On-chip isolator implementation method based on space-time symmetric directional coupler
By adopting an on-chip isolator implementation method based on parity-time symmetric directional coupler in the field of optical communication, the non-reciprocal nature of passive PT symmetric directional coupler is solved, and the problem of large size and suitable for strong signals is achieved is achieved with high efficiency non-reciprocal isolation effect suitable for any intensity signal.
Patent Information
- Application Number
- CN202311163775.6
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2023-09-11
- Publication Date
- 2025-03-11
AI Technical Summary
Existing isolators made of magneto-optical materials are large in size and incompatible with the CMOS process, while isolators made of nonlinear effects are only suitable for strong input signals.
Using the on-chip isolator implementation method based on parity-time symmetric directional coupler, using the non-reciprocal properties of the passive PT symmetric directional coupler, the loss is adjusted to achieve the amplitude and phase of a specific output port by constructing the Hamiltonian of a non-Hermi system, thereby setting a Y branch structure on the two output ports to achieve the non-reciprocality between the input and the output.
Achieve non-reciprocity for any intensity signal, avoiding the use of magneto-optical materials, reducing device size, making it easier to integrate at scale, and providing an isolation ratio of up to 10.4dB.
Smart Images
Figure CN119596448A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to a technology in the field of optical communication, and in particular to a method for realizing an on-chip isolator based on the non-reciprocal property of a parity-time (PT) symmetric directional coupler. Background Art
[0002] Non-Hermitian systems have been widely used in the field of optics due to their unique advantages. According to the relationship between coupling strength and gain-loss, a system with balanced gain and loss can be divided into PT symmetry state, PT symmetry breaking state and singular point (EP). On the other hand, traditional methods of generating non-reciprocity include using magneto-optical materials, nonlinear effects or time-varying materials. Summary of the invention
[0003] In view of the shortcomings of existing isolators made of magneto-optical materials, such as large size and incompatibility with CMOS technology, and the shortcomings of isolators made using nonlinear effects, such as being only applicable to strong input signals, the present invention proposes an on-chip isolator implementation method based on parity-time symmetric directional couplers. The on-chip isolator is implemented by utilizing the non-reciprocal properties of passive PT symmetric directional couplers, which can avoid the use of magneto-optical materials and is applicable to signals of any intensity.
[0004] The present invention is achieved through the following technical solutions:
[0005] The invention relates to an on-chip isolator realization method based on a parity-time symmetric directional coupler. The method comprises the following steps: constructing a Hamiltonian of a non-Hermitian system and solving an eigenvalue equation to obtain an input-output relationship of the directional coupler and an expression of its amplitude and phase characteristics; adjusting the amplitude and phase of two output ports by adjusting the loss to obtain a specific position so that light input through a lossless waveguide end is completely coupled to a lossy waveguide end, while light input from the lossy waveguide end is not completely coupled to the lossless waveguide; the specific position is used as the output end of the directional coupler; a Y-branch structure is arranged at the two output ends to combine two beams of output light together, thereby realizing non-reciprocity between input and output.
[0006] When the Hamiltonian of the non-Hermitian system satisfies the PT symmetry condition, the system has real eigenvalues.
[0007] The directional coupler is a passive PT symmetrical directional coupler, that is, only one of the two waveguides has loss, and the other waveguide has neither gain nor loss.
[0008] The present invention relates to an on-chip isolator obtained based on the above method, comprising: a directional coupler and a Y-branch structure, wherein: two waveguides of the directional coupler have the same width.
[0009] Preferably, the width of the two waveguides of the directional coupler is 0.6 μm, the gap between the two waveguides is 150 nm, and the length of the two waveguides matches the coupling period, that is, 49 μm.
