Method and system for preparing chirped fiber grating by triggering ultrafast laser pulse string output through displacement feedback counting hardware

Abstract

The invention discloses a method and a system for preparing a chirped fiber bragg grating by triggering ultrafast laser pulse string output through displacement feedback counting hardware, and belongs to the technical field of fiber bragg grating preparation. The system comprises a laser light source, a light beam transmission system, an objective lens, an optical fiber holder, an optical fiber, a linear motor displacement table, a driving control module and a computer; according to the method for preparing the chirped fiber grating, pulse signals fed back by a displacement platform grating ruler are counted through a hardware circuit, and when a pulse count value corresponding to a chirp period is reached, ultrafast laser direct writing is triggered to prepare the chirped fiber grating. According to the invention, the pulse string with low pulse energy is adopted to replace a single pulse with high pulse energy to realize the same modification effect on the refractive index of the material, and the material damage is reduced, so that the optical transmission loss is reduced. The method for preparing the chirped fiber grating has the characteristics of high grating period processing precision and controllability and low optical transmission loss.

Description

Technical Field

[0001] The invention belongs to the technical field of fiber grating preparation, and in particular relates to a method and a system for preparing chirped fiber grating by triggering ultrafast laser pulse train output through displacement feedback counting hardware. Background Art

[0002] As an important branch of fiber Bragg grating, chirped fiber Bragg grating plays a very important role in the fields of fiber sensing, fiber laser and fiber communication.

[0003] The chirped fiber grating is prepared by ultrafast laser single-point direct writing. The laser beam is focused in the fiber core through the objective lens, and the laser focus is scanned along the fiber transmission direction by the translation stage, with a relative variable speed motion of v(t). Therefore, each laser pulse will act on different positions of the fiber core in turn to modify the material and form a periodically changing grating structure. When the repetition frequency of the laser pulse output is f, the distance between two adjacent laser pulse action points is the chirp period of the chirped fiber grating. The chirp period can be changed by adjusting the scanning speed of the translation stage, thus achieving customized processing of chirped fiber gratings.

[0004] However, by regulating the chirp period of the fiber Bragg grating through variable speed scanning, the servo control tracking error in the motion control system will be coupled into the chirp period. Moreover, compared with uniform speed scanning, the acceleration required in variable speed scanning will further deteriorate the tracking error, resulting in low chirp period accuracy and poor controllability. In addition, the use of a single laser pulse to modify the refractive index of a single period of the fiber Bragg grating requires a high pulse energy. The high energy pulse directly acts on the fiber core and causes high damage to the material, resulting in a large transmission loss of the fiber Bragg grating.

[0005] Therefore, a method is needed to improve the accuracy and controllability of the chirp period of chirped fiber gratings prepared by ultrafast laser direct writing, and to reduce the light transmission loss, so as to obtain high-quality chirped fiber gratings. Summary of the invention

[0006] In view of the shortcomings of low chirp period accuracy and poor controllability in the preparation of chirped fiber gratings in the prior art, the present invention provides a method and system for preparing chirped fiber gratings by triggering ultrafast laser pulse train output through displacement feedback counting hardware, adopting a method of direct writing of ultrafast laser pulse output triggered by displacement feedback counting hardware instead of a variable speed scanning method to control the chirp period; adopting a method of modifying the refractive index of a material by using a pulse train of low pulse energy instead of a single pulse of high pulse energy to reduce damage to the material, thereby reducing light transmission loss.

[0007] The present invention is achieved through the following technical solutions:

[0008] A system for preparing chirped fiber gratings by triggering ultrafast laser pulse train output through displacement feedback counting hardware, comprising a laser light source 1, a beam transmission system 3, an objective lens 4, a fiber clamp 5, an optical fiber 6, a linear motor displacement stage 7, a drive control module 10 and a computer 12; the optical fiber 6 is fixed on the linear motor displacement stage 7 through the fiber clamp 5, and a photoelectric encoder 8 and a motor 9 are integrated on the linear motor displacement stage 7; the photoelectric encoder 8 is connected to an orthogonal encoding circuit 11 integrated on the drive control module 10 through a wire, the motor 9 is connected to the drive control module 10 through a wire, and the drive control module 10 is connected to the computer 12; the drive control module 10 is used to drive the motor 9 The linear motor translation stage 7 is controlled to move. The photoelectric encoder 8 is used to convert the real-time position information of the linear motor translation stage into an orthogonal analog signal or a digital signal, and then output it to the drive control module 10. The drive control module 10 processes the received orthogonal analog signal or digital signal to obtain a real-time feedback position count value. When the position count value preset in the memory in the drive control module 10 matches the real-time feedback position count value, the drive control module 10 sends a control signal to the modulator 2, thereby triggering the output of the ultrafast laser pulse train; the ultrafast laser pulse train emitted by the laser light source 1 passes through the beam transmission system 3 and the objective lens 4 in sequence and is focused on the core of the optical fiber 6 for pulse train direct writing.

