Patent Description:
Optical communication systems are continuously being miniaturized to integrate a large number of previously discrete optoelectronic devices with silicon-based integrated circuits to achieve on-chip optical interconnects for high performance computation. In particular, silicon photonics aims to integrate as many as possible optoelectronic functionalities based on CMOS compatible materials, in order to lower the cost without sacrificing performance.

Optical modulators and photodetectors are main building blocks of photonic systems.

These two types of devices operate based on very different mechanisms and consequently utilize different device geometries. They often have to be made of different materials that are difficult and costly to integrate with silicon photonics. Optical modulators are based on electro-optical or electro-absorptive effects in materials such as LiNbO<NUM>, germanium and compound semiconductor heterostructures. In silicon photonics, the dispersion effect induced by carrier injection or depletion is the most common method used to achieve integrated optical modulation, both in amplitude and phase. Typically this requires several millimeter long devices, but amplitude modulation can be also achieved with micron-scale devices, based on the Franz-Keldysh effect in SiGe compounds.

At the receiving end of optical links, photodetectors convert light back into electrical signals by absorbing photons and generating charges through photo-electric effects. Therefore, strong absorption and effective collection of photo-excited carriers are desired for efficient photo-detection. Because of these distinctive requirements, to date no device that can function as both a photodetector and a modulator, and whose role can be switched through external control, has been made with a single type of material. Such a simple yet multifunctional device, if implemented, not only can make integrated optical systems programmable and adaptable, but also can lead to novel applications such as optoelectronic oscillators and new schemes of optical computation and signal processing.

Because of its two-dimensional structure, graphene is ideally suited for integration with planar photonic devices and the performance of the devices benefits significantly from the elongated optical interaction length in coplanar configuration. With its remarkable optical and electrical properties, including absorption and dispersion, graphene has been exploited as a multifunctional optoelectronic material to produce, for example, highly tunable optoelectronic devices with high performance and adaptive controllability by electrostatic gating or chemical doping. Such devices include photodetectors, optical modulators, polarizers and saturable absorbers. Graphene optical modulators have been demonstrated to have very high speed (to date only limited by the RC constant of the electrodes) and very low energy consumption.

Indeed, graphene has been demonstrated to perform as a modulator on thin SOI waveguides with theoretical modulation speed of <NUM> which would be far beyond other technological platforms used e.g. in optical switches in data centres. The problem to be solved is how to structure and fabricate graphene modulators on thick SOI waveguides, so that the graphene is interacting with the optical field.

Photonics circuits based on micron-scale (<NUM> thick) silicon-on-insulator (SOI) waveguides have many advantages compared to standard submicron (<NUM> - <NUM> thick) silicon waveguide technology, but also some major limitations. Namely larger waveguide size implies devices with larger power consumption and, even more importantly, slower speed. This makes this thicker platform less attractive compared to others for most applications requiring high speed modulation and detection.

Prior art solutions to the problem include the use of submicron silicon waveguides for the whole circuit, which comes with many drawbacks, including high propagation losses, single-polarization operation, bad tolerances to fabrication errors, not to mention the requirement for very expensive deep UV fabrication tools. Fast devices have been demonstrated on thick SOI waveguides, where the volume of the devices was reduced by patterning sub-micron wide waveguides with a high aspect ratio, which makes the fabrication challenging and with a bad impact on yield. Only some types of fast detectors and modulators have been demonstrated with this approach, and no broadband modulators or phase modulators. Known prior art solutions are presented e.g. in <CIT>, <CIT>, and <CIT>.

It is an object of the present invention to create an interface between micro-scale waveguides and submicron waveguides, in order to exploit the advantages of both technologies. One of the advantages of submicron waveguides is that they interact much better with graphene layers. Important other advantages include the possibility to fabricate fast Ge detectors and SiGe Franz-Keldysh modulators.

The present invention aims to make the micron-scale SOI platforms more attractive in the implementation of photonic circuits, by changing the mode size into the submicron scale only when fast modulation and detection is needed, while keeping the advantages of micron-scale waveguides elsewhere.

<CIT> presents a waveguide mode expander using amorphous silicon. <CIT> presents waveguide photodetector and forming method thereof. <CIT> presents optical modulator including graphene. <CIT> presents mode transformation and loss reduction in silicon waveguide structures utilizing tapered transition regions. <CIT> epitaxial growth for waveguide tapering. <CIT> presents optical modulator with three-dimensional waveguide tapers.

According to some embodiments not being part of the claimed invention, the second waveguide acts as an intermediate waveguide, through which light is coupled into a third waveguide that is placed on top of the second waveguide. In this case light is adiabatically coupled from the second waveguide to the third waveguide, which forms the optical device. The third waveguide can be formed, for example, in a <NUM>-<NUM> thick layer of crystalline silicon or in a layer of III-V compound semiconductor material added on top of the second waveguide.

