Device and method for prolonging grating inscribing area based on femtosecond laser

Abstract

The invention discloses a device and method for prolonging a grating inscribing area based on femtosecond laser. According to the device, a three-axis displacement platform comprises a three-axis displacement platform controller, an upper computer, an X-axis translation electric displacement platform, a Y-axis translation electric displacement platform and a Z-axis lifting electric displacement platform; the lower end of the vertical support is fixedly connected to the X-axis translation electric displacement table. The gathering column lens is supported on the front side of the upper end of the vertical bracket; the one-dimensional displacement platform is mounted on the rear side of the upper end of the vertical bracket; the phase mask plate clamp is mounted on the one-dimensional displacement platform; the phase mask plate is arranged on the rear side of the gathering column lens and is clamped in the phase mask plate clamp; the lower supporting ends of the pair of optical fiber clamps are fixedly connected to the Z-axis lifting electric displacement table. The method comprises the following steps: sequentially arranging optical fibers of the focusing cylindrical lens and the phase mask plate in an inscribing light path of femtosecond laser; determining the monopulse energy and the basic moving speed of the optical fiber during inscribing; and carrying out first-time and second-time apodization operation. According to the device and the method, the manufacturing of the apodized fiber bragg grating can be efficiently completed.

Description

Technical Field

[0001] The invention belongs to the technical field of fiber grating production, and in particular relates to a device and method for extending a grating writing area based on a femtosecond laser. Background Art

[0002] Fiber Bragg grating is an optical passive device made by introducing a periodic refractive index modulation structure into an optical fiber. It has many advantages such as anti-electromagnetic interference, corrosion resistance, small size, and light weight, and is widely used in optical fiber communications, optical fiber sensing, and optical fiber lasers. At present, fiber Bragg gratings are mainly written by ultraviolet lasers. However, fiber Bragg gratings written by ultraviolet lasers have poor thermal stability and are easily erased in high temperature environments. In addition, optical fibers with poor photosensitivity require hydrogen pretreatment, which is not suitable for large-scale, high-temperature application environments.

[0003] Fiber Bragg gratings written by femtosecond lasers do not require additional hydrogen loading processes, and can also write high-reflectivity fiber Bragg gratings on optical fibers with poor photosensitivity. They have a large light refractive index modulation depth, high writing efficiency, and strong thermal stability. There are two commonly used femtosecond writing methods: phase mask method and direct writing method. The phase mask method of fiber Bragg grating writing technology has low requirements for the coherence of the light source, good stability and repeatability, high processing efficiency, and low loss, making it suitable for large-scale batch production.

[0004] Whether it is laser direct writing or phase mask writing, since the writing light source uses a femtosecond laser, the spot energy presents a Gaussian distribution during the transmission process, that is, the closer to the center of the spot, the higher the energy, and the farther away from the center of the spot, the lower the energy. After such Gaussian light is expanded or focused, the spot still has the characteristic of uneven energy distribution. This will result in a limited area of ​​the spot that can be used during writing, that is, the area in the spot where the energy meets the writing requirements can be written, while the rest of the spot fails to be written due to insufficient energy. Too short a deletion area will cause the grating to heat up severely due to heat accumulation during use, affecting the service life and stability of the device.

[0005] The current solution is to perform dynamic scanning writing by moving the optical fiber, thereby extending the grating writing area. This greatly increases the stability and accuracy requirements of the processing platform, and the writing cost also increases accordingly. In addition, the scanning speed of dynamic writing also has a great influence on the writing results. For this reason, it is urgent to provide a new device and method for extending the grating writing area based on femtosecond laser. Summary of the invention

[0006] In view of the problems existing in the above-mentioned prior art, the present invention provides a device and method for extending the grating writing area based on femtosecond laser. The device has a simple structure, good stability, convenient debugging process, and a wide range of applicability, and can quickly and efficiently complete the production of fiber gratings; the method has a simple implementation process, and can effectively extend the writing area without mobile scanning, and can efficiently and accurately complete the grating writing operation.

[0007] The method is simple to implement and can achieve the writing of fiber gratings with high refractive index modulation depth while effectively reducing the loss caused by uneven grating stripes.

