A filter based on a multimode subwavelength grating

Abstract

The application discloses a filter based on a multi-mode subwavelength grating. The filter comprises a mode demultiplexer for multiplexing and demultiplexing of a base mode and a high-order mode, a subwavelength grating for weakly binding regulation of a mode field of the base mode and the high-order mode, and a straight-through waveguide for output of the base mode. The application introduces a structure of the subwavelength grating and weakly binding regulation of the mode field, and by virtue of excellent flat dispersion regulation characteristics, the application realizes apodization of the grating while keeping consistent local refractive index of the grating, thereby obtaining the filter with high side mode suppression ratio and no free spectral range limitation, and the filter can realize flat and low-loss spectral response, meets the next-generation large-capacity and high-speed wavelength division multiplexing transmission demand, and has the advantages of simple structure, simple process, excellent performance and the like.

Description

Technical Field

[0001] This invention relates to a filter, to the field of integrated optoelectronic devices, and specifically to a filter based on a multimode subwavelength grating. Background Technology

[0002] Silicon-based optoelectronics technology has garnered increasing attention from academia and industry due to its advantages such as high bandwidth, high integration density, low power consumption, low cost, and CMOS (Complementary Metal Oxide Semiconductor) compatibility. It is considered one of the most promising and potentially cost-effective technologies, serving as a core supporting technology for next-generation optical communication, optical interconnect, and optical sensing systems. Among the many device structures in silicon-based optoelectronic systems, Bragg gratings are widely used in wavelength division multiplexing, spectral analysis, and nonlinear applications due to their large free spectral range (FSR), flattened spectrum, and high flexibility. Multimode waveguide gratings, in particular, offer a highly promising waveguide structure for on-chip optical filters on silicon substrates due to their extremely high flexibility, robustness, and functional scalability.

[0003] With the iterative upgrades of optical communication / optical interconnect systems and the rapid development of emerging fields such as optical sensing / optical measurement, higher demands are being placed on the FSR of filters. For example, for 400G CWDM8 (Coarse Wavelength Division Multiplexer) optical modules for next-generation data center networks, a new generation of 8-channel coarse wavelength division multiplexers with a channel spacing of 20nm is needed, with an operating bandwidth of up to 160nm, double that of CWDM4 optical modules. However, traditional multimode waveguide grating filters typically employ phase apodization to achieve high side-mode rejection ratios. Due to the coupling of forward and reverse fundamental modes introduced by the symmetrical grating, their FSR is limited to 120nm, far from meeting the performance requirements of next-generation optical communication and optical interconnects. Summary of the Invention

[0004] To address the problems existing in the background art, the present invention provides a filter based on a subwavelength multimode waveguide grating.

[0005] The technical solution adopted in this invention is:

[0006] The filter based on a multimode subwavelength grating of the present invention includes a mode demultiplexer for multiplexing and demultiplexing the fundamental mode and higher-order modes; a subwavelength grating for weakly confined mode field modulation of the fundamental mode and higher-order modes; and a through waveguide for the output of the fundamental mode.

[0007] The mode demultiplexer, subwavelength grating, and through waveguide are connected in sequence.

[0008] The subwavelength grating includes a front gradient subwavelength grating, a multimode subwavelength grating, and a rear gradient subwavelength grating connected in sequence. The input end of the front gradient subwavelength grating is connected to the output end of the mode demultiplexer, and the output end of the rear gradient subwavelength grating is connected to the input end of the through waveguide.

[0009] The input end of the front graded subwavelength grating serves as the input end of the subwavelength grating, and the output end of the rear graded subwavelength grating serves as the output end of the subwavelength grating.

[0010] The multimode subwavelength grating is a subwavelength grating structure, specifically a toothed structure consisting of a thin waveguide positioned at the center of the propagation direction of the filter input and a series of rectangular structures distributed on both sides thereon. The series of rectangular structures are arranged in parallel at intervals and perpendicular to the propagation direction of the filter input. The teeth and slots of each rectangular structure constitute a subwavelength grating unit, and every two adjacent subwavelength grating units constitute a Bragg grating unit.

[0011] The multimode subwavelength grating is achieved by distributing two subwavelength grating units in each Bragg grating unit at different sizes along the propagation direction perpendicular to the filter input, with the distribution following a gradually changing function.

[0012] The change function is a Gaussian function, a Hamming function, or a sine function, etc.

[0013] The multimode subwavelength grating satisfies the phase matching condition (n0+nm) / 2=λ / Λ, achieving reverse coupling of the fundamental mode to a higher-order mode at the Bragg wavelength, where n0 is the effective refractive index of the fundamental mode, nm is the effective refractive index of the m-th order mode, λ is the Bragg wavelength, and Λ is the grating tooth period.

