A method and system for the fabrication of a nonlinear chirped fiber grating

Abstract

The application discloses a nonlinear chirped fiber grating preparation method and system, and the method comprises the following steps: determining basic processing parameters of a fiber grating according to target parameters; based on the coupled mode theory, initially constructing a nonlinear periodic modulation function and a nonlinear apodization function, calculating a reflection spectrum and comparing the reflection spectrum with a target spectrum, and repeatedly adjusting the two functions for iteration optimization until the error reaches a standard; a periodic modulation selects a signal generator or a displacement table chirp, the former calculates a pulse delay according to a period value through a uniform displacement table, and the latter calculates a displacement table speed according to a period value through a uniform laser pulse; apodization modulation selects laser power or grating point spatial modulation, the former dynamically adjusts power to control the refractive index change, and the latter controls the laser focal point and the fiber core position; and the nonlinear chirped fiber grating is processed according to the optimized function and the selected mode. The application meets the application requirement of a complex optical system.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of optical fiber device preparation, and more particularly relates to a nonlinear chirped fiber grating preparation method and system. BACKGROUND

[0002] In the field of optical fiber communication, sensing, etc., the realization of wideband reflection function highly depends on the core device of chirped fiber grating. Chirped fiber grating is a periodic refractive index modulation structure arranged along the axial direction in the fiber core, and the grating period changes linearly with the position. During the processing, the refractive index of the fiber core processing point needs to be changed through a specific means. In order to improve the reflection spectrum characteristics, a tapering function (commonly used Gaussian function, tanh function) is introduced to modulate the effective refractive index envelope of the grating point in the traditional technology. Such grating is called a tapered grating.

[0003] However, the reflection spectrum of the traditional chirped fiber grating has significant defects, and there are obvious fluctuations at the top of the reflection spectrum, especially on both sides, which seriously limits the application range of the device. Although the tapering technology can be used to reduce the spectral fluctuation, this method has an irreconcilable contradiction: if the fluctuation is to be suppressed to the greatest extent, the effective length of the grating will be shortened, and the overall spectrum envelope will be deformed into a state of high middle and low both sides; if it is desired to maintain a rectangular reflection spectrum, the fluctuation in the middle part of the reflection spectrum cannot be avoided, especially in the weak reflection (reflection rate lower than 50%) scenario, this defect is more prominent, which cannot meet the application requirements of high stability of the reflection spectrum.

[0004] From the manufacturing process, the preparation method of the traditional chirped fiber grating also has many limitations. The ultraviolet laser mask method is the most widely used preparation method, but it needs to be hydrogen-loaded to improve the photosensitivity of the fiber, which is time-consuming, high-cost and has safety risks; at the same time, it is limited by the mask plate, and different parameters of the grating need to be matched with different mask plates, which significantly increases the design and preparation cost, and the prepared grating also has the risk of degradation, especially for weak reflection gratings. The method of cascading multiple Bragg gratings can achieve a similar chirp spectrum, but it is difficult to promote in actual scenarios due to high cost and poor reflection spectrum flatness. Although the femtosecond laser direct writing technology has the advantages of no mask plate and no hydrogen loading, the stability of the laser and the precision of the motion control are insufficient in the existing technology, which leads to poor processing consistency, and it is rarely used in complex optical systems.

[0005] In summary, the dual limitations of the traditional chirped fiber grating in spectrum design (reflection spectrum fluctuation, spectrum deformation) and manufacturing process (cost, safety, consistency) hinder the performance improvement and large-scale application of wideband reflection devices, and new technical solutions are needed to break through the existing bottlenecks. SUMMARY

[0006] The present application aims to solve the problems of traditional chirped fiber grating, such as large reflection spectrum fluctuation, easily deformed spectrum type, high cost, poor safety and insufficient consistency in manufacturing process, by designing a double nonlinear modulation function, optimizing the processing method and constructing a closed-loop detection mechanism, to prepare a nonlinear chirped fiber grating with strong adaptability to weak reflection and stable broadband reflection performance, meeting the needs of fiber communication and sensing fields.

[0007] In view of the above defects or improvement needs of the prior art, as a first aspect of the present application, the present application provides a nonlinear chirped fiber grating preparation method, comprising:

[0008] S1. Determine the basic processing parameters of the fiber grating according to the target parameters;

[0009] S2. Based on the coupled mode theory, initially construct a nonlinear periodic modulation function And a nonlinear apodization function , calculate the reflection spectrum and compare it with the target spectrum, and iteratively optimize by repeatedly adjusting the two functions until the error meets the requirements;

[0010] S3. Periodic modulation selects signal generator chirp or displacement table chirp; wherein the signal generator chirp calculates the pulse delay by keeping the displacement table at a uniform speed; the displacement table chirp calculates the displacement table speed by keeping the laser pulse uniform; the apodization modulation selects laser power modulation or grating point space modulation, wherein the laser power modulation controls the refractive index change by dynamically adjusting the power; the grating point space modulation is realized by controlling the position of the laser focal point and the fiber core;

[0011] S4. According to the optimized function and the selected implementation, a nonlinear chirped fiber grating is formed on the fiber core.

