A multimode interferometer

Abstract

The application relates to the technical field of multimode interferometer, and provides a multimode interferometer, which comprises a substrate layer, wherein an input waveguide structure, a multimode interference structure and an output waveguide structure are sequentially arranged on the substrate layer along the length direction, and the width of the end face of the multimode interference structure close to the output waveguide structure is greater than the width of the end face of the multimode interference structure close to the input waveguide structure. The multimode interferometer of the application improves the width of the end face close to the output waveguide structure, reduces the problems such as bridging and defects of the adjacent waveguide under the low-precision photoetching process, increases the process tolerance, avoids the length from being significantly increased due to the overall increase of the width of the multimode interference area, realizes the double optimization of the chip size and the process matching design, and is suitable for balancing the contradictory structure design between the output waveguide spacing and the multimode interference area length under the condition of the low-precision photoetching process.

Description

Technical Field

[0001] This invention relates to the field of multimode interferometer technology, specifically to a multimode interferometer. Background Technology

[0002] Indium phosphide-based multimode interferometers (MMIs), as core passive devices for optical power distribution and coupling, are widely used in photonic integrated circuits. A typical MMI can achieve a specific beam splitting ratio (e.g., 3 dB splitting in a 1×2 configuration) by rationally designing its width and length. However, in the actual design and manufacturing process of MMIs, the placement of the output waveguides is closely related to the geometry of their multimode interference region. Taking a 1×2 MMI as an example, the two output waveguides are typically located at specific proportions, such as one-quarter or one-sixth of the width at the end of the multimode interference region, to achieve precise alignment of self-image points. This design results in a narrow lateral spacing between the two output waveguides. When using low-precision lithography processes such as contact lithography, insufficient lithographic resolution between adjacent waveguides can easily lead to pattern transfer defects (such as waveguide sidewall bridging and line convergence), thus affecting device yield.

[0003] To alleviate the aforementioned problems, existing technologies often employ increasing the width of the multimode interference region, thereby proportionally increasing the spacing between the two output waveguides to meet the minimum line spacing requirements of photolithography. However, the working length of the multimode interference region is approximately proportional to the square of its width, meaning that increasing the width leads to a sharp increase in device length. This not only significantly increases the chip's footprint, hindering high-density integration, but also introduces additional transmission losses, limiting the device's application in large-scale photonic integrated circuits.

[0004] Therefore, there is an urgent need to design a multimode interferometer structure that can effectively increase the output waveguide spacing (to adapt to low-precision photolithography processes) while avoiding excessive increase in the length of the multimode interference region, thus achieving a reasonable balance between chip size and process tolerance. Summary of the Invention

[0005] The purpose of this invention is to provide a multimode interferometer that can at least solve some of the defects in the prior art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a multimode interferometer, comprising a substrate layer, wherein an input waveguide structure, a multimode interferometer structure, and an output waveguide structure are sequentially disposed on the substrate layer along the length direction, wherein the width of the end face of the multimode interferometer structure near the output waveguide structure is greater than the width of the end face of the multimode interferometer structure near the input waveguide structure.

[0007] Furthermore, the width of the multimode interference structure gradually increases along the direction from the input waveguide structure to the output waveguide structure.

[0008] Furthermore, the multimode interference structure includes multiple multimode interference blocks connected in sequence, and the maximum width of each multimode interference block increases sequentially along the direction from the input waveguide structure to the output waveguide structure.

[0009] Furthermore, there are two multimode interferometer blocks. Along the direction from the input waveguide structure to the output waveguide structure, the multimode interferometer block closer to the input waveguide structure is a multimode interferometer block of equal width, and the multimode interferometer block closer to the output waveguide structure is a multimode interferometer block with gradually increasing width.

[0010] Furthermore, the width of the end face of the multimode interferometer block closest to the output waveguide structure with the minimum width is equal to the width of one end face of the other multimode interferometer block, and the end face of the multimode interferometer block closest to the output waveguide structure with the minimum width is aligned with one end face of the other multimode interferometer block.

