Femtosecond laser processing method for rapidly preparing low-loss spiral cladding waveguide

Abstract

The invention discloses a femtosecond laser processing method for rapidly preparing a low-loss spiral cladding waveguide, and belongs to the technical field of femtosecond laser precision processing. The method specifically comprises the following steps: (1) leveling a transparent optical crystal; (2) a femtosecond laser Bessel beam vertically enters the transparent optical crystal and is focused on the contact surface of the sample and the six-dimensional electric platform, and a femtosecond laser modified nick with a reduced refractive index is generated; (3) a spiral curve machining program is operated to control the six-dimensional electric platform to move downwards along the incidence direction of the femtosecond laser along the track of a spiral line, and a spiral cladding waveguide is generated; and (4) optical polishing treatment. According to the femtosecond laser processing method, the spiral cladding waveguide structure can be simply and rapidly prepared in the transparent optical crystal, the side wall of the prepared cladding waveguide is smooth, and low-loss transmission of laser can be achieved.

Description

Technical Field

[0001] This invention relates to the field of femtosecond laser precision machining technology, and more specifically to a femtosecond laser machining method for rapidly fabricating low-loss helical cladding waveguides. Background Technology

[0002] Lithium triborate crystal is a high-performance nonlinear optical crystal material with an extremely wide transmission range (extending from the deep ultraviolet band (~160 nm) to the near-infrared band (~2600 nm)), a large nonlinear coefficient, an ultra-high laser damage threshold, a wide phase-matching tolerance angle, a very small dispersion angle, and stable chemical and mechanical properties. Therefore, lithium triborate crystal has important applications in laser frequency conversion, deep ultraviolet laser generation, and other fields.

[0003] Quartz crystal is a high-performance ultraviolet optical crystal material with excellent physical properties. It also possesses an extremely wide transmission range (extending from the deep ultraviolet band (~170 nm) to the near-infrared band (~2500 nm)), excellent birefringence and optical rotation properties, unique piezoelectric properties, a high laser damage threshold, a low coefficient of thermal expansion, and stable chemical and mechanical properties. Therefore, quartz crystal has important applications in fields such as electronics and information communication, time and frequency standards, precision polarization optics, imaging and sensing optics, and high-power laser systems.

[0004] By fabricating cladding waveguides with specific light-guiding areas in lithium triborate crystals and quartz crystals, low-loss transmission of ultraviolet light can be achieved while controlling the shape of the output light mode for input beams in the ultraviolet band. This lays the foundation for subsequent realization of efficient nonlinear frequency conversion of ultraviolet beams.

[0005] Femtosecond lasers exhibit unique processing characteristics due to their extremely high peak power and extremely short pulse duration. The extremely high peak power induces a series of nonlinear interactions within transparent optical materials, such as multiphoton absorption, tunneling ionization, and avalanche breakdown; while the extremely short pulse width effectively limits heat diffusion during processing, thus avoiding the formation of significant heat-affected zones. This makes precision processing at the micro- and nanoscale possible. Femtosecond laser direct writing technology possesses advantages such as high precision, high efficiency, and the ability to process real three-dimensional structures, and is currently widely used in various fields including biomedicine, materials engineering, and mechanical manufacturing. In the field of integrated optics, the fabrication process of waveguides (single-line waveguides, double-line waveguides, multi-line waveguides, ridge waveguides, cladding waveguides, etc.) using femtosecond laser Gaussian beams is already very mature. Taking cladding waveguides as an example, by setting appropriate femtosecond laser processing parameters, the transmission loss of traditional cladding waveguides fabricated with femtosecond laser Gaussian beams (where the refractive index reduction marks that make up the cladding waveguide are separated from each other and along the light guiding direction of the cladding waveguide) can be as low as 1 dB / cm. While achieving relatively efficient beam transmission, the output beam mode can be controlled by adjusting the area of ​​the light guiding region of the cladding waveguide. However, this type of cladding waveguide composed of parallel grooves has certain structural defects: on the one hand, the boundaries of the grooves are not "smooth" enough, that is, the direction of the groove extension is always along the incident direction of the femtosecond laser, and cannot always maintain the tangent direction of the equivalent circle it forms. When the incident beam undergoes total internal reflection inside the cladding waveguide, some light will be scattered due to the rough groove boundaries, reducing the intensity of the output beam; on the other hand, the spacing between the grooves will cause light leakage during the total internal reflection of the incident beam, further increasing the transmission loss of the cladding waveguide.

