Two-channel optical fiber rotary connector based on gradient refractive index lens

Abstract

The invention discloses a dual-channel optical fiber rotary connector based on a gradient refractive index lens, and belongs to the technical field of optical fibers. The dual-channel optical fiber rotary connector is composed of a rotor and a stator, the rotor comprises a first graded-index lens, an input collimator serving as a central channel, a second graded-index lens and an input collimator serving as a paraxial channel, and the distance between the central axis of the rotor and the central axis of the first graded-index lens is L; the stator comprises a right-angle prism used for turning a light beam of a center channel by 90 degrees, a third gradient refractive index lens serving as an output collimator of the center channel, a coaxial symmetric lens group composed of a convex lens and a concave lens and used for compensating a scanning light beam of a paraxial channel, and a fourth gradient refractive index lens serving as an output collimator of the paraxial channel. The invention provides a complete quantitative design scheme based on systematic analysis, and provides a reliable basis for manufacturing a compact and high-performance optical fiber rotary connector.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber technology, specifically relating to a dual-channel optical fiber rotary connector based on a graded refractive index lens, which is suitable for realizing bidirectional, parallel, and contactless transmission of optical signals at a rotating interface. Background Technology

[0002] A dual-channel fiber optic rotary connector (FORJ) is a passive optomechanical component that provides a continuous fiber optic connection between rotating and stationary equipment. It is a key device for enabling optical signal transmission through the interface between a continuously rotating platform and its fixed support structure. With the increasing demands for data throughput, transmission reliability, and environmental adaptability from high-end equipment, especially in space-constrained and complex operating scenarios such as robot joints, medical endoscopic imaging, and radar systems, more stringent requirements are being placed on the size, performance, and reliability of fiber optic rotary connectors.

[0003] Currently, the mainstream solutions for realizing dual-channel fiber optic rotary connectors are the derotation optical path design based on Dove Prism and the coaxial design based on graded refractive index lenses.

[0004] The core working principle of the derotation optical path design based on the Dowell prism is that when the rotational speed of the Dowell prism is half that of the rotary connector, the prism's image can remain stationary, thus maintaining the smooth flow of both signals even when the rotor rotates. However, this solution has the following technical problems:

[0005] A complex transmission mechanism is required: to ensure that the Dowell prism rotates at a precise half-speed, a sophisticated gear or differential mechanism must be installed. This not only increases the system complexity but also reduces its service life and reliability, making it difficult to meet the long-life requirements of space applications.

[0006] Alignment requirements are extremely high: the processing and assembly errors of the Daowei prism have a significant impact on the deflection and coupling loss of the emitted light, and this impact changes periodically during rotation, which seriously affects the transmission stability.

[0007] Large size: To ensure the normal operation of the Daowei prism, sufficient space needs to be reserved for its installation and rotation, resulting in a large overall size of the device, making it difficult to adapt to space-constrained application scenarios.

[0008] Limited channel isolation: When multiple channels share a single derotation prism, the assembly and adjustment of each channel affect each other. The measured channel isolation is approximately 31dB, which is insufficient to meet the signal purity requirements of high-end applications.

[0009] The core working principle of the coaxial design based on graded-index lenses is as follows: the optical signal transmitted through the outer channel single-mode fiber is first collimated by the first graded-index lens, the collimated beam passes through the second graded-index lens, and then is coupled into the fiber by the third and fourth graded-index lenses. The inner channel optical signal is first collimated by the second graded-index lens, and the collimated beam is coupled into the fiber by the third graded-index lens. However, this scheme has the following technical problems:

[0010] Signal crosstalk: The inner and outer channels adopt a coaxial optical path design, and the light from one channel may cross into the other channel, causing signal interference.

[0011] Low return loss: At the air-glass interface, if there is no high-transmittance antireflection coating, about 4% of the light will be reflected back, which will cause the laser to be unstable, generate noise, and affect the signal quality. Summary of the Invention

[0012] To address the above problems, this invention proposes a dual-channel fiber optic rotary connector based on a spatially multiplexed graded refractive index lens.

[0013] The technical solution of the present invention is: a dual-channel fiber optic rotary connector based on a spatially multiplexed graded refractive index lens, comprising a rotor and a stator coaxially arranged; the rotor can rotate around a central axis, while the stator is fixed.

