WAVEGUIDE ANTENNA DEVICE

An antenna device for performing off-chip light coupling comprising an array of radiating elements whose thickness is larger than λ/2, the radiating elements being chosen such that the length of the array is smaller than 10λ, where λ is the wavelength of light in the material chosen for the radiating elements. An advantage of this method is that, unlike in conventional waveguide grating antenna, by reducing the number of the radiating elements in the array, the dependence of the off-chip emission angle on the wavelength of light can be greatly reduced. Another advantage is that by using thick radiating elements the antenna efficiency can be greatly enhanced, thereby compensating for the reduced efficiency occurring as a consequence of using only a small number of radiating elements in the array.

FIELD OF THE INVENTION

The present disclosure generally relates to the field of optical waveguides. More particularly, examples of the disclosure relate to an antenna device for performing light emission from such waveguides or vice-versa to couple light into such waveguides.

BACKGROUND OF THE DISCLOSURE

The capability to couple light in and out of optical waveguides is a prerequisite for many applications areas of integrated optics, photonics, and optoelectronics, such as fiber-chip coupling and integrated optical phased arrays. Surface gratings are commonly used to perform light coupling in integrated photonics. Since grating operation requires a proper phase relation between the different periods to be satisfied (grating equation), the diffraction angle commonly depends strongly on the wavelength of light, especially in the high index contrast Silicon-On-Insulator (SOI) platform. Scattering efficiency also depends on wavelength. These dependencies impact the grating bandwidth, reducing the fiber-chip coupling efficiency when wavelength is detuned from the design point (which is problematic for example for Wavelength Division Multiplexing systems) and impacting the radiation pattern and angle accuracy of phased arrays.

Research in surface gratings has explored several approaches to extend their operational bandwidth, i.e. reduce the dependence of the diffraction angle and scattering efficiency on wavelength. One possible solution is attenuating the dispersion of the leaky Bloch mode by reducing the average refractive index of the grating. This has been achieved for example using silicon nitride (SiN) instead of a SOI platform. The same approach can also be pursued in a SOI platform by engineering the dispersion in the grating area through the integration of subwavelength metamaterials. Chirping the grating period can also be used to increase the bandwidth. Other approaches rely on reducing the effective interaction length of the grating. In the case of fiber-chip coupling, this requires using a fiber with smaller mode field diameter.

Such prior art approaches achieve only a limited increase of the bandwidth and suffer from a trade-off between bandwidth and overall efficiency. Decreasing the average effective index of the grating reduces the index contrast between the cladding materials and the grating, increasing light leakage into the cladding. Shorter gratings also suffer in terms of diffraction efficiency since in real-world applications it is usually difficult to obtain a large increase of the scattering strength in the grating region to compensate for the reduced interaction length.

Thus, it will be appreciated that the ability to reduce the dependence of the diffraction angle on the wavelength of light while maintaining a high diffraction efficiency of the antenna is a prerequisite for performing light emission from and to optical waveguides.

Any discussion of problems provided in this section has been included in this disclosure solely for the purposes of providing a background for the present invention, and should not be taken as an admission that any or all of the discussion was known at the time the invention was made.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The description of exemplary embodiments of the present disclosure provided below is merely exemplary and is intended for purposes of illustration only; the following description is not intended to limit the scope of the invention disclosed herein. Moreover, recitation of multiple embodiments having stated features is not intended to exclude other embodiments having additional features or other embodiments incorporating different combinations of the stated features.

As set forth below, the trade-off between diffraction efficiency and bandwidth can be overcome by combining a short effective interaction length with other methods of grating engineering to increase the scattering strength, such as core thickness optimization, apodization, and directionality enhancement by proper scattering block design or back-reflectors.

In one embodiment, off-chip light coupling (either to redirect optical waves from an in-plane direction to an out-of-plane direction or vice versa) is achieved using an antenna device comprising an array of radiating elements whose thickness is larger than λ/2, the number of radiating elements being chosen such that the length of the array is smaller than 10λ, where λ is the wavelength of the light in the material chosen for the radiating elements. By reducing the number of radiating elements in the array the dependence of the off-chip emission angle on the wavelength of light can be greatly reduced. Another advantage is that by using thick radiating elements the antenna efficiency can be greatly enhanced to compensate for reduced efficiency as a consequence of the small number of radiating elements in the array.

Therefore, according to an aspect there is provided . . .