[0006] To avoid the aforementioned structural defects and further reduce the transmission loss of cladding waveguides, researchers have discovered that by designing and optimizing femtosecond laser processing parameters, spiral cladding waveguides can be fabricated in transparent optical crystals, effectively reducing transmission loss. Compared to traditional cladding waveguides composed of parallel grooves with reduced refractive index, spiral cladding waveguides, by surrounding the light-guiding region with a continuous and smooth spiral groove, greatly avoid the problems mentioned above: on the one hand, the spiral structure makes the tangent direction at each point on the inner wall of the grooved cladding almost perpendicular to the radius of the equivalent circle, thereby reducing scattering loss of the beam during total internal reflection; on the other hand, by adjusting the pitch of the spiral, the groove can be made to spiral as much as possible to form a continuous cylindrical structure, thus avoiding light leakage during total internal reflection of the incident beam. Based on this processing approach, researchers have already fabricated spiral cladding waveguides with transmission losses as low as 0.12 dB / cm.

[0007] However, this method of fabricating helical cladding waveguides by transverse direct writing using a femtosecond laser Gaussian beam still has certain drawbacks. First, compared to traditional straight-line marking, each part of the helical marking structure has curvature, which significantly reduces the scanning speed and increases the time required for waveguide fabrication. With the same light-guiding area diameter (~100 μm) and waveguide length (~10 mm), fabricating a traditional cladding waveguide takes only a few minutes, while fabricating a helical cladding waveguide takes several hours, and the fabrication time increases with the increase in light-guiding area, transmission distance, and pitch. Therefore, the technical efficiency of fabricating helical cladding waveguides by transverse direct writing using a femtosecond laser Gaussian beam is very low. Second, the intensity distribution of the Gaussian beam gradually changes along the incident direction of the femtosecond laser, resulting in some undulations at the boundaries of the markings produced by the Gaussian beam. This also introduces scattering during total internal reflection, increasing transmission loss. Third, the helical marking process cannot be interrupted, therefore the femtosecond laser processing parameters are completely consistent at different depths from the surface. When the diameter of the cladding waveguide's guiding region is large (~100 μm), the size span of the helical structure along the femtosecond laser incident direction is significant. Due to the properties of the crystal material, the helical region far from the laser-matter interface may not be able to induce refractive index reduction modification. Simply increasing the pulse energy of the femtosecond laser cannot solve this problem, as excessively high pulse energy will increase the stress field in the guiding region, weakening its resistance to laser damage to some extent. Therefore, this cladding waveguide fabrication method places more stringent requirements on the femtosecond laser processing parameters and the properties of the optical crystal material.

[0008] Therefore, how to provide a spiral cladding waveguide structure element with relatively relaxed requirements for femtosecond laser processing parameters, shorter processing time, and simpler fabrication method, and achieve low-loss transmission of laser light, is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0009] In view of this, the purpose of this invention is to provide a femtosecond laser processing method for rapidly fabricating low-loss helical cladding waveguides, so as to overcome the shortcomings of the prior art.

[0010] To achieve the above objectives, the present invention adopts the following technical solution:

[0011] A femtosecond laser fabrication method for rapidly preparing low-loss helical cladding waveguides includes the following steps: (1) Place the transparent optical crystal to be processed on a six-dimensional electric platform and level it until the femtosecond laser incident end face reaches a horizontal state; (2) The femtosecond laser Gaussian beam is shaped into a femtosecond laser Bessel beam using a Bessel processing head, and then incident perpendicularly on the transparent optical crystal and focused onto the contact surface between the sample and the six-dimensional electric platform to produce femtosecond laser modified markings with reduced refractive index. (3) Design a spiral curve processing program in the computer, run the prepared spiral curve processing program to control the six-dimensional electric platform to move downward along the femtosecond laser incident direction in a spiral trajectory until the femtosecond laser Bessel beam completely leaves the femtosecond laser incident end face, and finally generate a spiral cladding waveguide that guides light along the femtosecond laser incident direction inside the transparent optical crystal. (4) The two end faces of the transparent optical crystal that are perpendicular to the direction of femtosecond laser transmission are optically polished to finally obtain a spiral cladding waveguide that runs through the transparent optical crystal along the light guiding direction.

