Ring-by-ring femtosecond laser lithography-based ring-shaped fiber grating efficient processing method

Abstract

The invention provides an annular fiber grating efficient processing method based on ring-by-ring femtosecond laser lithography. According to the method, the ring-by-ring femtosecond laser photoetching technology is utilized, the ring-shaped fiber bragg grating is accurately inscribed in the optical fiber, and the problems that in the prior art, the grating length is limited, inscribing uniformity is poor, and efficiency is low are effectively solved. By optimizing laser parameters and processing strategies, grating inscribing efficiency and refractive index distribution uniformity are remarkably improved, meanwhile, dependence on a high-precision three-dimensional displacement table is eliminated, and the high-quality preparation requirement of fiber gratings of any length can be met.

Description

Technical Field

[0001] The invention relates to a femtosecond laser photolithography optical fiber grating structure, belonging to the technical field of optical fiber sensors. Background Art

[0002] Optical fiber sensing technology has shown significant advantages in the measurement of parameters such as temperature, pressure and strain in extreme environments such as high temperature and high pressure, due to its small size, high precision, strong anti-electromagnetic interference ability and excellent environmental adaptability. Among them, the fiber grating sensor based on the Bragg reflection principle has attracted much attention due to its high sensitivity, easy multi-point measurement and relatively simple signal demodulation. By accurately detecting the drift of the central wavelength of the fiber grating reflection spectrum, high-precision sensing of physical quantities can be achieved.

[0003] Traditional fiber Bragg grating writing technologies mainly include standing wave method, lateral holographic interference method and phase mask method. However, these methods have certain limitations in terms of lithography flexibility, writing accuracy, sensor temperature tolerance and material universality. In order to break through these limitations, femtosecond laser lithography technology has been introduced into the field of fiber Bragg grating preparation. This technology can not only achieve nanometer-level precision grating writing and obtain high-quality grating structures, but also flexibly control parameters such as grating period, length, and refractive index modulation depth to achieve the preparation of complex structure gratings. In addition, femtosecond laser lithography technology is suitable for a variety of materials, including glass, crystals, polymers, etc., breaking through the traditional method's dependence on photosensitive materials. More importantly, the femtosecond laser has an extremely short action time with the material and a small heat-affected zone, which can effectively avoid material damage caused by thermal effects and improve the temperature tolerance of the sensor.

[0004] According to different processing methods, femtosecond laser grating engraving technology mainly includes femtosecond laser phase mask method, femtosecond laser holographic interferometry and femtosecond laser direct writing method. Among them, the femtosecond laser phase mask method uses a phase mask to generate interference fringes to form a periodic refractive index modulation inside the optical fiber. This method has the advantages of simple process, good repeatability and high yield. However, its grating period is limited by the phase mask, and the mask needs to be customized according to the refractive index of the optical fiber and the target wavelength. The grating position is difficult to control flexibly, the mask cost is high, and it is difficult to write complex structure gratings. The femtosecond laser holographic interferometry method uses the interference fringes of two coherent femtosecond lasers to form a periodic refractive index modulation in the optical fiber. This method does not require the removal of the optical fiber coating, the grating has high mechanical strength, and the grating period can be flexibly adjusted by adjusting the laser angle. However, the femtosecond laser pulse width is extremely narrow, the spatial matching of the two beams of light is difficult, the equipment repeatability and accuracy requirements are extremely high, and the engraving efficiency is low.

[0005] Femtosecond laser direct writing uses a focused femtosecond laser beam to induce material modification inside the optical fiber to form a grating structure. Depending on the writing path, it can be divided into point-by-point, line-by-line and surface-by-surface writing techniques. Point-by-point writing uses a high numerical aperture microscope to focus the femtosecond laser and flexibly writes the grating structure through point-to-point scanning. This method has high flexibility, but low writing efficiency. Line-by-line writing scans the femtosecond laser in a direction perpendicular to the optical fiber axis to form a grating structure that runs through the fiber core. This method improves the writing efficiency to a certain extent, but its flexibility is limited. Surface-by-surface writing performs two-dimensional scanning on the cross section of the optical fiber, which to a certain extent improves the problem of optical fiber core alignment in point-by-point and line-by-line writing. However, this method requires two-dimensional scanning inside the optical fiber, which has poor flexibility and is difficult to control the uniformity of the grating area.

[0006] Based on the above analysis, the existing fiber Bragg grating writing methods still have some shortcomings, mainly including:

[0007] 1. Limited grating length: It requires the assistance of a high-precision three-dimensional translation stage, and can only achieve grating writing of limited length;

[0008] 2. The uniformity of writing is difficult to ensure: the nonlinear interaction between femtosecond laser and optical fiber can easily lead to uneven writing depth and shape, affecting the grating performance; Summary of the invention

[0009] The purpose of the present invention is to provide a method for efficiently processing annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography, comprising the following steps:

[0010] 1) Assemble the optical fiber on the optical fiber clamping and transporting system of the femtosecond laser lithography system.

[0011] The processing system includes a femtosecond laser, a concave reflector, a spherical reflector, a fiber clamping system, a plane reflector, and a parabolic reflector.

