Large mode area (LMA) optical fiber with an Anti-resonant structure

Abstract

Disclosed is a large mode area (LMA) optical fiber, comprising, concentrically and radially outwards from a center axis of the optical fiber: - a core comprising at least a core material having a core refractive index, n core, wherein the core has a core radius, r core, of more than 5 microns, whereby the core is configured to guide a first light signal having an operational wavelength, λoperational, in a plurality of core modes; - a ring comprising a ring material contiguously surrounding the core and having a ring refractive index, n ring, and a thickness, t, whereby the ring is configured to guide a plurality of ring modes; and an inner cladding comprising an inner cladding material surrounding the ring and having an inner cladding refractive index, n clad; wherein the ring refractive index is defined such that the ring refractive index, n ring, is larger than the core refractive index, n core, and larger than the inner cladding refractive index, n clad,

Description

[0001]Large mode area (LMA) optical fiber with an anti-resonant structure Field of the invention The present disclosure relates generally to a large mode area (LMA) optical fiber, more specifically to an LMA optical fiber with an anti-resonant structure. Further, the present disclosure relates to an LMA optical fiber, which de- localizes high-order modes from a core of the LMA fiber when the optical fiber is bent. BackgroundLarge mode area (LMA) optical fibers are typically, by definition, optical fibers,which have an effective area for the fundamental mode that is larger than 90λ2, where λ is the wavelength of a signal guided in the fiber. The term “mode” refers to the transverse mode of an electromagnetic wave, i.e. the light signal propagating in the optical fiber. For a given optical power, LMA optical fibers imply reduced optical intensities, meaning that they exhibit weaker non-linear effects, and higher damage thresholds. Accordingly, LMA optical fibers are suitable for being used for amplification of intense pulses or single-frequency signals in fiber amplifiers. An LMA optical fiber that is configured to amplify a signal, may be an LMA optical fiber having a core being doped with a core material, such as a rare- earth material. Such an LMA optical fiber is known as active LMA optical fiber. A passive LMA optical fiber may not have a rare-earth doped core and may be suitable for simply guiding an amplified signal. An LMA optical fiber may be manufactured as a conventional step-index fiber, comprising a solid core surrounded by a solid cladding having a lower refractive index than the core. In such LMA optical fibers, it is known that, due to the large core diameter, the LMA optical fiber becomes multimode, which results in the propagation of higher order modes. Several solutions to de-localize these higher order modes from the core havebeen proposed over time.A recent solution, as proposed in US 2021 / 0242652, has been to provide araised index layer surrounding the core, wherein the raised index layer preserves a confinement of the core mode in the core. The raised index layerhas a refractive index between that of the core and the cladding. However, withthis solution, it has been found that the higher order modes in the core are not sufficiently suppressed. Another solution, as proposed in CN 117631133, has been to provide a double-layer anti-resonant ring around the core. The double-layer anti-resonant ring has a refractive index higher than the refractive index of the coreand the background material. The double-layer is made of a first ring and asecond ring in which the first ring is nested within the second ring. The double-layer anti-resonant ring provides coupling of higher order modes of the core tothe modes of the region between the first and second ring and the regionoutside the second ring. A disadvantage of this solution is that a double-layerring shape is more challenging to fabricate with respect to a single-layer shape.Another disadvantage is that, when introducing stress applying parts (SAP) tosuch fiber, the SAP will need to be positioned outside of the double-layer anti-resonant ring, thus far away from the core with respect to a single-layer design. When SAP are positioned far away from the core, their effectiveness ininducing the desired stress profile is significantly reduced, thus for examplediminishing the polarization maintaining properties of the fiber and / ordiminishing the general mechanical stress effect which might be employed toallow the fiber to be birefringent and polarization maintaining. Furthermore, the fiber of CN 117631133, due to its small diameter, around 125 µm for a 50 µm core diameter, would have high micro bend loss, thus unsuitable for being bended. Accordingly, it is desired to provide an LMA optical fiber that efficiently suppresses higher order modes in the core. Summary It is an objective of this disclosure to provide an LMA optical fiber, which de-localizes high-order modes from a core of the LMA fiber, particularly when theoptical fiber is bent.Further, it is an objective of this disclosure to provide an LMA optical fiber thatefficiently suppresses higher order modes in the core. These and other objectives have been solved by the supercontinuum systemas defined in the claims and as described below in the present disclosure.In one aspect of the disclosure, there is disclosed a large mode area (LMA)optical fiber, comprising, concentrically and radially