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Abstract
A compact 5-mode (de)multiplexer [(De)MUX] is proposed and experimentally demonstrated based on the principle of multi-phase matching. The proposed device comprises a cascaded asymmetric directional coupler (ADC) based on 3-mode phase-matching, a polarization beam combiner, and a taper waveguide connecting them. The multiple modes in the access waveguides are matched to different modes in the same bus waveguide, which eliminates the need for additional taper structures and results in a total coupling length of only 18.9 µm. Experimental results exhibit that the insertion losses of the five modes are below 3.4 dB, and the mode crosstalks are below -15 dB at the central wavelength. The 3-dB bandwidths of TM0, TM1, TE0, TE1, and TE2 modes are greater than 100 nm, 46 nm, 100 nm, 28 nm, and 37 nm, respectively. The proposed device can serve as a key functional component in highly integrated on-chip mode-division multiplexing systems.
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1. Introduction
Mode division multiplexing (MDM) technology is a potential remedy to alleviate the capacity limitations of conventional wavelength division multiplexing (WDM) communication technology [1]. MDM technology enables the multiplexing of multiple orthogonal modes on a single wavelength on the basis of WDM systems, achieving multidimensional hybrid multiplexing. MDM has been advocated as an approach to substantially enhance the capacity of optical communication, rendering it a compelling alternative for the next generation of communication technology [2]. On-chip MDM systems offer many advantages, including high integration, high stability, and high performance [3]. As a result, they have become a key technology for on-chip optical signal processing, optical interconnects, and optical communication. Recently, silicon-based on-chip MDM systems have gained widespread attention and research due to the advantages of silicon photonics, including the capability to achieve high-integration footprints through a high refractive-index difference, compatibility with mature complementary metal-oxide-semiconductor (CMOS) technology for fabrication, and cost-effectiveness [4,5].
The mode (de)multiplexer [(De)MUX] serves as a fundamental component for the development of MDM systems [6,7]. It enables the multiplexing of various modes from branches to trunks, or from trunks to branches, by efficiently combining or separating the modes. Various approaches and structures have been proposed to build a silicon-based mode (De)MUX, including the asymmetric directional coupler (ADC) [8,9], tapered ADC [10], bent ADC [11], triple-waveguide ADC [12], micro-ring resonator (MRR) [13], adiabatic coupler (AC) [14], multimode interference (MMI) coupler [15], Y-junction [16], densely packed waveguide arrays (DPWAs) [17], grating assisted couplers (GACs) [18], subwavelength metamaterials [19], and inverse design [20].
Of the above techniques mentioned, both AC and MMI solutions are capable of achieving a wide operating bandwidth, but they do require longer device sizes as they are based on the mode evolution principle. The Y-junction structure has the ability to achieve a wide operating bandwidth and low mode crosstalk. However, the sharp branching corners of the Y-junctions are challenging to fabricate accurately, which can result in fabrication errors and a reduction in device performance. The DPWA structure can achieve high extinction ratio, low loss, and large bandwidth. However, its size is typically larger compared to other structures due to the need for multiple waveguides. The mode (De)MUXs based on both the MRR and GACs exhibit lower crosstalk, but have a narrower operation bandwidth. The subwavelength metamaterials based mode (De)MUX can demonstrate an ultra-broad bandwidth, but its structure is relatively complex. The inverse-designed mode (De)MUXs can achieve ultra-compact footprints, but require more demanding fabrication facilities for the subwavelength units. The mode (De)MUX based on the ADC structure is capable of achieving a relatively compact device with a flexible structure. Additionally, it allows for more working mode numbers through cascading. However, the intermediate taper structure that connects two ADCs in a cascade is typically long, resulting in a relatively large size for the overall cascaded ADC structure. To address this issue, we have proposed and theoretically validated a 10-mode (De)MUX based on a cascaded-ADC structure incorporating with the multi-phase matching condition [21]. This approach enables multiple modes within different branch waveguides to be simultaneously matched to different modes within the same trunk waveguide, significantly reducing the demand for tapers and resulting in higher integration density.
