CN111190252B - Triangular lattice photonic crystal waveguide based on air column and lithium niobate air column structure - Google Patents
Triangular lattice photonic crystal waveguide based on air column and lithium niobate air column structure Download PDFInfo
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/122—Basic optical elements, e.g. light-guiding paths
- G02B6/1225—Basic optical elements, e.g. light-guiding paths comprising photonic band-gap structures or photonic lattices
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- G—PHYSICS
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- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
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- G02B2006/1204—Lithium niobate (LiNbO3)
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
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Abstract
The invention discloses a triangular lattice photonic crystal waveguide based on an air column and lithium niobate air column structure, wherein the triangular lattice photonic crystal forms guided waves for electromagnetic waves with corresponding frequencies by introducing photon local characteristics after defects, and the waveguide width of the triangular lattice photonic crystal is in the nanometer level, so that the triangular lattice photonic crystal waveguide can ensure high turning radius and simultaneously obtain extremely high transmission efficiency and transmission bandwidth by designing a 60-degree bent waveguide. The triangular lattice photonic crystal waveguide based on the air column and lithium niobate air column structure can solve the problems of waveguide miniaturization, high transmission bandwidth and low transmission loss in integrated optics, and provides an excellent solution for the design of a curved waveguide for future integrated optics.
Description
Technical Field
The invention relates to the technical field of integrated optics, in particular to a triangular lattice photonic crystal waveguide based on an air column and lithium niobate air column structure.
Background
Materials, energy and information are three main pillars of civilization in the world. The current society is developing rapidly towards informatization, information communication technology and application are quite wide, various electronic products appear in the aspect of our lives, and information growing rapidly fills in our lives, so that new requirements on the processing speed and the storage capacity of the information are provided. At the same time, the integration methods of higher efficiency, higher speed and miniaturization are continuously changed, and the chip size is continuously reduced. Since the semiconductor physics discipline was established in the last century, electrons became the main information carrier in integrated chips, but due to the coulomb effect among electrons, the improvement of the performance of integrated circuits must be contradictory to the reduction of the integration level. At the moment, the performance advantage of photons is highlighted, the loss can be reduced while the information transmission speed is greatly improved, and the integration level is continuously improved. The advantages of photons are more obvious by the proposal of the photonic crystal. Control of photonics, design and production of optical devices are possible, providing solutions for future all-optical communications, photonic computers, and the like.
The photonic crystal is a brand-new artificial microstructure material and has a plurality of excellent characteristics, such as photonic band gap, photonic local area, slow light effect and the like. The unique band gap and slow light effect provide possibility for designing optical communication devices with higher integration level and better performance, and meet the requirement of the development of optical communication on higher and higher integration level of the optical communication devices. The photonic crystal fundamentally solves the problem of light control of the micro-nano-sized optical device, provides a new way for realizing an ultra-dense integrated device, and enables photonic crystal waveguides, filters, modulators, beam splitters and the like based on the photonic crystal to have wide application prospects.
The waveguide is used as the most basic channel in integrated optics, the integration of the waveguide is always the core problem limiting the development of the integrated optics, how to design the integrated waveguide at a micro-nano level, and meanwhile, ensuring a certain transmission bandwidth and lower transmission loss is always the bottleneck of the development of the integrated optics. The waveguide manufactured by the traditional total reflection principle or the traditional ridge waveguide generally has the defects of small waveguide turning angle, high transmission loss, low transmission bandwidth, incapability of integration and the like.
Disclosure of Invention
The invention aims to provide a triangular lattice photonic crystal waveguide based on an air column and lithium niobate air column structure, and aims to solve the problems that the traditional waveguide is small in turning angle, high in transmission loss, low in transmission bandwidth and incapable of being integrated.
