Achromatic laser mode coupling system for hollow-core optical fiber fundamental mode excitation

Abstract

The invention relates to an achromatic laser mode coupling system for hollow-core optical fiber fundamental mode excitation, which belongs to the technical field of hollow-core optical fiber mode coupling, and comprises a first optical fiber collimator, a photonic crystal optical fiber, a second optical fiber collimator, a first lens, a hollow-core optical fiber, a second lens and a CCD (Charge Coupled Device) camera, mode matching is carried out on lasers with different wavelengths, meanwhile, it is guaranteed that the polarization state of the lasers is unchanged, and then the output lasers are coupled to enter a hollow-core optical fiber. Moreover, the achromatic optical fiber collimator and the achromatic lens are selected, so that the focal shift influence of the lens on the laser with different wavelengths is reduced, and the laser with different wavelengths enters the hollow-core optical fiber to be transmitted in a fundamental mode.

Description

Technical Field

[0001] The present invention relates to the technical field of hollow core optical fiber mode coupling, and in particular to a laser mode coupling system for hollow core optical fiber fundamental mode excitation, and in particular to a laser mode coupling system with achromatic characteristics for hollow core optical fiber fundamental mode excitation. Background Art

[0002] Atom interferometers use laser trapped atoms to obtain cold atomic clusters as sensitive units, which can achieve high-precision, high-sensitivity and mechanical wear-free acceleration vector measurement. In response to the needs of vector measurement in dynamic environments, hollow-core optical fibers are combined with atom interferometers to develop a new type of fiber-guided atom interferometer. Hollow-core optical fibers can confine the light field in the fiber core to achieve low-loss, long-distance transmission. The atoms guided into the optical fiber are constrained by the dipole light field in the radial direction of the optical fiber to inhibit their diffusion, and are guided over a long distance in the axial direction of the optical fiber, realizing the interferometer's measurement of physical quantities in the axial direction of the optical fiber. In a dynamic environment, the fiber-guided atom interferometer can effectively reduce the amplitude of the motion of atoms relative to the light field in the non-measurement direction, showing its great prospects in the fields of precision measurement and quantum sensing.

[0003] Fiber-guided atom interferometry requires the use of far-infrared detuned lasers to radially constrain atoms and achieve axial guidance of atoms. 87 Taking the D2 line of Rb atoms as an example, its resonant light wavelength is 780nm, while the commonly used guiding light wavelength is 1064nm. This requires that the hollow-core optical fiber needs to realize the fundamental mode transmission of lasers in these two bands at the same time.

[0004] Since the fundamental mode field diameter of hollow-core optical fiber is insensitive to wavelength, in order to ensure that lasers of different wavelengths can be coupled into the hollow-core optical fiber at the same time, existing solutions usually use multiple independent mode field matchers to transform the mode field diameters of lasers of different wavelengths respectively, so as to match the mode field diameter of the incident light field with the fundamental mode diameter of the hollow-core optical fiber. This independent coupling solution greatly increases the system complexity and debugging difficulty of mode coupling, making it difficult to achieve high-efficiency fundamental mode excitation of multi-wavelength light fields at the same time. Summary of the invention

[0005] In view of the above problems, the present invention proposes an achromatic laser mode coupling system for hollow-core optical fiber fundamental mode excitation. The present invention uses photonic crystal fiber as a mode matcher to perform mode matching on lasers of different wavelengths while ensuring that their polarization states remain unchanged, and then couples the output laser into the hollow-core optical fiber; and, an achromatic optical fiber collimator and an achromatic lens are selected to reduce the focal shift effect of the lens on lasers of different wavelengths, so that lasers of different wavelengths enter the hollow-core optical fiber for transmission in the fundamental mode, which is beneficial to achieve good fundamental mode excitation of lasers of each wavelength and maintain concentric spatial distribution.

[0006] The present invention provides an achromatic laser mode coupling system for hollow core fiber fundamental mode excitation, characterized in that it comprises a first fiber collimator 3, a photonic crystal fiber 4, a second fiber collimator 5 and a first lens 6, a hollow core fiber 7, a second lens 8 and a CCD camera 9;

[0007] The photonic crystal fiber 4 is connected to the first optical fiber collimator 3 and the second optical fiber collimator 5 respectively; a first lens 6 is arranged between the second optical fiber collimator 5 and the hollow core optical fiber 7; a second lens 8 is arranged between the hollow core optical fiber 7 and the CCD camera 9;

[0008] The first optical fiber collimator 3 and the second optical fiber collimator 5 are symmetrically arranged; the first lens 6 and the second lens 8 are symmetrically arranged.

