Mach-Zehnder modulator based on traveling wave electrode and lithium niobate waveguide structure

Abstract

The invention belongs to the technical field of photoelectric modulators, and particularly relates to a Mach-Zehnder modulator based on a traveling wave electrode and a lithium niobate waveguide structure. The modulator specifically comprises a multimode interference beam splitter / combiner, a push-pull traveling wave electrode, a film lithium niobate waveguide and the like. The multi-mode interference beam splitter / combiner is composed of three single-mode waveguides and a multi-mode waveguide and used for beam combination and beam splitting of light waves, traveling wave electrodes are of a grounding-electrode-grounding push-pull structure, the refractive index of the light waves in two phase modulation arms can be adjusted at the same time, and therefore the modulation voltage length product of the modulator is effectively reduced. The multimode interference beam splitter and combiner is connected with other components through a phase modulation arm, and the phase modulation arm changes the refractive index of light waves in the lithium niobate waveguide under the action of voltage, so that the phases of the light waves are changed. The modulator is low in electric signal attenuation amplitude, large in electric bandwidth, high in transmission speed and large in electro-optical response bandwidth, the thin-film lithium niobate waveguide is low in optical loss and small in overall size, and high integration and interconnection are easy to achieve.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical modulators, and particularly relates to a Mach-Zehnder modulator Background Art

[0002] With the continuous development of technology, optical communication technology has become an indispensable and important part of modern information society. Due to the significant advantages of optical communication systems in terms of high speed, long distance, low loss, etc., it has broad application prospects in fields such as data transmission and signal processing. As a core device in optical communication systems, the performance of electro-optic modulators directly determines the transmission efficiency and stability of the entire system. Therefore, the research and development of high-performance electro-optic modulators, especially Mach-Zehnder (MZ) modulators based on thin-film lithium niobate (LithiμmNiobate, LN) technology, have become a research hotspot in the current optical communication field

[0003] Lithium niobate materials are widely used in optoelectronic devices such as optical modulators, optical frequency combs, and optical waveguides due to their excellent electro-optic, nonlinear optical, and piezoelectric properties. Although traditional bulk lithium niobate modulators have stable and reliable performance, due to their waveguide mode limitations, complex processing technology, large size, high power consumption, etc., it is difficult to meet the requirements of modern optical communication for high speed, low power consumption, and miniaturization. In recent years, the rapid development of thin-film lithium niobate technology has provided a new solution for the miniaturization and high performance of electro-optic modulators. Thin-film lithium niobate materials not only inherit the excellent electro-optic performance of bulk lithium niobate, but also due to the introduction of thin-film technology, the modulator can achieve a more compact structure, lower half-wave voltage (Vπ), and higher modulation bandwidth. By combining thin-film lithium niobate waveguides with traveling-wave electrodes to design a Mach-Zehnder modulator, by optimizing the waveguide structure, electrode layout, and speed matching between microwaves and light waves, the modulation efficiency can be significantly improved, the insertion loss can be reduced, and it has stronger compatibility in photonic integrated circuits (PICs). Compared with traditional silicon-based waveguides, thin-film lithium niobate waveguides have a smaller refractive index difference and better mode matching ability, reducing optical loss and improving the overall performance of the device. At the same time, the excellent physical properties of thin-film lithium niobate, such as high Curie temperature, low optical absorption loss, etc., enable it to maintain stable working performance in harsh environments

[0004] In recent years, researchers at home and abroad have made many breakthroughs in thin-film lithium niobate Mach-Zehnder modulators. For example, ultraviolet lithography technology is used to achieve a high-precision waveguide structure, enabling the modulator to balance process controllability while maintaining low loss; the T-shaped traveling-wave electrode structure is adopted to improve the transmission efficiency of microwave signals and increase the modulation bandwidth of the device; a new type of SiO2 substrate and buffer layer design is introduced to achieve good impedance matching, further reducing the VπL (half-wave voltage-length product) and meeting the requirements of large-scale optical communication systems. Although the thin-film lithium niobate MZ modulator has significantly improved performance compared to traditional bulk materials, it still faces many challenges. For example, at high-frequency operation, the microwave loss and optical loss of the modulator still need to be further reduced; the 6.41 dB bandwidth of the electrical signal and the 3 dB bandwidth of the electro-optic response still need to be further expanded; how to optimize the coupling between the optical waveguide and the electrode, reduce the optical scattering loss, and improve the overall stability of the device; in addition, the process stability and cost control issues in large-scale manufacturing are also the focus of current research. Summary of the Invention