In order to achieve the purpose, the invention provides the following scheme:
an air column structure based triangular lattice photonic crystal waveguide, comprising: the waveguide structure comprises a 60-degree bent waveguide and an air column structure arranged at the bent waveguide of the 60-degree bent waveguide;
the 60-degree bending waveguide is obtained by introducing line defects into a two-dimensional photonic crystal structure of a triangular lattice; the two-dimensional photonic crystal structure of the triangular lattice comprises a first medium and a second medium; the first medium and the second medium are different media; one first medium is placed at each intersection point of the triangular lattice, and the second medium is filled between the first media to form a two-dimensional periodic structure in which the first media and the second media alternately appear;
the air column structure comprises 6 air columns; the 6 air columns are respectively a first air column, a second air column, a third air column, a fourth air column, a fifth air column and a sixth air column; the bent waveguide comprises a first waveguide, an inflection point and a second waveguide; the first waveguide and the second waveguide are symmetrical about a central line where the inflection point is located; the two first mediums which are positioned on the central line and adjacent to the inflection point are respectively an inner first medium and an outer first medium; the inner first medium is positioned at the inner side of the bent waveguide; the outer first medium is positioned outside the bent waveguide; a first medium which is positioned at the edge of the first waveguide and is respectively adjacent to the inflection point and the outer first medium is called a target first medium; the corner point, the triangular lattice where the outer side first medium and the target first medium are jointly located are first target triangular lattices; the second column of air is located at the first target trigonal lattice center; the fifth column of air and the second column of air are symmetrically disposed about the centerline; a triangular lattice located within the first waveguide and adjacent to the first target triangular lattice is referred to as a second target triangular lattice; the triangular lattice adjacent to the second target triangular lattice and with the inflection point is called a third target triangular lattice; a common edge of the second target triangular lattice and the third target triangular lattice is referred to as a target edge; the third air column is positioned at the center of the target edge; the sixth air column and the third air column are symmetrically arranged about the center line; the triangular lattice adjacent to the second target triangular lattice and in which the target first medium is located is called a fourth target triangular lattice; the first column of air is located at the fourth target trigonal lattice center; the fourth air column and the first air column are arranged symmetrically about the center line.
Optionally, the first medium is an air medium column; the second medium is a lithium niobate medium.
Optionally, the distance a between the centers of two adjacent first mediums is 500nm to 700 nm; the radius r of the first medium is 0.3a to 0.4 a.
Optionally, the radius r' of the second air column is 0.5 r; the third air column, the fifth air column, the sixth air column and the second air column have the same radius.
Optionally, the radius r ″ of the first air column is 0.46 r; the fourth column of air has the same radius as the first column of air.
A lithium niobate air column structure based triangular lattice photonic crystal waveguide, comprising: the waveguide structure comprises a 60-degree bent waveguide and a lithium niobate air column structure arranged at the bent waveguide of the 60-degree bent waveguide; the lithium niobate air column structure comprises 1 air column and 3 lithium niobate columns;
the 60-degree bending waveguide is obtained by introducing line defects into a two-dimensional photonic crystal structure of a triangular lattice; the two-dimensional photonic crystal structure of the triangular lattice comprises a first medium and a second medium; the first medium and the second medium are different media; one first medium is placed at each intersection point of the triangular lattice, and the second medium is filled between the first media to form a two-dimensional periodic structure in which the first media and the second media alternately appear;
the 3 lithium niobate columns are respectively a first lithium niobate column, a second lithium niobate column and a third lithium niobate column; the bent waveguide comprises a first waveguide, an inflection point and a second waveguide; the first waveguide and the second waveguide are symmetrical about a central line where the inflection point is located; the two first mediums which are positioned on the central line and adjacent to the inflection point are respectively an inner first medium and an outer first medium; the inner first medium is positioned at the inner side of the bent waveguide; the outer first medium is positioned outside the bent waveguide; the first lithium niobate column is positioned at the inflection point; the column of air is located at the inboard first medium;
a first medium which is positioned at the edge of the first waveguide and is respectively adjacent to the inflection point and the outer first medium is called a target first medium; the corner point, the triangular lattice where the outer side first medium and the target first medium are jointly located are first target triangular lattices; a central line passing through the inflection point among the three central lines of the first target triangular lattice is called a target central line; the second lithium niobate pillar is positioned on the target middle line, and the distance between the center of the second lithium niobate pillar and the center of the target first medium is 0.6 a; a is the distance between the centers of two adjacent first media; the third lithium niobate pillar and the second lithium niobate pillar are symmetrically arranged about the center line.
Optionally, the first medium is a lithium niobate medium column; the second medium is an air medium.
Optionally, the distance a between the centers of two adjacent first mediums is 580nm to 750 nm; the radius r of the first medium is 0.18a to 0.3 a.
Optionally, the radius r' of the air column is 0.9 r.
Optionally, the radius of the first lithium niobate pillar is r ″ -0.48 r; the second lithium niobate column and the third lithium niobate column have the same radius as the first lithium niobate column.
