Femtosecond laser and HC-ARF coupling method, system and variable-focus coupling device

Abstract

The present application relates to a kind of variable focusing coupling device for femtosecond laser and HC-ARF coupling, including first lens, second lens and focusing alignment component sequentially arranged along the direction of light path, the first lens and / or the second lens can be moved along the axis direction, so that the axial spacing between the first lens and the second lens is adjustable, in turn make the mode field diameter of the outgoing light beam of second lens adjustable;The focusing alignment component is used to focus the outgoing light beam of the second lens to the incident end surface of target optical fiber.In addition, it also relates to the femtosecond laser and HC-ARF coupling method and system based on the variable focusing coupling device.In the present application, the axial spacing between the first lens and the second lens is adjustable, can make outgoing light spot mode field diameter continuously adjustable within a certain range, a set of system can be adapted to different mode field diameter parameters of hollow-core antiresonant optical fiber, without replacing optical element, significantly reduce system switching cost and debugging time.

Description

Technical Field

[0001] This invention belongs to the field of laser fiber coupling technology, specifically relating to a variable magnification focusing coupling device for coupling femtosecond lasers and HC-ARF, as well as a method and system for coupling femtosecond lasers and HC-ARF based on the variable magnification focusing coupling device. Background Technology

[0002] Hollow-core anti-resonant fiber (HC-ARF) possesses advantages such as low nonlinearity, low dispersion, and high damage threshold, making it an ideal medium for flexible femtosecond laser transmission. When coupling a femtosecond laser into an HC-ARF, the following conditions must be met simultaneously: First, mode field matching: the mode field diameter (MFD) of the incident laser beam must precisely match the fundamental mode field of the HC-ARF (typically approximately 45 μm); otherwise, the coupling efficiency will significantly decrease. Second, lateral alignment: the core diameter of the HC-ARF is approximately 40–50 μm, requiring a lateral deviation of less than 2 μm between the beam and the core; otherwise, higher-order modes will be excited, introducing additional losses.

[0003] The current coupling schemes between femtosecond lasers and HC-ARF have the following problems: (1) Fixed magnification focusing lens groups cannot be adapted to optical fibers with different parameters. Existing solutions (such as CN224317815U, CN202010108804.9) use fixed focal length lenses or self-focusing lenses (GRIN), and the focusing magnification is not adjustable. When changing HC-ARF with different core diameters / numerical apertures, the lens group needs to be redesigned and replaced, which lacks versatility.

[0004] (2) Insufficient alignment accuracy and stability of mechanical translation stage. Conventional XY precision translation stage is driven by sliding guide rail. Mechanical friction causes degradation of repeatability (usually on the order of about 1μm). After long-term use, the guide rail wears and the positioning accuracy further decreases. Large-range movement and small-range fine adjustment are undertaken by the same mechanism, and coarse and fine adjustment are coupled and interfere with each other.

[0005] (3) The post-deflection scheme suffers from aberration coupling and accuracy degradation. The existing optical flat plate deflection scheme places the flat plate behind the focusing lens, and the beam has entered the strong convergence region (NA about 0.02~0.05) at the flat plate. This (post-deflection) architecture has fundamental defects: ① The formula for flat plate displacement in the converging beam no longer holds strictly, and the actual lateral displacement is affected by the convergence half angle, resulting in nonlinear deviation; ② The tilted flat plate introduces significant coma and astigmatism in the converging beam, which are directly projected onto the fiber end face, reducing coupling efficiency; ③ When adjusting the magnification, the lens displacement changes the convergence angle, causing the flat plate displacement sensitivity to change accordingly, and the magnification and alignment are mutually coupled.

[0006] (4) Lack of a system-level integrated solution for zoom and alignment. In existing patents, the zoom beam expansion system and the coupling alignment system are independent of each other, and there is no technical solution that deeply integrates zoom spot adjustment and precise lateral alignment functions at the optical path structure level. Especially for the femtosecond laser coupling scenario of HC-ARF, it is necessary to simultaneously meet the triple constraints of mode field matching, low nonlinearity constraint and submicron alignment, which existing technologies cannot solve systematically. Summary of the Invention

[0007] This invention relates to a variable magnification focusing coupling device for coupling femtosecond lasers with HC-ARF, as well as a method and system for coupling femtosecond lasers with HC-ARF based on the variable magnification focusing coupling device, which can at least solve some of the defects of the prior art.

[0008] This invention relates to a variable magnification focusing coupling device for coupling femtosecond lasers with HC-ARF, comprising a first lens, a second lens, and a focusing alignment assembly arranged sequentially along the optical path direction. The first lens and / or the second lens are axially movable, such that the axial distance between the first lens and the second lens is adjustable, thereby making the mode field diameter of the emitted beam from the second lens adjustable. The focusing alignment assembly is used to focus the emitted beam from the second lens onto the incident end face of the target optical fiber.