[0011] Furthermore, the input waveguide structure includes input waveguides arranged sequentially along the length direction. The input waveguides are connected to the central position of the end face of the multimode interference structure, or the input waveguides are connected to a position offset from the central position of the end face of the multimode interference structure by a certain distance.

[0012] Furthermore, the output waveguide structure includes multiple output waveguides arranged sequentially along the length direction, with each output waveguide arranged in parallel and spaced apart, and each output waveguide docking with the widest end face of the multimode interference structure.

[0013] Furthermore, there are two output waveguides, and the distance between the two output waveguides is half the width of the widest end face of the multimode interference structure.

[0014] Furthermore, both output waveguides are offset from the center of the widest end face of the multimode interference structure by one-quarter of the width of the widest end face in a direction away from each other.

[0015] Furthermore, both output waveguides are offset from the center of the widest end face of the multimode interference structure by one-sixth of the width of the widest end face in a direction away from each other.

[0016] Compared with the prior art, the beneficial effects of the present invention are: a multimode interferometer, by increasing the width of the end face near the output waveguide structure, reduces problems such as bridging and defects of adjacent waveguides under low-precision photolithography process, increases process tolerance, and at the same time avoids the significant increase in length caused by the overall increase in the width of the multimode interference region, thus achieving dual optimization of chip size and process matching design. It is suitable for structural design that balances the contradiction between the output waveguide spacing and the length of the multimode interference region under low-precision photolithography process conditions. Attached Figure Description

[0017] Figure 1 A top view schematic diagram of a conventional symmetrical indium phosphide multimode interferometer provided in an embodiment of the present invention;

[0018] Figure 2 A schematic cross-sectional view (section 1) of a conventional symmetrical indium phosphide multimode interferometer provided for an embodiment of the present invention;

[0019] Figure 3 A schematic cross-sectional view (section 2) of a conventional symmetrical indium phosphide multimode interferometer provided for an embodiment of the present invention;

[0020] Figure 4 A schematic cross-sectional view (section 3) of a conventional symmetrical indium phosphide multimode interferometer provided for an embodiment of the present invention;

[0021] Figure 5 A top view schematic diagram of a conventional limited interference indium phosphide multimode interferometer provided in an embodiment of the present invention;

[0022] Figure 6 This is a top view schematic diagram of a one-piece symmetrical interference gradient indium phosphide multimode interferometer provided in Embodiment 1 of the present invention;

[0023] Figure 7 A schematic diagram of optical power transmission of a conventional symmetrical indium phosphide multimode interferometer with a width of 14 μm provided in an embodiment of the present invention;

[0024] Figure 8 A schematic diagram of optical power transmission of a conventional symmetrical indium phosphide multimode interferometer with a width of 8μm is provided in one embodiment of the present invention.

[0025] Figure 9 This is a schematic diagram of optical power transmission in a single-stage gradually symmetrical indium phosphide multimode interferometer provided in Embodiment 1 of the present invention.

[0026] Figure 10 A schematic diagram of optical power transmission of a conventional limited interference indium phosphide multimode interferometer with a width of 15 μm provided in an embodiment of the present invention;

[0027] Figure 11 A schematic diagram of optical power transmission of a conventional limited interference indium phosphide multimode interferometer with a width of 8μm is provided in one embodiment of the present invention;

[0028] Figure 12 This is a schematic diagram of optical power transmission in a single-stage gradient-limited indium phosphide multimode interferometer provided in Embodiment 1 of the present invention.

