A large-bandwidth mode field converter based on a silica optical waveguide with a grating-type trident structure

Abstract

A kind of large bandwidth mode spot converter based on silicon dioxide optical waveguide grating type trident structure belongs to integrated optoelectronics technical field.From bottom to top, it is composed of substrate layer, lower cladding layer, core layer waveguide and upper cladding layer, lower cladding layer is located on substrate layer, core layer waveguide and upper cladding layer are located on lower cladding layer and core layer waveguide is covered by upper cladding layer;Core layer waveguide is composed of grating type trident input waveguide (composed of Core1, Core2 and Core3) and output waveguide Core4 of straight waveguide structure, Core1 and Core3 structure in grating type trident waveguide are completely same, composed of N similar small taper waveguide and uniformly arranged, Core2 is composed of N similar small inverted taper waveguide and uniformly arranged;Mode spot converter prepared by the application has the characteristics of high coupling efficiency, large bandwidth, compact structure, easy packaging and low process complexity, and has wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of integrated optoelectronics technology, specifically relating to a grating-type trident structure large-bandwidth mode converter based on a silicon dioxide optical waveguide. It can be used for edge coupling between silicon dioxide optical waveguides and single-mode optical fibers in optical path integration, and has important application value and development prospects in optical communication, high-performance computing, optical sensing and other fields. Background Technology

[0002] Photonic chips can be used in fields such as optical communication, big data computing, and artificial intelligence systems. Functional integration in optoelectronics refers to the creation of multifunctional, high-performance devices by integrating components with different functions. For example, photonic chips fabricate optical components (waveguides, modulators, switches, and detectors, etc.) and electronic components (field-effect transistors, etc.) on the same substrate, thereby reducing area, cost, and power consumption. In optoelectronic integration, low-loss connection between optical fibers and waveguides is a crucial prerequisite for fabricating high-performance chips and is also one of the key problems to be solved. Mode field converters are commonly used for coupling optical fibers to waveguide devices on photonic chips. Due to the significant differences in structure and size between waveguides and optical fibers, the coupling loss caused by differences in mode field size and shape becomes the main factor affecting insertion loss.

[0003] To achieve mode field matching between optical fiber and waveguide, the structure and fabrication method of the mode converter need to be optimized. By enhancing the effective refractive index matching and mode field matching between the fiber and the waveguide, coupling loss can be reduced while minimizing process complexity. Integrated optical devices based on silica materials have advantages such as low optical loss, large process tolerance, compatibility with CMOS processes, and good mode field matching with single-mode fibers, and are widely used in optical communication, optical interconnection, and integrated optics. Summary of the Invention

[0004] The purpose of this invention is to provide a high-bandwidth mode converter based on a silicon dioxide optical waveguide with a grating-type trident structure, which features high coupling efficiency, large bandwidth, compact structure, and low manufacturing complexity.

[0005] The present invention discloses a grating-type trident structure large bandwidth mode converter based on silicon dioxide optical waveguide, characterized in that:

[0006] 1. From bottom to top, it consists of a base layer 2, a lower cladding layer 3, a core waveguide 1, and an upper cladding layer 4. (Example) Figure 1 As shown, the lower cladding layer 3 is located above the substrate layer 2, the core waveguide 1 and the upper cladding layer 4 are both located above the lower cladding layer 3, and the core waveguide 1 is covered by the upper cladding layer 4.

[0007] 2. The lower cladding layer 3 and the upper cladding layer 4 are made of the same material, silicon dioxide, with a refractive index of 1.447; the core waveguide 1 is made of germanium-doped silicon dioxide with a refractive index of 1.481; the substrate layer 2 is a silicon wafer with a refractive index of 3.455.

