An on-chip integrated mid-infrared high-speed waveguide modulator and its design method

Abstract

To address the issue that existing mid-infrared modulators cannot simultaneously achieve high speed, low loss, and on-chip integration compatibility, this invention proposes an on-chip integrated mid-infrared high-speed waveguide modulator and its design method, belonging to the field of semiconductor optoelectronic device technology. The modulator includes an InP substrate, an epitaxial structure grown on the InP substrate, and a traveling-wave electrode structure fabricated on the epitaxial structure. The epitaxial structure, from bottom to top, includes an InP contact layer, an InAlAs lower optical confinement layer, an InP / InGaAs lower waveguide layer, an asymmetric multiple quantum well structure, an InP / InGaAs upper waveguide layer, an InAlAs upper optical confinement layer, and a Cap layer. The design method includes selecting a compatible material system, designing an asymmetric multiple quantum well structure, fabricating the epitaxial structure into a ridge waveguide structure, designing the traveling-wave electrode, and performing device characterization and testing. This invention is applicable to the design and fabrication of high-speed optical modulation devices in the mid-infrared band.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor optoelectronic device technology, specifically relating to an on-chip integrated mid-infrared high-speed waveguide modulator. Background Technology

[0002] The mid-infrared band (3-14μm), as a crucial "atmospheric window" and "molecular fingerprint region," possesses irreplaceable advantages in fields such as high-speed space communication and trace gas detection. With the upgrading of mid-infrared technology towards "integration and miniaturization," on-chip integrated high-speed optical modulators have become the core unit for constructing mid-infrared integrated chips. Their performance directly determines the signal transmission rate, optical loss, and compatibility of the integrated system. Electro-absorption modulators (EAMs), due to their compact structure, high modulation rate, and low driving voltage, have become the preferred modulation scheme for on-chip integration of mid-infrared signals.

[0003] Current mid-infrared modulator systems have significant technical limitations, making it difficult to meet on-chip integration requirements. They mainly fall into three categories: The first category is the Si / Ge electrorefractive index modulation system based on the plasma dispersion effect. Although it has achieved 2-4μm wavelength coverage and a high transmission rate of 80Gbit / s, this system has poor material compatibility with mainstream mid-infrared GaAs / InP quantum well devices (such as QCLs). Integration requires the introduction of heterogeneous material bonding processes, easily leading to increased interface optical loss (usually exceeding 2dB), and it cannot cover the key mid-infrared bands above 4μm. The second category is lithium niobate modulators based on the Pockels effect, with bandwidth exceeding 20GHz and wavelength coverage of 2-3μm. However, as an inorganic non-metallic material system, it is extremely difficult to integrate with semiconductor devices on-chip, and the devices are relatively large (usually millimeter-scale), making them unsuitable for the needs of miniaturized integrated chips. The third category is new modulators based on two-dimensional materials such as graphene and black phosphorus. Although they have the potential for wide wavelength coverage, currently... The 3dB bandwidth is only at the kHz level, which is far from meeting the needs of mid-infrared high-speed communication (which requires GHz-level bandwidth).

[0004] From the perspective of integration compatibility and technological maturity, GaAs / InP quantum well modulators are the optimal option for achieving low-loss on-chip integration. This system is highly compatible with the material processing of core devices such as mid-infrared QCLs and SOAs, and can be fabricated in an integrated manner using the same epitaxial growth technique, significantly reducing integration losses. However, currently only a few mid-infrared spatial photoelectric absorption modulators with quantum well structures have been reported in the industry, and these devices are all "free-space output" structures, rather than "waveguide-type" structures suitable for on-chip integration.

[0005] Crucially, core light source devices such as mid-infrared quantum well (QCL) generally employ strip waveguide structures to guide laser output. This necessitates that the integrated optical amplifier (EAM) also adopt a strip waveguide structure to achieve efficient coupling and low-loss transmission of optical signals. However, to date, there are no reports on waveguide-based mid-infrared quantum well (EAM) devices, either domestically or internationally. The core technological bottlenecks lie in three aspects: First, the active region design of the mid-infrared quantum well (EAM) must simultaneously meet the requirements of "high-speed modulation response" and "low waveguide loss." High-speed modulation requires a short carrier lifetime (<10 ps) in the active region, while low waveguide loss requires a high refractive index matching between the active region and the waveguide layer, creating a natural constraint between the two. Second, the lateral mode constraint capability and modulation bandwidth of the strip waveguide structure are difficult to optimize synergistically. Enhancing lateral mode constraint requires increasing waveguide width, which leads to increased parasitic capacitance and reduced modulation bandwidth. Thirdly, the epitaxial growth uniformity of mid-infrared GaAs / InP quantum well materials is poor, and the defect density at the active region quantum well interface is high, resulting in unstable electroabsorption coefficient and a modulation extinction ratio of less than 15dB, which cannot meet the requirements of high-performance integration.

