CN112526672A - Optical waveguide chiral mode conversion method and device - Google Patents
Optical waveguide chiral mode conversion method and device 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
- G02B6/12004—Combinations of two or more optical elements
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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
- G02B6/105—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type having optical polarisation effects
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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
- G02B6/126—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 using polarisation effects
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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
- G02B6/14—Mode converters
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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
- G02B2006/12133—Functions
- G02B2006/1215—Splitter
Abstract
The invention discloses a method and a device for converting an optical waveguide chiral mode, wherein the method comprises the following steps: constructing a parameter space of the double-waveguide structure parameters, and determining the position of a singular point in the parameter space; in the parameter space, making any closed loop around a singular point, and obtaining a structural parameter value of the closed loop based on a parameter equation; and preparing a multi-period sub-wavelength double-waveguide structure corresponding to the structural parameter values, and regulating the incident direction of an incident light signal in any mode of the double-waveguide structure to obtain the emergence of the chiral mode. The invention determines a micro-nano-scale double-waveguide structure through singular points, the structural parameters of the structure are continuously and slowly changed to form a closed path, when light enters from one end of the waveguide, no matter which mode the end is, the other end emits a symmetrical or anti-symmetrical mode; when light is incident from the other end, regardless of the mode at the end, the light exits at one end as an antisymmetric or symmetric mode, and chiral conversion is achieved in which the exiting mode depends on the propagation direction and is independent of the incident mode.
Description
Technical Field
The invention belongs to the field of micro-nano optics, and particularly relates to a method and a device for converting an optical waveguide chiral mode.
Background
Chiral materials are materials containing chiral bodies or microstructures, which have different response characteristics to left and right circularly polarized light. Chiral materials have gained wide attention and application in the optical field. In recent years, due to the research on the non-hermitian system, chiral mode conversion has been achieved in many systems even without the help of chiral materials. The chiral mode conversion is the asymmetric conversion of the mode, and has important significance for the research of optical isolators and nonreciprocal devices. At present, the method is realized mainly through a topological structure of space-time break and singular points. The former needs wave vector matching and is not beneficial to precise modulation; the latter has received extensive attention and research due to its strong robustness.
Using the topology of the singularities, researchers experimentally verified this chiral result in metal waveguides (j.doppler et al, Nature 537,76(2016)), opto-mechanical systems (h.xu et al, Nature 537,80(2016)) and ferromagnetic waveguides (x.l.zhang et al, phys.rev.x 8,021066 (2018)). However, these structures are not suitable for integration of optical devices due to the microwave operating band and the large size of the structures. How to realize chiral mode conversion on an optical chip is a technical problem to be solved urgently at present.
Disclosure of Invention
The invention provides an optical waveguide chiral mode conversion method and device, which are used for solving the technical problem that the conventional optical waveguide cannot effectively realize dual-mode chiral mode conversion on an optical chip due to the structural defect of the conventional optical waveguide.
The technical scheme for solving the technical problems is as follows: a method of optical waveguide chiral mode conversion, comprising:
constructing a parameter space of double-waveguide structure parameters, and determining a singular point position in the parameter space; in the parameter space, making any closed loop around the singular point, and obtaining a structural parameter value of the closed loop based on a parameter equation;
and preparing a multi-period sub-wavelength double-waveguide structure corresponding to the structural parameter values, and regulating the incident direction of any mode incident light signal of the double-waveguide structure to obtain the emergent of a chiral mode so as to realize chiral mode conversion.