[0012] The concave reflector has a central hole I;

[0013] After passing through the central hole I, the light beam output by the femtosecond laser is reflected by the spherical reflector, and then transmitted to the concave reflector, and is reflected by the reflective surface I of the concave reflector to form a ring-shaped femtosecond laser beam;

[0014] After being reflected by the concave reflector into a ring-shaped beam, the light beam moves forward to the plane reflector;

[0015] The plane reflector is tilted, and has a center hole II at its center for the annular optical fiber to pass through. The annular light beam is reflected by the plane reflector to the parabolic reflector above. The center of the parabolic reflector has a center hole III at its center for the annular optical fiber to pass through.

[0016] The optical fiber clamping and feeding system is used to stretch the optical fiber, and comprises an upper clamp located above the parabolic reflector and a lower clamp located below the plane reflector. The optical fiber is clamped between the upper clamp and the lower clamp and passes through the center hole II and the center hole III.

[0017] The annular femtosecond laser beam reflected by the parabolic reflector is focused inside the optical fiber between the plane reflector and the parabolic reflector.

[0018] 2) Start the optical fiber clamping system and stretch the optical fiber toward one end.

[0019] During the stretching process, the femtosecond laser outputs a femtosecond laser beam outward.

[0020] The femtosecond laser beam is converted into a ring-shaped femtosecond laser beam by the processing system, and then focused on the inside of the optical fiber between the plane reflector and the parabolic reflector, so as to perform photolithography on the inside of the optical fiber to form a ring-shaped fiber grating. The ring-shaped fiber grating includes at least the outer area of ​​the optical fiber that has not been photolithographically processed, the ring-shaped fiber grating area, and the inner core that has not been photolithographically processed from the outside to the inside;

[0021] The period of the ring fiber Bragg grating is determined by the quantitative relationship between the stretching speed of the fiber clamping system and the repetition frequency of the femtosecond laser.

[0022] Furthermore, the quantitative relationship between the stretching speed of the fiber clamping system and the repetition frequency of the femtosecond laser is shown as follows:

[0023]

[0024] Where C is the period of the annular fiber grating; v is the stretching speed of the fiber clamping system; and f is the repetition frequency of the femtosecond laser.

[0025] Furthermore, the length of the ring-shaped fiber Bragg grating is determined by the photolithography time and the stretching speed.

[0026] Furthermore, the grating thickness of the annular grating is controlled by the distance between the spherical mirror and the concave mirror in the lithography system.

[0027] Furthermore, the femtosecond laser lithography system also includes a horizontal displacement control system.

[0028] The spherical reflector is mounted on a slider on the horizontal displacement control system.

[0029] The distance between the spherical reflector and the concave reflector is adjusted by controlling the slider, thereby adjusting the diameter and width of the annular femtosecond laser beam reflected by the concave reflector.

[0030] Furthermore, the width of the annular grating unit is determined by the focused spot size of the femtosecond laser.

[0031] Furthermore, the femtosecond laser lithography system also includes a shaping and beam expansion system.

[0032] The shaping and beam expansion system is arranged in front of the femtosecond laser and is used to adjust the divergence angle of the femtosecond laser beam output by the femtosecond laser, thereby realizing the adjustment of the size of the annular femtosecond laser beam.

[0033] Furthermore, the femtosecond laser lithography system further comprises a flat mirror frame, and the spherical reflector is installed at the center of the mirror frame.

[0034] The mirror frame has an annular optical window. The annular light beam reflected by the concave reflector passes through the optical window and is then transmitted to the plane reflector.

[0035] Furthermore, in step 2), after the photolithography process is completed, the annular fiber grating is subjected to high temperature annealing.

[0036] Furthermore, the process of converting the femtosecond laser beam into a circular femtosecond laser beam by the processing system is as follows:

[0037] After passing through the central hole I, the light beam output by the femtosecond laser is reflected by the spherical reflector, and then transmitted to the concave reflector, and is reflected by the reflective surface I of the concave reflector to form a ring-shaped femtosecond laser beam;

[0038] After being reflected by the concave reflector into a ring-shaped beam, the light beam moves forward to the plane reflector;

[0039] After the annular femtosecond laser beam is transmitted to the plane mirror, it is reflected by the plane mirror to the parabolic mirror above;

[0040] The parabolic reflector reflects the annular femtosecond laser beam, so that the annular femtosecond laser beam is focused inside the optical fiber between the plane reflector and the parabolic reflector.

[0041] Furthermore, the optical fiber includes single crystal optical fiber and quartz optical fiber.

[0042] When the optical fiber is a single crystal optical fiber, the annular fiber grating includes from outside to inside the optical fiber peripheral area that has not been photolithographically processed, the optical fiber cladding area, the annular fiber grating area, and the inner core that has not been photolithographically processed; the optical fiber cladding area and the annular fiber grating area are obtained by photolithography.

[0043] When the optical fiber is a quartz optical fiber, the optical fiber peripheral area that is not photolithographically processed is the optical fiber cladding.

[0044] It is worth noting that the femtosecond laser is used to replace the CO in the laser-heated susceptor growth system. 2The laser is used to precisely control the diameter and width of the annular spot by using a shaping beam expansion system and a spherical reflector position adjustment. The annular spot is focused into the fiber through the optical path system and interacts with a specific area to induce material modification. By optimizing the matching relationship between the repetition frequency of the femtosecond laser and the speed of the stretching system, the preparation of annular grating structures with high efficiency, long distance, and uniform refractive index distribution in the fiber can be achieved.