outwards from a centeraxis of the optical fiber:^ a core comprising at least a core material having a core refractiveindex, ncore, wherein the core has a core radius, rcore, of more than 5 microns, whereby the core is configured to guide a first light signal having an operational wavelength, λoperational, in a plurality of core modes; ^a ring comprising a ring material contiguously surrounding thecore and having a ring refractive index, nring, and a thickness, t, whereby the ring is configured to guide a plurality of ring modes;and^ an inner cladding comprising an inner cladding materialsurrounding the ring and having an inner cladding refractive index, nclad; wherein the ring refractive index is defined such that the ring refractive index, nring, is larger than the core refractive index, ncore, and larger than the inner cladding refractive index, nclad. In a preferred embodiment, the ring becomes anti-resonant with the core modes at λoperationalsuch that: where v is an integer larger than 1 that defines the radial order of the ringmodes. This configuration may be ensured by selecting the ring refractiveindex, nring, the inner cladding refractive index, nclad, and the thickness of the ring, t, such that the equation is fulfilled. In other words, the ring may designed with specific properties to provide the anti-resonant behavior. The technical effect of the ring is as described here, to provide an anti-resonant element. The ring keeps the fundamental mode within the core even though the fiber is bent. In other words, the ring helps to prevent the mode from leaking out of the core, which ensures a higher modal overlap with the core even in a relatively tight bend configuration compared to a step index fiber only using step index confinement. In comparison to the fiber as described in US 2021 / 0242652, the presently disclosed LMA optical fiber differs from that by at least having a ring refractiveindex being defined such that the ring refractive index, nring, is larger than thecore refractive index, ncore, and larger than the inner cladding refractive index, nclad.In US 2021 / 0242652, the ring refractive index is defined such that the ringrefractive index, nring, is smaller than the core refractive index, ncore, and largerthan the inner cladding refractive index, nclad. The inventors of the present disclosure have seen a dramatic increase in thesuppression of higher order modes in the core – because of this difference -as will be described in the detailed description.In an embodiment of the invention, the ring is a single-layer ring and the LMAoptical fiber further comprises a pair of inclusions, wherein the pair of inclusions are enclosed in the inner cladding and each of the inclusions has an inclusion refractive index, ninc, that is smaller than the inner cladding refractive index, nclad. The technical effect of the single-layer ring and the inclusions is to delocalize at least one higher-order modes of the core in the area comprised between thesingle-layer ring and the inclusions when the fiber is bent. In other words, thearea comprised between the core and inclusions is hosting modes that areresonantly coupled to the LP11, when the LMA optical fiber is bent. Thus, theLMA fiber is efficiently suppressing higher order modes in the core. Brief description of the drawings The above and / or additional objects, features and advantages of the present disclosure, will be further described by the following illustrative and non-limiting detailed description of embodiments of the present disclosure, with reference to the appended drawing(s), wherein:Fig.1 shows results of simulations of one embodiment of the LMA optical fiberaccording to the present disclosure, together with results simulations of a fiberfrom the prior art, as described in US 2021 / 0242652, the results showing thecore overlap with different modes as a function of bend diameter. In the simulation for the results of Fig.1, the core is doped with at least one rare- earth dopant material, which provides amplification of the first light signal, and in this example the active part is equal to the core within the higher index ring surrounding the core. The core with the ring has in this example a diameter of 43 microns.Fig.2 shows results of simulations of one embodiment of the LMA optical fiberaccording to the present disclosure, together with results simulations of a fiber from the prior art, as described in US 2021 / 0242652, the results showing the core overlap with different modes as a function of bend diameter. In the simulation for the results of Fig.2, the core is doped with at least one rare- earth dopant material, which provides amplification of the first light signal, and in this example the active part is equal to 90% the core within the higher index ring surrounding the core. The core with the ring has in this example a diameter of 43 microns.Fig.3 shows results of simulations of one embodiment of the LMA optical fiberaccording to the present disclosure, together with results simulations of a fiber from the prior art, as described in US 2021 / 0242652, the results showing thecore overlap with different modes as a function of bend diameter. In thesimulation for the results of Fig.3, the core is doped with at least one rare- earth dopant material, which provides amplification of the first light signal, and in this example the active part is equal to 70% the core within the higher index ring surrounding the core. The core with the ring has in this example a diameter of 43 microns.Fig.4 shows an