[0012] This invention relates to a femtosecond laser processing method based on femtosecond laser Bessel beam longitudinal writing technology. It utilizes a femtosecond laser Bessel beam to generate long, deep modified indentations in a transparent optical crystal. Combined with the helical precession capability of a six-dimensional electrodynamic displacement platform, a helical line is scanned within the transparent optical crystal to directly obtain a helical cladding waveguide. During fabrication, by selecting appropriate femtosecond laser processing parameters, the longitudinal length of the femtosecond laser modified indentation reaches the hundreds of micrometers level. Simultaneously, the six-dimensional electrodynamic displacement platform is controlled by a designed program to ensure that the pitch of the helical line is approximately equal to the longitudinal length of the femtosecond laser modified indentation. Furthermore, after processing, the transparent optical crystal is polished to remove any incompletely closed portions at the beginning and end of the helical cladding waveguide. This femtosecond laser processing method enables simple and rapid fabrication of helical cladding waveguide structures in transparent optical crystals, and the fabricated cladding waveguide has relatively smooth sidewalls, achieving low-loss laser transmission.

[0013] The femtosecond laser processing method of this invention for fabricating helical cladding waveguides has a simple and convenient operation process. It only requires modifying and customizing an existing Bessel processing head on the market according to the center wavelength of the femtosecond laser used in the fabrication process. The femtosecond laser Gaussian beam is shaped into a femtosecond laser Bessel beam using the parameter-matched Bessel processing head. The structure file of the helical cladding waveguide is designed according to the software to make the helix have a suitable radius and pitch. Before fabricating the waveguide, suitable femtosecond laser direct-write parameters are found through processing tests, so that the adjacent periodic corresponding positions of the modified region of the helix obtained according to the program are almost exactly connected. This allows the helical cladding waveguide to transition into a cladding waveguide with a continuous interface, thereby achieving low-loss transmission of the laser beam.

[0014] The advantages of this invention are mainly reflected in the following three aspects: First, by utilizing the shaped femtosecond laser Bessel beam, a modified region with excellent edge smoothness can be obtained, reducing scattering during light transmission within the helical cladding waveguide and thus ensuring low-loss transmission of the laser beam through the cladding waveguide; Second, an ultra-long refractive index reduction modified region is obtained along the incident direction of the femtosecond laser Bessel beam, which helps to significantly shorten the fabrication time of the helical cladding waveguide, thereby enabling rapid and efficient fabrication of the helical cladding during femtosecond laser longitudinal direct writing and ensuring that corresponding regions in adjacent periods are precisely connected; Finally, this femtosecond laser processing method can fabricate low-loss helical cladding waveguide structures in any transparent crystal material, offering great freedom in material selection, especially in fabricating helical cladding waveguide structures in ultraviolet crystals, enabling low-loss transmission of ultraviolet lasers, and achieving high-efficiency, high-power, and high-stability nonlinear frequency conversion through phase-matching technology, thereby realizing the output of deep ultraviolet lasers.

[0015] Furthermore, in step (1) above, the transparent optical crystal is... z Cut lithium triborate crystals or xyz Cut quartz crystals.

[0016] Furthermore, in step (2) above, the air coke depth of the Bessel processing head is 8 mm.

[0017] Furthermore, in step (2) above, the length of the femtosecond laser modified marking along the laser incident direction is 200 μm.

[0018] Furthermore, in step (3) above, the light guiding direction of the spiral cladding waveguide is perpendicular to the incident end face of the femtosecond laser.

