Method and system for increasing higher-order mode suppression in large-mode-area ring fibers

Abstract

The embodiments of the present disclosure generally relate to methods for increasing high-order mode suppression in large mode area ring fibers. This approach may raise the transverse mode instability (TMI) threshold and enable further mode field diameter (MFD) scaling for higher power. Disclosed herein is an optical fiber comprising a core having a set of core characteristics and a cladding ring around the core, the optical fiber having a fundamental mode effective MFD between 14 microns and 40 microns, the optical fiber having a L-mode area ring fiber. HOM 4 shows the higher mode losses of the

Description

[Technical Field]

[0001] [Reference to Related Applications] This application claims priority to U.S. Provisional Patent Application No. 63 / 171,441 (“Increasing higher-order mode suppression in large-mode area ring fibers,” filed April 6, 2021), the disclosure of which is incorporated herein by reference in its entirety. [Background technology]

[0002] Embodiments of the present disclosure generally relate to higher-order mode suppression. Generally, a fiber laser may be a laser in which the active gain medium is an optical fiber doped with a rare-earth element, such as erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium, or holmium. Fiber lasers are related to doped fiber amplifiers, which provide optical amplification without lasing. Advances in fiber lasers have created opportunities for their use in a variety of applications and implementations. Fiber lasers are widely used in industrial laser processing applications requiring both high power and high beam quality, such as laser cutting and laser welding of metals and metal alloys. The core is typically pumped with pump radiation provided by multiple diode lasers. The diode lasers efficiently convert electrical power into optical power that can be directed to the gain fiber. In a "cladding-pumped" configuration, pump radiation is guided along the gain fiber within a pump cladding that coats the core. An outer cladding surrounds the pump cladding.

[0003] Specific applications of fiber lasers require specific power levels. To achieve the required power levels needed for some fiber laser applications, several lasers can be combined for increased power. Fiber lasers can be combined using spectral or coherent combining. The scaling of the output power of fiber lasers is limited by nonlinearities, such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), and self-phase modulation (SPM). In particular, fiber lasers designed for narrow linewidth operation are limited by SBS as the dominant nonlinearity. In contrast, fiber lasers designed for commercial applications that do not require narrow linewidth are often limited by SRS.

[0004] One way to reduce nonlinearity and increase output power is to increase the effective area of ​​the fundamental mode of the fiber. However, as the effective area of ​​the fiber increases, it becomes increasingly difficult to maintain single-mode operation of the fiber. At a certain point, as the effective area increases and higher-order mode (HOM) losses decrease, transverse mode instability (TMI) becomes the limiting factor in increasing output power, rather than nonlinearity.

[0005] TMI occurs when a thermally induced refractive index grating, created by quantum defect heating, couples the fundamental mode to a higher-order mode. Typically, the linearly polarized (LP) LP11 mode is the dominant HOM of interest. As the modes couple, the laser's output randomly fluctuates at kHz frequencies between the fundamental mode and the HOM, causing significant noise and degrading beam quality. The TMI threshold is typically increased by increasing the bending loss of the HOM, but this also increases the fundamental mode signal loss, reducing optical efficiency and limiting the achievable HOM loss.

[0006] Thus, there is a natural trade-off in designing fibers for high-power fiber lasers. Increasing the effective area increases the nonlinearity threshold but decreases the TMI threshold. Also, increasing the TMI threshold by increasing the HOM bend loss reduces optical efficiency. Various simple step-index profiles have been optimized to balance these limitations. The effective mode field diameter (MFD) of the fundamental mode (defined as 2 × (effective area / pi)^0.5) of these fibers is typically less than 20 microns, LPO1 bend-induced loss is kept below 2 dB / m, and higher-order mode loss is >200 dB / m. However, these designs are extremely sensitive to the low yields in fiber fabrication. Further scaling of output power beyond what is currently achievable with existing designs is not available. There is a need for new approaches to increase higher-order mode loss in these fibers. Summary of the Invention

[0007] Embodiments of the present disclosure generally relate to methods for increasing higher-order mode suppression in large-mode-area fibers with rings in the cladding. This approach can raise the transverse-mode instability (TMI) threshold and enable further mode-field diameter (MFD) scaling for higher power. In addition, this approach can also increase fiber manufacturing yields by broadening the range of refractive index profiles that can achieve the desired nonlinear and TMI thresholds.