embodiment of the LMA according to the present disclosure.Fig.5 shows a simulation of how the fundamental mode (LP01) propagates inthe LMA optical fiber when bent.Fig.6 shows a simulation of how a high order mode (HOM), in this case LP11,propagates in the LMA optical fiber when bent.Fig.7 shows a microscope image of a fabricated LMA according to the presentdisclosure.Fig.8 shows the loss of the LP01 and LP11 modes as function of coil diameterfor the fabricated LMA of Fig.7. Detailed description As described above, the active part of the core is in steps of 1, 0.9 and 0.7, and the core overlap is calculated as the overlap with the active part of the core for these steps, in Figs.1-3, respectively. As can be seen from the results of the simulations, shown in Figs.1-3, the core overlap is increased with the anti-resonant effect of the high index ring surrounding the core. This increase becomes more pronounced as the fiber is bent to smaller and smaller coil diameters. Accordingly, the presently disclosed LMA optical fiber keeps the fundamental mode within the core when the fiber is bent, and because of its difference from the design of US 2021 / 0242652, as described above, it does it more optimally, particularly with smaller coil diameters. Another important deviation between the presently disclosed LMA optical fiberand that of US 2021 / 0242652 is that the presently disclosed anti-resonantconfiguration enables a larger suppression or de-localization of the higherorder modes in the core for almost all applicable bend radii. This also means that the differential overlap, the difference in active core overlap between the fundamental mode and higher order modes, is larger for the presentlydisclosed anti-resonant fiber compared to a fiber from US 2021 / 0242652 withsimilar parameters. Accordingly, when the core is doped with at least one rare- earth dopant material, which provides amplification of the first light signal, thepresently disclosed LMA optical fiber allows more gain from the amplificationprocess into the fundamental mode instead of the higher order modes. Hence, when the core is doped with at least one rare-earth dopant material, a more efficient amplification is provided by the presently disclosed LMA optical fiber.Further details of the LMA optical fiber are disclosed in the following.The ring In one embodiment, nring, is larger than ncoreby more than 0.001, morepreferably by more than 0.002, such as around 0.0025. This embodimentexemplifies that the ring refractive index is defined such that the ring refractive index, nring, is larger than the core refractive index, ncore, and larger than the inner cladding refractive index, nclad. It has been found that when the ring refractive index is as here defined, then it may provide that the ring becomes anti-resonant with the core modes at λoperational such that: When in the anti-resonant configuration, as for example described just above,the ring is responsible for confining one or more fundamental core mode(s),both when the fiber is bent and straight. Preferably the fundamental core mode(s) is / are confined to have an overlap with the core by more than 40%. As examples of this, the fundamental core mode, such as those shown in thetop of Figs. 1-3, i.e. LP01 as indicated, for various bend diameters, overlapswith the core by more than 40% for bend diameters between 0.2 m and 0.5 min all the figures. The overlap is particularly clear in Fig.1 (as it is the overlapwith the active part being identical to the core diameter with the ring – of 43microns), where the overlap is more than 70%. In all the simulations used to produce Figs.1-3, the ring refractive index is selected to be larger than ncore by 0.0025. The core refractive index for all the simulations used to produceFigs.1-3 is selected to be ncore=1.4503. Further, the inner cladding refractiveindex is also selected to be nclad=1.45.In one embodiment, the ring is a single-layer ring, preferably having a ringrefractive index nring of 1.453. No additional rings are present within the innercladding of the optical fiber. In other words, the optical fiber may be free of anyadditional anti-resonant rings with a refractive index nring larger than the corerefractive index, ncore, and larger than the inner cladding refractive index, nclad.In another embodiment, the thickness of the ring, t, is between 1 and 10microns, preferably between 1 and 5 microns, more preferably between 1 and4 microns, such as around 3 microns. In all the simulations used to produceFigs.1-3, the thickness of the ring is 3 microns. As with the ring refractive index, it has also been found that when the thickness of the ring is as here defined, then it may provide that the ring becomes anti-resonant with the core modes at λoperationalsuch that: It is noted that this equation is valid when the core refractive index, ncore, isequal to the inner cladding refractive index, nclad, but to a first approximation, the equations still applies when ncore-nclad << nring-nclad. In other words, when the ring refractive index, nring, is smaller than the core refractive index, ncore, such as for example in the solution described in US 2021 / 0242652, the above equation is invalid, meaning that the ring in for example US 2021 / 0242652, is not anti-resonant in the same manner as described in the present disclosure. In most embodiments, the core refractive index is equal to the inner cladding refractive index. However, in some embodiments, the core refractive index is larger than the inner cladding refractive