Turning toFIG.1, an embodiment of waveguide antenna device suitable for off-chip light coupling is shown, comprising an input waveguide100, silicon substrate140, a buried oxide layer (BOX),110an array of silicon radiating elements120, and a silica upper cladding layer130.

Each radiating element of the antenna consists of a pillar120aof un-etched silicon, which in embodiments can be 300-nm-thick, and an L-shaped segment120b, which in embodiments can be a partial etch of 150 nm. The L-shape segment120bprovides blazing to increase the fraction of power diffracted upwards and improve the grating directionality while the pillar120areduces back-reflection by destructive interference. The thick (e.g. 300 nm) silicon layer increases the grating scattering strength of the radiating elements120and hence reduces the required number of elements to achieve a target efficiency.

The antenna embodiment shown inFIG.1results in vertical emission (i.e., with the center axis of the emitted beam being approximately perpendicular to the chip plane) and a technique such as described in D. Melati et al., “Mapping the global design space of nanophotonic components using machine learning pattern recognition,” Nature Commun, vol. 10, no. 1, pp. 1-9, 2019] for selecting the best values for each of the five segments L1-L5of the radiating elements120.

Simulations for the waveguide antenna device having five radiating elements120were performed using a 2D-FDTD simulator (for simplicity,FIG.1shows only two radiating elements). Light at a central wavelength of λ=1550 nm was launched into the antenna by the fundamental TE mode of the input waveguide100at one side of the antenna device. Silicon and silica refractive indices are 3.478 and 1.448 at λ=1550 nm, respectively, and dispersion was also taken into account. The silicon substrate100was included in the simulation even though its effect is normally negligible due to the high directionality of the grating originated from its vertical asymmetry. Optimal segment lengths were chosen as L=[112, 54, 188, 176, 171] nm, which resulted in a total length of 3.51 μm for the antenna comprising five radiating elements. This design provided high values for both the upward diffraction efficiency and directionality, being respectively ρu=0.876 and Γ=ρu/(ρu+ρd)=0.947, with ρd the fraction of the injected optical power diffracted downwards.

The antenna device ofFIG.1can also be used to couple light into (or from) an optical fiber placed vertically on top of the antenna. In this case, the mode of the fiber can be modeled with a Gaussian function with a mode field diameter of 3.2 μm at λ=1550 nm. The fiber coupling efficiency, calculated as η=ρu·ϕ where ϕ is the overlap integral between the diffracted field and the Gaussian function, can be as high as η=0.759 (−1.20 dB). At the same time, the 1-dB bandwidth of η can be about 133 nm and back-reflection (i.e., the fraction of power coupled to the counter-propagating fundamental TE mode of the input waveguide) can be lower than −20 dB.

In another embodiment, the requirement for a fully periodic structure for the antenna device ofFIG.1can be relaxed, allowing one radiating element120to have different segment lengths than the remaining four elements (apodization). For example, using the design procedure described above segment lengths L=[57, 93, 118, 90, 277, 143, 41, 225, 156, 171] nm can be selected, where the first five numbers are the lengths of the segments for the first radiating element, and the remaining numbers are the lengths of the segments for the other four identical radiating elements (i.e. 57 nm is the length of the first gap, 93 nm is the length of the pillar, 118 nm is the length of the second gap, 90 nm is the length of the partially etched part, and 277 nm is the length of the last un-etched segment, etc). This design can achieve an upward diffraction efficiency ρu=0.919, directionality Γ=0,979, fiber coupling efficiency η=0.813, back-reflections lower than −20 dB, and 1-dB bandwidth of 158 nm, further improving the performance of the periodic design.

The two antenna examples described above combine high upward diffraction efficiency, high fiber coupling efficiency, small back-reflection, large bandwidth, and a compact total length of only 3.51 μm and 3.58 μm, respectively.

Turning toFIG.2, a second embodiment of the waveguide antenna device is shown comprising an input waveguide200, a 300-nm-thick silicon waveguide core comprising a plurality of identical radiating elements210, a 1-μm buried oxide layer (BOX)220and 2-μm silica cladding230and silicon substrate240. Each of the radiating elements210comprises two sections of un-etched silicon segments210aand two sections of fully etched gaps210b. The use of two silicon segments reduces the back-reflection by destructive interference of the reflected mode (as for the first embodiment described above). The different sections of each radiating element have a length of L1, L2, L3and L4where L1and L3refer to the length of the fully etched regions, L2and L4the length of the un-etched silicon segments.

