CN113552670B

CN113552670B - Mach-Zehnder interferometer protected by topology

Info

Publication number: CN113552670B
Application number: CN202010338510.5A
Authority: CN
Inventors: 郭小伟; 杨沛霖; 蒋鹏鹏; 李绍荣; 杨承
Original assignee: University of Electronic Science and Technology of China
Current assignee: University of Electronic Science and Technology of China
Priority date: 2020-04-26
Filing date: 2020-04-26
Publication date: 2022-06-21
Anticipated expiration: 2040-04-26
Also published as: CN113552670A

Abstract

The present invention provides a topologically protected mach-zehnder interferometer. The interferometer is formed by integrating two-dimensional topological photonic crystal waveguides and comprises two valley photonic crystals with different topological indexes. The Mach-Zehnder interferometer has the characteristics of unidirectional transmission, immune backscattering, immune defects, compatibility with various substrate materials and the like, has high efficiency and robustness, can avoid interference caused by manufacturing defects while keeping high-efficiency output, and works stably. An efficient and stable implementation method of the photonic crystal MZI is provided for future all-optical networks, biosensing, spectral analysis and the like.

Description

Mach-Zehnder interferometer protected by topology

Technical Field

The invention relates to the technical field of photonic crystals, in particular to a Mach-Zehnder interferometer with topological boundary states.

Background

Mach-Zehnder interferometers (MZIs), one of the most widely used optical elements, are commonly used means for measuring the relative amount of change in relevant physical properties (refractive index or density, etc.) due to their high sensitivity to the surrounding medium. It has been widely used to implement various photonic devices such as optical switches, filters, intensity modulators, refractive index detectors, displacement gauges, and the like.

Conventional silicon-based MZIs using standard waveguides are a relatively mature technology, but the propagation constant perturbation variation Δ β of such MZIs is rather low. Because the phase change satisfies the relationship with the propagation constant variation Δ β and the waveguide length L: since Δ Φ is Δ β × L, the length of the waveguide needs to be long, and the length of the interference arm in the conventional silicon-based MZI usually needs to be half to several millimeters to satisfy the phase shift requirement. Although the sensitivity of the MZI can be improved using resonance enhancement, this approach limits the bandwidth of the device and makes it very sensitive to manufacturing defects.

MZIs using Photonic Crystal (PC) waveguides have a larger Δ β by introducing slow light effects. Thus, the PC-MZI can greatly shorten the length of the interference arm while preserving bandwidth. The length of the interference arm in the PC-MZI is typically only a few tens of microns, which is 10 times smaller than the silicon-based waveguide MZI. The short length of the interference arm not only makes the device more compact, but also can ensure low propagation loss, and avoid the coupling loss of the input and output ports in the traditional silicon-based waveguide MZI. However, based on PC-implemented MZI, errors inevitably occur in the manufacturing process, resulting in structural defects. As a result, strong backscattering of the light transmission process can occur, causing the power of the device to drop. Therefore, there is a strong need to design a photonic crystal waveguide with robustness so as to avoid the challenge of manufacturing defects to the reliability of the device.

Disclosure of Invention

The present invention is directed to overcoming the above-mentioned drawbacks of the prior art, and provides a topology protected mach-zehnder interferometer (TPMZI) which can ensure high efficiency output while avoiding interference from manufacturing defects and operating stably.

The TPMZI is formed by integrating two-dimensional topological photonic crystal waveguides and comprises a beam splitting waveguide, a wave combining waveguide, an input coupling waveguide, an output coupling waveguide, an interference arm waveguide, a reference arm waveguide and a middle interface waveguide.

The two-dimensional topological photonic crystal waveguide is based on a valley Hall quantum effect and consists of two Valley Photonic Crystals (VPCs) with unequal topological indexes, and the spliced boundary of the two valley photonic crystals is in a zigzag-zag type. Such "zig-zag" type boundaries of unequal topological indices, along which light can propagate unidirectionally, are called domain walls.