[0006] In summary, current mid-infrared modulator systems have significant shortcomings in terms of "high speed, low loss, and on-chip integration compatibility." In particular, the lack of waveguide-type quantum well EAMs adapted to QCL strip waveguide structures has become a core bottleneck restricting the development of high-speed integrated chips in the mid-infrared region. There is an urgent need to propose an innovative waveguide-type mid-infrared quantum well EAM design scheme to overcome the above-mentioned technical challenges. Summary of the Invention

[0007] This invention addresses the problem that existing mid-infrared modulators cannot simultaneously achieve high speed, low loss, and on-chip integration compatibility, especially the lack of technology for waveguide-type quantum well (EAM) modulators adapted to QCL strip waveguide structures. It proposes an on-chip integrated mid-infrared high-speed waveguide-type quantum well electroabsorption modulator, aiming to achieve the following objectives: adaptable to the mid-infrared band, high modulation rate (>10GHz), and fully compatible with the material and process of GaAs / InP QCL and SOA devices, enabling on-chip integrated design and providing a core modulation unit for mid-infrared high-speed integrated chips.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] In a first aspect, the present invention provides an on-chip integrated mid-infrared high-speed waveguide modulator, the modulator comprising an InP substrate, an epitaxial structure grown on the InP substrate, and a traveling wave electrode structure fabricated on the epitaxial structure.

[0010] The epitaxial structure, from bottom to top, includes an InP contact layer, an InAlAs lower optical confinement layer, an InP / InGaAs lower waveguide layer, an asymmetric multiple quantum well structure, an InP / InGaAs upper waveguide layer, an InAlAs upper optical confinement layer, and a Cap layer.

[0011] Secondly, the present invention also provides a design method for an on-chip integrated mid-infrared high-speed waveguide modulator. This design method is used to fabricate the aforementioned on-chip integrated mid-infrared high-speed waveguide modulator, and the design method includes the following steps:

[0012] Step 1: Select an InP / InGaAs / InAlAs material system compatible with InP-based quantum cascade lasers as the basis for modulator fabrication;

[0013] Step 2: Design an asymmetric multi-quantum-well structure to realize the electro-optic modulation function of the modulator;

[0014] Step 3: Apply a bias voltage to the asymmetric multi-quantum-well structure to change the energy difference between level 1 and level 2 in any two quantum wells;

[0015] Step 4: Fabricate the epitaxial structure into a ridge waveguide structure;

[0016] Step 5: Use the traveling wave electrode as the modulator electrode.

[0017] Furthermore, the above-mentioned asymmetric multi-quantum-well structure is prepared by layer-by-layer growth using molecular beam epitaxy. After the growth of a single layer of material is completed, it is first exposed to radiation with an appropriate group V beam for a specific duration before resuming the subsequent growth process.

[0018] Furthermore, after the asymmetric multi-quantum-well material is grown, a systematic analysis of its physical properties and optoelectronic performance is conducted through multi-dimensional characterization methods, thereby providing feedback to guide material optimization.

[0019] Furthermore, the energy level distribution and wave function morphology of the multiple quantum wells under external electric field modulation are calculated using an 8-band K•P perturbation algorithm as basic data. Based on the basic data, the dynamic evolution of exciton binding energy, absorption coefficient spectrum, modulation extinction characteristics, and chirp factor with electric field intensity is analyzed. The above-mentioned asymmetric multiple quantum well structure is verified through the dynamic evolution law.

[0020] Furthermore, the aforementioned ridge waveguide structure includes a high ridge waveguide and a low ridge waveguide;

[0021] A high-ridge waveguide is defined as one whose ridge height is greater than the sum of the thicknesses of the quantum well region, the upper cladding, and the electrode contact layer.

[0022] Furthermore, the above-mentioned traveling wave electrode design is specifically as follows:

[0023] The drive signal is input from one end of the modulator and transmitted synchronously with the optical signal to the other end of the modulator, and the output is configured with a matching load.

[0024] Furthermore, the above design method also includes modulator characterization testing steps, including static characteristic testing and frequency response characteristic testing.