[0012] Further, the target parameters in S1 include length, center wavelength, bandwidth, reflectivity and fluctuation range.

[0013] Further, the basic processing parameters in S1 include laser frequency, laser energy range and processing length.

[0014] Further, the nonlinear periodic modulation function in S2 Specifically,

[0015] ,

[0016] Wherein, is a periodic constant, is a periodic variation, is a modulation linear function, and its domain is , is the total length of the grating.

[0017] Furthermore, the modulation line function Flexible form spline curve form:

[0018] ,

[0019] in, for The parametric curve representation, Range of parameter values , As control points, for The basis functions of spline curves This represents the number of control points.

[0020] Furthermore, the nonlinear apodization function in S2 The range of functions selected includes Gaussian functions, Gaussian functions Power of 1 , , , , , , The function is included.

[0021] Furthermore, the specific process of iterative optimization in S2 by repeatedly adjusting the two functions is as follows:

[0022] A similarity calculation is performed between the calculated reflection spectrum and the target reflection spectrum to obtain the error between the reflection spectrum and the target; the apodization function is adjusted based on the error value. Spectral pattern; recalculate the error between the reflection spectrum and the target spectral pattern, and adjust the periodic modulation function. The error between the reflection spectrum and the target spectrum is calculated again; this process is repeated iteratively to obtain the final nonlinear apodization function. and nonlinear periodic modulation function .

[0023] Furthermore, the specific process of controlling the change in refractive index by dynamically adjusting the power during laser power modulation in S3 is as follows:

[0024] The laser power dynamically changes with the axial processing position. Based on the laser reference power, a smooth power envelope along the total length of the grating is constructed through the hyperbolic secant function. This envelope can avoid the abnormal refractive index modulation caused by power abrupt changes at both ends of the grating. At the same time, a cosine term is superimposed on the smooth envelope to achieve periodic fine-tuning, which is used to compensate for local spectral fluctuations caused by the nonlinear periodic modulation function. The characteristic attenuation parameter of the hyperbolic secant function is directly related to the shape of the nonlinear apodization function.

[0025] As a second aspect of the present application, a nonlinear chirped fiber grating preparation system is also provided, comprising:

[0026] A basic parameter determination unit is configured to determine basic processing parameters of the fiber grating according to the target parameters.

[0027] A function design optimization unit is configured to initially construct a nonlinear periodic modulation function and a nonlinear apodization function based on the coupled mode theory , calculate a reflection spectrum and compare it with a target spectrum, and iteratively optimize the two functions by repeatedly adjusting them until the error meets the requirements. A modulation mode selection unit is configured to select a signal generator chirp or a displacement stage chirp for periodic modulation.

[0028] The signal generator chirp is calculated by keeping the displacement stage at a constant speed and calculating the pulse delay according to the periodic value. The displacement stage chirp is calculated by keeping the laser pulse uniform and calculating the displacement stage speed according to the periodic value. The apodization modulation is selected by laser power modulation or grating point space modulation, wherein the laser power modulation controls the refractive index change by dynamically adjusting the power, and the grating point space modulation is achieved by controlling the position of the laser focal point and the fiber core.

[0029] A grating processing implementation unit is configured to process and form a nonlinear chirped fiber grating on the fiber core according to the optimized functions and the selected implementation mode.

[0030] As a third aspect of the present application, a computer readable storage medium is also provided, which stores a computer program that is executed by a processor to perform any step of the nonlinear chirped fiber grating preparation method.

[0031] Overall, the above technical solutions conceived by the present application can achieve the following beneficial effects compared with the prior art:

[0032] 1. The nonlinear chirped fiber grating preparation method of the present application constructs a nonlinear periodic modulation function and a nonlinear apodization function based on the coupled mode theory, and iteratively optimizes the two functions by repeatedly adjusting them until the reflection spectrum and the target spectrum meet the requirements. The traditional method uses linear periodic modulation and single apodization function, which is prone to reflection spectrum fluctuation or spectrum deformation, especially in weak reflection scenarios. The present method uses double nonlinear functions for collaborative optimization, compensates for the shortcomings of linear design by utilizing the coupling relationship between the two functions, effectively solves the contradiction between suppressing fluctuation and maintaining spectrum type in the traditional scheme, significantly improves the flatness of the grating reflection spectrum, controls the fluctuation range within the target range, and avoids the shortening of the effective length of the grating, thereby ensuring the wideband reflection performance.

