Apodization FBG circle-by-circle inscribing system and method based on variable radius method
The variable radius method of the toe-cut FBG turn-by-turn writing system solves the low SMSR problem of the traditional turn-by-turn FBG writing method, improves the spectral quality, and is suitable for mode division multiplexing systems and fiber optic sensing.
Patent Information
- Application Number
- CN202511226240.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-08-29
- Publication Date
- 2025-10-14
AI Technical Summary
The side mode suppression ratio (SMSR) of FBG written by the traditional turn-by-turn method in ring-core fiber is poor, which affects the demodulation of sensing signals and the spectral quality.
A toe-cut FBG turn-by-turn writing system based on the variable radius method is adopted. Through the laser beam shaping module, fiber trajectory control module and fiber real-time monitoring module, the fiber is controlled to move according to the sine function and cosine function in the Y-axis and Z-axis directions, forming a Gaussian-distributed refractive index modulation area, ensuring that the laser beam acts on the fiber core, and monitoring the side mode suppression ratio and reflectivity of the reflection spectrum in real time.
It significantly improves the side mode suppression ratio and reflectivity of FBG, improves the spectral quality, and is suitable for scenarios such as mode division multiplexing systems and optical fiber sensing.
Smart Images
Figure CN120779516A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present application relates to the technical field of fiber inscription, in particular to a variable radius method-based apodized FBG turn-by-turn inscription system and method. BACKGROUND
[0002] With the improvement of communication capacity and the progress of fiber drawing technology, various special core structure fibers such as large core, multi-core, spiral core and ring core are widely used. Among them, the unique ring core structure of the ring core fiber (RCF) significantly increases the internal mode interval, ensuring that different modes can maintain excellent stability in long-distance transmission, and becoming the preferred medium for signal transmission in wavelength division multiplexing (WDM) systems. At the same time, the fiber Bragg grating (FBG) plays a key role in filtering and signal enhancement in WDM systems, so it is of great significance to prepare high-quality FBG in RCF. Traditional FBG preparation techniques mainly include phase mask method and direct writing method, which can be further divided into point-by-point method, line-by-line method and face-by-face method.
[0003] In recent years, the turn-by-turn method has become the preferred technology for preparing FBG on RCF due to its excellent adaptability to RCF. However, although this method can achieve high signal-to-noise ratio and high reflectivity of FBG, its side mode suppression ratio (SMSR) is usually poor, which will adversely affect the demodulation of sensing signals, the spectral quality of fiber lasers and the signal crosstalk of wavelength division multiplexing systems. The key to solving this problem is to design the FBG refractive index modulation as a Gaussian-like distribution, so that the modulation amount along the fiber axis presents a high-middle and low-both-sides shape, i.e. apodization. However, as of now, there is no reported technology scheme for preparing apodized FBG based on the turn-by-turn method. SUMMARY
[0004] The purpose of the present application is to provide a variable radius method-based apodized FBG turn-by-turn inscription system and method, which can solve the low SMSR problem of traditional turn-by-turn FBG inscription and improve the spectral quality.
[0005] To achieve the above-mentioned purpose, the present application provides the following solutions:
[0006] In a first aspect, the present application provides a variable radius method-based apodized FBG turn-by-turn inscription system, comprising: a laser beam shaping module, a fiber track control module and a fiber real-time monitoring module; the laser beam shaping module is used to generate laser and shape the laser into a laser beam, and the laser beam is focused to form a refractive index modulation region; the fiber real-time monitoring module is used to monitor the side mode suppression ratio and reflectivity of the reflected spectrum in real time, and observe the real-time pose of the fiber, and control the pose of the fiber through the fiber track control module according to the real-time pose of the fiber, to ensure that the refractive index modulation region of the laser beam is always located on the core of the fiber; the fiber is a ring core fiber.
