Hollow-core anti-resonance optical fiber and preparation method thereof

Abstract

The invention discloses a hollow-core anti-resonance optical fiber and a preparation method thereof. The hollow-core anti-resonance optical fiber comprises an outer sleeve; the hollow capillary tube is arranged in the outer sleeve in a suspended manner; only one supporting tube is arranged, and the supporting tube is used for connecting the hollow capillary tube and the outer sleeve. The preparation method comprises the following steps: placing the hollow capillary tubes and the supporting tubes with different sizes in a triangular stacking device, and sintering and fixing to form a first stack; inserting a deposit I formed by the hollow capillary tube and the supporting tube into the outer sleeve I, and sintering and fixing again to form a deposit II; drawing the second deposit into a prefabricated middleware with a preset diameter in an optical fiber drawing tower; and sleeving the prefabricated middleware with an outer sleeve II to form a prefabricated rod, and drawing the prefabricated rod at a preset temperature to form the suspended hollow-core anti-resonance optical fiber. The suspension-core hollow-core anti-resonance optical fiber prepared by the invention has excellent high-order mode suppression performance.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fibers, and in particular to a hollow-core anti-resonance optical fiber and a preparation method thereof. Background Art

[0002] The 3-5μm mid-infrared band is an extremely important atmospheric transmission window. Laser transmission in this band is widely used in spectral analysis, biomedicine, infrared countermeasures and other fields. There are many ways to generate mid-infrared lasers. According to the different gain media, they can be divided into gas lasers, quantum cascade lasers and fiber lasers. Using optical fiber as the laser transmission medium can isolate most environmental factors from interference while ensuring flexible transmission, ensuring the quality, stability and confidentiality of laser transmission. Therefore, it is of great significance to study flexible optical fibers with excellent laser transmission characteristics in this band, which will greatly promote the development and application of these fields.

[0003] The current types of optical fibers include solid-core fibers and hollow-core fibers. However, the defects of current solid-core fibers in the mid-infrared band make it difficult to meet the requirements of low fiber loss and high laser damage threshold. Hollow-core fibers are usually divided into hollow-core photonic bandgap fibers and hollow-core antiresonant fibers. For hollow-core antiresonant fibers, their larger air core size will lead to the existence of a series of higher-order modes (HOMs). The existence of core higher-order modes not only easily leads to a decrease in output beam quality and an increase in power fluctuations, but also reduces the practicality of hollow-core antiresonant fibers in many applications. In addition, these core higher-order modes usually have relatively low losses and are easily excited by bending or external stress, which makes it difficult for hollow-core antiresonant fibers to obtain relatively stable pure single-mode light transmission.

[0004] Current research on suppressing higher-order modes either has a low degree of suppression or is mostly still in the stage of numerical simulation. It is difficult to convert it into actual fabricated structures for verification in the short term, so as to carry out large-scale mass production. Summary of the invention

[0005] The purpose of the present invention is to solve the shortcomings of the prior art and to propose a hollow core anti-resonance optical fiber and a preparation method thereof.

[0006] In order to achieve the above-mentioned object, the present invention adopts the following technical solution: a hollow core antiresonant optical fiber, comprising an outer sleeve;

[0007] A hollow capillary tube is suspended inside the outer sleeve;

[0008] Only one supporting tube is provided, and the supporting tube is used to connect the hollow capillary tube and the outer sleeve, so that the hollow capillary tube can be concentrically arranged inside the outer sleeve.

[0009] As a further description of the above technical solution, the hollow capillary is concentrically arranged with the outer sleeve.

[0010] As a further description of the above technical solution, the hollow capillary is eccentrically arranged inside the outer sleeve. The inner diameter of the hollow capillary is D, and the distance between the hollow capillary and the outer sleeve is d. The range of the structural ratio d / D is 0.316 < d / D < 0.4 to obtain better high-order mode suppression performance.

[0011] As a further description of the above technical solution, the support tube is also a hollow capillary. During use, the position of the hollow capillary is modified by adjusting the pressure entering the support tube.

[0012] As a further description of the above technical solution, the outer sleeve is a quartz tube. After the hollow capillary and the support tube are sintered and fixed to form a pre-stack, the stack is placed in the quartz tube and sintered for reinforcement.

