Patent Application
20250035871
The technology disclosed herein relates to fiber lasers and fiber-coupled lasers. More particularly, the disclosed technology relates to methods, apparatus, and systems for adjusting a near-field transverse spatial intensity distribution at an output of a fiber laser or fiber-coupled laser.
As described in U.S. Pat. No. 10,423,015, titled “Adjustable Beam Characteristics.” and its related patents, nLIGHT, Inc. developed technology for varying beam properties by controlling a spatial intensity distribution of a laser beam, i.e., adjusting its near-field intensity distribution.
The '015 patent describes a fiber operable to provide a laser beam having variable beam characteristics (VBC) that may reduce cost, complexity, optical loss, or other drawbacks of the conventional methods. This VBC device is configured to vary a wide variety of optical beam characteristics. Such beam characteristics can be controlled using the VBC device thus allowing users to tune various beam characteristics to suit the particular requirements of an extensive variety of laser processing applications. For example, a VBC device may be used to tune: beam diameter, divergence distribution, BPP, intensity distribution, M2 factor, NA, optical intensity, power density, radial beam position, radiance, spot size, or the like, or any combination thereof.
In some embodiments, the '015 patent describes adjusting the coupling of the beam into a so-called ring fiber, which has two or more guiding regions. A ring fiber has one or more annular cores optionally surrounding a central (non-annular) core, with low-index glass layers separating the cores so that light coupled into a core will be guided in that core. The angular intensity distribution of the beam need not be directly controlled and depends on various factors such as the laser divergence, bending of the upstream optical fibers, and overlap of the near-field transverse spatial intensity distribution with the guiding confinement regions of the ring fiber.
To achieve a variety of beam diameters and shapes using embodiments and techniques described in the '015 patent, it is possible to divide the spatial intensity distribution of a laser beam coupled to a ring fiber between two or more guiding zones of the fiber. This approach causes a portion of the beam to overlap with the low-index region(s) separating the zones, resulting in increased divergence. This increased divergence is undesirable for some applications, e.g., it can cause overheating of some process optics.
This disclosure describes embodiments for controlling and adjusting the near-field intensity distribution of a beam that emerges from an optical fiber train (i.e., coupled lengths of optical fiber) by controlling and adjusting its angular intensity distribution within a portion of the fiber train. The angular intensity distribution is preserved and propagated toward the end of the fiber train using a step-index fiber. A graded-index (GRIN) fiber of a certain length is spliced to the distal end of the step-index fiber to convert the angular intensity distribution to a near-field intensity distribution. The technique, therefore, need not use a ring fiber and thus avoids the aforementioned issue of divergence increase.
In one aspect, an optical beam delivery device includes multiple lengths of optical fiber arranged along an optical axis. The multiple lengths of optical fiber are configured to modify an angular intensity distribution of an input beam so that it is converted to an output beam having an adjustable near-field transverse spatial intensity distribution. The optical beam delivery device includes a first length of optical fiber having a first input and a first output. The first input is couplable to a source fiber and configured to receive therefrom the input beam that is azimuthally symmetric with respect to the optical axis. The first length of optical fiber has a first refractive index profile (RIP) configured to produce, in response to a controllable perturbation, a change in the angular intensity distribution from a first angular intensity distribution corresponding to the input beam to a second angular intensity distribution corresponding to a modified beam at the first output. The optical beam delivery device includes a second length of optical fiber having a second input and a second output. The second input is coupled to the first output of the first length of optical fiber and configured to receive therefrom the modified beam. The second length of optical fiber has a step-index RIP configured to preserve the second angular intensity distribution and provide at the second output the modified beam with a preserved angular intensity distribution. The optical beam delivery device includes a third length of optical fiber having a length of about ¼ pitch+N*½ pitch, where N is any positive integer and includes zero. The third length of optical fiber includes a third input and a third output. The third input is coupled to the second output of the second length of optical fiber and configured to receive therefrom the modified beam having the preserved angular intensity distribution. The third length of optical fiber having a graded index (GRIN) RIP configured to generate at the third output the output beam having the adjustable near-field transverse spatial intensity distribution corresponding to the preserved angular intensity distribution of the modified beam.