[0009] As one embodiment, the focusing alignment assembly includes a focusing lens and a beam alignment unit disposed between the second lens and the focusing lens. The beam alignment unit is used to adjust the position of the beam in the X and / or Y directions, and the focusing lens is used to focus the beam onto the incident end face of the target optical fiber.

[0010] As one embodiment, the beam alignment unit includes an X-axis alignment component, which includes an X-axis thick plate and an X-axis thin plate arranged sequentially along the optical path direction. Both the X-axis thick plate and the X-axis thin plate are optical flat plates, and the thickness of the X-axis thick plate is greater than the thickness of the X-axis thin plate. The X-axis thick plate is used to coarsely adjust the position of the beam in the X-axis direction, and the X-axis thin plate is used to finely adjust the position of the beam in the X-axis direction. And / or, the beam alignment unit includes a Y-axis alignment component, which includes a Y-axis thick plate and a Y-axis thin plate arranged sequentially along the optical path direction. Both the Y-axis thick plate and the Y-axis thin plate are optical flat plates, and the thickness of the Y-axis thick plate is greater than the thickness of the Y-axis thin plate. The Y-axis thick plate is used for coarse adjustment of the beam position in the Y-axis direction, and the Y-axis thin plate is used for fine adjustment of the beam position in the Y-axis direction.

[0011] As one embodiment, the thickness ratio of the X-direction thick plate to the X-direction thin plate is 3:1 to 10:1; the thickness ratio of the Y-direction thick plate to the Y-direction thin plate is 3:1 to 10:1.

[0012] As one embodiment, a sealed chamber is provided between the second lens and the focusing lens, the sealed chamber being a vacuum chamber or containing an inert gas; wherein, the first optical plate of the beam alignment unit also serves as the first axial window of the sealed chamber, and / or the focusing lens also serves as the last axial window of the sealed chamber.

[0013] As one implementation method, the axial distance between the first lens and the second lens is adjustable within the range of 10~80mm.

[0014] As one embodiment, the device further includes a coupling efficiency monitoring unit, which is used to collect the output optical power of the hollow anti-resonant optical fiber in real time, so as to control the axial spacing between the first lens and the second lens.

[0015] This invention also relates to a method for coupling a femtosecond laser with an HC-ARF, implemented based on the aforementioned variable magnification focusing coupling device, the method comprising: Adjust the axial distance between the first lens and the second lens to change the mode field diameter of the emitted beam from the second lens, so that it matches the mode field diameter of the target hollow anti-resonant fiber; The focusing alignment assembly focuses the emitted beam from the second lens onto the incident end face of the target hollow anti-resonant optical fiber.

[0016] Furthermore, the method also includes: Before focusing the emitted beam from the second lens onto the incident end face of the target hollow anti-resonant fiber, the position of the beam in the X and / or Y directions is adjusted by the focusing alignment assembly so that the focused beam can be aligned with the incident end face of the target hollow anti-resonant fiber.

[0017] This invention also relates to a femtosecond laser coupled with an HC-ARF system, comprising: Femtosecond laser source, used to output femtosecond pulsed laser; Hollow-core anti-resonant optical fiber with an incident end face having a set mode field diameter; And the aforementioned zoom-focusing coupling device is disposed between the femtosecond laser source and the hollow anti-resonant fiber, for coupling the femtosecond pulsed laser to the incident end face of the hollow anti-resonant fiber after zooming and focusing.

[0018] The present invention has at least the following beneficial effects: In this invention, the axial spacing between the first lens and the second lens is adjustable, which allows the diameter of the emitted light spot mode field to be continuously adjusted within a certain range. One system can be adapted to hollow anti-resonant optical fibers with different mode field diameter parameters without the need to replace optical components, significantly reducing system switching costs and debugging time.

[0019] The present invention further has the following beneficial effects: In this invention, the beam alignment unit is embedded in the pre-focusing region between the second lens and the focusing lens, achieving integrated zoom and alignment using the inherent space of the optical path. This eliminates the need for a separate beam alignment module, resulting in a more compact system and a shorter optical path. The optical plate of the beam alignment unit is placed within a weakly converging beam, ensuring the lateral displacement formula of the plate is approximately valid. The nonlinear correction term is significantly smaller than in the subsequent method, resulting in significantly better alignment accuracy and repeatability compared to the method placing the plate after the focusing lens. Simultaneously, the focusing lens exhibits an integral smoothing effect on residual aberrations introduced by the plate's tilt, leading to significantly better spot quality at the fiber end face compared to the subsequent method. This also minimizes the impact on the spatiotemporal characteristics of the femtosecond pulse and increases coupling efficiency. Furthermore, the plate displacement sensitivity is approximately independent of the distance between the first and second lenses, allowing for independent optimization of zoom adjustment and lateral alignment. This avoids the cross-coupling problem present in the subsequent method, resulting in faster closed-loop iterative convergence. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. 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.

