A broadband thin film lithium niobate electro-optic modulator

Abstract

The application discloses a broadband thin-film lithium niobate electro-optic modulator and belongs to the technical field of integrated optoelectronic devices. The device comprises, from bottom to top, a silicon substrate, a silicon oxide buffer layer, a thin-film lithium niobate optical waveguide layer, a silicon oxide cladding layer, and a metal modulation electrode. The optical waveguide layer is integrated with a Mach-Zehnder structure, and the metal modulation electrode comprises a middle traveling wave signal electrode and two traveling wave ground electrodes. In view of the defects that the silicon substrate has high-frequency microwave loss, conventional electrodes are difficult to realize optical-microwave speed matching, and traditional capacitive periodic loading structures rely on increasing capacitance to speed up microwave refractive index and have serious high-frequency loss deterioration, the application sets a periodic S-shaped bending slow wave structure on the traveling wave electrode, realizes low loss and optical-microwave speed matching, and the 3dB electro-optic bandwidth of the modulator can reach 150GHz, and the modulator has the advantages of CMOS process compatibility and low high-frequency loss.

Description

Technical Field

[0001] This invention relates to the field of integrated optoelectronic device technology, and more specifically to a broadband slow-light thin-film lithium niobate electro-optic modulator based on a silicon substrate and an apodized waveguide structure. Background Technology

[0002] With the rapid development of big data, cloud computing, and artificial intelligence, the demand for transmission rates in optical communication networks is growing exponentially. Electro-optic modulators, as core devices that convert electrical signals into optical signals, directly determine the upper limit of communication systems through their bandwidth. Thin-film lithium niobate (TFLN) platforms, due to their high electro-optic coefficient, compact mode field confinement, and excellent integration potential, have become the preferred material for next-generation high-performance modulators.

[0003] However, traditional traveling-wave electrode thin-film lithium niobate modulators face severe challenges of "velocity mismatch" and "microwave loss." To obtain high modulation bandwidth, the phase velocity of the microwave signal must be kept consistent with the group velocity of the optical signal. In conventional designs, the group refractive index (n...)... g The value is typically around 2.2. However, to achieve ultra-wideband matching, very stable electro-optic matching is often required at high frequencies; otherwise, electro-optic phase velocity mismatch will severely degrade the electro-optic matching bandwidth.

[0004] Therefore, there is an urgent need for a new type of modulator structure that can simultaneously solve the problem of limited modulation bandwidth, achieve precise speed matching on silicon substrates, and have low loss characteristics. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a broadband thin-film lithium niobate electro-optic modulator, aiming to solve the problems of narrow bandwidth, high high-frequency dielectric loss, and speed mismatch between microwave and slow light in existing electro-optic modulators, and to achieve an ultra-large electro-optic bandwidth of over 150 GHz.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A broadband thin-film lithium niobate electro-optic modulator includes, from bottom to top, a substrate, a silicon oxide buffer layer, a thin-film lithium niobate (TFLN) optical waveguide layer, a cladding layer, and a metal modulation electrode.

[0008] The characteristic feature is that the substrate is a silicon-based substrate;

[0009] The thin-film lithium niobate optical waveguide layer integrates a Mach-Zehnder structure for optical signal transmission and interference modulation. The Mach-Zehnder structure includes two optical waveguide arms, which have a periodic S-shaped bending structure along the optical transmission direction. The two optical waveguide arms have the same structure and are arranged in a mirror symmetrical manner.

[0010] The metal modulation electrode is used to apply microwave modulation signals, including a traveling wave signal electrode located between the two optical waveguide arms of the Mach-Zehnder structure, and two traveling wave grounding electrodes respectively disposed on the outer sides of the two optical waveguide arms.

[0011] The traveling wave signal electrode and the traveling wave ground electrode have the same periodic structure as the optical waveguide arm at their adjacent edges, so that the distance between the electrode and the corresponding optical waveguide arm remains consistent. By setting a smoothly bent S-shaped electrode structure, the microwave transmission path length is effectively increased, the electric field distribution is optimized and the surge in high-frequency parasitic capacitance is suppressed, and the refractive index velocity of the optical waveguide is matched with that of the optical waveguide, thereby achieving ultra-wideband electro-optic modulation performance.

[0012] Preferably, the S-shaped bending structure is a cosine bend, a circular arc bend, or a broken line bend.

[0013] Preferably, the period of the S-shaped bending structure is in the range of 30-60 μm.

[0014] Preferably, when the S-shaped bending structure is a zigzag bend, in each cycle, the optical waveguide arm first bends outward obliquely, then extends horizontally, and then bends inward obliquely to form an isosceles trapezoidal zigzag protrusion.

