LMA optical fiber with an annular refractive index reduction region within the core

Abstract

An active large-mode area (LMA) optical fiber is provided to mitigate transverse-mode instability (TMI) effects. The active LMA optical fiber comprises a core and one or more claddings surrounding the core. The core has a core diameter and an annular refractive index reduction region in the cross-section of the LMA optical fiber. The annular refractive index reduction region has an outer diameter smaller than the core diameter, and its local refractive index is lower than that of the rest of the core's cross-sectional area. The core has a core glass composition comprising one or more rare-earth dopants and at least one co-dopant. The core glass composition defines the radial refractive index profile in the core of the active LMA fiber, which includes a recess along the annular refractive index reduction region.

Description

Technical Field

[0001] This technical field generally relates to active large mode area optical fibers, and more particularly to fibers having a core with a radial refractive index profile that reduces transverse mode instability effects.

Background Art

[0002] Transverse mode instability (TMI) is a phenomenon that occurs in active large mode area (LMA) optical fibers when the heat load in the fiber exceeds a certain threshold. These optical fibers are often used in optical amplifiers and have a core doped with rare earth ions such as ytterbium. The heat load generally results from the quantum defect of radiative transitions in the core material initiated by optical pumping, but is also understood to result from background losses and photodarkening in the fiber. The instability causes an undesirable refractive index grating to form in the fiber core according to the interference between the fundamental transverse mode (LP 01 ) and the first higher-order mode (LP 11 ) and the former releasing some energy to the latter. Mode coupling occurring beyond a certain threshold causes beam distortion and pointing fluctuations in the amplifier output, thus restricting the use of these fibers in some applications.

[0003] TMI is known in the art as a significant obstacle to attempts to scale the power of active fiber-based lasers exceeding several kilowatts, as shown in Figure 1 (prior art). LMA fibers are known to support higher-order transverse modes in addition to the fundamental transverse mode. In fact, LMA fibers with a core diameter 50 times the wavelength of light propagating through them may not be considered single-mode, even with the most specialized designs known in the art. Such large core diameters are necessary to reduce the light intensity within the fiber core and avoid distortions induced by nonlinear effects such as stimulated Raman scattering. On the other hand, TMI can be prevented by using fibers with smaller core diameters, which, if possible, can support only the fundamental mode.

[0004] Various strategies for mitigating TMI effects have been reported in the art. For example, U.S. Patent No. 9,214,781 (Honea et al.) discloses an apparatus and method for suppressing mode instability in a fiber amplifier system. This apparatus includes a hybrid fiber with a smaller core at the initial length where the thermal load is highest, followed by a fiber with a larger core. U.S. Patent No. 9,972,961 (Sipes et al.) describes an optical fiber amplifier system including two stages composed of fibers with different core / cladding diameters, these stages connected via a double-mode adapter. Active methods for inhibiting the onset of TMI are also disclosed in the art, for example, U.S. Patent No. 9,235,106 (Jauregui Misas et al.) and U.S. Patent Application Publication No. 2017 / 0299900 (Montoya et al.). U.S. Patent No. 9,325,151 (Fini et al.) and U.S. Patent No. 10,263,383 (Headley) disclose TMI mitigation methods with specific layouts for fiber spools in laser systems. Finally, U.S. Patent Application No. 17 / 836.153 (Roy et al.) discloses an active LMA optical fiber having a core configured to mitigate the TMI effect. The core has a central core region and a peripheral core region, which are doped such that the radial refractive index profile of the core is substantially continuous and the temperature coefficient of the core material is lower in the peripheral region than in the central region.

[0005] TMI mitigation strategies that offer at least some improvements over conventional technologies are still needed in this field. [Overview of the Initiative]

[0006] According to one embodiment, an active large-mode area (LMA) optical fiber is provided for mitigating transverse-mode instability (TMI) effects, and the active LMA optical fiber is - Core and, - Including one or more cladding surrounding the core, An active LMA fiber having a core diameter, an annular refractive index reduction region in the cross-section of the LMA optical fiber, the annular refractive index reduction region having an outer diameter smaller than the core diameter, the core having a core glass composition comprising one or more rare earth dopants and at least one co-dopant, the core glass composition defining the radial refractive index profile in the core of the active LMA fiber, the radial refractive index profile having a recess along the annular refractive index reduction region.