[0008] An embodiment of the present disclosure also provides an optical fiber including a core having a set of core properties and a cladding ring around the core, wherein the optical fiber has a fundamental mode effective mode field diameter (MFD) between 14 microns and 40 microns, and the optical fiber has an L HOM In some implementations, the optical fiber may have a fundamental mode effective MFD between 14 microns and 37 microns.

[0009] An embodiment of the present disclosure also provides an optical fiber comprising a core having a set of core properties and a cladding ring around the core, the cladding ring starting between 3 microns and 15 microns from the edge of the core, wherein the optical fiber has a fundamental mode effective mode field diameter (MFD) between 14 microns and 40 microns, and the optical fiber has an L HOM Higher mode losses and P HOM 1 shows the higher order mode power overlap.

[0010] An embodiment of the present disclosure also provides a method for increasing higher-order mode suppression in a large mode area ring fiber, comprising the steps of providing an optical fiber having a core with a delta n of less than 2e-3 and a cladding ring around the core with an edge of the cladding ring core starting between 3 microns and 15 microns, the optical fiber having a fundamental mode effective mode field diameter (MFD) between 14 microns and 40 microns, and the optical fiber having a L HOM Higher mode losses and P HOM , which exhibits higher-order mode power overlap and propagates light through an optical fiber.

[0011] Thus, so that the above-cited features of the present disclosure can be understood in detail, more particular descriptions of the embodiments of the present disclosure can be had by reference to the accompanying drawings. It should be noted, however, that the accompanying drawings illustrate only exemplary embodiments encompassed within the scope of the present disclosure and should not be considered limiting, as the present disclosure may admit of other equally effective embodiments. [Brief explanation of the drawings]

[0012] [Figure 1A] 1 is a chart showing the design of an Yb-doped fiber. [Figure 1B] 1 is a chart illustrating an exemplary design of an Yb-doped fiber according to an embodiment of the present disclosure. [Figure 2A] 1 is a chart illustrating the relationship between modal loss and bend diameter according to an embodiment of the present disclosure. [Figure 2B]1 is a chart illustrating the relationship between modal loss and bend diameter for a fiber having a ring feature according to an embodiment of the present disclosure. [Figure 3A] 10 is a chart illustrating modal power overlap with the core for fundamental and higher order modes (HOMs) without a ring, according to an embodiment of the present disclosure. [Figure 3B] 10 is a chart illustrating modal power overlap of a HOM with a core and a ring for the fundamental mode, according to an embodiment of the present disclosure. [Figure 4] 1 is a plot illustrating a profile of a ring fiber according to an embodiment of the present disclosure. [Figure 5] 10 is a chart illustrating the relationship between LP11 loss at the spiral end and MFD, according to an embodiment of the present disclosure. [Figure 6] 1 is a flowchart illustrating a method for increasing higher-order mode suppression in a large mode area ring fiber, according to an embodiment of the present disclosure.

[0013] The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or claims. As used throughout this application, the term "may" is used in its permissive (i.e., potential) sense rather than its obligatory (i.e., required) sense. Similarly, the terms "include," "including," and "includes" mean including, but not limited to. For ease of understanding, like reference numerals have been used, where possible, to indicate like elements common to the figures. DETAILED DESCRIPTION OF THE INVENTION

[0014] Embodiments of the present disclosure generally relate to methods for increasing higher-order mode suppression in large-mode-area ring fibers. This approach can raise the transverse-mode instability (TMI) threshold and enable further mode-field diameter (MFD) scaling for higher power. In addition, this approach can also increase manufacturing yields by broadening the range of refractive index profiles that can achieve the desired nonlinear and TMI thresholds.

[0015] Exemplary embodiments described herein relate to cladding features added to high-power fiber laser fiber designs. According to the embodiments described herein, the cladding features can significantly increase higher-order mode losses while reducing higher-order mode overlap with the rare-earth doped fiber core for fibers with mode field diameters in the range of 14 microns and 40 microns, enabling higher power operation. In some implementations, the optical fiber can have an MFD between 14 microns and 37 microns.