According to the specific embodiment provided by the invention, the invention discloses the following technical effects:
the invention provides a triangular lattice photonic crystal waveguide based on an air column and lithium niobate air column structure, the triangular lattice photonic crystal forms guided waves for electromagnetic waves with corresponding frequencies by introducing photon local characteristics after defects, and the waveguide width of the triangular lattice photonic crystal is in a nanometer level, so that the triangular lattice photonic crystal waveguide can ensure high turning radius and simultaneously obtain extremely high transmission efficiency and transmission bandwidth by designing a 60-degree bent waveguide. The triangular lattice photonic crystal waveguide based on the air column and lithium niobate air column structure can solve the problems of waveguide miniaturization, high transmission bandwidth and low transmission loss in integrated optics, and provides an excellent solution for the design of a curved waveguide for future integrated optics.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without inventive exercise.
FIG. 1 is a schematic diagram of the basic structure of a triangular lattice two-dimensional photonic crystal provided by the present invention;
FIG. 2 is a schematic structural view of an unoptimized 60 degree curved waveguide provided by the present invention;
FIG. 3 is an optimized schematic diagram of a triangular lattice photonic crystal waveguide provided by the present invention; wherein FIG. 3(a) is a schematic skeletal diagram of a triangular lattice pattern; FIG. 3(b) is a schematic diagram of the structure of a perfect photonic crystal; FIG. 3(c) is a plan view of a two-dimensional perfect photonic crystal; FIG. 3(d) is a schematic illustration of the introduction of defects in a perfect photonic crystal; FIG. 3(e) is a schematic diagram of the structure of a 60 ° curved waveguide obtained after the introduction of a defect; FIG. 3(f) is a schematic diagram of an optimization of a 60 ° curved waveguide structure;
FIG. 4 is a schematic structural view of a 60 ° bend waveguide provided by the present invention;
FIG. 5 is a schematic structural diagram of a triangular lattice photonic crystal waveguide based on an air column structure according to the present invention;
FIG. 6 is a schematic diagram of a partial enlarged structure of a triangular lattice photonic crystal waveguide based on an air column structure according to the present invention;
FIG. 7 is a schematic structural diagram of a triangular lattice photonic crystal waveguide based on a lithium niobate air column structure provided by the present invention;
FIG. 8 is a schematic diagram of a partial enlarged structure of a triangular lattice photonic crystal waveguide based on a lithium niobate air column structure according to the present invention;
FIG. 9 is a schematic diagram of transmission modes in an unoptimized curved waveguide and an optimized curved waveguide provided by an embodiment of the present invention; fig. 9(a) is a schematic diagram of a transmission mode in an unoptimized curved waveguide, and fig. 9(b) is a schematic diagram of a transmission mode in an optimized curved waveguide.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The invention designs two curved thin film waveguides aiming at a two-dimensional triangular lattice photonic crystal taking lithium niobate and an air medium as base materials, wherein the two curved thin film waveguides are respectively a triangular lattice photonic crystal waveguide based on an air column structure and a triangular lattice photonic crystal waveguide based on a lithium niobate air column structure. The two waveguides respectively realize high-bandwidth and high-efficiency transmission for the light waves in the TM and TE modes of the near infrared band. The structural design of the two waveguides can realize waveguide miniaturization, and the waveguide can be applied to various integrated optical designs, so that the problems of small waveguide turning angle, high transmission loss, low transmission bandwidth, incapability of integration and the like are solved.
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.
The invention firstly selects a two-dimensional photonic crystal with triangular lattice as a basic structure, and the structure is shown in figure 1. Wherein the black circle is the first medium 1, the white background is the second medium 2, the first medium 1 is spread in the plane with 3 circles forming an equilateral triangle as a cycle, forming a two-dimensional periodic structure in which the first medium 1 and the second medium 2 appear alternately. The main parameters of the triangular lattice two-dimensional photonic crystal structure comprise: the distance between the centers of two adjacent first media 1, denoted by a, is also referred to as the period constant; the radius of the circle of the single first medium 1 is denoted by r. The ratio r/a of the two is a duty ratio, the change of the duty ratio enables the microstructure to generate a photon forbidden band, and the electromagnetic wave of the corresponding frequency band cannot be transmitted. By introducing line defects into the triangular lattice two-dimensional photonic crystal structure shown in fig. 1, that is, forming an unoptimized 60-degree curved waveguide shown in fig. 2, photon local regions are formed in the line defects, electromagnetic wave guided waves corresponding to forbidden band frequency bands are formed, a high transmission mode is formed, and the micro-nano thin film waveguide with high bandwidth and low transmission loss is realized.
