Polarization beam splitter-combiner
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- WUHAN POST & TELECOMM RES INST CO LTD
- Filing Date
- 2026-04-09
- Publication Date
- 2026-06-09
Smart Images
Figure CN122172381A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of optical communication devices, and more specifically to a polarization beam splitter / combiner. Background Technology
[0002] Polarization beam splitters (PBSs) are key passive devices for polarization multiplexing and demultiplexing in optical communication systems and are widely used in silicon-based photonic integrated circuits. Currently, on-chip silicon-based polarization beam splitters mostly employ multimode interferometers (MMIs) or directional couplers (DCs) to achieve polarization separation. Among these, DC-type PBSs are widely studied due to their simple structure and ease of integration.
[0003] However, traditional directional coupler-type polarization beamsplitters typically consist of two parallel waveguides of the same width and height. Their polarization separation relies on strict phase matching conditions, resulting in a narrow operating bandwidth that is difficult to meet the requirements of broadband optical communication systems. Furthermore, achieving effective polarization separation often requires a long coupling length, leading to a larger device footprint and hindering high-density integration. In addition, this type of structure is highly sensitive to geometric parameters such as waveguide gap, waveguide width, and etching depth, which significantly affect coupling efficiency and polarization extinction ratio. Under conditions of significant process variations, it is difficult to guarantee consistent device performance and low yield. Summary of the Invention
[0004] This application provides a polarization beam splitter / combiner that can balance broadband characteristics and high polarization extinction ratio.
[0005] In a first aspect, embodiments of this application provide a polarization beam splitter / combiner, including: Multiple optical ports, including a first optical port, a second optical port, and a third optical port; and, A polarization processing unit is optically connected between the first optical port and the second and third optical ports; The polarization processing unit includes a mode coupling region, a non-coupling region, and two mode selectors. The mode coupling region includes three waveguides arranged side by side with gradually varying widths, used for adiabatic mode coupling and separation between the transverse magnetic mode and the transverse electric mode. The uncoupled region is connected after the mode coupling region and is used to receive optical signals from the mode coupling region and eliminate residual coupling between waveguides. The two mode selectors are respectively located on the two output paths of the uncoupled region, used to filter and pass through the transverse magnetic mode and the transverse electric mode, and are respectively connected to the second optical port and the third optical port.
[0006] In conjunction with the first aspect, in one embodiment, the mode coupling region includes a first width-gradient waveguide, a second width-gradient waveguide, and a third width-gradient waveguide; The width of the first tapered waveguide narrows along the direction of light propagation, the width of the second tapered waveguide widens along the direction of light propagation, and the width of the third tapered waveguide narrows along the direction of light propagation.
[0007] In conjunction with the first aspect, in one implementation, when the first optical port is used as an input, the second and third optical ports are used as outputs to achieve polarization beam splitting.
[0008] In conjunction with the first aspect, in one embodiment, when the second optical port and the third optical port are used as input terminals, the first optical port is used as an output terminal to achieve polarization beam combining.
[0009] In conjunction with the first aspect, in one implementation, the width changes of the first and third width-gradient waveguides have at least one boundary that is a curved gradient.
[0010] In conjunction with the first aspect, in one embodiment, the uncoupled region includes: The first uncoupled waveguide has one end connected to the first output terminal of the mode coupling region and the other end connected to the first mode selector. A second uncoupled waveguide, one end of which is connected to the second tapered waveguide; and The third uncoupled waveguide has one end connected to the second output terminal of the mode coupling region and the other end connected to the second mode selector.
[0011] In conjunction with the first aspect, in one embodiment, the first width-gradient waveguide, the second width-gradient waveguide, and the third width-gradient waveguide are arranged in parallel in sequence, and the end of the first width-gradient waveguide is connected to the beginning of the first uncoupled waveguide, and the beginning of the third width-gradient waveguide is connected to the first optical port.
[0012] In conjunction with the first aspect, in one embodiment, the first uncoupled waveguide and / or the third uncoupled waveguide are S-shaped curved waveguides.
[0013] In conjunction with the first aspect, in one implementation, the second uncoupled waveguide is a waveguide whose width gradually changes along the propagation direction.
[0014] In conjunction with the first aspect, in one implementation, the mode selector is a multimode interference coupler.
[0015] The beneficial effects of the technical solutions provided in this application include: By employing a mode coupling region composed of three tapered waveguides, an uncoupled region for eliminating residual coupling, and a polarization processing unit consisting of two mode selectors for screening transverse magnetic and transverse electric modes respectively, the technical problem of traditional directional coupler-type polarization beamsplitters, which rely on strict phase matching, being unable to simultaneously achieve broadband operation and high polarization extinction ratio is solved. Attached Figure Description
[0016] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0017] Figure 1 This is a schematic diagram of a structure of an embodiment of the polarization beam splitter / combiner provided by the present invention; Figure 2 for Figure 1 Schematic diagram of the AA section; Figure 3 for Figure 1 Schematic diagram of the BB section; Figure 4 for Figure 1 Schematic diagram of the section view along line C1-C1; Figure 5 for Figure 1 Schematic diagram of cross-section C2-C2.
[0018] In the figure: 1. First optical port; 2. First width-gradient waveguide; 3. Second width-gradient waveguide; 4. Third width-gradient waveguide; 5. First uncoupled waveguide; 6. Second uncoupled waveguide; 7. Third uncoupled waveguide; 8. First mode selector; 9. Second mode selector; 10. Second optical port; 11. Third optical port. Detailed Implementation
[0019] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present application.
