Optical fibers and optical fiber devices

Abstract

An optical fiber having a first core and a second core, wherein the first core has an active dopant, the first core and the second core are separated by a first cladding, the first cladding has a first refractive index smaller than the refractive index of the first core and the second core, and when the optical fiber is composed of a straight line or a uniform radius of curvature, a first optical mode can propagate along the second core but is isolated from the active dopant, and when the optical fiber is perturbed, the first optical mode is coupled to a second optical mode that overlaps with the active dopant, the pump radiation propagates along the second core as the first optical mode, the perturbation selectively couples the first optical mode to the second optical mode, and the pump radiation coupled to the second optical mode is absorbed by the active dopant, which can amplify the signal mode guided by the first core by stimulated emission, thereby enabling control of at least one of the inversion ratio of the active dopant, optical gain characteristics, and thermal load.

Description

[Technical Field]

[0001] The present invention relates to optical fibers and optical fiber devices. The present invention has applications for increasing the available pulse energy from a fiber laser. The present invention is applicable to lasers used for cleaning, cutting, and welding. [Background technology]

[0002] Industrial fiber lasers have important applications in industrial processing of materials, including cleaning, marking, cutting, scribing, welding, sintering of metal powders using lasers in processes commonly known as additive manufacturing or 3D printing, and other material processing. Lasers are used in many industries, including consumer electronics, medical devices, automobiles, and aerospace. Lasers may be pulsed or continuous waves.

[0003] High pulse energies require fibers with high saturation energy. This necessitates a large core and a low radiation cross-section at the emission wavelength. Furthermore, to avoid parasitic spontaneous emission amplified light and parasitic laser oscillation, it is preferable that the laser gain peak be as close as possible to the emission wavelength.

[0004] High average power in fiber lasers is typically achieved through cladding pumping. Pump radiation is directed onto the cladding surrounding the core. The pump radiation can overlap with the core and be absorbed by active ions within the core.

[0005] As core size increases to achieve higher pulse energies, absorption of pump radiation within the core tends to increase. High pump absorption means a high inversion near the point of incidence of pump radiation, which shifts the laser gain peak to the shorter wavelength side where the radiation cross-section can be larger. This can cause parasitic spontaneous radiation amplification, parasitic laser oscillation, which can reduce the maximum pulse energy available from the laser. Furthermore, high inversion increases the core's tendency towards photodarkening, a well-known cause of laser performance degradation. A high pump absorption coefficient can also increase the thermal load on the fiber near the point of incidence of the pump, which can damage the fiber. Typical coating materials used in optical fibers have a limited temperature range and degrade if stored or operated outside this range. With a high pump absorption coefficient, the temperature range can be exceeded even with measures taken to maximize heat extraction, such as placing the fiber in a metal groove on a cooling plate.

[0006] For the reasons mentioned above, it is preferable to reduce pump absorption per unit length of fiber as the core size increases. While there are known methods to achieve this, they have clear drawbacks.

[0007] The simplest approach is to increase the cladding size to maintain the ratio of cladding area to core area. However, beyond a certain core size, it becomes impossible to give the fiber enough flexibility to be packaged and wound onto a spool while keeping the optical fiber diameter small enough. A 2mm diameter glass optical fiber is closer to a glass rod than a fiber.

[0008] Pump absorption can also be reduced by decreasing the concentration of the active dopant. However, this does not change the reversal ratio of the active dopant, which is governed solely by the pump strength and absorption cross-section. Since the reversal ratio is what determines the exit characteristics, this solution does not provide the important advantage of bringing the fiber's gain peak closer to the exit wavelength.

[0009] Pump absorption can be reduced by using different pump wavelengths and lowering the pump absorption cross-section. However, this is generally not practical. For example, moving to shorter wavelengths increases quantum defects (energy differences between the pump and signal photons), reducing laser efficiency and thus increasing the heat generated by the laser at the same output power. Efficient pump sources may not exist at the desired wavelength, or if feasible, the complexity and cost of the configuration increase significantly, such as the tandem excitation scheme used to excite fiber lasers near 1 μm. Cost and power efficiency are critical parameters in industrial laser systems.

[0010] Optical fibers and optical fiber devices are needed that can adjust pump absorption in several ways other than fiber size, doping concentration, or pump wavelength. [Overview of the project]

[0011] According to an unrestrictive embodiment of the present invention, an optical fiber having at least one first core and at least one second core is provided. • The first core has at least one active dopant, The first and second cores are separated by the first cladding. The first cladding has a first refractive index that is smaller than the refractive index of the first core and smaller than the refractive index of the second core. Optical fibers are If the optical fiber is composed of a straight or uniform radius of curvature, the first optical mode having a first propagation constant β1 can propagate along the second core, but is isolated from the active dopant. When the optical fiber is perturbed, the first optical mode can be coupled to a second optical mode having a second propagation constant β2 and overlapping with the active dopant. This enables the control of at least one of the following: the inversion rate of the active dopant along the optical fiber, the optical gain characteristics of the optical fiber, and the thermal load of the optical fiber. This is achieved by propagating pump radiation along the second core as a first optical mode, selectively coupling the first optical mode to the second optical mode by one or more perturbations, and absorbing the pump radiation selectively coupled to the second optical mode by an active dopant capable of amplifying one or more signal modes guided by the first core by stimulated emission.

[0012] The optical gain characteristics may be the wavelength dependence of the optical gain, or variations in the amplitude or wavelength distribution of the naturally radiated amplified light.

[0013] Optical fibers can be used in optical amplification devices such as fiber lasers or optical amplifiers where pump radiation is coupled to a second core. By combining the first cladding and perturbations, the proportion of pump radiation overlapping with the active dopant in the first core can be controlled by design as a function of distance along the fiber. This allows for control of the laser inversion rate, increasing the pulse energy of pulses emitted by the laser, decreasing the naturally radiated amplified light emitted by the laser, and reducing the thermal load on the optical fiber. This also provides the ability to change the pump absorption coefficient as a function of distance along the optical fiber, for example, by reducing the absorption coefficient closest to the pump incidence point where the pump intensity is highest, and thus better distributing the thermal load.

[0014] The perturbation can have a spatial frequency component Λ = (β1 - β2) / 2π.

[0015] The perturbation may have at least one spatial frequency component that couples together at least several first optical modes.

[0016] The perturbation may have spatial frequencies other than the spatial frequencies at which the signal modes couple together.

[0017] The perturbation can have at least one of a change in the radius of curvature, a change in the bending direction, a compressive force, a long-period grating, a change in the diameter of the optical fiber, a change in the cross-sectional shape of the optical fiber, a rotation of the optical fiber, and a change in the material composition of the optical fiber along the length of the optical fiber.

[0018] The active dopant can be selected from the group having ytterbium, erbium, neodymium, praseodymium, thulium, samarium, holmium, dysprosium, a transition metal, or a semiconductor. The active dopant may be erbium codoped with ytterbium.

[0019] The first optical mode propagating along the second core does not overlap with the first core because of the presence of the first cladding. The first cladding functions as a barrier such that only the second optical mode overlaps with the core and the first optical mode does not overlap with the core. For light starting from one of the first optical modes to overlap with the first core, it must first be coupled to one of the second optical modes by a perturbation. The ratio of the power coupled between modes per unit length is greater for a stronger perturbation than for a weaker perturbation. Examples of strong perturbations include a large change in the radius of curvature, a large change in the bending direction, and a stronger compressive force. The first cladding functions as a barrier layer that limits the coupling between the first optical mode and the second optical mode unless the optical fiber is perturbed by a perturbation having the following spatial frequency component Λ = (β1 - β2) / 2π.

[0020] Therefore, the amount of coupling can be controlled by selecting one or more of the strength of the perturbation, the number of perturbations, and the arrangement of the perturbations along the optical fiber.

[0021] Note that in order to increase the pulse energy of the laser output, it is necessary to increase the diameter of the first core. If there is no first cladding, it is also necessary to increase the size of the second core. When the required pulse energy exceeds 10 mJ, the overall diameter of the fiber rapidly exceeds 1 mm, making fiber packaging, cutting, splicing, and heat sinking difficult. The inclusion of the first cladding that separates the first core from the second core reduces the effective brightness of the pump radiation that overlaps with the second core. That is, the same amount of pump radiation travels along the smaller second core, but depending on the intensity, number, and arrangement of the perturbations, a lower percentage of the pump radiation can overlap with the active dopant within the first core.

[0022] Light rays that form an angle with the fiber axis smaller than the minimum divergence angle are confined (guided) to the second core with respect to the first cladding, and certain light rays with a higher divergence angle propagate along the second core, the first cladding, and the first core and can overlap and be absorbed by the active dopant. Therefore, the absorption rate of the pump radiation can be lower for a fiber with the first cladding than for a fiber without the first cladding. By including the first cladding, an optical fiber with a smaller diameter than an equivalent fiber providing the same output power and pulse energy can be realized. Also, by including the first cladding, the optical fiber can be operated at a lower temperature, thereby improving the reliability of the fiber coating.

[0023] The high-divergence pump radiation that overlaps with the first core can be absorbed by the active dopant, and thus the amount of pump radiation that overlaps with the active dopant in the first core is reduced. Although a large amount of pump radiation guided by the second core can still exist, the first cladding prevents this pump radiation from overlapping with the active dopant and being absorbed by the active dopant. Therefore, the pumping of the active dopant by the pump radiation can be suppressed compared to a fiber without the first cladding.

[0024] To maintain sufficient absorption along the length of the fiber, a mechanism is needed to supplement the high-divergence pump radiation. This supplementation is achieved by applying perturbations to the fiber at various points along its length, shifting the low-divergence pump radiation guided by the second core to high-divergence pump radiation that can overlap with the active dopant. Perturbations can be achieved by various means, such as changes in the radius of curvature or bending direction, compression or stress application of the optical fiber by mechanical grating, changes in the diameter of the optical fiber, long-period grating, rotation of the fiber, changes in the fiber shape such as local milling of the second core by a carbon dioxide laser, or changes in the material composition of the second core along the length of the optical fiber.

[0025] The combination of suppressing pump overlap with the first core due to the presence of the first cladding and a controlled increase in pump overlap with the first core due to the designed perturbation of the fiber allows for a design-based selection of the pump absorption rate along the fiber. The pump absorption rate can be more than twice as low, preferably more than three times lower, and even more preferably more than five times lower than in the absence of the first cladding. The rate of pump radiation absorbed per unit length of fiber can be independently controlled in different sections of the fiber by designing the intensity of the perturbation. Stronger perturbations (e.g., tighter bends, stronger compressive forces, stronger long-period gratings) couple more pump radiation and thus increase the rate of pump radiation absorbed in later sections of the fiber.

[0026] The ability to control the amount of pump radiation coupled from the second core to the first core allows a large amount of energy to be stored in the active dopant without causing excessive spontaneous radiation amplification and parasitic laser oscillations. This is because preventing pump radiation from coupling from the second core reduces the pump absorption rate along the optical fiber, thus enabling higher levels of pump radiation and the use of longer optical fibers. As shown below, the present invention enables nanosecond laser sources with pulse energies exceeding 100 mJ. Such lasers are difficult to design without very large fiber diameters, and because the stiffness of the fiber is highly dependent on the diameter, a very large radius of curvature is required to avoid fiber breakage, making it impractical to package them into reasonably sized modules.

[0027] An optical fiber can have multiple first cores.

[0028] Optical fibers can have multiple second cores.

