An apparatus and method for fabricating fiber Bragg gratings on optical fibers using femtosecond lasers

Abstract

The present invention discloses a device and method for processing fiber Bragg gratings on an optical fiber using femtosecond lasers. The device includes an optical path module, an imaging module, an optical fiber clamping module, a three-axis motion module, and a control module; on the output path of the femtosecond laser light source of the optical path module, a laser beam expander system, a power adjustment system, a laser switch system, a galvanometer system, a 4f system, and a laser beam focusing system are sequentially arranged; the laser emitted from the 4f system is transmitted through a dichroic mirror and then incident on the entrance of a microscope objective lens, and is focused into the optical fiber core through the microscope objective lens. The present invention realizes a galvanometer scanning type fiber Bragg grating processing system, which does not rely on the jumping motion of a three-axis motion platform. Only by making the three-axis platform move uniformly along the direction of the optical fiber, fiber Bragg gratings with good quality can be processed line by line.

Description

Technical Field

[0001] The present invention belongs to the technical field of fiber Bragg gratings, and particularly relates to a device and method for processing fiber Bragg gratings on an optical fiber using femtosecond laser. Background Art

[0002] The femtosecond laser processing method has been widely applied to the processing of fiber Bragg gratings, and such fiber Bragg gratings play a very important role in the fields of optical fiber communication, optical fiber sensing, etc.

[0003] Based on the characteristics of femtosecond laser processing, its refractive index modulation of the fiber core is permanent. Compared with the fiber Bragg gratings processed by ultraviolet exposure, the fiber Bragg gratings processed by femtosecond laser can withstand a high temperature of 1000 °C, and the parameters are easy to modulate during the processing, without the need for an expensive mask.

[0004] The femtosecond laser processing of fiber Bragg gratings is mainly divided into point-by-point processing, line-by-line processing, and surface-by-surface processing methods.

[0005] Among them, the point-by-point processing method is relatively simple, and only the relative movement of the fiber core relative to the femtosecond laser focus along the optical fiber needs to be controlled, and refractive index periodic modulation points are formed in the optical fiber by the single-pulse processing method of femtosecond laser.

[0006] Since the modified region of the fiber Bragg grating processed point by point usually only occupies a small part of the fiber mode field, the fiber Bragg grating processed point by point has the characteristic of serious scattering loss and cannot be used in the series application of fiber Bragg gratings or applications sensitive to loss.

[0007] The line-by-line and surface-by-surface processing methods of fiber Bragg gratings usually use a three-axis motion module to control the relative movement of the fiber core relative to the femtosecond laser focus in the transverse or longitudinal direction perpendicular to the optical fiber, and then the optical fiber jumps to the next modulation line / surface for another scan.

[0008] This method can expand the refractive index modulation region of the grating so that it occupies most of the fiber mode field, and the insertion loss of the fiber Bragg grating can be effectively reduced by using the surface-by-surface or line-by-line processing method.

[0009] The main steps of processing line-by-line / surface-by-surface fiber Bragg gratings based on a three-axis motion module are as follows:

[0010] (1) Determine the relative position of the femtosecond laser focus and the fiber core through the automatic / manual judgment of the imaging module;

[0011] (2) Move the femtosecond laser focus to one side of the fiber core by moving the three-axis motion module;

[0012] (3) By moving the three-axis motion module and opening the laser shutter, move the femtosecond laser focus from one side of the fiber core to the other side, and then close the laser shutter; in this way, one period unit in the fiber grating structure is realized.

[0013] (4) By moving the three-axis motion module, move the fiber along the fiber direction relative to the femtosecond laser focus by one period, and then move the femtosecond laser focus back to one side of the fiber core.

[0014] (5) Repeat steps 3-4 until the entire fiber grating structure is processed.

[0015] The existing processing method of fiber gratings line by line / area by area using a three-axis motion module has the following defects:

[0016] (1) The distance consistency between each period unit of the fiber grating structure greatly affects the quality of the fiber grating. In the above method, each time a period unit in the fiber grating structure is processed, the three-axis motion module needs to generate a micron-level jumping motion. Limited by the positioning accuracy of the three-axis motion module, it is impossible to ensure that the jumping motion intervals are strictly equal.