index. The inner cladding and the outer cladding In one embodiment, the LMA optical fiber further comprises an outer cladding comprising an outer cladding material surrounding the inner cladding and having an outer cladding refractive index, nout, that is equal or smaller than the inner cladding refractive index, nclad. As previously described, the inner cladding refractive index, nclad, may bearound 1.45.The operational wavelength and the resonant wavelengthIn one embodiment, the LMA optical fiber, and said refractive index thereof, is configured to guide the first light signal having the operational wavelength,λoperational, wherein the operational wavelength for the radial order v =2 isdefined to be: 2.2 ∗ ^^^^^^^^^ (^ = 2).In another embodiment, the LMA optical fiber, and said refractive index thereof, is configured to guide the first light signal having the operational wavelength, λoperational, wherein the operational wavelength for the radial orderv > 2 is defined to be: ^^^^^^^^^ (^ + 1) < ^^^^^^^^^^^^ < 0.9 ∗ ^^^^^^^^^ (^).The above embodiment may ensure that the fundamental mode(s) overlap with the core, such as by more than 40% of the core. In a preferred embodiment, the LMA optical fiber, and said refractive index thereof, is configured to guide the first light signal having the operational wavelength, λoperational, wherein the operational wavelength is between 1000 nm and 1100 nm, most preferably between 1030 nm and 1064 nm. In yet another preferred embodiment, the resonant wavelength for v = 2 isbetween 450 nm and 600 nm, preferably between 480 nm and 560 nm, morepreferably between 500 nm and 520 nm.Inclusions In one embodiment, the LMA optical fiber further comprises a pair of inclusions, wherein the pair of inclusions are enclosed in the inner cladding and each ofthe inclusions has an inclusion refractive index, ninc, that is smaller than theinner cladding refractive index, nclad. In a preferred embodiment, the pair of inclusions extend parallel to the core and each of the inclusions is located at a first distance, r1, from the center of the core, and at a second distance, r2, from an outer diameter of the anti- resonant ring, such that the inclusions do not overlap with the core and the anti-resonant ring. In a more preferred embodiment, the first distance, r1, and the second distance, r2, are selected to delocalize at least one mode of the first plurality of core modes, wherein the at least one mode is the lowest higher-order mode, the LP11mode, from the core and to the inner cladding, when the fiber is bent at a bending radius, rbend, preferably such that the LP11 mode has an overlapwith the core by less than 50%. In some embodiments related hereto, thebending radius, rbend, is between 0.04 m and 0.4 m, preferably between 0.04 m and 0.2 m. A bending radius as here defined has been found to increase the delocalization the at least one mode of the first plurality of core modes, wherein the at least one mode is the lowest higher-order mode, the LP11mode, from the core and to the inner cladding.In an even more preferred embodiment, the first distance, r1, is between 1.3rcoreand 2.6rcore.It has been found that these distances for r1provide optimaldelocalization the at least one mode of the first plurality of core modes, whereinthe at least one mode is the lowest higher-order mode, the LP11 mode, fromthe core and to the inner cladding. This is because the area comprisedbetween the core and inclusions is hosting modes that are resonantly coupledto the LP11 mode when the fiber is bent. In view of the above, and in a most preferred embodiment, the second distance, r2, defines an area in the inner cladding between the outer diameter of the anti-resonant ring and each of the inclusions, such that the area, in combination with the core refractive index, ncore, and the inclusion refractive index, ninc, resonantly couples and guides the LP11mode. The area betweenthe ring and the inclusions has been found to be important for obtaining the coupling and delocalizing of the higher order modes from within the ring to the cladding structure. In relation to the above embodiment, it has been found that in another most preferred embodiment, the area between each of the inclusions is further defined by an azimuthal angle that is larger than 40 degrees. Since the azimuthal angle defines the area, the angle larger than 40 degrees defines the area, where the higher order modes from within the ring are optimally couped and delocalized to the cladding structure. In most embodiments, the pair of inclusions is a pair of stress applying parts(SAPs), preferably doped with Boron. The SAPs may provide birefringence,but most importantly, bend robustness.In fact, not only the inclusions induce birefringence but can also be used toengineer a resonance to occur between the LP11 modes in the core and modes that are localized mostly in the area between the single-layer ring andthe inclusions. This resonance can be such that it occurs at a predefined benddiameter so that the HOMs loss is high when the fiber is coiled, contrary to CN117631133 where the resonant is designed to occur for a straight fiber. Theinclusions can also contribute to limit bend induced loss of the LP11, which isparticularly important for fibers with a large core. Finally, this approach givesimproved flexibility in the choice of the pump cladding diameter and outer fiberdiameter as these parameters have basically no impact on the modal properties of the core and the high loss induced by the resonance with