[0007] The optical fiber track control module is used for clamping the optical fiber and controlling the optical fiber to move in the Y-axis and Z-axis directions according to a sine function and a cosine function respectively, so as to form a single annular inscription track and control the optical fiber to step along the X-axis according to a preset grating period; the direction of the X-axis is the axial direction of the optical fiber, the direction of the Y-axis is the vertical upward direction of the cross section of the optical fiber, and the Z-axis is perpendicular to the plane formed by the X-axis and the Y-axis; the inscription radii of the annular inscription tracks change along the X-axis according to a Gaussian distribution, so as to realize the apodization distribution of the refractive index modulation region.
[0008] Optionally, the laser beam shaping module comprises a femtosecond laser, a beam shaper and an oil lens; the femtosecond laser is used to generate femtosecond laser, the beam shaper is used to shape the femtosecond laser into a laser beam, and the oil lens is used to converge the laser beam to form the refractive index modulation region.
[0009] Optionally, the parameters of the femtosecond laser are as follows: wavelength 515 nm, pulse energy 200 nJ, pulse width 290 fs and repetition frequency 200 kHz.
[0010] Optionally, the optical fiber real-time monitoring module comprises a CCD camera, a circulator, a spectrometer and a broadband light source; the CCD camera is confocal with the oil lens and is used to observe the real-time pose of the optical fiber; the broadband light source and the spectrometer are connected with the optical fiber through the circulator; the broadband light source inputs an optical signal into the optical fiber through the circulator, and the spectrometer receives the reflected optical signal from the optical fiber through the circulator and analyzes the side mode suppression ratio and reflectivity of the reflected spectrum.
[0011] Optionally, the laser beam shaping module further comprises an electrically controlled shutter; the electrically controlled shutter is arranged between the oil lens and the optical fiber and is used to control whether the laser passes through, so as to realize the preparation of FBG with different direct writing tracks.
[0012] Optionally, the optical fiber track control module adopts a high-precision three-dimensional displacement platform, and the optical fiber is fixed on the three-dimensional displacement platform through a clamp; the three-dimensional displacement platform drives the optical fiber to move in the Y-axis and Z-axis directions according to a sine function and a cosine function respectively, so as to form an annular inscription track and control the optical fiber to step along the X-axis according to a preset grating period.
[0013] Optionally, the motion equations of the three-dimensional displacement platform in the Y-axis and Z-axis directions are as follows: Y t =Y0+dR(n)×sinθ and Z t =Z0+dR(n)×cosθ, θ is an angle variable of 0-2π, dR(n) is a real-time inscription radius, Y0 and Z0 are the Y-axis and Z-axis coordinate values at an initial time, Y t and Z t are the Y-axis and Z-axis coordinate values at a time t.
[0014] Optionally, the radius of each annular inscription track varies along the X-axis according to a Gaussian-like distribution, specifically, the radius of the middle annular inscription track is the smallest, and gradually increases to r+Δr towards both sides, where Δr is the variation of the radius; the radius variation function according to the Gaussian-like distribution is:
[0015] dR(n)=r-Δr*A(n),0≤n≤N(1)
[0016] wherein dR(n) is the radius corresponding to each period, A(n) is a grating function, N is the total number of periods, and n is the current period number.
[0017] Optionally, the grating function adopts a Gaussian function, a linear function, a quadratic function or a sine function; the Gaussian function is as follows:
[0018] A(n)=e -(N / 2-n)2 / (4b2) ,0≤n≤N(2)
[0019] wherein b is the inflection point of the Gaussian function.
[0020] The sine function is as follows:
[0021] A(n)=sin(n*π / N),0≤n≤N(3)
[0022] The quadratic function is as follows:
[0023] A(n)=(-4n 2 / N+4*n) / N,0≤n≤N(4)
[0024] The linear function is as follows:
[0025] A(n)=1-|N-2n| / N,0≤n≤N (5).
[0026] In a second aspect, the application provides a variable-radius apodized FBG circle-by-circle inscription method, comprising the following steps:
[0027] The ring core optical fiber is fixed on the three-dimensional displacement platform through a clamp; in the coordinate system of the three-dimensional displacement platform, the X-axis is the optical fiber axial direction, the Y-axis is the direction perpendicular to the optical fiber section and upward, and the Z-axis is perpendicular to the plane formed by the X-axis and the Y-axis.
