A single-mode-to-multiple-mode optical fiber mode converter

Abstract

The application discloses a single-mode fiber to multi-mode fiber mode converter which comprises a single-mode fiber head assembly, a transmitting end C lens, a birefringent crystal sequence, a receiving end C lens and a multi-mode fiber head assembly arranged coaxially; an air gap is arranged between the light emitting end of the single-mode fiber head assembly and the transmitting end C lens, and an outer envelope tube of the transmitting end collimator is coaxially fixed to the outer circumferential side of the single-mode fiber head assembly and the transmitting end C lens; the single-mode fiber to multi-mode fiber mode converter can stably assemble each optical component and ensure the overall assembly precision by means of the outer envelope tubes of the transmitting end and receiving end collimators and the inner and outer envelope tubes of the crystal group; all the components are packaged by the overall outer envelope tube, the internal optical structure is effectively protected, and the firmness of the device structure is enhanced.

Description

Technical Field

[0001] This invention relates to the field of multimode fiber technology, specifically to a mode converter for converting single-mode fiber to multimode fiber. Background Technology

[0002] With the rapid development of optical fiber communication technology, single-mode fiber, with its advantages of low loss and high bandwidth, is widely used in long-distance backbone network transmission, while multimode fiber dominates in short-distance data centers, local area networks, and access networks due to its low cost and easy coupling. In the interconnection of heterogeneous optical fiber networks, mode converters from single-mode fiber to multimode fiber are needed to achieve mode matching and efficient coupling of optical signals.

[0003] Existing single-mode to multi-mode converters have the following main drawbacks:

[0004] (1) The method of using fiber optic disk to excite higher-order modes is unstable and requires a very long fiber disk to achieve effective mode conversion, resulting in high setup costs.

[0005] (2) The method of building a spatial light system requires the use of optical support rods and discrete optical components to assemble on an optical platform, which occupies a large space and cannot achieve miniaturized integration. Summary of the Invention

[0006] The purpose of this section is to outline some aspects of the embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0007] 1. Technical problems to be solved:

[0008] (1) High-order modes excited by fiber optic discs are unstable and require very long fiber optic discs, resulting in high setup costs;

[0009] (2) Using spatial light systems such as optical rods and optical components to build requires an optical platform and occupies a large space.

[0010] (3) This patent can process the single-mode to multi-mode solution into a miniaturized and portable device.

[0011] 2. Technical Solution:

[0012] To address the aforementioned technical problems, according to one aspect of the present invention, the present invention provides the following technical solution:

[0013] A single-mode fiber to multimode fiber mode converter includes a single-mode fiber head assembly, a transmitter C-lens, a birefringent crystal sequence, a receiver C-lens, and a multimode fiber head assembly arranged coaxially. An air gap is provided between the output end of the single-mode fiber head assembly and the transmitter C-lens. An outer casing for a transmitter collimator is coaxially fixed to the outer periphery of the single-mode fiber head assembly and the transmitter C-lens. An air gap is provided between the output end of the receiver C-lens and the multimode fiber head assembly. An outer casing for a receiver collimator is coaxially fixed to the outer periphery of the receiver C-lens and the multimode fiber head assembly. An inner casing and an outer casing for the crystal group are sequentially coaxially fitted onto the outer periphery of the birefringent crystal sequence. An integral outer casing is coaxially fitted onto the outer periphery of the transmitter collimator outer casing, the crystal group outer casing, and the receiver collimator outer casing.

[0014] As a preferred embodiment of the single-mode fiber to multimode fiber mode converter of the present invention, the single-mode fiber head assembly includes a single-mode fiber core, a single-mode fiber cladding, and a single-mode zirconia ceramic ferrule; the single-mode fiber core is coaxially disposed inside the single-mode fiber cladding, the single-mode fiber cladding passes through the central hole of the single-mode zirconia ceramic ferrule and is coaxially fixed by an optical fiber fixing adhesive layer, and the light-emitting end face of the single-mode zirconia ceramic ferrule is optically polished.

[0015] As a preferred embodiment of the single-mode fiber to multimode fiber mode converter of the present invention, the multimode fiber head assembly includes a multimode fiber core, a multimode fiber cladding, and a multimode zirconia ceramic ferrule; the multimode fiber core is coaxially disposed inside the multimode fiber cladding, the multimode fiber cladding passes through the central hole of the multimode zirconia ceramic ferrule and is coaxially fixed by an optical fiber fixing adhesive layer, and the light-incident end face of the multimode zirconia ceramic ferrule is optically polished.