[0013] A method for preparing a hollow anti-resonant fiber according to any one of the above, comprising:

[0014] S1, placing hollow capillaries and support tubes of different sizes in a triangular stacking device, sintering and fixing them to form Stack 1;

[0015] S2, inserting Stack 1 formed by the hollow capillary and the support tube into Outer Sleeve 1, and sintering and fixing it again to form Stack 2;

[0016] S3, drawing Stack 2 in an optical fiber drawing tower into a preform intermediate with a preset diameter;

[0017] S4, sleeving Outer Sleeve 2 on the preform intermediate to form a preform rod, and drawing the preform rod at a preset temperature into a suspended hollow anti-resonant fiber.

[0018] As a further description of the above technical solution, in step S3, the diameter of the preform intermediate is 1 / 10 of the original stacking size.

[0019] As a further description of the above technical solution, in step S4, the preset temperature is 2050 °C.

[0020] As a further description of the above technical solution, in step S3, the diameter of the preform intermediate is 1 - 2 mm.

[0021] As a further description of the above technical solution, in step S4, the final fiber structure is formed by controlling the air pressure in different regions and adjusting the feeding / drawing speed.

[0022] The present invention has the following beneficial effects:

[0023] 1. The suspended-core hollow-core antiresonant optical fiber prepared by the present invention has excellent high-order mode suppression performance. At the same time, the present application also proves that the single-mode performance of the suspended-core hollow-core antiresonant optical fiber does not depend on whether it is perfectly concentric or not, nor does it depend on the absolute physical size of the optical fiber. As long as the ratio is kept within a certain error range under a certain eccentric structure, it is still possible to prepare a suspended-core hollow-core antiresonant optical fiber that can achieve reliable single-mode transmission, which provides guidance for the subsequent preparation of suspended-core hollow-core antiresonant optical fibers. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A schematic diagram of the suspended core hollow core anti-resonant optical fiber proposed by the present invention;

[0025] Figure 2 (a) is the mode effective refractive index, Figure 2 (b) Figure 2 The effective refractive index of the localized mode in (a), Figure 2 (c) Partial mode loss and FOM 11 ;

[0026] Figure 3 (a) LPs subject to strong restrictions 01 Mode diagram; Figure 3 (b) and Figure 3 (c) LP resonantly coupled to the cladding mode 11 model;

[0027] Figure 4 is the calculated effective refractive index of the core and cladding modes in the wavelength range of 0.5-2μm;

[0028] Figure 5 (a) is the transmission spectrum, Figure 5 (b) and Figure 5 (c) is the result of attenuation measurement;

[0029] Figure 6 The near-field images of the optical fiber output end at different wavelengths;

[0030] Figure 7 The near-field image and energy intensity at the output end of the optical fiber at some wavelengths;

[0031] Figure 8 (a) is the M2 (beam quality) measurement at 550nm, Figure 8 (b) M2 (beam quality) measurement at 1100nm, Figure 8 (c) M2 (beam quality) measurement at 2020 nm;

[0032] Fig. 9 (a) is a simplified structural diagram when eccentric; Fig. 9 (b) To include LP 01 Schematic diagram of a pattern within a pattern, which is strongly constrained; Fig. 9 (c) and Fig. 9 (d) LP 11 mode, resonantly coupled with the cladding mode; Fig. 9 (e) is the variation of the effective refractive index of some modes with eccentricity; Fig. 9 (f) is the relationship between the effective refractive index and the ratio d / D when the degree of eccentricity of the core is equivalent to that of the manufactured optical fiber; Fig. 9 (g) is FOM 11 ,The gray shaded area shows the region where FOM11>30;

[0033] Fig.10 The near-field profiles at 1550 nm for different lateral incoupling positions. DETAILED DESCRIPTION

[0034] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] Embodiments of the present application provide a hollow anti-resonant fiber and a preparation method thereof, specifically a suspended-core hollow anti-resonant fiber. Currently, for an ideal suspended-core hollow anti-resonant fiber, its structure is that a capillary tube is coaxially suspended in a hollow glass outer tube without support. The core part can be regarded as a Fabry-Perot resonator. However, in the actual preparation process, due to the lack of support for the core part, this ideal state cannot be achieved. Currently, the method is to place an appropriate number of expandable support tubes at the air cladding position to connect the core wall and the outer cladding, which provides the possibility for the drawing and forming of the suspended-core hollow anti-resonant fiber. According to preparation experience, generally 3 - 4 or even more support tubes are used to ensure the concentricity between the core capillary tube and the outer tube. However, this will result in a non-circular core, and the node positions between the tubes will introduce some complex surface modes, affecting the characteristics of the core and cladding modes and bringing additional losses. To solve the above problems, the present application only sets one support tube to reduce and avoid the occurrence of a non-circular core, but this will increase the difficulty of concentricity control. After a series of preparations, the suspended-core hollow anti-resonant fiber prepared in the present application has a certain deviation in concentricity, but still has excellent high-order mode suppression performance. At the same time, it is also proved that the single-mode performance of this fiber does not depend on whether it is perfectly concentric, nor on the absolute physical size of the fiber. As long as the ratio is maintained within a certain error range under a certain eccentric structure, it is still possible to prepare a suspended-core hollow anti-resonant fiber that can achieve reliable single-mode transmission. This overcomes the technical prejudice that has always been considered that the closer to the ideal concentric situation, the better the high-order mode suppression performance for those skilled in the art.