[0028] The real-time pose of the optical fiber is observed through the optical fiber real-time monitoring module, and the pose of the optical fiber is controlled through the optical fiber track control module according to the real-time pose of the optical fiber, so as to ensure that the refractive index modulation region of the laser beam is always located on the fiber core of the optical fiber.
[0029] The laser beam shaping module generates laser and shapes the laser into a laser beam, and the laser beam is focused to form a refractive index modulation region.
[0030] The optical fiber is controlled to move in the Y-axis and Z-axis directions according to a sine function and a cosine function respectively by the optical fiber track control module, a single annular writing track is formed, and the optical fiber is controlled to step along the X-axis according to a preset grating period; in the writing process, the writing radius of each annular writing track changes along the X-axis according to a Gaussian-like distribution, the apodization distribution of the refractive index modulation region is realized, and the side mode suppression ratio and the reflectivity of the reflected spectrum are monitored in real time.
[0031] According to the specific embodiments provided in the application, the following technical effects are disclosed:
[0032] The application provides a variable-radius apodized FBG writing system and method, which comprises a laser beam shaping module, an optical fiber track control module and an optical fiber real-time monitoring module; in the laser beam shaping module, precise shaping and focusing of the laser beam can ensure that the laser energy forms a stable and effective refractive index modulation (RIM) region in the annular core region of the ring core optical fiber, laying a foundation for subsequent preparation of high-reflectivity FBG and avoiding the problem of too small or uneven modulation region caused by poor laser beam shape; in the optical fiber real-time monitoring module, by monitoring the side mode suppression ratio (SMSR) and reflectivity of the reflected spectrum in real time, the writing quality can be grasped in time, the refractive index modulation region deviating from the core caused by the fiber pose deviation in the writing process can be avoided, thereby preventing spectral degradation caused by geometric position asymmetry and ensuring the stability of the writing process and the spectral quality of the final FBG; at the same time, the observation and adjustment of the real-time pose of the optical fiber ensure that the laser beam always acts on the annular core of the ring core optical fiber, fully adapts to its special structure and improves the writing precision; in the optical fiber track control module, the annular writing track is synthesized by controlling the sine and cosine motion of the Y-axis and Z-axis, which can form an excellent adaptation with the annular core structure of the ring core optical fiber, ensure the modulation region coverage range of single writing, and avoid the low reflectivity problem caused by too small modulation region in the traditional point-by-point and line-by-line method; the design of the writing radius changing along the X-axis according to a Gaussian-like distribution can effectively realize the apodization distribution of the refractive index modulation region, solve the Fabry-Perot effect caused by uniform writing in the existing turn-by-turn writing (RbR), improve the side mode suppression ratio while maintaining a high reflectivity, significantly improve the spectral quality of the FBG, and make it more suitable for use in the scenarios of mode division multiplexing system, optical fiber sensing and optical fiber laser, etc. BRIEF DESCRIPTION OF DRAWINGS
[0033] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the drawings needed in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can be obtained by those skilled in the art without creative labor.
[0034] Figure 1 Schematic diagram of the traditional circle-by-circle method for writing FBG.
[0035] Figure 2 A schematic diagram of an implementation scenario of an apodized FBG turn-by-turn writing system based on a variable radius method provided in one embodiment of the present application.
[0036] Figure 3 A schematic diagram of circular trajectory writing in an apodized FBG circle-by-circle writing system based on a variable radius method provided in one embodiment of the present application.
[0037] Figure 4 A schematic diagram of the FBG morphology when the Gaussian function is used as the grating function in an apodized FBG turn-by-turn writing system based on a variable radius method provided in one embodiment of the present application.
[0038] Figure 5 A schematic diagram of the FBG morphology when a sine function is used as the grating function in an apodized FBG turn-by-turn writing system based on a variable radius method provided in one embodiment of the present application.
[0039] Figure 6 A schematic diagram of the FBG morphology when a quadratic function is used as the grating function in an apodized FBG turn-by-turn writing system based on a variable radius method provided in one embodiment of the present application.
