Bend-compensated optical fibers

Abstract

The present invention relates to bend-compensated optical fibers. Particularly, the present invention relates to a bend-compensated optic fibers with tailored core and cladding offering effective-single-mode or multi-mode operation that can be used in high power lasers. Such high power lasers can be used for material processing, recycling, remote sensing, satellite communication, and directed energy sources. The optical fibers bent at a suitable diameter provides multiple forms of fibers such as step-index fiber, single-trench, multi-trench, photonic crystal fiber and photonic bandgap fiber.

Description

[0001] BEND-COMPENSATED OPTICAL FIBERS FIELD OF INVENTION

[0001] The present invention relates to bend-compensated optical fibers. Particularly, the present invention relates to bend-compensated optic fibers with tailored core and cladding offering large mode area and effective single mode operation under coiled condition for high power lasers. BACKGROUND OF THE INVENTION

[0002] Over the last few decades, high-power fiber lasers have advanced at a fast pace, and have acquired many exceptional features over time, such as superb beam quality, good heat management, small footprint, and low operational cost. It has become the primary source for high-precision manufacturing, medical diagnosis and surgery, LIDAR (Light Detection and Ranging), and directed energy sources. High power fiber lasers require a large core diameter fiber so that the power density remain lower than the threshold of the non-linear effects (such as SRS, SBS, FWM, and SPM). These non-linear effects are detrimental to the power scaling. However, a large core diameter fiber offers multi-mode operation and leads to the poor beam quality. Therefore, a trade-off exists between beam quality and power level.

[0003] There is another trade-off between effective mode area and bend diameter of fiber due to bend-induced perturbations in waveguide. One needs to bend the fiber in order to achieve the compact device size. This trade-off limits the effective area of the fundamental mode. For very high power, a few thousands of the effective area of the fundamental mode is required. The current state-of-the-art fibers offer around 1,000 to 1,500 μm2effective area for the fundamental mode for a bend diameter of 30-40 cm.

[0004] There are several patent applications and research papers that disclose bend compensated optic fibers. For instance, US patent application US-9207395-B2 discloses a large mode area (LMA), single-mode optical fiber. This optical fiber comprises a core region, an inner cladding region surrounding the core region, and an outer cladding region surrounding the inner cladding region. The inner cladding region is configured to provide bend compensation. In one embodiment the index profile of the inner cladding region is graded with a slope of γncore / Rb, where ncoreis the refractive index of the core region, Rbis the bend radius, and γ=0.6-1.2. In addition, the inner cladding is annular and the ratio of its outer radius to its inner radius is greater than 2. In a preferred embodiment this ratio is greater than 3. The overall index profile may be symmetric or asymmetric. This cited prior art provides optics fibers with limited large Aeff(>1000 μm2). Moreover, it does not provide effective solution to issue of mode distortion under coiling. The core has a parabolic refractive index profile, which offers lower effective mode area than a step-index fiber, in general.

[0005] Another US Patent application: US-20150043879-A1 addresses trade-off between “effective mode area and bending”. The prior art discloses a fiber having a plurality of substantially parallel cores, the fiber including a straight section and a curved section; guiding signal light primarily in a second core in the straight section; guiding the signal light from the second core into a first core between the straight section and the curved section; and guiding the signal light primarily in the first core in the curved section. The proposed solution is fairly complex and requires micro-structurization of the core and cladding. This makes the fabrication process very complex. It's an asymmetrical fiber design. The major drawback of this technology is that the bending constraint is there, therefore it has to be bent in one direction only.

[0006] Therefore, in view of the problem associated with the state of the art, a need arises for optical fibers that can offer a large effective area with effective single mode operation and solve the issue of the natural tendency of mode shrinkage (effective mode-area reduction) often occurring due to bending of fibers. OBJECTIVES OF THE INVENTION

[0007] The primary objective of the present invention is to provide bend compensated optical fibers possessing tailored core and cladding.

