Anisotropic material-based phase-controlled waveguide structure and wavelength division multiplexer structure comprising same

Abstract

An anisotropic material-based phase-controlled waveguide structure and a wavelength division multiplexer structure comprising same. Phase-controlled waveguide portions of a wavelength division multiplexer are symmetrically designed along a special angle. On the basis of said method, also disclosed are an arrayed waveguide grating and a cascaded Mach-Zehnder interferometer structure, for realizing a wavelength division multiplexer on an anisotropic material. The design method and structure can effectively prevent the birefringence effect of anisotropic materials from bringing difficulties to the design of wavelength division multiplexers and generating negative impact on precise phase control.

Description

A phase-controlled waveguide structure based on anisotropic materials and its wavelength division multiplexer structure Technical Field

[0001] The present invention relates to the field of photonic integrated devices, in particular to a phase-controlled waveguide structure based on anisotropic materials and a wavelength division multiplexer structure thereof. Background Art

[0002] In recent years, global communication data traffic has grown exponentially, driving unprecedented advancements in communication technology. Optical communication, with its advantages of high capacity, strong anti-interference capabilities, and low construction costs, has become the predominant transmission method in modern communications. In optical communication systems, wavelength division multiplexing (WDM) significantly expands communication bandwidth compared to other multiplexing technologies by enabling the simultaneous transmission of multiple independent signals over the same optical fiber.

[0003] Arrayed waveguide gratings (AWGs) are one of the key components for on-chip wavelength division multiplexing (WDM) technology, offering comprehensive advantages such as strong stability, high resolution, and a large number of channels. Their primary structure consists of an input free propagation region, an arrayed waveguide, and an output free propagation region.

[0004] Cascaded MZIs are also a key component in wavelength division multiplexing technology. Compared to arrayed waveguide gratings, they offer advantages such as simpler fabrication, smaller chip footprint, and lower insertion loss. Their main structure includes an input waveguide, a 3dB directional coupler, a phase control waveguide, and an output waveguide.

[0005] Thin-film lithium niobate (TFLN), an emerging photonic integrated chip material, not only offers the advantages of low loss and a wide light transmission spectrum, but also possesses excellent electro-optical effects, large nonlinear optical coefficients, and stable chemical properties. Furthermore, compared to traditional bulk lithium niobate materials, thin-film lithium niobate has a higher refractive index contrast, enabling significant device size reduction.

[0006] Thin film lithium niobate wafers can be divided into X-cut, Y-cut and Z-cut. Among them, X-cut and Z-cut are the most common. In order to take advantage of the maximum electro-optic coefficient r of lithium niobate crystal 33 The transverse electric (TE) mode is selected as the waveguide transmission mode on X-cut wafers, while the transverse magnetic (TM) mode is selected as the waveguide transmission mode on Z-cut wafers. Compared with the TM mode, the TE mode has a higher overlap integral with the electric field, so X-cut lithium niobate films have better electro-optical performance.

[0007] When designing an arrayed waveguide grating (AWG), the optical path difference between the arrayed waveguide paths must be constant, which inevitably leads to curved waveguides at varying angles and straight waveguides in varying directions. However, the refractive index of X-cut thin-film lithium niobate is dependent on the waveguide orientation, making the design more difficult and placing high demands on the device fabrication process. Currently, the design of an AWG on X-cut thin-film lithium niobate has not yet been achieved worldwide.

[0008] When designing a cascaded MZI, each single-stage MZI must ensure a certain optical path difference between the upper and lower arms. Similarly, in X-cut thin-film lithium niobate, the design of the phase-control waveguide must also consider variations in the waveguide's refractive index in different directions. Currently, a common design employs a special structure that cancels out the curved waveguides in the upper and lower arms, with the optical path difference determined solely by the length and refractive index differences of the straight waveguides. However, cascaded MZIs designed using this approach often tend to be larger.

[0009] Summary of the Invention

[0010] The present invention aims to provide a phase-controlled waveguide structure based on anisotropic materials and a wavelength division multiplexer structure thereof. The main concept is to design the phase-controlled waveguide portion of the wavelength division multiplexer device symmetrically along a specific angle, thereby avoiding the difficulties caused by the birefringence effect of anisotropic materials in wavelength division multiplexer device design and the negative impact on precise phase control. The present invention specifically focuses on wavelength division multiplexer devices based on anisotropic materials.