[0033] 2. The nonlinear chirped fiber grating preparation method of the present application, by selecting signal generator chirp or displacement table chirp in the period modulation link, and selecting laser power modulation or grating point space modulation in the apodization modulation link. The traditional ultraviolet mask method needs hydrogen loading and depends on the mask plate, has poor flexibility and the grating is easy to degrade; the multi-grating cascade has high cost and poor flatness. The method relies on the processing characteristics of femtosecond laser, does not need mask plate and hydrogen loading process, and the signal generator chirp realizes period modulation through uniform displacement table and pulse delay coordination, the displacement table chirp achieves the target through uniform laser pulse and speed adjustment, cooperates with two apodization methods, improves the processing flexibility and reliability, reduces the cost, avoids the degradation risk of traditional process, and adapts to the application requirements of complex optical systems. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 A nonlinear chirped fiber grating preparation method flowchart of an embodiment of the present application;

[0035] Figure 2 A reflection spectrum schematic diagram of a traditional chirped fiber grating;

[0036] Figure 3 A traditional linear chirped fiber grating structure schematic diagram;

[0037] Figure 4 A traditional apodized linear chirped fiber grating refractive index change-position relationship diagram; wherein, represents the refractive index change of the fiber grating points;

[0038] Figure 5 An S-shaped grating point space modulation apodization schematic diagram of an embodiment of the present application;

[0039] Figure 6 A V-shaped grating point space modulation apodization schematic diagram of an embodiment of the present application;

[0040] Figure 7 A device reflection spectrum schematic diagram of an embodiment of the present application; wherein, nm represents wavelength unit nanometer; and dBm represents optical intensity unit decibel milliwatt;

[0041] Figure 8 A system unit diagram of an embodiment of the present application. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application is further described in detail below in combination with the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other.

[0043] Embodiment 1

[0044] Please refer to Figure 1 , this embodiment 1 provides a nonlinear chirped fiber grating preparation method, comprising:

[0045] S1. Determine the basic processing parameters of the fiber grating according to the target parameters;

[0046] S2. Based on the coupled mode theory, initially construct a nonlinear periodic modulation function And a nonlinear apodization function , calculate the reflection spectrum and compare it with the target spectrum, and iteratively optimize by repeatedly adjusting the two functions until the error meets the requirements;

[0047] S3. Periodic modulation selects signal generator chirp or displacement table chirp; wherein the signal generator chirp calculates the pulse delay by keeping the displacement table at a uniform speed; the displacement table chirp calculates the displacement table speed by keeping the laser pulse uniform; the apodization modulation selects laser power modulation or grating point space modulation, wherein the laser power modulation controls the refractive index change by dynamically adjusting the power; the grating point space modulation is realized by controlling the position of the laser focal point and the fiber core;

[0048] S4. According to the optimized function and the selected implementation, a nonlinear chirped fiber grating is formed on the fiber core.

[0049] Embodiment 1 discloses a configuration of a broadband reflection fiber device. The broadband reflection fiber device refers to a fiber grating device whose reflection bandwidth is significantly larger than that of a conventional fiber grating; the reflection bandwidth of a conventional fiber grating is about 0.5 nm, while the reflection bandwidth of a broadband reflection device is usually greater than 1 nm. Traditional schemes for achieving broadband reflection usually use chirped fiber gratings. Fiber gratings refer to a series of periodic refractive index modulation structures arranged axially in the fiber core; chirped fiber gratings refer to grating structures whose grating periods change linearly with position.

[0050] The reflection spectrum of a traditional chirped fiber grating is shown in Figure 2 , there are obvious fluctuations at the top of the reflection spectrum, especially on both sides, which seriously affects the application range of such gratings. The traditional method is to modulate the effective refractive index envelope function of the grating points, i.e. the apodization function, to achieve the effect of reducing spectral fluctuations. However, this method has some defects. To maximize the suppression of fluctuations, the effective length of the grating will be shortened, and the overall spectral envelope will be deformed to be high in the middle and low on both sides. If you want to maintain a rectangular reflection spectrum, the fluctuations in the middle part of the reflection spectrum will be difficult to avoid. Therefore, the traditional apodization method has great defects when making chirped fiber gratings, especially weak reflection (<50%) chirped fiber gratings.

