Dual-mode directional coupler based on inverted-cone-shaped sub-wavelength grating

Abstract

The invention discloses a dual-mode directional coupler based on inverted-cone-shaped sub-wavelength gratings. The dual-mode directional coupler mainly structurally comprises the two wide inverted-cone-shaped sub-wavelength gratings with opposite bottoms and a transition area composed of a sub-wavelength grating waveguide and a transverse four-trapezoid grating waveguide. By means of the characteristic of mode field regulation and control of the inverted-cone-shaped sub-wavelength grating, synchronous coupling of two modes of any order within one beat length is achieved, meanwhile, simultaneous regulation and control of the splitting ratio of the two modes are achieved, the processing efficiency of on-chip multimode signals is greatly improved, and the device has good application prospects in the fields of optical calculation, optical communication, optical interconnection and the like in the future.

Description

Technical Field

[0001] The present invention belongs to the field of optoelectronics, and relates to an optical signal processing device for different modes on a chip, and in particular to a dual-mode directional coupler based on an inverted tapered subwavelength grating. Background Art

[0002] Traditional wavelength division multiplexing (WDM) technology is limited to the 35nm spectral window in the C-band, resulting in fewer than 48 available wavelength channels and requiring independently tunable lasers. This leads to high system complexity and limited capacity expansion. While mode division multiplexing (MDM) technology can achieve linear capacity expansion by enabling single-wavelength parallel transmission through multiple modes, the significant difference in effective refractive index between different modes in the waveguide makes it difficult to achieve multi-mode synchronous control with conventional devices, hindering integration and application. Existing solutions, such as total internal reflection structures, adiabatic coupling systems, and mode converter arrays, suffer from large device size, while inverse design methods are limited by machining accuracy and scalability. To address these bottlenecks, this paper proposes an innovative solution based on an inverted tapered subwavelength grating. This solution achieves dual-mode synchronous coupling by unifying the coupling lengths of different modes. Combining this with a trapezoidal subwavelength grating shortens the coupling region to enhance integration, and optimizes the coupling region to enable flexible configuration of dual-mode power (equal / unequal splitting ratios). The resulting device offers a compact structure, low loss, high tolerance, and multimode expansion support, overcoming the core technical barriers to on-chip multimode synchronous control. Summary of the Invention

[0003] The purpose of the present invention is to provide a dual-mode directional coupler based on an inverted tapered subwavelength grating. By utilizing the different sensitivities of the evanescent fields of different order modes to the trapezoidal and ordinary grating structures in the inverted tapered subwavelength grating, a dual-mode directional coupler with consistent coupling lengths for the two modes is realized. At the same time, the coupling length of the device is greatly reduced, thereby reducing the device size and improving the integration of the integrated optical chip.

[0004] To achieve the above object, the present invention adopts the following technical solutions: The dual-mode directional coupler comprises: The busbar waveguide is sequentially connected by a first straight waveguide, a transition zone I, a first inverted tapered subwavelength grating waveguide I, a first inverted tapered curved subwavelength grating waveguide, a first inverted tapered subwavelength grating waveguide II, a transition zone II and the first straight waveguide II; The coupling waveguide is sequentially connected by a second inverted tapered sub-wavelength grating waveguide I, a second inverted tapered bent sub-wavelength grating waveguide, a second inverted tapered sub-wavelength grating waveguide II, a transition region III and a second straight waveguide; The first inverted tapered sub-wavelength grating waveguide I and the second inverted tapered sub-wavelength grating waveguide I are arranged in parallel to form a coupling region, and a coupling gap exists between the two; The first inverted tapered sub-wavelength grating waveguide I, the first inverted tapered bent sub-wavelength grating waveguide, the first inverted tapered sub-wavelength grating waveguide II, the second inverted tapered sub-wavelength grating waveguide I, the second inverted tapered bent sub-wavelength grating waveguide and the second inverted tapered sub-wavelength grating waveguide II are all inverted tapered sub-wavelength grating structures; The inverted tapered sub-wavelength grating structure is composed of a common sub-wavelength grating and a trapezoidal sub-wavelength grating, wherein: The ordinary subwavelength grating is located on the side away from the coupling gap; The trapezoidal subwavelength grating is located on a side close to the coupling gap, with its wide side facing the coupling gap; The duty cycle of the narrow side of the trapezoidal subwavelength grating is the same as that of the ordinary subwavelength grating; The narrow side width of the trapezoidal sub-wavelength grating is equal to the width of the ordinary sub-wavelength grating and is precisely aligned; The first inverted tapered curved sub-wavelength grating waveguide and the second inverted tapered curved sub-wavelength grating waveguide have the same curvature radius and are bent in a direction away from the other waveguide.