[0009] Furthermore, the modulator 2 is located inside the laser light source 1 or is independently arranged in the optical path.

[0010] Furthermore, the drive control module 10 includes an orthogonal encoding circuit 11, a subdivision circuit 13 and a counter 14; the orthogonal encoding circuit 11 is used to encode the orthogonal analog signal or digital signal output by the photoelectric encoder 8 to obtain an orthogonal digital signal, and send it to the subdivision circuit 13; the subdivision circuit 13 is used to subdivide the orthogonal digital signal output by the orthogonal encoding circuit 11, and output pulses to the counter 14; the counter 14 is used to count the pulses output by the subdivision circuit 13 to obtain a position count value.

[0011] Furthermore, the computer 12 is used to write a program, write the position count value corresponding to the designed chirp period into the memory of the drive control module 10, and write the instruction into the register.

[0012] Furthermore, the ultrafast laser pulse train output is calculated based on the laser repetition frequency and the number of pulse trains required to calculate the duration of the pulse train output when a single trigger is used.

[0013] On the other hand, the present invention provides a method for preparing a chirped fiber grating by triggering an ultrafast laser pulse train output through displacement feedback counting hardware, which specifically comprises the following steps:

[0014] A1, the computer 12 writes the position count value and related configuration corresponding to the designed chirp period into the memory and register in the drive control module 10;

[0015] A2, the computer 12 sends a command to the drive control module 10, the drive control module 10 controls the motor 9 to drive the linear motor displacement stage 7 to move linearly at a certain speed, and at the same time the photoelectric encoder 8 outputs the real-time position information of the linear motor displacement stage 7 to the drive control module 10;

[0016] A3. When the real-time feedback position count value matches the set position count value, the drive control module 10 sends a control signal to the modulator 2 to trigger the ultrafast laser pulse train output;

[0017] A4, the count value of counter 14 is cleared to 0, and a new round of counting is started, and step A3 is repeated. The number of repetitions is the number of grating structures in the chirped fiber grating, until the linear motion command of the linear motor translation stage 7 is executed, all grating structures are directly written, and the chirped fiber grating is prepared.

[0018] Further, in step A1, the position count value corresponding to the chirp period is obtained by the following design method, which specifically includes the following steps:

[0019] B1. Calculate the grating period ∧0, mλ corresponding to the test wavelength according to the reflection equation of the Bragg grating B =2n eff ∧, where n eff represents the effective refractive index of the fiber core, ∧ represents the grating period, and λ B represents the reflection wavelength, i.e. the test wavelength, and m represents the order of the grating;

[0020] B2. Compare the calculated grating period corresponding to the test wavelength with the focused spot diameter. Wherein, λ is the wavelength of the laser light source used for processing, and NA is the numerical aperture of the objective lens; the Bragg grating periods of different orders are calculated in ascending order, and compared with the focus spot diameter; if the order is close to the focus spot diameter, the high-order Bragg grating period is calculated in the order of increasing order, until the ratio of the calculated grating period to the focus spot diameter is within the range of two to five times, the grating order and the grating period are determined;

[0021] B3. Select the origin of coordinates on the optical fiber to be prepared; take the grating period corresponding to the test wavelength as the center, and calculate the grating period of other grating structures in the chirped fiber grating according to the arithmetic change relationship of the chirp rate; the calculation formula of the chirp period is Wherein, ∧(z) represents the chirp period at point z, c represents the chirp coefficient, and ∧0 is the grating period corresponding to the test wavelength;

[0022] B4. According to the proportional relationship between the position count value and the actual distance in the drive control module 10 The chirp period is converted into a position count value, and the decimal part is rounded off.

[0023] Furthermore, in step A2, the moving speed range of the linear motor translation stage 7 is 0.1 mm / s-5 mm / s.