[0007] In some implementations, at least one co-dopant is provided in the annular refractive index reduction region, and one or more refractive index reduction co-dopants are not present in the core outside the annular refractive index reduction region.

[0008] In some implementations, at least one co-dopant comprises one or more de-refractive-index co-dopants that are provided to the core at a higher concentration in the annular de-refractive-index region than outside the annular de-refractive-index region.

[0009] One or more refractive index-decreasing co-dopants may include at least one of fluorine and boron. In some implementations, at least one co-dopant may include one or more refractive index-raising co-dopants provided outside the cyclic refractive index-decreasing region and not present in the refractive index-decreasing region. In some implementations, at least one co-dopant may include one or more refractive index-raising co-dopants provided in the core at higher concentrations outside the cyclic refractive index-decreasing region than in the cyclic refractive index-decreasing region. One or more refractive index-raising co-dopants may include at least one of aluminum, phosphorus, and germanium.

[0010] In some implementations, the recesses of the radial refractive index profile have tapered side edges on the outside of the core.

[0011] In some implementations, the recesses of the radial refractive index profile have tapered side edges on the inside of the core.

[0012] In some implementations, the recesses in the radial refractive index profile have a uniform depth.

[0013] In some implementations, the core has a central region inside an annular refractive index reduction region, and the radial refractive index profile within the central region is uniform or non-uniform. In some modifications, the refractive index difference between the central region of the core and the annular refractive index reduction region may be smaller than the refractive index difference between the central region of the core and the innermost cladding of one or more claddings. In some modifications, the core may have an outer region extending between the refractive index reduction region and one or more claddings, and the refractive index within the outer region is different from that of the central region.

[0014] In some implementations, the core diameter d core It has a value of less than 100 μm.

[0015] In some implementations, the core has an numerical aperture (NA) of less than 0.10.

[0016] In some implementations, the width of the annular refractive index reduction region ranges from approximately 1 to approximately 10 times the wavelength of light propagating within the core.

[0017] In some implementations, the depth of the recess in the radial refractive index profile is approximately 10 -4 ~about 10 -3 It has the value of .

[0018] In some implementations, the active LMA optical fiber further comprises a stress-applying member that extends longitudinally along the core within one or more claddings.

[0019] In some implementations, one or more rare-earth dopants are concentrated in the central region of the core and include at least one of ytterbium, erbium, thulium, neodymium, holmium, and praseodymium.

[0020] Other features and advantages will be better understood by referring to the attached drawings and reading the detailed embodiments. [Brief explanation of the drawing]

[0021] [Figure 1] (Conventional Technology) This graph shows the laser slope efficiency exceeding the TMI threshold, measured using a conventional polarization-retaining 20 / 400 LMA optical fiber. [Figure 2] Figures 2A and 2B show the approximate cross-sectional area and radial refractive index profile of an active LMA optical fiber according to one embodiment, respectively. [Figure 3] Figures 3A and 3B show the approximate cross-sectional area and radial refractive index profile of an active LMA optical fiber according to one embodiment in which the fiber retains polarization, respectively. [Figure 4] (Prior technology) This graph shows the ratio of the LP11 mode field area to the LP01 mode field area of ​​a step refractive index optical fiber as a function of the normalized frequency V. [Figure 5A] This is an example of a radial refractive index profile of an LMA optical fiber core having annular refractive index reduction regions of varying depths and widths. [Figure 5B] This is an example of a radial refractive index profile of an LMA optical fiber core having annular refractive index reduction regions of varying depths and widths. [Figure 5C] This is an example of a radial refractive index profile of an LMA optical fiber core having annular refractive index reduction regions of varying depths and widths. [Figure 5D] This is an example of a radial refractive index profile of an LMA optical fiber core having annular refractive index reduction regions of varying depths and widths. [Figure 5E] This is an example of a radial refractive index profile of an LMA optical fiber core having annular refractive index reduction regions of varying depths and widths. [Figure 5F] This is an example of a radial refractive index profile of an LMA optical fiber core having annular refractive index reduction regions of varying depths and widths. [Figure 5G] This is an example of a radial refractive index profile of an LMA optical fiber core having annular refractive index reduction regions of varying depths and widths. [Figure 5H] This is an example of a radial refractive index profile of an LMA optical fiber core having annular refractive index reduction regions of varying depths and widths. [Figure 6A] These graphs show the radial intensity distribution of the LP01 mode (Figure 6A) and the radial intensity distribution of the LP11 mode (Figure 6B) for a 35 μm core fiber with a step refractive index profile (dotted line) and a 45 μm core fiber with an annular refractive index reduction region (solid line), respectively. [Figure 6B] These graphs show the radial intensity distribution of the LP01 mode (Figure 6A) and the radial intensity distribution of the LP11 mode (Figure 6B) for a 35 μm core fiber with a step refractive index profile (dotted line) and a 45 μm core fiber with an annular refractive index reduction region (solid line), respectively. [Figure 7] Figure 5A is a graph of the ratio of LP11 and LP01 mode field areas for step refractive index fibers (dashed line) and LMA optical fibers (solid line) having cores with radial refractive index profiles as shown. The normalized frequency of an LMA optical fiber as disclosed herein is determined from the parameters of an equivalent step refractive index fiber having an annular refractive index reduction region depth in the range of 0 to 8 × 10⁻⁴ (black diamond). [Figure 8A] The radial refractive index profiles of LMA optical fibers in various embodiments of multi-clad fibers are shown. [Figure 8B] The radial refractive index profiles of LMA optical fibers in various embodiments of multi-clad fibers are shown. [Figure 8C] The radial refractive index profiles of LMA optical fibers in various embodiments of multi-clad fibers are shown. [Figure 9]An exemplary radial refractive index profile of the core of an LMA optical fiber having an annular refractive index reduction region is shown, where the rare earth dopant (shown herein as a gray shaded region) is concentrated in the central region of the core. [Modes for carrying out the invention]