[0017] (2) Using a three-axis motion module, the efficiency of processing fiber gratings line by line is relatively low; in typical cases, when processing a line-by-line fiber grating with a length of about 1 mm, a period of 1.07 microns, and a single-period line length of 20 microns, considering the acceleration and deceleration times of the three-axis motion module, the total time required is approximately 3-5 minutes; it takes a long time. Summary of the Invention

[0018] The present invention provides a device and method for processing fiber gratings on an optical fiber using femtosecond laser to solve the defects existing in the prior art. It is a device and method for femtosecond laser line-by-line fiber gratings based on beam scanning.

[0019] The technical solution of the present invention is: a device for processing fiber gratings on an optical fiber using femtosecond laser, including an optical path module, an imaging module, an optical fiber clamping module, a three-axis motion module, and a control module, characterized in that:

[0020] The optical path module includes a femtosecond laser light source, and a laser beam expander system, a power adjustment system, a laser switch system, a galvanometer system, a 4f system, and a laser beam focusing system are sequentially arranged on the output path of the femtosecond laser light source;

[0021] The imaging module includes a dichroic mirror, an industrial camera, an imaging lens, and a bottom illumination light source;

[0022] The optical fiber clamping module includes an optical fiber fixture, an optical fiber positioning ferrule, and a glass sheet groove;

[0023] The glass slide groove holds a glass slide, which is used to support the optical fiber and the refractive index matching liquid;

[0024] The bottom illumination light source is fixed on the optical fiber fixture;

[0025] The three-axis motion module includes three motion components orthogonal in the X-axis, Y-axis, and Z-axis directions. The shafts of the X-axis and Y-axis are connected to each other and fixed to the bottom plate, which is a horizontal motion platform. The optical fiber clamping module is fixed on the horizontal motion platform; the shaft of the Z-axis is fixed on a plane perpendicular to the bottom plate, which is a vertical motion platform. The microscopic objective lens is fixed on the vertical motion platform;

[0026] The laser beam emitted by the 4f system passes through the dichroic mirror and is incident on the entrance of the microscopic objective lens, and is focused into the optical fiber core through the microscopic objective lens;

[0027] The illumination light emitted by the bottom illumination light source passes through the optical fiber and is collected by the microscopic objective lens, and is reflected by the dichroic mirror and then converged into the industrial camera by the imaging lens;

[0028] The control module includes a data acquisition card, which is used to generate an analog voltage signal with a high sampling frequency, input an array of analog voltage signals to be output, and output the analog voltage signal after outputting the number of analog signals per second;

[0029] The control module is connected to the power adjustment system, the laser switch system, and the galvanometer system.

[0030] Further, the power adjustment system and the laser switch system include an electro-optic half-wave plate, a Glan-Taylor prism, and a shutter.

[0031] Further, the power adjustment system and the laser switch system include an acousto-optic modulator.

[0032] Further, the optical fiber fixture is a manual flip-type optical fiber fixture or an electro-controlled pneumatic optical fiber fixture.

[0033] Further, the microscopic objective lens is an oil immersion objective lens, and the optical fiber and the top of the microscopic objective lens are immersed in the refractive index matching liquid during the processing.

[0034] Further, the optical fiber positioning ferrule is composed of two optical fiber ferrules fixed on the optical fiber flange and having a diameter slightly larger than the diameter of the optical fiber.

[0035] Further, the laser beam focusing system includes two or more mirrors. The first mirror is arranged on the optical path between the galvanometer system and the 4f system, and the second mirror is arranged on the optical path between the 4f system and the dichroic mirror.

[0036] Further, the 4f system is formed by the coincidence of the optical axes and foci of two lenses; the diameter of the parallel light beam emerging from the 4f system is equal to or slightly larger than the aperture of the microscopic objective lens.

[0037] Further, the galvanometer mirror system is a galvanometer-type galvanometer mirror. The galvanometer mirror reflects the incident parallel light beam and changes the angle of the emerging parallel light beam according to the rapid flipping of the galvanometer mirror. The parallel light beam emerging from the galvanometer mirror is imaged onto the aperture of the microscopic objective lens through the 4f system.

[0038] A method for processing fiber Bragg gratings on an optical fiber using femtosecond lasers, the steps of which include:

[0039] Step 1: According to the length of the line-by-line fiber Bragg grating to be processed, the three-axis motion module is controlled by a program to move to the two end points of the fiber Bragg grating to be processed. Based on the core pattern feedback by the microscopic imaging camera, the coordinate point positions of the cores at the two end points are respectively obtained; the connection line of the two coordinate points is the trajectory for the three-axis motion module to move during actual processing.