claddingmodes as opposed to CN 117631133, where the prescribed outer diameter ofthe second background area imposes stringent limitations. It is noted that the term “bend” and “coil” and “bend diameter” and “coil diameter” are used interchangeably throughout the application. The core In one embodiment, the core is doped with at least one rare-earth dopant material, which provides amplification of the first light signal. When the core is doped as here described, the LMA optical fiber becomes a so-called active LMA optical fiber. In some embodiments, the inner cladding and the outer cladding is separated by an air-cladding, which provides guidance of a pump signal during the amplification. In other embodiments, the at least one rare-earth dopant material is only in a central region of the core. In a preferred embodiment, the core and the ring are separated by a layer of material with refractive index, nclad. This layer may for example ease the manufacturing of the herein disclosed LMA optical fiber. In a most preferred embodiment, the core has a core radius, rcore, of more than 5 microns, preferably more than 10 microns, even more preferably more than 15 microns, most preferably more than 20 microns, such as around 21 microns. – One embodiment of the LMA optical fiber according to the presentdisclosure Fig. 4 shows an example of the LMA optical fiber 1 according to the presentdisclosure. The LMA optical fiber 1 comprises firstly a core 2 comprising atleast a core material having a core refractive index, ncore, wherein the core has acore radius, rcore, of more than 5 microns. The core 2 is configured to guidea first light signal having an operational wavelength, λoperational, in a plurality of core modes. As shown in Fig.4, the core 2 is doped with a rare-earth dopantmaterial 3, which provides amplification of the first light signal. As can also be seen from Fig.4, the rare-earth dopant material 3 is only in a central region ofthe core 2. Further, the LMA optical fiber 1 comprises a ring 4 comprising aring material contiguously surrounding the core 2 and having a ring refractiveindex, nring, and a thickness, t, whereby the ring 4 is configured to guide aplurality of ring modes. The ring 4 is a single-layer ring. In other words, the optical fiber 1 does not comprise more than one anti-resonant ring. Evenfurther, the LMA optical fiber 1 comprises an inner cladding 5 comprising aninner cladding material surrounding the ring 4 and having an inner claddingrefractive index, nclad. The ring refractive index is defined such that the ring refractive index, nring, is larger than the core refractive index, ncore, and larger than the inner cladding refractive index, nclad. In this example, the inner cladding refractive index, nclad, is around 1.45, corresponding to pure silica, when measured from 500 nm to 1500 nm at 20 degrees Celsius. Also in this example, the ring refractive index (being larger than the core) corresponds tothat of Germanium (Ge) or Aluminum (Al) doped silica.In this example, the LMA optical fiber 1 further comprises an outer cladding 6comprising an outer cladding material surrounding the inner cladding 5 andhaving an outer cladding refractive index, nout, that is equal or smaller than theinner cladding refractive index, nclad. The inner cladding 5 and the outercladding 6 is separated by an air-cladding 7, which provides guidance of apump signal during the amplification.Finally, the LMA optical fiber 1 further comprises a pair of inclusions 8, whereinthe pair of inclusions 8 are enclosed in the inner cladding 5 and each of theinclusions has an inclusion refractive index, ninc, that is smaller than the innercladding refractive index, nclad. In this example, each of the inclusionscomprises two separate parts. The pair of inclusions 8 extend parallel to thecore 2 and each of the inclusions is located at a first distance, r1, from thecenter of the core 2, and at a second distance, r2, from an outer diameter ofthe anti-resonant ring, such that the inclusions 8 do not overlap with the core2 and the anti-resonant ring 4. The pair of inclusions 8 is in this example a pairof stress applying parts (SAPs).Example 2 – One embodiment of the LMA optical fiber according to the presentdisclosure, showing how the fundamental mode propagates when bentFig.5 shows a simulation of how the fundamental mode (LP01) propagates inthe LMA optical fiber 1 when bent. In this example, the LMA optical fiber 1 isidentical to that described in Example 1 and shown in Fig.4. However, in Fig.5 is shown only the inner part of the LMA optical fiber of Fig. 4, showing thecore 2, the doped part of the core 3, the ring 4, and the SAPs 8. The LMAoptical fiber as shown in Fig.5 is simulated to be bend / coiled at a radius of 30cm. The LMA optical fiber is bend along the so-called slow axis, and thefundamental mode (LP01) as shown in Fig. 5 is also in the slow axis. As canbe seen from Fig.5, the fundamental LP01 mode (in the slow axis) overlaps with nearly all of the active part of the core 3, and all of the core 2. Accordingly, this mode is substantially unaffected by the LMA optical fiber being bent.Example 3 – One embodiment of the LMA optical fiber according to the presentdisclosure, showing how a high order mode propagates when bentFig.6 shows a simulation of how a high order mode (HOM), in this case LP11,propagates in the LMA optical fiber 1 when bent. In this example, the LMAoptical fiber 1 is identical to that described in Example and shown in Fig. 4.However, in Fig.6 is shown only the inner