[0029] The second core does not have to be circular.

[0030] The optical fiber may have a second cladding. The second cladding may surround a second core. The second core may have a second refractive index lower than the first refractive index.

[0031] The second cladding may not be circular.

[0032] The first and second cores can be surrounded by a second cladding.

[0033] The second cladding may be a polymer. The first and second cores may be separable from each other. This can be achieved, for example, by removing the second cladding from at least a portion of the optical fiber using chemical, thermal, or mechanical means. Advantageously, this allows the first core to be spliced ​​into a signal input fiber and the second core into a pump fiber.

[0034] The optical fiber may include a pump fiber that is in optical contact with a second core along its length. The second core may surround the first core. The second core and the pump fiber may be surrounded by a second cladding.

[0035] The second cladding may be a polymer. The optical fiber can be characterized by the fact that the pump fiber and the second core are separable from each other.

[0036] At least one of the first core and the second core may be a ring core.

[0037] The width of the first core can be greater than 50 μm, preferably greater than 100 μm, and more preferably greater than 250 μm.

[0038] The width of the first cladding can be greater than 5 μm, preferably greater than 10 μm, and more preferably greater than 15 μm.

[0039] The width of the second core can be greater than 50 μm, preferably greater than 100 μm, and more preferably greater than 150 μm.

[0040] The refractive index of the second core may be greater than 0.001, preferably greater than 0.004, and more preferably greater than 0.01, than the first refractive index of the first cladding.

[0041] The refractive index of the first core may be greater than the refractive index of the second core. The refractive index of the first core may be greater than the first refractive index of the first cladding by more than 0.015.

[0042] The active dopant can be uniformly distributed throughout the first core. Alternatively, the active dopant can be concentrated more towards the center of the first core than at the edges. This can improve the beam quality of the stimulated laser radiation emitted from the optical fiber.

[0043] The present invention also provides an optical fiber spool having the optical fiber of the present invention. The optical fiber spool may include a plurality of perturbations.

[0044] A perturbation can involve a change in the radius of curvature.

[0045] The perturbation may have a long-period grating.

[0046] Optical fibers can be placed inside grooves.

[0047] The second core of the optical fiber may have a width of less than 1 mm, preferably less than 0.9 mm, and more preferably less than 0.8 mm.

[0048] The optical fiber spool can be characterized by having a minimum radius of curvature of less than 20 mm, preferably less than 15 mm, and more preferably less than 10 mm.

[0049] The optical fiber spool can be characterized by having a cross-sectional width of less than 500 mm, preferably less than 400 mm, and more preferably less than 300 mm.

[0050] The present invention also provides an optical amplifier having an optical fiber spool and at least one pump source, which emits an output signal at a signal wavelength. The pump source can be connected to a second core such that pump radiation from the pump source is guided by the second core and perturbed to an active dopant. The output signal may have stimulated emission emitted from the active dopant. The stimulated emission is preferably guided by a first core. Multiple pump sources can be provided and emit at the same wavelength or different wavelengths.

[0051] The optical fiber can be characterized by its temperature rise relative to the substrate temperature. The intensity and arrangement of the perturbation can be such that the temperature rise is less than 100°C, preferably less than 70°C, and more preferably less than 50°C, when the optical fiber is pumped by the pump source.

[0052] The optical amplifier may have an inversion rate of active dopant equal to less than 10%, preferably less than 8%, and more preferably 7%, along at least 70% of the length of the optical fiber.

[0053] The first core can be characterized by its absorption length at the pump wavelength. The first core has a first core area, and the second core has a second core area. The optical fiber can be characterized by the ratio obtained by dividing the core area equal to the sum of the first and second core areas by the first core area. The length of the optical fiber can be at least twice, preferably at least three times, and more preferably at least five times, the product of the ratio of the absorption length to the core area.

[0054] The optical fiber has a fiber length at least twice that of an equivalent optical fiber, and is used selectively with an optical amplifier while absorbing the same amount of pump radiation along its length, without having a first cladding, wherein the first cladding of the optical fiber is replaced by a region within the equivalent optical fiber having the same refractive index as the second core of the optical fiber, thereby allowing the second core of the equivalent fiber to function as a pump cladding surrounding the first core.

[0055] The optical amplifier can be characterized by its pulse energy. The intensity and arrangement of the perturbation can result in a higher pulse energy when the optical fiber is pumped by a pump source than when the optical amplifier uses an equivalent optical fiber.

[0056] An optical amplifier can be characterized by its gain peak and gain peak wavelength. The intensity and arrangement of the perturbation can be such that the gain peak wavelength is longer when the optical fiber is pumped by a pump source than when the optical amplifier uses an equivalent optical fiber. The gain peak can be higher when using an optical fiber than when the optical amplifier uses an equivalent optical fiber.

[0057] The optical amplification device may include multiple pumps. One pump may emit pump radiation at a different pump wavelength than the other pumps.

[0058] The active dopant may contain ytterbium ions, and the gain peak may be 1060 nm.

[0059] The active dopant may have erbium or erbium codoped with ytterbium, and the gain peak wavelength may be in the range of 1555 nm to 1650 nm.

[0060] The active dopant may contain holmium, and the gain peak wavelength may be in the range of 1990 nm to 2150 nm. The optical amplifier may have multiple pumps. One pump can emit pump radiation at 915 nm. Another pump can emit pump radiation at 976 nm.

[0061] The active dopant 9 may contain thulium, and the gain peak wavelength may be in the range of 1900 nm to 2100 nm. Advantageously, the thulium ion can be pumped at 793 nm.

[0062] The difference between the signal wavelength and the gain peak wavelength may be less than 10 nm, preferably less than 5 nm, and more preferably less than 1 nm.

[0063] The optical amplification device can be configured such that pump radiation exceeding 1 kW, preferably exceeding 2 kW, and more preferably exceeding 3 kW, is guided by the second core.

[0064] The optical amplification device may include a seed laser connected to the first core such that the signal energy is guided by the first core and amplified along the optical fiber.

[0065] The seed laser may be a pulsed laser.

[0066] The seed laser can emit signal pulses with pulse widths ranging from 100 ps to 10 ms, and the optical amplifier can be configured to emit individual pulse energies exceeding 50 mJ, preferably exceeding 100 mJ, and more preferably exceeding 200 mJ.

[0067] The optical amplification device may have an optical feedback arrangement configured to promote light generation within the optical fiber and generate a laser.

[0068] The optical amplification device may have at least one reflector configured to reflect light energy back to the first core.

[0069] The optical amplifier may have an optical switch connected to the first core.

[0070] The present invention also provides a method for providing light emission, the method being The present invention provides an optical fiber having at least one first core and at least one second core, wherein the first core has at least one active dopant, the first core and the second core are separated by a first cladding, and the first cladding has a first refractive index lower than the refractive index of the first core and lower than the refractive index of the second core. • Pump radiation propagates along the second core in the form of a first optical mode having a first propagation constant β1. Select at least one perturbation intensity and configuration. • Perturb the optical fiber with at least one perturbation, and couple the pump radiation from the second core to a second optical mode having a second propagation constant β2 and overlapping with the active dopant. • The activated dopant absorbs the pump radiation, • Propagates one or more signal modes along the first core, • Amplify the signal mode by stimulated release using an activated dopant. • Outputs the amplified signal mode in the form of light emission. This includes, If the optical fiber is straight or has a uniform radius of curvature, the first optical mode can propagate along the second core, but it is isolated from the active dopant. It is characterized by the following:

[0071] This method may include the step of controlling at least one of the inversion rate of the active dopant along the optical fiber, the optical gain characteristics of the optical fiber, and the thermal load of the optical fiber. This can be achieved by selecting the intensity and arrangement of the perturbation.

[0072] The perturbation has a spatial frequency component Λ = (β1 - β2) / 2π.

[0073] The perturbation has at least one spatial frequency component that couples at least several first optical modes together.

[0074] The perturbation has spatial frequencies other than the spatial frequencies at which the signal modes couple together.

[0075] The perturbation includes at least one of the following: a change in radius of curvature, a change in bending direction, compressive force, long-period grating, a change in the diameter of the optical fiber, a change in the cross-sectional shape of the optical fiber, rotation of the optical fiber, and a change in the material composition of the optical fiber along its length.

[0076] This method may include the step of providing a signal mode by coupling the signal emission from a seed laser to a first core.

[0077] The method may include the step of providing an optical feedback arrangement for generating a laser by promoting light generation within an optical fiber. The optical feedback arrangement may have at least one reflector configured to reflect the light energy back to the first core. The reflector may have an optical fiber Bragg grating.

[0078] This method may include the step of providing an optical switch and connecting the optical switch to a first core.

[0079] The method of the present invention may have one or more steps required to utilize the above-described optional embodiments of the optical fiber, optical fiber spool, or optical amplifier of the present invention. [Brief explanation of the drawing]

[0080] Embodiments of the present invention will be described simply by reference to the accompanying drawings. [Figure 1] The optical fiber according to the present invention is shown. [Figure 2] This shows a ray of light that is coupled from the second core, absorbed by the active dopant, and guided by the first core. [Figure 3] This shows a group of optical modes guided by an optical fiber. [Figure 4] This shows an optical fiber with a non-circular second core. [Figure 5] This shows an optical fiber with a non-circular second cladding. [Figure 6] This shows an optical fiber with four second cores arranged in the second cladding. [Figure 7] This shows an optical fiber having four second cores arranged in the first cladding. [Figure 8] This shows an optical fiber having four first cores and three second cores arranged in the first cladding. [Figure 9] This shows an optical fiber having three first cores, each located in the second core and surrounded by the first cladding. [Figure 10] This diagram shows an optical fiber in which the second core is surrounded by the first cladding, and the first cladding is surrounded by the first core. [Figure 11] This diagram shows an optical fiber in which the first cladding surrounds the first core, and the first core and first cladding can be separated from the second core by removing the second cladding and pulling it. [Figure 12] The first cladding surrounds the second core, and by removing and pulling the second cladding, the first core is separated from the second core and the first cladding, revealing an optical fiber. [Figure 13] This shows an optical fiber containing a separate pump fiber that can be separated from the second core by removing and pulling the second cladding. [Figure 14] The present invention presents an optical amplifier having an optical fiber that is end-pumped and in which an amplifier is incorporated as a power amplifier for a master oscillator power amplifier. [Figure 15] This shows a bundle of pump fibers and signal fibers within a capillary that forms part of the output combiner. [Figure 16] Figure 15 shows a bundle of fibers with spliceable combiner fibers. [Figure 17] This shows a spool of optical fiber, which is composed of a flat spiral with straight edges and rounded corners. [Figure 18] This shows the optical fiber of the present invention wound onto a spool former. [Figure 19]The optical fiber of the present invention is shown, in which a perturbation that couples pump radiation from modes guided by a second core to other modes overlapping with the active dopant is provided by a long-period grating. [Figure 20] Figure 4 shows a laser with an optical fiber. [Figure 21] Figure 13 shows a laser with an optical fiber. [Figure 22] Figure 11 shows a master oscillator power amplifier with an optical fiber. [Figure 23] A master oscillator power amplifier having two amplifiers and the amplifier shown in Figure 14 is shown. [Figure 24] Figure 14 shows a Q-switched laser with an amplifier. [Figure 25] The absorption and emission cross-sections of ytterbium ions as a function of wavelength in ytterbium-doped optical fibers used in Examples 1 to 6 are shown. [Figure 26] The inversion ratio with respect to fiber length is shown for Examples 1 to 6. [Figure 27] The inversion ratio with respect to fiber length is shown for Examples 1 to 6. [Figure 28] Examples 1-6 show pump absorption with respect to fiber length. [Figure 29] Examples 1-6 show pump absorption with respect to fiber length. [Figure 30] The spectra of naturally emitted amplified light for Examples 1, 3, 4, 5, and 6 are shown. [Figure 31] The optical fiber used in Example 1 is shown. [Figure 32] An equivalent optical fiber not according to the present invention, which does not have a first cladding, is shown. [Modes for carrying out the invention]