[0040] Figure 7 A schematic diagram of the FBG morphology when a linear function is used as the grating function in an apodized FBG turn-by-turn writing system based on a variable radius method provided in one embodiment of the present application.
[0041] Figure 8 This is a comparison diagram of the effects of using and not using variable radius writing in an apodized FBG turn-by-turn writing system based on the variable radius method provided in one embodiment of the present application.
[0042] Figure 9 This is a comparison diagram of the effects of using different rasterization functions in an apodized FBG turn-by-turn writing system based on a variable radius method provided in one embodiment of the present application.
[0043] Figure 10 This is a comparison diagram of the effects of a non-apodized FBG array and a Gaussian FBG array using a variable radius writing method in an apodized FBG turn-by-turn writing system based on a variable radius method provided in one embodiment of the present application.
[0044] Figure 11 This is a flow chart of a method for writing apodized FBG turn by turn based on a variable radius method according to an embodiment of the present application. DETAILED DESCRIPTION
[0045] With reference to the drawings of the embodiments of the present application, the technical solutions in the embodiments of the present application will be described clearly and completely. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the scope of protection of the present application.
[0046] For the existing direct writing technology, the refractive index modulation region formed in the RCF is too small in a single writing, whether it is point-by-point or line-by-line, and the prepared FBG has extremely low reflectivity. Even if parallel multiple writing means is used, due to the special circular inner cladding, ring core and outer cladding structure of the RCF, parallel writing is achieved by rotating multiple writing, but the prepared FBG only has a certain improvement in the intensity of the reflection spectrum, and the transmission spectrum still has no obvious change, that is, the reflectivity improvement effect is poor. Although the angle of the rotating spiral clamp can be reduced to achieve more writing to increase the refractive index modulation region, multiple small-angle rotations undoubtedly increase the writing difficulty and reduce the writing efficiency, and inaccurate rotation will increase the asymmetry of the prepared FBG in the geometric position and deteriorate the spectrum.
[0047] The ring-by-ring (RbR) method, as a new writing method proposed in recent years, can prepare FBG with high reflectivity due to its excellent adaptability to the writing track of the RCF, as shown in FIG. 1. Figure 1 However, due to the strong Fabry-Perot effect caused by uniform writing on both sides of the grating region, the side mode suppression ratio is about 10 dB, which makes it unsuitable for use in WDM systems or fiber sensing systems. At present, there is no corresponding apodization method for this new writing method. In view of this, the present application first proposes variable radius writing to realize apodized ring-by-ring FBG writing to solve the low SMSR problem of traditional ring-by-ring FBG writing and improve the spectrum quality.
[0048] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the present application will be further described in detail below with reference to the drawings and specific embodiments.
[0049] The variable radius method-based apodized FBG ring-by-ring writing system provided by the embodiments of the present application, as shown in FIG. 2, comprises a laser beam shaping module, a fiber track control module and a fiber real-time monitoring module. Figure 2
[0050] The laser beam shaping module is configured to generate a laser beam and shape the laser beam into a refractive index modulation region. In the embodiment, the laser beam shaping module includes a femtosecond laser, a beam shaper, and an oil lens. The femtosecond laser is configured to generate a femtosecond laser beam. The beam shaper is configured to shape the femtosecond laser beam into a laser beam. The oil lens is configured to converge the laser beam to form the refractive index modulation region. The femtosecond laser has a wavelength of 515 nm, a pulse energy of 200 nJ, a pulse width of 290 fs, and a repetition frequency of 200 kHz. The femtosecond laser is a frequency-doubled regenerative amplified Yb:KGW femtosecond laser. The high frequency is set to improve the writing speed while ensuring the uniformity of the writing track, so that the writing track is a line circle rather than a point circle.
[0051] In one embodiment, the laser beam shaping module further includes an electrically controlled shutter. The electrically controlled shutter is arranged between the oil lens and the optical fiber and is configured to control whether the laser passes through to achieve different straight writing track preparation FBGs.