[0009] The first optical fiber collimator 3 and the second optical fiber collimator 5 are both achromatic optical fiber collimators;

[0010] The first lens 6 and the second lens 8 are both achromatic lenses.

[0011] The photonic crystal fiber comprises a fiber end 1; the first fiber collimator 3 is used to focus the guide laser and the resonance laser onto the fiber end face 1 for the first fiber coupling; the guide laser and the resonance laser are both collimated light;

[0012] The second fiber collimator 5 is used to collimate the guide laser and the resonance laser transmitted in the photonic crystal fiber 4 for the first time and then output them, so as to obtain the guide laser and the resonance laser after the first collimation.

[0013] The first lens 6 is used to focus the guide laser and the resonant laser after the first collimation onto the end face of the hollow core optical fiber 7 for the second optical fiber coupling;

[0014] The second lens 8 is used to collimate the guide laser and the resonance laser of the second fiber coupling in the hollow core fiber 7 for the second time, and adjust the second fiber coupling so that the output modes of the guide laser and the resonance laser are both fundamental mode; fundamental mode guide laser and fundamental mode resonance laser are obtained;

[0015] The light intensity distribution of the fundamental mode guided laser and the fundamental mode resonant laser comprises a concentric Gaussian distribution;

[0016] Exemplarily, after the second fiber coupling is adjusted to a suitable condition, the light intensity distributions of the fundamental mode guided laser and the fundamental mode resonant laser are both Gaussian distributions, and the two Gaussian distributions are concentric;

[0017] When the light intensity distributions of the fundamental mode guided laser and the fundamental mode resonant laser are concentric Gaussian distributions, it indicates that the regulation effect is optimal.

[0018] The first lens 6 includes a surface 6-1 with a small curvature radius and a surface 6-2 with a large curvature radius;

[0019] The small radius curvature surface 6-1 faces the second optical fiber collimator; the large radius curvature surface 6-2 faces the hollow core optical fiber 7;

[0020] The second lens includes a second surface 8 - 1 with a small curvature radius and a second surface 8 - 2 with a large curvature radius; the second surface 8 - 1 with a small curvature radius faces the CCD camera 9; and the second surface 8 - 2 with a large curvature radius faces the hollow core optical fiber 7.

[0021] A third lens 10 and a fourth lens 11 are respectively disposed in the first optical fiber collimator 3 and the second optical fiber collimator 5;

[0022] The third lens 10 and the fourth lens 11 are both achromatic lenses.

[0023] The third lens 10 includes a small curvature radius surface 10-1 and a large curvature radius surface 10-2; the small curvature radius surface 10-1 faces the side where the guide laser and the resonance laser are input; the large curvature radius surface 10-2 faces the photonic crystal fiber 4 side.

[0024] The fourth lens 11 includes a surface 11-1 with a small radius of curvature and a surface 11-2 with a large radius of curvature;

[0025] The small curvature radius surface 11-1 faces the side of the first lens 6; the large curvature radius surface 11-2 faces the side of the photonic crystal fiber 4.

[0026] The hollow core optical fiber 7 is a bare fiber; the packaging method of the photonic crystal optical fiber 4 is a jumper packaging.

[0027] The expression of the spot diameter of the guide laser 1 after the first collimation is:

[0028]

[0029] Wherein, D1 is the spot diameter of the guiding laser after the first collimation, f1 is the focal length of the first lens to the guiding laser after the first collimation, λ1 is the laser wavelength of the guiding laser; MFD is the mode field diameter of the photonic crystal fiber 4, which is a constant because it is insensitive to wavelength.

[0030] Similarly, the expression for the spot diameter of the resonant laser 2 after the first collimation is:

[0031]

[0032] Wherein, D2 is the spot diameter of the resonant laser after the first collimation, f2 is the focal length of the first lens to the resonant laser after the first collimation, λ2 is the laser wavelength of the resonant laser; MFD is the mode field diameter of the photonic crystal fiber 4, which is a constant because it is insensitive to wavelength.