[0025] Furthermore, the aforementioned static characteristic tests include near-field spot morphology observation, static modulation response, extinction ratio, driving voltage threshold, and insertion loss;

[0026] The near-field spot morphology observation specifically includes;

[0027] A distributed feedback quantum cascade laser is used as the excitation source. The laser is coupled into the ridge waveguide structure of the modulator through an optical lens system. The laser signal at the output of the ridge waveguide is imaged onto the target surface of the charge-coupled device camera through another set of lenses. The center scanning curve of the optical field is extracted by the display, and the uniformity of light intensity and the waveguide confinement effect are analyzed.

[0028] Furthermore, the above frequency response characteristic test specifically includes:

[0029] The mid-infrared laser emitted by the quantum cascade laser is collimated by a lens and coupled to the modulator ridge waveguide. At the same time, the microwave sweep signal output by the network analyzer is used as the modulation signal and applied to the modulator electrode.

[0030] The modulated optical signal is coupled to the mid-infrared photodetector through a lens to complete the photoelectric conversion. The weak electrical signal output by the mid-infrared photodetector is then boosted by a preamplifier and input to the receiver of the network analyzer.

[0031] The S12 scattering parameters, collected by a network analyzer, characterize the modulator's response capability at different frequencies.

[0032] The beneficial effects of this invention are as follows:

[0033] The on-chip integrated mid-infrared high-speed waveguide modulator designed in this invention ensures process compatibility and low coupling loss during on-chip integration by using a material system fully compatible with InP-based quantum cascade lasers (QCLs). Simultaneously, it innovatively designs an asymmetric multi-quantum-well structure. By utilizing the inherent differences in electronic energy level distribution within different well width regions, the energy difference between level 1 and level 2 in the two quantum wells is modulated under an applied bias voltage. This enables high-speed switching between the absorption and transmission states of the mid-infrared laser emission wavelength, meeting the high-speed modulation requirements of the mid-infrared band. This addresses the significant limitations of traditional InP-based InGaAs / InAlAs quantum well structures, whose inherent bandgap width is approximately 0.74 eV, far exceeding the photon energy in the mid-infrared band. Even under an applied bias voltage, they can only achieve absorption modulation of near-infrared light around 1.6 μm, failing to cover the critical mid-infrared application band.

[0034] Furthermore, to ensure the fabrication accuracy of the asymmetric multi-quantum-well structure is consistent with the theoretical design, this invention also employs molecular beam epitaxy (MBE) for material growth; simultaneously, to address the control of interface layer thickness and the segregation effect caused by interface atomic diffusion and exchange, an innovative interrupted growth process is introduced to construct the target interface.

[0035] Furthermore, to improve on-chip integration with other devices, the present invention also employs a ridge waveguide structure design.

[0036] This invention is applicable to the design and fabrication of high-speed optical modulation devices in the mid-infrared band. Its core application is in mid-infrared integrated optoelectronic chips, especially suitable for mid-infrared free-space optical communication (FSOC), mid-infrared spectral analysis, high-precision lidar and other fields. It can be integrated on-chip with devices such as quantum cascade lasers (QCL) and semiconductor optical amplifiers (SOA) to meet the system's comprehensive requirements for "high-speed modulation, low loss, miniaturization and high compatibility". Attached Figure Description

[0037] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0038] Figure 1 The diagram shows the structure of the on-chip integrated mid-infrared high-speed waveguide modulator EAM and the structure of the asymmetric multi-quantum well structure (stack modulation region) described in this invention.

[0039] Figure 2The diagram shows the electrode structure of the on-chip integrated mid-infrared high-speed waveguide modulator EAM according to the present invention, wherein Figure (a) is the lumped electrode structure and Figure (b) is the traveling wave electrode structure. Detailed Implementation

[0040] The specific implementation details of "an on-chip integrated mid-infrared high-speed waveguide modulator and its design method" provided in this specification are primarily intended for illustrative purposes rather than limiting definitions, aiming to help those skilled in the art thoroughly understand the principles and implementation of the invention. However, those skilled in the art should understand that these details represent only one feasible embodiment, and the core concept of the invention can be fully realized through other technical means or alternative solutions not described in detail, without departing from its spirit and essence. Furthermore, the omission of details of conventional experimental methods and apparatus known in the art in the specification is to avoid redundant information interfering with the understanding of the innovation points. This does not mean that these known technologies are not required during implementation, and those skilled in the art should be able to supplement and apply them based on their professional knowledge.

[0041] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings. The following examples will help those skilled in the art to further understand the present invention, but do not limit the present invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention.