In a design example for the embodiment ofFIG.2, an antenna device having three radiating elements was simulated using 2D-FDTD at wavelength λ=1550 nm, with parameters L1=116 nm, L2=50 nm, L3=185 nm, L4=399 nm, and a total length of the antenna of 2.25 μm.FIG.3ashows a diffraction efficiency of 56% achieved at the central wavelength of 1.55 μm.FIG.3bshows the simulated back-reflection including both the total reflected power and the fraction of power coupled to the counter-propagating fundamental TE mode of the input waveguide (modal reflection). The total back-reflection is seen to be below −20 dB and the modal reflectivity is approximately −21 dB at the central wavelength of 1.55 μm. Additionally, the simulated downward diffracted power and the residual power in the waveguide at the end of the antenna are seen to be 25% and 19%, respectively. The far field intensity as a function of polar angle θ is shown inFIG.4. The diffraction angle is 9° from the vertical. The full width half maximum (FWHM) of far field intensity along the polar coordinate is 23° at 1.55 μm.

Turning toFIG.5, a third embodiment of the waveguide antenna device is shown comprising an input waveguide300, a 300-nm-thick silicon waveguide core comprising a plurality of identical radiating elements310, a 1-μm buried oxide layer (BOX)320and 2-μm silica cladding530and silicon substrate340. Each of the radiating elements310comprises two sections of un-etched silicon segments310aand two sections of fully etched gaps310b. In this embodiment, a Bragg reflector (BR)350is placed at the end of the antenna device. The BR350behaves as a mirror for recirculating within the antenna the fraction of the optical power that remains un-diffracted, hence increasing the total upward diffraction efficiency. The G parameter represents the separation distance between the antenna termination and the BR350.

The Bragg reflector350in the embodiment ofFIG.5has three periods, with each period comprising a fully etched gap of length P1and un-etched silicon region of length P2. These parameters define the period (Λm=P1+P2) of the Bragg reflector350, which must satisfy the Bragg condition at the first order of diffraction, Λm=λ/2neff, where λ is the central wavelength 1.55 μm and neffis the effective refractive index of the Bloch mode of the grating. The optimized values for the structural parameters of Bragg reflector350are P1=152 nm and P2=148 nm. A reflectivity of 95% was achieved for the Bragg reflector350at 1.55 μm wavelength and a reflectivity above 94% for wavelengths ranging from 1.5 μm to 1.6 μm. The structural parameter values for the antenna part of the device are the same as the design example ofFIG.2(i.e. L1=116 nm, L2=50 nm, L3=185 nm, L4=399 nm) while G=337 nm. The total structure length (x-direction) was 3.487 μm.

With reference to the embodiment ofFIG.5,FIG.6ashows the diffraction efficiency for wavelengths ranging from 1.5 μm to 1.6 μm, with a diffraction efficiency of 64% at the central wavelength of 1.55 μm, indicating an improvement of 8% compared to the design example ifFIG.5without the Bragg reflector350.FIG.6bshows a modal back-reflection below −16 dB over the C optical communication band. The downward diffracted power and the residual power in the waveguide at the end of the Bragg reflector are about 30% and 0.5%, respectively. The far field intensity as a function of polar angle θ is shown inFIG.7, from which it will be noted that the diffraction angle is 8° from vertical. The full width half maximum (FWHM) of far field intensity along the polar coordinate is seen to be 23° at 1.55 μm.

Turning toFIG.8, a fourth embodiment of the waveguide antenna device is shown comprising an input waveguide800, a 300-nm-thick silicon waveguide core comprising a plurality of identical radiating elements810, and a 1-μm buried oxide layer (BOX)820and 2-μm silica cladding830. Each of the radiating elements810comprises two sections of un-etched silicon segments810aand two sections of fully etched gaps810b. Like the embodiment ofFIG.5, a Bragg reflector850is placed at the end of the antenna device. In the embodiment ofFIG.8, a bottom reflector860replaces the Si substrate to upwardly reflect the fraction of optical power diffracted downward by the antenna (e.g. 30%). This further increases the upward diffracted efficiency by having the reflected light constructively interfere with upward diffracted light. The parameters of the antenna embodiment ofFIG.8are L1=97 nm, L2=50 nm, L3=181 nm, L4=389 nm. For Bragg reflector850, the structural parameters are the same as the embodiment ofFIG.5(P1=152 nm, P2=148 nm.), and the total structure length (x-direction) was 3.384 μm.