[0022] In the following description, similar features in the drawings are given the same reference numerals. To avoid unduly obstructing the drawings, some elements may not be shown in some drawings if they have already been mentioned in preceding drawings. Also, please note that the elements in the drawings in this specification are not necessarily drawn to a fixed scale, and instead, the focus is on clearly illustrating the elements and structures of this embodiment.

[0023] The terms “a,” “an,” and “one” are defined herein to mean “at least one,” and thus, unless otherwise stated, these terms do not exclude multiple units. Terms such as “substantially,” “generally,” and “about” modifying values, conditions, or characteristics of the exemplary embodiment should be understood to mean that the values, conditions, or characteristics are defined within tolerances acceptable for the proper operation of this exemplary embodiment for its intended use.

[0024] Unless otherwise stated, the terms “connected” and “joined,” as well as their derivatives and variations, refer herein to any direct or indirect structural or functional connection or joining between two or more elements. For example, the connection or joining between elements may be mechanical, optical, electrical, logical, or any combination thereof.

[0025] In this specification, the terms “light” and “optical,” as well as their variations and derivatives, are used to refer to radiation in any suitable region of the electromagnetic spectrum. Therefore, the terms “light” and “optical” are not limited to visible light, but may also include, but are not limited to, the infrared or ultraviolet regions of the electromagnetic spectrum. Furthermore, those skilled in the art will understand that the definitions of spectral ranges, and their boundaries, for the ultraviolet, visible, and infrared regions may differ depending on the art or the definitions under consideration, and this does not mean to limit the scope of this technology.

[0026] To provide a more concise explanation, some of the quantitative expressions given herein may be modified with the term “about.” Whether the term “about” is explicitly used or not, all quantities given herein are meant to refer to an actual given value, and also to an approximation to such a given value that is reasonably inferred by those skilled in the art, including approximations by experimental and / or measurement conditions for such a given value.

[0027] In this specification, the term “approximately” means that a particular value is within an acceptable margin of error as determined by those skilled in the art, which in part depends on how the value is measured or determined, i.e., the limits of the measuring system. A measure of 10% precision is generally accepted as acceptable and encompasses the term “approximately.”

[0028] In this specification, when a wide range of numbers is provided, any possible narrower range within the boundaries of that wide range is also contemplated. For example, when a wide range of values ​​from 0 to 1000 is provided, any narrower range from 0 to 1000 is also contemplated. When a wide range of values ​​from 0 to 1 is mentioned, any narrower range between 0 and 1, i.e., decimal values, is also contemplated.

[0029] According to some embodiments, an active large-mode area (LMA) optical fiber is provided having a core with a refractive index profile that mitigates transverse-mode instability (TMI) effects.