[0036] For details, please refer to Figure 1 An embodiment provided by the present invention: A hollow anti-resonant fiber includes an outer tube 1, which is a quartz glass tube. Inside the outer tube 1, there is a hollow capillary tube 2, which is suspended inside the outer tube 1. In an ideal situation, the hollow capillary tube 2 and the outer tube 1 are concentrically arranged. Between the outer wall of the hollow capillary tube 2 and the inner wall of the outer tube 1, there is only one support tube 3, which can concentrically fix the hollow capillary tube 2 inside the outer tube 1 in an ideal state. At the same time, the present application provides another implementation manner, that is, the hollow capillary tube 2 is eccentrically arranged inside the outer tube 1, and only one support tube 3 is also provided to suspend the hollow capillary tube 2 inside the outer tube 1. It should be noted that after the hollow capillary tube 2 is fixed to the outer tube 1 through the support tube 3 and formed, the inner diameter of the hollow capillary tube 2 is D, and the distance between the hollow capillary tube 2 and the outer tube 1 is d. The range of the structural ratio d / D is 0.316 < d / D < 0.4. In this embodiment, although the hollow capillary tube 2 is not perfectly concentric, an effective high-order mode suppression effect can still be obtained.

[0037] Furthermore, the support tube 3 is also a hollow capillary tube 2 . After the support tube 3 is fixed at a corresponding position, the position of the hollow capillary tube 2 can be adjusted by adjusting the pressure entering the support tube 3 and then changing the diameter of the support tube 3 .

[0038] The present invention also provides a method for preparing a hollow-core antiresonant optical fiber, which specifically comprises:

[0039] Capillaries of different sizes, namely hollow capillaries 2 and support tubes 3, are placed in a self-made triangular stacking device and fixed by sintering to form a pre-stack. It should be noted that the support tube 3 is not a solid rod, but a relatively small capillary is used as the main support tube 3, so that the position of the core tube can be modified and adjusted by controlling the pressure in the tube during the optical fiber drawing process.

[0040] The entire stack, i.e., stack one, is inserted into a quartz outer sleeve 1, i.e., outer sleeve one, and then is also reinforced by sintering to maintain its structure, forming stack two.

[0041] It is drawn into a prefabricated intermediate piece with a diameter of 1 to 2 mm on a customized fiber drawing tower. After the first drawing, the structure of the suspended core hollow core antiresonant fiber is proportionally reduced to only 1 / 10 of the original stack size.

[0042] An additional quartz outer sleeve, namely outer sleeve 2, is added to the preformed intermediate piece to increase the cladding thickness to form the final preform rod, which is then drawn at a high temperature of 2050°C into a suspended core hollow core anti-resonant optical fiber.

[0043] During the preparation process, important structural parameters such as air filling fraction, fiber core and support tube 3 diameters are modified by controlling the active gas pressure in different areas and adjusting the feeding / drawing speed to form the final desired optical fiber structure.

[0044] Simulation experiment for ideal concentric circle design:

[0045] The finite element software COMSOL Multiphysics was used to accurately analyze the modal characteristics of the modes in different regions of the suspended core hollow core antiresonant fiber. An idealized design was simulated, with all tube walls having the same thickness, perfect concentric circle design and symmetry. The simulation calculations were performed at a wavelength of λ of 532nm, because the modes in the visible light band (short wavelength) have extremely close effective refractive indices and are expected to be most susceptible to multi-peak operation and more likely to produce multi-mode behavior.

[0046] The initial core wall thickness is set to 800 nm, because ideally the working wavelength of the optical fiber prepared with this wall thickness will cover some important transmission wavelengths, such as 532 nm, 1064 nm and 1550 nm.

[0047] According to the Sellmeier equation, the refractive index of fused silica is 1.4607 at 532nm, and the rest of the area is considered to be a vacuum area with a refractive index of 1. In addition, it is crucial to use a maximum element size (λ / 6) smaller than the wavelength size on the thin silica core wall to build a dense grid to obtain reliable results, while the maximum element size used in the rest of the air region is λ / 4. Finally, a 10μm thick perfectly matched layer and scattering boundary conditions are used around the cladding to set the boundaries.