[0005] The transition region I is composed of a first transverse quad-trapezoidal grating waveguide I and a first sub-wavelength grating waveguide I; The transition region II is composed of a first transverse quad-trapezoidal grating waveguide II and a first sub-wavelength grating waveguide II; The transition region III is composed of a second transverse quad-trapezoidal grating waveguide and a second sub-wavelength grating waveguide; The first and second transverse four-trapezoidal grating waveguides are both composed of four trapezoidal waveguides arranged in parallel with equal intervals, and their widths linearly change along the light transmission direction.

[0006] In the first inverted tapered sub-wavelength grating waveguide I, the duty cycle of the wide side of the trapezoidal sub-wavelength grating gradually increases from left to right, the duty cycle at the leftmost end is equal to the duty cycle of the first sub-wavelength grating waveguide I, and the duty cycle at the rightmost end is equal to the constant duty cycle of the first inverted tapered curved sub-wavelength grating waveguide; In the first inverted tapered subwavelength grating waveguide II, the duty cycle of the wide side of the trapezoidal subwavelength grating gradually decreases from top to bottom, the duty cycle at the top is equal to the constant duty cycle of the first inverted tapered curved subwavelength grating waveguide, and the duty cycle at the bottom is equal to the duty cycle of the first subwavelength grating waveguide II; In the second inverted tapered subwavelength grating waveguide II, the duty cycle of the wide side of the trapezoidal subwavelength grating gradually decreases from bottom to top, the duty cycle at the bottom is equal to the constant duty cycle of the second inverted tapered curved subwavelength grating waveguide, and the duty cycle at the top is equal to the duty cycle of the second subwavelength grating waveguide.

[0007] The width of the ordinary subwavelength grating is 1.5 μm; the width of the trapezoidal subwavelength grating is 1.2 μm. By adjusting the values ​​of the wide-side duty cycle and the narrow-side duty cycle of the trapezoidal subwavelength grating, the refractive index difference between the odd and even supermodes of the TE0 and TE1 modes is made equal, thereby achieving synchronous coupling of the TE0 and TE1 modes within a single beat length.

[0008] All waveguide components have the same width, and the first straight waveguide II is perpendicular to the first straight waveguide I, and the second straight waveguide is perpendicular to the busbar waveguide.

[0009] The dual-mode directional coupler supports synchronous coupling of any dual-mode combination, including: TE0 and TE1, TE0 and TE2, TE1 and TE2, or TE0 and TM0.

[0010] The dual-mode directional coupler is implemented based on an SOI material platform, and includes: a silicon substrate; a silicon dioxide buried oxide layer formed on the silicon substrate; and a top silicon layer formed on the buried oxide layer, wherein the busbar waveguide and the coupling waveguide are etched in the top silicon layer.

[0011] The dual-mode directional coupler based on the inverted tapered subwavelength grating waveguide of the present invention has the following advantages: 1. In traditional waveguides, different-order modes have different coupling lengths due to significant differences in effective refractive index, making it difficult for a single device to efficiently couple multiple modes simultaneously. However, the present invention utilizes an inverted tapered subwavelength grating structure to align the coupling lengths of different-order modes, enabling simultaneous dual-mode coupling from a single device. This solves the problem of traditional devices being unable to simultaneously process multiple modes.

[0012] 2. Traditional symmetrical directional couplers typically achieve power distribution of a single guided mode between two output waveguides by adjusting the coupling length based on phase matching between modes. However, their controllability is limited to the splitting ratio of a single mode. However, by optimizing the coupling region length, this invention enables any dual-mode combination to exhibit equal or unequal power distribution at the output, enabling simultaneous control of the splitting ratio of both modes, providing flexibility for dynamically reconfigurable optical networks.

[0013] 3. This paper proposes an innovative inverted-tapered subwavelength grating waveguide structure composed of a trapezoidal subwavelength grating and a conventional subwavelength grating. This composite grating structure reduces the waveguide's ability to confine the optical field and enhances the evanescent field, thereby reducing the coupling length and device size. This results in a dual-mode directional coupler with an extremely short coupling region, suitable for high-density photonic integrated circuits.