The invention has the beneficial effects that: the invention creatively adopts the sub-wavelength waveguide instead of the traditional continuous waveguide structure, can flexibly regulate and control the mode effective refractive index, and greatly shortens the size of the device. Secondly, the double-waveguide structure is designed based on the topological structure of the singular point, the double-waveguide structure has good robustness on working wavelength and structural defects, because the winding of the singular point in the parameter space only needs the winding track to contain the singular point, so the winding track is not very accurate, the width and the distance of the waveguide can be reflected on the waveguide structure, larger experimental error can be allowed in the preparation process, the accuracy is not very accurate, namely the robustness is very strong, and the traditional method needs to accurately control the structure, and has high requirement on the experimental accuracy. In addition, by adjusting the incident direction of the incident light signal of any mode of the dual-waveguide structure, for example, from one end or the other end of the dual-waveguide structure, the waveguide exit mode is only related to the incident direction, and the chiral mode conversion of two incident modes (two-end incident) can be realized. Specifically, the mode of the starting point is wound around the singular point in the parameter space, and non-adiabatic transition occurs in the evolution process, so that chiral mode output is caused, for example, when light enters from the left end of the waveguide, no matter a symmetric mode or an antisymmetric mode of the waveguide is excited, the emergent mode is a symmetric mode at the right end of the waveguide, namely an emergent port; in contrast, when light enters from the right end of the waveguide, the exit mode is an antisymmetric mode at the left end of the waveguide, regardless of whether a symmetric or antisymmetric mode of the waveguide is excited. Therefore, the invention utilizes the topological property of the singular point to simply and efficiently realize the method for converting the chiral mode.
On the basis of the technical scheme, the invention can be further improved as follows.
Further, the singular point is a position of degeneracy of a dual-waveguide eigenmode in the parameter space;
determining the position of the singular point in the parameter space specifically includes:
and determining the position of the singular point in the parameter space by calculating the waveguide loss and the coupling strength.
Further, the material of the double waveguide structure is silicon.
The invention has the further beneficial effects that: the invention selects the insulator silicon with low cost and wide application as the on-chip structure material. The invention adopts the sub-wavelength silicon waveguide structure, can flexibly regulate and control the effective refractive index of the mode, enhances the coupling, and greatly shortens the waveguide size.
Further, the dual waveguide structure parameters include: the width w (z) of one of the waveguides, and the spacing g (z) between the two waveguides, where z represents the distance from one end of the dual waveguide structure.
The invention has the further beneficial effects that: the broadband and the spacing can effectively reflect the dual-waveguide structure and the performance thereof.
Further, the width w of each silicon strip in the other waveguide in the dual-waveguide structures′＝w0+(–1)sδ, wherein w0δ is the fluctuation range, and s is the number of each silicon bar, which is 0.7 μm.
The invention has the further beneficial effects that: singular points appear in the non-hermite system, and in order to introduce loss in the structure, the width w' of one waveguide is designed to fluctuate around 0.7 μm, scattering loss is generated, and the fluctuation amplitude is delta. Wherein different δ correspond to different singular point positions g in the parameter spaceEPAnd thus determines the winding trajectory.
Further, the parametric equation includes: g (z) ═ g0+Δgsin(πz/L),w(z)＝w0±Δwsin(2πz/L)；
In the formula, g0Indicating the starting and stopping pointsThe waveguide spacing between two waveguides, w0Representing a waveguide width of the one of the waveguides at the start and stop point; Δ g denotes a modulation amplitude of a waveguide interval between two waveguides, Δ w denotes a modulation amplitude of a waveguide width of the one waveguide, and L denotes an overall length of the dual waveguide structure.
The invention has the further beneficial effects that: since when Δ g > gEP–g0Meanwhile, the circle locus contains singular points, so that any closed loop containing the singular points is easy to obtain based on the parameter equation.
Further, in the parameter space, making any closed loop around the singular point specifically includes:
determining an end point in the parameter space, continuously changing the two parameters by taking the end point as a starting point to form a closed winding path, returning the two parameters to the end point when a winding is finished, and enabling the closed winding path to wrap around a singular point, wherein the closed winding path is a closed loop.
The invention has the further beneficial effects that: the two modes of the end point are subjected to chiral conversion after being coiled; and designing a micro-nano structure for spatial modulation according to the waveguide width and the distance set by the winding path. Incident light enters from different ports of the grating coupler through the optical fiber to excite different modes of the double waveguides, and an emergent mode is only related to the incident direction of the light and is not related to the mode excited by the incident end.