[0010] Losses are introduced by depositing metal on the top of one of the waveguides of the directional coupler. The thickness of the metal is set to 300nm and the duty cycle is 0.2. Technical Effects
[0011] The present invention applies the non-reciprocal property under the PT symmetric state to the isolator. When the power distribution of the Y branch is 1:10, the isolation ratio of the present invention can reach its maximum value of 10.4dB. In order to meet the commercial demand for an isolation rate greater than 20dB, two such couplers need to be cascaded to work. Each coupler can provide an isolation ratio of about 10dB, and the insertion loss of each isolator is about 0.25dB. Compared with the prior art, the present invention can avoid the use of magneto-optical materials, thereby reducing the size of the device and making it easier to carry out large-scale integration. In addition, non-reciprocity can be achieved for light of any intensity, which is impossible to achieve with nonlinear effects. BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Figure 1 A schematic diagram of an existing system with balanced gains and losses;
[0013] In the figure: ω m = 0, the relationship between the real part (a) and imaginary part (b) of the system complex eigenfrequency and the normalized gain (loss) coefficient;
[0014] Figure 2 The schematic diagrams of the structures of PT symmetric directional couplers in two cases are shown;
[0015] In the figure: (a) the two waveguides have balanced gain and loss; (b) only one of the two waveguides has loss, and the other waveguide has neither gain nor loss;
[0016] Figure 3 It is the relationship diagram between phase and propagation distance;
[0017] In the figure: For a single-input system, when g / (2κ) = 0 (solid line), 0.4 (dashed line), 0.8 (dotted line) and 1 (dotted line), the relationship between the amplitude (a) of the output port b and the phase (b) and the propagation distance (when κ = 0.25);
[0018] Figure 4 It is the relationship diagram between phase and propagation distance;
[0019] In the figure: For two inputs with the same amplitude and phase, when g / (2κ) = 0 (solid line), 0.4 (dashed line), 0.8 (dotted line) and 1 (dotted line), the relationship between the amplitude (a) and phase (b) of the output port b and the propagation distance (when κ = 0.25);
[0020] Figure 5 (a) is a schematic diagram of the structure of a passive PT symmetric directional coupler with deposited metal; Figure 5 (b) is a schematic diagram of the simulation results of the relationship between the transmittance of the output port and the metal percentage;
[0021] Figure 6 The electric field distribution when light is input from a lossless waveguide (a) and a lossy waveguide (b);
[0022] Figure 7 Schematic diagram of the non-reciprocal characteristics of the PT symmetric directional coupler. DETAILED DESCRIPTION
[0023] like Figure 7 As shown, a method for realizing an on-chip isolator based on the non-reciprocal property of a PT-symmetric directional coupler involved in this embodiment is shown. By constructing the Hamiltonian of a non-Hermitian system and solving the eigenvalue equation, the input-output relationship of the directional coupler and the expressions of its amplitude and phase characteristics are obtained. By adjusting the loss, the amplitude and phase of the two output ports are adjusted to obtain a specific position so that the light input through the lossless waveguide end is completely coupled to the lossy waveguide end, while the light input from the lossy waveguide end is not completely coupled to the lossless waveguide. The specific position is used as the output end of the directional coupler, and a Y-branch structure is set at the two output ends to combine the two output light beams together, thereby realizing the non-reciprocity between the input and output.
[0024] The method specifically includes:
[0025] S1. Determine the Hamiltonian: Figure 2 (a) shows the schematic diagram of the structure of a PT-symmetric directional coupler with balanced gain and loss. When the magnitude of the gain and loss in the waveguide is g / 2 (g represents the difference between the gain and the loss), and the coupling coefficient between the two waveguides is κ, the Hamiltonian describing the directional coupler is Where: g is the difference between gain and loss, and κ is the coupling coefficient between the two waveguides.
[0026] S2. Solve the eigenvalue equation: The eigenvalue equation is Hψ=Eψ, where: the operator E represents The eigenstate ψ is (a,b) T , a and b represent the electric field intensities in waveguide a and waveguide b, respectively.
[0027] S3. Determine the input-output relationship: When the directional coupler works in the PT symmetric state, that is, κ>g / 2. By solving the above eigenvalue equation, the relationship between the input and output ports of the directional coupler is obtained, which is specifically: Where: θ = arcsin (g / 2κ), Z = κz, g is the difference between gain and loss, κ is the coupling coefficient, z is the distance of light propagation, a0 and b0 represent the electric field strength of the input signal in waveguide a and waveguide b respectively. For the case where the directional coupler works in the PT symmetry breaking state, the analysis method is similar.