[0042] Implementation Method 1: Combination Figure 1 and Figure 2 This embodiment addresses the problem that existing mid-infrared modulators cannot simultaneously achieve high speed, low loss, and on-chip integration compatibility, especially the lack of technology for waveguide-type quantum wells (EAMs) adapted to QCL strip waveguide structures. This embodiment provides an on-chip integrated mid-infrared high-speed waveguide modulator, which includes an InP substrate, an epitaxial structure grown on the InP substrate, and a traveling wave electrode structure fabricated on the epitaxial structure.

[0043] The epitaxial structure, from bottom to top, includes an InP contact layer, an InAlAs lower optical confinement layer, an InP / InGaAs lower waveguide layer, an asymmetric multiple quantum well structure, an InP / InGaAs upper waveguide layer, an InAlAs upper optical confinement layer, and a Cap layer.

[0044] The on-chip integrated mid-infrared high-speed waveguide modulator structure proposed in this embodiment is as follows: Figure 1As shown, the modulator uses a material system fully compatible with InP-based quantum cascade lasers (QCLs), namely the InP, InGaAs, and InAlAs series materials, to ensure process compatibility and low coupling loss during on-chip integration. Meanwhile, due to the significant limitations of traditional InP-based InGaAs / InAlAs quantum well structures, their inherent bandgap width is approximately 0.74 eV, much larger than the photon energy in the mid-infrared band. Even under an applied bias, they can only achieve absorption modulation of near-infrared light around 1.6 μm, failing to cover the critical mid-infrared application band. Therefore, an innovative asymmetric multi-quantum-well structure is incorporated into the modulator structure, namely… Figure 1 The stack modulation region of the asymmetric multi-quantum-well structure utilizes the inherent differences in electronic energy level distribution within different well width regions. Under an applied bias voltage, it modulates the energy difference between level 1 and level 2 in any two quantum wells, thereby achieving high-speed switching between the "absorption-transmission" state of the mid-infrared laser emission wavelength, meeting the high-speed modulation requirements of the mid-infrared band. Figure 1 As can be seen, the stack modulation region structure, from bottom to top, includes: InAlAs (20nm), InGaAs (2.4nm), InAlAs (1.6nm), InGaAs (6.8nm), InAlAs (20nm), InGaAs (2.4nm), InAlAs (1.6nm), and InGaAs (6.8nm). The InGaAs (6.8nm), InAlAs (1.6nm), InGaAs (2.4nm), and InAlAs (20nm) layers have 120 periods. This multi-period quantum well synergy addresses the insufficient absorption efficiency of mid-infrared modulation, enhancing the absorption efficiency of mid-infrared light. The electrode structure directly affects the modulation performance of the EAM. Commonly used lumped electrode structures include... Figure 2 As shown in (a), its equivalent length is small. Although it has a high modulation rate, it suffers from low extinction ratio, high driving voltage, and power saturation problems. Therefore, this embodiment adopts a traveling wave electrode structure, such as... Figure 2 As shown in (b), the driving signal enters from one end of the waveguide and is transmitted synchronously to the other end of the waveguide with the optical signal. At the output end, due to the presence of a matched load, the reflection rate of the driving signal is greatly reduced. The parasitic parameters of its equivalent circuit are distributed. Within each short segment in the direction of driving signal transmission, the parallel capacitance of the device is compensated by the series inductance, and together they form a microwave transmission line with a certain characteristic impedance. This overcomes the limitation of the RC time constant of the lumped electrode and, while ensuring the modulation rate, increases the saturation power and extinction ratio by increasing the length.

[0045] Implementation Method 2: This implementation method provides a design method for an on-chip integrated mid-infrared high-speed waveguide modulator. This design method is used to fabricate the on-chip integrated mid-infrared high-speed waveguide modulator described in Implementation Method 1 above. The design method includes the following steps:

[0046] Step 1: Select an InP / InGaAs / InAlAs material system compatible with InP-based quantum cascade lasers as the basis for modulator fabrication;

[0047] Step 2: Design an asymmetric multi-quantum-well structure to realize the electro-optic modulation function of the modulator;

[0048] Step 3: Apply a bias voltage to the asymmetric multi-quantum-well structure to change the energy difference between level 1 and level 2 in any two quantum wells;

[0049] Step 4: Fabricate the epitaxial structure into a ridge waveguide structure;

[0050] Step 5: Use the traveling wave electrode as the modulator electrode.