[0019] Furthermore, in step (3) above, the transmission loss of the helical cladding waveguide is as low as 0.5 dB / cm.

[0020] Furthermore, in step (3) above, the cross-section of the helical cladding waveguide (parallel to the laser incident end face) is circular, elliptical or any regular polygon, and the area of ​​the cross-section can be designed to different values ​​as needed.

[0021] Furthermore, in step (3) above, the maximum light transmission length of the spiral cladding waveguide is 1 cm.

[0022] Furthermore, in step (3) above, the pitch of the spiral cladding waveguide is 200 μm.

[0023] Furthermore, in step (4) above, each surface of the optical polishing process is polished by 0.5 mm.

[0024] This invention also claims protection for the application of the above-described femtosecond laser processing method in the fabrication of helical cladding waveguide structural elements.

[0025] As can be seen from the above technical solution, compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention utilizes a shaped femtosecond laser Bessel beam to prepare a longitudinally modified region inside a transparent optical crystal. By optimizing the femtosecond laser processing parameters, the length of the modified region induced by a single femtosecond laser Bessel beam reaches 200 μm along the light-guiding direction of the transparent optical crystal. Simultaneously, the sidewalls of this modified region prepared based on the femtosecond laser Bessel beam are sufficiently smooth, effectively reducing transmission loss when the beam undergoes total internal reflection inside the helical cladding waveguide based on this modification. This method of longitudinal direct writing of the Bessel beam effectively reduces the transmission loss of the beam from the cladding waveguide while improving processing efficiency.

[0026] 2. This invention utilizes a longitudinally written helical cladding waveguide based on a femtosecond laser Bessel beam in a transparent optical crystal, which can significantly reduce the transmission loss of the laser beam caused by the cladding waveguide. Compared to the modification caused by a femtosecond laser Gaussian beam, this invention uses a longitudinally written modification region by a femtosecond laser Bessel beam. This "elongation" shaping along the femtosecond laser transmission direction reduces the thermal effect of the modification region and makes the boundary of the modification region smoother. The helical cladding waveguide formed by this modification region facilitates total internal reflection of light, thereby reducing transmission loss. The modification region induced by the Bessel beam has a large depth (~200 μm) along the femtosecond laser transmission direction, compared to the modification length (~10 μm) produced by the Gaussian beam. The helical cladding waveguide based on the longitudinally written femtosecond laser Bessel beam, while ensuring that its helix is ​​tightly connected along the light guiding direction, can significantly reduce the number of cycles of the helical structure, thereby greatly shortening the processing time and improving processing efficiency. In addition, this invention uses... z The lithium triborate crystal and quartz crystal material platforms are characterized by their wide transmission spectrum, short ultraviolet cutoff edge, and strong resistance to light damage. Among these two materials, the helical cladding waveguide obtained by femtosecond laser Bessel beam longitudinal direct writing technology has lower transmission loss for ultraviolet beams, which is more conducive to subsequent nonlinear optical research in the ultraviolet band.

[0027] 3. The femtosecond laser processing method of this invention has the advantages of simple manufacturing process, fast preparation speed and good stability. The helical cladding waveguide prepared by this femtosecond laser processing method has the advantages of high integration, adjustable output mode, low transmission loss and good beam quality. In particular, it can realize low-loss transmission of ultraviolet laser in ultraviolet crystal, and will have wide applications in laser technology, integrated optics, nonlinear optics and other fields. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0029] Figure 1 This is a schematic diagram of the femtosecond laser processing method in Example 1; Figure 2 This is a flowchart of the femtosecond laser processing method in Example 1; Figure 3 This is a front view of the helical cladding waveguide structure element prepared in Examples 2-3; Figure 4 This is a top view of the helical cladding waveguide structure element prepared in Examples 2-3; Figure 5 The left view of the helical cladding waveguide structure element prepared in Examples 2-3; Figure 6 Microscopic images of the end faces of the conventional cladding waveguide in Comparative Example 1 and the spiral cladding waveguide in Example 2.