[0014] Multimode subwavelength gratings can be distributed in a symmetrical or antisymmetric structure.

[0015] Both the front and rear graded subwavelength gratings are subwavelength grating structures. Specifically, they consist of a graded waveguide positioned at the center along the propagation direction of the filter input and a series of rectangular structures distributed on both sides, forming a graded tooth structure. The series of rectangular structures are arranged in parallel at intervals and perpendicular to the propagation direction of the filter input. The height of the ends of the teeth of each rectangular structure on both sides along the propagation direction of the filter input is consistent. The graded waveguides of the front and rear graded subwavelength gratings are distributed according to a change function along the propagation direction and the reverse direction of the filter input, respectively. The depth of the bottom end of the tooth groove of each rectangular structure changes from shallow to deep, realizing low-loss conversion of the ordinary waveguide mode to the Bloch mode in the subwavelength grating waveguide.

[0016] The changing function is a linear function or a second-order function, etc.

[0017] The mode demultiplexer includes an input single-mode waveguide, a download single-mode waveguide, a mode demultiplexing working area, and a front-connected tapered waveguide. The output of the input single-mode waveguide and the input of the download single-mode waveguide are respectively connected to one end of the mode demultiplexing working area. The input of the input single-mode waveguide and the output of the download single-mode waveguide are free ends. The other end of the mode demultiplexing working area is connected to the input of the front-connected tapered waveguide. The output of the front-connected tapered waveguide is connected to the input of the front-tapered subwavelength grating of the subwavelength grating.

[0018] The input terminal of the input single-mode waveguide serves as the input port of the mode demultiplexer and also as the input port of the multimode subwavelength grating filter; the output terminal of the front-connected tapered waveguide serves as the output port of the mode demultiplexer; the output terminal of the download single-mode waveguide serves as the download port of the mode demultiplexer and also as the download port of the signal reflected by the multimode subwavelength grating filter; the output terminal of the through waveguide serves as the through port of the multimode subwavelength grating filter.

[0019] The through waveguide includes a rear-connected tapered waveguide and a through single-mode waveguide connected in sequence, with the input end of the rear-connected tapered waveguide connected to the output end of the rear-connected subwavelength grating.

[0020] The input end of the tapered waveguide is then connected to serve as the input end of the through waveguide, and the output end of the through single-mode waveguide serves as the output port of the through waveguide, which is also the through port of the multimode subwavelength grating filter.

[0021] The mode demultiplexer mentioned above is specifically an adiabatic graded coupling waveguide, an asymmetric directional coupling waveguide, or a grating-assisted coupling waveguide, etc.

[0022] The application of the multimode subwavelength grating filter in cascaded multichannel filters and tunable filters.

[0023] The beneficial effects of this invention are:

[0024] 1. By introducing a subwavelength grating structure and its control over the weak confinement of the mode field, this invention achieves a more flexible bandwidth adjustment range compared to traditional multimode waveguide grating filters, enabling filters with various bandwidth requirements and achieving a flattened, low-loss spectral response.

[0025] 2. By utilizing the excellent dispersion control mechanism of the subwavelength grating structure and the corresponding apodization method, this invention significantly reduces the change in the local refractive index of the grating during apodization, realizing a grating filter with high side-mode suppression ratio and enabling a filter with no FSR limitation, thus meeting the requirements of next-generation high-capacity, multi-channel wavelength division multiplexing transmission.

[0026] 3. This invention introduces a high-order mode reflection signal and mode multiplexer, which has the advantages of large tolerance and low loss, and realizes the interpolation and multiplexing of wavelength signals.

[0027] 4. This invention can be fabricated using planar integrated optical waveguide technology, requiring only one etching operation. The process is simple, low-cost, and has low loss. It is compatible with traditional CMOS technology and has the potential for large-scale production.

[0028] In summary, this invention, by introducing a multimode subwavelength grating and apodization technology, obtains an on-chip waveguide interpolation filter with a high flexible bandwidth adjustment range, high side-mode suppression ratio, low loss, and flat-top spectrum, which has the advantages of simple manufacturing process and excellent performance. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the overall structure of a multimode subwavelength grating filter;

[0030] Figure 2 This is a schematic diagram illustrating the working principle of a multimode subwavelength grating filter;

[0031] Figure 3 This is a schematic diagram of an apodized grating for a multimode subwavelength grating filter;

[0032] Figure 4 The image shows the simulation results of the multimode subwavelength grating of the grating filter in the embodiment.