[0045] Advantages of the present invention include:

[0046] 1. A high-precision stretching system is used to replace the high-precision three-dimensional translation stage in traditional technology to meet the writing requirements of gratings of any length;

[0047] 2. Using a circular femtosecond laser beam with uniform energy distribution to lithographically shape the grating structure can improve the writing efficiency and refractive index distribution uniformity of the fiber grating structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 This is the three-dimensional structure of the quartz fiber ring fiber Bragg grating and its cross-section.

[0049] Figure 2 This is a structural diagram of the femtosecond laser lithography system and the clamping system used in the present invention.

[0050] Figure 3 Schematic diagram of controlling the diameter and width of a circular femtosecond laser beam and obtaining a cladding structure within the optical fiber core.

[0051] Figure 4 Schematic diagram of the cross-sectional structure of the clad single crystal fiber ring grating.

[0052] In the figure: a femtosecond laser (1), a shaping and expanding system (2), a concave reflector (3), a central hole I (301), a reflecting surface I (302), a spherical reflector (4), an optical window (5), a mirror frame (6), a horizontal displacement control system (7), an upper clamp (8), a lower clamp (9), a support (9), an optical fiber (10), an optical fiber peripheral region (1002) that has not been photolithographically processed, an optical fiber cladding region (1004), an annular fiber grating region (1001), and an internal fiber core (1003) that has not been photolithographically processed; annular fiber grating period C, annular fiber grating radius R, annular fiber grating width D, annular fiber grating thickness H, cladding thickness P, a plane reflector (11), a central hole II (1101), a reflecting surface II (1102), a parabolic reflector (12), a central hole III (1201), and a reflecting surface III (1202). DETAILED DESCRIPTION

[0053] The present invention is further described below in conjunction with the embodiments, but it should not be understood that the above subject matter of the present invention is limited to the following embodiments. Without departing from the above technical ideas of the present invention, various substitutions and changes are made according to the common technical knowledge and customary means in the art, which should all be included in the protection scope of the present invention.

[0054] Embodiment 1:

[0055] See also Figures 1 to 4 , a highly efficient processing method of annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography, comprising the following steps:

[0056] 1) Assemble the optical fiber 10 on the optical fiber clamping and feeding system of the femtosecond laser lithography system.

[0057] The processing system includes a femtosecond laser 1 , a concave reflector 3 , a spherical reflector 4 , an optical fiber clamping system, a plane reflector 11 , and a parabolic reflector 12 .

[0058] The concave reflecting mirror 3 has a central hole I301;

[0059] The light beam output by the femtosecond laser 1 passes through the central hole I301, is reflected by the spherical reflector 4 to the concave reflector 3, is reflected by the reflective surface I302 to form a ring-shaped femtosecond laser beam, and then reaches the plane reflector 11;

[0060] The plane reflector 11 is tilted and has a center hole II1101 at its center for the optical fiber 10 to pass through. The annular light beam is reflected by the plane reflector 11 to the parabolic reflector 12 above. The parabolic reflector 12 has a center hole III1201 at its center for the optical fiber 10 to pass through.

[0061] The optical fiber clamping system is used to stretch the optical fiber 10, and includes an upper clamp 8 located above the parabolic reflector 12 and a lower clamp 9 located below the plane reflector 11. The optical fiber 10 is clamped between the upper clamp 8 and the lower clamp 9, and passes through the center hole II1101 and the center hole III1201.

[0062] The annular femtosecond laser beam reflected by the parabolic reflector 12 is focused inside the optical fiber 10 between the plane reflector 11 and the parabolic reflector 12 .

[0063] 2) Start the optical fiber clamping and feeding system to stretch the optical fiber 10 toward one end.

[0064] During the stretching process, the femtosecond laser 1 outputs a femtosecond laser beam to the outside.

[0065] The femtosecond laser beam is converted into a ring-shaped femtosecond laser beam by the processing system, and then focused on the inside of the optical fiber 10 between the plane reflector 11 and the parabolic reflector 12, so as to perform photolithography on the inside of the optical fiber 10 to form a ring-shaped fiber grating. The ring-shaped fiber grating at least includes, from the outside to the inside, an optical fiber peripheral area 1002 that has not been photolithographically processed, a ring-shaped fiber grating area 1001, and an inner fiber core 1003 that has not been photolithographically processed;

[0066] The period C of the annular fiber grating is determined by the quantitative relationship between the stretching speed of the fiber clamping system and the repetition frequency of the femtosecond laser.

[0067] The quantitative relationship between the pulling speed of the fiber clamping system and the repetition frequency of the femtosecond laser is shown below:

[0068]

[0069] Where C is the period of the annular fiber grating; v is the stretching speed of the fiber clamping system; and f is the repetition frequency of the femtosecond laser.

[0070] The length of the ring fiber Bragg grating is determined by the photolithography time and the stretching speed.

[0071] The grating thickness of the annular grating is controlled by the distance between the spherical reflector 4 and the concave reflector 3 in the photolithography system.

[0072] The femtosecond laser lithography system further includes a horizontal displacement control system 7 .

[0073] The spherical reflector 4 is mounted on a slider on the horizontal displacement control system 7 .

[0074] The distance between the spherical reflector 4 and the concave reflector 3 is adjusted by controlling the slider, thereby adjusting the diameter and width of the annular femtosecond laser beam reflected by the concave reflector 3 .

[0075] The width of the ring grating unit is determined by the focused spot size of the femtosecond laser.