[0028] S4. Determine the amplitude and phase characteristics: According to the relationship between the input and output ports of the directional coupler, the expressions of the amplitude and phase characteristics of the two output ports are obtained as follows: Where: θ = arcsin(g / 2κ), Z = κz, g is the difference between gain and loss, κ is the coupling coefficient, z is the distance of light propagation, a0 and b0 represent the electric field strength of the input signal in waveguide a and waveguide b respectively.
[0029] S5. Analyze the amplitude and phase characteristics when the signal is input from only one input port or when the signal is input from two ports at the same time, including: when the signal is input from only one input port, that is, a0=1, b0=0; when the signal is input from two ports at the same time, and the two signals have the same amplitude and phase, that is, a0=b0=1; the relationship between the amplitude, phase and propagation distance in these two cases is as follows: Figure 3 and Figure 4 As shown in the figure, the curves of different line types represent different g / (2κ) values. From the solid line in the figure, we can see that when g / (2κ)=0, the device is actually a traditional directional coupler. Since the two waveguides are completely symmetrical, energy will couple back and forth between the two waveguides. When the parameters of the device are fixed, the output is always the same. As the gain and loss in the waveguide increase, the symmetry of the two waveguides is gradually broken, and their coupling properties will change. From Figure 3 (a) and Figure 4 (a) It can be seen that as the gain and loss increase, the coupling period becomes longer and the range of amplitude change becomes larger. As for the phase characteristics, for case (1), the phase of the output port does not change with the change of gain and loss except for the sudden change of π (such as Figure 3 (b)), the phase difference between ports a and b is always ±π / 2; for case (2), the phases of the two output ports are no longer the same (as shown in Figure 4 (b)), and the phase difference varies with the gain and loss.
[0030] S6. Construct a passive PT symmetrical directional coupler: Set one waveguide in the directional coupler to have loss, while the other waveguide has neither gain nor loss, such as Figure 2 As shown in (b), its phase characteristics Expression of amplitude characteristics Where: e -zsinθ is the attenuation coefficient.
[0031] S7, adding a Y branch structure at the two output ports of the PT symmetric directional coupler obtained in step S6 to combine the two output lights together, such as Figure 7 As shown, specifically: when light is input only from the lossless waveguide, for forward transmission, the electric field intensity at the output port is When the power distribution ratio of the Y branch is r1:r2, for reverse transmission, the electric field strength at the output port is The decibel ratio of the forward transmission and reverse transmission electric field strengths, i.e. the isolation ratio Where: θ = arcsin(g / 2κ), Z = κz, g is the difference between gain and loss, κ is the coupling coefficient, z is the distance of light propagation, a0 and b0 represent the electric field strength of the input signal in waveguide a and waveguide b respectively, and r1:r2 is the power distribution ratio of the Y branch.
[0032] The maximum value of the isolation ratio It depends on the power distribution ratio of the Y branch. When the loss, length, and coupling strength of the PT symmetrical directional coupler satisfy Zcosθ-θ=π / 2, R can reach its maximum value. For example, when the waveguide width is 0.6μm and the gap between the two waveguides is 150nm, the required loss when the waveguide length is one, two, and three coupling cycles (49μm) is 641.14cm respectively. -1 、433.74cm -1 and 478.33cm -1 .
[0033] From the expressions of the amplitude and phase characteristics of the two output ports, it can be seen that when other parameters are fixed, the amplitude and phase characteristics of the two output ports are functions of the external loss in the lossy waveguide. Therefore, the amplitude and phase of the two output ports can be adjusted by adjusting the loss. This feature can be used in modulators or switches. This embodiment uses Lumerical FDTD Solutions simulation software to simulate a passive PT symmetric directional coupler. The structural diagram of the device is shown in Figure 5 (a) shown.
