Preparation method of dispersion fiber bragg grating with high spectral flatness

Abstract

The invention discloses a preparation method of a dispersion fiber bragg grating with high spectral flatness, and provides the preparation method of the dispersion fiber bragg grating with the high spectral flatness aiming at strong side lobes on two sides of a resonance peak caused by sudden change of refractive indexes at two ends in the writing process of the dispersion fiber bragg grating and short wave loss generated in the use process. The ultraviolet laser is expanded and shaped to achieve the purpose of homogenizing Gaussian light spots, and finally an ideal light field is obtained, so that the degree of refractive index mutation at the two ends of the dispersion fiber grating is reduced; the scanning speed of a stepping motor is precisely controlled to change the exposure time of each position of the dispersion fiber bragg grating, so that the refractive index modulation depths of different positions of the fiber bragg grating are controlled to eliminate the sudden change of refractive indexes at the two ends and compensate the problem that the reflectivity at the short wavelength is lower than the reflectivity at the long wavelength due to the short-wave loss phenomenon; and finally, the spectrum flatness of the dispersion fiber grating is greatly improved. Compared with the prior art, the method provided by the invention has the advantages of low cost, high flexibility, high stability and the like.

Description

Technical Field

[0001] The invention belongs to the field of ultrafast fiber lasers, and in particular relates to a method for preparing a dispersion fiber grating with high spectral flatness. Background Art

[0002] Dispersive fiber Bragg gratings must have a high degree of reflection spectrum flatness so that the laser light at each wavelength of the ultrafast laser seed source can be evenly reflected to the amplifier stage in the CPA system. However, in the preparation process of dispersive fiber Bragg gratings, there are two factors that will degrade its spectrum flatness. First, when writing dispersive fiber Bragg gratings, the refractive index at both ends of the fiber Bragg grating will change suddenly, which will lead to strong side lobes on both sides of its resonance peak, which will cause some non-working laser light to enter the amplifier stage and deteriorate the output pulse quality; second, because the light of the dispersive fiber Bragg grating enters from the long wavelength direction during use, the short-wave loss generated will cause the reflectivity of the grating at short wavelengths to decrease, which will lead to the degradation of the overall flatness of the reflection spectrum.

[0003] At present, there is basically no solution to the deterioration of spectral flatness caused by the combined effects of the side lobes on both sides of the dispersive fiber Bragg grating resonance peak and the short-wave loss. Therefore, a method that is easy to use and has a stable working state is needed to ensure the flatness of the reflection spectrum of the dispersive fiber Bragg grating. Summary of the invention

[0004] The purpose of the present invention is to provide a method for preparing a dispersion fiber grating with high spectral flatness, so as to improve the flatness of the reflection spectrum of the dispersion fiber grating. The method has the advantages of easy grating production, high stability and high multiplexing.

[0005] The technical solution to achieve the purpose of the present invention is: a method for preparing a dispersion fiber grating with high spectral flatness, the steps are as follows:

[0006] Step 1: Use the uniform ultraviolet light field control technology to expand and shape the ultraviolet laser, disperse its originally dense Gaussian energy to achieve the purpose of uniform Gaussian spot, and finally obtain the ideal writing light field; thereby reducing the refractive index mutation degree at both ends of the fiber grating during the ultraviolet writing scanning process and improving the side mode suppression ratio of the dispersive fiber grating.

[0007] Step 2: Use high side mode suppression ratio toe cutting technology to control the scanning speed V of the stepper motor a The exposure time of each position of the fiber grating is changed to control the refractive index modulation depth of different positions of the fiber grating to eliminate the refractive index mutation at both ends, and to obtain an apodized dispersion fiber grating with a side mode suppression ratio greater than or equal to 20dB.