[0011] The technical solution to achieve the objectives of the present invention is to arrange the phase control waveguide portion of the wavelength division multiplexing device along an axis that forms an angle of 45° or 135° with both crystal axes of the anisotropic material surface. Specifically, in the arrayed waveguide grating, the input free propagation region, arrayed waveguide, and output free propagation region are symmetrical about this 45° or 135° axis; in the cascaded MZI, each single-stage MZI is symmetrical about this 45° or 135° axis.

[0012] The present invention is achieved through the following technical solutions:

[0013] The present invention discloses a phase-controlled waveguide structure. The phase-controlled waveguide structure includes symmetrically arranged waveguide units on both sides. The waveguide units include connected straight waveguides and curved waveguides. The phase-controlled waveguide structure includes a straight waveguide A, a curved waveguide A, a curved waveguide B, and a straight waveguide B connected in sequence. The straight waveguide A, the curved waveguide A, the curved waveguide B, and the straight waveguide B are symmetrically arranged along an axis that forms an angle of 45° or 135° with two crystal axes on the surface of an anisotropic material.

[0014] As a further improvement, the straight waveguide A and the straight waveguide B described in the present invention have the same length, and the curved waveguide A and the curved waveguide B have the same bending radius and bending angle.

[0015] As a further improvement, the cross section of the phase control waveguide structure described in the present invention includes a buried oxide layer, a waveguide layer and a silicon dioxide upper cladding layer from bottom to top, and the anisotropic material is X-cut thin film lithium niobate.

[0016] As a further improvement, the radius of the curved waveguide described in the present invention is greater than or equal to 50 microns, and the waveguide layer is a ridge waveguide or a strip waveguide.

[0017] The present invention also discloses a wavelength division multiplexer structure based on anisotropic materials and including a phase-controlled waveguide structure based on an arrayed waveguide grating. The structure includes an input waveguide, an input end free propagation area, an input end tapered waveguide, an arrayed waveguide, an output end tapered waveguide, an output end free propagation area and an output waveguide connected in sequence.

[0018] As a further improvement, the input end free propagation area and the output end free propagation area, the input end tapered waveguide and the output end tapered waveguide, and the array waveguide itself described in the present invention are symmetrically arranged about an axis that forms an angle of 45° or 135° with the two crystal axes of the anisotropic material surface.

[0019] As a further improvement, the input-end tapered waveguide and the output-end tapered waveguide described in the present invention are used to reduce coupling loss between modes, and the input waveguide and the output waveguide are used for input and output of optical signals, respectively.

[0020] The present invention also discloses a wavelength division multiplexer structure based on anisotropic materials and a phase-controlled waveguide structure and based on a cascaded Mach-Zehnder interferometer. The structure is a binary tree structure composed of single-stage MZI units connected by a 3dB directional coupler. The single-stage MZI unit includes an upper arm and a lower arm of the phase-controlled waveguide structure. The upper arm and the lower arm are symmetrically arranged about an axis that forms an angle of 45° or 135° with the two crystal axes of the anisotropic material surface.

[0021] As a further improvement, the structure of the present invention includes an input waveguide, a single-stage MZI unit, a 3dB directional coupler connecting the single-stage MZI units for beam combining and beam splitting, and an output waveguide.

[0022] As a further improvement, each single-stage MZI unit described in the present invention includes two interference arms formed after beam splitting, namely an upper arm and a lower arm, and the upper arm and the lower arm have a length difference.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1) A wavelength division multiplexer based on an arrayed waveguide grating structure is designed on anisotropic materials, which overcomes the difficulties brought by the birefringence effect to precise phase control.

[0025] 2) The method for designing cascaded MZIs on anisotropic materials has been optimized, making the phase-controlled waveguide more efficient and compact.

[0026] 3) The present invention uses the anisotropic material X-cut thin film lithium niobate as an example to design a wavelength division multiplexing device. Compared with the traditional material silicon, it has the advantages of wide light transmission spectrum and low loss. At the same time, it also has excellent nonlinear and electro-optical properties, and has the potential to realize richer functions. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a schematic diagram of the design principle and phase control waveguide layout;

[0028] 1 is a straight waveguide A, 2 is a curved waveguide A, 3 is a curved waveguide B, and 4 is a straight waveguide B. The structure is symmetrical about an axis that forms an angle of 45° or 135° with both the Y and Z crystal axes.