The invention also provides an optical waveguide which is a dual-waveguide structure and is prepared based on the optical waveguide chiral mode conversion method.
The present invention also provides an optical waveguide chiral mode conversion device, comprising: a light source, a polarizer, an optical fiber, a grating coupler, an asymmetric directional coupler, an optical splitter, an optical power device, a spectrum analyzer, and an optical waveguide as described above;
wherein the light source is used for generating incident light; the polaroid is used for changing incident light into transverse electric polarization; the optical fiber is used for coupling transverse electric polarization to the grating coupler and transmitting light to the mode multiplexing demultiplexer through the grating coupler after the light exits from the optical waveguide; the mode multiplexing demultiplexer is used for exciting the mode of an incident end of the optical waveguide and sending different modes of an emergent end to the optical distributor, and the optical distributor is used for sending half of emergent light to the optical power device and sending the other half of the emergent light to the optical spectrum analyzer.
Further, the grating coupler adopts a one-dimensional chirp structure; the mode multiplexing demultiplexer is formed of asymmetric directional couplers.
Drawings
Fig. 1 is a flow chart of a method for converting a chiral mode of an optical waveguide according to an embodiment of the present invention;
fig. 2 is a schematic diagram of a structure of a sub-wavelength waveguide unit for optical waveguide chiral mode conversion and a singular point position according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of a closed loop of structural parameters provided in an embodiment of the present invention;
fig. 4 is a schematic diagram of an optical waveguide chiral mode conversion device according to an embodiment of the present invention;
FIG. 5 is a graph comparing the numerical and experimental transmission spectra obtained using the conversion apparatus and dual waveguide structure of FIG. 2 in accordance with an embodiment of the present invention;
fig. 6 is a diagram of a mode evolution process obtained by the dual-waveguide structure according to the embodiment of the present invention.
The same reference numbers will be used throughout the drawings to refer to the same or like elements or structures, wherein:
1. the device comprises a light source, 2, a polaroid, 3, an optical fiber, 4, a grating coupler, 5, a mode multiplexing demultiplexer, 6, an optical distributor, 7, an optical spectrum analyzer, 8 and an optical power device.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
Example one
A method for converting chiral modes of an optical waveguide, as shown in fig. 1, comprises:
and 120, preparing a multi-period and sub-wavelength double-waveguide structure corresponding to the structural parameter values, and regulating the incident direction of an incident light signal of any mode of the double-waveguide structure to obtain the emergent of a chiral mode so as to realize chiral mode conversion.
Compared with a method for realizing asymmetric transformation by utilizing space-time defects, the embodiment greatly enhances the robustness of the structure; compared with the existing method for realizing the asymmetric mode conversion by using the singular point topological structure, the method creatively adopts the sub-wavelength waveguide instead of the traditional continuous waveguide structure, thereby greatly shortening the size of the device and supporting the asymmetric conversion of two incident modes.
Specifically, based on the topological structure of the singular point, because the winding track only needs to contain the singular point based on the singular point winding in the parameter space, the winding track is not very accurate, so that the width and the distance of the waveguide reflected on the waveguide structure can allow a larger experimental error in the preparation process, and the winding track is not very accurate, namely the robustness is very strong. The traditional method needs to accurately control the structure and has high requirements on experimental precision, so that the embodiment has good robustness on working wavelength and structural defects.
In addition, by regulating the incident direction of the incident light signal of any mode of the dual-waveguide structure, for example, from one end or the other end of the dual-waveguide structure, the waveguide emergent mode is only related to the incident direction, i.e., the asymmetric conversion of two incident modes is realized. Specifically, the mode of the starting point circles around the singular point in the parameter space, and non-adiabatic transition occurs in the evolution process, resulting in chiral mode output: when light enters from the left end of the waveguide, no matter the symmetrical mode or the anti-symmetrical mode of the waveguide is excited, the emergent mode is a symmetrical mode at the right end of the waveguide, namely the emergent port; in contrast, when light enters from the right end of the waveguide, the exit mode is an antisymmetric mode at the left end of the waveguide, regardless of whether a symmetric or antisymmetric mode of the waveguide is excited. Therefore, the embodiment simply and efficiently realizes the method for converting the chiral mode by using the topological property of the singular point.