[0008] In order to achieve the above-mentioned object, the present invention provides a device for extending the grating writing area based on femtosecond laser, comprising a base, a phase mask support, a phase mask, an optical fiber displacement stage, an optical fiber clamp, a focusing cylindrical lens, a light spot homogenizing device and a CCD probe;

[0009] The phase mask support is vertically mounted on the left portion of the upper end of the base, and a phase mask fixture is disposed on the upper end thereof; the phase mask fixture has degrees of freedom in the left-right direction, the up-down direction, and the front-back direction relative to the phase mask support;

[0010] The phase mask is vertically mounted on the upper end of the phase mask bracket through a phase mask fixture;

[0011] The optical fiber displacement stage is located on the right side of the phase mask support and is vertically mounted on the right part of the upper end of the base;

[0012] The height of the pair of optical fiber clamps matches the height of the phase mask plate and is relatively distributed front to back. At the same time, the pair of optical fiber clamps are installed on the optical fiber translation stage and move in the up-down direction, left-right direction and front-back direction under the drive of the optical fiber translation stage.

[0013] The focusing cylindrical lens is supported on the left side of the phase mask plate through a supporting frame;

[0014] The light spot homogenizing device comprises a light spot homogenizing mirror and a homogenizing device bracket, wherein the light spot homogenizing mirror is arranged on the left side of the focusing cylindrical lens, and is used for homogenizing the light spot energy distribution of the pulsed laser through optical refraction and diffraction, so that the distribution changes from Gaussian distribution to flat-top distribution;

[0015] Among them, the light spot homogenizing device, the focusing cylindrical lens and the phase mask are arranged in sequence in the femtosecond writing light path;

[0016] The CCD probe is supported on one side above a pair of optical fiber clamps by a bracket, and its image collection window faces the space between the pair of optical fiber clamps.

[0017] Furthermore, in order to precisely adjust the displacement of the optical fiber clamp in three directions of X, Y and Z, the optical fiber displacement stage is a three-axis electric displacement stage.

[0018] Furthermore, in order to facilitate the use of a host computer to automatically control the action of the optical fiber displacement stage, a host computer is also included, and the host computer is connected to the optical fiber displacement stage.

[0019] As a preference, the host is a computer.

[0020] In the present invention, the phase mask plate clamp has freedom in the left-right direction, the up-down direction and the front-back direction relative to the phase mask plate bracket, so that the position of the phase mask plate relative to a pair of clamps and the focusing cylindrical lens can be flexibly adjusted, thereby ensuring that the positive and negative first-order diffracted light after passing through the phase mask plate can be better focused on the core of the optical fiber; a pair of optical fiber clamps are installed on the optical fiber displacement stage, and the pair of optical fiber clamps can be moved in the up-down direction, the left-right direction and the front-back direction under the drive of the optical fiber displacement stage, so that the position of the optical fiber to be written in the optical path can be easily adjusted, thereby further ensuring that the light spot can be better focused on the core of the optical fiber, and the femtosecond laser can be used to quickly and efficiently complete the production of optical fiber gratings. A spot homogenizing mirror is set on the upstream side of the focusing cylindrical lens. The spot energy distribution of the pulsed laser is homogenized through optical refraction and diffraction, changing it from a Gaussian distribution to a flat-top distribution. In this way, the spot energy distribution focused on the optical fiber is more uniform, which not only reduces the influence of the field blur caused by external stray light and the multi-mode characteristics of the laser, but also improves the average brightness of the spot. The entire spot can be used for writing, thereby extending the effectively utilized spot area without mobile scanning, that is, extending the grating writing area, thereby increasing the reflective performance of the high-reflection grating, reducing heat accumulation during use, extending the service life, and effectively utilizing the spot area.

[0021] The device has a simple structure, good stability, convenient debugging process, and a wide range of applications. It can quickly and efficiently complete the production of fiber gratings. The device can not only homogenize Gaussian light to extend the writing area in femtosecond laser phase mask writing, but also homogenize the light spot to extend the laser writing area in other systems involving femtosecond laser grating writing, such as femtosecond direct writing.

