An optical mode demultiplexer based on a cascaded silica multimode interferometer structure

Abstract

The application discloses an optical mode demultiplexer based on a cascaded silica multimode interferometer structure, and belongs to the technical field of silica integrated optics. The optical mode demultiplexer is composed of a substrate, a lower cladding layer, a core layer and an upper cladding layer from bottom to top, the core layer and the upper cladding layer are located above the lower cladding layer, the core layer is completely covered in the upper cladding layer, the materials of the upper cladding layer and the lower cladding layer are silica, the material of the core layer is germanium-doped silica, and the material of the substrate is silicon. The core layer is composed of three parts in cascade, the first part is two-stage Y-branch waveguides in cascade, the second part is a 4*4 multimode interferometer, and the third part is two straight waveguides and a 2*2 multimode interferometer. The device can output TE0 mode from four different ports when TE0, TE1, TE2 and TE3 modes are input respectively, thereby realizing the mode demultiplexing function. The device adopts a passive structure, and the phase of an optical signal in a waveguide is adjusted through a waveguide phase shifter, so that the mode demultiplexing function is realized.

Description

Technical Field

[0001] This invention belongs to the field of silicon dioxide integrated optical technology, specifically relating to an optical mode demultiplexer based on a cascaded silicon dioxide multimode interferometer structure. Background Technology

[0002] With the advancement of technology and the emergence of technologies such as 5G communication, artificial intelligence, and virtual reality, high demands are being placed on communication systems, requiring high communication speeds, high bandwidths, and large capacity. Optical fiber communication, due to its unique advantages (e.g., high speed and low latency), plays a crucial role in communication systems. To address the issue of single-mode fiber channel capacity approaching saturation, various multiplexing techniques have been proposed, such as wavelength division multiplexing (WDM), mode division multiplexing (MDM), and partial division multiplexing (PDM). However, due to the bandwidth limitations of fiber amplifiers and the inherent nonlinear effects of optical fibers, the transmission capacity and efficiency of WDM technology are difficult to overcome physical limits, hindering further significant expansion and improvement. Furthermore, each independent channel in a WDM system requires a laser source and corresponding driving devices, placing high demands on communication costs and complexity. PDM technology primarily uses two channels, TE and TM, and the number of channels is also difficult to increase effectively. In recent years, mode division multiplexing (MDM) technology has received widespread attention in order to expand the number of channels and data capacity, and is considered one of the most promising research directions for improving single-fiber communication capacity. In mode-division multiplexing systems, few-mode fiber is used as the transmission medium to transmit optical signals through different intrinsic modes within the fiber, effectively expanding channel capacity. This multiplexing technology offers a completely new perspective, introducing a new dimension to address the channel capacity issue in optical communication systems and further improving their transmission capabilities.

[0003] In mode division multiplexing (MDF) systems, mode multiplexing and demultiplexing are indispensable components; therefore, mode multiplexers and demultiplexers are crucial fundamental devices. Both have identical structures, differing only in their incident and exit directions. Utilizing the principle of optical path reversibility, they can achieve multiplexing and demultiplexing of multiple intrinsic modes in optical fiber communication, thereby enabling multi-channel transmission of optical signals in single-mode fibers. Common optical structures capable of multiplexing / demultiplexing include multimode interferometers, Mach-Zehnder modulators, asymmetric directional couplers, and grating couplers. Among these, multimode interferometers are widely used due to their advantages such as simple structure, large bandwidth, and high manufacturing tolerance.

[0004] The performance of mode multiplexers / demultiplexers is mainly measured by indicators such as insertion loss, crosstalk, and power consumption. To improve these indicators, it is necessary to select a suitable material platform for device fabrication. Silica material has advantages such as low loss, high process tolerance, and good mode field matching with single-mode fiber, which can meet the performance requirements of mode multiplexing / demultiplexing devices. Summary of the Invention

[0005] This invention aims to provide an optical mode demultiplexer based on a cascaded silicon dioxide multimode interferometer structure for mode division multiplexing systems. It can output the TE0 mode from four different ports when four modes (TE0, TE1, TE2, and TE3) are input respectively, thereby achieving mode demultiplexing. The device employs a passive structure, using a waveguide phase shifter to adjust the phase of the optical signal in the waveguide to achieve mode demultiplexing.

[0006] The optical mode demultiplexer based on a cascaded silicon dioxide multimode interferometer structure for a mode division multiplexing system, as described in this invention, consists of a substrate, a lower cladding layer, a core layer, and an upper cladding layer from bottom to top. The core layer and the upper cladding layer are both located on top of the lower cladding layer, and the core layer is completely enclosed within the upper cladding layer. Figure 1 As shown. The upper and lower cladding layers are made of the same material, silicon dioxide, while the core layer is made of germanium-doped silicon dioxide, and the substrate material is silicon. All core layers have the same thickness, h=4μm, but the width varies at different locations.

