Design of divergent distributions

Abstract

The fiber assembly includes a first lens, a divergent preservation fiber having a mode-distribution homogenization portion, and a second lens. This fiber assembly is configured to produce a flat-top divergent distribution. All embodiments include a junction within the divergent preservation fiber, which provides advantages in terms of manufacturability, maintainability, and / or performance. One or both of the core diameter and / or cladding diameter can be stepped up or down at this junction. In some embodiments, this structure is all-fiber (without free-space optical elements). In other embodiments, an inter-fiber coupler or switch is employed.

Description

[Technical Field]

[0001] Related applications

[0001] This application claims priority to U.S. Provisional Patent Applications No. 63 / 499,700, 63 / 499,701, 63 / 499,702, 63 / 499,703, and 63 / 499,705, filed on 2 May 2023, and to U.S. Provisional Patent Applications No. 63 / 591,051 and 63 / 591,053, filed on 17 October 2023. By using these patent applications as herein by reference, their entire contents are included in this application.

[0002] Technical field

[0002] The technologies disclosed herein relate to fiber lasers and fiber-coupled lasers. More specifically, the technologies disclosed relate to optical fiber assemblies using short-focus lenses. Prior Art

[0003] Background information

[0003] Some existing methods employ free-space optical elements (lenses, mirrors, etc.) to adjust the magnification of the laser beam. Free-space optical systems are plagued by several problems, which become more severe as the optical power level increases. They are expensive, they cause excessive losses (reducing efficiency), they are prone to mismatch, contamination, and damage, and their performance can be degraded by vibration, temperature fluctuations, other environmental factors, and changes in optical power (thermal lensing effect). In short, eliminating free-space optical elements reduces system cost and complexity, and improves system performance, stability, and reliability.

[0004]

[0004] As described in U.S. Patent No. 10,663,768 (issued to nLIGHT, Inc. on 26 May 2020) and related patents of Martinsen et al., nLIGHT, Inc. has developed a technique for varying beam properties by controlling the spatial intensity distribution of a laser beam, i.e., by adjusting its near-field intensity distribution. The '768 patent describes a fiber operable to supply a laser beam having variable beam characteristics (VBC), which can reduce cost, complexity, optical loss, or other drawbacks of conventional methods.

[0005]

[0005] Structured laser beams, such as flat-top (also called top-hat) and donut (ring) beams, are useful in some industrial laser applications, such as metal cutting, welding, brazing, heat treatment, and additive manufacturing. For example, in industrial laser processes such as cutting, welding, additive manufacturing, brazing, coating, and cleaning, advantages can be gained from the flat-top laser beam intensity distribution because the distribution of laser power across the entire cross-section of the beam is relatively uniform. A saddle beam shape (also called a saddle beam or saddle mode) is defined by an intensity profile of light that has an annular shape and lower non-zero intensity in the center. A pedestal beam shape (or pedestal shape or pedestal mode) is similar to a saddle shape, except that the intensity in the center is higher than the surrounding halo. A ring beam (also called a ring mode) has an annular shape with little to no power in the center. To the extent of this disclosure, the ring mode is therefore considered to be a unique type of saddle beam shape that has little to no power in the center of the beam. Flat-top, ring, saddle, and pedestal beam profiles (as well as other beam profiles) can be obtained at the output of a ring fiber, and this shape is determined by the relative power levels at each core of the ring fiber. Further details on generating beam shapes using a VBC laser are described in the '768 patent.

[0006]

[0006] U.S. Patents 9,250,390 and 9,823,422 by Muendel and Kliner describe optical transmission waveguides for material laser processing systems. These include a small lens at the output end of the transmission waveguide, which deforms the divergence of the laser beam inside the waveguide into a spot size and shape after passing through the lens. The waveguide has a single core (e.g., step-index) and the output spot size and shape can be continuously varied by changing one or both of the input focusing angle and / or emission angle of the laser beam emitted into the waveguide. This technique does not use multiple cores to produce different beam sizes and shapes, but instead uses a change in divergence, and the output lens maps the change in divergence to size and shape. [Overview of the Initiative] [Means for solving the problem]

[0007]

[0007] The embodiments described herein employ a multimode divergence-preserving fiber between short focal length lenses (e.g., distributed refractive index (GRIN) lenses) at the fiber input and output to transmit a laser beam. The input lens maps a position at the front focal plane to an angle in the divergence-preserving fiber, and the output lens maps this angle to a position at the back focal plane. In the disclosed embodiments, step-index cores and cladding of various sizes may be used depending on the application, and different lens designs may also be used (e.g., GRIN, curved end cap, or ball lens). For high-power applications, in one embodiment, a GRIN lens with an end cap (i.e., a 1 / 4 pitch length GRIN fiber) is employed at the output.

[0008]

[0008] By changing the source fiber coupled to the input lens, different all-fiber embodiments become possible, thereby providing desirable functionality for material processing and other industrial applications of lasers. Specifically, these embodiments enable: (1) an all-fiber method (i.e., without using free-space optical elements) for imaging and arbitrarily magnifying a laser beam. By combining this with a VBC fiber or ring fiber, it provides an image of a structured or tangible laser beam. (2) High optical power fiber transmission by either or both of longer fiber lengths and smaller output beam diameters than possible with conventional fibers due to limitations associated with the nonlinear process of the fiber. (3) Generating composite beam shapes, such as a central spot surrounded by one or more rings, without using multiple induction regions to maintain shape in the fiber. (4) Suppression of output divergence, or numerical aperture (NA) defined as sine (θ), where θ is the beam's half-angle divergence. (5) Generation of a flat-top near-field (divergent) distribution. (6) Transmitting tangible beam profiles from a fiber-to-fiber coupler (FFC) or fiber-to-fiber switch (FFS) without using multicore process fiber.

[0009]

[0009] A common feature of these embodiments is that they do not require the use of free-space optical elements or free-space beams. That is, a fiber assembly can be coupled to the output fiber of a fiber laser or other fiber-coupled laser to provide a full fiber path between the laser source(s) and the process optical elements (or other components downstream of the transmission fiber). By enabling a full fiber format, these embodiments not only address the limitations of the prior art employing free-space optical elements but also offer other advantages. (Of course, embodiments employing FFC or FFS naturally employ free-space optical elements, but laser systems upstream of the FFC or FFS do not require the use of free-space optical elements.)

[0010]

[0010] In one embodiment, a reduction in output divergence (compared to the use of ring fibers) is achieved. Suppressing the output beam divergence (for example, when combined with VBC fibers) makes it easier to meet performance specifications for industrial applications (e.g., cutting, welding, and additive manufacturing). By selecting the relative focal lengths of the two GRIN fiber segments (i.e., by adjusting the magnification), and by selecting the core diameter of the divergence-preserving fiber, the near-field diameter and far-field divergence or NA of the output beam can be optimized for a given application.

[0011]

[0011] End users of high-power fiber lasers sometimes use scanners to process optical elements having an NA of 0.10 to 0.14. For these applications, a VBC fiber system with a low NA beam that does not truncate excess power on an NA-limiting aperture is useful.

[0012]

[0012] The disclosed embodiments enable placement of a junction within a divergent storage fiber. This junction facilitates manufacturing and field service. That is, by adding (joining) an assembly downstream of the junction within a factory, different product configurations are enabled and / or this junction can serve as a field-replaceable unit (FRU) that can be joined to the system at a service logistics site or in the field.

[0013]

[0013] Also, an application may include a process fiber and FFC or FFS. The fiber downstream of the FFC or FFS (i.e., the process fiber) can be made an FRU.

[0014]

[0014] The present embodiment addresses the above-described industrial applications (and other applications) to avoid some of the drawbacks of the prior art. Specifically, the present embodiment provides one or more of the following advantages. (1) All-fiber technology. That is, elimination of free-space optical elements and the limitations and drawbacks inherent to them. (2) Elimination of fiber tapers that have manufacturing drawbacks. (3) Reduction or elimination of the use of relatively complex and expensive multi-core fibers. (4) Maintain compatibility with a range of input and output beam characteristics for use in various applications with minimal system complexity or number of components. (5) Compatibility with optical systems having NA limitations.

[0015]

[0015] In one embodiment, an image relay optical beam transmission system includes a source fiber configured to supply an input beam. This input beam is defined by a beam parameter product (BPP) and an azimuthal symmetric input transverse spatial intensity distribution. The system features a first GRIN lens configured to convert the input transverse spatial intensity distribution of the input beam into an intermediate beam having a divergent distribution. This first GRIN lens has a first effective focal length. A second GRIN lens coupled to the first GRIN lens receives the intermediate beam from the first GRIN lens and maps the divergent distribution to an output beam, so that the output transverse spatial intensity distribution represents an image of the input beam and maintains the BPP of the output beam. The second GRIN lens has a second effective focal length different from the first effective focal length. Both the first and second GRIN lenses have a 1 / 4 pitch length, which may include zero or greater multiples of 1 / 2 pitch.

[0016]

[0016] Yet another embodiment of the optical beam transmission system may include a difference of 1 / 4 pitch length between the first and second GRIN lenses. The effective focal lengths of the first and second lenses can be selected to either enlarge or reduce the output beam compared to the input beam. The lengths of the first and second effective focal lengths range from about 0.1 mn to about 2.0 mm. The input lateral spatial intensity distribution can be Gaussian, super-Gaussian, flat-top, ring-shaped, or saddle-shaped. In addition, the source fiber may be a ring fiber or a VBC fiber.

[0017]

[0017] In a related aspect, a method of relaying an input beam from a laser source includes inducing the input beam to pass through a source fiber and guiding it to a first GRIN lens. The first GRIN lens converts the input transverse spatial intensity distribution into an intermediate beam having a divergent distribution. Then, the intermediate beam is guided to a second GRIN lens. The second GRIN lens maps the divergent distribution to an output beam that represents an image of the input beam and maintains the BPP. The first and second GRIN lenses each have a 1 / 4 pitch length and may include multiples of its 1 / 2 pitch that are zero or greater.

[0018]

[0018] This method may further involve varying the 1 / 4 pitch lengths of the first and second GRIN lenses and selecting an effective focal length to either magnify or reduce the output beam with respect to the input beam. The focal lengths of these lenses range from about 0.1 mn to about 2.0 mm. Options for the input transverse spatial intensity distribution include Gaussian, super-Gaussian, flat-top, ring-shaped, or saddle-shaped, and the source fiber may be a ring fiber or a VBC fiber.

[0019]

[0019] In one aspect, an optical beam transmission system includes a source fiber configured to form a transverse spatial intensity distribution in an input beam and an image relay fiber assembly. This assembly includes a first lens that converts the input transverse spatial intensity distribution of the input beam into an intermediate beam having a divergent distribution. A divergence-preserving fiber receives this intermediate beam and maintains its divergent distribution along its length. A second lens receives the intermediate beam from the divergence-preserving fiber and generates an output beam as an image of the input beam.

[0020]

[0020] In another embodiment, the method is for relaying an input beam from a laser source, the input beam having an input lateral spatial intensity distribution. The method comprises the steps of: guiding the input beam to a first lens configured to map the input lateral spatial intensity distribution to the divergence distribution of an intermediate beam; guiding the intermediate beam with a divergence preservation fiber while preserving its divergence distribution along the length of the divergence preservation fiber; and guiding the intermediate beam to a second lens configured to map the divergence distribution to an output lateral spatial intensity distribution and supply the output beam as an image of the input beam. Options for this method may include the steps of coupling an output end cap or joining fibers to adjust the core and cladding diameters of the divergence preservation fiber.

[0021]

[0021] The optical beam transmission systems and methods described above may also include the option that the input lateral spatial intensity distribution is Gaussian, super-Gaussian, flat-top, ring-shaped, or saddle-shaped, and the source fiber may be a ring fiber. In addition, either or both of the first and second lenses may be GRIN lenses having a 1 / 4 pitch length, and may include zero or more multiples of 1 / 2 pitch, and may be coupled to the divergent storage fiber, or may be ball lenses or rounded end caps. The divergent storage fiber may be a step-index fiber, and both lenses may have a common effective focal length, or each lens may have a different effective focal length. The system may also include a variable or fixed input lateral spatial intensity distribution. Further improvements may include an output end cap coupled to the output of the second lens, a junction in the divergent storage fiber, or the incorporation of an FFC, FFS, or one or more cladding light strippers (CLS) along the divergent storage fiber.

[0022]

[0022] The method for modifying the aforementioned system may involve the steps of removing a portion of the divergent preservation fiber and joining a replacement divergent preservation fiber. The replacement divergent preservation fiber may include a GRIN lens, a ball lens, and a rounded end cap that function as a second lens. This method can also be part of a larger effort to enlarge or reduce the output beam.

[0023]

[0023] In one embodiment, the optical beam transmission system includes a source fiber having a first effective mode area and configured to supply an input beam. The system also includes a fiber assembly, which features a first lens that shifts the input beam from the first effective mode area to a larger second effective mode area. A subsequent divergence-preserving fiber maintains this second effective mode area along a core diameter larger than the core diameter of the source fiber. The second lens then refocuses the input beam to generate an output beam.

[0024]

[0024] The method associated with this system aims to reduce nonlinear optical effects by directing the input beam to pass through a first lens and transitioning it to a larger second effective mode area within the intermediate beam. This beam is then guided by a divergence-preserving fiber, which maintains the divergence distribution along its length, and is finally refocused by a second lens to form an output beam.

[0025]

[0025] The above systems and methods may also include variations in which the input lateral spatial intensity distribution is Gaussian, super-Gaussian, flat-top, ring-shaped, or saddle-shaped. The source fiber may be a ring fiber. Either or both lenses in this system may be GRIN lenses having a 1 / 4 pitch length and may include an extra 1 / 2 pitch multiple. Alternatively, they may be directly bonded to the divergence preservation fiber. Instead, these lenses may be ball lenses or rounded end caps. The divergence preservation fiber itself may be a step-index fiber.

[0026]

[0026] Furthermore, both lenses may share a common effective focal length, or they may have different effective focal lengths to expand or contract the output beam. The system may also feature an input lateral spatial intensity distribution that is either variable or fixed, and improvements may include an output end cap coupled to the second lens, a junction in the divergence-preserving fiber, or an incorporation along the fiber of an FFC, FFS, or CLS.

[0027]

[0027] A modification method for this system may involve the steps of removing a portion of the divergent preservation fiber and joining a replacement divergent preservation fiber. The replacement divergent preservation fiber includes, for example, a GRIN lens that functions as a second lens to change the magnification or to step up or down the refractive index of one or both of the core and cladding.

[0028]

[0028] In one embodiment, the optical beam transmission system includes a source fiber having a first effective mode area and is configured to supply an input beam. The system also features a fiber assembly, which includes a first pair of coupled GRIN lenses, namely a first GRIN lens having a first effective focal length and a second GRIN lens having a second effective focal length longer than the first effective focal length. Both the first and second GRIN lenses have a 1 / 4 pitch length, which may include zero or greater multiples of its 1 / 2 pitch. The first pair of coupled GRIN lenses is configured to transition the input beam from the first effective mode area to an intermediate beam in a second effective mode area that is larger than the first effective mode area. A step-index fiber guides the intermediate beam and features a second effective mode area and a core diameter larger than the core diameter of the source fiber. In addition, the second pair of coupled GRIN lenses comprises a third GRIN lens having a third effective focal length and a fourth GRIN lens having a fourth effective focal length different from the third effective focal length. Both the third and fourth GRIN lenses have a 1 / 4 pitch length, which may include a multiple of its 1 / 2 pitch greater than or equal to 0. This second pair of coupled GRIN lenses is configured to enlarge or reduce the intermediate beam to produce an output beam. The system may also include configurations in which the third effective focal length is longer or shorter than the fourth effective focal length. It may also be configured such that the first effective focal length is equal to the fourth effective focal length, and the second effective focal length is equal to the third effective focal length.

[0029]

[0029] In one embodiment, a related method for reducing the occurrence of nonlinear optical effects with respect to a desired length of process fiber or supply fiber suitable for guiding an input beam having a specified input effective mode area involves directing the input beam to a first pair of coupled GRIN lenses described above. This first pair transitions the beam from the input effective mode area to a larger intermediate mode area. After passing through a second GRIN lens, the intermediate beam, including this larger effective mode area, is guided by a step-index fiber and then directed to a second pair of coupled GRIN lenses. This second pair includes a third and a fourth GRIN lens with different effective focal lengths and is designed to expand or contract the intermediate beam to produce an output beam. Similar to the system configuration described above, this method may specify that the third effective focal length is longer or shorter than the fourth effective focal length, and may further feature focal length equivalence such that the first effective focal length is equal to the fourth effective focal length and the second effective focal length is equal to the third effective focal length.