[0076] The femtosecond laser lithography system further includes a beam shaping and expansion system 2 .

[0077] The shaping and beam expansion system 2 is arranged in front of the femtosecond laser 1 and is used to adjust the divergence angle of the femtosecond laser beam output by the femtosecond laser 1, thereby achieving the adjustment of the size of the annular femtosecond laser beam.

[0078] The femtosecond laser lithography system further comprises a flat mirror frame 6 . The spherical reflector 4 is installed at the center of the mirror frame 6 .

[0079] The mirror frame 6 has an annular optical window 5. The annular light beam reflected by the concave reflector 3 passes through the optical window 5 and is then transmitted to the plane reflector 11.

[0080] In step 2, after the photolithography process is completed, the annular fiber grating is also subjected to high temperature annealing.

[0081] The process of converting the femtosecond laser beam from the processing system into a circular femtosecond laser beam is as follows:

[0082] After passing through the central hole I301, the light beam outputted by the femtosecond laser 1 is reflected by the spherical reflector 4 to the concave reflector 3, and then reflected by the reflective surface I302 of the concave reflector 3 to form a ring-shaped femtosecond laser beam, and then reaches the plane reflector 11;

[0083] After the annular femtosecond laser beam is transmitted to the plane reflector 11, it is reflected by the plane reflector 11 to the parabolic reflector 12 above;

[0084] The parabolic reflector 12 reflects the annular femtosecond laser beam, so that the annular femtosecond laser beam is focused inside the optical fiber 10 between the plane reflector 11 and the parabolic reflector 12 .

[0085] The optical fiber includes single crystal optical fiber and quartz optical fiber.

[0086] like Figure 4 As shown, when the optical fiber 10 is a single crystal optical fiber, the annular fiber grating includes, from outside to inside, an optical fiber peripheral region 1002 that has not been photolithographically processed, an optical fiber cladding region 1004, an annular fiber grating region 1001, and an internal fiber core 1003 that has not been photolithographically processed; the optical fiber cladding region 1004 and the annular fiber grating region 1001 are obtained by photolithography.

[0087] like Figure 1 As shown, when the optical fiber 10 is a quartz optical fiber, the optical fiber peripheral region 1002 that has not been photolithographically processed is the optical fiber cladding.

[0088] Embodiment 2:

[0089] A highly efficient processing method for annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography comprises the following steps:

[0090] 1) Assemble the optical fiber 10 on the optical fiber clamping and feeding system of the femtosecond laser lithography system.

[0091] The processing system includes a femtosecond laser 1 , a concave reflector 3 , a spherical reflector 4 , an optical fiber clamping system, a plane reflector 11 , and a parabolic reflector 12 .

[0092] The concave reflector 3 has a central hole I301. After the light beam output by the femtosecond laser 1 passes through the central hole I301, it is reflected by the spherical reflector 4 to the concave reflector 3, and then reflected by the reflective surface I302 of the concave reflector 3 to form a ring-shaped femtosecond laser beam, and then reaches the plane reflector 11.

[0093] The plane reflector 11 is tilted and has a center hole II1101 at its center for the optical fiber 10 to pass through. The annular light beam is reflected by the plane reflector 11 to the parabolic reflector 12 above. The parabolic reflector 12 has a center hole III1201 at its center for the optical fiber 10 to pass through.

[0094] The optical fiber clamping system is used to stretch the optical fiber 10, and includes an upper clamp 8 located above the parabolic reflector 12 and a lower clamp 9 located below the plane reflector 11. The optical fiber 10 is clamped between the upper clamp 8 and the lower clamp 9, and passes through the center hole II1101 and the center hole III1201.

[0095] The annular femtosecond laser beam reflected by the parabolic reflector 12 is focused inside the optical fiber 10 between the plane reflector 11 and the parabolic reflector 12 .

[0096] 2) Start the optical fiber clamping and feeding system to stretch the optical fiber 10 toward one end.

[0097] During the stretching process, the femtosecond laser 1 outputs a femtosecond laser beam to the outside.

[0098] The femtosecond laser beam is converted into a ring-shaped femtosecond laser beam by the processing system, and then focused on the inside of the optical fiber 10 between the plane reflector 11 and the parabolic reflector 12, so as to perform photolithography on the inside of the optical fiber 10 to form a ring-shaped fiber grating. The ring-shaped fiber grating at least includes, from the outside to the inside, an optical fiber peripheral area 1002 that has not been photolithographically processed, a ring-shaped fiber grating area 1001, and an inner fiber core 1003 that has not been photolithographically processed;

[0099] The period C of the annular fiber grating is determined by the quantitative relationship between the stretching speed of the fiber clamping system and the repetition frequency of the femtosecond laser.

[0100] Embodiment 3:

[0101] A highly efficient processing method for annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography, the technical content of which is the same as that of Example 2, and further, the quantitative relationship between the stretching speed of the fiber clamping system and the repetition frequency of the femtosecond laser is as follows:

[0102]

[0103] Where C is the period of the annular fiber grating; v is the stretching speed of the fiber clamping system; and f is the repetition frequency of the femtosecond laser.

[0104] Embodiment 4:

[0105] A highly efficient processing method for annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography, the technical content of which is the same as any one of embodiments 2-3, and further, the length of the annular fiber Bragg grating is jointly determined by the lithography time and the stretching speed.