[0007] Along the direction of optical mode propagation, the core layer of this mode demultiplexer consists of three cascaded structural parts, such as... Figure 3As shown; the first part is a cascaded two-stage Y-branch waveguide, the second part is a 4×4 multimode interferometer (Ⅳ), and the third part is two straight waveguides and a 2×2 multimode interferometer (V); the two-stage Y-branch waveguide in the first part is formed by cascading the first Y-branch waveguide (I), the second Y-branch waveguide (II), and the third Y-branch waveguide (III); the first Y-branch waveguide (I) consists of input waveguide 1, the first straight waveguide 2, and the second straight waveguide 3; the second Y-branch waveguide (II) consists of the first straight waveguide 2, the third straight waveguide 4, and the fourth straight waveguide 5; the third Y-branch waveguide (III) consists of the second straight waveguide 3, the fifth straight waveguide 6, and the sixth straight waveguide 7; the input waveguide 1 has a width of W1 = 16 μm and can accommodate TE0, The transmission uses four modes: TE1, TE2, and TE3. After passing through the first Y-branch waveguide (I), the input waveguide 1 is divided into a first straight waveguide 2 and a second straight waveguide 3 with a width of W2 = 8 μm. After passing through the second Y-branch waveguide (II) and the third Y-branch waveguide (III), the first straight waveguide 2 and the second straight waveguide 3 with a width of W2 are respectively divided into a third straight waveguide 4, a fourth straight waveguide 5, a fifth straight waveguide 6, and a sixth straight waveguide 7 with a width of W3 = 4 μm. Among them, a phase-shifting region is set on the fourth straight waveguide 5 and the fifth straight waveguide 6. The phase-shifting region is formed by a first tapered waveguide 8 with a width that linearly increases from W3 = 4 μm to W4 = 6 μm, a seventh straight waveguide 9 with a width that linearly decreases from W4 = 6 μm, and a... The 4×4 multimode interferometer (Ⅳ) is composed of cascaded second tapered waveguides 10 with W3=4μm; the input terminals of the 4×4 multimode interferometer (Ⅳ) are provided with a first input tapered waveguide 11, a second input tapered waveguide 12, a third input tapered waveguide 13, and a fourth input tapered waveguide 14, and the output terminals of the 4×4 multimode interferometer (Ⅳ) are provided with a first output tapered waveguide 15, a second output tapered waveguide 16, a third output tapered waveguide 17, and a fourth output tapered waveguide 18; the first input tapered waveguide 11 and the first output tapered waveguide 15, the second input tapered waveguide 12 and the second output tapered waveguide 16, the third input tapered waveguide 13 and the third output tapered waveguide 17, the fourth input tapered waveguide 14 and the fourth output tapered waveguide 18 are related to the 4×4 multimode interferometer (Ⅳ). The waveguides are symmetrically arranged; the widths of the first input tapered waveguide 11, the second input tapered waveguide 12, the third input tapered waveguide 13, and the fourth input tapered waveguide 14 increase linearly from W3=4μm to W5=8μm, and the widths of the first output tapered waveguide 15, the second output tapered waveguide 16, the third output tapered waveguide 17, and the fourth output tapered waveguide 18 decrease linearly from W5=8μm to W3=4μm; the outputs of the third straight waveguide 4, the fourth straight waveguide 5, the fifth straight waveguide 6, and the sixth straight waveguide 7 are respectively connected to a 4×4 multimode interferometer (Ⅳ) with a width of W6=100μm through the first input tapered waveguide 11, the second input tapered waveguide 12, the third input tapered waveguide 13, and the fourth input tapered waveguide 14.The 4×4 multimode interferometer (Ⅳ) is connected to the first output straight waveguide 25, the eighth straight waveguide 19, the ninth straight waveguide 20, and the fourth output straight waveguide 28 via the first output tapered waveguide 15, the second output tapered waveguide 16, the third output tapered waveguide 17, and the fourth output tapered waveguide 18, respectively. The 2×2 multimode interferometer (Ⅴ) has a fifth input tapered waveguide 21 and a sixth input tapered waveguide 22 at its input end, and a fifth output tapered waveguide 23 and a sixth output tapered waveguide 24 at its output end. The fifth input tapered waveguide 21 and the fifth output tapered waveguide 23, the sixth input tapered waveguide 22 and the sixth output tapered waveguide 24 are symmetrically arranged about the 2×2 multimode interferometer (V); the widths of the fifth input tapered waveguide 21 and the sixth input tapered waveguide 22 linearly increase from W3=4μm to W5=8μm, and the widths of the fifth output tapered waveguide 23 and the sixth output tapered waveguide 24 linearly decrease from W5=8μm to W3=4μm; the eighth straight waveguide 19 and the ninth straight waveguide 20 pass through the fifth input tapered waveguide 21 and the sixth output tapered waveguide 24 respectively. Input waveguide 21 and the sixth input tapered waveguide 22 are connected to a 2×2 multimode interferometer (V) with a width of W7 = 30 μm. Output tapered waveguide 23 and the sixth output tapered waveguide 24 are connected to the second output straight waveguide 26 and the third output straight waveguide 27, respectively. The first output straight waveguide 25, the second output straight waveguide 26, the third output straight waveguide 27, and the fourth output straight waveguide 28 serve as the four output terminals Port1, Port2, Port3, and Port4 of the device. The length of the 4×4 multimode interferometer (Ⅳ) is L1 = 9 μm. The distance between the centers of the second and third input tapered waveguides 12 and 13 and the center of the 4×4 multimode interferometer (Ⅳ) is X1=11μm; the distance between the centers of the first and fourth input tapered waveguides 11 and 14 and the center of the 4×4 multimode interferometer (Ⅳ) is X2=40μm; the distance between the centers of the fifth and sixth input tapered waveguides 21 and the center of the 2×2 multimode interferometer (Ⅴ) is X1=11μm; the length of the 2×2 multimode interferometer (Ⅴ) is L2=1920μm.