The photonic crystal with the introduced defect directly forms a photonic crystal waveguide, an unoptimized thin film waveguide is shown in fig. 2, and the transmission efficiency is extremely low and guided waves cannot be formed due to the fact that the transmission modes of the light waves are not matched and the wave vectors are different before and after the bending of the waveguide. The invention performs local periodic constant change on the waveguide bending structure, so as to match the transmission mode of the bending waveguide and form a high transmission mode.
To facilitate understanding of the photonic crystal waveguide, the principle of the triangular lattice configuration will be briefly explained below. FIG. 3(a) shows a triangular lattice pattern skeleton, which is similar to (3) in the Archimedes two-dimensional periodic pattern6) The structure is the same, and both photonic crystal waveguides in the invention are designed based on the graph structure. The present invention places a circular medium at each triangular lattice intersection, named "first medium 1", represented by a black circle in fig. 3(b), and the rest is filled with another medium, named "second medium 2", as long as it is ensured that the first medium 1 and the second medium 2 are different media, i.e., a perfect photonic crystal is formed. After the black skeleton line in fig. 3(b) is erased, a plan view of a two-dimensional perfect photonic crystal can be obtained, as shown in fig. 3 (c).
In order to more clearly illustrate the design of the two waveguide structures of the present invention, the structure diagram with the skeleton line is used for the next explanation in fig. 3(d) -3 (e). It is understood here that all the portions covered by the skeleton lines do not allow electromagnetic waves of the corresponding frequency to pass through. In the structure of periodic alternation of lithium niobate and air, the wavelength of electromagnetic wave corresponding to frequency is about 2.2-2.6 times of the periodic constant, and the duty ratio influences the width of photonic band gap, which is the forbidden band characteristic of photonic crystal. Therefore, the invention needs to set a period constant of 500-700 nm, so that the forbidden band can be ensured to fall in a near infrared band (1300-1800 nm), and meanwhile, the forbidden band can accurately fall near 1550nm by adjusting the duty ratio r/a, and the band gap width is improved as much as possible.
In order to form a guided wave in the photonic crystal, the strict periodic structure of the perfect photonic crystal can be destroyed, so that the skeleton lines between the crystal lattices are also destroyed, as shown in fig. 3(d), the circular first medium in the dotted line in the figure is removed completely, and this step is called "introducing defects". Guided waves formed by the second medium 2 locally appear in the structure after the defect is introduced, the part is called photon local area, and the electromagnetic waves can only be transmitted along the guided waves formed by the photon local area because the electromagnetic waves can not be transmitted in the framework, namely the photonic crystal waveguide.
In the present invention, only 60 DEG bending waveguide is studied, and the structure thereof is shown in FIG. 3(e), and the waveguide width is
And the a is 500 nm-700 nm, so the width of the photonic crystal waveguide is 700 nm-980 nm.
Fig. 4 is a schematic structural diagram of a 60 ° curved waveguide provided by the present invention. As shown in fig. 4, the bending waveguide of the 60 ° bending waveguide of the present invention is divided into 3 parts, which are a first waveguide (waveguide 1), an inflection point, and a second waveguide (waveguide 2). Wherein due to the existence of the inflection point, the propagation modes in the waveguide 1 and the waveguide 2 are different, so that the electromagnetic wave cannot directly complete the process of coupling from the waveguide 1 into the waveguide 2. Therefore, the present invention can make the electromagnetic wave change the transmission mode at the inflection point to match the waveguide 1 with the waveguide 2 by changing the structure of the inflection point as shown in fig. 3(f) (the result in the figure is only the modulation of the schematic structure and does not represent the actual optimized structure), thereby forming lossless transmission.
FIG. 5 is a schematic structural diagram of a triangular lattice photonic crystal waveguide based on an air column structure according to the present invention; fig. 6 is a schematic diagram of a partial enlarged structure of a triangular lattice photonic crystal waveguide based on an air column structure provided by the present invention. Referring to fig. 5 and 6, the invention provides a triangular lattice photonic crystal waveguide based on an air column structure, which comprises: a 60 DEG curved waveguide and an air column structure disposed at the curved waveguide of the 60 DEG curved waveguide.
The 60-degree bending waveguide is obtained by introducing line defects into a two-dimensional photonic crystal structure of a triangular lattice. The two-dimensional photonic crystal structure of the triangular lattice comprises a first medium and a second medium; the first medium and the second medium are different media. One first medium 1 is placed at each intersection point of the triangular lattice, and the second medium 2 is filled between the first media 1 to form a two-dimensional periodic structure in which the first media 1 and the second media 2 alternately appear.