[0005] The purpose of the present invention is to provide a Mach-Zehnder modulator based on a traveling-wave electrode and a lithium niobate waveguide structure with low electrical signal attenuation amplitude, fast transmission speed, and large electro-optic response bandwidth to meet the development needs of highly integrated communication systems.

[0006] The Mach-Zehnder modulator based on a traveling-wave electrode and a lithium niobate waveguide structure provided by the present invention has a structure as shown in Figure 1 shown; specifically, it is composed of a multimode interference splitter / combiner connected to the phase modulation arm; where:

[0007] The splitter / combiner is divided into two parts with the same structure. A single device is sequentially composed of a single-mode trapezoidal silicon waveguide 1, a multimode interference rectangular silicon waveguide 2, and two identical and juxtaposed single-mode trapezoidal silicon waveguides 3, and the three have the same thickness; the two parts are symmetrically arranged above and below on both sides of the phase modulation arm; the upper part is a splitter with an entrance, and the lower part is a combiner with an exit;

[0008] The phase modulation arm is composed of a traveling-wave electrode 4, two trapezoidal lithium niobate ridge waveguides 6, two ground electrodes 5, a lithium niobate substrate 7, and a silicon dioxide substrate 8. The lithium niobate substrate 7 and the silicon dioxide substrate 8 are attached to each other up and down and have matching sizes; the traveling-wave electrode 4 is disposed along the phase modulation direction in the middle of the surface of the lithium niobate substrate 7. Two trapezoidal lithium niobate ridge waveguides 6 are symmetrically distributed on both sides of it with the center line of the traveling-wave electrode 4 as the axis of symmetry and at equal distances. The two ground electrodes 5 are respectively located at equal distances outside the corresponding trapezoidal lithium niobate ridge waveguides 6 and are on the same conductive layer as the traveling-wave electrode 4, and the three form a coplanar electrode structure; the extended widths of the traveling-wave electrode 4, the two trapezoidal lithium niobate ridge waveguides 6, and the two ground electrodes 5 are the same as the width of the lithium niobate substrate 7, so as to ensure that the effective lengths of the phase modulation regions are the same and achieve stable electro-optical modulation performance.

[0009] The traveling wave electrode 4 and the two ground electrodes 5 form a push-pull traveling wave electrode structure, that is, a design method of ground - electrical signal - ground.

[0010] The connection modes of the beam splitter, the combiner and the phase modulation arm are as follows: The two trapezoidal silicon waveguides 3 of the beam splitter are respectively connected to the upper ends of the two trapezoidal lithium niobate ridge waveguides 6 through the corresponding two strip silicon waveguides 9; The two trapezoidal silicon waveguides 3 of the combiner are respectively connected to the lower ends of the two trapezoidal lithium niobate ridge waveguides 6 through another two strip silicon waveguides 9; The structures and dimensions of the strip silicon waveguides 9 match the structures and dimensions of the trapezoidal silicon waveguides 3 and the trapezoidal lithium niobate ridge waveguides 6 they are connected to respectively.

[0011] In the phase modulation arm, the width of the traveling wave electrode 4 is denoted as d, and the distance between the traveling wave electrode 4 and the ground electrode 5 is denoted as h. As the width d of the electrode increases, the RF loss shows a decreasing trend, which is caused by the decrease in ohmic loss due to the increase in width. d is generally selected as 5 - 20 μm; The RF loss decreases as the electrode distance h increases because the resistivity between the electrodes decreases with the increase in distance; h is generally selected as 10 - 30 μm.

