Femtosecond point-by-point apodization FBG inscribing system for refractive index modulation area apodization

Abstract

The invention relates to the technical field of FBG preparation, and provides a refractive index modulation area apodization femtosecond point-by-point apodization FBG inscribing system, which comprises a femtosecond laser used for emitting femtosecond laser; the slit is used for cutting the femtosecond laser in the width direction of the slit, and the width direction of the slit is consistent with the radial direction of the optical fiber to be inscribed; the focusing objective lens is used for focusing the cut femtosecond laser into a fiber core of the optical fiber, and the width of a focused light spot is stretched in the cutting direction of the femtosecond laser due to the diffraction effect; and the control device is used for adjusting the width of the slit according to the inscribing position in the FBG inscribing process of the optical fiber, so that the change of the width of the light spot along the axial direction of the optical fiber meets Gaussian curve distribution, and Gaussian apodization of the FBG is realized. According to the invention, Gaussian apodization of the FBG is realized, non-destructive apodization can be carried out on the grating, and the high-quality FBG is prepared.

Description

Technical Field

[0001] The invention relates to the technical field of fiber grating preparation, and in particular to a femtosecond point-by-point apodization FBG writing system with refractive index modulation region apodization. Background Art

[0002] Fiber Bragg grating (FBG) has become a core component in the field of fiber-optic communication and sensing technology due to its compact size, wavelength tunability, immunity to electromagnetic interference, and compatibility with all-fiber systems, as well as its high sensitivity to temperature fluctuations and strain changes in its resonant wavelength. Since the rise of fiber Bragg grating technology in the 1970s, continuous scientific research and exploration have promoted the continuous advancement of its manufacturing process and the increasing scope of its application. Today, researchers can accurately customize fiber Bragg gratings with multiple parameter configurations to meet the growing and diverse needs in the field of fiber-optic communication and sensing, making it a leader in fiber-optic passive devices. In optical communication applications, fiber Bragg gratings are widely adopted as broadband or narrowband passband and bandstop filters due to their excellent wavelength selection performance and flexible design parameters, demonstrating their irreplaceable and important role. Among them, uniform FBG has a narrow reflection band, and gratings with different reflectivities can be prepared according to requirements. However, high-reflectivity gratings will produce large sidelobes, resulting in crosstalk between adjacent channels, increasing the difficulty of system demodulation, and reducing the selectivity of laser pulses. To solve this problem, a variety of fiber Bragg grating apodization techniques have been developed, such as apodization phase mask method, scanning method, multiple exposure method and ultraviolet pulse coherent writing method. However, due to practical conditions, most of these methods are based on holographic phase mask to produce fiber Bragg gratings, each with its own advantages and disadvantages, such as expensive mask, fixed grating period or complex and stable optical path. In addition, the ultraviolet induced gratings produced by these methods are usually longer than 5 mm, which is prone to chirping in the field of sensing due to excessive sensing area or uneven sensing area. Therefore, it is necessary to select a suitable apodization technology according to the actual application and parameter requirements. Femtosecond laser, with its ultrashort pulse width and high peak power, has become an ideal means of optical micro-nano processing and has been applied to fiber Bragg grating processing. Ultrafast femtosecond laser can induce permanent refractive index changes in transparent media, giving femtosecond fiber Bragg gratings excellent thermal stability. In practical applications, fiber apodization techniques mainly include tilt apodization and energy apodization. The tilted apodization method adjusts the lateral position of the refractive index modulation region so that the writing path forms an oblique line across the fiber core, thereby achieving the apodization effect. However, the formation mechanism of this method and the key parameters that affect the apodization effect have not been clearly explained. In addition, this method also requires real-time adjustment of the objective lens focusing depth or trajectory movement angle, which may not only introduce structural asymmetry, but also limit its application to grating apodization with different grating region lengths. In contrast, although the femtosecond laser energy apodization method has potential, its energy adjustable range is limited and the power stability requirement is extremely high, which makes it difficult for the apodization effect to reach the standard of Gaussian function apodization. Summary of the Invention

[0003] In order to solve the problem that in the prior art, the femtosecond laser energy apodization has a limited energy adjustable range and extremely high requirements for power stability, resulting in the apodization effect being difficult to meet the standard of Gaussian function apodization, the present invention provides a femtosecond point-by-point apodization FBG writing system with variable refractive index modulation region.