[0029] FIG2 is a schematic top view of a wavelength division multiplexer based on an arrayed waveguide grating structure on an anisotropic material according to the present invention;

[0030] 5 is the input waveguide, 6 is the input end free propagation area, 7 is the input end tapered waveguide, 8 is the array waveguide, 9 is the output end tapered waveguide, 10 is the output end free propagation area, and 11 is the output waveguide. When the anisotropic material is X-cut thin film lithium niobate, the Z direction is the Z crystal axis direction.

[0031] FIG3 is a top view of a wavelength division multiplexer based on a cascaded MZI structure on an anisotropic material according to the present invention;

[0032] 12 is an upper arm, 13 is a lower arm, 14 is an input waveguide, 15 is a 3dB directional coupler, and 16 is an output waveguide. When the anisotropic material is an X-cut thin film lithium niobate, the Z direction is the Z crystal axis direction.

[0033] FIG4 is a schematic diagram of the cross-sectional structure of a waveguide when the anisotropic material is an X-cut thin film lithium niobate according to the present invention;

[0034] 17 is a silicon dioxide upper cladding layer, 18 is a waveguide layer, and 19 is a buried oxide layer; wherein the X direction is the X crystal axis direction of the thin film lithium niobate. DETAILED DESCRIPTION

[0035] The present invention provides a phase-controlled waveguide structure and a wavelength division multiplexer structure thereof. These design methods and structures for implementing wavelength division multiplexing devices on anisotropic materials include a phase-controlled waveguide structure, a wavelength division multiplexer structure based on an arrayed waveguide grating, and a wavelength division multiplexer structure based on a cascaded Mach-Zehnder interferometer. The invention uses X-cut thin-film lithium niobate, an anisotropic material, as an example for specific illustration. Alternatively, the anisotropic material may be barium titanate.

[0036] Figure 1 is a schematic diagram of a phase-controlled waveguide path according to the present invention. The phase-controlled waveguide structure comprises symmetrically arranged waveguide units on both sides. The waveguide units comprise connected straight waveguides and curved waveguides, consisting of straight waveguide A1, straight waveguide B4, curved waveguide A2, and curved waveguide B3, with the four waveguides connected in sequence. The structure is symmetrical about an axis that is 45° with both the Y and Z crystal axes. That is, the straight waveguides A1 and B4 are of equal length, and the curved waveguides A2 and B3 have equal bending radii and bending angles. Straight waveguides A1, A2, B3, and B4 are symmetrically arranged along an axis that is 45° or 135° with both crystal axes on the surface of the anisotropic material. Straight waveguides A1 and B4 are of equal length, and the curved waveguides A2 and B3 have equal bending radii and bending angles.

[0037] Figure 2 shows a top view of a wavelength division multiplexer based on an arrayed waveguide grating structure using X-cut thin-film lithium niobate. This is a wavelength division multiplexer structure based on an arrayed waveguide grating that includes a phase-controlled waveguide structure. The arrayed waveguide path is the same as that in Figure 1 and is also composed of straight waveguides and curved waveguides. The structure includes an input waveguide 5, an input free propagation region 6, an input tapered waveguide 7, an arrayed waveguide 8, an output tapered waveguide 9, an output free propagation region 10, and an output waveguide 11, all connected in sequence. The input free propagation region 6, the output free propagation region 10, the input tapered waveguide 7, the output tapered waveguide 9, and the arrayed waveguide 8 are symmetrical about an axis that is 45° with both the Y and Z crystal axes. The input tapered waveguide 7 and the output tapered waveguide 9 are used to reduce coupling loss between modes. The input waveguide 5 and the output waveguide 11 are used for input and output of optical signals, respectively.

[0038] Figure 3 shows a top view of a wavelength division multiplexer based on a cascaded MZI structure using X-cut thin-film lithium niobate. This is a wavelength division multiplexer structure based on cascaded Mach-Zehnder interferometers and includes a phase-controlled waveguide structure. The phase-controlled waveguide path is the same as in Figure 1, also consisting of straight waveguides and curved waveguides. The structure is a binary tree consisting of single-stage MZI units connected by 3dB directional couplers 15. The single-stage MZI unit includes an upper arm 12 and a lower arm 13 of the phase-controlled waveguide structure. Both arms 12 and 13 are symmetrically arranged about an axis that forms an angle of 45° or 135° with both crystal axes of the anisotropic material plane.