[0012] Furthermore:

[0013] In the multimode interference beam splitter / combiner:

[0014] The trapezoidal silicon waveguide 1 is the incident waveguide, with a thickness of 0.20 - 0.24 μm, a length of 2.8 - 3.2 μm, a lower bottom width of 0.487 μm, an upper bottom width of 0.974 μm, and the angle between the waist side and the central axis is 80 - 85°.

[0015] The multimode interference rectangular silicon waveguide 2 has a length of 3 - 4 μm, a width of 1.5 - 2.5 μm, and a thickness of 0.2 - 0.24 μm.

[0016] The two trapezoidal silicon waveguides 3 are the output waveguides, with a thickness of 0.20 - 0.24 μm, a length of 1.5 - 2.5 μm, a lower bottom width of 0.4 μm, an upper bottom width of 0.971 μm, and the angle between the waist side and the central axis is 78 - 82°.

[0017] In the phase modulation arm structure:

[0018] The traveling wave electrode 4 has a width of 5 - 10 μm, a length of 5000 - 8000 μm, and a thickness of 0.8 - 1.5 μm;

[0019] The ground electrode 5 has a width of 100 - 200 μm, a length of 5000 - 8000 μm, and a thickness of 0.8 - 1.5 μm;

[0020] The distance between the traveling-wave electrode 4 and the ground electrode 5 is h: 10 - 30 μm;

[0021] The trapezoidal lithium niobate ridge waveguide 6 has a trapezoidal cross-section and is an integrated structure with the lithium niobate waveguide substrate 7 above and below. The length of the trapezoidal lithium niobate ridge waveguide 6 is 5000 - 8000 μm; its upper base width is 1.4 - 1.6 μm, height is 0.3 - 0.4 μm, the angle between the waist side and the central axis is 65 - 75 degrees, and the trapezoidal lithium niobate ridge waveguide 6 is obtained by etching.

[0022] The lithium niobate waveguide substrate 7 has a cuboid structure, with a length of 500 - 1000 μm, a width of 5000 - 8000 μm, and a thickness of 0.4 - 0.6 μm.

[0023] The silica substrate 8 has a cuboid structure, with the same length and width as the lithium niobate waveguide substrate 7 and a thickness of 2 - 3 μm.

[0024] The lithium niobate waveguide 7 is made of x-cut anisotropic lithium niobate material, and the relative dielectric constants in the x, y, and z directions are 27.9, 44.3, and 44.3.

[0025] The modulation process of this modulator is as follows:

[0026] When a beam of light is input from port 1 into the multimode interference beam splitter, after splitting, two light waves with the same frequency, the same phase, and the same amplitude are respectively input into two thin-film lithium niobate waveguides. The lithium niobate waveguide between the two electrodes serves as the phase modulation arm. Under the action of the modulation voltage at both ends, the refractive index of the lithium niobate material changes, thereby causing a change in the phase of the passing light wave. The other waveguide is subjected to the opposite voltage. Therefore, the phase changes of the two light waves are in opposite directions. The two light waves finally converge at the combiner and are output from output port 2. The push-pull structure used makes the phases of the two light waves change in opposite directions. When the phase difference reaches π, the two light waves cancel each other out when they converge.

[0027] The technical features and functional advantages of the present invention are mainly as follows:

[0028] The present invention creatively designs an electrode width and electrode spacing, greatly increasing the bandwidth of the microwave electrical signal and greatly reducing the loss, thereby increasing the electro-optic response bandwidth. By optimizing the beam splitter and combiner structures and introducing a trapezoidal structure, the loss caused by mode matching is smaller, and the insertion loss of the optical signal is greatly reduced (the insertion loss is only 0.45 dB), thereby improving the extinction ratio; using a lithium niobate waveguide, it has better electro-optic characteristics. Compared with a silicon-based Mach-Zehnder modulator, it has more excellent performance, a smaller size, an extinction ratio of 36.75 dB, and good modulation performance, and can meet the development needs of optical information transmission and processing systems. Description of the Drawings

[0029] Figure 1 Top view of the Mach-Zehnder modulator structure based on traveling-wave electrodes and lithium niobate waveguide structure according to the present invention.

[0030] Figure 2 Cross-sectional view of the Mach-Zehnder modulator structure based on traveling-wave electrodes and lithium niobate waveguide structure according to the present invention.