[0030] Among them, 1-Bessel processing head, 2-transparent optical crystal ( z Cutting lithium triborate crystals, xyz (3-Cut quartz crystal), 4-Helical cladding waveguide, 5-Six-dimensional electrodynamic platform. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] Terminology Explanation: Waveguide radius: The shortest distance from a point on the central axis of the helical cladding waveguide to the equivalent cylinder of the helical cladding waveguide.

[0033] Pitch: The axial distance between two points on the central axis corresponding to two adjacent cycles.

[0034] Helical direction: The direction of extension of the helical cladding waveguide, defined as the direction of propagation of the helical cladding being opposite to the propagation direction of the femtosecond laser Bessel beam. Hold the helical cladding with your right hand, with your four fingers bent in the same direction as the helix's rotation. When your thumb is in the same direction as the helical cladding's forward movement, the helical direction is left-handed; otherwise, it is right-handed.

[0035] Example 1 This embodiment discloses a femtosecond laser processing method for rapidly fabricating low-loss helical cladding waveguides. The processing schematic diagram and flowchart are shown below. Figure 1-2 As shown, the figure includes: Bezier head 1 for beam shaping; A transparent optical crystal material platform, wherein the light transmission direction of the crystal is along the crystal... e 2 directions; Transparent optical crystal 2 material has inscriptions along the e 3. Helical cladding waveguide in 2 directions; A six-dimensional electric platform 4 for carrying transparent optical crystals and achieving ultra-high precision six-dimensional motion.

[0036] The femtosecond laser processing method specifically includes the following steps: (1) Place the transparent optical crystal to be processed on a six-dimensional electric platform and level it until the femtosecond laser incident end face reaches a horizontal state; (2) The femtosecond laser Gaussian beam is shaped into a femtosecond laser Bessel beam using a Bessel processing head, and then incident perpendicularly on the transparent optical crystal and focused onto the contact surface between the sample and the six-dimensional electric platform to produce femtosecond laser modified markings with reduced refractive index. The Bessel processing head operates at a wavelength of 1031 nm to match the femtosecond laser used; the incident spot size (@1 / e) 2 The focal length is Ø10 μm; the air focal depth is 8 mm, which is used to meet the size requirements of the processed sample and the transmission distance of the spiral cladding waveguide. (3) Design a spiral curve processing program in the computer, run the prepared spiral curve processing program to control the six-dimensional electric platform to move downward along the femtosecond laser incident direction in a spiral trajectory until the femtosecond laser Bessel beam completely leaves the laser incident end face, and finally generate a spiral cladding waveguide that guides light along the laser incident direction inside the transparent optical crystal. Among them, the refractive index reduction modification region (the cladding region of the helical cladding waveguide) extends towards the crystal in a helical form. e The spiral extends in the opposite direction to the light guiding direction of the spiral cladding waveguide, either in a left-handed or right-handed direction. For linearly polarized incident light, the extension direction of these two spirals will not have a substantial impact on the transmission loss of the spiral cladding waveguide. (4) The two end faces of the processed transparent optical crystal that are perpendicular to the direction of femtosecond laser transmission are optically polished, and 0.5 mm is removed from each face to finally obtain a spiral cladding waveguide that runs through the transparent optical crystal along the light guiding direction.