[0008] Another objective of the present invention is to provide bend compensated optical fibers with tailored core and cladding, bending at a suitable diameter to provide multiple forms of fibers such as step-index fiber, single-trench, multi-trench, photonic crystal fiber and photonic bandgap fiber.

[0009] Another objective of the present invention is to provide bend compensated optical fibers possessing large effective mode area while providing effective single mode operation under coiled conditions.

[0010] Yet another objective of the present invention is to provide bend compensated optical fibers that resolve the issue of mode distortion under coiled conditions.

[0011] Yet another objective of the present invention is to provide bend compensated optical fibers that can be used to construct high power lasers with good beam quality.

[0012] Other objects and advantages of the present invention will become apparent from the following description taken in connection with the accompanying drawings, wherein, by way of illustration and example, the aspects of the present invention are disclosed. SUMMARY OF THE INVENTION

[0013] The present invention relates to optical fibers, which are bend compensated. The present invention relates to optic fibers comprising: core region; and cladding region, wherein the core and cladding has a bend-compensated tailored refractive index profile so that on coiling at an appropriate diameter, the right side of the refractive index becomes unperturbed standard refractive index profile offering undistorted mode profile. At this coiling diameter, the refractive index profile of the cladding is such that the core can offer effective single mode operation by offering high losses to the higher order modes.

[0014] These optical fibers can be utilized in high-power lasers. Furthermore, high power lasers can be used for material processing, recycling, remote sensing, and satellite communication. BRIEF DESCRIPTION OF DRAWINGS

[0015] An understanding of the present invention may be obtained by reference to the accompanying drawings, when taken in conjunction with the description herein and in which:

[0016] Figure 1(a) illustrates a diagram of the a triangular core fiber with tailored cladding with different labelled parameters and Figure 1(b) represent the bend equivalent refractive index profile at a bend radius of n(rc)*rc / 1.25*Δnc according to an embodiment of the present invention;

[0017] Figure 2 (a), (b), and (c) illustrates a graphical representation of calculated losses of the fundamental mode and least lossy higher-order mode and effective area of the fundamental mode. The parameters (a) = 0.001, = 0.00005, = 0.001, = 95 μm, = 200 μm, (b) = 0.0008, = 0.00005, = 0.0008, = 95 μm, = 200 μm, and (c) = 0.001, = 0.0001, = 0.0008, = 90 μm, = 200 μm according to an embodiment of the present invention;

[0018] Figure 3 (a) represents the bend compensated multi-trench fiber and Figure 3 (b) represents the loss of the least lossy higher order mode and the fundamental mode along with the effective area of the fundamental mode. Here the fiber parameters: Δnclad= 0.001, Δnt= 0.001, rc= 100 μm, t = 1.4 μm, bend radius = 28.5 cm, and λ = 1.06 μm according to an embodiment of the present invention;

[0019] Figure 4 represent the loss of the least lossy higher order mode and the fundamental mode along with the effective area of the fundamental mode. Here the fiber parameters: ^^^^^^^^^^^^= 0.001, ^^^^^^= 0.0012, ^^^^= 100 μm, t = 1.4 μm, bend radius = 23.5 cm, and λ = 1.06 μm according to an embodiment of the present invention;

[0020] Figure 5 represent the refractive index profile of the fiber. The refractive index is constant in y direction and changes in x direction only, according to an embodiment of the this invention. Such fiber offers much higher effective mode area than the standard optical fiber, where refractive index is a function of both (X and Y axes). These refractive index profiles can be implemented in optical fibers proposed in claim 1 can dramatically increase the effective mode area; and

[0021] Figure 6 (a) and (b) represent the 2D refractive index profile of the uncoiled and coiled fiber at 28.5 cm. Figure 6(c), (d), and (e) represent the 2D electric field intensity profile of the LP01 mode, first least lossy mode, and second least lossy mode fiber according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] The following description describes various features and functions of the disclosed system with reference to the accompanying figures. In the figures, similar symbols identify similar components, unless context dictates otherwise. The illustrative aspects described herein are not meant to be limiting. It may be readily understood that certain aspects of the disclosed system can be arranged and combined in a wide variety of different configurations, all of which have not been contemplated herein.