[0040] Step 2: According to the relevant parameters of the line-by-line fiber Bragg grating to be processed, calculate the array of analog voltage signals and the output rate required for the galvanometer mirror to work, as well as the moving speed of the three-axis motion module.

[0041] Step 3: Move the three-axis motion module to one of the end points of the fiber Bragg grating. After arrival, linearly move the three-axis motion module to the other end point of the fiber Bragg grating. At the same time, open the laser shutter and start the galvanometer mirror scanning simultaneously, so that the laser focus rapidly scans perpendicular to the fiber direction under the objective lens; according to the calculation relationship in Step 2, when the three-axis motion module reaches the other end point of the fiber Bragg grating, the three-axis motion module and the galvanometer mirror system should stop simultaneously. At this time, close the laser shutter; one line-by-line fiber Bragg grating is processed.

[0042] The relevant parameters include length, period, single-period line length, and processing power.

[0043] Or, without applying an analog voltage signal to the galvanometer mirror, set the femtosecond laser light source to the low repetition frequency mode, and complete the processing of the point-by-point fiber Bragg grating according to the above steps.

[0044] The present invention innovatively realizes a galvanometer mirror scanning type fiber Bragg grating processing system.

[0045] Compared with the three-axis motion platform scanning type for processing line-by-line fiber Bragg gratings, the beneficial technical effects of the present invention are:

[0046] (1)The jumping motion that does not rely on a three-axis motion platform only requires the three-axis platform to move uniformly along the direction of the optical fiber, effectively controlling the distance consistency between each periodic unit of the line-by-line fiber grating structure. High-quality line-by-line fiber gratings can be processed without the need for the high-precision absolute positioning accuracy provided by the most expensive air-bearing precision motion displacement stage.

[0047] (2)When processing the line-by-line fiber grating at a speed of 1 mm / s, there is an order-of-magnitude improvement compared to the speed of about 0.2 mm / min in the scanning processing of the motion platform. Description of the Drawings

[0048] Figure 1 is a structural diagram of an apparatus for processing a fiber grating on an optical fiber using a femtosecond laser.

[0049] Figure 2 is a structural diagram of the fiber clamping module.

[0050] Figure 3 is a processing flow chart.

[0051] Figure 4 is a spectrogram of a line-by-line fiber grating processed in a traditional manner using the same three-axis motion module.

[0052] Figure 5 is a spectrogram of a line-by-line fiber grating processed according to the present invention using the same three-axis motion module.

[0053] In the figures: 1 - femtosecond laser light source; 2 - laser beam expander system; 3 - power adjustment system and laser switch system; 4 - galvanometer system; 5 - first mirror; 6 - 4f system; 7 - second mirror; 8 - dichroic mirror; 9 - imaging lens; 10 - industrial camera; 11 - microscope objective; 12 - fiber clamping module; 13 - three-axis motion module; 14 - fiber fixture; 15 - bottom illumination light source; 16 - fiber positioning ferrule; 17 - glass sheet groove; 18 - fiber flange; 19 - fiber support. Detailed Embodiments

[0054] By referring to the exemplary embodiments, the objects and functions of the present invention and the methods for achieving these objects and functions will be clarified. However, the present invention is not limited to the exemplary embodiments disclosed below; it can be implemented in different forms. The essence of the specification is merely to assist those skilled in the relevant art in comprehensively understanding the specific details of the present invention.

[0055] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, the same reference numerals represent the same or similar components, or the same or similar steps.

[0056] As Figure 1As shown in the figure, a device for processing fiber Bragg gratings on optical fibers using femtosecond lasers consists of an optical path module, an imaging module, a fiber clamping module 12, a three-axis motion module 13, and a control module.

[0057] The optical path module includes the following parts:

[0058] A femtosecond laser light source 1;

[0059] A laser beam expander system 2 to adapt to the incident spot size required by the galvanometer;

[0060] A power adjustment system and a laser switch system 3 to achieve laser power adjustment and on / off during the processing;

[0061] A galvanometer system 4 and a 4f system 6 to map the beam angle change introduced by the galvanometer to the entrance pupil of the microscope objective 11;

[0062] A laser beam focusing system to achieve the focusing of the incident femtosecond laser beam.

