A polarization diversity lithium niobate thin film electro-optic modulator and monolithic integrated system

Abstract

This invention discloses a polarization diversity lithium niobate thin-film electro-optic modulator and its monolithic integrated system, relating to the field of integrated optoelectronic device technology. The system includes: an optical input coupling module, a polarization diversity processing module, an electro-optic modulation module, a polarization beam combining output module, an optical output coupling module, and an RF driving module. This invention employs a polarization diversity optical path architecture, coupled with a subwavelength grating strip waveguide structure polarization beam splitting and rotating unit, to decompose incident light of arbitrary polarization state into two optical components with the same polarization state. These components are then fed into corresponding electro-optic modulation arms for modulation, and finally, symmetrically arranged output polarization beam splitting and rotating units complete beam combining and polarization state restoration. From an architectural perspective, this frees the modulator from the strict requirements on the polarization state of the incident light. The subwavelength grating strip waveguide structure allows for flexible control of the waveguide's equivalent refractive index characteristics, utilizing its birefringence effect and low dispersion characteristics to achieve polarization beam splitting and polarization rotation functions.

Description

Technical Field

[0001] This invention relates to the field of integrated optoelectronic device technology, specifically to a polarization diversity lithium niobate thin-film electro-optic modulator and a monolithic integrated system. Background Technology

[0002] With the continuous expansion of the digital economy and the rapid growth of data traffic, high-speed optical communication networks, as the core carrier of data transmission, are constantly evolving towards higher speeds, greater bandwidth, and higher integration. Electro-optic modulators, as core functional devices in optical communication, directly determine the overall capability of optical transmission links, and market demand is steadily expanding with the iteration of optical modules. In the development of the optoelectronic information industry, optoelectronic devices are key components for development. The maturity of the lithium niobate-on-insulator material system has provided the foundation for the research and development and industrialization of a new generation of high-performance optical modulators. There is an urgent application demand in the industry for modulator chips with high integration and high adaptability.

[0003] However, in existing technologies, lithium niobate thin-film electro-optic modulators are generally polarization-sensitive due to the intrinsic anisotropy of the material, and can only achieve stable modulation of incident light with a specific polarization state. In actual optical transmission links, the polarization state of the optical signal changes randomly with the transmission environment. To ensure stable modulation performance, a separate polarization control unit and feedback system need to be configured in the link, which significantly increases complexity and hardware cost, and also limits the application of modulators in on-chip optical interconnect scenarios. Existing polarization diversity solutions mostly use conventional ridge or strip waveguides to construct polarization control devices, which are susceptible to waveguide dispersion and mode noise. The limitations imposed by polarization effects result in limited operating bandwidth and a relatively large overall size for the devices. Furthermore, the ridge waveguide requires extremely high precision in controlling the etching depth, making yield and cost control during mass production challenging. Simultaneously, existing modulation structures struggle to balance modulation efficiency and bandwidth. Conventional slow-light optimization schemes suffer from narrow operating bandwidth and large group velocity dispersion, and the velocity mismatch between light waves and microwaves further restricts the high-frequency performance of the devices. Moreover, existing polarization-independent modulation schemes are mostly simple splices of different functional devices, resulting in insufficient overall integration, high cascade losses, and poor process compatibility, leaving a significant gap before large-scale industrial applications. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a polarization diversity lithium niobate thin-film electro-optic modulator and its monolithic integrated system. This invention employs a polarization diversity optical path architecture, combined with a subwavelength grating strip waveguide structure polarization beam splitting and rotating unit, to decompose incident light of any polarization state into two optical components with the same polarization state. These components are then fed into corresponding electro-optic modulation arms for modulation, and finally, symmetrically arranged output polarization beam splitting and rotating units complete beam combining and polarization state restoration. From an architectural perspective, this frees the modulator from the strict requirements of the incident light's polarization state. The subwavelength grating strip waveguide structure allows for flexible control of the waveguide's equivalent refractive index characteristics, utilizing its birefringence effect and low dispersion characteristics to achieve polarization beam splitting and polarization rotation functions. The strip waveguide can be fabricated with only one etching process, making it fully compatible with the fabrication of the modulation arm waveguide. All functional units are monolithically integrated on the same substrate, reducing the additional losses caused by cascading discrete devices and simplifying the configuration complexity of the optical transmission link.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution: On the one hand, a polarization diversity lithium niobate thin film electro-optic modulator, the modulator comprising: a lithium niobate on insulator substrate and a waveguide structure and an electrode structure integrated on the lithium niobate substrate;

[0006] The waveguide structure includes an input polarization beam splitter rotation unit, a first electro-optic modulation arm, a second electro-optic modulation arm, and an output polarization beam splitter rotation unit connected in sequence by optical paths.

[0007] The two output ends of the input polarization beam splitting rotation unit are respectively connected to the input end of the first electro-optic modulation arm and the input end of the second electro-optic modulation arm. The output ends of the first electro-optic modulation arm and the second electro-optic modulation arm are connected to the input end of the output polarization beam splitting rotation unit.

[0008] Both the input polarization beam splitter rotation unit and the output polarization beam splitter rotation unit adopt a subwavelength grating strip waveguide structure.

[0009] The electrode structure is a coplanar traveling wave electrode, which is arranged in parallel along the transmission direction of the first electro-optic modulation arm and the second electro-optic modulation arm, and the same radio frequency modulation signal is applied to the first electro-optic modulation arm and the second electro-optic modulation arm.

