Patent Application
20230085272
This application claims priority under 35 USC § 119 to European Patent Application No. 21194821.1 filed on Sep. 3, 2021, which is hereby incorporated by reference in their entirety.
The invention relates to optical elements that comprise a plurality of perturbing centers arranged in a scattering plane.
Modern high-speed communication systems are entirely based on fiber-optic transmission technology. Depending on the link distance, we distinguish between two basic modulation techniques. Long-haul systems rely on spectrally efficient coherent modulation formats, where a signal is coded in amplitude and phase. By contrast, short-reach systems use intensity-modulated direct detection (IM-DD) scheme, in which bits are coded only in the signal amplitude. Currently, the internet traffic growth takes place predominantly in the short-distance inter- or intra-data center interconnect systems, where applications such as video streaming, online meetings, social media, virtual marketplaces etc. are being increasingly used. For that reason, the coherent formats are expected to enter the data center domain, since they allow for a better scalability and power efficiency than the traditional IM-DD.
The largest constraint on the way towards coherent formats for data center interconnects (DCIs) is the total cost of the required systems architecture. Several competing platforms promise for cost reductions by developing on-chip solutions for integrated coherent transmitters and receivers. The highest integration density so-far is offered by silicon photonics. A key advantage hides behind the co-integration of photonics and electronics on existing (Bi)CMOS foundries, by using a mature fabrication process flow. In silicon-based integrated transmitters and receivers, optical in-and out-coupling to a standard single-mode-fiber (SMF) supporting two orthogonal polarizations is advantageous. Optical coupling can face on the one hand issues such as coupling loss, polarization-dependent-loss (PDL), limited bandwidth, excitation of undesired modes, polarization crosstalk and on the other hand fabrication complexity, process variations intolerance, complex packaging and restriction on the wafer-scale characterization. The trade-off between these aspects depends on the coupling scheme of choice.
An objective of the present invention is to provide an optical element with improved optical characteristics.
An embodiment of the present invention relates to an optical element comprising a plurality of perturbing centers arranged in a scattering plane of the optical element and configured to effect out-of-plane diffraction of an optical wave propagating in the scattering plane to a diffraction direction having at least two different direction components, one of the direction components being directed perpendicular to the scattering plane. The optical element comprises at least two oriented groups of perturbing centers, wherein a group-individual orientation is assigned to each oriented group. The perturbing centers of each oriented group are oriented in accordance with the same group-individual orientation. The group-individual orientations are angled relatively to one another. An advantage of the above embodiment is that oriented perturbing centers provide additional design options to improve the device's performance. When an optical wave propagates through the optical element's plane, the scattering plane, a part of its power is redirected in nearly vertical direction out of this plane. The latter process is an out-of-plane diffraction and is associated with a propagation direction pointing away from the scattering plane, as governed by the laws of diffraction. In other words, the direction of the diffracted optical wave can be described as having at least two different direction components, one of which being directed perpendicular to the scattering plane. This results in a diffracted wave propagating at an angle to the scattering plane, which may for instance lie in the range between 70° and nearly 90°.
It is noted that, as usual in optics, the above-described configuration is also effective in the opposite direction of propagation. In other words, an incoming out-of-plane optical wave is diffracted to further propagate in-plane, i.e., in the optical element's plane.
In addition, a part of the optical power is scattered inside the optical element's plane: a process, which is called here in-plane scattering. While the out-of-plane diffraction is a desired effect, the in-plane scattering is not desired and in fact limits the light diffraction efficiency for the out-of-plane diffraction. The proposed optical element has two advantageous effects: 1) it ensures a good out-of-plane diffraction efficiency, 2) it suppresses the parasitic, in-plane scattered light.
In-plane scattering is stronger, when there is an array of periodic identical objects, which scatter light in the same in-plane direction. When we use objects (perturbing centers), which scatter light in different directions, a forwards-scattered wave can be compensated by a backwards-scattered wave within the optical element's plane. A precise design of the perturbing centers' dimensions may lead to a complete compensation of forward- and backwards-scattered waves, in particular by use of destructive interference. Oriented perturbing elements such as for instance elliptical or oval perturbing centers instead of or in addition to circular perturbing centers make it possible that adjacent perturbing centers have an abruptly different in-plane scattering pattern. Thus, we may avoid constructive superposition of in-plane scattered light. The reduced in-plane scattered light helps improving the out-of-plane diffraction efficiency.
A further advantage of the above embodiment in comparison with prior-art solutions is that polarizations of out-coupled optical waves are orthogonal to each other over a large range of wavelengths. Prior-art solutions, except for one specific wavelength, transform different input polarizations into non-orthogonal output polarizations.
The angle between adjacent group-individual orientations preferably equals 180° divided by the number of group-individual orientations. For instance, in case of three groups, the angle between adjacent group-individual orientations preferably equals 60°.
A first access side of the optical element may provide a first access port for inputting and/or outputting radiation along a first direction that lies in the scattering plane. The optical element may also have a second access port for inputting and/or outputting radiation along a second direction that differs from the first direction and also lies in the scattering plane.