[0029] Figure 13The phase simulation calculation results of a conventional limited interference indium phosphide multimode interferometer in one embodiment of the present invention are provided;

[0030] Figure 14 The phase simulation calculation results of a single-stage gradually limited indium phosphide multimode interferometer provided in Embodiment 1 of the present invention are shown below;

[0031] Figure 15 This is a top view schematic diagram of a two-segment symmetrical indium phosphide multimode interferometer provided in Embodiment 2 of the present invention;

[0032] Figure 16 This is a schematic diagram of optical power transmission in a two-segment symmetrical indium phosphide multimode interferometer provided in Embodiment 2 of the present invention. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] Please see Figure 6 and Figure 15 This invention provides a multimode interferometer, including a substrate layer. An input waveguide structure, a multimode interferometer structure, and an output waveguide structure are sequentially disposed on the substrate layer along its length. The width of the end face of the multimode interferometer structure near the output waveguide structure is greater than the width of the end face of the multimode interferometer structure near the input waveguide structure. In this embodiment, compared to conventional multimode interferometers, such as... Figure 1 and Figure 5As shown, its multimode interference structure has a uniform width. If the conventional method of increasing the width of the multimode interference region is used to solve the existing defects, it will lead to a sharp increase in device length. This not only significantly increases the chip's footprint, which is not conducive to high-density integration, but also introduces additional transmission losses, limiting the device's application in large-scale photonic integrated circuits. This embodiment cleverly designs the multimode interference structure with a non-uniform width structure. In this way, by increasing the width of the end face near the output waveguide structure, problems such as bridging and defects of adjacent waveguides under low-precision photolithography processes are reduced, and process tolerance is increased. At the same time, it avoids the significant increase in length caused by the overall increase in the width of the multimode interference region. This achieves dual optimization of chip size and process matching design, and is suitable for structural designs that balance the contradiction between the output waveguide spacing and the length of the multimode interference region under low-precision photolithography conditions. Specifically, the input and output waveguide structures are matched to the front and rear ends of this multimode interference structure, respectively. In the variable-width multimode interference region, since the traditional equal-width limitation is eliminated, the overall length can be optimized through calculation, allowing the output to be located at the self-image point. This effectively increases the waveguide spacing at the output end, increasing process tolerance, while simultaneously reducing the length of the multimode interference region and saving chip size. Furthermore, this multimode interference structure has a smaller width on the input waveguide side and a larger width on the output waveguide side to improve the output waveguide spacing. This multimode interferometer is fabricated on indium phosphide and its derivatives, including but not limited to indium phosphide and related ternary compounds. Each part of this multimode interferometer includes a core layer, an upper cladding layer, a lower cladding layer, and a substrate, with the substrates combined to form the substrate layer. Some low-refractive-index regions of this multimode interferometer can be replaced with other non-indium phosphide materials, including but not limited to silicon dioxide and organic polymers. The effective refractive index, width, height, and length of this multimode interference structure are all calculated and optimized to the theoretical optimal length. Without considering process errors, it can achieve near-lossless power distribution and power coupling functions.

[0035] Please see Figure 1 , Figure 1 This is a top view of a conventional symmetrical interferometric multimode interferometer, comprising the following parts: S14 is the substrate layer, mainly serving as a substrate to support the overall waveguide structure; S11 is the input waveguide; S13 consists of two output waveguides; S12 is the multimode interferometric structure, the main functional area of ​​the device; Figure 1 In the middle, section 1, section 2, section 3, corresponding to Figure 2 , Figure 3 and Figure 4 The three structures in; Figure 1 In the diagram, W11 is the width of the multimode interference structure, L11 is the length of the multimode interference structure, and D11 is the distance between the two output waveguides. Figure 1In this case, symmetrical interference requires the output waveguide to be located at a quarter of the distance from the center of the multimode interference structure, so D11 is directly determined by W11.

[0036] Please see Figure 5 This is a top view of a conventional limited interferometric multimode interferometer, which includes the following parts: S24 is the substrate layer, S21 is the input waveguide, S23 is the two output waveguides, and S22 is the multimode interferometric structure. Figure 5 In the diagram, W21 is the width of the multimode interference structure, L21 is the length of the multimode interference structure, and D21 is the distance between the two output waveguides. Figure 5 In the context of the interference constraint, the output waveguide is required to be located one-sixth of the distance from the center of the multimode interference structure. Therefore, D21 is directly determined by W21.