[0033] Therefore, the relationship between the spot diameters of the guide laser and the resonant laser after the first collimation is expressed as:

[0034]

[0035] Compared with the prior art, the present invention has at least the following beneficial effects:

[0036] (1) The photonic crystal fiber used in the present invention can ensure that lasers of different wavelengths have nearly the same mode field diameter, realize endless single-mode transmission, and achieve mode field diameter matching for coupling lasers of different wavelengths to hollow-core optical fibers;

[0037] (2) The present invention adopts an achromatic lens, which has a small change in focal shift for lasers of different wavelengths, and is beneficial to reducing the influence of the laser wavelength on fiber coupling, beam expansion and collimation;

[0038] (3) The present invention combines a photonic crystal fiber, an achromatic fiber collimator and an achromatic lens into a set of achromatic laser mode matchers, which can simultaneously achieve laser coupling of multiple wavelengths, simplify the system complexity, reduce the difficulty of debugging, and improve the system stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The drawings are only for the purpose of illustrating particular embodiments and are not to be construed as limiting the invention.

[0040] Figure 1 is a schematic diagram of a structure diagram of a laser mode coupling system in an embodiment of the present invention;

[0041] Figure 2 Schematic diagram of the relationship between the spot diameter of the collimated beam of the guide laser 1 and the spot diameter of the collimated beam of the resonance laser 2 in an embodiment of the present invention.

[0042] Reference numerals:

[0043] 1-guide laser, 2-resonant laser, 3-first fiber collimator, 3-1 fiber flange end face one, 4-photonic crystal fiber, 4-1 fiber end face one, 5-second fiber collimator, 6-first lens, 6-1 small curvature radius surface one, 6-2 large curvature radius surface one, 7-hollow core fiber, 8-second lens, 8-1 small curvature radius surface two, 8-2 large curvature radius surface two, 9-CCD camera, 10-third lens, 10-1 small curvature radius surface three, 10-2 large curvature radius surface three, 11-fourth lens, 11-1 small curvature radius surface four, 11-2 large curvature radius surface four. DETAILED DESCRIPTION

[0044] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. In addition, the present invention can also be implemented in other ways different from those described herein, and therefore, the protection scope of the present invention is not limited by the specific embodiments disclosed below.

[0045] A specific embodiment of the present invention, as Figure 1-Figure 2 , discloses an achromatic laser mode coupling system for hollow-core optical fiber fundamental mode excitation. In order to illustrate the effectiveness of the method proposed in the present invention, the above technical solution of the present invention is described in detail through a specific embodiment below. The specific implementation steps are as follows:

[0046] The present invention provides an achromatic laser mode coupling system for hollow core fiber fundamental mode excitation, comprising a first fiber collimator 3, a photonic crystal fiber 4, a second fiber collimator 5, a first lens 6, a hollow core fiber 7, a second lens 8 and a CCD camera 9; Figure 1 ;

[0047] Preferably, the photonic crystal fiber 4 is connected to the first fiber collimator 3 and the second fiber collimator 5 respectively; a first lens 6 is arranged between the second fiber collimator 5 and the hollow core fiber 7; a second lens 8 is arranged between the hollow core fiber 7 and the CCD camera 9;

[0048] The first optical fiber collimator 3 and the second optical fiber collimator 5 are symmetrically arranged; the first lens 6 and the second lens 8 are symmetrically arranged;

[0049] Furthermore, the first optical fiber collimator 3 and the second optical fiber collimator 5 are both achromatic optical fiber collimators;

[0050] A third lens 10 and a fourth lens 11 are respectively disposed in the first optical fiber collimator 3 and the second optical fiber collimator 5; the third lens and the fourth lens are both achromatic lenses;

[0051] Exemplarily, the achromatic lens includes a surface with a small curvature radius and a surface with a large curvature radius, and the preparation process of the achromatic lens is selected according to actual needs;

[0052] Furthermore, the achromatic lens is made by using a double bonding process;

[0053] Furthermore, the first fiber collimator 3 is used to focus the guide laser and the resonance laser onto the end face of the photonic crystal fiber 4 to perform the first fiber coupling; the guide laser and the resonance laser are both collimated light;

[0054] The second fiber collimator 5 is used to collimate the guide laser and the resonance laser transmitted in the photonic crystal fiber 4 for the first time and then output them, so as to obtain the guide laser and the resonance laser after the first collimation.