[0008] Step 3: Use progressive scanning writing technology to superimpose the progressive scanning speed V b, so that the scanning speed of the ultraviolet laser at the short wavelength position is V b1 , so that the scanning speed of the ultraviolet laser at the long wavelength position is V b2 , V b1 ≤V b2 , in order to increase the reflectivity of the dispersive fiber Bragg grating at the short wavelength position and reduce the reflectivity of the dispersive fiber Bragg grating at the long wavelength, thereby compensating for the problem that the reflectivity of the dispersive fiber Bragg grating at the short wavelength is lower than that at the long wavelength due to the short-wave loss phenomenon. Finally, the short-wave loss phenomenon is effectively suppressed, and a dispersive fiber Bragg grating with a spectral flatness less than or equal to 2% and a side mode suppression ratio greater than or equal to 20dB is obtained.

[0009] Compared with the prior art, the present invention has the following significant advantages:

[0010] (1) Taking into account the deterioration of spectral flatness caused by the combined effects of the side lobes and short-wave losses on both sides of the dispersive fiber Bragg grating resonance peak, the three key technologies of homogenized ultraviolet light field control, high side mode suppression ratio toe-cutting and progressive scanning writing are integrated to finally realize the preparation of dispersive fiber Bragg gratings with high spectral flatness and high side mode suppression ratio.

[0011] (2) Based on the existing UV mask writing technology, the beam is optimized, and the UV laser is expanded and homogenized and directly applied to the preparation of dispersive fiber gratings. The beam homogenization and shaping make the writing system have the advantages of low complexity and high flexibility.

[0012] (3) For the two major technologies of progressive grating scanning and high side mode suppression ratio toe-off, it is only necessary to establish the intrinsic mapping relationship between the refractive index exposure time (i.e., the stepper motor driven scanning function) and the reflectivity of the dispersed fiber grating in the writing process, so as to modulate the writing process. This has the advantages of being easy to use, stable in working state, and capable of real-time modulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 An optical path diagram is designed for the light beam homogenization module of the homogenized ultraviolet light field control technology described in the present invention.

[0014] Figure 2 Schematic diagram of the scanning writing system and real-time monitoring system of the UV lithography phase mask.

[0015] Figure 3 Schematic diagram of the Gaussian apodization function distribution of the exposure time at each position of the dispersive fiber grating and the Gaussian apodization function distribution of the scanning speed, wherein (a) is a schematic diagram of the Gaussian apodization function distribution of the exposure time, and (b) is a schematic diagram of the Gaussian apodization function distribution of the scanning speed.

[0016] Figure 4Schematic diagram of the Gaussian apodization function distribution of the exposure time at each position of the dispersion fiber Bragg grating when compensating for shortwave loss and the Gaussian apodization function distribution of the scanning speed, wherein (a) is a schematic diagram of the Gaussian apodization function distribution of the exposure time when compensating for shortwave loss, and (b) is a schematic diagram of the Gaussian apodization function distribution of the scanning speed when compensating for shortwave loss.

[0017] Figure 5 Schematic diagram of the reflection spectrum of the high spectral flatness dispersion fiber Bragg grating prepared by the present invention.

[0018] Figure 6 The present invention is a flow chart of a method for preparing a dispersion fiber grating with high spectral flatness. Specific implementation plan

[0019] The present invention is further described below in conjunction with the accompanying drawings and specific implementation methods. It should be understood that the specific implementation case methods described herein are only used to illustrate and explain the case of the present invention, and are not used to limit the embodiments of the present invention.

[0020] Combination Figure 6 , a method for preparing a dispersion fiber grating with high spectral flatness, the steps are as follows:

[0021] Step 1: Design and build the UV beam homogenization module 2-4, and use the homogenized UV light field control technology to expand and shape the UV laser, so as to disperse the energy originally densely distributed in the form of Gaussian light, so as to achieve the purpose of homogenizing the Gaussian light spot, and finally obtain the ideal writing light field; thereby reducing the refractive index mutation degree at both ends of the fiber grating during the UV writing scanning process, and improving the side mode suppression ratio of the dispersive fiber grating.