[0031] Figure 3 Transmission spectrum of the modulator in the "on" state with a modulation voltage of 0 V at a working wavelength of 1.5 μm to 1.6 μm.

[0032] Figure 4 Transmission spectrum of the modulator in the "off" state with a modulation voltage of 3.6 V at a working wavelength of 1.5 μm to 1.6 μm.

[0033] Figure 5 Attenuation of the electrical signal with frequency after a microwave electrical signal is applied to the traveling-wave electrode when the width of the phase modulation arm is 5000 μm, the length of the traveling-wave electrode is 7 μm, and the distance between the traveling-wave electrode and the ground electrode is 16 μm.

[0034] Figure 6 Steady-state field distribution diagram of the splitter / combiner at a working wavelength of 1.5 μm to 1.6 μm.

[0035] Reference numerals in the figure: 1 is a single-mode waveguide, 2 is a multimode waveguide, 3 is a single-mode waveguide, 4 is a traveling-wave electrode, 5 is a ground electrode, 6 is a lithium niobate waveguide (etching layer), 7 is a lithium niobate substrate, and 8 is a silica substrate. Detailed implementation mode

[0036] The present invention will be further described in detail below with reference to the accompanying drawings and specific implementation modes. However, it should be understood that the protection scope of the present invention is not limited by the specific implementation modes.

[0037] As Figure 1 、 Figure 2 shown, the Mach-Zehnder modulator based on traveling-wave electrodes and lithium niobate waveguide structure according to the present invention mainly includes a multimode interference splitter / combiner module (1, 2, 3), a push-pull traveling-wave electrode module (traveling-wave electrode 4, ground electrode 5), a thin-film lithium niobate waveguide module (trapezoidal lithium niobate ridge waveguide 6, lithium niobate waveguide substrate 7), a silica substrate module 8, and a waveguide 9 connecting the splitter / combiner and the phase modulation arm. The multimode interference splitter / combiner is composed of three single-mode waveguides (1, 3) and a multimode waveguide 2. Among them, the traveling-wave electrode adopts a push-pull structure of ground-electrode-ground. The traveling-wave electrode, thin-film lithium niobate waveguide, and silica substrate are stacked together from top to bottom to form a phase modulation arm, and the multimode interference splitter / combiner is connected through the phase modulation arm.

[0038] The specific parameters are as follows: The input waveguide 1 of the multimode interference splitter / combiner module is composed of a silicon waveguide. The thickness of the input silicon waveguide is 0.22 μm, the length is 2.94 μm, and the waveguide width gradually increases from 0.487 μm to 0.974 μm, presenting a tapered structure with a taper of 83.05°. The output waveguide of the multimode interference splitter / combiner module is composed of 2 identical silicon waveguides. The thickness of a single output silicon waveguide is 0.22 μm, the length is 2 μm, and the waveguide width gradually increases from 0.4 μm to 0.971 μm, presenting a tapered structure with a taper of 81.875°. The multimode interference region 2 of the multimode interference splitter / combiner module is composed of a rectangular silicon material with a length of 3.39 μm, a width of 1.942 μm, and a thickness of 0.22 μm. The phase modulation arm adopts a traveling-wave electrode structure, which successively includes a silica substrate 8, a lithium niobate thin-film substrate 7, a trapezoidal lithium niobate ridge waveguide 6, and a metal traveling-wave electrode layer (traveling-wave electrode 4 and symmetric ground electrode 5) from bottom to top. The traveling-wave electrode 4 is disposed on the surface of the lithium niobate thin film along the light propagation direction, and the two ridge waveguides 6 are equidistantly distributed with their centerlines as the symmetry axes; the ground electrodes 5 are respectively located outside the corresponding ridge waveguides. The specific parameters are as follows: Among them, the silica substrate has a thickness of 2 μm, a width of 5000 μm, and a length of 500 μm, which plays a role of mechanical support and optical isolation; the lithium niobate thin-film substrate has a thickness of 0.5 μm, and its transverse and longitudinal dimensions are the same as those of the silica substrate below. Two trapezoidal ridge waveguides are formed by etching on the thin-film surface. The ridge height is 0.3 μm, the upper base width is 1.6 μm, and the sidewall inclination angle is about 70°, which are symmetrically arranged on both sides of the centerline of the traveling-wave electrode, so that the total optical thickness of the ridge waveguide and the thin film reaches 0.8 μm. The metal thicknesses of the topmost traveling-wave electrode and the two ground electrodes are both 0.89 μm.