The air column structure comprises 6 air columns; the 6 air columns are respectively a first air column 301, a second air column 302, a third air column 303, a fourth air column 304, a fifth air column 305 and a sixth air column 306. As shown in fig. 4, the curved waveguide includes a first waveguide, an inflection point 4, and a second waveguide. As shown in fig. 6, the first waveguide and the second waveguide are symmetrical with respect to a center line where the inflection point 4 is located. The two first mediums located on the central line and adjacent to the inflection point 4 are an inner first medium 501 and an outer first medium 502, respectively; the inner first medium 501 is located inside the curved waveguide; the outer first medium 502 is located outside the curved waveguide. A first medium located at the edge of the first waveguide and adjacent to the inflection point 4 and the outer first medium 502, respectively, is referred to as a target first medium 503; the triangular lattice in which the inflection point 4, the outer first dielectric 502, and the target first dielectric 503 are located together is a first target triangular lattice 601.
The second column of air 302 is centered on the first target triangular lattice 601; the fifth column of air 305 and the second column of air 302 are symmetrically disposed about the center line.
The triangular lattice located within the first waveguide and adjacent to the first target triangular lattice 601 is referred to as a second target triangular lattice 602; the triangular lattice adjacent to the second target triangular lattice 602 and in which inflection point 4 is located is referred to as a third target triangular lattice 603. The common edge of the second target triangular lattice 602 and the third target triangular lattice 603 is referred to as the target edge. The third column of air 303 is located at the center of the target edge. The sixth air column 306 and the third air column 303 are arranged symmetrically with respect to the center line.
The triangular lattice adjacent to the second target triangular lattice 602 and in which the target first medium 503 is located is referred to as a fourth target triangular lattice 604. The first column of air 301 is centered on the fourth target triangular lattice 604. The fourth column of air 304 is disposed symmetrically with the first column of air 301 about the center line.
In the triangular lattice photonic crystal waveguide based on the air column structure, the first medium 1 is an air medium column; the second medium 2 is a lithium niobate medium.
According to the invention, the wavelength of a photonic crystal forbidden band is 2.2-2.6 times of a periodic constant, and the periodic constant is determined to be 500-700 nm for the photonic forbidden band to fall in 1350-1850 nm, so that in the triangular lattice photonic crystal waveguide structure based on the air column structure, the distance (namely the side length of the triangular lattice) a between the centers of two adjacent first media 1 is set to be 500-700 nm; the radius r of the first medium 1 is 0.3a to 0.4a, i.e., the duty ratio r/a is between 0.3 and 0.4. Therefore, the defect is introduced to form a bending guided wave with high transmission efficiency for TM mode electromagnetic wave with a wavelength of 1350nm to 1850 nm. Within the range of the duty ratio r/a of 0.3-0.4, the triangular lattice photonic crystal waveguide structure based on the air column structure can be applied to realize high output efficiency of 1350-1850 nm middle frequency bands.
The invention optimally designs a 60-degree bent waveguide structure, and the radius r' of the second air column 302 is 0.5r in 6 air columns added at the bent waveguide; the third column of air 303, the fifth column of air 305, the sixth column of air 306 and the second column of air 302 all have the same radius. Wherein a third column of air 303 and a sixth column of air 306 located in the waveguide are located on axis 1 and axis 2, respectively, as shown in fig. 5. The air columns (e.g., first air column 301 and second air column 302) at the waveguide edge form a local periodic constant with a' 1.54r with the target first medium 503.
The radius r ″ of the first air column 301 is 0.46 r; the fourth column of air 304 has the same radius as the first column of air 301.
Because the propagation directions of the electromagnetic waves relative to the waveguide 1 and the waveguide 2 are different, and the effect of constructive interference is formed in the waveguide 1 and the waveguide 2 at the same time, the propagation mode of the electromagnetic wave needs to be changed at the inflection point, and the purpose of the invention is to provide an air column structure at the bent waveguide of the 60-degree bent waveguide, namely, to change the propagation mode of the electromagnetic wave, so that the propagation modes in the waveguide 1 and the waveguide 2 are the same. The triangular lattice photonic crystal waveguide based on the air column structure has the greatest advantages of extremely high transmission efficiency, simple local structure optimization at an inflection point and easy preparation.