[0022] The present invention also provides a method for extending a grating writing area based on a femtosecond laser, and a device for extending a grating writing area based on a femtosecond laser comprises the following steps:

[0023] Step 1: Design the parameters of the light spot homogenizing mirror and manufacture the light spot homogenizing mirror;

[0024] S1: Calculate the energy A of the input beam according to formula (1);

[0025]

[0026] Where W is the cross-sectional waist radius of the Gaussian beam, X is the total area radius of the circular Gaussian light, R is the variable representing the radial length in the polar coordinate system, and another variable in the polar coordinate system is the angle θ, and the conversion relationship with the rectangular coordinate system is x = Rcosθ, y = Rsinθ, e is a natural constant, P is the maximum power of a Gaussian beam, which is considered a constant;

[0027] S2: transform formula (1) to obtain formula (2);

[0028]

[0029] S3: The output light is the energy B of the flat top light according to formula (3), wherein the flat top light presents a step distribution with a maximum value of H and a radius of S;

[0030] B=HπS 2 (3);

[0031] S4: Make the light energy in the circle surrounded by X in the input light spot equal to the light energy in the circle surrounded by S with a radius in the output light spot.

[0032] Let A = B, and we get formula (4);

[0033]

[0034] S5: Since the total energy of the entire input spot is equal to the total energy of the output spot with a radius of K, formula (5) is obtained;

[0035]

[0036] S6: Combining formula (4) and formula (5), the corresponding relationship between input point X and output point S is obtained, as shown in formula (6);

[0037]

[0038] S7: Use the simulation software Zemax to calculate the coordinate S of the output light at any input light coordinate X. At the same time, use the operand REAY in the evaluation function editor to determine the coordinate result of the output light corresponding to a set of input light coordinates. In the operand REAY, first determine the normalized radius of the input coordinate and its corresponding point on the image plane; use the ZPL macro program to automatically generate the evaluation function and optimize the spot homogenizer;

[0039] S8: Processing and producing the light spot homogenizing mirror according to the optimized parameters of the light spot homogenizing mirror;

[0040] Step 2: Install the spot homogenizing mirror to the upper end of the homogenizing device bracket; install the focusing cylindrical lens 3 to the upper end of the support frame; fix the phase mask plate bracket 8 to the left part of the upper end of the base 10, and install the phase mask plate 4 to the upper end of the phase mask plate bracket 8 through the phase mask plate clamp; then adjust the position so that the spot homogenizing mirror, the focusing cylindrical lens 3 and the bit mask plate 4 are arranged in sequence in the writing light path;

[0041] Step 3: The optical fiber displacement stage 9 is fixedly mounted on the right part of the upper end of the base 10, and a pair of optical fiber clamps 6 are mounted on the upper end of the optical fiber displacement stage 9, and then the optical fiber is clamped between the pair of optical fiber clamps 6; then, the position of the optical fiber is adjusted by the optical fiber displacement stage 9 so that the optical fiber is located in the writing optical path;

[0042] Step 4: Adjust the position of the phase mask plate 4 through the phase mask plate bracket 8 to ensure that the positive and negative first-order diffraction light generated by the laser passing through the spot homogenization device 2 and the focusing cylindrical lens 3 in sequence and then passing through the phase mask plate 4 is irradiated onto the optical fiber;

[0043] Step 5: Control the optical fiber displacement stage 9 to adjust the position of the optical fiber in the writing optical path so that the femtosecond laser is focused on the optical fiber core;

[0044] Step 6: Start the femtosecond laser and make it vertically incident on the spot homogenizer. At the same time, use the spot homogenizer to homogenize the light beam passing through, and make the homogenized light spot vertically incident on the center of the focusing cylindrical lens 3, and then focus on the optical fiber through the phase mask plate 4 after being focused by the focusing cylindrical lens 3;

[0045] Keep the frequency of the femtosecond laser output light pulse unchanged, gradually increase the energy of the single pulse of the femtosecond laser, and at the same time, use the CCD probe 7 to collect image data in real time, so as to observe the situation of the light spot focusing in the optical fiber in real time. When the light spot is focused on the interface between the core and the cladding of the optical fiber and obvious blue-purple fluorescence appears, further increase the energy of the single pulse to make the blue-purple fluorescence in the central area of ​​the focus position change to white fluorescence. At this time, the single pulse energy Q is used as the final writing energy, and the position of the optical fiber and the writing energy at this moment are recorded;

[0046] Step 7: Turn off the femtosecond laser, replace the new optical fiber to be engraved between a pair of optical fiber clamps, then turn on the femtosecond laser. White fluorescence gradually appears in the central area of ​​the focus position and the intensity gradually increases. The engraving begins and begins to spread to both ends of the area. When the white light in the central area is no longer enhanced, turn off the femtosecond laser to complete the engraving operation.