[0016] In a preferred embodiment of the single-mode fiber to multimode fiber mode converter of the present invention, the birefringent crystal sequence includes a first birefringent crystal, a second birefringent crystal, a third birefringent crystal, and a fourth birefringent crystal arranged coaxially in sequence; all four birefringent crystals are coaxially arranged in the internal cavity of the inner tube of the crystal group, and an outer crystal shell is coaxially sleeved on the outer periphery of the four birefringent crystals; an outer crystal sealing ring is coaxially fixed to the light input end of the first birefringent crystal and the light output end of the fourth birefringent crystal, respectively; and an intercrystalline fixing adhesive layer is provided between the first birefringent crystal, the second birefringent crystal, the third birefringent crystal, the fourth birefringent crystal, the outer crystal shell, and the outer crystal sealing ring.

[0017] As a preferred embodiment of the single-mode fiber to multimode fiber mode converter of the present invention, the optical axis of the first birefringent crystal is 22.5°, the optical axis of the second birefringent crystal is -22.5°, the optical axis of the third birefringent crystal is 67.5°, and the optical axis of the fourth birefringent crystal is 112.5°. The four crystals are arranged in series to decompose the incident single-mode Gaussian light into discrete light spots distributed in a 4×4 matrix.

[0018] In a preferred embodiment of the single-mode fiber to multimode fiber mode converter of the present invention, the numerical aperture of the transmitting end C lens is 0.15-0.20, which matches the numerical aperture of the single-mode fiber; the numerical aperture of the receiving end C lens is 0.1-0.20 larger than that of the multimode fiber, which is used to filter out the optical power of the smooth falling edge of the light spot, so that the falling edge of the actual received optical power is steeper, thereby improving the uniformity of the coupled beam energy; all birefringent crystals are tightly flat and the fixing adhesive layer is brushed on the side.

[0019] As a preferred embodiment of the single-mode fiber to multimode fiber mode converter of the present invention, a gap of 0.1-0.3mm is provided between the first birefringent crystal and the second birefringent crystal, and between the second birefringent crystal and the third birefringent crystal. The inter-crystal fixing adhesive layer fills the gap, and the thickness of the adhesive layer is the same as the thickness of the gap.

[0020] As a preferred embodiment of the single-mode fiber to multimode fiber mode converter of the present invention, the integral outer sleeve is made of stainless steel with an outer diameter of 5.5 mm and a total length of 35 mm; the input end of the integral outer sleeve is fixedly connected to an input fiber protective sleeve, and the output end is fixedly connected to an output fiber protective sleeve, both of which are made of rubber.

[0021] In a preferred embodiment of the single-mode fiber to multimode fiber mode converter of the present invention, the beveled end face of the transmitting end C lens faces the single-mode fiber head assembly, and the spherical end face faces the birefringent crystal sequence; the spherical end face of the receiving end C lens body faces the birefringent crystal sequence, and the beveled end face faces the multimode fiber head assembly, wherein the beveled end is an octave bevel.

[0022] As a preferred embodiment of the single-mode fiber to multimode fiber mode converter of the present invention, the birefringent crystal sequence can be replaced by a microlens array assembly, the microlens array assembly including a microlens array substrate and a 3×3 microlens unit array disposed on the microlens array substrate, each microlens unit having the same focal length.

[0023] 3. Beneficial effects:

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] This type of single-mode fiber to multimode fiber mode converter, through the coaxial arrangement of each component and the air gap between the transmitter and receiver, can ensure stable optical signal transmission and improve the reliability of mode conversion.

[0026] This type of single-mode fiber to multimode fiber mode converter, with the help of the outer sealing tubes of the collimators at the transmitter and receiver ends and the inner and outer sealing tubes of the crystal group, can stably assemble each optical component and ensure the overall assembly accuracy.

[0027] This single-mode fiber to multimode fiber mode converter uses an integrated outer sheath to encapsulate all components, which can effectively protect the internal optical structure and enhance the robustness of the device structure. Attached Figure Description

[0028] To more clearly illustrate the technical solutions of the embodiments of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and detailed embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0029] Figure 1 This is a cross-sectional view of the overall structure of a single-mode fiber to multimode fiber mode converter according to the present invention.

[0030] Figure 2 This is a schematic diagram of the overall structure of a single-mode fiber to multimode fiber mode converter according to the present invention.