[0023] Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope of invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.

[0024] Features that are described and / or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and / or in combination with or instead of the features of the other embodiments.

[0025] The terms and words used in the following description are not limited to the bibliographical meanings, but, are merely used to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustrative purpose only and not for the purpose of limiting the invention.

[0026] It is to be understood that the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.

[0027] It should be emphasized that the term “comprises / comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. Accordingly, the present invention discloses bend-compensated optical fibers. Particularly, the present invention relates to optical fibers having tailored core with a tailored cladding. The fibers comprises of a core region comprising silica doped with an index raising oxide and at least one oxide containing rare- earth ion and optionally at least one solubility enhancer; and a cladding region comprising silica doped with an index raising oxide selected from Ge2O3 or Al2O3 wherein; the core region having a triangular refractive index profile and the cladding region having a refractive index that linearly decreases with cladding radius; the bend optical fibers possessing large effective area of the fundamental mode under coiled conditions, wherein coiling of the bend compensated optical fiber provides unperturbed standard refractive index profile (in one direction) that provides undistorted mode, and offers effective single mode or multi-mode operation depending on the refractive profile of the cladding.

[0028] The solubility enhancer is required to avoid the quenching of the rare-earth ions and increase the laser slope efficiency. The solubility enhancer may be selected from but not limited to Al2O3 and P2O5.

[0029] The index raising oxide is selected from the group of Ge2O3 or Al2O3. These index raising oxides when added to silica increase refractive index of silica. The optical fibers having tailored core with a tailored cladding are fabricated by precisely controlling the flow of gases, temperature, and pressure to achieve precise control over refractive index profile during chemical vapour deposition (CVD) or vapour phase chelate deposition (VPCD) process. After preform fabrication, fiber drawing can be done over a draw tower.

[0030] In an embodiment, the process comprises: a. depositing a plurality of layers as a cladding layer comprising alumina or germania doped silica in the optical grade silica glass tube using anhydrous AlCl3 (or GeCl4) as a precursor in a carrier gas using a vapor-phase-chelate- delivery (VPCD) system, wherein the flow of the carrier gas is increased at each layer deposition to increase incorporation of the Al2O3or GeO2, comprising a plurality of layers of the cladding. b. depositing the core layer, i.e. silica doped with Alumina / Germania (or any other index raising element) and ytterbium / erbium / thulium oxide using precursors, such that the ratio of core having a triangular index profile and a titled cladding is kept in the range of 1: 2 to 1:3. c. collapsing the preform followed by etching it in a Hydrofluoric (HF) acid bath to reduce the outer diameter to remove the silica glass, and d. fabricating the preform by drawing over a draw tower to form different diameters of the optical fiber.

[0031] The process of fabricating preform may be carried out by a chemical vapour deposition (CVD) or a vapour phase chelate deposition (VPCD) process, and is not particularly limited. In the particular embodiment, VPCD process is used for depositing a plurality of layers of cladding and core. During the VPCD process in step (a), it is essential to control the flow of chelate precursors such as but not limited to AlCl3 or GeCl4 through carrier gases. The carrier gas for chelate deposition may be selected from He (for AlCl3) and O2(for GeCl4) having a flow rate in range from 20 to 150 sscm, at burner temperature range of 1700 to 2100 degree Celsius, burner speed 100 to 150 mm / minute, and at a pressure range of 0.7 to 1.5 atm inside the tube for layer by layer deposition to achieve the desired refractive index control on the scale of 0.00001 to 0.0001.