[0047] As a preference, in step six and step seven, the wavelength of the femtosecond laser is 800 nm.

[0048] The present invention discloses a method for extending the writing area of ​​a fiber grating based on a femtosecond laser. The parameters of a required spot homogenizer are determined according to the corresponding relationship between the input light coordinates and the output light coordinates, and then the spot homogenizer is manufactured, which can ensure that the spot homogenizer has a more ideal balancing effect on the light beam. Before the light beam enters the focusing cylindrical lens, the femtosecond laser is first homogenized by the spot homogenizer, which can convert the Gaussian light used for writing into a flat-top light, and then the spot energy distribution can be more uniform. After the beam is expanded and focused, the energy of the fine spot is also uniformly distributed, so that the energy of the spot focused into the optical fiber is also uniformly distributed, and the entire spot can be used for writing. The grating writing area is extended without moving scanning, so that the spot area that can be effectively used during writing is extended, the reflection performance of the high-reflection grating is increased, the heat accumulation during use is reduced, the service life is increased, and the writing failure and non-conformity caused by insufficient device accuracy and stability during scanning writing are avoided.

[0049] The method is simple to implement, and can effectively extend the writing area without moving and scanning, and can efficiently and accurately complete the grating writing operation. This method can not only homogenize Gaussian light to extend the writing area in femtosecond laser phase mask writing, but also homogenize the light spot to extend the laser writing area in other systems involving femtosecond laser grating writing, such as femtosecond direct writing. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is a schematic diagram of the structure of the present invention;

[0051] Figure 2 yes Figure 1 Top view of the .

[0052] In the figure: 1. Femtosecond laser; 2. Light spot homogenization device; 3. Focusing cylindrical lens; 4. Phase mask; 5. Optical fiber; 6. Optical fiber clamp; 7. CCD probe; 8. Phase mask holder; 9. Optical fiber translation stage; 10. Base; 11. Positive and negative first-order diffracted light. DETAILED DESCRIPTION

[0053] The present invention will be further described below.

[0054] like Figure 1 and Figure 2 As shown, the present invention provides a device for extending the grating writing area based on a femtosecond laser, comprising a base 10, a phase mask support 8, a phase mask 4, a fiber displacement stage 9, a fiber clamp 6, a focusing cylindrical lens 3, a light spot homogenizing device 2 and a CCD probe 7;

[0055] The phase mask support 8 is vertically mounted on the left portion of the upper end of the base 10, and a phase mask fixture is provided on the upper end thereof; the phase mask fixture has degrees of freedom in the left-right direction, the up-down direction, and the front-back direction relative to the phase mask support 8, so that it can be ensured that the positive and negative first-order diffracted light 11 after passing through the phase mask 4 can hit the optical fiber;

[0056] The phase mask plate 4 is vertically mounted on the upper end of the phase mask plate bracket 8 through a phase mask plate fixture;

[0057] The optical fiber displacement stage 9 is located on the right side of the phase mask support 8 and is vertically mounted on the right part of the upper end of the base 10;

[0058] The height of a pair of optical fiber clamps 6 is matched with the height of the phase mask plate 4, and they are relatively distributed front to back. At the same time, the pair of optical fiber clamps 6 are installed on the optical fiber displacement stage 9, and are moved in the up and down directions, left and right directions, and front and back directions under the drive of the optical fiber displacement stage 9; in this way, it is easy to adjust the position of the optical fiber to be written in the optical path, thereby ensuring that the light spot can be better focused on the core of the optical fiber.