[0052] The optical fiber real-time monitoring module is configured to monitor the side mode suppression ratio and reflectivity of the reflected spectrum in real time and observe the real-time pose of the optical fiber. The optical fiber real-time monitoring module is configured to control the pose of the optical fiber through the optical fiber track control module according to the real-time pose of the optical fiber to ensure that the refractive index modulation region of the laser beam is always located on the core of the optical fiber. The optical fiber is a ring core optical fiber.
[0053] In the embodiment, the optical fiber real-time monitoring module includes a CCD camera, a circulator, a spectrometer, and a broadband light source. The CCD camera is confocal with the oil lens and is configured to observe the real-time pose of the optical fiber. An LED illuminating lamp is arranged at the bottom of the optical fiber to provide a back light source. The broadband light source and the spectrometer are both connected to the optical fiber through the circulator. The broadband light source inputs an optical signal into the optical fiber through the circulator. The spectrometer receives the reflected optical signal from the optical fiber through the circulator and analyzes the side mode suppression ratio and reflectivity of the reflected spectrum.
[0054] The optical fiber track control module is configured to clamp the optical fiber and control the relative movement of the optical fiber in the Y-axis and Z-axis directions according to a sine function and a cosine function, respectively, to form a single ring writing track, and control the optical fiber to step along the X-axis according to a preset grating period. The X-axis direction is the axial direction of the optical fiber, the Y-axis direction is the vertical direction of the cross section of the optical fiber, and the Z-axis is perpendicular to the plane formed by the X-axis and the Y-axis. The writing radii of the ring writing tracks change along the X-axis according to a Gaussian distribution to achieve a Gaussian distribution of the refractive index modulation region.
[0055] Of course, Figure 2 The architecture shown is only exemplary, and in the implementation of different functions, according to the actual needs, one or at least two components in the system can be omitted Figure 2 The architecture shown is only exemplary, and in the implementation of different functions, according to the actual needs, one or at least two components in the system can be omitted
[0056] In this embodiment, the optical fiber trajectory control module uses a high-precision three-dimensional displacement platform. The optical fiber is fixed on the three-dimensional displacement platform by a clamp. In order to ensure stability during the experiment, the base of the three-dimensional displacement platform is a marble platform. The three-dimensional displacement platform drives the optical fiber to move relative to the Y-axis and Z-axis according to the sine function and cosine function respectively, forming a circular writing trajectory, and controls the optical fiber to step along the X-axis according to the preset grating period. Specifically, the motion equations of the optical fiber driven by the three-dimensional displacement platform in the Y-axis and Z-axis directions are: t =Y0+dR(n)×sinθ and Z t =Z0+dR(n)×cosθ, θ is the angle variable from 0 to 2π, dR(n) is the radius written at the real time, Y0 and Z0 are the Y-axis and Z-axis coordinate values at the initial time, Y t and Z t are the Y-axis and Z-axis coordinate values at time t.
[0057] The convergence point of the objective lens remains unchanged. Initially, it converges at (Y0, Z0). The optical fiber is moved relative to the objective lens by a translation stage, so that the convergence point changes with the angle θ (0 to 2π) (Y t ,Z t ), during which the femtosecond laser is continuously turned on, so if Figure 3 As shown in the figure, the motion trajectory (green dashed line) is the trajectory of the inscribed topography (red solid line). After each ring is inscribed, the X-axis jumps one cycle, which determines the central wavelength of the FBG. In this example, the radius r is set to 5 μm, the period Λ is set to 1.093 μm, and the inner and outer radii of the ring-core fiber are r1 = 3.5 μm and r2 = 7.5 μm.
[0058] In order to achieve the RbR method of apodized FBG, that is, the Gaussian distribution of the core's three-dimensional refractive index modulation, the writing radius is changed according to a Gaussian distribution, that is, the radius on both sides is large and the middle is small. The schematic diagram of the writing trajectory is shown as follows Figures 4-7 As shown in the figure, due to the limited field of view of the CCD camera, the entire FBG morphology cannot be observed, and since the changes are at the nanometer level, it is difficult to observe the changes within the limited field of view. Figures 4-7 Only 60 cycles are taken as an example.