[0031] Figure 3 This is a schematic diagram of the single-mode fiber head assembly structure of a single-mode fiber to multimode fiber mode converter according to the present invention.

[0032] Figure 4 This is a schematic diagram of the multimode fiber head assembly structure of a single-mode fiber to multimode fiber mode converter according to the present invention.

[0033] Figure 5 This is a schematic diagram of the birefringent crystal sequence structure of a mode converter from single-mode fiber to multimode fiber according to the present invention.

[0034] Figure 6 This is a schematic diagram of the intercrystalline fixation layer structure of a single-mode fiber to multimode fiber mode converter according to the present invention.

[0035] Figure 7 This is an end-face view of the birefringent crystal group of a single-mode fiber to multimode fiber mode converter according to the present invention;

[0036] Figure 8 This is an overall cross-sectional view of an alternative embodiment of a microlens array for a mode converter from single-mode fiber to multimode fiber according to the present invention.

[0037] Explanation of the labels in the diagram: 101, Single-mode fiber optic head assembly; 1011, Single-mode fiber core; 1012, Single-mode fiber cladding; 1013, Single-mode zirconia ceramic ferrule; 102, Transmitter C-lens; 103, Transmitter collimator outer sheath; 104, Transmitter air gap; 201, Receiver C-lens; 202, Multimode fiber optic head assembly; 2021, Multimode fiber core; 2022, Multimode fiber cladding; 2023, Multimode zirconia ceramic ferrule; 203, Receiver collimator outer sheath; 204, Receiver air gap. Gap; 301, outer sealing tube of crystal group; 302, inner sealing tube of crystal group; 303, birefringent crystal sequence; 3031, first birefringent crystal; 3032, second birefringent crystal; 3033, third birefringent crystal; 3034, fourth birefringent crystal; 3035, outer shell of crystal; 3036, outer sealing ring of crystal; 3037, inter-crystal fixing adhesive layer; 3038, discrete light spot; 401, overall outer sealing sleeve; 501, microlens array assembly; 502, microlens array substrate; 503, microlens unit. Detailed Implementation

[0038] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0039] This invention is described in detail with reference to the schematic diagrams. When describing the embodiments of this invention, for ease of explanation, the cross-sectional views illustrating the device structure may be partially enlarged, not according to the usual scale. Furthermore, the schematic diagrams are merely examples and should not be construed as limiting the scope of protection of this invention. In actual fabrication, the three-dimensional spatial dimensions of length, width, and depth should be included.

[0040] The orientation or positional relationship indicated in the terminology is based on the orientation or positional relationship shown in the accompanying drawings and is only for the convenience of describing the invention and simplifying the description, and is not intended to 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 the invention.

[0041] The term "connection method" should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be a connection within two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0042] The embodiments of the present invention will now be described in further detail with reference to the accompanying drawings.

[0043] This invention provides a schematic diagram of the overall structure of an embodiment of a single-mode fiber to multimode fiber mode converter, comprising:

[0044] Please see Figures 1-8 This embodiment of a single-mode fiber to multimode fiber mode converter includes a single-mode fiber head assembly 101, a transmitter C-lens 102, a birefringent crystal sequence 303, a receiver C-lens 201, and a multimode fiber head assembly 202, all coaxially arranged. A transmitter air gap 104 is provided between the light-emitting end of the single-mode fiber head assembly 101 and the transmitter C-lens 102. A transmitter collimator outer sealing tube 103 is coaxially fixed to the outer periphery of the single-mode fiber head assembly 101 and the transmitter C-lens 102. A receiver air gap 204 is provided between the light-emitting end of the receiver C-lens 201 and the multimode fiber head assembly 202. A receiver collimator outer sealing tube 203 is coaxially fixed to the outer periphery of the receiver C-lens 201 and the multimode fiber head assembly 202. An inner crystal group sealing tube 302 and an outer crystal group sealing tube 301 are sequentially coaxially sleeved on the outer periphery of the birefringent crystal sequence 303. An integral outer sleeve 401 is coaxially sleeved on the outer periphery of the transmitter collimator outer sleeve 103, the crystal group outer sleeve 301, and the receiver collimator outer sleeve 203.