[0008] The working principle of this mode demultiplexer is as follows:

[0009] When a TEO mode optical signal is input to the device's input terminal, the device's operating principle is as follows: Figure 4As shown in Figure a, the TE0 mode optical signal enters the input waveguide 1. When the initial phase is -π, the optical signal is evenly divided into two TE0 mode optical signals with a phase of -π by the first Y-branch waveguide (I). Subsequently, it is evenly divided into four TE0 mode optical signals with a phase of -π by the second Y-branch waveguide (II) and the third Y-branch waveguide (III). After being modulated by the phase shifting region in the fourth straight waveguide 5 and the fifth straight waveguide 6, the optical signals entering the first input tapered waveguide 11, the second input tapered waveguide 12, the third input tapered waveguide 13, and the fourth input tapered waveguide 14 have phases of -π, π / 4, -3π / 4, and -π, respectively. The phase shifting region is composed of a first tapered waveguide 8 with a width that linearly changes from W3=4μm to W4=6μm, a seventh straight waveguide 9 with a width of W4=6μm, and a second tapered waveguide 10 with a width that linearly changes from W4=6μm to W3=4μm, cascaded together. According to optical waveguide theory, changing the waveguide width can alter the rate of optical phase change within the waveguide, thereby modulating the optical phase. Furthermore, by altering the length of the seventh straight waveguide 9, which has a width of W4 = 6 μm, the optical phase within the waveguide can be changed to a specific value. The lengths of the seventh straight waveguide 9 in the phase-shifting regions of the fourth straight waveguide 5 and the fifth straight waveguide 6 are set to 1453 μm and 283 μm, respectively, thus changing the phase in the waveguide by 5π / 4 and π / 4, respectively. Therefore, the phase change of the optical signal entering the second input tapered waveguide 12 is -π + 5π / 4 = π / 4, and the phase change of the optical signal entering the third input tapered waveguide 13 is -π + π / 4 = -3π / 4. The optical signals entering the first input tapered waveguide 11 and the fourth input tapered waveguide 14, lacking phase-shifting modulation, retain the unchanged phase of -π. The optical signals in the first input tapered waveguide 11, the second input tapered waveguide 12, the third input tapered waveguide 13, and the fourth input tapered waveguide 14 are modulated by the phase of the 4×4 multimode interferometer (Ⅳ). According to the self-image principle of the multimode interferometer, the multiple optical signals are combined into a single TEO mode optical signal, which is output from Port4.