[0010] Furthermore, the input polarization beam splitting rotation unit sequentially includes an incident strip waveguide section, an input adiabatic tapered section, a subwavelength grating coupling section, and two output adiabatic tapered sections along the optical transmission direction. The incident strip waveguide section carries the incident optical signal, and the output end of the incident strip waveguide section is connected to the input end of the subwavelength grating coupling section via the input adiabatic tapered section. The subwavelength grating coupling section is configured with a direct transmission path and a cross transmission path, and the two output ends of the subwavelength grating coupling section are respectively connected to the input ends of the two output adiabatic tapered sections. The output polarization beam splitting rotation unit and the input polarization beam splitting rotation unit are arranged symmetrically in the optical path.

[0011] Furthermore, both the first and second electro-optic modulation arms adopt a Mach-Zehnder modulation structure. Each Mach-Zehnder modulation structure includes an input multimode interference coupler, two parallel herringbone-like grating modulation arm waveguides, and an output multimode interference coupler. The input end of the input multimode interference coupler is connected to the same polarization state optical signal output by the input polarization beam splitting rotation unit, and the two output ends of the input multimode interference coupler are respectively connected to the input ends of the two herringbone-like grating modulation arm waveguides. The coplanar traveling wave electrodes adopt a ground-signal-ground-signal-ground arrangement, and periodically loaded microstructures are set on the signal electrode.

[0012] On the other hand, a monolithic integrated system of a polarization diversity lithium niobate thin-film electro-optic modulator includes: an optical input coupling module, a polarization diversity processing module, an electro-optic modulation module, a polarization beam combining output module, an optical output coupling module, and a radio frequency driving module corresponding to the electro-optic modulation module, all of which are monolithically fabricated on the same lithium niobate substrate on an insulator.

[0013] The optical input coupling module is used to couple the optical signal between the external optical fiber and the on-chip waveguide, and output the incident optical signal to the polarization diversity processing module. The polarization diversity processing module is used to perform polarization beam splitting and polarization rotation of the incident optical signal, and output two same-polarization optical components to the two modulation channels of the electro-optic modulation module. The RF drive module is used to output a synchronous RF modulation signal to the electro-optic modulation module. The electro-optic modulation module is used to perform electro-optic modulation of the two same-polarization optical components, and output two modulated optical signals to the polarization beam combining output module. The polarization beam combining output module is used to perform polarization beam combining and polarization state restoration of the two modulated optical signals, and the combined optical signal is output through the optical output coupling module.

[0014] Furthermore, in the optical input coupling module, during optical signal coupling, the incident light signal output from the external optical fiber is transmitted to the end-face subwavelength grating section, which adapts the mode field distribution of the incident light signal. When designing the structural parameters of the end-face subwavelength grating section, calculations are performed using the equivalent refractive index correction formula for the subwavelength grating's fundamental TE mode. The incident light signal is transmitted through an adiabatic tapered transition section, where the waveguide width is gradually adjusted. The incident light signal is then transmitted to a strip waveguide section, which outputs the incident light signal to the input of the polarization diversity processing module. The equivalent refractive index correction formula for the subwavelength grating's fundamental TE mode is: ,in, The fundamental TE mode equivalent refractive index of the end-face subwavelength grating section waveguide is used for the structural design of end-face mode field matching. The ordinary optical refractive index of lithium niobate crystal is determined by the intrinsic optical properties of lithium niobate crystal combined with the operating wavelength, and its value ranges from 2.20 to 2.35. The refractive index of the silica cladding is determined by the intrinsic optical properties of the silica material combined with the operating wavelength, and its value ranges from 1.44 to 1.46. The nominal duty cycle of the subwavelength grating is determined by the structural design parameters of the end-face subwavelength grating segment, and its value ranges from 0.3 to 0.7. The duty cycle perturbation introduced by the waveguide sidewall tilt angle is determined by the sidewall tilt angle measured by the dry etching process of the lithium niobate waveguide, and the value ranges from -0.1 to 0.1.

[0015] Furthermore, in the polarization diversity processing module, the following operations are performed during optical signal polarization processing: First, the incident light signal enters the mode evolution region, where the transverse magnetic mode component in the incident light signal is gradually converted into a higher-order transverse electric mode; Second, the incident light signal enters the directional coupling region, where the higher-order transverse electric mode is converted into a fundamental transverse electric mode and separated to the cross-transmission channel; when designing the structural parameters of the directional coupling region, the polarization beam splitting length is determined using the polarization beam splitting length formula of the asymmetric directional coupler; Third, the transverse electric mode component in the incident light signal is transmitted along the direct transmission channel, and both same-polarization light components are output in the fundamental transverse electric mode to the two independent modulation channels of the electro-optic modulation module; the polarization beam splitting length formula of the asymmetric directional coupler is: ,in, The polarization beam splitting coupling length of the directional coupling region is used for the length structure design of the directional coupling region. The operating wavelength is determined by the communication band of the device's application scenario, and its value ranges from 1530nm to 1625nm. The equivalent refractive index of the fundamental TE mode of the through-path subwavelength grating waveguide. Let be the equivalent refractive index of the fundamental TE mode of the cross-path subwavelength grating waveguide. The difference between the two equivalent refractive indices of the fundamental TE mode is determined by the difference in the structural parameters of the two path subwavelength gratings.

