Cladding light filter based on chirp inclined grating and preparation method thereof

Abstract

The invention discloses a chirp inclined grating-based cladding light filter and a preparation method thereof, and relates to the technical field of cladding light filtering, the cladding light filter comprises an optical fiber core, an optical fiber cladding, a cladding light filtering layer, and a chirp inclined grating inscribed at the rear half section of the optical fiber cladding covered by the cladding light filtering layer; pumping cladding light of different wavebands is backwards coupled and reflected to the air, the pumping cladding light is filtered out, meanwhile, low-numerical-aperture pumping cladding light of all wavebands is reflected to become high-numerical-aperture pumping cladding light, the high-numerical-aperture pumping cladding light is reflected and scattered on the surface of the cladding light filtering layer for multiple times, and the high-numerical-aperture pumping cladding light is obtained. And filtering of low numerical aperture pumping cladding light is realized. According to the scheme, through the chirp inclined grating, the low NA pumping cladding light is converted into the reversely transmitted high NA pumping cladding light, so that the high NA and the low NA of the pumping cladding light can be easily filtered, and the stripping ratio of the cladding light filter is improved.

Description

Technical Field

[0001] The present application relates to the technical field of cladding light filtering, and in particular to a cladding light filter based on a chirped tilted grating and a preparation method thereof. Background Art

[0002] The cladding light filter (CLS) is one of the key components of high-power fiber lasers and amplifiers. It is responsible for removing excess cladding light in the system, thereby optimizing the beam quality and ensuring the stable operation of the laser system at high power output. At present, there are many methods for preparing cladding light filters, among which chemical etching is the mainstream technology, in addition to which there are also high refractive index glue method, soft metal method, laser micromachining method, etc. Although these methods are different in implementation, their working principles are the same: by treating the surface of the optical fiber cladding, the total reflection condition of the cladding light is destroyed, and the light is scattered into the air through the cladding interface, thereby filtering the cladding light. Based on this principle, the cladding light with a high numerical aperture (NA) has a large divergence angle θ, so that the light can intersect with the device cladding surface many times, experience reflection, refraction, and scattering, so it is easier to filter out; while the cladding light with a low numerical aperture (NA) has a small divergence angle θ, and the number of intersections between the light and the rough surface of the device cladding is reduced, or even may not intersect at all, resulting in difficulty in filtering. For example Figure 1 As shown in the figure, the light with a divergence angle of θ1 represents high-NA cladding light, while the light with a divergence angle of θ2 represents low-NA cladding light. Therefore, the existing cladding light filters generally have a good filtering effect on high-NA cladding light, but a poor filtering effect on low-NA cladding light, which results in a large amount of low-NA cladding light being difficult to be effectively stripped in high-power laser systems. In particular, for fiber lasers with the same band pumping structure, the part of the pump light coupled into the fiber cladding has a lower NA distribution, and the unabsorbed light part will be more difficult to filter out. If the stripping efficiency of the cladding light filter for low-NA cladding light is not improved, the beam quality and stability of the high-power fiber laser will not be guaranteed.

[0003] In the prior art, in order to enhance the filtering ability of the cladding light filter for low NA cladding light, a chemical etching method is usually used. First, hydrofluoric acid is used to reduce the remaining diameter of the cladding, and then glass etching paste is used to increase the surface roughness to obtain the following Figure 2 The cladding light filter structure shown in the figure increases the number of reflections and scattering rate of low NA cladding light on a unit length of optical fiber. Through this longer distance corrosion, the filtering efficiency of low numerical aperture cladding light and the stripping ratio of the device can be improved. However, this method requires the use of hydrofluoric acid to shrink the optical fiber, which is complicated and dangerous. In addition, in order to effectively improve the stripping ratio, the corrosion shrinkage of the optical fiber usually needs to reach more than 100 microns. This large-scale fiber shrinkage and long-distance chemical corrosion will significantly reduce the mechanical properties of the optical fiber, thereby reducing the reliability of the device.