[0004] The present invention provides a femtosecond point-by-point apodization FBG writing system with variable refractive index modulation region, and the system includes:

[0005] A femtosecond laser for emitting femtosecond laser;

[0006] A slit for cutting the femtosecond laser along the width direction of the slit, and the width direction of the slit is consistent with the radial direction of the optical fiber to be written;

[0007] A focusing objective lens for focusing the cut femtosecond laser into the core of the optical fiber, and the width of the focused light spot is stretched along the cutting direction of the femtosecond laser to achieve variable refractive index modulation area;

[0008] A control device for adjusting the width of the slit according to the writing position during the process of writing FBG on the optical fiber, so that the change of the width of the light spot along the axial direction of the optical fiber satisfies the Gaussian curve distribution, and realizes the Gaussian apodization of FBG.

[0009] Further, the system further includes:

[0010] A writing table for clamping the optical fiber and driving the optical fiber to move uniformly along the axial direction of the optical fiber at a preset speed;

[0011] The control device is specifically used for adjusting the width of the slit according to a preset slit width adjustment model during the process of the optical fiber moving along the axial direction,

[0012] wherein, the slit width adjustment model is:

[0013]

[0014] In the formula, w max is the maximum width of the slit, w min is the minimum width of the slit, σ w is the broadening factor for controlling the change of the slit width, L is the length of the grating writing area of the optical fiber, and v is the speed of the optical fiber moving along the axial direction.

[0015] Further, the control device is further used for adjusting the energy of the femtosecond laser incident on the slit according to a preset energy adjustment model during the process of the optical fiber moving along the axial direction,

[0016] Among them, the energy regulation model is as follows:

[0017]

[0018] In the formula, E max is the maximum energy of the femtosecond laser incident on the slit, and σ E is the broadening factor that controls the energy change of the femtosecond laser, where σ E = σ w .

[0019] Furthermore, the system further includes:

[0020] A half-wave plate, which is arranged between the femtosecond laser and the slit, and the polarization direction of the femtosecond laser can be changed by controlling the angle between the fast axis of the half-wave plate and the polarization direction of the femtosecond laser;

[0021] A beam splitter, which is arranged between the half-wave plate and the slit, and the light intensity of the femtosecond laser incident on the slit can be adjusted by controlling the angle between the polarization direction of the femtosecond laser and the projection axis of the beam splitter;

[0022] The control device is further configured to adjust the angle of the fast axis of the half-wave plate according to the width of the slit during the process of FBG inscription on the optical fiber, so that the change of the light intensity of the light spot along the axial direction of the optical fiber satisfies the Gaussian curve distribution.

[0023] Furthermore, the control device is specifically configured to adjust the angle of the fast axis of the half-wave plate according to a preset angle adjustment model during the process of the optical fiber moving along the axial direction, so as to adjust the light intensity of the femtosecond laser incident on the slit,

[0024] Among them, the angle adjustment model is as follows:

[0025]

[0026] Among them, σ is the broadening factor that controls the angle change.

[0027] Furthermore, the system further includes:

[0028] An optical shutter, which is arranged between the femtosecond laser and the half-wave plate. When the optical shutter is opened, the femtosecond laser is allowed to enter the half-wave plate, and when the optical shutter is closed, the femtosecond laser is blocked from entering the half-wave plate.

[0029] The control device is further configured to periodically open or close the optical shutter in cooperation with the grating writing position of the FBG during the process of the optical fiber moving along the axial direction.