[0059] The focusing cylindrical lens 3 is supported on the left side of the phase mask plate 4 through a supporting frame;

[0060] The light spot homogenizing device 2 comprises a light spot homogenizing mirror and a homogenizing device bracket, wherein the light spot homogenizing mirror is supported on the left side of the focusing cylindrical lens 3, and is used to homogenize the light spot energy distribution of the pulsed laser through optical refraction and diffraction, so that the distribution changes from Gaussian distribution to flat-top distribution;

[0061] Among them, the light spot homogenizing device 2, the focusing cylindrical lens 3 and the phase mask plate 4 are arranged in sequence in the femtosecond writing optical path;

[0062] The CCD probe 7 is supported by a bracket on one side above the pair of optical fiber clamps 6, and its image acquisition window faces the space between the pair of optical fiber clamps 6. In this way, the CCD probe 7 can collect the image of the position of the optical fiber in real time, and then observe the situation where the light spot is focused in the optical fiber;

[0063] In order to precisely adjust the displacement of the optical fiber clamp in the three directions of X, Y and Z, the optical fiber displacement stage 9 is a three-axis electric displacement platform.

[0064] In order to facilitate the use of the host computer to automatically control the action of the optical fiber displacement stage, a host computer is also included, and the host computer is connected to the optical fiber displacement stage 9.

[0065] As a preference, the host is a computer.

[0066] In the present invention, the phase mask plate clamp has freedom in the left-right direction, the up-down direction and the front-back direction relative to the phase mask plate bracket, so that the position of the phase mask plate relative to a pair of clamps and the focusing cylindrical lens can be flexibly adjusted, thereby ensuring that the positive and negative first-order diffracted light after passing through the phase mask plate can be better focused on the core of the optical fiber; a pair of optical fiber clamps are installed on the optical fiber displacement stage, and the pair of optical fiber clamps can be moved in the up-down direction, the left-right direction and the front-back direction under the drive of the optical fiber displacement stage, so that the position of the optical fiber to be written in the optical path can be easily adjusted, thereby further ensuring that the light spot can be better focused on the core of the optical fiber, and the femtosecond laser can be used to quickly and efficiently complete the production of optical fiber gratings. A spot homogenizing mirror is set on the upstream side of the focusing cylindrical lens. The spot energy distribution of the pulsed laser is homogenized through optical refraction and diffraction, changing it from a Gaussian distribution to a flat-top distribution. In this way, the spot energy distribution focused on the optical fiber is more uniform, which not only reduces the influence of the field blur caused by external stray light and the multi-mode characteristics of the laser, but also improves the average brightness of the spot. The entire spot can be used for writing, thereby extending the effectively utilized spot area without mobile scanning, that is, extending the grating writing area, thereby increasing the reflective performance of the high-reflection grating, reducing heat accumulation during use, extending the service life, and effectively utilizing the spot area.

[0067] The device has a simple structure, good stability, convenient debugging process, and a wide range of applications. It can quickly and efficiently complete the production of fiber gratings. The device can not only homogenize Gaussian light to extend the writing area in femtosecond laser phase mask writing, but also homogenize the light spot to extend the laser writing area in other systems involving femtosecond laser grating writing, such as femtosecond direct writing.

[0068] The present invention also provides a method for extending a grating writing area based on a femtosecond laser, and a device for extending a grating writing area based on a femtosecond laser comprises the following steps:

[0069] Step 1: Design the parameters of the light spot homogenizing mirror and manufacture the light spot homogenizing mirror;

[0070] S1: Calculate the energy A of the input beam according to formula (1);

[0071]

[0072] Where W is the cross-sectional waist radius of the Gaussian beam, X is the total area radius of the circular Gaussian light, R is the variable representing the radial length in the polar coordinate system, and another variable in the polar coordinate system is the angle θ, and the conversion relationship with the rectangular coordinate system is x = Rcosθ, y = Rsinθ, e is a natural constant, P is the maximum power of a Gaussian beam, which is considered a constant;

[0073] S2: transform formula (1) to obtain formula (2);

[0074]

[0075] S3: The output light is the energy B of the flat top light according to formula (3), wherein the flat top light presents a step distribution with a maximum value of H and a radius of S;