[0008] 3. As shown in Figure 2(a), the core waveguide 1 is located above the lower cladding 3 and is covered by the upper cladding 4; as shown in Figures 2(b) and 2(c), along the direction of light transmission, the core waveguide 1 consists of a grating-type trident input waveguide (including three parts: input waveguide Core1, input waveguide Core2, and input waveguide Core3) and an output waveguide Core4; the output waveguide Core4 is a straight waveguide structure; the bottom surfaces of the grating-type trident input waveguide and the output waveguide Core4 are located in the same plane and are both located on the upper surface of the lower cladding 3; the input waveguide Core1 and the input waveguide Core3 have the same structure. Along the direction of input light, the input waveguide Core1 and the input waveguide Core3 are composed of N=32 similar small conical waveguides that are uniformly arranged and whose outer edges are connected by straight lines. The length (X-axis direction) of each small conical waveguide is d=1.3μm, and the distance (X-axis direction) between every two small conical waveguides is d=1.3μm; the input waveguide Core1... The input width (Y-axis direction) of re1 and input waveguide Core3 is W1 = 1.8 μm, the output width (Y-axis direction) is W2 = 1.1 μm, the height (Z-axis direction) is H = 4 μm, and the length (X-axis direction) is L1 = 83.2 μm. Input waveguide Core2 consists of N = 32 similar small inverted conical waveguides arranged uniformly with straight lines connecting their outer edges. The length of each small inverted conical waveguide is d = 1.3 μm, and the distance between any two small inverted conical waveguides is d = 1.3μm; The input end width W3 of input waveguide Core2 is 2.4μm, the output end width W is 4μm, the height H is 4μm, and the length L1 is 83.2μm; The distance between the input and output ends of input waveguides Core1 and Core2, and input waveguides Core2 and Core3 is equal, with a gap of 1.3μm; The output straight waveguide Core4 has a width W of 4μm, a waveguide height H of 4μm, and a length L2 of 232μm.

[0009] 4. The effective optical mode area of ​​the input cross section (composed of input waveguide Core1, input waveguide Core2, and input waveguide Core3 and adjacent gaps) in the grating-type trident input waveguide matches the optical mode spot size of the optical fiber transmission field. The width of the output end of input waveguide Core2 is the same as the width of output waveguide Core4. The heights of input waveguide Core1, input waveguide Core2, input waveguide Core3, and output waveguide Core4 are always the same. The grating-type trident input waveguide has a tapered structure, and its width decreases from wide to narrow along the direction of light transmission (X-axis direction), thereby realizing the transformation of the optical mode spot size from large to small within the waveguide.

[0010] The working principle of this speckle converter is as follows:

[0011] The coupling efficiency between a waveguide and an optical fiber refers to the proportion of signal light energy coupled from the optical fiber into the waveguide to the total output light energy of the optical fiber (or the proportion of signal light energy coupled from the waveguide into the optical fiber to the total output light energy of the waveguide). The coupling loss between a single-mode optical fiber and a silica waveguide mainly refers to the mode mismatch loss caused by differences in structure, size, and effective refractive index between the waveguide and the optical fiber. According to the optical mode principle, as the width of the silica waveguide gradually decreases, the magnitude of the optical mode field it supports also decreases. When signal light from the optical fiber enters from the input end of the core waveguide grating-type trident structure, the signal mode field within the waveguide is gradually compressed. As the width of the trident cone structure changes, the optical mode field within the waveguide changes from a Gaussian-distributed circular fiber mode field (e.g., ...). Figure 3 As shown, the waveguide mode field gradually transforms into an elliptical waveguide mode field with a Hermitian-Gaussian distribution (as shown). Figure 4 (As shown); The mode field size supported by the input end cross-section of the grating-type trident input waveguide matches the optical mode field size supported by the optical fiber, which can effectively reduce optical loss caused by mode field mismatch; The overall width of the trident structure gradually decreases, and the mode spot size decreases accordingly; The grating structure is used to diffract the signal light, gradually shrinking it into the core waveguide; The trident structure, with Core1 and Core3 waveguides on both sides, can reduce signal light leakage, concentrate the signal light in Core2 waveguide, and achieve high-efficiency transmission of the signal to the output waveguide, that is, to achieve low-loss mode spot conversion from single-mode fiber signal light to single-mode silica waveguide signal light (e.g. Figure 5 (As shown).