[0045] It is worth noting that, in order to achieve precise collimation of the divergent light output from the single-mode fiber and ensure the uniformity of the subsequent beam splitting effect, the single-mode fiber head assembly 101 specifically includes a single-mode fiber core 1011, a single-mode fiber cladding 1012, and a single-mode zirconia ceramic ferrule 1013. The single-mode fiber core 1011 is coaxially disposed inside the single-mode fiber cladding 1012. The single-mode fiber cladding 1012 passes through the central hole of the single-mode zirconia ceramic ferrule 1013 and is coaxially fixed by the fiber optic adhesive layer. The light-emitting end face of the single-mode zirconia ceramic ferrule 1013 is treated with UPC optical polishing, and the surface roughness is less than 0.01μm. The air gap 104 at the transmitting end ranges from 80 to 150μm. By adjusting this gap, the Gaussian light with a divergence angle of approximately 14° output from the single-mode fiber can be precisely adjusted into collimated parallel light with a parallelism better than 1mrad.

[0046] Next, to achieve efficient coupling of the discrete spot 3037 to the multimode fiber and reduce insertion loss, the multimode fiber head assembly 202 specifically includes a multimode fiber core 2021, a multimode fiber cladding 2022, and a multimode zirconia ceramic ferrule 2023. The multimode fiber core 2021 is coaxially disposed inside the multimode fiber cladding 2022. The multimode fiber cladding 2022 passes through the central hole of the multimode zirconia ceramic ferrule 2023 and is coaxially fixed by the fiber optic adhesive layer. The light-incident end face of the multimode zirconia ceramic ferrule 2023 is treated with UPC optical polishing. The air gap 204 at the receiving end ranges from 60 to 120 μm and is used to precisely align the focused discrete spot 3037 with the center of the multimode fiber core 2021, achieving a coupling efficiency of over 90%.

[0047] Meanwhile, in order to uniformly decompose the single-mode Gaussian light into 16 discrete light spots 3037 with consistent energy and realize the core function of mode conversion, the birefringent crystal sequence 303 specifically includes a first birefringent crystal 3031, a second birefringent crystal 3032, a third birefringent crystal 3033, and a fourth birefringent crystal 3034 arranged coaxially in sequence. All four birefringent crystals are made of yttrium vanadate (YVO4), with a cross-sectional size of 2mm×2mm and a light-transmitting aperture of 1.5mm. The thickness of the first birefringent crystal 3031 is 1.0mm, the thickness of the second birefringent crystal 3032 is 1.5mm, the thickness of the third birefringent crystal 3033 is 1.5mm, and the thickness of the fourth birefringent crystal 3034 is 1.0mm. Four birefringent crystals are coaxially arranged within the internal cavity of the inner sealing tube 302 of the crystal assembly. A crystal outer shell 3035 is coaxially fitted around the outer periphery of each of the four birefringent crystals. An outer sealing ring 3036 is coaxially fixed to the light-incident end of the first birefringent crystal 3031 and the light-excising end of the fourth birefringent crystal 3034, respectively. An inter-crystal fixing adhesive layer 3036 fills the space between the outer crystal shell 3035 and the outer sealing ring 3036 of the four birefringent crystals.

[0048] Furthermore, to clearly demonstrate the correspondence between the optical axis orientation combination of the four crystals and the beam splitting effect, specifically, the optical axis orientation of the first birefringent crystal 3031 is 22.5°, the optical axis orientation of the second birefringent crystal 3032 is -22.5°, the optical axis orientation of the third birefringent crystal 3033 is 67.5°, and the optical axis orientation of the fourth birefringent crystal 3034 is 112.5°. After the four crystals are arranged in series, the incident single-mode Gaussian light is decomposed into discrete light spots 3038 distributed in a 4×4 matrix. The first birefringent crystal 3031 divides the light spot into a 1x2 light spot matrix, the second birefringent crystal 3032 divides the light into a 1x4 light spot matrix, the third birefringent crystal 3033 divides the light spot into a 2x4 light spot matrix, and the fourth birefringent crystal 3034 divides the light spot into a 4x4 matrix.

[0049] It is worth noting that, in order to achieve numerical aperture matched filtering, filter out low-energy divergent light at the edge of the light spot, and improve return loss, specifically, the planar end face of the transmitting end C lens 102 faces the single-mode fiber head assembly 101, and the spherical end face faces the birefringent crystal sequence 303, with a numerical aperture of 0.18, matching the numerical aperture (0.14) of the single-mode fiber. The spherical end face of the receiving end C lens 201 faces the birefringent crystal sequence 303, and the planar end face faces the multimode fiber head assembly 202, with a numerical aperture of 0.27, which is 0.1-0.20 larger than the numerical aperture (0.20) of the 50 / 125μm multimode fiber. This is used to filter out the optical power of the smooth falling edge at the edge of the light spot, making the falling edge of the actual received optical power steeper, improving the uniformity of the coupled beam energy, and effectively filtering out low-energy light with a divergence angle greater than 20° at the edge of the light spot, with a return loss greater than 45dB.

