Light beam regulation and control micro-lens array for multi-core optical fiber and preparation method of light beam regulation and control micro-lens array

Abstract

The invention relates to a light beam regulation and control micro-lens array for a multi-core optical fiber and a preparation method of the light beam regulation and control micro-lens array. The micro-lens array is embedded into the output end face of the multi-core optical fiber, and each fiber core corresponds to one cylindrical micro-lens unit. Each unit comprises a first combined lens and a second combined lens, and light beams sequentially pass through; the incident side of the first combined lens is a first lens curved surface, and the emergent side is a first refraction interface; the incident side of the second combined lens is a second refraction interface, and the emergent side of the second combined lens is a second lens curved surface; a cylindrical first cavity is formed between the first lens curved surface and the fiber core; the first and second refracting interfaces are parallel and inclined relative to the optical fiber end face, and a second cavity with a parallelogram section is formed between the first and second refracting interfaces. According to the micro lens array, the micro lens does not need to be directly aligned with the fiber core, the distance between the optical axes of the emergent light beams can be enlarged, the dual requirements of long-distance collimation and high-resolution focusing are met at the same time, and various light beam regulation and control functions such as collimation, deflection and shaping can be achieved.

Description

Technical Field

[0001] This application relates to the field of micro-nano optics and fiber optic devices, and in particular to a beam-tuning microlens array for multi-core optical fibers and its fabrication method. Background Technology

[0002] With the rapid development of fields such as optical interconnects, LiDAR, quantum information processing, and high-throughput bioimaging, the demand for high-density, parallel, and miniaturized optical systems is becoming increasingly urgent. Multi-core fiber (MCF) has become an ideal carrier for realizing multi-beam transmission due to its ability to integrate multiple independent optical channels in a single fiber. However, the spacing between the cores of an MCF is usually small (typically 25–50 μm). If the beam is directly output from the end face, not only will the dense beam easily generate crosstalk, but the physical space constraints also make it difficult to integrate high-performance micro-optical components to achieve effective beam manipulation (such as collimation, focusing, or spacing expansion).

[0003] To address these issues, researchers have attempted to integrate microlens arrays (MLAs) on the end face of optical fibers (MCFs) to improve beam quality and adapt to downstream optical systems. Current mainstream technologies include mechanical polishing, femtosecond laser processing, cemented pre-fabricated lenses, and micro / nano 3D printing. Among these, two-photon polymerization (TPP) is considered a cutting-edge method for achieving high-degree-of-freedom micro-optical integration on fiber end faces due to its sub-micron resolution and ability to fabricate arbitrary three-dimensional structures. However, even with TPP technology, if the microlenses are still directly aligned with the original fiber core arrangement, their aperture is still limited by the fiber core spacing, resulting in an excessively large numerical aperture and a short depth of focus, making it difficult to simultaneously meet the dual requirements of long-distance collimation and high-resolution focusing.

[0004] In their paper "Optically Aligned Molded Microlens Arrays on Multi-Core Fibers for Sub-Wavelength Focusing" (J. Lightwave Technol., Vol. 43, No. 10, pp. 4928–4933, 2025), Hajj et al. proposed a method for fabricating polymer microlens arrays on the end face of MCFs based on a mold replication process. This method utilizes hydrofluoric acid to selectively etch multi-core optical fibers with specific refractive index profiles to form tapered or parabolic mold cavities. These cavities are then filled with polydimethylsiloxane (PDMS) and optically aligned for replication, achieving subwavelength focusing (spot size close to 1.5 μm @ 1550 nm) on 4-core and 37-core optical fibers. This work has significant advantages in terms of process cost and batch consistency, and provides a practical path for MCF microlens integration. However, it still fails to solve the problem that the microlens are directly aligned with the original fiber core arrangement, and their aperture is still limited by the fiber core spacing, resulting in an excessively large numerical aperture and a short depth of focus. This makes it difficult to meet the dual requirements of long-distance collimation and high-resolution focusing at the same time. Furthermore, the function is limited, and it can only achieve focusing but cannot simultaneously meet the multiple beam control requirements such as collimation, deflection, and shaping. Summary of the Invention

[0005] Based on this, the purpose of this invention is to address the above-mentioned technical problems by providing a beam-tuning microlens array for multi-core optical fibers and its fabrication method. This allows the microlenses to be not directly aligned with the original fiber core arrangement, thereby expanding the spacing between the optical axes of the beams emitted from the fiber cores. This simultaneously meets the dual requirements of long-distance collimation and high-resolution focusing, and can also satisfy various beam-tuning requirements such as collimation, deflection, and shaping.