[0022] Combination Figure 1, the beam homogenization module 2-4 consists of three parts: a beam shaping structure, an optical path collimation structure, and a spot compression structure, and the coaxial axis is arranged in sequence. The beam shaping structure is composed of a hemispherical lens 1-1 and an aperture 1-2, wherein the hemispherical lens 1-1 has a diameter of 5mm and a convex radius of 2.5mm. Since the beam diameter of the writing ultraviolet laser is at the micron level, it is necessary to change the numerical aperture of the beam to achieve beam expansion, and disperse the energy originally densely distributed in the form of Gauss. And since the numerical aperture of the beam is proportional to the effective diameter of the lens and inversely proportional to the distance of the focus, if the numerical aperture of the laser beam is to be expanded, it is necessary to select a lens that is similar to the laser focal spot diameter and meets the minimum focal length. After the writing laser passes through the hemispherical lens 1-1, the laser energy distribution remains unchanged and is still a Gaussian distribution. However, since the hemispherical lens 1-1 expands the numerical aperture of the beam, the laser quickly diverges from one point after focusing, and the divergent spot is large, so it is convenient to shape the beam. This system adopts the simplest aperture method for beam shaping. By controlling the aperture size of the aperture 1-2, the middle part of the Gaussian beam is intercepted, and then a beam shaping effect close to a flat top can be achieved; in order to simplify the system, the plano-convex lens 1-3 constitutes an optical path collimation structure. The parameters are: diameter 25.4mm, radius of curvature 92mm, focal length 200.7mm, and center thickness 2.9mm. The plano-convex lens 1-3 is used to expand and collimate the shaped writing laser; in order to simplify the system, the cylindrical mirror 1-4 constitutes a spot compression structure. The parameters are: diameter 25.4mm, radius of curvature 114.6mm, focal length 238mm, and center thickness 4mm. The cylindrical mirror 1-4 is used to compress the collimated light obtained after beam expansion longitudinally to form a linear spot with uniform energy, which is directly used for grating writing.

[0023] Step 2: Use high side mode suppression ratio toe-cut technology to control the scanning speed V of the stepper motor a The exposure time of each position of the fiber grating is changed to control the refractive index modulation depth of different positions of the fiber grating to eliminate the refractive index mutation at both ends, and to obtain an apodized dispersion fiber grating with a side mode suppression ratio greater than or equal to 20dB.

[0024] Combination Figure 2First, a high spectral flatness dispersion fiber Bragg grating ultraviolet writing system and real-time monitoring system are built. The ultraviolet writing system consists of an ultraviolet laser 2-1, a stepper motor 2-2, an ultraviolet reflector 2-3, a beam homogenization module 2-4, a phase mask 2-5, an optical fiber bracket 2-6, and a six-dimensional adjustment frame 2-7. When writing the dispersion fiber Bragg grating, the ultraviolet laser passes through the ultraviolet reflector 2-3 and is folded 90° through the beam homogenization module 2-4 to homogenize the light field and compress the energy longitudinally to increase the power density, and then passes through the phase mask 2-5 to expose the optical fiber. During the writing process, the stepper motor 2-2 drives the ultraviolet reflector 2-3 and the beam homogenization module 2-4 to move along the grating area of ​​the phase mask 2-5 at the same time, realizing the scanning of the grating area of ​​the phase mask 2-5, thereby achieving the purpose of dispersion fiber Bragg grating writing; the real-time monitoring system mainly includes a broadband light source 2-8, a circulator 2-9, a spectrometer 2-10, and a high-reflection Bragg fiber Bragg grating 2-11. Among them, the fiber type of the high-reflection Bragg fiber grating 2-11 is PM980, the reflectivity is 99.9%, and the center wavelength is 1070nm. During the measurement process, port 1 of the circulator 2-9 is connected to the broadband light source 2-8, port 2 is connected to the high-reflection Bragg fiber grating 2-11, and the long-period end of the dispersion fiber grating is fused with the high-reflection Bragg fiber grating 2-11. When port 3 is connected to the spectrometer 2-10, the reflection spectrum intensity P1 of the dispersion fiber grating with a center wavelength near 1030nm and the reflection spectrum intensity P2 of the high-reflection Bragg fiber grating 2-11 with a center wavelength of 1070nm can be measured simultaneously. Then the reflectivity R of the dispersion fiber grating satisfies the following formula:

[0025]