[0048] Figure 2 Part (b) shows the suspended core hollow core antiresonant fiber fundamental mode (LP 01 The electric field intensity distribution of the mode is a perfect Gaussian distribution. In most cases, this mode has the highest effective refractive index and the lowest confinement loss. It should be clear that each mode simulated in this study is usually associated with a group of modes, such as the core LP with two degenerate states. 01 mold( and ), a core LP with four degenerate states 11 TM 01 TE 01 , and ) and so on. These modes have almost identical modal profiles but different polarization characteristics. Since the effective refractive index of the modes within a group is almost the same, the polarization state is not considered in this study, so the polarization effect can be ignored in the discussion. In addition, in order to make a clear distinction, AC is used in the subsequent discussion. lm Indicates the mode from the air cladding, while the core mode is still LP lm express.

[0049] (1) Simulation and analysis of high-order mode suppression performance under ideal concentric structure

[0050] The relationship characteristics of the effective refractive index of the six main core and cladding modes of suspended core hollow core antiresonant fiber at 532nm wavelength are numerically calculated. The results are as follows: Figure 2 shown. Figure 2 Part (a) is a plot of the effective refractive index of some of the main modes of suspended core hollow core antiresonant fibers versus the d / D ratio. It can be seen that the effective refractive index of the core mode decreases with increasing d / D, while the opposite is true for the cladding mode. Before further discussion, it should be clarified that mode coupling occurs when two modes have similar or identical effective refractive indices. When d / D < 0.63, LP 01 The effective refractive index of the mode is always higher than that of other modes. This non-intersection between the curves means that the modes will not be in phase, ensuring LP 01The LP mode does not resonantly couple with the mode in the fiber's air cladding. 11 The air cladding modes have the same effective refractive index at several specific structural ratios. This research focuses on the LP 11 When the mode is in phase with the first two cladding modes, that is, when d / D is close to 0.403 and 0.424, mode coupling occurs and the mixed mode exists in both the core and the cladding, such as Figure 3 As shown in parts b and c.

[0051] When the appropriate optical waveguide arrives, the light is not only confined to the core, but all leaks into the cladding, where a stable cladding mode is formed. This is why under certain coupling conditions, the core LP 11 The loss of the mode is significantly increased, forming a high loss peak in the loss spectrum, such as Figure 2 In addition, in order to quantify the core high-order mode LP 11 The degree of mode suppression introduces the high-order mode extinction ratio FOM lm As a quantitative standard, FOM lm =α lm / α 01-1 , where α lm LP lm The loss of the mode (in dB / m). lm Applied to the published results, the FOM of the conventional Kagome hollow core fiber was found to be 11 The theoretical FOM is about 2-10, which is the numerical optimization of the high-order mode suppression that can be achieved by a typical nodeless single-ring hollow-core antiresonant fiber within a limited wavelength range. 11 The value is between 50 and 600. Photonic bandgap hollow-core fibers with hollow leakage channels ("shunts") in the cladding can achieve FOMs of 400. 11 value. Figure 2 The results in part (c) show that when d / D≈0.403, FOM 11 The peak value can reach nearly 2000. The FOM near the intersection of the other core cladding modes 11 The value also increases significantly to at least 1000 or more. Such a huge coupling difference effectively suppresses LP 11Mode guidance. It is also verified that when the cladding space of the hollow-core fiber is reasonably used for mode formation, the mode coupling between the high-order modes in the core and the cladding modes is greatly enhanced. On the contrary, when various microstructures are introduced into the cladding, there is a large effective refractive index difference between the core and cladding modes, and the mode leaking from the core is no longer easily matched with the cladding mode. Therefore, the coupling is reduced. In the latter case, it is often necessary to modify the fiber cladding structure to a relatively extreme size parameter during the drawing process to regain the expected mode coupling efficiency, which is quite challenging in the actual preparation process.