In some embodiments of the invention, the first waveguide is covered with an etch-stop layer prior to depositing the second waveguide and/or said at least one layer of an optical material.

In some embodiments of the invention, the first waveguide is a strip waveguide made of crystalline silicon. According to the claimed invention, the second waveguide is made of amorphous silicon or hydrogenated amorphous silicon. The exact properties of the material naturally depend on the concentrations of any participating materials, such as germanium.

In some embodiments, the dielectric material may comprise aluminum oxide, silicon nitride or silicon dioxide, for example. The etch-stop layer may comprise silica, silicon nitride or thermally oxidized silicon dioxide SiO<NUM>, and the dielectric material comprise aluminum oxide, silicon nitride or silicon dioxide.

According to the claimed invention, the optical material of the photonic device is one of graphene, germanium or a silicon-germanium alloy.

In some embodiments of the invention, the contact to the layers of said at least one optical material layers are formed through etched openings in the dielectric material layers to enable contact to contact terminals patterned on the optical waveguide.

According to some embodiments, the thinner second waveguide is formed to the tapered shape having a cross-section in the horizontal plane of said substrate which is smaller at one end and larger at an opposite end of said tapered waveguide. In other embodiments, the second waveguide is formed to the tapered shape having a cross-section in the vertical plane of the substrate, which is smaller at one end and larger at an opposite end of said tapered waveguide.

The invention offers considerable benefits. In the case of detectors only a thin layer of Ge is needed, instead of the <NUM> thick Ge layer usually needed to make a detector on a <NUM> thick SOI. In this way the detector volume can be small and capacitance of vertical contacts can be low, paving the way to high speed devices. Also, unlike a <NUM> thick germanium layer grown on etched silicon, in the present invention Ge may be grown on high quality non-etched silicon surface, which makes the quality of the material much higher, with positive impact on sensitivity and dark current. Further, contacts for the devices can be implanted directly on flat a thick silicon surface. Similarly, a thin layer of SiGe alloy with suitable bandgap can be deposited to realize fast Franz-Keldysh modulators with smaller volume than existing devices in microscale silicon platforms. 2D materials, such as graphene, can be easily integrated and sandwiched between waveguides of amorphous and crystalline silicon, with suitable dielectric insulating layers.

Photonic circuits built according to the present invention are potentially much faster than present modulators based on thick SOI waveguides.

The inventive technology may be used and applied in monolithic integration of thin and thick waveguides, i.e. the possibility to fabricate thick and thin waveguides within the same fabrication process, making available a platform with the advantages of both types of waveguides. Integrated optics is an enabling technology, with a long list of possible applications, from integrated optical modulators and photodetectors for high speed optical switching in telecommunications data centres to gas sensing, and from medical imaging to LIDAR systems.

Franz-Keldysh modulator - an electro-absorption modulator for controlling the intensity of a laser light via an electric voltage based on the Franz-Keldysh effect, i.e. a change in the absorption spectrum caused by an applied electric field changes the bandgap energy optical material - a material consisting of graphene, germanium or a silicon-germanium alloy, which is optically active, i.e. forms a controllable photonic device, i.e. a modulator thick waveguide - a waveguide having a thickness of <NUM>-<NUM>, and a refractive index in the range of <NUM>-<NUM>. The waveguide may consist of, for example, crystalline silicon, indium phosphide, gallium arsenide, or any other high-refractive index transparent material that receives input light from one direction and may feed an optical waveguide with that light in another direction. The material may be designed to be a waveguide in itself. thin waveguide - a submicron scale low-loss waveguide having a thickness of <NUM>-<NUM> and a refractive index in the range of <NUM> - <NUM>. The waveguide consists of amorphous silicon or hydrogenated amorphous silicon.

<FIG> illustrate the phenomenon of adiabatic light transfer. In <FIG> is shown a schematic top view a system of three waveguides consisting of two circularly bent outermost waveguides L and R, and one straight central waveguide C. The minimum distance between waveguides is given by x<NUM>, and the z distance between the centers of the curved waveguides is defined by δ. The radius of curvature of the outermost waveguides L, R may be <NUM>, the spatial delay δ=<NUM>, for example, and the minimum separation between waveguides x<NUM>=<NUM>, for example.

<FIG> shows a side view and a top view of an exemplary waveguide, where a simulation shows how the light is transferred from a lower thick silicon waveguide <NUM> made of crystalline silicon Si to an upper tapered (<NUM>) and thin hydrogenated amorphous silicon (a-Si:H) waveguide <NUM>, with a refractive index higher than that of crystalline silicon. The cross-section "a" to the left further illustrate the light distribution at the left end of the waveguide and the cross-section "b" to the right the same situation at the right end.