[0030]

[0030] In one embodiment, the optical beam transmission system includes a source fiber configured to supply an input beam characterized by an input lateral spatial intensity distribution along a first optical axis. The system also includes a fiber assembly having a second optical axis, the second optical axis being centrally located and radially offset laterally from the first optical axis. The assembly includes a first lens configured to convert the input lateral spatial intensity distribution into a divergence distribution of an intermediate beam, a divergence-preserving fiber that receives the intermediate beam and preserves its divergence distribution along its length, and a second lens configured to receive the intermediate beam from the divergence-preserving fiber and convert the divergence distribution into an output lateral spatial intensity distribution in the output beam. The output beam includes a structured beam shape. The system may also feature the option that the structured beam shape is ring-shaped, saddle-shaped, or pedestal-shaped. In addition, the source fiber may include a central longitudinal axis radially offset from the first optical axis. Furthermore, the system can be designed such that the diameter of the structured beam shape is proportional to the amount of offset between the first and second optical axes. This may also include a fiber bundle comprising a source fiber and at least one other fiber offset laterally from the second optical axis. In one configuration, the source fiber is a multicore fiber, with at least one core offset laterally from the second optical axis. The system can also be configured such that the first optical axis is the central longitudinal axis of a single central core, and the source fiber is coupled to the first lens by an offset splice. In addition, the source fiber may consist of a single core defining the first optical axis. The input lateral spatial intensity distribution of this system can be Gaussian, super-Gaussian, flat-top, ring-shaped, saddle-shaped, or pedestal-shaped. It may also include a ring fiber as the source fiber. Furthermore, the system may include a first source fiber and a first fiber assembly together with a second fiber assembly for a second source fiber, where each source fiber has independently controllable power and forms a composite shape.

[0031]

[0031] In another embodiment, a method for generating an output beam having an output transverse spatial intensity distribution that defines a composite beam shape is determined by the superposition of a first intensity distribution and a second intensity distribution. This method includes the step of generating a first intensity distribution by converting the input transverse spatial intensity distribution of the first beam into a first divergent distribution and converting this first divergent distribution into a first part of the output transverse spatial intensity distribution, the first part of which may be ring-shaped or saddle-shaped. This method also includes the step of inducing a second beam and converting its second divergent distribution into a second part of the output transverse spatial intensity distribution, and combining the first and second divergent distributions in a fused fiber combiner. This method may also specify that the second part of the output transverse spatial intensity distribution is located in the center of the ring-shaped or saddle-shaped first part, or that the second part does not overlap with the first part. This may also involve a first laser source for the first beam and a second laser source for the second beam, each laser source having independently controllable power. Additional steps may include receiving a first beam in a fiber assembly including a first lens and a divergent preservation fiber, receiving a second beam in a step-index fiber, and combining the outputs from the fiber assembly and the step-index fiber to produce an intermediate beam. The intermediate beam is converted by the second lens into an output transverse spatial intensity distribution. Further modifications may include receiving different beams in different fiber assemblies, each having an input lens and an associated divergent preservation fiber, and combining the outputs from each to produce an intermediate beam. The intermediate beam is converted by the second lens into an output transverse spatial intensity distribution. Alternatively, this method may include receiving the first and second beams in a fiber assembly, where the first beam is offset from the second beam, and this assembly includes a first lens, a divergent preservation fiber, and a second lens.The first lens is configured to generate first and second divergence distributions of the intermediate beam, the divergence-preserving fiber is configured to receive the intermediate beam and preserve its divergence distribution along the length of the divergence-preserving fiber, and the second lens is configured to generate the output lateral spatial intensity distribution.

[0032]

[0032] A system for structured beam shapes, or a method for composite beams, may include one or both of the first and second lenses as GRIN lenses having a pitch length of 1 / 4 + 1 / 2 × n, where n is zero or any positive integer. Alternatively, these may be GRIN lenses, ball lenses, or rounded end caps joined to the divergent preservation fiber. In addition, the divergent preservation fiber may be a step-index fiber. The system or method may specify that the first and second lenses have a common effective focal length, or that the first lens has a first effective focal length and the second lens has a second effective focal length different from the first effective focal length. In yet another configuration, an output end cap coupled to the output of the second lens and a junction in the divergent preservation fiber, or an FFC or FFS in the divergent preservation fiber, so that the input divergent preservation fiber and the output divergent preservation fiber are placed with the FFC or FFC in between. In addition, one or more CLS may be placed along the divergent preservation fiber. This assembly may also include a mode scrambling section. A method for modifying this system may involve removing at least a portion of the divergent preservation fibers from the fiber assembly, and then joining replacement divergent preservation fibers onto the remaining portion and changing the magnification again, or stepping up or down the refractive index.

[0033]

[0033] In one aspect, the optical beam transmission system includes a maximum numerical aperture (NA). maxThe system is configured to supply an output beam having the NA of the output beam. The system includes a source fiber that supplies the input beam and a fiber assembly featuring a divergence-preserving fiber having a specified core diameter. This fiber is configured to receive an intermediate beam corresponding to the input beam and preserve its divergence distribution. An output lens having an effective focal length is configured to convert the divergence distribution into an output beam. max This is determined by the ratio of the core diameter to the effective focal length. The system may also include the option that the input lateral spatial intensity distribution of the input beam is Gaussian, super-Gaussian, flat-top, ring-shaped, saddle-shaped, or pedestal-shaped, and that the source fiber may be a ring fiber. In addition, the fiber assembly may include an input lens coupled to the source fiber that converts the input beam into an intermediate beam. The divergent preservation fiber may be a step-index fiber. Furthermore, the input lateral spatial intensity distribution may be variable or fixed. Additional features may include an output end cap coupled to the output of the output lens, a junction within the divergent preservation fiber, and an FFC or FFS within the divergent preservation fiber, thereby setting the input and output divergent preservation fibers. Optionally, one or more CLSs arranged along the divergent preservation fiber may also be included. A method for modifying the optical beam transmission system may involve the steps of removing at least a portion of the divergent preservation fiber from the fiber assembly and bonding a replacement divergent preservation fiber onto the remaining portion.

[0034]

[0034] In other embodiments, NA max A method for supplying an output beam having NA involves the step of receiving an input beam in an optical beam transmission device. This input beam passes through a divergence-preserving fiber and an output lens. This fiber guides an intermediate beam while preserving its divergence distribution in a core having a specific radius, and an output lens having an effective focal length transforms this distribution to NA max It generates an output beam having NA. maxThis is determined by the ratio of the core diameter to the effective focal length. Furthermore, this method includes the step of transmitting the output beam to a process optical element, correcting the original numerical aperture of the input beam, and NA. max This ensures that truncation on the process optical element configured accordingly is prevented. Additional options for this method include having an input lateral spatial intensity distribution as Gaussian, super-Gaussian, flat-top, ring-shaped, saddle-shaped, or pedestal-shaped, and using a ring fiber as the source. The divergent storage fiber may also be a step-index fiber, and the optical beam transmission device may further include an input lens coupled to the source fiber and converting the input beam into an intermediate beam. The input lateral spatial intensity distribution may be variable or fixed. Further features may include coupling an output end cap to the output of the output lens, joining the divergent storage fibers such that the joint adjusts either the core or cladding diameter, and providing segments of the divergent storage fiber having potentially different core and cladding sizes by including an FFC or FFS within the divergent storage fiber. This method may also include one or more CLS along the divergent storage fiber or near the joint.

[0035]

[0035] NA maxRegarding, the optical beam transmission system or method may further include a configuration in which one or both of the input and output lenses are GRIN lenses having a length of 1 / 4 + 1 / 2×n pitch, where n is zero or any positive integer, or a configuration in which the lens is joined to a divergence-preserving fiber, a configuration in which the lens is a ball lens, or a configuration in which the lens is a rounded end cap. Additionally, the input and output lenses may have a common effective focal length, or alternatively, the input lens and the output lens may have different effective focal lengths selected to expand or contract the output beam. Similarly, the method may include steps of aligning with these configurations, and potentially changing the NA max based on the new core diameter or effective focal length of the replacement divergence-preserving fiber and the accompanying lens. The accompanying lens may be a GRIN lens, a ball lens, or a rounded end cap. Further consideration may involve steps of adjusting the dimensions of the core or cladding of the divergence-preserving fiber to individually form the optical properties as desired.

[0036]

[0036] In one aspect, the optical beam transmission system is designed to supply an output beam having an NA max This system features a source fiber that transmits the input beam, and this source fiber includes a first portion with less divergence and a second portion with more divergence. The system also includes a fiber assembly comprising a step-index fiber having a core and a cladding, an input lens, and a CLS. The input lens can be a single or double GRIN lens having a length of 1 / 4 pitch and possible multiples of 1 / 2 pitch, and is positioned between the source fiber and the step-index fiber. This directs the first portion of the light into the core and the second portion into the cladding. Then, the CLS removes the second portion from the cladding and directs the first portion within the core to an NA maxThis allows the output beam to be used as having the following characteristics. In addition, this configuration may also include a second GRIN lens having a smaller core size than the first GRIN lens and positioned between the first GRIN lens and the step-index fiber. The core of the step-index fiber may be larger than that of the source fiber. The system may also feature an input lens and CLS housed together in a single mechanical package. Further technical details are presented in the accompanying drawings, description, and claims.

[0037]

[0037] In another embodiment, an optical beam transmission system is designed to form a flat-top divergent distribution. This system includes a source fiber that supplies an input beam, which is first received by a first lens. This lens converts the input beam into an intermediate beam. A divergent preservation fiber equipped with a mode distribution homogenization section then modifies the intermediate beam to produce a flat-top intensity distribution. A second lens then deforms this distribution into an output beam having a desired flat-top divergent distribution. In addition, the system may include various configurations in the mode distribution homogenization section, such as a skewed junction, a mode-scrambling section that can extend throughout the fiber, or a non-circular core for the same purpose. The divergent preservation fiber may also include one or more of a circular core, micro-bends, coils, or macro-bends, each contributing to the homogenization of the mode distribution.

[0038]

[0038] The method relating to this system involves several important steps. The input beam is directed to a first lens, which maps the input lateral spatial intensity distribution of the beam to the divergence distribution of the intermediate beam. A divergence-preserving fiber then guides the beam through its homogenized portion to obtain a flat-top intensity distribution. The flat-top intensity distribution is converted by a second lens to an output lateral spatial intensity distribution, forming an output beam with a specified divergence. The method also involves firing the modified beam toward a process head, ensuring that the beam maintains its shape on the workpiece at various distances.

[0039]

[0039] Furthermore, this system can be adapted to input beams having various lateral spatial intensity distributions, such as Gaussian or flat-top types. It can also use GRIN lenses, ball lenses, or rounded end caps in its optical configuration. Additional elements may include lenses with different effective focal lengths to enlarge or reduce the beam, output end caps, or splines in divergence-preserving fibers to adjust the diameter of the core and cladding.

[0040]

[0040] In one embodiment, the system can be configured to transmit a divergent distribution using a step-index fiber having an azimuthal asymmetric core shape. A lens receives the initial beam from this fiber and converts it into a modified beam having a divergent distribution. The asymmetric core may be square, rectangular, or may include multiple circular cores. A collimator lens may also be part of the system to deform the modified beam into a parallelized beam having a specific near-field shape and a defined large depth of focus.

[0041]

[0041] Other technical features will also be readily apparent to those skilled in the art from the following figures, description and claims. Other objectives, features, and advantages will become even more apparent from the following detailed description. The detailed description proceeds with reference to the accompanying drawings, which may not be drawn at the same scale.

[0042]

[0042] In any discussion of any particular element or act, for ease of identification, one or more of the most significant digits of the reference number shall indicate the figure number in which the element is first introduced. [Brief explanation of the drawing]

[0043] [Figure 1] A side view of an optical beam transmission system according to the first embodiment of an image relay. [Figure 2] Annotated block diagram of an optical beam transmission system according to a second embodiment of an image relay fiber assembly. [Figure 3] An annotated cross-sectional view of an optical beam transmission system showing an end view of an input fiber option according to a third embodiment of the image relay. [Figure 4] A table showing a comparison of measured output BPP for an example of an image relay implementation. [Figure 5] This is an annotated cross-sectional view of an optical beam transmission system (top) according to a first embodiment of a nonlinear optical effect reduction fiber assembly, along with a graph of the effective mode area for each fiber segment in the system (bottom). [Figure 6] A pair of side views comparing a conventional step-index transmission fiber with a nonlinear optical effect reduction fiber assembly, along with their corresponding near-field spatial profiles (right), demonstrating that, according to one embodiment, each forms a near-field beam diameter of approximately 50 microns. [Figure 7]Figure 6 shows a plot of experimental results for the system, demonstrating a reduction in stimulated Raman scattering (SRS) power for the fiber assembly compared to conventional methods. [Figure 8] An annotated cross-sectional view of an optical beam transmission system showing end views of input and intermediate fiber options according to a second embodiment of a nonlinear optical effect reduction fiber assembly. [Figure 9] A block diagram of a transmission fiber and process head according to one embodiment. [Figure 10] Figure 1 shows two near-spatial profiles (left) and their corresponding divergence distributions (enclosed power vs. NA, right) obtained by two embodiments of the optical beam transmission system. [Figure 11] Side view of an optical beam transmission system according to a divergence limiting embodiment. [Figure 12] Figure 11 shows a set of induction near-field and far-field intensity distributions, as well as a one-dimensional profile, for the optical beam transmission system. [Figure 13] Figure 11 shows a blot (divergence distribution) of the encapsulation power versus NA for the optical beam transmission system. [Figure 14] Side view of an optical beam transmission system according to another divergence limiting embodiment. [Figure 15] Figure 14 shows a set of induction near-field and far-field intensity distributions, as well as a one-dimensional profile, for a light beam transmission system. [Figure 16] Figure 14 shows a plot of the enclosed power versus NA (divergence distribution) for the optical beam transmission system. [Figure 17] Side view of an optical beam transmission system according to a ring beam generation embodiment. [Figure 18] Figure 17 shows an end view of the source fiber. [Figure 19] A partial side view of an optical beam transmission system according to another ring beam generation embodiment. [Figure 20]A set of ray-tracing simulations based on the embodiment shown in Figure 19, illustrating the effect of fiber misalignment between the axes of the input fiber and fiber assembly. [Figure 21] End view of a fiber bundle according to a composite beam shape generation embodiment. [Figure 22] End view of a fiber bundle according to another composite beam shape generation embodiment. [Figure 23] End view of a multicore fiber according to another composite beam shape generation embodiment. [Figure 24] Another set of ray-tracing simulations based on one of the embodiments shown in Figures 20-23 when two cores are active. [Figure 25] A side view of an optical beam transmission system that selectively generates different composite beam shapes according to one embodiment. [Figure 26] Figure 25 shows the experimentally measured near-field intensity distribution for the optical beam transmission system, illustrating the two-dimensional (X,Y) intensity distribution (upper plot) and the corresponding one-dimensional (X,Y=0) beam profile (lower plot) at the system output when only the first input fiber is guiding the beam. [Figure 27] Figure 25 shows the experimentally measured near-field intensity distribution for the optical beam transmission system, illustrating the two-dimensional (X,Y) intensity distribution at the system output when only the second input fiber is guiding the beam (upper plot) and the corresponding one-dimensional (X,Y=0) beam profile (lower plot). [Figure 28] Figure 25 shows the experimentally measured near-field intensity distribution for the optical beam transmission system, illustrating the two-dimensional (X,Y) intensity distribution at the system output when only the third input fiber is guiding the beam (upper plot) and the corresponding one-dimensional (X,Y=0) beam profile (lower plot). [Figure 29]Figure 25 shows the experimentally measured near-field intensity distribution for the optical beam transmission system, illustrating the two-dimensional (X,Y) intensity distribution at the system output when only the fourth input fiber is guiding the beam (upper plot) and the corresponding one-dimensional (X,Y=0) beam profile (lower plot). [Figure 30] Figure 25 shows the experimentally measured near-field intensity distribution for the optical beam transmission system, illustrating the two-dimensional (X,Y) intensity distribution at the system output (upper plot) and the corresponding one-dimensional (X,Y=0) beam profile (lower plot) when only the first and fourth input fibers are inducing beams at different power levels. [Figure 31] Figure 25 shows the experimentally measured near-field intensity distribution for the optical beam transmission system, illustrating the two-dimensional (X,Y) intensity distribution at the system output (upper plot) and the corresponding one-dimensional (X,Y=0) beam profile (lower plot) when only the first and fourth input fibers are inducing beams at different power levels. [Figure 32] A block diagram of a transmission fiber and process head according to one embodiment. [Figure 33] The images on the left and right show the outputs of image relay fiber assemblies with a single circular core, as shown in Figures 1, 2, and 5, and a non-circular core, as shown in Figure 34, respectively. [Figure 34] A side view of an optical beam transmission system that generates a flat-top divergence distribution according to one embodiment. [Figure 35] A side view of an optical beam transmission system that generates a flat-top divergence distribution according to one embodiment. [Figure 36] A side view of an optical beam transmission system that generates a flat-top divergence distribution according to one embodiment. [Figure 37] A side view of an optical beam transmission system that generates a flat-top divergence distribution according to one embodiment. [Figure 38]This is a side view of an optical beam transmission system including a fiber assembly that generates a rectangular near-field beam shape, and shows a free-space lens that images and magnifies the fiber output so that the near-field beam has the intensity distribution present in the fiber core(s). [Figure 39] The images show the results of ray tracing simulations (left) and experiments (right), with the upper and lower columns (respectfully) representing the near-field and far-field spatial profiles of the octagonal beam shape in the far field. [Modes for carrying out the invention]

[0044] Introduction

[0082] Where used in this application specification and claims, the singular forms "a," "an," and "the" also include the plural form unless the context clearly indicates otherwise. In addition, the term "includes" means "comprises." Furthermore, the term "coupled" does not exclude the presence of intermediate elements between coupled items.