[0026] Combination Figure 3 , based on the high spectral flatness dispersion fiber Bragg grating ultraviolet writing system and real-time monitoring system to prepare the side mode suppression ratio ≥ 20dB toe-cut dispersion fiber Bragg grating. The fiber type is PM980, the scanning length is 100mm, the writing laser power is 30mW, and the phase mask template chirp rate is 3nm / cm. The high side mode suppression ratio toe-cut technology is used to adjust the scanning speed V of the stepper motor 2-2 during the writing process. a To change the exposure time of each position of the dispersion fiber grating, the Gaussian apodization effect is achieved. The Gaussian apodization function of the exposure time with respect to the position is:

[0027]

[0028] Where L is the grating length, set to 100mm; k Gaussian is the apodization coefficient, set to 0.7; T is the exposure time before apodization; z is the real-time position of exposure. Figure 3 (a) is a schematic diagram of the Gaussian apodization function distribution of the exposure time at each position of the dispersion fiber grating. Figure 3(b) is a schematic diagram of the Gaussian apodization function distribution of the scanning speed. The scanning speed is inversely proportional to the exposure time. Considering the movement accuracy of the stepper motor 2-2, the entire speed change process is divided into 50 sections; at the grating position corresponding to the Gaussian apodization function of 1, the stepper motor 2-2 is at the minimum scanning speed. At this time, the scanning speed of the stepper motor 2-2 is set to 0.009 mm / s, and the fiber Bragg grating is written with the scanning speed distribution of the Gaussian apodization.

[0029] Step 3: Using progressive scanning writing technology, superimpose progressive scanning speed V b , so that the scanning speed of the ultraviolet laser at the short wavelength position is The scanning speed of the ultraviolet laser at the long wavelength position is In order to improve the reflectivity of the dispersive fiber grating at the short wavelength position and reduce the reflectivity of the dispersive fiber grating at the long wavelength, thereby compensating for the problem that the reflectivity of the dispersive fiber grating at the short wavelength is lower than that at the long wavelength due to the short-wave loss phenomenon, and finally effectively suppress the short-wave loss phenomenon, and obtain a dispersive fiber grating with a spectral flatness less than or equal to 2% and a side mode suppression ratio greater than or equal to 20dB.

[0030] Combination Figure 4 , using progressive scanning writing technology. When short-wavelength light passes through a long-period grating structure, part of the light will meet the mode coupling condition with the higher-order mode, resulting in short-wave loss in the dispersion fiber grating reflection spectrum. Short-wave loss will reduce the spectral flatness of the dispersion fiber grating, degrade the spectral effect of broadening the pulse, and reduce the output performance of the ultrafast laser. Introduce progressive scanning writing in the preparation process of the dispersion fiber grating. Set the scanning speed of the stepper motor 2-2 at the short wavelength position Stepper motor (2-2) scanning speed at long wavelength position Finally, the reflectivity in the short-wavelength region is improved, thereby compensating for the short-wave loss and improving the spectral flatness of the dispersion fiber grating. By further introducing the linear function:

[0031]

[0032] where k Compensation is the compensation factor, in s / cm, which is the change in exposure time per unit length of the grating. Gaussian is the initial value of exposure time distribution, set k Compensation =-0.005s / cm to compensate for short-wave loss. The compensation factor k Compensation , which allows the introduction of progressive scanning writing during the preparation process of CFBG, thereby increasing the reflectivity of the short-wavelength region relative to the long-wavelength region to improve the flatness of the CFBG spectrum and compensate for the short-wave loss.

[0033] Combination Figure 5 , finally, the spectrum flatness of the prepared dispersion fiber grating should be effectively tested. The specific process is as follows:

[0034] 1) Use a spectrometer with a resolution of 0.02nm to measure the reflection spectrum within the range of ±6nm of the central wavelength of the dispersed fiber grating, and measure the highest peak value P of the five reflection spectrum powers within this range. P1 To P P5 , and the lowest valley value P of the 5 reflected spectral powers V1 To P V5 ;

[0035] 2) Calculate the spectral flatness η of the dispersion fiber Bragg grating according to the following formula;

[0036]

[0037] 3) Repeat the measurement ten times and record them, and calculate the average value of the ten measurements as the calibration result of the spectral flatness.