In this paper, we present the design, optimization, fabrication, and testing of a 5-mode (De)MUX based on our original multi-phase matching principle. The phase-matching is achieved between the fundamental modes of three input waveguides and the TM1, TE1, and TE2 modes of the same bus waveguide using this approach. A taper is used to connect this three-mode (De)MUX to a polarization beam combiner (PBC), resulting in a 5-mode (De)MUX capable of multiplexing TM0, TE0, TM1, TE1, and TE2 modes. To determine the multi-phase matching conditions, both the finite-element method (FEM) and three-dimensional full-vectorial finite-difference time-domain (3D-FV-FDTD) method are utilized, and the 5-mode (De)MUX is optimized accordingly. The optimized mode (De)MUX is fabricated on a silicon-on-insulator (SOI) platform, and its performance is characterized using an in-house testing platform.
2. Structure and operating principle
The schematic diagram of the proposed silicon five-mode (De)MUX is shown in Fig. 1, consisting of a 3-mode (De)MUX based on the multi-phase matching, a PBC based on a triple-waveguide coupler (TWC), and a taper waveguide in the middle. The access waveguides have five input ports labeled I1-I5 with corresponding waveguide widths of WTE2, WTE1, Wa, Wa, and WTM, respectively. The PBC is indicated by the purple region with a spacing of g between the three waveguides and a coupling length of Lp. The 3-mode (De)MUX is shown in the red region with a bus waveguide width of Wb. The coupling lengths of TE1, TE2, and TM1 modes are denoted by LTE1, LTE2, and LTM1, respectively, and the spacing between the access waveguides and the bus waveguide are represented by gTE1, gTE2, and gTM1, respectively. It can be observed from Fig. 1 that the TE1, TE2, and TM1 modes in the same bus waveguide and the fundamental modes in the three access waveguides are phase-matched simultaneously. In this work, the process of determining waveguide parameters based on the multi-phase matching condition is as follows: firstly, the width of the access waveguide for TM0 mode is determined as WTM = 400 nm, which is the typical width of single-mode silicon waveguides; then, the width, Wb of the bus waveguide for demultiplexing TM1 mode is determined based on the phase-matching condition of two parallel waveguides; subsequently, the effective refractive indices of the TE1 and TE2 modes within this bus waveguide are calculated; finally, the widths (WTE1 and WTE2) of the access waveguides for TE1 and TE2 modes can be obtained based on the phase-matching conditions of two parallel waveguides. Compared with the traditional cascaded ADCs, the structure proposed in this paper can greatly reduce the size of the entire device by eliminating two intermediate taper waveguides through the multi-phase matching.
Fig. 1. Schematic diagram of the proposed 5-mode hybrid (de)multiplexer based on multi-phase matching.
The multi-phase matching condition for the TM1, TE1, and TE2 modes is calculated using the FEM, and the results are presented in Fig. 2 for both the bus and access waveguides in isolation. Here, the thicknesses of all silicon waveguides are 220 nm, and a silica cladding layer is arranged on the top of the device. In the simulations, the operating wavelength is 1550 nm, and the refractive indices of Si and SiO2 are set to be 3.47548 and 1.46, respectively. The effective index of the TM0 in the access waveguide is calculated to be 1.71, and is indicated by a red dot. The phase-matched width for the TM1 mode in the bus waveguide is determined to be 1.07 µm. Subsequently, the effective indices of the TE1 mode (2.48) and TE2 mode (1.95) in the bus waveguide can be obtained, and the phase-matched widths for the two TE-access waveguides are determined to be 520 nm and 323 nm, respectively. Based on the mode analysis of isolated waveguides, the multi-phase matching condition is achieved for all three modes. As shown in Fig. 2, when the TM1 mode is phase-matched (green point in Fig. 2), the bus waveguide cannot support the TM2 mode. Therefore, the mode (De)MUX proposed in our work cannot utilize the TM2 mode. If we increase the width of bus waveguide to simultaneously support both the TM1 and TM2 modes (orange point in Fig. 2), it would lead to a smaller phase difference between the TE1 and TE2 modes, inevitably resulting in increased crosstalk between these two modes. In this case, to ensure that the crosstalk between different modes remains at a low level, we only consider two TM modes and do not utilize the TM2 mode. Additionally, it is possible to achieve the (de)multiplexing of higher-order TM modes by cascading different bus waveguides.