[0030] The femtosecond point-by-point apodized FBG writing system with refractive index modulation region tapering provided by the present invention includes a femtosecond laser, a slit, a focusing objective lens, a writing table and a control device. The femtosecond laser is used to emit femtosecond laser. The width direction of the slit is consistent with the radial direction of the optical fiber to be written, and the slit can cut the femtosecond laser along the width direction of the slit. The focusing objective lens is used to focus the cut femtosecond laser into the core of the optical fiber. Due to the existence of the diffraction effect, the width of the focused light spot is stretched along the cutting direction of the femtosecond laser, that is, along the radial direction of the optical fiber. During the process of writing FBG on the optical fiber, the control device adjusts the width of the slit according to the writing position, so that the change of the width of the light spot along the axial direction of the optical fiber satisfies the Gaussian curve distribution, so as to realize the Gaussian apodization of FBG. By precisely controlling the width of the slit, the present invention changes the width of the light spot focused on the core of the optical fiber, and then realizes the arbitrary adjustment of the width of the grating writing area, realizes the Gaussian apodization of FBG, and can apodize the grating without damage to prepare high-quality fiber gratings. Description of the Drawings

[0031] By reading the following detailed description of the preferred embodiments, various other advantages and benefits will become clear to those of ordinary skill in the art. The drawings are only for the purpose of showing the preferred embodiments and are not considered to be a limitation of the present invention. Moreover, throughout the drawings, the same reference numerals are used to represent the same components. In the drawings:

[0032] Figure 1 is a simplified schematic diagram of the femtosecond point-by-point apodized FBG writing system with refractive index modulation region tapering provided by the embodiment of the present invention;

[0033] Figure 2 is a structural schematic diagram of the femtosecond point-by-point apodized FBG writing system with refractive index modulation region tapering provided by a specific embodiment of the present invention;

[0034] FIG. 3 is a schematic principle diagram of femtosecond point-by-point apodized FBG writing with refractive index modulation region tapering based on an adjustable slit according to an embodiment of the present invention, wherein

[0035] FIG. 3(a) is a schematic diagram of the corresponding relationship between the slit width and the area of the refractive index modulation region inside the core according to an embodiment of the present invention;

[0036] FIG. 3(b) is a structural schematic diagram of an FBG that realizes Gaussian function apodization according to an embodiment of the present invention;

[0037] Figure 4 is a structural schematic diagram of the slit device provided by the embodiment of the present invention;

[0038] Figure 5 is a graph of the coordinated adjustment of the RIM tapering by the half-wave plate prism and the adjustable slit according to an embodiment of the present invention. Detailed implementation manners

[0039] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part rather than all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0040] In the prior art, the demodulation of the center wavelength of a fiber Bragg grating is usually carried out by utilizing the reflection characteristics of the fiber Bragg grating, and the center wavelength of the fiber Bragg grating is determined based on the peak value in the reflection spectrum of the fiber Bragg grating. The present invention utilizes the bidirectional transmission characteristics of an optical fiber, inputs optical signals at both ends of the fiber Bragg grating simultaneously and receives the optical intensity signals output from both ends of the fiber Bragg grating, forms a bidirectional measurement optical path of the optical fiber, locates the waveform characteristics of the reflection spectrum of the fiber Bragg grating through the bidirectional measurement signals, and realizes the high-speed measurement of the fiber Bragg grating signals.

[0041] Figure 1 Fig. shows a simplified schematic diagram of a femtosecond point-by-point apodized FBG writing system with refractive index modulation area apodization according to an embodiment of the present invention. Figure 2 Fig. shows a structural schematic diagram of a femtosecond point-by-point apodized FBG writing system with refractive index modulation area apodization according to a specific embodiment of the present invention. The following will be combined with Figure 1 and Figure 2 to introduce in detail the femtosecond point-by-point apodized FBG writing system with refractive index modulation area apodization provided by the present invention.

[0042] As Figure 1 shown, the femtosecond point-by-point apodized FBG writing system with refractive index modulation area apodization provided by the embodiment of the present invention at least includes: a femtosecond laser 1, a slit 4, a focusing objective lens 8, and a control device. Among them, the femtosecond laser 1 is used to emit femtosecond laser; the slit 4 is used to cut the femtosecond laser along the width direction of the slit 4, and the width direction of the slit 4 is consistent with the radial direction of the optical fiber to be written; the focusing objective lens 8 is used to focus the cut femtosecond laser into the core of the optical fiber, and due to the diffraction effect, the width of the focused light spot is stretched along the cutting direction of the femtosecond laser, realizing the apodization of the refractive index modulation area; the control device is used to adjust the width of the slit 4 according to the writing position during the process of writing the FBG on the optical fiber, so that the change of the width of the light spot along the axial direction of the optical fiber satisfies the Gaussian curve distribution, so as to realize the Gaussian apodization of the FBG.

