Polarization independent electro-optic modulator based on dual optical mode modulation and method of making the same

Abstract

The application discloses a polarization-independent electro-optical modulator based on double optical mode modulation and a preparation method thereof. The polarization-independent electro-optical modulator comprises a substrate layer, a thin film lithium niobate flat plate layer arranged on the top of the substrate layer, a first end face coupler, a polarization rotation beam splitter, a first waveguide cross structure, a Mach-Zehnder interferometer, a second waveguide cross structure, a polarization rotation beam combiner, a second end face coupler, a 1*2 multimode interferometer, a first TE0-TE1 mode conversion structure, a first TE1-TE0 mode conversion structure, a 2*1 multimode interferometer, a second TE0-TE1 mode conversion structure, a second TE1-TE0 mode conversion structure, a first thermal phase shifter, a second thermal phase shifter and a traveling wave electrode. The embodiment of the application can improve the modulation efficiency, is compact in structure and can be widely applied to the technical field of electro-optical modulators.

Description

Technical Field

[0001] This invention relates to the field of electro-optic modulator technology, and in particular to a polarization-independent electro-optic modulator based on dual optical mode modulation and its fabrication method. Background Technology

[0002] Electro-optic modulators play a crucial role in optoelectronic links such as optical communications, data centers, and wireless communication systems. Thin-film lithium niobate modulators offer advantages such as low loss, low drive voltage, and ultra-high electro-optic bandwidth, and are smaller in size than traditional bulk lithium niobate modulators, and are currently experiencing rapid development.

[0003] However, the performance of most electro-optic modulators is currently dependent on the input polarization state. To ensure the stability and reliability of electro-optic modulators, complex polarization control devices need to be placed near them in practical applications. This increases the complexity, size and cost of the system. In addition, it may cause the co-packaged or integrated laser source to be placed in a high-temperature environment, which significantly reduces the laser performance and the reliability of the overall solution.

[0004] To address this issue, polarization-insensitive modulators are needed to replace current polarization-sensitive modulators. Polarization-insensitive modulators can handle input signals with arbitrary polarization states, improving their versatility and making them applicable to optical signals with various polarization states, thus providing greater flexibility. Furthermore, in practical applications of optical communication and optical networks, the polarization state may change due to variations in the transmission path and environment of the optical signal. Polarization-insensitive modulators simplify system integration, reducing the need for complex polarization control devices around the modulator, thereby improving system stability and reliability. Simultaneously, since polarization-insensitive modulators do not require complex polarization control systems, the overall system cost can be reduced.

[0005] Among related technologies, polarization-independent electro-optic modulators based on thin-film lithium niobate suffer from low modulation efficiency or insufficiently compact structure. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a polarization-independent electro-optic modulator based on dual optical mode modulation and its fabrication method, which improves modulation efficiency and has a compact structure.

[0007] On one hand, embodiments of the present invention provide a polarization-independent electro-optic modulator based on dual optical mode modulation, comprising a substrate layer, a thin-film lithium niobate plate layer disposed on top of the substrate layer, and a first end-face coupler, a polarization rotating beam splitter, a first waveguide cross structure, a Mach-Zehnder interferometer, a second waveguide cross structure, a polarization rotating beam combiner, a second end-face coupler, a 1×2 multimode interferometer, a first TE0-TE1 mode conversion structure, a first TE1-TE0 mode conversion structure, a 2×1 multimode interferometer, a second TE0-TE1 mode conversion structure, a second TE1-TE0 mode conversion structure, a first thermally modulated phase shifter, a second thermally modulated phase shifter, and a traveling wave electrode; the output end of the first end-face coupler is connected to the input end of the polarization rotating beam splitter, and the output end of the polarization rotating beam combiner is connected to the input end of the second end-face coupler, wherein...

[0008] The first output terminal of the polarization rotating beam splitter is connected to the input terminal of the Mach-Zehnder interferometer through a first waveguide cross structure, and the output terminal of the Mach-Zehnder interferometer is connected to the second input terminal of the polarization rotating beam combiner through a second waveguide cross structure.