3. Optimizations and fabrication tolerances
Although the multi-phase matching conditions can be obtained through mode analysis for individual waveguides, these conditions may vary in the coupling structures composed of two waveguides. The variations in the phase matching conditions are primarily due to mode interactions between two parallel waveguides. The results shown in Fig. 2 are obtained based on the mode analysis, where the effective refractive index of each waveguide mode is calculated individually. This method does not consider the mode interaction between different waveguides. When two parallel waveguides come close together and form a directional coupler, the mode interactions would be introduced due to the evanescent fields. The size of the evanescent field varies with different waveguide widths, leading to changes in mode coupling strength. Additionally, the spacing between the waveguides also affects the variation in coupling strength. Therefore, it is necessary to consider both waveguide width and spacing to achieve the best mode conversion efficiency. Consequently, there may be some differences in the phase matching conditions obtained between the two analysis methods. The phase matching conditions for the directional coupler formed by two waveguides are based on the transmission analysis, which is closer to the actual situation. On the other hand, the phase matching conditions obtained from mode analysis of two separate waveguides can serve as a reference for transmission analysis. Therefore, it is necessary to perform the transmission analysis for such structures to obtain more accurate multi-phase matching conditions. In this study, the 3D-FV-FDTD method is utilized to conduct transmission analysis for our proposed 3-mode (De)MUX, and the optimized results are shown in Fig. 3. During the simulations, the refractive indices of silicon and silica materials are obtained with reference to Palik's book [22].
Fig. 3. (a) Variations of mode-conversion efficiency with both the coupling length (y-axis) and width of bus waveguide (x-axis) for gTM1 = 200 nm. Variations of mode-conversion efficiency (left y-axis) and optimized coupling-length (right y-axis) with the gap for (b) TM1, (c) TE1, and (d) TE2 modes, respectively.
Firstly, the optimal phase-matching condition between the TM1 mode in the bus waveguide and the TM0 mode in the access waveguide is obtained based on the transmission analysis. When the waveguide spacing is gTM1 = 200 nm, the 3D-FV-FDTD method is used to calculate the relationship between the mode conversion efficiency, coupling length, and the width of the bus waveguide, as shown in Fig. 3(a). The optimum mode conversion efficiency of TM1 mode can be achieved by simultaneously optimizing the width of the bus waveguide and the coupling length. It can be noted that the maximum mode conversion efficiency of 98.9% can be achieved when the coupling length and the width of the bus waveguide are chosen to be 3.1 µm and 990 nm, respectively. Once the width of the bus waveguide is determined, we can calculate the effective refractive indices for TE1 and TE2 modes within this bus waveguide. Then, the optimum mode conversion efficiencies for these two TE modes can be achieved by simultaneously optimizing the coupling length and widths of access waveguides. However, different waveguide spacings would result in different mode-coupling strengths, which in turn affect the length of the coupling length and mode conversion efficiency. Therefore, the transmission analysis needs to be carried out for different waveguide spacings to obtain the optimal mode conversion efficiency and corresponding structure parameters. Variations of TM1 mode conversion efficiency (left y-axis) and optimized coupling-length (right y-axis) with the gap are shown in Fig. 3(b). Here, each data point is obtained by plotting a result graph similar to Fig. 3(a), and then the optimal coupling length could be obtained. It can be observed from Fig. 3(b) that the optimal coupling length increases with the increase of the waveguide spacing, while the mode conversion efficiency first increases with the increase of the spacing and then reaches saturation. To minimize the device size, the minimum coupling length should be chosen. Therefore, taking into account both the mode conversion efficiency and the size of the coupling length, we ultimately choose a waveguide spacing of gTM1 = 200 nm, with the corresponding optimal coupling length of LTM1 = 3.1 µm and the width of the bus waveguide of Wb = 990 nm.