The optical beam delivery device may also include the first length of optical fiber having a GRIN optical fiber segment.
The optical beam delivery device may also include the second length of optical fiber as a step-index optical fiber segment having a length configured to azimuthally scramble the modified beam.
The optical beam delivery device may also include the third length of optical fiber configured to generate a ring-shaped beam as the output beam.
The optical beam delivery device may also include the third length of optical fiber having a different effective focal length than that of the first length of optical fiber.
The optical beam delivery device may also include the first length of optical fiber being configured to change the first angular intensity distribution by increasing an angular width of the input beam.
The optical beam delivery device may also include the first length of optical fiber having a first portion and a second portion coupled to the first portion. The first portion includes a GRIN optical fiber segment. The second portion including divergence structures having a refractive index configured to increase divergence.
The optical beam delivery device may also include the first length of optical fiber having an input GRIN portion configured to collimate the input beam to provide a collimated beam, a central GRIN portion configured to shift the collimated beam in response to the controllable perturbation so as to provide a shifted beam, and an output GRIN portion configured to focus the shifted beam to provide the modified beam having an angular offset that is different from that of the input beam.
The optical beam delivery device may also include the GRIN optical fiber segment being responsive to the controllable perturbation configured to change a bend radius of the GRIN optical fiber segment.
The optical beam delivery device may also include the different effective focal length being attributable to a different gradient constant selected to impart magnification of the output beam.
The optical beam delivery device may also include the second length of optical fiber configured to preserve the angular width of the modified beam.
The optical beam delivery device may also include the first length of optical fiber being responsive to the controllable perturbation applied to the first length of optical fiber as a microbend.
The optical beam delivery device may also include the GRIN optical fiber segment having a length of about N*½ pitch, where N is any positive integer.
An optical beam delivery system may include the optical beam delivery device and a ring fiber acting as the source fiber coupled thereto. A variable beam characteristic (VBC) device may include the ring fiber.
An optical beam delivery system may include the optical beam delivery device having a step-index fiber acting as the source fiber coupled thereto.
Other technical features may be readily apparent to one skilled in the art from the following figures, descriptions, and claims.
The foregoing and other objects, features, and advantages will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures, which may not be drawn to scale.
To easily identify the discussion of any particular element or act, the most significant digit or digits in a reference number refer to the figure number in which that element is first introduced.
As used in this application and in the claims, the singular forms “a,” “an,” and “the” include the plural forms unless the context clearly dictates otherwise. Additionally, the term “includes” means “comprises.” Further, the term “coupled” does not exclude the presence of intermediate elements between the coupled items.
The systems, apparatuses, and methods described herein should not be construed as limiting in any way. Instead, the present disclosure is directed toward all novel and non-obvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. The disclosed systems, methods, and apparatuses are not limited to any specific aspect or feature or combinations thereof, nor do the disclosed systems, methods, and apparatuses require that any one or more specific advantages be present, or problems be solved. Any theories of operation are to facilitate explanation, but the disclosed systems, methods, and apparatuses are not limited to such theories of operation.
Although the operations of some of the disclosed methods are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed systems, methods, and apparatuses can be used in conjunction with other systems, methods, and apparatuses. Additionally, the description sometimes uses terms like “produce” and “provide” to describe the disclosed methods. These terms are high-level abstractions of the actual operations that are performed. The actual operations that correspond to these terms will vary depending on the particular implementation and are readily discernible by skilled persons.
In some examples, values, procedures, or apparatuses are referred to as “lowest,” “best,” “minimum,” or the like. It will be appreciated that such descriptions are intended to indicate that a selection among many used functional alternatives can be made, and such selections need not be better, smaller, or otherwise preferable to other selections. Examples are described with reference to directions indicated as “above,” “below,” “upper,” “lower,” and the like. These terms are used for convenient description, but do not imply any particular spatial orientation. Moreover, 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 mechanical, electrical, and optical elements necessary for operation.