[0009] The second output of the polarization rotating beam splitter is connected to the input of a 1×2 multimode interferometer. The first output of the 1×2 multimode interferometer is connected to the first input of the first TE0-TE1 mode conversion structure via a first waveguide cross structure. The second output of the 1×2 multimode interferometer is connected to the second input of the second TE0-TE1 mode conversion structure. Both the second input and output of the first TE0-TE1 mode conversion structure are connected to the upper arm waveguide of a Mach-Zehnder interferometer. Both the first input and output of the second TE0-TE1 mode conversion structure are connected to the lower arm waveguide of a Mach-Zehnder interferometer. The first output of the first TE1-TE0 mode conversion structure is connected to the first input of a 2×1 multimode interferometer. The second output of the second TE1-TE0 mode conversion structure is connected to the second input of the 2×1 multimode interferometer via a second waveguide cross structure. The output of the 2×1 multimode interferometer is connected to the first input of the polarization rotating beam combiner.

[0010] The first thermally adjustable phase shifter is used to control the bias phase of the first mode, and the second thermally adjustable phase shifter is used to control the bias phase of the second mode.

[0011] The traveling wave electrode acts on the Mach-Zehnder interferometer.

[0012] Optionally, the first TE0-TE1 mode conversion structure or the second TE0-TE1 mode conversion structure includes a 1×1 multimode interferometer directional coupler and a directional coupler connected in sequence, and the first TE1-TE0 mode conversion structure or the second TE1-TE0 mode conversion structure includes a directional coupler and a 1×1 multimode interferometer directional coupler connected in sequence.

[0013] Optionally, the Mach-Zehnder interferometer includes a 1×2 multimode interferometer and a 2×1 multimode interferometer, and the 1×2 multimode interferometer and the 2×1 multimode interferometer are connected by two optical waveguides.

[0014] Optionally, the first or second thermally adjustable phase shifter includes a resistor, a positive electrode, and a negative electrode.

[0015] Optionally, the traveling wave electrode structure includes a GSG electrode structure, and both waveguides of the Mach-Zehnder interferometer are disposed between the G traveling wave electrode and the S traveling wave electrode.

[0016] Optionally, the traveling wave electrode adopts a T-type capacitively loaded periodic electrode structure.

[0017] Optionally, the first end-face coupler or the second end-face coupler includes a deeply etched waveguide portion and two shallowly etched waveguide portions on the top and bottom.

[0018] Optionally, tapered optical fibers are provided at the input end of the first end-face coupler or the output end of the second end-face coupler.

[0019] Optionally, the polarization rotating beam splitter includes an inverted conical mode converter, a directional coupler, and a 1×1 multimode interferometer connected in sequence; the polarization rotating beam combiner includes a 1×1 multimode interferometer, a directional coupler, and an inverted conical mode converter connected in sequence.

[0020] On the other hand, embodiments of the present invention provide a method for fabricating a polarization-independent electro-optic modulator based on dual optical mode modulation, comprising:

[0021] A substrate layer and a thin-film lithium niobate planar layer are prepared. A device waveguide structure is prepared on the thin-film lithium niobate planar layer. The device waveguide structure includes a first end-face coupler, a polarization rotating beam splitter, a first waveguide cross structure, a Mach-Zehnder interferometer, a second waveguide cross structure, a polarization rotating beam combiner, a second end-face coupler, a 1×2 multimode interferometer, a first TE0-TE1 mode conversion structure, a first TE1-TE0 mode conversion structure, a 2×1 multimode interferometer, a second TE0-TE1 mode conversion structure, and a second TE1-TE0 mode conversion structure.

[0022] The thin-film lithium niobate plate layer with the device waveguide structure is cleaned, a capping layer is grown, photoresist is coated, and photolithography is performed on the photoresist to prepare the first thermally modulated phase shifter, the second thermally modulated phase shifter, and the traveling wave electrode.

[0023] A polished end face is fabricated to form a polarization-independent electro-optic modulator based on dual optical mode modulation.

[0024] The implementation of this invention includes the following beneficial effects: In this embodiment, arbitrary polarized input light enters from the first end-face coupler and is converted into two beams of the same mode by a polarization rotating beam splitter. One mode of light is converted into another mode by the first TE0-TE1 mode conversion structure and the second TE0-TE1 mode conversion structure. The two different modes of light are transmitted in a Mach-Zehnder interferometer and modulated by a set of traveling-wave electrodes. The modulated transmitted light is then converted again by the first TE1-TE0 mode conversion structure and the second TE1-TE0 mode conversion structure to restore the original light mode. The two modes of input light are combined by a polarization rotating beam combiner and output through the second end-face coupler. That is, the polarization-independent electro-optic modulator uses a Mach-Zehnder interferometer, a set of traveling-wave electrodes to modulate the two optical modes, and then polarizes and combines the beams. While maintaining high modulation efficiency, the overall structure is also very compact. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the structure of a polarization-independent electro-optic modulator based on dual optical mode modulation provided in an embodiment of the present invention;