[0024] The present invention provides a method and system for preparing chirped fiber gratings by triggering ultrafast laser pulse output through displacement feedback counting hardware. The principle is as follows:

[0025] The present invention adopts a method of directly writing chirped fiber grating by triggering ultrafast laser pulse output through displacement feedback counting hardware, mainly through the process of laser focus scanning inside the fiber core, the hardware circuit counts the real-time displacement feedback value of the displacement stage. When the count value matches the respective count values ​​in the memory corresponding to the respective chirp cycle distances of the fiber grating, the hardware circuit triggers the laser light source to emit a laser pulse. The above-mentioned displacement stage position feedback is mainly realized by a linear displacement encoder, i.e., a grating ruler, which directly outputs an orthogonal analog or digital signal to an orthogonal encoding circuit, and the orthogonal encoding circuit outputs an orthogonal digital signal after encoding, and the subdivision circuit subdivides the orthogonal digital signal and outputs a pulse. The counter counts the pulses and converts them into a count value representing the current real-time position of the displacement stage. Among them, before the laser direct writing chirp cycle, the position count values ​​corresponding to the respective chirp cycle distance values ​​need to be written into the memory in advance, and when the real-time feedback count value matches the respective position count values ​​pre-stored in the memory during the scanning process, the laser pulse output is triggered.

[0026] The ultrafast laser acts on the quartz glass material of the optical fiber core to modify the refractive index of the material, and the pulse energy needs to meet a certain energy threshold. After reaching the energy threshold, the adjustment of the refractive index change in the region can be carried out by adjusting the pulse energy or by adjusting the number of laser pulses acting on the region. The latter uses lower energy laser pulses to act on the region, which produces a cumulative effect on the refractive index change caused by multiple pulses, thereby achieving the same refractive index change as a single laser pulse with a single pulse energy; and the material stress damage generated inside the material by the lower pulse energy is lower, so that the material produces a lower scattering effect on light. Therefore, the present invention uses displacement feedback counting hardware to trigger a laser pulse train rather than a single laser pulse to perform direct writing modification processing on the optical fiber core.

[0027] Compared with the prior art, the advantages of the present invention are as follows:

[0028] (1) The system and method for preparing chirped fiber gratings proposed in the present invention effectively avoid the coupling of the following error in servo motion control into the chirp period and the further influence of the acceleration of variable speed scanning on the following error;

[0029] (2) Real-time counting and triggering of laser pulse output through dedicated hardware circuit, short response time, and higher trigger positioning accuracy;

[0030] (3) The position count values ​​corresponding to each chirp cycle distance value can be written into the memory in advance, and the position count values ​​can be arbitrarily configured;

[0031] (4) A pulse train with low pulse energy is used instead of a single pulse with high pulse energy, which reduces the optical transmission loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for the specific embodiments or the description of the prior art. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn according to the actual scale.

[0033] Figure 1 A schematic diagram of the structure of a system for preparing a chirped fiber Bragg grating by triggering ultrafast laser pulse output through displacement feedback counting hardware of the present invention;

[0034] Among them, laser light source 1, modulator 2, beam transmission system 3, objective lens 4, fiber holder 5, optical fiber 6, linear motor translation stage 7, photoelectric encoder 8, point machine 9, drive control module 10, orthogonal encoding circuit 11, computer 12, subdivision circuit 13, counter 14;

[0035] Figure 2 A schematic flow chart of a method for preparing a chirped fiber Bragg grating by triggering ultrafast laser pulse train output through displacement feedback counting hardware of the present invention;

[0036] Figure 3 Partial timing diagram of the fiber position and laser pulse output of the chirped fiber grating prepared in the embodiment of the present invention and schematic diagrams of the chirped fiber grating structure prepared by different methods;

[0037] Among them, (a) is a partial timing diagram of the fiber position and laser pulse output of this embodiment, (b) is a schematic diagram of the partial structure of the chirped fiber grating corresponding to (a), and (c) is a schematic diagram of the partial structure of the chirped fiber grating when the chirped fiber grating is prepared by uniformly variable speed motion through a translation stage under the same design, and the following error of the servo system is coupled into the chirp period, and the error is coupled into the chirp period randomly. DETAILED DESCRIPTION

[0038] In order to clearly and completely describe the technical solution and its specific working process of the present invention, the specific implementation methods of the present invention are as follows in conjunction with the accompanying drawings of the specification:

[0039] Example 1

[0040] like Figure 1 As shown, this embodiment provides a system for preparing chirped fiber Bragg gratings by triggering ultrafast laser pulse train output through displacement feedback counting hardware, comprising a laser light source 1, a beam transmission system 3, an objective lens 4, a fiber clamp 5, an optical fiber 6, a linear motor displacement stage 7, a drive control module 10 and a computer 12; the optical fiber 6 is fixed on the linear motor displacement stage 7 through the fiber clamp 5, and the linear motor displacement stage 7 is integrated with a photoelectric encoder 8 and a motor 9; the photoelectric encoder 8 is connected to an orthogonal encoding circuit 11 integrated on the drive control module 10 through a wire, the motor 9 is connected to the drive control module 10 through a wire, and the drive control module 10 is connected to the computer 12; the drive control module 10 is used to The driving motor 9 controls the movement of the linear motor translation stage 7. The photoelectric encoder 8 is used to convert the real-time position information of the movement of the linear motor translation stage into an orthogonal analog signal or a digital signal, and then output it to the driving control module 10. The driving control module 10 processes the received orthogonal analog signal or digital signal to obtain a real-time feedback position count value. When the preset position count value in the memory in the driving control module 10 matches the real-time feedback position count value, the driving control module 10 sends a control signal to the modulator 2, thereby triggering the output of the ultrafast laser pulse train; the ultrafast laser pulse train emitted by the laser light source 1 passes through the beam transmission system 3 and the objective lens 4 in turn and is focused on the core of the optical fiber 6 for pulse train direct writing.

[0041] The modulator 2 is located inside the laser light source 1 or is independently arranged in the optical path.

[0042] The drive control module 10 includes an orthogonal encoding circuit 11, a subdivision circuit 13 and a counter 14; the orthogonal encoding circuit 11 is used to encode the orthogonal analog signal or digital signal output by the photoelectric encoder 8 to obtain an orthogonal digital signal, and send it to the subdivision circuit 13; the subdivision circuit 13 is used to subdivide the orthogonal digital signal output by the orthogonal encoding circuit 11, and output pulses to the counter 14; the counter 14 is used to count the pulses output by the subdivision circuit 13 to obtain a position count value.

[0043] The computer 12 is used to write a program, write the position count value corresponding to the designed chirp period into the memory of the drive control module 10, and write the instruction into the register.

[0044] The ultrafast laser pulse train output is based on the laser repetition frequency and the number of pulse trains required to calculate the duration of the pulse train output when a single trigger is used.

[0045] The laser light source 1 used in this embodiment is a CARBIDE-CB3-80W-2mJ laser light source 1 produced by Light Conversion Company. The typical output wavelength of the laser light source 1 is 1030nm, which is converted to 515nm after frequency doubling, with a repetition frequency of 500kHz and a single pulse energy of 8μJ. After modulation by the beam transmission system 3, the oil-immersed objective lens 4 focuses the femtosecond laser onto the core of the optical fiber 6, and the single pulse energy acting on the core of the optical fiber 6 is 100nJ; the optical fiber 6 used is a single-mode optical fiber (CORNING-28e).

[0046] The drive control module 10 in this embodiment is programmed using Aerotech motion controller Ndrive ML and A3200 Motion Composer, and the position synchronous output (PSO) function in the motion control system of A3200 is a control method embodiment of the present invention. This embodiment uses an oil immersion objective lens with a numerical aperture of 1.42.

[0047] The orthogonal encoding circuit used in Aerotech motion controllers to process position feedback in real time has a response time of several hundred nanoseconds to a few microseconds, while the real-time performance of computer software processing is usually in the millisecond range. Therefore, the former has higher real-time performance, thereby achieving higher trigger positioning accuracy, and thus the laser acting on the fiber core during fiber Bragg grating processing has higher positioning accuracy.

[0048] The error of the fiber Bragg grating prepared by the method provided in this embodiment is not affected by the following error of the servo system, and the fiber Bragg grating as a whole is only affected by the positioning error and mechanical error of the displacement stage, which can reach a minimum of ±5nm. That is, the method of preparing chirped fiber Bragg grating by triggering ultrafast laser pulse train output through displacement feedback counting hardware can obtain a higher precision chirp period.