[0052] (2) Fiber structure scalability and broadband guidance verification

[0053] To verify whether the specific structural ratio d / D = 0.403 provides the best LP for all operating wavelengths of the fiber 11 To suppress the mode, an analytical model was established, and the relationship between the modal characteristics and wavelength when d / D=0.403 was simulated numerically. The results are as follows Figure 4 As shown. It can be seen that within the entire calculated transmission window, the fiber core LP 01 The mode maintains the highest effective refractive index, and as the wavelength increases, the effective refractive index separation between the modes becomes clearer. 11 Mode and cladding AC 01 The effective refractive index of the modes is still very close or almost the same. This means that if the high-order mode suppression is optimized for short wavelengths where multimode guidance is easy to achieve, the suppression of high-order modes will become clearer as the effective refractive index separation between modes becomes clearer as the wavelength increases. Therefore, the suspended core hollow core antiresonant fiber design provides theoretically stable broadband single-mode guidance. In addition, Figure 4 Also shown are three core LP 01 The three deep and narrow loss bands caused by the first, second and third order resonances of the mode and the core wall. The m-order core wall resonance can be accurately calculated by the formula, where for the core wall thickness t = 800nm ​​and the refractive index of fused silica glass nglass = 1.44, λ1 = 1.66μm, λ2 = 0.83μm, λ3 = 0.55μm. This is consistent with Figure 4 The simulation results show that the resonant wavelength position is highly consistent (in the simulation calculation process, the core LP close to the resonant wavelength position 01 modes will show indistinguishable mode profiles or exhibit strong leakage towards the cladding direction due to resonance effects).

[0054] For easier understanding, the present application provides an embodiment of experimental characterization test of the prepared eccentric suspended core hollow core antiresonant optical fiber, as follows:

[0055] The end face of the prepared optical fiber was characterized by using SEM. Figure 1 As shown. Its structural parameters were measured, the core diameter is 85μm, the core tube wall thickness t is 3.43μm, and the node thickness formed by the core tube and the support tube leads to the expected eccentricity, and the eccentricity distance is 6.16μm. Obviously, the prepared optical fiber deviates from the ideal situation assumed in the calculation in terms of concentricity and symmetry, and the single support tube leads to the expected eccentricity. For the convenience of discussion, the average value of the air cladding spacing d is taken as 27.67μm, and d / D=0.326 is obtained. Therefore, the optimal d / D value for high-order mode suppression calculated under the ideal concentric structure is expected to no longer apply to the eccentric structure. Please continue to refer to the following.

[0056] Whether the prepared optical fiber can be effectively guided by the antiresonance of the core mode and the core wall is the primary concern. For this purpose, an experimental test platform was built to test and characterize the transmission window and broadband loss spectrum of the suspended core hollow core antiresonant optical fiber. Among them, the test light source used two broadband supercontinuum light sources (SuperKCompact, NKTPhotonics, Denmark, and ElectroMIR4.8, LEUKOS, France). Each light source was coupled into a suspended core hollow core antiresonant optical fiber of at least 1m through a suitable plano-convex lens, and then coupled into two spectrum analyzers (AQ6374, Yokogawa, Japan, and AQ6377, Yokogawa, Japan) to obtain the transmission spectrum of the optical fiber. The results are shown in Figure 1. Figure 5 shown.

[0057] like Figure 5 As shown in (a), the transmission spectrum of the optical fiber covers a comprehensive wavelength of more than 4300nm. Dense and clear resonances were observed in the visible to near-infrared band, which is attributed to the resonance occurring in the thicker core wall (3.43μm). The corresponding resonance wavelength can be accurately obtained by substituting the core wall thickness and node thickness obtained from SEM analysis into the formula. The final calculated resonance peak position, including the resonance spectrum broadening caused by the thicker area at the contact point between the core tube wall and the support tube, is also clearly observed, which is consistent with the analysis of the spectral measurement.

[0058] The mid-infrared band is often referred to as the "spectral fingerprint" region because many gas molecules exhibit fundamental absorption resonances at these wavelengths. During the preparation of the optical fiber, the hollow part of the optical fiber is filled with nitrogen, but the ends are not sealed and exposed to the laboratory atmosphere. Therefore, it is inevitable that atmospheric gases (partly driven by the laboratory's thermal waves) gradually enter the optical fiber core.

[0059] In the spectral range studied in this work, there is a strong absorption band near the wavelength of 2.8μm due to the concentration of CO2 and H2O in the atmosphere, and a weak absorption band up to about 3.5μm due to the concentration of H2O. Part of the H2O absorption band overlaps with the high-loss resonance region of the optical fiber, so it is difficult to determine the exact absorption cutoff position, but the position of the H2O absorption band in silica is known. After a transmission window, a clear strong CO2 absorption band is then presented around 4.2μm. In addition, some rotation absorption bands are found between 3.2-3.8μm, which are attributed to the hydrochloric acid residue in the process of synthesizing silica glass. Although HCl can be removed by careful purging based on some preparation experience, HCl may be present throughout the entire length of the optical fiber.