[0014] 4. The present invention utilizes an inverted tapered subwavelength grating structure. This geometric form gives the structure greater adaptability to inherent errors such as sidewall angle fluctuations in the manufacturing process. Specifically, even in the face of certain process fluctuations, the inverted tapered subwavelength grating structure can more effectively maintain its key optical performance near the design target, ensuring that the optical signal is propagated and regulated according to the expected path, intensity, phase, and other characteristics. This tolerance for increased process errors directly translates into advantages in the manufacturing process: it significantly reduces the phenomenon of device performance substandard due to process deviations, thereby effectively improving the overall manufacturing yield of the product. At the same time, because the device performance is less sensitive to changes in process parameters, its performance in mass production and actual working environments is more consistent and reliable, ultimately achieving enhanced overall device stability and facilitating the large-scale mass production application of this technology.

[0015] 5. The dual-mode directional coupler based on an inverted tapered subwavelength grating proposed in this invention has excellent mode scalability. By adjusting the duty cycle and width of the inverted tapered subwavelength grating, the coupling lengths of different modes can be made the same, thereby achieving not only specific dual-mode directional coupling, but also any combination of dual-mode directional coupling. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a structural schematic diagram of the dual-mode directional coupler based on the inverted tapered subwavelength grating of the present invention.

[0017] Figure 2 Schematic diagram of the transition zone in the present invention.

[0018] Figure 3 This is a curve showing the variation of the third-order mode supermode refractive index difference with the wide-side duty ratio of the trapezoidal subwavelength grating in the inverted tapered subwavelength grating of the present invention when the narrow-side duty ratio of the trapezoidal subwavelength grating takes different values, where the width of the ordinary subwavelength grating is 1.5 μm and the width of the trapezoidal subwavelength grating is 1.2 μm.

[0019] Figure 4 It is a structural schematic diagram of the present invention taking SOI material as an example.

[0020] Figure 5 This is an implementation of the present invention taking a dual-mode directional coupler that simultaneously couples TE0 and TE1 as an example.

[0021] Figure 6 This is an implementation of the present invention taking a dual-mode directional coupler that simultaneously couples TE0 and TE2 as an example.

[0022] In the figure: 1-first straight waveguide I, 2-transition zone I, 201-first transverse four-trapezoidal grating waveguide I, 202-first sub-wavelength grating waveguide I, 3-first inverted tapered sub-wavelength grating waveguide I, 4-first inverted tapered bent sub-wavelength grating waveguide, 5-first inverted tapered sub-wavelength grating waveguide II, 6-transition zone II, 601-first transverse four-trapezoidal grating waveguide II, 602-first sub-wavelength grating waveguide II, 7-first straight waveguide II, 8-second inverted tapered sub-wavelength grating waveguide I, 9-second inverted tapered bent sub-wavelength grating waveguide, 10-second inverted tapered sub-wavelength grating waveguide II, 11-transition zone III, 111-second transverse four-trapezoidal grating waveguide, 112-second sub-wavelength grating waveguide, 12-second straight waveguide. DETAILED DESCRIPTION

[0023] The present invention will be further explained below with reference to specific embodiments.

[0024] like Figure 1 As shown in the figure, the main structures of the present invention are a straight waveguide, a transition region, an inverted tapered sub-wavelength grating waveguide, and an inverted tapered curved sub-wavelength grating waveguide. Among them, the inverted tapered sub-wavelength grating waveguide and the inverted tapered curved sub-wavelength grating waveguide are both composed of a normal sub-wavelength grating and a trapezoidal sub-wavelength grating.

[0025] The following describes the specific design process of a dual-mode directional coupler, taking a dual-mode directional coupler that simultaneously couples TE0 and TE1 as an example.

[0026] First, the overall structure of the directional coupler is analyzed. All subwavelength grating waveguides must meet the subwavelength condition, which determines the period of the subwavelength grating. The coupling region must ensure that the TE0 and TE1 modes are simultaneously coupled from the busbar waveguide to the coupling waveguide at a single beat length. Therefore, the widths of the trapezoidal subwavelength grating and the conventional subwavelength grating in the inverted-tapered subwavelength grating waveguide, as well as the duty cycle of the wide and narrow sides of the trapezoidal subwavelength grating, must be optimized. Ultimately, the width of the inverted-tapered subwavelength grating waveguide and the duty cycle of the trapezoidal subwavelength grating on both sides within the coupling region are determined.

