The invention relates to photonically integrated chips, to optical elements having such chips and to methods for producing them. The term “photonically integrated chips” is understood as meaning integrated chips which have a substrate and material layers situated on said substrate (for example grown on or deposited) and in which one or more photonic components (for example waveguides, couplers, etc.) are integrated in one or more of the material layers.
When developing optical components, in particular integrated optical components, the problem often arises of light having to be transmitted from one component to another, for example from a laser to a waveguide on a chip or from the chip to a fiber. In this case, it is fundamentally possible, on the one hand, to place the two components beside one another and to couple the light horizontally in the plane of the waveguide, also called butt coupling. On the other hand, the components can be placed on top of one another in order to transmit the light vertically or virtually vertically with respect to the plane of the waveguide. In the latter variant, the light striking the waveguide at a small angle with respect to the surface normal is generally deflected into the waveguide via a grating coupler and is guided further in the waveguide.
When very divergent or convergent radiation is vertically coupled in a waveguide, the current methods entail great losses because the grating couplers which are usually used have only a limited angular acceptance. These other optical components likewise have an angular acceptance when coupling light out of a waveguide into other optical components, for example fibers (for example glass or polymer fibers). The portions of the radiation which are incident outside the angular acceptance are not coupled into the waveguide or the fiber, for example, and are lost. These losses are greater, the more divergent or convergent the incident light. On account of the beam divergence, the coupling losses may increase with greater distance between the coupling elements if the aperture of the target coupling element does not suffice. The upper material layers of optical elements, also called “backend of line” of the element in technical terminology, having five metal layers, for example, have a thickness of approximately 20 μm. During the propagation of a divergent light beam over this distance, its beam diameter increases significantly.
In the case of a very divergent or convergent light source, nowadays a fiber is usually interposed between the light source and the grating coupler of the waveguide. The light is first of all coupled into the fiber and is coupled out of the fiber at the other fiber end and is coupled into the waveguide via the grating coupler. This is associated with great manufacturing effort, additional components and coupling losses at the entrance and exit facets of the fiber [1].
Another approach is to use micro-optics, for example lenses, as separate components which are fastened on the element (also called “chip” for short below in technical terminology in the case of integrated elements) above the grating coupler and are intended to collimate or focus the vertically incident light. This method also requires a large amount of manufacturing effort with additional components (for example injection molding or glass micro-lenses), manufacturing steps and associated tolerances and poor scalability [2].
Another approach is to use lenses which are etched into the exit facet of a laser in order to collimate or focus the emitted light before it emerges from the laser [3].
A photonically integrated chip having the features according to the precharacterizing clause of patent claim 1 is known from the publication “A polarization-diversity wavelength duplexer circuit in silicon-on-insulator photonic wires” (Wim Bogaerts, Dirk Taillaert, Pieter Dumon, Dries Van Thourhout, Roel Baets; Feb. 19, 2007/Vol. 15, no. 4/OPTICS EXPRESS 1567).
Proceeding from the last-mentioned prior art, the invention is based on the object of easily improving the coupling efficiency which can be achieved in the chip.
This object is achieved, according to the invention, by means of a photonically integrated chip having the features according to patent claim 1. Advantageous configurations of the chip according to the invention are stated in subclaims.
According to this, the invention provides for an optical diffraction and refraction structure to be integrated in a material layer of the chip above or below the optical grating coupler or in a plurality of material layers above or below the optical grating coupler or on the rear side of the substrate, which diffraction and refraction structure carries out beam shaping of the radiation before it is coupled into the waveguide or after it has been coupled out of the waveguide.
As a result of the diffraction and refraction structure provided according to the invention, the wave front of the incident light can be transformed into any desired wave front of the emerging light. The invention makes it possible, for example, to collimate and focus the incident light if the diffraction and refraction structure is implemented according to the principle of a discretized lens or Fresnel lens. This makes it possible, for example, to reduce the beam divergence of the incident light to such an extent that the entire beam propagates within the acceptance angle of the grating coupler and can be coupled into the waveguide only with very low losses. In addition, the diffraction and refraction structure also means that the diameter of the incident light is adapted to the aperture of the grating coupler, thus minimizing losses caused by beam parts which do not strike the grating coupler. In this case, the incident light may come, for example, both from a fiber (for example glass or polymer fiber), a further photonically integrated chip, and directly from a laser (for example HCSEL, VCSEL). Furthermore, it is possible to couple light out of upper material layers of the chip (the so-called “backend of line”) into a second optical component, for example a fiber, a further photonically integrated chip, a photodetector or micro-optics, via the diffraction and refraction structure. For this purpose, the diffraction and refraction structure may be adapted in such a manner that beam divergence of the emergent light for the most efficient possible coupling into the target component is achieved.