[0060] Figure 5 (b) and Figure 5 (c) is the fiber attenuation from the cutoff measurement. There are some transmission windows with lower losses in the spectral range from visible light to near infrared, about 1-2 dB / m, with a minimum loss of 0.71 dB / m at 1115 nm. However, the losses are relatively high in the long-wave range dominated by material losses. When the material absorption exceeds 50 dB / m, the contribution of material absorption losses to the total loss is comparable to leakage losses at wavelengths starting from 3 μm. When the material absorption losses exceed 600 dB / m (about 3.7 μm), material losses become the main loss mechanism. At wavelengths of 3.7 μm and above, the losses of suspended core hollow core antiresonant fibers are expected to be higher due to the material absorption of silica, which makes light transmission in this spectral region using current suspended core hollow core antiresonant fiber designs not yet competitive with some alternative technologies. Therefore, optimizing the losses of optical fibers, especially in the mid-infrared band, will be the subject of future work. In addition, the absorption peaks of various gases in the mid-infrared band are consistent with the results in the transmission spectrum. The absorption spectrum of some HCl is compared with the known gas absorption spectrum database

[27] , and it is found that the peak wavelength and relative intensity of HCl are very consistent.

[0061] (5) Suspended core hollow core antiresonant fiber offset excitation test

[0062] The simulation results in parts (1) and (2) above show that the suspended core hollow core antiresonant fiber design has excellent potential for achieving stable broadband single-mode transmission. Therefore, in order to test the ability and stability of the suspended core hollow core antiresonant fiber in achieving broadband single-mode, the light output from the supercontinuum source is coupled into optical fibers of different lengths through a plano-convex lens, and the effects of aligned excitation and lateral misalignment on the output mode profile of the fiber are explored while maintaining a constant distance between the end faces. In order to avoid the possibility that the single-mode behavior observed in the end is due to the natural loss leakage of the core high-order mode caused by the excessive length of the test fiber, the test fiber length used at short wavelengths (<2000nm) is close to 1m, and the long wavelength is tested on a fiber length of only 21cm. The near-field mode profile at the output surface is tracked using a microscope objective, 550nm, 700nm, 1100nm, 1550nm, 2000nm, 3250nm, 3750nm and 4000nm bandpass filters, and three-band cameras. At maximum transmission, that is, when the fiber is fully aligned, observe whether a clean fundamental mode can be collected. The result of alignment excitation is as follows Figure 6 shown.

[0063] The near-field images collected at the fiber output demonstrate the ability of suspended-core hollow-core antiresonant fibers to achieve broadband single-mode transmission. From the visible to the mid-infrared, when the fiber is fully aligned to achieve maximum transmission, clean fundamental mode output can be observed, with each mode having a strong central intensity maximum and no visible local intensity peaks or sidelobes. At positions close to the resonant wavelength, such as 3250nm, the low-intensity fundamental mode is still clearly visible at higher losses.

[0064] In addition to proving that suspended core hollow core antiresonant fibers can achieve broadband single-mode transmission, the stability of the single-mode performance of such large core hollow core fibers can be tested by adjusting the fiber displacement platform to achieve misaligned input at different lateral misalignment distances, that is, by checking the reconstructed near-field profiles under different coupling conditions to test the stability of the fiber single-mode operation when the laser beam and the fiber core are misaligned. The experiment also used the bandpass filter used for alignment excitation, and recorded the power and output of the near-field profile while scanning the input fiber position along the horizontal and vertical axes. The fiber length used in the test was consistent with the fiber length used for alignment excitation. The scan span along the two axes was 60μm, that is, Δy = ±30μm, and the origin corresponded to the calibrated maximum transmission power.

[0065] exist Figure 7 The recorded near-field images of the offset excitation at some wavelengths and the intensity distribution of each mode field are shown in the figure. The results are sufficient to show that at 6060μm 2The near-field profile remains basically the same in the region of . It is worth noting that some of the color diagrams are not completely symmetrical due to the incomplete light coupling at the fiber input, the expected reduction in the launch efficiency caused by the misalignment, and the change of the mode profile at some positions. However, relative to the larger core diameter of about 85μm, such a large horizontal / vertical span still does not find obvious core high-order mode excitation, indicating the stable single-mode performance of the fiber.