<FIG> show a photonic device <NUM> comprising a thin submicron (t<NUM> =about <NUM> thick, for example) low-loss waveguide made of a hydrogenated amorphous silicon (a-Si:H) layer <NUM> having a refractive index higher than that of crystalline silicon. The waveguide <NUM> is deposited on top of a thick Si strip waveguide 33a, which may be covered by a very thin (about <NUM>, for example) etch-stop layer <NUM> made of silica or silicon nitride, for example. As shown in <FIG>, the a-Si:H layer <NUM> is etched into a waveguide with tapered width W<NUM>, to adiabatically transfer the light from the thick Si waveguide 33a to the thin a-Si waveguide <NUM>, located mainly on top of the Si waveguide portion 33a. The bottom layer <NUM> is an insulating part of a silicon substrate for the waveguide, and is here an SOI buried oxide (BOX) layer. The BOX layer <NUM> has a lower refractive index and a thickness that will optically separate the waveguide from a higher-index silicon substrate (not shown), located underneath the BOX layer.

The wide end of the tapered a-Si waveguide <NUM>, having a width of W<NUM>, is butt-coupled to an optical material that comprises a photonic active device <NUM> deposited at the same height as the waveguide <NUM> and having a comparable submicron thickness t<NUM>. The joint and the device <NUM> is also shown in <FIG>. The device <NUM> may be located on the Si layer <NUM>, which forms a silicon substrate and bottom cladding 33b for the device <NUM>.

The device <NUM> can be made of the very same amorphous silicon material as waveguide <NUM>, e.g. as a pn-implanted waveguide for phase modulation. It may also be a waveguide grown on top of a <NUM>-dimensional (2D) material like graphene, for example. The material may also be a high refractive index material like germanium (Ge) and the device may then be used as a detector, for example, or a SiGe alloy in a Franz-Keldysh modulator, for example.

The different widths W<NUM> and W<NUM> as a result from a tapered shape of the waveguide in the horizontal plane of the silicon substrate <NUM> are not the only way to increase the cross-section of a thin waveguide <NUM>. Alternatively, the shape may be tapered in the vertical plane. The critical feature for the waveguide is to have is to have a smaller volume in the area where the light enters the waveguide and a bigger volume in the exit area. The shape of the waveguide may be selected according to various design criteria, and it need not to be linear and/or planar, i.e. tapered as shown. Alternatively, the waveguide cross-sections could be kept constant, whereas the refractive indexes within a waveguide may vary, i.e. having a refractive index gradient, to achieve similar adiabatic light transfer. Clearly a combination of waveguide cross-section change and refractive index change can be also used. The general criterion to efficiently move the light from one waveguide to the other is to adiabatically change from a condition where the effective index of the mode of the thick waveguide (neff1) is significantly higher than that of the second waveguide (neff2), to a condition where the opposite is true (neff2> neff1). This can be achieved by playing with the waveguide geometry (smaller waveguide corresponding to lower effective index) and/or with the material refractive index.

Depending on the type of device, the device can be either coupled back to a further silicon waveguide <NUM> through a second a-Si taper as shown in <FIG>, e.g. for modulation, or it can be just terminated as in <FIG>, e.g. for Ge detectors. The high refractive index layer <NUM> may consist of hydrogenated amorphous silicon (a-Si:H), or of any transparent material with refractive index higher than that of silicon, e.g. a SiGe alloy.

In <FIG> are shown a similar, but double-ended photonic device <NUM> as in <FIG>, with thin double tapered waveguide 42a and 42b on each side of an optically active material comprising a photonic active device <NUM>, all deposited on a silicon waveguide 43a, and the etch-stop layer <NUM>. As in <FIG>, the device <NUM> is located on an extension of a generic thick Si substrate <NUM>, forming a silicon substrate and bottom cladding 43b for the device <NUM>. Also a BOX layer <NUM> is implied.

In <FIG> is shown an embodiment of the present invention, where a photonic circuit <NUM> has layers <NUM> of an optical 2D material, like graphene for example, which with intervening dielectric layers <NUM>, <NUM> constitute an optically active photonic device. The submicron hydrogenated amorphous silicon (a-Si:H) waveguide layer consists in this embodiment of three waveguide portions 52a - 52c deposited on top of a thick Si strip waveguide <NUM>, having three portions 53a -53c. Two thin waveguide portions 52a and 52b are tapered and deposited on the thick silicon waveguides 53a, 53b, as explained in connection with <FIG>. The third and central thin waveguide portion 52c is formed as a square on top of the photonic active device <NUM>, <NUM> and a silicon cladding waveguide portion 53c.

An advantageous feature of the configuration shown in <FIG> is that the optically active photonic device may be deposited on the silicon waveguide 53a - 53c prior to the deposition of layer 52a - 52c, consisting of amorphous silicon.