[0012] Compared with existing devices, the advantages of this invention are as follows: Compared with traditional continuous tapered mode converters, the silicon dioxide waveguide mode converter of this invention has a discontinuous trident structure at the input end, employing a grating-type waveguide to diffract the signal light and gradually compress it into the core waveguide; the trident structure reduces signal light leakage at the Core1 and Core3 on both sides, concentrating the signal light in the middle Core3 section; the overall tapered structure enhances the mode field matching between the silicon dioxide waveguide and the output optical signal from the optical fiber. Through the grating-type trident input waveguide structure, low-loss transformation from Gaussian mode field to Hermitian-Gaussian mode field transmitted in the optical fiber can be achieved, which is beneficial for integration and packaging. Among different fiber-waveguide coupling structures, this structure has the characteristics of high coupling efficiency, large bandwidth, compact structure, easy packaging, and low process complexity, and has broad application prospects. Attached Figure Description

[0013] Figure 1This is a schematic diagram of a large bandwidth mode converter based on a silicon dioxide optical waveguide grating-type trident structure, as described in this invention. The parts are named as follows: substrate 2, lower cladding 3, core waveguide 1, and upper cladding 4. Core waveguide 1 and upper cladding 4 are located on the lower cladding 3, and core waveguide 1 is encased in the upper cladding 4.

[0014] Figure 2 is a cross-sectional view (a), a top view (b), and a side view (c) of the input end of a large bandwidth mode converter based on a grating-type trident structure of a silicon dioxide optical waveguide according to the present invention.

[0015] Figure 3 It is a Gaussian-distributed circular fiber mode field diagram transmitted in the input end of a single-mode fiber and a grating-type trident input waveguide;

[0016] Figure 4 It is a Hermitian-Gaussian distributed elliptical waveguide mode field diagram transmitted in the output waveguide Core4;

[0017] Figure 5 It is a low-loss optical mode field diagram transmitted by the entire grating-type trident input waveguide (Core1, Core2 and Core3) and output waveguide Core4 (mode converter);

[0018] Figure 6(a) shows the relationship between the distance Y from the center of the core waveguide 1 to the vertical axis z and the coupling efficiency (fiber-waveguide alignment tolerance) when the signal wavelength is 1550 nm, calculated using the Beam Propagation Method (BPM); Figure 6(b) shows the relationship between the distance Z from the center of the core waveguide 1 to the horizontal axis y and the coupling efficiency (fiber-waveguide alignment tolerance) when the signal wavelength is 1550 nm, calculated using the Beam Propagation Method (BPM).

[0019] Figure 7 This is a curve showing the coupling efficiency of a single-mode fiber with the mode converter described in this invention as a function of optical wavelength;

[0020] Figure 8 This is a process flow diagram of the fabrication process of the large bandwidth mode converter based on the grating-type trident structure of silicon dioxide optical waveguide described in this invention; including the following steps: cleaning silicon wafer 1, thermal oxidation deposition of low refractive index silicon dioxide lower cladding layer 2, deposition of high refractive index silicon dioxide core layer 3, etching to form high refractive index silicon dioxide waveguide 4, and deposition of SiO2 upper cladding layer 5. Detailed Implementation

[0021] The present invention will now be described in further detail with reference to specific embodiments and accompanying drawings.

[0022] Example 1

[0023] 1. First, determine the width W of the output waveguide, and the height H of the grating-type trident input and output waveguides (mode converters). Based on the relationship between the effective refractive index and the waveguide width, it is determined that when the output waveguide width W = 4μm, the waveguide outputs in single mode; according to the existing PLC platform technology, when H = 4μm, the waveguide has polarization insensitivity characteristics.

[0024] 2. Determine the input width W1 and output width W2 of Core1 and Core3, and the input width W3 of Core2 in the grating-type trident input waveguide of the core waveguide. Since the core diameter of the single-mode fiber is 8.3 μm, the supported optical mode fields are as follows: Figure 3 As shown; the optical mode field distribution supported by the SiO2 single-mode waveguide Core4 is as follows. Figure 4 As shown. Based on the beam propagation method and the finite-time difference method (Huei-Min Yang, Sun-Yuan Huang, Chao-Wei Lee, et al. High-Coupling Tapered Hyperbolic Fiber Microlens and Taper Asymmetry Effect. IEEE Journal of Lightwave Technology, 2004, 22(5): 1395~1401), the input width W1 = 1.8 μm and output width W2 = 1.1 μm of Core1 and Core3, and the input width W3 = 2.4 μm of Core2 in the grating-type trident structure of the core waveguide are determined.

[0025] 3. Determine the number N and length d of the small tapered waveguides in Core1, Core2, and Core3 of the grating-type trident structure of the core waveguide, as well as the distance d between every two small tapered waveguides. Using the beam propagation method, it was determined that when the number of small tapered waveguides N = 32 in Core1, Core2, and Core3, the optical diffraction efficiency is highest. Choosing the minimum dimension d = 1.3 μm for the length of each small tapered waveguide and its spacing effectively enhances the mode matching between the waveguide mode field and the fiber mode field of the mode converter described in this invention, achieving the conversion from a Gaussian distributed circular fiber mode field to a Hermitian-Gaussian distributed elliptical waveguide mode field.