Preferably, the singular point is a degenerate position of the eigenmode of the dual-waveguide in the parameter space; and determining the position of the singular point in the parameter space by calculating the waveguide loss and the coupling strength.
Preferably, the material of the dual waveguide structure is silicon.
And selecting low-cost and widely-applied silicon as the on-chip structure material. Because the refractive index of silicon in a communication waveband is greatly different from that of air, an optical field is strongly bound in the silicon waveguide, so that the coupling strength of the two silicon waveguides is very weak, the embodiment adopts a sub-wavelength silicon waveguide structure, the effective refractive index of a mode can be flexibly regulated and controlled, the coupling is enhanced, and the waveguide size is greatly shortened.
Preferably, the dual waveguide structure parameters include: the width w (z) of one of the waveguides, and the spacing g (z) between the two waveguides, where z represents the distance from one end of the dual waveguide structure.
The broadband and the spacing can effectively reflect the dual-waveguide structure and the performance thereof.
Preferably, the width w of each silicon strip in the other of the waveguides in the dual waveguide structures′＝w0+(–1)sδ, wherein w0δ is the fluctuation range, and s is the number of each silicon bar, which is 0.7 μm.
As shown in fig. 2, in the dual waveguide shown in the left diagram, considering the transverse electric modes in the waveguide, their distribution satisfies the coupling equation:
wherein A is1,2Representing the amplitude, alpha, of the mode in each waveguide1,2Is the propagation constant of the mode, gamma1,2To spread the lossAnd C represents the coupling strength. When alpha is1＝α2And C ═ γ1–γ2I/2, the eigenvalues and eigenmodes of the Hamiltonian H degenerate simultaneously, this point being the singular point. In addition, the right diagram of fig. 2 shows the variation of the coupling strength C with the waveguide spacing g and the loss γ2Changes with the fluctuation range delta, when delta is 0.1 mu m, gamma is20.017, then the singular point condition C ═ γ1–γ2Finding out the position g of the corresponding singular point, |/2 ═ 0.017EP＝0.52μm。
In this embodiment, a commercial SOI chip with a silicon thickness of 220nm and a silicon dioxide thickness of 3 μm is used for each waveguide, the sub-wavelength waveguide period is Λ 0.3 μm, and the duty cycle f is 0.565.
Singular points appear in the non-hermite system, and in order to introduce loss in the structure, the width w' of one waveguide is designed to fluctuate around 0.7 μm, scattering loss is generated, and the fluctuation amplitude is delta. Wherein different δ correspond to different singular point positions g in the parameter spaceEPAnd thus determines the winding trajectory. While in order to reduce the overall system losses, γ, are introduced only in a single waveguide 10 and gamma2Not equal to 0. Loss gamma2Introduced by silicon strips of different widths: w ═ w0+(–1)sδ, wherein w0δ is the fluctuation amplitude, and s represents the number of each bar, 0.7 μm.
Selecting delta to be 100nm, and finding out the position of the singular point in the parameter space according to the requirement of realizing the singular point: w is aEP＝0.7μm，gEP0.52 μm. After the position of the singular point is determined, the circle is wrapped around the singular point in the parameter space. The end point of the coil is selected as (w)0,g0) Using the end point as a starting point, two parameters are expressed by a parameter equation g (z) g0+Δgsin(πz/L),w(z)＝w0. + -. Δ wsin (2 π z/L). When the parameter z is varied from 0 to L, the width w and the spacing g will return to the end points, forming a closed winding path (as shown in fig. 3). The +/-signs correspond to clockwise/counterclockwise windings in the parameter space. If Δ g > gEP–g0The singular point is surrounded by the winding path, so that the two modes of the end point will generate chiral modes after passing through the windingAnd (4) converting the formula. The two modes of the end point are subjected to chiral conversion after being coiled; and designing a micro-nano structure for spatial modulation according to the waveguide width and the distance set by the winding path. Incident light enters from different ports of the grating coupler through the optical fiber to excite different modes of the double waveguides, and an emergent mode is only related to the incident direction of the light and is not related to the mode excited by the incident end.