As used herein, optical radiation refers to electromagnetic radiation at wavelengths of between about 100 nm and 10 μm, and typically between about 500 nm and 2 μm. Examples based on available laser diode sources and optical fibers generally are associated with wavelengths of between about 800 nm and 2,000 nm. In some examples, propagating optical radiation is referred to as one or more beams having diameters, asymmetric fast and slow axes, beam cross-sectional areas, and beam divergences that can depend on beam wavelength and the optical systems used for beam shaping. For convenience, optical radiation is referred to as light or beams in some examples and need not be at visible wavelengths. Forward-propagating light or optical beams or beam portions refer to light, beams, or beam portions that propagate in a direction of normal emission. Backward-propagating light or optical beams or beam portions refer to light, beams, or beam portions that propagate in an opposite direction of normal emission.
To understand the characteristics of embodiments described in the present disclosure, it is helpful to first understand the underlying components and physical phenomena exploited with the components. Accordingly, the following paragraphs summarize optical fibers and lenses.
Optical fibers: An optical fiber is a thin, flexible strand of material (typically glass) that guides light within a core that is surrounded by a cladding. The core has a higher refractive index than the cladding, causing light to be guided by total internal reflection. The refractive index profile (RIP), i.e., the refractive index as a function of position transverse to the fiber axis, determines many of the important properties of the fiber. Many but not all fibers are cylindrically symmetric (i.e., the index is independent of azimuthal angle about the fiber axis). The optical fibers, therefore, may have circular, square, rectangular, polygonal, oval, elliptical, or other cross-sections. In some embodiments, the core and claddings are approximately concentric. In other examples, one or more of the core and claddings are decentered, and in some examples, core and cladding orientation and/or displacement vary along a waveguide length.
Optical fibers are typically formed of silica (glass) that is doped (or undoped) to provide predetermined RIPs. In some examples, fibers or other waveguides are made of other materials such as fluorozirconates, fluoroaluminates, fluoride or phosphate glasses, chalcogenide glasses, or crystalline materials such as sapphire, depending on wavelengths and other properties of interest. Refractive indices of silica and fluoride glasses are typically about 1.5, but refractive indices of other materials such as chalcogenides can be 3 or more. In still other examples, optical fibers can be formed in part or completely of plastics (polymers).
In some examples, a doped waveguide core such as a fiber core provides optical gain in response to pumping. In the examples disclosed herein, a waveguide core such as an optical fiber core is doped with a rare earth element such as Nd, Yb, Ho, Er, or other active dopants or combinations thereof. Such actively doped cores can provide optical gain in response to optical or other pumping. As disclosed below, waveguides having such active dopants can be used to form optical amplifiers, or, if provided with suitable optical feedback such as reflective layers, mirrors, Bragg gratings, or other feedback mechanisms, such waveguides can generate laser emissions. Optical pump radiation can be arranged to co-propagate and/or counter-propagate in the waveguide with respect to a propagation direction of an emitted laser beam or an amplified beam.
Step-index fibers: The most common RIP is known as “step index,” in which the core has a uniform index and is surrounded by the cladding with a uniform (lower) index. In a step-index fiber, light launched into the fiber core in a given location tends to spread radially and azimuthally to fill the core, i.e., the core does not preserve spatial information about the location of the launched beam. Light rays launched at a particular angle with respect to the fiber axis tend to exit the fiber core at the same angle (in a cone because of azimuthal scrambling), i.e., step-index fibers typically preserve angular information. In particular, the preservation of launch and the output angles (i.e., the far-field) is a property leveraged by embodiments of this disclosure. (Note that the terms far- and near-field intensity distribution mean, respectively, an angular intensity distribution (angular as with respect to the optical axis) and a transverse spatial intensity distribution.) Various effects can cause imperfect preservation of the launch angle (e.g., causing higher divergence for the output than the input rays), and multiple studies have investigated methods to minimize this effect.
GRIN fibers: Some fibers have a non-uniform RIP in the core and (less often) in the cladding. A common, non-uniform core RIP is known as “graded index” or “GRIN,” in which the index value decreases monotonically from the center of the core to the cladding. In many cases, the GRIN profile is parabolic. The size of a beam coupled into a parabolic GRIN fiber may oscillate periodically as the beam propagates along the fiber, with the oscillation period known as the “pitch” of the fiber. A parabolic GRIN fiber with an appropriate length can act as a lens that focuses or collimates a beam. This property is also used in free-space GRIN lenses, i.e., it is not limited to fibers.