[0016] TMI generally prevents power scaling in fiber lasers. TMI involves power transfer between the fundamental mode and LP11 higher-order modes (HOMs) facilitated by thermally induced refractive index gratings. Increasing the mode field diameter (MFD) can result in higher nonlinear thresholds, lower HOM loss, and associated lower TMI thresholds. Nonlinear thresholds can include stimulated Brillouin scattering (SBS), Raman, and four-wave mixing (FWM) thresholds. Four-wave mixing (FWM) is an intermodulation phenomenon in nonlinear optics, where the interaction between two or three wavelengths generates two or one new wavelength. TMI impacts both commercial fiber lasers and directed-energy fiber laser programs. One approach to suppressing TMI is to increase HOM loss. Increasing HOM loss becomes more difficult for larger mode field diameters. Increasing HOM loss while maintaining MFD allows for increased manufacturing yields for current operating power levels and increased efficiency by operating at lower LP01 loss while maintaining high LP11 loss. This also allows scaling to larger effective areas, reducing nonlinearities, and increasing operating power levels. Increasing the gain dopant concentration can reduce nonlinearities by decreasing the fiber length, but this can be detrimental due to increased photodarkening, which reduces the TMI threshold.

[0017] According to exemplary embodiments, cladding rings or the like can be added to the refractive index profile. Adding cladding rings can increase the HOM bending loss due to resonance, etc. The symmetry of the LP11 mode with respect to bending can include parallel symmetry and orthogonal symmetry. Parallel symmetry typically has higher bending loss. Orthogonal symmetry typically has lower bending loss. In some implementations, at resonance, the parallel symmetry LP11 mode has lower loss than orthogonal symmetry at some bending diameters. In some implementations, quantum defect-induced heating generated during operation in an amplifier can maximize the benefits of the rings because the refractive index profile of the fiber is modified through the thermo-optic coefficient.

[0018] In some implementations, for example, in a 19-micron MFD Yb-doped fiber and a 16-micron MFD fiber with a graded-index core, keeping the core the same and adding a ring increases the HOM bend loss over a wide bend diameter range. Adding a ring can substantially reduce the HOM mode overlap with the core without affecting the fundamental mode overlap. In some embodiments, a ring design for a given refractive index profile may be optimized and then applied to other measured fiber refractive index profiles. The ring design may be optimized for a single profile but may also give an increase in HOM bend loss over a wide range of fiber designs and MFDs. The ring design may be robust to core changes.

[0019] FIG. 1A is a chart illustrating a Yb-doped fiber design 100a. FIG. 1B is a chart illustrating an exemplary Yb-doped fiber design 100b according to an embodiment of the present disclosure. In some embodiments of the present disclosure, a solution to increasing the HOM loss without increasing the fundamental mode loss is to add a new structure to the cladding near the core. This structure may be called a ring. FIG. 1A illustrates a high-power Yb-doped fiber design, and FIG. 1B illustrates an additional ring structure for HOM suppression. Exemplary design parameters shown in FIG. 1B include the starting radius, delta n, and width. This may be optimized to interact primarily with higher-order modes, with the index of refraction of the ring kept low enough to avoid significant perturbations to the fundamental mode. Additionally, more than one ring may be used.

[0020] FIG. 2A is a chart 200a illustrating the relationship between modal loss and bend diameter without a ring feature according to an embodiment of the present disclosure. FIG. 2B is a chart 200b illustrating the relationship between modal loss and bend diameter for a fiber with a ring feature according to an embodiment of the present disclosure. The curves on the chart show the fundamental mode loss as a function of bend diameter and the parallel and orthogonal symmetric LP11 modes. As shown in FIG. 2B, adding a ring increases the HOM loss. The LP01 loss also increases, increasing the ratio of LP11 / LP01 loss. In some embodiments, higher LP01 bend losses can be accommodated by moving the operating point to a larger bend diameter. The core and ring design can also provide a desirable bend radius.