Another great advantage is the extremely low manufacturing tolerance and therefore alignment accuracy of the diffraction and refraction structure with respect to the grating coupler in comparison with conventional methods with separate components. The reason is that the diffraction and refraction structure is produced, for example, using lithographic production methods with a very high degree of precision and positioning accuracy as a result of lithographic alignment methods instead of mechanical positioning and adhesive bonding of individual components. A silicon-on-insulator (SOI) substrate can be used, for example, as the material system for producing photonically integrated chips.
In the chip according to the invention, there is advantageously no need for any separate components with associated packaging effort. In addition, the components to be coupled can be placed closer together, thus making it possible to reduce scattering losses and apertures of the coupling structures. The integrated production enables considerably better scalability, for example when producing a plurality of couplers on a photonically integrated chip. In this case, there is no repeated effort needed to position and adhesively bond additional individual components.
It is considered to be particularly advantageous if the optical diffraction and refraction structure forms a lens, a beam splitter or a polarization separator.
The optical diffraction and refraction structure is preferably formed by steps in one or more material layers of the chip above or below the optical grating coupler or at least also comprises such steps.
The waveguide is preferably a ridge waveguide which comprises a ridge formed in a wave-guiding material layer of the chip. In such a configuration, the optical diffraction and refraction structure is preferably integrated in one or more layers of the chip above or below the ridge.
The substrate of the chip is preferably a semiconductor material, for example silicon.
The chip is particularly preferably based on SOI (silicon on insulator) material. In the case of such a material system, it is considered to be advantageous if the ridge waveguide is formed in a silicon covering layer of an SOI material, and the optical diffraction and refraction structure is integrated in one or more layers of the chip above the silicon covering layer.
The grating coupler may be a one-dimensional or two-dimensional grating coupler. The grating coupler is preferably a Bragg grating or preferably also at least comprises such a Bragg grating.
The diffraction and refraction structure is preferably two-dimensional and is preferably in a plane parallel to the wave-guiding material layer(s).
With regard to an optimum coupling efficiency, it is considered to be particularly advantageous if the diffraction and refraction structure is location-dependent in two dimensions, specifically in a dimension dependent on the location along the longitudinal direction of the waveguide and in a dimension perpendicular thereto dependent on the location perpendicular to the longitudinal direction of the waveguide.
The diffraction and refraction structure preferably forms a two-dimensional Fresnel lens.
The waveguide is preferably an SOI ridge waveguide having a ridge which is formed in a wave-guiding silicon layer of an SOI material on a silicon dioxide layer and the longitudinal direction of which extends along the direction of propagation of the radiation guided in the SOI ridge waveguide.
With regard to an optimum coupling efficiency, it is considered to be particularly advantageous if the diffraction and refraction structure is two-dimensional and is in a plane parallel to the wave-guiding silicon layer, the diffraction and refraction structure being location-dependent in two dimensions, specifically in a dimension dependent on the location along the longitudinal direction of the ridge of the SOI waveguide and in a dimension perpendicular thereto dependent on the location perpendicular to the longitudinal direction of the ridge of the SOI waveguide.
Webs are preferably situated beside the ridge, the layer height of which webs is lower than that of the ridge.
An alternative, but likewise preferred, configuration provides for at least sections of the wave-guiding silicon layer to have been removed beside the ridge.
The invention also relates to an optical element which has a photonically integrated chip.
Such an element preferably comprises a fiber, the fiber end of which is coupled to the optical diffraction and refraction structure on that side of the latter which faces away from the grating coupler, the longitudinal direction of the fiber being oriented virtually perpendicularly to the wave-guiding layer(s) of the chip in the region of the fiber end. In this case, the term “virtually perpendicular” is understood as meaning an angular range between 70° and 90°.
Alternatively or additionally, the optical element may comprise a radiation emitter which is coupled to the optical diffraction and refraction structure on that side of the latter which faces away from the grating coupler, the radiation direction of the radiation emitter being oriented virtually perpendicularly to the wave-guiding layer(s) of the chip.