Multi-clad fibers: Another example of a non-uniform RIP is a fiber with multiple claddings (“double-clad fiber,” “triple-clad fiber,” etc.). As with other non-uniform RIPs, propagation of light in a multi-clad fiber depends on the specific RIP and the launch conditions. Light launched into the highest-index guiding region may not spread into all the cladding regions, but light launched into a cladding layer will spread into all the higher index regions.
Multi-core fibers: Some fibers have multiple cores, i.e., multiple high-index guiding regions separated by surrounding low-index regions. The cores can have different sizes and shapes. Light launched into a given core will typically spread out to fill that core (as in a fiber with a single core) but will not spread into the other cores because the surrounding low-index region prevents propagation between the cores. Multi-core fibers are generally more expensive than more standard fibers, especially step-index fibers. The low-index regions separating the guiding regions generate highly divergent light if a portion of the beam is coupled into them. The downstream optics must be designed to accommodate this excess divergence, which typically adds cost, complexity, and/or optical loss (reduced efficiency). If not properly managed, the excess divergence can cause overheating or damage of the optics. The dimensions of the guiding regions are fixed when the fiber is manufactured. The beam dimensions thus cannot be continuously varied (although the power in each region can be continuously varied). Furthermore, if an application prefers different dimensions (e.g., a larger diameter ring), a new multi-core fiber must be fabricated, which can lead to a proliferation of product designs (which complicates and adds cost to manufacturing). Splicing of multi-core fibers in the factory and field is complicated by the need to align multiple guiding regions, which each have fabrication tolerances. Achieving the desired splicing performance can require excessively tight fiber tolerances (increasing cost), low splice yields, and/or performance degradation upon splicing (e.g., from coupling light into the low-index regions).
Note that the above discussion of the different types of optical fibers primarily pertains to multimode fibers, in which the ray picture of light propagation is accurate. Single-mode or near-single-mode fibers introduce other phenomena that are less relevant to the embodiments described in this disclosure.
Lenses: Lenses are ubiquitous in optical systems. A lens is conventionally thought of as a piece of glass with one or both surfaces curved, and the direction of propagation of a light ray is influenced by refraction at the surfaces. Other implementations of lenses are possible, including a GRIN lens (as mentioned above) in which the RIP of the material varies with radial position (rather than employing curved surfaces). The end of a fiber or an end cap attached to a fiber can have a curved surface to function as a lens. A design property of a lens is its focal length. A collimated input beam is focused to a point one focal length away from the lens (in the ray picture). Similarly, a point source located one focal length away from a lens is collimated at the lens output. Stated more generally, a lens maps between position and angle, i.e., a lens converts the angle of a ray at the input focal plane to position at the output focal plane, and it converts the input position of a ray at the input focal plane to angle at the output focal plane.
In one embodiment of
Source fiber 114 is spliced to a first length of optical fiber 118, which has a first input 120 and a first output 122. Thus, first input 120 is coupled to a source fiber 114 and is configured to receive therefrom input beam 108 that is azimuthally symmetric with respect to optical axis 104.
First length of optical fiber 118 has a first RIP 124. First RIP 124 is configured to produce, in response to a controllable perturbation (see, e.g, bending shown in
In the example of
where k is the gradient constant determined by the refractive index profile of the GRIN fiber. The refractive index profile of the GRIN fiber is defined as set forth in W. J. Smith, Modern Optical Engineering, Third Edition, McGraw-Hill, Inc. (2000), p. 286:
where n0 is the axial refractive index of the GRIN fiber.