[0055] The first lens 6 is used to focus the guide laser and the resonant laser after the first collimation onto the end face of the hollow core optical fiber 7 for the second optical fiber coupling;

[0056] The second lens 8 is used to collimate the guide laser and the resonance laser of the second fiber coupling in the hollow core fiber 7 for the second time, and adjust the second fiber coupling so that the output modes of the guide laser and the resonance laser are both fundamental mode; fundamental mode guide laser and fundamental mode resonance laser are obtained;

[0057] The light intensity distribution of the fundamental mode guided laser and the fundamental mode resonant laser comprises a concentric Gaussian distribution;

[0058] Exemplarily, after the second fiber coupling is adjusted to a suitable condition, the light intensity distributions of the fundamental mode guided laser and the fundamental mode resonant laser are both Gaussian distributions, and the two Gaussian distributions are concentric;

[0059] When the light intensity distributions of the fundamental mode guided laser and the fundamental mode resonant laser are concentric Gaussian distributions, it indicates that the regulation effect is optimal.

[0060] Furthermore, both the first lens 6 and the second lens 8 are achromatic lenses;

[0061] The first lens and the second lens each include a diverging convex surface and a converging convex surface;

[0062] Further, the first lens 6 includes a surface 6-1 with a small curvature radius and a surface 6-2 with a large curvature radius;

[0063] Exemplarily, the small radius curvature surface 6-1 faces the second optical fiber collimator; the large radius curvature surface 6-2 faces the hollow core optical fiber 7;

[0064] The second lens includes a second small curvature radius surface 8-1 and a second large curvature radius surface 8-2; the second small curvature radius surface 8-1 faces the CCD camera 9; the second large curvature radius surface 8-2 faces the hollow core optical fiber 7;

[0065] A third lens 10 and a fourth lens 11 are respectively disposed in the first optical fiber collimator 3 and the second optical fiber collimator 5;

[0066] The third lens 10 and the fourth lens 11 are both achromatic lenses.

[0067] The third lens 10 includes a small curvature radius surface 10-1 and a large curvature radius surface 10-2; the small curvature radius surface 10-1 faces the side where the guide laser and the resonance laser are input; the large curvature radius surface 10-2 faces the photonic crystal fiber 4 side.

[0068] The fourth lens 11 includes a surface 11-1 with a small radius of curvature and a surface 11-2 with a large radius of curvature;

[0069] The small curvature radius surface 11-1 faces the side of the first lens 6; the large curvature radius surface 11-2 faces the side of the photonic crystal fiber 4.

[0070] Furthermore, the photonic crystal fiber 4 includes a fiber end face 1 4 - 1 and a fiber end face 2;

[0071] The optical fiber end face 1 4-1 is connected to the optical fiber flange end 1 3-1 of the first optical fiber collimator; the optical fiber end face 2 is connected to the optical fiber flange end 2 of the second optical fiber collimator;

[0072] The third lens 10 is arranged in the first optical fiber collimator at an end away from the optical fiber flange end 3-1; the optical fiber end face 4-1 is located at the focal length of the third lens;

[0073] The fourth lens 11 is arranged at an end of the second optical fiber collimator away from the second optical fiber flange end, and the second optical fiber end face is located at the focal length of the fourth lens;

[0074] Furthermore, the packaging method of the photonic crystal fiber 4 is jumper packaging; the mode field diameter of the photonic crystal fiber 4 is insensitive to the wavelength.