[0030] Referring to Figures 2A and 2B, the schematic cross-sectional area and corresponding radial refractive index profile 26 of an active LMA optical fiber 20 according to one embodiment are shown. Thus, the profile 26 in Figure 2B shows how the local refractive index of the optical fiber changes along an arbitrary given radial line passing through the center of the fiber's cross-sectional area.

[0031] The active LMA optical fiber 20 includes a core 22 and one or more claddings 40a, 40b, ... surrounding the core 22. In some modifications, the active LMA optical fiber 20 may have a single cladding 40a. In other embodiments, the active LMA optical fiber 20 may be a multi-clad optical fiber including two or more claddings 40a, 40b, .... As is well known in the art, a polymer jacket or other coating (not shown) may be provided around the outermost cladding 40b. The example shown in Figure 2A shows a double-clad fiber including a first cladding 40a and a second cladding 40b, but it will be readily apparent that in other embodiments, more cladding may be provided. Multi-clad fibers can be used to amplify light in devices such as fiber lasers and amplifiers. Such devices are used in a wide range of applications in fields such as medical and surgical, scientific instruments, semiconductor device manufacturing, military technology, and industrial material processing.

[0032] In some variations, as shown in Figures 3A and 3B, for example, the active LMA optical fiber may include stress-applying members 42a, 42b that extend longitudinally along the core 22 into at least one of one or more claddings 40a, 40b, ... to maintain the polarization of the light in a linear state. In the embodiment shown in Figure 3A, the stress-applying members 42a, 42b extend into the first cladding 40a. Optical fibers with stress-applying members are commonly known in the art as panda or bowtie polarization-retaining fibers. Such special fibers are used in fiber lasers and amplifiers in some of the above fields, for example, when a particular application requires the linear polarization of the light. For example, the generation of laser frequency harmonics using nonlinear optical crystals is one example where linear polarization is required, and chirp pulse amplification using a lattice-pair pulse compressor in ultrafast lasers is another example.

[0033] The parameters of the active LMA optical fiber 20, such as the composition, size, and configuration of the core 22 and one or more claddings 40a, 40b, ..., can be selected considering the intended application of the fiber. As is known in the art, the core 22 is typically configured to support the propagation of an optical beam amplified in the basic core mode of a minority-mode active LMA optical fiber 20, while the claddings 40a, 40b, ..., are configured to confine the optical beam within the core 22. When the active LMA optical fiber 20 is used in a cladding pump implementation, at least one of the claddings 40a, 40b, ..., may also be configured to support and guide the optical pump beam in one or more cladding modes.

[0034] Returning to Figures 2A and 2B, the core 22 has a core diameter d coreIt has. The core 22 includes an annular (ring-shaped) refractive index lowering region 24 in any given cross-section P of the LMA optical fiber 20 corresponding to the plane of the page in the representation of FIG. 2A. The annular refractive index lowering region 24 is depicted in black in FIG. 2A. As well shown in FIG. 2B, the local refractive index of the region 24 is lower than that of the remaining portion of the cross-sectional area of the core 22. The annular refractive index lowering region 24 has an outer diameter d ring_out and an inner diameter d ring_in and half of the difference between them defines the width t ring of the region 24. In a typical implementation, the outer diameter d ring_out of the annular refractive index lowering region 24 is smaller than the core diameter d core and the inner diameter d ring_in is greater than zero. Thus, the core has a central region 23 inside the refractive index lowering region 24 and an outer region 25 extending between the refractive index lowering region 24 and one or more claddings 40.

[0035] The core 22 has a core glass composition including a glass host such as silica glass and one or more rare earth dopants such as ytterbium (Yb 3+ ), neodymium (Nd 3+ ), erbium (Er 3+ ), thulium (Tm 3+ ), praseodymium (Pr 3+ ), and holmium (Ho 3+ ). As will be readily understood by those skilled in the art, the rare earth dopant provides amplification of light propagating within the core when pumped by an optical pump beam of appropriate energy and defines the "active" characteristics of the active LMA optical fiber 20. By way of example, light having a wavelength in the range from about 1020 nm to about 1080 nm can be initiated by a 976 nm optical pump light having a numerical aperture (NA) of less than 0.46 and amplified from the stimulated emission of radiation incident on one or more of the claddings before being absorbed by the core of the active LMA optical fiber (doped with ytterbium in this example).