[0016] Furthermore, in the electro-optic modulation module, the following operations are performed during electro-optic modulation of the optical signal: First, each same-polarization light component is split into two beams by an input multimode interference coupler, and transmitted to two herringbone-like grating modulation arm waveguides respectively; Second, an RF modulation signal is applied to the coplanar traveling wave electrode corresponding to the herringbone-like grating modulation arm waveguide to perform phase modulation on the light component transmitted within the herringbone-like grating modulation arm waveguide; when designing the structural parameters of the herringbone-like grating modulation arm waveguide, the length of the modulation arm is determined by the half-wave voltage-length product formula of the slow photoelectric modulator; Third, the two phase-modulated light components are combined by an output multimode interference coupler to form two modulated optical signals output to the polarization combining output module; the half-wave voltage-length product formula of the slow photoelectric modulator is: ,in, It is the product of the half-wave voltage of the Mach-Zehnder modulation structure and the effective operating length of the herringbone-like grating modulation arm waveguide. The operating wavelength is determined by the communication band of the device's application scenario, and its value ranges from 1530-1625nm. The spacing between the signal electrode and the adjacent ground electrode of the coplanar traveling wave electrode is determined by the structural design parameters of the coplanar traveling wave electrode. The unusual optical refractive index of lithium niobate crystal is determined by combining the intrinsic optical properties of lithium niobate crystal with the operating wavelength. The longitudinal electro-optic coefficient of lithium niobate crystal is determined by its intrinsic electro-optic properties. The optical overlap integral factor of the herringbone-like grating modulated arm waveguide is obtained by calibrating the two-dimensional overlap relationship between the optical field distribution inside the herringbone-like grating modulated arm waveguide and the radio frequency electric field distribution of the coplanar traveling wave electrodes. The group refractive index of the herringbone-like grating modulation arm waveguide is determined by the herringbone-like grating structure and ranges from 2 to 10.

[0017] Furthermore, in the radio frequency driving module, the radio frequency modulation signal output by the radio frequency signal source is transmitted to the impedance matching network, and the impedance matching network adjusts the impedance of the radio frequency modulation signal. The radio frequency modulation signal after impedance adjustment is divided into two synchronous branch signals, which are transmitted to two signal branches of the coplanar traveling wave electrode respectively. The two signal branches load the branch signals into the two modulation channels of the electro-optic modulation module respectively, and the transmission path lengths of the two branch signals are the same.

[0018] Furthermore, the polarization beam combining output module and the polarization diversity processing module are arranged symmetrically in the optical path. The two input terminals of the polarization beam combining output module are respectively connected to the two modulated optical signals output by the electro-optic modulation module, and the output terminal of the polarization beam combining output module is connected to the input terminal of the optical output coupling module. The optical output coupling module and the optical input coupling module are symmetrically structured, and the mode field parameters of the output terminal of the optical output coupling module correspond to the mode field parameters of the external single-mode optical fiber.

[0019] Compared with existing technologies, this polarization diversity lithium niobate thin-film electro-optic modulator and monolithic integrated system have the following advantages:

[0020] I. This invention employs a polarization diversity optical path architecture, coupled with a subwavelength grating strip waveguide structure polarization beam splitting and rotation unit, to decompose incident light of any polarization state into two optical components with the same polarization state. These components are then fed into corresponding electro-optic modulation arms for modulation, and finally combined and polarization state restored by symmetrically arranged output polarization beam splitting and rotation units. From an architectural perspective, this frees the modulator from the strict requirements on the polarization state of the incident light. The subwavelength grating strip waveguide structure allows for flexible control of the waveguide's equivalent refractive index characteristics. It utilizes its own birefringence effect and low dispersion characteristics to achieve polarization beam splitting and polarization rotation functions. The strip waveguide can be fabricated with only one etching process and is fully compatible with the fabrication of the modulation arm waveguide. All functional units are monolithically integrated on the same substrate, reducing the additional losses caused by the cascading of discrete devices and simplifying the configuration complexity of the optical transmission link.

[0021] II. This invention employs a herringbone-like grating modulation arm waveguide in the Mach-Zehnder modulation structure of the electro-optic modulation arm. By controlling the band structure of the waveguide, a flat slow-light effect is obtained, extending the interaction time between the optical carrier and the radio frequency electric field, thus optimizing the effect of electro-optic modulation. The coplanar traveling wave electrodes adopt a ground-signal-ground-signal-ground arrangement, and periodically loaded microstructures are set on the signal electrodes to synchronously control the transmission speed of the radio frequency signal, achieving wave velocity matching between the optical carrier and the radio frequency signal, thus optimizing modulation efficiency while ensuring modulation bandwidth. The two electro-optic modulation arms share the same set of traveling wave electrodes to load the same radio frequency modulation signal, ensuring the modulation synchronization and consistency of the two optical components. Combined with a symmetrical polarization beam combiner structure, the final output is a modulated optical signal with the same incident polarization state, adapting to the optical transmission link requirements in different scenarios.