[0006] To achieve the above-mentioned objectives, the first aspect of this application provides a beam-tuning microlens array for multi-core optical fibers, comprising: The microlens array is embedded in the output end face of the multi-core optical fiber. The microlens array includes several cylindrical microlens units, each of which corresponds to one of the cores of the multi-core optical fiber. Each microlens unit includes a first combined lens and a second combined lens. The beam emitted from the fiber core of a single microlens unit first passes through the first combined lens and then through the second combined lens. The beam incident side of the first combined lens is the first lens surface, and the beam exit side is the first refractive interface. The beam incident side of the second combined lens is the second refractive interface, and the beam exit side is the curved surface of the second lens. The first lens surface and the output end face of the fiber core are opposite each other, and there is a first cavity between the first lens surface and the output end face of the fiber core. The cross-section of the first cavity is cylindrical, with one end being the first lens surface and the other end being the output end face of the fiber core. The first refractive interface and the second refractive interface are opposite surfaces and parallel to each other. The first refractive interface and the second refractive interface have a preset tilt angle relative to the output end face of the fiber core. There is a second cavity between the first refractive interface and the second refractive interface. The cross-section of the second cavity is a parallelogram.

[0007] Preferably, the two first cavities of two adjacent microlens units are connected by two adjacent sides parallel to the fiber core axis.

[0008] Preferably, the two second cavities of two adjacent microlens units are connected by two adjacent sides parallel to the fiber core axis.

[0009] Preferably, the first lens surface and the second lens surface include aspherical lens surfaces.

[0010] Preferably, the aspherical lens surface of the first lens surface includes a concave lens, and the aspherical lens surface of the second lens surface includes a convex lens.

[0011] Preferably, the concave lens is used to increase the laser divergence angle within the lens, shorten the length of the laser beam to the exit aperture, and thus reduce the length of the microlens array; The convex lens is used for wavefront modulation to collimate or focus the laser beam passing through the second refractive interface.

[0012] Preferably, the sidewalls of the first cavity and the second cavity of each microlens unit are provided with at least one through hole.

[0013] Preferably, the diffusion angle of the laser beam emitted from the second combined lens satisfies the Gaussian beam divergence angle formula:

[0014] in, It's the wavelength. It is the waist radius.

[0015] To achieve the objective of this invention, a second aspect of this application provides a method for fabricating a beam-tuning microlens array for multi-core optical fibers, used to fabricate the beam-tuning microlens array for multi-core optical fibers described in the above-mentioned technical solution. The method employs two-photon polymerization technology to fabricate the microlens array in situ at the output end face of the multi-core optical fiber, and includes the following steps: S1: Polish and clean the output end face of the multi-core optical fiber, and then coat the surface of the output end face with a photosensitive resin suitable for two-photon polymerization to form a uniform photoresist layer. S2: Based on a preset three-dimensional microstructure model, the femtosecond laser beam is controlled to scan layer by layer within the photoresist layer. Through the two-photon polymerization effect, the three-dimensional microstructure model of the photoresist layer is directly written in situ, completing the structural aggregation of several microlens units including a first combined lens, a second combined lens, a first cavity, and a second cavity. S3: The polymerized structure is developed, rinsed, and post-cured to obtain a microlens array integrated into the output end face of the multi-core optical fiber.

[0016] Preferably, the sidewalls of the first cavity and the second cavity of the microlens unit constructed in step S2 are provided with through holes. The through holes are used to allow the developer to smoothly enter the cavity during the development process, so as to completely remove the unpolymerized photoresist, thereby ensuring structural integrity and optical transparency.