The VPC primitive cell medium column material is silicon, the background is air, and the VPC primitive cell medium column material is arranged according to a honeycomb-shaped lattice period. When the radii of the medium columns in the VPC protocell are equal, the energy band structure of the VPC protocell is in a Dirac cone shape, degeneracy occurs at nonequivalent valleys K and K' of the Brillouin zone, and a double degenerated Dirac point is formed. At the moment, the VPC topological index is equal to 0, and a topological peaceful state is presented; when the radius of a medium column in a VPC primitive cell is adjusted, a Dirac point in an energy band can be opened by destroying space inversion symmetry, and topological phase change is realized. At this time, the VPC topological index is not equal to 0, and the topological non-trivial state is presented. The two types of topological indexes, namely the radius difference delta r >0 and delta r <0 of two dielectric columns in the VPC primitive cell are opposite (+ -1) and are respectively marked as VPC1 and VPC 2.

The two-dimensional topological photonic crystal waveguide has two domain wall types, namely an S type and an L type, which respectively correspond to the condition that VPC1 is above the boundary of a splicing part and VPC2 is above the boundary of the splicing part.

In the above scheme, the beam splitting waveguide and the wave combining waveguide are in a Y-shaped structure, and the beam splitting angle and the wave combining angle are 120 degrees.

In the above scheme, the intermediate interface waveguide is different from other waveguide domain wall types.

Compared with the prior art, the MZI provided by the invention has the following excellent properties:

firstly, the efficiency is high: the MZI is protected by topology, light propagates unidirectionally along a domain wall, and power loss caused by backward reflection is avoided.

Secondly, defective immunity: the MZI is protected by topology, immune disorder, blockage and other manufacturing defects, the working stability of the device is ensured, and the requirements on the manufacturing technology are reduced.

Thirdly, the structure is compact: the MZI inherits the advantages of a traditional PC-MZI, has small structure size and can meet the requirement of high integration level.

Fourthly, the use is flexible: through the change of the structure size, the device can work at any wavelength and frequency, including radio, microwave, terahertz, infrared, visible and ultraviolet bands.

Drawings

FIG. 1 is a schematic diagram of a TPMZI structure according to the present invention.

FIG. 2 is a schematic diagram of a two-dimensional topological photonic crystal waveguide structure and a projection diagram of an energy band thereof.

FIG. 3 is a diagram of the transmission of light without a fill probe in the probe arm of the MZI.

FIG. 4 shows the propagation of light in the case of defects in the MZI.

Detailed Description

The technical solution of the present invention is described in detail below with reference to the accompanying drawings by specific embodiments.

As shown in fig. 1, the TPMZI of the present invention is integrated by two-dimensional topological photonic crystal waveguides, and includes a splitting waveguide SP1, a combining waveguide SP2, input and output waveguides, an interference arm waveguide, a reference arm waveguide, and an intermediate interface waveguide. The lengths of the input coupling waveguide and the output coupling waveguide are both 36a, the lengths of the interference arm waveguide and the reference arm waveguide are both 34a, and the distances of the split beam and the wave combination of the split beam and wave combination waveguides are both 17 a.

The beam splitting waveguide and the wave combining waveguide are Y-shaped structures, and the beam splitting angle and the wave combining angle are 120 degrees.

As shown in FIG. 2(a), the two-dimensional topological photonic crystal waveguide is composed of two VPCs with different topological indexes, and the spliced boundary of the VPCs is in a zigzag-zag shape to form a domain wall capable of guiding unidirectional transmission of light. The VPC primitive cell medium column material is silicon, the background is air, the VPC primitive cell medium column material is arranged according to a honeycomb-shaped crystal lattice period, and the crystal lattice constant a is 451 nm. The radius of the dielectric pillars in the VPC primitive cell was adjusted so that the difference in radius | Δ r | -0.06 a, and VPC1 and VPC2 corresponded to the cases where Δ r >0 and Δ r <0, respectively. There are two domain wall types for two-dimensional topological photonic crystal waveguides, denoted as "S-type" and "L-type", corresponding to the situation where VPC2 is above the boundary at the splice and VPC1 is above the boundary at the splice, respectively. The edge states propagating within the different types of domain walls also differ, as shown in FIG. 2(b), where the normalized frequency range of the edge states in the projected energy band for "S-type" domain walls is 0.266-0.302c/a, and the normalized frequency range of the edge states in the projected energy band for "L-type" domain walls is 0.246-0.282 c/a.