[0051] In practical operation, the first step in this implementation is the design of the band structure of an asymmetric multi-quantum-well Stark effect modulator. Designing the band structure and function of a waveguide-type electro-absorption modulator based on the quantum-confined Stark effect is a key technological path for achieving rapid modulation of laser signals. To reduce optical interface reflection loss and ensure high-quality compatibility with mid-infrared integrated chips, the modulator must use a material system fully compatible with InP-based quantum cascade lasers (QCLs), namely InP, InGaAs, and InAlAs series materials, to ensure process compatibility and low coupling loss during on-chip integration. However, traditional InP-based InGaAs / InAlAs quantum well structures have significant limitations. Their inherent bandgap width is approximately 0.74 eV, much larger than the photon energy in the mid-infrared band. Even under an applied bias, they can only achieve absorption modulation of near-infrared light around 1.6 μm, failing to cover the critical mid-infrared application band. Therefore, to overcome this material limitation and achieve effective modulation in the mid-infrared band, this implementation innovatively designs an asymmetric multi-quantum-well structure. Specifically, using an 8-band K•P perturbation algorithm, the energy level distribution and wave function morphology of multiple quantum wells under external electric field modulation are accurately calculated. The dynamic evolution of exciton binding energy, absorption coefficient spectrum, modulation extinction characteristics, and chirp factor with electric field strength is systematically analyzed. By utilizing the inherent differences in electronic energy level distribution within different well width regions, the energy difference between level 1 and level 2 in two quantum wells is controlled under an external bias voltage, thereby achieving high-speed switching of the "absorption-transmission" state of mid-infrared laser emission wavelength to meet the high-speed modulation requirements of the mid-infrared band.

[0052] Secondly, the epitaxy and device fabrication of the mid-infrared high-speed waveguide modulator (EAM) structure are discussed. The consistency between the fabrication precision and theoretical design of the asymmetric multi-quantum-well structure is a core prerequisite for ensuring the high modulation performance of the EAM. To precisely control the alloy composition uniformity and the thickness accuracy of each film layer in the asymmetric multi-quantum-well material, this implementation uses molecular beam epitaxy (MBE) for material growth. To address the control of interface layer thickness and the segregation effect caused by interface atomic diffusion and exchange, an innovative interrupted growth process is introduced to construct the target interface. Specifically, after completing the growth of a single layer, the material is first exposed to radiation with a suitable group V beam for a specific duration before resuming the subsequent growth process. Previous experiments have verified that this process can effectively form an atomically flat interface and significantly reduce the interface defect density. After the growth of high-quality quantum well materials is completed, a systematic analysis of the material's physical properties and photoelectric performance is required through multi-dimensional characterization methods to guide material optimization. Specifically, X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and Raman spectroscopy are used to characterize the material's crystal quality, microstructure, and lattice matching degree. Photoluminescence (PL), absorption photocurrent spectroscopy, and electromodulation absorption spectroscopy are used to analyze the carrier transition law and radiative and non-radiative recombination mechanism in the active region of the material. Through comparative analysis of characterization data and theoretical design, the epitaxial growth parameters are iteratively optimized to finally obtain high-quality epitaxial materials that meet the requirements for waveguide-type EAM fabrication.

[0053] To improve on-chip integration with other devices, the waveguide modulator EAM designed in this embodiment adopts a ridge waveguide structure design, specifically divided into two types: high-ridge waveguide and low-ridge waveguide. The high-ridge structure is defined as having a ridge height greater than the sum of the thicknesses of the quantum well region, the upper cladding (InAlAs optical confinement layer), and the electrode contact layer. The device fabrication process is as follows: substrate cleaning, deep etching of the modulator region, secondary cleaning, photolithography to define the device pattern, growth of high-resistivity material for electrical isolation, etching of the high-resistivity material growth area, tertiary cleaning, epitaxial growth of the complete EAM structure, ridge waveguide forming, secondary deep etching of the modulator region, benzocyclobutene (BCB) passivation process, electrode region isolation etching, substrate thinning, and evaporation of the back electrode. To address the critical impact of electrode structure on EAM modulation performance, this embodiment abandons the traditional lumped electrode. While such electrodes can achieve high modulation rates due to their short equivalent length, they suffer from inherent defects such as low extinction ratio, high driving voltage, and power saturation. Therefore, the waveguide modulator EAM designed in this embodiment adopts a traveling wave electrode design. Specifically, the driving signal is input from one end of the waveguide and transmitted synchronously with the optical signal to the other end. A matched load is configured at the output end to significantly reduce driving signal reflection. The parasitic parameters of its equivalent circuit exhibit distributed characteristics. Within each short segment of the driving signal transmission direction, the parallel capacitor of the device is compensated by the series inductor, forming a microwave transmission line with a specific characteristic impedance. This effectively overcomes the RC time constant limitation of the lumped electrode, ensuring a high modulation rate while simultaneously increasing the electrode length to achieve a synergistic improvement in saturation power and extinction ratio.