[0021] Figure 1 This is a schematic diagram of the variable magnification focusing coupling device provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the zoom optical path provided in an embodiment of the present invention; Figure 3 A schematic diagram illustrating the tilt displacement of an optical flat plate in a weakly converging beam, as provided in an embodiment of the present invention. Figure 4 This is a schematic diagram of an optical flat plate and a focusing lens that also serve as a sealing chamber window, provided in an embodiment of the present invention. Detailed Implementation

[0022] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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.

[0023] Example 1

[0024] like Figure 1This invention provides a variable magnification focusing coupling device for coupling femtosecond lasers with HC-ARF, comprising a first lens 101, a second lens 102, and a focusing alignment assembly arranged sequentially along the optical path. The first lens 101 and / or the second lens 102 are axially movable, such that the axial distance between the first lens 101 and the second lens 102 is adjustable, thereby making the mode field diameter of the emitted beam from the second lens 102 adjustable. The focusing alignment assembly is used to focus the emitted beam from the second lens 102 onto the incident end face of the target optical fiber.

[0025] The femtosecond laser emitted from the femtosecond laser source passes sequentially through the first lens 101, the second lens 102, and the focusing alignment assembly before being focused onto the target hollow anti-resonant optical fiber.

[0026] Preferably, when adjusting the axial distance d between the first lens 101 and the second lens 102, the waist diameter of the emitted beam from the second lens 102 is used as the adjustment target so as to match the mode field diameter of the hollow anti-resonant fiber.

[0027] When the axial distance d between the first lens 101 and the second lens 102 is adjusted, the equivalent beam expansion ratio of the device changes accordingly, and the mode field diameter of the emitted beam from the second lens 102 is continuously adjustable. Specifically, the waist diameter of the emitted beam from the second lens 102... ,in Let the waist radius be the incident beam. The effective magnification of the lens pair composed of the first lens 101 and the second lens 102 is a function of the axial spacing d. When the axial spacing d increases, the emitted light spot becomes smaller (matching small mode field diameter optical fibers), and when the axial spacing d decreases, the emitted light spot becomes larger (matching large mode field diameter optical fibers). Figure 2 The optical path diagrams are shown for two cases with focused spot radii of 33μm and 20μm, which conform to the above-mentioned change of increasing axial spacing d → decreasing output spot size.

[0028] In one embodiment, the axial spacing d is adjustable in the range of 10~300mm, and more preferably in the range of 10~80mm; the scaling ratio is continuously adjustable from 1× to 5×, which can match different hollow anti-resonant optical fibers with a mode field diameter in the range of 10~45μm.

[0029] Alternatively, either the first lens 101 or the second lens 102 can be designed as an axially movable lens; or both the first lens 101 and the second lens 102 can be designed as axially movable lenses. This approach can improve the operational flexibility and adjustment accuracy of the zoom adjustment.

[0030] The first lens 101 and the second lens 102 mentioned above are preferably made of low-dispersion materials (fused silica or calcium fluoride) and achromatic design; the focusing lens 3 in the focusing alignment assembly is also preferably made of low-dispersion materials (fused silica or calcium fluoride) and achromatic design; the three work together to ensure that the group velocity dispersion introduced at the 1030nm working wavelength is less than 30fs², which does not affect the time domain characteristics of the femtosecond pulse.

[0031] Based on the above design, with the mode field diameter of the target hollow anti-resonant fiber fixed, the axial spacing between the first lens 101 and the second lens 102 can be adjusted to accommodate different input beam sizes. When the beam radius incident on the first lens 101 changes (e.g., by changing the laser source or adjusting the pre-amplifier ratio), only the spacing between the first lens 101 and the second lens 102 needs to be adjusted accordingly to maintain the final focused spot mode field diameter matching the target hollow anti-resonant fiber mode field diameter, without replacing any optical components. In a specific simulation application: with the MFD of the hollow anti-resonant fiber fixed at 45μm, when the input beam radius changes from 1.1mm to 2.0mm, the distance between the first lens 101 and the input port (i.e., the incident end face of the fiber or the focusing lens 3) exhibits an approximately linear mapping relationship (slope approximately 220~250mm / mm). That is, for every 1mm increase in the input beam radius, the first lens 101 moves back approximately 22~25cm, and the coupling efficiency remains greater than 88%~92%.

[0032] In one embodiment, such as Figure 1 The focusing alignment assembly includes a focusing lens 3 and a beam alignment unit 2 disposed between the second lens 102 and the focusing lens 3. The beam alignment unit 2 is used to adjust the position of the beam in the X and / or Y directions, and the focusing lens 3 is used to focus the beam onto the incident end face of the target optical fiber.