Preferably, the parametric equation comprises: g (z) ═ g0+Δgsin(πz/L),w(z)＝w0. + -. Δ wsin (2 π z/L); in the formula, g0Denotes the waveguide spacing, w, between two waveguides at the start and stop points0Indicating the waveguide width of the one of the waveguides at the start and stop point; Δ g denotes a modulation amplitude of a waveguide interval between two waveguides, Δ w denotes a modulation amplitude of a waveguide width of the above-mentioned one of the waveguides, and L denotes an overall length of the dual waveguide structure.
Since when Δ g > gEP–g0Meanwhile, the circle locus contains singular points, so that any closed loop containing the singular points is easy to obtain based on the parameter equation.
Example two
An optical waveguide is a dual-waveguide structure, which is manufactured based on the optical waveguide chiral mode conversion method described in the first embodiment. The related technical solution is the same as the first embodiment, and is not described herein again.
EXAMPLE III
An optical waveguide chiral mode conversion device comprising: a light source, a polarizer, an optical fiber, a grating coupler, an asymmetric directional coupler, an optical splitter, an optical power device, a spectrum analyzer, and an optical waveguide as described in example two above;
wherein, the light source 1 is used for generating incident light; the polarizing plate 2 is used for changing incident light into transverse electric polarization; the optical fiber 3 is used for coupling transverse electric polarization to the grating coupler 4 and transmitting light to the mode multiplexing demultiplexer 5 through the grating coupler after the light exits from the optical waveguide; the mode multiplexing demultiplexer 5 is used for exciting the mode of the incident end of the optical waveguide and sending the mode with different emergent ends to the optical distributor 6, and the optical distributor 6 is used for sending half of the emergent light to the optical power device 8 and sending the other half of the emergent light to the optical spectrum analyzer 7.
If the fiber is coupled to the first port (port 1) or the third port (third port) of the grating coupler, the odd mode of the incident end of the optical waveguide is excited, and the even mode of the incident end of the modulation waveguide is excited when the fiber is incident to the second port (port 2) or the fourth port (port 4). The mode is converted into an odd mode or an even mode after passing through the optical waveguide, and is demodulated by a mode multiplexing demultiplexer 5 at an emergent end and then coupled to an optical power meter 8 and an optical spectrum analyzer 7 through a grating coupler 4 and an optical fiber 3.
Preferably, the grating coupler adopts a one-dimensional chirp structure; the mode multiplexing demultiplexer is formed of asymmetric directional couplers.
As shown in fig. 4, a diagram of a measuring apparatus for realizing the winding waveguide (i.e., the dual waveguide structure obtained in the second embodiment) is shown. Light of a communication band emitted from a light source 1 is converted into a transverse electric wave (or transverse magnetic wave) by a polarizing plate 2, and then vertically coupled to a grating coupler 4 at an input end by a single-mode optical fiber 3. After passing through the double waveguide, the incident light is vertically coupled to the single-mode fiber 3 from the grating coupler 4 at the output end. Finally, the optical fiber is divided into two beams by a commercial 3dB beam splitter 6, and one beam enters an optical power meter to be used for monitoring and adjusting the alignment of the optical fiber and the optical grating; the other beam is connected to the spectrometer 7 for measuring the transmission spectrum in the mode.
As in the chip structure of fig. 4, the on-chip structure has an auxiliary optical circuit connecting both ends thereof in addition to the winding waveguide. The line mainly comprises a mode multiplexing demultiplexer 5 and a grating coupler 4 at the input/at the output. The grating coupler 4 adopts a one-dimensional chirp design, so that the mode fields of the grating coupler and the single-mode fiber are overlapped, and the coupling loss is reduced. The coupling angle of the fiber was chosen to be 10 degrees.
