Integrated Polarisation Splitter

Abstract

The invention relates to an integrated polarisation splitter based on a sub-wavelength multimode interference coupler (110), in other words, a multimode interference coupler (110) with an anisotropic multimode waveguide region formed by a plurality of sections of core material (210) and a plurality of sections of a cladding material (230) alternately arranged in a periodic way, with a period () smaller than the wavelength of a light propagated through said anisotropic region. The core material sections (210) are rotated an angle () greater than zero with respect to a perpendicular with an input waveguide (120) to increase the anisotropic character of the multimode waveguide region.

Claims

1. An integrated polarisation splitter comprising a multimode interference coupler (110) which in turn comprises: an input waveguide (120) intended to feed a transverse electric (TE) mode and a transverse magnetic (TM) mode of an optical signal to the multimode interference coupler (110), a first output waveguide (130) intended to transmit the transverse electric (TE) mode of the optical signal, a second output waveguide (140) intended to transmit the transverse magnetic (TM) mode of the optical signal, the splitter being characterised in that the multimode interference coupler (110) comprises an anisotropic multimode waveguide region formed by a plurality of sections of a core material (210) and a plurality of sections of a cladding material (230), respectively alternately arranged in a periodic way with a period () smaller than the wavelength of a light propagated through said anisotropic region; said anisotropic region generating a Talbot self-image of the transverse electric (TE) mode at a first distance (L.sub.TE), and a Talbot self-image of the transverse magnetic (TM) mode at a second distance (L.sub.TM), said second distance (L.sub.TM) being a whole number times greater than the first distance (L.sub.TE); and forming interfaces between the plurality of sections of core material (210) and the plurality of sections of cladding material (230) an angle () greater than zero with a plane perpendicular to the input waveguide (120), the first output waveguide (130) and the second output waveguide (140).

2. The polarisation splitter according to claim 1, characterised in that the second distance (L.sub.TM) is approximately two times greater than the first distance (L.sub.TE).

3. The polarisation splitter according to claim 2, characterised in that the anisotropic region of the multimode interference coupler (110) has a length that is approximately three times greater than the second distance (L.sub.TM).

4. The polarisation splitter according to any of the preceding claims, characterised in that the angle () is comprised between 5 and 25.

5. The polarisation splitter according to claim 4, characterised in that the angle () is comprised between 10 and 20.

6. The polarisation splitter according to any one of the preceding claims, characterised in that the plurality of sections of core material (210) comprise a central gap completed by cladding material (230).

7. The polarisation splitter according to any one of the preceding claims, characterised in that it comprises: a first taper (121) connected to the input waveguide (120); a second taper (131) connected to the first output waveguide (130); and a third taper (141) connected to the second output waveguide (140); wherein each one of: the first taper (121), the second taper (131) and the third taper (141), respectively, comprise: a waveguide with variable width, and sections of core material (210) and sections of cladding material (230) arranged in an alternating and periodic way.

8. The polarisation splitter according to claim 7, characterised in that the first taper (121), the second taper (131), the third taper (141), respectively, comprise a central bridge made of core material (210) that joins the sections of core material (210), the width of each central bridge being inversely proportional to the width of the respective taper (121, 131, 141).

9. The polarisation splitter according to any one of the claim 7 or 8, characterised in that the sections of core material (210) of the first taper (121), the second taper (131) and the third taper (141) are adapted to rotate from a plane perpendicular to the input waveguide (120), the first output waveguide (130) and the second output waveguide (140), to an angle (), said angle () being greater than zero.

10. The polarisation splitter according to any one of the preceding claims, characterised in that the anisotropic region of the multimode interference coupler (110) has constant geometric properties throughout the length thereof.

11. The polarisation splitter according to any one of claims 1 to 10, characterised in that the anisotropic region of the multimode interference coupler (110) comprises a variable geometry throughout the length thereof.

12. The polarisation splitter according to any one of the preceding claims, characterised in that at least one waveguide selected among: the input waveguide (120), the first output waveguide (130) and the second output waveguide (140) is arranged with an angle different than 90 in an interface with the multimode interference coupler (110).

13. The polarisation splitter according to any one of the preceding claims, characterised in that the core material (210) is silicon.

14. The polarisation splitter according to claim 13, characterised in that the core material (210) is arranged on an insulator layer.

