CN117590514A - Method and device for improving transmission spectrum depth of femtosecond laser inscription fiber grating - Google Patents

Abstract

The invention provides a method and a device for improving the transmission spectrum depth of a femtosecond laser inscribing fiber grating, which inscribes the fiber grating on a double-clad fiber by utilizing the femtosecond laser, wherein the double-clad fiber comprises a fiber core, an inner cladding and an outer cladding, the cross section of the inner cladding is polygonal, and at least one corner in the polygon is an arc-shaped corner; the femtosecond laser is incident to the fiber core from the arc-shaped corner of the inner cladding to inscribe the fiber grating, so that the transmission spectrum depth of the inscribed fiber grating can be improved. Based on the invention, the optimal incidence angle of the femtosecond laser when the optical fiber grating is inscribed on the double-clad optical fiber can be determined, and the optical fiber grating with good high-efficiency inscription performance can be realized.

Description

Method and device for improving transmission spectrum depth of femtosecond laser inscription fiber grating

Technical Field

The invention mainly relates to the technical field of fiber bragg grating inscription, in particular to a method and a device for improving the transmission spectrum depth of a femtosecond laser inscription fiber bragg grating.

Background

The femtosecond laser phase mask method for inscribing the Fiber Bragg Grating (FBG) has the advantages of short preparation period, good performance, suitability for large-scale production and the like, and has wide application prospect. In addition, the method has no requirement on photosensitivity of the optical fiber, and the fiber grating can be directly prepared in the gain optical fiber.

In order to improve the absorption efficiency of the core gain medium to the pump light, the shape of the inner cladding of many gain fibers is not a standard circle, but an octagon, a hexagon, a D shape and the like. In actual production, the vertex of the fiber cladding forms a circular arc structure, as shown in fig. 1, which is an octagonal fiber (the most commonly used cladding structure in high power situations). At this time, the femtosecond laser is only incident from a specific angle and can be just focused in the fiber core, if the angle deviates, the position and the light intensity of the focus can be influenced due to factors such as aberration, and the optical performance of the written fiber grating can be seriously influenced, even the fiber grating cannot be written. Therefore, it is very difficult to write the fiber grating in the optical fiber with the cladding having the shape other than the standard circular shape, but no method is available at present to determine the relative angle relation between the femtosecond laser and the double-cladding optical fiber, which results in different grating performances of each writing, extremely low yield and no suitability for the requirement of mass production.

Disclosure of Invention

Aiming at the technical problems in the prior art, the invention provides a method and a device for improving the transmission spectrum depth of a femtosecond laser inscribing fiber grating.

In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:

in one aspect, the invention provides a method for improving the transmission spectrum depth of a femtosecond laser inscribing fiber grating, which inscribes the fiber grating on a double-clad fiber by using the femtosecond laser, wherein the double-clad fiber comprises a fiber core, an inner cladding and an outer cladding, the cross section of the inner cladding is polygonal, and at least one corner in the polygon is an arc-shaped corner; the femtosecond laser is incident to the fiber core from the arc-shaped corner of the inner cladding to inscribe the fiber grating, so that the transmission spectrum depth of the inscribed fiber grating can be improved.

Further, the double-clad optical fiber is a double-clad doped optical fiber, and rare earth ions are doped in the fiber core of the double-clad doped optical fiber.

Further, the cross section of the inner cladding is hexagonal or octagonal or D-shaped.

Further, the femtosecond laser is incident to the fiber core from the center position of the contour line at the arc-shaped corner of the inner cladding.

In another aspect, the present invention provides a method for writing a fiber grating with a femtosecond laser, including:

taking a double-clad optical fiber, wherein the double-clad optical fiber comprises a fiber core, an inner cladding and an outer cladding, the cross section of the inner cladding is polygonal, and at least one corner in the polygon is an arc-shaped corner;

and the femtosecond laser is incident to the fiber core from the arc-shaped corner of the inner cladding, so that the inscription of the fiber grating is completed.

Further, the femtosecond laser is incident to the fiber core from the center position of the contour line at the arc-shaped corner of the inner cladding.

On the other hand, a device for writing fiber gratings by femtosecond laser is provided, which comprises a femtosecond laser, a laser scanning galvanometer, a cylindrical lens, a phase mask plate, double-clad fiber, a fiber clamp and a three-dimensional moving platform, wherein a pair of fiber clamps are arranged on the three-dimensional moving platform;

the double-clad optical fiber is clamped and fixed by a pair of optical fiber clamps, the double-clad optical fiber comprises a fiber core, an inner cladding and an outer cladding, the cross section of the inner cladding is polygonal, and at least one corner in the polygon is an arc-shaped corner;

the clamping position and the clamping angle of the double-cladding optical fiber are adjusted through the three-dimensional moving platform and the optical fiber clamp, so that the femtosecond laser output by the femtosecond laser enters the fiber core from the arc-shaped corner of the inner cladding for writing of the fiber grating after passing through the laser scanning galvanometer, the cylindrical lens and the phase mask plate.