[0081] Figure 1 shows an optical fiber 10 having at least one first core 2 and at least one second core 12. • The first core 2 has at least one active dopant 9, The first core 2 and the second core 12 are separated by the first cladding 3. The first cladding 3 has a first refractive index 5 which is smaller than the refractive index 4 of the first core 2 and smaller than the refractive index 14 of the second core 12. Optical fiber 10 is If the optical fiber 10 is composed of a straight or uniform radius of curvature, the first optical mode 101 having a first propagation constant β1 can propagate along the second core 12, but is isolated from the active dopant 9. When the optical fiber 10 is subjected to a perturbation 29 as shown in Figure 2, the first optical mode 101 can be coupled to a second optical mode 102 which has a second propagation constant β2 and overlaps with the active dopant 9. This allows the pump radiation to propagate along the second core 12 as a first optical mode 101, the first optical mode 101 to be selectively coupled to the second optical mode 102 by one or more perturbations 29, and the pump radiation selectively coupled to the second optical mode 102 to be absorbed by an active dopant 9 that can amplify one or more signal modes 31 guided by the first core 2 by stimulated emission, thereby enabling control of at least one of the inversion rate of the active dopant 9 along the optical fiber 10, the optical gain characteristics of the optical fiber 10, and the thermal load of the optical fiber 10.

[0082] The coupling from the first optical mode 101 to the second optical mode 102 may be direct or indirect. An example of indirect coupling is that perturbation 29 can couple the first optical mode 101 to a skew mode that does not overlap with the active dopant 9. Additional coupling can occur through one of the perturbations, another perturbation, or an asymmetry designed in the optical fiber 10, which can couple the skew mode to the second optical mode 102 that overlaps with the active dopant 9.

[0083] The perturbation 29 can include at least one of the following: a change in the radius of curvature, a change in the bending direction, a compressive force, a long-period grating, a change in the diameter of the optical fiber 10, a change in the cross-sectional shape of the optical fiber 10, a rotation of the optical fiber 10, and a change in the material composition of the optical fiber along the length of the optical fiber 10.

[0084] The active dopant 9 can be selected from the group having ytterbium, erbium, neodymium, praseodymium, thulium, samarium, holmium, and dysprosium. Furthermore or alternatively, the active dopant may be a transition metal or a semiconductor. Multiple active dopant 9s may be selected. For example, the active dopant 9 may have ytterbium and co-doped erbium.

[0085] The optical gain characteristics may be the wavelength dependence of the optical gain, or variations in the amplitude or wavelength distribution of the naturally radiated amplified light.

[0086] In rare-earth doped fiber amplifiers, the upper and lower energy levels of the active dopant 9 have a number of slightly different energy levels that are strongly coupled to each other, and therefore population inversion in the sense of over 50% is not required to obtain optical amplification. At long wavelengths where emission is usually much stronger than absorption, gain can be achieved even at low excitation levels. For this reason, the inversion ratio N, i.e., the ratio of ions or atoms of the active dopant 9 in the excited state compared to those in the lower energy state, is used to model the fiber. It is the inversion ratio that enables optical amplification by stimulated emission, and the higher the inversion ratio, the higher the optical gain. The higher the inversion ratio, the higher the level of spontaneous emission, as the active dopant 9 relaxes to the lower energy state by emitting photons. When high optical gain is combined with high levels of spontaneous emission, the level of spontaneously amplified light (ASE) becomes high, which is generally undesirable as it reduces the amount of gain available for the signal, and ASE is usually an undesirable output signal.

[0087] The optical gain g in an optical amplifier is determined by the inversion ratio and the (wavelength-dependent) emission and absorption cross-sections.

number

[0088] When energy is extracted from the amplifier, the inversion rate decreases, and consequently the gain also decreases. The saturation energy E of the laser gain medium. sat The gain is 1 / e of the initial value. 2 This is the pulse energy of the incident signal pulse, which reduces it to approximately 37%.

number

[0089] The maximum pulse energy that can be extracted from an optical amplifier (for very long pulses) is usually considered to be about 10 times the saturation energy. However, for pulses with a pulse width of 100 ns to 1000 ns, the actual limit is closer to 3 or 4 times the saturation energy.

[0090] In the case of a ytterbium-doped fiber, the emission cross-section σ em In the case of 1030nm, it is about three times larger than in the case of 1065nm, while the absorption cross-section σ abs σ at both wavelengths em Because it is considerably smaller, the saturation energy E sat The pulse energy is approximately three times greater for a 1065nm signal compared to a 1030nm signal. Therefore, if maximizing pulse energy is important, it is preferable to operate with a 1065nm signal.

[0091] The gain peak wavelength is the wavelength of light that yields the highest gain as it passes through the amplifier. As can be inferred from the equation for g, the gain peak wavelength depends on both the cross-section and inversion, and is therefore controllable by controlling the pump absorption along the fiber. Inversion can change along the length of the amplifier and with time. Therefore, the gain peak wavelength is typically applied to the time-averaged gain integrated along the entire length of the amplifier.

[0092] Spontaneous emission amplified light (ASE) is broadband light resulting from random spontaneous emission within an amplifier. Spontaneous emission, like intentionally introduced optical signals, undergoes optical gain. Because the initial input is broadband and different wavelengths undergo different gains, the ASE spectrum at the amplifier output is typically maximum at or near the amplifier's gain peak wavelength.

[0093] Unlike signals, which can be pulsed, ASE is always emitted while the amplifier is pumping. If the signal is at a wavelength far from the gain peak wavelength, the signal will have a lower gain than the ASE, and as a result, the ASE will account for a larger proportion of the amplifier's total output. To maximize pulse generation efficiency (and minimize the light level between pulses), it is preferable that the signal has a wavelength close to the gain peak wavelength. The difference between the signal wavelength and the gain peak wavelength may be less than 10 nm, preferably less than 5 nm, and more preferably less than 1 nm.

[0094] The optical fiber 10 shown in Figure 1 can be used in a fiber laser or optical amplifier in which pump radiation is coupled to a second core 12. Note that to increase the pulse energy of the laser output, it is necessary to increase the diameter 6 of the first core 2. If the first cladding 3 is not present, this also requires an increase in the size of the second core 12. When the required pulse energy exceeds 10 mJ, the overall diameter of the fiber rapidly exceeds 1 mm, making fiber packaging, cutting, splicing, and heat sinking difficult. The inclusion of the first cladding 3, which separates the first core 2 from the second core 12, reduces the effective brightness of the pump radiation traveling along the second core 12. That is, the same amount of pump radiation travels along a smaller second core 12, but depending on the perturbation 29, a lower proportion of the pump radiation may overlap with the active dopant 9. Therefore, the absorption rate of the pump radiation may be lower. With the inclusion of the first cladding 3, the optical fiber 10 can have a smaller diameter than an equivalent fiber that provides the same output power and pulse energy. Furthermore, the optical fiber 10 can be operated at a lower temperature, which improves the reliability of the fiber coating.

[0095] The ability to control the amount of pump radiation coupled from the second core 12 to the first core 2 allows a large amount of energy to be stored in the active dopant 9 without causing excessive spontaneous emission amplification and parasitic laser oscillation. As shown below, the present invention enables nanosecond laser sources with pulse energies exceeding 100 mJ. Such lasers are difficult to design unless they have a very large fiber diameter, and because the stiffness of the fiber is highly dependent on the diameter, a very large radius of curvature is required to avoid fiber breakage, making it impractical to package them into reasonably sized modules.

[0096] The optical fiber 10 may have a second cladding 13 having a second refractive index 15 that is smaller than the refractive index 14 of the second core 12. Preferably, the second refractive index 15 is smaller than the first refractive index 5 of the first cladding 3. The second cladding 13 may be glass or a polymer coating. The second cladding 13 may be air or a vacuum, both of which have a refractive index of 1 and can be treated as cladding in waveguide theory. In the absence of the second cladding 13, optical modes propagating within the second core 12 may leak out.

[0097] The first core 2 can form part of a signal waveguide which may have additional cores and cladding. The first core 2 can be positioned along the central axis of the optical fiber 10 or offset from the central axis. The second core 12 can form part of a pump waveguide which may have additional cores and cladding.

[0098] The optical fiber 10 may also have an optional third cladding 18 having a third refractive index 19. The third cladding 18 may be glass or a polymer coating. The third cladding 18 may be air or vacuum, both of which have a refractive index of 1 and can be treated as cladding in waveguide theory.

[0099] The first refractive index 5 may be greater than the second refractive index 15. The first refractive index 5 may be less than the second refractive index 15 and greater than the third refractive index 19.

[0100] The optical modes of a multimode waveguide can be modeled using rays. Figure 2 shows the first optical mode 101, represented by a ray 21 propagating along the second core 12 of the optical fiber 10. Ray 21 is characterized by an angle θ122 with respect to the local long axis 20 of the optical fiber 10. Ray 21 is a low-divergence ray that is totally internally reflected at the boundary between the second core 12 and the first cladding 3, and at the boundary between the second core 12 and the second cladding 13. Ray 21 does not interact with the active dopant 9. The optical fiber 10 is perturbed as shown by a perturbation 29. The perturbation 29 may be a change in radius of curvature, application of a compressive force, or some other form of perturbation. The perturbation 29 can couple the first optical mode 101 to the second optical mode 102, represented by a ray 23. Ray 23 is characterized by an angle θ224 with respect to the local long axis 20 of the optical fiber 10. The angle θ224 is greater than the angle θ122, and is large enough that ray 23 is not totally internally reflected by the first cladding 3. Therefore, the second optical mode 102 represented by ray 23 is not guided by the second core 12. The indicated ray 23 represents a highly divergent pump emission traversing the first core 2 and is absorbed by the active dopant 9. Spontaneous or stimulated emission from the active dopant 9 then emits a signal mode 31 represented by ray 25, characterized by an angle θ326 with respect to the local long axis 20 of the optical fiber 10. Ray 25 undergoes totally internal reflection at the boundary between the first core 2 and the first cladding 3 and is guided by the first core 2. The angle θ326 of ray 25 propagating along the first core 2 is smaller than the angle θ224 of ray 23 traversing the first core 2.