[0033] Further optimize the beam alignment unit 2 described above, such as... Figure 1 The beam alignment unit 2 includes an X-axis alignment assembly, which includes an X-axis thick plate 211 and an X-axis thin plate 212 arranged sequentially along the optical path direction. Both the X-axis thick plate 211 and the X-axis thin plate 212 are optical flat plates, and the thickness of the X-axis thick plate 211 is greater than the thickness of the X-axis thin plate 212. The X-axis thick plate 211 is used to coarsely adjust the position of the beam in the X-axis direction, and the X-axis thin plate 212 is used to finely adjust the position of the beam in the X-axis direction. And / or, the beam alignment unit 2 includes a Y-axis alignment component, which includes a Y-axis thick plate 221 and a Y-axis thin plate 222 arranged sequentially along the optical path direction. Both the Y-axis thick plate 221 and the Y-axis thin plate 222 are optical flat plates, and the thickness of the Y-axis thick plate 221 is greater than the thickness of the Y-axis thin plate 222. The Y-axis thick plate 221 is used for coarse adjustment of the position of the beam in the Y-axis direction, and the Y-axis thin plate 222 is used for fine adjustment of the position of the beam in the Y-axis direction.

[0034] Preferably, both the X-axis alignment component and the Y-axis alignment component are set simultaneously to ensure beam alignment accuracy.

[0035] The aforementioned X-axis alignment component is used to apply X-axis displacement to the beam, and the aforementioned Y-axis alignment component is used to apply Y-axis displacement to the beam. Since the beam alignment unit 2 is located between the second lens 102 and the focusing lens 3, that is, in the weakly converging optical path section (pre-focusing zone), the numerical aperture NA of the beam in this section is less than 0.01, and the lateral displacement formula of the flat plate approximately holds under weak converging conditions; for example... Figure 3 When the optical plate is tilted, the transmitted light beam does not change its propagation direction, but it undergoes a lateral displacement parallel to itself. The precise displacement formula is:

[0036] Where d is the thickness of the plate, n is the refractive index, and θ is the tilt angle.

[0037] In the above scheme, the thick plate 211 and the thin plate 212 in the X direction are arranged coaxially, and the rotation axes of the thick plate 211 and the thin plate 212 in the X direction are parallel to the Y direction; the thick plate 221 and the thin plate 222 in the Y direction are arranged coaxially, and the rotation axes of the thick plate 221 and the thin plate 222 in the Y direction are parallel to the X direction.

[0038] Preferably, the rotation of the X-direction thick plate 211 and the rotation of the X-direction thin plate 212 are independent; the rotation of the Y-direction thick plate 221 and the rotation of the Y-direction thin plate 222 are also independent. The driving of each optical plate includes, but is not limited to, using piezoelectric ceramic actuators, with a rotation resolution better than 0.001°.

[0039] Alternatively, the rotation axis of each optical plate may employ a flexible hinge structure to eliminate mechanical backlash.

[0040] In one embodiment, the thickness ratio of the X-direction thick plate 211 to the X-direction thin plate 212 is 3:1 to 10:1; the thickness ratio of the Y-direction thick plate 221 to the Y-direction thin plate 222 is 3:1 to 10:1. Optionally, the thickness of the X-direction thick plate 211 and the Y-direction thick plate 221 is 3 to 8 mm (typically 5 mm), and the thickness of the X-direction thin plate 212 and the Y-direction thin plate 222 is 0.5 to 2 mm (typically 1 mm).

[0041] The aforementioned thick plate 211 in the X direction and thick plate 221 in the Y direction are used for large-scale rapid optimization, preferably with the sensitivity controlled within the range of 0.162~0.432μm / 0.01°, and a typical sensitivity is 0.27μm / 0.01°. The aforementioned thin plate 212 in the X direction and thin plate 222 in the Y direction are used for submicron-level precision locking, preferably with the sensitivity controlled within the range of 0.027~0.108μm / 0.01°, and a typical sensitivity is 0.054μm / 0.01°. Specifically, in the coarse scan stage, the thick plate is tilted for scanning, and the thin plate is locked (θ=0) to search for the optimal coupling position over a large range; in the fine-tuning stage, the thick plate is locked (θ fixed), and the thin plate is tilted for fine-tuning to optimize the coupling efficiency at the submicron level; in the maintenance stage, both the thick and thin plates are locked for long-term stable operation.

[0042] Preferably, each optical plate is made of a low-dispersion optical material, such as fused silica or calcium fluoride.

[0043] In this embodiment, the beam alignment unit 2 is placed in the pre-focusing area between the second lens 102 and the focusing lens 3. The beam in this section is in a weakly converging state. This is not an arbitrary position selection, but a core decision of the entire technical solution, which solves the fundamental defects of the post-deflection scheme in the background art. (1) The displacement is approximately linear

[0044] In a strongly converging beam (rear-mounted scheme), the actual displacement is corrected by the convergence half-angle α:

[0045] The correction term varies with the axial distance between the first lens 101 and the second lens 102, resulting in the coupling of displacement sensitivity and magnification parameter, and the correction amount is difficult to calibrate accurately.

[0046] In weakly converging beams, the formula for the lateral displacement of the flat plate is approximately valid, and the displacement error is much smaller than that of the post-processor scheme, reducing the correction term by more than an order of magnitude.