[0063] As Figure 1 shown, the laser beam focusing system includes two reflectors. The first reflector 5 is arranged on the optical path between the galvanometer system 4 and the 4f system 6, and the second reflector 7 is arranged on the optical path between the 4f system 6 and the dichroic mirror 8.

[0064] Figure 1 The reflectors in [[ ]] are only for illustration to facilitate drawing. In the actual optical path construction process, there can be more than two reflectors.

[0065] The 4f system 6 is composed of two convex lenses with coincident optical axes and coincident focal points; after the beam passes through the two convex lenses with coincident focal points, an expanding or contracting effect will occur according to the different focal length ratios of the convex lenses; usually, the two convex lenses in the system satisfy the expanding effect (the focal length of convex lens 1 < the focal length of convex lens 2) or the normal effect (the focal length of convex lens 1 = the focal length of convex lens 2); the final result is that the diameter of the parallel light emerging from the 4f system 6 should be equal to or slightly larger than the entrance pupil of the microscope objective 11.

[0066] I The galvanometer system 4 is a galvanometer-type galvanometer, and its function is to introduce an electrically controllable and fast angular deflection to the incident beam.

[0067] The galvanometer will reflect the incident parallel beam and change the angle of the emerging parallel beam according to the rapid flipping of the galvanometer; since the final parallel beam needs to be focused by the microscope objective 11, the beam emerging from the galvanometer needs to accurately enter the entrance pupil of the microscope objective 11, and at the same time, the parallel state of this beam itself still needs to be maintained. Therefore, it is necessary to image the beam emerging from the galvanometer to the entrance pupil of the microscope objective 11 through the 4f system 6.

[0068] Specifically, according to the selection of the lens focal length of the 4f system 6, the spot size of the light beam incident on the microscope objective 11 can be changed.

[0069] The power adjustment system and the laser switch system 3 include two implementation schemes.

[0070] In Scheme A, the power adjustment system and the laser switch system 3 include an electro-optic half-wave plate, a Glan-Taylor prism, and a shutter; the electro-optic half-wave plate + Glan-Taylor prism adjusts the laser power, and the shutter adjusts the laser on / off.

[0071] In Scheme B, the power adjustment system and the laser switch system 3 include an acousto-optic modulator; the laser power and on / off are adjusted simultaneously.

[0072] The laser emitted from the 4f system 6 is transmitted through the dichroic mirror 8 and then incident on the entrance of the microscope objective 11, and is focused into the fiber core through the microscope objective 11.

[0073] The microscope objective 11 uses an oil-immersion objective, and during the processing, the fiber and the top of the microscope objective 11 are immersed in a refractive index matching liquid to eliminate the spherical aberration caused by the air-fiber interface during the laser focusing process.

[0074] The illumination light emitted from the bottom illumination source 15 passes through the fiber and is collected by the microscope objective 11, and is reflected by the dichroic mirror 8 and then converged into the industrial camera 10 by the imaging lens 9.

[0075] The imaging module realizes real-time microscopic imaging of the fiber, and the real-time microscopic images collected by the computer can be used for manual or automatic positioning of the fiber.

[0076] During the process of building the optical path, it is necessary to adjust the focal length of the imaging lens 9 to ensure that the imaging plane of the microscopic system and the focusing point of the processing beam are in the same plane, that is, the femtosecond laser focusing point can be observed most clearly through the microscope objective 11. At this time, the imaging plane and the processing plane are in a conjugate state. The plane with the clearest imaging during the subsequent automatic detection process can be considered as the processing depth.

[0077] As Figure 2 shown, the fiber clamping module 12 includes a fiber fixture 14, a fiber positioning ferrule 16, a glass slide groove 17, and a fiber support 19.

[0078] The fiber fixture 14 includes two implementation schemes.

[0079] A. Manual flip-type fiber fixture; corresponding to the manual operation device.

[0080] B. Electrically controlled pneumatic fiber fixture; corresponding to the automatic operation device.

[0081] The optical fiber positioning ferrule 16 is composed of two optical fiber ferrules fixed on the optical fiber flange 18 with a diameter slightly larger than the diameter of the optical fiber. The optical fiber passes through the two ferrules simultaneously to achieve the preliminary positioning of the optical fiber. The glass sheet for supporting the optical fiber and the refractive index matching liquid is placed in the glass sheet groove 17. This glass sheet can keep the whole optical fiber to be processed on the same horizontal plane to reduce the micro-offset of the optical fiber during the processing.