With reference to the embodiment ofFIG.8,FIG.9ashows a diffraction efficiency of 95% at the central wavelength of 1.55 urn.FIG.9bshows total back-reflection power below −21 dB and modal reflectivity of −27 dB at the central wavelength of 1.55 urn. The far field intensity as a function of polar angle θ is shown inFIG.10. The diffraction angle is 4° from the vertical. The full width half maximum (FWHM) of the far field intensity along the polar coordinate is 23° at 1.55 μm.

Turning toFIG.11, a fifth embodiment of the waveguide antenna device is shown comprising an input waveguide1100, a 300-nm-thick silicon waveguide core comprising a plurality of identical radiating elements1110, a 1-μm buried oxide layer (BOX)1120, 2-μm silica cladding1130and silicon substrate1140. Each of the radiating elements1110comprises two sections of un-etched silicon segments1110aand two sections of fully etched gaps1110b. As in the embodiment ofFIG.5, a Bragg reflector1150is placed at the end of the antenna device. In the embodiment ofFIG.11, only two radiating elements1110are provided, exploiting the fact the Bragg reflector1150at the end of the antenna allows the number of radiating elements to be reduced because the un-diffracted power is recirculated by the Bragg reflector1150in the antenna device. In other respects, the parameters are the same as the antenna device embodiments ofFIG.5having three radiating elements1110and Bragg reflector1150. The total structure length (x-direction) is 2.737 μm.

With reference to the embodiment ofFIG.11,FIG.12ashows a diffraction efficiency of 62% at the central wavelength of 1.55 μm, representing only a 2% reduction compared to the antenna with three radiating elements despite a reduction in the length of the full device of more than 1 μm.FIG.12bshows total back-reflection power below −12.7 dB and modal reflectivity of −13 dB at the central wavelength of 1.55 μm. The downward diffracted power and the residual power in the waveguide at the end of the Bragg reflector350are about 28% and 0.8%, respectively. The far field intensity as a function of polar angle θ is shown inFIG.13. The diffraction angle is 2° from the vertical. The full width half maximum (FWHM) of far field intensity along the polar coordinate is 32° at 1.55 μm, representing an increment of 9° compared to an antenna device with three radiating elements.

Turning toFIG.14, a sixth embodiment of the waveguide antenna device is shown comprising an input waveguide1400, a 300-nm-thick silicon waveguide core comprising a radiating element1410, a 1-μm buried oxide layer (BOX)1420, 2-μm silica cladding1430and silicon substrate1440. The radiating element1410comprises two sections of un-etched silicon segments1410aand two sections of fully etched gaps1410b. As in the embodiment ofFIG.5, a Bragg reflector1450is placed at the end of the antenna device. In the embodiment ofFIG.14, only a single radiating element1410is provided, exploiting the fact the Bragg reflector1450at the end of the antenna allows the number of radiating elements to be reduced because the un-diffracted power is recirculated by the Bragg reflector1450in the antenna device. In other respects, the parameters are the same as the antenna device embodiment ofFIGS.5and11, while the total structure length (x-direction) is 1.987 μm.

With reference to the embodiment ofFIG.14,FIG.15ashows a diffraction efficiency of 41% at the central wavelength of 1.55 μm.FIG.15bshows total back-reflection power below −6 dB and modal reflectivity of −6 dB at the central wavelength of 1.55 μm. The downward diffracted power and the residual power in the waveguide at the end of the Bragg reflector350are about 20% and 1%, respectively. The diffraction angle is −8° from the vertical (FIG.16). The full width half maximum (FWHM) of the far field intensity along the polar coordinate is 40° at 1.55 μm.

Contemplated applications of embodiments set forth herein include optical communication systems such as waveguide-to-fiber and chip-to-chip interfaces for telecommunications, data communications, optical interconnects, WDM systems, dense fiber arrays, multi-channel systems and multi-core fiber interfaces. Additional examples of applications include optical (phased) arrays, single-pixel imaging, Lidars for remote sensing and navigation, autonomous cars and drones navigation, free-space optical communication, and power efficient transceivers.

The present invention has been described above with reference to a number of exemplary embodiments and examples. It should be appreciated that the particular embodiments shown and described herein are illustrative of the invention and its best mode and are not intended to limit in any way the scope of the invention as set forth in the claims. The features of the various embodiments may stand alone or be combined in any combination. Further, unless otherwise noted, various illustrated steps of a method can be performed sequentially or at the same time, and not necessarily be performed in the order illustrated. It will be recognized that changes and modifications may be made to the exemplary embodiments without departing from the scope of the present invention. These and other changes or modifications are intended to be included within the scope of the present invention, as expressed in the following claims.