[0022] Other advantages, objectives and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination or study, or may be learned from the practice of the invention. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0024] Figure 1 A flowchart of a monolithic integrated system of a polarization diversity lithium niobate thin-film electro-optic modulator;

[0025] Figure 2 This is a framework diagram of the polarization diversity processing module in a monolithic integrated system of a polarization diversity lithium niobate thin-film electro-optic modulator;

[0026] Figure 3 This is a framework diagram of the electro-optic modulation module in a monolithic integrated system of a polarization diversity lithium niobate thin-film electro-optic modulator. Detailed Implementation

[0027] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0028] Example:

[0029] In long-distance, high-speed coherent optical communication transmission scenarios in the 1530-1625nm C / L band, the polarization state of the optical signal transmitted inside the optical fiber is continuously and randomly shifted due to factors such as fiber bending stress, ambient temperature changes, and equipment mechanical vibration. Traditional lithium niobate thin-film electro-optic modulators can only stably process incident light with a single fixed polarization. When applied, discrete polarization control optics must be added externally to the optical link, which not only increases the overall insertion loss of the optical link but also increases the overall size and assembly complexity of the optical module. This embodiment provides a polarization diversity lithium niobate thin-film electro-optic modulator and its monolithic integrated system, which can be directly integrated into the 400G and 800G commercial coherent optical modules as the core optical chip, natively adapting to incident light with any polarization state, without the need for external polarization control components.

[0030] Structure of a polarization diversity lithium niobate thin-film electro-optic modulator:

[0031] In this embodiment, the polarization diversity lithium niobate thin-film electro-optic modulator uses a lithium niobate-on-insulator substrate as the carrier substrate. The substrate consists of a lithium niobate single-crystal thin film layer, a silicon dioxide buried oxide layer, and a single-crystal silicon substrate from top to bottom. All waveguide structures and electrode structures of the modulator are integrated on the same lithium niobate-on-insulator substrate. The waveguide structure is arranged along the optical transmission direction as follows: an input polarization beam splitting rotation unit, a first electro-optic modulation arm, a second electro-optic modulation arm, and an output polarization beam splitting rotation unit. The input polarization beam splitting rotation unit has two output terminals, which are respectively connected to the input terminals of the first and second electro-optic modulation arms. The output terminals of the first and second electro-optic modulation arms are connected to the input terminal of the output polarization beam splitting rotation unit, forming a symmetrical polarization diversity optical path architecture.

[0032] Both the input polarization beam splitter rotation unit and the output polarization beam splitter rotation unit adopt a subwavelength grating strip waveguide structure, and the two are arranged symmetrically in the optical path. Taking the input polarization beam splitter rotation unit as an example, it is arranged along the optical transmission direction in sequence as an incident strip waveguide section, an input adiabatic tapered section, a subwavelength grating coupling section, and two output adiabatic tapered sections. The incident strip waveguide section is the optical input port of the modulator, used to receive the incident optical signal. The incident strip waveguide section smoothly transitions to the subwavelength grating coupling section through the input adiabatic tapered section to avoid additional losses caused by mode mismatch. Within the subwavelength grating coupling section... The unit is equipped with a direct transmission path and a cross transmission path. Relying on the equivalent refractive index modulation capability and birefringence effect of the subwavelength grating, it realizes the polarization beam splitting and polarization rotation functions of the incident light. The two output adiabatic tapered sections correspond to the output ends of the direct transmission path and the cross transmission path, respectively, smoothly transitioning the grating waveguide mode to the conventional strip waveguide mode, and respectively connecting to the subsequent first electro-optic modulation arm and second electro-optic modulation arm. The internal structure of the output polarization beam splitting and rotation unit is completely symmetrical with that of the input polarization beam splitting and rotation unit, and is used to perform reverse polarization beam combining and polarization state restoration operations.

[0033] Both the first and second electro-optic modulation arms employ Mach-Zehnder modulation structures. Each Mach-Zehnder modulation structure contains an input multimode interference coupler, two parallel herringbone-like grating modulation arm waveguides, and an output multimode interference coupler. The input of the input multimode interference coupler receives the same-polarization optical signal output from the input polarization beam splitting and rotation unit, and splits the same-polarization optical signal into two beams, which are then fed into the two herringbone-like grating modulation arm waveguides. The herringbone-like grating modulation arm waveguides have periodic grating structures along the optical transmission direction. By bandgap modulation, a flat slow light effect is obtained, extending the interaction time between the optical carrier and the electric field. The modulation efficiency is optimized; the output multimode interference coupler combines the optical signals output from the two fishbone-like grating modulation arm waveguides into one beam to complete the interference modulation process; the electrode structure of the modulator is a coplanar traveling wave electrode, which adopts a ground-signal-ground-signal-ground arrangement and is arranged in parallel along the transmission direction of the first electro-optic modulation arm and the second electro-optic modulation arm, so that the same radio frequency modulation signal can be loaded onto the first electro-optic modulation arm and the second electro-optic modulation arm; the signal electrode surface of the coplanar traveling wave electrode is provided with periodically loaded microstructures to regulate the transmission speed of the radio frequency signal, realize the wave speed matching between the radio frequency wave and the optical carrier, and ensure the high-frequency modulation performance of the modulator.

[0034] Monolithic integrated system of polarization diversity lithium niobate thin-film electro-optic modulator

[0035] The monolithic integrated system of polarization diversity lithium niobate thin-film electro-optic modulator based on the modulator is constructed by cascading an optical input coupling module, a polarization diversity processing module, an electro-optic modulation module, a polarization beam combining output module, and an optical output coupling module in sequence along the optical transmission direction. At the same time, an RF driving module corresponding to the electro-optic modulation module is set up. All functional modules are monolithically fabricated on the same lithium niobate substrate on an insulator, eliminating the need for discrete device splicing and assembly.