part of the LMA optical fiber of Fig.4, showing the core 2, the doped part of the core 3, the ring 4, and the SAPs8. The LMA optical fiber as shown in Fig.6 is simulated to be bend / coiled at aradius of 30 cm. The LMA optical fiber is bend along the so-called slow axis,and the high order mode (LP11) as shown in Fig.6 is also in the slow axis. Ascan be seen from Fig.6, the high order LP11 mode (in the slow axis) does not fully overlap with the active part of the core 3, as was the case with the fundamental LP01 mode. In this example, much of the high order LP11 modeis now delocalized from the core 2 and guided in the area between the ring 4and the SAP 8 on the right side. As desired, the presently disclosed LMA optical fiber de-localizes high-ordermodes from the core of the LMA fiber when the LMA optical fiber is bent.Accordingly, the present example demonstrates an LMA optical fiber that efficiently suppresses higher order modes in the core.Fig. 7 shows amicroscope image of a fabricated fiber. The core 2 has a radius rcore ofapproximately 21.5 µm and is doped with a rare-earth dopant material, in thisexample Ytterbium, which provides amplification of the first light signal, and inthis example the active part including Ytterbium is filling the whole region ofthe core 2.. In other words, the microscope image shows a single-layer ring 4contiguously surrounding the core 2 which is all doped with Yitterbium. Therefractive index of the core ncore is approximately 1.4503. The single-layer ring4 has a thickness of approximately 5 µm. In this example the ring is doped withGermanium to increase its refractive index to approximately 1.453. Themicroscope image shows a pair of SAPs located at a first distance, r1 ofapproximately 45 µm, from the center of the core, and at a second distance, r2of approximately 17 µm from the outer diameter of the anti-resonant ring. Inthis example, the SAPs are doped with Boron. It can be observed by the imagethat the single-layer ring 4 has a slightly elongated form along the axis of theSAPs with respect to the y-axis. This undesired egg-like shape of the ring isoften difficult to control during fabrication and might lead to increased loss inthe fundamental mode LP01 in the core, due to resonance with modessupported by the ring. Likewise, for some batches of the fabricated opticalfibers, locally induced wave-like fluctuations of the ring have been observed,which might further cause increased loss of the LP01 mode and decreasedoperation bandwidth due to unwanted resonances between the LP01 mode ofthe core and modes supported by the single-layer ring 4. Thus, a design withmore than one layer of rings in the structure might be a disadvantage, due touncontrolled asymmetries in the ring shape and wave-like local fluctuations ofthe anti-resonant ring. In fact, controlling these fluctuations in two or more rings would give a further degree of uncertainty with respect to controlling the fluctuations for a single ring. In this example, the inventors have found that for optimal LP11 delocalization,r1 is preferably around 2.09 time the core radius. This ensures that the areacomprised between the core and SAPs is hosting modes that are resonantlycoupled to the LP11 when the LMA optical fiber is bent. As desired, the presently disclosed LMA optical fiber de-localizes high-ordermodes, such as LP11 modes from the core of the LMA fiber when the LMAoptical fiber is bent. Accordingly, the present example demonstrates an LMA optical fiber that efficiently suppresses higher order modes in the core.Fig. 8 shows the results of loss measurements for the fiber of Fig. 7. Fig. 8shows the loss of the LP01 and LP11 modes as function of coil diameter of thefiber. Both the LP01 and LP11 modes have considerable losses at lower coildiameters. At a coil diameter of 25 cm, the loss of the LP01 mode is slightlybelow 1 dB / m, while the loss of the LP11 mode is above 12 dB / m. By increasing the coil diameter, both the losses of the LP01 and LP11 decrease,however the LP11 loss still far exceeds the LP01 loss when the coil diameteris below 50, allowing for a strong suppression of the LP11 modes. In thisexample the most suitable coil diameter for achieving a low loss of the LP01 mode and at the same time an efficient suppression of the LP11 mode is between 30 and 45 cm. Further details are provided in the following embodiments. Embodiments: 1. A large mode area (LMA) optical fiber, comprising, concentrically andradially outwards from a center axis of the optical fiber:^ a core comprising at least a core material having a core refractiveindex, ncore, wherein the core has a core radius, rcore, of more than 5 microns, whereby the core is configured to guide a first light signal having an operational wavelength, λoperational, in a plurality of core modes; ^a ring comprising a ring material contiguously surrounding thecore and having a ring refractive index, nring, and a thickness, t, whereby the ring is configured to guide a plurality of ring modes;and ^an inner cladding comprising an inner cladding materialsurrounding the ring and having an inner cladding refractive index, nclad; wherein the ring refractive index is defined such that the ring refractive index, nring, is larger than the core refractive index, ncore, and larger than the inner cladding refractive index, nclad, whereby the ring becomes anti-resonant with the core modes at where v is an integer larger than 1 that defines the radial orderof the ring modes. 