[0037] Example 2 This embodiment discloses a method in z Spiral cladding waveguide structures fabricated in lithium triborate crystals, their x , y , z The directions correspond to Figure 1 In e 1. e 2. e The three views of this structural element are shown in the following diagrams. Figure 3-5 As shown, the size is 5 ( x ) 3 ( y ) 8 ( z ) mm 3 tangential direction z Cut, light transmission direction along y The preparation method of the present invention includes the following steps: (1) The material to be processed z The lithium triborate crystal was placed on a six-dimensional electric platform and leveled until the femtosecond laser incident end face reached a horizontal state. (2) The femtosecond laser Gaussian beam is shaped into a femtosecond laser Bessel beam using a Bessel machining head, along the... z Cut lithium triborate crystals y perpendicular incidence z A femtosecond laser was used to cut a lithium triborate crystal and focus it onto the contact surface between the sample and the six-dimensional electric platform to generate a femtosecond laser-modified indentation with a reduced refractive index. The Bessel processing head operates at a wavelength of 1031 nm to match the femtosecond laser used; the incident spot size (@1 / e 2) The focal length is Ø10 μm; the air focal depth is 8 mm, which is used to meet the size requirements of the processed sample and the transmission distance of the spiral cladding waveguide; The femtosecond laser pulse width is 439 fs, the wavelength is 1031 nm, the repetition rate is 2.5 MHz, the single pulse energy is 4 μJ, and the femtosecond laser scanning speed is 0.1 mm / s. (3) Design a spiral curve machining program in the computer, and run the prepared spiral curve machining program to control the six-dimensional electric platform to follow the spiral trajectory. z Cut lithium triborate crystals y Move downwards until the femtosecond laser Bessel beam completely leaves the femtosecond laser incident end face, and finally atz Cutting the interior of lithium triborate crystals generates along... z Cut lithium triborate crystals y A spiral cladding waveguide for guiding light; Among them, the refractive index reduction modification region (the cladding region of the helical cladding waveguide) is helical in shape, towards... z Cut lithium triborate crystals y Extending in the opposite direction to the light guiding direction of the helical cladding waveguide, it can be in a left-handed or right-handed direction; in this embodiment, the helical cladding waveguide is composed of a right-handed helix. The radius of the helical cladding waveguide is set to 25 μm, the pitch of the helix is ​​200 μm, and the helix extends in a right-handed direction. (4) z Two cuts of lithium triborate crystal y end face (i.e. with) z Cut lithium triborate crystals y Optical polishing was performed on the two perpendicular end faces, removing 0.5 mm from each face. The sample was then further cleaned with a mixture of ethanol and acetone to obtain a final product that penetrates along the light-guiding direction. z Spiral cladding waveguides for cutting lithium triborate crystals.

[0038] Example 3 This embodiment discloses a method in xyz Helical clad waveguide structures fabricated in quartz crystals, their... x , y , z The directions correspond to Figure 1 In e 1. e 2. e The three views of this structural element are shown in the following diagrams. Figure 3-5 As shown, the size is 5 ( x ) 3( y ) 8 ( z ) mm 3 tangential direction xyz Cut, light transmission direction along z The preparation method of the present invention includes the following steps: (1) The material to be processed xyz The quartz crystal was placed on a six-dimensional electric platform and leveled until the femtosecond laser incident end face reached a horizontal state. (2) The femtosecond laser Gaussian beam is shaped into a femtosecond laser Bessel beam using a Bessel machining head, along the... xyz Cutting quartz crystals z perpendicular incidence xyzA quartz crystal is cut and focused onto the contact surface between the sample and the six-dimensional electric platform to generate a femtosecond laser-modified indentation with a reduced refractive index. The Bessel processing head operates at a wavelength of 1031 nm to match the femtosecond laser used; the incident spot size (@1 / e 2) The focal length is Ø10 μm; the air focal depth is 8 mm, which is used to meet the size requirements of the processed sample and the transmission distance of the spiral cladding waveguide; The femtosecond laser pulse width is 439 fs, the wavelength is 1031 nm, the repetition rate is 200 kHz, the single pulse energy is 6 μJ, and the femtosecond laser scanning speed is 0.1 mm / s. (3) Design a spiral curve machining program in the computer, and run the prepared spiral curve machining program to control the six-dimensional electric platform to follow the spiral trajectory. xyz Cutting quartz crystals z Move downwards until the femtosecond laser Bessel beam completely leaves the femtosecond laser incident end face, and finally at xyz Cutting the interior of a quartz crystal produces along xyz Cutting quartz crystals z A spiral cladding waveguide for guiding light; Among them, the refractive index reduction modification region (the cladding region of the helical cladding waveguide) is helical in shape, towards... xyz Cutting quartz crystals z Extending in the opposite direction to the light guiding direction of the helical cladding waveguide, it can be in a left-handed or right-handed direction; in this embodiment, the helical cladding waveguide is composed of a right-handed helix. The cross-section (parallel to the laser incident end face) of the helical cladding waveguide is circular; The radius of the helical cladding waveguide is set to 25 μm, the pitch of the helix is ​​200 μm, and the helix extends in a right-handed direction. (4) xyz Two cuts of a quartz crystal z end face (i.e. with) xyz Cutting quartz crystals z Optical polishing was performed on the two perpendicular end faces, removing 0.5 mm from each face. The sample was then further cleaned with a mixture of ethanol and acetone to obtain the final product. z To penetrate xyz Spiral cladding waveguide for cutting quartz crystals.