[0045]

[0083] The systems, apparatus, and methods described herein should not be construed as limitations. Rather, this disclosure directs all aspects of the various embodiments disclosed, whether individually, in various combinations, or in subcombinations thereof, to novel and non-obvious features. The disclosed systems, methods, and apparatus are not limited to any particular embodiment, feature, or combination thereof, and the disclosed systems, methods, and apparatus do not require the existence of any particular advantage or the solution of any particular problem. Any operating theories provided are for illustrative purposes only, but the disclosed systems, methods, and apparatus are not limited to such operating theories.

[0046]

[0084] In some methods of disclosure, the operations are described in a specific order for ease of presentation; however, it should be understood that this style of description also includes reordering unless a specific ordering is required by the specific wording expressed below. For example, operations described sequentially may, in some cases, be reordered or performed simultaneously. Furthermore, for simplification, accompanying drawings may not show various ways in which the disclosed systems, methods, and apparatus can be used in conjunction with other systems, methods, and apparatus. In addition, the description may sometimes use terms such as “produce” and “provide” to describe the disclosed methods. These terms are higher-level abstractions of the actual operations performed. The actual operations corresponding to these terms vary depending on the individual embodiment, but are readily identifiable to those skilled in the art.

[0047]

[0085] In some examples, values, procedures, or devices may be referred to as "lowest," "best," "minimum," etc. It should be noted that such descriptions are intended to indicate that a choice can be made among many functional alternatives, and that such a choice does not necessarily have to be superior, smaller, or otherwise preferable to other choices. When describing examples, directions are referred to as "above," "below," "upper," "lower," etc. These terms are used for explanatory convenience but do not imply any specific spatial orientation. Furthermore, in the following examples, laser components and assemblies are described at a high level of abstraction and do not include a complete description of all the mechanical, electrical, and optical elements necessary for their operation.

[0048]

[0086] As used herein, optical radiation refers to electromagnetic radiation at wavelengths between approximately 100 nm and 10 μm, and typically between approximately 200 nm and 2 μm. Examples based on available laser diodes and fiber laser sources, as well as optical fibers, are typically associated with wavelengths between approximately 800 nm and 2,000 nm. In some examples, propagating optical radiation is referred to as one or more beams, which may have diameter, asymmetric high-speed and low-speed axes, beam cross-section, and beam divergence, depending on the beam wavelength and the optical system used for beam shaping. For convenience, in some examples, optical radiation is referred to as light or a beam, and does not necessarily have to be at a visible wavelength. Forward-propagating light or optical beam or beam portion refers to light, a beam, or beam portion propagating in the direction of normal emission. Backward-propagating light or optical beam or beam portion refers to light, a beam, or beam portion propagating in the opposite direction to normal emission.

[0049] Overall view of the components

[0087] To understand the characteristics of the embodiments described in this disclosure, it is helpful to first understand the underlying components and the physical phenomena that are utilized with these components. Therefore, the following chapters summarize the various designs and properties of optical fibers and lenses.

[0050]

[0088] Optical Fiber: An optical fiber is a thin, flexible, thread-like material (usually glass) that guides light into a core surrounded by cladding. The core has a higher refractive index than the cladding and guides light by total internal reflection. The refractive index profile (RIP), i.e., the refractive index as a function of position across the fiber axis, determines many of the important properties of the fiber. Many, though not all, fibers are cylindrically symmetric (i.e., the refractive index is independent of the azimuthal angle around the fiber axis). Optical fibers can also have circular, square, rectangular, polygonal, oval, elliptical, or other cross-sections. In some embodiments, the core and cladding are nearly concentric. In other examples, one or more of the core and cladding are eccentric, and in some examples, the orientation and / or displacement of the core and cladding vary along the waveguide length.

[0051]

[0089] By generating and transmitting a laser beam within an optical fiber, fiber lasers not only eliminate free-space optical elements but also their associated mounts, adjustment mechanisms, beam tubes, and purge gas systems. A key trend in the ongoing development of fiber lasers has been the integration of additional functions within the optical fiber. An early example is the fiber plug grating, an in-fiber mirror used to construct a monolithic laser resonator without using any free-space beam (among its applications). More recently, fiber components have been developed to enable the merging of multiple fiber lasers for power adjustment up to multi-kW levels, polarization control, and beam size and shape control, i.e., tuning. All of these fiber components can be joined (fused) within the laser system to provide a sealed, stable, and matching-free optical system. While some advantages can be gained by replacing some free-space components with fiber components, the development of a monolithic, all-fiber system is required to maximize the practicality, reliability, and performance advantages of fiber lasers.

[0052]

[0090] Optical fibers are typically formed from silica (glass) with dopants added to increase or decrease the refractive index. The dopant concentration is varied laterally to produce the desired RIP (Rapid Isolation Process). In some cases, fibers or other waveguides are made from other materials such as zirconate fluoride, aluminate fluoride, fluoride glass or phosphate glass, chalcogenide glass, or crystalline materials such as sapphire, depending on the wavelength and other desired properties. The refractive index of silica and fluoride glass is typically around 1.5, while the refractive index of other materials such as chalcogenides can be 3 or higher. In yet another example, optical fibers can be formed partially or entirely from plastic (polymer).

[0053]

[0091] In some cases, a doped waveguide core, such as a fiber core, provides optical gain in response to excitation. In the examples disclosed herein, a waveguide core, such as an optical fiber core, is doped with one or more rare earth elements, such as Nd, Yb, Ho, Er, Tm, or other active dopants, or combinations thereof. Such an active-doped core can obtain optical gain in response to photoexcitation. Waveguides having such active dopants can be used to form optical amplifiers, or, provided that suitable optical feedback is provided, such as a reflective layer, mirrors, plug gratings, or other feedback mechanisms, such waveguides can generate laser emission. With respect to the propagation direction of the emitted laser beam or amplified beam, the photoexcitation radiation can be tuned to propagate in the same direction and / or in opposite directions within the waveguide.

[0054]

[0092] A feeding fiber is an optical fiber assembly joined to a laser, with a connector at its distal output end. This connector is typically plugged into a socket at the input to a processing optical element (e.g., a cutting or welding head, scanner, etc.). The processing optical element shapes the beam and transmits it toward the workpiece being processed. Alternatively, the beam from the feeding fiber can be emitted into a process fiber, which has connectors at both ends. A process fiber is an optical fiber assembly that connects the feeding fiber to the process optical element.

[0055]

[0093] The connection between the supply fiber and the process fiber can be made via an inter-fiber coupler (FFC, a device with integrated free-space optical elements that enables coupling between two optical fibers of the same or different dimensions) or an inter-fiber switch (FFS, a device with integrated free-space optical elements that allows programming to couple between one input fiber and two or more output optical fibers of the same or different dimensions).

[0056]

[0094] Step-Index Fibers: The most common type of RIP is known as a "step-index" fiber, in which the core has a uniform refractive index and is surrounded by cladding with a uniform (lower) refractive index. Within a step-index fiber, light emitted into the fiber core at a given position tends to diffuse radially and azimuthally, filling the core. That is, the core does not store spatial information about the position of the emitted beam. It is known that a ray emitted at a particular angle with respect to the fiber axis tends to emit from the fiber core at the same angle (conical due to azimuthal scrambling). That is, step-index fibers typically store angular information. Specifically, the storage of the emission angle (i.e., far-field divergence profile) at the output of a step-index fiber is a property utilized by embodiments of this disclosure. (Note that the terms far-field and near-field intensity distribution refer to the angular intensity distribution (with respect to the optical axis) and the transverse spatial intensity distribution, respectively). Various effects can impair the conservation of the emission angle (for example, by making the divergence of the output ray greater than that of the input ray), and several studies have investigated ways to minimize this effect.

[0057]

[0095] GRIN Fibers: Some fibers have non-uniform RIPs in the core and cladding (less frequently). A common non-uniform core RIP is known as a "refractive index distribution type" or "GRIN," where the refractive index decreases monotonically from the core center to the cladding. Often, the GRIN profile is parabolic. A parabolic GRIN fiber, if of appropriate length, can act as a lens to focus or parallelize a beam. This property is also used in free-space GRIN lenses, i.e., it is not limited to fibers. The size of a beam coupled to a parabolic GRIN fiber may fluctuate periodically as the beam propagates along the fiber, and this fluctuating period is known as the fiber's "pitch." The GRIN pitch is the physical length of the GRIN fiber over which the input ray completes its fluctuating period within the fiber. The fiber length corresponding to one sinusoidal period is defined in Equation 1.

[0058]

number

[0059] The radially symmetric RIP of a GRIN fiber can be described by this equation 2.

number

[0060]

[0096] Multi-clad fibers: Another example of non-uniform RIPs is a fiber with multiple claddings ("double-clad fibers," "triple-clad fibers," etc.). As with other non-uniform RIPs, the propagation of light within a multi-clad fiber varies depending on the specific RIP and emission or coupling conditions. Light emitted within a given region diffuses into all adjacent regions with higher refractive indices, but not into all adjacent regions with lower refractive indices (which act as cladding to the regions with higher refractive indices).

[0061]

[0097] Here, a single fiber design, referred to as a ring fiber, has multiple cores or induction regions. Specifically, a ring fiber has one or more annular cores concentrically surrounding a central (non-annular) core, with low-refractive-index glass layers separating the cores so that light coupled within a given core is guided within that core. The central core is surrounded by low-refractive-index (non-inductive) annular cladding, and the ring-shaped induction regions are bounded on the inner and outer circumferences by low-refractive-index annular cladding.

[0062]

[0098] Multicore Fibers: Some fibers have multiple cores, i.e., multiple high-refractive-index inductive regions separated by surrounding low-refractive-index regions. These cores can have different sizes and shapes. The ring fiber described earlier is an example of a multicore fiber. Light emitted into a given core of a multicore fiber typically diffuses outward and fills that core (as in a fiber with a single core), but does not diffuse into other cores. This is because the surrounding low-refractive-index regions (cladding) hinder propagation between cores. Multicore fibers can be used to generate different beam shapes (i.e., near-spatial profiles) by coupling different powers into each core. Compared to standard fibers (especially step-index fibers), multicore fibers have several disadvantages, including: (1) Multicore fibers are usually more expensive than standard fibers. (2) The low-refractive-index regions separating the inductive regions produce highly divergent light when a portion of the beam is coupled to them. Downstream optical elements must be designed to accommodate this excessive divergence, which typically increases costs, complexity, and / or optical losses (decreases efficiency). If not properly managed, this excessive divergence can cause overheating or damage to optical elements or other components. (3) The dimensions of the induction region are fixed when the fiber is manufactured; that is, the beam dimensions cannot be continuously varied (although the power in each region can be continuously varied). Furthermore, if different dimensions (e.g., a larger diameter ring) are preferred for a particular application, a new multicore fiber must be manufactured, which can lead to a significant increase in product design complexity (manufacturing becomes more complex and manufacturing costs increase). (4) The need to align multiple induction regions, each with its own manufacturing tolerance, complicates the joining of multicore fibers. Achieving the desired bonding performance may necessitate excessively tight fiber tolerances (leading to increased costs), low bonding yield, and / or performance degradation during bonding (e.g., increased divergence due to coupling light to a low refractive index region).

[0063]

[0099] It should be noted that previous discussions of different types of optical fibers mainly relate to multimode fibers, and that ray pictures of optical propagation are highly accurate. Single-mode or pseudo-single-mode fibers introduce other phenomena, but these phenomena are of little relevance to the embodiments described herein.

[0064]

[0100] Lenses: Lenses are present everywhere in optical systems. Traditionally, a lens has been considered to be a single piece of glass with one or both surfaces curved, and the propagation angle of light rays is affected by the refraction at these surfaces. Other embodiments of lenses are possible, including GRIN lenses (as described above) where the RIP of the material varies with radial position (instead of employing curved surfaces). The ends of a fiber, or the ends of an end / cap attached to a fiber, can have curved surfaces to function as lenses. One of the design properties of a lens is its focal length. A parallelized input beam is focused on a point one focal distance from the lens (in ray photography). Similarly, a point source located one focal distance from the lens is parallelized at the lens output. More generally, a lens performs a mapping between position and angle. That is, a lens converts the angle of a ray at the input focal plane to its position at the output focal plane, and further converts the input position of a ray at the input focal plane to its angle at the output focal plane.

[0065] Image relay for structured laser beam profiles

[0101] Industrial lasers with composite beam profiles (i.e., beams generated by coupling power to two or more induction regions of a multicore fiber) and / or variable beam profiles (i.e., beams with a variable lateral spatial intensity distribution) sometimes use ring fibers as feed fibers. In systems incorporating FFC or FFS, the process fiber is intended to preserve at least some aspects of the beam shape from the feed fiber. For example, if the feed fiber has a central core and a surrounding ring, the process fiber often also has a central core and a surrounding ring (perhaps of different dimensions than the feed fiber), and the light propagating through the core and ring of the feed fiber is coupled into the core and ring of the process fiber, respectively. When the FFC / FFS optical elements image the beam on the input of the process fiber (magnification ~1), they can be designed to preserve the size of the induction regions to a certain extent, or they can also enlarge or reduce the beam (by corresponding changes in divergence). Process fiber RIPs are designed to adapt to the beam shape and divergence characteristics from the supply fiber, accommodate any variations imparted by the FFC / FFS optical elements, and further address any drawbacks associated with fiber tolerances and the optical performance and matching of the FFC / FFS optical elements. Ring fibers with multiple NAs and dimensional tolerances can be difficult to produce, and cumulative tolerances and system drawbacks (e.g., mismatches) may unnecessarily alter the beam characteristics at the process fiber output. For example, when the output of a first ring fiber (supply fiber) has an FFC or FFS and a ring beam is desired at the output of the system (FFC / FFS and process fiber), sometimes the second ring fiber (process fiber) may deviate relative to the beam from the first ring fiber, resulting in truncation on the low refractive index (e.g., fluorosilicate) ring of the second ring fiber. This can unnecessarily increase the divergence or NA of the beam emerging from the second ring fiber.This increase in the numerical aperture (NA) of the output laser beam hinders the use of several conventional process optics, thus significantly reducing the number of potential applications for such beams. In some cases, apertures may be used to reduce the NA of the output beam. However, this method reduces the output power of the beam, which is undesirable in certain applications. In addition, the light may be coupled from one core of the supply fiber to different cores of the process fiber (e.g., from the central core to the ring, or vice versa), thus unnecessarily altering the beam shape.

[0066]

[0102] Figure 1 shows an optical beam transmission system 100 including a laser source 102 and a fiber assembly 104. Figure 1 shows different options for the laser source 102, including a step-index fiber 106, a free-space optical element 108 (e.g., FFC or FFS), a fiber bundle 110, or a ring fiber 112. When the fiber assembly 104 is a fiber coupled to the source fiber, the fiber assembly 104 acts as a feed fiber (i.e., attached to the source). When the beam is emitted from free space into the fiber assembly 104, the fiber assembly 104 acts like a process fiber (i.e., detachable from the optical element). Of course, the fiber assembly 104 can also be a fiber coupled to a conventional fiber at the output of an FFC / FFS, in which case the fiber assembly 104 becomes part of the process fiber. The term fiber coupled includes not only direct physical connections but also somewhat spaced connections, i.e., connections of one Rayleigh length or within that range. Rayleigh length is the distance along the beam's propagation direction from the waist to the point where the cross-sectional area doubles.