In some embodiments, first length of optical fiber 118 and an initial section of a second length of optical fiber 134 are bent a variable amount (or not at all) around a mandrel (not shown) or other perturbation device shown and described in
Second length of optical fiber 134, which includes a second input 136 and a second output 138, is coupled to first output 122 and configured to receive therefrom modified beam 130. Second length of optical fiber 134 has a step-index RIP 140 configured to preserve second angular intensity distribution 128. Modified beam 130 enters a step-index fiber core 142 (e.g., core diameter of 300 μm) and propagates while substantially preserving the angular offset with respect to optical axis 104. Second angular intensity distribution 128 is preserved, i.e., its ray angles relative to optical axis 104 (i.e., the fiber core) remain substantially constant from second input 136 to second output 138. Although it is largely preserved, for purposes of clarity (and recognizing that there may be some minor variation in the distributions) this disclosure refers to second angular intensity distribution 128 at second output 138 as a preserved angular intensity distribution 144.
In some embodiments, second length of optical fiber 134 may be about five meters to about 50 meters long so as to transport modified beam 130 a significant distance from the source (e.g., to a processing head or tool). As modified beam 130 travels through second length of optical fiber 134, second angular intensity distribution 128 is azimuthally averaged (i.e., smoothed or symmetrized) due to normal routing (coiling or bending) of this delivery fiber. Thus, phi ray angles (azimuthal) are assumed to be scrambled in second length of optical fiber 134. The angular width of input beam 108 results from rays having slightly different angles. These rays take different paths (generally helical) and end up being scrambled azimuthally (cylindrically symmetric, phi invariant), although for simplicity
Finally,
The bottom axis of
Through simulations, GRIN pitch, fiber length, and bending profile may be tailored to achieve a desired angular offset of input beam 108 with a minimal change in divergence. For example, to maximize angular offset without increasing divergence, a specific length of GRIN fiber could be used as the first length of optical fiber and subjected to bending to create a desired angular offset. The pitch of the GRIN may be selected to increase the beam diameter, thus lowering its divergence width and maximizing the ratio between angular offset and width. Specifically, for a given input beam divergence and spot size, a focal length of a ¼-pitch GRIN fiber may be chosen to decrease the divergence of the beam exiting the GRIN fiber.
For example, an input GRIN having a gradient constant of 2.0 and output GRIN having a gradient constant of 0.5 results in an approximately 2× magnification of the near field beam diameter. Likewise, an input GRIN having a gradient constant of 0.5 and output GRIN having a gradient constant of 2.0 results in an approximately 0.5× magnification of the near-field beam diameter. Gradient constants ranging between 0.1 and 50 are also possible, depending on the application.
Note that the GRIN design pitch can be selected to size the output near-field diameters and divergences. Through simulations, GRIN pitch, fiber length, and bending profile may be tailored to achieve a desired output.
Skilled persons will appreciate that various techniques and fibers may be used to increase divergence in response to perturbation of first length of optical fiber 902. For example, first length of optical fiber 902 may be a step-index fiber configured to respond to a non-adiabatic microbend (see, e.g., micro-bend 2404 in FIG. 24 of the '015 patent).
In another embodiment,
Skilled persons will appreciate in light of this disclosure that both optical beam delivery device 1100 and optical beam delivery device 100 (
In block 1202, method 1200 adjusts, in response to controllable perturbation applied to a first length of optical fiber, the angular intensity distribution of the input beam from a first angular intensity distribution at a first input of the first length of optical fiber to a second angular intensity distribution of a modified beam at a first output of the first length of optical fiber, the first angular intensity distribution being azimuthally symmetric with respect to an optical axis of the first length of optical fiber.
In block 1204, method 1200 relays, through a second length of optical fiber coupled to the first length of optical fiber, the modified beam having the second angular intensity distribution. In some cases, the second length of optical fiber largely preserves the second angular intensity distribution as the beam propagates through the fiber.
In block 1206, method 1200 converts, with a third length of optical fiber coupled to the second length of optical fiber, from the second angular intensity distribution to a near-field transverse spatial intensity distribution provided at an output of the third length of optical fiber, the third length of optical fiber including a GRIN optical fiber segment.
Having described and illustrated the general principles of examples of the presently disclosed technology, it should be apparent that the examples may be modified in arrangement and detail without departing from such principles. Skilled persons, therefore, will appreciate that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.
This application claims the benefit of U.S. Provisional Patent Application No. 63/264,574, filed Nov. 24, 2021, the disclosure of which is hereby incorporated by reference herein in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2022/080460 | 11/23/2022 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63264574 | Nov 2021 | US |