FIG. 7 is a schematic structural diagram of a triangular lattice photonic crystal waveguide based on a lithium niobate air column structure provided by the present invention; fig. 8 is a schematic diagram of a local enlarged structure of a triangular lattice photonic crystal waveguide based on a lithium niobate air column structure provided by the present invention. Referring to fig. 7 and 8, the triangular lattice photonic crystal waveguide based on the lithium niobate air column structure according to the present invention includes: the waveguide structure comprises a 60-degree bent waveguide and a lithium niobate air column structure arranged at the bent waveguide of the 60-degree bent waveguide. The lithium niobate air column structure comprises 1 air column 7 and 3 lithium niobate columns. The 3 lithium niobate columns are respectively a first lithium niobate column 801, a second lithium niobate column 802 and a third lithium niobate column 803.
The 60-degree bending waveguide is obtained by introducing line defects into a two-dimensional photonic crystal structure of a triangular lattice; the two-dimensional photonic crystal structure of the triangular lattice comprises a first medium 1 and a second medium 2; the first medium 1 and the second medium 2 are different media. One first medium 1 is placed at each intersection point of the triangular lattice, and the second medium 2 is filled between the first media 1 to form a two-dimensional periodic structure in which the first media 1 and the second media 2 alternately appear.
The bent waveguide comprises a first waveguide, an inflection point 4 and a second waveguide. The first waveguide and the second waveguide are symmetrical about a center line where the inflection point 4 is located. The first lithium niobate pillar 801 is located at the inflection point 4.
The two first mediums located on the central line and adjacent to the inflection point 4 are an inner first medium 501 and an outer first medium 502, respectively; the inner first medium 501 is located inside the curved waveguide; the outer first medium 502 is located outside the curved waveguide. The air column 7 replaces the original inner first medium 501 and is located at the inner first medium 501.
A first medium located at the edge of the first waveguide and adjacent to the inflection point 4 and the outer first medium 502, respectively, is referred to as a target first medium 503; the triangular lattice in which the inflection point 4, the outer first dielectric 502, and the target first dielectric 503 are located together is a first target triangular lattice 601. The center line of the three center lines of the first target triangular lattice 601 passing through the inflection point 4 is referred to as a target center line. As shown in fig. 8, the second lithium niobate pillar 802 is located on the target centerline, and the distance b between the center of the second lithium niobate pillar 802 and the center of the target first medium 503 is 0.6 a; a is the distance between the centers of two adjacent first media 1. The third lithium niobate pillar 803 and the second lithium niobate pillar 802 are symmetrically disposed about the center line.
In the triangular lattice photonic crystal waveguide based on the lithium niobate air column structure, the first medium 1 is a lithium niobate medium column; the second medium 2 is an air medium.
According to the invention, the wavelength of a photonic crystal forbidden band is 2.2-2.6 times of a periodic constant, and the periodic constant is determined to be 580-750 nm in order to enable the photonic forbidden band to fall within 1400-1800 nm, so that in the triangular lattice photonic crystal waveguide based on the lithium niobate air column structure, the distance a between the centers of two adjacent first media 1 is set to be 580-750 nm; the radius r of the first medium 1 is 0.18a to 0.3a, i.e., the duty ratio r/a is between 0.18 and 0.3. After the defect is introduced, the bending guided wave with high transmission efficiency can be formed for TE mode electromagnetic wave with the wavelength of 1400nm to 1800 nm. In the range of the duty ratio r/a of 0.18-0.3, the structure of the triangular lattice photonic crystal waveguide based on the lithium niobate air column structure can be applied to realize the high output efficiency of 1400-1800 nm middle frequency bands.
The invention optimally designs a 60-degree bent waveguide structure, and 3 lithium niobate columns and 1 air column 7 are added at the bent waveguide. Wherein the radius r' of the air column 7 is 0.9 r. The radius of the first lithium niobate column 801 is 0.48r ″; the second lithium niobate pillar 802 and the third lithium niobate pillar 803 have the same radius as the first lithium niobate pillar 801. The second lithium niobate pillar 802 located at the waveguide edge forms a local periodic constant of a' 2.14r with the target first medium 503.
The maximum advantage of the triangular lattice photonic crystal waveguide based on the lithium niobate air column structure of the present invention is that the local structure optimization at the inflection point is simple except that the triangular lattice photonic crystal waveguide has extremely high transmission efficiency, so that the triangular lattice photonic crystal waveguide is easy to prepare.