[0017] Compared with the prior art, the beneficial effects of this invention are: The microlens array of this invention forms a beam translation structure through a first refractive interface, a second refractive interface, and a second cavity, enabling the beams emitted from each fiber core to achieve lateral displacement in free space. This effectively expands the spacing between the optical axes of the output beams. The wider optical axis spacing provides a physical space basis for designing aspherical lens surfaces with larger radii, allowing for an increase in lens aperture and significantly improving the collimation distance or reducing the focusing spot size. This invention also allows for flexible configuration of the parameters of the second lens surface according to actual needs, enabling different types of beam control. The microlens array is fabricated through integrated molding, which improves the long-term stability and compactness of the system, while simplifying the manufacturing process and reducing production costs. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the cross-section of two adjacent microlens units and a multi-core optical fiber in the microlens array of this application; Figure 2 This is a schematic diagram of the fabrication process of a microlens array in one embodiment; Figure 3 This is a schematic diagram of the overall structure of the microlens array and multi-core optical fiber prepared in one embodiment; Figure 4 This is a simulation diagram illustrating the collimation effect of the microlens array of this application and the multi-core fiber of a traditional single-lens array.

[0019] In the figure: 1-microlens unit; 2-first combined lens; 3-second combined lens; 4-first cavity; 5-second cavity; 6-beam translation structure; 7-laser beam; 8-optical axis; 9-multi-core fiber; 10-fiber core; 11-fiber cladding; 12-photoresist layer; 13-through hole. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of the invention. The following embodiments are used to illustrate the invention but are not intended to limit its scope.

[0021] In the description of this invention, it should be noted that the terms "center", "longitudinal", "lateral", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", and "outer" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention 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. Therefore, they should not be construed as limitations on this invention.

[0022] Example 1 Embodiment 1 of this application provides a beam-tuning microlens array for multi-core optical fibers, such as... Figure 1 As shown ( Figure 1 The diagram shows the radial cross-section of any two adjacent fiber cores of the microlens array and their corresponding microlens units. The multi-core fiber 9 is composed of fiber cores 10 and fiber cladding 11. The output end face of the multi-core fiber 9 is embedded in the microlens array. The microlens array includes several cylindrical microlens units 1, and each microlens unit 1 corresponds to each fiber core 10 of the multi-core fiber 9. Each microlens unit 1 includes a first combined lens 2 and a second combined lens 3. The laser beam 7 of the fiber core 10 corresponding to a single microlens unit 1 passes through the first combined lens 2 and then through the second combined lens 3. The beam incident side of the first combined lens 2 is the first lens surface 21, and the beam exit side is the first refractive interface 22. The beam incident side of the second combined lens is the second refractive interface 31, and the beam exit side is the second lens surface 32. The first lens surface 21 and the output end face of the fiber core 10 are opposite to each other, and the first cavity 4 is between the first lens surface 21 and the output end face of the fiber core 10. The cross-section of the first cavity 4 is cylindrical, with one end being the first lens surface 21 and the other end being the output end face of the fiber core 10. The first refractive interface 22 and the second refractive interface 31 are opposite surfaces and parallel to each other. The first refractive interface 22 and the second refractive interface 31 have a preset tilt angle relative to the output end face of the fiber core 10. The second cavity 5 is located between the first refractive interface 22 and the second refractive interface 31. The cross-section of the second cavity 5 is a parallelogram.

[0023] The first refractive interface 22, the second refractive interface 31, and the second cavity 5 form a beam translation structure 6. Its function is to cause the laser beam emitted from the fiber core to undergo lateral refraction displacement as it passes through this structure, thereby increasing the optical axis spacing of the laser beams from different fiber cores 10 passing through the first combined lens 2 and the second combined lens 3. When the beam spacing is too small, crosstalk easily occurs between adjacent channels, especially prominent in high-density multi-core fiber systems. Increasing the beam spacing not only provides more flexibility for the layout of subsequent optical components, but more importantly, it effectively reduces the probability of crosstalk between channels, ensuring signal purity and system reliability. This is particularly important for applications with extremely high signal-to-noise ratio requirements, such as automotive LiDAR and optoelectronic co-packaged CPO.