[0067]

[0103] In the example shown in Figure 1, the laser source 102 is a ring fiber 112 coupled to an image relay fiber input 114, and the fiber assembly 104 supplies the output optical beam 118 at its image relay fiber output 116. The output optical beam 118 has an image of the output of the laser source 102 (i.e., it maintains the beam shape of the laser source 102 and includes scaling up or down as needed).

[0068]

[0104] As noted earlier, the ring fiber 112 may be a component of a VBC laser (perturbation device and other fiber components not shown), which supplies the input optical beam 122 at its output 120. The input optical beam 122 has a lateral spatial intensity distribution (as well as other beam shapes) in the form of a variable-structured near-field profile, e.g., a tunable saddle mode or ring mode, and this distribution is generally azimuthal symmetric about the optical axis. To supply a variable beam, the ring fiber 112 includes a centrally located core 124, a first coaxial anti-guiding (cladding) region 126, and a first annular core 128. The ring fiber 112 also includes a second coaxial anti-guiding (cladding) region 130, which may include an optional CLS. Further details of the VBC fiber are described in the '768 patent. The input optical beam 122 may be supplied by the ring fiber 112, but the ring fiber is not essential. In other embodiments, the laser source 102 may also supply a fixed, non-adjustable beam.

[0069]

[0105] The fiber assembly 104 includes a divergent preservation fiber 132 (also called an angular or divergent distribution preservation fiber), or a series of such fibers. The divergent preservation fiber 132 is joined to both sides of a subsequent GRIN fiber lens, i.e., between the first GRIN fiber 134 and the second GRIN fiber 136. The output 138 of the first GRIN fiber 134 is coupled to the input 140 of the divergent preservation fiber 132. The output 142 of the divergent preservation fiber 132 is coupled to the input 144 of the second GRIN fiber 136. Note that multiple segments or sections of the fiber assembly 104 may include combinations of different parts to achieve the desired function. For example, as will be explained in more detail below, the divergent preservation fiber 132 may include a joint, which has different parts (i.e., different RIPs or dimensions) on each side.

[0070]

[0106] The spatial distribution of the input optical beam 122 incident on the image relay fiber input 114 is converted to a divergent distribution by the first GRIN fiber 134 (by lens properties and design as described below) to form an intermediate beam 146. The intermediate beam 146 is fed to the input 140 of the divergent storage fiber 132. The divergent storage fiber 132 carries the intermediate beam 146 (along with the roughly stored divergent distribution) to the output 142, which is incident on the input 144 of the second GRIN fiber 136. The second GRIN fiber 136 then converts the angular distribution of the intermediate beam 146 to a spatial distribution and effectively refocuses it to form a refocused beam 148. The refocused beam 148 is fed to the image relay fiber output 116. The refocused beam 148 is an image scrambled in the azimuthal direction of the input optical beam 122, and therefore, in this disclosure, the fiber assembly 104 is referred to as the image relay fiber. In other words, the refocused beam 148 is formed by injecting the input optical beam 122, which has known or identifiable near-field and far-field distributions, into the step-index core 150. At this point, the angular distribution of the step-index core 150 with respect to the optical axis is preserved, while the spatial and azimuthal angular distributions are scrambled as it propagates.

[0071]

[0107] If the first GRIN fiber 134 and the second GRIN fiber 136 each have lengths equal to (1 / 4 + 1 / 2n) pitch (where n is zero or any positive integer), the fiber assembly 104 acts similarly to a relay imaging telescope (by azimuthal scrambling of the input beam), thereby forming an azimuthal scrambled image of the input optical beam 122 in the output optical beam 118. If the first GRIN fiber 134 and the second GRIN fiber 136 have the same effective focal length (see, for example, the symbol "f" attached to the second GRIN fiber 136), the output spatial profile is an inverted and azimuthal scrambled image of the input spatial profile. If the spatial profile of the input optical beam 122 is azimuthal symmetric, the spatial profile of the output optical beam 118 is an inverted image of the input beam (similar to the function of a free-space optical telescope). In other embodiments, the beam profile at the image relay fiber output 116 can also be imaged as an enlarged or reduced version of the input by using a second GRIN fiber 136 with a shorter or longer effective focal length, respectively. For example, an input GRIN with a gradient constant of 2.0 and an output GRIN with a gradient constant of 0.5 can yield a near-field beam diameter approximately twice as large. Similarly, an input GRIN with a gradient constant of 0.5 and an output GRIN with a gradient constant of 2.0 can yield a near-field beam diameter approximately 0.5 times larger. Depending on the application, gradient constants between 0.1 and 50 are also possible.

[0072]

[0108] For the divergent preservation fiber 132, if the ratio of the cladding diameter to the core diameter is 2 or greater, mode coupling in the fiber is reduced, leading to improved preservation of the angular distribution along the length of the fiber. In one embodiment, the diameters of the cores (not shown) of the first GRIN fiber 134 and the second GRIN fiber 136 are selected to match those of the step-index core 150 of the divergent preservation fiber 132. In other embodiments, however, the core diameters of the divergent preservation fiber and the GRIN core diameter do not need to match. For example, in one embodiment, if the step-index core 150 is smaller than that of the output GRIN core, imaging is improved (aberrations are reduced).

[0073]

[0109] The first GRIN fiber 134 and the second GRIN fiber 136 can have different core diameters while maintaining the same gradient constant. For example, in one embodiment, the GRIN with the larger diameter may be used for output. In other embodiments, when both the gradient constant and core diameter are the same, the two GRIN fibers can have the same NA.

[0074]

[0110] In one embodiment, the lengths of both the first GRIN fiber 134 and the second GRIN fiber 136 are each equal to 1 / 4 of the pitch of the GRIN lens. Alternatively, both the first GRIN fiber 134 and the second GRIN fiber 136 may include lengths of 1 / 4 pitch and any integer multiple of 1 / 2, i.e., (0.25 + 0.5 × n) pitch, where n is zero or any positive integer (0, 1, 2, ...).

[0075]

[0111] Figure 1 also shows examples of RIPs for a first GRIN fiber 134, a divergent preservation fiber 132, and a second GRIN fiber 136. For example, the first GRIN fiber 134 and the second GRIN fiber 136 each have a parabolic RIP 512. This parabolic RIP 152 may be similar for each fiber subject to typical manufacturing tolerances and imaging design optimization. The divergent preservation fiber 132 includes a step-index RIP 154 formed by a high refractive index step-index core 150 surrounded by cladding 156. In other embodiments (not shown), the divergent preservation fiber 132 includes a step-index RIP formed by a high refractive index core surrounded by a low-doped, uninductive annular region and an outer cladding region.

[0076]

[0112] As shown in the partial diagram of Figure 1, an optional junction 158 is included within the divergent preservation fiber 132. Including the junction within the divergent preservation fiber 132 is advantageous for manufacturing and field service. For example, it is common to assemble subassemblies and join them together to form an optical beam transmission system 100. Different products or configurations can correspond to different sets of subassemblies. That is, different combinations of laser sources 102 and fiber assemblies 104 (first GRIN fiber 134, divergent preservation fiber 132, and second GRIN fiber 136) will result in different output beam sizes and divergences. Including an optional junction 158 within the fiber makes it possible to generate different output beam parameters by selecting different output "pigtails" (i.e., divergent preservation fiber 160 and second GRIN fiber 136). An example of an expanded pigtail will be described later with reference to Figure 2. More generally, a large number of possible combinations are possible with a minimum number of subassemblies.

[0077]

[0113] Furthermore, the optional joint 158 ​​allows for relatively inexpensive repairs, minimizing yield loss. For example, if the pigtail (i.e., the divergent storage fiber 160 and the second GRIN fiber 136) is damaged, only the pigtail needs to be replaced, rather than the entire fiber assembly 104. Similarly, including the optional joint 158 ​​is advantageous for service (on-site or in the warehouse). Burning of the end cap surface on the supply fiber is relatively common. As noted earlier, the cost and complexity of the parts that need to be replaced are minimized by including the optional joint 158 ​​within the divergent storage fiber 132 so that the FRU becomes the pigtail. Then, if the customer or end-user wants to change their laser configuration (different spot size, divergent, or both), the FRU can be replaced relatively easily and inexpensively on-site at the customer's or end-user's location.

[0078]

[0114] In addition, the optional splicer 158 facilitates core diameter step-up or step-down, allowing for adjustment of splice tolerances and fiber tolerances, simplifying the splicing process and increasing yield. Step-up means that the core diameter of the divergent preserved fiber 160 is larger than that of the divergent preserved fiber 132. Step-down means that the core diameter of the divergent preserved fiber 160 is smaller than that of the divergent preserved fiber 132. Fiber core diameters may vary slightly between batches and within batches. If this variation causes a step-down at the splice, it may result in increased optical loss or divergence (due to the coupling of light from the core to the cladding at the splice). Similarly, the splicer has a matching tolerance, which may cause imperfect matching of the fiber core at the splice, which may also result in increased optical loss or divergence. By incorporating a step-up in the core diameter at the optional joint 158, these problems are minimized or eliminated, ensuring a high yield at the joint and minimizing any degradation of optical performance or reliability from the joint. For example, a step-up of approximately 2–10 μm is typical for some applications, but larger step-ups are also possible and may be desirable (depending on the desired power beam characteristics).

[0079]

[0115] In other embodiments, in addition to or separately from any change in core diameter, the cladding diameter can also be stepped up or down at an optional joint 158. Stepping up means that the outer diameter of the divergent preservation fiber 160 is larger than that of the divergent preservation fiber 132. Stepping down means that the outer diameter of the divergent preservation fiber 160 is smaller than that of the divergent preservation fiber 132. The inner diameter is determined by the core size and can also be adjusted at the optional joint 158 ​​as previously described. For example, stepping down the cladding can be desirable for reliability because light reflected backward (e.g., from the workpiece) remains in the cladding and is not lost at the optional joint 158, or is bonded to the polymer (i.e., fiber buffer or potting around the optional joint 158, not shown), and light reflected backward can cause damage.

[0080]

[0116] In one embodiment, light emitted into the cladding 156 of the divergent storage fiber 132 is removed by an optional CLS 162. This technique enables the removal of high NA light in the optical beam transmission system 100, rather than in downstream optical elements (e.g., process heads, collimators, or scanners). In one embodiment, an optional CLS 164 is included at the input 140 of the divergent storage fiber 132. Another option is to include the CLS 166 at the output of the laser source 102. And yet another option is to include the CLS 168 on one or both sides of an optional junction 158 or an optional FFC / FFS 170. Depending on the presence of cladding light at different locations, various CLS options can also be combined.

[0081]

[0117] An optional FFC / FFS 170 may be placed along the divergent preservation fiber 132 (e.g., two step-index fibers separated by a coupler or switch). The two divergent preservation fibers at the input and output of the FFC / FFS may have the same or different core diameters and cladding diameters. The output characteristics of the beam transmitted in this configuration can be controlled by the selection of the FRU, i.e., by the design of the fiber assembly downstream of the FFC / FFS (specifically, the core diameter of the divergent preservation fiber and the focal length of the GRIN lens). This fiber assembly can also function as a process fiber. This embodiment using the FFC / FFS enables the transmission of a tangible beam without the need to image the beam shape onto a process fiber input and without requiring a multicore process fiber (e.g., from a multicore fiber or other sources described below). This offers significant advantages in fiber manufacturing tolerances, beam matching tolerances, and the performance of the free-space optical elements in the FFC / FFS, and prevents the generation of high NA tails in the output beam from the process fiber. Furthermore, fiber assemblies comprising process fibers (i.e., divergent preservation fibers + GRIN lenses + optional end caps) may include junctions that step up or down the core diameter and / or cladding diameter, thereby providing some of the advantages discussed earlier in the context of junctions in the supply fibers.

[0082]

[0118] As explained earlier, the first GRIN fiber 134 maps position to angle. Similarly, the second GRIN fiber 136 maps angle to position. These two fibers, together with the divergence-preserving fiber 132, form a monolithic fiber assembly with various use cases, which will be described in more detail below. Other options for lenses used in other monolithic fiber assemblies are shown in Figure 1, such as rounded end caps and ball lenses. Figure 1 also shows that free-space optical elements may be used as substitutes for monolithic fiber assemblies.

[0083]

[0119] Figure 2 shows another embodiment of the optical beam transmission system 200. The optical beam transmission system 200 includes a source fiber 202 and an image relay fiber assembly 204 which can be directly bonded to the source fiber 202, as already described. The image relay fiber assembly 204 includes components similar to those shown in fiber assembly 104, and also includes an end cap 206. In both embodiments, a 1 / 4 pitch GRIN fiber can also be directly bonded to the end cap 206. The purpose of the end cap 206 is to increase the power handling capacity of the supply fiber or process fiber (by expanding the beam before it encounters the glass / air interface), and the output surface of the end cap may be coated with an anti-reflective coating. The input near-field beam profile 208 shown in Figure 2 can also be formed by a ring fiber.

[0084]

[0120] Figure 2 also shows examples of beam intensity profiles 210 at various Z-axis positions along the optical axis of the image relay fiber assembly 204. Each of the beam intensity profiles 210 shows the intensity as a function of the X position at the selected Z-axis position.

[0085]

[0121] The input near-field beam profile 208 includes a structured near-field intensity profile with a centrally located optical power 212, surrounded by a non-inductive annular region, i.e., cladding (see, e.g., the first coaxial non-inductive region 126 in Figure 1), and a centrally located core (see, e.g., the centrally located core 124 in Figure 1). Annularly defined optical power 214 is supplied in the annular core (see, e.g., the first annular core 128 in Figure 1). Since the annularly arranged optical power 214 is more intense than the centrally located optical power 212, the input near-field beam profile 208 becomes a so-called saddle mode. However, from the viewpoint of this disclosure, it will be recognized to those skilled in the art that a saddle or ring beam is shown as an example. Other input beam profiles are also possible, including beams from single-core and other multi-core fibers. Furthermore, beam shapes generated by the VBC system of the '768 patent may also be used for the beam intensity profile 210. If the fiber assembly is designed to expand the beam, it can result in a reduction in divergence.

[0086]

[0122] As already explained, the input near-field beam profile 208 is transformed by the first GRIN fiber 216 into an intermediate profile 218 (shown as Gaussian shape, this is the case when the input angular distribution to the first GRIN fiber 216 is Gaussian). As the intermediate profile 218 propagates through the divergence-preserving fiber 222, it can spatially expand into other intermediate profiles 220 (e.g., by diffusing to fill the core and / or by scrambling in the azimuthal direction), but the divergence distribution is largely preserved. The intermediate profile 220 is coupled to the second GRIN fiber 224, which refocuses the intermediate profile 220 to form the output beam profile 226. In this example, the output beam profile 226 is an expanded version of the input near-field beam profile 208, but expansion is optional in other embodiments. Furthermore, the input near-field beam profile 208 may be stationary or variable depending on the input beam.

[0087]

[0123] As already described, the downstream portion of the joint 228 can also be used as a field-replaceable part. For example, in some VBC systems, the end caps of the ring fibers may become damaged. If the ring fibers have a supply fiber (i.e., are bonded to the laser output) and are directly coupled to a process optical element (i.e., there is no replaceable process fiber), damage to the end caps of the ring fibers may necessitate replacing the entire VBC system because it is difficult to accurately align and bond the two ring fibers together without causing undesirable changes or degradation to the beam properties. Therefore, the image relay fiber assembly 204 and the joint 228 in the divergent storage fiber 222 facilitate the bonding of step-index fibers, which can be performed as a field operation without significantly altering the beam properties. Furthermore, a CLS (not shown) may be included in the divergent storage fiber 222. It will be recognized by those skilled in the art that optional joints, CLS, or both structures can also be included in any other embodiments described herein. Similarly, as already explained, by incorporating FFC or FFS along the divergent storage fiber 222, in-situ replacement of the process fiber (downstream of the FFC or FFS) becomes possible, and this too can be performed as a field operation.