[0004] Therefore, developing a cladding light filter with high stripping ratio, excellent low numerical aperture cladding light filtering efficiency, high power resistance, excellent mechanical properties and strong reliability is crucial to ensure the beam quality and stability of high-power laser systems. Summary of the invention

[0005] The purpose of the present application is to provide a cladding light filter based on a chirped tilted grating and a preparation method thereof, which can improve the stripping ratio of the cladding light filter.

[0006] To achieve the above objectives, this application provides the following solutions: In a first aspect, the present application provides a cladding light filter based on a chirped tilted grating, comprising: an optical fiber core, an optical fiber cladding, and a cladding light filtering layer obtained after a total reflection condition destruction treatment is performed on the surface of the optical fiber cladding; a chirped tilted grating is engraved on the rear half of the optical fiber cladding covered by the cladding light filtering layer; the chirped tilted grating comprises a plurality of tilted gratings with a continuously changing period; the tilted grating is used to back-couple and reflect the pump cladding light of the corresponding band into the air to filter the pump cladding light; the tilted grating is also used to reflect the low numerical aperture pump cladding light of the corresponding band, so that the low numerical aperture pump cladding light is converted into high numerical aperture pump cladding light, and the high numerical aperture pump cladding light is reflected and scattered multiple times on the surface of the cladding light filtering layer to filter the low numerical aperture pump cladding light.

[0007] Optionally, the chirped tilted grating may completely cover the optical fiber cladding or partially cover the optical fiber cladding.

[0008] Optionally, the chirped tilted grating may cover the optical fiber core or may not cover the optical fiber core.

[0009] Optionally, the relationship between the tilted grating and the wavelength band of the low numerical aperture pump cladding light that can be reflected is as shown in the following formula: .

[0010] in, λ is the wavelength band of the low numerical aperture pump cladding light that the tilted grating can reflect, n clad is the effective refractive index of the optical fiber cladding, n air is the refractive index of air, Λ is the central period of the tilted grating, θ is the tilt angle of the tilted grating.

[0011] Optionally, the cladding light filtering layer is a structure obtained by destroying the total reflection condition of the optical fiber cladding surface by chemical etching, high refractory resin method, laser micromachining method or soft metal method.

[0012] Optionally, the cladding light filtering layer includes a plurality of etched rough sections; the roughness of the plurality of etched rough sections increases from front to back.

[0013] In a second aspect, the present application provides a method for preparing a cladding light filter based on a chirped tilted grating, comprising the following steps: A chirped tilted grating is engraved in an optical fiber by combining a femtosecond laser with a phase mask. The optical fiber includes an optical fiber cladding and an optical fiber core; the chirped tilted grating includes a plurality of tilted gratings with a continuously changing period; the tilted grating is used to reflect the pump cladding light of the corresponding band back-coupled into the air to filter out the pump cladding light; the tilted grating is also used to reflect the low numerical aperture pump cladding light of the corresponding band, so that the low numerical aperture pump cladding light is converted into a high numerical aperture pump cladding light.

[0014] The optical fiber with the chirped tilted grating is processed by chemical etching to obtain a cladding light filtering layer. The cladding light filtering layer is used to multiple reflect and scatter the high numerical aperture pump cladding light reflected to the cladding light filtering layer on the surface of the cladding light filtering layer, thereby filtering the high numerical aperture pump cladding light.

[0015] The optical fiber with the cladding light filtering layer is packaged to obtain a cladding light filter.

[0016] Optionally, a femtosecond laser combined with a phase mask method is used to write a chirped tilted grating in an optical fiber, specifically comprising: when writing the chirped tilted grating, the femtosecond laser and the cylindrical lens remain stationary, the phase mask is controlled to move slowly along the x-axis to increase the grating bandwidth, and the optical fiber is controlled to periodically move obliquely in the xy plane to obtain a chirped tilted grating written in the optical fiber.