[0020] Polarization beam splitters (PBSs) are key passive devices for polarization multiplexing and demultiplexing in optical communication systems and are widely used in silicon-based photonic integrated circuits. Currently, on-chip silicon-based polarization beam splitters mostly employ multimode interferometers (MMIs) or directional couplers (DCs) to achieve polarization separation. Among these, DC-type PBSs are widely studied due to their simple structure and ease of integration.
[0021] However, traditional directional coupler-type polarization beamsplitters typically consist of two parallel waveguides of the same width and height. Their polarization separation relies on strict phase matching conditions, resulting in a narrow operating bandwidth that is difficult to meet the requirements of broadband optical communication systems. Furthermore, achieving effective polarization separation often requires a long coupling length, leading to a larger device footprint and hindering high-density integration. In addition, this type of structure is highly sensitive to geometric parameters such as waveguide gap, waveguide width, and etching depth, which significantly affect coupling efficiency and polarization extinction ratio. Under conditions of significant process variations, it is difficult to guarantee consistent device performance and low yield.
[0022] To address the above problems, this invention provides a polarization beam splitter / combiner that can balance broadband characteristics and a high polarization extinction ratio.
[0023] Please refer to Figures 1 to 5 This invention proposes a polarization beam splitter / combiner, comprising multiple optical ports and a polarization processing unit. The multiple optical ports include a first optical port 1, a second optical port 10, and a third optical port 11. The polarization processing unit is optically connected between the first optical port 1 and the second optical port 10 and the third optical port 11. The polarization processing unit includes a mode coupling region, a decoupling region, and two mode selectors. The mode coupling region includes three waveguides arranged side-by-side with gradually varying widths, used for adiabatic mode coupling and separation between transverse magnetic modes and transverse electric modes. The decoupling region is connected after the mode coupling region and is used to receive optical signals from the mode coupling region and eliminate residual coupling between the waveguides. The two mode selectors are respectively disposed on the two output paths of the decoupling region, used to filter and pass through the transverse magnetic mode and the transverse electric mode, and are respectively connected to the second optical port 10 and the third optical port 11.
[0024] In the technical solution of this application, by employing a polarization processing unit composed of a mode coupling region consisting of three tapered waveguides, a non-coupling region for eliminating residual coupling, and two mode selectors for screening transverse magnetic modes and transverse electric modes respectively, the technical problem of traditional directional coupler-type polarization beam splitters in related technologies, which rely on strict phase matching, is difficult to achieve both broadband operation and high polarization extinction ratio at the same time.
[0025] Among them, a width-gradient waveguide refers to a waveguide whose width gradually changes along the direction of light propagation. This gradient structure enables adiabatic evolution of optical modes, thereby maintaining low loss and high efficiency during mode switching or separation.
[0026] Adiabatic mode coupling and decoupling refers to the ability of an optical mode to smoothly transition from one mode to another, or to couple from one waveguide to another, and vice versa, under conditions of slow changes in waveguide structural parameters, with minimal mode energy loss during this process. The realization of adiabatic mode coupling relies on the continuous variation of the structural parameters (i.e., waveguide width) of the tapered waveguide along the light propagation path. When the change in waveguide structure satisfies the requirement that the optical field has sufficient spatial periodicity to adapt to the new waveguide structure, the input optical mode will always smoothly follow the local eigenmode evolution of the waveguide system, without exciting unwanted modes or causing unintended intermode power conversions due to small, continuous changes in structural parameters.
[0027] Specifically, in this invention, by designing the width variation curves of the first, second, and third tapered waveguides, the effective refractive index and field distribution of the system's intrinsic modes corresponding to the transverse magnetic mode (TM) and transverse electric mode (TE) undergo different and controllable evolutions with the propagation position. Under adiabatic conditions, the energy of the TM and TE modes will be deterministically guided to different output waveguides along their respective differentiated intrinsic mode paths, thereby achieving efficient polarization separation.
[0028] The key is that this adiabatic evolution process does not rely on the strict phase-matching conditions required by traditional directional couplers, which can only be satisfied at specific wavelengths. This polarization separation process can be stably achieved with high efficiency and low crosstalk over a wide wavelength range. This is the fundamental physical principle behind this invention's ability to overcome traditional technological bottlenecks and significantly increase the operating bandwidth.
[0029] The decoupled region is set after the mode coupling region. Its function is to receive the optical signal from the mode coupling region and further eliminate any residual coupling that may exist between waveguides, ensuring that optical signals of different modes or different paths can be transmitted independently and avoiding crosstalk.
[0030] A mode selector is an optical device that can selectively allow light signals of a specific mode to pass through, while suppressing or blocking light signals of other modes, based on the polarization state or other mode characteristics of light.
[0031] This embodiment provides a polarization beam splitter / combiner, which mainly consists of multiple optical ports and a polarization processing unit.
[0032] This polarization beam splitter / combiner is configured with multiple optical ports, specifically including a first optical port 1, a second optical port 10, and a third optical port 11. These optical ports can be designed as fiber optic coupling interfaces, on-chip waveguide interfaces, or other forms of optical input / output terminals. For example, these optical ports can be connected to external optical fibers using edge couplers or grating couplers, or directly connected to the waveguides of other on-chip photonic devices. In practical applications, these optical ports can be designated as input or output terminals according to specific requirements.
[0033] The polarization processing unit is optically connected between the first optical port 1 and the second optical port 10 and the third optical port 11. This connection ensures that an optical signal can enter the polarization processing unit from any input optical port and exit from the corresponding output optical port after processing. For example, an optical signal can be input from the first optical port 1, pass through the polarization processing unit, and be output from the second optical port 10 or the third optical port 11 according to its polarization state. Conversely, an optical signal can also be input from the second optical port 10 and the third optical port 11, be combined in the polarization processing unit, and be output from the first optical port 1.
