Higher order mode filter and method of making the same

Abstract

The application provides a high-order mode filter and a preparation method thereof. The high-order mode filter comprises a substrate, a flat plate groove, a first stray light absorption area and a reflection area. The substrate comprises a base, an insulating layer and a semiconductor top layer arranged in sequence. The flat plate groove is arranged in the semiconductor top layer at intervals, and a ridge waveguide is formed between two adjacent flat plate grooves. The first stray light absorption area is distributed in the semiconductor top layer on both sides of the ridge waveguide and is used for absorbing stray light in incident light. The reflection area is arranged in the flat plate groove on both sides of the ridge waveguide. The length direction of the reflection area has an inclination angle with the light propagation direction in the ridge waveguide, and the inclination angle is less than 90 degrees, and the reflection area is used for reflecting light incident from the outside of the reflection area to the ridge waveguide. The application can greatly block stray light reflected by the side wall of the flat plate groove, prevent the stray light from re-entering the waveguide, and greatly increase the attenuation of the stray light.

Description

Technical Field

[0001] This invention belongs to the field of integrated optical device design and manufacturing technology, and in particular relates to a high-order mode filter and its fabrication method. Background Technology

[0002] In today's field of optical communication and photonic integration, as technology advances towards higher performance and smaller size, photonic integrated circuits (PICs) play a central role. PICs aim to integrate multiple optical functional devices onto a single chip to achieve complex and efficient optical signal processing.

[0003] During optical signal transmission, waveguides, serving as the transmission channel, inevitably introduce higher-order optical modes. The presence of these higher-order modes negatively impacts the performance of optical devices on photonic integrated chips. Firstly, these higher-order modes cause significant fluctuations in the transmitted light spectrum, reducing the spectral purity of the signal and making it difficult to meet the stringent requirements of applications demanding spectral accuracy, such as high-resolution spectral analysis and precision optical sensing. Secondly, and more critically, interference from higher-order modes causes unstable output power. In practical applications such as optical communication links and optical sensing systems, stable output power is a fundamental requirement for reliable system operation; unstable power output can lead to serious consequences such as signal misinterpretation and communication interruptions.

[0004] To address this issue, high-order mode filters (HOMFs) have emerged. Their core function is to precisely eliminate high-order mode optical signals from the waveguide, thereby ensuring that optical devices on photonic integrated chips can operate in a relatively stable optical environment, minimizing fluctuations in the transmitted light spectrum and stable output power. However, existing traditional HOMFs have significant drawbacks. Extensive experimental and practical application feedback has shown that traditional HOMFs can only provide a very limited attenuation capability of 20 dB / mm. This means that in complex photonic integrated chip applications with extremely high power stability requirements, such a low attenuation level is far from meeting the system's power stability specifications. For example, in some high-precision optical sensing devices, even slight changes in the optical signal can lead to deviations in detection results. Traditional HOMFs struggle to effectively filter out power fluctuations caused by high-order mode optical signals, significantly limiting the detection accuracy and reliability of such devices. Similarly, in long-distance optical communication optical transmission and reception modules, accumulated high-order mode interference, due to insufficient attenuation by traditional filters, easily causes signal degradation and reduces communication quality. In conclusion, to meet the growing demand for high-performance photonic integrated chips, it is essential to develop a higher-order mode filter with better attenuation performance.

[0005] It should be noted that the above introduction to the technical background is only for the purpose of providing a clear and complete explanation of the technical solutions of this application and facilitating understanding by those skilled in the art. It should not be assumed that these technical solutions are known to those skilled in the art simply because they have been described in the background section of this application. Summary of the Invention

[0006] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a high-order mode filter and its preparation method, so as to solve the problem of insufficient attenuation performance of high-order mode filters in the prior art.

[0007] To achieve the above and other related objectives, the present invention provides a high-order mode filter, comprising: a substrate, including a base, an insulating layer, and a semiconductor top layer disposed sequentially; planar trenches disposed at intervals in the semiconductor top layer, with a ridge waveguide formed between adjacent planar trenches; a first stray light absorption region distributed in the semiconductor top layer on both sides of the ridge waveguide for absorbing stray light in incident light; and a reflection region disposed in the planar trenches on both sides of the ridge waveguide, wherein the length direction of the reflection region has an inclined angle with the light propagation direction in the ridge waveguide, the inclined angle being less than 90 degrees, for reflecting light incident from outside the reflection region toward the ridge waveguide.

[0008] Optionally, the reflective region is configured as a reflective trench, which is disposed in the flat plate trenches on both sides of the ridge waveguide and extends from the bottom surface of the flat plate trenches into the interior of the substrate.

[0009] Optionally, the depth of the reflective trench is greater than or equal to the depth of the first stray light absorption region, and the depth of the reflective trench is less than or equal to the thickness of the semiconductor top layer.

[0010] Alternatively, the filler for the reflective trench may include either air or silica.

[0011] Optionally, the higher-order mode filter includes multiple reflection regions configured to be spaced apart along the direction of light propagation in the ridge waveguide.

[0012] Optionally, the tilt angles corresponding to multiple reflective zones are equal.