[0014] Furthermore, the rotor includes a first graded refractive index lens (center channel input) and a second graded refractive index lens (paraxial channel input); the optical axis of the second graded refractive index lens is parallel to the central axis of the rotor and has an eccentricity L;

[0015] Furthermore, the stator includes a right-angle prism, a third graded-index lens, a coaxial symmetrical lens group, and a fourth graded-index lens; the right-angle prism and the third graded-index lens constitute the receiving optical path of the central channel; the coaxial symmetrical lens group and the fourth graded-index lens constitute the receiving optical path of the off-axis channel.

[0016] Furthermore, the optical path of the central channel is as follows: the optical signal is emitted from the first graded refractive index lens at the center of the rotor, transmitted through the air gap, and then deflected by the right-angle prism of the stator by 90°, before entering the third graded refractive index lens and being coupled to the output optical fiber.

[0017] Furthermore, the off-axis channel optical path is as follows: the optical signal is emitted from the eccentric second graded refractive index lens, forming a scanning beam that rotates around the central axis; after being compensated by the coaxial symmetrical lens group of the stator, the beam maintains a paraxial parallel state, and is finally received by the fourth graded refractive index lens and coupled to the output optical fiber;

[0018] Furthermore, the coaxially symmetrical lens group consists of convex and concave lenses. Due to its axial symmetry characteristics, it can achieve accurate compensation for off-axis beam scanning regardless of the angle at which the incident beam enters.

[0019] Furthermore, the coupling loss between the center channel and the off-axis channel of the optical fiber connector is mainly composed of mode mismatch loss, Fresnel reflection loss, and assembly deviation loss.

[0020] Furthermore, the off-axis channel of the fiber optic connector achieves physical isolation through spatial multiplexing technology, and the beam of the optical beam does not overlap with the beam of the central channel in physical space, thus achieving ultra-high isolation between channels.

[0021] Furthermore, at all the interfaces between the optical fibers and the graded-index lenses, as well as on the free end face of the graded-index lenses, an 8° bevel angle (APC physical contact) is used and an antireflection coating with a transmittance of ≥99.5% is deposited to achieve higher return loss.

[0022] The beneficial effects of this invention are:

[0023] (1) No transmission mechanism, high reliability: The Dow prism and its necessary transmission mechanism are completely eliminated. Static optical elements are used to realize dual-channel transmission, which fundamentally eliminates the reliability risks brought about by the transmission mechanism.

[0024] (2) Excellent optical performance: By rationally selecting the parameters of the dual-channel fiber optic rotary connector, the link coupling loss of the entire system can be reduced to as low as 2.24 dB (better than the level of 3 dB of commercial products); the return loss can reach as high as 73.5 dB (far exceeding the level of 55 dB of commercial products); and the channel isolation can reach 70 dB (far exceeding the level of 50 dB of commercial products).

[0025] (3) High independence between channels: The optical paths of the central channel and the off-axis channel are independent of each other, and optical alignment and assembly adjustment can be performed separately, which greatly reduces the difficulty of assembly and improves production efficiency.

[0026] (4) Small size: The overall outer diameter of the system is controlled within 3 mm, which meets the miniaturization requirements of space-constrained scenarios such as high-end robot joints and medical endoscope probes. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the dual-channel fiber optic rotary connector based on a graded refractive index lens according to the present invention.

[0028] Figure 2 This is the optical path diagram of the central channel of the present invention;

[0029] Figure 3 This is the optical path diagram of the off-axis channel of the present invention; Detailed Implementation

[0030] The embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0031] The specific definitions of abbreviations and key terms are as follows:

[0032] FORJ: Fiber Optic Rotary Joint, used to achieve contactless transmission of optical signals at a rotating interface;

[0033] Gradient-Index Lens: A gradient-index lens has a continuously changing refractive index along the radial direction and a flat end face, which facilitates direct coupling with optical fibers.

[0034] Spatial multiplexing: Dividing different channels in physical space to prevent beam paths from overlapping and fundamentally eliminate crosstalk.

[0035] APC: Angled Physical Contact, which typically uses an 8° bevel angle to deflect reflected light from the optical path.

[0036] like Figure 1 As shown, this invention provides a dual-channel fiber optic rotary connector based on spatial multiplexing, comprising a rotor and a stator coaxially arranged. The rotor can rotate around a central axis, while the stator remains stationary.