[0026] Figure 2 This is a first schematic diagram of a polarization-independent electro-optic modulator based on dual optical mode modulation provided in an embodiment of the present invention;

[0027] Figure 3 This is a second schematic diagram of a polarization-independent electro-optic modulator based on dual optical mode modulation provided in an embodiment of the present invention;

[0028] Figure 4 This is a schematic diagram of a TE1-TE0 mode conversion structure and a TE0-TE1 mode conversion structure provided in an embodiment of the present invention;

[0029] Figure 5 This is a schematic diagram of the structure of a Mach-Zehnder interferometer provided in an embodiment of the present invention;

[0030] Figure 6 This is a schematic diagram of a waveguide cross structure provided in an embodiment of the present invention;

[0031] Figure 7 This is a schematic diagram of the structure of a thermally adjustable phase shifter provided in an embodiment of the present invention;

[0032] Figure 8 This is a schematic diagram of the structure of a traveling wave electrode provided in an embodiment of the present invention;

[0033] Figure 9 This is a schematic diagram of a three-dimensional structure of a polarization-independent electro-optic modulator based on dual optical mode modulation provided in an embodiment of the present invention;

[0034] Figure 10 This is a schematic diagram of the structure of an end-face coupler provided in an embodiment of the present invention;

[0035] Figure 11 This is a schematic diagram of the structure of a polarization rotating beam splitter and a polarization rotating beam combiner provided in an embodiment of the present invention;

[0036] Figure 12 This is a schematic flowchart illustrating the steps of a method for fabricating a polarization-independent electro-optic modulator based on dual optical mode modulation, as provided in an embodiment of the present invention. Detailed Implementation

[0037] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. The step numbers in the following embodiments are only for ease of explanation and do not limit the order of the steps. The execution order of each step in the embodiments can be adapted according to the understanding of those skilled in the art.

[0038] See Figure 1 This invention provides a polarization-independent electro-optic modulator based on dual optical mode modulation, comprising a substrate layer, a thin-film lithium niobate plate layer disposed on top of the substrate layer, a first end-face coupler (2), a polarization rotating beam splitter (3), a first waveguide cross structure (4), a Mach-Zehnder interferometer (5), a second waveguide cross structure (6), a polarization rotating beam combiner (7), a second end-face coupler (8), a 1×2 multimode interferometer (9), a first TE0-TE1 mode conversion structure (10), a first TE1-TE0 mode conversion structure (11), a 2×1 multimode interferometer (12), a second TE0-TE1 mode conversion structure (13), a second TE1-TE0 mode conversion structure (14), a first thermally modulated phase shifter (15), a second thermally modulated phase shifter (16), and a traveling wave electrode (17); wherein, light of any polarization state is input through the first end-face coupler (2) and then enters the polarization rotating beam splitter (3);

[0039] The first output end of the polarization rotating beam splitter (3) is connected to the input end of the Mach-Zehnder interferometer (5) through the first waveguide cross structure (4). The output end of the Mach-Zehnder interferometer (5) is connected to the second waveguide cross structure (6), enters the second input end of the polarization rotating beam combiner (7), and is output through the second end face coupler (8).

[0040] The second output of the polarization rotating beam splitter (3) is connected to the input of the 1×2 multimode interferometer (9). The first output of the 1×2 multimode interferometer (9) is connected to the first input of the first TE0-TE1 mode conversion structure (10) through the first waveguide cross structure (4). The second output of the 1×2 multimode interferometer (9) is connected to the second input of the second TE0-TE1 mode conversion structure (13). After the TE1 mode converted from TE0 is transmitted in the two arms of the Mach-Zehnder interferometer (5), it is connected to the first TE1-TE0 mode conversion structure (11) and the second TE1-TE0 mode conversion structure (14) respectively. The first output end of the first TE1-TE0 mode conversion structure (11) is connected to the first input end of the 2×1 multimode interferometer (12). The second output end of the second TE1-TE0 mode conversion structure (14) is connected to the second input end of the 2×1 multimode interferometer (12) through the second waveguide cross structure (6). The output end of the 2×1 multimode interferometer (12) is connected to the first input end of the polarization rotation beam combiner (7) and output through the second end face coupler (8).