Alternatively or additionally, the optical element may comprise a radiation detector which is coupled to the optical diffraction and refraction structure on that side of the latter which faces away from the grating coupler, the active reception surface of the radiation detector being oriented parallel to the wave-guiding layer(s) of the chip.
The invention also relates to a method for producing a photonically integrated chip which comprises a substrate and a plurality of material layers applied to a top side of the substrate, wherein, in the method, an optical waveguide is integrated in one or more wave-guiding material layers of the chip, and a grating coupler is formed in the optical waveguide and causes beam deflection of radiation guided in the waveguide in the direction out of the layer plane of the wave-guiding material layer(s) or causes beam deflection of radiation to be coupled into the waveguide in the direction into the layer plane of the wave-guiding material layer(s).
With respect to such a method, the invention provides for an optical diffraction and refraction structure to be integrated in a material layer above or below the waveguide or in a plurality of material layers of the chip above or below the waveguide or on the rear side of the substrate, which diffraction and refraction structure carries out beam shaping of the radiation before it is coupled into the grating coupler or after it has been coupled out of the grating coupler.
With respect to the advantages of the method according to the invention, reference is made to the statements above in connection with the chip according to the invention.
It is advantageous if a lens, a beam splitter or a polarization separator is produced as the optical diffraction and refraction structure.
The production of the optical diffraction and refraction structure is preferably carried out by etching steps in one or more material layers of the chip above or below the optical grating coupler or preferably at least also comprises etching of steps.
In order to be able to carry out the etching steps with optimum positioning, one or more lithography steps for applying one or more etching masks are preferably carried out in advance.
Depending on the demand imposed on the coupling efficiency of the diffraction and refraction structure, the number of etching steps and therefore the steps of graduated depth can be kept low, as a result of which the production costs can remain low. Even if only a single etching step is used, it is possible to implement a binary diffraction and refraction structure, also called a phase plate, which, with the same aperture, achieves a slightly lower coupling efficiency, however, than a diffraction and refraction structure having a plurality of steps. If a sufficient aperture on the chip can be achieved, a sufficient coupling efficiency can also be readily achieved, however, with a binary structure.
In order to achieve any desired transformations of the incident wave front, the individual steps of the optical diffraction and refraction structure produced can be made independently of one another in both spatial directions of the plane of the substrate.
Suitably selecting the spatial distribution of the etching steps makes it possible to spatially separate the incident light beam into individual separated partial beams which can be guided further independently of one another. Such separation can also be implemented using different polarization directions of the separated partial beams.
The invention is explained in more detail below using exemplary embodiments; in this case, by way of example,
For the sake of clarity, the same reference symbols are always used for identical or comparable components in the figures.
The photonically integrated chip 2 comprises a substrate 20, on the top side 21 of which a plurality of material layers are arranged. A silicon dioxide layer 30, inter alia, is thus situated on the top side 21 of the substrate 20, on which silicon dioxide layer a wave-guiding silicon layer 40 is in turn arranged. The substrate 20, the silicon dioxide layer 30 and the wave-guiding silicon layer 40 may be formed by a so-called SOI (silicon on insulator) material which is commercially available in prefabricated form.
A ridge waveguide 50 is provided in the wave-guiding silicon layer 40 and can be formed, for example, by etching the wave-guiding silicon layer 40. A grating coupler 60 in the form of a Bragg grating is connected to the ridge waveguide 50 and has preferably likewise been produced by etching the wave-guiding silicon layer 40.
In the exemplary embodiment according to
A diffraction and refraction structure 100, which is not illustrated in any more detail in
The optical element 1 according to
The radiation-emitting component 3 produces a divergent light beam Pe, the curved wave front 200 of which has a divergence α. The divergent light beam Pe strikes the diffraction and refraction structure 100 which, in the exemplary embodiment according to
The diffraction and refraction structure 100 transforms the incident wave front 200 of the divergent light beam Pe into a planar wave front 201 which then strikes the grating coupler 60 and is coupled into the ridge waveguide 50 via said coupler. The light guided in the ridge waveguide 50 is identified using the reference symbol Pa in
In summary, the diffraction and refraction structure 100 in the exemplary embodiment according to
The Fresnel lens 300 formed by the etched sections 101 and unetched sections 102 of the diffraction and refraction structure 100 is shown in more detail in a plan view in
In the exemplary embodiment according to
The photonically integrated chip 2 has a substrate 20, a buried silicon dioxide layer 30, a wave-guiding silicon layer 40, an intermediate layer 70 and an upper covering layer 80 in which a diffraction and refraction structure 100a is provided. A ridge waveguide 50 and a grating coupler 60 are integrated in the wave-guiding silicon layer 40, preferably by means of etching.