[0043] Furthermore, in the specific embodiments of the present invention, the length direction of the slit 4 is consistent with the axial direction of the optical fiber, that is, consistent with the x-axis direction, and the length of the slit 4 > the spot diameter. The width of the slit 4 is consistent with the radial direction of the optical fiber, that is, consistent with the y-axis direction. It should be noted that the "consistent" mentioned in the embodiments of the present invention refers to the consistency of the optical path direction, not the consistency in the pure physical sense.

[0044] Furthermore, in the cross-section of the optical fiber (y-z plane), the width of the refractive index modulation region in the y direction and the height in the z direction are mainly determined by the waist radius. The modulation region in the y direction is mainly also determined by the diffraction effect of the slit 4. Therefore, by appropriately adjusting the width of the slit 4, when the femtosecond laser passes through the slit 4, the transverse light intensity distribution (i.e., the width in the y direction) of its Gaussian beam is cropped, changing the wavefront shape. After the cropped beam passes through the focusing objective 8, the diffraction effect will cause the deformation of the focal plane spot. Since the slit 4 restricts the beam width in the y direction, the diffraction in the y direction is enhanced, and the spot is stretched in the y direction, forming an elliptical spot. Specifically, as shown in the corresponding relationship between the width of the slit 4 and the area of the refractive index modulation region inside the fiber core in Fig. 3(a), when there is no slit 4, the width of the spot in the y direction is the narrowest. When the slit 4 is small, the width of the spot in the y direction is stretched, and the width of the spot in the y direction is in a proportional relationship with the width of the slit 4.

[0045] Based on the diffraction effect of the spot, the present invention sets the slit 4 as an adjustable slit 4, and its slit 4 structure can be as Figure 4 shown in the adjustable slit 4 structure. The slit 4 includes two symmetric sliders and a rotating shaft 44. When the bottom surface of the first slider 41 is uniformly pushed by a stepper motor, the rotating shaft 44 is pushed upward through the slide rail 43 below the first slider 41. The middle of the rotating shaft 44 pushes the second slider 42 to move downward along the slide rail 43 through a rotating pin. Therefore, when the stepper motor makes the first slider 41 displace upward by △x mm, at this time, the second slider 42 displaces downward by △x mm, and the width of the slit 4 becomes narrower. Similarly, when the first slider descends by △x mm and the second slider ascends by △x mm, the width of the slit 4 becomes wider by 2△x mm. The adjustable width range of the slit 4 designed in the present invention is 0.1 - 12 mm. It is placed behind the half-wave plate 2 and the polarization beam splitter 3, and the width of the slit 4 is adjusted by a stepper motor.

[0046] Furthermore, during the process of grating engraving and the axial movement of the optical fiber, by adjusting the width of the slit 4 in real time, the Gaussian function apodization of the optical fiber is realized.

[0047] Specifically, as Figure 2As shown, in a specific embodiment of the present invention, the optical fiber to be inscribed is clamped by an inscription stage 7. The inscription stage 7 is a high-precision displacement stage with three axes of X, Y, and Z. When preparing for inscription, the inscription stage 7 accurately positions the core in the XY plane to ensure that the femtosecond laser can be precisely focused into the core of the optical fiber through an oil-immersion objective lens. And during the inscription process, it drives the optical fiber to move uniformly along the axial direction of the optical fiber at a preset speed;

[0048] Further, as shown in Fig. 3(b), which is the correspondence between the width of the slit 4 and the area of the refractive index modulation region inside the core. Combining with the correspondence between the width of the slit 4 and the area of the refractive index modulation region inside the core in Fig. 3(a), to achieve Gaussian function apodization, let the length of the grating inscription region of the optical fiber be L, and the displacement speed of the inscription stage 7 be v, that is, the speed at which the optical fiber moves along the axial direction, and the inscription position x changes with time as:

[0049]

[0050] Then the width w(x) of the slit 4 is the smallest w at the center min , and symmetrically increases to w on both sides max , and its distribution conforms to the reverse Gaussian function:

[0051]

[0052] Among them, σ w is the broadening factor that controls the change in the width of the slit 4, w min is the minimum width at the center, w max is the maximum width at the edge. The broadening factor σ can be optimized through curve fitting and process iteration. For example, parameters such as waveform symmetry, peak position, and full width at half maximum can be used to reverse-derive its value. Its core role is to quantify the distribution characteristics of energy and the width of the slit 4 in the grating inscription direction and precisely control the grating apodization effect.

[0053] Further, in actual control, the width of the slit 4 is controlled based on time. Therefore, by substituting the above position x with x(t) = v·t - L / 2, the width adjustment model of the slit 4 can be obtained:

[0054]

[0055] Further, the control device of the embodiment of the present invention is specifically used to adjust the width of the slit 4 according to the above width adjustment model of the slit 4 during the process of the optical fiber moving along the axial direction. Thus, the apodization effect shown in Fig. 3(b) can be achieved.

[0056] Further, since when the slit 4 is narrower, less laser beam passes through the slit 4, in a preferred embodiment of the present invention, to ensure effective refractive index modulation within the fiber core, the narrower the slit 4, the greater the energy required to pass through the slit 4. Therefore, the control device of the present invention is further configured to adjust the energy of the femtosecond laser incident on the slit 4 according to a preset energy adjustment model during the axial movement of the optical fiber. Specifically, as Figure 5 shown in the quasi-Gaussian curve of the coordinated adjustment of the slit 4 width and the writing energy, so as to achieve the purpose of Gaussian apodization.

[0057] Specifically, the energy E(x) is maximum at the center (E max ), and symmetrically decreases towards both sides. Its distribution conforms to the forward Gaussian function and satisfies the energy-slit 4 inverse relationship with the width of the slit 4:

[0058]

[0059] Further, by replacing the position x with x(t) = v·t - L / 2, the energy adjustment model can be obtained:

[0060]

[0061] In the formula, E max is the maximum energy of the femtosecond laser incident on the slit 4, σ E is the broadening factor that controls the energy change of the femtosecond laser, where σ E = σ w .

[0062] Further, in a specific embodiment of the present invention, the combination of the half-wave plate 2 and the beam splitter 3 is used to achieve energy adjustment. Specifically, the half-wave plate 2 is arranged between the femtosecond laser 1 and the slit 4. By controlling the angle between the fast axis of the half-wave plate 2 and the polarization direction of the femtosecond laser, the polarization direction of the femtosecond laser can be changed; the beam splitter 3 is arranged between the half-wave plate 2 and the slit 4. By controlling the angle between the polarization direction of the femtosecond laser and the projection axis of the beam splitter 3, the light intensity change of the light spot along the axial direction of the optical fiber satisfies the Gaussian curve distribution; at this time, the control device is further configured to adjust the angle of the fast axis of the half-wave plate 2 according to the width of the slit 4 during the FBG writing process of the optical fiber, and the light intensity of the femtosecond laser incident on the slit can be adjusted.

[0063] Specifically, the half-wave plate 2 is a phase retarder that can rotate the polarization direction of linearly polarized light by a certain angle. Let the angle between the polarization direction of the incident light and the fast axis of the half-wave plate 2 be θ, then the polarization direction of the outgoing light will rotate by 2θ. The beam splitting characteristic of the beam splitter 3 is that the transmission axis (P axis) allows the polarized light (P light) parallel to the transmission axis to pass through. The reflection axis (S axis) reflects the polarized light (S light) perpendicular to the transmission axis. By adjusting the angle between the polarization direction of the incident light and the transmission axis of the beam splitter 3 The energy ratio of the transmitted light and the reflected light can be controlled. By rotating the half-wave plate 2 by an angle θ, the included angle of the polarization direction incident on the beam splitter 3 is changed. According to Malus' Law, the transmitted light intensity is:

[0064] I trans = I0cos 2 (2θ) (6)

[0065] The reflected light intensity is:

[0066] I refl = I0sin 2 (2θ) (7)

[0067] Therefore, by rotating the half-wave plate 2 by an angle θ, the ratio of the transmitted energy (or reflected energy) can be continuously adjusted. By rotating θ ∈ [0°, 45°], continuous adjustment of the transmitted energy from 100% to 0% can be achieved. The femtosecond laser has a relatively wide spectrum. It is necessary to use the half-wave plate 2 and the beam splitter 3 to ensure stable polarization control in the entire spectral range, and high-precision adjustment of the femtosecond laser energy can be achieved. The advantages of this method are no mechanical loss, fast response, and applicability to high-power laser systems.

[0068] Furthermore, since in actual control, the control device controls the angle θ of the half-wave plate 2, in the specific embodiment of the present invention, the control device is specifically configured to adjust the angle of the fast axis of the half-wave plate 2 according to a preset angle adjustment model during the process of the optical fiber moving along the axial direction, so as to adjust the energy of the femtosecond laser incident on the slit 4, where the angle adjustment model is:

[0069]

[0070] In the above formula, σ is the broadening factor for controlling the angle change. In the present invention, through the inverse Gaussian distribution of the slit 4 width and energy, the refractive index modulation depth of the grating is enhanced in the central region (high energy, narrow slit 4) and weakened in the edge region (low energy, wide slit 4), thereby suppressing the side lobes and improving the grating performance.

[0071] Furthermore, the system of the embodiment of the present invention further includes an optical shutter disposed between the femtosecond laser 1 and the half-wave plate 2. When the optical shutter is opened, the femtosecond laser is allowed to enter the half-wave plate 2, and when the optical shutter is closed, the femtosecond laser is blocked from entering the half-wave plate 2; the control device is further configured to periodically open or close the optical shutter in cooperation with the grating writing position of the FBG during the process of the optical fiber moving along the axial direction. The optical shutter periodically modulates the output of the incident light, reduces the thermal accumulation of the optical fiber coating layer, and thus realizes the writing of high-strength and high-spectral-quality fiber gratings without damage while penetrating the coating layer.

[0072] Specifically, the control device of the embodiment of the present invention specifically includes a computer 6 and a controller 5. The controller 5 is respectively connected to the half-wave plate 2 and the slit 4, and the computer 6 is connected to the controller 5 for controlling the half-wave plate 2 and the slit 4 through the controller 5 during the grating engraving process. The femtosecond laser 1 used in the invention is produced by Coherent Corporation, with a central wavelength of 800 nm, a pulse width of 35 fs, a repetition frequency of 1 KHz, and a maximum power of 6 W.

[0073] Further, during the actual grating engraving process, first wipe the single-mode optical fiber of the polyimide coating layer clean, apply a pre-tightening force and clamp it on the optical fiber fixture and the glass slide, and inject 2 - 3 drops of optical fiber refractive index matching liquid between the optical fiber and the oil immersion objective lens; then accurately position the core position in the XY plane through the XYZ three-axis high-precision displacement stage to ensure that the femtosecond laser can be accurately focused into the core of the optical fiber through the oil immersion objective lens. Then, using the slit 4, the controller 5 and the computer 6 software, set parameters such as the rotation angle of the half-wave plate 2, the width of the slit 4, the grating area length, and the period required for writing the FBG. Click the writing button on the software to complete the writing of the refractive index modulation region apodized grating, and use a spectrometer to record and analyze the spectrum during the writing process.

[0074] Compared with the existing solutions, the femtosecond point-by-point apodized FBG writing system with refractive index modulation region apodization provided by this embodiment has the following advantages:

[0075] (1) Aiming at the problems of the traditional tilt apodization method that cannot achieve the Gaussian apodization effect and the asymmetric apodization problem, the present invention adopts the cooperative control of the half-wave plate 2 prism combination and the variable slit 4. By adjusting the rotation angle and speed of the half-wave plate 2 through the upper computer program to control the width of the variable slit 4, the effective volume of the femtosecond light spot is stretched in the core along the vertical direction of the optical fiber, increasing the RIM range, so that the RIM modulation in the core shows the effect of Gaussian function apodization, thereby preparing a grating with a high side mode suppression ratio.