[0036] The optical input coupling module is located at the front end of the system, serving as the connection structure between the external optical fiber and the on-chip waveguide, and completing the coupling and input of the incident optical signal. The polarization diversity processing module corresponds to the function of the input polarization beam splitting and rotation unit, receiving the incident optical signal output from the optical input coupling module, completing polarization beam splitting and polarization rotation, and outputting two same-polarization optical components to the two independent modulation channels of the electro-optic modulation module. The electro-optic modulation module corresponds to the functions of the first and second electro-optic modulation arms, receiving the two same-polarization optical components and completing electro-optic modulation. The RF drive module provides a synchronous RF modulation signal to the electro-optic modulation module, ensuring the signal consistency of the two modulation channels. The polarization beam combining output module corresponds to the function of the output polarization beam splitting and rotation unit, receiving the two modulated optical signals and completing polarization beam combining and polarization state restoration. The optical output coupling module is located at the end of the system, coupling the combined modulated optical signal to the external optical fiber link. The waveguide structure of all modules uses the same lithium niobate thin film material, and the modules are directly cascaded through on-chip strip waveguides, eliminating the cascading loss of discrete components.

[0037] The optical input coupling module employs a subwavelength grating reverse tapered structure, consisting sequentially of an end-face subwavelength grating section, an adiabatic tapered transition section, and a strip waveguide section along the optical transmission direction. During operation, the incident light signal from the external single-mode fiber is first transmitted to the end-face subwavelength grating section. This section uses periodic microstructures to adjust the waveguide's equivalent refractive index, matching the mode field distribution of the external fiber and reducing coupling loss due to mode field mismatch. Subsequently, the incident light signal is transmitted through the adiabatic tapered transition section. This section gradually narrows the waveguide width, confining the optical field within the waveguide and smoothly transitioning to a conventional strip waveguide mode. Finally, the incident light signal is output to the input of the polarization diversity processing module via the strip waveguide section. Figure 1 As shown;

[0038] During the structural design phase, the structural parameters of the end-face grating were determined using the subwavelength grating fundamental TE mode equivalent refractive index correction formula to accommodate the sidewall tilt angle effect caused by actual processing. The intrinsic optical parameters of the materials were first determined: at room temperature and a wavelength of 1550 nm, the ordinary refractive index of lithium niobate crystal was taken as 2.28, and the refractive index of silicon dioxide cladding was taken as 1.445. Secondly, the nominal duty cycle of the subwavelength grating was set to 0.5, and the grating period was 280 nm. Then, combined with the measured 71° waveguide sidewall tilt angle from actual processing, the duty cycle perturbation was calculated to be -0.06. Substituting this into the subwavelength grating fundamental TE mode equivalent refractive index correction formula, the fundamental TE mode equivalent refractive index of the end-face subwavelength grating segment was calculated to be 1.86. The subwavelength grating fundamental TE mode equivalent refractive index correction formula is as follows: ,in, The equivalent refractive index of the fundamental TE mode of the subwavelength grating segment waveguide at the end face. The ordinary refractive index of lithium niobate crystal is... The refractive index of the silica cladding is... This is the nominal duty cycle of the subwavelength grating. The duty cycle perturbation is introduced to the waveguide sidewall tilt angle. Based on the calculation results, a three-dimensional simulation model is constructed, and the length and envelope function of the adiabatic tapered transition section are iteratively optimized. Finally, a Gaussian tapered structure with a length of 100μm is selected. At this time, the mode field overlap efficiency can reach 76%, which meets the design goal of low-loss coupling. The optical output coupling module and the optical input coupling module have a symmetrical structure, and the working process is completely reversed with the design logic to ensure consistent mode field adaptability of input and output.

[0039] The polarization diversity processing module utilizes an asymmetric subwavelength grating waveguide structure to achieve polarization processing. The process consists of two stages: mode evolution and directional coupling. After the incident light signal enters the mode evolution region, the transverse magnetic mode component gradually transforms into a higher-order transverse electric mode as the waveguide width gradually adjusts, completing the first step of polarization rotation. Subsequently, the incident light signal enters the asymmetric directional coupling region, where the higher-order transverse electric mode is transformed into a fundamental transverse electric mode through directional coupling and separated for output via the cross-transmission channel. The original transverse electric mode component in the incident light signal remains unchanged throughout the process and is directly output along the straight-through transmission channel. Finally, both optical signals are output in their fundamental transverse electric modes, forming two identically polarized light components, which are then fed into two independent modulation channels of the electro-optic modulation module. Figure 2 As shown;

[0040] During the structural design phase, the physical dimensions of the coupling section were determined using the asymmetric directional coupler polarization beam-splitting length formula. With 1550nm as the central operating wavelength, asymmetric subwavelength grating waveguide models for both the through path and the cross path were constructed. The widths of the two waveguides were set to 1.2μm and 0.9μm, respectively, creating a refractive index difference. The equivalent refractive index of the fundamental TE mode of both waveguides was calculated, yielding a refractive index difference of 0.24. Substituting this into the asymmetric directional coupler polarization beam-splitting length formula, the theoretical polarization beam-splitting coupling length was calculated to be 322μm. The asymmetric directional coupler polarization beam-splitting length formula is as follows: ,in, The polarization beam splitting coupling length of the directional coupling region. The working light wavelength, The equivalent refractive index of the fundamental TE mode of the through-path subwavelength grating waveguide. The equivalent refractive index of the fundamental TE mode of the cross-path subwavelength grating waveguide is given. Based on this calculation result, a complete optical path simulation model is built to verify the complete process of mode evolution and directional coupling. The coupling lengths of 300μm, 320μm, and 340μm are compared and verified. Finally, the coupling length parameter of 320μm is selected. At this time, the polarization extinction ratio obtained by simulation is >22dB and the insertion loss is <1.2dB, which meets the device performance design requirements. The polarization beam combining output module and the polarization diversity processing module are arranged symmetrically in the optical path and perform reverse mode conversion and beam combining operations to restore the two fundamental transverse electric mode optical signals to polarized optical signals consistent with the incident state.

