WAVEGUIDE, WAVEGUIDE PRODUCTION METHOD, AND POLARISATION SPLITTER USING SAID WAVEGUIDE

Abstract

The invention relates to a waveguide and a polarisation splitter based on said waveguide, in which a rotation of an angle greater than zero is applied to a plurality of sections of a core material and a plurality of sections of a covering material, thereby achieving an independent control of the refractive indices of a zero-order transverse electric mode and a zero-order transverse magnetic mode. This document also describes a manufacturing method of said waveguide which allows the birefringence of the light that passes through the waveguide.

Claims

1. A polarisation splitter comprising: a first waveguide comprising a first plurality of sections of a core material and a first plurality of sections of a covering material alternately arranged in a periodic way with a period smaller than a wavelength of a light guided through the first waveguide; a second waveguide with at least a second portion thereof adjacently arranged at a distance from at least a first portion of the first waveguide, the second waveguide comprising a second plurality of sections of a core material and a second plurality of sections of a covering material alternately arranged in a periodic way with a period smaller than the wavelength of the light guided through the second waveguide; and a central structure arranged between at least the first and second portions of the first and second waveguides, the central structure comprising a third plurality of sections of a core material and a third plurality of sections of a covering material alternately arranged in a periodic way with a period smaller than the wavelength of the light guided through the central structure; wherein the first and second pluralities of sections of core material and the first and second pluralities of sections of covering material respectively have constant geometric properties at least throughout the length of the first and second portions of the first and second waveguides; wherein interfaces defined between each section of core material and each section of covering material of at least one of the first, second and third pluralities of sections of core material and the respective plurality of sections of covering material form an angle greater than zero with respect to a plane perpendicular to a propagation direction of the light guided by at least one of the first and second waveguides.

2. The polarisation splitter of claim 1, wherein the angle is selected such that a first effective index of a zero-order transverse electric mode is different from a second effective index of a zero-order transverse magnetic mode.

3. The polarisation splitter of claim 1, wherein the angle is selected such that a first effective index of a zero-order transverse electric mode is equal to a second effective index of a zero-order transverse magnetic mode.

4. The polarisation splitter of claim 1, wherein the first waveguide comprises a first modal adapter at an input of the first waveguide, the first modal adapter comprising a plurality of sections of core material and sections of covering material arranged in an alternated and periodic way with a period smaller than the wavelength of the light guided through the first modal adapter.

5. The polarisation splitter of claim 4, wherein the first and second waveguides comprise second and third modal adapters, respectively, at respective outputs of the first and second waveguides, each of the second and third modal adapters comprising a plurality of sections of core material and sections of covering material arranged in an alternated and periodic way with a period smaller than the wavelength of the light guided through the second and third modal adapters, respectively.

6. The polarisation splitter of claim 5, wherein the sections of core material of at least one of the first, second and third modal adapters have a variable width.

7. The polarisation splitter of claim 1, wherein the interfaces of the first plurality of sections of core material and the first plurality of sections of covering material form the angle greater than zero.

8. The polarisation splitter of claim 1, wherein the interfaces of at least one of the second and third pluralities of sections of core material and the respective plurality of sections of covering material form the angle greater than zero.

9. The polarisation splitter of claim 1, wherein a duty cycle of the central structure is different from a duty cycle of one or both of the first and second waveguides.