[0082] The three-axis motion module 13 includes three motion components orthogonal in the X-axis, Y-axis, and Z-axis directions. Among them, the shafts of the X-axis and Y-axis are connected to each other and fixed to the equipment bottom plate, which is called the horizontal motion platform. The optical fiber clamping module 12 is fixed thereon; the shaft of the Z-axis is fixed on the plane perpendicular to the equipment bottom plate, which is called the vertical motion platform. The microscopic objective lens 11 is fixed thereon.

[0083] The control module mainly includes a data acquisition card, which can generate analog voltage signals with a high sampling frequency. The array of analog voltage signals to be output can be input through software, as well as the number of analog signals output per second, and then the analog voltage signals are output according to the actual processing requirements, and the real-time position of the galvanometer scanning and the on / off state of the laser are controlled thereby.

[0084] The analog voltage signals output by the data acquisition card include the following three channels: the galvanometer X-axis signal, the galvanometer Y-axis signal, and the laser on / off level signal.

[0085] The galvanometer X-axis signal and the galvanometer Y-axis signal are directly connected to the drive board of the galvanometer system 4. The deflection angle of the galvanometer is controlled according to the magnitude of the analog voltage value (-10 V to 10 V), and the real-time position of the galvanometer scanning is controlled thereby.

[0086] The laser on / off signal is directly connected to the control interface of the femtosecond laser. The on / off state of the laser is directly controlled by the high and low levels of this signal, and the on / off control of the laser at the microsecond level can be achieved.

[0087] The control module is connected to the power adjustment system, the laser switch system 3, and the galvanometer system 4.

[0088] A method for processing line-by-line fiber Bragg gratings on an optical fiber using a femtosecond laser: The steps are as follows:

[0089] Step 1: According to the length of the line-by-line fiber Bragg grating to be processed, the three-axis motion module 13 is controlled by a program to move to the two end points of the optical fiber grating to be processed. Based on the core pattern feedback by the microscopic imaging camera, the coordinate point positions of the cores at the two end points are obtained respectively through an automatic detection algorithm; the line connecting the two coordinate points is the moving trajectory of the three-axis motion module 13 during actual processing;

[0090] Step 2: According to the relevant parameters of the line-by-line fiber grating to be processed, such as length, period, single-period line length, processing power, etc.; calculate the array of analog voltage signals and output rate required for the galvanometer to work according to the parameters, as well as the moving speed of the three-axis motion module 13.

[0091] Step 3: Move the three-axis motion module 13 to one of the endpoints; after arrival, linearly move the three-axis motion module 13 to the other endpoint, and at the same time open the laser shutter and start the galvanometer scanning, so that the laser focus quickly scans perpendicular to the fiber direction under the microscope objective 11; according to the calculation relationship in Step 2, when the three-axis motion module 13 reaches the other endpoint, the three-axis motion module 13 and the galvanometer system 4 should stop at the same time, and then close the laser shutter. One section of the line-by-line fiber grating is processed.

[0092] If no analog voltage signal is applied to the galvanometer and the femtosecond laser source is set to the low repetition frequency mode, this system can also complete the processing of the point-by-point fiber grating according to the above steps.

[0093] The embodiment describes the processing flow of a line-by-line fiber grating with a length of L = 1 mm and a period of P = 1.07 μm.

[0094] The processing flow is as Figure 3 shown.

[0095] Thread the optical fiber through the fiber fixture 14, the fiber positioning ferrule 16, pass over the glass sheet and then thread it through the fiber positioning ferrule 16 and the fiber fixture 14.

[0096] Adjust the three-axis motion platform so that the microscopic system can image the optical fiber into the camera.

[0097] Make the three-axis motion platform move leftward along the axis of the optical fiber, denoted as the X direction, by L / 2 = 0.5 mm, and consider this as the processing end point; at this time, it is not guaranteed that the optical fiber is exactly on the plane with the clearest imaging, that is, the laser focusing position is not guaranteed to be at the exact center of the fiber core.

[0098] Locate the position of the exact center of the fiber core, and move the optical fiber to the laser focusing position through the three-axis motion platform, and record the coordinates here as P2, and P2 is the termination point.