[0041] The electro-optic modulation module contains two independent Mach-Zehnder modulation channels, each corresponding to an electro-optic modulation arm. During operation, the single-path same-polarization light component first enters the input multimode interference coupler and is split into two beams, which are then transmitted to the two herringbone-like modulation arm waveguides. The radio frequency modulation signal is applied to the coplanar traveling-wave electrode corresponding to the herringbone-like modulation arm waveguide. Utilizing the electro-optic effect of the lithium niobate crystal, the electric field changes the refractive index of the waveguide, causing a phase difference between the light components transmitted within the two herringbone-like modulation arm waveguides. The two beams carrying the phase difference are finally combined and interfered by the output multimode interference coupler, converting phase modulation into intensity modulation to form a modulated optical signal output, such as... Figure 3 As shown, the slow light effect brought about by the fishbone grating structure can prolong the dwell time of the optical carrier in the modulation region, obtain better modulation efficiency with the same electrode length, and effectively reduce the overall size of the device.

[0042] During the structural design phase, the effective operating length of the herringbone-like grating modulation arm waveguide was determined using the half-wave voltage-length product formula for slow photoelectric modulators. The design first determined the basic parameters: the distance between the signal electrode and ground electrode of the coplanar traveling wave electrode was set to 6 μm; the unusual light refractive index of the lithium niobate crystal at a wavelength of 1550 nm at room temperature was 2.20; and the longitudinal electro-optic coefficient was 30.8 pm / V. Secondly, the group refractive index of the herringbone-like grating modulation arm waveguide was determined through band structure simulation. When the grating period was set to 420 nm, the group refractive index was 4.2, and the group velocity dispersion was less than 2 ps. 2 / mm, possessing flat slow-light characteristics; then, through field distribution simulation, the internal optical field distribution of the herringbone-like grating modulation arm waveguide and the RF electric field distribution of the coplanar traveling-wave electrode are obtained. Based on the two-dimensional overlap relationship of the two sets of field distributions, the electro-optic overlap integral factor is calibrated to be 0.72; then, substituting into the formula for calculating the half-wave voltage-length product of the slow-light electro-optic modulator, the half-wave voltage-length product of the herringbone-like grating modulation arm waveguide is calculated to be 1.18V・cm, corresponding to an effective operating length of 3.5mm for the herringbone-like grating modulation arm waveguide; the formula for the half-wave voltage-length product of the slow-light electro-optic modulator is: ,in, It is the product of the half-wave voltage of the Mach-Zehnder modulation structure and the effective operating length of the herringbone-like grating modulation arm waveguide. The working light wavelength, The distance between the signal electrode of the coplanar traveling wave electrode and the adjacent ground electrode. The unusual optical refractive index of lithium niobate crystals The longitudinal electro-optic coefficient of lithium niobate crystal is . The waveguide optical overlap integral factor for the modulation arm of the fishbone grating. The group refractive index of the fishbone grating modulated arm waveguide is optimized; the periodic loading microstructure size of the coplanar traveling wave electrode is simultaneously optimized, and the electrode loading period is set to 150μm to match the radio frequency wave velocity with the slow light group velocity, thus ensuring the high-frequency modulation bandwidth of the modulator.

[0043] The RF drive module consists of an RF signal source, an impedance matching network, and a coplanar traveling wave electrode. During operation, the RF modulation signal output from the RF signal source is first transmitted to the impedance matching network, which performs impedance adjustment to achieve a 50Ω characteristic impedance match between the signal source and the coplanar traveling wave electrode, reducing signal reflection loss. The impedance-adjusted RF modulation signal is divided into two synchronous branch signals, which are transmitted to the two signal branches of the coplanar traveling wave electrode respectively. The two signal branches correspond to the two modulation channels of the electro-optic modulation module, synchronously loading the modulation signal into the herringbone-like grating modulation arm waveguide within the channel. The transmission path lengths of the two branch signals are kept consistent, with the length difference controlled within ±5μm, ensuring that the phase difference between the two modulation signals is less than 1°, ensuring that the modulation states of the two modulation channels are completely synchronized, and avoiding signal distortion after beam combining.

[0044] The complete optical signal transmission and modulation process of the monolithic integrated system of the polarization diversity lithium niobate thin-film electro-optic modulator is as follows: An incident optical signal with arbitrary polarization state output from an external single-mode fiber is coupled into the on-chip waveguide structure via a low-loss optical input coupling module, and then transmitted to the polarization diversity processing module. Within the polarization diversity processing module, the incident optical signal undergoes two stages: mode evolution and directional coupling, decomposing into two identically polarized optical components, each in a fundamental transverse electric mode. These components then enter two independent modulation channels of the electro-optic modulation module. Each identically polarized optical component completes the complete modulation process of beam splitting, phase modulation, and beam combining interference within the Mach-Zehnder modulation structure. The RF drive module synchronously loads consistent RF modulation signals onto the two modulation channels, ensuring complete synchronization of the modulation amplitude and phase of the two optical signals. The two modulated optical signals are then transmitted to the polarization beam combining output module. After reverse mode conversion and beam combining, the polarization state is restored to the same state as the incident light. Finally, the combined modulated optical signal is coupled and output to the external single-mode fiber via the optical output coupling module, achieving stable electro-optic modulation functionality independent of the incident light's polarization state throughout the entire process.