[0013] Optionally, the ratio of the projected length of the reflective area in the width direction of the flat plate groove to the width of the flat plate groove is 0.5 to 1.

[0014] Optionally, the tilt angle is 20 degrees to 85 degrees.

[0015] Optionally, the width of the flat plate trench is 10 micrometers to 50 micrometers, and the width of the first stray light absorption region is 15 micrometers to 60 micrometers.

[0016] Optionally, the higher-order mode filter further includes a second stray light absorption region, which is disposed in the first stray light absorption region, and the doping concentration of the second stray light absorption region is greater than that of the first stray light absorption region.

[0017] Optionally, the dopants in the first and second stray light absorption regions include one of boron, arsenic, boron difluoride, phosphorus, and antimony, and the doping concentration in the first stray light absorption region is 1 × 10⁻⁶. 18 / cm 3 ~5×10 19 / cm 3 The doping concentration of the second stray light absorption region is 5 × 10⁻⁶. 18 / cm 3 ~1×10 20 / cm 3 .

[0018] The present invention also provides a photonic integrated chip, which includes a high-order mode filter according to any of the above schemes.

[0019] This invention also provides a method for fabricating a high-order mode filter. The method includes: providing a substrate, the substrate comprising a base, an insulating layer, and a semiconductor top layer disposed sequentially; forming first stray light absorption regions spaced apart in the semiconductor top layer for absorbing stray light in incident light; etching spaced planar trenches in the semiconductor top layer, forming a ridge waveguide between two adjacent planar trenches, the ridge waveguide being located between two adjacent first stray light absorption regions; forming a reflection region in the planar trenches on both sides of the ridge waveguide, the length direction of the reflection region having an inclination angle less than 90 degrees with the light propagation direction in the ridge waveguide, for reflecting light incident from outside the reflection region toward the ridge waveguide.

[0020] Optionally, forming a reflective region in the planar trenches on both sides of the ridge waveguide includes: defining a reflective region pattern using a photolithography process; etching the semiconductor top layer based on the reflective region pattern to form a reflective trench in the planar trenches on both sides of the ridge waveguide, the reflective trench extending from the bottom surface of the planar trench into the interior of the substrate, and the depth of the reflective trench being less than or equal to the thickness of the semiconductor top layer.

[0021] Optionally, multiple reflective region patterns are defined by photolithography. These multiple reflective region patterns are configured to be arranged at intervals along the light propagation direction in the ridge waveguide. The top layer of the semiconductor is etched based on the reflective region patterns to form multiple reflective trenches in the planar trenches on both sides of the ridge waveguide.

[0022] Optionally, the tilt angles corresponding to the multiple reflective grooves are equal.

[0023] Optionally, the preparation method further includes the step of filling the reflective trench with silicon dioxide.

[0024] Optionally, the ratio of the projected length of the reflective groove in the width direction of the flat plate groove to the width of the flat plate groove is 0.5 to 1.

[0025] Optionally, the tilt angle is 20 degrees to 85 degrees.

[0026] Optionally, the width of the flat plate trench is 10 micrometers to 50 micrometers, and the width of the first stray light absorption region is 15 micrometers to 60 micrometers.

[0027] Optionally, a second stray light absorption region is formed in the first stray light absorption region by ion implantation. The doping concentration of the second stray light absorption region is greater than that of the first stray light absorption region. The dopants in the first and second stray light absorption regions include one of boron, arsenic, boron difluoride, phosphorus, and antimony. The doping concentration of the first stray light absorption region is 1 × 10⁻⁶. 18 / cm 3 ~5×10 19 / cm 3 The doping concentration of the second stray light absorption region is 5 × 10⁻⁶. 18 / cm 3 ~1×10 20 / cm 3 .

[0028] As described above, the high-order mode filter and its fabrication method of the present invention have the following beneficial effects:

[0029] The present invention designs reflection zones on both sides of the ridge waveguide. The length direction of the reflection zone is inclined at an angle to the light propagation direction in the ridge waveguide. The reflection zone can reflect light rays incident from the outside of the reflection zone toward the ridge waveguide. In particular, it can block stray light rays reflected by the sidewall of the flat groove to a great extent, preventing stray light rays from re-entering the waveguide.

[0030] This invention further increases stray light absorption by designing a highly doped second stray light absorption region in the groove region of the flat plate, thereby greatly increasing the attenuation of stray light.

[0031] The high-order mode filter of this invention is crucial for mitigating the harmful effects of high-order modes in complex photonic circuits. The design of the reflection region in this invention effectively eliminates the downstream propagation of high-order modes, preventing high-order modes from interacting with the fundamental modes. This results in better power stability of the integrated photonic chip in terms of wavelength and temperature. The high-order mode filter of this invention has an attenuation greater than 40 dB / mm, which is two orders of magnitude higher than that of traditional filter designs. This gives the integrated photonic chip a higher signal-to-noise ratio, thereby greatly improving the sensitivity of the integrated photonic chip.