[0099] The specific adjustment method is as follows:

[0100] Adjust the three-axis motion platform to translate leftward along the optical fiber by half of the processing length L, and automatically or manually detect and locate the fiber core, and adjust the three-axis motion platform to move the center of the fiber core to the laser focus position preset by the program; record the coordinates of the three-axis motion platform here as the P2 termination point.

[0101] Adjust the three-axis motion platform to translate the processing length L to the right along the optical fiber, and automatically or manually detect and position the optical fiber core. Adjust the three-axis motion platform to move the center of the optical fiber core to the laser focus position preset in the program; record the coordinates of the three-axis motion platform here as the starting point P1.

[0102] Make the three-axis motion platform move 1 mm to the right relative to the optical fiber. Consider this point as the starting point of processing. Since the assembly of the optical fiber does not necessarily make the optical fiber strictly parallel to the X-axis of the three-axis motion platform, the laser focusing position is still not guaranteed to be exactly at the center of the optical fiber core after the movement. Use the automatic detection algorithm again to locate the center position of the optical fiber core, and move the optical fiber to the laser focusing position through the three-axis motion platform. Record the coordinates here as P1, and P1 is the starting point.

[0103] According to the set line length of the fiber Bragg grating to be processed line by line, the number of laser pulses required per micron of the line, and the preset laser repetition frequency, calculate the moving speed V of the displacement stage, the array A of the analog voltage signals required to control the galvanometer, and the output rate R of the array A during the processing.

[0104] In the software, control the three-axis motion platform to move uniformly at a speed of V from the starting point P1 to the ending point P2, and control the data acquisition card to synchronously output the analog voltage signal A at the output rate R.

[0105] When the three-axis motion platform moves to the ending point P2, the output of the data acquisition card ends simultaneously; the processing is completed.

[0106] Embodiment: The line length along the optical fiber core during the line processing is 20 microns. It is considered that 5 pulsed processing points are required per micron of the line; that is, 100 laser pulses are required in this line. The laser repetition frequency R is an adjustable item of the laser, which can be continuously adjusted from 1 Hz to 1 MHz. Here, it is temporarily adjusted to 100 kHz; that is, the laser outputs 100,000 pulses per second. Therefore, the processing time required for each line is 1 millisecond.

[0107] Test in advance according to the analog voltage signal V input to the galvanometer control system and the moving distance dx of the laser focus on the focusing plane. Its mathematical relationship basically satisfies the linear relationship dx = Gx×V, where Gx is the proportionality coefficient.

[0108] The line length of the optical fiber core is 20 microns, which corresponds to the laser focus swinging between +10 microns and -10 microns. According to the above Gx proportionality coefficient, the peak and valley values of the required analog voltage signal can be obtained, similar to a triangular wave; construct the array of the required analog voltage signal in the control program, specify the output rate of the analog voltage signal as 100 kHz, and send the task to the data acquisition card.

[0109] The period of the processed line-by-line fiber grating is 1.07 um. That is, the three-axis displacement stage is set to start running from the starting point P1 to the ending point P2 at a speed of 1.07 um / ms (1.07 mm / s); the control program synchronously controls the data acquisition card to output an array of analog voltage signals; that is, the scanning process of the laser focus is synchronously controlled during the forward movement of the optical fiber; finally, the line-by-line fiber grating is obtained.

[0110] The automatic detection algorithm is as follows:

[0111] After the displacement stage moves the optical fiber into position, there are horizontal and vertical deviations between the center of the fiber core and the laser focus. The optical fiber still shows an axisymmetric characteristic when observed in the imaging system.

[0112] Since the laser focus and the imaging system are in a conjugate state, we can know the pixel position of the laser focus in the camera image when the analog voltage signal received by the galvanometer scanner is 0V. Recorded as (px0, py0).

[0113] Horizontal direction correction: The relationship between the number of pixels dp corresponding to the actual object size dx is obtained in advance by calibration as dx = C×dp, where C is the proportionality coefficient; according to the camera image, it can be judged by the algorithm that the pixel offset between the center pixel coordinate p of the fiber core in the horizontal direction and the laser focus is (p - px0), that is, the actual offset value dx = C×(p - px0) is calculated through the pixel offset value. Let the Y-axis of the displacement stage, where the Y-axis is the axis perpendicular to the optical fiber in the horizontal direction, carry the optical fiber and move this value, then the fiber core can be moved to the position of the laser focus in the horizontal direction.