[0066] (6) Suspended core hollow core anti-resonant optical fiber output beam quality test

[0067] In order to more quantitatively evaluate the single-mode purity of the drawn optical fiber, the output beam quality of the 1m long suspended core hollow core antiresonant fiber at different wavelengths was tested by measuring the M2 parameter. To this end, two commercial beam quality analyzers (DatarayWCD-IR-BB-7.5, 2-16μm; OphirBGS-USB3-LT665, 190-1100nm) and a motorized displacement platform were used. The output beam of the suspended core hollow core antiresonant fiber is collimated by an infrared compatible aspheric lens, and the beam is focused on the spot at the center of the beam profile travel range through a plano-convex lens. The beam diameter is then scanned at different positions along the Z axis by a camera. The size of the beam is obtained from the Gaussian fit of the lateral intensity distribution of all measurement points, and the standard of 1 / e2 of the maximum intensity is adopted. Subsequently, the beam size measured along the propagation distance is fitted with the Gaussian beam profile containing the parameter M2, and the formula is as follows:

[0068]

[0069] Where λ is the test wavelength, z0 is the relative position of the beam waist along the z-axis, and w0 is the spot radius at the beam waist. The fitting results of the measured data under the three sets of wavelengths are shown in Figure 8 The three sets of M2 measurement results at 550nm, 1100nm and 2020nm are 1.049, 1.028 and 1.055 respectively. These results show that due to the high-order mode loss caused by mode coupling in the suspended core hollow core antiresonant fiber, the output light of the fiber is in the fundamental mode and has a beam quality close to the diffraction limit.

[0070] (7) Simulation and analysis of high-order mode suppression performance under non-ideal eccentric structures

[0071] Although the prepared suspended core hollow core antiresonant fiber is eccentric with the assumed ideal model in terms of core concentricity, symmetry and structure ratio d / D, the experimental characterization results of the fiber still show that the suspended core hollow core antiresonant fiber has excellent single-mode performance. In order to explain this phenomenon, the ideal model of the suspended core hollow core antiresonant fiber was reconstructed, such as Fig. 9(a) shows the fiber structure characteristics under non-ideal eccentricity. First, the mode characteristics under different eccentricity are analyzed, and the results are plotted in Fig. 9 (e). By analyzing the dependence of the core-cladding mode characteristics on different eccentric positions, the influence and tolerance of suspended-core hollow-core antiresonant fibers on the core eccentricity can be studied. Obviously, within the calculated eccentricity range, the effective refractive index of the core mode has almost no change, among which the LP01 mode is still always higher than other modes, which ensures that even in the eccentric structure, the LP01 mode has the prerequisite of not coupling with other modes. On the contrary, the eccentric core structure has a strong influence on the cladding mode. The uneven distribution of the air cladding effectively changes the effective refractive index and morphology of the corresponding mode. The effective refractive index of most cladding modes increases with the increase of eccentricity. This means that the optimal phase matching condition obtained in the ideal concentric structure, that is, when the eccentric distance is 0, is no longer applicable to the eccentric fiber structure. The original highly in-phase core high-order mode and cladding mode are no longer close or equal, and the stable single-mode guiding conditions under the eccentric structure need to be re-examined.

[0072] Fig. 9 (f) The calculated results of the effective refractive index of the mode when the structural parameters and the degree of eccentricity are equivalent to those of the prepared fiber are plotted. The optimal phase matching condition between the LP11 mode and the AC01 mode is reduced to 0.347. This is attributed to the increase in the single-side area of ​​the cladding caused by the eccentricity, and the difference in the core area is effective. This change leads to an effective increase in the effective refractive index of the cladding mode, which in turn increases the probability of phase matching between a specific core high-order mode and the cladding mode, making coupling between modes more likely to occur. The influence of only the fundamental mode and other higher-order core modes with a larger effective refractive index than the cladding mode cannot be achieved in unstructured fibers such as Kagome fibers because the cladding air holes are much smaller than the core. This difference in area makes it difficult to adjust the effective refractive index so that all core high-order modes fall within the range of the cladding mode. The diameter of the cladding mode air hole is two to three times smaller than the diameter of the corresponding core. This leads to a four to nine-fold difference in the air hole area, which is the reason for the observation of multimode guiding. For example, the UV-guided Kagome fiber of reference

[80] consists of a 30 μm core and cladding holes with a diameter of only 7 μm. This large diameter difference (the cladding holes cover only 5% of the core area) actually reduces the effective refractive index of the first cladding mode. As a result, a series of core high-order modes cannot match the cladding modes, and strong multimode guidance is observed. In suspended-core fibers, the core diameter is only 2-4 times the diameter of the cladding on one side. For this reason, the first core high-order mode is already located in the vicinity of the cladding mode region, so that even the prepared fiber with a relatively large core diameter of 85 μm can effectively show single-mode guidance.