[0021] Table 1 below shows the calculated LP11 loss at bend diameters where LP01 loss = 1 dB / m. Adding the ring increases the bend diameter at LP01 = 1 dB / m loss from 7.7 cm to 9 cm. However, at that diameter, the LP11 loss increases from 59 dB / m to 1380 dB / m with the addition of the ring. The mode field diameter of the fiber does not change substantially with the addition of the ring. [Table 1]

[0022] The ring increases the calculated HOM loss of the fiber, which can result in an increase in the TMI threshold. Bending losses can be calculated by a mode solver based on the refractive index of the fiber. In low-index coated fibers, high bending losses in the core mean high coupling to cladding modes, but not the power coupled into those cladding modes remaining guided by the fiber. The calculated loss is a proxy for how well the HOM samples the glass-coating interface.

[0023] Due to the issue of bend loss in low-index coated fibers, it is also useful to consider how much the HOM overlaps with the fiber's gain-doped region. The ring can cause the LP11 to extend its energy into the cladding. Therefore, the overlap between the HOM and the fiber's core decreases when a ring is added to the index profile. This is another benefit of fiber lasers because the smaller the overlap with the gain-doped region, the less gain the HOM will have, further increasing the TMI threshold. In many cases, the gain dopant is only present throughout the entire core. In cases where the gain dopant is confined to a portion of the core or extends beyond the core, the modal power overlap should take into account the fiber's gain-doped region.

[0024] FIG. 3A is a chart 300a showing the modal power overlap of the fundamental mode and higher-order modes (HOMs) with the core without a ring. FIG. 3B is a chart 300b showing the modal power overlap of the fundamental mode with the core and the HOM with a ring, according to an embodiment of the present disclosure. FIGS. 3A and 3B show the calculated modal overlap of the fundamental mode with the core and the HOM with and without a ring for a particular refractive index profile design. The shaded area on the chart indicates the operating diameter typically used when a 10-m length of Yb-doped fiber is spirally wound. While the fundamental mode overlap with the core does not change over the expected operating diameter range, the higher-order mode overlap with the core decreases dramatically. The operating diameter shifts to a slightly larger diameter with the addition of the ring, which can be compensated for during the core design phase.

[0025] In some implementations, the benefits provided by the ring are robust to the details of the ring index profile. However, because the interaction between the HOM and the ring is based on resonance, and the ring diameter, ring width, and delta n are maximized at points within a particular ring design, significant increases in loss and decreases in core overlap are maintained over a wide range of designs. This makes the ring design robust and can increase fiber yield. There may be regions within the design space that optimize for both high LP11 loss and low modal overlap with the core.

[0026] In some embodiments of the present disclosure, the ring works because the fiber exhibits a low NA, resulting in relatively weak core confinement and high loss for the LP11 mode even without the ring. Adding a ring to such a sensitive design promotes leakage of HOM into the cladding.

[0027] Figures 1A, 1B, 2A, 2B, 3A, and 3B show data related to fibers with step-index-like cores. The rings can perform equally well with graded-index cores, such as those used in commercial fiber lasers. The rings can also perform equally well with cores that deviate from the idealized step-index fiber and exhibit peaks or dips in the profile.

[0028] The calculations described herein can be performed on a fiber's refractive index profile measured, for example, at room temperature. When operating in an amplifier, heating caused by quantum defects between the pump and the signal can cause substantial changes in the refractive index profile due to the thermo-optic effect. Taking this effect into account during the design phase can further improve the performance of the fiber in high-power amplifiers.

[0029] The above discussion considers the bend diameter of a coiled fiber, which varies along the length of the fiber. In some implementations, this is advantageous because TMI may be most severe at the signal input end of the fiber, where HOM bend losses may be higher, even at the expense of locally higher LPO1 losses. In some implementations, it is desirable to manage HOM bend losses or core overlap averaged along the gain fiber length. In some implementations, path-averaged characteristic-based optical power may be used. This may be relevant when applied to passive fibers that do not produce gain and must avoid nonlinear effects, etc.

[0030] Although a coil configuration is described herein, in some cases the fiber may be wound with an essentially uniform bend diameter, such as held in a ring or wound on a cylinder. This may be made possible by the ring-based design feature, in which the HOM loss and LP11 / LP01 loss ratio vary less with bend diameter than ring-less designs. The relative insensitivity of bend loss with bend diameter for low-NA, large fiber designs may be highly advantageous for both high performance and improved packageability.

[0031] The properties described herein can be important near the threshold for deleterious effects such as nonlinearity and TMI that arise when the device is operating at high power and therefore under high thermal load. Because the refractive index of a fiber changes with temperature, it can be important to compensate the fiber design to produce the desired properties when operated over the target operating temperature range.

