ E and n H , ⁇ i4 being respectively the effective refractive indices of the TE mode and the mode TM of the upper refractive index guiding layer ("high")
• n B , ⁇ E and n B , ⁇ M being respectively the effective refractive indices of the TE mode and the TM mode of the fluid or the filling material of the recesses of lower refractive index (“low”).
• n 2 is strictly equal to the surrounding index n in v (in the case where the structured guide layer is placed in a liquid or in the most general case, in the air or in any gas) , then n B , ⁇ E and n B , ⁇ M will be taken equal to n in v
• the structuring of the guiding layer may be carried out by lithography, then etching of said layer.
• the production method further comprises, after the structuring step, a step of depositing a coating layer on the structured guide layer, said coating layer having a refractive index different from that of the guiding layer.
• the production method further comprises, before the step of depositing the coating layer on the structured guiding layer, a step of forming a first conductive electrode on the guiding layer and, after the step of depositing the coating layer on the structured guide layer, a step of forming a second conductive electrode on the coating layer.
• the conductive electrodes may be electrically conductive or thermally conductive. The formation of the electrodes may for example be obtained by deposition of an electrically conductive layer. It is specified that the first electrode may be formed before or after the structuring of the guiding layer.
• the support substrate comprises a substrate having an index of refraction n 3 and a layer having an index of refraction n 4 in contact with the guiding layer, n 4 being lower than the refractive index and the guiding layer.
• the support substrate comprises a silicon substrate and a silicon dioxide layer, and the guiding layer is a silicon layer.
• FIG. 1 represents an example of an optical waveguide structure seen in cross-section according to the invention
• FIGS. 2A to 2D represent the steps of the method of manufacturing an optical waveguide structure according to a first embodiment of the invention
• FIGS. 3A and 3B show a view from above of the structure represented in FIG.
• FIG. 4 represents a waveguide structure seen in cross section with two electrodes
• FIGS. 5A to 5D show the steps of the method of manufacturing an optical waveguide structure according to a second embodiment of the invention
• FIG. 6 is a graph showing the birefringence calculations as a function of the silicon filling rate by simulation using software (RSOFT) and with the real sample;
• FIG. 7 is a graph showing the effective indices of the states; polarization TE and TM according to the silicon filling rate of a waveguide structure according to the invention.
• optical guide structure 1 An example of optical guide structure 1 according to the invention is illustrated in FIG. 1.
• the structure 1 consists of an optical guide, comprising a single structured guidance layer 200 with a periodic recess network. A, resting on a support substrate 3.
• the waveguide of the waveguide structure according to the invention may also consist of several stacked layers, at least of these layers comprising lateral nanostructuring.
• SOI substrate silicon on insulator
• a layer of silicon dioxide 300 of about 1 micrometer thick (Figure 2A).
• a polycrystalline silicon layer 20 is then deposited on the silicon dioxide layer 300.
• This silicon layer 20 has a thickness of between 200 nm and 1 ⁇ m (FIG. 2B).
• the polycrystalline silicon layer 20 is then structured: a structured layer 200 is then obtained.
• the lateral structuring of the waveguide can be done according to several geometries (in the form of slots or holes), the structuring being able to be mixed (holes and slots), periodic or non-periodic.
• the structuring is a periodic array of parallel slots created throughout the thickness of the polycrystalline silicon layer 20.
• the structuring is obtained for example by performing a lithography of the layer 20, then a RIE or ICP etching. . Lithography can be performed using UV, deep UV, focused ion beam or electron beam.
• the recesses of the structured guide layer 200 may be left empty (Fig. 2C), but may also be filled.
• the recesses of the structured guide layer can thus be filled with a solid or a fluid, the fluid can be a liquid or a gas.
• the gas may for example be ambient air.
• the structured guiding layer 200 is covered with a coating layer 5 which will fill the slots of the guiding layer.
• a silicon dioxide coating layer 5 having a thickness of 1.5 micrometers is deposited on the structured guide layer 200, then this coating layer 5 is planarized by chemical mechanical polishing. The final thickness of the coating layer 5 can be controlled and adjusted between 100 nm and 1 micrometer.
• a waveguide structure is thus obtained comprising a support substrate 3 and a waveguide 4 consisting of a structured guide layer 200 and a coating layer 5.
• the coating layer 5 is made of a material identical to the material of the layer underlying the guiding layer, that is to say the layer 300.
• a coating layer of a material having a different refractive index for example a lower refractive index
• that of the guiding layer for example a resin or a dielectric material.
• a slot ID network is etched throughout the thickness of the silicon layer at a period of 120 nm.
• Such a micronanotructured waveguide is adapted to serve as a waveguide for a light wave having a wavelength of 1.55 microns.
• FIGS. 3A and 3B show a view from above of the structure illustrated in FIG. 2C: the structured guide layer 200 is seen, as are the underlying substrate (layer 300) and the recesses A.
• the structuring parameters (not between the recesses, number of recesses 7), one can increase, decrease or cancel the birefringence of the waveguide, but once the structuring is performed, the birefringence of the waveguide is fixed.
• this birefringence can be modified by changing the material included in the recesses of the guiding layer. It is then preferable to use a fluid to fill and empty the recesses more easily.
• a reconfigurable waveguide is obtained over time.
• two electrodes (6, 7) are placed on either side of the coating layer 5 (FIG. 4) so as to dynamically change the refractive index of the coating layer and / or the parameters opto-geometric structuring of the structured guide layer 200.
• the variation can be done electro-optically by applying an electrical voltage between the electrodes (6, 7) (to modify the refractive index) or thermally by producing local heating between the electrodes (the heat variation causes expansion or narrowing of the guide layer and changes the gap between the recesses).
• the coating layer 5 is made of a material different from the layer 300 underlying the structured guide layer 200.