[0034] After specific simulation, the directional coupler parameters are: the width of the two waveguides is 0.6μm, the gap between the two waveguides is 150nm, and the length of the two waveguides is one coupling period (49μm). Assume that the light is input from the waveguide a0 port. When no loss is added, the transmittance of the output port a is 0.003, and the transmittance of the output port b is 0.995. The method of introducing loss is to deposit metal on the top of the waveguide, and the thickness of the metal is set to 300nm. The amount of loss introduced can be measured by the duty cycle of the metal (the percentage of the metal length to the waveguide length). When the loss is less than (greater than) 641.14cm -1 (The corresponding duty cycle is about 0.2), the device works in the PT symmetry (PT symmetry breaking) state. The simulation results of the relationship between the transmittance and loss of the two output ports are shown in Figure 5 (b) is shown. When the metal duty cycle is changed, the transmittance of the two output ports will change. When the metal duty cycle is 0.2, the transmittance of output port a is 0.883, and the transmittance of output port b is 0.020. Since the transmittance changes with the change of the external loss, when the metal duty cycle is between 0 and 0.2, the PT symmetric directional coupler can be used as a switch.
[0035] Since the loss only exists in one of the waveguides, the symmetry of the loss distribution between the two waveguides is broken, so when the light is input from the lossless waveguide and the lossy waveguide, the output of the device is different. A special position can be found where the light input from the lossless waveguide is fully coupled to the lossy waveguide, however, the light input from the lossy waveguide is not fully coupled to the lossless waveguide (such as Figure 6 This is the origin of the non-reciprocal characteristics in the PT symmetric state.
[0036] When the power distribution of the Y branch is r1:r2=1:10, the isolation ratio can reach its maximum value R max =10.4dB. In order to meet the commercial demand for an isolation rate greater than 20dB, two such couplers need to be cascaded to work. Each coupler can provide an isolation ratio of about 10dB, and the insertion loss of each isolator is about 0.25dB. Therefore, compared with the prior art, the beneficial effects of this embodiment are: the method proposed in this embodiment to use the concept of PT symmetry to realize an on-chip isolator can avoid the use of magneto-optical materials, thereby reducing the size of the device and making it easier to carry out large-scale integration. In addition, non-reciprocity can be achieved for light of any intensity, which is impossible to achieve with nonlinear effects.
[0037] The following table compares this approach with isolators proposed in other literature.
[0038] Compared with the prior art, the present invention adopts a lossy directional coupler in structure, avoiding the use of magneto-optical materials and the introduction of an external magnetic field, thereby reducing the size of the device. In principle, the non-reciprocal property of PT symmetry is adopted to overcome the shortcomings of traditional non-reciprocity and achieve large-scale integration. By deriving the amplitude and phase characteristics of the PT symmetric directional coupler, and based on the non-reciprocity of passive PT symmetric directional coupling, a method for realizing an on-chip isolator is proposed. It can be seen from the theoretical derivation and simulation results that the device has the advantages of avoiding the use of magneto-optical materials, avoiding the introduction of gain, and having non-reciprocity for light of any intensity.
[0039] The above-mentioned specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principle and purpose of the present invention. The protection scope of the present invention shall be based on the claims and shall not be limited by the above-mentioned specific implementation. Each implementation scheme within its scope shall be subject to the constraints of the present invention.
Claims
1. A method for realizing an on-chip isolator based on a parity-time symmetric directional coupler, characterized in that: By constructing the Hamiltonian of a non-Hermitian system and solving the eigenvalue equation, the input-output relationship of the directional coupler and its amplitude and phase characteristic expressions are obtained; by adjusting the loss to adjust the amplitude and phase of the two output ports, a specific position is obtained so that the light input through the lossless waveguide end is completely coupled to the lossy waveguide end, while the light input from the lossy waveguide end is not completely coupled to the lossless waveguide. The specific position is used as the output end of the directional coupler, and a Y-branch structure is set at the two output ends to combine the two output light beams together, thereby realizing the non-reciprocity between input and output.
2. The on-chip isolator implementation method based on parity-time symmetric directional coupler according to claim 1 is characterized in that: When the Hamiltonian of the non-Hermitian system satisfies the PT symmetry condition, the system has real eigenvalues.
3. The on-chip isolator implementation method based on parity-time symmetric directional coupler according to claim 1, characterized in that: The directional coupler adopts a passive PT symmetrical directional coupler, that is, only one of the two waveguides has loss, and the other waveguide has neither gain nor loss.