2. The LMA optical fiber according to embodiment 1, nring, is larger thanncoreby more than 0.001, more preferably by more than 0.002, such as around 0.0025.The LMA optical fiber according to any of the previous embodiments,wherein the LMA optical fiber further comprises an outer cladding comprising an outer cladding material surrounding the inner cladding and having an outer cladding refractive index, nout, that is equal or smaller than the inner cladding refractive index, nclad.The LMA optical fiber according to any of the previous embodiments,wherein the ring, due to its anti-resonant configuration, is responsible for confining one or more fundamental core mode(s), both when the fiber is bent and straight, preferably wherein the fundamental core mode(s) is / are confined to have an overlap with the core by more than 40%.The LMA optical fiber according to any of the previous embodiments,wherein the thickness, t, is between 1 and 10 microns, preferably between 1 and 5 microns, more preferably between 1 and 4 microns, such as around 3 microns.The LMA optical fiber according to any of the previous embodiments,wherein the inner cladding refractive index, nclad, is around 1.45 at 1030 nm.The LMA optical fiber according to any of the previous embodiments,wherein the LMA optical fiber, and said refractive index thereof, is configured to guide the first light signal having the operational wavelength, λoperational, wherein the operational wavelength for the radialorder v =2 is defined to be: ^^^^^^^^^ (^ = 2) < ^^^^^^^^^^^^ < 2.2 ∗^^^^^^^^^ (^ = 2).8. The LMA optical fiber according to any of the previous embodiments,wherein the LMA optical fiber, and said refractive index thereof, is configured to guide the first light signal having the operational wavelength, λoperational, wherein the operational wavelength for the radial order v > 2 is defined to be: ^^^^^^^^^ (^ + 1) < ^^^^^^^^^^^^ < 0.9 ∗ 9. The LMA optical fiber according to any of the previous embodiments,wherein the LMA optical fiber, and said refractive index thereof, is configured to guide the first light signal having the operational wavelength, λoperational, wherein the operational wavelength is between 1000 nm and 1100 nm, most preferably between 1030 nm and 1064 nm. 10.The LMA optical fiber according to any of the previous embodiments, wherein the resonant wavelength for v = 2 is between 450 nm and 600 nm, preferably between 480 nm and 560 nm, more preferably between 500 nm and 520 nm.11.The LMA optical fiber according to any of the previous embodiments, wherein the LMA optical fiber further comprises a pair of inclusions, wherein the pair of inclusions are enclosed in the inner cladding and each of the inclusions has an inclusion refractive index, ninc, that is smaller than the inner cladding refractive index, nclad. 12.The LMA optical fiber according to embodiment 11, wherein the pair of inclusions extend parallel to the core and each of the inclusions is located at a first distance, r1, from the center of the core, and at a second distance, r2, from an outer diameter of the anti-resonant ring, such that the inclusions do not overlap with the core and the anti-resonant ring.13.The LMA optical fiber according to embodiment 12, wherein the firstdistance, r1, and the second distance, r2, are selected to delocalize at least one mode of the first plurality of core modes, wherein the at least one mode is the lowest higher-order mode, the LP11mode, from the core and to the inner cladding, when the fiber is bent at a bending radius, rbend, preferably such that the LP11 mode has an overlap with the core by less than 50%. 14.The LMA optical fiber according to embodiment 12, wherein the first distance, r1, is between 1.3rcore and 2.6rcore.15.The LMA optical fiber according to embodiment 12, wherein the seconddistance, r2, defines an area in the inner cladding between the outer diameter of the anti-resonant ring and each of the inclusions, such that the area, in combination with the core refractive index, ncore, and the inclusion refractive index, ninc, resonantly couples and guides the LP11 mode. 16.The LMA optical fiber according to embodiment 15, wherein the area between each of the inclusions is further defined by an azimuthal angle that is larger than 40 degrees. 17.The LMA optical fiber according to embodiment 13, wherein the bending radius, rbend, is between 0.04 m and 0.4 m, preferably between 0.04 m and 0.2 m. 18.The LMA optical fiber according to any of the embodiment 11-17, wherein the pair of inclusions is a pair of stress applying parts (SAPs), preferably doped with Boron. 19.The LMA optical fiber according to any of the previous embodiments, wherein the core is doped with at least one rare-earth dopant material, which provides amplification of the first light signal. 20.The LMA optical fiber according to embodiment 19, wherein the inner cladding and the outer cladding is separated by an air-cladding, which provides guidance of a pump signal during the amplification. 21.The LMA optical fiber according to embodiment 19, wherein the at least one rare-earth dopant material is only in a central region of the core. 22.The LMA optical fiber according to any of the preceding embodiments, wherein the core and the ring are separated by a layer of material with refractive index, nclad. 23.The LMA optical fiber according to any of the preceding embodiments, wherein the core has a core radius, rcore, of more than 5 microns, preferably more than 10 microns, even more preferably more than 15 microns, most preferably more than 20 microns, such as around 21 microns.