[0033] In the diagram: 1. Mode demultiplexer; 2. Subwavelength grating; 3. Through waveguide; 01. Input single-mode waveguide; 02. Download single-mode waveguide; 03. Mode demultiplexing working area; 04. Front tapered waveguide; 05. Front tapered subwavelength grating; 06. Multimode subwavelength grating; 07. Rear tapered subwavelength grating; 08. Rear connected tapered waveguide; 09. Through single-mode waveguide. Detailed Implementation

[0034] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0035] like Figure 1 As shown, the filter based on a multimode subwavelength grating of the present invention includes a mode demultiplexer 1 for multiplexing and demultiplexing the fundamental mode and higher-order modes, a subwavelength grating 2 for weakly confined mode field modulation of the fundamental mode and higher-order modes, and a through waveguide 3 for the output of the fundamental mode. The mode demultiplexer 1, the subwavelength grating 2, and the through waveguide 3 are connected in sequence. The mode demultiplexer 1 is specifically an adiabatic graded coupling waveguide, an asymmetric directional coupling waveguide, or a grating-assisted coupling waveguide, etc.

[0036] Subwavelength grating 2 includes a front-gradient subwavelength grating 05, a multimode subwavelength grating 06, and a rear-gradient subwavelength grating 07 connected in sequence. The input of the front-gradient subwavelength grating 05 is connected to the output of the mode demultiplexer 1, and the output of the rear-gradient subwavelength grating 07 is connected to the input of the through waveguide 3. The input of the front-gradient subwavelength grating 05 serves as the input of subwavelength grating 2, and the output of the rear-gradient subwavelength grating 07 serves as the output of subwavelength grating 2.

[0037] The multimode subwavelength grating 06 is a subwavelength grating structure, specifically a toothed structure consisting of a thin waveguide positioned at the center along the propagation direction of the filter input and a series of rectangular structures distributed on both sides thereof. The series of rectangular structures are arranged in parallel at intervals and perpendicular to the propagation direction of the filter input. The teeth and slots of each rectangular structure constitute a subwavelength grating unit, and every two adjacent subwavelength grating units constitute a Bragg grating unit, such as... Figure 3 As shown.

[0038] The multimode subwavelength grating 06 achieves grating apodization by distributing two subwavelength grating elements in each Bragg grating element along the propagation direction perpendicular to the filter input with a staggered size distribution, which is based on a gradually changing function, such as a Gaussian function, a Hamming function, or a sine function.

[0039] The multimode subwavelength grating 06 satisfies the phase-matching condition (n0 + nm) / 2 = λ / Λ, achieving reverse coupling of the fundamental mode to a higher-order mode at the Bragg wavelength. Here, n0 is the effective refractive index of the fundamental mode, nm is the effective refractive index of the m-th order mode, λ is the Bragg wavelength, and Λ is the grating tooth period. The multimode subwavelength grating 06 can be distributed in a symmetrical or antisymmetric structure.

[0040] Both the front-gradient subwavelength grating 05 and the rear-gradient subwavelength grating 07 are subwavelength grating structures. Specifically, they consist of a gradient waveguide positioned at the center along the propagation direction of the filter input and a series of rectangular structures distributed on both sides, forming a gradient tooth structure. The series of rectangular structures are arranged in parallel at intervals and perpendicular to the propagation direction of the filter input. The height of the ends of the teeth of each rectangular structure on both sides of the propagation direction of the filter input is consistent. The gradient waveguides of the front-gradient subwavelength grating 05 and the rear-gradient subwavelength grating 07 are gradually distributed according to a change function along the propagation direction and the reverse direction of the filter input, respectively. The depth of the bottom end of the tooth groove of each rectangular structure changes from shallow to deep, realizing low-loss conversion of the ordinary waveguide mode to the Bloch mode in the subwavelength grating waveguide. The change function is a linear function or a second-order function, etc.

[0041] The mode demultiplexer 1 includes an input single-mode waveguide 01, a download single-mode waveguide 02, a mode demultiplexing working area 03, and a front-connecting tapered waveguide 04. The output of the input single-mode waveguide 01 and the input of the download single-mode waveguide 02 are respectively connected to one end of the mode demultiplexing working area 03. The input of the input single-mode waveguide 01 and the output of the download single-mode waveguide 02 are free ends. The other end of the mode demultiplexing working area 03 is connected to the input of the front-connecting tapered waveguide 04. The output of the front-connecting tapered waveguide 04 is connected to the input of the front-connecting tapered subwavelength grating 05 of the subwavelength grating 2. The input terminal of the input single-mode waveguide 01 serves as the input port of mode demultiplexer 1, and also as the input port of the multimode subwavelength grating filter; the output terminal of the preceding tapered waveguide 04 serves as the output port of mode demultiplexer 1; the output terminal of the download single-mode waveguide 02 serves as the download port of mode demultiplexer 1, and also as the download port of the signal reflected by the multimode subwavelength grating filter; the output terminal of the through waveguide 3 is the through port of the multimode subwavelength grating filter.