[0032] The high-order mode filter of this invention is compatible with traditional process flows and can be combined with doping processes to achieve further attenuation, thus having a good application foundation and a wide range of applications. Attached Figure Description

[0033] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the embodiments of this application and to illustrate the implementation of this application, together with the textual description, to explain the principles of this application. Obviously, the drawings described below are merely some embodiments of this application.

[0034] Figures 1-4 The diagram shows the structural schematics of each step in the fabrication method of the high-order mode filter according to an embodiment of the present invention. Figures 1-4 The diagram includes a top view of the structure (above) and a cross-sectional view of the structure at point A-A'. Figure 3 The diagram shown is a schematic representation of a high-order mode filter with a reflection region according to an embodiment of the present invention. Figure 4 The diagram shown is a schematic of a high-order mode filter with a reflection region and a high-concentration stray light absorption region according to an embodiment of the present invention.

[0035] Figure 5 This is a schematic diagram of the optical signal path of an existing high-order mode filter structure.

[0036] Figure 6 The diagram shows the optical signal path principle of the high-order mode filter structure according to an embodiment of the present invention.

[0037] Component designation explanation

[0038] 101 base

[0039] 102 insulation layer

[0040] 103 Semiconductor Top Layer

[0041] 104 First stray light absorption region

[0042] 105 flat groove

[0043] 106 Ridge Waveguide

[0044] 107 Reflective Trench

[0045] 108 Second Stray Light Absorption Region

[0046] 201 light signal

[0047] 202 reflected light Detailed Implementation

[0048] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0049] It should be emphasized that the term "including / comprises" as used herein refers to the presence of a feature, whole, step, or component, but does not exclude the presence or addition of one or more other features, wholes, steps, or components.

[0050] Features described and / or illustrated for one embodiment may be used in the same or similar manner in one or more other embodiments, combined with features in other embodiments, or substituted for features in other embodiments.

[0051] In the detailed description of embodiments of the present invention, for ease of explanation, the cross-sectional views illustrating the device structure may be partially enlarged and not to scale. Furthermore, the schematic diagrams are merely examples and should not limit the scope of protection of the present invention. In actual fabrication, the three-dimensional spatial dimensions of length, width, and depth should be included.

[0052] For ease of description, spatial relation terms such as “below,” “under,” “lower than,” “below,” “above,” and “upper” may be used herein to describe the relationship between one element or feature shown in the accompanying drawings and other elements or features. It will be understood that these spatial relation terms are intended to include directions other than those depicted in the drawings for devices in use or operation. Furthermore, when a layer is referred to as being “between” two layers, it may be the only layer between the two layers, or there may be one or more layers in between.

[0053] In the context of this application, the structure described above the first feature may include embodiments in which the first and second features are formed in direct contact, or embodiments in which additional features are formed between the first and second features, such that the first and second features may not be in direct contact.

[0054] It should be noted that the illustrations provided in this embodiment are only schematic representations of the basic concept of the present invention. Therefore, the illustrations only show the components related to the present invention and are not drawn according to the actual number, shape and size of the components in the actual implementation. In the actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0055] like Figure 3 As shown, where, Figure 3The top view (top) and cross-sectional view (bottom) of the high-order mode filter are shown in this embodiment. The high-order mode filter includes a substrate, a planar trench 105, a first stray light absorption region 104, and a reflection region.

[0056] like Figure 3 As shown, the substrate may include a base 101, an insulating layer 102, and a semiconductor top layer 103 arranged sequentially.

[0057] In one embodiment, the substrate 101 can be a silicon substrate, a germanium substrate, a silicon-germanium substrate, a silicon carbide substrate, etc., the insulating layer 102 can be a suitable dielectric layer, such as a silicon dioxide layer, and the semiconductor top layer 103 can be a silicon top layer. The thickness of the silicon top layer can be 2.5 micrometers to 5 micrometers, specifically 3 micrometers.

[0058] like Figure 3 As shown, planar trenches 105 are spaced apart in the semiconductor top layer 103, and a ridge waveguide 106 is formed between two adjacent planar trenches 105.

[0059] In one embodiment, the width z of the planar trench 105 can be 10 micrometers to 50 micrometers, and the depth of the planar trench 105 can be 1.1 micrometers to 1.3 micrometers. The depth of the planar trench 105 can be set according to the required thickness of the ridge waveguide 106, and is not limited to the examples listed herein. The shape of the planar trench 105 can be adjusted according to the required shape of the ridge waveguide 106. For example, when the ridge waveguide 106 is a rectangular waveguide, the planar trench 105 can be set as a rectangular trench so that a rectangular waveguide is constructed between adjacent planar trenches 105; while when the ridge waveguide 106 is conical, the planar trench 105 can be set as a trapezoid so that a conical waveguide is constructed between adjacent planar trenches 105.

[0060] like Figure 3 As shown, the first stray light absorption region 104 is distributed at intervals in the semiconductor top layer 103 on both sides of the ridge waveguide 106 to absorb stray light in the incident light. The first stray light absorption region 104 can be continuously distributed or discontinuously distributed in the extension direction of the ridge waveguide 106. In this embodiment, the first stray light absorption region 104 is set to be continuously distributed in the extension direction of the ridge waveguide 106.