FIG. 3 is a schematic of the output power of the TPMZI. The transmission of the TPMZI at the SP1 and SP2 ends is shown in fig. 3(a), where the selected operating frequency range is covered with rectangular shading. It can be seen that the transmittance of the output end of the TPMZI is almost the same as that of the SP1 end, broadband high-efficiency transmission is realized in the wavelength range of 1500nm-1600nm, and light can be transmitted without loss. Fig. 3(b) - (d) are graphs of electric fields at different wavelengths of the TPMZI in the operating frequency range, and it can be seen that light waves can propagate without loss at places with sharp corners and no backscattering occurs. This property is particularly attractive for the development of efficient optical and photonic devices. For example, photonic integrated circuits (e.g., filters, wavelength demultiplexers or channel interleavers, etc.) for use in high-speed Wavelength Division Multiplexing (WDM) systems are implemented.

FIG. 4 shows the propagation of light in the case of a TPMZI containing typical defects, such as blocking and disorderly defects. The disordered defects are generated by that the size of a photonic crystal forming a detection arm interface is not uniform due to the fact that a simulation manufacturing process is not perfect, the diameter of an upper row of dielectric columns is increased by 3nm, and the diameter of a lower row of dielectric columns is reduced by 3nm, as shown in fig. 4 (a); the blocking defect was created by adding a column of silicon media in the probe arm, simulating the situation where particles block the light propagation channel when chemical and biological sensing tests were performed, as shown in fig. 4 (c). As can be seen from fig. 4(b), in the case of a blocking defect, the input and output power of the MZI are almost equal, due to the light bypassing the defect without back reflection, so the optical path is only slightly increased. In the presence of a disordered defect, however, it can be found that the power in the probe arm is lower than the power in the reference arm (as shown in figure 4 (d)), because the propagation constant in the probe arm changes, and is equal to the incident waveguide. Thus, when the light is split at SP1, more energy enters the reference arm closer to the incident waveguide propagation constant. However, neither the blocking defects nor the disordered defects are of the valley-mixed type, and therefore the edge state mode propagating along the domain wall is not destroyed and can still propagate without back reflection, as shown in fig. 4(d), confirming the immunity of the topologically protected edge state to defects.

The above description is only for the specific embodiments of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art can easily conceive of the changes or substitutions within the technical scope of the present invention, and all the changes or substitutions should be covered within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the appended claims.

Claims

1. A topologically protected mach-zehnder interferometer, wherein:

the Mach-Zehnder interferometer protected by topology is formed by integrating two-dimensional topological photonic crystal waveguides, comprises a beam splitting waveguide, a wave combination waveguide, an input coupling waveguide, an output coupling waveguide, an interference arm waveguide, a reference arm waveguide and an intermediate interface waveguide, and is characterized in that:

the two-dimensional topological photonic crystal waveguide is based on a valley Hall quantum effect and consists of two valley photonic crystals with unequal topological indexes, the valley photonic crystals are formed by periodically arranging medium columns with circular sections in a honeycomb mode, the medium columns are made of silicon, and the background is air; in the primitive cells of the two valley photonic crystals, the radius difference Δ r of the two dielectric columns at the same position is greater than 0 and Δ r is less than 0, so that the topological indexes of the two valley photonic crystals are opposite numbers, and the two valley photonic crystals are respectively marked as a valley photonic crystal 1 and a valley photonic crystal 2.

2. The topologically protected mach-zehnder interferometer of claim 1, wherein the splitting waveguide and the combining waveguide are Y-shaped structures and the splitting and combining angles are 120 °.

3. The topologically protected mach-zehnder interferometer of claim 1, wherein the boundary at which the two valley photonic crystals are spliced is of a "zig-zag" type, forming a domain wall capable of guiding unidirectional transmission of light.

4. A topologically protected mach-zehnder interferometer as claimed in claim 3, characterized in that the intermediate interface waveguide is of a different domain wall type than the other waveguides.

5. A topologically protected mach-zehnder interferometer according to claim 3, characterized in that there are two domain wall types, denoted "S-type" and "L-type", corresponding to the case of valley photonic crystals 2 above the boundary of the splice and valley photonic crystals 1 above the boundary of the splice, respectively.