[0038] In summary, the present invention provides a method for preparing a dispersion fiber grating with high spectral flatness. By combining the homogenized ultraviolet light field control technology, the high side mode suppression ratio apodization technology and the progressive scanning writing technology, the dispersion fiber grating with a spectral flatness of less than or equal to 2% and a side mode suppression ratio of greater than or equal to 20dB can be prepared.

Claims

1. A method for preparing a dispersion fiber grating with high spectral flatness, characterized in that: Here are the steps: Step 1: Use the homogenized ultraviolet light field control technology to expand and shape the ultraviolet laser beam, disperse its originally dense Gaussian energy to achieve the purpose of homogenizing the Gaussian spot, and finally obtain the ideal writing light field; thereby reducing the refractive index mutation degree at both ends of the fiber grating during the ultraviolet writing scanning process, and improving the side mode suppression ratio of the dispersive fiber grating; Step 2: Use high side mode suppression ratio toe cutting technology to control the scanning speed V of the stepper motor a The exposure time of each position of the fiber grating is changed, so as to control the refractive index modulation depth of different positions of the fiber grating to eliminate the refractive index mutation at both ends thereof, and obtain an apodized dispersion fiber grating with a side mode suppression ratio greater than or equal to 20 dB; Step 3: Use progressive scanning writing technology to superimpose the progressive scanning speed V b , so that the scanning speed of the ultraviolet laser at the short wavelength position is V b1 , so that the scanning speed of the ultraviolet laser at the long wavelength position is V b2 , V b1 ≤V b2 , in order to increase the reflectivity of the dispersive fiber Bragg grating at the short wavelength position and reduce the reflectivity of the dispersive fiber Bragg grating at the long wavelength, thereby compensating for the problem that the reflectivity of the dispersive fiber Bragg grating at the short wavelength is lower than that at the long wavelength due to the short-wave loss phenomenon. Finally, the short-wave loss phenomenon is effectively suppressed, and a dispersive fiber Bragg grating with a spectral flatness less than or equal to 2% and a side mode suppression ratio greater than or equal to 20dB is obtained.

2. The method for preparing a dispersion fiber Bragg grating with high spectral flatness according to claim 1, characterized in that: In step 1, the UV laser is expanded and shaped by using the homogenized UV light field control technology to disperse the originally dense Gaussian energy to achieve the purpose of homogenizing the Gaussian spot and finally obtain the ideal writing light field. The specific implementation is as follows: S1-1. Design of beam homogenization module (2-4): The beam homogenization module (2-4) comprises a beam shaping structure, an optical path collimation structure, and a light spot compression structure which are arranged in sequence on a common optical axis; The beam shaping structure includes a hemispherical lens (1-1) and an aperture (1-2); the optical path collimation structure includes a plano-convex lens (1-3); and the light spot compression structure includes a cylindrical mirror (1-4); A hemispherical lens (1-1), an aperture (1-2), a plano-convex lens (1-3) and a cylindrical mirror (1-4) are arranged in sequence on a common optical axis; S1-2, using the beam homogenization module (2-4) to homogenize the ultraviolet light field: 1-2-1), the writing laser first passes through the beam shaping structure, and after passing through the hemispherical lens (1-1), the laser energy distribution remains unchanged and is still a Gaussian distribution; but since the hemispherical lens (1-1) expands the numerical aperture of the light beam, the laser quickly diverges from a point after being focused, and the divergent light spot is convenient for beam shaping; then it passes through the aperture (1-2), controls the aperture size of the aperture (1-2), intercepts the middle part of the Gaussian beam, achieves a flat-top beam shaping effect, and obtains the shaped writing laser; 1-2-2), expanding and collimating the shaped writing laser through a plano-convex lens (1-3) to obtain a collimated laser; 1-2-3) The collimated laser passes through a cylindrical mirror (1-4) and is longitudinally compressed to form a linear light spot with uniform energy, which is directly used for grating writing.