[0088]

[0124] Custom beam shapes for enhancing cutting and welding performance are shown. Image relay fiber assemblies utilizing distributed refractive index (GRIN) fibers (see, e.g., Figures 1 and 2) are shown to shape the output near-field and far-field beam intensity distributions to suit specific laser processing applications. For example, a 2.1 × GRIN relay assembly (i.e., input GRIN, step-index fiber, and output GRIN) is shown to increase the maximum cutting speed of 1-inch mild steel compared to a conventional 100 μm laser using both inert and fusion cutting methods. It should be noted that mode mixing in the step fiber between the two GRIN fibers results in an increase in the laser output beam parameter product (BP). This may affect cutting performance at certain metal thicknesses and may not be optimal for non-cutting applications (e.g., remote welding). In some embodiments, when the CLS truncates some of the image of the highly diverging input beam to maintain a reasonably low BPP, this typically results in a power loss of about 1–3%.

[0089]

[0125] Figure 3 shows another optical beam transmission system 300 that avoids the aforementioned power loss while maintaining BPP. This achieves expansion and contraction of the input beam without causing significant changes in the shape of the intensity distribution. In previous embodiments, an intermediate step-index fiber facilitates azimuthal scrambling. In some other applications (e.g., when the input beam is already azimuthal symmetric), however, the optical beam transmission system 300 employs an input beam 302 supplied from a source fiber 304 (e.g., a step-index fiber 306 or a ring fiber 308) to a first GRIN lens 310. The first GRIN lens 310 is directly coupled to a second GRIN lens 312, i.e., they are back-to-back lenses. Alternatively, the input beam 302 can be obtained from a VBC fiber.

[0090]

[0126] The first and second GRIN fibers 310 and 312 are similar to those already described with reference to Figures 1 and 2, and act to expand or contract the near-field intensity distribution of the input beam 302 while preserving the input beam BPP at the output of the second GRIN lens 312. For example, both the first GRIN lens 310 and the second GRIN lens 312 have a pitch length of 0.25 + 0.5 × n. When these GRIN fibers are given different effective focal lengths (EFLs) (for example, ranging from approximately 0.1 mm to approximately 2.0 mm), the input beam to the first GRIN lens 310 is expanded or contracted, from the smallest change to the shape of the input intensity distribution. To expand the near-field intensity distribution, the effective focal length of the second GRIN lens 312 is made longer than that of the first GRIN lens 310. To contract the near-field intensity distribution, the effective focal length of the first GRIN lens 310 is made longer than that of the second GRIN lens 312. Furthermore, it is possible to achieve magnification ranges of less than approximately 0.5x and greater than approximately 3.0x.

[0091]

[0127] The advantage of this type of in-fiber adjustment is that, for example, in the case of in-fiber magnification, it can be used as a substitute for a relatively large magnification cutting head, in which case a smaller cutting head can be used. The advantage of a smaller cutting head is that its mass is inevitably reduced, and the mechanical forces generated when rapidly accelerating and stopping while moving during laser processing are also reduced. Other examples of applications where the miniaturization of the near spot size achieved by reduction is useful include cutting thin metals and drilling metals.

[0092]

[0128] Figure 4 shows Table 400 of experimentally measured output BPP values ​​for each of the step-index fiber 402, optical beam transmission system 100, and optical beam transmission system 300 in a 2.1x expansion configuration. For optical beam transmission system 100, the BPP is expanded by approximately 1.5 times (from 3.6 to 5.5), while optical beam transmission system 300 retains 3.6 BPP from the input source.

[0093]

[0129] In one embodiment, the optical beam transmission system 100 generates a flat-top divergence distribution, which has several useful applications. In contrast, the optical beam transmission system 300 maintains (expands or contracts) the original shape of the divergence distribution. Furthermore, because the optical beam transmission system 100 has a core of 125 μm, it generates less SRS. This will be explained below with reference to Figure 5.

[0094] SRS reduction

[0130] High-power industrial fiber lasers and fiber-coupled lasers are susceptible to nonlinear optical effects within the fiber, such as SRS, stimulated Brillouin scattering (SBS), four-wave mixing (FWM), self-phase modulation (SPM), and thermal mode instability (TMI). These nonlinear optical effects can negatively impact the performance of the laser system by reducing laser signal power, broadening the laser linewidth, or inducing fluctuations in power, near-field beam profile, and / or divergence profile. For example, SRS (often the most detrimental nonlinear optical effect in high-power industrial fiber lasers) shifts a portion of the light beam to longer wavelengths. This is often unhelpful for the process and can even be detrimental to the process or optical elements.

[0095]

[0131] The proportion of power converted to SRS wavelength increases with increasing power and fiber length, as well as decreasing core diameter, effectively reducing the power × length / diameter value. To avoid excessive SRS power at a given laser power, the transmission fiber length must be limited (which can create practical problems in tool design) or the core diameter must be increased (which can result in reduced processing capability due to decreased power density or brightness). SRS is becoming increasingly problematic in industrial fiber and fiber-coupled laser systems employing high power (multiple kW), small beams (often less than 100 microns), and / or long distances between the laser and process optics (usually longer than 10 m, but sometimes less). For industrial laser applications of high-power lasers, it is sometimes desirable to use long feed or process fibers with relatively small cores. Because optical fibers have small core diameters, nonlinear optical processes reduce the fiber's ability to transmit light over long distances. For example, large laser cutting tools may employ feed fibers longer than 20 meters. A 20kW laser with a 100μm core supply fiber, however, may generate significant SRS (Sampling Reduction Syndrome).

[0096]

[0132] The critical SRS threshold is defined as the input power at which the SRS signal power equals the laser signal power at the fiber output. As described in *Nonlinear Fiber Optics* by G. Agrawal (6th edition, page 301), the critical threshold power for generating SRS in a fiber laser is given by the effective mode area (A) as shown in Equation 3. eff ) and effective fiber length (L eff It is proportional to the ratio of ).

[0097]

number

[0098]

[0133] A effThis is obtained from the transverse spatial integral with the beam intensity distribution and is equivalent to the area of ​​a top-hat beam that produces the same SRS power as the actual beam profile. eff This includes the effects of beam amplification, absorption, and scattering, based on the length of the fiber over which the SRS process occurs. Parameter A eff and L eff These are directly related to the fiber core diameter and length, respectively. In other words, the threshold power for SRS generation increases as the effective mode area (core size) increases and decreases as the fiber length increases.

[0099]

[0134] This threshold definition is higher than what is acceptable in most industrial fiber laser applications. The maximum SRS tolerance may be 10% or less, or even 1% or less depending on the application. Certain applications have other critical SRS thresholds (i.e., fractional values ​​of the maximum allowable SRS power), e.g., 0.1, 0.01, or less. Regardless of the numerical value of the SRS threshold, the threshold is A, as shown in Equation 3. eff and L eff It increases or decreases along with [other factors]. In practice, the SRS ratio is usually measured experimentally using a spectrometer or optical filter. In high-power CW fiber lasers used in industrial applications, SRS is a particular concern, resulting in practical design limits for certain combinations of output power, fiber length, and core diameter (near-field beam size). Similarly, for pulsed lasers with high peak intensity, peak power, fiber length, or core size are often limited due to SRS.

[0100]

[0135] Fiber tapers are used to increase the achievable laser power or transmission fiber length before excessive SRS occurs. In contrast to most fibers, which have a constant diameter along the fiber axis, fiber tapers gradually increase or decrease the fiber diameter to lengthen or shorten it, respectively. Increasing the fiber core diameter increases the diameter of the beam propagating within this core, and correspondingly decreases beam divergence. Similarly, a down-taper shortens the beam diameter and correspondingly increases beam divergence. If the taper is "adiabatic" (i.e., sufficiently progressive and defect-free), the product of beam diameter and divergence (known as "luminance") is conserved. For non-adiabatic tapers, luminance decreases. If a transmission fiber gradually increases at its input end (near the laser source) and gradually decreases at its output end (near the process optics), and the taper is adiabatic (or nearly adiabatic), the beam diameter increases over most of the fiber, but the beam diameter and brightness are conserved (or nearly conserved) at the fiber output. As the beam diameter increases, the value of power × length / diameter decreases, allowing for either or both increased power and / or increased fiber length before excessive SRS occurs, according to Equation 3. Disadvantages of this method include the potential difficulty in manufacturing the taper (higher cost and / or reduced reliability), and the fact that the minimum bending radius increases with increasing fiber diameter (which can be a problem in practice).

[0101]

[0136] As an example where a taper is not used, Figure 5 shows an optical beam transmission system 500. This system 500 replaces a typical supply fiber having an input effective mode area with a fiber assembly 502 in which the core size is increased along its entire length and the effective mode area is widened, effThis increases the SRS threshold (thus increasing the SRS threshold compared to conventional transmission fibers). This technique allows for longer supply fibers or smaller output beam diameters before nonlinear effects become a problem. This is useful in both pulsed and CW lasers, in either supply or process fibers. For example, when fiber assembly 502 is joined to a source fiber, it acts as a supply fiber (i.e., attached to the source) and increases the SRS threshold. When the beam is emitted from free space into fiber assembly 502, it acts like a process fiber (i.e., detachable from the optical element) and increases the SRS threshold. Of course, fiber assembly 502 can also be joined to a conventional fiber at the output of an FFC / FFS to increase the SRS threshold, but in this case, fiber assembly 502 becomes part of the process fiber. In any of these scenarios, this technique is suitable for applications using high peak or average power, long supply or process fibers, and / or small core diameters.

[0102]

[0137] The fiber assembly 502 includes a first GRIN fiber 504, a second GRIN fiber 506, and a divergent storage fiber 508 joined between the first GRIN fiber 504 and the second GRIN fiber 506. Thus, the fiber assembly 502 has a structure similar to that of the fiber assembly 104 (Figure 1) and the image relay fiber assembly 204 (Figure 2). The integration of the fibers in the fiber assembly 502 reproduces the input beam diameter from the source fiber 510 at the output 512 of the second GRIN fiber 506, which can be connected to the end cap 514. However, in contrast to the ring fiber as a source fiber as described above, the optical beam transmission system 500 includes a single-core (e.g., step-index) source fiber 510, whose output 516 is joined (or free-space coupled) to the input 518 of the first GRIN fiber 504. In other embodiments, a ring fiber or other multicore fiber may be used as the sous fiber 510.

[0103]

[0138] Each of the first GRIN fiber 504 and the second GRIN fiber 506 can be split and joined at a 1 / 4 pitch (typically less than 10 mm, for example), and each can also be optimized for imaging (a power law distribution with an alpha parameter of approximately 2, where alpha is shown as the value 2 in the exponent of Equation 2). Alternatively, each GRIN fiber length may have a pitch of 1 / 2n + 1 / 4, where n is zero or any positive integer (0, 1, 2, ...).

[0104]

[0139] The divergent storage fiber 508 has a larger core diameter compared to the source fiber 510, which expands the effective mode area. In other words, the embodiment in Figure 5 acts to increase the SRS threshold compared to conventional transmission fibers. For example, a core diameter of 200 μm may be a suitable size for the divergent storage fiber 508 (Figure 5) when the diameter of the source fiber 510 is 50 μm.

[0105]

[0140] Other core diameters are also possible for the input fiber and divergence storage fiber. As mentioned above, for step-index fibers, a ratio of cladding diameter to core diameter of 2 or more is also employed in some embodiments. eff In other words, since the SRS threshold power increases or decreases with the square of the core diameter, the ratio of the core diameter of the divergent storage fiber to the core diameter of the input fiber should be 1.4 or greater for supply fibers of similar length to achieve at least a twofold increase in SRS threshold power. Other design optimizations in cladding and core size are also possible.

[0106]

[0141] Compared to conventional output fibers, the effective mode area expands along the length of the divergent storage fiber 508 (considering the majority of typical supply or process fiber lengths). As shown in Graph 520, the first GRIN fiber 504 transitions from the first (input) effective mode area 522 of the source fiber 510 to the second effective mode area 524 of the divergent storage fiber 508. The divergent storage fiber 508 has a larger core diameter 526 than the core diameter 528 of the source fiber 510 (e.g., ranging from approximately 50 μm to approximately 800 μm). For a GRIN fiber with a 1 / 4 pitch focal length f, fθ > w in If so, the divergence angle θ and waist size w in A Gaussian beam input having A eff To increase the output waist w. out This is shown by fθ, and the input waist w in It is larger than A. eff is the ratio (w out / w in- ) 2 The beam is increased and has a larger waist. Therefore, the second GRIN fiber 506 is configured to refocus this modified beam. When these two GRIN fibers have the same focal length, the diameter of the output beam is approximately the same as the diameter of the input beam, but the beam is larger between the two GRIN fibers.

[0107]

[0142] In one embodiment, the core diameter 526 is made large enough to generate minimal cladding light. Optionally, a cladding light stripper may be incorporated into the divergence-storing fiber 508. Furthermore, the core diameter 526 and the focal length of the second GRIN fiber 506 are selected so that the output divergence is sufficiently low for the desired final application, as will be explained later with reference to Equation 4 and Figures 9 and 10.

[0108]

[0143] The first GRIN fiber 504 expands the beam (but not to a greater extent than the core diameter 526 of the step-index divergence-preserving fiber 508), and the second GRIN fiber 506 reduces its diameter before the end cap 514. The first GRIN fiber 504 and the second GRIN fiber 506 can have the same gradient constant (focal length) to reproduce the input beam diameter, or they can have different gradient constants to introduce expansion or reduction of the input beam into the output beam supplied at output 512. If the two GRIN lenses have the same focal length, the output beam diameter at the back focal plane of the second GRIN lens will be approximately equal to the beam diameter at the fiber laser output, but the beam will be larger between the two GRIN lenses (see Figure 5), and the SRS threshold will increase. The second GRIN lens can have a longer or shorter focal length than the first GRIN lens, respectively, to lengthen or shorten the output beam diameter (resulting in changes corresponding to beam divergence), but the SRS reduction effect will still be present. Thus, this method reduces SRS without requiring the fabrication of fiber tapers and provides flexibility in selecting the output beam diameter for a given input beam diameter.

[0109]

[0144] The divergent storage fiber 508 may also include a joint in which one or both of the core diameter and / or cladding diameter can be stepped up or down, as described in relation to the optional joint 158 ​​in Figure 1. As with the optional joint 158, this joint can provide manufacturing, service, and performance advantages. In other embodiments, the divergent storage fiber 508 includes an FFC / FFS (as shown in Figure 1).

[0110]

[0145] Figure 6 shows a comparative inspection system 600 featuring SRS reduction achieved by a beam transmission system similar to that shown in Figure 5. The comparative inspection system 600 includes a conventional 30-meter 50 μm step-index feed fiber assembly 602 and a 30-meter feed fiber assembly 604. Similar to the fiber assembly shown in Figure 5, the feed fiber assembly 604 includes three fibers joined together: an input 1 / 4-pitch GRIN fiber with an effective focal length of 0.5 mm, a 30-meter-long 125 μm core step-index fiber, and an output 1 / 4-pitch GRIN fiber with an effective focal length of 0.5 mm. The feed fiber assembly 604 is similar to that of the optical beam transmission system 500 (Figure 5), both of which have divergent storage fibers with a core diameter larger than the core diameter of the fiber laser 606.

[0111]

[0146] At each input, a 5kW fiber laser 606 is joined to supply fiber assembly 602 and supply fiber assembly 604. The core diameter of the input to each supply fiber is approximately 40 μm. In both configurations, the diameter of the output near-field beam 608 was approximately 50 μm. In both cases, there was an end cap joined to the output fiber.

[0112]

[0147] As shown in the test results 700 (Figure 7) for the comparative test system 600, when the total output power is 5.1 kW, the output SRS signal decreased by approximately 15 dB, from 3.5% with the conventional supply fiber assembly 602 to 0.1% with the supply fiber assembly 604. For all power levels, the percentage of SRS is lower with the supply fiber assembly 604 than with the standard step-index supply fiber. Test results 700 demonstrate a significant reduction in the percentage of SRS due to the increased core size of the input fiber. This is consistent with the reduction in SRS achieved by the optical beam transmission system 500 shown in Figure 5.

[0113]

[0148] The SRS reduction techniques described above can be implemented in various laser system configurations, such as pulsed fiber lasers (e.g., Q-switched, mode-locked, QCW), CW fiber lasers, non-fiber lasers using optical transmission fibers (e.g., direct diode lasers, disk lasers, or rod lasers), multimode lasers, and laser systems using FFC or FFS.