[0047] (2) Minimal aberrations

[0048] In a strongly converging beam (rear-mounted scheme), the coma wavefront introduced by the tilted plate is proportional to NA²×d×θ, and the coma magnitude can reach λ / 10~λ / 4, directly affecting the quality of the focused spot.

[0049] In weakly converging beams, NA < 0.01, the coma wavefront is negligible (less than λ / 100), and only a very small higher-order term exists.

[0050] (3) Aberration self-healing effect

[0051] Even if the plate introduces a small amount of aberration, the focusing process of the focusing lens 3 has an integral smoothing effect on the incident wavefront. When the tilted beam is refocused by the focusing lens 3, the local wavefront distortion introduced by the plate is averaged at the focal point, and the quality of the end face spot is significantly better than that of the rear-mounted scheme.

[0052] (4) Approximate decoupling of zoom and alignment

[0053] Adjusting the axial spacing between the first lens 101 and the second lens 102 changes the size of the beam cross-section in the prefocusing area, but does not change the sensitivity of the plate displacement. Therefore, the magnification adjustment and the lateral alignment are almost completely decoupled. During the closed-loop optimization process, the two can be adjusted independently, and there is no cross-coupling problem as in the post-scheme.

[0054] In one embodiment, such as Figure 4 A sealed chamber is provided between the second lens 102 and the focusing lens 3. The sealed chamber is a vacuum chamber or contains an inert gas. The first optical plate of the beam alignment unit 2 also serves as the first axial window of the sealed chamber, and / or the focusing lens 3 also serves as the last axial window of the sealed chamber.

[0055] In the beam alignment unit 2, which includes an X-axis thick plate 211, an X-axis thin plate 212, a Y-axis thick plate 221, and a Y-axis thin plate 222, the X-axis thick plate 211 also serves as the first axial window of the sealed chamber. The other three flat plates (X-axis thin plate 212, Y-axis thick plate 221, and Y-axis thin plate 222) are located inside the sealed chamber and do not serve as the sealing interface with the atmospheric environment; they are only used as internal dimming elements.

[0056] In the above scheme, since the optical plate needs to rotate and tilt, it is preferable to design a dynamic sealing connection between the first optical plate and the sealed chamber to ensure that the first optical plate remains sealed even when rotating. Optionally, such as Figure 4 The first optical plate is fixed in a plate support by an O-ring, and the periphery of the plate support is connected to the adjacent sealed chamber wall by an annular elastic diaphragm. In another alternative, an annular metal bellows can also be used to connect the plate support and the sealed chamber wall.

[0057] Based on the above scheme, by setting a sealed cavity between the second lens 102 and the focusing lens 3, the optical path between the second lens 102 and the focusing lens 3 is sealed in a vacuum or inert gas environment. This eliminates the risk of self-focusing / ionization breakdown of high-power femtosecond pulses in air and suppresses the absorption of water molecules in the air at 1030nm in the coupling section. Therefore, it can effectively improve the reliability and safety of the coupling between the femtosecond laser and the hollow-core anti-resonant fiber. At the same time, using an optical flat plate and the focusing lens 3 as the axial windows at the beginning and end of the sealed cavity results in high integration, eliminates the need for additional window elements, reduces the number of optical interfaces, reduces reflection loss and aberration introduction, simplifies the mechanical structure, and improves the system's structural compactness.

[0058] After being focused by focusing lens 3, the diameter of the light spot at the focal point is... Determined by the Gaussian beam transmission matrix (ABCD matrix): After the complex beam parameters of the outgoing beam from the second lens 102 are propagated in free space (the free space between the second lens 102 and the focusing lens 3) and transformed by the thin lens of the focusing lens 3, the q parameter at the focal point is obtained, and thus:

[0059] Where, q f λ represents the parameters of the complex beam at the focal point (beam waist); λ is the center wavelength of the laser.

[0060] It should be noted that the emitted beam from the second lens 102 is not strictly collimated (but weakly converged). The actual focal point is not strictly located at the back focal plane f3 of the focusing lens 3, but is offset by the combined influence of the distance between the second lens 102 and the focusing lens 3, and the radius of curvature of the emitted beam from the second lens 102. The focal point offset can be rigorously calculated using the Gaussian beam ABCD matrix (ray transmission matrix). Under the collimation approximation (when the emitted beam from the second lens 102 is nearly collimated), the diameter of the light spot at the focal point... Approximately equal to:

[0061] Where f3 is the focal length of focusing lens 3; λ is the center wavelength of the laser; ω out(d) The diameter of the light spot on the focusing lens 3 is the diameter of the light beam emitted from the second lens.

[0062] The error of this approximation is negligible under small NA conditions. Based on the above scheme, within the axial spacing adjustment range of 10~80mm between the first lens 101 and the second lens 102, the focusing position offset of the focusing lens 3 is less than 10μm (less than 1 / 2 of the core radius of the hollow anti-resonant fiber).