[0039] The phase-control waveguide structures of each MZI stage are symmetrical about axes with angles of 135° to both the Y and Z crystal axes. A single-stage MZI upper arm 12 and a single-stage MZI lower arm 13 are used to generate an optical path difference. A 3dB directional coupler 15 is used for MZI beam splitting and combining. Input waveguides 14 and output waveguides 16 are used for optical signal input and output, respectively.

[0040] The structure includes an input waveguide 14, a single-stage MZI unit, a 3dB directional coupler 15 connecting the single-stage MZI units for beam combining and splitting, and an output waveguide 16. Each single-stage MZI unit includes two interferometer arms formed after beam splitting, namely an upper arm 12 and a lower arm 13, and the upper arm 12 and the lower arm 13 have a length difference.

[0041] As shown in Figure 4, a cross-section of a phase-controlled waveguide structure consists, from bottom to top, of a buried oxide layer 19, a waveguide layer 18, and a silica upper cladding layer 17. The anisotropic material is X-cut thin-film lithium niobate. The curved waveguide radius is greater than or equal to 50 microns, and the waveguide layer is a ridge waveguide or strip waveguide. In arrayed waveguide gratings and cascaded MZIs, the refractive index of the waveguide layer 18 is greater than that of both the silica upper cladding layer 17 and the buried oxide layer 19. This confines the light field within the waveguide layer 19 and ensures stable transmission.

[0042] In order to reduce bending loss and avoid mode hybridization, the bending radius of the curved waveguide in Figures 2 and 3 is not less than 50 microns.

[0043] The cross-sectional structure of the thin film lithium niobate waveguide in FIG4 is a ridge waveguide or a strip waveguide.

[0044] When arrayed waveguide 8 is in use, multi-wavelength optical signals travel from the optical fiber through the input waveguide 5 into the input free propagation zone 6, where they undergo diffraction. The optical energy, with a Gaussian distribution, enters the input tapered waveguide 7 at the entrance of each arrayed waveguide 8. The arrayed waveguide 8 is designed to ensure a constant optical path difference between adjacent waveguides. When the multi-path optical signals output from the arrayed waveguide 8 enter the output free propagation zone 10, the different wavelengths of the optical signals interfere and superimpose in the output free propagation zone 10 due to their different phase differences. After this, the optical signals are focused to different locations and then output by the output waveguide 11, achieving wavelength-specific light separation.

[0045] When a cascaded MZI is used, a multi-wavelength optical signal is input via an optical fiber through an input waveguide 14. After passing through a 3dB directional coupler 15, the light is split into two coherent beams of equal intensity. These two beams propagate through the two interferometer arms of the MZI. Because the upper arm 12 and lower arm 13 in a single-stage MZI have different lengths, a certain optical path difference is generated. This causes superposition interference in the 3dB directional coupler 15, and light of specific wavelengths is amplified at the corresponding ports. The light is then output through different output waveguides 16, thus achieving light separation at different wavelengths. Furthermore, cascading multiple MZI structures can optimize the filtering curve, while the binary tree structure can multiplex more wavelengths of light.

[0046] To further clarify the objectives, technical solutions, and advantages of the present invention, the present invention is further described below in conjunction with the accompanying drawings and examples. The specific embodiments described herein are intended only to illustrate the present invention and are not intended to limit it. This article describes the design method and wavelength division multiplexing device structure, using X-cut thin-film lithium niobate material as an example.

[0047] Example 1

[0048] An embodiment of the present invention provides a wavelength division multiplexer (WDM) structure design based on a novel X-cut thin-film lithium niobate arrayed waveguide grating. As shown in Figures 2 and 4, the device sequentially comprises the input waveguide 5, the input free propagation region 6, the input tapered waveguide 7 at the arrayed waveguide inlet, the arrayed waveguide 8, the output tapered waveguide 9, the output free propagation region 10, and the output waveguide 11, as shown in Figure 2. A cross-section of this waveguide structure is shown in Figure 4, comprising a thin-film lithium niobate waveguide layer 19, a silicon dioxide upper cladding layer 17, and a buried oxide layer 19.