[0039] The lithium niobate waveguide is composed of an x-cut anisotropic lithium niobate material, and the relative dielectric constants in the x, y, and z directions are 27.9, 44.3, and 44.3. The refractive index of silicon is 3.48 near the wavelength of 1550 nm, and the refractive index of silica is 1.44.

[0040] The modulation method of the modulator is as follows: When a beam of light (1550 nm) is input from port 1 to the multimode interference beam splitter, the insertion loss of the beam splitter is 0.45 dB. After splitting, it is respectively input into two lithium niobate waveguides. The lithium niobate waveguide between the two electrodes serves as the phase modulation arm. Under the action of the modulation voltage at both ends, its refractive index changes, thereby causing a change in the phase of the passing light wave. The push-pull structure changes the phase of both beams of light at the same time. The two beams of light finally converge in the multimode interference combiner and are output from port 2. If the phase difference between the two light beams is 0, the two lights are added together, as Figure 3 shown. At this time, the output loss of the modulator is 0.4 dB, showing a "through" state; if the phase difference between the two light beams is π, the two lights cancel each other out, showing a "cut-off" state, as Figure 4As shown, the optical intensity loss at the output end of the modulator is 37.15 dB, and the extinction ratio is 36.75 dB at this time. By controlling the on and off of the light wave, signal modulation is achieved. Figure 5 It is a graph of the relationship between the electrical bandwidth and frequency of the modulator. When the microwave refractive index matches the optical wave group refractive index, the electrical bandwidth of -6.41 dB corresponds to -3 dB of the electro-optic response. Figure 6 It is the mode field diagram of the multimode interference beam splitter. A light wave is split into two light waves with exactly the same phase and intensity, and the overall loss is as low as 0.45 dB.

[0041] The above are only the preferred embodiments of the present invention, and the present invention is not limited to the content of the embodiments. For those skilled in the art, various changes and modifications can be made within the scope of the technical solution of the present invention, and any changes and modifications made are within the protection scope of the present invention.

Claims

1. A Mach-Zehnder modulator based on a traveling-wave electrode and a lithium niobate waveguide structure, characterized in that Its structure is composed of a multimode interference splitter / combiner connected to a phase modulation arm; among which: The splitter / combiner is divided into two parts with the same structure. A single device is composed of a single-mode trapezoidal silicon waveguide (1), a multimode interference rectangular silicon waveguide (2), and two identical and juxtaposed single-mode trapezoidal silicon waveguides (3) connected in sequence. The thicknesses of the three are the same. The two parts are symmetrically arranged above and below on both sides of the phase modulation arm. The upper part is a splitter with an input port, and the lower part is a combiner with an output port; The structure of the phase modulation arm is composed of a traveling-wave electrode (4), two lithium niobate waveguides (6), two ground electrodes (5), a lithium niobate substrate (7), and a silica substrate (8); among which, the lithium niobate substrate (7) and the silica substrate (8) are laminated on top of each other with matching sizes. The traveling-wave electrode (4) is arranged along the phase modulation direction in the middle of the surface of the lithium niobate substrate (7). The two trapezoidal lithium niobate waveguides (6) are symmetrically distributed on both sides of it with the center line of the traveling-wave electrode (4) as the axis of symmetry and at equal distances. The two ground electrodes (5) are located at equal distances outside the corresponding trapezoidal lithium niobate ridge waveguides (6) and are on the same conductive layer as the traveling-wave electrode (4). The three form a coplanar electrode structure. The widths of the traveling-wave electrode (4), the two trapezoidal lithium niobate ridge waveguides (6), and the two ground electrodes (5) are the same as the width of the lithium niobate substrate (7), so as to ensure that the effective length of the phase modulation region is the same and achieve stable electro-optical modulation performance; The signal electrode (4) and the two ground electrodes (5) form a push-pull traveling-wave electrode structure, that is, a design method of ground - electrical signal - ground; The connection method between the splitter, the combiner and the phase modulation arm is: the two trapezoidal silicon waveguides (3) of the splitter are respectively connected to the upper ends of the two lithium niobate waveguides (6) through the corresponding two strip silicon waveguides (9); the two trapezoidal silicon waveguides (3) of the combiner are respectively connected to the lower ends of the two lithium niobate waveguides (6) through the other two strip silicon waveguides (9); the structures and dimensions of the strip silicon waveguides (9) match the structures and dimensions of the trapezoidal silicon waveguides (3) and the lithium niobate waveguides (6) they are connected to respectively.