[0061] In one embodiment, the dopant of the first stray light absorption region 104 includes one of boron, arsenic, boron difluoride, phosphorus, and antimony. The specific dopant may depend on design requirements, such as the wavelength of the light to be absorbed.

[0062] Specifically, the size of the pattern of the first stray light absorption region 104 can be adjusted according to the width of the planar trench 105 of the ridge waveguide 106 used. For a planar trench 105 with a width between 10 micrometers and 50 micrometers, the width of the first stray light absorption region 104 can be between 15 micrometers and 60 micrometers, and the depth of the first stray light absorption region 104 can be between 2.5 micrometers and 3 micrometers. In this embodiment, the doping concentration of the first stray light absorption region is 1×10⁻⁶. 18 / cm 3 ~5×10 19 / cm 3 .

[0063] like Figure 3 As shown, the reflective region is disposed in the flat grooves 105 on both sides of the ridge waveguide 106. The length direction of the reflective region has an inclined angle θ with the light propagation direction in the ridge waveguide 106, and the inclined angle θ is less than 90 degrees, which is used to reflect the light incident from the outside of the reflective region toward the ridge waveguide 106. In one embodiment, the inclined angle θ is 20 degrees to 85 degrees, preferably 30 degrees.

[0064] In one embodiment, the reflective region is configured as a reflective trench 107, which is disposed in the planar trenches 105 on both sides of the ridge waveguide 106. The reflective trench 107 extends from the bottom surface of the planar trench 105 into the interior of the substrate. The depth of the reflective trench 107 is less than or equal to the thickness of the semiconductor top layer 103 to avoid damage to the insulating layer 102. For example, the depth of the reflective trench 107 can be between 2.5 micrometers and 3 micrometers. The ratio of the projected length of the reflective region in the width direction of the planar trench 105 to the width of the planar trench 105 is 0.5 to 1, that is, the width of the reflective trench 107 can be designed to cover most of the width of the planar trench 105 as much as possible to improve its reflection probability of stray light. For a planar trench 105 with a width between 10 micrometers and 50 micrometers, the actual length of the reflective trench 107 can be between 10 micrometers and 100 micrometers. The width of the reflective trench 107 can be between 3 micrometers and 10 micrometers. Because there is a significant difference in refractive index between the semiconductor top layer 103 on one side of the reflection trench 107 and the filling material (such as air or silicon dioxide) in the emission trench, on the one hand, when stray light or higher-order modes from the outside irradiate or are emitted from the outer wall of the planar trench 105 to the reflection trench 107, the stray light or higher-order modes will be reflected by the sidewall of the reflection trench 107, thereby preventing the stray light or higher-order modes from entering the ridge waveguide 106, thus greatly improving the attenuation of stray light or higher-order modes. On the other hand, when stray light or higher-order modes moving out of the waveguide irradiate the inner side of the reflection trench 107, they can be reflected multiple times by the sidewall of the adjacent reflection trench 107, eventually moving them out to the outer side of the reflection trench 107, further improving the attenuation of stray light or higher-order modes.

[0065] In one embodiment, the reflective groove 107 can be a rectangular groove, an inverted trapezoidal groove, a U-shaped groove, a V-shaped groove, etc., and can be set according to design requirements or process requirements, and is not limited to the examples listed here.

[0066] In one embodiment, the filling material of the reflective trench 107 includes one of air and silicon dioxide. Air and silicon dioxide have low refractive indices, which can effectively increase the probability of total internal reflection of the sidewalls of the reflective trench 107, thereby increasing the reflectivity of the reflective trench 107 for stray light or higher-order modes. Of course, the filling material can also be other low-refractive-index dielectric materials, and is not limited to the examples listed herein.

[0067] In one embodiment, the higher-order mode filter includes multiple reflective regions configured to be spaced apart along the light propagation direction in the ridge waveguide 106. The tilt angle θ corresponding to the multiple reflective regions can be equal, meaning the length directions of the multiple reflective regions are parallel to each other. This simplifies the design and manufacturing process while ensuring the reflection probability of stray light or higher-order modes by the multiple reflective regions. Alternatively, the tilt angle θ corresponding to the multiple reflective regions can be configured to gradually decrease along the light propagation direction in the ridge waveguide 106 to increase the reflection probability of subsequent reflective regions for stray light or higher-order modes from the outside. Of course, in other instances, the tilt angle θ rule of the multiple reflective regions can be designed according to actual conditions and is not limited to the examples listed here.

[0068] like Figure 4 As shown, the high-order mode filter may further include a second stray light absorption region 108, which is disposed in the first stray light absorption region 104, and the doping concentration of the second stray light absorption region 108 is greater than the doping concentration of the first stray light absorption region 104.

[0069] In one embodiment, the second stray light absorption region 108 is configured to surround the inner and outer sides of the reflection region to facilitate the absorption of stray light incident or reflected from the inner and outer sides of the reflection region.

[0070] In one embodiment, the dopant of the second stray light absorption region 108 includes one of boron, arsenic, boron difluoride, phosphorus, and antimony.