[0010] When a TE1 mode optical signal is input to the device's input terminal, the device's operating principle is as follows: Figure 4As shown in b, the TE1 mode optical signal enters the input waveguide 1. When the initial phase is -π, the optical signal is split into two TE0 mode optical signals with a phase difference of π by the first Y-branch waveguide (I). The optical signal in the first straight waveguide 2 has the same phase as the initial phase, -π, while the optical signal in the second straight waveguide 3 has a phase of 0. After passing through the second Y-branch waveguide (II) and the third Y-branch waveguide (III), the optical signals in the third straight waveguide 4, the fourth straight waveguide 5, the fifth straight waveguide 6, and the sixth straight waveguide 7 are then... The phases are -π, -π, 0, 0 respectively; after modulation by the phase-shifting regions in the fourth straight waveguide 5 and the fifth straight waveguide 6, the phases of the optical signals entering the first input tapered waveguide 11, the second input tapered waveguide 12, the third input tapered waveguide 13, and the fourth input tapered waveguide 14 are -π, -3π / 4, -3π / 4, 0 respectively. The phase-shifting regions are the first tapered waveguide 8 with a width linearly changing from W3=4μm to W4=6μm, the seventh straight waveguide 9 with a width of W4=6μm, and the... The system consists of 10 cascaded second tapered waveguides with a width W4=6μm linearly changing to W3=4μm. According to optical waveguide theory, changing the width of the waveguide alters the rate of phase change of light within it, thus modulating the phase. Furthermore, by changing the length of the seventh straight waveguide 9 (W4=6μm), the phase in the waveguide is altered by a specific value. The lengths of the seventh straight waveguide 9 in the phase-shifting regions of the fourth straight waveguide 5 and the fifth straight waveguide 6 are set to 283μm and 1453μm, respectively, to achieve this. The phase changes by π / 4 and 5π / 4 respectively. Therefore, the phase change of the optical signal entering the second input tapered waveguide 12 is -π + π / 4 = -3π / 4, and the phase change of the optical signal entering the third input tapered waveguide 13 is 0 + 5π / 4 = 5π / 4 - 2π = -3π / 4. The optical signal entering the first input tapered waveguide 11 is modulated without a phase shift region, and the phase remains unchanged at -π. The optical signal entering the fourth input tapered waveguide 14 is modulated without a phase shift region, and the phase remains unchanged at 0. The optical signals in the first input tapered waveguide 11, the second input tapered waveguide 12, the third input tapered waveguide 13, and the fourth input tapered waveguide 14 are modulated by the phase of a 4×4 multimode interferometer (Ⅳ). The phases of the optical signals output from the eighth straight waveguide 19 and the ninth straight waveguide 20 are π / 2 and 0, respectively. After the phase of the optical signal is modulated by a 2×2 multimode interferometer (Ⅴ), according to the self-image principle of the multimode interferometer, the multiple optical signals are merged into a TEO mode optical signal, which is output from Port3.

[0011] When a TE2 mode optical signal is input to the device's input terminal, the device's operating principle is as follows: Figure 4As shown in Figure c, the TE2 mode optical signal enters the input waveguide 1. When its initial phase is -π, the optical signal is split into two TE1 mode optical signals with a phase difference of π by the first Y-branch waveguide (I). The phase of the optical signal in the first straight waveguide 2 is the same as the initial phase, which is -π, and the phase of the optical signal in the second straight waveguide 3 is 0. Subsequently, it is split into two TE0 mode optical signals with a phase difference of π by the second Y-branch waveguide (II) and the third Y-branch waveguide (III). At this time, the third straight waveguide 4 and the fourth straight waveguide 5 are split into two TE0 mode optical signals with a phase difference of π. The phases of the TE0 mode optical signals in waveguide 5, the fifth straight waveguide 6, and the sixth straight waveguide 7 are -π, 0, 0, and -π, respectively. After modulation by the phase-shifting regions in the fourth straight waveguide 5 and the fifth straight waveguide 6, the phases of the optical signals entering the first input tapered waveguide 11, the second input tapered waveguide 12, the third input tapered waveguide 13, and the fourth input tapered waveguide 14 are -π, -3π / 4, π / 4, and -π, respectively. The phase-shifting regions linearly change from a width of W3 = 4 μm to W4 = 6 μm in the first tapered waveguide. The waveguide consists of waveguide 8, a seventh straight waveguide 9 with a width of W4=6μm, and a second tapered waveguide 10 with a width that linearly changes from W4=6μm to W3=4μm. According to optical waveguide theory, changing the width of the waveguide can change the rate of phase change of the light in the waveguide, thereby achieving phase modulation of the light in the waveguide. Furthermore, changing the length of the seventh straight waveguide 9 with a width of W4=6μm can change the phase of the waveguide by a specific value. Phase shifting regions are respectively set in the fourth straight waveguide 5 and the fifth straight waveguide 6. The lengths of the seventh straight waveguide 9, 1453 μm and 283 μm, cause phase changes of 5π / 4 and π / 4 respectively. Therefore, the phase change of the optical signal entering the second input tapered waveguide 12 is 0 + 5π / 4 = 5π / 4 - 2π = -3π / 4, and the phase change of the optical signal entering the third input tapered waveguide 13 is 0 + π / 4 = π / 4. The optical signals entering the first input tapered waveguide 11 and the fourth input tapered waveguide 14 are modulated without phase shift, and their phase remains unchanged at -π. After the optical signals in the first input tapered waveguide 11, the second input tapered waveguide 12, the third input tapered waveguide 13, and the fourth input tapered waveguide 14 are modulated by the phase of the 4×4 multimode interferometer (Ⅳ), according to the self-image principle of the multimode interferometer, the multiple optical signals are combined into a single TE0 mode optical signal, which is output from Port1.