[0114]

[0149] In other embodiments, an optical beam transmission system 300 (Figure 3) can also be employed to reduce SRS. As shown in Figure 8, the first pair of junctioned GRIN fibers 802 of the optical beam transmission system 300 expands the near-field intensity distribution of the beam (i.e., the EFL of the second GRIN lens 312 is greater than that of the first GRIN lens 310), and couples the expanded beam to the core 804 of a step-index fiber 806. The step-index fiber 806 is similar to the divergence-preserving fiber 508 in that its core 804 is larger than the core 808 of the source fiber 304. The size of the core 804 is approximately consistent with the spread of the expanded beam at the output of the second GRIN lens 312 (e.g., within about 15 μm). This intermediate beam is then expanded or contracted (e.g., reduced) by a second pair of junctioned GRIN fibers 810 (including the third GRIN lens 812 and the fourth GRIN lens 814) to obtain the final target output beam diameter. The second pair of bonded GRIN fibers 810 may be considered identical to the first pair of bonded GRIN fibers 802, except that they have an inverted EFL arrangement for reduction.

[0115]

[0150] There exists a specific configuration in which the FEL of the first GRIN lens 310 is equal to that of the fourth GRIN lens 814, and the EFL of the second GRIN lens 312 is equal to that of the third GRIN lens 812. In this configuration, a nearly accurate image of the input beam to the first GRIN lens 310 is obtained at the output of the fourth GRIN lens 814.

[0116]

[0151] As already explained, the intermediate beam in core 804 has a lower average intensity (or a larger effective mode area) compared to the beam in core 808, and harmful nonlinear effects such as SRS are reduced compared to a system of the same length consisting only of source fiber 304. This allows for extension of the supply fiber, less heat generation, and less power loss from the primary laser wavelength. In contrast to optical beam transmission system 500 (Figure 5), optical beam transmission system 800 preserves the shape of the input near-field intensity distribution at the output of each pair of GRIN lenses. For fiber laser inputs with a flat-top intensity distribution, optical beam transmission system 800 provides improved laser BPP preservation at the output compared to optical beam transmission system 500.

[0117]

[0152] The configuration shown in Figure 8 can represent a supply fiber or a process fiber (typically having a length of 5 to 30 meters). A process fiber of a given magnification can be easily replaced with other process fibers of a different magnification. A supply fiber of a given magnification can also be replaced with other supply fibers of a different magnification by splicing in the field.

[0118] Default maximum NA

[0153] Industrial fiber lasers are combined with external process optics that have a fixed numerical aperture. These process optics include beam couplers, beam switches, collimators, cutting heads, scanners, and zoom optics. These process optics have limited ability to accept light outside the device's numerical aperture range, and are typically limited by the heat generated by the components. Often, the intrinsic output beam divergence of the fiber laser can exceed the allowable aperture of the process optics, hindering compatibility. This problem occurs particularly in multimode fiber laser systems, where divergence is inherently large.

[0119]

[0154] Figure 9 shows an optical beam transmission system 900. The optical beam transmission system 900 includes a transmission fiber (i.e., a supply or process fiber) 902 that supplies an output beam 904 to a process head 906. The process head 906 generates a parallelized beam 908 and then focuses it onto (or near) the surface of a workpiece 910 (other configurations of process heads are also possible, including additional lenses, multiple lenses, zoom optics, reflective optics, scanners, etc.). The process head 906 typically has a maximum numerical aperture (NA) that it can accept. As shown in Figure 9, the maximum NA can be determined by an aperture 912 in front of this optics element. The maximum NA may also be determined by other apertures within the optical array, by lens mounts, by the physical dimensions of lenses or other optical components, or by some other structure of the optical system (not shown in Figure 9). Within the output beam 904, optical power that falls outside the maximum NA range of the process head (sometimes referred to as the "high NA tail" 914 of the output beam 904) is either truncated by the aperture 912 or otherwise lost from the output beam 904. This can cause undesirable heat generation, damage, or other problems within the process head 906.

[0120]

[0155] nLIGHT's previous method for limiting high divergence in lasers relied on the use of low core NA fibers, allowing light to leak into the cladding before a cladding light stripper. An example is shown in U.S. Patent Application US2023 / 0106619A1. This method has two drawbacks: (1) Producing and reproducing fibers with specific NAs is challenging, and typically the product will have some deviation from the desired truncation level. (2) Losses to cladding in low NA fibers can depend on fiber length and fiber routing, which can vary from laser to laser, resulting in further variability in power loss and divergence truncation levels.

[0121]

[0156] In contrast, when fiber assembly 104 is used as the transmission fiber 902, the output beam 904 has a clearly defined maximum NA and therefore passes through the process optics without excessive truncation. The maximum NA is designed and incorporated into the fiber to produce a strict cutoff that does not result in a high NA tail (i.e., no high divergent light 914). This is because the far-field divergence profile of the optical beam transmission system 100 (Figure 1) is determined by the spatial distribution of light in the divergence-preserving fiber 132 and the focal length of the second GRIN fiber 136. The ray with the maximum NA is generated at the outer edge of the step-index core 150. Therefore, the maximum NA (NA) max ) is designed as shown in Equation 4 below and is incorporated into fiber assembly 104.

[0122] At small angles,

number

[0123]

[0157] In some applications, a high NA tail with relatively low total power (e.g., 2%) can still pose problems related to compatibility with process optics, particularly with higher laser powers. The maximum allowable NA, and the maximum allowable power above that NA, vary depending on the application and design of the process optics. A quantitative example of a maximum NA specification for a process optics is at least 99.5% of the laser power at an NA of 0.10 or less. However, those skilled in the art will acknowledge that other percentages and NA values ​​are possible depending on the design of the process optics. In a typical laser system, the power at the high NA tail may vary from laser to laser, and for a given laser, it can be assumed to vary as a function of laser power and time. These variations result in inconsistent laser performance or inconsistent compatibility of the laser with process optics. The clearly defined NA for a laser system is given by Equation 4. max Therefore, compatibility between the laser system and the process optical elements can be ensured.

[0124]

[0158] Properly controlled power divergence is useful in industrial fiber laser applications. Improving (reducing) divergence allows for use with external process optics with smaller numerical apertures, opening up new applications in welding, cutting, and additive manufacturing. Furthermore, improving the control of divergence truncation levels and avoiding the dissipation of excess power due to divergence truncation saves costs and improves laser reliability.

[0125]

[0159] Figure 10 shows how NA max This shows an example of how to design and incorporate it into a fiber assembly. On the left side of Figure 10, two near-field intensity distributions are shown for two different variants of the optical beam transmission system 100 (i.e., two different second GRIN fibers 136). The first intensity distribution 1002 corresponds to the first variant with the first GRIN focal length, and the second intensity distribution 1004 corresponds to the second variant with the second GRIN focal length being 1.2 times that of the first GRIN focal length. The corresponding plots 1006 of encapsulated power versus NA show how different maximum power NAs were designed and incorporated into the two variants. When using GRIN #2, the near-field beam diameter is approximately 20% larger, and the NA is different compared to the result when using GRIN #1. max This is reduced by approximately 20%. Both of these fiber assemblies were examined using an input source 106 with a divergence of 80 mrad (NA=0.08), shown by the dashed line in plot 1006.

[0126]

[0160] Those skilled in the art will recognize that Equation 4 is applicable regardless of whether the fiber assembly includes a first lens (e.g., the first GRIN fiber 134 in Figure 1). In other words, based on the core diameter of the divergence-preserving fiber 132 and the effective focal length of the second lens (e.g., the second GRIN fiber 136 in Figure 1), the NA is calculated. max This can be adapted to downstream process optical elements or process heads and further optimized for them.

[0127]

[0161] Figure 11 shows the optical beam transmission system 1100 configured as a divergence suppression fiber assembly 1102. This is an all-fiber device and can be incorporated into a fiber laser 1104 to dissipate any highly divergent source light 1106 in the CLS 1108 before the output 1110 of the transmission fiber.

[0128]

[0162] In the optical beam transmission system 1100, a relatively short section (e.g., less than 10 millimeters) of the GRIN fiber 1112 is joined between the input step-index fiber 1114 and the output step-index fiber 1116. The GRIN fiber 1112 acts to redirect the direction of the highly divergent source light 1106 so that it is incident on the cladding 1118 of the output step-index fiber 1116. The highly divergent source light 1106 can then be filtered out by the CLS 1108. In this way, the CLS 1108 filters out the highly divergent source light 1106 in a controlled manner, while the low-divergent light 1120 is coupled to the core 1122 of the output step-index fiber 1116.

[0129]

[0163] In one configuration, the length of the GRIN fiber 1112 is set to a 1 / 4 pitch so that the fiber acts as a conventional lens. In such a configuration, the output NA is as described in Equation 4. This is similar to the configuration shown in Figure 1, where the input step-index fiber 1114 corresponds to the step-index fiber 106, the GRIN fiber 1112 corresponds to the first GRIN fiber 134, the CLS 1108 corresponds to the CLS 164, and the output step-index fiber 1116 corresponds to the divergence preservation fiber 132. The GRIN fiber 1112 may be joined before the CLS 1108, or it may be placed together with the CLS 1108 in a mechanical package.

[0130]

[0164] The input step-index fiber 1114 and the output step-index fiber 1116 may have the same or different core diameters. When the core diameter is step-up, a GRIN with a specified effective focal length can act to suppress the divergence of the laser beam with little or no additional loss in the CLS 1108. Also, as shown in Figure 1, the input step-index fiber 1114 may have a CLS (see, for example, CLS 166 in Figure 1) to remove any cladding light at the input.

[0131]

[0165] Figures 12 and 13 show the simulated near-field and far-field intensity distributions, as well as a one-dimensional profile diagram, for the optical beam transmission system 1100.

[0132]

[0166] In Figure 12, the upper column shows the beam properties of the input source fiber. A typical source used is a 100 μm diameter fiber with a second moment divergence of 80 mrad. The lower column shows the output fiber beam properties of the CLS (i.e., output 1110).

[0133]

[0167] Figure 13 shows the encapsulation power versus NA plot (divergence distribution) for input step-index fiber 1114 and output step-index fiber 1116.

[0134]

[0168] Figure 14 shows another optical beam transmission system 1400 configured as a divergence suppression fiber assembly 1402. Divergence suppression fiber assembly 1402 is similar to divergence suppression fiber assembly 1102, except that instead of a single GRIN fiber 1112, divergence suppression fiber assembly 1402 includes back-to-back GRIN fibers 1404 (first GRIN fiber 1406 and second GRIN fiber 1408). As already described in Figure 11, divergence suppression fiber assembly 1402 includes an input step-index fiber 1410, an output step-index fiber 1412, a laser source 1414, an output 1416, and a CLS 1418.

[0135]

[0169] Figure 15 shows that the optical beam transmission system 1400 preserves the near field even while discarding high NA light. This is because the first GRIN fiber 1406 and the second GRIN fiber 1408 in this back-to-back GRIN fiber 1404 have different core sizes. The second GRIN fiber 1408 has a smaller core size and refocuses the light with less divergence to preserve its near field.

[0136]

[0170] Figure 16 shows the plots (divergence distribution) of encapsulated power versus NA for the input and output fibers in Figure 14.

[0137] Ring beam shape generation

[0171] Figure 17 shows another embodiment of the optical beam transmission system 1700, including a source fiber 1702 and a fiber assembly 1704. Fiber assembly 1704 is similar to fiber assembly 104 (see, for example, the RIP in Figure 1), image relay fiber assembly 204 (Figure 2), and fiber assembly 502 (Figure 5). For example, fiber assembly 1704 includes a first GRIN fiber 1706, a second GRIN fiber 1708, and a divergent storage fiber 1710 coupled between the first GRIN fiber 1706 and the second GRIN fiber 1708 (i.e., coupled or positioned at a distance of one Rayleigh length, or within a range of one Rayleigh length).

[0138]

[0172] The source fiber 1702 is coupled to the first GRIN fiber 1706 and includes a fiber core 1712 that defines the first optical axis 1714. The first optical axis 1714 has a radial offset 1802 (Figure 18) with respect to both the second optical axis 1716 located at the center of the fiber assembly 1704 and the longitudinal axis 1718 located at the center of the source fiber 1702.

[0139]

[0173] When beam 1720 is transmitted by fiber core 1712 to the first GRIN fiber 1706, the first GRIN fiber 1706 contributes angular displacement to form an intermediate beam 1722, which is then transmitted to the divergence-preserving fiber 1710. The angular distribution of the intermediate beam 1722 is preserved by the divergence-preserving fiber 1710, which also introduces azimuthal scrambling into the intermediate beam 1722. The output beam of the divergence-preserving fiber 1710 is then refocused by the second GRIN fiber 1708 to form a ring beam at the output of the second GRIN fiber 1708. Thus, this method generates a ring beam in the entire fiber system without employing ring fibers and mechanical actuator components on fibers that are sensitive to bending or other disturbances.

[0140]

[0174] Furthermore, in the embodiments described in Figures 17 and 18, as well as in the embodiments described below, it will be apparent to those skilled in the art that the input beam is guided without angular displacement relative to the input GRIN axis. As a result, the first GRIN acts to convert spatial displacement into angular displacement.

[0141]

[0175] Figure 19 shows a portion of another optical beam transmission system 1900. In the example in Figure 19, the source fiber 1902 includes a centrally located fiber core 1904. The fiber core 1904 is coaxially aligned with the central optical axis 1906 of the source fiber 1902. However, since the source fiber 1902 is joined to the first GRIN fiber 1706, the centrally located fiber core 1904 is radially offset from the second optical axis 1716 of the first GRIN fiber 1706. Similar to the optical beam transmission system 1700, the embodiment in Figure 19 also generates a ring beam in the entire fiber system without employing ring fibers and mechanical actuator components (i.e., the radial offset shown in Figure 19 provides the same function as the radially offset core in Figures 17 and 18).

[0142]

[0176] Figure 20 shows an example of an output near-field beam profile 2002 obtained from a ray-trace simulation performed on the input beam 2004 using various radial displacements in the optical beam transmission system 1700 (Figure 17). The centrally located input beam supplies an output beam with a similar shape to the input beam (with optional expansion or contraction). The radially displaced input beam supplies a ring-shaped output beam, and the ring shape in the output near-field beam profile 2002 expands as an additional displacement is introduced into the input beam 2004.

[0143] Generation of composite beam shapes

[0177] Depending on the application, it has been found useful to use a laser beam shape that includes a central spot (typically about 50-100 μm in diameter) surrounded by a halo or pedestal (typically about 150-600 μm in diameter). This beam shape can be generated by using a dual-core transmission fiber. The dual-core transmission fiber constitutes a ring fiber, i.e., a ring fiber, which is a central induction region (also typically step-indexed) surrounded by an annular (ring-shaped) induction region (typically step-indexed). This beam shape is determined by the dimensions of the central core and the annular core, and by the relative power induced in each region. To generate the desired beam shape, one or more free-space beams can be emitted into the dual-core fiber. Alternatively, a fused fiber combiner can be used to couple different fiber lasers or fiber-coupled input lasers to the central and annular induction regions. The latter method has the advantage of allowing a fully fiber-based design, i.e., there are no free-space optical elements between the output fibers of the individual lasers and the output of the dual-core fiber. However, this has the drawback that the maximum power (and therefore the maximum power beam shape) in each region is suppressed at the time of manufacturing. The maximum laser power can only be obtained with one beam shape, and different beam shapes cannot be obtained without reducing the power of one or more input lasers. Another drawback is that in order to provide different core diameters and ring diameters and thicknesses, it is necessary to manufacture ring fibers of different dimensions as desired.

[0144]

[0178] Using all fiber embodiments and techniques described in the '768 patent, the spatial intensity distribution of a laser beam coupled to a ring fiber can be split between two or more induction zones of the fiber to obtain various beam diameters and shapes. This technique allows for obtaining full power at all beam shapes because the distribution of the laser system's output power between the induction zones can be altered. However, a drawback of this technique is that a portion of the beam may extend into the low refractive index region (one or more) separating the induction zones, potentially leading to increased beam divergence (high NA tail). This increased divergence is undesirable in some applications, for example, it can cause overheating of certain process optical elements or apertures, limiting potential end-use applications, particularly when used in laser beam scanners.

[0145]

[0179] Embodiments in Figures 21-23 utilize multiple input beam positions within a fiber assembly such as 104, 204, or 502 to address the various problems previously described with reference to the '768 patent. Firstly, by excluding the aforementioned low refractive index region that separates the induction region in the multicore fiber, the risk of increased divergence due to coupling to the low refractive index induction region is eliminated, and furthermore, tight matching tolerances for factory and field joining are not required. Secondly, in conventional methods, changing the dimensions of the central core and / or annular core requires the fabrication of a new dual-core (ring) fiber and a corresponding fused fiber combiner. The new method allows the parameters of the output beam to be changed while always maintaining the same step-index fiber by changing the dimensions of the bundle or by changing the relative focal lengths of the two GRIN lenses (and even when using different step-index fibers, it is less costly and time-consuming than producing a new dual-core fiber).