[0059] In an exemplary embodiment of the present application, the writing radius of each annular writing track varies along the X-axis according to a quasi-Gaussian distribution. Specifically, the writing radius r of the central annular writing track is the smallest and gradually increases to r+Δr on both sides, where Δr is the change in writing radius. The radius variation function of the quasi-Gaussian distribution is:
[0060] dR(n)=r-Δr*A(n), 0≤n≤N (1)
[0061] wherein dR(n) is the writing radius corresponding to each period, A(n) is the grating function, N is the total number of periods, and n is the current period index. In an exemplary embodiment, r=7 μm, N=60, and Δr=2 μm.
[0062] The grating function can be a Gaussian function, a sinusoidal function, a quadratic function, or a linear function.
[0063] In an exemplary embodiment, when the grating function is a Gaussian function, the profile of the FBG obtained by writing is as shown in FIG. 2. The Gaussian function is as shown in the following equation: Figure 4
[0064]
[0065] wherein b is the inflection point of the Gaussian function.
[0066] When the grating function is a sinusoidal function, the profile of the FBG obtained by writing is as shown in FIG. 3. The sinusoidal function is as shown in the following equation: Figure 5
[0067] A(n) = sin(n*π / N), 0≤n≤N (3)
[0068] When the grating function is a quadratic function, the profile of the FBG obtained by writing is as shown in FIG. 4. The quadratic function is as shown in the following equation: Figure 6
[0069] A(n) = (-4n 2 / N + 4*n) / N, 0≤n≤N (4)
[0070] When the grating function is a linear function, the profile of the FBG obtained by writing is as shown in FIG. 5. The linear function is as shown in the following equation: Figure 7
[0071] A(n) = 1- |N-2n| / N, 0≤n≤N (5).
[0072] In actual writing, the total number of periods N is increased to 2800 (period interval of 1.093 μm, 2800 periods, and the length of the written grating region is about 3 mm), and other parameters remain unchanged.
[0073] The apodized FBG writing method proposed in this embodiment is compared with uniform writing, Figure 8 to visually compare the effect of using variable-radius writing, Figure 8 the middle red dashed line is the Gaussian function apodization result, and it can be seen that the SMSR is increased from 10 dB to 25 dB by using variable-radius writing; in addition, Figure 9 The SMSR, the FWHM and the R of the Gaussian apodized FBG are the smallest, the largest and the highest, respectively. Therefore, it is concluded that the RbR method can effectively apodize the FBG and the Gaussian apodization function is the best.
[0074] To prove the effectiveness of the RbR method in the fabrication of the apodized FBG array on the RCF, four Gaussian apodized FBGs and a non-apodized FBG with a grating length of 3 mm were inscribed on two independent RCFs. Figure 10 The reflection spectra of the FBG array composed of four non-apodized FBGs (as shown in Figure 10 ) and the Gaussian apodized FBGs (as shown in Figure 10 ) inscribed by the RbR method are shown. It can be seen that the FBG array with the Gaussian apodization has better spectrum and higher SMSR, and the Bragg wavelengths of the four apodized FBGs are 1580.55 nm, 1582.75 nm, 1584.48 nm and 1586.87 nm, respectively, and the corresponding 3dB bandwidth is about 0.55 nm. The experimental results show that the RbR method can be used to fabricate the apodized FBG array on the RCF.
[0075] Based on the same inventive concept, the embodiment of the present application also provides a method for implementing the above-mentioned apodized FBG inscribing system based on the RbR method. The implementation scheme for solving the problem provided by the method is similar to the implementation scheme described in the above-mentioned system. In an exemplary embodiment, as shown in Figure 11 , an apodized FBG inscribing method based on the RbR method is provided, including the following steps:
[0076] Step 1, fix the ring core fiber on the three-dimensional displacement platform through the clamp; in the coordinate system of the three-dimensional displacement platform, the X axis is the fiber axial direction, the direction of the fiber cross section perpendicular to the upward direction is the Y axis, and the Z axis is perpendicular to the plane formed by the X axis and the Y axis.