[0034] The internal structure of this polarization processing unit includes a mode coupling region, a decoupling region, and two mode selectors. These components work together to perform polarization processing on the optical signal. For example, the mode coupling region is responsible for initial polarization mode conversion or separation, the decoupling region ensures mode purity, and the mode selectors perform the final selection of specific polarization modes.
[0035] Specifically, the mode coupling region comprises three waveguides arranged side-by-side, with their widths gradually varying along the direction of light propagation. This gradually varying width design enables adiabatic mode coupling and separation between transverse magnetic and transverse electric modes as the optical signal passes through this region. For example, when an optical signal enters the mode coupling region, its internal transverse magnetic and transverse electric modes can be gradually guided to different output paths or exchange energy between different waveguides, depending on the change in waveguide width. This adiabatic process is insensitive to wavelength changes, ensuring that mode switching or separation can be performed efficiently and stably over a wide wavelength range.
[0036] The decoupling region follows the mode-coupled region. Its main function is to receive optical signals from the mode-coupled region and further eliminate residual coupling between waveguides. For example, after the initial mode separation is completed in the mode-coupled region, weak cross-coupling may still exist between optical signals on different paths. The decoupling region, through its specific structural design, such as using sufficiently long straight waveguides or appropriately curved waveguides, ensures that these residual couplings are effectively suppressed, thereby guaranteeing the purity of the output signal.
[0037] Two mode selectors are positioned on the two output paths of the decoupled region, respectively. These mode selectors filter and allow the transverse magnetic mode and transverse electric mode to pass through, and are connected to the second optical port 10 and the third optical port 11, respectively. For example, one mode selector can be designed to allow only the transverse magnetic mode to pass through and guide it to the second optical port 10; while the other mode selector can allow only the transverse electric mode to pass through and guide it to the third optical port 11. The mode selectors can be implemented using various structures, such as those based on directional couplers, multimode interferometers, or photonic crystal structures. With this configuration, the mixed polarized light signal entering the polarization processing unit can be effectively separated into pure transverse magnetic mode and transverse electric mode, and output from different optical ports respectively.
[0038] When the optical signal enters the polarization processing unit, it first reaches the mode coupling region. This region consists of three waveguides arranged side-by-side with gradually varying widths. Due to the gradually varying waveguide widths, the transverse magnetic and transverse electric modes in the optical signal undergo an adiabatic mode evolution process as they propagate along the waveguides. Specifically, the transverse magnetic and transverse electric modes are adiabatically guided to different waveguide paths based on changes in their effective refractive index. This adiabatic process avoids non-adiabatic transitions between modes, thus significantly reducing transmission loss and improving mode separation efficiency. Unlike traditional directional couplers that rely on strict phase matching, this adiabatic mode coupling region has better tolerance to small fluctuations in waveguide geometry parameters because the mode evolution is gradual and less susceptible to changes in local parameters.
[0039] After initial polarization separation is completed in the mode coupling region, the optical signal enters the decoupling region. The decoupling region is connected to the output of the mode coupling region. Its function is to receive the optical signal from the mode coupling region and further eliminate residual coupling between waveguides. For example, the two optical paths output from the mode coupling region may still have weak cross-coupling. The decoupling region, through its structural design, such as using appropriate waveguide spacing and length, ensures that the optical signals on different paths are completely decoupled, thereby guaranteeing the purity of subsequent mode selection. This step is crucial for obtaining a high polarization extinction ratio.
[0040] Subsequently, the two output paths of the decoupled region are connected to two mode selectors. One mode selector is configured to allow only the transverse magnetic mode to pass through and guide it to the second optical port 10; the other mode selector is configured to allow only the transverse electric mode to pass through and guide it to the third optical port 11. These mode selectors can efficiently filter out the target polarization mode while suppressing other polarization modes. In this way, the mixed polarization light signal input to the first optical port 1 is successfully separated into pure transverse magnetic and transverse electric modes, and output from the second optical port 10 and the third optical port 11, respectively.
[0041] Therefore, this polarization beamsplitter / combiner achieves efficient and low-loss polarization beam splitting of mixed-polarization optical signals through adiabatic mode coupling and separation in the mode coupling region, residual coupling elimination in the uncoupled region, and precise screening by the mode selector. The entire process utilizes the adiabatic properties of the width-gradient waveguide, enabling the device to maintain good performance over a wide wavelength range, thus solving the narrow bandwidth problem of traditional directional couplers. Simultaneously, the adiabatic mode evolution has low sensitivity to process parameters, improving the device's process tolerance and yield.
[0042] Furthermore, the adiabatic coupling and decoupling characteristics significantly reduce the device's sensitivity to geometric parameters such as waveguide width, gap, and etching depth. In actual manufacturing, process variations are unavoidable, but the solution in this embodiment better tolerates these process errors, thereby ensuring consistent device performance and a high yield. This effectively solves the problem in existing technologies where device performance is highly sensitive to process parameters, making it difficult to guarantee performance consistency.
[0043] Through the synergistic effect of the mode coupling region, the uncoupled region, and the mode selector, the polarization beamsplitter in this embodiment achieves high polarization extinction ratio and broadband operation. The thermal insulation properties of the mode coupling region make its performance independent of strict phase matching at a specific wavelength, thus laying the foundation for broadband operation. The uncoupled region further eliminates residual coupling, ensuring mode purity. Finally, the mode selector precisely selects the target polarization mode. This step-by-step and efficient processing mechanism enables this embodiment to provide a silicon-based polarization beamsplitter solution with high extinction ratio, insensitivity to process errors, and suitability for broadband operation, effectively overcoming the challenge of simultaneously achieving broadband characteristics, high polarization extinction ratio, and good process tolerance in existing technologies.