[0024] like Figure 1 As shown, after the optical axis 8 of the laser beam 7 passes through the beam translation structure 6, it is laterally shifted to a new position, increasing the optical axis spacing between different fiber cores. This increased optical axis spacing allows for the use of larger aperture lenses to achieve longer collimation distances or smaller focused spots, even when the beams do not overlap. The collimation mode employs a large-aperture lens, achieving collimation distances of hundreds of micrometers while maintaining a low divergence angle; the focusing mode utilizes a large-aperture, short-focal-length lens, enabling the acquisition of extremely small spot diameters on the subwavelength scale (e.g., 1.1–1.3 μm & 1550 nm).

[0025] In this embodiment 1, the first lens surface 21 and the second lens surface 32 are aspherical lens surfaces; wherein, the first lens surface 21 is configured as a concave lens to increase the laser divergence angle inside the lens and shorten the length of the laser beam expansion to the exit aperture, thereby reducing the length of the microlens array; the second lens surface 32 is configured as a convex lens for wavefront modulation to collimate or focus the laser beam after passing through the beam translation structure 6.

[0026] The laser beam 7 emitted from fiber core 10 has a diffusion angle that satisfies the Gaussian beam divergence angle formula:

[0027] in, It's the wavelength. It is the waist radius.

[0028] like Figure 2As shown, each of the microlens units 1 has at least one through hole 13 on the sidewall of the first cavity 4 and the second cavity 5.

[0029] Example 2 Embodiment 2 of this application, based on Embodiment 1, provides a method for fabricating a beam-tuning microlens array for multi-core optical fibers, such as... Figure 2 As shown, the method employs two-photon polymerization technology to fabricate the microlens array in situ at the output end face of the multi-core optical fiber 9, and includes the following steps: like Figure 2 As shown in Figure A, S1: Polish and clean the output end face of the multi-core optical fiber 9, and then coat the surface of the output end face with a photosensitive resin suitable for two-photon polymerization to form a uniform photoresist layer 12. like Figure 2 As shown in B, S2: Based on the preset three-dimensional microstructure model, the femtosecond laser beam is controlled to scan layer by layer in the photoresist layer 12, and the three-dimensional microstructure model is directly written in situ on the photoresist layer 12 through the two-photon polymerization effect, so as to complete the structural aggregation of several microlens units 1 including the first combined lens 2, the second combined lens 3, the first cavity 4 and the second cavity 5. In step S2, the sidewalls of the first cavity 4 and the second cavity 5 of the microlens unit 1 are provided with through holes 13. The through holes 13 are used to allow the developer to smoothly enter the cavity during the development process, so as to completely remove the unpolymerized photoresist, thereby ensuring structural integrity and optical transparency.

[0030] like Figure 2 As shown in C, S3: The polymerized structure is developed, rinsed, and post-cured to obtain a microlens array integrated into the output end face of the multi-core optical fiber 9, as shown in Figure C. Figure 3 As shown, the green part is the microlens array, the gray part is the optical fiber, and the blue part is the fiber core.

[0031] The method for fabricating a beam-tuning microlens array for multi-core optical fibers in Embodiment 2 can flexibly configure the parameters of the second lens surface 32 according to actual needs to achieve different types of beam tuning: Collimation function: suitable for scenarios requiring long-distance, low-loss transmission, such as optical communication, LiDAR, and parallel optical tweezers; Focusing function: Applicable to fields such as super-resolution imaging, biosensing, and quantum manipulation; Potential extended functions: By designing asymmetric lens surfaces, beam deflection or special shaping can also be achieved (such as generating ring spots, Bessel beams, etc.).