[0050] Finally, to improve the device's resistance to vibration and temperature changes and ensure stable operation in harsh environments, the overall outer sleeve 401 is made of 304 stainless steel with an outer diameter of 5.5mm and a total length of 35mm. The input end of the overall outer sleeve 401 is fixedly connected to an input fiber optic protective sleeve, and the output end is fixedly connected to an output fiber optic protective sleeve. Both the input and output fiber optic protective sleeves are made of silicone rubber to protect the fiber roots from bending damage and can withstand more than 1000 90° bends without damage.

[0051] Meanwhile, to broaden the scope of protection of this invention and cover more beam splitting implementation methods, another specific embodiment of this invention differs from the above embodiment in that the birefringent crystal sequence 303 is replaced with a microlens array assembly 501. The microlens array assembly 501 includes a microlens array substrate 502 and an array of microlens units 503 disposed on the microlens array substrate 502. Each microlens unit 503 has the same focal length of 2.0 mm. The collimated Gaussian light output from the single-mode fiber is also decomposed into 16 discrete light spots 3038 with uniformly distributed energy after passing through the microlens array, and then coupled into the multimode fiber via the receiving end C lens 201.

[0052] Combination Figures 1-8 The specific usage process of the single-mode fiber to multimode fiber mode converter of this embodiment is as follows:

[0053] 1. Select a mode converter model that matches the operating wavelength of the communication system according to the actual usage. Confirm that the input end is a standard single-mode fiber optic interface and the output end is a standard multimode fiber optic interface. Prepare the corresponding fiber optic connectors, optical power meters, bit error rate testers, fiber optic cleaning pens, and other supporting tools.

[0054] 2: Perform pre-installation cleaning. Use lint-free paper dipped in anhydrous ethanol to wipe the fiber end faces and connector ceramic ferrules at both ends of the mode converter. Then use a special fiber cleaning pen to rotate and clean the end faces to ensure that the end faces are free of dust, oil, scratches and fingerprints, so as to avoid increasing optical signal reflection and loss.

[0055] 3: Complete the physical connection of the devices. Align the single-mode fiber optic connector at the input end of the mode converter with the output port of the optical transmitter, gently insert and rotate the connector locking nut until it is tight. Connect the multimode fiber optic connector at the output end to the input port of the optical receiver in the same way. Avoid end face collision during the connection process to prevent the ceramic ferrule from chipping.

[0056] 4. Perform system optical power debugging: Connect the power supply of the optical transmitter and optical receiver, connect the optical power meter in series to the output fiber optic link, and observe the real-time optical power reading; fine-tune the mating angle and axial position of the fiber optic connector to make the received optical power reach the maximum value, at which time the insertion loss of the mode converter is minimized.

[0057] 5: Verify the system transmission performance. Connect the bit error rate tester to both ends of the communication system, set the test rate and test pattern that match the system, and test continuously for a sufficient duration to confirm that the system bit error rate meets the communication standard requirements and that there is no packet loss or bit error.

[0058] 6. The system can be put into normal use after debugging and long-term stable operation. The invention adopts a fully fixed and non-adjustable structure, which can automatically maintain a stable mode switching effect without manual intervention during operation.

[0059] 7. Perform routine maintenance and troubleshooting. Regularly check the cleanliness and connection status of the fiber optic connectors. If an abnormal increase in insertion loss is found, first clean the connector end face. If the loss cannot be restored after cleaning, check whether the fiber is excessively bent or broken, and replace the damaged fiber or connector in time.

[0060] Example 1 (Basic Example of Birefringent Crystal Beam Splitting)

[0061] It is worth noting that, in order to clearly demonstrate the core structural components of the present invention, the following basic parameter embodiments for the 1310nm wavelength band are provided, specifically:

[0062] The single-mode fiber head assembly 101 uses G.652D standard single-mode fiber with a core diameter of 9μm and a cladding diameter of 125μm; the single-mode zirconia ceramic ferrule 1013 has a concentricity error of no more than 1μm, and the light-emitting end face is treated with UPC optical polishing.