[0032] In an exemplary embodiment, the preform fabrication comprises: 1) An optical-quality silica tube as a base on a VPCD lathe is used. In an exemplary embodiment, the optical-grade silica glass tube with dimensions 14 x 11 mm (or 20 x 16 mm), from Heraeus with an internal octagonal shape is taken; 2) On the optical quality / grade silica glass tube is deposited several layers of alumina / germania-doped silica using a vapor-phase-chelate-delivery (VPCD) system. Here, anhydrous AlCl3(or GeCl4) is used as a precursor. The anhydrous AlCl3 is placed in a high-temperature cabinet and Helium gas can be used as a carrier gas. The GeCl4 is placed in a bubbler and oxygen gas can be used as a carrier gas. 3) With each layer deposition the flow of the Helium / oxygen gas is increased to increase the incorporation of the Al2O3 / GeO2 in the cladding, leading to a linearly increasing refractive index of the cladding. The expect of 30 to 50 layers of deposition with an increase of 1 to 2 cm3in gas flow per minute for each layer.

[0033] Afterward that deposit, the core is doped with both alumina and ytterbium / erbium / thulium ions. The precursor used for ytterbium oxide may be Ytterbium tris (2,2,6,6- tetramethyl-3,5-heptanedionate), however it is not particularly limited. The core so fabricated has a refractive index that linearly decreases with cladding radius.

[0034] The aim is to deposit a ~2 mm core with a triangular index profile and ~4 to 5 mm titled cladding. After collapsing the preform, it is etched in a Hydrofluoric (HF) acid bath to reduce the outer diameter to remove the silica glass. Further, the fabricated preform is drawn over a draw tower for different diameters.

[0035] In an embodiment, as shown in Figure 1(a), the fiber design has a triangular core with a tailored cladding (where refractive index linearly decreases with cladding radius) that can offer large effective mode area in the range of 3,000 to 5,000 μm2at a coiled diameter of 20 to 30 cm, while ensuring effective single mode operation. And Figure 1(b) represents the bend equivalent refractive index profile, which looks similar to the step-index fiber, almost like an unperturbed fiber. This feature helps in avoiding bend-induced mode distortion. This optic fibers’ design can dramatically scale the output power level of the fiber lasers to 10s of kilowatt output power level in the continuous laser regime and larger than GWs of output power level in pulsed laser regime with good beam quality and stability.

[0036] The refractive index profile may be modelled and bending losses may be calculated using different commercially available software. In an exemplary embodiment, the COMSOL Multiphysics software module based on finite element method has been used for calculations. A mesh size equal to the wavelength of the operation has been used to ensure the accuracy. A perfectly matched layer (PML) around the cladding has been used to calculate the leakage and bending losses. The bending effect of these fibers has been modeled using standard conformal mapping with additional stress perturbations for the equivalent index model. The curved fiber is equivalent to a straight fiber with an effective refractive index distribution: where n(r) is the index profile of the straight fiber; R is the radius of curvature, φ is the azimuthal angle, r is the distance from the fiber axis in the plane of the bend and ρ (here fixed to 1.25) has been included to take account of the stress factor.