[0027] In the busbar waveguide, the first straight waveguide I, transition region I, first inverted-tapered subwavelength grating waveguide I, first inverted-tapered curved subwavelength grating waveguide, first inverted-tapered subwavelength grating waveguide II, transition region II, and first straight waveguide II are sequentially connected. Transition region I consists of the first transverse quad-trapezoidal grating waveguide I and the first subwavelength grating waveguide I, while transition region II consists of the first transverse quad-trapezoidal grating waveguide II and the first subwavelength grating waveguide II. In the coupling waveguide, the second inverted-tapered subwavelength grating waveguide I, second inverted-tapered curved subwavelength grating waveguide, second inverted-tapered subwavelength grating waveguide II, transition region III, and second straight waveguide are sequentially connected. Transition region III consists of the second subwavelength grating waveguide and the second transverse quad-trapezoidal grating waveguide. Both the busbar waveguide and the coupling waveguide should support transmission in both TE0 and TE1 modes. Because the directional coupler utilizes a symmetrical design, the widths of the busbar waveguide and the coupled waveguide are equal to the width of the inverted-tapered subwavelength grating waveguide. Since the duty cycle of the narrow side of the trapezoidal subwavelength grating is consistent with that of a regular subwavelength grating, and the narrow side of the trapezoidal subwavelength grating trapezoidal waveguide is equal in width and precisely aligned with the regular subwavelength grating rectangular waveguide, the duty cycle of all regular subwavelength gratings, including the regular subwavelength grating in the inverted-tapered subwavelength grating waveguide, is identical.

[0028] The wide-side duty cycle of the leftmost trapezoidal subwavelength grating in the first inverted tapered subwavelength grating waveguide I is equal to that of the first subwavelength grating waveguide I. The wide-side duty cycle of the trapezoidal subwavelength grating gradually increases from left to right until it approaches the coupling region, where the wide-side duty cycle remains unchanged. At this time, the wide-side duty cycle is equal to that of the trapezoidal subwavelength grating in the first inverted tapered curved subwavelength grating waveguide; the duty cycles on both sides of the trapezoidal subwavelength grating in the first inverted tapered curved subwavelength grating waveguide are constant; the wide-side duty cycle of the uppermost trapezoidal subwavelength grating in the first inverted tapered subwavelength grating waveguide II is equal to that of the first inverted tapered curved subwavelength grating waveguide, and the wide-side duty cycle of the trapezoidal subwavelength grating gradually decreases from top to bottom until it is equal to that of the first subwavelength grating waveguide II. Similarly, the duty ratios of the trapezoidal subwavelength gratings on both sides of the second inverted tapered subwavelength grating waveguide are equal to those of the first inverted tapered subwavelength grating waveguide and remain constant. The duty ratio of the wide side of the trapezoidal subwavelength grating at the bottom of the second inverted tapered subwavelength grating waveguide II is equal to that of the second inverted tapered subwavelength grating waveguide. The wide side duty ratio of the trapezoidal subwavelength grating gradually decreases from bottom to top until it equals the duty ratio of the second subwavelength grating waveguide. The purpose of gradually changing the wide side duty ratio of the trapezoidal subwavelength grating in the inverted tapered subwavelength grating waveguide is to achieve a smooth transition between the subwavelength grating waveguide and the inverted tapered subwavelength grating waveguide in the transition region, ensuring that the optical signal completes energy transfer with low loss.

[0029] The transition zone is composed of a transverse four-trapezoidal grating waveguide and a sub-wavelength grating waveguide. The purpose of the transverse four-trapezoidal grating waveguide is to achieve the transition between the straight waveguide and the sub-wavelength grating waveguide. For example, the transition zone I where the TE0 and TE1 modes are coupled from the first straight waveguide I to the first sub-wavelength grating waveguide I is shown in FIG. Figure 2 As shown in Figure 1, the first transverse quad-trapezoidal grating waveguide I is located in the middle of the first subwavelength grating waveguide I. Four identical transverse trapezoidal waveguides are arranged in parallel with equal spacing, and the waveguide width decreases linearly from left to right. The leftmost end is connected to the first straight waveguide I, and the rightmost end is connected to the first inverted tapered subwavelength grating waveguide I, thus achieving low-loss coupling of optical signals from the straight waveguide to the subwavelength grating waveguide.