[0101] Figure 3 shows groups of multiple modes that can propagate along the optical fiber 10. A "signal mode" 31 is guided by the boundary between the first core 2 and the first cladding 3. The signal mode 31 has an effective refractive index 32 between the first refractive index 5 of the first cladding 3 and the refractive index 4 of the first core 2. The signal mode 31 is shown as having an effective refractive index mode spacing 33 between different modes. • A first optical mode 101, or "A mode," guided by the boundary between the second core 12 and the first and second claddings 3 and 13. The first optical mode 101 has an effective refractive index 35 that is less than the refractive index 14 of the second core 12 and greater than the greater of the first refractive index 5 of the first cladding 3 and the second refractive index 15 of the second cladding 13. The first optical mode 101 is shown as having an effective refractive index mode spacing 36 between different modes. • A “B-mode” 37 guided by the boundary between the second core 12 and the second cladding 13. The B-mode 37 has an effective refractive index 38 that is less than the first refractive index 5 of the first cladding 3 and higher than the second refractive index 15 of the second cladding 13. The B-mode 37 is shown as having an effective refractive index mode spacing 39 between different modes. Some of the B-modes 37 may be skew modes that do not overlap with the active dopant 9. Therefore, not all of the B-modes 37 are the first optical mode 101 in Figure 1. • A "C-mode" 45 guided by the boundary between the second cladding 13 and the third cladding 18. The C-mode 45 has an effective refractive index 46 smaller than the lower of the first refractive index 5 of the first cladding 3 and the second refractive index 15 of the second cladding 13. The C-mode 45 is shown as having an effective refractive index mode spacing 47 between different modes. Some of the C-modes 45 may be skewed modes that do not overlap with the active dopant 9.

[0102] The effective refractive index mode spacings 32, 36, 39, and 47 have been shown to be uniform within each mode group. In practice, there is some variation within each group.

[0103] The description of the above mode suggests that when the effective refractive index of the core is lower than that of the adjacent cladding, the core does not guide the mode. This phenomenon is called cutoff. At an effective refractive index below cutoff, the mode cannot exist as a properly guided mode. However, the propagation of the wave does not abruptly stop below cutoff. Instead, many modes can convert themselves into leaky waves that can propagate along the core for a considerable distance with relatively low loss. For the purposes of this discussion, the modes guided by the core also include leaky modes or leaky rays that are effectively guided by the core of the optical fiber when the optical fiber is held straight. Such modes can be coupled to other optical modes of the optical fiber 10 that overlap with the active dopant 9 when the optical fiber 10 is perturbed by the perturbation 29. "Effectively guided" means that the attenuation is less than 0.1 dB / m.

[0104] When the first refractive index 5 of the first cladding 3 is higher than the second refractive index 15 of the second cladding 13 (as shown in FIG. 3), the first optical mode 101 guided by the second core 12 can be coupled by the perturbation 29 to the B mode 37 and / or the C mode 45. A part of the B mode 37 and the C mode 45 overlaps with the active dopant 9 in the first core 2 that can absorb the energy from the mode. Spontaneous emission or stimulated emission from the active dopant 9 can emit or amplify the signal mode 31 guided by the first core 2.

[0105] When the perturbation has a spatial frequency component Λ = (β1 - β2) / 2π = (neff1 - neff2) / λ, where λ is the wavelength of the optical radiation and neff1 and neff2 are the effective refractive indices of the first and second modes to be coupled, the perturbation can couple the first mode having the propagation constant β1 to the second mode having the propagation constant β2. For example, when the perturbation is a sinusoidal variation of an optical fiber with a period of 1 mm, the spatial frequency is 1000 cycles / m. For a wavelength λ = 1 μm, the perturbation will couple modes with an effective refractive index difference of 0.001 or 10 -3The modes are coupled. The coupled power, with phase matching at each fluctuating position in the optical fiber, constructively increases along the length of the optical fiber. Similarly, if the period of the sinusoidal fluctuation is 10 mm, the spatial frequency is 100 cycles / m. At a wavelength λ = 1 μm, the perturbation is such that the difference in effective refractive index is 0.0001 or 10 -4 It combines the modes.

[0106] The bending stiffness of an optical fiber increases proportionally to the fourth power of its glass diameter. This means that, in a moderately large diameter fiber, mechanically coupling modes with a small difference in effective refractive index is far easier than coupling modes with a large difference in effective refractive index, because the latter requires a higher spatial frequency in the perturbation that couples the modes. For example, if an optical fiber has a diameter of 0.5 mm, it is easier to achieve a sinusoidal variation with a spatial frequency of 100 cycles / m than to achieve the same variation with 1000 cycles / m. The dependence of mode coupling on the fiber diameter is very strong. Optical fibers act as low-pass filters, removing perturbations from bending forces with high spatial frequencies.

[0107] As shown in Figure 3, the dependence of mode coupling on the effective refractive index mode spacing between different modes can be utilized. Preferably, the mode spacing 36 between consecutive mode groups in the second core 12 is smaller than the mode spacing 33 between consecutive mode groups in the first core 2. This is because it is desirable for the perturbation 29 to couple the light emission from the first optical mode 101 propagating through the second core 12 to mode B 37. A portion of mode B 37 overlaps with and is absorbed by the active dopant 9, thereby amplifying the light emission propagating through the first core 2 as signal mode 31 by stimulated emission. It is often preferable that the perturbation 29 does not couple signal mode 31 to mode B 37, as this leads to losses and reduced efficiency. Therefore, it is preferable that (i) the spacing 33 is larger than the spacing 36, and / or (ii) the spatial frequency of the perturbation is selected to couple the first optical mode 101 to mode B 37 rather than coupling signal mode 31 to mode B 37.

[0108] The average spacing between the effective refractive indices of consecutive mode groups is approximately inversely proportional to the width or diameter of the core. Therefore, the mode spacing 33 between consecutive mode groups in the first core 2 can be designed to be larger than the mode spacing 36 between consecutive mode groups in the second core 12, by ensuring that the diameter 6 of the first core 2 is smaller than the diameter 17 of the second core 12. This is the case for the optical fiber 10 shown in Figure 1. Differences in the refractive index profiles of the first core 2 and the second core 12 can also be used to influence the difference in the spacing between the effective refractive indices of the modes.

[0109] Perturbation 29 couples or scatters the first optical modes 101 together and couples the first optical modes 101 to mode B 37, which overlaps with the active dopant 9. To couple the first optical modes 101 together, the length of perturbation 29 is preferably shorter than the adiabatic length, which is the minimum length of perturbation required to allow the coupling between modes to be ignored. For a typical step-index fiber with a core diameter 17 of 660 μm, the adiabatic length is approximately 10 mm. Therefore, to achieve mode coupling between the first optical modes 101, the optical fiber needs to be perturbed by bending, stress, taper, refractive index variations, and / or other variations, with longitudinal spatial frequencies of less than 10 mm. This can be achieved by mechanically bending the optical fiber.

[0110] The adiabatic length is approximately proportional to the width of the core. Therefore, the adiabatic length of a typical first core 2 with a width 6 of 170 μm is approximately 2.6 mm. It is more difficult to generate a bend with a spatial frequency component of 2.6 mm in a fiber with a diameter of 660 μm than to generate a bend with a spatial frequency component of 10 mm. Thus, the first optical mode 101 propagating through the second core 12 can be coupled or scattered together much more easily than the signal mode 31 propagating through the first core 2.

[0111] Similarly, if the diameter of the second core doubles from 17 to 1320 μm, the adiabatic length increases to approximately 20 mm, and the bending stiffness increases 16-fold. Such optical fibers are difficult to package in a spool unless the spool diameter is not very large, or the stress inside the fiber can be released, for example, by melting the glass with a flame or electric arc.

[0112] Therefore, the perturbation 29 can be designed to couple the first optical mode 101 to the second optical mode 102 by selecting its spatial frequency component Λ as Λ=(β1-β2) / 2π.

[0113] However, this typically only combines the first optical mode 101, which has an effective refractive index very similar to that of the second optical mode 102. Therefore, it is preferable that one or more perturbations 29 have at least one spatial frequency component that combines at least some of the first optical modes 101.

[0114] The beam quality of a signal is typically a critical parameter for a laser or amplifier. Higher beam quality is provided by the signal mode 31 having the highest effective refractive index 32. As previously mentioned, the mode spacing 33 of signal mode 31 in Figure 3 is greater than the mode spacing 36 of the first optical mode 101. Therefore, the beam quality of signal mode 31 propagating along the optical fiber 10 can be maintained by designing the perturbation 29 to have a spatial frequency other than the spatial frequency at which the signal modes 31 are coupled together. The design parameters can be obtained theoretically or experimentally, as shown in Example 1.

[0115] The ray 23 shown in Figure 2 is a meridian ray that intersects the first core 2. Many optical modes have skew rays that do not intersect the first core 2. Skew rays or meridian rays may not intersect the first core 2 if the core is offset from the center of the optical fiber. If skew rays cannot couple to rays that intersect the first core 2, they do not overlap and cannot be absorbed by the active dopant 9. Therefore, to facilitate coupling between skew rays and rays that overlap with the active dopant 9, it is preferable that at least one of the first cladding 3, the second core 12, and the second cladding 13 is non-circular.

[0116] The second core 12 may be a regular polygon or an irregular polygon, as in the optical fiber 40 shown in Figure 4, which has an octagonal second core 12. The polygon may be a Reuleaux polygon. The optical fiber 40 may have an optional third cladding 18. The second core 12 may have a D-shaped cross-section with one plane or multiple planes. Furthermore or alternatively, the second cladding 13 may be non-circular. The second cladding 13 may be a regular polygon or an irregular polygon, as in the optical fiber 50 shown in Figure 5, which has an octagonal second cladding 13. The polygon may be a Reuleaux polygon. The second cladding 13 may have a D-shaped cross-section with one plane or multiple planes. By removing circular symmetry from the second core 12 and / or the second cladding 13, for example, in the case of a non-circularly symmetric second core 12, coupling between different first optical modes 101 (A to A mode coupling) and coupling between different B modes 37 (if present) (B to B mode coupling) can be increased, and in the case of a non-circularly symmetric second cladding 13, coupling between different C modes 45 (if present) (C to C mode coupling) can be increased. By removing circular symmetry, coupling with the active dopant 9 can be increased, thereby improving the efficiency of optical amplification devices such as optical amplifiers or lasers made from the optical fiber 10.

[0117] Figure 6 shows an optical fiber 60 having four second cores 12. The four second cores 12 are located within a common second cladding 13. The first cores 2 are surrounded by the first cladding 3, and the first cladding 3 is surrounded by the second cladding 13. It is shown that the second refractive index 15 of the second cladding 13 is greater than the first refractive index 5 of the first cladding 3. At least one of the second cores 12 and / or the second cladding 13 may be non-circular. An optical fiber having multiple second cores 12 helps to maintain the brightness of the pump radiation along the optical fiber 60. The second optical mode 102 is guided by the boundary between the second cladding 13 and the third cladding 18.

[0118] Figure 7 shows an optical fiber 70 having four second cores 12 within a first cladding 3. The first cores 2 are surrounded by cladding 71, which is surrounded by the first cladding 3. Cladding 71 has a refractive index 72 that is higher than the first refractive index 5 of the first cladding 3. The optical fiber having multiple second cores 12 helps to maintain the brightness of the pump radiation along the optical fiber 70. The second optical mode 102 is guided by the boundary between the first cladding 3 and the third cladding 18.

[0119] Figure 8 shows an optical fiber 80 having four first cores 2 and three second cores 12, all surrounded by a first cladding 3. The second cladding 13 surrounds the first cladding 3. The first refractive index 5 is greater than the second refractive index 15. The second optical mode 102 is guided by the boundary between the first cladding 3 and the second cladding 13.

[0120] Figure 9 shows an optical fiber 90 having three first cores 2, each surrounded by a first cladding 3. The three first claddings 3 are surrounded by second cores 12. The second cores 12 are surrounded by a second cladding 13. The first refractive index 5 of the first claddings 3 is greater than the second refractive index 15 of the second cladding 13. The second optical mode 102 is guided by the boundary between the second cores 12 and the second cladding 13.