• a layer of silicon dioxide 3000 is deposited on a III-V layer of a III-V heterostructure consisting of a III-V 8 substrate, a III-V 9 sacrificial layer and a III-V layer. 20 stacked ( Figure 5A).
• the III-V materials forming the heterostructure may be for example GaAs or InP.
• This heterostructure is transferred to a stack consisting of a silicon substrate 30 and a silicon dioxide layer 300 by a molecular bonding method, the adhesion occurring between the silicon dioxide layer 3000 of the heterostructure and the silicon dioxide layer 300 of the stack (FIG. 5B). Then, the substrate is removed
• III-V 8 by lapping, polishing and selective chemical etching until reaching the sacrificial layer 9. Then the sacrificial layer 9 is removed by selective chemical etching.
• the surface of the III-V layer can be flattened ( Figure 5C).
• recesses are made in the III-V layer throughout its thickness, for example by performing a lithography followed by an RIE or ICP etching of the III-V layer.
• the recesses may be a periodic network of parallel slots for example.
• Birefringence B is obtained according to a formula which depends on a factor of the index material nf (filling factor ff) and refractive indices n, TE , n, TM , n 2 , ⁇ and n 2 , ⁇ M- This formula comes from an analytical model that allows to model the behavior of thin layer type slit structures. In this analytical model, we have:
• ax is a corrective term of the first order, a being a constant, ⁇ being the period and ⁇ the wavelength; n e ff, ⁇ i4 and n e ff, ⁇ E being the effective refractive index of the TM mode and the TE mode, respectively; n H , ⁇ i4 and n H , ⁇ E being the high refractive index of the TM mode and the TE mode, respectively; n B , ⁇ i4 and n B , ⁇ E being the low refractive index of the TM mode and the TE mode, respectively.
• ni and n 2 are different, there is necessarily one that will be “high” and the other "low".
• n B , ⁇ E and n B , ⁇ M will be taken to be equal to n env .
• the structuring parameters are connected to the filling factor ff in a manner known per se.
• the width of a recess is equal to (l-ff) * ⁇ and the spacing between two adjacent recesses is equal to ff * ⁇ , where ⁇ is the period of the structured guide network.
• the waveguide structure 1 studied in this example consists of a layer of silicon having a thickness of 320 nm, a width of 5 microns and a length of 300 microns, which forms the waveguide, and a substrate 3 ( Figure 1).
• the refractive index of the structured silicon layer is 3.5.
• This silicon layer comprises a periodic network of slots A parallel to the length of the layer, made throughout the thickness of the layer and organized in a period of 100 nm.
• the substrate 3 may for example consist of a silicon substrate and a silica layer having a thickness of 2 microns. The assembly formed by this substrate and the 220 nm silicon layer then forms a micronanostructured SOI substrate.
• the layer thus structured 200 is immersed in a homogeneous medium of silicon nitride Si 3 N 4 having a refractive index of 2.2.
• a Gaussian beam is generated at one of the two edges of the structured layer 200 so that the beam propagates both in a direction parallel to the plane of the layer and parallel to the slots of the layer.
• the beam is chosen so that it has a lateral extension smaller than the width of the structured layer so as to have optimal overlap with the optical mode.
• n n H and n 2 is n B and we have:
• Figure 6 shows the calculations of birefringence versus silicon fill rate by simulation using software (RSOFT) (straight line) and from the actual sample (round). It can be seen that the results obtained by simulation and the real sample (obtained using the analytical model) are close. The analytical model is therefore a good way to know the fill rate according to the desired birefringence.
• FIG. 7 presents the effective indices calculated for the two states of polarization TE and TM, depending on the silicon fill factor of the structured layer.
• the index difference between the two states TE and TM is greater than 0.1, the effective index for the TE mode being 3.21976 and the effective index for the TM mode being 3.0863.
• the unstructured waveguide thus has a significant birefringence. This is explained by the fact that the waveguide has a strong asymmetry at its cross section.
• a filling factor of 87% corresponds in our example to slots spaced from each other by 13 nm in a period of 100 nm, the slots being filled with silicon nitride. With a fill rate of 87%, there is therefore in the end a periodic succession of 87 nm wide silicon "bars" and "bars" of silicon nitride 13 nm wide.
• the electric field feels nano-structuring as a homogeneous medium of average index while in TE polarization the field distribution is locally disturbed.
• the amplitude of the electric field is maximum in the area of low index, in this case in the nitride; it is decreased by a factor approximately equal to ⁇ S i / £ Si3N4 in the zone of strong index, that is to say in silicon.
• This disturbance of the electric field and its location, mainly in areas where n is low, are the two factors responsible for the strong alteration of the effective index in TE polarization as the ffs ⁇ filler factor decreases.
• an optical waveguide structure actually makes it possible to control the birefringence, namely to reduce, cancel or precisely reverse the birefringence.
• This control is determined by the choice of the parameters of the lateral structuring of the guide layer of the waveguide structure. Depending on this choice, we have a control of the birefringence over a wide range.
• the waveguide that is to say the layer or layers resting on the support substrate, can have any section.
• the control of the birefringence can be exercised on waveguides of section and report of form arbitrary (rectangular waveguides, square, circular ).
• planar optical waveguides of submicron height can be manufactured which can easily be integrated into optical circuits, the substrate of the optical circuits forming the support substrate of the waveguides.
• the waveguide according to the invention can thus have a small footprint.
• These waveguides can be used in optoelectronic components to integrate in a compact way, and on the same chip, complex optical beam management functions such as multiplexing, demultiplexing, modulation, spectral routing ... These functions can be dedicated to optical links very short distances, at the millimeter scale (as for example for intra-chip communication), as for very long distances, of the order of several kilometers (for example for a communication in a metropolitan network).

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