Description

DESCRIPTION OF THE FIGURES

[0022] In order to assist in a better understanding of the characteristics of the invention according to a preferred practical exemplary embodiment thereof and to complement this description, the following figures, of illustrative and non-limiting nature, are attached:

[0023] FIG. 1 shows a multimode interference coupler with rotated sections according to a preferred embodiment of the invention.

[0024] FIG. 2 illustrates the main geometric parameters of a sub-wavelength structure.

[0025] FIG. 3 exemplifies the propagation principles of ordinary and extraordinary beams in an anisotropic structure.

[0026] FIG. 4 illustrates the effect of the rotation angle on the anisotropic properties of a SWG structure.

PREFERRED EMBODIMENT OF THE INVENTION

[0027] It must be noted that the invention is described with the device operating as a polarisation splitter. However, the same device can operate in a reciprocal way as a polarisation multiplexer, in other words, combining two orthogonal polarisation signals of two input waveguides in a single output waveguide, by simply reversing the operation direction of the device.

[0028] However, it must be noted that the polarisation splitter object of the invention is preferably implemented in silicon on insulator (SOI) to thus benefit from the high rate of contrast of SOI; however, particular embodiments could be implemented in other, different photonic platforms. In other words, all the waveguides of the device are preferably made by means of a silicon core, deposited on an insulator layer, such as for example silicon dioxide. The cladding 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.

[0029] With regard to the manufacture of the devices proposed, it must be noted that the sub-wavelength structures (SWG), 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 manufacture of conventional waveguides. In other words, all of the structures used by the polarisation splitter of the invention can be manufactured in a single etching step at a complete depth of any conventional microelectronic etching technique, for example by means of e-beam or deep-UV lithography.

[0030] FIG. 1 shows a first preferred embodiment of the invention based on a multimode interference coupler (110), more specifically a multimode interference coupler (110) provided with sub-wavelength grating (SWG) structures, the multimode waveguide region of which has anisotropic characteristics generated by a periodic arrangement of sections of a core material (210) and sections of cladding material (230) with a period () smaller than the wavelength of a light propagated through said anisotropic region. The polarisation splitter of the invention further comprises an input waveguide (120) that receives the transverse electric (TE) mode and the transverse magnetic (TM) mode, a first output waveguide (130) intended to transmit the transverse electric (TE) mode and a second output waveguide (140) intended to transmit the transverse magnetic (TM) mode. The input waveguide (130) is located on one end of the anisotropic region of the SWG multimode interference coupler (110), facing the first output waveguide (130), which is located on the opposite end of the SWG multimode interference coupler (110), while the second output waveguide (140) is also located on said opposite end of the multimode interference coupler (110), laterally separated from the first output waveguide (130).

[0031] With the aim of reducing the reflection losses in the interfaces of the multimode interference coupler (110), the polarisation splitter comprises a first taper (121), also known as mode adapter, connected to the input waveguide (120), a second taper (131) connected to the first output waveguide (130) and a third taper (141) connected to the second output waveguide (140). The first taper (121), the second taper (131) and the third taper (141) have SWG structures with the same period () and duty cycle (f) as the multimode interference coupler (110), while the width thereof progressively varies from the width of a single mode waveguide to a final width of the taper, preferably greater than the width of the single mode waveguide, and therefore, the SWG structures preferably have an incremental width. The first taper (121), the second taper (131) and the third taper (141) likewise comprise a central bridge, meaning a small connector made of core material (210) in the centre of the sections of cladding material (230). The width of the central bridge is reduced as the total width of the respective taper (121,131,141) increases, completely disappearing in the interface with the multimode region of the multimode interference coupler (110). It must be noted that the specific geometry of the taper (130) can vary among implementations, as long as smooth modal transition can be guaranteed, minimising reflections in the interfaces between the multimode interference coupler (110), the input waveguide (120) the first output waveguide (130) and the second output waveguide (140).

[0032] FIG. 2 shows the main parameters of any SWG structure in more detail, which can be adjusted by means of photonic simulations in order to engineer the refractive index and dispersion. Specifically, said SWG parameters include the width of the waveguide (W), the height of the waveguide (H), the period () and the duty cycle (f). The duty cycle, also known as fill factor, is the relationship between a length (a) of the section of core material (210), and a length (b) of the section of cladding material (230), within a period (). Both the sections of core material (210) as well as the sections of cladding material (230) are arranged on at least one layer of insulator material (220).