[0121] Figure 10 shows an optical fiber 100 in which the first core 2 is a ring core surrounding the second core 12. The first core 2 and the second core 12 are separated by the first cladding 3. The second optical mode 102 is guided by the boundary between the first core 2 and the second cladding 13.

[0122] Figure 11 shows an optical fiber 110 in which a first cladding 3 and a second core 12 are surrounded by a second cladding 13. Preferably, the second core 12 is in optical contact with the first cladding 3 along its length. The second core 12 is shown as circular, but may alternatively be non-circular. The second core 12 can have a larger cross-sectional area than the first core 2, so that the effective refractive index interval 36 for the first optical mode 101 is smaller than the effective refractive index interval 32 for the signal mode 31 shown in Figure 3. A larger cross-sectional area also makes it possible to obtain higher optical power from the first core 2. The second cladding 13 may be a polymer. The first core 2 and the second core 12 are separable from each other by removing the second cladding 13. This allows the end of the second core 12 to be separated from the first core 2 and connected to a pump source.

[0123] Figure 12 shows an optical fiber 120 similar to the optical fiber 110 in Figure 11, except that the first cladding 3 is arranged around the second core 12.

[0124] Figure 13 shows an optical fiber 130 having a pump fiber 131 that is preferably in optical contact with a second core 12 along its length. The pump fiber 131 and the second core 12 are surrounded by a second cladding 13. The refractive index 132 of the pump fiber 131 is preferably equal to the refractive index 14 of the second core 12. The second cladding 13 is preferably a polymer. The pump fiber 131 is preferably a silica fiber having a cross-sectional area larger than the cross-sectional area of ​​the second core 12. The pump fiber 131 is shown as circular, but may alternatively be non-circular. The pump fiber 131 and the second core 12 are separable from each other by removing the second cladding 13. This allows the end of the pump fiber 131 to be separated from the second core 12 and connected to a pump source.

[0125] The light radiation incident on the pump fiber 131 can propagate along the pump fiber 131 in mode 133, which has an effective refractive index 134 greater than the first refractive index 5 of the first cladding 3. Mode 133 can be coupled laterally with the first optical mode 101 of the second core 12 along its length. The first optical mode 101 guided by the second core 12 can be coupled with the second optical mode 102 of the optical fiber 130, which overlaps with the active dopant 9 when the optical fiber 130 is perturbed. The second optical mode 102 has an effective refractive index 135 smaller than the first refractive index 5.

[0126] With respect to the optical fibers 10, 40, 50, 60, 70, 80, 90, 100, 110, 120, and 130 shown in Figures 1 and 4-13, the first cladding 3 functions as a barrier layer that prevents the coupling of optical power between the first core 2 and the second core 12. The first core 2 may be centrally located as shown, or offset from the central axis of the fiber.

[0127] When used in optical amplification devices such as lasers or amplifiers, optical fibers 10, 40, 50, 60, 70, 80, 90, 100, 110, 120, and 130 can be compared to conventional double-clad fibers that do not have a barrier layer provided by the first cladding 3. The absorption length of a double-clad fiber used in such a device is approximately equal to the absorption length of the pump radiation when coupled only to the first core 2, multiplied by the sum of the cross-sectional areas of the first core 2 and the second core 12, and divided by the cross-sectional area of ​​the first core 2. By including the first cladding 3, the coupling of pump radiation from the second core 12 that guides the pump radiation to the first core 2 that guides the signal can be significantly reduced. As a result, the length of optical fiber 10 can be made typically at least twice, preferably at least three times, and more preferably at least five times longer than the product of the absorption length and the area of ​​the second core 12 divided by the area of ​​the first core 2.

[0128] The perturbation 29, shown with reference to Figure 2, couples the first optical mode 101 guided by the second core 12 to the second optical mode 102, which overlaps with the active dopant 9. Pump radiation propagating along the second core 12 as the first optical mode 101 can be selectively coupled to the second optical mode 102 by one or more perturbations 29 and absorbed by the active dopant 9. The active dopant 9 can amplify one or more signal modes 31 guided by the first core 2 by stimulated emission.

[0129] The second optical mode 102 has an effective refractive index 135 that is smaller than the first refractive index 5 of the first cladding 3. The refractive index 4 of the first core 2 may be higher than, equal to, or lower than the refractive index 14 of the second core 12. Generally, it is preferable that the refractive index 4 is higher than the refractive index 14 of the second core 12, but it may be lower than or equal to the refractive index 14.

[0130] The diameter or width 6 of the first core 2 of the optical fibers 10, 40, 50, 60, 70, 80, 90, 100, 110, 120, and 130 shown in Figures 1 and 4 to 13 may be greater than 50 μm, preferably greater than 100 μm, and more preferably greater than 250 μm.

[0131] The first core 2 and second core 12 of the optical fibers 10, 40, 50, 60, 70, 80, 90, 100, 110, 120, and 130 shown in Figures 1 and 4 to 13 are separated by a width 7 shown in Figure 1, which is greater than 5 μm, preferably greater than 10 μm, and more preferably greater than 15 μm. A width 7 of 10 μm is stronger than a width 7 of 5 μm in separating modes propagating along the second core 12 from other modes that overlap with the active dopant 9. Preferably, 5% or less, more preferably 1% or less of the optical power propagating along the second core 12 of a 1 m long straight fiber is coupled to other modes that overlap with the active dopant 9. Preferably, the first cladding 3 reduces the overlap of light radiation incident on the second core 12 by at least 2 times, preferably at least 10 times, compared to an equivalent optical fiber without the first cladding 3.

[0132] The preferred width 16 of the second core 12 may be the minimum width to which sufficient pump light can be coupled to achieve the desired incident power while primarily incident light on the first optical mode 101. The width 16 of the second core 12 may be greater than 50 μm, preferably greater than 100 μm, and more preferably greater than 150 μm. In other cases, a smaller second core 12 may be desirable.

[0133] The refractive index 14 of the second core 12 may be greater than the first refractive index 5 of the first cladding 3 by more than 0.001, preferably more than 0.004, and even more preferably more than 0.01.

[0134] The refractive index 4 of the first core 2 may be greater than the refractive index 14 of the second core 12. The refractive index 4 of the first core 2 may be greater than the first refractive index 5 of the first cladding 3 by more than 0.015.

[0135] The active dopant 9 may be uniformly distributed throughout the first core 2. Alternatively, the active dopant 9 may be more concentrated around the center of the first core 2 than at the edges of the first core 2 to improve the beam quality of the stimulated laser radiation emitted from the first core 2 compared to the beam quality of the stimulated laser radiation emitted from an optical fiber in which the active dopant 9 is uniformly distributed throughout the first core 2.

[0136] The second core 12 and / or the second cladding 13 may be circular or non-circular.

[0137] Optical fibers 80, 90, 110, 120, and 130 may have a third cladding 18, as shown with reference to Figure 1. The third cladding 18 may be circular or non-circular.

[0138] The optical fibers 10, 40, 50, 60, 70, 80, 90, 100, 110, 120, and 130 shown in Figures 1 and 4 to 13 are shown having a circular first core 2 and a circular first cladding 3. The first core 2 and / or the first cladding 3 may be non-circular.

[0139] Optical fibers 10, 40, 50, 60, 70, 80, 90, 100, 110, 120, and 130 may have silica, silicate glass, phosphate glass, or soft glass belonging to, for example, fluoride or chalcogenide glass.

[0140] Figure 14 shows an optical amplifier 140 having an optical fiber 10 and at least one pump 141 that provides a pump radiation 209 having at least one pump wavelength 2320. Pump 141 is coupled to a second core 12 of the optical fiber 10 via a pump fiber 142 and an output combiner 143. The output combiner 143 also couples the first core 2 to an output fiber 144 that outputs an output signal 145. An optional combiner fiber 146 splices the optical fiber 10 to the combiner 143 via a splice 147. The output fiber 144 may be terminated with an end cap 1415 to protect the output end 1414 of the output fiber 144 from damage by the output signal 145. The end cap 1415 may have fused silica. The output signal 145 may be parallelized by a collimating lens 1416 and focused to a material 1418 that is processed by a focusing lens 1417.

[0141] Amplifier 140 can be used to amplify the input signal 1411, which is emitted from the seed laser 1410, input to the input end 171 of the optical fiber 10, amplified by the optical fiber 10, and output from the output end 179 of the optical fiber 10. The input signal 1411 can be coupled to the first core 2 of the optical fiber 10 via the input fiber 149 and splice 148. The seed laser 1410 may be a fiber laser, disk laser, solid-state laser, slab laser, or semiconductor laser. The seed laser 1410 may be a continuous-wave laser, Q-switched laser, main oscillator output amplifier laser, or mode-locked laser. Controller 1412 may be provided to control the pump 141, preferably in synchronization with the seed laser 1410. Pump 141 can be turned on before the seed laser 1410 emits the input signal 1411 so that the active dopant 9 is excited before the input signal 1411 reaches the optical fiber 10.

[0142] The output combiner 143 may have a fiber bundle 150 as shown in Figure 15. Six pump fibers 142 and a central output fiber 144 are bundled together in a capillary 151. The capillary 151 may be silica, may contain silica, or may be fluorine-doped silica. The capillary 151 may be another form of glass, such as soft glass. The capillary 151 may have a single bore as shown, or may have multiple bores. The fiber bundle 150 can be coupled to the combiner fiber 146, or it can be coupled directly to the optical fiber 10. The coupling is preferably done using a fusion splice, but can also be provided by coupling means such as a butt splice or imaging optics. The capillary 151 can grip the pump fiber 142 by being crushed by heating and tension, and the fiber bundle can be tapered and the sizes of the pump fiber 142 and central output fiber 144 can be adjusted to fit the dimensions of the spliced ​​fibers, which may be combiner fibers 146 or optical fibers 10.

[0143] The output fiber 144 preferably has a core 152 surrounded by a cladding 153, with a refractive index of 158, which is higher than the refractive index 159 of the cladding 153. The core 152 may or may not be silica, and the cladding 153 may be fluorine-doped silica. The inclusion of the cladding 153 isolates the core 152 from the pump radiation 209 propagating along the pump fiber 142 in the output combiner 143.

[0144] Furthermore, or alternatively, the pump fiber 142 may have a core 155 with a refractive index 1510, higher than the refractive index 1511 of the cladding 156, surrounded by an optional cladding 156. The core 155 may have or be silica, and the cladding 156 may be fluorine-doped silica. The inclusion of the cladding 156 allows the core 152 of the output fiber 144 to be isolated from the pump radiation 209 propagating along the pump fiber 142. The cladding 156 can also isolate the core 155 of the pump fiber 142 from the capillary 151, thereby helping to preserve the brightness of the pump radiation 209 propagating along the pump fiber 142.

[0145] The combiner fiber 146 may be a combiner fiber 160 as shown in Figure 16, having a central core 161 surrounded by a ring core 162. Preferably, the central core 161 is surrounded by an inner cladding 163 having a refractive index 167 that is lower than the refractive index 165 of the central core 161 and the refractive index 166 of the ring core 162. This choice allows the pump radiation 209 that connects from the pump fiber 142 to the ring core 162 to be isolated from the central core 161 and thus connected to the second core 12 of the optical fiber 10, but not to the first core 2 of the optical fiber 10. The width 1611 of the inner cladding 163 may be greater than or equal to the width 7 of the first cladding 3 shown in Figure 1. The ring core 162 may be surrounded by an outer cladding 164 having a refractive index 168 that is lower than the refractive index 166 of the ring core 162. The outer cladding 164 may have a glass or polymer coating. The central core 161 may have silica, or may be silica, and the inner cladding 163 may be fluorine-doped silica.