[0106] Embodiment 5:

[0107] A highly efficient processing method for annular fiber gratings based on ring-by-ring femtosecond laser lithography, the technical content of which is the same as any one of embodiments 2-4, and further, the grating thickness of the annular grating is controlled by the distance between the spherical reflector 4 and the concave reflector 3 in the lithography system.

[0108] Embodiment 6:

[0109] A highly efficient processing method for annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography, the technical content of which is the same as any one of embodiments 2-5, and further, the femtosecond laser lithography system also includes a horizontal displacement control system 7.

[0110] The spherical reflector 4 is mounted on a slider on the horizontal displacement control system 7 .

[0111] The distance between the spherical reflector 4 and the concave reflector 3 is adjusted by controlling the slider, thereby adjusting the diameter and width of the annular femtosecond laser beam reflected by the concave reflector 3 .

[0112] like Figure 3 As shown, the spherical reflector is located at different positions ( Figure 3 Left side) results in different diameters and widths of the annular femtosecond laser beam ( Figure 3 middle), which results in different lithography effects of the annular grating in the core cross section ( Figure 3 on the right).

[0113] Embodiment 7:

[0114] A highly efficient processing method for annular fiber gratings based on ring-by-ring femtosecond laser lithography, the technical content of which is the same as any one of embodiments 2-6, and further, the width of the annular grating unit is determined by the focus spot size of the femtosecond laser.

[0115] Embodiment 8:

[0116] A highly efficient processing method for annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography, the technical content of which is the same as any one of embodiments 2-7, wherein the femtosecond laser lithography system further includes a shaping and beam expansion system 2.

[0117] The shaping and beam expansion system 2 is arranged in front of the femtosecond laser 1 and is used to adjust the divergence angle of the femtosecond laser beam output by the femtosecond laser 1, thereby achieving the adjustment of the size of the annular femtosecond laser beam.

[0118] Embodiment 9:

[0119] A highly efficient processing method for annular fiber Bragg grating based on ring-by-ring femtosecond laser lithography, the technical content of which is the same as any one of embodiments 2-8, further, the femtosecond laser lithography system further comprises a flat mirror frame 6. The spherical reflector 4 is installed at the center of the mirror frame 6.

[0120] The mirror frame 6 has an annular optical window 5. The annular light beam reflected by the concave reflector 3 passes through the optical window 5 and is then transmitted to the plane reflector 11.

[0121] Embodiment 10:

[0122] A highly efficient processing method for annular fiber Bragg grating based on ring-by-ring femtosecond laser lithography, the technical content of which is the same as any one of embodiments 2-9, and further, in step 2, after the lithography processing is completed, the annular fiber Bragg grating is also subjected to high-temperature annealing.

[0123] Embodiment 11:

[0124] A highly efficient processing method for annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography, the technical content of which is the same as any one of embodiments 2-10, and further, the process of converting the femtosecond laser beam from the processing system into an annular femtosecond laser beam is as follows:

[0125] The light beam output by the femtosecond laser 1 passes through the central hole I301, is reflected by the spherical reflector 4, and then is transmitted to the concave reflector, and is reflected by the reflective surface I302 of the concave reflector 3 to form a ring-shaped femtosecond laser beam;

[0126] After being reflected by the concave reflector 3 into a ring-shaped beam, the light beam reaches the plane reflector 11;

[0127] After the annular femtosecond laser beam is transmitted to the plane reflector 11, it is reflected by the plane reflector 11 to the parabolic reflector 12 above;

[0128] The parabolic reflector 12 reflects the annular femtosecond laser beam, so that the annular femtosecond laser beam is focused inside the optical fiber 10 between the plane reflector 11 and the parabolic reflector 12 .

[0129] Embodiment 12:

[0130] A highly efficient processing method for annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography, the technical content of which is the same as any one of embodiments 2-11, and further, the femtosecond laser lithography system also includes a beam shaping and expansion system arranged in front of the femtosecond laser. The beam shaping and expansion system is used to precisely control the divergence angle of the light beam output by the femtosecond laser, thereby achieving precise control of the diameter and focusing depth of the annular femtosecond laser beam.

[0131] Embodiment 13:

[0132] A highly efficient processing method for annular fiber Bragg grating based on ring-by-ring femtosecond laser lithography, the technical content is the same as any one of embodiments 2-12, and further, the femtosecond laser lithography system also includes a horizontal displacement control system. The spherical reflector is mounted on a slider on the horizontal displacement control system. By controlling the slider to adjust the distance between the spherical reflector and the concave reflector, the diameter and width of the annular femtosecond laser beam reflected by the concave reflector are controlled, thereby adjusting the grating area thickness of the annular grating.

[0133] Embodiment 14:

[0134] A highly efficient processing method for annular fiber Bragg grating based on ring-by-ring femtosecond laser lithography, the technical content of which is the same as any one of embodiments 2-13, further, the femtosecond laser lithography system also includes a flat mirror frame. The spherical reflector is installed at the center of the mirror frame. The mirror frame has an annular optical window. The annular femtosecond laser beam reflected by the concave reflector passes through the optical window.

[0135] Embodiment 15:

[0136] A highly efficient processing method for annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography, the technical content of which is the same as any one of Examples 2-13, and further, the optical fiber is a single crystal optical fiber or a quartz optical fiber.