2. The Mach-Zehnder modulator according to claim 1, characterized in that, In the multimode interference splitter / combiner: The trapezoidal silicon waveguide (1) is an incident waveguide, with a thickness of 0.20 - 0.24 μm, a length of 2.8 - 3.2 μm, a lower bottom width of 0.487 μm, an upper bottom width of 0.974 μm, and an angle between the waist side and the central axis of 80 - 85°; The multimode interference rectangular silicon waveguide (2) has a length of 3 - 4 μm, a width of 1.5 - 2.5 μm, and a thickness of 0.2 - 0.24 μm; The two trapezoidal silicon waveguides (3) are output waveguides, with a thickness of 0.20 - 0.24 μm, a length of 1.5 - 2.5 μm, a lower bottom width of 0.4 μm, an upper bottom width of 0.971 μm, and an angle between the waist side and the central axis of 78 - 82°; 3. The Mach-Zehnder modulator according to claim 2, wherein, In the structure of the phase modulation arm: The signal electrode (4) has a width of 5 - 10 μm, a length of 5000 - 8000 μm; and a thickness of 0.8 - 1.5 μm; The grounding electrode (5) has a width of 100 - 200 μm, a length of 5000 - 8000 μm, and a thickness of 0.8 - 1.5 μm; The distance between the traveling - wave electrode (4) and the grounding electrode (5) is h: 10 - 30 μm; The lithium niobate waveguide (6) has a trapezoidal cross - sectional structure. The lithium niobate waveguide (6) and the lithium niobate waveguide substrate (7) are an integrated structure from top to bottom. The length of the lithium niobate waveguide (6) is 5000 - 8000 μm; its upper - base width is 1.4 - 1.6 μm, its height is 0.3 - 0.4 μm, the angle between the waist side and the central axis is 65 - 75 degrees, and the trapezoidal lithium niobate waveguide (6) is obtained by etching; The lithium niobate waveguide substrate (7) has a cuboid structure, with a length of 500 - 1000 μm, a width of 5000 - 8000 μm, and a thickness of 0.4 - 0.6 μm; The silica substrate (8) has a cuboid structure, with its length and width being the same as those of the lithium niobate waveguide substrate (7), and a thickness of 2 - 3 μm.

4. The Mach-Zehnder modulator according to claim 2, wherein, The lithium niobate waveguide (7) is made of x - cut anisotropic lithium niobate material, and the relative dielectric constants in the x, y, and z directions are 27.9, 44.3, and 44.

3.

5. The Mach-Zehnder modulator according to any one of claims 1-3, characterized in that, The modulation process is as follows: When a beam of light is input from the input port into the multimode interference beam splitter, after splitting, two light waves with the same frequency, the same phase, and the same amplitude are respectively input into two thin - film lithium niobate waveguides. The lithium niobate waveguide between the two electrodes serves as the phase - modulation arm. Under the action of the modulation voltage at both ends, the refractive index of the lithium niobate material changes, thereby causing a change in the phase of the passing light wave. The other waveguide is subjected to the opposite voltage. Therefore, the directions of the phase changes of the two light waves are opposite. The two light waves finally converge at the combiner and are output from the output port. The push - pull structure adopted enables the phases of the two light waves to change in opposite directions. When the phase difference reaches π, the two light waves cancel each other out when they converge.

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