Thus the optical device <NUM>, <NUM> becomes embedded inside the optical mode, which enables an easy and large overlap of the top waveguide 52a - 52c compared to standard approaches, where the mode graphene interacts mainly or only with the evanescent tail of the optical mode, see <FIG> and <FIG>.

<FIG> shows for example how a bilayer <NUM> of graphene can be embedded in the thin waveguide 52c, sandwiched between three dielectric thin films <NUM> and <NUM>, consisting of e.g. SiO<NUM>, Si<NUM>N<NUM>, or Al<NUM>O<NUM>. The large overlap with the graphene bilayer is obtained thanks to the relatively low index contrast between amorphous and crystalline silicon.

According to the claimed invention, the waveguide 52a - 52c is deposited on the thick waveguide portion 53c so as to wholly or partially overlap the photonic device, as shown best in <FIG>, which photonic device has a layered structure, e.g., as described above. A BOX layer <NUM> is also implied, see discussion above.

In some embodiments not being part of the claimed invention, a high index contrast between amorphous and crystalline silicon is wanted, for example when bends with micron-scale bending radii are used to build micro-ring resonators with a free-spectral range as large as possible. <FIG> show a photonic device <NUM> with high index contrast thin waveguides <NUM>, <NUM> located on a thick silicon waveguide 63a and the cladding extension 63b of the Si substrate <NUM>, respectively.

The waveguide <NUM> has a silica bottom cladding <NUM> formed in the cladding 63b by selectively etching silicon away, and replace it with silica <NUM>. In this region the submicron waveguide <NUM> will deposit direct on top of the silica cladding which leads to a high index contrast waveguide suitable for tight bends.

Taken further, as shown in <FIG>, it is possible to couple light into photonic circuits <NUM> fabricated on SOI wafers with submicron silicon layer that are bonded directly onto a photonic circuit <NUM> on top of a thin amorphous silicon waveguide 72a - 72c, having a silica bottom cladding <NUM>. In this fashion, two different device platforms may be coupled at and around the marked area <NUM> through a suitable combination of inverse tapers. Thus, light can be coupled back and forth between devices with thick SOI waveguides 73a and devices <NUM> that are based on standard submicron silicon waveguide technologies. A silicon substrate and cladding portion 73b and BOX layers <NUM> are also indicated.

In <FIG> is shown a light beam <NUM> entering a device <NUM>, like the one in <FIG>. The light <NUM> enters at one end of a thick SOI waveguide 83a at arrow A. As have been described in connection with <FIG>, the light escalates through adiabatic transfer from the thick waveguide 83a to the thin waveguide 82c, because, when wide enough, the effective refractive index of the thin upper waveguide 82c becomes sufficiently high to effect the adiabatic light transfer. The light <NUM> proceeds in the waveguide 82c - 82a, until it again escalates to a photonic circuit <NUM> in the area <NUM>, which corresponds to the area <NUM> in <FIG>. As in <FIG>, a silicon substrate and cladding portion 83b and a BOX layer <NUM> is also indicated.

In the reverse direction, from a photonic circuit to waveguides, modulated or otherwise processed light may be led out from the photonic circuit by optical coupling to a submicron waveguide, and further by adiabatic transfer to thicker micron-scale silicon-on-insulator (SOI) waveguides.

Furthermore, the described features, structures or characteristics may be combined in any suitable manner in one or more embodiments. In the description numerous specific details are provided to provide a thorough understanding of embodiments of the invention.

Claim 1:
A photonic circuit, comprising:
- a first waveguide (<NUM>) on a silicon substrate (<NUM>), said waveguide having a thickness of <NUM> - <NUM> and a refractive index of <NUM> - <NUM>;
- a second waveguide (52a-52c) having a thickness of <NUM> - <NUM> and a refractive index of <NUM> - <NUM>, said waveguide having a tapered shape with a cross-section that is smaller at one end and larger at the opposite end at least in one direction;
wherein the photonic circuit further comprises:
- a photonic device (<NUM>) comprising:
- at least one layer (<NUM>) of an optical material deposited on said first waveguide and arranged to optically interface with said second waveguide, wherein said optical material is one of graphene, germanium ora silicon-germanium alloy;
- a layer (<NUM>) of a dielectric material deposited on each layer of said optical material;
said end of said tapered second waveguide having a smaller cross-section is interfaced with said first waveguide (<NUM>) to provide adiabatic light transfer between said first and second waveguides, and wherein said photonic device is interfaced with said end (W3) of said second waveguide (52b) having a larger cross-section to provide optical coupling between said second waveguide and said photonic device, characterized in that said second waveguide (52a-52c) is deposited partly on said first waveguide and partly on the uppermost dielectric layer, and
said second waveguide (52a - 52c) is a waveguide made of amorphous silicon or hydrogenated amorphous silicon.