[0114] According to the actual imaging principle, when the fiber core deviates from the imaging plane of the microscope objective, the interface between the central fiber core and the cladding will be highlighted. According to whether it is above or below the imaging plane, it will appear white or black, and when the fiber core is exactly at the imaging plane of the microscope objective, the interface between the fiber core and the cladding disappears in the camera imaging pattern and the two cannot be distinguished, that is, the standard deviation of the pixel intensity of the overall area of the fiber core and the cladding in the image is the smallest at this time.

[0115] Therefore, a square box with a side length of 400 pixels centered on the laser focus is taken. The Z-axis of the displacement stage is used to move the microscope objective up and down. By calculating the standard deviation of all pixels in the image of the square box in real time, the actual position z0 of the Z-axis when the minimum standard deviation is obtained is obtained. Move the Z-axis of the displacement stage to z0, then the laser focus can be made to be at the center of the fiber core.

[0116] The above method can realize the automatic alignment of the actual positions of the fiber core center and the laser focus, and the displacement stage can also be manually controlled by software according to the above logic to manually align the fiber core center and the laser focus.

[0117] Such as Figure 4As shown, it is the reflection spectrum diagram of a line-by-line fiber grating processed by the traditional method measured using a spectral analyzer. The processed grating length is 1 mm, its side mode suppression ratio is about 11 dB, the reflected light intensity is about -32.5 dBm, the length is 1 mm, and the time consumption is about 5 minutes.

[0118] As Figure 5 shown, it is the reflection spectrum diagram of a line-by-line fiber grating processed according to the method of the present invention measured using a spectral analyzer. The processed grating length is 1 mm, its side mode suppression ratio is about 24 dB, the reflected light intensity is about -31 dBm, the length is 1 mm, and the time consumption is about 15 seconds.

[0119] Compare the line-by-line fiber grating spectra processed by the traditional method and the method described in the present invention using the same three-axis motion module 13; in contrast, the method described in the present invention avoids the influence of the position error of the precision displacement stage, the processed line-by-line fiber grating spectrum is sharper, the side mode suppression ratio is greatly improved, and both the spurious peaks and the background noise are significantly suppressed; it has beneficial effects on the subsequent applications of fiber gratings, such as the cavity mirror of a fiber laser or the demodulation of sensing signals, etc. <{

[0120] Combined with the description and practice of the present invention disclosed herein, other embodiments of the present invention will be readily conceivable and understandable to those skilled in the art. The description and embodiments are only considered to be exemplary, and the true scope and gist of the present invention are defined by the claims.

Claims

1. An apparatus for fabricating fiber Bragg gratings on an optical fiber using femtosecond lasers, comprising an optical path module, an imaging module, an optical fiber clamping module, a three-axis motion module, and a control module, characterized in that: The optical path module includes a femtosecond laser light source (1), and a laser beam expander system (2), a power adjustment system and a laser switch system (3), a galvanometer scanner system (4), a 4f system (6), and a laser beam focusing system are sequentially arranged on the output path of the femtosecond laser light source (1); The imaging module includes a dichroic mirror (8), an industrial camera (10), an imaging lens (9), and a bottom illumination light source (15); The optical fiber clamping module (12) includes an optical fiber fixture (14), an optical fiber positioning ferrule (16), and a glass sheet groove (17); The glass sheet groove (17) is for placing a glass sheet, and the glass sheet is used to support the optical fiber and the refractive index matching liquid; The bottom illumination light source (15) is fixed on the optical fiber fixture (14); The three-axis motion module (13) includes three motion components orthogonal in the X-axis, Y-axis, and Z-axis directions. The X-axis and Y-axis bodies are connected to each other and fixed to the bottom plate, and the bottom plate is a horizontal motion platform. The optical fiber clamping module (12) is fixed on the horizontal motion platform; the Z-axis body is fixed on a plane perpendicular to the bottom plate, and the plane perpendicular to the bottom plate is a vertical motion platform. A microscope objective lens (11) is fixed on the vertical motion platform; The laser emitted from the 4f system (6) is transmitted through the dichroic mirror (8) and then incident on the entrance pupil of the microscope objective lens (11), and is focused into the optical fiber core through the microscope objective lens (11); The illumination light emitted by the bottom illumination light source passes through the optical fiber and is collected by the microscope objective lens (11), and is reflected by the dichroic mirror (8) and then converged by the imaging lens (9) into the industrial camera (10); The control module includes a data acquisition card, which is used to generate an analog voltage signal with a high sampling frequency, and outputs the analog voltage signal after inputting an array of analog voltage signals to be output and the number of analog signals output per second; The control module is connected to the power adjustment system and the laser switch system (3), and the galvanometer scanner system (4); The galvanometer scanner system (4) is a galvanometer-type scanner. The scanner reflects the parallel light beam incident on it, and changes the angle of the outgoing parallel light beam according to the rapid flipping of the scanner. The parallel light beam emitted by the scanner is imaged onto the entrance pupil of the microscope objective lens (11) through the 4f system (6); The data acquisition card controls the real-time position of the scanner scanning and the on / off state of the laser; The analog voltage signal output by the data acquisition card includes the following three channels: the scanner X-axis signal, the scanner Y-axis signal, and the laser on / off level signal.