10. The polarisation splitter of claim 9, wherein the first waveguide has a same duty cycle as the second waveguide.

11. A polarisation splitter comprising: a first waveguide, the first waveguide comprising a plurality of sections of a core material and a plurality of sections of a covering material alternately arranged in a periodic way with a period smaller than a wavelength of a light guided through the waveguide; a subwavelength grating, SWG, structure adjacent to at least a portion of the first waveguide, the SWG structure comprising a plurality of sections of a core material and a plurality of sections of a covering material alternately arranged in a periodic way with a period smaller than a wavelength of the light guided through the central structure; and a second waveguide with at least a portion thereof adjacent to the SWG waveguide, the second waveguide comprising a plurality of sections of a core material and a plurality of sections of a covering material alternately arranged in a periodic way with a period smaller than the wavelength of the light guided through the waveguide; wherein interfaces defined between each section of core material and each section of covering material of the plurality of sections of core material of the second waveguide or SWG structure and the plurality of sections of covering material of the second waveguide or SWG structure form an angle greater than zero with respect to a plane perpendicular to a propagation direction of the light guided by at least one of the first and second waveguides; wherein the plurality of sections of core material and the plurality of sections of covering material of at least one of the first and second waveguides have constant geometric properties throughout at least part of a length thereof.

12. The polarisation splitter of claim 11, wherein the angle is selected such that a first effective index of a zero-order transverse electric mode is different from a second effective index of a zero-order transverse magnetic mode.

13. The polarisation splitter of claim 11, wherein the angle is selected such that a first effective index of a zero-order transverse electric mode is equal to a second effective index of a zero-order transverse magnetic mode.

14. The polarisation splitter of claim 11, wherein the first waveguide comprises a first modal adapter at an input of the first waveguide and a second modal adapter at an output of the first waveguide, the first and second modal adapters each comprising a plurality of sections of core material and sections of covering material arranged in an alternated and periodic way with a period smaller than the wavelength of the light guided through the first modal adapter.

15. The polarisation splitter of claim 14, wherein the second waveguide comprises a third modal adapter at an output of the second waveguide, the third modal adapter comprising a plurality of sections of core material and sections of covering material arranged in an alternated and periodic way with a period smaller than the wavelength of the light guided through the third modal adapter.

16. The polarisation splitter of claim 11, wherein at least the interfaces of the plurality of sections of core material and the plurality of sections of covering material of the SWG structure form the angle greater than zero.

17. The polarisation splitter of claim 16, wherein the interfaces of the plurality of sections of core material and the plurality of sections of covering material of the second waveguide form the angle greater than zero.

18. The polarisation splitter of claim 11, wherein a duty cycle of the central structure is different from a duty cycle of one or both of the first and second waveguides.

19. A polarisation splitter comprising: a first waveguide, the first waveguide comprising a plurality of sections of a core material and a plurality of sections of a covering material alternately arranged in a periodic way with a period smaller than a wavelength of a light guided through the waveguide; and a second waveguide with at least part of a length thereof adjacent to the SWG waveguide, the second waveguide comprising a plurality of sections of a core material and a plurality of sections of a covering material alternately arranged in a periodic way with a period smaller than the wavelength of the light guided through the waveguide; wherein interfaces defined between each section of core material and each section of covering material of the plurality of sections of core material and the plurality of sections of covering material of either the first waveguide or the second waveguide form an angle greater than zero with respect to a plane perpendicular to a propagation direction of the light guided by the first waveguide; wherein the plurality of sections of the core material and the plurality of sections of the covering material of at least one of the first and second waveguides have constant geometric properties throughout at least part of a length thereof.

20. The polarisation splitter of claim 19, further comprising a third waveguide with at least part of a length thereof adjacent to the second waveguide; wherein an end of the second waveguide or the third waveguides is adapted to output a zero-order transverse magnetic mode when zero-order transverse electric and magnetic modes are inputted at an end of the first waveguide.

Description

DESCRIPTION OF THE FIGURES

[0031] With the aim of helping improve the comprehension of the features of the invention according to a preferred practical embodiment thereof, and to complement this description, the following figures are attached as an integral part of the same, with an illustrative and non-limiting character:

[0032] FIG. 1 schematically shows, in a perspective view, birefringence waveguides modified by means of the rotation of sub-wavelength grating structures according to a preferred embodiment of the invention, illustrating the main geometric parameters of said structure.

[0033] FIG. 2 illustrates, in an upper view, a preferred embodiment of the modified birefringence waveguide of the invention, wherein the rotation angle is selected such that a polarisation-independent behaviour is obtained.