[0073] In addition, similar to the ideal concentric structure, the FOM near the intersection point of the mode curves 11 also increases significantly, as shown in Fig. 9 (g). The range where FOM 11 is greater than 30 is represented by the gray shaded area in the figure (0.316 < d / D < 0.4). This explains why the fabricated fiber does not match the optimal phase matching value obtained from the modeling but still exhibits excellent high-order mode suppression performance (d / D = 0.326, FOM11 = 51). The above results indicate that the single-mode performance of this fiber highly depends neither on perfect ideal concentricity nor on the absolute physical size of the fiber. This means that as long as the ratio d / D is maintained within a certain error range under a certain eccentric structure, it is possible to fabricate a suspended-core hollow antiresonant fiber that can achieve reliable single-mode transmission.

[0074] To more comprehensively understand and evaluate the high-order mode suppression performance of the suspended-core hollow antiresonant fiber, it is necessary to perform mode excitation under some extreme conditions. In this regard, the offset excitation of the suspended-core hollow antiresonant fiber at 1550 nm was tested with an extremely short fiber length of only 13 cm. Generally, large-core hollow fibers usually retain most of the core modes at such a short length, and the test results are as shown in Fig.10 . Each image was appropriately digitally post-processed to enhance visibility. As expected, when the misalignment amount is too large, the input laser beam overlaps at some positions on the fiber core tube wall, resulting in mode excitation within the core wall. Theoretically, if high-order modes are excited, the LP 11 mode with the lowest loss should be observed first, followed by the LP 21 mode with the second lowest loss. However, the fact is that the excitation of the LP 21 mode is more obvious, and the excitation of the LP 11 mode is not observed at any lateral misalignment value, indicating that the suspended-core hollow antiresonant fiber acts as an extremely effective LP 11 mode filter, which is consistent with the simulation results of Fig. 9 .

[0075] Finally, it should be noted that the above description is only the preferred embodiment of the present invention and is not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A hollow core antiresonant optical fiber, characterized in that: Comprising an outer sleeve; A hollow capillary tube, suspended inside the outer sleeve; Only one support tube is provided, and the support tube is used to connect the hollow capillary tube and the outer sleeve, so that the hollow capillary tube can be concentrically arranged inside the outer sleeve.

2. A hollow core antiresonant optical fiber according to claim 1, characterized in that: The hollow capillary tube is concentrically arranged with the outer sleeve.

3. The hollow core antiresonant optical fiber according to claim 1, characterized in that: The hollow capillary tube is eccentrically arranged inside the outer sleeve. The inner diameter of the hollow capillary tube is D, and the distance between the hollow capillary tube and the outer sleeve is d. The range of the structural ratio d / D is 0.316 < d / D < 0.4 to obtain better high-order mode suppression performance.

4. The hollow core antiresonant optical fiber according to claim 1, characterized in that: The support tube is also a hollow capillary tube. During use, the position of the hollow capillary tube is modified by adjusting the pressure entering the support tube.

5. The hollow core antiresonant optical fiber according to claim 1, characterized in that: The outer sleeve is a quartz tube. After the hollow capillary tube and the support tube are sintered and fixed to form a pre-stack, the stack is placed inside the quartz tube and sintered for reinforcement.

6. A method for preparing a hollow core antiresonant optical fiber according to any one of claims 1 to 5, characterized in that: Comprising: S1, placing hollow capillary tubes and support tubes of different sizes in a triangular stacking device, sintering and fixing them to form Stack 1; S2, inserting Stack 1 formed by the hollow capillary tube and the support tube into Outer Sleeve 1, and sintering and fixing it again to form Stack 2; S3, drawing Stack 2 in an optical fiber drawing tower into a preform intermediate with a preset diameter; S4, sleeving Outer Sleeve 2 on the preform intermediate to form a preform rod, and drawing the preform rod at a preset temperature into a suspended hollow anti-resonant optical fiber.

7. The method for preparing a hollow core antiresonant optical fiber according to claim 6, characterized in that: In step S3, the diameter of the preform intermediate is 1 / 10 of the original stacked size.

8. The method for preparing a hollow-core antiresonant optical fiber according to claim 6, characterized in that: In step S4, the preset temperature is 2050 °C.

9. The method for preparing a hollow core antiresonant optical fiber according to claim 6, characterized in that: In step S3, the diameter of the preform intermediate is 1 - 2 mm.

10. The method for preparing a hollow core antiresonant optical fiber according to claim 6, characterized in that: In step S4, the final optical fiber structure is formed by controlling the air pressure in different regions and adjusting the feeding / drawing speed.

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