[0076] B=HπS 2 (3);

[0077] S4: Make the light energy in the circle surrounded by X in the input light spot equal to the light energy in the circle surrounded by S with a radius in the output light spot, that is, let A = B, and get formula (4);

[0078]

[0079] S5: Since the total energy of the entire input spot is equal to the total energy of the output spot with a radius of K, formula (5) is obtained;

[0080]

[0081] S6: Combining formula (4) and formula (5), the corresponding relationship between input point X and output point S is obtained, as shown in formula (6);

[0082]

[0083] S7: Use the simulation software Zemax to calculate the coordinate S of the output light at any input light coordinate X. At the same time, use the operand REAY in the evaluation function editor to determine the coordinate result of the output light corresponding to a set of input light coordinates. In the operand REAY, first determine the normalized radius of the input coordinate and its corresponding point on the image plane. Compared with the cumbersome manual input of operands, this application uses the ZPL macro program to automatically generate the evaluation function and optimize the spot homogenizer.

[0084] S8: Processing and producing the light spot homogenizing mirror according to the optimized parameters of the light spot homogenizing mirror;

[0085] Step 2: Install the spot homogenizing mirror to the upper end of the homogenizing device bracket; install the focusing cylindrical lens 3 to the upper end of the support frame; fix the phase mask plate bracket 8 to the left part of the upper end of the base 10, and install the phase mask plate 4 to the upper end of the phase mask plate bracket 8 through the phase mask plate clamp; then adjust the position so that the spot homogenizing mirror, the focusing cylindrical lens 3 and the bit mask plate 4 are arranged in sequence in the writing light path;

[0086] Step 3: The optical fiber displacement stage 9 is fixedly mounted on the right part of the upper end of the base 10, and a pair of optical fiber clamps 6 are mounted on the upper end of the optical fiber displacement stage 9, and then the optical fiber is clamped between the pair of optical fiber clamps 6; then, the position of the optical fiber is adjusted by the optical fiber displacement stage 9 so that the optical fiber is located in the writing optical path;

[0087] Step 4: adjust the position of the phase mask plate 4 through the phase mask plate bracket 8 to ensure that the positive and negative first-order diffraction light 11 generated by the laser passing through the spot homogenization device 2 and the focusing cylindrical lens 3 in sequence and then passing through the phase mask plate 4 is irradiated onto the optical fiber;

[0088] Step 5: Control the optical fiber displacement stage 9 to adjust the position of the optical fiber in the writing optical path so that the femtosecond laser is focused on the optical fiber core;

[0089] Step 6: Start the femtosecond laser and make it vertically incident on the spot homogenizer. At the same time, use the spot homogenizer to homogenize the light beam passing through, and make the homogenized light spot vertically incident on the center of the focusing cylindrical lens 3, and then focus on the optical fiber through the phase mask plate 4 after being focused by the focusing cylindrical lens 3;

[0090] Keep the frequency of the femtosecond laser output light pulse unchanged, gradually increase the energy of the single pulse of the femtosecond laser, and at the same time, use the CCD probe 7 to collect image data in real time, so as to observe the situation of the light spot focusing in the optical fiber in real time. When the light spot is focused on the interface between the core and the cladding of the optical fiber and obvious blue-purple fluorescence appears, further increase the energy of the single pulse to make the blue-purple fluorescence in the central area of ​​the focus position change to white fluorescence. At this time, the single pulse energy Q is used as the final writing energy, and the position of the optical fiber and the writing energy at this moment are recorded;

[0091] Step 7: Turn off the femtosecond laser, replace the new optical fiber to be engraved between a pair of optical fiber clamps, then turn on the femtosecond laser. White fluorescence gradually appears in the central area of ​​the focus position and the intensity gradually increases. The engraving begins and begins to spread to both ends of the area. When the white light in the central area is no longer enhanced, turn off the femtosecond laser to complete the engraving operation.

[0092] As a preference, in step six and step seven, the wavelength of the femtosecond laser is 800 nm.