[0037] In an embodiment, as shown in Figure 2(a) represent a fiber design acquire on coling the shape of the a step index fiber with parameters ^^^^c= 0.001, ^^^^o= 0.00005, ^^^^cladd= 0.001, ^^c= 95 μm, and ^^clad = 200 μm, here an effective mode area of 3,160 μm2has been achieved at 11.5 cm bend radius. The loss of the fundamental mode is 0.0037 dB / m and the loss of the first higher order mode (HOM) is 9.02 dB / m. Figure 2(b) represent a fiber design with parameters ^^^^c= 0.0008, ^^^^o = 0.00005, ^^^^cladd = 0.0008, ^^c = 95 μm, and ^^clad = 200 μm, here an effective mode area of 3,480 μm2has been achieved at 14.3 cm bend radius. The loss of the fundamental mode is 0.0061 dB / m and the loss of the first higher order mode (HOM) is 15.1 dB / m and Figure 2(c) represent a fiber design with parameters ^^^^c= 0.001, ^^^^o= 0.0001, ^^^^cladd= 0.0008, ^^c= 90 μm, and ^^clad= 200 μm, here an effective mode are a of 2,613 μm2has been achieved at 14.3 cm bend radius. The loss of the fundamental mode is 0.107 dB / m and the loss of the first higher order mode (HOM) is 9.36 dB / m. The above equation 1 and 2 shows the bend equivalent refractive index equation for the coiled fiber using standard conformal mapping with additional stress perturbations. Implementing these equations in Figure 1(a), provides Equation 4 shows the bend radius required to convert the core’s linearly decreasing refractive index to a constant value. Higher is the Δnc, smaller would be the bend radius. This is one of the major achievements. A small bend radius can be achieved for a large core. This can resolve the issue of bend effective area reduction. Applying the same for cladding region:

[0002] Higher is the Δnclad than Δnc, larger is the clad radius that can be afforded for a perfect SIF. The effective mode area can be scaled further by employing different fiber designs such as multi- trench fiber, photonic crystal fiber, large pitch fiber, Bragg fiber, 2D Photonic Bandgap Fiber, etc.

[0038] In an embodiment, as represented in Figure 3 (a) shows the schematic bend compensated multi-trench fiber. Figure 3 (b) shows the calculated losses of the fundamental mode and least lossy HOM at a fixed bend radius of 28.5 cm (57 cm bend diameter) for Δnclad= 0.001, Δnt = 0.001, rc = 100μm, t = 1.4 μm for varying resonant ring thickness (d).The effective mode area exceeds 4,000 μm2at 57 cm bend diameter, with very high loss discrimination ratio between the least lossy mode and fundamental mode (~900).

[0039] In another embodiment, as shown in Figure 4, shows the calculated losses of the fundamental mode and the least lossy higher order mode. In order to reduce the bend diameter, the Δntwas increased to 0.0012, the loss of the fundamental mode is 0.05095 dB / m and the least lossy HOM is 27.79 dB / m, the effective mode area of the fundamental mode is 4,112 μm2.at resonant ring thickness of 32 μm.

[0040] In an embodiment, as shown in Figure 5 for scaling the effective area of fundamental mode of the bend compensated fiber, a different design is also being invented. The refractive index of the bend compensated optic- fibers varies only in one direction (such as X-axis), while the refractive index remains constant in other direction (such as Y-axis). Such fiber offers much higher effective mode area than the standard optical fiber, where refractive index is a function of both (X and Y axes). These refractive index profiles can be implemented in optical fibers can dramatically increase the effective mode area.

[0041] So, the refractive index remains constant in y direction and changes in x direction only. For parameters ^^^^c= 0.001, ^^^^o= 0.00005, ^^^^cladd= 0.001, ^^c= 100 μm, and ^^clad= 200 μm, an effective mode area of 8,494 μm2was achieved at 11.5 cm bend radius. The loss of the fundamental mode is 0.039 dB / m, the loss of the first higher order mode (HOM) is 0.11 dB / m, and the loss of second least lossy mode is 5.23 dB / m. It is to note that, a hybrid model can also be implemented, where core has index variation only with respect to x and cladding has index variation with respect to r (spatial radius).

[0042] In an embodiment the same index variation can also be applied to Multi Trench or any other fiber. Figure 6 (a) and (b) shows the 2D refractive index profile of the uncoiled and coiled fiber at 28.5 cm. Figure 6(c), (d), and (e) shows the 2D electric field intensity profile of the LP01 mode, first least lossy mode, and second least lossy mode. The loss of the fundamental mode is 0.02 dB / m and the effective mode area is 9109 μm2, while the loss of the least lossy mode is 0.07 dB / m. The coil radius is 28.5 cm and the operational wavelength is ~28.5 cm.