[0012] When a TE3 mode optical signal is input to the device's input terminal, the device's operating principle is as follows: Figure 4As shown in diagram d, the TE3 mode optical signal enters input waveguide 1. When its initial phase is -π, the optical signal is split into two TE1 mode optical signals with a phase difference of -π by the first Y-branch waveguide (I). Subsequently, it is split into two TE0 mode optical signals with a phase difference of π by the second Y-branch waveguide (II) and the third Y-branch waveguide (III). At this time, the phases of the TE0 mode optical signals in the third straight waveguide 4, the fourth straight waveguide 5, the fifth straight waveguide 6, and the sixth straight waveguide 7 are -π, 0, -π, and 0, respectively. After modulation by the phase shifting regions in the fourth straight waveguide 5 and the fifth straight waveguide 6, the optical signals entering the first input tapered waveguide 11, the second input tapered waveguide 12, the third input tapered waveguide 13, and the fourth input tapered waveguide 14 have phases of -π. The phase-shifting region is composed of a first tapered waveguide 8 with a width that linearly changes from W3=4μm to W4=6μm, a seventh straight waveguide 9 with a width of W4=6μm, and a second tapered waveguide 10 with a width that linearly changes from W4=6μm to W3=4μm, cascaded together. According to optical waveguide theory, the rate of change of the phase of light in the waveguide can be changed by changing the width of the waveguide, thereby achieving phase modulation of light in the waveguide. Furthermore, by changing the length of the seventh straight waveguide 9 with a width of W4=6μm, the phase in the waveguide can be changed by a specific value. The lengths of the seventh straight waveguide 9 in the phase-shifting region of the fourth straight waveguide 5 and the fifth straight waveguide 6 are set to 283μm and 1453μm, respectively, so that the phase in the waveguide can be changed by π / 4 and 5π / 4, respectively. Therefore, the phase change of the optical signal entering the second input tapered waveguide 12 is 0 + π / 4 = π / 4, the phase change of the optical signal entering the third input tapered waveguide 13 is -π + 5π / 4 = π / 4, the optical signal entering the first input tapered waveguide 11 has no phase shift modulation and its phase remains unchanged at -π, and the optical signal entering the fourth input tapered waveguide 14 has no phase shift modulation and its phase remains unchanged at 0. After the optical signals in the first input tapered waveguide 11, the second input tapered waveguide 12, the third input tapered waveguide 13 and the fourth input tapered waveguide 14 are phase-modulated by the 4×4 multimode interferometer (Ⅳ), the phases of the optical signals output from the eighth straight waveguide 19 and the ninth straight waveguide 20 are 0 and π / 2, respectively. After the phase is modulated by the 2×2 multimode interferometer (Ⅴ), according to the self-image principle of the multimode interferometer, the multiple optical signals are combined into a single TE0 mode optical signal, which is output from Port2.

[0013] When a TEO mode optical signal is input to the device's input terminal, the simulation results of the device are as follows: Figure 5 As shown in Figure a, the device mode demultiplexing output efficiency is 97.2% at this time. According to the calculation formula for optical loss in the device performance index: IL = -10log(P out / P in ), where IL is the optical loss of the device, P out For the output efficiency of the device, Pin Based on the input efficiency of the device, the optical loss can be calculated to be 0.12 dB; and according to the extinction ratio calculation formula in the device performance index: ER=10log[(P out ) min / (P out ) max ], where ER is the extinction ratio of the device, (P out ) min The minimum output efficiency of the device's output port (P) out ) max Based on the maximum output efficiency of the device's output port, the extinction ratio can be calculated to be 20.60 dB. When a TE1 mode optical signal is input to the device's input, the simulation results are as follows: Figure 5 As shown in b, the device's mode demultiplexing output efficiency is 94.7%, and the calculated optical loss is 0.24 dB, with an extinction ratio of 27.48 dB. When a TE2 mode optical signal is input to the device's input terminal, the simulation results are as follows: Figure 5 As shown in Figure c, the device's mode demultiplexing output efficiency is 95.2% at this time, and the calculated optical loss is 0.21 dB, with an extinction ratio of 24.35 dB. When a TE3 mode optical signal is input to the device's input terminal, the simulation results are as follows: Figure 5 As shown in d, the output efficiency of the device mode demultiplexing is 94.7% at this time, and the optical loss of the device can be calculated to be 0.24dB, and the extinction ratio is 27.21dB.

[0014] Figure 6 The figure shows the bandwidth characteristics of the device. The bandwidth range where the normalized output efficiency of the device is greater than 90% is 11nm. Attached Figure Description

[0015] Figure 1 This is a cross-sectional structural diagram of the mode demultiplexer;

[0016] Figure 2 It is a curve showing the effective refractive index of silicon dioxide as a function of width;

[0017] Figure 3 This is a top view of the core structure of the device;

[0018] Figure 4 a, Figure 4 b、 Figure 4 c. Figure 4 d represents the optical phase diagrams at various locations in the core layer of the device when optical signals in modes TE0, TE1, TE2, and TE3 are input to the device.

[0019] Figure 5 a, Figure 5 b、 Figure 5 c. Figure 5 d represents the simulation output results of the device core layer when the device is input with optical signals in TE0, TE1, TE2, and TE3 modes.