3. The method for preparing a dispersion fiber Bragg grating with high spectral flatness according to claim 2, characterized in that: In step 2, the high side mode suppression ratio apodization technology is used to control the scanning speed V of the stepper motor. a By changing the exposure time of each position of the fiber Bragg grating, the refractive index modulation depth of the fiber Bragg grating at different positions is controlled to eliminate the refractive index mutation at both ends, and the apodized dispersion fiber Bragg grating with a side mode suppression ratio greater than or equal to 20 dB is obtained, as follows: S2-1. Build a high spectral flatness dispersion fiber Bragg grating UV writing system: 2-1-1), a high spectral flatness dispersion fiber Bragg grating ultraviolet writing system comprises an ultraviolet laser (2-1), a stepper motor (2-2), an ultraviolet reflector (2-3), a beam homogenization module (2-4), a phase mask (2-5), an optical fiber bracket (2-6), and a six-dimensional adjustment frame (2-7); the ultraviolet reflector (2-3) and the beam homogenization module (2-4) are arranged on the stepper motor (2-2), the phase mask (2-5) and the optical fiber bracket (2-6) are arranged on the six-dimensional adjustment frame (2-7), and the optical fiber is fixed on the optical fiber bracket (2-6); An ultraviolet laser (2-1), a stepping motor (2-2), an ultraviolet reflector (2-3), a beam homogenization module (2-4), a phase mask (2-5), and a dispersion fiber grating are sequentially arranged along the optical path; 2-1-2), when writing a dispersive fiber grating with high spectral flatness, the ultraviolet laser emitted by the ultraviolet laser (2-1) is deflected 90 degrees through the ultraviolet reflector (2-3), and the light field is homogenized by the beam homogenization module (2-4) and the energy is longitudinally compressed to increase the power density, and the optical fiber on the optical fiber bracket (2-6) is exposed through the phase mask (2-5); during the writing process, the stepper motor (2-2) drives the ultraviolet reflector (2-3) and the beam homogenization module (2-4) to move along the grating area of ​​the phase mask (2-5) at the same time, so as to scan the grating area of ​​the phase mask (2-5), thereby achieving the purpose of writing the dispersive fiber grating; S2-2. Build a real-time monitoring system for dispersion fiber Bragg gratings with high spectral flatness: 2-2-1), a real-time monitoring system for dispersion fiber Bragg grating with high spectral flatness comprises a broadband light source (2-8), a circulator (2-9), a spectrometer (2-10) and a high reflection Bragg fiber Bragg grating (2-11); 2-2-2), when the central wavelength, reflectivity and bandwidth of the dispersion fiber grating are monitored in real time during the writing process, the fiber type of the high-reflection Bragg fiber grating (2-11) is PM980, the reflectivity is 99.9%, and the central wavelength is 1070nm; port 1 of the circulator (2-9) is connected to the broadband light source (2-8), port 2 is connected to one end of the high-reflection Bragg fiber grating (2-11), and one end of the fiber on the fiber bracket (2-6) is fused with the other end of the high-reflection Bragg fiber grating (2-11); port 3 of the circulator (2-9) is connected to the spectrometer (2-10) to simultaneously measure the reflection spectrum intensity P1 of the dispersion fiber grating with a central wavelength near 1030nm and the reflection spectrum intensity P2 of the high-reflection Bragg fiber grating (2-11) with a central wavelength of 1070nm, then the reflectivity R of the dispersion fiber grating satisfies the following formula: S2-3. Using high side mode suppression ratio apodization technology to prepare apodized dispersion fiber Bragg gratings with side mode suppression ratio greater than or equal to 20dB: 2-3-1) Based on the ultraviolet writing system and the real-time monitoring system, a toe-cut dispersion fiber Bragg grating with a side mode suppression ratio greater than or equal to 20 dB is produced; the fiber type is PM980, the scanning length is 100 mm, the writing laser power is 30 mW, and the phase mask template chirp rate is 3 nm / cm; during the writing process, the scanning speed V of the stepper motor (2-2) is adjusted a To change the exposure time of each position of the dispersion fiber grating to achieve the effect of Gaussian toe cutting; 2-3-2), at the grating position corresponding to the Gaussian apodization function of 1, the stepper motor (2-2) is at the minimum scanning speed, and the scanning speed of the stepper motor (2-2) is set to 0.009mm / s at this time, and the fiber grating is started to be written with the scanning speed distribution of Gaussian apodization.