[0137] When the optical fiber 10 is a single crystal optical fiber, the annular fiber grating includes, from outside to inside, an optical fiber peripheral region 1002 that has not been photolithographically processed, an optical fiber cladding region 1004, an annular fiber grating region 1001, and an inner core 1003 that has not been photolithographically processed; the optical fiber cladding region 1004 and the annular fiber grating region 1001 are obtained by photolithography.

[0138] When the optical fiber 10 is a silica optical fiber, the optical fiber peripheral region 1002 that is not photolithographically processed is the optical fiber cladding.

[0139] Embodiment 15:

[0140] A highly efficient processing method for annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography comprises the following steps:

[0141] 1) Use alcohol cotton to clean the surface of the optical fiber, and then fix the optical fiber in the clamping device of the writing system so that the optical fiber is in a vertical state.

[0142] A femtosecond laser lithography system including a femtosecond laser, a concave reflector, a spherical reflector, a fiber clamping system, a plane reflector, and a parabolic reflector is arranged in front of the fiber clamping system.

[0143] The concave reflecting mirror 3 has a central hole I301. After the light beam output by the femtosecond laser 1 passes through the central hole I301, it is reflected by the spherical reflecting mirror 4, and then transmitted to the concave reflecting mirror, and is reflected by the reflecting surface I302 of the concave reflecting mirror 3 to form a ring-shaped femtosecond laser beam.

[0144] After being reflected by the concave reflector 3 into a ring-shaped beam, the light beam reaches the plane reflector 11;

[0145] The plane reflector is placed at an angle, and has a center hole II at its center for the optical fiber to pass through. The annular femtosecond laser beam is reflected by the plane reflector to the parabolic reflector above. The center of the parabolic reflector has a center hole III at its center for the optical fiber to pass through.

[0146] The optical fiber clamping and feeding system is used to stretch the optical fiber, and comprises an upper clamp located above the parabolic reflector and a lower clamp located below the plane reflector. The optical fiber is clamped between the upper clamp and the lower clamp and passes through the center hole II and the center hole III.

[0147] The annular femtosecond laser beam reflected by the parabolic reflector is focused inside the optical fiber between the plane reflector and the parabolic reflector.

[0148] 2) Control the stretching speed of the optical fiber according to the design parameters such as the period and unit width of the annular grating, and write the annular fiber grating in the optical fiber.

[0149] It is worth noting that the key parameters of the ring fiber grating, such as period, thickness, unit width and total length, need to be designed according to requirements. Among them, the grating thickness of the ring grating is controlled by the distance between the spherical reflector and the concave reflector in the lithography system. The period Λ of the ring grating is determined by the repetition frequency f of the femtosecond laser and the speed v of the stretching system:

[0150]

[0151] The width of the ring grating unit is mainly determined by the focus spot size of the femtosecond laser, and the total length of the ring grating is determined by the stretching system speed and the lithography time;

[0152] 3) Start the system, stretch the optical fiber toward one end, and write a ring grating structure in the optical fiber.

[0153] 4) The photolithographically completed annular fiber Bragg grating is taken out and placed in a high temperature furnace for high temperature annealing to eliminate the unstable structure and internal stress in the annular fiber Bragg grating.

[0154] Embodiment 16:

[0155] An efficient processing method for ring-shaped fiber Bragg gratings based on ring-by-ring femtosecond laser lithography is as follows:

[0156] See also Figure 1 In this embodiment, a ring-shaped fiber grating is precisely written in a single crystal optical fiber to form a Figure 4 As shown, the optical fiber has an annular fiber grating region 1001 inside. The outer periphery of the annular fiber grating region 1001 has a cladding 1004, and the inner periphery of the annular fiber grating region has a core 1003. The outer periphery of the cladding 1002 is the optical fiber outer region 1002 which has not been photolithographically processed.

[0157] The processing includes the following steps:

[0158] 1) Assembling the single crystal optical fiber 10 on the single crystal optical fiber clamping and conveying system;

[0159] In front of the single crystal fiber clamping and delivering system is a femtosecond laser lithography system including a femtosecond laser 1, a concave reflector 3, a spherical reflector 4, a single crystal fiber clamping and delivering system, a plane reflector 11, and a parabolic reflector 12;

[0160] The concave reflecting mirror 3 has a central hole I301. After the light beam output by the femtosecond laser 1 passes through the central hole I301, it is reflected by the spherical reflecting mirror 4, and then transmitted to the concave reflecting mirror, and is reflected by the reflecting surface I302 of the concave reflecting mirror 3 to form a ring-shaped femtosecond laser beam.

[0161] After being reflected by the concave reflector 3 into a ring-shaped beam, the light beam reaches the plane reflector 11;

[0162] The plane reflector 11 is tilted, and has a center hole II1101 at its center for the single crystal optical fiber 10 to pass through; the annular femtosecond laser beam is reflected by the plane reflector 11 to the parabolic reflector 12 above; the center of the parabolic reflector 12 has a center hole III1201 for the single crystal optical fiber 10 to pass through;

[0163] The optical fiber clamping and conveying system is used to stretch the single crystal optical fiber 10, and includes an upper clamp 8 located above the parabolic reflector 12 and a lower clamp 9 located below the plane reflector 11; the single crystal optical fiber 10 is clamped between the upper clamp 8 and the lower clamp 9, and passes through the center hole II1101 and the center hole III1201;

[0164] The annular femtosecond laser beam reflected by the parabolic reflector 12 is focused inside the single crystal optical fiber 10 between the plane reflector 11 and the parabolic reflector 12;

[0165] 2) According to the design parameters such as the period and unit width of the annular grating, the stretching speed of the single crystal fiber is controlled, and the annular single crystal fiber grating is engraved in the single crystal fiber.