[0034] FIG. 3 shows an integrated polarisation splitter based on directional coupling with a modified birefringence waveguide, according to a preferred embodiment of the invention.

[0035] FIG. 4 illustrates the effect of the rotation angle on the anisotropic properties of a SWG structure.

[0036] FIG. 5 illustrates a chart showing the variation of the first effective index (n.sub.TE) for the zero-order transverse electric mode (TE.sub.0) and a second effective index (n.sub.TM) for the zero-order transverse magnetic mode (TM.sub.0) based on the rotation angle (α) of the SWG structure.

PREFERRED EMBODIMENT OF THE INVENTION

[0037] Note that the preferred embodiments of the device (modified birefringence single mode waveguide) and of the associated system thereof (polarisation splitter based on said guide) object of the invention is preferably implemented in silicon on insulator (SOI) to thus benefit from the high contrast of the SOI index. However, specific embodiments could be implemented in other different photonic platforms. In other words, all of the waveguides of the device are preferably made by means of a silicon core, deposited on an insulating layer, such as silicon dioxide. The covering material can vary for different embodiments of the invention, some of the possibilities being silicon dioxide, polymers or air, without this list limiting the use of other possible options.

[0038] Likewise, it must be noted that the preferred embodiments of the polarisation splitter of the invention are described with the system operating as a polarisation splitter. However, the same system can operate in a reciprocal manner as a polarisation multiplexer, in other words, combining two orthogonal polarisation signals of two input guides in a single output guide, simply by reversing the operating direction of the device.

[0039] With regard to the manufacturing of the devices proposed, it must be noted that the sub-wavelength grating (SWG) structures, independent of the rotation angle thereof with respect to the optical axis, do not increase the difficulty or the number of steps with respect to the manufacturing of conventional waveguides. In other words, all of the structures used by the polarisation splitter of the invention can be manufactured in a single exposure step at a complete depth with any conventional microelectronic etching technique, for example by means of exposure to e-beams or deep-UV.

[0040] FIG. 1 shows a schematic view of a preferred embodiment of a waveguide (100), preferably single mode, which allows the birefringence of the light that passes through at least part of said waveguide (100) object of a first aspect of the invention to be modified, likewise used in a preferred embodiment of the polarisation splitter (200) object of a second aspect of the invention, as well as the main geometric parameters thereof.

[0041] The waveguide (100) is formed by a periodic alternative arrangement of sections of a core material (110) and sections of a covering material (120) with a period (Λ) smaller than the wavelength of a light propagated by said periodic arrangement of the waveguide (100); further having, arranged between the sections of core material (110) and sections of a covering material (120), interfaces that are rotated an angle (a) with respect to a plane perpendicular to the propagation direction of the light (meaning, perpendicular to the main axis of the waveguide). The core material sections (110) of the wave guide (100) have a height (H) and a width (W) and are arranged on a support material (130) which acts as an insulating layer. On this structure, a filling factor or duty cycle (f) is defined as the ratio between the proportion of covering material (a) and the proportion of core material (b) within a period (A). The specific values of all of these geometric parameters are defined prior to the manufacturing of the device by means of numeric simulations according to that described by the method of the invention.

[0042] Specifically, the waveguide (100) is preferably modeled as a rotated tensor (ñ) according to the following expression:

[00006]$\tilde{n}=\left[\begin{array}{ccc}{\tilde{n}}_{xx}& 0& {\tilde{n}}_{xz}\\ 0& {\tilde{n}}_{yy}& 0\\ {\tilde{n}}_{xz}& 0& {\tilde{n}}_{zz}\end{array}\right]$

where:

[00007]${\tilde{n}}_{xx}=\sqrt{n_{xx}^{2}co{s}^2\left(\alpha \right)+n_{zz}^{2}si{n}^2\left(\alpha \right)},$

[00008]${\tilde{n}}_{yy}=n_{yy},$

[00009]${\tilde{n}}_{zz}=\sqrt{n_{xx}^{2}si{n}^2\left(\alpha \right)+n_{zz}^{2}co{s}^2\left(\alpha \right)},$