[0032] Based on two-photon polymerization technology, the fabrication method of this application can print arbitrary three-dimensional curved surfaces in situ with nanoscale precision, including but not limited to parabolic surfaces, hyperboloids, and freeform surfaces. This high degree of design freedom allows optical engineers to precisely optimize the geometric parameters of microlenses according to specific application scenarios, ensuring optimal optical performance. The microlens array and the fiber output end face are integrally formed in situ, eliminating the need for additional gluing or alignment operations. This fundamentally eliminates the defects of gluing prefabricated microlens arrays or discrete component assembly methods, such as large alignment errors, thermal expansion mismatch, and interface reflections, which affect the long-term stability of the system. This not only significantly improves the mechanical strength, thermal stability, and vibration resistance of the system, but also greatly simplifies the manufacturing process and reduces production costs.

[0033] Example 3 This embodiment 3, based on embodiments 1 and 2, provides a specific simulation scheme for a beam-tuning microlens array integrated into the end face of a multi-core optical fiber: This embodiment 3 provides a beam-tuning microlens array integrated on the end face of a multi-core optical fiber, which aims to achieve efficient optical coupling between two multi-core optical fibers at a free space distance of 2400 micrometers. It is suitable for scenarios such as inter-board optical interconnection and co-packaging of optoelectronics (CPO).

[0034] 1. Structural design parameters Multi-core fiber: 4-core single-mode fiber with a core spacing of 30 μm and an operating wavelength of λ = 1550 nm; Microlens unit (one per fiber core): Beam translation structure: It consists of two parallel and inclined refractive interfaces with an inclination angle of 12° (relative to the fiber end face normal), which is used to laterally translate the output beam and increase the optical axis spacing of the output beam. Lens surfaces: The radius of curvature of the first lens surface is 60 μm, the radius of curvature of the second lens surface is 107, and the total height of the overall microlens unit structure is 250 μm; Material: Photosensitive polymer (refractive index n ≈ 1.52); Output beam spacing: After translation, it is expanded from the original 30 μm to about 45 μm, providing space for large-aperture lens design.

[0035] 2. Simulation Methods Three-dimensional full-wave electromagnetic simulation was performed using COMSOL Multiphysics software: The model includes a transmitter MCF + microlens array + 2400 μm free space + microlens array + receiver MCF; Each fiber core at the transmitter excites the fundamental mode (LP). 01 ); The receiver fiber core is set to the power integration domain; The boundary uses a perfectly matched layer (PML) to absorb scattered light; The maximum mesh size is ≤ λ / 8, and the mesh is locally refined in the lens surface area.

[0036] 3. Simulation Results and Technical Effects At a free-space transmission distance of 2400 μm, the beams of each channel can still be efficiently coupled to the corresponding fiber core at the receiving end; The average coupling transmittance is as high as 94.7% (defined as the ratio of the receiver core capture power to the transmitter core output power). The simulation results of the microlens array in Example 3 are as follows: Figure 4 As shown in the intensity distribution in (A), the beam maintains good collimation during propagation, with no significant broadening or crosstalk (crosstalk between adjacent channels – 47 dB).

[0037] 4. Control group (traditional regimen) If the beam translation structure of this application is not adopted, such as Figure 4 As shown in (B), limited by the 30 μm fiber core spacing, at the same distance of 2400 μm, even if the lens group height is increased to 300 μm, the coupling transmittance still drops to about 47.1%, and the crosstalk increases significantly (-16.3 dB).

[0038] 5. Conclusion This embodiment 3 successfully expands the optical axis spacing of the output beam by introducing a 12° tilted beam translation structure, enabling the integration of a large-aperture collimating lens with a curvature radius of 107 μm and a total height of 250 μm. This achieves a high coupling efficiency of 94.7% at a distance of 2400 μm, which is significantly better than the traditional solution, verifying the practical value of the present invention in long-distance free-space optical interconnection.

[0039] In summary, this invention provides a beam-tuning microlens array for multi-core optical fibers and its fabrication method. By introducing a beam translation structure, the beam spacing is expanded, creating conditions for designing lens surfaces with larger radii, thereby significantly improving the collimation distance or reducing the focusing spot size. Furthermore, the microlens array and its fabrication method of this application possess numerous advantages, including multifunctional integration, high design freedom, monolithic molding, and strong process scalability, providing an effective solution for key components in future high-density parallel optical systems.