[0071] Stray light absorption increases exponentially with doping concentration; the higher the doping concentration, the more significant the suppression effect on stray light. However, when the doping concentration exceeds a threshold of approximately 5 × 10¹⁸ / cm², the absorption decreases. 3 At this point, stray light absorption will begin to deviate from the exponential relationship because the doping concentration will cause a refractive index perturbation in the top layer 103 of the semiconductor, which will cause stray light to be reflected back into the waveguide. In one embodiment, the doping concentration of the second stray light absorption region is 5 × 10⁻⁶. 18 / cm3 ~1×10 20 / cm 3 This design ensures that the second stray light absorption region 108 strongly absorbs stray light and / or higher-order modes entering its region. In general, even though the interface between the second stray light absorption region 108 and the first stray light absorption region 104 has some reflection due to the difference in refractive index, the actual difference in refractive index between the two regions is not significant, resulting in a low probability of total internal reflection at the interface. Therefore, a large portion of the stray light or higher-order modes can still enter and be absorbed by the second stray light absorption region 108. The depth of the second stray light absorption region 108 can be set to be less than half or less than one-third of the depth of the first stray light absorption region 104, depending on actual design requirements. In some embodiments, a concentration gradient region or concentration transition region may be provided between the second stray light absorption region 108 and the first stray light absorption region 104. The doping concentration of the concentration gradient region or concentration transition region gradually increases from the first stray light absorption region 104 toward the second stray light absorption region 108, thereby avoiding further increase in stray light and higher-order modes entering the second stray light absorption region 108 from the first stray light absorption region 104.

[0072] This embodiment also provides a photonic integrated chip, which includes a high-order mode filter as described in the above embodiment. The photonic integrated chip may also include other optical components, such as photodetectors, avalanche photodiodes, optical waveguides, optical beam splitters, optical input / output couplers, optical resonators, etc., and is not limited to the examples listed herein.

[0073] like Figures 1-3 As shown in the figure, this embodiment also provides a method for fabricating a high-order mode filter, the method comprising:

[0074] like Figure 1 As shown, step 1) is performed first, a substrate is provided, which includes a substrate 101, an insulating layer 102 and a semiconductor top layer 103 arranged sequentially.

[0075] In one embodiment, the substrate 101 can be a silicon substrate, a germanium substrate, a silicon-germanium substrate, a silicon carbide substrate, etc., the insulating layer 102 can be a suitable dielectric layer, such as a silicon dioxide layer, and the semiconductor top layer 103 can be a silicon top layer. The thickness of the silicon top layer can be 2.5 micrometers to 5 micrometers, specifically 3 micrometers.

[0076] like Figure 1 As shown, then step 2) is performed, forming a first stray light absorption region 104 spaced apart in the semiconductor top layer 103 to absorb stray light in the incident light.

[0077] In one embodiment, forming the spaced-apart first stray light absorption regions 104 includes:

[0078] The photoresist layer is rotated onto the substrate and patterned using a photolithography process to remove photoresist areas that require stray light doping. Then, dopants (such as boron, arsenic, boron difluoride, phosphorus, and antimony) are implanted into the substrate, followed by high-temperature annealing to allow the dopants to diffuse from the implanted areas into the semiconductor top layer 103, so that the final contour almost reaches the underlying insulating layer 102.

[0079] Specifically, the size of the pattern of the first stray light absorption region 104 can be adjusted according to the width of the planar trench 105 of the ridge waveguide 106 used. For a planar trench 105 with a width z between 10 micrometers and 50 micrometers, the width x of the first stray light absorption region 104 can be between 15 micrometers and 60 micrometers, and the depth y of the first stray light absorption region 104 can be between 2.5 micrometers and 3 micrometers. In this embodiment, the doping concentration of the first stray light absorption region is 1×10⁻⁶. 18 / cm 3 ~5×10 19 / cm 3 .

[0080] like Figure 2 As shown, then step 3) is performed, in which spaced planar trenches 105 are etched in the semiconductor top layer 103, and a ridge waveguide 106 is formed between two adjacent planar trenches 105. The ridge waveguide 106 is located between two adjacent first stray light absorption regions 104.

[0081] In one embodiment, etching spaced planar trenches 105 in the semiconductor top layer 103 includes:

[0082] The photoresist layer is rotated onto the substrate and patterned using a photolithography process to define the etching area of ​​the flat trench 105.

[0083] Anisotropic dry etching is used to etch planar trenches 105 in the top layer 103 of a semiconductor.

[0084] In one embodiment, the width z of the planar trench 105 can be 10 micrometers to 50 micrometers, and the depth of the planar trench 105 can be 1.1 micrometers to 1.3 micrometers. The depth of the planar trench 105 can be set according to the required thickness of the ridge waveguide 106, and is not limited to the examples listed herein. The shape of the planar trench 105 can be adjusted according to the required shape of the ridge waveguide 106. For example, when the ridge waveguide 106 is a rectangular waveguide, the planar trench 105 can be set as a rectangular trench so that a rectangular waveguide is constructed between adjacent planar trenches 105; while when the ridge waveguide 106 is conical, the planar trench 105 can be set as a trapezoid so that a conical waveguide is constructed between adjacent planar trenches 105.