[0146]

[0180] The novel method described with reference to Figures 21-23 introduces new design possibilities that are difficult or impossible with conventional methods. Composite beam shapes having an arbitrary central spot and two or more annular rings of different diameters can be generated by fabricating a bundle of fibers arranged at different radial displacements from a central fiber (one or more). This possibility can coexist with conventional multicore methods, but it becomes increasingly difficult and expensive as the number of annular cores increases due to the tolerances for fabricating multicore fibers and junction matching. Furthermore, the method shown in Figures 21-23 can generate composite beam shapes that are not possible with conventional methods. These composite beam shapes include a central core surrounded by overlapping annular beams, and / or two or more overlapping annular beams (with or without a central core). These beam shapes can also be generated by appropriately positioning peripheral fibers to obtain the desired beam overlap. Such beam overlap is not possible with conventional methods because it requires separation between cores within a multicore fiber.

[0147]

[0181] Figure 21 shows an example of a fiber bundle 2100 that can be used as a substitute for source fiber 1702 (Figure 17). Fiber bundle 2100 includes fused or unfused bundles of individual fibers. It is shown that some of these fibers propagate the beam, while others do not (usually any number of input fibers can be used to propagate the beam). The outer fiber 2102 is radially offset from the second optical axis 1716 (Figure 17), and the central fiber 2104 is optionally axially aligned with the second optical axis 1716. The first GRIN fiber 1706 (acting as the first GRIN lens) maps the central fiber 2104 to a low propagation angle in the divergent preservation fiber 1710 and the outer fiber 2102 to a larger propagation angle. Next, the second GRIN fiber 1708 (acting as the second GRIN lens) converts the low-angle beam from the central laser into a central spot and the high-angle beam from the peripheral laser into a peripheral ring at the back focal plane of the second GRIN lens. (The peripheral beam forms a ring due to azimuthal scrambling as the beam propagates through the divergence-preserving fiber 1710.) It should be noted that although the various beams are not spatially separated within the step-index supply fiber (in contrast to the situation with the prior art dual-core fiber), a composite beam shape is still generated. As in the SRS reduction embodiment and other embodiments, the two GRIN lenses can have the same or different focal lengths, which affects the size and divergence of the output beam.

[0148]

[0182] In this example, all fiber positions are connected to different laser sources. Each laser source can have independent power control, allowing for fine-grained control of the resulting beam profile output from the second GRIN fiber 1708. In this way, by changing the amount of light incident on either the outer fiber 2102 or the central fiber 2104, the intensity distribution of the output from the second GRIN fiber 1708 can be modified, enabling the generation of various beam shapes similar to, but without, the drawbacks of, conventional methods (dual-core fibers).

[0149]

[0183] Figures 21 and 23 (described later) show examples where all of the outer fibers (Figure 21) or outer core (Figure 23) are roughly at the same radial distance from the central axis. In contrast, Figure 22 shows an example of a fiber bundle 2200 (or a multicore fiber in other embodiments) having an outer fiber ring 2202, an inner fiber ring 2204, and a central fiber 2206. Thus, the excited outer fiber 2208 has a greater radial distance than the excited inner fiber 2210. The fiber bundle 2200 can generate various near-field intensity profiles depending on the power associated with each laser source and the radial displacement of the source fibers within the bundle.

[0150]

[0184] In one embodiment, the fiber bundle 2200 can be used in place of a signal combiner when using multiple fiber laser sources. A signal combiner is a fused fiber component in which multiple input fibers are joined to an output fiber. The embodiment in Figure 22 similarly connects multiple input fibers to an output fiber, but with the addition of spatial beam control.

[0151]

[0185] One method for creating a fiber bundle is to melt the bundle together while simultaneously pulling the bundle to reduce its diameter (i.e., tapering it). Therefore, in some embodiments, the input bundle may have a taper to set the diameter and distance (radial position) of the input core.

[0152]

[0186] As an alternative to the fiber bundle (Figures 21 and 22), Figure 23 shows a multicore fiber 2300 that can be used as a replacement for the source fiber 1702 (Figure 17), where the outer core 2302 is radially offset from the second optical axis 1716 (Figure 17), and the central core 2304 can be optionally axially aligned with the second optical axis 1716. The first GRIN fiber 1706 (acting as the first GRIN lens) maps the central input port (central core 2304) to a low propagation angle in the divergent preservation fiber 1710 and the peripheral input port (outer core 2302) to a larger propagation angle. Then, the second GRIN fiber 1708 (acting as the second GRIN lens) converts the low-angle rays from the central laser to a central spot and the high-angle rays from the peripheral laser to a peripheral ring at the back focal plane of the second GRIN lens. (The peripheral beam forms a ring due to azimuthal scrambling as the beam propagates through the divergence-preserving fiber 1710.) Note that the various beams are not spatially separated within the step-index fiber (in contrast to the situation with the prior art dual-core fiber). As in the SRS reduction embodiment and other embodiments, the two GRIN lenses can have the same or different focal lengths, which affects the size and divergence of the output beam.

[0153]

[0187] In this example, all fiber positions are connected to different laser sources. Each laser source can have independent power control, allowing for fine-grained control of the resulting beam profile output from the second GRIN fiber 1708. In this way, by changing the amount of light incident on either the outer core 2302 or the central core 2304, the intensity distribution of the output from the second GRIN fiber 1708 can be modified, enabling the generation of various beam shapes similar to, but without, the drawbacks of, conventional methods (dual-core fibers).

[0154]

[0188] Figure 24 shows an example of an output near-beam profile 2402 when two input sources (e.g., two fibers in a bundle) are active, demonstrating the generation of a composite beam shape according to one embodiment. In this example, the composite output beam is a superposition of the two input sources, each of which can be independently controlled with respect to power level, power modulation (e.g., pulse setting), or more generally, output waveform (power versus time). The size of the ring or saddle shape can be selected based on the radial displacement of the input source(s), so different combinations of nested or overlapping ring, saddle, or pedestal shapes are also possible.

[0155]

[0189] Figure 25 shows another variant of the optical beam transmission system 2500, which uses separately controllable laser sources, each coupled to a different input fiber 2502, to generate a composite beam shape. In this example, the first laser source is coupled to the first input fiber 2504, the second laser source to the second input fiber 2506, the third laser source to the third input fiber 2508, and the fourth laser source to the fourth input fiber 2510. In some embodiments, there may be a different number (or different types) of input fibers.

[0156]

[0190] The fused fiber combiner 2512 couples each input fiber 2504, 2506, 2508, and 2510 to the divergent storage fiber 2514. The divergent storage fiber 2514 is then coupled to the output GRIN fiber 2516. As already described, the divergent storage fiber 2514 is a step-index fiber, and the output GRIN fiber 2516 acts as a lens, i.e., has a pitch of 1 / 4 + 1 / 2n, where n is zero or any positive integer (0, 1, 2, ...). Other types of lenses are also possible.

[0157]

[0191] In the example in Figure 25, the first input fiber 2504 is a step-index fiber, or another type of fiber with one induction zone. Thus, coupled with the divergence-storage fiber 2514 and the output GRIN fiber 2516, the output beam supplied by the first input fiber 2504 has a central beam whose near-field diameter is proportional to the divergence of the source laser. An example of this output is shown in Figure 26.

[0158]

[0192] The second input fiber 2506, the third input fiber 2508, and the fourth input fiber 2510 are similar to those of the optical beam transmission system 1700, with each input fiber comprising (1) a step-index input fiber with an arbitrary radial displacement, (2) a GRIN lens, (3) a divergent storage fiber with a junction to a divergent storage fiber 2514, and (4) an output GRIN fiber 2516. In this example, the radial displacements of the second input fiber 2506, the third input fiber 2508, and the fourth input fiber 2510 are 0 μm, 50 μm, and 85 μm, respectively, and the final outputs for each fiber are as shown in Figures 27, 28, and 29. Specifically, without radial displacement, the second input fiber 2506 supplies a central beam at the output of the output GRIN fiber 2516, where the near-field diameter is proportional to the diameter of the source laser. With a small radial misalignment of 50 μm, the third input fiber 2508 forms a ring shape with a relatively small diameter. With a larger radial misalignment of 85 μm, the fourth input fiber 2510 forms a ring shape with an even larger diameter. The specific misalignments and beam diameters shown in this example are representative, and other dimensions and beam properties are achievable without departing from the scope of the present invention.

[0159]

[0193] When multiple laser sources are simultaneously coupled to the input channels of a combiner, the combined output beam becomes a superposition of the individual channel output beams. In this technique, the system controller can superimpose the power combinations of multiple input sources within the same near-field beam profile, independently select different intermediate spatial profiles between the spot and the ring, and superimpose them within the same near-field beam profile.

[0160]

[0194] Since the beam source can be controlled separately for each input fiber of the optical beam transmission system 2500 (e.g., adjustable continuous or pulsed power, on / off pulse duration, output waveform, etc.), different fibers and fiber combinations can propagate beams, and these beams can be combined to generate a composite beam shape at the output of the output GRIN fiber 2516. For example, Figure 30 shows the output when the first input fiber 2504 and the fourth input fiber 2510 are supplying beams, and the fourth input fiber 2510 is supplying four times more power than the first input fiber 2504. Similarly, Figure 31 shows a power ratio of 13:1.

[0161]

[0195] With respect to the workpiece, different input fibers or combinations of input fibers can be temporally modulated so that the beam transmitted over time is an average of different inputs (for example, by rapidly switching to inject power into different input fibers at different points in time). This switching between each output beam shape can be performed at frequencies from 1 Hz to many Hz or more. In general, any time waveform can be injected into any of the input beams.

[0162]

[0196] In many laser welding and additive manufacturing applications, such as those using two-axis or three-axis scanners, the laser beam needs to have sufficiently low divergence to avoid overheating and damage to process optical elements. As a result, the optical beam transmission system 2500 described herein can be optimized to generate low divergence and have a second moment beam diameter in the range of approximately 50 to 300 μm. By independently varying the power in the central beam and the ring beam, welding properties such as welding mode (conduction vs. keyhole), welding depth, and characteristics related to the dynamics of the weld pool (smoke, spatter, porosity, process stability) can be optimized for a given application.

[0163]

[0197] Furthermore, it will be acknowledged to those skilled in the art that the different radial deviations obtained by three separate step-index fibers can, in some embodiments, also be obtained by a single fiber bundle or multicore fiber, as previously described. Additionally, the input and output GRIN lenses may have different effective focal lengths to accommodate magnification or reduction. Other design optimizations previously described can also be implemented, such as optional step-up or step-down junctions, the use of FFC / FFS, and various CLSs. In this example, CLS2518 is shown at the input of the input step-index fiber and divergence-preserving fiber. Further system customization is possible, including any of the following: Optional CLS may be added to the step-index fiber at the output of the fused fiber combiner 2512. A junction to a second step-index fiber (optionally having a different core or cladding diameter) may be added before the output GRIN fiber 2516. FFC / FFS may be used between the output step-index fiber of the fused fiber combiner 2512 and the step-index transmission fiber housing the output GRIN fiber 2516.

[0164] Mapping of divergent distributions

[0198] Some industrial fiber and fiber-coupled lasers form a flat-top or nearly flat-top near-field intensity distribution. When such beams are imaged or focused, they typically maintain a flat-top shape within approximately ±1 Rayleigh length of the beam waist. Outside this range (i.e., beyond approximately ±1 Rayleigh length from the beam waist), the beam intensity distribution is no longer flat-top and often approaches a Gaussian or super-Gaussian shape, which is not ideal depending on the application.

[0165]

[0199] In certain laser applications (e.g., welding, cutting, brazing, soldering, heat treatment, and additive manufacturing), working away from the laser's focal point can be a useful advantage, for example, to adjust the beam size on the workpiece while maintaining a flat-top intensity distribution. This capability, however, is not available with current lasers because the beam does not maintain its flat-top shape as it propagates and moves away from the beam waist. While the size of the flat-top (near-field) beam can be changed using a zoom lens (and by working closer to the beam waist), this method is costly, complex, and can reduce the performance and reliability of the tool compared to a process head with fixed (non-zoom) optical elements.

[0166]

[0200] The application requirements described above can be met by forming a flat-top far-field divergence distribution such that the intensity distribution outside approximately ±1 Rayleigh length of the beam waist maintains a flat-top intensity distribution (see, for example, the right side of Figure 33). As already explained, the second GRIN fiber 136 (Figure 1) maps the position in the divergence-preserving fiber 132 (or in the optional fiber 160 after the junction 158) to an angle, so the output divergence distribution from the fiber assembly 104 is determined by the intensity distribution received in the second GRIN fiber 136. For example, a flat-top far-field divergence profile is obtained at the output of the fiber assembly 104 from the flat-top intensity profiles in fibers 132 and 160. The divergence distribution is largely independent of the beam characteristics (including divergence) of the input beam(s). A stable and reproducible divergence profile designed in this way is desirable for many applications.

[0167]

[0201] Figure 32 shows an optical beam transmission system 3200 that includes a transmission fiber (i.e., a supply or process fiber) 3202 that supplies a beam 3206 having a flat-top divergence distribution to a process head 3204. Examples of the transmission fiber 3202 are shown below with reference to Figures 34 to 37.

[0168]

[0202] The process head 3204 includes a parallelizing lens 3208 and a focusing lens 3210. Other configurations of the process head 3204 are also possible, including additional lenses, multiple lenses, zoom optical elements, reflective optical elements, scanners, etc.

[0169]

[0203] In the example shown in Figure 32, the process head 3204 generates a parallelized beam 3212 from the beam 3206 and then focuses the parallelized beam 3212 toward the beam waist 3214. Because the beam 3206 has a flat-top divergence distribution, the output beam 3216 maintains its flat-top shape when it reaches the surface of the workpiece 3218 located far away from the beam waist 3214. The output beam 3216 maintains its flat-top shape even as it moves away from the focal point, so that the area of ​​the workpiece 3218 onto which the output beam 3216 is incident can be easily expanded or contracted by changing the working distance (WD) 3220 to move away from or closer to the beam waist 3214, without any change in its relative intensity distribution.

[0170]

[0204] Figure 33 shows camera measurements of the far-field divergence distribution 3300. The first measurement 3302 is from an image relay fiber assembly having only circular core fibers, as already described with reference to Figures 1 and 2. The second measurement 3304 is from the output 3402 (Figure 34) of an optical beam transmission system 3400 that includes non-circular core mode scrambling fibers 3404. In the second measurement 3304, the far-field distribution is more uniform and has a flatter top shape compared to that of the first measurement 3302.

[0171]

[0205] A flat-top beam is defined by ISO 13694:2005 as having high flatness (1 for an ideal flat-top). The table below shows that in Figure 33, the distribution on the left (first measurement 3302) has a flatness of 0.45, and the distribution on the right (second measurement 3304) has a flatness of 0.79. A standard 13.5% truncation level limit was used when analyzing these beams. For some laser processing applications, a flat-top divergence distribution corresponds to a flatness greater than 0.6. Note that other metrics or specifications may be used to define the flat-top beam shape depending on the application requirements.

[0172] [Table 1]

[0173]

[0206] Figure 34 shows an optical beam transmission system 3400. The optical beam transmission system 3400 includes an input fiber 3406 coupled to a continuous fiber assembly 3408 to form a flat-top divergence distribution at the output 3402 (see, for example, the image on the right in Figure 33). In various embodiments, the input beam can be coupled from a multimode fiber, a single-mode fiber, a multicore fiber, a ring fiber, a fiber bundle, or free space.

[0174]

[0207] In general terms, fiber assembly 3408 has the same configuration as already described in the far-field image relay configuration (e.g., fiber assembly 104 and image relay fiber assembly 204). In the example of Figure 34, however, the divergent preservation fiber 3410, deployed between the first GRIN fiber 3412 and the second GRIN fiber 3414, also includes a mode distribution homogenization portion 3416. For example, the divergent preservation fiber 3410 includes a non-circular core mode-scrambling fiber 3404.

[0175]

[0208] ISO 11145:2018 defines a "circular power density distribution," i.e., an azimuthally symmetric beam, as having an ellipticity of 0.87 or greater. When a circular aperture is desired, an optional circular core divergence-preserving fiber 3418 is coupled between the non-circular core mode-scrambling fiber 3404 and the second GRIN fiber 3414. Otherwise, the flat-top far-field intensity distribution may not be circular (for example, an octagonal fiber 3404 will have an octagonal shape). Such non-circular apertures may be desirable depending on the application.