[0037] Please see Figure 6 Along the direction from the input waveguide structure to the output waveguide structure, the width of the multimode interference structure gradually increases. In this embodiment, the form of the multimode interference structure of Embodiment 1 is shown, and the overall structure is a one-piece gradient structure with a gradually increasing width. Preferably, the output waveguide structure includes multiple output waveguides arranged sequentially along the length direction, with each output waveguide arranged in parallel and spaced apart, and each output waveguide abutting the widest end face of the multimode interference structure. There are two output waveguides, and the distance between the two output waveguides is half the width of the widest end face of the multimode interference structure. Both output waveguides are offset from the center position of the widest end face of the multimode interference structure by one-quarter of the width of the widest end face in a direction away from each other. Wherein, S31 is the input waveguide, S32 is the multimode interference structure, S34 is the substrate layer, S33 is the output waveguide, W31 is the width of the first end of the multimode interference structure, W32 is the width of the tail end of the multimode interference structure, L31 is the overall length of the multimode interference structure, and D31 is the distance between the two output waveguides. Symmetrical interference requires the output waveguide to be located one-quarter of the way off from the center of the multimode interference structure; therefore, D31 is directly determined by W32. In another embodiment, the interference requirement is defined as the output waveguide being located one-sixth of the way off from the center of the multimode interference structure, and both output waveguides are deviated from the center of the widest end face of the multimode interference structure by one-sixth of the width of the widest end face, starting from the center position of the widest end face away from each other. Therefore, D31 is also directly determined by W32. The deviation is taken from the center position of the widest end face, i.e., the width of W32, as the starting point. Figure 6 and Figure 15As shown, the cases illustrate symmetrical interference, where the upper output waveguide is offset upwards by one-quarter of the width of the widest endface, starting from the center of the widest endface, while the lower output waveguide is offset downwards by one-quarter of the width of the widest endface, starting from the center of the widest endface. For the case of constrained interference, the upper output waveguide is offset upwards by one-sixth of the width of the widest endface, starting from the center of the widest endface, while the lower output waveguide is offset downwards by one-sixth of the width of the widest endface, starting from the center of the widest endface.

[0038] Please see Figure 15 The multimode interference structure includes multiple multimode interference blocks connected in sequence. Along the direction from the input waveguide structure to the output waveguide structure, the maximum width of each multimode interference block increases sequentially. There are two multimode interference blocks. Along the direction from the input waveguide structure to the output waveguide structure, the multimode interference block closer to the input waveguide structure is of equal width, while the multimode interference block closer to the output waveguide structure is of progressively wider width. The width of the end face of the multimode interference block closest to the output waveguide structure is equal to the width of one end face of the other multimode interference block, and the end face of the multimode interference block closest to the output waveguide structure is aligned with one end face of the other multimode interference block. In this embodiment, the form of the multimode interference structure of Embodiment Two is shown, and the overall structure is a two-segment cross-section structure. Of course, this embodiment is not limited to only two segments; more segments can be set according to the width-to-length ratio. In this embodiment, S41 is the input waveguide, W41 is the width of the first multimode interferometer block (e.g., 8 μm), S42 is the first multimode interferometer block, S44 is the second multimode interferometer block, S43 is the output waveguide, W42 is the widest end face of the second multimode interferometer block (e.g., 14 μm), L41 is the length of the first multimode interferometer block, L42 is the length of the second multimode interferometer block, and D41 is the spacing between the two output waveguides. This embodiment also illustrates symmetrical interference, with the two output waveguides offset in the same way as described above. Figure 6 The two output waveguides are offset in the same way, except that the offset distance is one-quarter of that of W42. For the case of limited interference, the offset distance is one-sixth of that of W42, and the offset method is the same as described above. Figure 6 Consistent. Figure 16 for Figure 15The optical field transmission simulation diagram is shown. In this case, the waveguide center spacing is 7 μm, and the overall length is 91 μm. Compared to a single-segment gradient structure, the length of the two-segment structure is further reduced. The comparison demonstrates that the gradient multimode interferometer in this embodiment can effectively reduce the overall device size while ensuring sufficient waveguide width (lower requirements for process precision), thus achieving the main objective of this invention. Furthermore, the multi-segment multimode interferometer can further optimize device performance.