Next, we optimize the phase matching conditions for the TE1 and TE2 modes using the similar process. However, unlike the TM1 mode, the width of the bus waveguide is fixed for these modes, and the width of the access waveguide and the coupling length are varied to obtain the optimal mode conversion efficiency. The relationships between the mode conversion efficiency, the optimal coupling length, and the waveguide spacing are shown in Figs. 3(c) and 3(d) for TE1 and TE2 modes, respectively. To ensure the minimum coupling length and considering the fabrication process accuracy of at least 100 nm, a waveguide spacing of 100 nm is chosen. Therefore, we obtain the optimal coupling lengths of LTE1 = 15 µm and LTE2 = 0.8 µm for the TE1 and TE2 modes, respectively, along with the corresponding phase-matched widths of access waveguides of 480 nm and 300 nm, and the mode conversion efficiencies of 98.8% and 95%, respectively. As shown in Fig. 3, the mode conversion efficiency in case of TE2 mode is around 96%, whereas that for TE1 and TM1 modes is close to 99%. Here, the widths of the access waveguides for (de)multiplexing TE1 and TM1 modes are optimized and chosen to be WTE1 = 480 nm and WTM = 400 nm, respectively. However, the optimum width of the access waveguide for (de)multiplexing TE2 mode is set to be only 300 nm. Due to the smaller waveguide width, WTE2, there is a relatively larger evanescent field for the TE0 mode within the access waveguide, resulting in a higher propagation loss compared to the other two modes. Therefore, the results in Fig. 3 indicate that the mode conversion efficiency for TE2 mode is lower than that for the other two modes.
After optimizing the structural parameters of the 3-mode (De)MUX, the propagation fields of the three modes are simulated using the 3D-FV-FDTD method, as shown in Fig. 4. It can be seen that when the TM, TE, and TE fundamental modes are respectively input from the I5, I2, and I1 ports, they are converted to the TM1, TE1, and TE2 modes in the bus waveguide through the rightmost ADC, leftmost ADC, and middle ADC, respectively. The transmission between each mode is independent and not affected by the coupling structure. The simulated results demonstrate that the optimized device efficiently multiplexes/demultiplexes three different modes in the same waveguide. To illustrate the losses in the S-bends within our proposed structure, we conduct theoretical simulations on the transmission characteristics of these bends at the operating wavelength of 1550 nm. The simulated results show that the losses for these three types of bends are calculated to be 0.009 dB, 0.0007 dB, and 0.014 dB for (de)multiplexing TM1, TE1, and TE2 modes, respectively. It is evident that the theoretical losses in the S-bends within the proposed structure can be considered negligible. While it's expected that experimentally fabricated bends may be affected by sidewall roughness, it can be anticipated that their transmission losses still remain at very low levels.
Fig. 4. Propagation fields along the z-direction for multiplexing and (de)multiplexing TM1, TE1, and TE2 modes.
After optimizing the structural parameters of the 3-mode (De)MUX, we assess its performance under fabrication tolerances and varying operating bandwidth using the 3D-FV-FDTD method, as shown in Fig. 5. It can be noted from Fig. 5(a) that when the fabrication error varies by ± 20 nm, the transmittance of the TM1 mode remains relatively stable, while the insertion losses of the TE1 and TE2 modes are both below 1.5 dB and 1.2 dB, respectively. We can note from Fig. 5(a) that the transmission of TM1 mode is more fabrication tolerant compared to the other two TE modes. The reason for this phenomenon is that the coupling strength of the TM mode is greater compared to the other two TE modes. Because the TM mode has a larger evanescent field, its mode interaction strength is stronger, and it is less affected by fabrication errors. It can also be noted from Fig. 5(a) that the rapid fall in TE1 mode transmission as the fabrication error is above zero, whereas the TE2 mode transmission gradually decreases as fabrication error is positive or negative. The reason for this phenomenon is the difference in coupling lengths of the TE1 and TE2 modes. The coupling length of TE1 mode is LTE1 = 15 µm, while the coupling length of TE2 mode is only LTE2 = 0.8 µm. For the TE1 mode, when the fabrication error is above zero, the spacing between the access and bus waveguides decreases, leading to an increase in mode coupling strength. This would result in a shorter coupling length, and causes the power of the TE1 mode coupled to the bus waveguide to re-couple back to the access waveguide, which leads to a rapid fall in TE1 mode transmission. However, due to the short coupling length of the TE2 mode, it is less affected by the fabrication error, resulting in a more gradual decrease in its transmission. As shown in Fig. 5(b), when the operating bandwidth is over 100 nm from 1.5 µm to 1.6 µm, the insertion losses of the TM1, TE1, and TE2 modes are below 0.13 dB, 0.61 dB, and 1.1 dB, respectively.