Further, the double-clad optical fiber is a double-clad doped optical fiber, and rare earth ions are doped in the fiber core of the double-clad doped optical fiber; the device also comprises an ASE light source, a pattern matcher, a spectrometer, a filter and a fluorescence spectrometer, wherein one end of the double-clad optical fiber is connected with the first spectrometer through the filter to detect the fluorescence spectrum excited by the femtosecond laser in the fiber core, the other end of the double-clad optical fiber is connected with the ASE light source and the spectrometer through the pattern matcher, and the second spectrometer is used for detecting the reflection spectrum or the transmission spectrum of the carved fiber grating.

Further, the device for writing the fiber grating by the femtosecond laser further comprises a tunable attenuator and a diaphragm, wherein the femtosecond laser output by the femtosecond laser is incident to the scanning galvanometer through the tunable attenuator and the diaphragm; the optical fiber clamps are rotatable optical fiber clamps.

Further, when the device for writing the fiber grating by the femtosecond laser writes the fiber grating, the femtosecond laser is kept in a low power state, so that the output femtosecond laser power is lower than a fluorescence excitation threshold value, and the double-clad fiber clamped on the fiber clamp is rotated through a three-dimensional displacement platform, so that the femtosecond laser output by the femtosecond laser can be incident to a fiber core from an arc-shaped corner of the inner cladding after passing through a laser scanning galvanometer, a cylindrical lens and a phase mask plate, and the initial adjustment of the clamping position and the clamping angle of the double-clad fiber is completed;

then, increasing the power of the femtosecond laser to ensure that the output femtosecond laser power is higher than a fluorescence excitation threshold and lower than a grating writing threshold; detecting fluorescence spectrum excited by the femtosecond laser in the fiber core by utilizing the first spectrometer, rotating the double-cladding optical fiber in the contour line range of the arc-shaped corner of the inner cladding, and finely adjusting the clamping angle of the double-cladding optical fiber to ensure that the fluorescence detected by the first spectrometer is strongest, thereby finishing fine adjustment of the clamping position and the clamping angle of the double-cladding optical fiber;

and then continuously increasing the power of the femtosecond laser to ensure that the output femtosecond laser power is higher than the grating writing threshold value, thus finishing the writing of the fiber grating.

The invention has the beneficial effects that:

for the optical fiber with the inner cladding not standard circular in shape, the femtosecond laser can be incident from the arc corner or plane contour of the inner cladding, the focusing positions of the two states and the light intensity distribution at the focus are greatly different, and the carved gratings have different performances and cannot meet the requirement of mass production. The invention uses the femtosecond laser to write the fiber grating on the double-cladding fiber, and by measuring the fluorescence intensity excited by the femtosecond laser in the fiber core and combining the rotation and the movement of the fiber, the relative position of the femtosecond laser and the fiber can be accurately adjusted, so that the femtosecond laser is incident to the fiber core from the arc-shaped corner of the inner cladding for writing the fiber grating. The intensity variation of the femtosecond laser at the focal point is more severe due to refocusing effect. Therefore, under the same femtosecond laser power, the grating etching threshold value of the femtosecond laser is easier to reach when the laser is incident from the arc-shaped corner, and the refractive index modulation depth of the etched fiber grating is larger. The fiber bragg grating has the advantages of strict and controllable performance parameters and good stability, is suitable for large-scale preparation, and can be widely applied to fiber lasers.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and other drawings may be obtained according to the structures shown in these drawings without inventive effort for a person skilled in the art.

FIG. 1 is a schematic diagram of an exemplary ytterbium-doped fiber (with an inner cladding resembling a regular octagon);

FIG. 2 is a schematic view of the structure of several inner cladding layers, wherein (a) is an inner cladding layer having a regular octagon-like shape, (b) is an inner cladding layer having a regular hexagon-like shape, and (c) is an inner cladding layer having a regular quadrilateral-like shape;

FIG. 3 is a schematic diagram of a focusing simulation structure of a femtosecond laser in an ytterbium-doped fiber;

FIG. 4 is a graph showing the effect of the curvature of the laser entrance face on the focal position according to one embodiment;

FIG. 5 is a schematic view of a refractive index change region of a femtosecond laser according to one embodiment;

FIG. 6 is a schematic structural diagram of an apparatus for writing fiber gratings with femtosecond laser according to one embodiment;

FIG. 7 is a schematic structural diagram of an apparatus for writing fiber gratings with femtosecond laser according to one embodiment;

FIG. 8 is a graph of the spectrum detected by the fluorescence spectrometer when the femtosecond laser single pulse energy is 60 muJ and the pulse repetition frequency is 10KHz in one embodiment;

FIG. 9 is a schematic diagram showing the relative incidence angles of YDF and femtosecond laser and scanning writing in an optimal writing state according to one embodiment;

FIG. 10 is a graph showing the variation of fluorescence intensity with rotation angle when the optical fiber is rotated by about 45 degrees according to one embodiment;

FIG. 11 is a high/low reflectivity FBG transmission spectrum inscribed in a 20/400 μm ytterbium-doped double-clad fiber in an embodiment.