[0039] Please see Figure 6 and Figure 15 The input waveguide structure includes input waveguides arranged sequentially along its length. The input waveguides are aligned with the center of the end face of the multimode interference structure, or they are aligned with a position offset from the center of the end face of the multimode interference structure by a certain distance. In this embodiment, Figure 6 and Figure 15 All examples illustrate symmetrical interference, with their input waveguides aligned with the center of the end face of the multimode interference structure. In the defined interference case, the input waveguides are aligned at a position offset from the center of the end face of the multimode interference structure by a certain distance. Specifically, this offset could be from the center of the widest end face of the multimode interference structure, moving upwards or downwards by one-sixth of the width of the widest right end face. In this way, the input waveguides are aligned with one of the output waveguides, on the same straight line along their length.

[0040] Please see Figure 7 , it is Figure 1 The simulation diagram shows the optical field transmission when W11 is 14 μm in the structure. It can be seen that when the width of the multimode interference region is 14 μm, the length of a conventional symmetrical multimode interferometer is 190 μm. The center-to-center spacing of the output waveguides is 7 μm, which is sufficient to meet the requirements of most contact lithography methods.

[0041] Please see Figure 8 , it is Figure 1 The simulation diagram shows the optical field transmission when W11 is 8 μm in the structure. It can be seen that when the width of the multimode interference region is 8 μm, the length of a conventional symmetrical multimode interferometer is 62 μm. In this case, although the length is significantly reduced, the center-to-center spacing of the output waveguides is only 4 μm. If the waveguide width is large, this approaches the limit of contact lithography.

[0042] Please see Figure 9 , it is Figure 6 The optical field transmission simulation diagram shows that W31 is 8 μm and W32 is 14 μm in the structure. This is a one-piece graded-symmetric interferometric multimode interferometer. It can be seen that the length of the multimode interference region is 107 μm, and the center-to-center spacing of the output waveguides is 7 μm. The total length is less than... Figure 7In this situation, the output waveguide spacing is greater than Figure 8 This approach satisfies both the waveguide spacing and device length optimization requirements.

[0043] Please see Figure 10 ,for Figure 5 Conventional limited interferometric multimode interferometers in China Figure 10 for Figure 5 In the structure, W21 is a simulation diagram of optical field transmission at 15μm. At this time, the required multimode interferometer length is 291μm and the center-to-center spacing of the output waveguide is 5μm.

[0044] Please see Figure 11 , it is Figure 3 In the structure, W21 represents the optical field transmission simulation at 8 μm, requiring a multimode interferometer length of 82 μm and an output waveguide center-to-center spacing of 2.67 μm. Considering the waveguide width limitation, this value exceeds the limit of contact lithography and is difficult to achieve in terms of fabrication.

[0045] For constrained interference, a one-stage gradient multimode interferometer can also be used for optimization. Figure 12 This is a simulated optical field transmission diagram with an 8μm front end and a 15μm rear end. In this case, the multimode interferometer length is 154μm, and the center-to-center spacing of the output waveguides is 5μm. The total length is less than... Figure 10 In this situation, the output waveguide spacing is greater than Figure 11 This approach satisfies both the waveguide spacing and device length optimization requirements.

[0046] It is important to note that the two output waveguides in the interference must meet the requirement of a phase difference of π / 2. To further confirm that the graded multimode interferometer meets the requirement, a simulation comparison of the phase results was also performed. Figure 13 The middle part shows the phase simulation results of the two output ends of the conventional limited interference. It can be seen that the phase difference between the two ports is close to π / 2. Figure 14 The phase simulation results of the phase-controlled gradually limited interferometer in the implementation phase show that the phase difference between the two ports is still close to π / 2, which meets the requirements.