Claims

AMENDED CLAIMS received by the International Bureau on 16 December 2025 (16.12.2025)1. A large mode area (LMA) optical fiber, comprising, concentrically and radially outwards from a center axis of the optical fiber:- a core comprising at least a core material having a core refractive index, nCOre, wherein the core has a core radius, rCore, of more than 5 microns, whereby the core is configured to guide a first light signal having an operational wavelength, / operational, in a plurality of core modes;- a ring comprising a ring material contiguously surrounding the core and having a ring refractive index, / 7ring, and a thickness, t, whereby the ring is configured to guide a plurality of ring modes; and an inner cladding comprising an inner cladding material surrounding the ring and having an inner cladding refractive index, nciaci; wherein the ring refractive index is defined such that the ring refractive index, / 7ring, is larger than the core refractive index, ncore, and larger than the inner cladding refractive index, / ?ciad, whereby the ring becomes anti-resonant with the core modes at / operational SUCh that:where v is an integer larger than 1 that defines the radial order of the ring modes; and wherein the ring is a single-layer ring.

2. The LMA optical fiber according to claim 1 , / 7ring, is larger than nCote by more than 0.001 , more preferably by more than 0.002, such as around 0.0025.

3. The LMA optical fiber according to any of the previous claims, wherein the LMA optical fiber further comprises an outer cladding comprising an outer cladding material surrounding the inner cladding and having an outer cladding refractive index, nOut, that is equal or smaller than the inner cladding refractive index, / ?ciad.

4. The LMA optical fiber according to any of the previous claims, wherein the thickness, t, is between 1 and 10 microns, preferably between 1 and 5 microns, more preferably between 1 and 4 microns, such as around 3 microns.

5. The LMA optical fiber according to any of the previous claims, wherein the LMA optical fiber, and said refractive index thereof, is configured to guide the first light signal having the operational wavelength, / operational, wherein the operational wavelength for the radial order v =2 is defined to be.

6. The LMA optical fiber according to any of the previous claims, wherein the LMA optical fiber, and said refractive index thereof, is configured to guide the first light signal having the operational wavelength, / operational, wherein the operational wavelength for the radial order v > 2 is defined to be.

7. The LMA optical fiber according to any of the previous embodiments, wherein the LMA optical fiber, and said refractive index thereof, is configured to guide the first light signal having the operationalwavelength, / operational, wherein the operational wavelength is between 1000 nm and 1100 nm, most preferably between 1030 nm and 1064 nm.

8. The LMA optical fiber according to any of the previous embodiments, wherein the resonant wavelength for v = 2 is between 450 nm and 600 nm, preferably between 480 nm and 560 nm, more preferably between 500 nm and 520 nm.

9. The LMA optical fiber according to any of the previous claims, wherein the LMA optical fiber further comprises a pair of inclusions, wherein the pair of inclusions are enclosed in the inner cladding and each of the inclusions has an inclusion refractive index, nine, that is smaller than the inner cladding refractive index, nciad.

10. The LMA optical fiber according to claim 9, wherein the pair of inclusions extend parallel to the core and each of the inclusions is located at a first distance, ri , from the center of the core, and at a second distance, f2, from an outer diameter of the anti-resonant ring, such that the inclusions do not overlap with the core and the anti-resonant ring.11 . The LMA optical fiber according to claim 10, wherein the first distance, n, is between 1 .3rCOre and 2.6rCOre.

12. The LMA optical fiber according to any of the claims 9-11 , wherein the pair of inclusions is a pair of stress applying parts (SAPs), preferably doped with Boron.

13. The LMA optical fiber according to any of the previous claims, wherein the core is doped with at least one rare-earth dopant material, which provides amplification of the first light signal.

14. The LMA optical fiber according to claim 13, wherein the inner cladding and the outer cladding is separated by an air-cladding, which provides guidance of a pump signal during the amplification.

15. The LMA optical fiber according to claim 13, wherein the at least one rare-earth dopant material is only in a central region of the core.

16. The LMA optical fiber according to any of the previous claims, wherein the core and the ring are separated by a layer of material with refractive index, nciad.

17. The LMA optical fiber according to any of the preceding claims, wherein the core has a core radius, rCore, of more than 5 microns, preferably more than 10 microns, even more preferably more than 15 microns, most preferably more than 20 microns, such as around 21 microns.

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