[00010]${\tilde{n}}_{xz}=\sqrt{\left(n_{zz}^2-n_{zz}^2\right)sin\left(\alpha \right)cos\left(\alpha \right)},$

[n.sub.xx, n.sub.yy, n.sub.zz] being components of a diagonal tensor (n) of the homogeneous anisotropic medium equivalent to the SWG structure without rotating (meaning, with interfaces perpendicular to the main axis of the waveguide). This modeling can provide a final value of the angle (a), or serve as a first approximation, refined in a second calculation step by means of a complete simulation (meaning, without approximations of SWG structures as homogeneous means) of the structure.

[0043] FIG. 2 shows a schematic upper view of a preferred embodiment of the waveguide (100) of the invention (meaning, a view on a plane parallel to the interface between the support (130) and the core (110), and in which the light therefore propagates following the horizontal direction of the figure). The waveguide (100) has an effective index (n.sub.TE) for the zero-order transverse electric mode (TE.sub.0) and a second effective index (n.sub.TM) for the zero-order transverse magnetic mode (TM.sub.0) for each wavelength (A). Depending on the angle (a), for specific geometric and material conditions, the first effective index (n.sub.TE) can be equal to the second effective index (n.sub.TM), or significantly different from said second effective index (n.sub.TM). The necessary difference in order for it to be considered a “significantly different” value depends on the specific application of the device, but in general, this expression must be understood as a difference of propagation characteristics that allows for differential processing of the TE.sub.0 y TM.sub.0 modes. For example, in the case of using the waveguide (100) for implementing a polarisation splitter (200), “significantly different” is understood as an effective index difference that gives rise to the lateral coupling of the TM.sub.0 mode between adjacent guides, while said coupling does not occur in the TE.sub.0 mode (due to a greater first effective index of said TE.sub.0 mode with respect to the second effective index of the TM.sub.0 mode and, thus, a smaller modal diameter).

[0044] With the aim of reducing the reflection losses in the interfaces of the waveguide (100), said waveguide (100) can comprise a modal adapter (140) on each end. Each modal adapter (140) has at least one SWG structure with the same period (Λ) and duty cycle (f) as the waveguide (100), while the width thereof progressively varies from the width of a single mode waveguide to a final width of a modal adapter (140), preferably greater than the width of the single mode waveguide, and thus, preferably, the SWG structures of the modal adapter (140) have an incremental width. Each modal adapter (140) likewise comprises a central bridge (141), which is a small connector made of core material in the center of the sections of covering material (120). The width of the central bridge reduces as the total width of the respective modal adapter (140) increases, completely disappearing in the interface with the rotated SWG structure. Note that the specific geometry of the modal adapter (140) can vary between implementations, as long as a progressive and smooth modal transition is guaranteed.

[0045] FIG. 3 shows a specific embodiment of a polarisation splitter (200) corresponding to a second aspect of the invention and comprises a directional coupler which, in turn, comprises, preferably adjacently arranged, a first single mode guide (210) and a second single mode guide (220), said second single mode guide (220) being a specific embodiment of the waveguide (100) of the first aspect of the invention. In this case, the first single mode guide (210) is a SWG waveguide with interfaces perpendicular to the axis of the guided light, with modal adapters (140) located on the ends thereof, although in other embodiments of the invention, the first single mode guide (210) can be a continuous waveguide or comprise SWG structures with other alternative geometries.

[0046] Additionally, the polarisation splitter (200) can comprise a central structure (230) arranged between the first single mode guide (210) and the second single mode guide (220), which allows the polarisation-selective properties of the system to be enhanced and the features thereof to be improved. In this example, the central structure (230) is simply a rectangular segment, preferably homogeneous of the same material as the sections of core material (110). However, in other specific embodiments, the central structure (230) can be implemented by means of other alternative geometries and/or materials.