[0049] Example 2

[0050] This embodiment provides a method for preparing a chirped fiber Bragg grating by triggering ultrafast laser pulse train output through displacement feedback counting hardware, which specifically includes the following steps:

[0051] Step 1: The computer 12 writes the position count value corresponding to the designed chirp cycle into the memory of the drive control module 10, that is, setting the distance array [$Index] = ROUND (3461 + (0.00607 * $Index)), where $Index is the index of the array, the array length is 9390, ROUND is the rounding instruction, 3461 is the position count value corresponding to the first chirp cycle, and the position count values ​​in the distance array are statistically arithmetic after rounding;

[0052] The specific steps of designing the position count value corresponding to the chirp period are as follows:

[0053] a) By focusing the spot diameter Analyze the processing resolution, where λ represents the laser wavelength of 515nm, NA represents the objective lens numerical aperture of 1.42, and the calculated processing resolution is about 442.46nm; calculate the grating period ∧0 corresponding to the test wavelength according to the reflection equation of the Bragg grating, mλ B =2n eff ∧, where n eff represents the effective refractive index of the fiber core, which is 1.45, ∧ represents the grating period, and λ B Represents the reflection wavelength, m represents the order of the grating, and thus the test wavelength is calculated, that is, when the reflection wavelength is 1550nm, the period of the first-order Bragg grating is 534.48nm; this period is close to the processing resolution. In order to avoid the aliasing problem of adjacent grating structures, the second-order grating period is calculated and the calculated period is 1064.96nm. This period is significantly larger than the estimated processing resolution, so the second-order grating is used with a period of 1064.96nm;

[0054] b) According to the period expression of linear chirped grating Calculate the period of other grating structures, where ∧(z) represents the chirp period at point z, c represents the chirp coefficient, which is designed to be 50nm / cm, the grating length is designed to be L=10mm, the grating midpoint is selected as the origin z=0, and the chirp period of the left endpoint is calculated Chirp period at the right end According to the arithmetic progression calculation formula, the fiber contains 9390 grating structures, and the chirp period increases arithmetic progression from 1056.34nm to 1073.58nm.

[0055] c) In the motion control system, the position count value corresponding to the actual distance of 1mm is set to 3276800. According to this proportional relationship, The calculated count value is rounded to an integer. The position count value corresponding to the grating period of 1056.34nm is 3461, and the position count value corresponding to the grating period of 1073.58nm is 3518. The position count value corresponding to the chirp period of the chirped fiber grating designed in this embodiment is 3461 as the first item, 3518 as the last item, and the ... The rounded values ​​of the arithmetic sequence with the common difference constitute an arithmetic sequence statistically.

[0056] The pulse train with low pulse energy replaces the single pulse with high pulse energy. The moving speed of the translation stage is 0.5 mm / s. When the laser pulse repetition frequency is 500 kHz, in order to avoid the aliasing of adjacent grating structures, the number of pulse trains n ranges from 2 to 90. The corresponding laser pulse output time calculation process is as follows: time The duration of emitting 2 pulses is 4us, and the duration of emitting 90 pulses is 180us. When a single trigger is performed, the drive control module 10 sends a signal to the modulator 2 to control the duration of the laser pulse output.

[0057] Before the chirp cycle is processed by laser direct writing, the position count values ​​corresponding to the distance values ​​of each chirp cycle are pre-written into the memory. The position count values ​​in the memory can be configured arbitrarily, so the chirp cycle of the chirped fiber grating prepared by ultrafast laser pulse output triggered by displacement feedback counting hardware is more controllable. Using a pulse train with low pulse energy instead of a single pulse with high pulse energy can achieve the same effect of modifying the refractive index of the material, and the material stress damage generated inside the material is lower, so that the material has a lower scattering effect on light, reducing the light transmission loss.

[0058] Step 2: Configure other parameters of the control system. Set the trigger mode to distance trigger, and the trigger distance is the value in the distance array; set the trigger output mode to pulse output, where the drive control module 10 outputs a signal to the modulator 2 in pulse output. Each time the trigger is triggered, the number of output pulses is 1, and the pulse width is 2nμs; configure the drive control module 10 to track the photoelectric encoder of the x-axis;

[0059] Step 3: Output command to make the linear motor displacement stage 7 move linearly, with a moving distance of 10 mm and a moving speed of 0.5 mm / s;

[0060] Step 4: Start the tracking photoelectric encoder at the same time;

[0061] Step 5: After the linear motor displacement stage 7 has finished its linear motion, the pulse generator is turned off;