[0049] In order to allow light of different wavelengths to form interference superposition at the corresponding output waveguides, it is necessary to design the adjacent array waveguides to produce equal phase differences.

[0050] X-cut lithium niobate thin film material exhibits birefringence, and the effective refractive index and group refractive index of the waveguide are both related to the waveguide orientation (related to the Z-axis angle). Arrayed waveguides include straight waveguides, curved waveguides, and tapered waveguides in various directions, making it challenging to maintain phase differences between adjacent arrayed waveguides.

[0051] According to the simulation results of finite element simulation software, the effective refractive index and group refractive index of the straight waveguide, tapered waveguide and slab waveguide based on X-cut thin film lithium niobate all satisfy the relationship (where θ is the angle between the waveguide direction and the Z crystal axis): n = n x cos 2 θ+n y sin 2 θ

[0052] Based on this relationship, in the embodiment of the present invention, the phase control waveguide portion is designed with the 45° or 135° axis as the symmetry axis. The principle is explained below using Figure 1 as an example: the straight waveguide A1 and the straight waveguide B4 are symmetrical about the 45° axis, S is the length of the straight waveguide, and the optical path of the straight waveguide A1 and the straight waveguide B4 are: L1 = (n x cos 2 θ+n y sin 2 θ)×S L4=(n x cos 2 (90°-θ)+n y sin 2(90°-θ))×S

[0053] The sum of the two is: L1+L4=(n x +n y )×S

[0054] It can be seen that the optical path after addition has nothing to do with the straight waveguide path, but only with its length.

[0055] Similarly, the optical path lengths of the curved waveguides 2 and 3 are calculated by integration as follows:

[0056] Where R is the waveguide bending radius, θ is the waveguide bending angle, and the sum of the two is: L2+L3=(n x +n y )×Rθ

[0057] It can be seen that the optical path after addition has nothing to do with the curved waveguide path, but is only related to the curved waveguide radius and bending angle.

[0058] The above conclusions can be extended to structures such as slab waveguides and tapered waveguides. Specifically, when the waveguide structure is symmetrical along a 45° or 135° axis, the optical path difference between adjacent arrayed waveguides can be kept constant, regardless of the path of each waveguide, as long as the total length difference of the waveguide paths is kept constant. This design approach cleverly avoids the effects of birefringence and converts an anisotropic device design into an isotropic one.

[0059] When the optical path difference between adjacent array waveguides is kept constant at ΔL, light of a certain wavelength can form the main interference maximum at the corresponding output waveguide position: I=I0+I1e jΔL +I2e j2ΔL +...+I n e jnΔL

[0060] If the energy in each array waveguide is equal (for example only), the interference maximum can be obtained as follows:

[0061] Output waveguides are placed at different positions on the image plane to obtain light of different wavelengths, thereby realizing wavelength division multiplexing function.

[0062] Example 2

[0063] An embodiment of the present invention provides a wavelength division multiplexer (WDM) structure design based on an X-cut thin-film lithium niobate cascaded MZI. As shown in Figures 3 and 4, the device sequentially comprises the upper arm 12 and lower arm 13 of the single-stage MZI in Figure 3, along with an input waveguide 14, a 3dB directional coupler 15, and an output waveguide 16. A cross-section of this waveguide structure is shown in Figure 4, comprising a thin-film lithium niobate waveguide layer 19, a silicon dioxide upper cladding layer 17, and a buried oxide layer 19.

[0064] As shown in Figure 1, when the waveguide path is symmetrical along a 45° or 135° axis, the waveguide optical path length is independent of the waveguide path and is only related to the waveguide length and equivalent refractive index. Based on this conclusion, each set of phase-controlled waveguides in the cascaded MZI provided in Example 2 is symmetrical along a 135° axis, and the phase-controlled waveguide length is designed based on the designed optical path difference. Compared to traditional MZI structures, the phase-controlled waveguide can be designed along any path, which can make the structure more efficient and compact to a certain extent.

[0065] The light is split into two coherent beams of equal intensity by the 3dB directional coupler in Figure 4. The two beams differ in phase by π / 2. After passing through the phase-controlled waveguide, the lower arm 13 lags behind the upper arm 12 by βΔL. After passing through the second 3dB directional coupler, the lower arm 13 lags behind the upper arm by π / 2. According to the principle of interference enhancement, the wavelengths of the optical signals output by the upper and lower arms 12 and 13 respectively satisfy: βΔL=2kπ βΔL=(2k+1)π

[0066] Therefore, based on the single-stage MZI structure, light splitting at different wavelengths can be easily achieved.