[0043] While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

Claims

CLAIMS:

1. Bend compensated optic-fibers comprising: a core region comprising silica doped with an index raising oxide and at least one oxide containing rare-earth ion and optionally at least one solubility enhancer; and a cladding region comprising silica doped with an index raising oxide selected from Ge2O3or Al2O3wherein; the core region having a triangular refractive index profile and the cladding region having a refractive index that linearly decreases with cladding radius; the bend optical fibers possessing large effective mode area under coiled conditions, wherein coiling of the bend compensated optical fiber provides unperturbed standard refractive index profile, and offers effective single mode or multi-mode operation depending on the refractive profile of the cladding; the solubility enhancer is selected from Al2O3and P2O5; said bend compensated fibers having linearly decreasing refractive index from the center of the fiber with single or multiple trenches; said optical fibers bend at a suitable diameter to provide unperturbed refractive index of fibers selected from any one of single / multi-trench, photonic crystal fiber and photonic bandgap fiber.

2. The optic-fibers as claimed in claim 1, wherein refractive index of the bend compensated optic- fiber varies in one direction (such as X-axis), and remains constant in other direction (such as Y-axis).

3. The optic-fibers as claimed in claim 1, wherein the bend compensated step-index optical fiber provides effective mode area in the range of 3,000-5,000 μm2at a coiled diameter of 10 to 30 cm, while ensuring effective single mode operation.

4. The optic fibers as claimed in claim 1, wherein the fiber acquires on coiling the shape of a step-index fiber, possess loss of the fundamental mode is 0.0037 dB / m and the loss of the first higher order mode (HOM) is 9.02 dB / m with parameters ^^^^c= 0.001, ^^^^o= 0.00005, ^^^^cladd = 0.001, ^^c = 95 μm, and ^^clad = 200 μm, an effective mode area of 3,160 μm2at bend radius of 11.5 cm.

5. The optic fibers as claimed in claim 1, wherein the fiber acquires on coiling the shape of a step-index fiber, loss of the fundamental mode is 0.0061 dB / m and the loss of the first higher order mode (HOM) is 15.1 dB / m with parameters ^^^^c= 0.0008, ^^^^o= 0.00005, ^^^^cladd= 0.001, ^^c= 95 μm, and ^^clad= 200 μm, an effective mode area of 3,480 μm2at bend radius of 14.3 cm.

6. The optic fibers as claimed in claim 1, wherein the fiber acquires on coiling the shape of a step-index fiber possess the loss of the fundamental mode is 0.107 dB / m and the loss of the first higher order mode (HOM) is 9.36 dB / m with parameters ^^^^c= 0.001, ^^^^o= 0.0001, ^^^^cladd= 0.0008, ^^c= 90 μm, and ^^clad= 200 μm, an effective mode area of 2,613 μm2at bend radius of 14.3 cm.

7. The optic fibres as claimed in claim 1, wherein the bend compensated multi-trench fiber possess loss discrimination ratio of larger than 500 between the least lossy mode and fundamental mode at a fixed bend radius of 23.5 cm and effective area of 4,112 μm2at 47 cm bend diameter for Δnclad = 0.001, Δnt = 0.0012, ^^c = 100 μm, t = 1.4 μm, and d = 32 μm.

8. The optic fibres as claimed in claim 2, wherein for a bend compensated fiber having parameters ^^^^c= 0.001, ^^^^o= 0.00005, ^^^^cladd= 0.001, ^^c= 100 μm, and ^^clad= 200 μm, an effective mode area of 8,494 μm2is achieved at 11.5 cm bend radius, wherein loss of the fundamental mode is 0.039 dB / m, the loss of the first higher order mode (HOM) is 0.11 dB / m, and the loss of second least lossy mode is 5.23 dB / m.

9. The optic fibers as claimed in claim 1 and 2, wherein the fibers can provide an output power level of up to 10 kilowatt and larger than GWs in continuous and pulse regime respectively.

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