The invention designs and fabricates an ultra-broadband mode size converter based on an on-chip Luneburg lens, which relates to a technology in the field of integrated photonics.
In the photonic integrated circuits, in order to achieve ultra-broad operation bandwidth and small insertion loss, it is necessary to design optical devices with compact footprints and high coupling efficiency. One of the important devices is the mode size converter.
The mode size converter is used to match the different mode sizes. It can convert the mode size to achieve low-loss coupling between waveguides of different widths.
Silicon-based photonic devices have the characteristics of strong mode field confinement and the advantages of being compatible with complementary metal oxide semiconductor (CMOS) processes, making them as an ideal choice for photonic integrated circuits.
In view of the complex design of the existing tapered waveguide structures and the difficulty in the manufacturing for focused ion beam etching or gray-scale exposure technology, an ultra-broadband mode size converter based on an integrated Luneburg lens is proposed. The refractive index distributions required by the Luneburg lens are achieved by the gradient index structures based on the metamaterial, which is combined with the silicon waveguides. Finally, a matching of the mode size in the waveguides of different widths is realized.
The invention is implemented through the following technical solutions:
The invention includes: an integrated Luneburg lens, input and output silicon waveguides, where the input and the output waveguides are respectively arranged on both sides of the Luneburg lens.
The silicon waveguide includes: an input waveguide named first waveguide and an output waveguide named second waveguide.
The width of the first waveguide is greater than the width of the second waveguide.
The structure of the on-chip Luneburg lens is a metamaterial layer whose upper and lower cladding layers are both silicon dioxide, and the metamaterial layer is a silicon periodic nanorods array structure with gradient index profiles.
The Luneburg lens has a radial duty ratio distribution, and the refractive index distribution satisfies: n(R)=ne √{square root over (2−(R/Rlens)2)}, where: ne is the edge refractive index, Rlens is the radius of the Luneburg lens, R is the radial distance from the center of the Luneburg lens, The length of the lens is L=2Rlens.
The relationship between the maximum refractive index and the minimum refractive index in the Luneburg lens is nmax=√{square root over (2)}nmin, where nmin refers to the minimum refractive index value in the Luneburg lens, and nmax, refers to the maximum refractive index value in the Luneburg lens.
The equivalent material refractive index of the Luneburg lens is nmeta(R)2=δ(R)·nSi2+[1−δ(R)]·nSiO
Results
The invention realizes the mode size conversion, coupling the light in the wide waveguide to the narrow waveguide in the silicon-based chip with extremely low loss. compared with the reported mode size converters, the mode size conversion can be achieved in the wavelength of 1.26 μm˜2 μm with the conversion loss of <1 dB, and a length of 11.2 μm, which exhibits excellent performance.
In the
As shown in
The silicon waveguide 2 includes: a first waveguide 5 and a second waveguide 6, where the first waveguide 5 is arranged on the input position 3, and the second waveguide 6 is arranged on the output position 4.
The structure of the Luneburg lens 1 is a metamaterial layer with both upper and lower cladding layers of silicon dioxide, where the metamaterial layer is a silicon periodic nanorod array with gradient index profiles, and the effective refractive index depends on the duty cycle of the silicon nanorods array. The period of the nanorods is P. The metamaterial layer realizes the function of the Luneburg lens, reducing the footprint with low loss.
The width of the first waveguide 5 and the diameter of the Luneburg lens 1 can be adjusted according to practical applications.
The width of the first waveguide 5 is greater than the width of the second waveguide 6, and the expansion ratio of the first waveguide 5 and the second waveguide 6 is 20:1, and the expansion ratio can be adjusted according to practical applications.
The Luneburg lens 1 has a radial duty cycle distribution, and the refractive index distribution satisfies n(R)=ne √{square root over (2−(R/Rlens)2)}, where ne is the edge refractive index, Rlens is the radius of the Luneburg lens 1, and R is the radial distance from the center of the Luneburg lens 1. The length of Luneburg lens 1 is L=2Rlens.
The relationship between the maximum refractive index and the minimum refractive index in the Luneburg lens 1 is nmax=√{square root over (2)}nmin.
The refractive index of the equivalent material of the Luneburg lens 1 is nmeta(R)2=δ(R)·nSi2+[1−δ(R)]·nSiO
The solutions simulation relates to an ultra-broadband mode size converter, which includes the following steps:
Step 1: Set simulation parameters;
The thickness of the silicon layer on the SOI platform is 220 nm, the thickness of the buried oxide layer is 3 μm, and the thickness of the cladding silicon dioxide layer is 1 μm. The width of the first waveguide 5 and the second waveguide 6 are set to 10 μm and 0.5 μm, respectively. The minimum duty cycle of the nanorods is set to 15% so that the minimum effective refractive index is 1.84. The maximum duty ratio is set to 81% so that the maximum refractive index is 2.6. The period is 246 nm, and the length of the lens is L=2Rlens=11.2 μm.
Step 2: Calculate the insertion loss and operation bandwidth according to the simulation parameters.
As shown in
Step 3: Changing the parameters of the first waveguide 5, the second waveguide 6 and the Luneburg lens 1, and calculating the effective refractive index of the TM fundamental mode under different wavelengths.
As shown in
In the experiments, under normal room temperature, the C- and O-band laser are employed as input light source, the width of the first waveguide 5 and the second waveguide 6 are 10 μm and 0.5 μm, respectively. The minimum duty cycle of the nanorods is set to 15% so that the minimum effective refractive index is 1.84. The maximum duty cycle is set to 81% so that the maximum refractive index is 2.6. The period is 246 nm, the length of the lens is L=2Rlens=11.2 μm. The mode size conversion is lower than 1 dB in the wavelength of 1260 nm˜1360 nm and 1507 nm˜1607 nm.
Compared with the reported mode size converter, this device can realize the conversion from 1.26 μm to 2 μm with a bandwidth of 740 nm, which is better than the performance of the existing taper structure. The conversion loss of the mode size is lower than 1 dB, which is better than the performance of the existing Hollow taper. The length of the device is 11.2 μm, and the footprint is more compact than a flat lens and other lens structures.
The above-mentioned specific implementations can be locally adjusted by different ways without departing from the principle and purpose of the invention. The protection scope of the invention is subject to the claims and is not limited by the above-mentioned specific implementations. All implementation schemes within the scope are bound by the invention.
Number | Date | Country | Kind |
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202011208110.9 | Nov 2020 | CN | national |
This application is the US continuation application of International Application No. PCT/CN2021/096618 filed on 28 May 2021 which designated the U.S. and claims priority to Chinese Application No. CN202011208110.9 filed on 3 Nov. 2020, the entire contents of each of which are hereby incorporated by reference.
Number | Date | Country | |
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Parent | PCT/CN2021/096618 | May 2021 | US |
Child | 17448186 | US |