[0017] Optionally, a chemical etching method is used to process the optical fiber engraved with a chirped tilted grating to obtain a cladding light filtering layer, specifically comprising: applying glass etching paste on the surface of the optical fiber cladding of the optical fiber engraved with the chirped tilted grating, and performing a total reflection condition destruction treatment on the surface of the optical fiber cladding to obtain a cladding light filtering layer; the chirped tilted grating is located in the rear half of the optical fiber cladding covered by the cladding light filtering layer.

[0018] Optionally, the chirped tilted grating can be written in the optical fiber by using the face-by-face method in the femtosecond laser direct writing method.

[0019] According to the specific embodiments provided in this application, this application discloses the following technical effects: The present application provides a cladding light filter based on a chirped tilted grating and a preparation method thereof. The cladding light filter comprises: an optical fiber core, an optical fiber cladding, and a cladding light filtering layer obtained by performing a total reflection condition destruction treatment on the surface of the optical fiber cladding; a chirped tilted grating is engraved on the rear half of the optical fiber cladding covered by the cladding light filtering layer; pump cladding light of different bands is back-coupled and reflected into the air to filter the pump cladding light, and low numerical aperture pump cladding light of each band is reflected to become high numerical aperture pump cladding light, which is reflected and scattered multiple times on the surface of the cladding light filtering layer to filter the low numerical aperture pump cladding light. The above scheme of the present application places a chirped tilted grating with refractive index modulation extending from the core to the cladding in the second half of the cladding light filter, so that the low-NA pump cladding light is converted into the reverse-propagating high-NA pump cladding light, so that the pump cladding light can be easily filtered out regardless of its high or low NA, thereby improving the stripping ratio of the cladding light filter. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0021] Figure 1 It is a schematic diagram of the principle of a common cladding light filter in the prior art.

[0022] Figure 2 It is a schematic diagram of the principle of a cladding light filter optimized for low-NA cladding light in the prior art.

[0023] Figure 3 A schematic structural diagram of a cladding light filter based on a chirped tilted grating provided in one embodiment of the present application.

[0024] Figure 4 A schematic diagram of the principle of a cladding light filter based on a chirped tilted grating provided in an embodiment of the present application for filtering high numerical aperture pump cladding light.

[0025] Figure 5 A schematic diagram of the principle of a cladding light filter based on a chirped tilted grating provided in an embodiment of the present application for filtering low numerical aperture pump cladding light.

[0026] Figure 6 A flow chart of a method for preparing a cladding light filter based on a chirped tilted grating provided in one embodiment of the present application.

[0027] Figure 7A method for preparing a cladding light filter based on a chirped tilted grating provided in an embodiment of the present application is a schematic diagram of writing a chirped tilted grating in an optical fiber.

[0028] Figure 8 A top view of the moving trajectory of a femtosecond laser on the surface of an optical fiber when a chirped tilted grating is written in a method for preparing a cladding light filter based on a chirped tilted grating provided in an embodiment of the present application.

[0029] Fig. 9 A schematic diagram of a method for preparing a cladding light filter based on a chirped tilted grating provided in one embodiment of the present application, in which a plurality of chirped tilted gratings symmetrically distributed about the optical fiber core are inscribed using an optical fiber rotation axis inscription method.

[0030] Reference numerals: 1-fiber core; 2-fiber cladding; 3-cladding light filtering layer; 4-chirped tilted grating. DETAILED DESCRIPTION

[0031] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0032] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0033] In an exemplary embodiment, Figure 3 As shown, a cladding light filter based on a chirped tilted grating is provided, comprising: an optical fiber core 1, an optical fiber cladding 2, and a cladding light filtering layer 3 obtained by destroying the total reflection condition on the surface of the optical fiber cladding 2; a chirped tilted grating 4 is engraved on the rear half of the optical fiber cladding 2 covered by the cladding light filtering layer 3; the cladding light filtering layer 3 can directly filter out the incident high numerical aperture cladding light, such as Figure 4 On the other hand, the low numerical aperture cladding light reflected back by the chirped tilted grating 4 can be filtered out by scattering, as shown in FIG. Figure 5 shown.