[0015] Preferably, when the S-shaped bending structure is a cosine bend or a circular arc bend, the minimum bending radius ranges from 60 to 100 μm.

[0016] Preferably, the height of the isosceles trapezoid ranges from 5 to 10 μm, the length of the upper base ranges from 10 to 20 μm, and the length of the lower base ranges from 20 to 50 μm.

[0017] Preferably, the distance between the traveling wave signal electrode and the traveling wave ground electrode is 3-6 μm; the distance between the optical waveguide arm and the electrode is 0.5-1 μm.

[0018] This invention selects silicon as the substrate material. Silicon substrates possess excellent mechanical stability, high integration potential, and compatibility with CMOS processes, meeting the fabrication and application requirements of large-scale optoelectronic integrated systems. However, silicon substrates have a high dielectric constant, which easily introduces significant microwave losses at high frequencies. Furthermore, conventional electrode structures struggle to simultaneously achieve low-loss transmission and optical-microwave velocity matching on a silicon-based platform, limiting the high-frequency modulation performance of the device.

[0019] Traditional periodic loading structures typically rely on increasing capacitance to enhance microwave refractive index. However, excessive parasitic capacitance can lead to severe dielectric loss, impedance degradation, and microwave signal attenuation at high frequencies. To address this, this invention introduces a periodic S-shaped bending or folding slow-wave structure into the traveling-wave electrode. This structure differs from traditional linear electrodes and finger-shaped or rectangular tooth-shaped capacitive loading structures. By periodically bending / folding along the transmission direction, it extends the microwave geometric propagation path, increasing the equivalent series inductance per unit length of the transmission line, rather than simply relying on capacitance loading. This achieves the slow-wave effect while reducing transmission loss caused by high-frequency capacitance effects. Furthermore, its continuous and smooth transition characteristics facilitate fabrication and avoid the strong local electric field concentration and excessive edge capacitance associated with traditional sharp loading structures, thus suppressing the deterioration of high-frequency parasitic parameters.

[0020] Based on the analysis of equivalent circuit theory, this structure achieves precise control of microwave phase velocity by appropriately increasing the equivalent inductance L and suppressing excessive increase of parasitic capacitance C. Combined with the reasonable design of the S-shaped electrode period and the size of the periodic unit, the effective refractive index of microwave can be stably adjusted to close to the refractive index of the thin-film lithium niobate optical waveguide group of 2.256, thus achieving speed matching between light waves and microwaves.

[0021] Based on this, the present invention can significantly reduce optical-microwave phase detachment under high-speed modulation and improve high-frequency electro-optic response; the smooth bending of the S-shaped structure can optimize the spatial electric field distribution, reduce the local enhancement of the electric field at sharp corners, reduce the additional loss and mode disturbance caused by local electric field peaks, and enable the microwave signal to maintain stable and continuous transmission within the electrodes.

[0022] Ultimately, this invention enables stable speed matching of microwaves and light waves on a silicon substrate, allowing the modulator to maintain high modulation efficiency in the ultra-high frequency band and obtain an electro-optic bandwidth of 3dB above 150GHz, with a maximum of 200GHz, making it suitable for ultra-high-speed optical communication and broadband microwave photonics system applications.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] First, it balances silicon-based integration with high-speed modulation performance: Silicon substrates offer excellent process compatibility and integration potential, but traditional silicon-based modulators are susceptible to high-frequency microwave losses and speed mismatch limitations. This invention overcomes the problems of high-frequency losses and speed matching difficulties on silicon substrates through an S-shaped slow-wave electrode, giving silicon-based thin-film lithium niobate modulators greater practical value.

[0025] Second, it reduces high-frequency losses caused by traditional capacitive slow-wave loading: Conventional finger-shaped or rectangular tooth-shaped slow-wave structures mainly achieve slow waves by enhancing capacitance, but this easily leads to excessive edge capacitance, increased dielectric loss, and impedance degradation. The S-shaped bent electrode of this invention mainly achieves slow waves by increasing the microwave propagation path and equivalent inductance, reducing the dependence on strong capacitive loading, thereby reducing losses caused by high-frequency capacitive effects.

[0026] Third, achieve stable optical-microwave speed matching: by adjusting the period of the S-shaped bent electrode and the size parameters of the periodic unit, the effective refractive index of the microwave can be continuously controlled to make it stably close to the optical group refractive index of 2.256, thereby improving the electro-optic modulation efficiency over a wide frequency range.