[0051] Conventional manufacturing methods for chirped fiber gratings generally use the following process:

[0052] UV-laser mask method for preparing chirped fiber gratings; UV mask method is the most commonly used method for preparing chirped fiber gratings. However, there are many disadvantages in using UV mask. First, UV mask method requires hydrogen loading on the optical fiber to increase its photosensitivity, which is time-consuming, high-cost and dangerous. Second, due to the limitation of the mask, the UV mask method is not flexible, different parameters need to be matched with different mask plates, and the design and preparation cost is high. In addition, the grating made by UV mask method has the risk of degradation, especially when making weak reflection grating.

[0053] Multiple Bragg grating cascades; the method of multiple fiber grating cascade can achieve a spectrum similar to chirp, but its cost and flatness of reflection spectrum limit its application in practice.

[0054] Femtosecond laser direct writing technology; femtosecond laser direct writing has the most flexible processing means, without the need for a mask plate and hydrogen loading. However, the stability of the laser and the precision of the motion control in the prior art seriously limit the consistency of laser processing, resulting in its less application in complex optical systems.

[0055] The above is the manufacturing method of chirped fiber grating; in addition to the manufacturing method, there is also the configuration of chirped fiber grating. Figure 3 is a schematic diagram of the structure of the traditional chirped fiber grating. Fiber grating, as the name implies, is a grating processed in the fiber core. Chirped fiber grating refers to a fiber grating with uniform variation of grating point period. The change of grating point with position is generally represented by the function: (period modulation function). Generally changes linearly, that is . Processing means that the refractive index of the core processing point position is changed from to . If the refractive index change is taken as the vertical axis and the position of the processing direction as the horizontal axis, then is obtained. Figure 4 The refractive index of the grating point will change with the grating position, and such grating is called apodized grating, and the dashed line in the figure is the apodization function . At present, commonly used apodization functions include: Gaussian function, function. As mentioned earlier, although the reflection spectrum of the chirped grating is improved after adding the apodization technology, the fluctuations of the effective length and the spectral center position still cannot be well improved.

[0056] Further, it is known that the traditional chirped fiber grating design has many problems, especially in the case of weak reflection, it is difficult to get a reliable reflection spectrum with small fluctuations. Therefore, the embodiment 1 provides an improved method, and the specific process will be further described below.

[0057] (1) Basic parameter determination

[0058] Under the demand for broadband reflector devices in the field of optical fiber communication and sensing, the traditional chirped fiber grating is difficult to meet the application requirements due to its large reflection spectrum fluctuation and limited manufacturing process. According to the target parameters, the basic processing parameters of the fiber grating are determined, which is the starting link of the preparation process, and it meets the core requirements of the device performance in the background.

[0059] The target parameter determination needs to address the pain points of the traditional technology in the background: the grating length adapts to the system installation space, avoiding the application limitations caused by the size deviation of the traditional process; the center wavelength locks the core wavelength position of the reflection signal, matching the wavelength demand of the communication and sensing system, solving the problem of wavelength shift of the traditional grating; the bandwidth covers the effective reflection wavelength interval required by the application, making up for the defect of narrow bandwidth of the traditional grating; the reflectivity controls the signal reflection intensity, improving the unstable performance of the traditional weak reflection (reflectivity less than 50%) grating; the fluctuation range limits the reflection spectrum stability, responding to the core defect of large reflection spectrum fluctuation of the traditional grating. These parameters constitute the performance benchmark of the device, and after being determined, the core indicators that need to be achieved in the subsequent preparation are clear.

[0060] When deriving the basic processing parameters based on the target parameters, the laser frequency is combined with the target bandwidth and the subsequent period modulation function design logic to ensure accurate control of the grating period formation rhythm; the laser energy range matches the target reflectivity, adapting to the refractive index change corresponding to different reflectivities; the processing length corresponds to the grating length in the target parameters, limiting the actual processing physical range, avoiding the influence of processing range deviation on the grating performance. This process converts the performance requirements in the background into executable processing parameters, and sets constraints for the subsequent preparation steps.

[0061] (2) Function design optimization

[0062] Function design optimization is the core link to realize the low fluctuation and high adaptability of the nonlinear chirped fiber grating reflection spectrum. Based on the coupled mode theory, the key functions are constructed and iteratively optimized to ensure that the function form accurately matches the processing requirements and performance targets. The specific process is as follows: first, construct the nonlinear period modulation function and the nonlinear apodization function , obtain the corresponding reflection spectrum through theoretical calculation, compare it with the preset target reflection spectrum, quantify the error between the two through similarity calculation, and then repeatedly adjust the parameters and form of the two functions according to the error feedback, continuously iteratively optimize until the reflection spectrum error meets the design requirements.