[0077] Step 2, observe the real-time pose of the fiber through the fiber real-time monitoring module, control the pose of the fiber through the fiber trajectory control module according to the real-time pose of the fiber, and ensure that the refractive index modulation region of the laser beam is always located on the fiber core. Specifically, the fiber placement is observed through the oil lens combined with the CCD camera, and the three-dimensional displacement table is adjusted to ensure that the fiber always keeps consistent with the center position of the field of view of the CCD camera when moving along the axial direction.
[0078] Step 3, control the laser beam shaping module to generate laser and shape the laser into a laser beam, and pass the laser beam through focusing to form a refractive index modulation region. The writing energy is set to 200nJ, and the laser frequency is set to 200KHz. The high frequency is set to improve the writing speed and ensure the uniformity of the writing track, so that the writing track is a line circle, not a point circle.
[0079] Step 4, control the optical fiber track control module to control the optical fiber to move in the Y-axis and Z-axis directions according to the sine function and the cosine function respectively, form a single annular writing track, and control the optical fiber to step along the X-axis according to the preset grating period; during the writing process, the writing radius of each annular writing track changes along the X-axis according to a Gaussian distribution, so as to realize the apodization distribution of the refractive index modulation region; the side mode suppression ratio and the reflectivity of the reflected spectrum are monitored in real time.
[0080] The technical features of the above embodiments can be combined in any way. To make the description concise, not all possible combinations of the technical features in the above embodiments are described, but as long as the combinations of the technical features do not exist contradictions, they should be considered as the scope of the present application.
[0081] The principles and implementation modes of the present application are described by using specific examples in this paper, and the above embodiments are only used to help understand the method and its core idea of the present application; at the same time, for those skilled in the art, according to the idea of the present application, the specific implementation mode and application range will be changed. In conclusion, the content of the present application should not be understood as a limitation.
Claims
1. A turn-by-turn apodized FBG writing system based on a variable radius method, characterized in that: include: Laser beam shaping module, optical fiber trajectory control module and optical fiber real-time monitoring module; The laser beam shaping module is used to generate laser light and shape the laser light into a laser beam, and to focus the laser beam to form a refractive index modulation region; the optical fiber real-time monitoring module is used to monitor the side mode suppression ratio and reflectivity of the reflection spectrum in real time, and to observe the real-time position of the optical fiber. The optical fiber trajectory control module controls the position of the optical fiber according to the real-time position of the optical fiber to ensure that the refractive index modulation region of the laser beam is always located on the core of the optical fiber; the optical fiber is a ring-core optical fiber; The fiber trajectory control module is used to clamp the optical fiber and control the relative movement of the optical fiber in the Y-axis and Z-axis directions according to the sine function and cosine function respectively, forming a single annular writing trajectory, and controlling the optical fiber to step according to a preset grating period along the X-axis; the direction of the X-axis is the axial direction of the optical fiber, the direction of the Y-axis is the vertical upward direction of the optical fiber cross section, and the Z-axis is perpendicular to the plane formed by the X-axis and the Y-axis; the writing radius of each annular writing trajectory varies along the X-axis according to a Gaussian-like distribution, realizing a toe-cut distribution in the refractive index modulation area.
2. The apodized FBG turn-by-turn writing system based on the variable radius method according to claim 1, characterized in that: The laser beam shaping module includes a femtosecond laser, a beam shaper and an oil immersion lens; the femtosecond laser is used to generate femtosecond laser light, the beam shaper is used to shape the femtosecond laser light into a laser beam, and the oil immersion lens is used to converge the laser beam to form a refractive index modulation area.
3. The apodized FBG turn-by-turn writing system based on the variable radius method according to claim 2, characterized in that: The parameters of the femtosecond laser are: wavelength 515 nm, pulse energy 200 nJ, pulse width 290 fs, and repetition frequency 200 kHz.
4. The apodized FBG turn-by-turn writing system based on the variable radius method according to claim 2, characterized in that: The optical fiber real-time monitoring module includes a CCD camera, a circulator, a spectrometer and a broadband light source; the CCD camera is confocal with the oil immersion lens and is used to observe the real-time position of the optical fiber; the broadband light source and the spectrometer are both connected to the optical fiber through the circulator, the broadband light source inputs an optical signal into the optical fiber through the circulator, and the spectrometer receives the optical signal reflected back from the optical fiber from the circulator and analyzes it to obtain the side mode suppression ratio and reflectivity of the reflection spectrum.