[0062] Among them, during the linear motion of the linear motor displacement stage 7, the signal transmission process inside the control system is as follows:

[0063] 1) The photoelectric encoder 8 feeds back the real-time displacement information of the linear motor displacement stage 7 and sends an electrical signal to the orthogonal encoding circuit 11;

[0064] 2) The orthogonal encoding circuit 11 of the driving control module 10 encodes the electrical signal fed back by the translation stage 7 and outputs the orthogonal digital signal to the subdivision circuit 13. The subdivision circuit 13 subdivides the orthogonal digital signal output by the orthogonal encoding circuit 11 and outputs a pulse. The counter 13 receives the pulse and starts counting from 0;

[0065] 3) When the real-time count value matches the set position count value in the memory, that is, the value in the distance array, the drive control module 10 outputs a pulse to the modulator 2, the pulse width is 2nμs, and the laser light source 1 outputs n pulse trains with a single pulse energy of 100nJ;

[0066] 4) The count is reset and starts again from 0, the distance array index is increased by 1, and steps 1), 2), and 3) are repeated to compare with the next position count value in the distance array until the linear motion ends.

[0067] like Figure 3 As shown, (a) is a partial timing diagram of the fiber position and the laser pulse output, (b) is a partial structural schematic diagram of the chirped fiber grating prepared in this embodiment, and (c) is a partial structural schematic diagram of the chirped fiber grating when the chirped fiber grating is prepared by uniformly variable speed motion through the displacement stage under the same design, and the following error of the servo system is coupled into the chirp period. The error is coupled into the chirp period randomly. The fiber grating prepared in this embodiment contains nearly 10,000 grating structures, and the grating structure is randomly affected by the following error. The error of tens of nanometers will have a great impact on the bandwidth and other performance of the chirped fiber grating. When the scanning speed of the displacement stage changes, the fluctuation of the scanning speed causes the position of the laser pulse acting on the fiber core to shift to varying degrees, which will also cause the chirp period to fluctuate. The method for preparing chirped fiber grating proposed in the present invention uses a method of directly writing ultrafast laser pulse train output triggered by displacement feedback counting hardware instead of a variable speed scanning method to control the chirp period, effectively avoiding the coupling of the following error in servo motion control into the chirp period and the further influence of the acceleration of the variable speed scanning on the following error. Triggering the laser pulse by a displacement feedback device can also eliminate the influence of speed fluctuations on the laser pulse position in the preparation of chirped fiber grating by variable speed scanning. It can be seen that the chirp period accuracy of the chirped fiber grating prepared by the method proposed in the present invention is higher and the controllability is stronger.

[0068] The preferred embodiments of the present invention are described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, a variety of simple modifications can be made to the technical solution of the present invention, and these simple modifications all belong to the protection scope of the present invention.

[0069] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0070] In addition, various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A system for preparing chirped fiber Bragg gratings by triggering ultrafast laser pulse train output through displacement feedback counting hardware, characterized in that: The invention comprises a laser light source (1), a beam transmission system (3), an objective lens (4), a fiber holder (5), an optical fiber (6), a linear motor displacement stage (7), a drive control module (10) and a computer (12); the optical fiber (6) is fixed on the linear motor displacement stage (7) through the fiber holder (5); a photoelectric encoder (8) and a motor (9) are integrated on the linear motor displacement stage (7); the photoelectric encoder (8) is connected to an orthogonal encoding circuit (11) integrated on the drive control module (10) through a wire; the motor (9) is connected to the drive control module (10) through a wire; the drive control module (10) is connected to the computer (12); the drive control module (10) is used to drive the motor (9) to control the linear motor The displacement stage (7) moves, and the photoelectric encoder (8) is used to convert the real-time position information of the linear motor displacement stage into an orthogonal analog signal or a digital signal, and then output it to the drive control module (10). The drive control module (10) processes the received orthogonal analog signal or digital signal to obtain a real-time feedback position count value. When the position count value preset in the memory in the drive control module (10) matches the real-time feedback position count value, the drive control module (10) sends a control signal to the modulator (2), thereby triggering the output of an ultrafast laser pulse train; the ultrafast laser pulse train emitted by the laser light source (1) passes through the beam transmission system (3) and the objective lens (4) in sequence, and then is focused on the core of the optical fiber (6) to perform pulse train direct writing.