[0044] Furthermore, the mode coupling region includes a first width-gradient waveguide 2, a second width-gradient waveguide 3, and a third width-gradient waveguide 4; the width of the first width-gradient waveguide 2 narrows along the light propagation direction, the width of the second width-gradient waveguide 3 widens along the light propagation direction, and the width of the third width-gradient waveguide 4 narrows along the light propagation direction.
[0045] The first tapered waveguide 2, the second tapered waveguide 3, and the third tapered waveguide 4 are the core components constituting the mode coupling region. Their widths change along the light propagation direction, aiming to achieve effective coupling or separation between different polarization modes (such as transverse magnetic modes and transverse electric modes) through thermal insulation. These waveguides can be silicon-based waveguides, formed through photolithography and etching processes; they can also be silicon nitride waveguides, formed through deposition and etching processes; or they can be polymer waveguides, formed through direct writing or imprinting. The width of the first tapered waveguide 2 narrows along the light propagation direction. This design helps guide the light field distribution of specific modes. For example, during mode coupling, by gradually reducing the waveguide width, the light field can be gradually transferred from one waveguide to another, or its effective refractive index can be changed, thereby achieving mode conversion. The sidewalls of the waveguides can be designed as straight inclined lines, forming a conical structure; or they can be designed as gradually tapered curves, such as parabolic or S-shaped curves, to optimize thermal insulation and reduce reflection loss. The second tapered waveguide 3 widens along the light propagation direction. Similar to the narrowing waveguide, this widening design is also used to precisely control the light field distribution and effective refractive index. In the mode coupling region, it may be used to receive or output light of a specific mode, or as an intermediate waveguide, its width variation working in conjunction with other waveguides to maintain thermal insulation. The sidewalls of the waveguide can be designed with a straight, inclined shape, forming an inverse conical structure; or with a gradually tapered curve, such as a parabola or S-curve, to optimize thermal insulation and reduce reflection loss. The third tapered waveguide 4 narrows along the light propagation direction. Its function is similar to that of the first tapered waveguide 2, typically used to further separate or guide modes on the other side of the mode coupling region. The sidewalls of the waveguide can also be designed with a straight, inclined shape, forming a conical structure; or with a gradually tapered curve, such as a parabola or S-curve, to optimize thermal insulation and reduce reflection loss.
[0046] The scheme in this application achieves adiabatic mode coupling and separation of transverse magnetic modes and transverse electric modes through the synergistic effect of the first tapered waveguide 2, the second tapered waveguide 3, and the third tapered waveguide 4 in the mode coupling region of the polarization processing unit. Specifically, when the optical signal enters the mode coupling region, the widths of the three waveguides undergo a specific gradient along the light propagation direction—the widths of the first tapered waveguide 2 and the third tapered waveguide 4 become narrower, while the width of the second tapered waveguide 3 becomes wider. This carefully designed width change leads to a gradient change in the effective refractive index within the waveguides. By precisely controlling the rate of change of the widths and the relative positions of these waveguides, it can be ensured that the optical signal always meets the adiabatic conditions during propagation, allowing the light energy of different polarization modes to be smoothly coupled or separated between different waveguides without drastic mode jumps or unnecessary reflections. For example, in polarization beam splitting applications, incident mixed-polarized light passing through this structure is guided to different output waveguides under adiabatic conditions by the transverse magnetic mode and the transverse electric mode, based on their differences in effective refractive index, thereby achieving efficient and low-loss polarization separation. This structural combination optimizes the mode conversion process, reduces mode crosstalk, and improves the efficiency of beam splitting and combining.
[0047] In some implementations, when the first optical port 1 is used as the input, the second optical port 10 and the third optical port 11 are used as the output to achieve polarization beam splitting.
[0048] When the first optical port 1 is designated as the input, the incident optical signal (which may contain transverse magnetic and transverse electric modes) enters the polarization processing unit through the first optical port 1. In the mode coupling region, a structure consisting of a first-width tapered waveguide 2, a second-width tapered waveguide 3, and a third-width tapered waveguide 4 utilizes the tapered characteristics of their widths along the light propagation direction to perform adiabatic mode coupling and separation of the transverse magnetic and transverse electric modes in the incident light. Specifically, the geometry of these tapered waveguides is designed so that optical signals with different polarization states generate different effective refractive indices during propagation, thereby guiding them to propagate along different paths. Subsequently, these initially separated optical signals enter the uncoupled region, which is designed to eliminate any residual coupling between waveguides and further purify the polarization states. Finally, two mode selectors located on the two output paths in the uncoupled region filter the optical signals, ensuring that one output path passes only the transverse magnetic mode and the other output path passes only the transverse electric mode. These filtered pure polarized optical signals are output from the second optical port 10 and the third optical port 11, respectively. Through this explicit port configuration and the synergistic effect of the polarization processing unit, the polarization beam splitter and combiner can effectively separate the incident mixed polarized light signal into two light signals with a single polarization state, thereby achieving precise polarization beam splitting function.