[0042] The through waveguide 3 includes a rear-connected tapered waveguide 08 and a through single-mode waveguide 09 connected in sequence. The input of the rear-connected tapered waveguide 08 is connected to the output of the rear-connected tapered subwavelength grating 07 of the subwavelength grating 2. The input of the rear-connected tapered waveguide 08 serves as the input of the through waveguide 3, and the output of the through single-mode waveguide 09 serves as the output port of the through waveguide 3, which is also the through port of the multimode subwavelength grating filter.

[0043] like Figure 2 As shown, the working principle of this invention is illustrated. The mode demultiplexer 1, subwavelength grating 2, and direct-through waveguide 3 are connected sequentially. The upper left port is the input port, the lower left port is the download port, and the right port is the direct-through port. Taking operation in transverse electric mode (TE) as an example, the subwavelength grating 2 reflects the TE1 mode. The TE0 mode input from the left end of the mode demultiplexer 1 can be output as TE0 mode from the right end without loss, and vice versa; while the TE1 mode input from the right end can be coupled into TE0 mode and output as TE0 mode from the lower left port. The multimode subwavelength grating satisfies the phase matching condition between the TE0 mode and the inverse TE1 mode. As an antisymmetric grating, the TE0 mode input to the multimode subwavelength grating can be reverse-coupled into TE1 mode near the Bragg wavelength resonance condition. The reflected TE1 mode is output from the lower left port through the mode demultiplexer 1, while the unreflected TE0 mode is output from the direct-through port, thus achieving interleaving and multiplexing of wavelength signals. By optimizing the overall width, grating misalignment size, and grating period of the subwavelength grating 2, filters with different center wavelengths and bandwidths can be obtained.

[0044] Multimode subwavelength gratings achieve spectral sideband suppression through apodization techniques, thereby obtaining a square spectral response with a high sidemode suppression ratio. For example... Figure 3The diagram illustrates the configuration of an apodized subwavelength grating. Within a Bragg grating unit composed of two subwavelength gratings, apodization is achieved by adjusting the misalignment (δ) of the two gratings along the perpendicular propagation direction. δ exhibits a gradually changing distribution along the propagation direction, with a trend of small-large-small. Figure 3 As shown, the trend of δ is defined using a Gaussian function, i.e., δ=δ0exp[-b(zL / 2)]. 2 / L 2 ], where δ0 is the grating misalignment value with the largest misalignment value in the middle of the grating, b is the apodization intensity, z is the grating position along the propagation direction, and L is the length of the multimode subwavelength grating. Choosing a larger apodization intensity b can obtain a spectrum with a higher side-mode suppression ratio.

[0045] Specific embodiments of the present invention are as follows:

[0046] A silicon nanowire optical waveguide based on silicon insulator (SOI) material was selected: its core layer is silicon material with a thickness of 220 nm and a refractive index of 3.4744; its lower and upper cladding materials are both SiO2, with the lower cladding SiO2 having a thickness of 2 μm and the upper cladding SiO2 having a thickness of 1 μm and a refractive index of 1.4404.

[0047] For the mode demultiplexer 1 of the multimode subwavelength grating filter, an adiabatic graded coupling waveguide structure is adopted.

[0048] For the antisymmetric multimode subwavelength grating structure of the multimode subwavelength grating filter, the selected parameters are as follows: the total width of the multimode subwavelength grating is 1200nm, the maximum grating misalignment value of subwavelength grating 2 is 90nm, the period of subwavelength grating 2 is 206nm, the number of periods of subwavelength grating 2 is 400, the duty cycle of subwavelength grating 2 is 0.5, the number of periods of the front / back gradient subwavelength gratings 05 and 07 is 40, the width of the middle thin waveguide is 100nm, the apodization is Gaussian apodization, and the apodization intensity is 10.

[0049] The multimode subwavelength grating filter was simulated and verified using a three-dimensional finite-difference time-domain algorithm, such as... Figure 4 The figure shows the simulation results. As can be seen from the figure, the center wavelength of the device of the present invention is approximately 1550 nm, the 1 dB bandwidth is 20 nm, the loss is less than 0.1 dB, the side-mode rejection ratios on the left and right sides of the spectrum are 25 dB and 27 dB, respectively, the spectrum has a flattened channel and a high kurtosis, exhibiting a relatively ideal square spectral response, and its free spectral range (FSR) is >350 nm, breaking the previous 120 nm FSR of traditional multimode waveguide gratings. Therefore, the device of the present invention can achieve an on-chip optical filter with low loss, high side-mode rejection ratio, and an ultra-large free spectral range.