The arrangement of the perturbing centers in the scattering plane is preferably axially symmetric with respect to the first and second direction. Additionally or alternatively, the arrangement of the perturbing centers in the scattering plane may be axially symmetric with respect to a mirror axis that mirrors the first and second direction with respect to one another.
The first and second direction are preferably angled by an angle between 80° and 90°.
According to a first preferred variant, the perturbing centers may form an array of perturbing centers where the distance is equal.
According to a second preferred variant, the scattering centers may form an array of perturbing centers where the distance between adjacent perturbing centers varies.
According to a third preferred variant, the scattering centers may form an array of perturbing centers where the distance between adjacent perturbing centers is smaller in the array's center than at the array's edge.
According to a fourth preferred variant, the perturbing centers may form an array of perturbing centers where the distance between adjacent perturbing centers increases from the array's center towards the array's edge.
According to a fifth preferred variant, the perturbing centers form different segments. In each segment the perturbing elements are equally sized and at an equal mutual distance. However, the size and mutual distance of the perturbing centers increase with each segment, starting with respective lowest values from any of the access ports and increasing with every next segment at increasing distance from the given access port.
All of the perturbing centers that belong to the same group are preferably identically shaped.
The perturbing centers of the oriented groups are preferably rotationally asymmetric or non-circular.
The oriented perturbing centers are preferably axially symmetric with respect to the group-individual orientation of their group.
All perturbing centers of the oriented groups are preferably identically shaped and/or identically sized.
The oriented perturbing centers are preferably elongated along the respective group-individual orientation.
The perturbing centers of at least one group of perturbing centers are preferably elliptical, or rhombic, or oval.
The optical element preferably also comprises at least one un-oriented group of un-oriented (e. g. symmetric, preferably circular or starlike) perturbing centers.
According to an exemplary embodiment, the optical element comprises a first group of perturbing centers and a second group of perturbing centers, wherein the perturbing centers of the first group are oriented along a first orientation, wherein the perturbing centers of the second group are oriented along a second orientation, and wherein the first orientation and the second orientation are angled by 90°. The optical element preferably also comprises a third group of perturbing elements without any orientation.
According to an another exemplary embodiment, the optical element comprises:
The latter embodiment preferably also comprises a fifth group of un-oriented perturbing centers.
The oriented groups of perturbing centers may overlap each other in space such that individual perturbing centers of different groups are arranged in an interweaved manner. For instance, two oriented groups of perturbing centers may overlap in that the individual perturbing centers of the groups are arranged alternately to form pairs of neighboring perturbing centers.
In particular, where there are two interweaved oriented groups, the orientations of neighboring perturbing centers in the direction of propagation of the optical wave may alternate such that the perturbing centers are oriented at non-orthogonal angles with respect to the direction of propagation, wherein the perturbing centers of a given group may all have the same orientation.
Furthermore, where oriented groups of perturbing centers are interweaved, a first interweaved pair of the oriented groups of perturbation centers forms a first segment having a two-dimensional geometrical shape of perturbation centers having at least three edges, the geometrical shape being in particular a rectangular shape or a square shape. A second and any further interweaved pair of the oriented groups of perturbation centers adds a respective angled fringe segment extending the geometrical shape of the first segment along two edges of the first segment. A spatial repetition period of the perturbation centers and a size of the individual perturbation centers decreases with increasing order number of the segments.
A first access side, which provides access from directions out of the optical element's plane may provide an access port for inputting radiation having a first mode with a given polarization state. A second access side of the optical element may provide a first and second access port, which are aligned with the optical element's plane. The perturbing centers are preferably arranged such that each of the latter ports of the second access side outputs radiation having said first polarization and at least a second polarization in response to the radiation that is input at the first access side. The connectivity between the first access side and the two ports of the second access side is enabled by an out-of-plane diffraction.
The optical element may be a polarization multiplexer or de-multiplexer. In the case of a multiplexer, a first access side of the optical element preferably provides a first access port for inputting and/or outputting radiation having a first polarization, and a second access port for inputting and/or outputting radiation having a second polarization that differs from the first polarization. Both ports are aligned with the optical element's plane. A second access side of the optical element preferably provides an access port for inputting and/or outputting radiation of both, the first and second polarization.
The first polarization is preferably perpendicular to the second polarization.
In order that the manner in which the above-recited and other advantages of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are therefore not to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail by the use of the accompanying drawings in which:
The preferred embodiments of the present invention will be best understood by reference to the drawings. It will be readily understood that the present invention, as generally described and illustrated in the figures herein, could vary in a wide range. Thus, the following more detailed description of the exemplary embodiments of the present invention, as represented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the invention.