[0049] For example, when it is necessary to separate a beam of mixed polarized light containing transverse magnetic and transverse electric modes, the mixed polarized light can be connected to the first optical port 1 via an optical fiber. At this time, the polarization beam splitter / combiner is configured in polarization beam splitting mode. The incident light signal enters the mode coupling region, where specific width variations in the first tapered waveguide 2, the second tapered waveguide 3, and the third tapered waveguide 4 cause the transverse magnetic and transverse electric modes to gradually separate during propagation. For example, the transverse magnetic mode may be primarily guided to one output path, while the transverse electric mode is guided to the other output path. Subsequently, the decoupling region further ensures complete separation of the two polarization modes. Finally, two mode selectors positioned on the output path of the decoupling region are used; for example, one mode selector is designed to allow only the transverse magnetic mode and guide it to the second optical port 10; the other mode selector is designed to allow only the transverse electric mode and guide it to the third optical port 11. In this way, the incident mixed polarized light is effectively separated into two pure polarized beams, output from the second optical port 10 and the third optical port 11, respectively.
[0050] In some implementations, when the second optical port 10 and the third optical port 11 are used as inputs, the first optical port 1 is used as the output to achieve polarization beam combining. Defining the second optical port 10 and the third optical port 11 as inputs and the first optical port 1 as the output means changing the operating mode of the polarization beam combiner. This can be achieved by coupling the optical signals to be combined into the second optical port 10 and the third optical port 11 respectively during system design, and receiving the combined optical signal from the first optical port 1. For example, an external fiber optic connector can be used to guide the optical signal output from an external light source or modulator into the second optical port 10 and the third optical port 11, while connecting the first optical port 1 to a photodetector or subsequent optical path. Achieving polarization beam combining refers to combining two or more optical signals with different polarization states (e.g., transverse magnetic mode and transverse electric mode) into a single optical signal output through optical devices. This can be achieved by utilizing the optical reciprocity of the polarization processing unit, i.e., the propagation path and mode conversion process of the optical signal in the device are reversible.
[0051] The scheme of this application defines the second optical port 10 and the third optical port 11 as input terminals and the first optical port 1 as the output terminal, thereby enabling the polarization beam splitter / combiner to achieve polarization beam combining. Specifically, when optical signals with different polarization states (e.g., a transverse magnetic mode optical signal and a transverse electric mode optical signal) are input from the second optical port 10 and the third optical port 11 respectively, they will pass through their respective connected mode selectors in reverse. The mode selectors ensure that only optical signals with specific polarization states can pass through and enter the uncoupled region. Subsequently, these optical signals enter the mode coupling region, which is composed of a first width-gradient waveguide 2, a second width-gradient waveguide 3, and a third width-gradient waveguide 4, the width of which gradually changes along the direction of light propagation (in this case, in reverse). In the mode coupling region, due to the width-gradient characteristics of the waveguides, optical signals with different polarization states entering from different input ports can undergo adiabatic mode coupling and be guided into the waveguide connected to the first optical port 1. Finally, the two optical signals with different polarization states are combined into a single optical signal output at the first optical port 1. This operating mode effectively utilizes the inherent optical properties of the polarization processing unit, cleverly applying the structure originally used for beam splitting to beam combining, thereby solving the problem of how to efficiently combine optical signals.
[0052] Through the above technical solution, the polarization beamsplitter and combiner of this application can achieve polarization beam combining function by simply changing the input and output port definitions of the optical signal. This allows a single device to support both polarization beam splitting and polarization beam combining operating modes simultaneously, greatly improving the device's versatility and application range, avoiding the need to design and manufacture a separate device for polarization beam combining function, thereby reducing system complexity and cost. Simultaneously, due to the inherent optical reciprocity of the same polarization processing unit, the beam combining process can maintain high efficiency and low loss, ensuring the quality of the combined optical signal and effectively solving the technical problem of how to efficiently combine optical signals.
[0053] In some embodiments described above in this application, a polarization beam splitter / combiner is proposed. Its polarization processing unit includes a mode coupling region composed of a first width-gradient waveguide 2, a second width-gradient waveguide 3, and a third width-gradient waveguide 4, used for adiabatic mode coupling and separation between transverse magnetic and transverse electric modes. However, in practical implementation, if the boundaries of these width-gradient waveguides only employ simple linear gradients, it may lead to poor mode conversion efficiency or require a longer device length to meet the adiabatic requirements, thus limiting the device's performance and integration density.
[0054] In this regard, this application further proposes that the width changes of the first width-gradient waveguide 2 and the third width-gradient waveguide 4 have at least one boundary that is a curved gradient.
[0055] A gradient waveguide refers to a waveguide width transition that is not a simple straight line, but rather follows a smooth transition according to a nonlinear function (e.g., parabola, sigmoid curve, exponential curve). This design aims to optimize the evolution of optical modes within the waveguide to achieve more efficient and lower-loss mode switching. One implementation is to use a parabolic boundary, thus achieving effective mode matching within a finite length. Another implementation is to use an sigmoid boundary, which has zero slope at the start and end points of the waveguide, providing a smoother mode transition and further reducing reflection and radiation losses during mode switching.
[0056] The scheme in this application achieves adiabatic mode coupling and separation between transverse magnetic modes and transverse electric modes through a mode coupling region. This mode coupling region includes a first width-gradient waveguide 2, a second width-gradient waveguide 3, and a third width-gradient waveguide 4. The width of the first width-gradient waveguide 2 narrows along the light propagation direction, the width of the second width-gradient waveguide 3 widens along the light propagation direction, and the width of the third width-gradient waveguide 4 narrows along the light propagation direction. When at least one boundary of the width change of the first width-gradient waveguide 2 and the third width-gradient waveguide 4 is designed as a curved gradient, this nonlinear width change can more precisely control the effective refractive index distribution of the waveguide and its gradient along the propagation direction. Through a carefully designed curved shape, the adiabatic conditions can be better met when light energy is converted between different modes. This means a smoother mode conversion process, reducing mode crosstalk and unnecessary radiation loss, thereby improving the efficiency of mode coupling and the extinction ratio. This curved gradient design enables the device to achieve high-performance polarization beam combining and splitting functions within a relatively short physical length, effectively overcoming the performance bottleneck that may arise from simple linear gradients.