[0093] The present invention discloses a method for extending the writing area of ​​a fiber grating based on a femtosecond laser. The parameters of a required spot homogenizer are determined according to the corresponding relationship between the input light coordinates and the output light coordinates, and then the spot homogenizer is manufactured, which can ensure that the spot homogenizer has a more ideal balancing effect on the light beam. Before the light beam enters the focusing cylindrical lens, the femtosecond laser is first homogenized by the spot homogenizer, which can convert the Gaussian light used for writing into a flat-top light, and then the spot energy distribution can be more uniform. After the beam is expanded and focused, the energy of the fine spot is also uniformly distributed, so that the energy of the spot focused into the optical fiber is also uniformly distributed, and the entire spot can be used for writing. The grating writing area is extended without moving scanning, so that the spot area that can be effectively used during writing is extended, the reflection performance of the high-reflection grating is increased, the heat accumulation during use is reduced, the service life is increased, and the writing failure and non-conformity caused by insufficient device accuracy and stability during scanning writing are avoided.

[0094] The method is simple to implement, and can effectively extend the writing area without moving and scanning, and can efficiently and accurately complete the grating writing operation. This method can not only homogenize Gaussian light to extend the writing area in femtosecond laser phase mask writing, but also homogenize the light spot to extend the laser writing area in other systems involving femtosecond laser grating writing, such as femtosecond direct writing.

Claims

1. A device for extending a grating writing area based on a femtosecond laser, comprising a base (10); characterized in that: It also includes a phase mask support (8), a phase mask (4), an optical fiber displacement stage (9), an optical fiber clamp (6), a focusing cylindrical lens (3), a light spot homogenization device (2) and a CCD probe (7); The phase mask support (8) is vertically mounted on the left portion of the upper end of the base (10), and a phase mask fixture is provided on the upper end thereof; the phase mask fixture has degrees of freedom in the left-right direction, the up-down direction, and the front-back direction relative to the phase mask support (8); The phase mask plate (4) is vertically mounted on the upper end of the phase mask plate bracket (8) via a phase mask plate clamp; The optical fiber displacement stage (9) is located on the right side of the phase mask support (8) and is vertically mounted on the right part of the upper end of the base (10); The height of the pair of optical fiber clamps (6) matches the height of the phase mask plate (4) and is relatively distributed front to back. At the same time, the pair of optical fiber clamps (6) are installed on the optical fiber displacement stage (9) and move in the up-down direction, left-right direction and front-back direction under the drive of the optical fiber displacement stage (9); The focusing cylindrical lens (3) is supported on the left side of the phase mask plate (4) via a supporting frame; The light spot homogenizing device (2) comprises a light spot homogenizing mirror and a homogenizing device bracket, wherein the light spot homogenizing mirror is supported on the left side of the focusing cylindrical lens (3), and is used to homogenize the light spot energy distribution of the pulsed laser through optical refraction and diffraction, so that the distribution changes from Gaussian distribution to flat-top distribution; Wherein, the light spot homogenizing device (2), the focusing cylindrical lens (3) and the phase mask plate (4) are arranged in sequence in the femtosecond writing light path; The CCD probe (7) is supported on one side above the pair of optical fiber clamps (6) by a bracket, and its image acquisition window faces the space between the pair of optical fiber clamps (6).

2. The device for extending the grating writing area based on femtosecond laser according to claim 1, characterized in that: The optical fiber displacement platform (9) is a three-axis electric displacement platform.

3. The device for extending the grating writing area based on femtosecond laser according to claim 2, characterized in that: It also includes a host computer, which is connected to the optical fiber displacement platform (9).

4. The device for extending the grating writing area based on femtosecond laser according to claim 3 is characterized in that: The host is a computer.