2. A system for preparing chirped fiber Bragg gratings by triggering ultrafast laser pulse train output through displacement feedback counting hardware as claimed in claim 1, characterized in that: The modulator (2) is located inside the laser light source (1) or is independently arranged in the optical path.

3. A system for preparing chirped fiber Bragg gratings by triggering ultrafast laser pulse train output through displacement feedback counting hardware as claimed in claim 1, characterized in that: The drive control module (10) comprises an orthogonal encoding circuit (11), a subdivision circuit (13) and a counter (14); the orthogonal encoding circuit (11) is used to encode the orthogonal analog signal or digital signal output by the photoelectric encoder (8) to obtain an orthogonal digital signal, and send it to the subdivision circuit (13); the subdivision circuit (13) is used to subdivide the orthogonal digital signal output by the orthogonal encoding circuit (11) and output pulses to the counter (14); the counter (14) is used to count the pulses output by the subdivision circuit (13) to obtain a position count value.

4. A system for preparing chirped fiber Bragg gratings by triggering ultrafast laser pulse train output through displacement feedback counting hardware as claimed in claim 1, characterized in that: The computer (12) is used to write a program, write the position count value corresponding to the designed chirp cycle into the memory of the drive control module (10), and write the instruction into the register.

5. A system for preparing chirped fiber Bragg gratings by triggering ultrafast laser pulse train output through displacement feedback counting hardware as claimed in claim 1, characterized in that: The ultrafast laser pulse train output is calculated based on the laser repetition frequency and the number of required pulse trains.

6. A method for preparing chirped fiber Bragg gratings by triggering ultrafast laser pulse train output through displacement feedback counting hardware, which is implemented by the system of claim 1, characterized in that: The specific steps include: A1, the computer (12) writes the position count value corresponding to the designed chirp period and related configuration into the memory and register in the drive control module (10); A2, the computer (12) sends a command to the drive control module (10), and the drive control module (10) controls the motor (9) to drive the linear motor displacement stage (7) to move linearly at a certain speed, and at the same time, the photoelectric encoder (8) outputs the real-time position information of the movement of the linear motor displacement stage (7) to the drive control module (10); A3. When the position count value fed back in real time matches the set position count value, the drive control module (10) sends a control signal to the modulator (2) to trigger the output of the ultrafast laser pulse train; A4, the count value of the counter (14) is cleared to 0, and a new round of counting is started, and step A3 is repeated. The number of repetitions is the number of grating structures in the chirped fiber grating, until the linear motion command of the linear motor translation stage (7) is executed, all grating structures are directly written, and the chirped fiber grating is prepared.

7. The method for preparing chirped fiber Bragg grating by triggering ultrafast laser pulse train output by displacement feedback counting hardware as claimed in claim 6, characterized in that: In step A1, the position count value corresponding to the chirp period is obtained by the following design method, which specifically includes the following steps: B1. Calculate the grating period ∧0, mλ corresponding to the test wavelength according to the reflection equation of the Bragg grating B =2n eff ∧, where n eff represents the effective refractive index of the fiber core, ∧ represents the grating period, and λ B represents the reflection wavelength, i.e. the test wavelength, and m represents the order of the grating; B2. Compare the calculated grating period corresponding to the test wavelength with the focused spot diameter. Wherein, λ is the wavelength of the laser light source used for processing, and NA is the numerical aperture of the objective lens; the Bragg grating periods of different orders are calculated in ascending order, and compared with the focus spot diameter; if the order is close to the focus spot diameter, the high-order Bragg grating period is calculated in the order of increasing order, until the ratio of the calculated grating period to the focus spot diameter is within the range of two to five times, the grating order and the grating period are determined; B3. Select the origin of coordinates on the optical fiber to be prepared; take the grating period corresponding to the test wavelength as the center, and calculate the grating period of other grating structures in the chirped fiber grating according to the arithmetic change relationship of the chirp rate; the calculation formula of the chirp period is Wherein, ∧(z) represents the chirp period at point z, c represents the chirp coefficient, and ∧0 is the grating period corresponding to the test wavelength; B4. According to the drive control module (10) The chirp period is converted into a position count value, and the decimal part is rounded off.

8. The method for preparing chirped fiber Bragg grating by triggering ultrafast laser pulse train output by displacement feedback counting hardware as claimed in claim 6, characterized in that: In step A2, the moving speed range of the linear motor translation stage (7) is 0.1 mm / s-5 mm / s.

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