Fig. 9 is a schematic diagram of transmission modes in an unoptimized curved waveguide and an optimized curved waveguide provided by an embodiment of the present invention. Wherein FIG. 9(a) is a transmission mode in an unoptimized curved waveguide, FIG. 9(b) is a transmission mode in an optimized curved waveguide, and FIG. 9(b) has x-axis and z-axis lengths on the abscissa and ordinate, respectively, in microns; "contact Map of Hx" represents a Hx Contour Map, i.e., the wave form shown by the waveguide in the figure is the component of the magnetic field H along the x-axis. From fig. 9(a) it can be seen that the non-optimized curved waveguide cannot be coupled due to the difference in transmission modes in waveguide 1 and waveguide 2. The waveguide (fig. 9(b) is the waveguide optimized by the lithium niobate air column structure of the invention) optimized by the principle of the invention has good coupling efficiency at the inflection point, and the lithium niobate air column structure is added near the inflection point, so that the local period constant is changed, the propagation direction of the light beam is changed, the matching of the transmission modes of the waveguide 1 and the waveguide 2 is achieved, and the coupling efficiency is as high as 99.7%.
Therefore, by adopting the triangular lattice photonic crystal waveguide based on the air column structure or the lithium niobate air column structure, the photonic crystal forms guided waves for electromagnetic waves with corresponding frequencies by introducing the photon local characteristics after defects, and the waveguide width of the photonic crystal is in the nanometer level, so that the high turning radius can be ensured and the extremely high transmission efficiency and transmission bandwidth can be obtained by optimally designing the bent waveguide. The triangular lattice photonic crystal waveguide design based on the air column structure or the lithium niobate air column structure solves the problems of waveguide miniaturization, high transmission bandwidth and low transmission loss in integrated optics, and provides an excellent solution for the optimal design of a bent waveguide for future integrated optics.
Compared with the existing waveguide structure, the two optimized waveguide structures of the invention also have the following advantages:
(1) the triangular lattice photonic crystal waveguide forms a guided wave mode through a photon forbidden band, and compared with the waveguide manufactured by the traditional total reflection principle, the triangular lattice photonic crystal waveguide has higher transmission efficiency and transmission bandwidth.
(2) The width of the triangular lattice photonic crystal waveguide is 500 nm-750 nm, while the conventional ridge waveguide is generally 10000nm in size, so that the triangular lattice photonic crystal waveguide has smaller width and can realize integration.
(3) Because both high duty cycle and low duty cycle can cause structural collapse and increase the complexity of preparation, the design of the two waveguides of the invention takes the preparation requirements into consideration and controls the duty cycle between 0.18 and 0.4.
Because the column size of the medium 1 in the photonic crystal is small, if the medium 1 is lithium niobate, if the duty ratio is too small, the medium column is too thin and is easy to damage; if the medium 1 is an air column, if the duty ratio is too large, the surrounding medium column 2 (lithium niobate) becomes too thin, and the entire structure is easily damaged. In general, a stable structure is obtained when the duty ratio is between 0.18 and 0.4.
The main reason for the complex structure is that the cylinders are added and deleted at the inflection points, only 6 cylinders are added under the air column structure, and only 4 cylinders are added under the lithium niobate air column structure, so that the waveguide structure can reduce the complexity of preparation (compared with other optimized waveguides). And because the duty ratio is controlled to be about 0.2-0.4, the waveguide structure is in a stable state.
The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other.
The principles and embodiments of the present invention have been described herein using specific examples, which are provided only to help understand the method and the core concept of the present invention; meanwhile, for a person skilled in the art, according to the idea of the present invention, the specific embodiments and the application range may be changed. In view of the above, the present disclosure should not be construed as limiting the invention.
Claims (10)
1. An air column structure based triangular lattice photonic crystal waveguide, comprising: the waveguide structure comprises a 60-degree bent waveguide and an air column structure arranged at the bent waveguide of the 60-degree bent waveguide; the 60-degree bending waveguide is obtained by introducing line defects into a two-dimensional photonic crystal structure of a triangular lattice; the two-dimensional photonic crystal structure of the triangular lattice comprises a first medium and a second medium; the first medium and the second medium are different media; one first medium is placed at each intersection point of the triangular lattice, and the second medium is filled between the first media to form a two-dimensional periodic structure in which the first media and the second media alternately appear;
characterized in that the air column structure comprises 6 air columns; the 6 air columns are respectively a first air column, a second air column, a third air column, a fourth air column, a fifth air column and a sixth air column; the bent waveguide comprises a first waveguide, an inflection point and a second waveguide; the first waveguide and the second waveguide are symmetrical about a central line where the inflection point is located; the two first mediums which are positioned on the central line and adjacent to the inflection point are respectively an inner first medium and an outer first medium; the inner first medium is positioned at the inner side of the bent waveguide; the outer first medium is positioned outside the bent waveguide; a first medium which is positioned at the edge of the first waveguide and is respectively adjacent to the inflection point and the outer first medium is called a target first medium; the corner point, the triangular lattice where the outer side first medium and the target first medium are jointly located are first target triangular lattices; the second column of air is located at the first target trigonal lattice center; the fifth column of air and the second column of air are symmetrically disposed about the centerline; a triangular lattice located within the first waveguide and adjacent to the first target triangular lattice is referred to as a second target triangular lattice; the triangular lattice adjacent to the second target triangular lattice and with the inflection point is called a third target triangular lattice; a common edge of the second target triangular lattice and the third target triangular lattice is referred to as a target edge; the third air column is positioned at the center of the target edge; the sixth air column and the third air column are symmetrically arranged about the center line; the triangular lattice adjacent to the second target triangular lattice and in which the target first medium is located is called a fourth target triangular lattice; the first column of air is located at the fourth target trigonal lattice center; the fourth air column and the first air column are arranged symmetrically about the center line.