[0047] During the operation as a polarisation splitter (200), the TE.sub.0 and TM.sub.0 modes are introduced in the same input of the first single mode waveguide (210). While the TM.sub.0 mode couples to the second single mode waveguide (220), following the general behaviour of any directional coupler, the TE.sub.0 mode is confined to the first single mode guide (210), thereby obtaining a highly efficient polarisation split in an extremely reduced size.

[0048] The manufacturing process of the polarisation splitting system (200) is preferably carried out by means of a photonic simulation process consisting of two steps. In a first step, a first approximation of the design is obtained by a simulation of the SWG structure as a homogeneous and anisotropic material. The anisotropic material used is defined by a dielectric tensor, obtained by means of Rytov approximation. Establishing these starting parameters, photonic simulations are carried out for a sweep of rotation angles (a), modeling the effect of said rotation by means of the expressions described for the waveguide (100). This first approximation allows simulations to be carried out in three dimensions reducing the computational time and cost of the simulation.

[0049] Once the anisotropic properties are modeled at different angles for the specific geometry and platform under analysis, said modeling is applied to a first optimisation by means of a sweep of different parameters, such as the width of the multimodal zone, the duty cycle of the corresponding real SWG structure or the length of the polarisation splitter. Said first optimisation process is done by executing sweeps of the aforementioned parameters and imposing as objectives the minimization of total insertion losses. As a result of this optimisation, an approximation of the initial design parameters (angle, separation between guides, period, duty cycle and length of the device) is obtained.

[0050] From the design parameters obtained in the first step, the physical modeling of the complete SWG structure (without approximations to the homogeneous medium) is then carried out, which provides us with the final design of the polarisation splitter (200). Although this process can be done maintaining the degrees of freedom of all the design parameters, it is recommended that this second optimisation process be simplified in order to reduce the computational load of the same, establishing the width of the multimodal zone, the duty cycle, and the period (always imposing that said period be outside the Bragg regime, determined by the proportionality ratio between the period of the SWG structure and the effective wavelength of the light propagated by said structure). This second optimisation process is preferably done using three-dimensional finite difference time domain methods (FDTD), although other photonic computational techniques may likewise be applied to the same.

[0051] FIG. 4 shows a variant of the polarisation splitter (200) of the invention, wherein the waveguide (100) with which the second waveguide (220) is implemented comprises a plurality of central attachments (140) that connect the sections of core material (110) of the SWG structure. The plurality of central attachments (140), and specifically the selection of the width thereof provides an additional degree of freedom in the design of the polarisation splitter (200), increasing the tuning of the first effective index (n.sub.TE) for the zero-order transverse electric mode (TE.sub.0) and of the second effective index (n.sub.TM) for the zero-order transverse magnetic mode (TM.sub.0).

[0052] Lastly, FIG. 5 schematically shows a variation of the first effective index (n.sub.TE) for the zero-order transverse electric mode (TE.sub.0) and a second effective index (n.sub.TM) for the zero-order transverse magnetic mode (TM.sub.0) based on the rotation angle (α) of the SWG structure. In spite of the fact that the specific dependencies of these indices with the angle (a) vary based on the materials used and the rest of the geometric properties, it must be mentioned that the effect of said rotation is greater for the TE.sub.0 mode than for the TM.sub.0 mode, and furthermore the variation thereof have opposite signs. This allows, on the one hand, the properties of the first effective index (n.sub.TE) to be altered without hardly modifying the response of the second effective index (n.sub.TM), thereby developing polarisation-selective devices; and on the other hand, determining an angle for polarisation independence (α.sub.ind) in which it is verified that both the first effective index (n.sub.TE) and the second effective index (n.sub.TM) are equal to each other, taking a value of the polarisation-independent effective index (n.sub.ind).

[0053] In light of this description and figures, a person skilled in the art will be able to understand that the invention has been described according to some preferred embodiments of the same, but multiple variations may be introduced in said preferred embodiments, without detracting from the object of the invention as claimed.