Detailed Description

The following description of the embodiments of the present invention will be made clearly and fully with reference to the accompanying drawings, in which it is evident that the embodiments described are only some, but not all embodiments of the invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

The shape of the inner cladding of the current gain fiber is not a standard circle, but an octagon, a hexagon, a D shape, etc. For example, ytterbium-doped fiber (YDF) is the most commonly used gain fiber in the field of high-power fiber laser, if fiber gratings can be directly written in ytterbium-doped fiber and laser output can be formed, the melting point in the high-power fiber laser system can be effectively reduced, and the robustness of the high-power fiber laser system can be effectively improved. The ytterbium-doped fiber is used as a gain fiber, and in order to improve the absorption efficiency of pump light, the shape of the inner cladding of the ytterbium-doped fiber is generally not a round shape of a common fiber, but an octagonal, hexagonal, D-shaped fiber and the like, and the complex inner cladding structure has influence on the focusing of the femtosecond laser, so that the fiber grating with excellent performance can be written in the fiber core only when the femtosecond laser is incident from a specific angle.

In one embodiment, a method for improving transmission spectrum depth of a femtosecond laser writing fiber grating is provided, wherein the femtosecond laser is utilized to write the fiber grating on a double-clad fiber, the double-clad fiber comprises a fiber core, an inner cladding and an outer cladding, the cross section of the inner cladding is polygonal, and at least one corner in the polygon is an arc-shaped corner; the femtosecond laser is incident to the fiber core from the arc-shaped corner of the inner cladding to inscribe the fiber grating, so that the transmission spectrum depth of the inscribed fiber grating can be improved. When the optical fiber with the arc-shaped corner inner cladding is used for writing the optical fiber grating, the performance of the written optical fiber grating is different and the difference is larger when the optical fiber grating is incident from different positions of the inner cladding under the same femtosecond laser power. Under the same femtosecond laser power, the grating threshold value of the femtosecond laser is easier to reach when the laser is incident from the arc corner, and the refractive index modulation depth of the inscribed fiber grating is larger.

The type of the double-clad optical fiber is not limited, preferably the double-clad optical fiber is a double-clad doped optical fiber, and the fiber core of the double-clad doped optical fiber is doped with one or more rare earth ions, and the type of the rare earth ions is not limited.

The inner cladding has a shape which is not limited, and the cross section of the inner cladding which is commonly common at present is hexagonal, octagonal or D-shaped, and the like, and at least one corner on the cross section of the inner cladding is an arc-shaped corner.

Because the arc-shaped corner is provided with a section of arc outline, when the power of the femtosecond laser is unchanged during the writing of the fiber bragg grating, the performance of the written FBG is different and the refractive index modulation depth of the written fiber bragg grating is also different when the femtosecond laser is incident from different positions on the arc-shaped corner. Preferably, the femtosecond laser is incident to the fiber core from the central position of the contour line at the arc-shaped corner of the inner cladding, so that the optimal transmission spectrum depth of the fiber grating can be obtained.

Taking ytterbium-doped fiber as an example, the structure of the ytterbium-doped fiber is shown in fig. 1, the ytterbium-doped fiber includes a fiber core 701, an inner cladding 702, an outer cladding 703 and a coating 704, the inner cladding 702 of the ytterbium-doped fiber is not a standard round but is a regular octagon-like inner cladding, wherein eight corners are arc-shaped transitions, i.e. eight corners of the inner cladding 702 are arc-shaped corners. When the femtosecond laser light is incident from different angles, the incident surface may be either a circular arc (when incident from a corner of the arc) or a straight line (when incident from a plane). The ytterbium-doped optical fiber has an inner cladding not limited to the inner cladding having a regular octagon-like shape shown in FIG. 1, as shown in FIG. 2, in which FIG. 2 (a) is an inner cladding having a regular octagon-like shape, FIG. 2 (b) is an inner cladding having a regular octagon-like shape, and FIG. 2 (c) is an inner cladding having a regular tetragon-like shape

The femtosecond laser is incident into the ytterbium-doped optical fiber based on the optical design software ZEMAX, and a focusing simulation structure diagram of the femtosecond laser in the ytterbium-doped optical fiber is shown in FIG. 3. The femtosecond laser output by the femtosecond laser is incident to the fiber core after passing through the laser scanning galvanometer, the cylindrical lens and the phase mask plate, the diameter of the incident femtosecond laser spot is 8mm, the central wavelength is 515nm, and the bandwidth is 10nm; the cylindrical lens selects a non-cylindrical lens AYL2520-A of the company of Soxhlet Lei Bo, the focal length is 20mm, and the center thickness is 7.5mm; the phase mask can be approximately a parallel plate with the thickness of 4mm; the fiber is the most commonly used 20/400 μm octagonal ytterbium-doped fiber, and the radius of curvature of the circular arc at the vertex is approximately 100 μm.