[0146] The pump power 209 injected into the pump fiber 142 is preferably mainly coupled to the second core 12 of the optical fiber 10. The larger the proportion of pump power 209 coupled to the second core 12 compared to the total pump power coupled to the optical fiber 10, the less pump power is absorbed by the active dopant 9 in the first core 2 before the perturbation 29. This is beneficial because absorbed pump power can generate heat, and this heat may need to be removed if it causes an undesirable temperature rise in the optical fiber 10. To maximize this proportion, The diameter 157 of the core 155 of the pump fiber 142 shown in Figure 15 is preferably smaller than or equal to the width 169 of the ring core 162 shown in Figure 16. The width 169 of the ring core 162 is preferably smaller than or equal to the width 16 of the second core 12 shown in Figure 1. • Preferably, the centerlines and outer diameters of each core are aligned so that the light radiation does not couple to the cladding region surrounding the core. The numerical aperture of each core receiving the pump radiation 209 is preferably greater than or equal to the numerical aperture of the pump radiation 209 emitted from the previous core. The numerical aperture of the pump radiation 209 can be controlled by the incidence conditions on the fiber. Alternatively, the numerical aperture of the pump radiation 209 can be controlled by selecting the numerical aperture of the fiber core relative to the cladding.

[0147] It is preferable that the signal radiation emitted from the first core 2 of the optical fiber 10 efficiently passes through and combines with the central core 161 of the combiner fiber 160 and the core 152 of the output fiber 144 (if present). To maximize this efficiency, The diameter 6 of the first core 2 shown in Figure 1 is preferably smaller than or equal to the diameter 1610 of the central core 161 of the combiner fiber 160 shown in Figure 16 (if present). The diameter 1610 of the central core 161 of the combiner fiber 160 is preferably smaller than or equal to the diameter 154 of the core 152 of the output fiber 144 shown in Figure 15. The centerlines and outer diameters of each core should be aligned so that signal radiation does not couple to the cladding region surrounding the core. The numerical aperture of each core receiving the signal radiation is greater than or equal to the numerical aperture of the signal radiation emitted from the previous core. The numerical aperture of the signal radiation can be controlled by the incidence conditions to each fiber. Alternatively, the numerical aperture of the signal radiation can be controlled by selecting the numerical aperture of each core relative to the cladding.

[0148] The signal waveguide having the first core 2 and first cladding 3 shown in Figure 1 may be multimode. The input fiber 149 may be single-mode or a multimode fiber operating in fundamental mode. The splice 148 may include a taper. A cladding mode stripper (not shown) may be included in the splice 148, the input fiber 149, or the seed laser 1410. This arrangement allows the input fiber 149 and / or the cladding mode stripper to function as a mode filter, reducing the energy of back-reflected radiation reaching the seed laser 1410, thereby reducing the problem of reflections from the workpiece being amplified by the amplifier 140 and damaging the seed laser 1410. Note that a taper can increase the divergence of light rays propagating along the core. Therefore, care should be taken to avoid tapering that couples an amount of light radiation to the core that would cause an undesirable thermal load on the optical fiber 10 during operation.

[0149] The optical amplifier 140 shown in Figure 14 is back-excited, i.e., the pump radiation 209 is incident on the output end 179, i.e., the end of the optical fiber 10 from which the output signal 145 is emitted. Back-excitement generally has the advantage of increasing output power and pulse energy. The optical amplifier 140 can alternatively be forward-excited, i.e., the pump radiation 209 can be incident on the input end 171 of the optical fiber 10, or both back-excitement and forward-excitement can be performed. Pump radiation 209 exceeding 1 kW, preferably exceeding 2 kW, and more preferably exceeding 3 kW can be guided by the second core 12.

[0150] The optical fiber 10 shown in Figure 14 can be configured within the spool 170 shown in Figure 17. The optical fiber 10 is inserted into a helical groove 173 of the substrate 175. The groove 173 may have sections with a constant or variable radius of curvature. The length of the bending transition of the optical fiber 10 can be controlled by making the width 174 of the groove 173 larger than the width or diameter 1710 of the fiber 10.

[0151] The substrate 175 may be a heatsink that can be air-cooled using a fan or water-cooled using channels within the substrate 175. The groove 173 has a width 174 that can be designed to guide the optical fiber 10 into a desired trajectory for optimizing mode coupling. The greater the difference between the width 174 of the groove 173 and the width 1710 of the fiber 10, the less the optical fiber 10 is constrained by the curvature of the groove 173. When placed in the groove 173, the fiber 10 relaxes into a shape that experiences the least stress. This means that a fiber in a wider groove will change its radius of curvature over a larger distance when subjected to a change in the radius of curvature of the groove, resulting in a perturbation 29 with a longer effective transition length or a lower spatial frequency. The width 174 may be only 25 μm to 150 μm larger than the width 1710. Other widths 174 may also be used.

[0152] The optimal radius of curvature and transition region can be found experimentally by varying the perturbation 29 of the optical fiber 10, the difference between the width 1710 of the optical fiber 10 and the width 174 of the groove 173, and measuring the amount of cross-coupling between different mode groups. Alternatively, the optimal radius of curvature and transition region can be theoretically optimized as described with reference to Example 1.

[0153] The spool 170 may be a rounded regular or irregular polygon with sides 176 having a radius of curvature greater than the radius of curvature of the corners 177. The sides 176 may be straight. Examples include rounded polygons with 3 to 12 sides and curved corners. The rounded polygon may be a rounded rectangle, a rounded square, a superellipse, or a squirkle. For ease of manufacture, the spool 170 shown in Figure 17 is a rounded square with straight sides 176 and rounded corners 177. The rounded corners 177 have the same length and radius as each other and are offset from each other to form a helix. Pump radiation can be coupled to modes or rays absorbed by the active dopant 9 from the second core 12 at the transitions 178 between each side 176 and each corner 177 of the fiber 10. Having multiple transitions 178 allows the pump radiation 209 to be coupled in a controlled manner along the length of the optical fiber 10.

[0154] The mode coupling from the first optical mode 101 to mode B 37 in Figure 3 is induced by a transition 178 between sections of the optical fiber 10 having different radii of curvature. The sharper the transition, the higher the spatial frequency component of the perturbation 29. The sharpness of the transition is related to the rate of change of curvature of the optical fiber 10 along the length of the transition, limited by the glass diameter 8 of the optical fiber 10, and therefore the stiffness of the optical fiber. In Figure 1, the glass diameter 8 is shown as the diameter of the second cladding 13. If the third cladding 18 is glass and not polymer, the glass diameter 8 is the diameter or width of the third cladding 18. Similarly, if the second cladding 13 is polymer, the glass diameter 8 is the diameter or width 17 of the second core 12.

[0155] Referring to Figure 17, it is preferable that the radius of curvature of the optical fiber 10 at angle 177 and transition 178 be selected so as to couple the first optical mode 101 in Figure 3 to mode B 37, without coupling signal mode 31 to mode B 37. This can be achieved because the effective mode interval 33 of signal mode 31 in the optical fiber 10 shown in Figures 1 and 3 is greater than the effective mode interval 36 of the first optical mode 101. Signal mode 31 is coupled together by a perturbation having a higher spatial frequency (sharper transition region) than the perturbation that couples together the first optical mode 101. Therefore, by avoiding these high spatial frequencies, the first optical mode 101 can couple together and to mode B 37, while simultaneously minimizing the coupling of signal mode 31 to other signal modes 31 and mode B 37. Thus, the power of the first optical mode 101 can be scattered at each transition 178 from the second core 12 to the mode overlapping with the active dopant 9.

[0156] Since coupling between signal modes 31 can degrade the optical beam quality of the output signal 145 as described with reference to Figure 14, the radii of curvature of the angles 177 and transitions 178 of the optical fiber 10 can also be selected so as not to cause coupling between signal modes 31. This can be advantageous for certain cutting and micro-welding applications. Alternatively, the radii of curvature of the angles 177 and transitions 178 of the optical fiber 10 can be selected to cause coupling between signal modes 37 in order to produce an output signal 145 that is multimode and has relatively low beam quality. This can be advantageous for certain welding or cleaning applications. Mode coupling between signal modes 31 is preferably smaller than mode coupling between the first optical modes 101.

[0157] The optical amplification process (absorption and stimulated emission) causes heat generation. The output end 179 of the spool 170 (i.e., the end of the optical fiber 10 emitting the output signal 145) preferably does not have abrupt transitions between sections of the fiber having different radii of curvature, and therefore mode coupling is reduced compared to angles 177 and transitions 178. The optical fiber 10 can preferably be characterized by its temperature rise relative to the temperature of the substrate 175, which acts as a heat sink. The intensity and arrangement of the perturbation 29 are preferably such that the temperature rise is less than 100°C, preferably less than 70°C, and more preferably less than 50°C when the optical fiber 10 is pumped by the pump 141.

[0158] The radius of curvature and / or transition 178 may be the same at each angle 177 of the spool 170. Alternatively, at least one angle of the spool 170 may have a different radius of curvature from the other angles. The latter may be advantageous for coupling different mode groups together.

[0159] The radius of curvature of the optical fiber 10 at the output end 179, which is the end of the optical fiber connected to the output fiber 144, may be greater than the radius of curvature of the optical fiber 10 at the input end 171, which is the end of the optical fiber 10 connected to the seed laser 1410. This can be advantageous at high signal powers by reducing the coupling between the first optical mode 101 and the second optical mode 102, thereby reducing subsequent absorption by the active dopant 9. Alternatively, the coupling can be increased at low signal powers.

[0160] The spool 170 is shown with the optical fiber 10 restricted by the helical groove 173. Alternatively, the optical fiber 10 may be restricted by adhesive and / or bosses, pins, or pegs to achieve a desired bending transition.

[0161] Figure 18 shows a spool 180 in which the optical fiber 10 is wound around a spool former 181 having straight sides and rounded corners. The shape of the spool 181 is preferably a polygon with rounded corners. The shape of the spool 180 may be similar to the various spool shapes described with reference to Figure 17.

[0162] The optical fiber 10 used in spool 170 or spool 180 may have a glass diameter 8 of less than 1 mm, preferably less than 0.9 mm, and more preferably less than 0.8 mm.

[0163] The optical fiber spool 170 or 180 can be characterized by having a minimum radius of curvature at angle 177 and transition 178 of less than 20 mm, preferably less than 15 mm, and more preferably less than 10 mm.

[0164] The optical fiber spool 170 or 180 can be characterized by having a cross-sectional width 172 of less than 500 mm, preferably less than 400 mm, and more preferably less than 300 mm. An optical fiber spool made from an optical fiber having optical properties equivalent to that of the optical fiber spool 170 and without the first cladding 3 typically requires an optical fiber having a larger glass diameter 8.

[0165] The optical fiber 10 of spool 170 or spool 180 may be optical fibers 40, 50, 60, 70, 80, 90, 100, 110, 120, or 130, as shown in Figures 4 to 13, respectively.