[0020] Figure 6 It is a bandwidth characteristic diagram of the device;

[0021] Figure 7 This is a flowchart of the device fabrication process. Detailed Implementation

[0022] The present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0023] Example 1

[0024] 1. See Figure 1 The silicon dioxide waveguide four-mode demultiplexer consists of a substrate, a lower cladding, a core layer, and an upper cladding, from bottom to top. The core layer and upper cladding are both located on top of the lower cladding, and the core layer is completely enclosed within the upper cladding. According to optical waveguide mode theory, when the refractive index difference between the silicon dioxide core layer and the cladding is 2% (the upper and lower cladding are made of the same material, silicon dioxide, with a refractive index of 1.4447; the core layer is made of germanium-doped silicon dioxide with a refractive index of 1.4741, and the core layer thickness h = 4 μm), in order to reduce the polarization dependence of the single-mode waveguide, according to... Figure 2 The relationship between the effective refractive index and the waveguide width is shown in the figure. The width of the core layer is set to W = 4 μm (i.e., Figure 3 The width of the first output straight waveguide 25 is W3 = 4 μm.

[0025] 2. See Figure 3 The core layer of the mode demultiplexer, along the direction of light propagation, consists of two cascaded Y-branch waveguides, a 4×4 multimode interferometer, two straight waveguides, and a 2×2 multimode interferometer, all located in the same plane and parallel to the substrate.

[0026] 3. First, determine the width of input waveguide 1. Based on the TE3 mode input and output first output straight waveguide 25 width W3=4μm condition of device function requirements, determine the width of input waveguide 1 W1=16μm. After passing through two stages of Y-branch waveguides, determine the width of the first straight waveguide 2 and the second straight waveguide 3 W2=8μm, and the width of the third straight waveguide 4, the fourth straight waveguide 5, the fifth straight waveguide 6 and the sixth straight waveguide 7 W3=4μm.

[0027] 4. By determining the phase-shifting region width W4 = 6 μm in the fourth and fifth straight waveguides 5 and 6 using the beam propagation method, phase modulation can be achieved within a relatively small size. When a TE0 mode optical signal is input to the device, the lengths of the seventh straight waveguide 9 in the phase-shifting region of the fourth and fifth straight waveguides 5 and 6 are 1453 μm and 283 μm, respectively; when a TE1 mode optical signal is input to the device, the lengths of the seventh straight waveguide 9 in the phase-shifting region of the fourth and fifth straight waveguides 5 and 6 are 283 μm and 1453 μm; when a TE2 mode optical signal is input to the device, the lengths of the seventh straight waveguide 9 in the phase-shifting region of the fourth and fifth straight waveguides 5 and 6 are 1453 μm and 283 μm; when a TE3 mode optical signal is input to the device, the lengths of the seventh straight waveguide 9 in the phase-shifting region of the fourth and fifth straight waveguides 6 are 283 μm and 1453 μm.

[0028] 5. The width and length of the 4×4 multimode interferometer (Ⅳ) are determined to be W6=100μm and L1=9800μm respectively by the beam propagation method. The distance between the center of the first input tapered waveguide 11 and the fourth input tapered waveguide 14 and the center of the 4×4 multimode interferometer (Ⅳ) is equal to X2=40μm. The distance between the center of the second input tapered waveguide 12 and the third input tapered waveguide 13 and the center of the 4×4 multimode interferometer (Ⅳ) is equal to X1=11μm. The width and length of the 2×2 multimode interferometer (Ⅴ) are determined to be W7=30μm and L2=1920μm respectively.

[0029] Example 2: Figure 7 The fabrication process of the device is demonstrated, and the specific steps are as follows:

[0030] 1. Silicon wafers as substrates after cleaning: Single-crystal silicon wafers are selected as substrates and ultrasonically cleaned with acetone, ethanol and deionized water in sequence until the surface of the silicon wafer is smooth and free of impurities.

[0031] 2. Thermal oxidation deposition of low refractive index (refractive index 1.4447) silicon dioxide lower cladding: At an ambient temperature of 1000℃, a silicon dioxide thin film is grown on a clean silicon wafer substrate as a lower cladding through a wet thermal oxidation process. By controlling the water vapor flow rate, substrate temperature and reaction time, the silicon dioxide lower cladding is grown to a thickness of 10μm.

[0032] 3. Deposition of high refractive index (refractive index 1.4741) silicon dioxide core layer: Germanium-doped high refractive index silicon dioxide was deposited by plasma-enhanced chemical vapor deposition. The flow rates of the reaction gases GeCl4, SiH4 and N2O were controlled to be 32 sccm, 20 sccm and 40 sccm, respectively. The radio frequency power was 50W and the substrate temperature was 200℃. By controlling the reaction time, a high refractive index silicon dioxide core layer with a thickness of h = 4μm was deposited on the cladding surface at a low refractive index.