4. Fabrication and experimental measurements
The optimized 5-mode (De)MUX is fabricated on the SOI platform. In this case, the PBC structure is referred to our designed parameters in our earlier work [21], which includes Wa = 400 nm, Wp = 460 nm, g = 200 nm, and Lp = 7 µm. The taper length between the PBC and the 3-mode (De)MUX is set to be Lt = 1.5 µm. The SOI wafer used in this work has a silicon-waveguide thickness of 220 nm, and includes a 3-µm thick buried-oxide (BOX) layer to isolate the silicon waveguide and substrate layers. A two-step etching process was employed to sequentially fabricate the I/O grating-couplers and silicon waveguides. First, the grating-coupler pattern was transferred onto the wafer using electron beam lithography (EBL) lithography, followed by etching it using inductively coupled plasma (ICP) to achieve a depth of 70 nm. Next, a second EBL lithography step was used to transfer the entire waveguide pattern onto the wafer, which was then etched using ICP to a depth of 220 nm. Finally, a 1-µm thick layer of silicon dioxide was deposited on top of the waveguides using plasma-enhanced chemical vapor deposition (PECVD) to serve as the top cladding layer. Two 5-mode (De)MUXs are connected back-to-back to form an MDM-link for testing purposes, with one functioning as a multiplexer and the other as a demultiplexer. The TE and TM referenced waveguides are also fabricated to normalize the coupling loss at each port. The scanning electron microscope (SEM) image of the fabricated 5-mode MDM-link is shown in Fig. 6(a). The enlarged view of the 5-mode multiplexer is shown in Fig. 6(b), with the 3-mode multiplexer based on multi-phase matching and a PBC based on the TWC. The zoom-in view of coupling regions is shown in Fig. 6(c). The enlarged picture of ADC structure for multiplexing the TE1 mode is shown in Fig. 6(d).
Fig. 6. (a) SEM image of the fabricated MDM-link and referenced waveguides. (b) Enlarged image of five-mode (De)MUX. (c) Zoom-in image of coupling regions. (d) Enlarged picture of ADC structure for multiplexing the TE1 mode.
The fabricated MDM-Link chip is evaluated using an in-house testing setup that includes an optical testing circuit, an adjustment system, and an observation system. The optical testing circuit consists of a tunable laser source (Agilent 81640B), a high-sensitivity photodetector (Agilent 81634B), and a three-ring polarization controller. The adjustment system comprises six-axis micro-positioning stages on the left and right sides of the device-under-test (DUT). The observation system includes horizontal and vertical cameras with corresponding three-dimensional adjustment mechanisms. The input and output signal lights are coupled using vertical optical fibers and grating couplers.
The transmission spectra of the five output ports are measured and are shown in Fig. 7. The TE0 light is input at the port I1, resulting in the multiplexing and demultiplexing process between the TE0 mode in the access and the TE2 mode in the bus waveguide, and the output spectra of the five ports are shown in Fig. 7(a). At the center wavelength of 1550 nm, the insertion loss and mode crosstalk are found to be 2.4 dB and -15.0 dB, respectively. The 3-dB bandwidth is 37 nm, ranging from 1528 nm to 1565 nm, and the corresponding mode crosstalk is below -13.0 dB. The TE2 mode bandwidth covers the commonly used C-band in optical communications, making it suitable for hybrid multiplexing systems for WDM and MDM.
Fig. 7. Measured transmission spectra at five output ports, O1∼O5 when light is launched at the input ports (a) I1, (b) I2, (c) I3, (d) I4, and (d) I5.
The TE0 light input into the port I2 undergoes the process of multiplexing and demultiplexing between the TE0 mode in the branch and the TE1 mode in the bus guide. The transmission spectra of the five output ports are shown in Fig. 7(b). It can be noted that when operating at the center wavelength of 1550 nm, the insertion loss and mode crosstalk are 1.9 dB and -16.6 dB, respectively. The 3-dB bandwidth is 28 nm, ranging from 1535 nm to 1563 nm, and the corresponding mode crosstalk is lower than -16.5 dB. The shorter working bandwidth of the TE1 mode may be due to its longer coupling length, which can be more susceptible to horizontal fabrication errors, resulting in decreased coupling efficiency.
When TM0 light is launched into the port I3, it corresponds to the TM0 mode inside the PBC being multiplexed into the bus waveguide through the TWC, without any mode conversion. The transmission spectra of the five ports under this condition are shown in Fig. 7(c). It can be seen that at the center wavelength of 1550 nm, the insertion loss and mode crosstalk are 0.9 dB and -16.0 dB, respectively. The 3-dB bandwidth can cover 100 nm, and the bandwidth with mode crosstalk below -15 dB is over 70 nm. It can be noted that the high-performance TM0 mode multiplexing and transmission can be achieved using the TWC structure. Since there is no mode conversion, the working bandwidth of this mode is relatively large, and the loss and crosstalk are small.