[0033] FIG. 3 schematically illustrates the behaviour of any anisotropic material (300), applicable to the comprehension of the anisotropic multimode waveguide region of the multimode interference coupler (110) of the invention. When an incident beam (310) reaches an interface with the anisotropic material (300) with an angle of incidence (.sub.i), one part of the optical power is reflected, generating a single conventional reflected beam (320), with an angle of reflection (.sub.r). However, the power transmitted to the anisotropic material (300) does not result in a single beam, as occurs in conventional isotropic materials, rather it generates two beams with different angles: an ordinary beam (330) with polarisation parallel to the interface (in other words, TE polarisation), and a first angle of transmission (.sub.TE); and an extraordinary beam (340) with polarisation perpendicular to the interface (in other words, TM polarisation), and a second angle of transmission (.sub.TM). The difference between the first angle of transmission (.sub.TE) and the second angle of transmission (.sub.TM), as well as the difference of effective index perceived by the two modes, are used in the anisotropic multimode waveguide region of the multimode interference coupler (110) of the invention for selecting the proportionality relationship between the distances at which the Talbot self-imaging of the modes are generated.

[0034] In other words, the anisotropic SWG region of the multimode interference coupler (110) generates a Talbot self-image of the transverse electric (TE) mode at a first distance (L.sub.TE), and a Talbot self-image of the transverse magnetic (TM) mode at a second distance (L.sub.TE). Selecting the geometric parameters of the SWG structure of said multimode interference coupler (110) by means of photonic simulation, in other words, the height of the waveguide (H), the period () and the duty cycle (f), results in the second distance (L.sub.TM) being a whole number times greater than the first distance (L.sub.TE). In this preferred embodiment, the second distance (L.sub.TM) can be approximately two times greater than the first distance (L.sub.TM). By imposing a total length of the multimode interference coupler (110) a whole number times greater than the second distance (L.sub.TM), the TE mode is focalised on the first output waveguide (130) and the TM mode on the second output waveguide (140). Furthermore, the greater the anisotropic behaviour of the material, the more unequal the relationship between the first distance (L.sub.TE) and the second distance (L.sub.TM) will be, and therefore, the more compact the resulting multimode interference coupler (110) will be.

[0035] The anisotropic behaviour of the multimode interference coupler (110) of the invention (in other words, the difference between the effective refractive indices for the TE and TM modes of order 0 and order 1) can be controlled by rotating the SWG structure of the anisotropic region an angle (). In other words, interfaces between the plurality of sections of the core material (210) and the plurality of sections of cladding material (230) form an angle () greater than zero with a plane that is perpendicular to the optical axis of the system, meaning, to the axis defined by the direction of the input waveguide (120), the first output waveguide (130) and the second output waveguide (140) at the input to the multimode interference coupler (110). The section of an angle () greater than zero allows the beat lengths of the TE and TM modes to be adjusted so that they are proportional in a double factor, thereby optimising the length of the multimode interference coupler (110).

[0036] It must be noted that in alternative embodiments of the invention, the input waveguide (120), the first output waveguide (130) and the second output waveguide (140) can have angles different than 90 in the interface thereof with the multimode interference coupler (110). In other words, the device of FIG. 4, the angle of incidence (.sub.i) with the average anisotrope is 0, corresponding to a perpendicular incidence. However, in other embodiments, said angle of incidence (.sub.i) can be increased by rotating the input waveguide (120), thereby modifying the first angle of transmission (.sub.TE) and the second angle of transmission (.sub.TM). By modifying said angles, also modified are the first distance (L.sub.TE) and the second distance (L.sub.TM) at which Talbot self-images are generated, enabling to thereby reduce the size of the resulting device.

[0037] Likewise, it is possible to implement the SWG anisotropic region of the multimode interference coupler (110) including one or more gaps in the sections of the core material (210), said gaps being filled by the cladding material (230). For example, in a preferred embodiment, each section of the sections of core material (210) schematically shown in the FIGS. 1 and 4 can be divided into two sections, separated by a central gap filled by the cladding material (230). The number and arrangement of said gaps can vary between implementations.