The diffraction and refraction structure 100a in the covering layer 80 is formed by a single-step stepped profile or a binary step filter which comprises etched sections 101 and unetched sections 102.
The optical element 1 according to
A light beam Pe which is guided in the ridge waveguide 50 reaches the grating coupler 60 which couples out the light beam Pe and deflects it in the direction of the radiation-receiving component 4. The deflected beam preferably has a planar wave front 201.
The planar wave front 201 reaches the diffraction and refraction structure 100a which carries out beam shaping and converts the previously planar wave front 201 into a convergent wave front 203 with a divergence β. The resulting convergent light beam is identified using the reference symbol Pa in
An exemplary embodiment of a diffraction and refraction structure 100a which can be used in the photonically integrated chip 2 according to
The substrate 20, on the top side 21 of which a plurality of material layers are arranged, is seen in
A ridge waveguide 50 is provided in the wave-guiding silicon layer 40; the ridge width of the ridge 51 is identified using the reference symbol B in
Further material layers, for example in the form of the intermediate layer 70 and the upper covering layer 80, are situated on the wave-guiding silicon layer 40.
The diffraction and refraction structure 100 is integrated in the covering layer 80, is two-dimensional and carries out beam shaping in two axes, namely both along the arrow direction or along the direction of propagation of the light beam Pa according to
The arrangement of the etched sections 101 and unetched sections 102 is selected, for example, in such a manner that the diffraction and refraction structure 100 forms a two-dimensional Fresnel lens 300 or a Fresnel lens 300 which operates in two axes. The Fresnel lens 300 formed by the etched sections 101 and unetched sections 102 of the diffraction and refraction structure 100 is shown in more detail in a plan view in
It goes without saying that the diffraction and refraction structure 100 may also have multiple steps along the arrow direction Y, as has been explained in connection with
The substrate 20, on the top side 21 of which a plurality of material layers are arranged, is seen in
A ridge waveguide 50 is provided in the wave-guiding silicon layer 40; the ridge width of the ridge 51 is identified using the reference symbol B in
In summary, in the above exemplary embodiments, a lithographically produced optical diffraction and refraction structure 100 is introduced onto one or more upper material layers, preferably onto the uppermost material layer (covering layer 80), of the photonically integrated chip 2, that is to say the so-called “backend of line” region of the photonically integrated chip, for the purpose of subjecting light to beam shaping. For this purpose, step-like structures are preferably etched into the uppermost material layer or one or more upper material layers. Depending on the number of etching steps used, which may be limited by the number of available exposure masks for example, structures having one or more steps of graduated depth can be achieved. These structures function, as a whole, as a refractive and diffractive beam shaping element for a particular wavelength range by spatially varying the refractive index in a targeted manner. The etched and unetched regions have different refractive indices. The times of flight and directions of propagation of light waves through these different regions are therefore different, with the result that the wave front of the incident light wave is deformed after propagation through the diffraction and refraction structure. This effect can be used, for example, to collimate or even focus the light beam before it strikes the grating coupler 60 in the wave-guiding material layer in a deeper layer of the chip 2, the so-called “frontend of line” region of the chip. With a greater number of steps in the diffraction and refraction structure 100, the diffraction and refraction behavior of a perfect lens can be approximated. The diffraction and refraction structure 100 is preferably produced by means of a photolithographic exposure and etching process, which can also be combined with a plasma etching process, or else by means of ion beam etching. This process usually takes place at the end of the complete processing of the chip.
Although the invention was described and illustrated more specifically in detail by means of preferred exemplary embodiments, the invention is not restricted by the disclosed examples and other variations can be derived therefrom by a person skilled in the art without departing from the scope of protection of the invention.
Number | Date | Country | Kind |
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10 2014 219 663.9 | Sep 2014 | DE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/DE2015/200463 | 9/25/2015 | WO | 00 |