[0027] Fourth, the S-shaped bending structure improves the electric field distribution and reduces local field concentration. It has a smooth transition characteristic, which can avoid the electric field spikes and local energy accumulation commonly found in traditional sharp loading units, making the electric field distribution more uniform. This is beneficial for reducing additional losses and improving the stability of high-frequency transmission.

[0028] Fifth, it has the potential for ultra-large electro-optic bandwidth: Since the structure reduces high-frequency loss and improves speed matching at the same time, the present invention can break through the bandwidth limitation of traditional silicon-based thin-film lithium niobate modulators and achieve an electro-optic bandwidth of more than 150 GHz, or even up to 200 GHz, with a 3 dB electro-optic bandwidth. It can support applications such as ultra-high-speed data transmission, millimeter-wave / terahertz photonic links and high-speed test systems.

[0029] In summary, this invention introduces a periodic S-shaped low-loss slow-wave electrode into a silicon-based thin-film lithium niobate MZM electro-optic modulator. The electrode bending increases the microwave propagation path and equivalent series inductance, while simultaneously suppressing high-frequency losses caused by strong edge capacitance in traditional periodic loading structures. This structure achieves a smooth match between the effective microwave refractive index and the optical group refractive index on the silicon substrate, reducing the high-frequency response degradation caused by light-microwave velocity mismatch and improving the high-frequency electric field distribution. Compared to traditional finger-shaped or rectangular tooth-shaped slow-wave electrodes, this invention maintains the advantages of silicon-based integration while also possessing low loss, wide bandwidth, high modulation efficiency, and high-speed data transmission capabilities, making it suitable for ultra-high-speed thin-film lithium niobate electro-optic modulators and related integrated optoelectronic systems. Attached Figure Description

[0030] Figure 1 This is an overall structural view of a broadband thin-film lithium niobate electro-optic modulator according to the present invention.

[0031] Figure 2 This is a cross-sectional schematic diagram of a broadband thin-film lithium niobate electro-optic modulator according to the present invention.

[0032] Figure 3This is a view of the electrode structure of a broadband thin-film lithium niobate electro-optic modulator according to the present invention.

[0033] Figure 4 The microwave effective refractive index n of a broadband thin-film lithium niobate electro-optic modulator in this embodiment is in the frequency range of 130–150 GHz. rf The simulation results.

[0034] Explanation of reference numerals in the attached figures: 1. Silicon substrate, 2. Silicon oxide buffer layer, 3. Thin-film lithium niobate optical waveguide layer, 31. Optical waveguide arm, 4. Cladding, 5. Modulation electrode. Detailed Implementation

[0035] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0036] This embodiment provides a broadband thin-film lithium niobate electro-optic modulator, such as... Figure 1 As shown in the figure, the structure includes, from bottom to top, a substrate, a silicon oxide buffer layer, a thin-film lithium niobate (TFLN) optical waveguide layer, a cladding layer, and a metal modulation electrode.

[0037] The substrate is a silicon-based substrate, compatible with CMOS processes, and adapted to the needs of integrated optoelectronic device fabrication.

[0038] The thin-film lithium niobate optical waveguide layer integrates a Mach-Zehnder structure for optical signal transmission and interference modulation. The Mach-Zehnder structure includes two optical waveguide arms, which are arranged in a periodic S-shaped zigzag structure along the optical transmission direction. The two optical waveguide arms have the same structure and are arranged in a mirror image symmetrically. In each period, the optical waveguide arm first bends outward obliquely, then extends horizontally, and then bends inward obliquely to form an isosceles trapezoidal zigzag protrusion. In this embodiment, the width of the optical waveguide arm is 1 μm, and the dimensions of the isosceles trapezoid are: height 5 μm, upper base length 20 μm, and lower base length 50 μm.

[0039] To smoothly match the optical group refractive index of 2.256, this invention implements a special modulation electrode structure on a silicon substrate. The metal modulation electrode, used to apply microwave modulation signals, includes a traveling-wave signal electrode located between two optical waveguide arms of a Mach-Zehnder structure, and two traveling-wave ground electrodes respectively disposed on the outer sides of the two optical waveguide arms. The edges of the adjacent optical waveguide arms of the traveling-wave signal electrode and the traveling-wave ground electrodes have the same periodic structure as the optical waveguide arms, ensuring consistent spacing between the electrodes and their corresponding optical waveguide arms. In this embodiment, the spacing between the electrodes is 5 μm.