[0041] The first thermally adjustable phase shifter (15) is used to control the bias phase of the first mode (TE0 mode), and the second thermally adjustable phase shifter (16) is used to control the bias phase of the second mode (TE1 mode).

[0042] The traveling wave electrode (17) is applied to the Mach-Zehnder interferometer (5).

[0043] It should be noted that the substrate layer includes a substrate layer and a buried oxide layer arranged sequentially, and the upper layer of the buried oxide layer is a thin-film lithium niobate plate layer and a silicon dioxide layer.

[0044] See Figure 2The input light of any polarization state (including TE0 mode and TM0 mode) is input to the polarization rotating beam splitter (3) through the first end face coupler (2). The polarization rotating beam splitter (3) converts the input light into two light components with the same polarization mode (TE0 mode), and then transmits them through two waveguides respectively: the light converted from TM0 mode to TE0 mode enters the upper waveguide A, is transmitted through the first waveguide cross structure (4) and then connects to waveguide B to enter the Mach-Zehnder interferometer (5). After being output, it is connected to the second waveguide cross structure (6) through waveguide B' and then enters waveguide A'. Subsequently, waveguide A' is connected to the second input end of the polarization rotating beam combiner (7). The polarization rotating beam combiner (7) converts the TE0 mode input at the second input end into TM0 mode and outputs it through the second end face coupler (8).

[0045] See Figure 3 The light, maintained in TE0 mode, enters the lower waveguide C and is split into two beams by a 1×2 multimode interferometer (9). These beams are then transmitted through two separate waveguides: the first beam passes through waveguide D, the cross-shaped waveguide cross structure (4), waveguide E, and the first TE0-TE1 mode conversion structure (10). The first TE0-TE1 mode conversion structure (10) converts the TE0 mode to the TE1 mode, thus transmitting the light in TE1 mode into the upper arm waveguide of the Mach-Zehnder interferometer (5). Subsequently, it passes through the first TE1-TE0 mode conversion structure (11), which converts the TE1 mode in the upper arm waveguide of the Mach-Zehnder interferometer (5) to the TE0 mode. The light is transmitted through waveguide F' into the first input terminal of the 2×1 multimode interferometer (12); the second beam of light is transmitted through waveguide F and the second TE0-TE1 mode conversion structure (13), and the second TE0-TE1 mode conversion structure (13) converts the TE0 mode to the TE1 mode. Therefore, the light enters the lower arm waveguide of the Mach-Zehnder interferometer (5) in TE1 mode and is transmitted. Then, after passing through the second TE1-TE0 mode conversion structure (14), the second TE1-TE0 mode conversion structure (14) converts the TE1 mode in the lower arm waveguide of the Mach-Zehnder interferometer (5) to the TE0 mode. The light is transmitted through waveguide E' into the cross-shaped waveguide cross structure (6), and then connected to waveguide D' to enter the second input terminal of the 2×1 multimode interferometer (12). The output of the 2×1 multimode interferometer (12) is connected to the waveguide C' and transmitted into the first input of the polarization rotating beam combiner (7). The polarization rotating beam combiner (7) keeps the TE0 mode input at the first input as TE0 mode and outputs it through the second end face coupler (8).

[0046] An arbitrary input polarization state (including TE0 and TM0 modes) is first converted into two beams of light with the same polarization mode (TE0 mode) by a polarization beam splitter. Then, one beam, maintaining the TE0 mode, is input into the Mach-Zehnder interferometer, while the other beam is converted to the TE1 mode via a TE0-TE1 mode conversion structure and input into the Mach-Zehnder interferometer. Traveling wave electrodes achieve electro-optic modulation of the two optical modes (TE0 and TE1 modes) transmitted in the Mach-Zehnder interferometer, enabling polarization-independent modulation of any input polarization state on a lithium niobate thin film.

[0047] Optionally, the first TE0-TE1 mode conversion structure or the second TE0-TE1 mode conversion structure includes a 1×1 multimode interferometer directional coupler and a directional coupler connected in sequence, and the first TE1-TE0 mode conversion structure or the second TE1-TE0 mode conversion structure includes a directional coupler and a 1×1 multimode interferometer directional coupler connected in sequence.