[0054] Finally, the waveguide modulator design method proposed in this embodiment also includes modulator characterization and testing steps, including static characteristic testing and frequency response characteristic testing. The static characteristics of the electroabsorption type optical modulator are key indicators for evaluating its basic performance, mainly including parameters such as near-field spot morphology observation of the modulator output, static modulation response, extinction ratio, driving voltage threshold, and insertion loss. Near-field spot morphology observation uses a distributed feedback (DFB) quantum cascade laser as the excitation source, and the laser is precisely coupled into the modulator's ridge waveguide structure through an optical lens system. The laser signal at the ridge waveguide output is imaged onto the target surface of a charge-coupled device (CCD) camera through another set of lenses. The spot shape at the ridge waveguide end face is observed in real time on a display, and simultaneously, a scanning curve passing through the center of the optical field is acquired and extracted using an oscilloscope. The uniformity of the light intensity distribution is analyzed through this curve to determine the constraint effect of the waveguide structure on the optical field. Frequency response characteristic testing is used to verify the high-speed modulation capability of the modulator. The specific process is as follows: The mid-infrared laser emitted by the quantum cascade laser (QCL) is collimated by a lens and coupled to the modulator ridge waveguide in a low-loss manner; the modulation signal is generated by the signal output port of the network analyzer and applied to the modulator electrodes using a microwave sweep signal as the driving source; the modulated optical signal is exported through the lens coupling system, transmitted to the mid-infrared photodetector, and photoelectric conversion is completed, converting the optical signal into the corresponding electrical signal; the weak electrical signal output by the detector is boosted by a preamplifier and then input to the signal receiving port of the network analyzer; finally, the S12 scattering parameters collected by the network analyzer are used to directly characterize the response capability of the optical modulator at different frequencies, thereby determining its core high-frequency performance indicators such as the -3dB modulation bandwidth.

[0055] Implementation Method 3: This implementation method provides specific steps for an on-chip integrated mid-infrared high-speed waveguide modulator, used to comprehensively explain the design method described in the above implementation methods, specifically including the following steps:

[0056] 1. Design steps for the band structure of an asymmetric multi-quantum-well Stark effect modulator:

[0057] 1.1 Determine the material system: Select the InP / InGaAs / InAlAs material system that is compatible with InP-based quantum cascade lasers (QCLs) to ensure process compatibility and low coupling loss during on-chip integration, and reduce optical interface reflection loss.

[0058] 1.2 Analysis of the limitations of traditional structures: The defects of traditional InP-based InGaAs / InAlAs quantum wells (with an inherent band gap of 0.74 eV) are identified, which can only modulate near-infrared light at around 1.6 μm and cannot cover the mid-infrared band, providing a direction for innovative designs.

[0059] 1.3 Design of asymmetric multi-quantum-well structure: Innovatively construct an asymmetric multi-quantum-well structure, and lay the structural foundation for subsequent energy level control through differentiated well width design.

[0060] 1.4 Energy Level and Wave Function Calculation: The 8-band K•P perturbation algorithm is used to accurately calculate the energy level distribution and wave function morphology of multiple quantum wells under external electric field modulation, and obtain basic data.

[0061] 1.5 Key Parameter Analysis: Based on the calculation results, the dynamic evolution of exciton binding energy, absorption coefficient spectrum, modulation extinction characteristics and chirp factor with electric field strength is analyzed to verify the feasibility of the structure.

[0062] 1.6 Modulation Mechanism Optimization: Utilizing the difference in electronic energy level distribution in different well width regions, an external bias control scheme is designed—by changing the energy difference between the 1st and 2nd energy levels in the two quantum wells through bias, a high-speed switching of the mid-infrared laser “absorption-transmission” state is achieved.

[0063] 2. EAM structure epitaxy and device fabrication steps:

[0064] 2.1 Epitaxial Growth Preparation: Using semi-insulating InP as the substrate, determine the technical parameters of molecular beam epitaxy (MBE), including growth temperature (500-520℃) and growth rate (0.5-1nm / s), to ensure the accuracy of material growth.

[0065] 2.2 Asymmetric Multi-Quantum-Well Epitaxial Growth: Asymmetric multi-quantum-well structures were grown using MBE technology, with an interruption process introduced during growth. After the growth of a single layer of material was completed, the material was exposed to radiation with an adapted Group V beam for a specific duration, and then growth resumed to form an atomically smooth interface, reducing the interface defect density.