Consider the effect of the incident surface curvature on focusing with the ytterbium-doped fiber core position unchanged. As shown in fig. 4, the effect of the curvature of the laser light incident surface on the focal position is shown. When the curvature radius r=200 μm of the incident surface, the corresponding focusing position is point a; when the incident position is a plane, the focal position is a point B; when the radius of curvature of the incident surface is 100 μm, the focal position is point C. The distance D between A and C is about 40 μm and the distance D between A and B is about 100 μm as calculated by ZEMAX simulation.

In the actual grating writing process, the difference of incidence focusing positions of the femtosecond laser from the arc-shaped corner or plane of the ytterbium-doped optical fiber is more than 100 mu m, and the difference is far more than the diameter of the fiber core. It can also be seen from fig. 4 that the energy density of the femtosecond laser at the focal point is greater due to refocusing effect when incident from the arc corner. Therefore, under the same femtosecond laser power, the incidence from the arc corner can more easily reach the grating etching threshold value of the femtosecond laser, and the transmission spectrum depth of the fiber grating is larger. That is, assuming that the femtosecond laser power is unchanged, the performance of the etched fiber grating is different and the gap is large when the laser power is incident from different positions. The effectiveness of the invention has been demonstrated experimentally: in one embodiment, for a 20/400 μm dual-clad ytterbium-doped fiber, the inner cladding is regular octagon, and the eight corners of the inner cladding are all arc-shaped transitions, i.e., the eight corners of the inner cladding are all arc-shaped corners, and the radius of curvature of the arc-shaped corners is approximately 100 μm. The power of the femtosecond laser is kept to be 200mW, the single-point writing is performed for 100s, the depth of the transmission spectrum of the FBG which is written by the laser from the arc-shaped corner of the inner cladding of the double-cladding ytterbium-doped optical fiber is higher than 10dB, wherein the depth of the transmission spectrum of the femtosecond laser which is incident to the fiber core from the center of the contour line at the arc-shaped corner of the inner cladding of the double-cladding ytterbium-doped optical fiber can be up to 20dB, and the depth of the transmission spectrum is lower than 10dB when the laser is incident from other plane contours.

In one embodiment, a method for writing a fiber grating by using a femtosecond laser is provided, which includes:

taking a double-clad optical fiber, wherein the double-clad optical fiber comprises a fiber core, an inner cladding and an outer cladding, the cross section of the inner cladding is polygonal, and at least one corner in the polygon is an arc-shaped corner;

and the femtosecond laser is incident to the fiber core from the arc-shaped corner of the inner cladding, so that the inscription of the fiber grating is completed.

Referring to fig. 6, an apparatus for writing a fiber grating with a femtosecond laser according to an embodiment includes a femtosecond laser 1, a tunable attenuator 2, a diaphragm 3, a laser scanning galvanometer 4, a cylindrical lens 5, a phase mask 6, a double-clad fiber 7, a fiber clamp (not shown), and a three-dimensional moving platform (not shown), where a pair of fiber clamps (not shown) are disposed on the three-dimensional moving platform (not shown).

The double-clad optical fiber 7 is clamped and fixed by a pair of optical fiber clamps, the double-clad optical fiber 7 comprises a fiber core, an inner cladding and an outer cladding, the cross section of the inner cladding is polygonal, and at least one corner in the polygon is an arc-shaped corner.

The clamping position and the clamping angle of the double-cladding optical fiber are adjusted through the three-dimensional moving platform and the optical fiber clamp, so that the femtosecond laser output by the femtosecond laser 1 is incident to the laser scanning galvanometer 4 through the tunable attenuator 2 and the diaphragm 3, and then is incident to the fiber core from the arc-shaped corner of the inner cladding for inscription of the optical fiber grating after passing through the cylindrical lens 5 and the phase mask plate 6.

When the device for writing the fiber grating by the femtosecond laser writes the fiber grating, the femtosecond laser is kept in a low power state, so that the output femtosecond laser power is lower than a fluorescence excitation threshold value, and the femtosecond laser output by the femtosecond laser can be incident to a fiber core from an arc-shaped corner of an inner cladding after passing through a laser scanning galvanometer, a cylindrical lens and a phase mask plate through a three-dimensional displacement platform and rotating a double-cladding optical fiber clamped on the optical fiber clamp, so that the initial adjustment of the clamping position and the clamping angle of the double-cladding optical fiber is completed;

and then increasing the power of the femtosecond laser to ensure that the output femtosecond laser power is higher than the grating writing threshold value, thus finishing the writing of the fiber grating.