2. The air column structure-based triangular lattice photonic crystal waveguide of claim 1, wherein the first medium is a column of air medium; the second medium is a lithium niobate medium.
3. The air column structure-based triangular lattice photonic crystal waveguide of claim 1, wherein a distance a between centers of two adjacent first media is 500nm to 700 nm; the radius r of the first medium is 0.3a to 0.4 a.
4. The air column structure-based triangular lattice photonic crystal waveguide of claim 3, wherein the radius r' of the second air column is 0.5 r; the third air column, the fifth air column, the sixth air column and the second air column have the same radius.
5. The air column structure-based triangular lattice photonic crystal waveguide of claim 3, wherein the radius r "of the first air column is 0.46 r; the fourth column of air has the same radius as the first column of air.
6. A triangular lattice photonic crystal waveguide based on a lithium niobate air column structure, comprising: the waveguide structure comprises a 60-degree bent waveguide and a lithium niobate air column structure arranged at the bent waveguide of the 60-degree bent waveguide; the lithium niobate air column structure comprises 1 air column and 3 lithium niobate columns;
the 60-degree bending waveguide is obtained by introducing line defects into a two-dimensional photonic crystal structure of a triangular lattice; the two-dimensional photonic crystal structure of the triangular lattice comprises a first medium and a second medium; the first medium and the second medium are different media; one first medium is placed at each intersection point of the triangular lattice, and the second medium is filled between the first media to form a two-dimensional periodic structure in which the first media and the second media alternately appear;
the 3 lithium niobate columns are respectively a first lithium niobate column, a second lithium niobate column and a third lithium niobate column; the bent waveguide comprises a first waveguide, an inflection point and a second waveguide; the first waveguide and the second waveguide are symmetrical about a central line where the inflection point is located; the two first mediums which are positioned on the central line and adjacent to the inflection point are respectively an inner first medium and an outer first medium; the inner first medium is positioned at the inner side of the bent waveguide; the outer first medium is positioned outside the bent waveguide; the first lithium niobate column is positioned at the inflection point; the column of air is located at the inboard first medium;
a first medium which is positioned at the edge of the first waveguide and is respectively adjacent to the inflection point and the outer first medium is called a target first medium; the corner point, the triangular lattice where the outer side first medium and the target first medium are jointly located are first target triangular lattices; a central line passing through the inflection point among the three central lines of the first target triangular lattice is called a target central line; the second lithium niobate pillar is positioned on the target middle line, and the distance between the center of the second lithium niobate pillar and the center of the target first medium is 0.6 a; a is the distance between the centers of two adjacent first media; the third lithium niobate pillar and the second lithium niobate pillar are symmetrically arranged about the center line.
7. The triangular lattice photonic crystal waveguide based on a lithium niobate air column structure of claim 6, wherein the first medium is a lithium niobate dielectric column; the second medium is an air medium.
8. The lithium niobate air column structure-based triangular lattice photonic crystal waveguide of claim 6, wherein a distance a between centers of two adjacent first media is 580nm to 750 nm; the radius r of the first medium is 0.18a to 0.3 a.
9. The lithium niobate air column structure-based triangular lattice photonic crystal waveguide of claim 8, wherein the radius r' of the air column is 0.9 r.
10. The lithium niobate air column structure-based triangular lattice photonic crystal waveguide of claim 8, wherein the radius of the first lithium niobate column is r ═ 0.48 r; the second lithium niobate column and the third lithium niobate column have the same radius as the first lithium niobate column.
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