[0048] See Figure 4 The TE0-TE1 mode conversion structure includes a first TE0-TE1 mode conversion structure (10) and a second TE0-TE1 mode conversion structure (13). The TE0-TE1 mode conversion structure specifically includes a 1×1 multimode interferometer directional coupler (1001 / 1301) and a directional coupler (1002 / 1302) connected in sequence. The TE1-TE0 mode conversion structure includes a first TE1-TE0 mode conversion structure (11) and a second TE1-TE0 mode conversion structure (14). The TE1-TE0 mode conversion structure specifically includes a directional coupler (1101 / 1401) and a 1×1 multimode interferometer directional coupler (1102 / 1402) connected in sequence. Wherein, Q represents the output terminal, P represents the input terminal, A1 represents the first input terminal of the first TE0-TE1 mode conversion structure (10), A2 represents the second input terminal of the first TE0-TE1 mode conversion structure (10), A3 represents the first input terminal of the second TE0-TE1 mode conversion structure (13), A4 represents the second input terminal of the second TE0-TE1 mode conversion structure (13), B1 represents the first output terminal of the first TE1-TE0 mode conversion structure (11), B2 represents the second output terminal of the first TE1-TE0 mode conversion structure (11), B3 represents the first output terminal of the second TE1-TE0 mode conversion structure (14), and B4 represents the second output terminal of the second TE1-TE0 mode conversion structure (14).

[0049] Optionally, the Mach-Zehnder interferometer includes a 1×2 multimode interferometer and a 2×1 multimode interferometer, which are connected by two optical waveguides.

[0050] See Figure 5The Mach-Zehnder interferometer (5) includes a 1×2 multimode interferometer (501) as an optical beam splitter and a 2×1 multimode interferometer (502) as an optical beam combiner. The 1×2 multimode interferometer (501) and the 2×1 multimode interferometer (502) are connected by two optical waveguides. The optical waveguides can support the transmission of TE0 and TE1 optical modes simultaneously. The upper waveguide arm (503) of the two optical waveguides overlaps with the lower waveguide of the directional coupler in the first TE0-TE1 mode conversion structure (10) and the first TE1-TE0 mode conversion structure (11). The lower waveguide arm (504) overlaps with the upper waveguide of the directional coupler in the second TE0-TE1 mode conversion structure (10) and the second TE1-TE0 mode conversion structure (11). The upper waveguide arm (503) and the lower waveguide arm (504) are symmetrical about the 1×2 multimode interferometer (501), the first TE0-TE1 mode conversion structure (10) and the second TE0-TE1 mode conversion structure (13) are symmetrical about the 1×2 multimode interferometer (501), and the first TE1-TE0 mode conversion structure (11) and the second TE1-TE0 mode conversion structure (14) are symmetrical about the 1×2 multimode interferometer (501).

[0051] See Figure 6 The cross-shaped waveguide cross structure (4) is used to connect waveguide A and waveguide B laterally and waveguide D and waveguide E longitudinally. The cross-shaped waveguide cross structure (6) is used to connect waveguide A' and waveguide B' laterally and waveguide D' and waveguide E' longitudinally.

[0052] Optionally, the first or second thermally adjustable phase shifter includes a resistor, a positive electrode, and a negative electrode.

[0053] See Figure 7 The thermally modulated phase shifter is used to adjust the phase bias point of the modulator. The first thermally modulated phase shifter (15) is located on the right side of the second TE1-TE0 mode conversion structure (14) and on the top right side of the lower arm waveguide (504) of the Mach-Zehnder interferometer. It includes a nickel-chromium resistor (1501) and positive metal electrodes (1502) and negative metal electrodes (1503) located on both sides of the nickel-chromium resistor. It is used to control the bias phase of the first mode (TE0 mode). The second thermally modulated phase shifter (16) is located on the top of the waveguide F'. It includes a nickel-chromium resistor (1601) and positive metal electrodes (1602) and negative metal electrodes (1603) located on both sides of the nickel-chromium resistor. It is used to control the bias phase of the second mode (TE1 mode).

[0054] The position of the first thermally modulated phase shifter (15) can be adjusted to the right of the first TE1-TE0 mode conversion structure (11) and the top right of the upper arm waveguide (503) of the Mach-Zehnder interferometer, the left of the first TE0-TE1 mode conversion structure (10) and the top left of the upper arm waveguide (503) of the Mach-Zehnder interferometer, the left of the second TE0-TE1 mode conversion structure (13) and the top left of the lower arm waveguide (504) of the Mach-Zehnder interferometer. The position of the second thermally modulated phase shifter (16) can be adjusted to the top of waveguide D', the top of waveguide E', the top of waveguide D, the top of waveguide E, or the top of waveguide F.