[0045] The monolithic integrated system of the polarization diversity lithium niobate thin-film electro-optic modulator prepared in this embodiment was subjected to complete optoelectronic performance testing. A 1550nm tunable laser was used as the test light source, and 20 different incident polarization angles were continuously switched with a polarization controller. Each angle was spaced 18° apart to simulate the random polarization fluctuations in a real fiber optic link. The test results showed that the amplitude fluctuation of the system output signal was less than 0.3dB, the polarization-dependent loss was less than 0.4dB, and the extinction ratio was stable above 18dB with no significant performance drop, verifying the stable polarization-independent modulation characteristics of the system. In terms of core optical and electrical performance, the on-chip insertion loss was less than 1.6dB, the overall insertion loss including fiber coupling was less than 2.5dB, the 3dB electro-optic modulation bandwidth was greater than 80GHz, the half-wave voltage was about 3.4V, and the deviation of each indicator from the design simulation value was less than 8%.

[0046] In summary, this embodiment fully presents the structure, working principle, design logic, and performance of a polarization diversity lithium niobate thin-film electro-optic modulator and its monolithic integrated system. It utilizes a subwavelength grating strip waveguide to construct a full-chip polarization diversity optical path, and optimizes modulation performance with a herringbone-like slow-light structure and wave velocity-matched coplanar traveling-wave electrodes. The monolithic integration of the entire module eliminates the need for external polarization devices, directly adapting to mass production applications of high-speed coherent optical communication chips. This effectively solves the problems of polarization sensitivity, insufficient integration, and difficulty in balancing performance and size found in similar devices.

[0047] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A polarization diversity lithium niobate thin-film electro-optic modulator, characterized in that, The modulator includes: a lithium niobate on an insulator substrate and a waveguide structure and an electrode structure integrated on the lithium niobate substrate; The waveguide structure includes an input polarization beam splitter rotation unit, a first electro-optic modulation arm, a second electro-optic modulation arm, and an output polarization beam splitter rotation unit connected in sequence by optical paths. The two output ends of the input polarization beam splitting rotation unit are respectively connected to the input end of the first electro-optic modulation arm and the input end of the second electro-optic modulation arm. The output ends of the first electro-optic modulation arm and the second electro-optic modulation arm are connected to the input end of the output polarization beam splitting rotation unit. Both the input polarization beam splitter rotation unit and the output polarization beam splitter rotation unit adopt a subwavelength grating strip waveguide structure. The electrode structure is a coplanar traveling wave electrode, which is arranged in parallel along the transmission direction of the first electro-optic modulation arm and the second electro-optic modulation arm, and the same radio frequency modulation signal is applied to the first electro-optic modulation arm and the second electro-optic modulation arm.

2. The polarization diversity lithium niobate thin-film electro-optic modulator according to claim 1, characterized in that, The input polarization beam splitting rotation unit includes, in sequence along the optical transmission direction, an incident strip waveguide section, an input adiabatic tapered section, a subwavelength grating coupling section, and two output adiabatic tapered sections. The incident strip waveguide section carries the incident optical signal. The output end of the incident strip waveguide section is connected to the input end of the subwavelength grating coupling section via the input adiabatic tapered section. The subwavelength grating coupling section is configured with a direct transmission path and a cross transmission path. The two output ends of the subwavelength grating coupling section are respectively connected to the input ends of the two output adiabatic tapered sections. The output polarization beam splitting rotation unit and the input polarization beam splitting rotation unit are arranged symmetrically in the optical path.

3. A polarization diversity lithium niobate thin-film electro-optic modulator according to claim 1, characterized in that, Both the first and second electro-optic modulation arms adopt Mach-Zehnder modulation structures. Each Mach-Zehnder modulation structure includes an input multimode interference coupler, two parallel herringbone-like grating modulation arm waveguides, and an output multimode interference coupler. The input end of the input multimode interference coupler is connected to the same polarization state optical signal output by the input polarization beam splitting rotation unit, and the two output ends of the input multimode interference coupler are respectively connected to the input ends of the two herringbone-like grating modulation arm waveguides. The coplanar traveling wave electrodes adopt a ground-signal-ground-signal-ground arrangement, and periodically loaded microstructures are set on the signal electrodes.

4. A monolithic integrated system for a polarization diversity lithium niobate thin-film electro-optic modulator, the system being applicable to the polarization diversity lithium niobate thin-film electro-optic modulator according to any one of claims 1-3, characterized in that, The system includes: an optical input coupling module, a polarization diversity processing module, an electro-optic modulation module, a polarization beam combining output module, an optical output coupling module, and an RF driving module set up in sequence along the optical transmission direction. All modules are monolithically fabricated on the same lithium niobate substrate on an insulator. The optical input coupling module is used to couple the optical signal between the external optical fiber and the on-chip waveguide, and output the incident optical signal to the polarization diversity processing module. The polarization diversity processing module is used to perform polarization beam splitting and polarization rotation of the incident optical signal, and output two same-polarization optical components to the two modulation channels of the electro-optic modulation module. The RF drive module is used to output a synchronous RF modulation signal to the electro-optic modulation module. The electro-optic modulation module is used to perform electro-optic modulation of the two same-polarization optical components, and output two modulated optical signals to the polarization beam combining output module. The polarization beam combining output module is used to perform polarization beam combining and polarization state restoration of the two modulated optical signals, and the combined optical signal is output through the optical output coupling module.