4. The method for preparing a dispersion fiber Bragg grating with high spectral flatness according to claim 3, characterized in that: Gaussian apodization function T of exposure time with respect to position Gaussian for: Where L is the grating length, k Gaussian is the apodization coefficient, T is the exposure time before apodization; z is the real-time exposure position.

5. The method for preparing a dispersion fiber Bragg grating with high spectral flatness according to claim 4, characterized in that: L is set to 100 mm, k Gaussian It is set to 0.7, and the scanning speed is inversely proportional to the exposure time. Considering the movement accuracy of the stepper motor (2-2), the entire speed change process is divided into 50 sections.

6. The method for preparing a dispersion fiber Bragg grating with high spectral flatness according to claim 4, characterized in that: In step 3, the progressive scanning writing technology is used to superimpose the progressive scanning speed V b , so that the scanning speed of the ultraviolet laser at the short wavelength position is V b1 , so that the scanning speed of the ultraviolet laser at the long wavelength position is V b2 , V b1 ≤V b2 , in order to improve the reflectivity of the dispersive fiber Bragg grating at the short wavelength position, reduce the reflectivity of the dispersive fiber Bragg grating at the long wavelength, and then compensate for the problem that the reflectivity of the dispersive fiber Bragg grating at the short wavelength is lower than that at the long wavelength due to the short-wave loss phenomenon, and finally effectively suppress the short-wave loss phenomenon, and obtain a dispersive fiber Bragg grating with a spectrum flatness of less than or equal to 2% and a side mode suppression ratio of greater than or equal to 20dB, as follows: S3-1. Preparation of dispersive fiber Bragg gratings with spectral flatness less than or equal to 2% and side mode suppression ratio greater than or equal to 20dB by progressive scanning writing technology: Set the scanning speed of the stepper motor (2-2) at the short wavelength position to V b1 , the scanning speed of the stepper motor (2-2) at the long wavelength position is V b2 , V b1 <V b2 , ultimately increasing the reflectivity in the short-wavelength region, thereby compensating for short-wave losses and improving the spectral flatness of the dispersive fiber grating; Further introduce the linear function T Compensation , to compensate for the short-wave loss, thereby achieving the purpose of improving the spectral flatness of the dispersive fiber Bragg grating; S3-2, Dispersive Fiber Bragg Grating Spectral Flatness Measurement, the specific process is as follows: 3-2-1) Use a spectrometer (2-10) with a resolution of 0.02nm to measure the reflection spectrum within the range of ±6nm of the central wavelength of the dispersed fiber grating, and measure the highest peak value P of the 5 reflection spectrum powers within this range. P1 To P P5 , and the lowest valley value P of the 5 reflected spectral powers V1 To P V5 ; 3-2-2) Calculate the spectral flatness η of the dispersion fiber Bragg grating; 3-2-3) Repeat the measurement ten times and record them, and calculate the average value of the ten measurements as the calibration result of the spectral flatness.

7. The method for preparing a dispersion fiber Bragg grating with high spectral flatness according to claim 6, characterized in that: V b1 =0.015mm / s,V b2 =0.018mm / s。 8. The method for preparing a dispersion fiber Bragg grating with high spectral flatness according to claim 6, characterized in that: In S3-1, the linear function T Compensation as follows: where k Compensation is the compensation coefficient, in s / cm, which is the change in exposure time per unit length of the grating; T Gaussian is the initial value of exposure time distribution, T is the exposure time before apodization, z is the real-time exposure position, and L is the grating length.

9. The method for preparing a dispersion fiber Bragg grating with high spectral flatness according to claim 8, characterized in that: Setting k Compensation =-0.005s / cm.

10. The method for preparing a dispersion fiber Bragg grating with high spectral flatness according to claim 6, characterized in that: 3-2-2), calculate the spectral flatness η of the dispersive fiber Bragg grating; Among them, P P1 To P P5 Indicates the highest peak of the five reflected spectral powers, P V1 To P V5 Indicates the lowest valley value of the five reflected spectral powers.

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