5. A device for extending a grating writing area based on a femtosecond laser, using the device for extending a grating writing area based on a femtosecond laser according to any one of claims 1 to 4, characterized in that: The following steps are involved: Step 1: Design the parameters of the light spot homogenizing mirror and manufacture the light spot homogenizing mirror; S1: Calculate the energy A of the input beam according to formula (1); Where W is the cross-sectional waist radius of the Gaussian beam, X is the total area radius of the circular Gaussian light, R is the variable representing the radial length in the polar coordinate system, and another variable in the polar coordinate system is the angle θ, and the conversion relationship with the rectangular coordinate system is x = Rcosθ, y = Rsinθ, e is a natural constant, P is the maximum power of a Gaussian beam, which is considered a constant; S2: transform formula (1) to obtain formula (2); S3: The output light is the energy B of the flat top light according to formula (3), wherein the flat top light presents a step distribution with a maximum value of H and a radius of S; B=HπS 2 (3); S4: Make the light energy in the circle surrounded by X in the input light spot equal to the light energy in the circle surrounded by S with a radius in the output light spot, that is, let A = B, and get formula (4); S5: Since the total energy of the entire input spot is equal to the total energy of the output spot with a radius of K, formula (5) is obtained; S6: Combining formula (4) and formula (5), the corresponding relationship between input point X and output point S is obtained, as shown in formula (6); S7: Use the simulation software Zemax to calculate the coordinate S of the output light at any input light coordinate X. At the same time, use the operand REAY in the evaluation function editor to determine the coordinate result of the output light corresponding to a set of input light coordinates. In the operand REAY, first determine the normalized radius of the input coordinate and its corresponding point on the image plane; use the ZPL macro program to automatically generate the evaluation function and optimize the spot homogenizer; S8: Processing and producing the light spot homogenizing mirror according to the optimized parameters of the light spot homogenizing mirror; Step 2: Install the spot homogenizing mirror to the upper end of the homogenizing device bracket; install the focusing column lens (3) to the upper end of the support frame; fix the phase mask plate bracket (8) to the left part of the upper end of the base (10), and install the phase mask plate (4) to the upper end of the phase mask plate bracket (8) through the phase mask plate clamp; then adjust the position so that the spot homogenizing mirror, the focusing column lens (3) and the bit mask plate (4) are arranged in sequence in the writing light path; Step 3: The optical fiber displacement stage (9) is fixedly mounted on the right part of the upper end of the base (10), and a pair of optical fiber clamps (6) are mounted on the upper end of the optical fiber displacement stage (9), and the optical fiber is clamped between the pair of optical fiber clamps (6); then, the position of the optical fiber is adjusted by the optical fiber displacement stage (9) so that the optical fiber is located in the writing optical path; Step 4: adjusting the position of the phase mask plate (4) by means of the phase mask plate bracket (8) to ensure that the positive and negative first-order diffraction light (11) generated by the laser light passing through the light spot homogenizing device (2) and the focusing cylindrical lens (3) in sequence and then passing through the phase mask plate (4) is irradiated onto the optical fiber; Step 5: Control the optical fiber displacement stage (9) to adjust the position of the optical fiber in the writing optical path so that the femtosecond laser is focused on the optical fiber core; Step 6: Start the femtosecond laser and make it vertically incident on the spot homogenizing mirror. At the same time, use the spot homogenizing mirror to homogenize the light beam passing through, and make the homogenized light spot vertically incident on the center of the focusing cylindrical lens (3), and then focus on the optical fiber through the phase mask plate (4) after being focused by the focusing cylindrical lens (3); Keeping the frequency of the femtosecond laser output light pulses unchanged, gradually increase the energy of the single pulse of the femtosecond laser, and at the same time, using a CCD probe (7) to collect image data in real time, so as to observe in real time the situation of the light spot being focused in the optical fiber. When the light spot is focused at the interface between the core and the cladding of the optical fiber and obvious blue-purple fluorescence appears, further increase the energy of the single pulse so that the blue-purple fluorescence in the central area of ​​the focus position changes to white fluorescence. At this time, the single pulse energy Q is used as the final writing energy, and the position of the optical fiber and the writing energy at this moment are recorded; Step 7: Turn off the femtosecond laser and replace the new optical fiber to be written between a pair of optical fiber clamps (6), then turn on the femtosecond laser. White fluorescence gradually appears in the central area of ​​the focus position and the intensity gradually increases. The writing begins and begins to spread to both ends of the area. When the white light in the central area is no longer strengthened, turn off the femtosecond laser and complete the writing operation.

6. The method for realizing apodization of low-loss gratings based on femtosecond laser writing according to claim 5, characterized in that: In step six and step seven, the wavelength of the femtosecond laser is 800 nm.

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