[0026] 4. Determine the overall length L1 of the grating-type trident structure (Core1, Core2, and Core3) of the core waveguide and the length L2 of the output waveguide Core4. Considering that excessively large device size is not conducive to integration and packaging, the overall length L1 of the grating-type trident structure (Core1, Core2, and Core3) is determined to be 81.9 μm, and the length L2 of the output waveguide Core4 is determined to be 232 μm using the beam propagation method.

[0027] 5. Figure 5 The diagram shows the optical field transmitted along the silica waveguide mode converter of the grating-type trident structure when the signal light enters the grating and diffracts in the grating, exiting from the output waveguide Core4. It is evident that the mode field size at the input of the grating-type trident structure in the core waveguide matches the signal light mode size in the fiber, effectively improving optical coupling efficiency. The overall tapered profile of the grating-type trident structure enables low-loss conversion from a large-size optical mode field to a single-mode waveguide mode field within Core4, and Cores 1 and 3 on both sides prevent light leakage from both sides. This demonstrates that the structure can achieve low-loss conversion from a Gaussian-distributed fiber mode field to a Hermitian-Gaussian-distributed waveguide mode field, effectively reducing coupling loss caused by optical mode field mismatch.

[0028] 6. Determine the fiber-waveguide alignment tolerance of the device. Ideally, the coupling efficiency of the three-dimensional edge coupler can reach 89.3% when the signal light wavelength is 1550nm. Given the alignment problem between the fiber and waveguide, it is necessary to increase the alignment tolerance to reduce the manufacturing difficulty. Figure 6(a) shows the relationship between the distance Y from the center of the core waveguide to the vertical axis z and the coupling efficiency when the signal light wavelength is 1550nm, calculated using the beam propagation method. The figure shows that the coupling efficiency is highest (89.3%) when the distance Y from the center of the core waveguide to the vertical axis z is 0; when the distance Y is within the range of -3μm to 3μm, the coupling efficiency is greater than 71.5%. Figure 6(b) shows the relationship between the distance Z of the core waveguide center position from the horizontal axis y and the coupling efficiency when the signal wavelength is 1550nm, calculated using the beam propagation method. The figure shows that the coupling efficiency is highest (89.3%) when the distance Z is 0; and when the distance Z is within the range of -3μm to 3μm, the coupling efficiency is greater than 71.5%. This demonstrates that the mode converter has low requirements for vertical and horizontal alignment accuracy, effectively reducing the manufacturing complexity.

[0029] 7. Figure 7 The figure shows the coupling efficiency of the high-bandwidth mode-spot converter with single-mode fiber in the grating-type trident structure described in this invention as a function of optical wavelength. The results show that the coupling efficiency is 89.3% when the signal wavelength is 1550 nm; within the wavelength range of 870 nm to 1600 nm, the coupling efficiency of this mode-spot converter is greater than 50%, and the bandwidth is greater than 730 nm, making it a high-bandwidth mode-spot converter.

[0030] Example 2

[0031] The following is combined with Figure 8 The specific preparation method of this invention is described in detail below, with the following steps:

[0032] 1. Cleaning the silicon wafer as the substrate layer 2: Select a monocrystalline silicon wafer as the substrate layer 2, and use acetone, ethanol and deionized water to ultrasonically clean the silicon wafer in sequence to remove impurities on the silicon wafer surface;

[0033] 2. Thermal oxidation deposition of low refractive index silicon dioxide lower cladding layer 3: At 1000℃, a silicon dioxide thin film is grown on the cleaned silicon wafer as lower cladding layer 2 using a wet thermal oxidation process. By controlling the water vapor flow rate, substrate temperature and reaction time, the thickness of the grown low refractive index silicon dioxide lower cladding layer 2 is 10μm.

[0034] 3. Deposition of high refractive index silicon dioxide core layer: Plasma-enhanced chemical vapor deposition (PECVD) is used to deposit germanium-doped (Ge) high refractive index silicon dioxide. The flow rates of the reaction gases GeCl4, SiH4 and N2O are controlled to be 32 sccm, 20 sccm and 40 sccm respectively, the radio frequency power is 50W, the substrate temperature is 200℃, and the reaction time is controlled to form a high refractive index silicon dioxide layer with H=4μm.