[0036] In some implementations, the rare-earth dopant may be confined within the central region 23 of the core 22. This is done, for example, to increase gain saturation by the fundamental mode and minimize overlap of higher-order modes with the rare-earth dopant. The rare-earth dopant may be circumscribed, for example, within a region bounded by an annular refractive index reduction region 24. This is because the portion of the cross-sectional area of ​​the rare-earth doped core has an outer diameter d ring_out Smaller than, inner diameter d ring_in The diameter d may be larger than dopants This means that it is demarcated by [a specific region]. As an example, Figure 9 shows an example of the radial refractive index profile 27 of the core of an LMA optical fiber in which the rare earth dopant (gray shaded) is concentrated in the central region 23 of the core. In other variations, it will be easily understood that the region of the core in which the rare earth dopant is concentrated extends either within or outside the annular refractive index reduction region 24.

[0037] Referring to Figures 2A, 2B, 3A, and 3B, the core glass composition further comprises at least one co-dopant, which may be embodied, for example, aluminum, phosphorus, fluorine, or a combination thereof. The rare earth dopant and the at least one co-dopant have respective concentrations and distributions that determine the radial refractive index profile 27 of the core 22, i.e., a portion of the entire radial refractive index profile 26 of the active LMA optical fiber 20 located within the core 22. As will be further described below, the radial refractive index profile 27 of the core 22 includes recesses 28 along an annular refractive index reduction region 24.

[0038] The molar refractive index values ​​of dopants are reported in the scientific literature and are most frequently cited for oxide species (e.g., Al2O3, P2O5, ...). In this specification, rare earth dopants and co-dopants may be referred to using the chemical element names of the corresponding oxides. For example, "aluminum" or "phosphorus" co-dopant and "ytterbium" dopant can be referred to. Those skilled in the art will understand that such elements exist in optical fibers in the form of their oxides. Therefore, the terms "aluminum" and "aluminum oxide" co-dopant can be used interchangeably. Similarly, the terms "phosphorus" and "phosphorus oxide" co-dopant can be used interchangeably. For example, with respect to rare earth dopants such as ytterbium, the terms "ytterbium" and "Yb" can be used interchangeably. 3+ "Ytterbium oxide" and "Yb2O3" are used interchangeably.

[0039] As described above, TMI occurs in active LMA optical fibers when the thermal load of the optical fiber exceeds a certain threshold. In one embodiment, the core structure and composition of the LMA optical fiber disclosed herein may be configured to increase this threshold to a greater output power, thereby mitigating or preventing the occurrence of TMI. In some implementation embodiments, this is achieved with linearly polarized LP 11 Mode and linearly polarized LP guided by LMA optical fiber 01 This can be achieved by adjusting the radial refractive index profile of the active LMA optical fiber 20 to substantially reduce the lateral spatial overlap with the modes, and, in embodiments where the rare earth elements are confined to the center of the core, with the gain region.

[0040] For illustrative purposes only, LP 11 and LP 01 Mode field area ratio A_L P11 / A LP01Using this, the influence of the physical parameters of the LMA optical fiber on the spatial overlap between these modes can be shown. Referring to Figure 4 (prior art), this ratio is plotted for a step refractive index fiber against a normalized frequency V defined as follows: V=πd core NA) / λ Here d core θ is the core diameter, NA is the core numerical aperture as described above, and λ is the wavelength of light in a vacuum. This ratio A LP11 / A LP01 It can be seen that as the normalized frequency V increases, the ratio A decreases to an asymptotic value of approximately 1.3. When V exceeds 4, the ratio A LP11 / A LP01 This does not change much. As a result, core diameter d greater than approximately 20 μm core LP in LMA fiber having 11 and LP 01 Reducing spatial overlap between modes (or ratio A LP11 / A LP01 Increasing it is difficult.

[0041] As described above, and as shown in both Figures 2B and 3B, the core glass composition is such that the radial refractive index profile 27 of the core includes a recess 28 along the annular refractive index reduction region in the embodiment of the active LMA optical fiber described herein. In some implementations, at least one co-dopant may be provided in the annular refractive index reduction region and include one or more refractive index reduction dopants that are absent or present at lower concentrations in the rest of the core. In another variation, at least one co-dopant may be introduced outside the annular refractive index reduction region or include one or more refractive index elevation dopants that are provided at lower concentrations in the annular refractive index reduction region than in the rest of the core. In some implementations, the refractive index elevation dopants may be selected from aluminum, phosphorus, and germanium, while the refractive index reduction dopants may be selected from fluorine and boron.