[0063] In one embodiment, the device further includes a coupling efficiency monitoring unit, which is used to collect the output optical power of the hollow anti-resonant optical fiber in real time to control the axial spacing between the first lens 101 and the second lens 102. Furthermore, when the aforementioned beam alignment unit 2 is provided, the coupling monitoring unit is also used to control the tilt angle of each optical plate. The feedback control method involved is a conventional design and will not be detailed here.

[0064] Example 2

[0065] This invention provides a method for coupling a femtosecond laser with an HC-ARF, implemented based on the variable magnification focusing coupling device provided in Embodiment 1 above. The method includes: Adjust the axial distance between the first lens 101 and the second lens 102 to change the mode field diameter of the emitted beam of the second lens 102 so that it matches the mode field diameter of the target hollow anti-resonant fiber. The focusing alignment assembly focuses the emitted beam from the second lens 102 onto the incident end face of the target hollow anti-resonant optical fiber.

[0066] Furthermore, the method also includes: Before focusing the emitted beam from the second lens 102 onto the incident end face of the target hollow anti-resonant fiber, the position of the beam in the X and / or Y directions is adjusted by the focusing alignment assembly so that the focused beam can be aligned with the incident end face of the target hollow anti-resonant fiber.

[0067] The relevant adjustment methods have been described in the above embodiment one, and will not be repeated here.

[0068] Example 3

[0069] This invention provides a femtosecond laser coupled with an HC-ARF system, comprising: Femtosecond laser source, used to output femtosecond pulsed laser; Hollow-core anti-resonant optical fiber with an incident end face having a set mode field diameter; The variable magnification focusing coupling device provided in Embodiment 1 is disposed between the femtosecond laser source and the hollow anti-resonant fiber, and is used to couple the femtosecond pulse laser to the incident end face of the hollow anti-resonant fiber after magnification and focusing.

[0070] The following are some specific examples: Example 1: Coupling of 1030nm femtosecond laser with HC-ARF Laser parameters: wavelength 1030nm, pulse width 300fs, single pulse energy 2μJ, average power 20W.

[0071] Fiber optic parameters: HC-ARF core diameter 40μm, mode field diameter 45μm, NA < 0.02, fiber length 5m.

[0072] Coupler parameters: First lens 101: focal length -100mm, fused silica material; Second lens 102: focal length +100mm, fused silica material; Focusing Lens 3: Focal length 75mm, fused silica material; The distance between the first lens 101 and the second lens 102 can be adjusted from 10 to 80 mm, with an adjustment accuracy better than 0.01 mm. X-axis thick plate 211: 5mm thick, fused silica material (n=1.449@1030nm). X-axis thin plate 212: 1mm thick, fused silica material; Y-direction thick plate 221: 5mm thick, fused silica material; Y-axis thin plate 222: 1mm thick, fused silica material; Operating procedures: Step 1, Initial state: All optical plates are tilted to zero, and the first lens 101 and the second lens 102 are at the nominal distance.

[0073] The second step is coarse adjustment in the X direction: the thick plate 211 in the X direction scans around the Y axis within a range of ±5° to search for the position in the X direction that maximizes the coupling efficiency and locks the tilt angle of the thick plate 211 in the X direction.

[0074] The third step is fine-tuning in the X direction: After the tilt angle of the thick plate 211 in the X direction is locked, the thin plate 212 in the X direction is finely adjusted within ±2° around the Y axis to perform submicron-level fine-tuning in the X direction until the coupling efficiency is optimal.

[0075] Step 4, coarse adjustment in the Y direction: The thick plate 221 in the Y direction scans around the X-axis within a range of ±5° to search for the position in the Y direction that maximizes the coupling efficiency and locks the tilt angle of the thick plate 221 in the Y direction.

[0076] Fifth step, fine adjustment in the Y direction: After the tilt angle of the thick plate 221 in the Y direction is locked, the thin plate 222 in the Y direction is finely adjusted within ±2° around the X axis to perform submicron-level fine adjustment in the Y direction until the coupling efficiency is optimal.

[0077] (Coarse and fine adjustments in the X and Y directions can be performed sequentially or simultaneously, with the two being orthogonally decoupled and not interfering with each other.)

[0078] Step 6, Magnification Optimization: Fine-tune the axial relative spacing between the first lens 101 and the second lens 102 to precisely match the mode field diameter of the focused spot with the mode field diameter (45μm) of the HC-ARF, thereby further improving the coupling efficiency.

[0079] Step 7, Closed-loop monitoring: The optical power at the output end of the HC-ARF is collected in real time through the coupling efficiency monitoring unit, and the above parameters are adjusted in feedback and iteratively optimized until the coupling efficiency converges.

[0080] Based on the above method, the coupling efficiency is greater than 92%, the alignment accuracy is less than 0.1 μm, and the zoom-alignment coupling degree is less than 0.5%.