The optical fiber clamp is a rotatable optical fiber clamp, and although the optical fiber can be conveniently rotated by utilizing the optical fiber clamp, so that the femtosecond laser is incident to the fiber core from the arc-shaped corner of the inner cladding, the focus of the femtosecond laser is difficult to be accurately positioned in the fiber core. Because the refractive index change region of the femtosecond laser after focusing the femtosecond laser cannot completely cover the core for the gain fiber in which the cross section of the inner cladding is polygonal and at least one corner of the polygon is an arc-shaped corner. As for the ytterbium-doped fiber shown in fig. 1, it has an inner cladding similar to a regular octagon, in which eight corners are all arc-shaped transitions, i.e., the eight corners of the inner cladding are all arc-shaped corners, and the refractive index change region of the focused femtosecond laser may be approximately elliptical in fig. 5, which does not cover the core completely. In order to coincide the focus position with the core, the double-clad fiber clamping position and clamping angle must be precisely adjusted.

Referring to fig. 7, an apparatus for writing a fiber grating with a femtosecond laser according to an embodiment includes a femtosecond laser 1 (not shown), a tunable attenuator 2 (not shown), a diaphragm 3 (not shown), a laser scanning galvanometer 4, a cylindrical lens 5, a phase mask 6, a double-clad fiber 7, a fiber clamp 8, a three-dimensional moving platform (not shown), an ASE light source 12, a pattern matcher 11, a second spectrometer 13, a filter 9, and a first spectrometer 10; a pair of optical fiber clamps 8 are arranged on a three-dimensional moving platform; the double-clad optical fiber 7 is a double-clad doped optical fiber, and rare earth ions are doped in the fiber core of the double-clad doped optical fiber; one end of the double-clad optical fiber 7 is connected to a first spectrometer 10 after passing through a filter 8, the first spectrometer 10 is a fluorescence spectrometer and is used for detecting fluorescence spectrum excited by femto-second laser in the fiber core of the double-clad doped optical fiber, the other end of the double-clad optical fiber 7 is connected with an ASE light source 12 and a second spectrometer 13 through a pattern matcher 11, and the second spectrometer 12 is used for detecting reflection spectrum or transmission spectrum of the inscribed fiber grating. The optical fiber clamps 8 are rotatable optical fiber clamps.

When the device for writing the fiber grating by the femtosecond laser writes the fiber grating, the femtosecond laser is kept in a low power state, so that the output femtosecond laser power is lower than a fluorescence excitation threshold value, and the femtosecond laser output by the femtosecond laser can be incident to a fiber core from an arc-shaped corner of an inner cladding after passing through a laser scanning galvanometer, a cylindrical lens and a phase mask plate through a three-dimensional displacement platform and rotating a double-cladding optical fiber clamped on the optical fiber clamp, so that the initial adjustment of the clamping position and the clamping angle of the double-cladding optical fiber is completed;

then, increasing the power of the femtosecond laser to ensure that the output femtosecond laser power is higher than a fluorescence excitation threshold and lower than a grating writing threshold; detecting fluorescence spectrum excited by the femtosecond laser in the fiber core by utilizing the first spectrometer, rotating the double-cladding optical fiber in the contour line range of the arc-shaped corner of the inner cladding, and finely adjusting the clamping angle of the double-cladding optical fiber to ensure that the fluorescence detected by the first spectrometer is strongest, thereby finishing fine adjustment of the clamping position and the clamping angle of the double-cladding optical fiber;

and then continuously increasing the power of the femtosecond laser to ensure that the output femtosecond laser power is higher than the grating writing threshold value, thus finishing the writing of the fiber grating.

In one embodiment, the center wavelength of the femtosecond laser is 515nm, the power and the pulse repetition frequency are adjustable, and the output light spot is circular after collimation. The laser scanning galvanometer is a laser scanning galvanometer, and the rotation angle can be controlled by a computer. The cylindrical lens is a cylindrical/non-cylindrical lens and is used for focusing the collimated femtosecond laser in one direction, so that the light spot width at the focal line is smaller than that of the optical fiber core, and the focal length of the cylindrical lens can be selected from 10-25mm according to the diameter of the optical fiber core.

The + -1 level diffraction light of the phase mask is strongest, and the 0 level diffraction intensity and other higher-order diffraction intensities are weaker. The + -1-level diffraction light generates interference fringes after the phase mask, and the period of the interference fringes is 1/2 of the period of the phase mask, so that the period of the FBG is also 1/2 of the period of the phase mask.

The double-clad optical fiber is a double-clad doped optical fiber, the fiber core is doped with rare earth ytterbium, and the cladding is in a round shape or other geometric structures, such as octagon, hexagon, D shape and the like. The optical fiber clamp is a pair of high-coaxiality optical fiber rotating clamps with adjustable angles and a high-precision three-dimensional displacement table. Fluorescence is generated by the interaction of the femtosecond laser and ytterbium ions doped in the fiber core of the double-cladding doped fiber, the spectrum is positioned at 1 μm, the peak value is 976nm, and the threshold value of fluorescence generation is lower than the threshold value of inscribing FBG. The intensity of the generated fluorescence is positively correlated with the power density of the femtosecond laser. When the focal position of the femtosecond laser is in the fiber core, the higher the single pulse energy of the femtosecond laser, the higher the pulse repetition frequency, and the higher the fluorescence intensity. The detection range of the fluorescence spectrometer should cover the wave band of 0.4-1.1 mu m, and the filter plate can filter most of femtosecond laser.