When the TE0 light is input at the port I4, the output spectra of the five ports are shown in Fig. 7(d). The TE0 mode in the PBC is transmitted directly to the bus waveguide without undergoing any mode conversion. It can be noted that at the central wavelength of 1550 nm, the insertion loss and modal crosstalk are 0.37 dB and -22.1 dB, respectively. The 3-dB bandwidth covers 100 nm, and the wavelength range with mode crosstalk below -15 dB is over 80 nm. It can be seen that the fundamental TE mode in the PBC does not undergo mode conversion and directly passes through the TWC structure. Compared with the fundamental TM mode shown in Fig. 7(c), this mode has a wider operating bandwidth, lower insertion loss, and mode crosstalk.
When the TM0 light is input into the port I5, it undergoes multiplexing and demultiplexing with the TM1 mode. The output spectra of the five ports are shown in Fig. 7(d). The output spectra of the five ports are illustrated in Fig. 7(d), indicating an insertion loss and mode crosstalk of 3.4 dB and -16.6 dB, respectively, at the center wavelength of 1550 nm. The 3-dB and 5-dB bandwidths are 46 nm and 100 nm, respectively, and the wavelength range with mode crosstalk less than -13 dB is over 80 nm. Although the ADC for the TM1 mode is located at the innermost position of the MDM-link and is basically unaffected by other ADC structures, the short coupling length of this mode makes it susceptible to longitudinal fabrication-errors, which may lead to the performance degradation.
Table 1 presents a comparison for experimental performance of the reported mode (De)MUXs. It can be observed that compared to previously reported structures such as ADCs, AC, MMI, Y-junction, DPWAs, and subwavelength gratings, our proposed multi-phase matching condition based mode (De)MUX can offer a shorter device length, enabling higher integration in on-chip MDM systems. While our proposed device has a longer length compared to the inverse-designed one, it exhibits a lower mode crosstalk and a higher robustness. In contrast to the MRR structure, our device features a larger operating bandwidth and a lower loss. Therefore, our proposed multi-phase matching condition based mode (De)MUX not only offers ultra-high integration but also competitive performance. The proposed cascaded-ADC structure based on the multi-phase matching condition can greatly reduce the overall size of the mode (De)MUX. The experimental results above demonstrate that the 5-mode (De)MUX exhibits high performance, although the bandwidth of the TE1 mode is relatively narrow due to the impact of fabrication errors. Subsequently, further improvements in the accuracy of the fabrication process can lead to higher performance in mode multiplexing and demultiplexing.
Table 1. Experimental performance of reported mode (de)multiplexers
5. Conclusions
In conclusion, we have presented and experimentally demonstrated a highly integrated 5-mode (De)MUX based on a cascaded ADC structure utilizing the multi-phase matching condition. The proposed design requires only a 1.5 µm-long taper waveguide connecting to a PBC to form the 5-mode (De)MUX. By simultaneously matching three modes to the same bus waveguide, the use of two taper structures can be avoided, resulting in a total coupling length of only 18.9 µm. The layout of the proposed mode (De)MUX has been designed and optimized using the FEM and 3D-FV-FDTD method, and fabricated on the SOI platform to form the MDM-link chip. Experimental results have indicated that at the central wavelength, the insertion loss of the 5 modes is below 3.4 dB, and the mode crosstalk is below -15.0 dB. The proposed device has demonstrated working bandwidths larger than 100 nm, 46 nm, 100 nm, 28 nm, and 37 nm for the TM0, TM1, TE0, TE1, and TE2 modes, respectively. Furthermore, the device performance can be improved by further enhancing the fabrication accuracy. The proposed multi-phase matching principle could provide a feasible technical solution for constructing high-performance MDM systems with high-integration mode-controlling devices.
Funding
National Natural Science Foundation of China (11904178, 62275128); State Key Laboratory of Advanced Optical Communication Systems and Networks Shanghai Jiao Tong University, China (2023GZKF015); The Startup Foundation for Introducing Talent of NUIST.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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