[0032] Some fibers may have a ring-like structure as well as a trench with a lower refractive index than the rest of the pump cladding. A combination of ring and trench structures may be used to further increase the HOM loss.

[0033] Exemplary embodiments of the present invention may include design parameters related to high-power fiber lasers. In the context of high-power fiber lasers for commercial and directed energy applications, these parameters may include, for example, a solid core with a solid cladding fiber to differentiate microstructured fiber approaches, an MFD greater than 14 microns, and an MFD less than about 40 microns. In some embodiments, designs may include an MFD of 25 microns, with some advantages. Above 40 microns, the ring approach to HOM suppression may be more challenging.

[0034] Some embodiments of the present disclosure may use low-delta-n fibers. As described herein, the fiber core design may be tuned to a point where LP11 bend loss is significant and can interact with the ring, allowing the ring to function. For example, the core delta-n may be limited to <2e-3, etc. In some embodiments, a ring with a delta-n substantially smaller than the core may be used. As the ring delta-n approaches the core delta-n, the fundamental mode may become lossy. In this example, the ring delta-n may be limited to <70% of the core delta-n. Other parameters may include a ring that begins at least 2 microns from the core edge and no more than 15 microns from the core edge. Rings that are too far from the core narrow the design space, making it difficult to simultaneously achieve high loss and low HOM core overlap. Rings that are too close to the core may be difficult to fabricate. Other parameters may include high HOM loss, for example, LP11 > 300 dB / m.

[0035] In some implementations, fiber design based on a multidimensional optimization routine has found ring-based designs with HOM losses that exceed those of 19-micron fibers with MFDs up to 23.5 microns. According to exemplary embodiments described herein, the design algorithm determines designs at 25-micron MFDs with HOM losses >200 dB / m. In some implementations, the designs allow for higher values ​​of HOM loss at larger MFDs, with MFDs >200 or even >300 dB / m above 25 microns or even 30 microns.

[0036] The designs discussed herein may show a single rectangular-shaped ring. While the ring appears rectangular in appearance, the sides of the ring may acquire a slope during stretching due to diffusion. Other ring shapes, such as triangular or graded index, may also offer benefits. There may also be advantages to using multiple rings in the cladding. The ring may be continuous, with constant or azimuthally varying inner and outer radii, or may consist of separate segments.

[0037] According to exemplary embodiments of the present disclosure, an optical fiber can be designed and manufactured. In some implementations, the optical fiber can include a core and a ring. The optical fiber has properties including a core with delta n<2e-3 and a ring starting between 3 and 15 microns from the edge of the core, where the ring has delta n<0.7*delta n; the fiber has a fundamental effective MFD between 14 microns and 30 microns; the fiber has a fundamental mode loss <1 dB / m occurring at a bend diameter between 5 cm and 30 cm; and the fiber has higher-order mode losses >300 dB / m at a bend diameter where fundamental mode loss = 1 dB / m.

[0038] FIG. 4 is a plot 400 illustrating a profile of a ring fiber according to an embodiment of the present disclosure. Plot 400 illustrates delta n versus radius (microns) for a ring fiber according to an embodiment of the present disclosure. The fiber may include a cladding ring according to an embodiment of the present disclosure. The cladding ring may be added to increase the TMI threshold, etc. According to an exemplary embodiment, the manufactured ring fiber may be used.

[0039] FIG. 5 is a chart 500 illustrating the relationship between LP11 loss at the spiral end and MFD, according to an embodiment of the present disclosure. In some implementations, the ring fiber design for pulsed Yb fiber amplifiers is scalable to mode fields as large as 37 microns. Chart 500 shows a compilation of fabricated fibers according to embodiments of the present disclosure, demonstrating that at larger mode fields, the ring fiber achieves significantly higher HOM loss than conventional step-index designs. For example, at a 25-micron MFD, the design has a higher HOM loss than a 19-micron MFD, step-index fiber. At a 37-micron MFD, the design has a loss of over 40 dB / m, which may be sufficient to support over 1 kW of TMI-free signal power. As the MFD is scaled, delta n may decrease and the operating diameter may increase. In some implementations, at a 37-micron MFD, the operating diameter may be approximately 30 cm.