[0166] Figure 19 shows a perturbation 29 in the form of a long-period grating 190 written on the second core 12 of the optical fiber 100, as described with reference to Figure 10. The long-period grating 190 has a region 191 whose refractive index is altered by ultraviolet irradiation. Region 191 has a spatial frequency equal to 1 / pitch 192 selected to couple the modes guided by the second core 12 to other modes of the optical fiber 100 that overlap with the active dopant 9 of the first core 2. The modes guided by the second core 12 are coupled or scattered together by the long-period grating 190 and coupled or scattered to other modes of the optical fiber 100. The pitch 192 is preferably equal to the pump wavelength λ divided by the effective refractive index mode spacing 36. This arrangement is useful for optical fibers such as optical fiber 100, where the effective refractive index mode spacing 33 between the signal modes 31 guided by the first core 2 is smaller than the effective refractive index mode spacing 36 of the first optical mode 101 guided by the second core 12. This is because, when using bending and bending transitions as shown in Figure 17, it is more difficult to couple the first optical mode 101 together without coupling the signal mode 31 together in these fibers.

[0167] The long-period grating can also be used to couple the first optical mode 101 guided by the second core 12 to the second optical mode 102, which overlaps with the active dopant 9 of the optical fibers 10, 40, 50, 60, 70, 80, 90, 110, 120, and 130.

[0168] Other forms of long-period gratings can also be used, including those based on applying periodic bending or pressure to the optical fiber 100.

[0169] Figure 20 shows an end-pumped fiber laser 200 having an optical fiber 40, an optional combiner fiber 160 between the optical fiber 40 and its respective output combiner 143, an optional input fiber 204, and an optional output fiber 144. Reflectors 201 and 202 are positioned at both ends of the optical fiber 40 and form a laser cavity 203. The reflectors 201 and 202 are preferably optical fiber Bragg gratings that can be written to the combiner fiber 160. Alternatively, the optical fiber Bragg grating can be written to the optical fiber 40, or to the input fiber 204 and output fiber 144.

[0170] The pump source 141 is coupled to the second core 12 of the optical fiber 40 via a pump fiber 142, an output combiner 143 as described with reference to Figures 14-16, and an optional combiner fiber 160. The optional combiner fiber 160 as described with reference to Figure 16 preferably has a ring core 162 having the same shape and size as the second core 12 of the optical fiber 40. Alternatively, the ring core 162 may be circular and smaller than the second core 12 to ease splicing requirements.

[0171] The pump source 141 may have a laser diode, a laser diode module having multiple laser diodes, or a laser diode bar. The pump source 141 may have one or more fiber lasers. The pump source 141 may have multiple pump sources 207 whose outputs are combined with each other in a pump combiner 208. The pump sources 207 may be individual laser diodes, laser diode modules each having multiple laser diodes, a laser diode bar, or a fiber laser. The pump combiner 208 may be a fusion spliced ​​fiber pump combiner. The pump sources 207 may emit pump radiation 209 having the same pump wavelength 2320. Alternatively, different pump sources 207 may emit pump radiation 209 having different pump wavelengths 2320. The ability to select different pump wavelengths 2320 can be advantageous when pump absorption by the active dopant 9 differs at these wavelengths.

[0172] The optical fiber 40 is perturbed by one or more perturbations 29 to couple the pump radiation 209 emitted from the second core 12 by the pump 141 to the active dopant 9. The optical fiber 40 can be constructed in the same or similar manner as the optical fiber 10 described with reference to Figures 17 to 19. The optical fiber 40 can be replaced by the optical fibers 10, 50, 60, 70, 80, 90, or 100 described with reference to Figures 1 and 5 to 10, respectively.

[0173] The fiber laser 200 can be connected to an optional beam delivery fiber 206, which can be terminated by an end cap 1415. A collimating lens 1416 and a focusing lens 1417 can be provided to focus the output signal 145 onto the workpiece 1418.

[0174] Figure 21 shows a side-pumped laser 210 having an optical fiber 130 as described with reference to Figure 13. The second cladding 13 is removed from each end of the optical fiber 130, and the pump fiber 131 and the second core 12 are separated from each other. Pump radiation 209 from at least one pump source 141 is coupled to the pump fiber 131 via the optical fiber 142. The pump radiation 209 is coupled to the second core 12 and then to the active dopant 9 of the first core 2 by perturbation 29. Reflectors 201 and 202 may be dichroic mirrors or gratings configured to facilitate photogeneration in the optical fiber 130 by reflecting light energy back to the first core 2. Reflectors 201 and 202 are preferably optical fiber Bragg gratings. The resulting output signal 145 is the output via an optional beam delivery fiber 206. Alternatively, the laser 210 can be end-pumped by providing an output combiner 143, as described with reference to Figure 14, and coupling the pumped radiation 209 to the second core 12.

[0175] Figure 22 shows a side-pumped laser 220 in the form of a master oscillator power amplifier similar to the laser described with reference to Figure 14. The side-pumped laser 220 has an optical fiber 110 as described with reference to Figure 11. A second core 12 is connected to at least one pump 141. A first core 2 is connected to a seed laser 1410 via an optical fiber 221. An input signal 1411 emitted by the seed laser 1410 is amplified by an active dopant 9 in the first core 2, and the resulting output signal 145 is the output via a beam delivery fiber 206. The optical fiber 110 can be replaced with optical fiber 110 or 130 as shown in Figures 11 and 13, respectively. If optical fiber 130 is used instead of optical fiber 110, the second core 12 of optical fiber 130 can perform both end-pumping and side-pumping as described with reference to Figure 14.

[0176] Fiber lasers 200, 210, and 220 are shown to be subjected to both back-excitation and forward-excitation. If fiber lasers 200, 210, and 220 were only back-excited, higher pulse energies could be obtained. Only one amplification stage is shown. However, a laser can have multiple amplification stages.

[0177] Figure 23 shows how the amplifier 140 of Figure 14 is incorporated as a power amplifier in a master oscillator power amplifier (MOPA) 230. The MOPA 230 has at least one seed source 231 that provides seeding radiation 2315, and a first preamplifier 232 for amplifying the seeding radiation 2315. A second preamplifier 2313 may also be provided.

[0178] The optical isolator 233 can be placed between the seed source 231 and the first preamplifier 232, between the first preamplifier 232 and the second preamplifier 2313, and between the second preamplifier 2313 and the amplifier 140. The optical isolator 233 protects the seed source 231 and the first and second preamplifiers 232 and 2313 from the reverse-direction radiation 2310 emitted from the amplifier 140.

[0179] The optical isolator 233 may also be placed at the output of the amplifier 140 to attenuate light radiation reflected or emitted from the workpiece.

[0180] Seed source 231 may have a Fabry-Perot semiconductor laser.

[0181] The seed source 231 may have a super light-emitting diode.

[0182] The seed source 231 can emit signal pulses 2319 having pulse widths in the range of 100 ps to 10 ms, and the optical amplifier can be configured to emit individual pulse energies greater than 50 mJ, preferably greater than 100 mJ, and more preferably greater than 200 mJ. It is believed that pulse energies of pulses having pulse widths in the range of 100 ps to 1000 ns can reach and exceed 1 J.

[0183] The reflector 2311 can be configured to reflect a portion of the seeding radiation 2315 back to the seed source 231. This helps prevent the formation of random pulses within the MOPA 230 that could cause catastrophic damage to the MOPA. The reflector 2311 may also be an optical fiber Bragg grating.

[0184] The seeding radiation 2315 may have an optical pulse 2319 characterized by a signal wavelength 2316. The amplifier 140 may have an optical gain 2318 that varies with wavelength and has a gain peak 2321 at a gain peak wavelength 2317. The seed source 231 may be selected such that the signal wavelength 2316 is within 10 nm, preferably within 2 nm, and more preferably within 1 nm, of the gain peak wavelength 2317.

[0185] MOPA230 may have a depolarizer 2312 between the seed source 231 and the first preamplifier 232. The depolarizer 2312 may be a Lyot-type depolarizer. A Lyot-type depolarizer may have two polarization-maintaining optical fibers of two lengths, one of which is twice the length of the other, spliced ​​together. When using a single seed source 231, it has been found that the depolarizer scrambles the polarization of the seeding radiation before it enters the first preamplifier 232, and thus the peak power of the output signal 145 can be increased before nonlinear effects such as stimulated Brillouin scattering become a problem. When using two or more seed sources 231, the polarization can be scrambled by aligning the input polarizations of the signals from the two seed sources 231 at an angle to each other so that they enter the coupler 234. The angle may be 45 degrees or 90 degrees.

[0186] MOPA230 may have at least two seed sources 231. The outputs from the seed sources 231 can be combined by a coupler 234. The seed sources 231 may have the same signal wavelength 2316. The coupler 234 may be a polarizing beam combiner. The seed sources 231 may have different signal wavelengths 2316. The coupler 234 may be a wavelength division multiplexer. The coupler 234 is preferably a fusion splice fiber coupler. Furthermore or alternatively, the optical fiber 10 may have a plurality of first cores 2, and the outputs from each seed source 231 can be coupled to one of each different first core 2. Such optical fibers are described with reference to Figures 8 and 9. Arrangements having a plurality of first cores 2 may be advantageous in certain welding applications where multiple laser spots are projected onto the workpiece.

[0187] The MOPA230 may have an optical switch 2314. The optical switch 2314 may also be an acousto-optic modulator. The optical switch 2314 can be used to reduce the naturally radiated amplified light ASE emitted from the first preamplifier 232 during pulse 2319, thereby reducing the ASE amplified by the amplifier 140. The optical switch 2314 can also be used to reshape pulse 2319 if the seed source 231 does not output the desired pulse shape. This can be advantageous when a pulse with a gentle ramp is desired, which can be difficult to generate from semiconductor laser diodes.

[0188] The optical switch 2314 may be placed after the second preamplifier 2313 or the amplifier 140. However, it may be necessary to have a higher power rating for the optical switch 2314, which increases costs.

[0189] The MOPA230 includes a visible laser diode 236 and a coupler 237, the coupler 237 may be configured to integrate the visible light radiation 238 emitted from the visible laser diode 236 and the light radiation 239 emitted from the optical amplifier 140. The coupler 237 is preferably a wavelength division multiplexer. The wavelength division multiplexer may be a fusion spliced ​​fiber coupler or a dichroic mirror.

[0190] The device may include a seed source 231, a first preamplifier 232, a second preamplifier 2313 (if installed), an optical switch 2314 (if installed), and a controller 1412 for controlling the amplifier 140. When amplifying the optical pulse 2319, it is preferable that the first and second preamplifiers 232 and 2313 are turned on (i.e., supplied with energy by the pumps) before the amplifier 140 is turned on, in order to protect the amplifier 140 from damage. The amplifier 140 may be turned on before the arrival of the first pulse 2319, for example by the controller 1412, to ensure that the first pulse is amplified as well as subsequent pulses.

[0191] Figure 24 shows the amplifier 140 of Figure 14 incorporated as a power amplifier into the Q-switched laser 240. The Q-switched laser 240 comprises the reflector 201 of Figures 20 and 21, the amplifier 140, the Q-switch 241, and the output coupler 242. The Q-switch 241 may be a free-space optical Q-switch. The light radiation emitted from the amplifier 140 can be coupled to the output coupler 242 via the collimating lens 244 and the Q-switch 241. To prevent the light radiation from damaging the end face of the output fiber 144, it is preferable that an end cap 1415 is provided at the end of the output fiber 144. The controller 249 synchronizes the pumping of the optical fiber 10 with the opening of the Q-switch 241 to facilitate light generation within the optical fiber 10. The generated output signal 145 can be focused by the output lens 245 and output via the beam delivery fiber 206. The beam delivery fiber 206 may be a solid glass fiber or a fiber having a microstructure or a hollow core. The Q-switch 241 may be an acousto-optic modulator, an electro-optic switch, or a mechanical Q-switch having a rotating mirror or prism. Note that the amplifier 140 can be replaced with other optical amplification arrangements including the amplifiers shown in Figures 20 to 23, and any of the optical fibers 10, 40, 50, 60, 70, 80, 90, 100, 110, 120, and 130 shown in Figures 1 and 4 to 13 can be used.