[0067] However, the passband characteristics of a single-stage MZI filter are sinusoidal, resulting in an effective bandwidth that is too narrow to meet the requirements of practical applications. To achieve a flat-top spectral distribution, a cascaded MZI structure is generally used in practical applications. Furthermore, to multiplex and demultiplex optical signals with a greater number of wavelengths, a binary tree structure can be introduced, as shown in Figure 3, enabling the multiplexing and demultiplexing of four channels.

[0068] The above description is not intended to limit the present invention. It should be noted that a person skilled in the art may make several changes, modifications, additions or substitutions without departing from the essential scope of the present invention. Such improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A phase control waveguide structure based on anisotropic materials, characterized in that: The phase-controlled waveguide structure comprises symmetrically arranged waveguide units on both sides, the waveguide unit comprises a connected straight waveguide and a curved waveguide, the phase-controlled waveguide structure comprises a straight waveguide A, a curved waveguide A, a curved waveguide B, and a straight waveguide B connected in sequence, and the straight waveguide A, the curved waveguide A, the curved waveguide B, and the straight waveguide B are symmetrically arranged along an axis that forms an angle of 45° or 135° with the two crystal axes on the surface of the anisotropic material.

2. The phase control waveguide structure according to claim 1, characterized in that: The straight waveguide A and the straight waveguide B have the same length, and the curved waveguide A and the curved waveguide B have the same bending radius and bending angle.

3. The phase control waveguide structure according to claim 1 or 2, characterized in that: The cross section of the phase-controlled waveguide structure comprises, from bottom to top, a buried oxide layer, a waveguide layer and a silicon dioxide upper cladding layer, and the anisotropic material is X-cut thin-film lithium niobate.

4. The phase control waveguide structure according to claim 3, characterized in that: The radius of the curved waveguide is greater than or equal to 50 microns, and the waveguide layer is a ridge waveguide or a strip waveguide.

5. A wavelength division multiplexer structure based on anisotropic materials and including a phase-controlled waveguide structure based on an arrayed waveguide grating, characterized in that: The structure comprises an input waveguide, an input end free propagation zone, an input end tapered waveguide, an array waveguide, an output end tapered waveguide, an output end free propagation zone and an output waveguide which are connected in sequence.

6. The wavelength division multiplexer structure based on arrayed waveguide grating including a phase control waveguide structure according to claim 4, characterized in that: The input free propagation area and the output free propagation area, the input tapered waveguide and the output tapered waveguide, and the array waveguide itself are symmetrically arranged about an axis that forms an angle of 45° or 135° with both crystal axes of the anisotropic material surface.

7. The wavelength division multiplexer structure based on arrayed waveguide grating including a phase control waveguide structure according to claim 5, characterized in that: The input end tapered waveguide and the output end tapered waveguide are used to reduce coupling loss between modes. The input waveguide and the output waveguide are used for input and output of optical signals respectively.

8. A wavelength division multiplexer structure based on a cascaded Mach-Zehnder interferometer including a phase-controlled waveguide structure based on anisotropic materials, characterized in that: The structure is a binary tree structure formed by connecting single-stage MZI units through 3dB directional couplers. The single-stage MZI unit includes an upper arm and a lower arm of a phase control waveguide structure. The upper arm and the lower arm are symmetrically arranged about an axis that is at an angle of 45° or 135° to the two crystal axes on the surface of the anisotropic material.

9. The wavelength division multiplexer structure based on cascaded Mach-Zehnder interferometers including a phase-controlled waveguide structure according to claim 7, characterized in that: The structure comprises an input waveguide, a single-stage MZI unit, a 3dB directional coupler for beam combining and beam splitting and connecting the single-stage MZI units, and an output waveguide.

10. The wavelength division multiplexer structure based on cascaded Mach-Zehnder interferometers including a phase-controlled waveguide structure according to claim 8, characterized in that: Each single-stage MZI unit includes two interference arms formed after beam splitting, namely an upper arm and a lower arm, and the upper arm and the lower arm have a length difference.

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