[0057] This application further proposes that the first width-gradient waveguide 2, the second width-gradient waveguide 3, and the third width-gradient waveguide 4 are arranged in parallel in sequence, and the end of the first width-gradient waveguide 2 is connected to the beginning of the first uncoupled waveguide 5, and the beginning of the third width-gradient waveguide 4 is connected to the first optical port 1.
[0058] "Parallel arrangement" refers to the three waveguides maintaining a geometrically parallel relationship along the light propagation direction within the mode coupling region and being arranged adjacently in a specific order. This arrangement helps maintain a constant or controllable spacing between the waveguides, thereby achieving stable mode coupling. Its function is to ensure effective coupling of the optical signal between the three waveguides and to allow precise control of the strength and efficiency of mode coupling through gradual changes in waveguide width, enabling adiabatic mode coupling and separation of transverse magnetic and transverse electric modes. Besides a strict parallel arrangement, a slightly angled arrangement can also be used to optimize coupling efficiency, or in some designs, a non-parallel but gradually changing spacing arrangement can be employed to achieve specific coupling characteristics.
[0059] This application constructs a polarization beamsplitter with a clear structure and well-defined optical path by explicitly defining the parallel arrangement of three tapered waveguides within the mode coupling region and their precise connections to the uncoupled region and the input optical port. Specifically, when the optical signal is input from the first optical port 1, it is first precisely guided to the beginning of the third tapered waveguide 4. Since the first tapered waveguide 2, the second tapered waveguide 3, and the third tapered waveguide 4 are arranged in parallel, the input optical signal can effectively couple with adjacent waveguides via evanescent waves within the mode coupling region. Combined with the tapered design of each waveguide width along the light propagation direction, this parallel arrangement ensures adiabatic mode coupling and separation between transverse magnetic and transverse electric modes, avoiding mode crosstalk and energy loss. After mode separation, the optical signal is precisely guided from the output end of the mode coupling region (e.g., the end of the first tapered waveguide 2) to the beginning of the first uncoupled waveguide 5. This direct and explicit connection ensures that the optical signal output from the mode coupling region can enter the uncoupled region efficiently and without loss, thereby eliminating residual coupling between waveguides. Through this precise structural positioning and connection, the optical path of the entire polarization processing unit is optimized, ensuring smooth transmission and efficient processing of optical signals between different functional regions.
[0060] In some embodiments described above in this application, a polarization beam splitter / combiner is proposed, which includes a decoupled region for receiving optical signals from a mode-coupled region and eliminating residual coupling between waveguides. However, in practical implementation, how to effectively and accurately eliminate residual coupling between waveguides to ensure pure separation or beam combining of optical signals is a problem that needs further refinement and resolution.
[0061] In this regard, this application further proposes that the uncoupled region includes a first uncoupled waveguide 5, a second uncoupled waveguide 6, and a third uncoupled waveguide 7. One end of the first uncoupled waveguide 5 is connected to the first output terminal of the mode coupling region, and the other end is connected to the first mode selector 8. One end of the second uncoupled waveguide 6 is connected to the second width-gradient waveguide 3. One end of the third uncoupled waveguide 7 is connected to the second output terminal of the mode coupling region, and the other end is connected to the second mode selector 9.
[0062] The decoupling region plays a crucial role in the polarization processing unit. Its main function is to receive optical signals from the mode coupling region and eliminate residual coupling between waveguides that may occur during mode coupling. If this residual coupling is not effectively eliminated, it will lead to crosstalk between different polarization modes, affecting the performance of the device. The decoupling region aims to maximize the isolation of different optical paths by optimizing the waveguide structure and layout, ensuring that the subsequent mode selector can receive pure polarization mode signals. This can be achieved by precisely controlling the waveguide spacing, waveguide geometry, or introducing specific optical isolation structures. As part of the decoupling region, the first decoupling waveguide 5 is responsible for transmitting the optical signal from the first output end of the mode coupling region to the first mode selector 8. Its design goal is to further eliminate any residual coupling that may exist during transmission and maintain the integrity of the optical signal. The structure of this waveguide can take various forms; for example, it can be a straight waveguide segment with a specific length, or the optical transmission path can be optimized by introducing specific bends or shape changes. Also as part of the decoupling region, the third decoupling waveguide 7 is responsible for transmitting the optical signal from the second output end of the mode coupling region to the second mode selector 9. Its function is similar to that of the first uncoupled waveguide 5, aiming to ensure that the polarization state of the optical signal is fully decoupled and isolated before it is transmitted to the mode selector. The waveguide structure can also be a straight waveguide section, or other geometric designs can be used to achieve optimal decoupling. The first and second output terminals of the mode coupling region are the physical interfaces for outputting optical signals after the mode coupling and decoupling operations are completed. They are the starting points for the uncoupled region to receive optical signals. The first mode selector 8 and the second mode selector 9 are located at the ends of the uncoupled waveguide, respectively, and are used to receive optical signals from the uncoupled waveguide and filter them according to preset polarization characteristics, allowing only specific transverse magnetic or transverse electric modes to pass through.