[0063] Nonlinear periodic modulation function The modular construction approach is adopted, with the core form being... .in, As a reference period constant, it determines the fundamental periodic characteristics of the grating; It is a periodic change quantity used to regulate the amplitude of periodic changes; The modulation line shape function is defined in the domain of the total grating length. This is the key to achieving periodic nonlinear changes.

[0064] To adapt to different processing scenarios and performance requirements, The selection is highly flexible: it can be chosen Error function, hyperbolic tangent function Classical functions, such as those mentioned above, are stable and easy to calculate, making them suitable for conventional nonlinear periodic designs.

[0065] Alternatively, Parametric curves such as spline curves, by adjusting the number and position of control points and the order of basis functions, can achieve precise customization of complex nonlinear periodic shapes to adapt to special bandwidth or spectral requirements; for example, in a preferred embodiment, its The spline curve takes the following form:

[0066] ,

[0067] in, for The parametric curve representation, The parameter value range is [0,1]. As control points, for The basis functions of spline curves This represents the number of control points.

[0068] The target periodic curve can also be approximated using approximate expansion methods such as Taylor expansion, balancing computational efficiency and morphological accuracy. It should be noted that regardless of the method used, Undetermined coefficients in (e.g.) The coordinates of the control points of the spline curve, the coefficients of the Taylor expansion, etc., all need to form a coupled optimization relationship with the subsequent apodization modulation method to ensure that the function design and the actual processing parameters can be directly converted to avoid the disconnect between design and processing.

[0069] Nonlinear apodization function Its core function is to modulate the effective refractive index envelope of the grating dots, suppress reflection spectrum fluctuations, and optimize spectral integrity. Its selection range covers various classical apodization function types, including Gaussian functions and other similar functions. power, Bartlett function, Blackman function, Connes function, cosine function, Hamming function, Hanning function, tanh function, etc. Different functions have different characteristics: Gaussian function and its power can realize smooth envelope modulation and effectively reduce spectral line sidelobes; Blackman, Hamming and other window functions perform well in suppressing sidelobes and maintaining spectral width balance; tanh function can realize gradual transition of refractive index envelope, avoiding spectral distortion caused by refractive index mutation at both ends of the grating. In actual design, according to the fluctuation control requirements, bandwidth requirements, etc. of the target reflection spectrum, the appropriate apodization function type can be selected, or the performance can be further optimized by combining and modifying the function form.

[0070] In the iterative optimization process, the nonlinear periodic modulation function and the nonlinear apodization function are not adjusted independently, but form a cooperative optimization relationship: the change of the periodic modulation function will affect the bandwidth and center wavelength stability of the reflection spectrum, and the apodization function mainly controls the spectral line fluctuation and sidelobe suppression effect. The adjustment of the parameters of the two functions needs to be based on the cross feedback of the reflection spectrum error to ensure that the final function combination can meet the needs of periodic nonlinear modulation and achieve low fluctuation and high regularity of the reflection spectrum through apodization optimization, providing accurate and feasible theoretical basis for subsequent processing links.

[0071] Specifically, the specific process of iterative optimization by repeatedly adjusting the two functions is as follows:

[0072] Similarity calculation is performed on the calculated reflection spectrum and the target reflection spectrum to obtain the error of the reflection spectrum and the target; the apodization function is adjusted according to the error value The error of the reflection spectrum and the target spectrum is calculated again, and the periodic modulation function is adjusted The error of the reflection spectrum and the target spectrum is calculated again, and the periodic modulation function is adjusted .

[0073] (3) Modulation mode selection

[0074] The optimization of the periodic modulation function and the apodization function of all embodiments is similar. The difference lies in the implementation of the periodic modulation function and the apodization function. There are two ways to implement the periodic modulation function: signal generator chirp, displacement table chirp; there are two ways to implement the apodization modulation function: laser power modulation, grating point space modulation;

[0075] The specific implementation includes but is not limited to four: signal generator chirp + laser power modulation, signal generator chirp + grating point space modulation, displacement table chirp + laser power modulation, displacement table chirp + grating point space modulation.

[0076] 3.1 Signal generator chirp; the signal generator can send a signal pulse at any time interval, and the laser will send a laser pulse after receiving the signal. First, give the displacement table a uniform speed in the fiber processing direction According to the period value of the periodic modulation function The delay required at this point can be determined Send the next pulse, so according to the periodic modulation function, the pulse sequence of the signal generator can be calculated, and inputting this pulse sequence into the signal generator can achieve the chirp effect under the condition of uniform speed of the displacement table.