[0047] This invention provides a method for fabricating a multimode interferometer, wherein the core layer material and the upper cladding material are InP and its quaternary compounds. The core layer contains an MQW structure or a bulk quaternary compound material, which is the main region for optical power distribution; the upper cladding confines the optical field and provides protection for the waveguide core layer. All epitaxial structures can be obtained directly through a single epitaxial growth process, without the need for secondary epitaxy or other steps.

[0048] The specific manufacturing process is as follows:

[0049] Step 1): Epitaxial fabrication in one step: grow the lower cladding, core layer, and upper cladding layer in sequence. The core layer contains multiple quantum wells or bulk materials with a specific PL wavelength. Epitaxy can be fabricated using methods such as MOCVD or MBE.

[0050] Step 2): The input / output waveguide and the multimode interference region waveguide can be fabricated through photolithography, development, and etching. The input / output waveguides have the same width, allowing for the use of tapered sections to reduce losses. The multimode interference region has a wider width than the waveguides, and the relative positions of the input / output waveguides and the multimode interference region waveguide are calculated.

[0051] Preferably, the multimode interferometer can be fabricated by spin-coating polymer materials, growing silicon oxide by PECVD, or other methods to create an outer cladding layer.

[0052] According to the design in this embodiment, the multimode interferometer can achieve uniform power output from both output waveguides when operating at the optimal wavelength. Considering factors such as material absorption, the power of each output waveguide will be similar, but cannot reach 50%. This embodiment mainly discusses structural optimization issues, therefore power analysis is not performed.

[0053] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A multimode interferometer, comprising a substrate layer, characterized in that: An input waveguide structure, a multimode interference structure, and an output waveguide structure are sequentially arranged along the length of the substrate. The width of the end face of the multimode interference structure near the output waveguide structure is greater than the width of the end face of the multimode interference structure near the input waveguide structure.

2. The multimode interferometer as described in claim 1, characterized in that: The width of the multimode interference structure gradually increases along the direction from the input waveguide structure to the output waveguide structure.

3. The multimode interferometer as described in claim 1, characterized in that: The multimode interference structure includes multiple multimode interference blocks connected in sequence, and the maximum width of each multimode interference block increases sequentially along the direction from the input waveguide structure to the output waveguide structure.

4. A multimode interferometer as described in claim 3, characterized in that: There are two multimode interferometer blocks. Along the direction from the input waveguide structure to the output waveguide structure, the multimode interferometer block closer to the input waveguide structure is a multimode interferometer block of equal width, and the multimode interferometer block closer to the output waveguide structure is a multimode interferometer block with gradually increasing width.

5. The multimode interferometer as described in claim 4, characterized in that: The width of the end face of the multimode interferometer block closest to the output waveguide structure is equal to the width of one end face of the other multimode interferometer block, and the end face of the multimode interferometer block closest to the output waveguide structure is aligned with one end face of the other multimode interferometer block.

6. The multimode interferometer as described in claim 1, characterized in that: The input waveguide structure includes input waveguides arranged sequentially along the length direction. The input waveguides are connected to the center position of the end face of the multimode interference structure, or the input waveguides are connected to a position offset from the center position of the end face of the multimode interference structure by a certain distance.

7. The multimode interferometer as described in claim 1, characterized in that: The output waveguide structure includes multiple output waveguides arranged sequentially along the length direction. The output waveguides are arranged in parallel and spaced apart, and each output waveguide is connected to the widest end face of the multimode interference structure.

8. The multimode interferometer as described in claim 7, characterized in that: The output waveguide has two strands, and the distance between the two output waveguides is half the width of the widest end face of the multimode interference structure.

9. The multimode interferometer as described in claim 8, characterized in that: Both output waveguides are offset from the center of the widest end face of the multimode interference structure by one-quarter of the width of the widest end face in a direction away from each other.

10. The multimode interferometer as described in claim 8, characterized in that: Both output waveguides are offset from the center of the widest end face of the multimode interference structure by one-sixth of the width of the widest end face in a direction away from each other.

Images