[0076] (2) Aiming at the problem of limited adjustable range in the traditional energy apodization method, the cooperative control of the half-wave plate 2 prism combination and the variable slit 4 is used to avoid the limitations of the small energy adjustable range of the variable slit 4 and the extremely high requirement for the power stability of the femtosecond laser 1.

[0077] In addition, those skilled in the art can understand that although some embodiments herein include certain features included in other embodiments rather than other features, the combination of features of different embodiments means that it is within the scope of the present invention and forms different embodiments. For example, any one of the claimed embodiments can be used in any combination.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A femtosecond point-by-point apodized FBG writing system with refractive index modulation region apodization, characterized in that, The system includes: A femtosecond laser for emitting femtosecond laser; A slit for clipping the femtosecond laser along the width direction of the slit, and the width direction of the slit is consistent with the radial direction of the optical fiber to be inscribed; A focusing objective lens for focusing the clipped femtosecond laser into the core of the optical fiber, and the width of the focused light spot is stretched along the clipping direction of the femtosecond laser to achieve apodization of the refractive index modulation area; A control device for adjusting the width of the slit according to the inscription position during the process of inscribing FBG on the optical fiber, so that the change of the width of the light spot along the axial direction of the optical fiber satisfies the Gaussian curve distribution, and Gaussian apodization of the FBG is achieved.

2. The system according to claim 1, characterized in that, The system further includes: An inscription stage for clamping the optical fiber and driving the optical fiber to move uniformly along the axial direction of the optical fiber at a preset speed; The control device is specifically used for adjusting the width of the slit according to a preset slit width adjustment model during the process of the optical fiber moving along the axial direction, wherein, the slit width adjustment model is: where w max is the maximum width of the slit, w min is the minimum width of the slit, σ w is the broadening factor for controlling the change in the slit width, L is the length of the grating region of the optical fiber, and v is the velocity of the optical fiber moving in the axial direction.

3. The system according to claim 2, characterized in that, The control device is further used for adjusting the energy of the femtosecond laser incident on the slit according to a preset energy adjustment model during the process of the optical fiber moving along the axial direction, wherein, the energy adjustment model is: Where, E max is the maximum energy of the femtosecond laser incident on the slit, and σ E is the broadening factor for controlling the energy variation of the femtosecond laser, where σ E = σ w .

4. The system according to claim 2, wherein The system further includes: A half-wave plate disposed between the femtosecond laser and the slit, and the polarization direction of the femtosecond laser can be changed by controlling the angle between the fast axis of the half-wave plate and the polarization direction of the femtosecond laser; A beam splitter disposed between the half-wave plate and the slit, and the light intensity of the femtosecond laser incident on the slit can be adjusted by controlling the angle between the polarization direction of the femtosecond laser and the projection axis of the beam splitter; The control device is further used for adjusting the angle of the fast axis of the half-wave plate according to the width of the slit during the process of inscribing FBG on the optical fiber, so that the change of the light intensity of the light spot along the axial direction of the optical fiber satisfies the Gaussian curve distribution.

5. The system according to claim 4, wherein The control device is specifically used for adjusting the angle of the fast axis of the half-wave plate according to a preset angle adjustment model during the process of the optical fiber moving along the axial direction to adjust the light intensity of the femtosecond laser incident on the slit, wherein, the angle adjustment model is: wherein, σ is a broadening factor for controlling the angle change.

6. The system according to claim 4, characterized in that, The system further includes: An optical shutter disposed between the femtosecond laser and the half-wave plate, allowing the femtosecond laser to enter the half-wave plate when the optical shutter is open, and blocking the femtosecond laser from entering the half-wave plate when the optical shutter is closed; The control device is further used for periodically opening or closing the optical shutter in cooperation with the grating inscription position of the FBG during the process of the optical fiber moving along the axial direction.

Images