[0077] 3.2 Displacement table chirp; displacement table chirp requires uniform laser pulse emission. According to the period value of the periodic modulation function And the uniform pulse interval of the laser The moving speed of the displacement table Can be obtained. In this way, the speed of each grating point position is calculated, and the displacement table is controlled to move at this speed, and the chirp effect can be obtained.

[0078] 3.3 Laser power modulation apodization; the refractive index change of the fiber grating grating point is positively related to the power of the femtosecond laser, and the higher the power of the femtosecond laser, the greater the refractive index change. According to the apodization modulation function, we can rely on the optical system to dynamically adjust the laser power to achieve any apodization function spectrum.

[0079] Specifically, the specific process of laser power modulation to control the refractive index change through dynamic adjustment of power is as follows:

[0080] The laser power dynamically changes with the axial processing position, and the laser reference power is used as the basis to construct a smooth power envelope along the total length of the grating through the hyperbolic secant function. This envelope can avoid abnormal refractive index modulation at both ends of the grating due to sudden changes in power; at the same time, on the basis of the smooth envelope, a cosine term is superimposed to realize periodic fine adjustment, which is used to compensate for local spectral fluctuations caused by nonlinear periodic modulation functions, wherein the characteristic decay parameter of the hyperbolic secant function is directly related to the shape of the nonlinear apodization function.

[0081] 3.4 Spatial modulation apodization; Spatial modulation apodization can also be called tilt apodization. It refers to the method of controlling the effective refractive index change of the grating point by controlling the relative position of the laser processing point and the core. When laser processing, if the laser focus point is exactly in the middle of the fiber core, the effective refractive index change is the largest. If the laser focus point is offset from the center during processing, the effective refractive index will decrease. The farther the laser focus point position is from the core position, the smaller the effective refractive index change will be. Using this characteristic, we can design and implement any apodization function.

[0082] Specifically, spatial modulation of the grating point is achieved by controlling the relative position of the laser focus point and the fiber core, and the process is as follows:

[0083] Taking the center axis of the fiber core as the position reference, the radial offset of the laser focus point relative to the center of the core will dynamically change with the axial processing position. The offset direction is symmetrically switched by the sign function: when the processing position is in the first half of the total length of the grating, the focus point is offset in one direction; when it is in the second half, the focus point is offset in the opposite direction, ensuring the symmetry of the offset law.

[0084] On the basis of symmetric offset, periodic fine tuning is introduced to compensate for the local refractive index modulation deviation caused by the nonlinear periodic modulation function. At the same time, the optimized nonlinear apodization function is directly used as the target envelope of the effective refractive index change of the grating point, and the radial offset of the laser focus point is related to this apodization function.

[0085] The effective refractive index change of the grating point is related to the radial offset of the focus point: when the laser focus point is located at the center of the core, the refractive index change reaches the maximum value; as the focus offset increases, the effective refractive index change gradually decays, and the decay law is related to the radius of the fiber core.

[0086] Figure 5 And Figure 6 Two methods of spatial modulation apodization are shown: Figure 5 S-type, which means that the grating point position gradually changes from one side of the core to the other side of the fiber, and the two sides are symmetrically centered. Figure 6 V-type, which means that the grating points are all located on one side of the core, and the two sides are axially symmetric.

[0087] By designing the functional relationship between the distance of the grating point from the center and the processing length We can get the apodization envelope. Commonly used Including but not limited to linear, Gaussian function, etc.

[0088] Four specific implementations can be obtained by combining the implementation of the two periodic modulation functions and the implementation of the apodization modulation function. Please refer to Figure 7Through the above embodiments, we can obtain the spectral diagram of the above broadband reflection device. It can be found that the device reflection band fluctuation is small, and the effect is greatly improved compared with the traditional technology.

[0089] (4) Grating processing implementation

[0090] After the iterative optimization of the nonlinear periodic modulation function and the nonlinear apodization function is completed, and the implementation mode of the periodic modulation (signal generator chirp or displacement table chirp) and the apodization modulation (laser power modulation or grating point space modulation) is selected, the actual processing of the fiber grating can be carried out.

[0091] During processing, the optimized nonlinear periodic modulation function is used as the basis. If the signal generator chirp is selected, the displacement table is kept uniform and the pulse delay is set according to the period value; if the displacement table chirp is selected, the laser pulse is kept uniform and the displacement table speed is adjusted according to the period value, so as to realize dynamic control of the grating period. At the same time, the selected apodization modulation mode is combined: if it is laser power modulation, the laser power is adjusted according to the dynamic change law of the power with the processing position (a hyperbolic secant function is constructed to build a smooth envelope, and a cosine term is added for fine adjustment), and the refractive index change amount of the corresponding position of the fiber core is controlled through the power change; if it is grating point space modulation, the laser focal point position is adjusted according to the radial offset law of the focal point relative to the center of the fiber core (the offset direction is symmetrically switched, and a sinusoidal term is added for fine adjustment), and the effective refractive index change amount of the corresponding position of the fiber core is controlled through the focal point offset. Finally, a nonlinear chirped fiber grating that meets the design of the optimized function is formed on the fiber core.