[0040] By setting a smoothly bent S-shaped electrode structure, the microwave transmission path length is effectively increased, the electric field distribution is optimized, and the surge in high-frequency parasitic capacitance is suppressed, achieving velocity matching with the optical group refractive index of the optical waveguide, thereby realizing ultra-wideband electro-optic modulation performance.

[0041] Unlike traditional microstructures (such as sawtooth or T-type structures) that generate a large amount of interdigital parasitic capacitance between adjacent electrodes, the smooth bending feature of the S-type structure not only greatly increases the geometric length of the microwave signal transmission path, effectively increasing the series inductance L per unit length of the transmission line; at the same time, this smooth transition structure effectively avoids the local peak accumulation of the electric field, strictly controls and suppresses the surge of unnecessary parallel parasitic capacitance C at high frequencies, thus cleverly eliminating the serious dielectric loss caused by the high-frequency capacitance effect.

[0042] By introducing an appropriate equivalent inductance L and optimizing the control of the high-frequency equivalent capacitance C, the microwave phase velocity v is effectively reduced. m Thus, the effective refractive index n of microwaves is increased on a silicon-based substrate. m The frequency response was steadily and smoothly increased to around 2.256. This not only achieved the required perfect slow-wave speed matching but also ensured low-loss transmission of microwave signals in the UHF band.

[0043] Figure 4 The microwave effective refractive index n of a broadband thin-film lithium niobate electro-optic modulator in this embodiment is in the frequency range of 130–150 GHz. rf Simulation results show that as the frequency increases, the effective refractive index of the microwave generally increases slowly from approximately 2.2808 to 2.2832, with a change of less than 0.003, reaching a maximum value near 148 GHz, and then slightly decreasing. This indicates that the slow-wave electrode exhibits low dispersion and relatively stable microwave propagation characteristics within the target frequency band, which is beneficial for reducing the velocity mismatch between the microwave signal and the optical signal, thereby improving the high-frequency response and operating bandwidth of the electro-optic modulator.

Claims

1. A broadband thin-film lithium niobate electro-optic modulator, comprising, from bottom to top, a substrate, a silicon oxide buffer layer, a thin-film lithium niobate optical waveguide layer, and a cladding layer, and further comprising a metal modulation electrode. Its features are, The substrate is a silicon-based substrate; The thin-film lithium niobate optical waveguide layer integrates a Mach-Zehnder structure for optical signal transmission and interference modulation. The Mach-Zehnder structure includes two optical waveguide arms, which have a periodic S-shaped bending structure along the optical transmission direction. The two optical waveguide arms have the same structure and are arranged in a mirror symmetrical manner. The metal modulation electrode is used to apply microwave modulation signals, including a traveling wave signal electrode located between the two optical waveguide arms of the Mach-Zehnder structure, and two traveling wave grounding electrodes respectively disposed on the outer sides of the two optical waveguide arms. The traveling wave signal electrode and the traveling wave ground electrode have the same periodic structure as the optical waveguide arm at their adjacent edges, so that the distance between the electrode and the corresponding optical waveguide arm remains consistent. By setting an S-shaped bending structure, the refractive index velocity of the optical waveguide is matched with that of the optical waveguide, thereby achieving ultra-wideband electro-optic modulation performance.

2. The broadband thin-film lithium niobate electro-optic modulator as described in claim 1, characterized in that, The S-shaped bending structure is a cosine bend, a circular arc bend, or a broken line bend.

3. A broadband thin-film lithium niobate electro-optic modulator as described in claim 2, characterized in that, The period of the S-shaped bending structure ranges from 30 to 60 μm.

4. A broadband thin-film lithium niobate electro-optic modulator as described in claim 3, characterized in that, When the S-shaped bending structure is a zigzag bend, in each cycle, the optical waveguide arm first bends outward at an angle, then extends horizontally, and then bends inward at an angle, forming an isosceles trapezoidal zigzag protrusion.

5. A broadband thin-film lithium niobate electro-optic modulator as described in claim 3, characterized in that, When the S-shaped bending structure is a cosine bend or a circular arc bend, the minimum bending radius ranges from 60 to 100 μm.

6. A broadband thin-film lithium niobate electro-optic modulator as described in claim 4, characterized in that, The height of the isosceles trapezoid ranges from 5 to 10 μm, the length of the upper base ranges from 10 to 20 μm, and the length of the lower base ranges from 20 to 50 μm.

7. A broadband thin-film lithium niobate electro-optic modulator as described in any one of claims 3-6, characterized in that, The distance between the traveling wave signal electrode and the traveling wave ground electrode ranges from 3 to 6 μm; the distance between the optical waveguide arm and the electrode ranges from 0.5 to 1 μm.

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