The mode multiplexing demultiplexer 5 is formed of asymmetric directional couplers (asymmetric directional couplers) including a narrow single mode waveguide and a wide multimode main waveguide. The asymmetric optical coupler converts the TE0 mode of the narrow single-mode waveguide into the TE0/TE1 mode of the wide multi-mode main waveguide and vice versa. The asymmetric optical coupler of the TE0 mode has a length of 46.2 μm and consists of a single mode waveguide 0.5 μm wide and a main waveguide 1.05 μm wide. The asymmetric optical coupler of the TE1 mode has a length of 57.5 μm and consists of a single mode waveguide 0.5 μm wide and a main waveguide 1.55 μm wide. The spacing between the two waveguides in both asymmetric optical couplers was 130 nm. These asymmetric couplers are cascaded together to implement the two lowest order TE mode division multiplexers. The main waveguides of different widths are connected by an adiabatic taper structure of sufficient length to avoid inter-band mode coupling and radiation loss. Finally, the main waveguide is widened to 2 μm with a width close to that of the winding waveguide. When multimode light enters the ring waveguide, a mirrored line demultiplexes the different modes into the TE0 modes of the different channels. The grating coupler couples the light of each output channel into a single mode optical fiber, which is ultimately input into an optical power meter 7 and a spectrum analyzer 8.
In fig. 5, the two graphs in the first row are numerically calculated transmittances at 1545-1565nm, and the two graphs in the second row are experimental results measured by the apparatus shown in fig. 4 in this embodiment, which shows that the experimental and numerical calculation results are good. Due to the influence of Fabry-perot resonance, the experimentally measured transmission spectrum has a small oscillation. Smn(S′mn) Represents the transmission coefficient from mode m to mode n at the time of forward (backward) incidence of light. m, n-1 represents a symmetric (TE0) mode, and m, n-2 represents an asymmetric (TE1) mode. For clockwise winding has S12–S119dB and S22–S219dB (the horizontal line represents minus, the wavy line represents approximately equal) indicates that the emergent mode is mainly asymmetric; for counter-clockwise winding there is S'11–S′1210dB and S'21–S′2210dB, indicating that the exit mode is mainly a symmetric mode. Chiral mode conversion is achieved.
The left graph and the right graph in fig. 6 respectively show the dynamic evolution process of the incident mode in the winding waveguide. The solid line in the figure represents the evolution of the effective refractive index of the eigenmode of the waveguide in the propagation direction, while the dotted line represents the dynamic field distribution ψ (z) in the transient eigenfield phi1,2(z) projection onto. The dotted line represents the values:
wherein | b1,2(z)|2＝|∫∫φ* 1,2(z)ψ(z)dxdy|2. Therefore, when clockwise winding, namely light is incident from the left side, the asymmetric mode is generated after being modulated by the winding waveguide regardless of whether the symmetric mode or the asymmetric mode is excited at an incident end; when the circle is anticlockwise, namely light is incident from the right, a symmetric mode is emitted at the incident end regardless of whether the symmetric mode or the asymmetric mode is excited. It is noted that the propagation process is consistent with the transient evolution when the symmetric mode is incident in the forward direction or the anti-symmetric mode is incident in the reverse direction, and the winding process is adiabatic, so that the mode inversion is generated. Adiabatic evolution requires the mode to be in a lower loss state during the winding process. When the antisymmetric mode is incident in the forward direction or the symmetric mode is incident in the reverse direction, we see that the mode transitions to another state in the middle of propagation, and adiabatic evolution is broken because the incident mode is in a state with higher loss and it transitions to a state with lower loss.