[0075] Exemplarily, the photonic crystal fiber 4 is connected to the first optical fiber collimator 3 and the second optical fiber collimator 5 respectively through jumper packaging;

[0076] In one embodiment of the present invention, a collimated guiding laser 1 and a collimated resonant laser 2 are obtained, and the guiding laser 1 and the resonant laser 2 are transmitted to a first optical fiber collimator 3, and are focused to an end face of a photonic crystal fiber 4 via a third lens in the first optical fiber collimator 3, and are transmitted to a second optical fiber collimator 5 after a first optical fiber coupling via the photonic crystal fiber 4, and are collimated for the first time via a fourth lens, and the guiding laser and the resonant laser after the first collimation are obtained, and are then focused to an end face of a hollow core fiber 7 via a first lens 6, and are coupled for the second time in the hollow core fiber to obtain guiding laser and resonant laser;

[0077] After the guiding laser and the resonant laser are collimated for the second time by the second lens 8, they are transmitted to the CCD camera 9 to obtain the guiding laser and the resonant laser after the second collimation; the CCD camera 9 can be used to observe the modes of the guiding laser and the resonant laser after the second collimation so as to facilitate real-time adjustment to obtain the optimal coupling efficiency and mode.

[0078] The modes of the guide laser 1 and the resonant laser 2 after the second collimation are observed by a CCD camera to assist in achieving the function of real-time adjustment of coupling;

[0079] When the guide laser and the resonant laser after the second collimation are fundamental mode guide laser and resonant guide laser, and the light intensity distribution is a concentric Gaussian distribution, it indicates that the adjustment effect is optimal;

[0080] Furthermore, the first fiber coupling focuses the guide laser 1 and the resonance laser 2 onto the end face of the photonic crystal fiber 4, so that the beam waist sizes of the guide laser 1 and the resonance laser 2 match the mode field diameter of the photonic crystal fiber 4;

[0081] The second fiber coupling focuses the guide laser 1 and the resonance laser 2 onto the end face of the hollow-core optical fiber 7, so that the beam waist sizes of the guide laser 1 and the resonance laser 2 after the first collimation output match the mode field diameter of the hollow-core optical fiber 7;

[0082] The present application realizes the transformation of the mode field diameters of the guide laser 1 and the resonant laser 2 in the photonic crystal fiber and the hollow-core fiber based on the proportional relationship between the focal length of the fourth lens in the second fiber collimator and the focal length of the first lens; the mode field diameters of the photonic crystal fiber and the hollow-core fiber are different, and the collimated light of the guide laser 1 and the resonant laser 2 emitted from the photonic crystal fiber through the second fiber collimator is focused again by the first lens and coupled into the hollow-core fiber at a suitable beam waist radius size.

[0083] Furthermore, the hollow-core optical fiber 7 is a bare fiber; the mode field diameter of the hollow-core optical fiber 7 is insensitive to wavelength.

[0084] In the present application, the focal lengths of the third and fourth lenses in the optical fiber collimator are determined according to the mode field diameter of the selected photonic crystal fiber; the focal lengths of the first and second lenses are determined according to the mode field diameters of the photonic crystal fiber and the hollow core fiber.

[0085] Exemplarily, the guide laser 1 and the resonant laser 2 are respectively transmitted to the second fiber collimator 5 after the first fiber coupling through the first fiber collimator 3 and the photonic crystal fiber 4, and are first collimated through the fourth lens in the second fiber collimator 5. The guide laser and the resonant laser after the first collimation are obtained and then passed through the first lens 6 to realize the mode field diameter transformation of the photonic crystal fiber, and the transformation ratio is independent of the wavelength.

[0086] The present invention uses an achromatic lens to reduce the focal shift of the lens to lasers of different wavelengths, thereby ensuring excellent coupling effect of the two lasers and ensuring that the fundamental mode is excited in the hollow-core optical fiber 7 at the same time;

[0087] The photonic crystal fiber, the second fiber collimator and the first lens of the present invention constitute a laser mode matcher. The lens and the first lens arranged in the achromatic fiber collimator have little effect on the focal shift of lasers of different wavelengths, so that lasers of different wavelengths have similar focusing effects under the same optical system, ensuring that both lasers obtain higher fiber coupling efficiency and are collimated for output at the same time.

[0088] Furthermore, the mode field diameters of the guide laser 1 and the resonant laser 2 in the photonic crystal fiber 4 are the same, thereby achieving polarization-maintaining and single-mode transmission; and the photonic crystal fiber 4 plays a role in matching the mode field diameter.

[0089] The guide laser 1 is far-red detuned from the resonant laser 2, and its wavelength is greater than the wavelength of the resonant laser 2. When the two lasers are coupled, the spot size of the collimated beam of the guide laser 1 is different, and the spot size of the collimated beam of the guide laser 1 is greater than the spot size of the collimated beam of the resonant laser 2.