In one embodiment, the device for writing the fiber grating by using the femtosecond laser shown in fig. 7 is used for writing the fiber grating, wherein the center wavelength of the femtosecond laser output by the femtosecond laser is 515nm, the pulse width is 270fs, and the pulse repetition frequency can be adjusted between 1Hz and 10 KHz. The laser sequentially passes through a tunable attenuator, a diaphragm and a laser scanning galvanometer, the shaped femtosecond laser vertically irradiates on a cylindrical lens with the focal length of 25mm, and is then diffracted by a phase mask plate and focused in an optical fiber. After the phase mask, the + -1-order diffraction light forms double-beam interference, the interference fringe period is about 0.72 mu m, and the center wavelength of the carved FBG is 1.08 mu m. The optical fiber may be various types of ytterbium-doped double-clad optical fibers, such as octagons, hexagons, D-shapes, etc. The ytterbium-doped double-clad fiber in this embodiment has an octagonal inner cladding of 20/400 μm, and specifically, the eight corners of the inner cladding are all arc-shaped transitions, i.e., the eight corners of the inner cladding are all arc-shaped corners, and the radius of curvature of the arc-shaped corners is approximately 100 μm.

The cleaned ytterbium-doped double-clad fiber was placed and connected in the configuration shown in fig. 7. The two ends of the ytterbium-doped double-clad optical fiber are fixed by a pair of rotatable optical fiber clamps, and the optical fiber clamps are fixed on a three-dimensional moving platform. One end of the ytterbium-doped double-clad fiber is filtered by a filter and then connected to a fluorescence spectrometer. The filter is a band-stop filter, the central wavelength is 515nm, the bandwidth is 20nm, and the band-stop filter is used for filtering femtosecond laser in the optical fiber; the detection wavelength of the fluorescence spectrometer is 200nm-1100nm, and the resolution is less than 0.5nm. The other end of the ytterbium-doped double-clad fiber is respectively connected with an ASE light source and a spectrometer through a pattern matcher, a circulator and the like and is used for detecting the reflection spectrum or the transmission spectrum of the FBG.

The pulse repetition frequency of the femtosecond laser is adjusted to 10KHz, the femtosecond laser is turned on to output the femtosecond laser, an attenuator is adjusted, the femtosecond laser is kept in a low power state, the output femtosecond laser power is lower than a fluorescence excitation threshold value, the laser power reaching the ytterbium-doped double-clad optical fiber is lower than 5mW in the embodiment, and the condition of light spots and the optical fiber can be observed by a human eye through a light screen arranged on one side of the ytterbium-doped double-clad optical fiber. The height of the optical fiber is adjusted through the three-dimensional moving platform, so that the diffraction light spot shape is not changed when the femtosecond laser scans left and right, and the shadow of the ytterbium-doped double-cladding optical fiber on the light screen always coincides with the center of the diffraction light spot. The double-clad optical fiber clamped on the optical fiber clamp is rotated, so that the femtosecond laser output by the femtosecond laser can be incident to the fiber core from the arc-shaped corner of the inner cladding after passing through the laser scanning galvanometer, the cylindrical lens and the phase mask plate, and the initial adjustment of the clamping position and the clamping angle of the double-clad optical fiber is completed.

The laser repetition frequency is kept unchanged, the attenuator is adjusted, the power of the femtosecond laser is increased, and the power of the femtosecond laser is higher than the fluorescence excitation threshold and lower than the grating inscription threshold. In this embodiment, the single pulse energy of the femtosecond laser is adjusted to 60 μj. Under the power output, the femtosecond laser can excite stronger fluorescence in the ytterbium-doped double-clad fiber, and the single pulse energy is lower than the threshold value of inscribing the FBG. And detecting the fluorescence spectrum excited by the femtosecond laser in the fiber core by using the first spectrometer, rotating the double-cladding optical fiber in the contour line range at the arc-shaped corner of the inner cladding, and finely adjusting the clamping angle of the double-cladding optical fiber to ensure that the fluorescence detected by the first spectrometer is strongest, wherein the spectrum of the fluorescence spectrometer is shown in figure 8, and the small peak with the peak value at 976nm and 515nm is the femtosecond laser spectrum. Thus, the fine adjustment of the clamping position and the clamping angle of the double-clad optical fiber is completed.

And observing the position relation between the optical fiber shadow and the diffraction light spot on the light screen, wherein the optical fiber shadow can be positioned above or below the diffraction light spot or in the middle. The following is discussed in some cases:

in the first case, the fiber shadow is in the middle of the diffraction spot;

the fiber was rotated approximately 10 ° clockwise or counterclockwise to see if the diffraction spot height was significantly changed.