In the embodiment, a micro-nano dual-waveguide structure is processed on a silicon wafer through an etching process, and two structural parameters of the waveguide continuously and slowly change to form a closed path. The method comprises the steps of setting up an experimental device, wherein the experimental device comprises a grating coupler, a mode multiplexing demultiplexer, a polarization controller, a spectrum analyzer and the like, when light enters from the left end of a waveguide, no matter an odd mode or an even mode of an output port is excited, the right end of the waveguide is an output port, and the output mode is the odd mode; on the contrary, when light enters from the right end of the waveguide, the exit mode is an even mode at the left end of the waveguide regardless of whether the odd mode or the even mode of the waveguide is excited. This achieves a chiral conversion of the exit mode independent of the incident mode depending on the propagation direction. The design is based on a topological structure of singular Points (Exceptional Points), and Robustness (Robustness) of the device to incident wavelength disturbance and structural defects is enhanced.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (10)
1. A method for chiral mode conversion in an optical waveguide, comprising:
constructing a parameter space of double-waveguide structure parameters, and determining a singular point position in the parameter space; in the parameter space, making any closed loop around the singular point, and obtaining a structural parameter value of the closed loop based on a parameter equation;
and preparing a multi-period sub-wavelength double-waveguide structure corresponding to the structural parameter values, and regulating the incident direction of any mode incident light signal of the double-waveguide structure to obtain the emergent of a chiral mode so as to realize chiral mode conversion.
2. The method according to claim 1, wherein the singular point is a position in the parameter space where the eigen-mode of the dual waveguide is degenerate;
determining the position of the singular point in the parameter space specifically includes:
and determining the position of the singular point in the parameter space by calculating the waveguide loss and the coupling strength.
3. The method according to claim 1, wherein the material of the double waveguide structure is silicon.
4. The method of claim 1, wherein the parameters of the dual waveguide structure comprise: the width w (z) of one of the waveguides, and the spacing g (z) between the two waveguides, where z represents the distance from one end of the dual waveguide structure.
5. The method as claimed in claim 4, wherein the width w of each silicon stripe in the other waveguide is equal to the width w of each silicon stripe in the dual waveguide structures′＝w0+(–1)sδ wherein w0δ is the fluctuation range, and s is the number of each silicon bar, which is 0.7 μm.
6. The method of claim 5, wherein the parametric equation comprises: g (z) ═ g0+Δgsin(πz/L),w(z)＝w0±Δwsin(2πz/L)；
In the formula, g0Denotes the waveguide spacing between two waveguides at the start and stop points, w0The waveguide width of one of the waveguides at the starting point and the stopping point is shown; Δ g denotes a modulation amplitude of a waveguide interval between two waveguides, Δ w denotes a modulation amplitude of a waveguide width of the one waveguide, and L denotes an overall length of the dual waveguide structure.
7. The method according to any one of claims 1 to 6, wherein in the parameter space, any closed loop is formed around the singular point, specifically:
determining an end point in the parameter space, continuously changing the two parameters by taking the end point as a starting point to form a closed winding path, returning the two parameters to the end point when a winding is finished, and enabling the closed winding path to wrap around a singular point, wherein the closed winding path is a closed loop.
8. An optical waveguide having a dual waveguide structure, which is produced based on the optical waveguide chiral mode conversion method according to any one of claims 1 to 7.
9. An optical waveguide chiral mode conversion device, comprising: a light source, a polarizer, an optical fiber, a grating coupler, an asymmetric directional coupler, an optical splitter, an optical power device, a spectrum analyzer, and an optical waveguide according to claim 8;
wherein the light source is used for generating incident light; the polaroid is used for changing incident light into transverse electric polarization; the optical fiber is used for coupling transverse electric polarization to the grating coupler and transmitting light to the mode multiplexing demultiplexer through the grating coupler after the light exits from the optical waveguide; the mode multiplexing demultiplexer is used for exciting the mode of an incident end of the optical waveguide and sending different modes of an emergent end to the optical distributor, and the optical distributor is used for sending half of emergent light to the optical power device and sending the other half of the emergent light to the optical spectrum analyzer.
10. The device of claim 9, wherein the grating coupler is configured as a one-dimensional chirped structure; the mode multiplexing demultiplexer is formed of asymmetric directional couplers.
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