[0038] Regardless of the particular geometry of the multimode interference coupler (110), when the SWG structure of the anisotropic region is rotated an angle (), it is recommendable to modify the first taper (121), the second taper (131) and the third taper (141), with respect to the case of the perpendicular SWG structure shown in FIG. 1, with the aim of optimising the transitions between said multimode interference coupler (110), the input waveguide (120), the first output waveguide (130) and the second output waveguide (140). To do so, said first taper (121), second taper (131) and third taper (141) comprise SWG structures that are progressively rotated between 0 at the end farthest from the multimode interference coupler (110), and the angle () at the end in contact with said multimode interference coupler (110).

[0039] The design of the device is preferably done by means of a photonic simulation process in two stages. In the first stage, a first approximation of the design is obtained by means of the simulation of the SWG structure as a homogenous and anisotropic material. The anisotropic material used is defined by a dielectric tensor, obtained by means of Rytov approximation for the effective indices of the ordinary ray (330) and the extraordinary ray (340). In particular, preferably selected is a multimodal region of an initial width approximately three times greater than that of the taper and a waveguide height (H) defined by the photonic platform on which the device is implemented. Establishing these starting parameters, photonic simulations are carried out for a sweep of rotation angles (), thereby modelling the evolution of the anisotropic properties of the MMI for the particular case of the application of the device. This first approximation allows simulations to be carried out in three dimensions reducing the computational time and cost of the simulation. Although the use of waveguide heights of 260 nm are recommended in order to facilitate the confinement of the transverse magnetic (TM) mode, it must be noted that the device of the invention can adapt to other heights of the waveguide (H).

[0040] Once the anisotropic properties are modelled at different angles for the specific geometry and platform under analysis, said modelling is applied to a first optimisation by means of a sweep of different parameters, such as the width of the multimodal region, 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 minimisation of total insertion losses, as well as the verification of a proportionality factor as close as possible to 2 between the beat lengths of the transverse electric (TE) and transverse magnetic (TM) modes. As a result of this optimisation, an approximation of the initial design parameters (angle, width of the multimodal region, period, duty cycle and length of the device) is obtained.

[0041] From the design parameters obtained in the first stage, the physical modelling of the complete SWG structure (without approximations to the homogenous medium) is then carried out, which provides us with the final design of the polarisation splitter. Although this process can be done maintaining the degrees of freedom of all of 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 region (for example at 4 microns), the duty cycle (for example 0.5 so as to simplify the manufacturing), and the period (always imposing that said period is outside the Bragg regime, determined by the proportionality relationship between the period of the SWG structure; and the effective wavelength of the light propagated through said structure).

[0042] The multiplicity between the beat lengths is then optimised by means of fine tuning (in other words, by means of sweeps of less variability than in the first stage) of the rotation angle and the number of periods (and in the case of not having been established according to the recommendation of the previous step, of the duty cycle, the width of the multimodal region, and/or the SWG period). As an objective of this optimisation, a minimisation of the beat length deviation of the transverse magnetic mode is imposed with respect to twice the beat length of the transverse electric mode. This second optimisation process is preferably done using three-dimensional finite difference time domain method (FDTD), although other photonic computation techniques can likewise be applied to the same.

[0043] Although the exact effect of the rotation of the SWG structure depends on the properties of the materials that make up the same, as well as the rest of the geometric parameters, FIG. 4 exemplifies this effect for the case of a SWG structure of silicon on insulator with a wavelength height of 260 nm, a width of 4 microns, a period of 220 nm and a fill factor of 0.5, for a wavelength of 1550 nm. FIG. 4a shows the effective refraction indices (n.sub.eff) as a function of the angle () for a TE mode of the order 0 (TE0) and for a TE mode of the order 1 (TE1). FIG. 4b shows the effective refraction indices (n.sub.eff) as a function of the angle () for a TM mode of the order 0 (TM0) and for a TM mode of the order 1 (TE1). It must be noted that an increase in the rotation angle () reduces the effective indices (n.sub.eff) of the TE0 and TE1 modes and increases the effective indices (n.sub.eff) of the TM0 and TM1 modes, the effect on the TE polarisation being also significantly greater than on the TM polarisation.