[0030] Then the specific parameters of the directional coupler are analyzed. In the coupling region, when the width of the trapezoidal subwavelength grating of the inverted tapered subwavelength grating waveguide increases to meet the transmission of TE0, the influence of the ordinary subwavelength grating width on TE0 will be weaker than TE1. Therefore, the coupling strength of TE0 and TE1 can be adjusted by changing the width, so that the two modes can be coupled into the same waveguide at the same length. According to the coupled mode theory, the difference between the odd and even supermode refractive index of TE1 and the difference between the odd and even supermode refractive index of TE0 are calculated. If the two are the same, the coupling length of the two modes is also the same, thereby achieving simultaneous coupling of TE0 and TE1 under one beat length. In addition, the coupling length can be reduced by increasing the difference between the odd and even supermodes, thereby reducing the device size. The consistency of the coupling length can also be achieved by changing the duty cycle. The narrow side duty cycle of the trapezoidal subwavelength grating is defined as D1, the wide side duty cycle is defined as D2, and the effective refractive index of the even supermode is N even , the effective refractive index of the odd supermode is N odd When the width of the ordinary subwavelength grating is 1.5 μm and the width of the trapezoidal subwavelength grating is 1.2 μm, the curve of the difference in the refractive index of the third-order mode supermode with D2 is obtained when D1 takes different values ​​in the inverted tapered subwavelength grating, as shown in Figure 2. Figure 3As shown in the figure, adjusting the duty cycle can change the difference between the odd and even supermodes, making the refractive index difference between the odd and even supermodes the same, which means that their coupling lengths are consistent. This determines the waveguide parameters of the inverted tapered subwavelength grating in the coupling region. When D1 is fixed in the inverted tapered subwavelength grating, the supermode refractive index difference of the TE0, TE1, and TE2 third-order modes exhibits a differential distribution as D2 changes. When the curves for the TE0 and TE2 modes intersect, the difference in refractive indices between the odd and even supermodes is the same, achieving synchronous coupling of the TE0 and TE2 modes. When the curves for the TE1 and TE2 modes intersect, the difference in refractive indices between the odd and even supermodes is the same, achieving synchronous coupling of the TE1 and TE2 modes. When the curves for the TE0 and TE1 modes intersect, the difference in refractive indices between the odd and even supermodes is the same, achieving synchronous coupling of the TE0 and TE1 modes. The intersection points are clearly marked in the figure with circles, and their abscissas correspond to the D2 values ​​for achieving synchronous coupling of the three mode pairs mentioned above. The intersection points, in order of increasing D2, are the TE0-TE2 coupling point, the TE1-TE2 coupling point, and the TE0-TE1 coupling point. As D1 increases in the inverted tapered subwavelength grating, the abscissas of the intersection points gradually increase, and the D2 values ​​for synchronous coupling of the three mode pairs also increase. Therefore, by adjusting the duty cycles D1 and D2, the coupling lengths of any two modes can be precisely matched, achieving synchronous coupling of selected mode pairs within a single beat length.

[0031] The following takes a dual-mode directional coupler that couples TE0 and TE1 as an example to explain the device's operating mode. Figure 5 As shown, light first inputs the TE0 and TE1 modes from the first straight waveguide I, and is coupled to the first subwavelength grating waveguide I through the first transverse four-trapezoidal grating waveguide I in the transition zone I. Then, the light gradually transitions to the coupling zone from the first inverted tapered subwavelength grating waveguide I. In the coupling zone, the light goes from the bus waveguide to the coupling waveguide at one beat length, passes through the second inverted tapered curved subwavelength grating waveguide and enters the second inverted tapered subwavelength grating waveguide II. The duty cycle of the trapezoidal subwavelength grating on the wide side of the second inverted tapered subwavelength grating waveguide II gradually changes, so that the light gradually and smoothly transitions to the second subwavelength grating waveguide, and then is coupled to the second straight waveguide through the second transverse four-trapezoidal grating waveguide in the transition zone III, and the TE0 and TE1 modes are output from the second straight waveguide.

[0032] The present invention has mode scalability. By optimizing the parameters of the inverted tapered subwavelength grating, changing the width of the trapezoidal subwavelength grating, the duty ratio on both sides of the trapezoidal subwavelength grating in the coupling region, and the width of the ordinary subwavelength grating, a dual-mode directional coupler can be realized in which two modes of any order are simultaneously coupled, for example: TE0-TE2, TE1-TE2, TE0-TM0, etc.