[0166] It is worth noting that the fiber clamping system continuously stretches the single crystal fiber 10; the grating area thickness of the annular fiber grating is determined by the distance between the spherical reflector and the concave reflector in the lithography system, the period of the annular grating is determined by the stretching speed and the repetition frequency of the femtosecond laser, and the total length of the annular grating is determined by the stretching speed and the total lithography time. The method of the present invention utilizes the ring-by-ring femtosecond laser lithography technology to accurately write the annular single crystal fiber grating in the single crystal fiber. After the femtosecond laser pulse with ultra-high peak power and ultra-short pulse duration is focused inside the single crystal fiber, it triggers strong nonlinear effects such as multi-photon ionization, tunneling ionization and avalanche ionization, generating a large amount of free electron plasma. These plasmas are then rapidly quenched and solidified, causing permanent changes in the physical properties such as the refractive index of the writing area. The ring-by-ring femtosecond laser lithography technology is not limited by the high-precision translation stage, and a ring grating structure with a uniform refractive index distribution can be efficiently written in the single crystal fiber.

[0167] The ring light formation principle of this application is as follows Figure 2 As shown in the figure, after passing through the spherical reflector, the light beam is converted into annular light, and the annular light is then reflected by a concave reflector to reach a plane reflector 11; the plane reflector 11 reflects to the parabolic reflector 12 above, and after being reflected by the parabolic reflector 12, an annular light spot focused on a plane is formed.

[0168] The annular light beam reflected by the parabolic reflector 12 is focused inside the single crystal optical fiber 10 between the plane reflector 11 and the parabolic reflector 12. In the embodiment, the single crystal optical fiber clamping and feeding system stretches the single crystal optical fiber 10 upward or downward. Since the annular light spot is focused inside the single crystal optical fiber 10, photolithography of the focused area inside the single crystal optical fiber 10 can be achieved, forming a Figure 4 The annular cladding 1004 and the annular fiber grating region 1001 are shown. The periphery of the annular fiber grating region 1001 is the cladding 1004, and the interior of the annular fiber grating region has a fiber core 1003. The periphery of the cladding 1002 is the fiber peripheral region 1002 that has not been photolithographically processed.

[0169] Embodiment 17:

[0170] The main structure of this embodiment is the same as that of embodiment 16, and further comprises a shaping beam expansion system 2 arranged in front of the femtosecond laser 1. The shaping beam expansion system 2 can precisely control the divergence angle of the femtosecond laser beam output by the femtosecond laser 1, thereby realizing precise control of the width of the annular femtosecond laser beam and the focusing depth in the single crystal optical fiber.

[0171] In the present invention, the optical fiber comprises:

[0172] 1. Quartz optical fiber

[0173] 2. Single crystal sapphire (Al 2 O3 )optical fiber,

[0174] 3. Single crystal lutetium oxide (Lu 2 O 3 )optical fiber

[0175] 4. Single crystal yttrium aluminum garnet (Y 3 Al 5 O 12 )optical fiber

[0176] 5. Single crystal lithium niobate (LiNbO 3 )optical fiber

[0177] 6. Single crystal yttrium oxide (Y 2 O 3 )optical fiber

[0178] 7. Single crystal tantalum oxide (Ta 2 O 5 )optical fiber

[0179] 8. Single crystal lithium tantalate (LiTaO 3 )optical fiber

[0180] The main parameters that need to be controlled during the ring fiber Bragg grating writing process include:

[0181] Central wavelength, pulse energy and repetition rate of femtosecond laser

[0182] In the embodiment, the following parameters may be used:

[0183] Related parameters of ring fiber Bragg gratings by ring-by-ring femtosecond laser lithography

[0184]

[0185] Embodiment 18:

[0186] The main structure of this embodiment is the same as that of Embodiment 16 or 17, and further includes a horizontal displacement control system 7. The spherical reflector 4 is mounted on a slider on the horizontal displacement control system 7. The distance between the spherical reflector 4 and the concave reflector 3 is adjusted by controlling the slider to control the diameter of the annular light beam reflected by the concave reflector 3. Further, the spherical reflector 4 is mounted at the center of the mirror frame 6. The mirror frame 6 has an annular optical window 5. The annular light beam reflected by the concave reflector 3 passes through the optical window 5. In the embodiment, the mirror frame 6 is in the shape of a flat plate, and its lower end is fixed to the horizontal displacement control system 7 by a bracket 9. The substrate of the flat plate is made of a transparent material, and its surface is partially coated with an opaque coating, that is, the rear of the spherical reflector 4 is an opaque coating, and the spherical reflector 4 is surrounded by an uncoated annular optical window 5, and the inner and outer sides of the optical window 5 are coated, so that the light beam passing through the annular optical window 5 is a horizontally reversed annular light beam.