In general, when light is incident on an object, two effects can result—diffraction and scattering. Prior art grating couplers are generally considered as diffracting structures only, i.e. it is expected that incident light in the grating plane is completely diffracted out of the grating plane and vice versa. However, when we look at typical two-dimensional grating couplers according to prior art (hereinafter referred to as “prior art 2D GC”), we see that every row consists of discrete elements with sizes smaller than the light spot size of the waveguide mode. For that reason, in-plane scattering will occur in parallel with out-of-plane diffraction. If we have a periodical array of identical objects, the in-plane scattering strength increases due to the identical local in-plane scattering direction and the constructive superposition of the fields scattered by every single element. Thus, in-plane scattered power in a prior art 2D GC is not negligible. There are several consequences of in-plane scattering for prior art 2D GCs. First, increased in-plane scattering reduces the out-of-plane diffraction efficiency. Therefore, the prior art 2D GCs suffer from higher insertion loss. Second, in-plane scattering is able to convert the polarization of an incident wave, which explains the high cross-polarization in prior art 2D GCs. Cross-polarization is inevitably related to polarization crosstalk and can cause that initially orthogonal polarizations become non-orthogonal. Third, the cross-polarization causes higher-order mode coupling, because of its field distribution. In single-mode waveguides, the higher-order modes will be filtered, which leads again to higher insertion loss. Fourth, due to the random polarization rotation in a SMF cross-polarization in the waveguides is not necessarily in phase with the target signal's polarization. Superposition of target- and cross-polarization with different phase relations in the two waveguides can lead to significant PDL.
In connection with
The waveguides 11a and 11b are preferably SOI (silicon-on-insulator) ridge waveguides. The perturbing centers SC are preferably formed by holes etched inside the upper silicon layer 12 of the SOI material.
The embodiment of
The perturbing centers SC of a second group G2 are also oval but oriented vertically (again with respect to their longitudinal axis and the page's orientation of
In the exemplary embodiment of
The optical element 10 of
A second access port P2 of the optical element 10 of
The arrangement of the perturbing centers SC in the scattering plane SP is axially symmetric with respect to a mirror axis M that mirrors the first and second direction D1 and D2 with respect to one another. In the exemplary embodiment of
A third access port P3 of the optical element 10 of
As depicted in an exemplary fashion in
The perturbing centers SC may transform the electromagnetic fields such that the first access port P1 outputs radiation with TE00x-mode and X-polarization. The second access port P2 outputs radiation in TE00y-mode and with Y-polarization. X and Y refer to coordinates of a x-y-z-coordinate system.
Of course, the optical element 10 of
In other words, the optical element 10 may operate as a polarization multiplexer or de-multiplexer in both directions.
The angle between the two group-individual orientations O1 and O2 preferably equals 180° divided by the number of group-individual orientations. If the embodiment of
The perturbing unit SCU of
The concentration and arrangement of the circular perturbing centers SC influences the conversion of radiation with respect to the modes. Therefore, the circular perturbing centers SC may be added to achieve other conversion behaviors than the one discussed above with reference to
The exemplary embodiment according to
The following table lists the geometric details and simulation results regarding a reference 2D GC (with only circular holes), and the proposed optical element according to
The following table lists the geometric details and simulation results regarding a reference 2D GC (with only circular holes), and the proposed optical element according to
The simulation results are shown in
The reference structure and the proposed structure according to
The following table lists performance results regarding the reference 2D GC with only circular holes, and the proposed optical element according to
The following table lists performance results regarding the reference 2D GC with only circular holes, and the proposed optical element according to
In the exemplary embodiment of
Furthermore, the array's edges are circularly bent at the access ports P1 and P2 in order to enable mode coupling via shorter adjacent tapers 20.
On a first access side S1 of the optical element 10, an access port P11 is connected to a waveguide 11a. The access port P11 allows inputting radiation R11 with a first mode. In the exemplary embodiment of
A second access side S2 of the optical element 10 provides a first access port P21 and second access port P22. Each of the latter ports P21 and P22 outputs radiation R21/R22 that comprises the first mode, e. g. said TE00-mode, and at least a second mode in response to the radiation R11 that is inputted at the first access side S1. In the exemplary embodiment of
The ports P21 and P22 are connected to waveguides 10b and 10c. The waveguides 10a, 10b, and 10c as well as the perturbing centers SC lie in the scattering plane SP.
The size, orientation and arrangement of perturbing centers SC determines the conversion and the conversion ratios of the modes. The different sizes of the arrows in
The optical elements 10 described above in connection with
The perturbing centers SC discussed above in the context of the exemplary embodiments of
The exemplary embodiments described above in connection with
The various embodiments and aspects of embodiments of the invention disclosed herein are to be understood not only in the order and context specifically described in this specification, but to include any order and any combination thereof. Whenever the context requires, all words used in the singular number shall be deemed to include the plural and vice versa. Whenever the context requires, all options that are listed with the word “and” shall be deemed to include the word “or” and vice versa, and any combination thereof.
In the drawings and specification, there have been disclosed a plurality of embodiments of the present invention. The applicant would like to emphasize that each feature of each embodiment may be combined with or added to any other of the embodiments in order to modify the respective embodiment and create additional embodiments. These additional embodiments form a part of the present disclosure and, therefore, the applicant may file further patent claims regarding these additional embodiments at a later stage of the prosecution.
Further, the applicant would like to emphasize that each feature of each of the following dependent claims may be combined with any of the present independent claims as well as with any other (one or more) of the present dependent claims (regardless of the present claim structure). Therefore, the applicant may direct further patent claims towards other claim combinations at a later stage of the prosecution.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21194821.1 | Sep 2021 | EP | regional |