[0042] Figures 5A to 5H show examples of radial refractive index profiles 27 of the core under various implementation configurations. All radial refractive index profiles shown in Figures 5A to 5H relate to optical fiber structures that can be assumed to have rotational symmetry with respect to the core center position located at X=0. Each vertical axis in these figures corresponds to the incremental refractive index Δn with respect to the refractive index of the first cladding 40a directly surrounding the core 22. This choice means that each radial refractive index profile decreases to zero at the outer edge of the core.

[0043] The recess 28 of the radial refractive index profile 27 of the core is located at the core diameter d core The width may be narrow or wide depending on the core numerical aperture NA. In some examples, the width t of the refractive index recess 28 is... ring For example, in the case of a small core diameter, it may be as small as a few times the wavelength of light, in other examples, the width of the recess 28 may be as large as 10 times the wavelength of light, and in the case of a very large core, it may be even larger.

[0044] The recess 28 also has a core diameter d core And depending on the core NA, it may be shallow or deep. The depth of the refractive index recess 28 is such that n is approximately 10 -4 ~10 -3 It may be within the range of . In some implementations, the recess 28 may have a uniform depth, as shown, for example, in Figures 5A, 5E, 5F, and 5G. In other implementations, the recess may have a radially varying depth that defines tapered side edges 30a or 30b on the inside, outside, or both (see, for example, Figures 5B, 5C, 5D, and 5H). Furthermore, the refractive index of the core's central region 23 may be uniform (see Figures 5A-5E and 5H), or it may be a non-uniform profile, such as a step (or staircase) profile (see Figure 5F) or a dome-shaped profile (see Figure 5G). The refractive index contrast or difference between the core's central region and the annular refractive index reduction region is preferably lower than the refractive index contrast between the core's central region and the first cladding. Therefore, the refractive index of the outer region of the core may differ from that of the central region.

[0045] In some implementations, the refractive index contrast between the outer region 25 of the core 22 and the annular refractive index reduction region 24 is reduced when the fiber is bent. 01 While it can be adjusted to avoid excessive propagation loss in modes, LP 11 Modes and LP 01 This effectively reduces spatial overlap with modes. This is generally possible when the refractive index of the outer region 25 is lower than the refractive index of the central region 23, as shown in Figures 5E to 5H. At some point the trade-off is satisfied, i.e., the refractive index of the outer region 25 is approximately the same as the refractive index of the central region 23, LP 11 Modes and LP 01 While the spatial overlap between modes decreases, LP 01 The bending-induced propagation loss of the modes also increases (and vice versa, since the refractive index of the outer region 25 is set much lower than that of the central region 23).

[0046] The embodiments shown in Figures 5A to 5H all have the same core diameter d core It has a thickness of 45 μm, but the refractive index contrast between the core and cladding is 550 μm. 2 It is adjusted to have a mode field area of ​​. For comparison, core diameter d core The core is 35 μm and has a core NA of 0.065 (refractive index contrast is approximately 1.5 × 10⁻⁶). -3 A step refractive index fiber of ) yields the same mode field area. It will be readily understood that the specific numbers mentioned herein are meant to serve as examples. In some embodiments, the active LMA optical fiber 20 preferably has a core diameter d of less than about 100 μm. core It can have such a feature. Similarly, the active LMA optical fiber 20 can preferably have a core NA of less than about 0.10.

[0047] Referring further to the embodiments shown in Figures 5A to 5H, the refractive index contrast between the annular refractive index reduction region and the central region of the core is, for example, about 3 × 10 for a core diameter of about 45 μm. -4~5×10 -4 This is shown. In some implementations, this refractive index difference may be lower or higher for different core diameters and core NAs. In some modifications, the refractive index difference between the refractive index reduction region and the central region of the core is LP 01 Modes and LP 11 The mode may be selected to minimize spatial overlap with other modes. In a step refractive index fiber, LP 11 The mode field corresponding to the mode is LP 01 It extends further from the core's central region than the mode field corresponding to the mode. In an active LMA optical fiber having a refractive index reduction region as described herein, LP 11 The mode field corresponding to the mode spreads out even further away from the center, LP 01 The mode field corresponding to the mode becomes more confined. This can be observed, for example, in the graphs of Figures 6A and 6B, for a 35 μm core fiber with a step refractive index profile (dotted line) and a 45 μm core fiber with an annular refractive index reduction region (solid line) LP 01 (Figure 6A) and LP 11 (Figure 6B) This figure shows the radial intensity distribution of the mode.