[0063] The solution in this application explicitly sets up a first uncoupled waveguide 5 and a third uncoupled waveguide 7 within the uncoupled region, connecting them respectively to the first and second output terminals of the mode coupling region, as well as the first mode selector 8 and the second mode selector 9. This structure allows two optical signals output from the mode coupling region to be transmitted through their respective independent uncoupled waveguides. By providing a dedicated uncoupled waveguide for each output path, the geometry, length, or spacing of the waveguides can be specifically designed and optimized to minimize residual coupling on each path. For example, the bending radius or width of the waveguides can be adjusted to ensure that crosstalk between different polarization modes is effectively suppressed during optical signal transmission. This separate and dedicated transmission path design avoids mutual interference between different optical paths, thereby improving the isolation of the polarization beam splitter / combiner, ensuring that the mode selector can receive pure polarization mode signals, and thus improving the overall performance of the device.
[0064] In some embodiments described above, a polarization beamsplitter / combiner is proposed. Its polarization processing unit includes a mode coupling region, an uncoupled region, and a mode selector. The uncoupled region includes a first uncoupled waveguide 5, a second uncoupled waveguide 6, and a third uncoupled waveguide 7, used to guide optical signals from the mode coupling region to the corresponding mode selectors. However, in practical implementation, if the first uncoupled waveguide 5 and the third uncoupled waveguide 7 adopt a straight-line structure, the optical path layout may not be compact enough, or it may be difficult to effectively eliminate residual coupling between waveguides within a limited space, thereby affecting the device's integration and performance.
[0065] In this regard, this application further proposes the above-mentioned polarization beam splitter and combiner, wherein the first uncoupled waveguide 5 and / or the third uncoupled waveguide 7 are S-shaped curved waveguides.
[0066] An S-shaped curved waveguide is an optical waveguide structure whose geometry resembles an S-curve. This type of waveguide is typically composed of two curved segments in opposite directions connected together, allowing the optical signal to be laterally displaced in the propagation direction while maintaining effective signal transmission. The implementation of an S-shaped curved waveguide can be, but is not limited to, forming it by connecting two circular arc segments of the same or different radii in opposite directions; or by designing it using smooth spline curves (such as Bézier curves) to ensure that signal loss at the bend is minimized. Furthermore, an S-shaped curved waveguide can also be composed of multiple S-curves connected in series to achieve larger lateral displacements or more complex path planning.
[0067] By employing the aforementioned technical solution, designing the first uncoupled waveguide 5 and / or the third uncoupled waveguide 7 as S-shaped curved waveguides effectively addresses the challenges of optical path layout and residual coupling elimination in compact integration environments. The introduction of the S-shaped curved waveguide enables precise and compact lateral displacement of the optical signal during transmission from the mode coupling region to the mode selector, thereby providing sufficient physical isolation for the mode selector within a limited chip area and significantly reducing crosstalk between waveguides. Furthermore, the smooth transition characteristics of the S-shaped curved waveguide help further reduce optical signal transmission loss and optimize the elimination of any weak residual coupling that may exist in the output optical signal of the mode coupling region, thus improving the overall performance and integration of the polarization beam splitter / combiner.
[0068] This application further proposes that the second uncoupled waveguide 6 is a waveguide whose width gradually changes along the propagation direction.
[0069] In this context, a waveguide with a gradually changing width along the propagation direction refers to a waveguide whose width gradually changes along the direction of optical signal propagation. This gradual change can be linear, nonlinear (such as exponential or parabolic), or piecewise. Its main function is to achieve effective waveguide mode matching, reduce reflection loss, and improve the transmission efficiency and mode purity of the optical signal. By precisely designing the waveguide's width gradient curve, effective control of the optical field mode can be achieved, such as smoothly transitioning the optical field from one waveguide mode to another, or efficiently transferring energy between waveguides of different widths. One implementation method is linear gradient, where the waveguide width changes linearly from the starting point to the ending point. This method is relatively simple to design and suitable for scenarios where mode matching requirements are not so stringent. Another implementation method is nonlinear gradient, such as exponential or parabolic gradient. This method provides smoother mode transitions, effectively suppressing losses and reflections during mode transitions, and is particularly suitable for applications requiring high efficiency and high mode purity.
[0070] By employing the aforementioned technical solution, designing the second uncoupled waveguide 6 as a waveguide with a gradually varying width along the propagation direction significantly improves the performance of the uncoupled region in the polarization beamsplitter and combiner. This gradually varying waveguide design enables more effective mode matching of the optical signal, reducing reflection loss and mode conversion loss during transmission in the uncoupled region, thereby minimizing any residual coupling that may exist in the mode coupling region. This not only ensures higher mode purity of the optical signal entering the mode selector but also improves the overall optical signal transmission efficiency and polarization extinction ratio of the polarization beamsplitter and combiner. Especially when the uncoupled waveguide is an S-shaped curved waveguide, the gradually varying width design effectively compensates for mode mismatch caused by the curvature, further optimizing the compactness and performance of the device, resulting in superior performance of the polarization beamsplitter and combiner in achieving high-efficiency and high-purity polarization separation or combining.
[0071] This application further proposes that the mode selector is a multimode interference coupler.