[0063] The transmitting end C lens 102 is made of BK7 optical glass with a focal length of 2.0mm and a numerical aperture of 0.18. Both end faces are coated with a 1310nm single-band anti-reflection coating. The transmitting end air gap 104 is designed to be 100μm to collimate the divergent light output from the single-mode fiber into parallel light. The birefringent crystal sequence 303 consists of four optical-grade yttrium vanadate (YVO4) crystals with a cross-sectional size of 2mm×2mm and a light-transmitting aperture of 1.5mm. The first crystal is 1.0mm thick (optical axis 22.5°), the second crystal is 1.5mm thick (optical axis -22.5°), the third crystal is 1.5mm thick (optical axis 67.5°), and the fourth crystal is 1.0mm thick (optical axis 112.5°). All crystals are coated with a 1310nm anti-reflection coating on their light-transmitting end faces. The inter-crystal gap is 0.2mm and is fixed with UV-curable optical adhesive.

[0064] The receiver C lens 201 is made of BK7 optical glass with a focal length of 1.8mm and a numerical aperture of 0.27. Both ends are coated with 1310nm anti-reflection coating. The receiver air gap 204 is designed to be 80μm, which is used to focus and couple the discrete spot 3038 into the multimode fiber.

[0065] The overall outer sheath 401 is made of 304 stainless steel, with an outer diameter of 5.5mm and a total length of 35mm; the silicone rubber protective sleeves at both ends are used to protect the fiber root from bending damage.

[0066] Example 2 (Alternative Example of Microlens Array Beam Splitting)

[0067] It is worth noting that, in order to demonstrate an alternative implementation of the present invention, the following microlens array beam splitting embodiment is provided. Specifically, the difference between this embodiment and embodiment 1 is only that the birefringent crystal sequence 303 is replaced with the microlens array assembly 501.

[0068] The microlens array assembly 501 uses a fused silica substrate with a thickness consistent with the total thickness of the original birefringent crystal sequence 303. The microlens unit 503 array is fabricated on the substrate. Each microlens unit 503 is a spherical lens with a focal length of 2.0 mm and a center-to-center distance of 0.6 mm between adjacent microlenses. The surface of the microlenses is coated with a 1310 nm / 1550 nm dual-band antireflection coating.

[0069] This embodiment can also decompose single-mode Gaussian light into 16 discrete light spots 3038, which is suitable for application scenarios with special requirements for polarization characteristics.

[0070] Although the present invention has been described above with reference to embodiments, various modifications can be made and components can be replaced with equivalents without departing from the scope of the invention. In particular, as long as there is no structural conflict, the features in the disclosed embodiments can be combined with each other in any manner. The lack of an exhaustive description of these combinations in this specification is merely for the sake of brevity and resource conservation. Therefore, the present invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A mode converter for converting single-mode fiber to multimode fiber, characterized in that, The device includes a single-mode fiber optic head assembly (101) coaxially arranged, a transmitter C-lens (102), a birefringent crystal sequence (303), a receiver C-lens (201), and a multimode fiber optic head assembly (202). An transmitter air gap (104) is provided between the light-emitting end of the single-mode fiber optic head assembly (101) and the transmitter C-lens (102). A transmitter collimator outer sealing tube (103) is coaxially fixed to the outer periphery of the single-mode fiber optic head assembly (101) and the transmitter C-lens (102). The light-emitting end of the receiver C-lens (201) is connected to the multimode fiber optic head assembly (202). An air gap (204) is provided between the fiber head assembly (202). The outer periphery of the receiver C lens (201) and the multimode fiber head assembly (202) is coaxially fixed with the outer sealing tube (203) of the receiver collimator. The outer periphery of the birefringent crystal sequence (303) is coaxially fitted with the inner sealing tube (302) of the crystal group and the outer sealing tube (301) of the crystal group. The outer periphery of the outer sealing tube (103) of the transmitter collimator, the outer sealing tube (301) of the crystal group and the outer sealing tube (203) of the receiver collimator is coaxially fitted with an integral outer sealing tube (401).