4. The on-chip isolator implementation method based on parity-time symmetric directional coupler according to any one of claims 1 to 3, characterized in that: include: S1. Determine the Hamiltonian: Schematic diagram of the structure of a PT-symmetric directional coupler with balanced gain and loss; When the gain and loss in the waveguide are g / 2 (g represents the difference between gain and loss), and the coupling coefficient between the two waveguides is κ, the Hamiltonian describing the directional coupler is Where: g is the difference between gain and loss, κ is the coupling coefficient between the two waveguides; S2. Solve the eigenvalue equation: The eigenvalue equation is Hψ=Eψ, where: the operator E represents The eigenstate ψ is (a,b) T , a and b represent the electric field intensity in waveguide a and waveguide b respectively; S3. Determine the input-output relationship: When the directional coupler works in the PT symmetric state, that is, κ>g / 2; by solving the above eigenvalue equation, the relationship between the input and output ports of the directional coupler is obtained, which is specifically: Where: θ = arcsin(g / 2κ), Z = κz, g is the difference between gain and loss, κ is the coupling coefficient, z is the distance of light propagation, a0 and b0 represent the electric field strength of the input signal in waveguide a and waveguide b respectively; S4. Determine the amplitude and phase characteristics: According to the relationship between the input and output ports of the directional coupler, the expressions of the amplitude and phase characteristics of the two output ports are obtained as follows: Where: θ = arcsin(g / 2κ), Z = κz, g is the difference between gain and loss, κ is the coupling coefficient, z is the distance of light propagation, a0 and b0 represent the electric field strength of the input signal in waveguide a and waveguide b respectively; S5. Analyze the amplitude and phase characteristics when the signal is input from only one input port or when the signal is input from two ports at the same time, specifically including: when the signal is input from only one input port, that is, a0=1, b0=0; when the signal is input from two ports at the same time, and the two signals have the same amplitude and phase, that is, a0=b0=1; S6. Construct a passive PT symmetrical directional coupler: Set one waveguide in the directional coupler to have loss, while the other waveguide has neither gain nor loss. The expression of amplitude characteristic Amp(a)= Where: e -Z sinθ is the attenuation coefficient; S7, add a Y branch structure at the two output ports of the PT symmetric directional coupler obtained in step S6 to combine the two output lights together. Specifically, when the light is input only from the lossless waveguide, for forward transmission, the electric field intensity at the output port is When the power distribution ratio of the Y branch is r1:r2, for reverse transmission, the electric field strength at the output port is The decibel ratio of the forward transmission and reverse transmission electric field strengths, i.e. the isolation ratio Where: θ = arcsin(g / 2κ), Z = κz, g is the difference between gain and loss, κ is the coupling coefficient, z is the distance of light propagation, a0 and b0 represent the electric field strength of the input signal in waveguide a and waveguide b respectively, and r1:r2 is the power distribution ratio of the Y branch.
5. The on-chip isolator implementation method based on parity-time symmetric directional coupler according to claim 4 is characterized in that: The maximum value of the isolation ratio It depends on the power distribution ratio of the Y branch; When the loss, length, and coupling strength of the PT symmetrical directional coupler satisfy Zcosθ-θ=π / 2, R reaches its maximum value.
6. The on-chip isolator implementation method based on parity-time symmetric directional coupler according to claim 4 is characterized in that: The amplitude and phase of the two output ports are adjusted by adjusting the loss.
7. An on-chip isolator obtained based on the method according to any one of claims 1 to 6, characterized in that: include: A directional coupler and a Y-branch structure, wherein: the two waveguides of the directional coupler have the same width.
8. The on-chip isolator according to claim 7, characterized in that: The width of the two waveguides of the directional coupler is 0.6 μm, the gap between the two waveguides is 150 nm, and the length of the two waveguides matches the coupling period, that is, 49 μm.
9. The on-chip isolator according to claim 7, characterized in that: Losses are introduced by depositing metal on the top of one of the waveguides of the directional coupler. The thickness of the metal is set to 300nm and the duty cycle is 0.2.