[0039] Comparative Example 1 The difference between this comparative example and Example 2 is that the former uses a Gaussian beam and is fabricated using the same spiral cladding waveguide structure prepared in Example 2.

[0040] Comparative Example 2 The difference between this comparative example and Example 3 is that the former uses a Gaussian beam and is fabricated using the same spiral cladding waveguide structure prepared in Example 3.

[0041] Performance testing 1. Through Comparative Example 1 z The helical cladding waveguide prepared in lithium triborate crystal and Example 2 z Comparative experiments were conducted on the helical cladding waveguide structure element prepared in lithium triborate crystal to test the transmission loss of the helical cladding waveguides in Example 2 and Comparative Example 1, respectively.

[0042] In Comparative Example 1, the longitudinal length of the grooves produced by the Gaussian beam is relatively short, making the processing operation more complex and time-consuming, on the order of hours (typically greater than 1 hour), and significantly increasing with the increase in the length of the light guide direction and the decrease in the pitch. In contrast, the processing time of the helical cladding waveguide in Example 2 can be controlled within 10 minutes. Furthermore, because Comparative Example 1 uses a femtosecond laser Gaussian beam, the modified area at the edge of the cladding waveguide is rougher. For ease of observation of the end face, the fabrication of the helical cladding waveguide using the Bessel beam in Example 2 and the fabrication of the ordinary cladding waveguide using the Gaussian beam in Comparative Example 1 are used as examples, such as under a metallographic microscope. Figure 6 As shown, (a) is a microscopic image of the end face of the conventional cladding waveguide in Comparative Example 1, with a transmission loss of approximately 2.03 dB / cm; (b) is a microscopic image of the end face of the helical cladding waveguide in Example 2. The comparison reveals that the edge smoothness of the Bessel beam-induced modified region is significantly better than that of the Gaussian beam, which facilitates total internal reflection of light within the waveguide. Therefore, the transmission loss of the waveguide in Comparative Example 1 is significantly higher than that of the waveguide in Example 2.

[0043] Experiments show that this invention innovatively utilizes the shaped femtosecond laser Bessel beam in... z A longitudinally oriented helical cladding waveguide is constructed within a lithium triborate crystal. By selecting appropriate femtosecond laser direct-writing parameters and setting suitable cladding waveguide structure parameters, adjacent periods of the helical cladding can be precisely connected along the light-guiding direction. This enhances the beam confinement during total internal reflection, thereby efficiently reducing transmission loss of the incident beam. Due to the crystal's wide transmission range and excellent nonlinear optical properties, the helical cladding waveguide within the crystal can achieve efficient frequency conversion and beam transmission for incident beams of specific wavelengths. For crystals with specific chamfers and incident beams of specific wavelengths, when the beam obtained after nonlinear effects (e.g., frequency doubling) is in the ultraviolet band, the helical cladding waveguide within the crystal can further ensure low-loss transmission of the generated ultraviolet beam.

[0044] 2. Through Comparative Example 2 in xyz The helical cladding waveguide prepared in a quartz crystal and Example 3 xyz Comparative experiments were conducted on the helical cladding waveguide structure elements prepared in quartz crystals to test the transmission loss of the helical cladding waveguides in Example 3 and Comparative Example 2, respectively.

[0045] In Comparative Example 2, the longitudinal length of the grooves produced by the Gaussian beam is relatively short, making the processing operation more complex and time-consuming, on the order of hours (typically greater than 1 hour), and significantly increasing with the increase of the beam guide length and the decrease of the pitch. In contrast, the processing time of the helical cladding waveguide in Example 3 can be controlled within 10 minutes. Furthermore, because Comparative Example 2 uses a femtosecond laser Gaussian beam, the modified region at the edge of the cladding waveguide is rougher, resulting in a significantly higher transmission loss for the waveguide in Comparative Example 2 compared to the waveguide in Example 2.