[0048] In some implementations, LP 01 Modes and LP 11 To minimize spatial overlap with the modes, the intermediate diameter d of the annular refractive index reduction region 24 ring It is also possible to adjust this. For illustrative purposes only, in the embodiments shown in Figures 5A to 5H, ratio d ring / d core This ratio is set to approximately 0.65. core And it can be adjusted according to the core NA. This ratio d ring / d core , LP 01 and LP 11 As the mode becomes more restricted toward the central region of the core, i.e., the core diameter d coreAnd / or as the core NA is set to be greater than 45μm and / or 0.065 respectively, it takes a smaller value. On the other hand, the core diameter d core And / or when the core NA is smaller than 45μm and / or 0.065, the ratio d ring / d core takes a larger value. Generally, an excessively high ratio d ring / d core brings little benefit, while an excessively low ratio d ring / d core causes distortion in the intensity distribution of LP 01 and as a result, it can be said that the laser beam at the output of the fiber is no longer considered diffraction-limited.

[0049] As an example, the change in the ratio of the mode field area A LP11 / A LP01 is shown in FIG. 7 for a specific embodiment shown in FIG. 5A (solid line) as a function of the depth of the refractive index reduction region, and all other parameters remain unchanged. The ratio A LP11 / A LP01 is found to exceed almost twice the ratio obtained for the step-index fiber (dashed line). Similar results (not shown) are obtained for other embodiments shown in FIGS. 5B to 5H. Similarly, an optical fiber having a core with a refractive index reduction region different from that shown in FIGS. 5A to 5H can produce similar results. 01 eq eq

[0050] The normalized frequency V of the solid line curve plotted in FIG. 7 is the same effective refractive index and the same mode field diameter as that of an equivalent step-index fiber, that is, a fiber having the radial refractive index profile shown in FIG. 5A and having an LP 01 mode, and the core diameter d eq and the core numerical aperture NA eqThe calculations were performed under the assumption that the normalized frequency, determined according to the parameters of the equivalent step refractive index fiber described above, decreases as the depth of the refractive index reduction region increases, as shown in Figure 7. Therefore, as the fiber approaches single-mode cutoff, the mode field area also decreases, and a trade-off can be expected as smaller normalized frequencies are reached.

[0051] Active LMA optical fibers as described herein can be manufactured according to known methods and techniques. The addition of a refractive index-decreasing co-dopant within an annular refractive index-decreasing region, or similarly, an refractive index-increasing co-dopant outside an annular refractive index-decreasing region, may be carried out in proportion to the molar refractive index of the refractive index-decreasing (or refractive index-increasing) co-dopant, taking into account the molar concentrations of other dopants and co-dopans present in the core NA and optical fiber matrix. The tuned refractive index profile intended herein may require good control of the fiber manufacturing process. More specifically, in the core region where the refractive index is not uniform, the molar concentration of the co-dopant varies according to a given radial gradient. The concentration gradient is first calculated according to the refractive index profile and the molar refractive index of the dopant. In the polarization-retaining embodiment shown in Figure 3A, stress birefringence can be induced in the active LMA optical fiber, for example, by introducing a boron-doped rod along the core before fiber drawing. If the boron-doped stress rod is sized accordingly, 1 × 10⁻⁶ -4 ~4×10 -4 Birefringence within this range can be achieved in this way.

[0052] As described above, the active LMA optical fiber described herein may have multiple claddings 40 surrounding the core. Figures 8A to 8C show examples of radial refractive index profiles 26 of active LMA optical fibers according to various multi-cladding embodiments. In some implementations, the innermost (first) cladding has a refractive index 32 lower than the average refractive index of the core 34, and the core NA is governed by the refractive index contrast between the core and the innermost cladding. The outermost cladding is generally composed of a fluorinated acrylate polymer having a refractive index 36 of about 1.37 and an numerical aperture of 0.46 or greater. The outermost cladding may also be composed of fluorine-doped silica glass, in which case the refractive index 36 is about 1.43 and the resulting numerical aperture is 0.22 or greater. Any intermediate cladding (see Figure 8B) may have a refractive index 39 higher or lower than the refractive index 32 of the first cladding. For example, the intermediate cladding may be made of pure silica glass, or silica glass doped with at least one refractive index-increasing dopant and / or one refractive index-decreasing dopant. The inner edge of the outer cladding can form a circle with a diameter in the range of 80 μm to 600 μm.