[0040] FIG. 6 is a flowchart illustrating a method 600 for increasing higher-order mode suppression in a large-mode-area ring fiber, according to an embodiment of the present disclosure. Method 600 can begin in step 602, where core properties or parameters are set. Core properties may include, for example, delta n<2e-3. The method can proceed to step 604, where ring parameters are set. Ring parameters may include, for example, a ring starting between 3 and 15 microns from the edge of the core, with the ring having a delta n<0.7*delta n. Defining the core design in terms of delta n and core radius in step 602 can substantially define the MFD and operating bend diameter. Defining the ring design in step 604 can determine the HOM loss and fine-tune the operating diameter. The fiber parameters may include, for example, a fiber with a fundamental mode effective MFD between 14 microns and 30 microns, a fundamental mode loss <1 dB / m occurring at a bend diameter between 5 cm and 30 cm, and higher order mode losses >300 dB / m at bend diameters where the fundamental mode loss = 1 dB / m. In step 606, light may be propagated through the fiber, etc.

[0041] For simplicity and clarity of illustration, the accompanying drawings depict general structural schemes, and omit detailed descriptions of features and techniques well known in the art to avoid unnecessarily obscuring the discussion of exemplary embodiments of the present disclosure. Further, components in the accompanying drawings are not necessarily drawn to scale. For example, sizes may be exaggerated to aid in understanding the exemplary embodiments of the present disclosure.

[0042] The exemplary embodiments of the present disclosure described herein are described herein with the understanding that they may be operated in an order different from that illustrated. Where a method is described herein as comprising a series of steps, the sequence of those steps suggested herein is not necessarily the order in which those steps may be performed.

[0043] The terms used in this disclosure are intended to describe exemplary embodiments, not to limit the disclosure. In this disclosure, the singular includes the plural unless expressly stated otherwise. As used in this disclosure, components, steps, operations, and / or elements referred to by the terms "comprise" and / or "comprising" do not exclude the presence or addition of one or more other components, steps, operations, and / or elements.

[0044] In the above, the present disclosure has been described with reference to its exemplary embodiments. All exemplary embodiments and conditional descriptions disclosed in the present disclosure have been described with the intention of helping those skilled in the art to understand the principles and concepts of the present disclosure. Therefore, those skilled in the art will understand that the present invention can be modified and implemented within the scope of the present invention without departing from the spirit and scope of the present invention. Although numerous embodiments having various features have been described herein, combinations of such various features in other combinations not discussed herein are also contemplated to be within the scope of the embodiments of the present disclosure.

Claims

1. a core having a set of core properties; an optical fiber comprising an inner cladding around the core and an outer cladding around the inner cladding, The optical fiber has an effective mode field diameter (MFD) of the fundamental mode between 14 microns and 40 microns, and the optical fiber has a higher order mode loss (L ＨＯＭ ) Optical fiber.

2. The optical fiber of claim 1 , further comprising a second outer cladding around said outer cladding.

3. 10. The optical fiber of claim 1, further comprising a second outer cladding around said outer cladding and a trench.

4. The L of the optical fiber ＨＯＭ 10. The optical fiber of claim 1, wherein is at least 1.5 times greater than an optical fiber having said set of core properties without said outer cladding.

5. The higher-order mode power (P ＨＯＭ 2. The optical fiber of claim 1, wherein λ / 2 is at least 30% less than an optical fiber having said set of core properties without said outer cladding.

6. 2. The optical fiber of claim 1, wherein the set of core properties comprises a relative refractive index difference (Δn) between the core and the inner cladding, the relative refractive index difference being less than 0.

002.

7. 10. The optical fiber of claim 1, wherein the outer cladding begins between 3 and 15 microns from the edge of the core.

8. 2. The optical fiber according to claim 1, wherein the relative refractive index difference between the outer cladding and the inner cladding is smaller than 0.7 times the relative refractive index difference between the core and the inner cladding.

9. 10. The optical fiber of claim 1 having an effective MFD of the fundamental mode between 14 microns and 37 microns.

10. 10. The optical fiber of claim 1, having a fundamental mode loss of less than 1 dB / m occurring at a bend diameter between 5 cm and 30 cm.

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