Claims

1. A highly efficient processing method for annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography, characterized in that: The following steps are involved: 1) assembling the optical fiber (10) on the optical fiber clamping and transporting system of the femtosecond laser lithography system; The processing system comprises a femtosecond laser (1), a concave reflector (3), a spherical reflector (4), an optical fiber clamping system, a plane reflector (11), and a parabolic reflector (12); The concave reflecting mirror (3) has a central hole I (301); The light beam output by the femtosecond laser (1) passes through the central hole I (301), is reflected by the spherical reflector (4), and then is transmitted to the concave reflector, and is reflected by the reflective surface I (302) of the concave reflector (3) to form a ring-shaped femtosecond laser beam; After being reflected by the concave reflector (3) into a ring-shaped beam, the light beam moves forward to the plane reflector (11); The plane reflector (11) is tilted and has a center hole II (1101) at its center for the optical fiber (10) to pass through; the annular light beam is reflected by the plane reflector (11) to the parabolic reflector (12) above; the parabolic reflector (12) has a center hole III (1201) at its center for the optical fiber (10) to pass through; The optical fiber clamping and delivery system is used for stretching an optical fiber (10), and comprises an upper clamp (8) located above a parabolic reflector (12) and a lower clamp (9) located below a plane reflector (11); the optical fiber (10) is clamped between the upper clamp (8) and the lower clamp (9), and passes through a center hole II (1101) and a center hole III (1201); The annular femtosecond laser beam reflected by the parabolic reflector (12) is focused inside the optical fiber (10) between the plane reflector (11) and the parabolic reflector (12); 2) starting the optical fiber clamping and feeding system to stretch the optical fiber (10) toward one end; During the stretching process, the femtosecond laser (1) outputs a femtosecond laser beam to the outside; A femtosecond laser beam is converted into an annular femtosecond laser beam by a processing system, and then focused on the inside of an optical fiber (10) located between a plane reflector (11) and a parabolic reflector (12), thereby performing photolithography on the inside of the optical fiber (10) to form an annular fiber grating; the annular fiber grating comprises at least an optical fiber peripheral region (1002) that has not been photolithographically processed, an annular fiber grating region (1001), and an internal fiber core (1003) that has not been photolithographically processed from the outside to the inside; The period of the ring fiber Bragg grating is determined by the quantitative relationship between the stretching speed of the fiber clamping system and the repetition frequency of the femtosecond laser.

2. The method for efficiently processing annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography according to claim 1, characterized in that: The quantitative relationship between the pulling speed of the fiber clamping system and the repetition frequency of the femtosecond laser is shown below: Where C is the period of the annular fiber grating; v is the stretching speed of the fiber clamping system; and f is the repetition frequency of the femtosecond laser.

3. The method for efficiently processing annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography according to claim 1, characterized in that: The length of the ring fiber Bragg grating is determined by the photolithography time and the stretching speed.

4. The method for efficiently processing annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography according to claim 1, characterized in that: The grating thickness of the annular grating is controlled by the distance between the spherical reflector (4) and the concave reflector (3) in the photolithography system.

5. The method for efficiently processing annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography according to claim 4, characterized in that: The femtosecond laser lithography system further comprises a horizontal displacement control system (7); The spherical reflector (4) is mounted on a slider on a horizontal displacement control system (7); The distance between the spherical reflector (4) and the concave reflector (3) is adjusted by controlling the slider, thereby regulating the diameter and width of the annular femtosecond laser beam reflected by the concave reflector (3).

6. The method for efficiently processing annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography according to claim 1, characterized in that: The width of the ring grating unit is determined by the focused spot size of the femtosecond laser. The femtosecond laser lithography system also includes a beam shaping and expansion system (2); The shaping and beam expansion system (2) is arranged in front of the femtosecond laser (1) and is used to adjust the divergence angle of the femtosecond laser beam output by the femtosecond laser (1), thereby achieving the adjustment of the size of the annular femtosecond laser beam.

7. The method for efficiently processing annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography according to claim 1, characterized in that: The femtosecond laser lithography system further comprises a flat mirror frame (6); the spherical reflector (4) is mounted at the center of the mirror frame (6); The mirror frame (6) has an annular optical window (5); the annular light beam reflected by the concave reflector (3) passes through the optical window (5) and is then transmitted to the plane reflector (11).

8. The method for efficiently processing annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography according to claim 1, characterized in that: In step 2), after the photolithography process is completed, the annular fiber grating is also subjected to high temperature annealing.

9. The method for efficiently processing annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography according to claim 1, characterized in that: The process of converting the femtosecond laser beam from the processing system into a circular femtosecond laser beam is as follows: The light beam output by the femtosecond laser (1) passes through the central hole I (301), is reflected by the spherical reflector (4), and then is transmitted to the concave reflector, and is reflected by the reflective surface I (302) of the concave reflector (3) to form a ring-shaped femtosecond laser beam; After being reflected by the concave reflector (3) into a ring-shaped beam, the light beam moves forward to the plane reflector (11); After the annular femtosecond laser beam is transmitted to the plane reflector (11), it is reflected by the plane reflector (11) to the parabolic reflector (12) above; The parabolic reflector (12) reflects the annular femtosecond laser beam, so that the annular femtosecond laser beam is focused inside the optical fiber (10) located between the plane reflector (11) and the parabolic reflector (12).

10. The method for efficiently processing annular fiber Bragg gratings based on ring-by-ring femtosecond laser lithography according to claim 1, characterized in that: The optical fiber (10) includes single crystal optical fiber and quartz optical fiber.

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