[0085] like Figure 3 Then, step 4) is performed, in which a reflection region is formed in the flat groove 105 on both sides of the ridge waveguide 106. The length direction of the reflection region has an inclination angle θ with the light propagation direction in the ridge waveguide 106, and the inclination angle θ is less than 90 degrees. This is used to reflect light rays incident on the ridge waveguide 106 from outside the reflection region. In one embodiment, the inclination angle θ is 20 degrees to 85 degrees, preferably 30 degrees.

[0086] In one embodiment, forming a reflection region in the planar trench 105 on both sides of the ridge waveguide 106 includes:

[0087] The pattern of the reflective area is defined using photolithography.

[0088] Based on the reflective region pattern, the top semiconductor layer 103 is etched to form reflective trenches 107 in the planar trenches 105 on both sides of the ridge waveguide 106. The reflective trenches 107 extend from the bottom surface of the planar trenches 105 into the interior of the substrate. The depth of the reflective trenches 107 is greater than or equal to the depth of the first stray light absorption region 104, and the depth of the reflective trenches 107 is less than or equal to the thickness of the top semiconductor layer 103.

[0089] In one embodiment, multiple reflective region patterns are defined by photolithography. These patterns are arranged at intervals along the light propagation direction in the ridge waveguide 106. The semiconductor top layer 103 is etched based on the reflective region patterns to form multiple reflective trenches 107 in the planar trenches 105 on both sides of the ridge waveguide 106. The tilt angles θ corresponding to the multiple reflective regions can be equal, meaning the length directions of the multiple reflective regions are parallel to each other. This simplifies the design and manufacturing process while ensuring the reflection probability of stray light or higher-order modes from the multiple reflective regions. Alternatively, the tilt angles θ corresponding to the multiple reflective regions can be configured to gradually decrease along the light propagation direction in the ridge waveguide 106 to increase the reflection probability of subsequent reflective regions for stray light or higher-order modes from the outside. Of course, in other examples, the rules governing the tilt angles θ of the multiple reflective regions can be designed according to actual conditions and are not limited to the examples listed here.

[0090] The depth of the reflective trench 107 is less than or equal to the thickness of the semiconductor top layer 103 to avoid damage to the insulating layer 102. For example, the depth of the reflective trench 107 can be between 2.5 micrometers and 3 micrometers. The ratio of the projected length of the reflective area in the width direction of the planar trench 105 to the width of the planar trench 105 is 0.5 to 1, that is, the width of the reflective trench 107 can be designed to cover most of the width of the planar trench 105 as much as possible to improve its reflection probability of stray light. For a planar trench 105 with a width between 10 micrometers and 50 micrometers, the actual length of the reflective trench 107 can be between 10 micrometers and 100 micrometers. The width of the reflective trench 107 can be between 3 micrometers and 10 micrometers. Because there is a significant difference in refractive index between the semiconductor top layer 103 on one side of the reflection trench 107 and the filling material (such as air or silicon dioxide) in the emission trench, on the one hand, when stray light or higher-order modes from the outside irradiate or are emitted from the outer wall of the planar trench 105 to the reflection trench 107, the stray light or higher-order modes will be reflected by the sidewall of the reflection trench 107, thereby preventing the stray light or higher-order modes from entering the ridge waveguide 106, thus greatly improving the attenuation of stray light or higher-order modes. On the other hand, when stray light or higher-order modes moving out of the waveguide irradiate the inner side of the reflection trench 107, they can be reflected multiple times by the sidewall of the adjacent reflection trench 107, eventually moving them out to the outer side of the reflection trench 107, further improving the attenuation of stray light or higher-order modes.

[0091] In one embodiment, the reflective groove 107 can be a rectangular groove, an inverted trapezoidal groove, a U-shaped groove, a V-shaped groove, etc., and can be set according to design requirements or process requirements, and is not limited to the examples listed here.

[0092] In one embodiment, the fabrication method further includes the step of filling the reflection trench 107 with silicon dioxide. Specifically, when filling the reflection trench 107 with silicon dioxide, it can be deposited simultaneously with the upper cladding (silicon dioxide layer) of the ridge waveguide in subsequent processes to save process steps. Of course, other processes (such as controlling deposition conditions or filling with a sacrificial layer) can also be used to keep the reflection trench 107 filled with air to ensure a low refractive index inside the reflection trench 107.

[0093] like Figure 4 As shown, the preparation method may further include: forming a second stray light absorption region 108 in the first stray light absorption region 104 by an ion implantation process, wherein the doping concentration of the second stray light absorption region 108 is greater than the doping concentration of the first stray light absorption region 104, and the dopants of the first stray light absorption region 104 and the second stray light absorption region 108 include one of boron, arsenic, boron difluoride, phosphorus and antimony.