[0033] 4. Etching to prepare the silicon dioxide core layer: Photoresist is spin-coated onto the surface of the deposited high-refractive-index silicon dioxide layer. Ultraviolet lithography is performed using a photolithography machine to transfer the core layer pattern on the photomask to the photoresist surface. Subsequently, development is performed (for positive photoresist, the light-shielding area of ​​the core layer pattern on the photomask is the same as the core layer structure, and the positive photoresist without photomask shielding is exposed, and the exposed photoresist is removed after development; for negative photoresist, the light-shielding area of ​​the core layer pattern on the photomask is complementary to the core layer structure, and the negative photoresist without photomask shielding is exposed, and the unexposed photoresist is removed after development). Inductively coupled plasma etching is then used to remove the high-refractive-index silicon dioxide without photoresist protection, forming a high-refractive-index silicon dioxide core layer after photoresist removal.

[0034] 5. Deposition of low-refractive-index silica top cladding: A low-refractive-index silica top cladding is deposited on the surface of the low-refractive-index silica bottom cladding and the high-refractive-index silica core layer using chemical vapor deposition. By adjusting the gas flow rate, reactant ratio, radio frequency power and time, the deposition thickness of the silica top cladding on the low-refractive-index silica bottom cladding is controlled to be 20 μm. Then, the surface flatness of the top cladding is controlled by chemical mechanical polishing.

Claims

1. An optical mode demultiplexer based on a cascaded silicon dioxide multimode interferometer structure for a mode division multiplexing system, characterized in that: The structure consists of a substrate, a lower cladding, a core layer, and an upper cladding from bottom to top. The core layer and the upper cladding are both located on top of the lower cladding, and the core layer is completely enclosed within the upper cladding. The upper and lower cladding are made of the same material, silicon dioxide, while the core layer is made of germanium-doped silicon dioxide. The substrate is made of silicon. The core layer has a uniform thickness of h along the optical mode propagation direction and is composed of three cascaded parts. The first part is a cascaded two-stage Y-branch waveguide, the second part is a 4×4 multimode interferometer (Ⅳ), and the third part is two straight waveguides and a 2×2 multimode interferometer (V). The two-stage Y-branch waveguide in the first part consists of a first Y-branch waveguide (I), a second Y-branch waveguide (II), and a third Y-branch waveguide. (III) Cascaded; the first Y-branch waveguide (I) is composed of the input waveguide (1), the first straight waveguide (2), and the second straight waveguide (3); the second Y-branch waveguide (II) is composed of the first straight waveguide (2), the third straight waveguide (4), and the fourth straight waveguide (5); the third Y-branch waveguide (III) is composed of the second straight waveguide (3), the fifth straight waveguide (6), and the sixth straight waveguide (7); the input waveguide (1) has a width of W1 and can accommodate the transmission of four modes: TE0, TE1, TE2, and TE3; after passing through the first Y-branch waveguide (I), the input waveguide (1) is divided into the first straight waveguide (2) and the second straight waveguide (3) with a width of W2; and then through the second Y-branch... After waveguide (II) and the second Y-branch waveguide (III), the first straight waveguide (2) and the second straight waveguide (3) with a width of W2 are respectively divided into the third straight waveguide (4) and the fourth straight waveguide (5), the fifth straight waveguide (6) and the sixth straight waveguide (7) with a width of W3; a phase shifting region is provided on the fourth straight waveguide (5) and the fifth straight waveguide (6), which is composed of a first tapered waveguide (8) with a width that linearly increases from W3 to W4, a seventh straight waveguide with a width of W4, and a second tapered waveguide (10) with a width that linearly decreases from W4 to W3; the input end of the 4×4 multimode interferometer (Ⅳ) is provided with a first input tapered waveguide (11) and a second input tapered waveguide (12). The output terminals of the 4×4 multimode interferometer (Ⅳ) are provided with a first output tapered waveguide (15), a second output tapered waveguide (16), a third output tapered waveguide (17), and a fourth output tapered waveguide (18); the first input tapered waveguide (11) and the first output tapered waveguide (15), the second input tapered waveguide (12), the second output tapered waveguide (16), the third input tapered waveguide (13) and the third output tapered waveguide (17), the fourth input tapered waveguide (14), and the fourth output tapered waveguide (18) are symmetrically arranged about the 4×4 multimode interferometer (Ⅳ);The widths of the first input tapered waveguide (11), the second input tapered waveguide (12), the third input tapered waveguide (13), and the fourth input tapered waveguide (14) increase linearly from W3 to W5; the widths of the first output tapered waveguide (15), the second output tapered waveguide (16), the third output tapered waveguide (17), and the fourth output tapered waveguide (18) decrease linearly from W5 to W3; the output terminals of the third straight waveguide (4), the fourth straight waveguide (5), the fifth straight waveguide (6), and the sixth straight waveguide (7) are respectively connected through the first input tapered waveguide (11), the second input tapered waveguide (12), the third input tapered waveguide (13), the fourth input tapered waveguide (14), the fifth input tapered waveguide (15), the sixth straight waveguide (16), the seventh input tapered waveguide (17), and the eighth input tapered waveguide (18). The three-input tapered waveguide (13) and the fourth-input tapered waveguide (14) are connected to a 4×4 multimode interferometer (Ⅳ) with a length of L1 and a width of W6; the 4×4 multimode interferometer (Ⅳ) is connected to the first output straight waveguide (25), the eighth straight waveguide (19), the ninth straight waveguide (20), and the fourth output straight waveguide (28) respectively through the first output tapered waveguide (15), the second output tapered waveguide (16), the third output tapered waveguide (17), and the fourth output tapered waveguide (18); the input end of the 2×2 multimode interferometer (Ⅴ) is provided with a fifth input tapered waveguide (21) and a sixth input tapered waveguide (2 2) The output terminals of the 2×2 multimode interferometer (V) are provided with a fifth output tapered waveguide (23) and a sixth output tapered waveguide (24). The fifth input tapered waveguide (21) and the fifth output tapered waveguide (23), the sixth input tapered waveguide (22) and the sixth output tapered waveguide (24) are symmetrically arranged about the 2×2 multimode interferometer (V). The widths of the fifth input tapered waveguide (21) and the sixth input tapered waveguide (22) change linearly from W3 to W5, and the widths of the fifth output tapered waveguide (23) and the sixth output tapered waveguide (24) change linearly from W5 to W3. The eighth straight waveguide (19) The ninth straight waveguide (20) is connected to a 2×2 multimode interferometer (V) with a length of L2 and a width of W7 via the fifth input tapered waveguide (21) and the sixth input tapered waveguide (22), respectively. The fifth output tapered waveguide (23) and the sixth output tapered waveguide (24) are connected to the second output straight waveguide (26) and the third output straight waveguide (27), respectively. The first output straight waveguide (25), the second output straight waveguide (26), the third output straight waveguide (27), and the fourth output straight waveguide (28) serve as the four output terminals Port1, Port2, Port3, and Port4 of the device.