[0055] Optionally, the structure of the traveling wave electrode includes a GSG electrode structure, with both waveguides of the Mach-Zehnder interferometer positioned between the G traveling wave electrode and the S traveling wave electrode.

[0056] See Figure 8 The traveling wave electrode (17) adopts the GSG electrode structure, and one optical waveguide (503) in the Mach-Zehnder interferometer (5) is set between the G traveling wave electrode (1701) and the S traveling wave electrode (1702), and another optical waveguide (504) is set between the G traveling wave electrode (1703) and the S traveling wave electrode (1702).

[0057] Optionally, the traveling wave electrode adopts a T-type capacitively loaded periodic electrode structure.

[0058] See Figure 8 The traveling wave electrode (17) adopts a T-type capacitor-loaded periodic electrode structure.

[0059] Optionally, the first end-face coupler or the second end-face coupler includes a deeply etched waveguide portion and two shallowly etched waveguide portions on the upper and lower sides.

[0060] See Figure 9 The substrate layer includes a substrate (101) and a buried oxide layer (102) arranged sequentially from bottom to top. A thin film lithium niobate plate layer (103) is disposed above the buried oxide layer (102). A silicon dioxide layer (104) covers the thin film lithium niobate plate layer (103). A first thermally modulated phase shifter (15), a second thermally modulated phase shifter (16), and a traveling wave electrode (17) are disposed above the silicon dioxide layer (104).

[0061] See Figure 10 The first end coupler (2) supports dual-polarization optical coupling input and includes a deeply etched waveguide section (201) and two shallowly etched waveguide sections (202) on the upper and lower sides; the second end coupler (8) supports dual-polarization optical coupling input and includes two shallowly etched waveguide sections (801) on the upper and lower sides and a deeply etched waveguide section (802). Among them, the deeply etched waveguide section (201) is the same as (802), and the two shallowly etched waveguide sections (202) on the upper and lower sides are the same as (801).

[0062] Optionally, tapered optical fibers are provided at the input end of the first end-face coupler or the output end of the second end-face coupler.

[0063] See Figure 10 Tapered optical fibers are coupled to the input end of the first end face coupler (2) and the output end of the second end face coupler (8).

[0064] Optionally, the polarization rotating beam splitter includes an inverted conical mode converter, a directional coupler, and a 1×1 multimode interferometer connected in sequence; the polarization rotating beam combiner includes a 1×1 multimode interferometer, a directional coupler, and an inverted conical mode converter connected in sequence.

[0065] See Figure 11 The polarization rotating beam splitter (3) includes an inverted conical mode converter (301), a directional coupler (302), and a 1×1 multimode interferometer (303) connected in sequence; the polarization rotating beam combiner (7) includes a 1×1 multimode interferometer (703), a directional coupler (702), and an inverted conical mode converter (701) connected in sequence. The inverted conical mode converter (301) is the same as (701), the directional coupler (302) is the same as (702), and the 1×1 multimode interferometer (303) is the same as (703). B5 represents the first output terminal of the polarization rotating beam splitter (3), B6 ​​represents the second output terminal of the polarization rotating beam splitter (3), A5 represents the first input terminal of the polarization rotating beam combiner (7), and A6 represents the second input terminal of the polarization rotating beam combiner (7).

[0066] See Figure 12 This invention provides a method for fabricating a polarization-independent electro-optic modulator based on dual optical mode modulation, comprising:

[0067] S100. Fabricate a substrate layer and a thin-film lithium niobate planar layer. Fabricate a device waveguide structure on the thin-film lithium niobate planar layer. The device waveguide structure includes a first end-face coupler, a polarization rotating beam splitter, a first waveguide cross structure, a Mach-Zehnder interferometer, a second waveguide cross structure, a polarization rotating beam combiner, a second end-face coupler, a 1×2 multimode interferometer, a first TE0-TE1 mode conversion structure, a first TE1-TE0 mode conversion structure, a 2×1 multimode interferometer, a second TE0-TE1 mode conversion structure, and a second TE1-TE0 mode conversion structure.

[0068] S200. Clean the thin film lithium niobate plate layer with the device waveguide structure, grow a capping layer, coat it with photoresist, and photolithographically ...