[0037] The rotor includes a first graded-index lens (center channel input) and a second graded-index lens (paraxial channel input). The second graded-index lens has an off-center distance (L=1.0 mm), and its optical axis is parallel to the central axis of the rotor.

[0038] The stator includes a right-angle prism (center channel turning element), a third graded refractive index lens (center channel output), a coaxial symmetrical lens group composed of convex and concave lenses (paraxial channel compensation element), and a fourth graded refractive index lens (paraxial channel output).

[0039] In this embodiment of the invention, the central channel optical path is as follows: Figure 2 As shown, it carries downlink control commands (wavelength 1310 nm). The optical signal is emitted from the input fiber at the center of the rotor and collimated into a parallel beam by the first graded-index lens. After being transmitted through the air gap, the beam reaches the right-angle prism fixed to the stator. The right-angle prism deflects the beam by 90°, and then the beam enters the third graded-index lens, is refocused, and coupled to the output fiber.

[0040] Key parameter design:

[0041] Gradient-index lens diameter: 1.8 mm; center refractive index = 1.608; self-focusing constant = 0.323mm⁻¹; Length: 4.863 mm.

[0042] Single-mode fiber mode field diameter: 9.2 μm (1310 nm); numerical aperture: 0.12.

[0043] End face tilt angle: 8° (APC physical contact); Anti-reflection membrane permeability: 99.5%.

[0044] In this embodiment of the invention, the off-axis channel optical path is as follows: Figure 3 As shown, the optical signal (wavelength 1550nm) is transmitted back from the upstream sensor. The optical signal is collimated into a parallel beam by a second graded-index lens (off-center distance L = 1.0 mm). This beam generates a circumferential scan around the central axis as the rotor rotates.

[0045] To stabilize the reception of the scanning beam, a coaxially symmetrical lens group (2.5 mm in diameter, 15 mm focal length for the convex lens, and -15 mm focal length for the concave lens) consisting of a convex lens and a concave lens is set on the central axis. Due to its axial symmetry, the lens group can keep the outgoing beam parallel to the axis regardless of the angle at which the incident beam enters, thereby achieving precise compensation for off-axis beam scanning.

[0046] The compensated parallel beam is finally received by the fourth graded refractive index lens fixed on the axis and focused and coupled to the output fiber. The lens structure data is shown in Table 1.

[0047] Table 1

[0048]

[0049] Comprehensive application of key technological measures:

[0050] To achieve high performance, the present invention adopts the following technical measures in combination:

[0051] Spatial multiplexing design: The beam of the central channel is concentrated near the central axis, while the beam of the off-axis channel rotates near the off-center distance L=1.0 mm. The two beams do not overlap in physical space, fundamentally eliminating crosstalk between channels.

[0052] 8° Angle-Cut Physical Contact (APC): An 8° angle-cut is used at the interface between all optical fibers and graded-index lenses, causing the reflected light from the interface to deviate from the optical path at a larger angle, thus achieving high return loss.

[0053] High-efficiency antireflective coating across all surfaces: Antireflective coatings with a reflectivity of less than 0.5% (transmittance ≥ 99.5%) are deposited on all optical surfaces to reduce the intensity of reflected light at the source.

[0054] Collimation and beam expansion design: By optimizing the parameters of the graded refractive index lens (self-focusing constant, length) and the fiber-to-lens gap (0.2 mm), a collimated beam with a large beam waist diameter (>100 μm) is obtained, which significantly improves the system's tolerance to assembly errors and rotational wobbling.

[0055] Precision tolerance control: Based on theoretical analysis, the lateral deviation is controlled within 1.5 mm, the axial deviation does not exceed 0.025 mm, and the angular deviation is less than 0.06°, ensuring that the coupling loss is lower than the design specifications.

[0056] The core technology of this invention lies in achieving physical isolation between the two channels through spatial multiplexing. Specifically:

[0057] Dual-channel spatial separation design: the central channel transmits coaxially, while the off-axis channel is arranged eccentrically (1.0 mm), and the two beam paths do not overlap in physical space.

[0058] Off-axis beam scanning compensation method: Utilizing the axial symmetry characteristics of a coaxially symmetrical lens group, off-axis beams incident at different angles can all exit in a near-axial parallel state, thus achieving stable reception in a rotating state without any moving parts.

[0059] Comprehensive reflection suppression technology: a triple measure of 8° APC oblique angle physical contact + antireflection coating of ≥99.5% across the entire interface + collimated beam expansion propagation, achieving a high return loss of ≥73.5 dB.