[0050] The above embodiments are used to explain and illustrate the present invention, but not to limit the present invention. Any modifications and changes made to the present invention within the spirit and scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A filter based on a multimode subwavelength grating, characterized in that: Includes a pattern demultiplexer for multiplexing and demultiplexing of fundamental and higher-order modes (1); Including subwavelength gratings for weakly confined mode field modulation of the fundamental mode and higher-order modes (2); Includes a through waveguide for the output of the fundamental mode (3); The mode demultiplexer (1), the subwavelength grating (2), and the through waveguide (3) are connected in sequence; The subwavelength grating (2) includes a front gradient subwavelength grating (05), a multimode subwavelength grating (06) and a rear gradient subwavelength grating (07) connected in sequence. The input end of the front gradient subwavelength grating (05) is connected to the output end of the mode demultiplexer (1), and the output end of the rear gradient subwavelength grating (07) is connected to the input end of the through waveguide (3). The multimode subwavelength grating (06) is a subwavelength grating structure, specifically a toothed structure consisting of a waveguide positioned at the center of the propagation direction of the filter input and a series of rectangular structures distributed on both sides thereon. The series of rectangular structures are arranged in parallel at intervals and perpendicular to the propagation direction of the filter input. The teeth and grooves of each rectangular structure constitute a subwavelength grating unit, and every two adjacent subwavelength grating units constitute a Bragg grating unit. The multimode subwavelength grating (06) achieves grating apodization by displacing two subwavelength grating units in each Bragg grating unit along the propagation direction perpendicular to the input of the filter, with the distribution following a gradual change function. The change function is a Gaussian function, a Hamming function, or a sine function; The multimode subwavelength grating (06) satisfies the phase matching condition (n0+nm) / 2=λ / Λ, and realizes the fundamental mode reverse coupling to a higher-order mode at the Bragg wavelength, where n0 is the effective refractive index of the fundamental mode, nm is the effective refractive index of the m-th order mode, λ is the Bragg wavelength, and Λ is the grating tooth period. The front gradient subwavelength grating (05) and the rear gradient subwavelength grating (07) are both subwavelength grating structures. Specifically, they are gradient tooth structures consisting of a gradient waveguide set at the center position along the propagation direction of the filter input and a series of rectangular structures distributed on both sides thereon. The series of rectangular structures are arranged in parallel at intervals and perpendicular to the propagation direction of the filter input. The height of the ends of the teeth of each rectangular structure on both sides along the propagation direction of the filter input is consistent. The gradient waveguides of the front gradient subwavelength grating (05) and the rear gradient subwavelength grating (07) are distributed according to a change function along the propagation direction and the opposite direction of the filter input, respectively, so as to realize the conversion of the ordinary waveguide mode into the Bloch mode in the subwavelength grating waveguide. The changing function is a linear function or a second-order function.

2. The filter based on a multimode subwavelength grating according to claim 1, characterized in that: The mode demultiplexer (1) includes an input single-mode waveguide (01), a download single-mode waveguide (02), a mode demultiplexing working area (03), and a front-connected tapered waveguide (04). The output end of the input single-mode waveguide (01) and the input end of the download single-mode waveguide (02) are respectively connected to one end of the mode demultiplexing working area (03). The input end of the input single-mode waveguide (01) and the output end of the download single-mode waveguide (02) are free ends. The other end of the mode demultiplexing working area (03) is connected to the input end of the front-connected tapered waveguide (04). The output end of the front-connected tapered waveguide (04) is connected to the input end of the front-connected tapered subwavelength grating (05) of the subwavelength grating (2).

3. The filter based on a multimode subwavelength grating according to claim 1, characterized in that: The straight waveguide (3) includes a rear-connected tapered waveguide (08) and a straight single-mode waveguide (09) connected in sequence. The input end of the rear-connected tapered waveguide (08) is connected to the output end of the rear-tapered subwavelength grating (07) of the subwavelength grating (2).

4. The filter based on a multimode subwavelength grating according to claim 1, characterized in that: The mode demultiplexer (1) is specifically an adiabatic graded coupling waveguide, an asymmetric directional coupling waveguide, or a grating-assisted coupling waveguide.

5. The application of the filter based on a multimode subwavelength grating according to any one of claims 1-4, characterized in that: The application of the multimode subwavelength grating filter in cascaded multichannel filters and tunable filters.

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