[0033] The following takes a dual-mode directional coupler that couples TE0 and TE2 at the same time as an example to explain the device's operating mode in detail. Figure 6As shown, light first inputs the TE0 and TE2 modes from the first straight waveguide I, and is coupled to the first subwavelength grating waveguide I through the first lateral four-trapezoidal grating waveguide I in the transition zone I. Then, the duty cycle of the wide side of the trapezoidal subwavelength grating in the first inverted tapered subwavelength grating waveguide I gradually changes, allowing the light to gradually transition to the coupling zone with low loss. In the coupling zone, the light enters the coupling waveguide from the busbar waveguide at one beat length, passes through the second inverted tapered bent subwavelength grating waveguide, and is coupled to the second inverted tapered subwavelength grating waveguide II. The duty cycle of the wide side of the trapezoidal subwavelength grating in the second inverted tapered subwavelength grating waveguide II gradually changes, allowing the light to gradually and smoothly transition to the second subwavelength grating waveguide, and then couples to the second straight waveguide through the second lateral four-trapezoidal grating waveguide in the transition zone III, and outputs the TE0 and TE2 modes from the second straight waveguide.

[0034] The dual-mode directional coupler proposed in this paper innovatively incorporates an inverted tapered subwavelength grating waveguide structure. Compared to traditional symmetrical directional couplers, this achieves synchronous coupling of different-order modes at the same beat length. Furthermore, by optimizing the coupling region length, any dual-mode combination can achieve equal or unequal power distribution at the output, enabling simultaneous control of the splitting ratio of the two modes, providing flexibility for dynamically reconfigurable optical networks. Therefore, the dual-mode directional coupler proposed in this paper has promising application prospects.

[0035] The dual-mode directional coupler proposed in the present invention is realized based on the SOI platform. The SOI platform refers to the silicon-on-insulator (SOI) material on an insulating substrate, in which a layer of silicon dioxide is embedded between the top silicon and the backing silicon as a buried oxide layer. Figure 4 As shown. SOI materials have good compatibility with CMOS processes. From an optical perspective, the refractive index difference between the SiO2 cladding layer and the Si core layer in SOI materials is significant. This large refractive index difference gives it a strong ability to confine the light field, and light propagates through subwavelength gratings with relatively low loss. Therefore, devices manufactured based on this material have significant advantages in power consumption, integration, cost, and integration with electrical components. In the future, this invention is expected to enable on-chip simultaneous multi-mode processing, promoting the development of highly integrated mode-multiplexed optical communication systems.

[0036] Based on the same underlying principle, this device is not limited to a specific material platform and has the possibility of being implemented on other material platforms. However, the specific structural parameters set for the dual-mode directional coupler of the present invention are closely related to the inherent properties of the selected materials. Although the basic working principle of the device remains unchanged when implemented on different material platforms, the factors that need to be considered when designing the parameters of each part of the device are also roughly the same. However, due to the differences in the properties of different materials, these differences will have a direct impact on the performance of the device. Therefore, when choosing different materials to manufacture the device, it is necessary to combine the characteristics of the actual materials used, take the device principle as the fundamental starting point, and re-optimize and adjust the structural parameters of each part of the device to ensure that the device can achieve the expected performance indicators on different material platforms.

Claims

1. A dual-mode directional coupler based on an inverted tapered subwavelength grating, characterized in that: include: The busbar waveguide is sequentially connected by a first straight waveguide I (1), a transition region I (2), a first inverted tapered sub-wavelength grating waveguide I (3), a first inverted tapered curved sub-wavelength grating waveguide (4), a first inverted tapered sub-wavelength grating waveguide II (5), a transition region II (6) and a first straight waveguide II (7); The coupling waveguide is sequentially connected by a second inverted tapered sub-wavelength grating waveguide I (8), a second inverted tapered bent sub-wavelength grating waveguide (9), a second inverted tapered sub-wavelength grating waveguide II (10), a transition region III (11) and a second straight waveguide (12); The first inverted tapered sub-wavelength grating waveguide I (3) and the second inverted tapered sub-wavelength grating waveguide I (8) are arranged in parallel to form a coupling region, and a coupling gap exists between the two. The first inverted tapered sub-wavelength grating waveguide I (3), the first inverted tapered bent sub-wavelength grating waveguide (4), the first inverted tapered sub-wavelength grating waveguide II (5), the second inverted tapered sub-wavelength grating waveguide I (8), the second inverted tapered bent sub-wavelength grating waveguide (9) and the second inverted tapered sub-wavelength grating waveguide II (10) are all inverted tapered sub-wavelength grating structures; The inverted tapered sub-wavelength grating structure is composed of a common sub-wavelength grating and a trapezoidal sub-wavelength grating, wherein: The ordinary subwavelength grating is located on the side away from the coupling gap; The trapezoidal subwavelength grating is located on a side close to the coupling gap, with its wide side facing the coupling gap; The duty cycle of the narrow side of the trapezoidal subwavelength grating is the same as that of the ordinary subwavelength grating; The narrow side width of the trapezoidal sub-wavelength grating is equal to the width of the ordinary sub-wavelength grating and is precisely aligned; The first inverted tapered curved sub-wavelength grating waveguide (4) and the second inverted tapered curved sub-wavelength grating waveguide (9) have the same radius of curvature and are bent in a direction away from the other waveguide.