[0094] Stray light absorption increases exponentially with doping concentration; the higher the doping concentration, the more significant the suppression effect on stray light. However, when the doping concentration exceeds a threshold of approximately 5 × 10¹⁸ / cm², the absorption decreases. 3 At this point, stray light absorption will begin to deviate from the exponential relationship because the doping concentration will cause a refractive index perturbation in the top layer 103 of the semiconductor, which will cause stray light to be reflected back into the waveguide. In one embodiment, the doping concentration of the second stray light absorption region is 5 × 10⁻⁶. 18 / cm 3 ~1×10 20 / cm 3 This design ensures that the second stray light absorption region 108 strongly absorbs stray light and / or higher-order modes entering its region. In general, even though the interface between the second stray light absorption region 108 and the first stray light absorption region 104 has some reflection due to the difference in refractive index, the actual difference in refractive index between the two regions is not significant, resulting in a low probability of total internal reflection at the interface. Therefore, a large portion of the stray light or higher-order modes can still enter and be absorbed by the second stray light absorption region 108. The depth of the second stray light absorption region 108 can be set to be less than half or less than one-third of the depth of the first stray light absorption region 104, depending on actual design requirements. In some embodiments, a concentration gradient region or concentration transition region may be provided between the second stray light absorption region 108 and the first stray light absorption region 104. The doping concentration of the concentration gradient region or concentration transition region gradually increases from the first stray light absorption region 104 toward the second stray light absorption region 108, thereby avoiding further increase in stray light and higher-order modes entering the second stray light absorption region 108 from the first stray light absorption region 104.

[0095] In one embodiment, the second stray light absorption region 108 is configured to surround the inner and outer sides of the reflection region to facilitate the absorption of stray light incident or reflected from the inner and outer sides of the reflection region.

[0096] Figure 5 The diagram shows a top view and a cross-sectional view of an existing high-order mode filter structure. After entering the ridge waveguide 106, the optical signal 201 (such as the TE10 optical signal) is removed from the ridge waveguide 106 due to the diffraction effect. However, some of the optical signal 201 is reflected by the sidewall of the flat groove 105 and then re-enters the ridge waveguide 106. Figure 6The diagram shows a top view and a cross-sectional view of a high-order mode filter structure according to an embodiment of the present invention. After entering the ridge waveguide 106, the optical signal 201 (such as the TE10 optical signal) is diffracted and then removed from the ridge waveguide 106. However, some of the optical signal 201, after being reflected by the sidewall of the planar trench 105, is reflected again by the reflection region to the outside of the ridge waveguide 106. The reflection region effectively prevents the TE10 optical signal from re-entering the ridge waveguide 106. Furthermore, after multiple reflections, part of the reflected light 202 is directly removed from the planar trench 105, while the other part is repeatedly absorbed by the first stray light absorption region 104 and the second stray light absorption region 108, thereby greatly improving the attenuation of the optical signal.

[0097] As described above, the high-order mode filter and its fabrication method of the present invention have the following beneficial effects:

[0098] The present invention designs reflection zones on both sides of the ridge waveguide. The length direction of the reflection zone is inclined at an angle to the light propagation direction in the ridge waveguide. The reflection zone can reflect light rays incident from the outside of the reflection zone toward the ridge waveguide. In particular, it can block stray light rays reflected by the sidewall of the flat groove to a great extent, preventing stray light rays from re-entering the waveguide.

[0099] This invention further increases stray light absorption by designing a highly doped second stray light absorption region in the groove region of the flat plate, thereby greatly increasing the attenuation of stray light.

[0100] The high-order mode filter of this invention is crucial for mitigating the harmful effects of high-order modes in complex photonic circuits. The design of the reflection region in this invention effectively eliminates the downstream propagation of high-order modes, preventing high-order modes from interacting with the fundamental modes. This results in better power stability of the integrated photonic chip in terms of wavelength and temperature. The high-order mode filter of this invention has an attenuation greater than 40 dB / mm, which is two orders of magnitude higher than that of traditional filter designs. This gives the integrated photonic chip a higher signal-to-noise ratio, thereby greatly improving the sensitivity of the integrated photonic chip.

[0101] The high-order mode filter of this invention is compatible with traditional process flows and can be combined with doping processes to achieve further attenuation, thus having a good application foundation and a wide range of applications.

[0102] Therefore, this invention effectively overcomes the various shortcomings of the prior art and has high industrial application value.

[0103] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A high-order mode filter, characterized in that, The higher-order mode filter includes: The substrate includes a base, an insulating layer, and a semiconductor top layer arranged sequentially. Planar trenches are spaced apart in the top layer of the semiconductor, and a ridge waveguide is formed between two adjacent planar trenches. The first stray light absorption region is distributed in the semiconductor top layer on both sides of the ridge waveguide and is used to absorb stray light in the incident light. A reflection zone is disposed in the flat grooves on both sides of the ridge waveguide. The length direction of the reflection zone has an inclined angle with the light propagation direction in the ridge waveguide. The inclined angle is less than 90 degrees. The reflection zone is used to reflect light rays incident from the outside of the reflection zone toward the ridge waveguide. The higher-order mode filter further includes a second stray light absorption region, which is disposed within the first stray light absorption region. The doping concentration of the second stray light absorption region is greater than that of the first stray light absorption region. A concentration gradient region or concentration transition region is disposed between the second stray light absorption region and the first stray light absorption region. The doping concentration of the concentration gradient region or concentration transition region gradually increases from the first stray light absorption region toward the second stray light absorption region.