Claims

1. A beam-tuning microlens array for multi-core optical fibers, characterized in that, The output end face of the multi-core optical fiber (9) is embedded with the microlens array, which includes several cylindrical microlens units (1), each microlens unit (1) corresponding to each fiber core (10) of the multi-core optical fiber (9); Each of the microlens units (1) includes a first combined lens (2) and a second combined lens (3). The laser beam (7) of the fiber core (10) corresponding to a single microlens unit (1) passes through the first combined lens (2) and then through the second combined lens (3). The beam incident side of the first combined lens (2) is the first lens surface (21), and the beam exit side is the first refractive interface (22). The beam incident side of the second combined lens is the second refractive interface (31), and the beam exit side is the second lens surface (32). The first lens surface (21) and the output end face of the fiber core (10) are opposite to each other, and the first cavity (4) is between the first lens surface (21) and the output end face of the fiber core (10). The cross-section of the first cavity (4) is cylindrical, with one end being the first lens surface (21) and the other end being the output end face of the fiber core (10). The first refractive interface (22) and the second refractive interface (31) are opposite surfaces and parallel to each other. The first refractive interface (22) and the second refractive interface (31) have a preset tilt angle relative to the output end face of the fiber core (10). The second cavity (5) is between the first refractive interface (22) and the second refractive interface (31). The cross section of the second cavity (5) is a parallelogram.

2. The microlens array according to claim 1, characterized in that, The two first cavities (4) of two adjacent microlens units (1) are connected by two adjacent sides parallel to the central axis of the fiber core (10).

3. The microlens array according to claim 2, characterized in that, The two second cavities (5) of two adjacent microlens units (1) are connected by two adjacent sides parallel to the central axis of the fiber core (10).

4. The microlens array according to claim 1, characterized in that, The first lens surface (21) and the second lens surface (32) include aspherical lens surfaces.

5. The microlens array according to claim 4, characterized in that, The aspherical lens surface of the first lens surface (21) includes a concave lens, and the aspherical lens surface of the second lens surface (32) includes a convex lens.

6. The microlens array according to claim 5, characterized in that, The concave lens is used to increase the laser divergence angle inside the lens, shorten the length of the laser beam (7) to the exit aperture, and thus reduce the length of the microlens array; The convex lens is used for wavefront modulation to collimate or focus the laser beam (7) that passes through the second refractive interface (31).

7. The microlens array according to claim 1, characterized in that, Each microlens unit (1) has at least one through hole (13) on the sidewall of the first cavity (4) and the second cavity (5).

8. The microlens array according to claim 1, characterized in that, The diffusion angle of the laser beam (7) emitted from the second combined lens (3) satisfies the divergence angle formula of a Gaussian beam: in, It's the wavelength. It is the waist radius.

9. A method for fabricating a beam-tuning microlens array for multi-core optical fibers, used to fabricate a beam-tuning microlens array for multi-core optical fibers as described in any one of claims 1 to 8, characterized in that, The method employs two-photon polymerization technology to fabricate the microlens array in situ at the output end face of a multi-core optical fiber (9), and includes the following steps: S1: Polish and clean the output end face of the multi-core optical fiber (9), and then coat the surface of the output end face with a photosensitive resin suitable for two-photon polymerization to form a uniform photoresist layer (12). S2: Based on the preset three-dimensional microstructure model, the femtosecond laser beam is controlled to scan layer by layer in the photoresist layer (12), and the three-dimensional microstructure model of the photoresist layer (12) is directly written in situ through the two-photon polymerization effect, so as to complete the structural aggregation of several microlens units (1) including the first combined lens (2), the second combined lens (3), the first cavity (4) and the second cavity (5); S3: The polymerized structure is developed, rinsed and post-cured to obtain a microlens array integrated into the output end face of the multi-core optical fiber (9).

10. The method according to claim 9, characterized in that, The sidewalls of the first cavity (4) and the second cavity (5) of the microlens unit (1) constructed in step S2 are provided with through holes (13). The through holes (13) are used to allow the developer to smoothly enter the cavity during the development process, so as to completely remove the unpolymerized photoresist, thereby ensuring structural integrity and optical transparency.

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