5. The monolithic integrated system of a polarization diversity lithium niobate thin-film electro-optic modulator according to claim 4, characterized in that, In the optical input coupling module, during optical signal coupling, the incident optical signal output from the external optical fiber is transmitted to the end-face subwavelength grating section, which adapts the mode field distribution of the incident optical signal. When designing the structural parameters of the end-face subwavelength grating section, the equivalent refractive index correction formula for the subwavelength grating's fundamental TE mode is used. The incident optical signal is transmitted through an adiabatic tapered transition section, where the waveguide width is gradually adjusted. The incident optical signal is then transmitted to a strip waveguide section, which outputs the incident optical signal to the input of the polarization diversity processing module. The equivalent refractive index correction formula for the subwavelength grating's fundamental TE mode is: ,in, The equivalent refractive index of the fundamental TE mode of the subwavelength grating segment waveguide at the end face. The ordinary refractive index of lithium niobate crystal is... The refractive index of the silica cladding is... This is the nominal duty cycle of the subwavelength grating. The duty cycle perturbation introduced to the waveguide sidewall tilt angle.

6. The monolithic integrated system of a polarization diversity lithium niobate thin-film electro-optic modulator according to claim 4, characterized in that, In the polarization diversity processing module, the following operations are performed when polarizing the optical signal: First, the incident optical signal enters the mode evolution region, and the transverse magnetic mode component in the incident optical signal is gradually converted into a higher-order transverse electric mode; Second, the incident optical signal enters the directional coupling region, and the higher-order transverse electric mode is converted into a basic-order transverse electric mode and separated into the cross-transmission channel. When designing the structural parameters of the directional coupling region, the polarization beam splitting length is determined using the formula for the polarization beam splitting length of the asymmetric directional coupler. In the third step, the transverse electric mode component of the incident light signal propagates along the direct transmission channel, and both same-polarization light components are output in the fundamental transverse electric mode to the two independent modulation channels of the electro-optic modulation module. The formula for the polarization beam splitting length of the asymmetric directional coupler is: ,in, The polarization beam splitting coupling length of the directional coupling region. The working light wavelength, The equivalent refractive index of the fundamental TE mode of the through-path subwavelength grating waveguide. The equivalent refractive index of the fundamental TE mode of the cross-path subwavelength grating waveguide is given.

7. The monolithic integrated system of a polarization diversity lithium niobate thin-film electro-optic modulator according to claim 4, characterized in that, In the electro-optic modulation module, the following operations are performed during electro-optic modulation of the optical signal: First, each same-polarization optical component is split into two beams by an input multimode interference coupler, and transmitted to two herringbone-like grating modulation arm waveguides respectively; Second, an RF modulation signal is applied to the coplanar traveling-wave electrode corresponding to the herringbone-like grating modulation arm waveguide to phase-modulate the optical components transmitted within the herringbone-like grating modulation arm waveguide; when designing the structural parameters of the herringbone-like grating modulation arm waveguide, the length of the modulation arm is determined using the half-wave voltage-length product formula of the slow-electro-optic modulator; Third, the two phase-modulated optical components are combined by an output multimode interference coupler to form two modulated optical signals output to the polarization combining output module; the half-wave voltage-length product formula of the slow-electro-optic modulator is: ,in, It is the product of the half-wave voltage of the Mach-Zehnder modulation structure and the effective operating length of the herringbone-like grating modulation arm waveguide. The working light wavelength, The distance between the signal electrode of the coplanar traveling wave electrode and the adjacent ground electrode. The unusual optical refractive index of lithium niobate crystals The longitudinal electro-optic coefficient of lithium niobate crystal is . The waveguide optical overlap integral factor for the modulation arm of the fishbone grating. The group refractive index of the fishbone-like grating modulation arm waveguide.

8. The monolithic integrated system of a polarization diversity lithium niobate thin-film electro-optic modulator according to claim 4, characterized in that, In the radio frequency driving module, the radio frequency modulation signal output by the radio frequency signal source is transmitted to the impedance matching network. The impedance matching network adjusts the impedance of the radio frequency modulation signal. The radio frequency modulation signal after impedance adjustment is divided into two synchronous branch signals, which are transmitted to two signal branches of the coplanar traveling wave electrode respectively. The two signal branches load the branch signals into the two modulation channels of the electro-optic modulation module respectively, and the transmission path lengths of the two branch signals are the same.

9. A monolithic integrated system of a polarization diversity lithium niobate thin-film electro-optic modulator according to claim 4, characterized in that, The polarization beam combining output module and the polarization diversity processing module are arranged symmetrically in the optical path. The two input terminals of the polarization beam combining output module are respectively connected to the two modulated optical signals output by the electro-optic modulation module. The output terminal of the polarization beam combining output module is connected to the input terminal of the optical output coupling module. The optical output coupling module and the optical input coupling module are symmetrical in structure. The mode field parameters of the output terminal of the optical output coupling module correspond to the mode field parameters of the external single-mode fiber.

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