[0072] A multimode interference coupler (MMC) is a passive optical device based on the self-imaging effect of the optical field in a multimode waveguide. It typically consists of an input waveguide, a multimode region (or multimode interference region), and one or more output waveguides. When an optical signal enters the multimode region from the input waveguide, these modes propagate with different propagation constants and interfere, as the multimode region supports multiple modes. At specific locations within the multimode region, these modes recombine to form multiple self-images or inverse images of the input optical field, thereby enabling functions such as beam splitting, beam combining, or mode selection of optical power. The waveguide structure of a MMC can be formed by etching or depositing a material with a specific refractive index on a substrate. The multimode region of the MMC is typically designed as rectangular or trapezoidal, with its length and width precisely calculated and optimized according to the required beam splitting ratio, mode selection function, and operating wavelength. For example, silicon-based waveguide technology can be used to fabricate silicon waveguides on a silicon wafer through photolithography and etching processes, utilizing the high refractive index difference of silicon to achieve strong optical field confinement and a compact device size. For polarization mode selection, multimode interferometric couplers can be designed to be polarization-sensitive. For example, by adjusting the geometry of the multimode region (such as width and length) and / or introducing anisotropic materials, different polarization states (such as transverse magnetic modes and transverse electric modes) can have different self-imaging periods or output positions within the multimode region. In this way, at a specific output port in the multimode region, effective filtering and output of specific polarization modes can be achieved.
[0073] The mode selector can be a multimode interference coupler implemented using silicon-based waveguide technology. For example, a multimode region of length L and width W can be designed, with both its input and output waveguides using single-mode waveguides. When an optical signal (potentially containing TM and TE modes) from the uncoupled region enters the multimode region of the multimode interference coupler, by optimizing the geometric parameters of the multimode region, the transverse magnetic mode can form a self-image at a specific location within the multimode region and couple to one of the output waveguides, while the transverse electric mode can form a self-image at another specific location within the multimode region and couple to the other output waveguide. For example, an asymmetric multimode interference coupler can be designed, with its multimode region width and length precisely calculated to utilize the difference in effective refractive index between the TM and TE modes in the silicon waveguide to achieve a polarization-sensitive self-image effect. The output waveguide can be connected to the second optical port 10 and the third optical port 11 to output the filtered TM and TE modes, respectively.
[0074] The above technical solution implements the mode selector as a multimode interference coupler, effectively utilizing the inherent self-imaging effect and polarization sensitivity of the multimode interference coupler to achieve efficient and accurate selection of transverse magnetic and transverse electric modes. Compared to other mode selection schemes, the multimode interference coupler has advantages such as compact structure, low insertion loss, wide bandwidth, and relatively high tolerance to manufacturing errors. This enables the entire polarization beam splitter / combiner to achieve a higher polarization extinction ratio and lower crosstalk when performing polarization beam splitting or combining functions, thereby improving the overall performance and reliability of the device.
[0075] In the description of this application, it should be noted that the terms "upper," "lower," etc., indicating the orientation or positional relationship are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. Unless otherwise expressly specified and limited, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication between two elements. For those skilled in the art, the specific meaning of the above terms in this application can be understood according to the specific circumstances.
[0076] It should be noted that in this application, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.
[0077] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.
Claims
1. A polarization beam splitter / combiner, characterized in that, include: Multiple optical ports, including a first optical port, a second optical port, and a third optical port; and, A polarization processing unit is optically connected between the first optical port and the second and third optical ports; The polarization processing unit includes a mode coupling region, a non-coupling region, and two mode selectors. The mode coupling region includes three waveguides arranged side by side with gradually varying widths, used for adiabatic mode coupling and separation between the transverse magnetic mode and the transverse electric mode. The uncoupled region is connected after the mode coupling region and is used to receive optical signals from the mode coupling region and eliminate residual coupling between waveguides. The two mode selectors are respectively located on the two output paths of the uncoupled region, used to filter and pass through the transverse magnetic mode and the transverse electric mode, and are respectively connected to the second optical port and the third optical port.
2. The polarization beam splitter and combiner according to claim 1, characterized in that, The mode coupling region includes a first width-gradient waveguide, a second width-gradient waveguide, and a third width-gradient waveguide; The width of the first tapered waveguide narrows along the direction of light propagation, the width of the second tapered waveguide widens along the direction of light propagation, and the width of the third tapered waveguide narrows along the direction of light propagation.
3. The polarization beam splitter and combiner as described in claim 2, characterized in that, When the first optical port is used as the input, the second and third optical ports are used as the output to achieve polarization beam splitting.
4. The polarization beam splitter and combiner as described in claim 2, characterized in that, When the second and third optical ports are used as inputs, the first optical port is used as the output to achieve polarization beam combining.
5. The polarization beam splitter and combiner as described in claim 2, characterized in that, The width changes of the first and third tapered waveguides have at least one boundary that is a curved gradient.
6. The polarization beam splitter and combiner as described in claim 2, characterized in that, The uncoupled region includes: The first uncoupled waveguide has one end connected to the first output terminal of the mode coupling region and the other end connected to the first mode selector. A second uncoupled waveguide, one end of which is connected to the second tapered waveguide; and The third uncoupled waveguide has one end connected to the second output terminal of the mode coupling region and the other end connected to the second mode selector.
7. The polarization beam splitter and combiner as described in claim 6, characterized in that, The first, second, and third width-gradient waveguides are arranged in parallel sequence, with the end of the first width-gradient waveguide connected to the beginning of the first uncoupled waveguide, and the beginning of the third width-gradient waveguide connected to the first optical port.
8. The polarization beam splitter and combiner as described in claim 6, characterized in that, The first uncoupled waveguide and / or the third uncoupled waveguide are S-shaped curved waveguides.
9. The polarization beam splitter / combiner as described in claim 6 or 8, characterized in that, The second uncoupled waveguide is a waveguide whose width gradually changes along the propagation direction.
10. The polarization beam splitter and combiner as described in claim 1, characterized in that, The mode selector is a multimode interference coupler.