2. The mode converter from single-mode fiber to multimode fiber according to claim 1, characterized in that, The single-mode fiber head assembly (101) includes a single-mode fiber core (1011), a single-mode fiber cladding (1012), and a single-mode zirconia ceramic ferrule (1013). The single-mode fiber core (1011) is coaxially disposed inside the single-mode fiber cladding (1012). The single-mode fiber cladding (1012) passes through the central hole of the single-mode zirconia ceramic ferrule (1013) and is coaxially fixed by the fiber fixing adhesive layer. The light-emitting end face of the single-mode zirconia ceramic ferrule (1013) is optically polished.

3. The mode converter from single-mode fiber to multimode fiber according to claim 1, characterized in that, The multimode fiber head assembly (202) includes a multimode fiber core (2021), a multimode fiber cladding (2022), and a multimode zirconia ceramic ferrule (2023). The multimode fiber core (2021) is coaxially disposed inside the multimode fiber cladding (2022). The multimode fiber cladding (2022) passes through the central hole of the multimode zirconia ceramic ferrule (2023) and is coaxially fixed by the fiber fixing adhesive layer. The light-incident end face of the multimode zirconia ceramic ferrule (2023) is optically polished.

4. The mode converter from single-mode fiber to multimode fiber according to claim 1, characterized in that, The birefringent crystal sequence (303) includes a first birefringent crystal (3031), a second birefringent crystal (3032), a third birefringent crystal (3033), and a fourth birefringent crystal (3034) arranged coaxially in sequence. The four birefringent crystals are all coaxially arranged in the internal cavity of the inner sealing tube (302) of the crystal group. The outer periphery of the four birefringent crystals is coaxially fitted with a crystal outer sealing shell (3035). The light-incident end of the first birefringent crystal (3031) and the light-excising end of the fourth birefringent crystal (3034) are respectively coaxially fixed with crystal outer sealing rings (3036). A crystal fixing adhesive layer (3037) is provided between the four birefringent crystals, the crystal outer sealing shell (3035), and the crystal outer sealing ring (3036).

5. The single-mode fiber to multimode fiber mode converter according to claim 4, characterized in that, The optical axis of the first birefringent crystal (3031) is 22.5°, the optical axis of the second birefringent crystal (3032) is -22.5°, the optical axis of the third birefringent crystal (3033) is 67.5°, and the optical axis of the fourth birefringent crystal (3034) is 112.5°. The four crystals are arranged in series to decompose the incident single-mode Gaussian light into discrete light spots (3038) with a 4×4 matrix distribution.

6. The mode converter from single-mode fiber to multimode fiber according to claim 5, characterized in that, The numerical aperture of the transmitting end C lens (102) is 0.15-0.20, which matches the numerical aperture of the single-mode fiber; the numerical aperture of the receiving end C lens (201) is 0.1-0.20 larger than that of the multimode fiber, which is used to filter out the optical power of the smooth falling edge of the light spot, making the falling edge of the actual received optical power steeper and improving the uniformity of the coupled beam energy. All birefringent crystals are tightly flat and the fixing adhesive layer (3037) is brushed on the side.

7. The mode converter from single-mode fiber to multimode fiber according to claim 6, characterized in that, A gap of 0.1-0.3 mm is provided between the first birefringent crystal (3031) and the second birefringent crystal (3032), and between the second birefringent crystal (3032) and the third birefringent crystal (3033). The inter-crystal fixing adhesive layer (3037) fills the gap, and the thickness of the adhesive layer is the same as the thickness of the gap.

8. The single-mode fiber to multimode fiber mode converter according to claim 7, characterized in that, The integral outer sleeve (401) is made of stainless steel, with an outer diameter of 5.5 mm and a total length of 35 mm. The input end of the integral outer sleeve (401) is fixedly connected to an input fiber optic protective sleeve, and the output end is fixedly connected to an output fiber optic protective sleeve. Both the input and output fiber optic protective sleeves are made of rubber.

9. The single-mode fiber to multimode fiber mode converter according to claim 8, characterized in that, The beveled end face of the transmitting end C lens (102) faces the single-mode fiber head assembly (101), and the spherical end face faces the birefringent crystal sequence (303); the spherical end face of the receiving end C lens (201) body faces the birefringent crystal sequence (303), and the beveled end face faces the multimode fiber head assembly (202), and the beveled end face is an octave bevel.

10. The single-mode fiber to multimode fiber mode converter according to claim 9, characterized in that, The birefringent crystal sequence (303) can be replaced by a microlens array assembly (501), which includes a microlens array substrate (502) and a 3×3 array of microlens units (503) disposed on the microlens array substrate (502), each microlens unit (503) having the same focal length.

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