[0046] Experiments show that this invention innovatively utilizes the shaped femtosecond laser Bessel beam in... xyz A longitudinally direct-write helical cladding waveguide inside a quartz crystal is employed. By selecting appropriate femtosecond laser direct-write parameters and setting suitable cladding waveguide structure parameters, adjacent periods of the helical cladding can be precisely connected along the light-guiding direction. This enhances the beam confinement effect during total internal reflection, thereby efficiently reducing transmission loss of the incident beam. Due to the extremely wide light transmission range of quartz crystals, especially when the incident light is in the ultraviolet band, the helical cladding waveguide inside the crystal can achieve ultra-low loss transmission of ultraviolet beams.

[0047] The various embodiments in this specification are described in parallel. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant details can be found in the method section.

[0048] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A femtosecond laser processing method for rapidly fabricating low-loss helical cladding waveguides, characterized in that, Specifically, the following steps are included: (1) Place the transparent optical crystal to be processed on a six-dimensional electric platform and level it until the femtosecond laser incident end face reaches a horizontal state; (2) The femtosecond laser Gaussian beam is shaped into a femtosecond laser Bessel beam using a Bessel processing head, and then incident perpendicularly on the transparent optical crystal and focused onto the contact surface between the sample and the six-dimensional electric platform to produce femtosecond laser modified markings with reduced refractive index. (3) Design a spiral curve processing program in the computer, run the prepared spiral curve processing program to control the six-dimensional electric platform to move downward along the femtosecond laser incident direction in a spiral trajectory until the femtosecond laser Bessel beam completely leaves the femtosecond laser incident end face, and finally generate a spiral cladding waveguide that guides light along the femtosecond laser incident direction inside the transparent optical crystal. (4) The two end faces of the transparent optical crystal that are perpendicular to the direction of femtosecond laser transmission are optically polished to finally obtain a spiral cladding waveguide that runs through the transparent optical crystal along the light guiding direction.

2. The femtosecond laser processing method for rapidly fabricating low-loss helical cladding waveguides according to claim 1, characterized in that, In step (1), the transparent optical crystal is z Cut lithium triborate crystals or xyz Cut quartz crystals.

3. The femtosecond laser processing method for rapidly fabricating low-loss helical cladding waveguides according to claim 1, characterized in that, In step (2), the air coke depth of the Bessel processing head is 8 mm.

4. The femtosecond laser processing method for rapidly fabricating low-loss helical cladding waveguides according to claim 1, characterized in that, In step (2), the length of the femtosecond laser modified marking along the laser incident direction is 200 μm.

5. The femtosecond laser processing method for rapidly fabricating low-loss helical cladding waveguides according to claim 1, characterized in that, In step (3), the light guiding direction of the spiral cladding waveguide is perpendicular to the incident end face of the femtosecond laser.

6. The femtosecond laser processing method for rapidly fabricating low-loss helical cladding waveguides according to claim 1, characterized in that, In step (3), the transmission loss of the spiral cladding waveguide is at least 0.5 dB / cm.

7. The femtosecond laser processing method for rapidly fabricating low-loss helical cladding waveguides according to claim 1, characterized in that, In step (3), the cross-section of the spiral cladding waveguide is circular, elliptical, or any regular polygon.

8. The femtosecond laser processing method for rapidly fabricating low-loss helical cladding waveguides according to claim 1, characterized in that, In step (3), the maximum light transmission length of the spiral cladding waveguide is 1 cm.

9. The femtosecond laser processing method for rapidly fabricating low-loss helical cladding waveguides according to claim 1, characterized in that, In step (3), the pitch of the spiral cladding waveguide is 200 μm.

10. The application of the femtosecond laser processing method as described in any one of claims 1-9 in the fabrication of helical cladding waveguide structural elements.

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