[0060] Based on the above theoretical analysis, this embodiment optimizes the parameters of the graded-index lens. To achieve a large beam waist output and a short working distance, the following parameters of the graded-index lens need to be designed synergistically: self-focusing constant. The central refractive index is controlled between 0.1-0.5 mm⁻¹. The length should be controlled between 1.5 and 1.7. With pitch The concentration should be controlled between 0.24P and 0.28P.

[0061] The beneficial effects of this invention are:

[0062] Extremely low coupling loss: The central channel has a total system link coupling loss of only 2.24 dB at a wavelength of 1310 nm.

[0063] Extremely high return loss: After adopting an 8° APC bevel angle and a 99.5% antireflection coating, the system return loss reaches 73.5dB.

[0064] Ultra-high channel isolation: Due to the complete physical separation of the two channel beams, the channel isolation reaches 70 dB.

[0065] Ample assembly tolerance: The beam expansion design allows the output beam waist diameter to be greater than 100μm, and the system has a high tolerance for lateral, axial and angular deviations: lateral deviation ≤1.5 mm, axial deviation ≤0.025 mm, and angular deviation ≤0.06°.

[0066] Alternative variations:

[0067] Wavelength extension: The design method and parameter selection principles of this invention are also applicable to other operating wavelengths (such as 850nm or the mid-infrared band). Simply recalculate and optimize the specific parameters according to the new wavelength to obtain a low-loss connector suitable for that wavelength.

[0068] Optical fiber type replacement: For applications that require maintaining polarization state (such as optical fiber sensing and coherent communication), polarization-maintaining fiber can be used to replace ordinary single-mode fiber, and polarization matching can be achieved by adjusting the axial rotation angle of the fiber.

[0069] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the principles of the invention, and should be understood that the scope of protection of the invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical teachings disclosed in this invention without departing from the spirit of the invention, and these modifications and combinations are still within the scope of protection of this invention.

Claims

1. A dual-channel fiber optic rotary connector based on a graded-index lens, characterized in that, This includes the central channel and the off-axis channel, which are spaced L apart on their central axes; The central channel carries downlink control commands, and the operating wavelength is selected from the 1310nm band with extremely low dispersion. The optical path is designed as follows: the optical signal is output from the first single-mode fiber at the center of the rotor, and the beam is collimated into a parallel beam propagating along the rotation axis by the first graded refractive index lens; the parallel beam is transmitted through the air gap for a preset working distance and then reaches the right-angle prism fixed on the stator; the right-angle prism deflects the beam by 90°, turning it to propagate perpendicular to the rotation axis, and the deflected beam enters the fixed third graded refractive index lens, is refocused and coupled to the output fiber, and the signal transmission is completed. The off-axis channel carries the uplink sensor signal return, with an operating wavelength of 1550nm. The optical path design is as follows: the optical signal is collimated into a parallel beam by a second graded-index lens. When the rotor rotates, the parallel beam becomes an off-axis beam that deviates from the central axis of the off-axis channel. The off-axis beam is precisely compensated into a parallel beam on the central axis of the off-axis channel by a coaxial symmetrical lens group (composed of a convex lens and a concave lens). The compensated parallel beam is received by a fourth graded-index lens and focused and coupled to the output optical fiber to achieve continuous signal transmission during rotation.

2. The dual-channel fiber optic rotary connector based on a graded-index lens according to claim 1, characterized in that, The central channel beam is concentrated near the central optical axis, while the off-axis channel beam is near the off-axis optical axis. The two beams do not overlap in physical space, fundamentally eliminating crosstalk between channels and achieving ultra-high isolation.

3. The dual-channel fiber optic rotary connector based on a graded-index lens according to claim 2, characterized in that, The front face of the graded refractive index lens is cut at an 8° angle, and an anti-reflection coating with a reflectivity of less than 0.5% (transmittance ≥ 99.5%) is coated on all optical surfaces to reduce the intensity of reflected light from the source and further improve the return loss.

4. The dual-channel fiber optic rotary connector based on a graded-index lens according to claim 1, characterized in that, The parameters (center refractive index, self-focusing constant, lens length) of the four graded refractive index lenses can be optimized to obtain a collimated beam with a large beam waist diameter, which can improve the system's tolerance to assembly errors and rotational wobbling.

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