[0034] Specifically, the chirped tilted grating 4 includes a plurality of tilted gratings with continuously changing periods; the tilted gratings are used to couple the pump cladding light of the corresponding wavelength band back into the air, such as Figure 5The medium pump cladding light enters the air on the upper surface of the cladding light filter layer 3, and the pump cladding light is directly filtered out; the tilted grating is also used to reflect the low numerical aperture pump cladding light of the corresponding band, so that the low numerical aperture pump cladding light is converted into high numerical aperture pump cladding light, and the high numerical aperture pump cladding light is reflected and scattered multiple times on the surface of the cladding light filter layer 3, such as Figure 5 The high numerical aperture pump cladding light converted from the low numerical aperture pump cladding light is reflected and scattered multiple times on the upper and lower surfaces of the cladding light filtering layer 3, thereby filtering out the low numerical aperture pump cladding light.

[0035] Specifically in this embodiment, the relationship between the tilted grating and the wavelength band of the low numerical aperture pump cladding light that can be reflected is shown in the following formula: .

[0036] in, λ is the wavelength band of low numerical aperture pump cladding light that can be reflected by the tilted grating. n clad is the effective refractive index of the optical fiber cladding 2, n air is the refractive index of air, Λ is the central period of the tilted grating, θ is the tilt angle of the tilted grating.

[0037] In this embodiment, the chirped tilted grating 4 is located in the second half of the cladding light filter layer 3, most of the high NA pump cladding light has been filtered out, and the remaining small part of the low NA pump cladding light is reflected by the tilted grating, which can not only ensure the high stripping ratio of the cladding light filter, but also improve the power tolerance of the cladding light filter.

[0038] As a variety of options, in the cladding light filter provided in this embodiment, the chirped tilted grating 4 can completely cover the optical fiber cladding 2 or partially cover the optical fiber cladding 2, the difference is that the numerical aperture range of the pump cladding light that can be acted on is different, and when the optical fiber cladding 2 is completely covered, the pump cladding light of all numerical apertures can be reflected by the chirped tilted grating 4. Similarly, in the cladding light filter provided in this embodiment, the chirped tilted grating 4 can cover the optical fiber core 1 or not cover the optical fiber core 1, the difference is that when the chirped tilted grating 4 covers the optical fiber core 1, the preparation method is simpler, and when it does not cover the optical fiber core 1, the signal insertion loss of the device is smaller.

[0039] The cladding light filter layer 3 in the cladding light filter is a structure obtained by destroying the total reflection condition of the optical fiber cladding 2 surface by chemical etching, high refractive index glue method, laser micromachining method or soft metal method. The grating structure can be flexibly selected according to the device performance requirements.

[0040] In an exemplary embodiment, the cladding light filtering layer 3 in the cladding light filter includes several etched rough sections; the roughness of the several etched rough sections increases from front to back, so that each section of the device heats evenly and the withstand power is improved.

[0041] The cladding light filter based on the chirped tilted grating provided in the embodiment of the present application has the characteristics of high stripping ratio, high low numerical aperture light filtering efficiency, high power tolerance, good mechanical properties, and strong reliability.

[0042] Based on the same inventive concept, the present application also provides a method for preparing the cladding light filter based on the chirped tilted grating 4 mentioned above. In an exemplary embodiment, Figure 6 As shown, a method for preparing a cladding light filter based on a chirped tilted grating 4 is provided, comprising the following steps: A1. A chirped tilted grating is engraved in an optical fiber by using a femtosecond laser combined with a phase mask method. The optical fiber includes an optical fiber cladding 2 and an optical fiber core 1; the chirped tilted grating 4 includes a number of tilted gratings with a continuously changing period; the tilted grating is used to couple the corresponding band of pump cladding light back into the air to filter out the pump cladding light; the tilted grating is also used to reflect the low numerical aperture pump cladding light of the corresponding band, so that the low numerical aperture pump cladding light becomes a high numerical aperture pump cladding light.