[0035] 4. Etching to form silicon dioxide core waveguide 1: Photoresist is spin-coated on the surface of the generated high refractive index silicon dioxide layer. The core waveguide pattern is transferred to the photoresist using ultraviolet lithography. After the photoresist is developed, excess high refractive index silicon dioxide without photoresist protection is removed by etching using inductively coupled plasma (ICP) to obtain the silicon dioxide core waveguide 1 with the structure described in this invention.

[0036] 5. Deposition of SiO2 upper cladding 4: Low-refractive-index silicon dioxide is deposited on the surface of the silicon dioxide core waveguide 1 obtained in step 4 using the PECVD method as the upper cladding 4. The thickness of the silicon dioxide upper cladding is controlled by adjusting the gas flow rate, reactant ratio, radio frequency power and time. Then, chemical mechanical polishing is used to control the thickness of the SiO2 upper cladding 4 to 20 μm. That is, the total thickness of the upper and lower silicon dioxide claddings is 10 + 20 = 30 μm.

Claims

1. A high-bandwidth mode converter based on a grating-type trident structure of silicon dioxide optical waveguide, characterized in that: From bottom to top, it consists of a base layer (2), a lower cladding layer (3), a core waveguide (1), and an upper cladding layer (4). The lower cladding layer (3) is located above the base layer (2). The core waveguide (1) and the upper cladding layer (4) are both located above the lower cladding layer (3), and the core waveguide (1) is covered by the upper cladding layer (4). Along the direction of light transmission, the core waveguide (1) is composed of a grating-type trident input waveguide and a straight waveguide output waveguide Core4. The bottom surfaces of the grating-type trident input waveguide and the output waveguide Core4 are located in the same plane and are both located on the upper surface of the lower cladding layer (3). The grating-type trident input waveguide... It consists of three parts: input waveguide Core1, input waveguide Core2, and input waveguide Core3. Input waveguide Core1 and input waveguide Core3 have identical structures. Along the direction of the input light, input waveguide Core1 and input waveguide Core3 are composed of N similar small conical waveguides that are evenly arranged and whose outer edges are connected by straight lines. Input waveguide Core2 is composed of N similar small inverted conical waveguides that are evenly arranged and whose outer edges are connected by straight lines. The grating-type trident input waveguide has a conical structure as a whole, and its width decreases from wide to narrow along the direction of light transmission, thereby realizing the transformation of the optical mode size from large to small within the waveguide.

2. The wideband mode converter based on a grating-type trident structure of a silicon dioxide optical waveguide as described in claim 1, characterized in that: The input and output ends of input waveguides Core1 and Core2, and input waveguides Core2 and Core3 are equidistant; the width of the output end of input waveguide Core2 is the same as the width of output waveguide Core4; and the heights of input waveguides Core1, Core2, Core3, and Core4 are the same.

3. A high-bandwidth mode converter based on a grating-type trident structure of a silicon dioxide optical waveguide as described in claim 1, characterized in that: The lower cladding (3) and upper cladding (4) are both silicon dioxide with a refractive index of 1.447; the core waveguide (1) is germanium-doped silicon dioxide with a refractive index of 1.481; and the substrate (2) is a silicon wafer with a refractive index of 3.

455.

4. A high-bandwidth mode converter based on a grating-type trident structure of a silicon dioxide optical waveguide as described in claim 1, characterized in that: N=32. The length of each small conical waveguide in input waveguides Core1 and Core3 is d=1.3μm, and the distance between any two small conical waveguides is d=1.3μm. The input width of Core1 and Core3 is W1=1.8μm, the output width is W2=1.1μm, the height is H=4μm, and the length is L1=83.2μm. The length of each small inverted conical waveguide in input waveguide Core2 is d=1.3μm, and the distance between any two small inverted conical waveguides is d=1.3μm. The input width of Core2 is W3=2.4μm, the output width is W=4μm, the height is H=4μm, and the length is L1=83.2μm. The distance between the input and output ends of Core1 and Core2, and between Core2 and Core3, is gap=1.3μm. The width of the output straight waveguide Core4 is W=4μm, the waveguide height is H=4μm, and the length is L2=232μm.

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