[0069] S300, fabricate polished end faces to form a polarization-independent electro-optic modulator based on dual optical mode modulation.

[0070] In a specific embodiment, the fabrication process of the polarization-independent electro-optic modulator based on dual optical mode modulation is as follows:

[0071] S1. The upper waveguide of the device waveguide structure is fabricated using photolithography and etching techniques. The device waveguide structure includes a first end-face coupler (2) as the input waveguide, a polarization rotating beam splitter (3), a first waveguide cross structure (4), a Mach-Zehnder interferometer (5), a second waveguide cross structure (6), a polarization rotating beam combiner (7), a second end-face coupler (8), a 1×2 multimode interferometer (9), a first TE0-TE1 mode conversion structure (10), a first TE1-TE0 mode conversion structure (11), a 2×1 multimode interferometer (12), a second TE0-TE1 mode conversion structure (13), and a second TE1-TE0 mode conversion structure (14).

[0072] S2. Photolithography is used to fabricate the lower waveguides of the first end-face coupler (2) and the second end-face coupler (8) on the sample obtained in step S1.

[0073] S3. Clean the sample obtained in step S2 to remove residual photoresist and mask;

[0074] S4. Grow a capping layer on the sample obtained in step S3 at high temperature;

[0075] S5. Spin coat the sample obtained in step S4 with photoresist, and then photolithographically prepare the mask for the nickel-chromium resistor (1501) in the first thermally modulated phase shifter (15) and the nickel-chromium resistor (1601) in the second thermally modulated phase shifter (16) on the photoresist.

[0076] S6. Electron beam evaporation is used to deposit a nickel-chromium layer on the sample obtained in step S5.

[0077] S7. Spin coat the sample obtained in step S6 with photoresist, and photolithographically prepare the masks for electrodes (1502)(1503) in the first thermally modulated phase shifter (15), electrodes (1602)(1603) in the first thermally modulated phase shifter (16), and traveling wave electrode (17) on the photoresist.

[0078] S8. Apply an adhesion layer and a metal electrode to the sample obtained in step S7 using electron beam evaporation, and then remove excess metal using a metal stripping process.

[0079] S9. The sample obtained in step S8 is polished by grinding and polishing to form a complete polarization-independent electro-optic modulator.

[0080] The implementation of this invention includes the following beneficial effects: In this embodiment, arbitrary polarized input light enters from the first end-face coupler and is converted into two beams of the same mode by a polarization rotating beam splitter. One mode of light is converted into another mode by the first TE0-TE1 mode conversion structure and the second TE0-TE1 mode conversion structure. The two different modes of light are transmitted in a Mach-Zehnder interferometer and modulated by a set of traveling-wave electrodes. The modulated transmitted light is then converted again by the first TE1-TE0 mode conversion structure and the second TE1-TE0 mode conversion structure to restore the original light mode. The two modes of input light are combined by a polarization rotating beam combiner and output through the second end-face coupler. That is, the polarization-independent electro-optic modulator uses a Mach-Zehnder interferometer, a set of traveling-wave electrodes to modulate the two optical modes, and then polarizes and combines the beams. While maintaining high modulation efficiency, the overall structure is also very compact.

[0081] The above is a detailed description of the preferred embodiments of the present invention. However, the present invention is not limited to the embodiments described. Those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of the present invention. All such equivalent modifications or substitutions are included within the scope defined by the claims of this application.