[0092] The embodiment 1 has significant application potential in the field of optical fiber communication, and can be adapted to high-speed and wide-band signal transmission scenarios. The current optical fiber communication system continuously improves the stability and bandwidth demand of signal reflection. The traditional chirped fiber grating has large reflection spectrum fluctuation, poor adaptability in weak reflection scenarios, and is easy to cause signal transmission loss or bit error rate increase. The nonlinear chirped fiber grating prepared by the embodiment 1 can realize low fluctuation and wide frequency band stable reflection through double nonlinear function optimization and precise processing control, and can be used as a core component such as a broadband reflector and a dispersion compensator in an optical fiber communication system. Especially in long-distance trunk communication and dense wavelength division multiplexing systems, it can effectively improve the signal transmission quality and system stability, and meet the needs of communication capacity expansion and transmission distance extension.

[0093] In the field of optical fiber sensing, the present embodiment 1 also has broad application space. Optical fiber sensing systems often need to detect parameters such as temperature, pressure and strain in complex environments (such as industrial high temperature, oil and gas exploration underground) with high precision. The stability of the reflection spectrum of the grating device and the environmental adaptability are strict. The traditional grating is easy to cause the detection precision to decrease due to the spectrum fluctuation, and the limitation of the manufacturing process makes it difficult to batch adapt to different sensing scene requirements. The grating prepared by the present embodiment 1 not only has controllable reflection spectrum fluctuation, but also can be customized with flexible center wavelength, bandwidth and other characteristics by adjusting the target parameters. It can be used as a high-sensitivity sensing probe in structural health monitoring, environmental parameter detection and other scenes. At the same time, its simplified processing flow and closed-loop detection mechanism can reduce the production cost of the sensor device, and promote the large-scale application of optical fiber sensing technology in civil, industrial and other fields.

[0094] Embodiment 2

[0095] For reference Figure 8 , the present embodiment 2 also provides a nonlinear chirped fiber grating preparation system, comprising:

[0096] A basic parameter determination unit is configured to determine the basic processing parameters of the fiber grating according to the target parameters.

[0097] A function design optimization unit is configured to initially construct a nonlinear periodic modulation function and a nonlinear apodization function based on the coupled mode theory, calculate the reflection spectrum and compare it with the target spectrum, and iteratively optimize the two functions by repeatedly adjusting them until the error meets the requirements.

[0098] A modulation mode selection unit is configured to select the signal generator chirp or the displacement table chirp for periodic modulation. The signal generator chirp calculates the pulse delay by keeping the displacement table at a constant speed according to the period value. The displacement table chirp calculates the displacement table speed by keeping the laser pulse uniform according to the period value. The apodization modulation selects laser power modulation or grating point space modulation, wherein the laser power modulation controls the refractive index change by dynamically adjusting the power, and the grating point space modulation is realized by controlling the position of the laser focal point and the fiber core.

[0099] A grating processing implementation unit is configured to process and form a nonlinear chirped fiber grating on the fiber core according to the optimized function and the selected implementation mode.

[0100] Embodiment 3

[0101] The present embodiment 3 also provides a computer readable storage medium having a computer program stored thereon, and the computer program can implement any step of a nonlinear chirped fiber grating preparation method when executed by a processor.

[0102] The computer readable storage medium can include a U disk, a mobile hard disk, a Read-Only Memory (ROM), a Random Access Memory (RAM), a magnetic disk or an optical disk, and various media capable of storing program codes.

[0103] For the computer readable storage medium provided in the present application, refer to the above method embodiments, which will not be repeated herein.