If there is no significant change, then the best inscription situation is one that satisfies the following conditions: the fluorescence intensity is strongest, the optical fiber shadow is positioned at the center of the diffraction light spot, and the diffraction light spot does not change in height when the optical fiber rotates at a small angle. In this case, as shown in fig. 9, the black ellipse in the drawing is the region where the femto-second laser focusing intensity is the largest, and the overlapping area of the femto-second laser focusing intensity and the optical fiber core determines the fluorescence intensity. In addition, in practical use, the vertex of the octagonal optical fiber can be generally approximated to an arc with smaller curvature, and the laser light is incident from the point of incidence, so that the writing effect is the best.

If there is a significant change, the fiber is rotated to the maximum fluorescence intensity, and then rotated clockwise or counterclockwise by about 22.5 ° to bring the fiber shadow again in the middle of the diffraction spot. The optical fiber is moved back and forth to make the fluorescence intensity strongest, which is the best writing condition.

In the second case, the fiber shadow is in the middle of the diffraction spot;

the fiber shadow is above the diffraction spot: the fiber is moved downward so that the fiber shadow is at the center of the diffraction spot. The optical fiber is moved back and forth and rotated to maximize the fluorescence intensity. According to the relative position of the diffraction light spots, the optimal writing condition can be generated after a plurality of operations.

The optical fiber shadow is similar to the case of being below and above the diffraction spot, and will not be described again here. After the adjustment, the optical fiber was rotated, and it was found that the fluorescence intensity was periodically changed, and the fluorescence intensity was recovered to the maximum value every 45 ° of rotation, as shown in fig. 10.

And finally, regulating the repetition frequency of the femtosecond laser to be 1kHz, increasing the laser power to be higher than the grating writing threshold value, and preparing the grating. In this embodiment, the single pulse energy of the femtosecond laser is adjusted to 200 μj.

FIG. 11 is a transmission spectrum of a high-reflection and low-reflection FBG inscribed in a 20/400 μm ytterbium-doped double-clad fiber of the invention, wherein the high-reflection FBG has a center wavelength of 1079.5nm, a transmission spectrum depth of greater than 20dB, a corresponding peak reflectivity of greater than 99.9%, and a bandwidth of greater than 4nm; the low anti-FBG has the central wavelength of 1079.6nm, the transmission spectrum depth of about 0.6dB and the bandwidth of 2nm, has good uniformity, and meets the requirements of high/low reflectivity cavity mirrors in the optical fiber oscillator.

By detecting the intensity of fluorescence of 1 mu m excited by the femtosecond laser in the fiber core of the ytterbium-doped fiber, and combining with the rotation of the rotating clamp to find the optimal incidence angle, the medium-high-efficiency FBG inscribing of the ytterbium-doped fiber with a special cladding structure can be realized.

By detecting the fluorescence intensity excited by the femtosecond laser in the optical fiber, the optimal angle and position of the grating can be accurately found by combining the rotation of the optical fiber, and the FBG with excellent performance can be prepared in ytterbium-doped optical fibers with various geometric structures. The grating performance parameters (center wavelength, reflectivity and the like) are strictly controllable, the operation is easy, the preparation period is short, the repeatability is high, the problem of low grating engraving efficiency in the ytterbium-doped optical fiber at present is effectively solved, the method can be directly applied to fiber laser to realize laser output without melting point, the production efficiency is improved, and the industrialization is easy to realize.

In a conventional fiber laser oscillator, germanium-doped fibers (GDFs) with high-reflection fiber gratings and low-reflection fiber gratings are welded and carved at two ends of ytterbium-doped fibers, respectively. Fusion of the two fibers is difficult because the cladding of ytterbium-doped fibers is not regular circular, whereas the cladding of GDF fibers is circular. In addition, since the cladding contains a large amount of pump light, the melting point of the ytterbium-doped optical fiber and the GDF is easy to generate heat and burn under the condition of high power, and is a weak link of the whole system. The method provided by the invention can be used for inscribing the fiber bragg grating with excellent optical parameters in the ytterbium-doped fiber, and based on the method, the integrated fiber laser oscillator can be realized, the number of the melting points of the fiber laser oscillator is effectively reduced, the robustness of a fiber laser system is improved, and the production efficiency of a laser can also be improved.

The invention is not a matter of the known technology.

The technical features of the above embodiments may be arbitrarily combined, and all possible combinations of the technical features in the above embodiments are not described for brevity of description, however, as long as there is no contradiction between the combinations of the technical features, they should be considered as the scope of the description.

The above examples merely represent a few embodiments of the present application, which are described in more detail and are not to be construed as limiting the scope of the invention. It should be noted that it would be apparent to those skilled in the art that various modifications and improvements could be made without departing from the spirit of the present application, which would be within the scope of the present application. Accordingly, the scope of protection of the present application is to be determined by the claims appended hereto.