[0043] A2. The optical fiber with the chirped tilted grating is processed by chemical etching to obtain a cladding light filtering layer. The cladding light filtering layer 3 is used to reflect and scatter the high numerical aperture pump cladding light reflected to the cladding light filtering layer 3 multiple times on the surface of the cladding light filtering layer 3, thereby filtering the high numerical aperture pump cladding light.

[0044] A3. Encapsulate the optical fiber with the cladding light filtering layer to obtain a cladding light filter.

[0045] The traditional ultraviolet grating method cannot write gratings in the cladding, so it is not applicable. The existing femtosecond grating method for writing tilted gratings generally uses a rotating mask to obtain the tilt angle. This method has extremely low writing efficiency and low grating reflectivity, which cannot meet the device performance requirements.

[0046] Specifically in this embodiment, step A1 is as follows: when writing the chirped tilted grating 4, Figure 7 As shown, the femtosecond laser and the cylindrical lens remain stationary, the phase mask is controlled to move slowly along the x-axis to increase the grating bandwidth, and the optical fiber is controlled to periodically move obliquely in the xy plane to write a chirped tilted grating 4 in the optical fiber.

[0047] The optical fiber periodically performs oblique motion in the xy plane. Specifically, the optical fiber needs to be scanned in the x and y directions at the same time: the movement of the optical fiber on the x and y axes is combined into an oblique motion. At this time, the femtosecond laser emits light, and the writing forms a tilt angle of θ The grating, , generally the tilt angle is 3-8°; after completing one oblique movement, the laser is turned off, the fiber returns to the initial position in the y-axis direction, and moves forward a certain distance in the x-axis direction (generally less than the effective spot size), and then the laser emits light to start the next cycle of scanning. The scanning motion trajectory of the laser on the optical fiber is shown in the figure below. Figure 8 As shown, this can form a grating tilt angle, increasing the grating length and grating transmission depth.

[0048] In the writing process of the chirped tilted grating 4, the period of the phase mask should be adapted to the wavelength of the pump cladding light and isolated from the laser signal wavelength. In order to make the chirped tilted grating 4 cover the fiber core 1 and the fiber cladding 2, the scanning range in the y-axis direction should be larger than the size of the fiber core 1, and the focal depth of the femtosecond laser should be It should be larger than the fiber core 1. Here ,in, is the wavelength of the femtosecond laser, f is the focal length of the cylindrical lens, is the femtosecond laser spot size, is the effective refractive index of the cladding.

[0049] As another possible embodiment, the chirped tilted grating 4 can also be written using the face-by-face method in the femtosecond laser direct writing method. It should also be noted that during the face-by-face scanning process, the scanning range should be appropriately expanded to ensure that the grating modulation area covers the optical fiber cladding 2.

[0050] When the chirped tilted grating 4 is only written on the optical fiber cladding 2, the optical fiber rotation axis writing method can also be used in step A1 to write multiple cladding grating structures symmetrically distributed about the fiber core, such as Fig. 9 shown.

[0051] In this embodiment, step A2 is specifically as follows: applying glass etching paste on the surface of the optical fiber cladding 2 of the optical fiber engraved with the chirped tilted grating 4, and performing a total reflection condition destruction treatment on the surface of the optical fiber cladding 2 to obtain a cladding light filtering layer 3; the chirped tilted grating 4 is located in the rear half of the optical fiber cladding 2 covered by the cladding light filtering layer 3. The cladding light filtering layer 3 generally adopts a one-stage etching, and the etching length is 10-15cm, wherein the chirped tilted grating 4 is located 2-4cm in the rear section of the etching area. Of course, a segmented etching method can also be used, with the roughness increasing from the front to the back, so that each section of the device heats evenly and the tolerable power is improved.

[0052] When performing total reflection condition destruction treatment, as a technical means to replace the chemical etching method, the high-refractive resin method, laser micromachining method or soft metal method can also be used to process the optical fiber engraved with the chirped tilted grating 4 to obtain the cladding light filtering layer 3.