[0104] Those skilled in the art can easily understand that the above description is only a preferred embodiment of the present application, and is not intended to limit the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for fabricating a nonlinear chirped fiber grating, characterized in that, include: S1. Determine the basic fabrication parameters of the fiber Bragg grating based on the target parameters; S2. Based on coupled-mode theory, an initial nonlinear periodic modulation function is constructed. and nonlinear apodization function The reflection spectrum is calculated and compared with the target spectrum. The two functions are iteratively optimized by repeatedly adjusting them until the error meets the requirements. S3. Periodic modulation selects either the signal generator chirp or the shift stage chirp; The signal generator chirp keeps the displacement stage moving at a constant speed and calculates the pulse delay based on the period value; the displacement stage chirp keeps the laser pulse uniform and calculates the displacement stage speed based on the period value; the apodization modulation selects either laser power modulation or grating space modulation, where laser power modulation controls the amount of refractive index change by dynamically adjusting the power; and grating space modulation is achieved by controlling the laser focus and the fiber core position. S4. According to the optimized function and the selected implementation method, a nonlinear chirped fiber grating is fabricated on the fiber core; Specifically, the iterative optimization process by repeatedly adjusting the two functions is as follows: A similarity calculation is performed between the calculated reflection spectrum and the target reflection spectrum to obtain the error between the reflection spectrum and the target; the apodization function is adjusted based on the error value. Spectral pattern; recalculate the error between the reflection spectrum and the target spectral pattern, and adjust the periodic modulation function. The error between the reflection spectrum and the target spectrum is calculated again; this process is repeated iteratively to obtain the final nonlinear apodization function. and nonlinear periodic modulation function .

2. The method for fabricating a nonlinear chirped fiber grating according to claim 1, characterized in that, The target parameters in S1 include length, center wavelength, bandwidth, reflectivity, and fluctuation range.

3. The method for fabricating a nonlinear chirped fiber grating according to claim 1, characterized in that, The basic processing parameters in S1 include laser frequency, laser energy range, and processing length.

4. The method for fabricating a nonlinear chirped fiber grating according to claim 1, characterized in that, The nonlinear periodic modulation function in S2 Specifically: ， in, It is a periodic constant. It is a periodic change. The modulation linear function has the following domain: , This is the total length of the grating.

5. The method for fabricating a nonlinear chirped fiber grating according to claim 4, characterized in that, The modulation line function The flexible form is spline curve form: ， in, for The parametric curve representation, Range of parameter values , As control points, for The basis functions of spline curves This represents the number of control points.

6. The method for fabricating a nonlinear chirped fiber grating according to claim 1, characterized in that, The nonlinear apodization function in S2 The range of functions selected includes Gaussian functions, Gaussian functions Power of 1 , , , , , , The function is included.

7. The method for fabricating a nonlinear chirped fiber grating according to claim 1, characterized in that, The specific process of laser power modulation in S3, which dynamically adjusts the power to control the change in refractive index, is as follows: The laser power dynamically changes with the axial processing position. Based on the laser reference power, a smooth power envelope along the total length of the grating is constructed through the hyperbolic secant function. This envelope can avoid the abnormal refractive index modulation caused by power abrupt changes at both ends of the grating. At the same time, a cosine term is superimposed on the smooth envelope to achieve periodic fine-tuning, which is used to compensate for local spectral fluctuations caused by the nonlinear periodic modulation function. The characteristic attenuation parameter of the hyperbolic secant function is directly related to the shape of the nonlinear apodization function.

8. A nonlinear chirped fiber grating fabrication system, characterized in that, include: The basic parameter determination unit is used to determine the basic processing parameters of the fiber grating based on the target parameters. The function design optimization unit is used to initially construct a nonlinear periodic modulation function based on coupled-mode theory. and nonlinear apodization function The reflection spectrum is calculated and compared with the target spectrum. The two functions are iteratively optimized by repeatedly adjusting them until the error meets the requirements. A modulation mode selection unit is used to select either the periodic modulation signal generator chirp or the shift stage chirp. The signal generator chirp keeps the displacement stage moving at a constant speed and calculates the pulse delay based on the period value; the displacement stage chirp keeps the laser pulse uniform and calculates the displacement stage speed based on the period value; the apodization modulation selects either laser power modulation or grating space modulation, where laser power modulation controls the amount of refractive index change by dynamically adjusting the power; and grating space modulation is achieved by controlling the laser focus and the fiber core position. A grating fabrication implementation unit is used to fabricate a nonlinear chirped fiber grating on an optical fiber core according to an optimized function and a selected implementation method. Specifically, the iterative optimization process by repeatedly adjusting the two functions is as follows: A similarity calculation is performed between the calculated reflection spectrum and the target reflection spectrum to obtain the error between the reflection spectrum and the target; the apodization function is adjusted based on the error value. Spectral pattern; recalculate the error between the reflection spectrum and the target spectral pattern, and adjust the periodic modulation function. The error between the reflection spectrum and the target spectrum is calculated again; this process is repeated iteratively to obtain the final nonlinear apodization function. and nonlinear periodic modulation function .

9. A computer-readable storage medium having a computer program stored thereon, characterized in that, The computer program is executed by a processor according to any one of claims 1-7, a method for fabricating a nonlinear chirped fiber grating.

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