The above description is only of the preferred embodiments of the present invention and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (10)

1. The method for improving the transmission spectrum depth of the femtosecond laser inscribing fiber grating is characterized in that the femtosecond laser inscribes the fiber grating on a double-clad fiber, wherein the double-clad fiber comprises a fiber core, an inner cladding and an outer cladding, the cross section of the inner cladding is polygonal, and at least one corner in the polygon is an arc-shaped corner; the femtosecond laser is incident to the fiber core from the arc-shaped corner of the inner cladding to inscribe the fiber grating, so that the transmission spectrum depth of the inscribed fiber grating can be improved.

2. The method for improving the transmission spectrum depth of a femtosecond laser inscription fiber grating according to claim 1, wherein the double-clad fiber is a double-clad doped fiber, and rare earth ions are doped in a fiber core of the double-clad doped fiber.

3. The method for improving the transmission spectrum depth of the femtosecond laser inscription fiber grating according to claim 1 or 2, wherein the cross section of the inner cladding is hexagonal or octagonal or rectangular or D-shaped.

4. The method of claim 1, wherein the femtosecond laser is incident to the fiber core from a center position of a contour line at an arc-shaped corner of the inner cladding.

5. The method for writing the fiber grating by the femtosecond laser is characterized by comprising the following steps:

taking a double-clad optical fiber, wherein the double-clad optical fiber comprises a fiber core, an inner cladding and an outer cladding, the cross section of the inner cladding is polygonal, and at least one corner in the polygon is an arc-shaped corner;

and the femtosecond laser is incident to the fiber core from the arc-shaped corner of the inner cladding, so that the inscription of the fiber grating is completed.

6. The method of writing a fiber grating with a femtosecond laser according to claim 5, wherein the femtosecond laser is incident to the fiber core from a center position of a contour line at an arc corner of the inner cladding.

7. The device for writing the fiber grating by the femtosecond laser is characterized by comprising a femtosecond laser, a laser scanning galvanometer, a cylindrical lens, a phase mask plate, double-clad fiber, a fiber clamp and a three-dimensional moving platform, wherein a pair of fiber clamps are arranged on the three-dimensional moving platform;

the double-clad optical fiber is clamped and fixed by a pair of optical fiber clamps, the double-clad optical fiber comprises a fiber core, an inner cladding and an outer cladding, the cross section of the inner cladding is polygonal, and at least one corner in the polygon is an arc-shaped corner;

the clamping position and the clamping angle of the double-cladding optical fiber are adjusted through the three-dimensional moving platform and the optical fiber clamp, so that the femtosecond laser output by the femtosecond laser enters the fiber core from the arc-shaped corner of the inner cladding for writing of the fiber grating after passing through the laser scanning galvanometer, the cylindrical lens and the phase mask plate.

8. The apparatus for writing a fiber grating with a femtosecond laser according to claim 7, wherein the double-clad fiber is a double-clad doped fiber, and rare earth ions are doped in a fiber core of the double-clad doped fiber; the device also comprises an ASE light source, a pattern matcher, a spectrometer, a filter and a fluorescence spectrometer, wherein one end of the double-clad optical fiber is connected with the first spectrometer through the filter to detect the fluorescence spectrum excited by the femtosecond laser in the fiber core, the other end of the double-clad optical fiber is connected with the ASE light source and the spectrometer through the pattern matcher, and the second spectrometer is used for detecting the reflection spectrum or the transmission spectrum of the carved fiber grating.

9. The apparatus for writing a fiber grating with a femtosecond laser according to claim 8, further comprising a tunable attenuator and a diaphragm, wherein the femtosecond laser output by the femtosecond laser is incident to a scanning galvanometer through the tunable attenuator and the diaphragm; the optical fiber clamps are rotatable optical fiber clamps.

10. The device for writing the fiber grating by the femtosecond laser according to claim 8 or 9, wherein when the fiber grating is written, the femtosecond laser is kept in a low power state, so that the output femtosecond laser power is lower than a fluorescence excitation threshold value, and the double-clad fiber clamped on the fiber clamp is rotated through a three-dimensional displacement platform, so that the femtosecond laser output by the femtosecond laser can be incident to a fiber core from an arc-shaped corner of the inner cladding after passing through a laser scanning vibrating mirror, a cylindrical lens and a phase mask plate, and initial adjustment of the clamping position and the clamping angle of the double-clad fiber is completed;

then, increasing the power of the femtosecond laser to ensure that the output femtosecond laser power is higher than a fluorescence excitation threshold and lower than a grating writing threshold; detecting fluorescence spectrum excited by the femtosecond laser in the fiber core by utilizing the first spectrometer, rotating the double-cladding optical fiber in the contour line range of the arc-shaped corner of the inner cladding, and finely adjusting the clamping angle of the double-cladding optical fiber to ensure that the fluorescence detected by the first spectrometer is strongest, thereby finishing fine adjustment of the clamping position and the clamping angle of the double-cladding optical fiber;

and then continuously increasing the power of the femtosecond laser to ensure that the output femtosecond laser power is higher than the grating writing threshold value, thus finishing the writing of the fiber grating.