[0053] The cladding light filter layer 3 can be used alone as a cladding light filter (traditional method), and combined with the chirped tilted grating written in the present application, a higher stripping ratio of the device can be achieved. Similarly, the chirped tilted grating 4 can also be used alone as a cladding light filter. In this case, the chirped tilted grating 4 should cover the entire optical fiber cladding 2, and is generally selected when the filtering power is not high.

[0054] The cladding light filter based on chirped tilted grating and the preparation method thereof provided in the above embodiments of the present application proposes an innovative and novel device solution to solve the problem of difficulty in filtering low NA cladding light. The device can achieve high-power, high-stripping ratio, and high-reliability cladding light filtering. The key technologies are mainly the following: 1) Low-NA light filtering structure of cladding light filter. This application innovatively uses a chirped tilted grating engraved in the optical fiber cladding, which can directly couple the cladding mode back into the air for filtering, and on the other hand, can reflect the low-NA cladding light back into high-NA cladding light, and then use the cladding light filtering layer to achieve more thorough scattering filtering, thus achieving efficient low-NA cladding light filtering performance of the cladding light filter.

[0055] 2) The cladding light filter has a high stripping ratio, high power filtering and high reliability structure. The cladding light filter described in this application adopts a high stripping ratio and high power cladding light filtering structure that combines a cladding light filtering layer and a chirped tilted grating. The two main parts complement each other and achieve the effect of 1+1>2. Specifically, the general traditional cladding light stripper must increase the device stripping ratio by increasing the device length so that more low NA light has the opportunity to interact with the device cladding surface, and the improvement effect is limited. The cladding light stripper described in the present application can effectively filter out low-NA light without increasing the length, and effectively improve the stripping ratio per unit length of the device; the cladding light filtering layer can be located in the entire device section, directly filtering out the incident high-NA cladding light, and at the same time filtering out the low-NA cladding light reflected by the tilted grating, with good temperature characteristics, and can achieve the high filtering power of the traditional cladding light filter; the chirped tilted grating structure is located in the second half of the cladding light filtering layer, and mainly processes the low-NA cladding light that has not passed through the cladding light filtering layer, and reflects it into high-NA light so that it is filtered by the cladding light filtering layer. The chirped tilted grating itself bears a low cladding power, and there is no need to consider the problem of additional heating. The combination of the two greatly improves the stripping ratio, that is, the filtering efficiency, while maintaining a high filtering power. At the same time, this solution does not require the use of large-scale diameter reduction using hydrofluoric acid, which greatly improves the mechanical properties and reliability of the device.

[0056] 3) Preparation method of femtosecond laser tilted grating for cladding light filter. The most critical preparation method of the cladding light filter described in this application is the preparation of the chirped tilted grating. Since the tilted grating must be engraved in the optical fiber cladding, the traditional ultraviolet grating writing method is not applicable, and a femtosecond grating method must be used. At the same time, the femtosecond writing of the chirped tilted grating is also difficult. The oblique scanning of the optical fiber in this application, combined with the longitudinal scanning of the laser, is innovative and novel. It solves the problems of low writing efficiency and low reflectivity caused by the traditional femtosecond mask method of writing tilted gratings using a rotating mask. In addition, the preparation of this structure can be achieved by pre-setting the preparation and writing program in a commercial femtosecond grating system to realize automated writing operations, and the process is simple, convenient and safe.

[0057] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0058] This article uses specific examples to illustrate the principles and implementation methods of this application. The description of the above embodiments is only used to help understand the method and core ideas of this application. At the same time, for those skilled in the art, according to the ideas of this application, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting this application.

Claims

1. A cladding light filter based on a chirped tilted grating, comprising: An optical fiber core, an optical fiber cladding, and a cladding light filtering layer obtained by destroying the total reflection condition on the surface of the optical fiber cladding; characterized in that a chirped tilted grating is engraved on the rear half of the optical fiber cladding covered by the cladding light filtering layer; the chirped tilted grating includes a plurality of tilted gratings with a continuously changing period; the tilted grating is used to backward couple and reflect the pump cladding light of the corresponding band into the air to filter the pump cladding light; the tilted grating is also used to reflect the low numerical aperture pump cladding light of the corresponding band to convert the low numerical aperture pump cladding light into high numerical aperture pump cladding light, and the high numerical aperture pump cladding light is reflected and scattered multiple times on the surface of the cladding light filtering layer to filter the low numerical aperture pump cladding light.