[0066] 2.3 Multidimensional characterization of materials:

[0067] 2.3.1 Physical property characterization: X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and Raman spectroscopy were used to detect the crystal quality, microstructure, and lattice matching degree of the material.

[0068] 2.3.2 Photoelectric performance characterization: The active region carrier transition law and radiative and non-radiative recombination mechanism were analyzed by photoluminescence (PL), absorption photocurrent spectroscopy and electromodulation absorption spectroscopy.

[0069] 2.3.3 Parameter optimization feedback: By comparing the characterization data with the theoretical design values, the MBE growth parameters are iteratively adjusted to obtain high-quality epitaxial materials that meet the requirements for waveguide-type EAM fabrication.

[0070] 2.4 Fabrication of ridge waveguide structure:

[0071] 2.4.1 Pretreatment: Substrate cleaning, deep etching of the modulator area, and secondary cleaning are carried out sequentially;

[0072] 2.4.2 Pattern Definition and Isolation: The device pattern is defined by photolithography, a high-resistivity material for electrical isolation is grown, and the high-resistivity material growth area is etched and then cleaned three times.

[0073] 2.4.3 Structure Forming: Epitaxial growth of the complete EAM structure, fabrication of ridge waveguides (divided into high ridge / low ridge, high ridge is defined as ridge height > the sum of the thickness of the quantum well region + upper cladding + electrode contact layer), and secondary deep etching of the modulator region.

[0074] 2.5 Passivation and Electrode Fabrication:

[0075] 2.5.1 Passivation process: Benzocyclobutene (BCB) is used for passivation treatment to protect the device structure;

[0076] 2.5.2 Electrode fabrication: Etching the isolation layer in the electrode area, thinning the substrate, and evaporating the back electrode;

[0077] 2.5.3 Electrode Optimization: Abandoning the traditional lumped electrode, a traveling wave electrode is adopted. The driving signal is input from one end of the waveguide and transmitted synchronously with the optical signal. The output end is equipped with a matching load. A microwave transmission line is formed by "parallel capacitor + series inductor compensation", which breaks through the limitation of RC time constant.

[0078] 3. EAM device characterization and testing procedures:

[0079] 3.1 Static Characteristic Test:

[0080] 3.1.1 Near-field spot observation: Using a distributed feedback (DFB) quantum cascade laser as the light source, the laser is coupled to the EAM ridge waveguide through an optical lens system. The output light is imaged onto the target surface of a CCD camera through a lens. The shape of the spot is observed on a display, and the center scanning curve of the light field is extracted by an oscilloscope to analyze the uniformity of light intensity and the waveguide confinement effect.

[0081] 3.1.2 Core parameter testing: Detect static modulation response, extinction ratio, drive voltage threshold and insertion loss, and record basic performance data.

[0082] 3.2 Frequency response characteristic test:

[0083] 3.2.1 Optical path setup: The mid-infrared laser emitted by the quantum cascade laser (QCL) is collimated by a lens and then coupled to the modulator EAM ridge waveguide with low loss;

[0084] 3.2.2 Signal Loading and Conversion: The microwave sweep signal output by the network analyzer is used as a modulation signal and loaded onto the EAM electrode; the modulated optical signal is coupled to the mid-infrared photodetector through a lens to complete the photoelectric conversion;

[0085] 3.2.3 Signal Amplification and Analysis: The weak electrical signal output by the detector is amplified by the preamplifier and then input to the receiver of the network analyzer.

[0086] 3.2.4 Performance Determination: The S12 scattering parameters collected by the network analyzer are used to characterize the modulator's response capability at different frequencies and determine high-frequency performance indicators such as the -3dB modulation bandwidth.

[0087] 3.3 Test Result Verification: Compare the test data with the design goals (such as modulation rate > 10GHz, extinction ratio > 20dB) to verify whether the device performance meets the requirements of the mid-infrared integrated chip. If it does not meet the requirements, it will be fed back to step 1-2 for parameter optimization.

[0088] In the above description, it should be understood that, when used in this specification and the appended claims, the term "comprising" indicates the presence of the described features, integrals, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0089] The above description of the technical solution provided by the present invention through several specific embodiments is intended to highlight the advantages and benefits of the technical solution provided by the present invention. However, the above-described specific embodiments are not intended to limit the present invention. Any reasonable modifications and improvements to the present invention, reasonable combinations of implementation methods and equivalent substitutions based on the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A mid-infrared high-rate waveguide-type modulator that can be integrated on a chip, characterized in that, The modulator comprises an InP substrate, an epitaxial structure grown on the InP substrate, and a traveling wave electrode structure prepared on the epitaxial structure; The epitaxial structure comprises, from bottom to top, an InP contact layer, an InAlAs lower light confinement layer, an InP / InGaAs lower waveguide layer, an asymmetric multi-quantum well structure, an InP / InGaAs upper waveguide layer, an InAlAs upper light confinement layer, and a Cap layer. The asymmetric multi-quantum well structure comprises, from bottom to top, InAlAs, InGaAs, InAlAs, InGaAs, InAlAs, InGaAs, InAlAs, and InGaAs.