5. The apodized FBG turn-by-turn writing system based on the variable radius method according to claim 2, characterized in that: The laser beam shaping module further includes an electrically controlled shutter, which is disposed between the oil immersion lens and the optical fiber and is used to control whether the laser passes through, thereby realizing the preparation of FBGs with different direct writing trajectories.
6. The apodized FBG turn-by-turn writing system based on the variable radius method according to claim 1, characterized in that: The optical fiber trajectory control module adopts a high-precision three-dimensional displacement platform, and the optical fiber is fixed on the three-dimensional displacement platform by a clamp; The three-dimensional displacement platform drives the optical fiber to move relative to the Y-axis and Z-axis according to the sine function and cosine function respectively, forming a circular writing trajectory, and controls the optical fiber to step along the X-axis according to the preset grating period.
7. The apodized FBG circle-by-circle writing system based on the variable radius method according to claim 1 or 6, characterized in that: The motion equations of the optical fiber driven by the three-dimensional displacement platform in the Y-axis and Z-axis directions are: t =Y0+dR(n)×sinθ and Z t =Z0+dR(n)×cosθ, θ is the angle variable from 0 to 2π, dR(n) is the radius written at the real time, Y0 and Z0 are the Y-axis and Z-axis coordinate values at the initial time, Y t and Z t are the Y-axis and Z-axis coordinate values at time t.
8. The apodized FBG turn-by-turn writing system based on the variable radius method according to claim 1, characterized in that: The writing radius of each of the annular writing tracks varies along the X-axis according to a quasi-Gaussian distribution. Specifically, the writing radius r of the central annular writing track is the smallest and gradually increases to r+Δr on both sides, where Δr is the change in writing radius. The radius variation function of the quasi-Gaussian distribution is: dR(n)=r-Δr*A(n), 0≤n≤N (1) Where dR(n) is the writing radius corresponding to each period, A(n) is the rasterization function, N is the total number of periods, and n is the current period number.
9. The apodized FBG circle-by-circle writing system based on the variable radius method according to claim 8, characterized in that: The rasterization function adopts a Gaussian function, a linear function, a quadratic function or a sine function; the Gaussian function is shown in the following formula: Where b is the inflection point of the Gaussian function; The sine function is shown below: A(n)=sin(n*π / N), 0≤n≤N (3) The quadratic function is shown below: A(n)=(-4n 2 / N+4*n) / N, 0≤n≤N (4) The linear function is shown below: A(n)=1-|N-2n| / N, 0≤n≤N (5).
10. A method for writing FBG circle by circle based on variable radius method, characterized in that: include: Fixing the ring-core optical fiber on the three-dimensional displacement platform through a clamp; In the coordinate system of the three-dimensional displacement platform, the X-axis is the axial direction of the optical fiber, the vertical upward direction of the optical fiber cross section is the Y-axis, and the Z-axis is perpendicular to the plane formed by the X-axis and the Y-axis; Observing the real-time position of the optical fiber through the optical fiber real-time monitoring module, and controlling the position of the optical fiber through the optical fiber trajectory control module according to the real-time position of the optical fiber, so as to ensure that the refractive index modulation area of the laser beam is always located on the core of the optical fiber; Controlling the laser beam shaping module to generate laser light and shape the laser light into a laser beam, and focusing the laser beam to form a refractive index modulation area; The fiber trajectory control module controls the relative motion of the optical fiber in the Y-axis and Z-axis directions according to the sine function and cosine function, respectively, to form a single annular writing trajectory, and controls the optical fiber to step along the X-axis according to a preset grating period. During the writing process, the writing radius of each annular writing trajectory varies along the X-axis according to a Gaussian-like distribution, realizing a toe-cut distribution in the refractive index modulation area. The side mode suppression ratio and reflectivity of the reflection spectrum are monitored in real time.