[0081] Example 2: Adaptation of HC-ARF with different parameters

[0082] Based on the above Example 1, the target fiber is replaced with HC-ARF with a core diameter of 45μm and a mode field diameter of 35μm (other parameters remain unchanged).

[0083] Operation: Simply increase the axial relative distance between the first lens 101 and the second lens 102 to reduce the mode field diameter of the emitted beam from the second lens 102 from 45μm to 35μm to complete the adaptation.

[0084] No optical components need to be replaced throughout the process. Due to the decoupling characteristics of the pre-focusing area, zoom adjustment does not change the locked tilt angle of each optical plate, that is, it does not affect the completed two-dimensional lateral alignment position. Only a fine adjustment of the axial distance between the first lens 101 and the second lens 102 after zoom adjustment is needed to restore the coupling efficiency to its optimal state.

[0085] In contrast, in the post-deflection scheme, the convergence angle changes during zoom adjustment, causing a corresponding change in the plate displacement sensitivity. This results in a shift in the locked alignment position (3-5 μm), requiring a complete iterative alignment of all four plates. Therefore, in this scheme, because zoom and alignment are relatively independent, the adaptation efficiency is significantly improved.

[0086] Example 3: In this example, under the same lens parameters and beam conditions, the coupling performance of the pre-focusing deflection scheme and the rear deflection scheme (the flat plate is placed after the focusing lens 3) of the present invention were tested respectively.

[0087] Test conditions: The same laser parameters, fiber parameters, and lens parameters as in Example 1 were used.

[0088] Test results:

[0089] Analysis shows that: Regarding coupling efficiency, the coupling efficiency of the rear-mounted scheme decreases significantly (from 88% to 75%) as the tilt angle of the plate increases. This is mainly because the tilted plate introduces coma and astigmatism on the order of λ / 10 to λ / 4 into the strongly converging beam, which directly projects onto the fiber end face, leading to a deterioration in the beam quality. In contrast, in this invention, the plate is placed in the pre-focusing region (NA < 0.01), where the coma wavefront is less than λ / 100 and can be ignored. In addition, the aberration self-healing effect of the focusing lens 3 ensures that the coupling efficiency remains above 90% at various tilt angles.

[0090] Regarding zoom-alignment coupling, in the post-projection scheme, zoom adjustment changes the convergence angle, leading to a change in the plate displacement sensitivity. This results in a 3-5 μm shift in the locked alignment position, requiring re-iteration. In this invention, zoom only changes the beam cross-section in the pre-focusing area, without altering the plate displacement sensitivity (the displacement depends only on the thickness d, tilt angle θ, and refractive index n), resulting in an alignment shift of less than 0.3 μm.

[0091] In terms of iteration efficiency, the present invention can converge in only 2 to 3 iterations due to the near decoupling of scaling and alignment; the post-implementation scheme requires 4 to 6 iterations due to the two-axis coupling.

[0092] In terms of long-term stability, the PZT+ flexible hinge structure of the rear-mounted solution has no mechanical friction and drifts by 0.8μm in 24 hours; the structure of this invention is the same, but the plate is located in the weak convergence region and is less affected by temperature drift, drifting by 0.4μm in 24 hours.

[0093] Example 4: Adaptive Matching of Multiple Input Spots

[0094] This embodiment is used to illustrate the adaptability of the present invention to different input beam radii. That is, after changing the laser source or adjusting the pre-stage beam expander ratio, there is no need to change the optical components. The optimal coupling can be restored by simply adjusting the axial distance between the first lens 101 and the second lens 102.

[0095] Test conditions: Target fiber: HC-ARF with a core diameter of 40μm and a mode field diameter of 45μm (fixed). Lens configuration: First lens 101 focal length -100mm, second lens 102 focal length +100mm, focusing lens 3 focal length 75mm; Input beam radius: varies from 1.1mm to 2.0mm; Operation method: For each input beam radius value, the axial positions of the first lens 101 and the second lens 102 are readjusted to ensure that the beam pattern diameter after focusing by the focusing lens 3 matches the target fiber pattern diameter (45μm). The alignment positions of the four optical plates remain locked during the zoom adjustment process, without the need for realignment. Simulation data are shown in the table below:

[0096] The above data indicates that when the input beam radius increases from 1.1 mm to 2.0 mm, in order to maintain high coupling efficiency on the HC-ARF, it is necessary to coordinately adjust the axial positions of the first lens 101 and the second lens 102 (both need to be far away from the focusing lens 3), and simultaneously change the axial distance between them. The specific parameter changes are as follows: The distance between the first lens 101 and the focusing lens 3 needs to be increased from 77.496mm to 233.92mm; The distance between the second lens 102 and the focusing lens 3 needs to be increased from 45.547mm to 216.872mm; The axial distance between the first lens 101 and the second lens 102 needs to be gradually reduced from 31.949 mm to 17.048 mm.