2. The apparatus for processing fiber Bragg gratings on an optical fiber using femtosecond laser according to claim 1, characterized in that The power adjustment system and the laser switch system (3) include an electro-optic half-wave plate, a Glan-Taylor prism, and a shutter.

3. The device for processing fiber Bragg gratings on an optical fiber using femtosecond laser according to claim 1, characterized in that The power adjustment system and the laser switch system (3) include an acousto-optic modulator.

4. A device for fabricating a fiber grating on an optical fiber using femtosecond laser according to claim 1, characterized in that The optical fiber fixture (14) is a manual flip-type optical fiber fixture or an electro-controlled pneumatic optical fiber fixture.

5. The apparatus for processing fiber Bragg gratings on an optical fiber using femtosecond lasers according to claim 1, characterized in that The microscope objective lens (11) is an immersion objective lens, and the optical fiber and the top of the microscope objective lens (11) are immersed in the refractive index matching liquid during the processing.

6. The device for processing fiber Bragg gratings on an optical fiber using femtosecond laser according to claim 1, characterized in that The optical fiber positioning ferrule (16) consists of two fiber ferrules fixed on the optical fiber flange (18) with a diameter slightly larger than the diameter of the optical fiber.

7. A device for fabricating a fiber grating on an optical fiber using femtosecond laser according to claim 1, characterized in that The laser beam focusing system includes two or more mirrors. The first mirror (5) is arranged on the optical path between the galvanometer system (4) and the 4f system (6), and the second mirror (7) is arranged on the optical path between the 4f system (6) and the dichroic mirror.

8. A device for processing fiber Bragg gratings on an optical fiber using femtosecond laser according to claim 1, characterized in that The 4f system (6) is composed of two convex lenses with coincident optical axes and coincident focal points; the diameter of the parallel light emitted from the 4f system (6) is equal to or slightly larger than the aperture of the microscope objective (11).

9. A method for processing fiber Bragg gratings on an optical fiber using femtosecond laser by means of the device according to any one of claims 1-8, characterized in that the steps Including: Step 1: According to the length of the line-by-line fiber grating to be processed, control the three-axis motion module (13) to the two end points of the fiber grating to be processed through the program. Based on the core pattern feedback by the industrial camera (10), obtain the coordinate point positions of the cores at the two end points respectively; the line connecting the two coordinate points is the trajectory of the three-axis motion module during actual processing. Step 2: According to the relevant parameters of the line-by-line fiber grating to be processed, calculate the array of analog voltage signals and the output rate required for the galvanometer to work, as well as the moving speed of the three-axis motion module (13). Step 3: Move the three-axis motion module (13) to one end point of the fiber grating. After arrival, linearly move the three-axis motion module (13) to the other end point of the fiber grating, and at the same time open the laser shutter and start the galvanometer scanning simultaneously, so that the laser focus quickly scans perpendicular to the fiber direction under the objective lens; according to the calculation relationship in Step 2, when the three-axis motion module (13) reaches the other end point of the fiber grating, the three-axis motion module (13) and the galvanometer system (4) stop simultaneously, and close the laser shutter; one line-by-line fiber grating is processed. The relevant parameters include length, period, single-period line length, and processing power. Or do not apply an analog voltage signal to the galvanometer system (4), set the femtosecond laser light source (l) to the low repetition frequency mode, and complete the processing of the point-by-point fiber grating according to the above steps.

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