Claims

1. A polarization-independent electro-optic modulator based on dual optical mode modulation, characterized in that, The system includes a substrate layer, a thin-film lithium niobate plate layer disposed on top of the substrate layer, and a first end-face coupler, a polarization rotating beam splitter, a first waveguide cross structure, a Mach-Zehnder interferometer, a second waveguide cross structure, a polarization rotating beam combiner, a second end-face coupler, a 1×2 multimode interferometer, a first TE0-TE1 mode conversion structure, a first TE1-TE0 mode conversion structure, a 2×1 multimode interferometer, a second TE0-TE1 mode conversion structure, a second TE1-TE0 mode conversion structure, a first thermally modulated phase shifter, a second thermally modulated phase shifter, and a traveling wave electrode disposed on the thin-film lithium niobate plate layer. The output end of the first end-face coupler is connected to the input end of the polarization rotating beam splitter, and the output end of the polarization rotating beam combiner is connected to the input end of the second end-face coupler. The first output terminal of the polarization rotating beam splitter is connected to the input terminal of the Mach-Zehnder interferometer through a first waveguide cross structure, and the output terminal of the Mach-Zehnder interferometer is connected to the second input terminal of the polarization rotating beam combiner through a second waveguide cross structure. The second output of the polarization rotating beam splitter is connected to the input of a 1×2 multimode interferometer. The first output of the 1×2 multimode interferometer is connected to the first input of the first TE0-TE1 mode conversion structure through a first waveguide cross structure. The second output of the 1×2 multimode interferometer is connected to the second input of the second TE0-TE1 mode conversion structure. The second input and output of the first TE0-TE1 mode conversion structure are both connected to the upper arm waveguide of the Mach-Zehnder interferometer. The first input and output of the second TE0-TE1 mode conversion structure are both connected to the lower arm waveguide of the Mach-Zehnder interferometer. The first output of the first TE1-TE0 mode conversion structure is connected to the first input of a 2×1 multimode interferometer. The second output of the second TE1-TE0 mode conversion structure is connected to the second input of the 2×1 multimode interferometer through a second waveguide cross structure. The output of the 2×1 multimode interferometer is connected to the first input of the polarization rotating beam combiner. The first thermally adjustable phase shifter is used to control the bias phase of the first mode, and the second thermally adjustable phase shifter is used to control the bias phase of the second mode. The traveling wave electrode acts on the Mach-Zehnder interferometer.

2. The electro-optic modulator according to claim 1, characterized in that, The first TE0-TE1 mode conversion structure or the second TE0-TE1 mode conversion structure includes a 1×1 multimode interferometer directional coupler and a directional coupler connected in sequence. The first TE1-TE0 mode conversion structure or the second TE1-TE0 mode conversion structure includes a directional coupler and a 1×1 multimode interferometer directional coupler connected in sequence.

3. The electro-optic modulator according to claim 1, characterized in that, The Mach-Zehnder interferometer includes a 1×2 multimode interferometer and a 2×1 multimode interferometer, which are connected by two optical waveguides.

4. The electro-optic modulator according to claim 1, characterized in that, The first or second thermally adjustable phase shifter includes a resistor, a positive electrode, and a negative electrode.

5. The electro-optic modulator according to claim 1, characterized in that, The structure of the traveling wave electrode includes a GSG electrode structure, and both waveguides of the Mach-Zehnder interferometer are disposed between the G traveling wave electrode and the S traveling wave electrode.

6. The electro-optic modulator according to claim 1, characterized in that, The traveling wave electrode adopts a T-type capacitively loaded periodic electrode structure.

7. The electro-optic modulator according to claim 1, characterized in that, The first end-face coupler or the second end-face coupler includes a deeply etched waveguide portion and two shallowly etched waveguide portions on the top and bottom.

8. The electro-optic modulator according to claim 1, characterized in that, Tapered optical fibers are provided at the input end of the first end face coupler or the output end of the second end face coupler.

9. The electro-optic modulator according to claim 1, characterized in that, The polarization rotating beam splitter includes an inverted conical mode converter, a directional coupler, and a 1×1 multimode interferometer connected in sequence; the polarization rotating beam combiner includes a 1×1 multimode interferometer, a directional coupler, and an inverted conical mode converter connected in sequence.

10. A method for fabricating a polarization-independent electro-optic modulator based on dual optical mode modulation, characterized in that, include: A substrate layer and a thin-film lithium niobate planar layer are prepared. A device waveguide structure is prepared on the thin-film lithium niobate planar layer. The device waveguide structure includes a first end-face coupler, a polarization rotating beam splitter, a first waveguide cross structure, a Mach-Zehnder interferometer, a second waveguide cross structure, a polarization rotating beam combiner, a second end-face coupler, a 1×2 multimode interferometer, a first TE0-TE1 mode conversion structure, a first TE1-TE0 mode conversion structure, a 2×1 multimode interferometer, a second TE0-TE1 mode conversion structure, and a second TE1-TE0 mode conversion structure. The thin-film lithium niobate plate layer with the device waveguide structure is cleaned, a capping layer is grown, photoresist is coated, and photolithography is performed on the photoresist to prepare the first thermally modulated phase shifter, the second thermally modulated phase shifter, and the traveling wave electrode. A polished end face is fabricated to form a polarization-independent electro-optic modulator based on dual optical mode modulation.

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