2. The optical mode demultiplexer based on a cascaded silicon dioxide multimode interferometer structure for a mode division multiplexing system as described in claim 1, characterized in that: h=4μm, W1=16μm, W2=8μm, W3=4μm, W4=6μm, W5=8μm, W6=100μm, W7=30μm, L1=9800μm, L2=1920μm.

3. The optical mode demultiplexer based on a cascaded silicon dioxide multimode interferometer structure for a mode division multiplexing system as described in claim 1, characterized in that: The distance between the center of the second input tapered waveguide (12) and the center of the third input tapered waveguide (13) and the center of the 4×4 multimode interferometer (Ⅳ) is X1=11μm. The distance between the center of the fifth input tapered waveguide (21) and the center of the sixth input tapered waveguide (22) and the center of the 2×2 multimode interferometer (Ⅴ) is X1=11μm. The distance between the center of the first input tapered waveguide (11) and the fourth input tapered waveguide (14) and the center of the 4×4 multimode interferometer (Ⅳ) is X2=40μm.

4. The optical mode demultiplexer based on a cascaded silicon dioxide multimode interferometer structure for a mode division multiplexing system as described in claim 1, characterized in that: When a TE0 mode optical signal is input to the device input terminal, the TE0 mode optical signal enters the input waveguide (1). The lengths of the seventh straight waveguide (9) in the phase-shifting region of the fourth straight waveguide (5) and the fifth straight waveguide (6) are 1453μm and 283μm, respectively, and the TE0 mode optical signal is output from Port4. When a TE1 mode optical signal is input to the device input terminal, the TE1 mode optical signal enters the input waveguide (1). The lengths of the seventh straight waveguide (9) in the phase-shifting region of the fourth straight waveguide (5) and the fifth straight waveguide (6) are 283μm and 1453μm, respectively, and the TE0 mode optical signal is output from Port3. When a TE2 mode optical signal is input to the device input terminal, the TE2 mode optical signal enters the input waveguide (1). The lengths of the seventh straight waveguide (9) in the phase-shifting region of the fourth straight waveguide (5) and the fifth straight waveguide (6) are 1453μm and 283μm, respectively. The TE0 mode optical signal is output from Port1. When a TE3 mode optical signal is input to the device input terminal, the TE3 mode optical signal enters the input waveguide (1). The lengths of the seventh straight waveguide (9) in the phase-shifting region of the fourth straight waveguide (5) and the fifth straight waveguide (6) are 283μm and 1453μm, respectively. The TE0 mode optical signal is output from Port2.

5. The optical mode demultiplexer based on a cascaded silicon dioxide multimode interferometer structure for a mode division multiplexing system as described in claim 1, characterized in that: The thickness of the lower cladding layer is 10 μm, and the thickness of the upper cladding layer above the lower cladding layer is 20 μm; the refractive index of the upper and lower cladding layers is 1.4447, and the refractive index of the core layer is 1.4741.

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