[0176]

[0209] As already explained, the first GRIN fiber 3412 acts as a lens to deform the near-field intensity distribution of the input beam (not shown) into a far-field angular distribution. Next, the non-circular core mode-scrambling fiber 3404 acts to scramble the modes and homogenize the intensity distribution within fiber 3404 without significantly altering the divergence distribution. Then, to ensure that the output flat-top divergence distribution from the fiber assembly has a circular aperture, a circular core divergence-preserving fiber 3418 is joined behind the non-circular core mode-scrambling fiber 3404 (i.e., to generate the flat-top intensity distribution) and before the second GRIN fiber 3414. By homogenizing the near-field intensity distribution before it is incident on the second GRIN fiber 3414, the second GRIN fiber 3414 acts as a lens to deform the flat-top intensity distribution within the circular core divergence-preserving fiber 3418 into a flat-top far-field angular distribution.

[0177]

[0210] In one embodiment, the non-circular core mode-scrambling fiber 3404 is a step-index fiber. The non-circular core mode-scrambling fiber 3404 is shown having an octagonal core. In other embodiments, the mode-scrambling fiber may have a core that is rectangular, hexagonal, D-shaped, or other non-circular shape that can homogenize the mode distribution within the fiber. The non-circular core mode-scrambling fiber 3404 may also incorporate a CLS, as already described with reference to CLS162 (Figure 1). The length of the fiber 3404 is typically about 0.1 to 2 m without the CLS and about 2 to 5 m with the CLS, but other lengths are also possible.

[0178]

[0211] In one embodiment, the non-circular core mode-scrambling fiber 3404 can be excluded, and this structure still forms a flat-top far-field divergence distribution depending on the characteristics of the input source and the firing conditions to the first GRIN fiber 3412.

[0179]

[0212] In one embodiment, the circular core divergent storage fiber 3418 is a step-index fiber. As shown in Figure 34, the circular core divergent storage fiber 3418 has approximately the same core diameter (or a slightly larger diameter) as the non-circular core mode-scrambling fiber 3404 to ensure that cladding loss at the junction is minimized. The difference in core size between the circular core divergent storage fiber 3418 and the input fiber 3406 depends on the focal length of the first GRIN fiber 3412.

[0180]

[0213] In one embodiment, the first GRIN fiber 3412 can be excluded, and the output divergence distribution remains flat-top. In this embodiment, however, the output near-field intensity distribution is no longer an image of the input near-field intensity distribution.

[0181]

[0214] Figures 35–37 illustrate other embodiments for achieving the desired mode scrambling, i.e., a flat-top far-field divergence distribution (i.e., without using a non-circular core fiber).

[0182]

[0215] For example, Figure 35 shows an optical beam transmission system 3500 having a fiber assembly 3502. The fiber assembly 3502 includes one or more coils 3504 within the divergent storage fiber 3506 to distribute the mode scrambling of the intermediate beam induced by the divergent storage fiber 3506 into a mode distribution homogenization section 3508. Otherwise, the optical beam transmission system 3500 is functionally similar to the optical beam transmission system 3400 (Figure 34) with respect to mode scrambling. In one embodiment, one or more coils 3504 are formed by winding the divergent storage fiber 3506 around a fixture.

[0183]

[0216] In another example, Figure 36 shows an optical beam transmission system 3600 having a fiber assembly 3602. In the fiber assembly 3602, a microbend device 3604 is used to slightly bend the divergent preservation fiber 3606 so as to distribute the mode scrambling of the intermediate beam induced by the divergent preservation fiber 3606 into the mode distribution homogenization portion 3608. Otherwise, the optical beam transmission system 3600 is functionally similar to the optical beam transmission system 3400 (Figure 34) with respect to mode scrambling. In one embodiment, the microbend device 3604 includes one or more structures that perturb the fiber.

[0184]

[0217] In another example, Figure 37 shows an optical beam transmission system 3700 having a fiber assembly 3702. In the fiber assembly 3702, a mandrel 3704 (or other device) is used to bend the divergent preservation fiber 3706 by a small amount so as to distribute mode scrambling in the intermediate beam induced by the divergent preservation fiber 3706 into the mode distribution homogenization portion 3708. Otherwise, the optical beam transmission system 3700 is functionally similar to the optical beam transmission system 3400 (Figure 34) with respect to mode scrambling. In one embodiment, the mandrel 3704 includes one or more structures that perturb the fiber to create one or more macrobends, for example. The one or more macrobends may have the same or different bending radii.

[0185]

[0218] Furthermore, it will be acknowledged to those skilled in the art that other mode distribution homogenization techniques may be employed. For example, a misaligned junction may be used to excite more modes or different combinations of modes than a junction without misalignment (located in the center). The intensity distribution in the second fiber behind the junction will differ depending on the firing conditions of the first fiber. The intensity distribution of the output fiber will be affected by changing the input beam properties, firing angle (junction angle), or radial misalignment. In junctions with increasing core diameter, adding radial misalignment between the fiber core can further flatten the output intensity distribution.

[0186]

[0219] Most laser beams have a featureless shape at the far field (i.e., at a distance of at least several Rayleigh lengths from the beam waist), and the far-field beam shape is often nearly Gaussian or predominantly Gaussian. In some industrial applications, particularly those where the workpiece is positioned at the far field (see, for example, Figure 32), a distinctive and consistent far-field beam shape would be useful. As discussed earlier with reference to Figure 33, if a short focal length lens is properly positioned at the output of a step-index fiber, it maps the near-field intensity distribution to a divergent distribution (i.e., the far-field beam shape). By appropriately setting the mode of the step-index fiber, the intensity distribution at the output end of the fiber becomes uniform, and therefore the lens maps this beam shape to the far-field beam shape. If the core of the step-index fiber is circular, the far-field beam shape will also be circular (and flat-topped).

[0187]

[0220] In contrast, Figure 38 shows an example of how the optical beam transmission system 3800, including the fiber assembly 3802, can form other flat-top shapes (square, octagon, etc.) based on the selection of the corresponding core shape for the step-index fiber 3804. Specifically, the step-index fiber 3804 includes an azimuthal asymmetric core shape 3806, which in this example is a square core 3808. While Figure 38 shows a square core 3808, this method is general-purpose and can be used with any core shape and / or multiple cores.

[0188]

[0221] The step-index fiber 3804 generates or propagates a specific beam shape, after which a short-focus lens 3810, for example, a GRIN fiber 3812, maps this beam shape to a divergent distribution 3814, that is, to a far-field beam shape corresponding to an azimuthal asymmetric core shape 3806. The short-focus lens 3810 is typically a 1 / 4-pitch length GRIN fiber joined to the step-index fiber 3804, but other embodiments are possible.

[0189]

[0222] Figure 38 also shows how the collimator 3816 is used to deform the far-field shape to the same near-field shape (a rectangular intensity profile 3818 with optional magnification) by positioning the free-space lens 3820 downstream of the fiber assembly 3802 (near the focal point, approximately ±1 Rayleigh length) so that the short-focus lens 3810 and the free-space lens 3820 form a telescope that images the beam shape determined by the intensity distribution within the rectangular core 3808. The free-space lens 3820 can be refractive (including multiplets) or reflective.

[0190]

[0223] The advantage of this method is that, because the workpiece is positioned near the beam waist, the near beam has a larger depth of focus (process window). In laser applications, depth of focus relates to how precisely the laser must be positioned relative to the workpiece in order to obtain effective results. The optical definition of depth of focus is 2x Rayleigh length. The parallelized beam in Figure 38 has a much larger depth of focus / Rayleigh length. Specifically, depth of focus / Rayleigh length increases with the square of the magnification. For the fiber laser configuration in this disclosure, the parallelized beam may have a depth of focus between 0.1 and 10 meters. A larger depth of focus ensures that the laser is effective over a wide distance range, especially when dealing with surfaces that are not perfectly flat, or when the laser must maintain its effect while moving across the surface of the material. Furthermore, the beam, when magnified, becomes relatively larger, and a consistent near-field size is obtained for a given combination of lenses (i.e., the magnification is equal to the ratio of the focal length of the free-space lens 3820 to that of the short-focus lens 3810).

[0191]

[0224] In other configurations, with such a designed far-field beam shape (i.e., the designed divergent profile), it becomes possible to change or optimize the tangible beam size on the workpiece by changing the far-field position while maintaining the same beam shape. In other words, various other non-circular shapes can be deployed instead of the circular shape shown in Figure 32.

[0192]

[0225] Figure 39 illustrates how other flat-top shapes (square, octagon, etc.) are obtained by selecting the corresponding core shape for step-index fiber 3804 (Figure 38). Step-index fiber 3804 with a non-circular core (e.g., square, hexagonal, or octagonal) produces the corresponding far-field beam shapes (square, hexagonal, or octagonal, respectively). An example of an octagonal far-field beam is shown in Figure 39.

[0193]

[0226] When heat-treating or brazing a surface, if the beam shape is square or rectangular, optimal use of available laser power is possible by minimizing the overlap of laser beam passages that occur sequentially across different areas of the surface. In other embodiments, a beam consisting of multiple spots from different cores in an MCF or bundle (optionally, the different spots having different sizes and / or intensities) would also improve some welding and brazing applications. A step-index fiber 3804 having two or more circular or non-circular cores that are offset from each other would produce the corresponding far-field beam shape (i.e., multiple spots).

[0194]

[0227] A pedestal beam shape (where the central spot is surrounded by a less intense halo) minimizes adverse effects such as spatter during welding. Pedestal beams can also be used to maximize cutting speed. While methods exist for generating near-field pedestal beams, these beams typically cannot maintain their shape at far distances. That is, obtaining pedestal beams of different sizes involves changing the magnification of the process optics by altering the lens focal length, using a zoom head, or using different fibers (possibly including fiber-to-fiber couplers or switches). All of these methods are costly, complex, and increase the number of failure modes. A step-index fiber 3804, with a central core surrounded by a ring-shaped core and a certain power coupled to both cores, can generate pedestal or saddle beams at far distances.

[0195]

[0228] Ring beams and saddle-shaped beams (i.e., rings with low strength in the center) are useful for maximizing productivity and material quality in additive manufacturing tools. While methods exist to generate near-field ring or saddle beams, similar to pedestal beams, these shapes are not maintained far-field. A step-index fiber 3804 with a ring-shaped core would generate a ring-shaped far-field beam.

[0196] Conclusion

[0229] The overall principles of the examples of the technologies disclosed herein have been described and illustrated above, but it should be apparent that these examples can be modified in configuration and detail without departing from such principles. For example, an optional joint 158 ​​(Figure 1) or an optional FFC / FFS 170 may be included in any of the embodiments. A non-circular core mode-scrambling fiber 3404 (or other mode-dispersion homogenization device) may also be included in any of the embodiments. Furthermore, the embodiments described above employ a divergent-preserving fiber, which is an intermediate section having a core with a higher refractive index than the first cladding, but in some other embodiment the fiber may have multiple concentric claddings. Thus, it will be apparent to those skilled in the art that many modifications can be made to the details of the embodiments described above without departing from the fundamental principles of the present invention. Accordingly, the scope of the present invention is defined solely by the following claims.

Claims

1. A light beam transmission system configured to form a flat-top divergence distribution, A source fiber configured to supply an input beam, A fiber assembly, A first lens coupled to the source fiber receives the input beam from it and converts it into an intermediate beam, A divergent preservation fiber including a mode-distribution homogenization portion is used to generate a flat-top intensity distribution from the aforementioned intermediate beam. A second lens configured to convert the flat-top intensity distribution into an output beam having the flat-top divergence distribution, A fiber assembly comprising, A light beam transmission system equipped with the following features.

2. A method for forming a flat-top divergence distribution, A step of guiding an input beam to a first lens, wherein the first lens is configured to map the input lateral spatial intensity distribution to the divergence distribution of an intermediate beam. The steps include: generating a flat-top intensity distribution by guiding the intermediate beam through a divergence-preserving fiber and passing it through a mode distribution homogenization portion; The second lens converts the flat-top intensity distribution into an output lateral spatial intensity distribution to generate an output beam having the flat-top divergence distribution. Methods that include...

3. A method according to claim 2, further comprising the step of firing the modified beam toward a process head such that the far-field beam shape is maintained on the workpiece, wherein the beam size varies as a function of the distance to the workpiece.

4. An optical beam transmission system or method according to claim 1 or claim 2, wherein the mode-distribution homogenization portion includes a skewed junction.

5. An optical beam transmission system or method according to claim 1, wherein the mode-distribution homogenization portion includes a mode-scrambling portion.

6. An optical beam transmission system or method according to claim 5, wherein the divergent storage fiber includes a non-circular core configured to act as the mode-scrambling portion.

7. An optical beam transmission system or method according to claim 5, wherein the mode-scrambling portion extends over the entire length of the divergent storage fiber.

8. An optical beam transmission system or method according to claim 1 or claim 2, wherein the divergent storage fiber includes a circular core that defines a circular aperture for the output beam.

9. An optical beam transmission system or method according to claim 1 or claim 2, wherein the divergent preservation fiber includes one or more microbends configured to act as the mode-distribution homogenization portion.

10. An optical beam transmission system or method according to claim 1 or claim 2, wherein the divergent storage fiber comprises one or more coils configured to act as the mode-distribution homogenization portion.

11. An optical beam transmission system or method according to claim 1 or claim 2, wherein the divergent preservation fiber includes one or more macrobends configured to act as the mode-distribution homogenization portion.

12. An optical beam transmission system or method according to claim 1 or claim 2, wherein the input lateral spatial intensity distribution of the input beam is Gaussian, predominantly Gaussian, flat-top, ring-shaped, saddle-shaped, pedestal-shaped, variable, or fixed.

13. An optical beam transmission system or method according to claim 1 or claim 2, wherein the source fiber is a ring fiber.

14. The optical beam transmission system or method according to claim 1 or claim 2, wherein one or both of the first and second lenses are GRIN lenses having a pitch length of 1 / 4 + 1 / 2 × n, where n is zero or equal to any positive integer.

15. An optical beam transmission system or method according to claim 1 or claim 2, wherein one or both of the first and second lenses are ball lenses or rounded end caps.

16. An optical beam transmission system or method according to claim 1 or claim 2, wherein the divergent storage fiber is a step-index fiber.

17. An optical beam transmission system or method according to claim 1 or claim 2, wherein the first lens has a first effective focal length, and the second lens has a second effective focal length different from the first effective focal length for expanding or reducing the output beam.

18. An optical beam transmission system according to claim 1 or a method according to claim 2, further comprising an output end cap coupled to the output of the second lens.

19. An optical beam transmission system according to claim 1 or a method according to claim 2, further comprising a junction in the divergent storage fiber, wherein the junction steps up or down the diameter of one or both of the core and cladding of the divergent storage fiber.

20. A light beam transmission system configured to form a divergent distribution mapped from an azimuthal asymmetric core shape, A step-index fiber configured to supply an initial beam, the step-index fiber having the azimuthal asymmetric core shape, A lens coupled to the step-index fiber, which receives the initial beam from there and converts it into a modified beam having the divergent distribution, A light beam transmission system equipped with the following features.

21. A method for generating a divergence distribution mapped from an azimuthal asymmetric core shape, Steps include: guiding the initial beam with the azimuthal asymmetric core shape using a step-index fiber; In a lens coupled to the step-index fiber, the steps include receiving the initial beam, receiving the initial beam from there, and converting the initial beam into a modified beam having the divergent distribution, Methods that include...

22. The optical beam transmission system or method according to claim 20 or claim 21, wherein the lens is a GRIN lens having a pitch length of 1 / 4 + 1 / 2 × n, where n is zero or equal to any positive integer.

23. An optical beam transmission system or method according to claim 20 or claim 21, wherein the azimuthal asymmetric core shape includes a rectangular shape.

24. An optical beam transmission system or method according to claim 20 or claim 21, wherein the azimuthal asymmetric core shape includes a rectangular shape.

25. An optical beam transmission system or method according to claim 20 or claim 21, wherein the azimuthal asymmetric core shape includes a plurality of circular cores spaced apart from each other.

26. An optical beam transmission system or method according to claim 20 or claim 21, wherein the azimuthal asymmetric core shape includes a ring shape.

27. An optical beam transmission system according to claim 20 or a method according to claim 21, further comprising a collimator lens that receives the modified beam and converts it into a parallelized beam, wherein the parallelized beam has a near-field shape corresponding to the azimuthal asymmetric core shape and a depth of focus ranging from about 0.1 to about 10 meters.

Images