[0053] Naturally, numerous additional modifications can be made to the embodiments described above without departing from the scope of protection defined in the attached claims.

Claims

1. An active large-mode area (LMA) optical fiber for mitigating transverse-mode instability (TMI) effects, wherein the active LMA optical fiber is - Core and, - comprising one or more cladding surrounding the core, An active LMA optical fiber, wherein the core has a core diameter and comprises an annular refractive index reduction region in the cross-section of the LMA optical fiber, the annular refractive index reduction region having an outer diameter smaller than the core diameter, the core has a core glass composition comprising one or more rare earth dopants and at least one co-dopant, the core glass composition defines the radial refractive index profile in the core of the active LMA fiber, and the radial refractive index profile comprises a recess along the annular refractive index reduction region.

2. The active LMA optical fiber according to claim 1, wherein the at least one co-dopant is provided in the annular refractive index reduction region and includes one or more refractive index reduction co-dopants that are not present in the core outside the annular refractive index reduction region.

3. The active LMA optical fiber according to claim 1, wherein the at least one co-dopant comprises one or more de-refractive-index co-dopants provided to the core at a higher concentration in the annular de-refractive-index region than outside the annular de-refractive-index region.

4. The active LMA optical fiber according to claim 1 or 2, wherein the one or more refractive index-reducing co-dopants include at least one of fluorine and boron.

5. The active LMA optical fiber according to any one of claims 1 to 3, wherein the at least one co-dopant is provided outside the annular refractive index reduction region and includes one or more refractive index increasing co-dopants that are not present in the refractive index reduction region.

6. The active LMA optical fiber according to any one of claims 1 to 3, wherein the at least one co-dopant comprises one or more refractive index-raising co-dopants provided in the core at a higher concentration outside the annular refractive index-reducing region than in the annular refractive index-reducing region.

7. The active LMA optical fiber according to claim 5 or 6, wherein the one or more refractive index-enhancing co-dopants include at least one of aluminum, phosphorus, and germanium.

8. The active LMA optical fiber according to any one of claims 1 to 7, wherein the recess of the radial refractive index profile has a tapered side edge on the outside of the core.

9. The active LMA optical fiber according to any one of claims 1 to 8, wherein the recess of the radial refractive index profile has a tapered side edge on the inside of the core.

10. The active LMA optical fiber according to any one of claims 1 to 7, wherein the recess in the radial refractive index profile has a uniform depth.

11. The active LMA optical fiber according to any one of claims 1 to 10, wherein the core has a central region inside the annular refractive index reduction region, and the radial refractive index profile within the central region is uniform.

12. The active LMA optical fiber according to any one of claims 1 to 10, wherein the core has a central region inside the annular refractive index reduction region, and the radial refractive index profile within the central region is non-uniform.

13. The active LMA optical fiber according to claim 11 or 12, wherein the refractive index difference between the central region of the core and the annular refractive index reduction region is lower than the refractive index difference between the central region of the core and the innermost cladding of the one or more claddings.

14. The active LMA optical fiber according to claim 11 or 12, wherein the core has an outer region extending between the refractive index reduction region and one or more claddings, and the refractive index in the outer region is different from the refractive index in the central region.

15. The core diameter d ｃｏｒｅ The active LMA optical fiber according to any one of claims 1 to 14, wherein the value is less than 100 μm.

16. The core has an numerical aperture NA of less than 0.10, as described in any one of claims 1 to 15.

17. The active LMA optical fiber according to any one of claims 1 to 16, wherein the width of the annular refractive index reduction region is in the range of about 1 to about 10 times the wavelength of light propagating within the core.

18. The depth of the recess in the radial refractive index profile is approximately 10 －４ ~about 10 －３ An active LMA optical fiber according to any one of claims 1 to 17, having the value of .

19. The active LMA optical fiber according to any one of claims 1 to 18, further comprising a stress-applying member extending longitudinally within one or more claddings along the core.

20. The active LMA optical fiber according to any one of claims 11 to 19, wherein the one or more rare earth dopants are concentrated in the central region of the core and include at least one of ytterbium, erbium, thulium, neodymium, holmium, and praseodymium.

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