[0090] The expression of the spot diameter of the guide laser 1 after the first collimation is:

[0091]

[0092] Wherein, D1 is the spot diameter of the guiding laser after the first collimation, f1 is the focal length of the first lens to the guiding laser after the first collimation, λ1 is the laser wavelength of the guiding laser; MFD is the mode field diameter of the photonic crystal fiber 4, which is a constant because it is insensitive to wavelength.

[0093] Similarly, the expression for the spot diameter of the resonant laser 2 after the first collimation is:

[0094]

[0095] Wherein, D2 is the spot diameter of the resonant laser after the first collimation, f2 is the focal length of the first lens to the resonant laser after the first collimation, λ2 is the laser wavelength of the resonant laser; MFD is the mode field diameter of the photonic crystal fiber 4, which is a constant because it is insensitive to wavelength.

[0096] Therefore, the relationship between the spot diameters of the guide laser and the resonant laser after the first collimation is expressed as:

[0097]

[0098] In the present invention, the first fiber collimator, the second fiber collimator, the first lens and the second lens all use achromatic lenses. Based on the spot size of the collimated light beams of the two lasers and the ratio of the two laser wavelengths, the mode field diameter of the multi-wavelength laser in the two optical fibers can be transformed.

[0099] Exemplarily, when the first fiber collimator 3 and the second fiber collimator 5 are used, the focal length f1 of the first lens 6 for the guide laser after the first collimation is approximately equal to the focal length f2 of the first lens 6 for the resonance laser after the first collimation;

[0100] That is, the ratio of the spot diameters of the guide laser and the resonant laser after the first collimation is considered to be approximately equal to the ratio of the laser wavelengths of the guide laser and the resonant laser after the first collimation, and the expression is:

[0101]

[0102] Exemplarily, the focal length of the achromatic lens is 50 mm;

[0103] Calculated by numerical method, the focal shift of the achromatic lens for the guide laser and the resonant laser in the 780nm and 1064nm bands is 53.72μm.

[0104] Comparative Example 1

[0105] The first lens and the second lens in the embodiment are replaced by plano-convex lenses using an N-BK7 glass substrate, and the rest of the technical solutions remain unchanged;

[0106] Obtain the focal length difference of 780nm and 1064nm laser wavelengths using a plano-convex lens;

[0107] The refractive index n(λ) of the plano-convex lens is expressed as:

[0108]

[0109] Wherein, λ is the wavelength in μm. Assume that the wavelength of the guide laser 1 is 1.064 μm, and the corresponding refractive index n1 is 1.511; the wavelength of the resonance laser 2 is 0.78 μm, and the corresponding refractive index n2 is 1.507.

[0110] The focal length f(λ) of the plano-convex lens is expressed as:

[0111]

[0112] Where R is the radius of curvature of the plano-convex lens.

[0113] The curvature radius of the plano-convex lens is selected to be R=25.8mm, and the focal length f1 of the guide laser 1 after the first collimation is 50.924mm, and the focal length f2 of the resonant laser 2 is 50.471mm. It can be seen that the focal length difference of the non-achromatic plano-convex lens with a curvature radius of 25.8mm for the two wavelength lasers of 780nm and 1064nm is 453μm. Such a focal shift of the order of hundreds of μm is very unfavorable for precise fiber coupling.

[0114] The focal shift of the achromatic lens for the guide laser and the resonant laser in the 780nm and 1064nm bands is 53.72μm, which is 453μm compared to the focal shift of the non-achromatic plano-convex lens for the 780nm and 1064nm wavelength lasers in Comparative Example 1. The focal shift in the embodiment is reduced by an order of magnitude. It can be seen that the achromatic lens plays an important role in reducing the focal shift and improving the coupling efficiency of the two wavelength lasers to the optical fiber.