2. The cladding light filter based on chirped tilted grating according to claim 1, characterized in that: The chirped tilted grating completely covers the optical fiber cladding or partially covers the optical fiber cladding.

3. The cladding light filter based on chirped tilted grating according to claim 1, characterized in that: The chirped tilted grating covers the optical fiber core or does not cover the optical fiber core.

4. The cladding light filter based on chirped tilted grating according to claim 1, characterized in that: The relationship between the tilted grating and the wavelength band of the low numerical aperture pump cladding light that can be reflected is shown in the following formula: ； in, λ is the wavelength band of the low numerical aperture pump cladding light that the tilted grating can reflect, n clad is the effective refractive index of the optical fiber cladding, n air is the refractive index of air, Λ is the central period of the tilted grating, θ is the tilt angle of the tilted grating.

5. The cladding light filter based on chirped tilted grating according to claim 1, characterized in that: The cladding light filtering layer is a structure obtained by destroying the total reflection condition of the optical fiber cladding surface by chemical etching, high-refractive resin method, laser micromachining method or soft metal method.

6. The cladding light filter based on chirped tilted grating according to claim 1, characterized in that: The cladding light filtering layer includes a plurality of etched rough sections; the roughness of the plurality of etched rough sections increases from front to back.

7. A method for preparing a cladding light filter based on a chirped tilted grating, characterized in that: include: A chirped tilted grating is engraved in an optical fiber by using a femtosecond laser combined with a phase mask method; the optical fiber comprises an optical fiber cladding and an optical fiber core; the chirped tilted grating comprises a plurality of tilted gratings with a continuously changing period; the tilted grating is used to couple the pump cladding light of the corresponding band back into the air to filter out the pump cladding light; the tilted grating is also used to reflect the low numerical aperture pump cladding light of the corresponding band to convert the low numerical aperture pump cladding light into a high numerical aperture pump cladding light; The optical fiber with the chirped tilted grating is processed by chemical etching to obtain a cladding light filtering layer; the cladding light filtering layer is used to reflect and scatter the high numerical aperture pump cladding light reflected to the cladding light filtering layer on the surface of the cladding light filtering layer for multiple times, thereby filtering the high numerical aperture pump cladding light; The optical fiber with the cladding light filtering layer is packaged to obtain a cladding light filter.

8. The method for preparing a cladding light filter based on a chirped tilted grating according to claim 7, characterized in that: A chirped tilted grating is engraved in an optical fiber by using a femtosecond laser combined with a phase mask method, specifically comprising: when the chirped tilted grating is engraved, the femtosecond laser and the cylindrical lens remain stationary, the phase mask is controlled to move slowly along the x-axis to increase the grating bandwidth, and the optical fiber is controlled to periodically move obliquely in the xy plane to obtain a chirped tilted grating engraved in the optical fiber.

9. The method for preparing a cladding light filter based on a chirped tilted grating according to claim 7, characterized in that: The optical fiber engraved with the chirped tilted grating is processed by a chemical etching method to obtain a cladding light filtering layer, specifically comprising: applying glass etching paste on the surface of the optical fiber cladding of the optical fiber engraved with the chirped tilted grating, and performing a total reflection condition destruction treatment on the surface of the optical fiber cladding to obtain a cladding light filtering layer; the chirped tilted grating is located in the rear half of the optical fiber cladding covered by the cladding light filtering layer.

10. The method for preparing a cladding light filter based on a chirped tilted grating according to claim 7, characterized in that: The chirped tilted grating can also be written in the optical fiber using the face-by-face method in the femtosecond laser direct writing method.

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