[0097] The above position parameters have an approximately piecewise linear mapping relationship with the input beam radius: In the range of 1.1 to 1.5 mm, the displacement slope of the first lens 101 is approximately 107 mm / mm; In the 1.5~2.0mm range, the slope increases to approximately 226mm / mm.

[0098] This characteristic means that after changing the laser source or adjusting the pre-amplifier beam ratio, no lenses need to be replaced. Optimal coupling can be quickly restored simply by readjusting the axial distance between the first lens 101 and the second lens 102 and their relative positions to the focusing lens 3 according to the aforementioned mapping rules. Compared to a fixed magnification scheme (which requires redesigning or replacing the lens group), the adaptive adjustment capability of this invention significantly reduces system maintenance and debugging costs.

[0099] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A variable magnification focusing coupling device for coupling femtosecond lasers with HC-ARF, characterized in that, The device includes a first lens, a second lens, and a focusing alignment assembly arranged sequentially along the optical path. The first lens and / or the second lens can be moved axially so that the axial distance between the first lens and the second lens is adjustable, thereby making the mode field diameter of the emitted beam from the second lens adjustable. The focusing alignment assembly is used to focus the emitted beam from the second lens onto the incident end face of the target optical fiber.

2. The variable magnification focusing coupling device as described in claim 1, characterized in that: The focusing alignment assembly includes a focusing lens and a beam alignment unit disposed between the second lens and the focusing lens. The beam alignment unit is used to adjust the position of the beam in the X and / or Y directions, and the focusing lens is used to focus the beam onto the incident end face of the target optical fiber.

3. The variable magnification focusing coupling device as described in claim 2, characterized in that, The beam alignment unit includes an X-axis alignment component, which includes an X-axis thick plate and an X-axis thin plate arranged sequentially along the optical path direction. Both the X-axis thick plate and the X-axis thin plate are optical flat plates, and the thickness of the X-axis thick plate is greater than the thickness of the X-axis thin plate. The X-axis thick plate is used to coarsely adjust the position of the beam in the X-axis direction, and the X-axis thin plate is used to finely adjust the position of the beam in the X-axis direction. And / or, the beam alignment unit includes a Y-axis alignment component, which includes a Y-axis thick plate and a Y-axis thin plate arranged sequentially along the optical path direction. Both the Y-axis thick plate and the Y-axis thin plate are optical flat plates, and the thickness of the Y-axis thick plate is greater than the thickness of the Y-axis thin plate. The Y-axis thick plate is used for coarse adjustment of the beam position in the Y-axis direction, and the Y-axis thin plate is used for fine adjustment of the beam position in the Y-axis direction.

4. The variable magnification focusing coupling device as described in claim 3, characterized in that: The thickness ratio of the X-direction thick plate to the X-direction thin plate is 3:1 to 10:1; the thickness ratio of the Y-direction thick plate to the Y-direction thin plate is 3:1 to 10:

1.

5. The variable magnification focusing coupling device as described in claim 3, characterized in that: A sealed chamber is provided between the second lens and the focusing lens. The sealed chamber is a vacuum chamber or contains an inert gas. The first optical plate of the beam alignment unit also serves as the first axial window of the sealed chamber, and / or the focusing lens also serves as the last axial window of the sealed chamber.

6. The variable magnification focusing coupling device as described in claim 1, characterized in that: The axial distance between the first lens and the second lens can be adjusted within the range of 10 to 80 mm.

7. The variable magnification focusing coupling device as described in claim 1, characterized in that: The device further includes a coupling efficiency monitoring unit, which is used to collect the output optical power of the hollow anti-resonant optical fiber in real time, so as to control the axial spacing between the first lens and the second lens.

8. A method for coupling a femtosecond laser with an HC-ARF, characterized in that, The method, implemented based on the variable magnification focusing coupling device according to any one of claims 1 to 7, comprises: Adjust the axial distance between the first lens and the second lens to change the mode field diameter of the emitted beam from the second lens, so that it matches the mode field diameter of the target hollow anti-resonant fiber; The focusing alignment assembly focuses the emitted beam from the second lens onto the incident end face of the target hollow anti-resonant optical fiber.

9. The femtosecond laser and HC-ARF coupling method as described in claim 8, characterized in that, The method further includes: Before focusing the emitted beam from the second lens onto the incident end face of the target hollow anti-resonant fiber, the position of the beam in the X and / or Y directions is adjusted by the focusing alignment assembly so that the focused beam can be aligned with the incident end face of the target hollow anti-resonant fiber.

10. A femtosecond laser coupled with an HC-ARF system, characterized in that, include: Femtosecond laser source, used to output femtosecond pulsed laser; Hollow-core anti-resonant optical fiber with an incident end face having a set mode field diameter; And the variable magnification focusing coupling device as described in any one of claims 1 to 7, disposed between the femtosecond laser source and the hollow anti-resonant fiber, for coupling the femtosecond pulse laser to the incident end face of the hollow anti-resonant fiber after magnification and focusing.

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