2. A dual-mode directional coupler based on an inverted tapered subwavelength grating according to claim 1, characterized in that: The transition region I (2) is composed of a first transverse four-trapezoidal grating waveguide I (201) and a first sub-wavelength grating waveguide I (202); The transition region II (6) is composed of a first transverse four-trapezoidal grating waveguide II (601) and a first sub-wavelength grating waveguide II (602); The transition region III (11) is composed of a second transverse four-trapezoidal grating waveguide (111) and a second sub-wavelength grating waveguide (112); The first and second transverse four-trapezoidal grating waveguides (201, 601, 111) are both composed of four equally spaced and parallel-arranged trapezoidal waveguides, and their widths linearly change along the light transmission direction.

3. A dual-mode directional coupler based on an inverted tapered sub-wavelength grating according to claim 1, characterized in that: In the first inverted tapered subwavelength grating waveguide I (3), the duty cycle of the wide side of the trapezoidal subwavelength grating gradually increases from left to right, the duty cycle at the leftmost end is equal to the duty cycle of the first subwavelength grating waveguide I, and the duty cycle at the rightmost end is equal to the constant duty cycle of the first inverted tapered curved subwavelength grating waveguide; In the first inverted tapered subwavelength grating waveguide II (5), the duty cycle of the wide side of the trapezoidal subwavelength grating gradually decreases from top to bottom, the duty cycle at the top is equal to the constant duty cycle of the first inverted tapered curved subwavelength grating waveguide, and the duty cycle at the bottom is equal to the duty cycle of the first subwavelength grating waveguide II; In the second inverted tapered subwavelength grating waveguide II (10), the duty cycle of the wide side of the trapezoidal subwavelength grating gradually decreases from bottom to top, the duty cycle at the bottom is equal to the constant duty cycle of the second inverted tapered curved subwavelength grating waveguide, and the duty cycle at the top is equal to the duty cycle of the second subwavelength grating waveguide.

4. A dual-mode directional coupler based on an inverted tapered sub-wavelength grating according to claim 1, characterized in that: The width of the ordinary subwavelength grating is 1.5 μm; the width of the trapezoidal subwavelength grating is 1.2 μm. By adjusting the values ​​of the wide-side duty cycle and the narrow-side duty cycle of the trapezoidal subwavelength grating, the refractive index difference between the odd and even supermodes of the TE0 and TE1 modes is made equal, thereby achieving synchronous coupling of the TE0 and TE1 modes within a single beat length.

5. The dual-mode directional coupler based on an inverted tapered sub-wavelength grating according to claim 1, wherein: All waveguide components have the same width, and the first straight waveguide II (7) is perpendicular to the first straight waveguide I (1), and the second straight waveguide (12) is perpendicular to the busbar waveguide.

6. A dual-mode directional coupler based on an inverted tapered sub-wavelength grating as claimed in claim 1, characterized in that: The dual-mode directional coupler supports synchronous coupling of any dual-mode combination, including: TE0 and TE1, TE0 and TE2, TE1 and TE2, or TE0 and TM0.

7. The dual-mode directional coupler based on an inverted tapered sub-wavelength grating according to claim 1, wherein: The dual-mode directional coupler is implemented based on an SOI material platform, and includes: a silicon substrate; a silicon dioxide buried oxide layer formed on the silicon substrate; and a top silicon layer formed on the buried oxide layer, wherein the busbar waveguide and the coupling waveguide are etched in the top silicon layer.

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