2. The high-order mode filter according to claim 1, characterized in that: The reflective region is configured as a reflective trench, which is disposed in the planar trenches on both sides of the ridge waveguide. The reflective trench extends from the bottom surface of the planar trench into the interior of the substrate, and the depth of the reflective trench is less than or equal to the thickness of the semiconductor top layer.

3. The high-order mode filter according to claim 2, characterized in that: The filling material of the reflective trench includes one of air and silicon dioxide.

4. The high-order mode filter according to claim 1, characterized in that: The higher-order mode filter includes a plurality of reflection regions, which are configured to be spaced apart along the light propagation direction in the ridge waveguide.

5. The high-order mode filter according to claim 4, characterized in that: The tilt angles corresponding to the multiple reflection zones are equal.

6. The high-order mode filter according to claim 1, characterized in that: The ratio of the projected length of the reflective area in the width direction of the flat plate groove to the width of the flat plate groove is 0.5~1.

7. The high-order mode filter according to claim 1, characterized in that: The tilt angle is 20 degrees to 85 degrees.

8. The high-order mode filter according to claim 1, characterized in that: The width of the flat plate trench is 10 micrometers to 50 micrometers, and the width of the first stray light absorption region is 15 micrometers to 60 micrometers.

9. The high-order mode filter according to claim 1, characterized in that: The dopants in the first and second stray light absorption regions include one of boron, arsenic, boron difluoride, phosphorus, and antimony, and the doping concentration of the first stray light absorption region is 1 × 10⁻⁶. 18 / cm³~5×10 19 / cm³, the doping concentration of the second stray light absorption region is 5×10⁻⁶. 18 / cm³~1×10 20 / cm³.

10. A photonic integrated chip, characterized in that, The photonic integrated chip includes a high-order mode filter as described in any one of claims 1 to 9.

11. A method for fabricating a high-order mode filter, characterized in that, The preparation method includes: A substrate is provided, the substrate comprising a base, an insulating layer, and a semiconductor top layer disposed sequentially thereon; A first stray light absorption region is formed in the top layer of the semiconductor at intervals to absorb stray light in the incident light; Spacingd planar trenches are etched in the top layer of the semiconductor, and a ridge waveguide is formed between two adjacent planar trenches. The ridge waveguide is located between two adjacent first stray light absorption regions. A reflection zone is formed in the flat groove on both sides of the ridge waveguide. The length direction of the reflection zone has an inclination angle with the light propagation direction in the ridge waveguide. The inclination angle is less than 90 degrees. The reflection zone is used to reflect light rays incident from the outside of the reflection zone toward the ridge waveguide. A second stray light absorption region is formed in the first stray light absorption region by ion implantation. The doping concentration of the second stray light absorption region is greater than that of the first stray light absorption region. A concentration gradient region or concentration transition region is provided between the second stray light absorption region and the first stray light absorption region. The doping concentration of the concentration gradient region or concentration transition region gradually increases from the first stray light absorption region to the second stray light absorption region.

12. The method for fabricating a high-order mode filter according to claim 11, characterized in that: Forming reflection regions in the flat grooves on both sides of the ridge waveguide includes: The pattern of the reflective area is defined using photolithography. The semiconductor top layer is etched based on the reflective region pattern to form reflective trenches in the planar trenches on both sides of the ridge waveguide. The reflective trenches extend from the bottom surface of the planar trenches into the interior of the substrate, and the depth of the reflective trenches is less than or equal to the thickness of the semiconductor top layer.

13. The method for fabricating a high-order mode filter according to claim 12, characterized in that: Multiple reflective area patterns are defined by photolithography. These multiple reflective area patterns are arranged at intervals along the light propagation direction in the ridge waveguide. The semiconductor top layer is etched based on the reflective area patterns to form multiple reflective trenches in the planar trenches on both sides of the ridge waveguide.

14. The method for fabricating a high-order mode filter according to claim 13, characterized in that: The tilt angles corresponding to the multiple reflective grooves are equal.

15. The method for fabricating a high-order mode filter according to claim 12, characterized in that: The preparation method further includes the step of filling the reflective trench with silicon dioxide.

16. The method for fabricating a high-order mode filter according to claim 12, characterized in that: The ratio of the projected length of the reflective groove in the width direction of the flat plate groove to the width of the flat plate groove is 0.5~1.

17. The method for fabricating a high-order mode filter according to claim 11, characterized in that: The tilt angle is 20 degrees to 85 degrees.

18. The method for fabricating a high-order mode filter according to claim 11, characterized in that: The width of the flat plate trench is 10 micrometers to 50 micrometers, and the width of the first stray light absorption region is 15 micrometers to 60 micrometers.

19. The method for fabricating a high-order mode filter according to claim 11, characterized in that: The dopants in the first and second stray light absorption regions include one of boron, arsenic, boron difluoride, phosphorus, and antimony, and the doping concentration of the first stray light absorption region is 1 × 10⁻⁶. 18 / cm³~5×10 19 / cm³, the doping concentration of the second stray light absorption region is 5×10⁻⁶. 18 / cm³~1×10 20 / cm³.

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