[0115] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. An achromatic laser mode coupling system for hollow-core fiber fundamental mode excitation, It is characterized in that It comprises a first optical fiber collimator (3), a photonic crystal optical fiber (4), a second optical fiber collimator (5), a first lens (6), a hollow core optical fiber (7), a second lens (8) and a CCD camera (9); The photonic crystal optical fiber (4) is respectively connected to the first optical fiber collimator (3) and the second optical fiber collimator (5); a first lens (6) is arranged between the second optical fiber collimator (5) and the hollow core optical fiber (7); and a second lens (8) is arranged between the hollow core optical fiber (7) and the CCD camera (9); The first optical fiber collimator (3) and the second optical fiber collimator (5) are symmetrically arranged; the first lens (6) and the second lens (8) are symmetrically arranged.

2. The achromatic laser mode coupling system according to claim 1, It is characterized in that The first optical fiber collimator (3) and the second optical fiber collimator (5) are both achromatic optical fiber collimators; The first lens (6) and the second lens (8) are both achromatic lenses.

3. The achromatic laser mode coupling system according to claim 2, It is characterized in that The photonic crystal optical fiber (4) comprises an optical fiber end face one (4-1); The first optical fiber collimator (3) is used to focus the guide laser and the resonance laser onto the optical fiber end face one (4-1) to perform the first light coupling; the guide laser and the resonance laser are both collimated light; The second optical fiber collimator (5) is used to collimate the guide laser and the resonance laser transmitted in the photonic crystal optical fiber (4) for the first time and then output them, so as to obtain the guide laser and the resonance laser after the first collimation.

4. The achromatic laser mode coupling system according to claim 3, It is characterized in that The first lens (6) is used to focus the guide laser and the resonant laser after the first collimation onto the end face of the hollow core optical fiber (7) to perform a second optical fiber coupling; The second lens (8) is used to collimate the guide laser and the resonance laser of the second fiber coupling in the hollow core optical fiber (7) for the second time, so that the output modes of the guide laser and the resonance laser are both fundamental mode, thereby obtaining fundamental mode guide laser and fundamental mode resonance laser; The light intensity distributions of the fundamental mode guided laser and the fundamental mode resonant laser include concentric Gaussian distributions.

5. The achromatic laser mode coupling system according to claim 2, It is characterized in that The first lens (6) comprises a surface with a small curvature radius (6-1) and a surface with a large curvature radius (6-2); The small curvature radius surface 1 (6-1) faces the second optical fiber collimator; the large curvature radius surface 1 (6-2) faces the hollow core optical fiber; The second lens (8) comprises a second surface with a small curvature radius (8-1) and a second surface with a large curvature radius (8-2); the second surface with a small curvature radius (8-1) faces the CCD camera (9); and the second surface with a large curvature radius (8-2) faces the hollow core optical fiber (7).

6. The achromatic laser mode coupling system according to claim 2, It is characterized in that A third lens (10) and a fourth lens (11) are respectively arranged in the first optical fiber collimator (3) and the second optical fiber collimator (5); The third lens and the fourth lens are both achromatic lenses.

7. The achromatic laser mode coupling system according to claim 6, It is characterized in that The third lens (10) comprises a small curvature radius surface three (10-1) and a large curvature radius surface three (10-2); the small curvature radius surface three (10-1) faces the side where the guide laser and the resonance laser are input; and the large curvature radius surface three (10-2) faces the side of the photonic crystal optical fiber (4).

8. The achromatic laser mode coupling system according to claim 6, It is characterized in that The fourth lens (11) comprises a surface four (11-1) with a small curvature radius and a surface four (11-2) with a large curvature radius; The small curvature radius surface four (11-1) faces one side of the first lens (6); and the large curvature radius surface four (11-2) faces one side of the photonic crystal optical fiber (4).

9. The achromatic laser mode coupling system according to claim 2, It is characterized in that The hollow core optical fiber (7) is a bare fiber; the packaging method of the photonic crystal optical fiber (4) is jumper packaging.

10. The achromatic laser mode coupling system according to claim 3, It is characterized in that The expression of the spot diameter of the guide laser and the resonant laser after the first collimation is: Among them, D 1 is the spot diameter of the guide laser after the first collimation, f 1 is the focal length of the first lens on the guide laser after the first collimation, λ 1 is the laser wavelength of the guided laser; MFD is the mode field diameter of the photonic crystal fiber, D 2 is the spot diameter of the resonant laser after the first collimation, f 2 is the focal length of the first lens for the resonant laser after the first collimation, λ 2 is the laser wavelength of the resonant laser.

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