2. A design method of a mid-infrared high-rate waveguide modulator that can be integrated on a chip, the design method being used to produce the mid-infrared high-rate waveguide modulator of claim 1, characterized in that, The method comprises: Step 1: Selecting an InP / InGaAs / InAlAs material system compatible with an InP-based quantum cascade laser as the basis for preparing the modulator; Step 2: Designing an asymmetric multi-quantum well structure for realizing the electro-optical modulation function of the modulator; Step 3: Applying a bias voltage to the asymmetric multi-quantum well structure to change the energy difference between the 1 energy level and the 2 energy level in any two quantum wells; The preparation of the asymmetric multi-quantum well structure adopts molecular beam epitaxy technology to grow layer by layer, and when the growth of a single layer of material is completed, the subsequent growth process is resumed after a specific time of exposure to a V-group beam. Step 4: Processing the epitaxial structure into a ridge waveguide structure; Step 5: Using a traveling wave electrode as the electrode of the modulator.

3. The method of claim 2, wherein the method is for designing a mid-infrared high-speed waveguide modulator that can be integrated on a chip. After the growth of the asymmetric multi-quantum well material, a multi-dimensional characterization method is used to systematically analyze the physical properties and photoelectric performance of the material, thereby providing feedback for material optimization.

4. The method of claim 2, wherein the method is used for designing a mid-infrared high-speed waveguide modulator that can be integrated on a chip. The 8-band K•P perturbation algorithm is used to calculate the energy level distribution and wave function morphology of the multi-quantum well under the modulation of an applied electric field, which serves as the basic data; based on the basic data, the dynamic evolution law of the exciton binding energy, absorption coefficient spectrum, modulation extinction characteristic, and chirp factor with respect to the electric field intensity is systematically analyzed, and the feasibility of the asymmetric multi-quantum well structure is verified based on the dynamic evolution law.

5. The method of claim 2, wherein the method is used for designing a mid-infrared high-speed waveguide modulator that can be integrated on a chip. The ridge waveguide structure comprises a high-ridge waveguide and a low-ridge waveguide. The high-ridge waveguide is defined as a ridge height greater than the sum of the thicknesses of the quantum well region, the upper cladding layer, and the electrode contact layer.

6. The method of claim 2, wherein the method is used for designing a mid-infrared high-speed waveguide modulator that can be integrated on a chip. The traveling wave electrode is designed as follows: The driving signal is input from one end of the modulator and is transmitted synchronously with the optical signal to the other end of the modulator, and a matching load is configured at the output end.

7. The method of claim 2, wherein the method is used for designing a mid-infrared high-speed waveguide modulator that can be integrated on a chip. The method further comprises a characterization test step of the modulator, including a static characteristic test and a frequency response characteristic test.

8. The method of claim 7, wherein the method is a design method of a mid-infrared high-speed waveguide modulator that can be integrated on a chip. The static characteristic test comprises near-field spot morphology observation, static modulation response, extinction ratio, driving voltage threshold, and insertion loss. The near-field spot morphology observation is specifically as follows: A distributed feedback quantum cascade laser is used as an excitation light source, and an optical lens system is used to couple the laser into the ridge waveguide structure of the modulator; the laser signal at the output end of the ridge waveguide is imaged on the target surface of a charge-coupled device camera through another set of lenses, and the light field center scanning curve is extracted through a display to analyze the light intensity uniformity and waveguide confinement effect.

9. The method of claim 7, wherein the method is a design method of a mid-infrared high-speed waveguide modulator that can be integrated on a chip. The frequency response characteristic test is specifically as follows: After the mid-infrared laser emitted by the quantum cascade laser is collimated by a lens, it is coupled into the modulator ridge waveguide, and a microwave sweep signal output by a network analyzer is loaded as a modulation signal to the modulator electrode. The modulated light signal is coupled to the mid-infrared light detector through a lens to complete photoelectric conversion, and the weak electric signal output by the mid-infrared light detector is input into a network analyzer receiving end after gain promotion by a preamplifier; The S12 scattering parameters collected by the network analyzer represent the response capability of the modulator at different frequencies.

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