[0192] In the following examples, MOPA230, shown in Figure 23, was modeled for multiple types of optical fibers 10. MOPA230 had one seed source 231 and the amplifier 140 shown in Figure 14. The optical fiber 10 used in Examples 1 and 3-6 was the optical fiber 310 shown in Figure 31, similar to the optical fiber 40 in Figure 4 but without the third cladding 18. The second cladding 13 was a polymer.

[0193] The optical fiber used in Example 2 was an optical fiber 320 equivalent to optical fiber 310, except that it lacked the first cladding 3. The design of the first core 2 was identical, having the same refractive index 4, the same core diameter 6, and the same active dopant concentration 9. The design of the second core 12 of the equivalent optical fiber 320 extends to the outer circumference of the first core 2 and is in optical contact with the outer circumference of the first core 2. The refractive index 14 of the second core 12 is the same in optical fiber 310 and the equivalent optical fiber 320. As described below, when the inventive optical fiber 310 is used in an optical amplification device such as an amplifier or laser, it has at least twice the length of the equivalent optical fiber and can absorb the same amount of pump radiation along its length. By selecting the intensity and arrangement of the perturbation 29 along the optical fiber 310, when the optical amplification device uses optical fiber 310, the pulse energy can be increased, the gain peak wavelength 2317 can be moved to a longer wavelength side, and the gain of the gain peak 2321 at the signal wavelength 2316 can be increased compared to when the optical amplification device uses the equivalent optical fiber 320.

[0194] The bending of the optical fiber restricted to groove 173 in Figure 17 was modeled using Cossellat rod theory, a method for modeling the bending and other deformations of thin rods. This was used to create a plot of the radius of curvature against distance along the optical fiber 10.

[0195] Pump absorption was modeled using beam propagation methodology (BPM), which simulates the propagation of the optical field within a defined refractive index structure. The radius of curvature, determined by the Cossellat model, was expressed by applying a gradient to the refractive index value along one of the cross-sectional directions of the fiber model. The absorption characteristics of active dopant 9 for pump emission 209 were represented by the imaginary refractive index component calculated from the selected dopant concentration and the absorption cross-section of the selected dopant.

[0196] Using a rate equation model, the signal gain and spontaneous emission amplified light characteristics of optical fiber 10 were determined by pump absorption calibrated with the results of a BPM absorption model. Similar to the spontaneous emission amplified light generated by power amplifier 140, the model also considered the spontaneous emission amplified light generated by the first and second preamplifiers 232 and 2313 in Figure 23, which can be input to and amplified by amplifier 140. The input amount of spontaneous emission amplified light depends on the characteristics of preamplifiers 232 and 2313 and any (intentional or accidental) wavelength filtering effects of isolator 233, and therefore varies with the details of the laser system.

[0197] The thermal load of optical fiber 10 was calculated. The thermal load is caused by pump absorption and quantum defects. Quantum defects arise from the energy difference between the pump and the signal photon. The maximum temperature of optical fiber 10 was calculated at the location along the fiber with the highest thermal load, using a two-dimensional thermal model representing the cross-section of the fiber and the substrate 175. It was assumed that the space between fiber 10 and substrate 175 was filled with polyacrylate. Typical thermal properties of silica for the fiber, polyacrylate for the fiber coating and filling, and aluminum for the substrate 175 were used. The value of the maximum temperature rise is given in the following example, pointing to the highest temperature rise point in polyacrylate. Thermal degradation of the fiber coating when operating fiber lasers at high optical power and resulting high fiber temperatures is a well-known problem.

[0198] BPM was also used to simulate signal perturbations within fiber 10. In the active-doped region of fiber 10, gain was applied in the form of an imaginary refractive index component to simulate amplification. At certain gains and perturbations, it was found that the beam quality of the output signal 239 emitted from amplifier 140 was only slightly affected by the beam quality of the input signal 1411 input to amplifier 140.

[0199] The input signal 1411 was multimode and had a field diameter matching the diameter 6 of the first core 2 of fiber 10. The beam quality of the input signal 1411 was characterized by a beam parameter product BPP of 5.0. The BPP of the output signal 239 was 6.1 for a groove width 174 of 0.9 mm and 5.0 for a groove width 174, compared to approximately 5.5 for a spool 170 with a groove width 174. By comparison, the pump absorption varied from 0.5 dB / m for a spool with a groove width 174 of 1.3 mm to 1.6 dB / m for a spool with a groove width 174, demonstrating that pump absorption can be controlled to a considerable extent while maintaining good beam quality for the output signal 239.

[0200] The modeling was performed using different designs and configurations for the optical fiber 10, based on the apparatus shown in Figure 14. In all cases, the input signal 1411 fed into amplifier 140 had an average power of 20W.

[0201] Example 1 is a preferred arrangement using an optical fiber 310 having a first cladding 3 that isolates the active dopant 9 from the pump radiation 209 incident only on the second core 12. The active dopant 9 was a ytterbium ion. The optical fiber 310 was perturbed using a helical round square spool configuration as shown in Figure 17, but the perturbation was very gentle at the output end 179 (where the pump radiation 209 was incident) to minimize coupling of the pump radiation 209 with the active dopant 9 and thereby minimize the thermal load on the optical fiber 310. The function of the first cladding 3 and perturbation 29 was demonstrated in Example 2 by comparison with an identical configuration (fiber design, perturbation 29, and physical layout) except that it did not have the first cladding 3. The absorption was so high that the fiber length had to be shortened. The fiber in Example 3 was the same as in Example 2 except that the concentration of the active dopant 9 was reduced. The fiber in Example 4 was the same as in Example 1, except that the perturbation 29 at the output end 179 was stronger. The fiber in Example 5 was also the same as in Example 1, except that the pump radiation at the output end 179 was incident on both the first core 2 and the second core 12. Finally, in Example 6, the arrangement of Example 1 was reproduced, except that the variation in the radius of curvature was reduced to reduce the coupling of the pump radiation 209 from the second core 12.

[0202] Example 1. Referring to Figure 23, optical fiber 10 was optical fiber 310 of Figure 31, having a circular first core 2 with a first core diameter 6 of 170 μm. The active dopant 9 was ytterbium, and the dopant ion concentration was 7.775 × 10⁻¹⁶. 25 / m 3 The first core 2 was co-doped with phosphorus and aluminum in appropriate proportions to avoid photodarkening. The second core 12 was an octagonal core with a second core diameter 17 of 660 μm. The first cladding 3 consisted of fluorine-doped silica to reduce the refractive index and had a thickness 7 of 10 μm. The second cladding 13 was a polymer coating with a diameter of 850 μm. There was no third cladding 18.

[0203] The first cladding 3 had a first refractive index 5 that was 0.017 lower than the refractive index 4 of the first core 2 and 0.004 lower than the refractive index 14 of the second core 12.

[0204] The second cladding 13 had a second refractive index 15 that was 0.07 lower than the refractive index 14 of the second core 12.

[0205] The optical fiber 10 was formed in the spool 170 by inserting the optical fiber 10 into the helical groove 173 of the substrate 175. The groove 173 had a round-square configuration as shown in Figure 17 and a width 174 of 1 mm. The groove 173 had a minimum radius of curvature of 100 mm at the corner 177 of the spool 170 and a maximum radius of curvature of over 10 m along the side 176.

[0206] The pump wavelength 2320 was 960 nm, and the signal wavelength 2316 was 1065 nm.

[0207] The optical amplifier 140 is back-excited as shown with reference to Figure 14. Pump radiation 209 is introduced into the optical fiber 310 from the combiner fiber 160, as described with reference to Figure 16, and the pump radiation 209 is restricted to the ring core 162. There was no pump radiation 209 coupled to the central core 161. As a result, all pump radiation 209 was incident on the second core 12 of the optical fiber 310. The input signal 1411 is introduced to the other end of the optical fiber 310 via a step-index fiber having a core size such that all input signal 1411 is incident on the first core 2 of the optical fiber 310.

[0208] By increasing the pump power, it was possible to obtain an output signal 145 having pulses with a pulse repetition frequency of 20 kHz, an average output of 2458 W, and a pulse energy of 121 mJ. By increasing the diameter 6 of the first core, it was possible to obtain pulse energies exceeding 200 mJ.

[0209] The average pump absorption along optical fiber 310 was 1.0 dB / m, the maximum pump absorption was 1.4 dB / m, and the maximum thermal load for an average output power of 2.4 kW was 91 W / m.

[0210] The maximum temperature rise relative to the temperature of substrate 175 was 45°C.

[0211] The gain peak wavelength 2317, corresponding to the gain peak of amplifier 140 at 2318, was 1060 nm. The wavelength at which the amplified spontaneous emission spectrum peaked was also 1060 nm. There was 41 W of amplified spontaneous emission against an average output power of 2458 W, representing slightly less than 2% of the total output power.

[0212] Example 2. The fiber design and configuration of Example 2 were the same as in Example 1, except that the optical fiber 10 was replaced with an equivalent optical fiber 320 that does not have the first cladding 3 separating the first core 2 and the second core 12, as described with reference to Figure 32. The first core 2 had the same dimensions and design. The second core 12 had the same octagonal shape as shown in Figure 4, except that its inner diameter was equal to the outer diameter of the first core 2. Thus, the second core 12 functioned as the pump cladding of a prior art double-clad fiber. Example 2 is provided not according to the present invention, but to demonstrate the advantages of the first cladding 3 and perturbation 29.

[0213] Due to the absence of the first cladding 3, the pump radiation 209 coupled with the active dopant 9 at a higher rate per unit length of fiber. The average pump absorption was 4.2 dB / m, and the maximum pump absorption was 4.6 dB / m. The length of the equivalent fiber 320 was shortened from 15 m to 3.9 m to maintain in the equivalent fiber 320 the same total pump absorption as obtained in the optical fiber 310 of Example 1.

[0214] An output signal 145 was obtained with pulses having a pulse repetition frequency of 20 kHz, an average output power of 2371 W, and a pulse energy of 94 mJ. The gain peak wavelength 2317 was 1028 nm. There was 460 W of amplified spontaneous emission against an average output power of 2.4 kW, representing 19% of the total power. It was found that without the first cladding 3, high pump absorption would cause a very large amount of ASE, resulting in pulse energy loss and a risk of parasitic laser oscillation near the gain peak wavelength 2317, which can degrade performance and damage the laser.

[0215] The high pump absorption also resulted in a very high peak thermal load of 276 W / m. The maximum temperature rise of the optical fiber 320 relative to the temperature of the substrate 175 was 137°C.

[0216] Example 3. The fiber design and configuration of Example 3 is such that the active dopant 9 (ytterbium ion) is 1.944 × 10⁻¹⁴ 25 / m 3 It was the same as Example 2 except that it had a low dopant concentration. Example 3 was not according to the present invention because the optical fiber 320 did not have a first cladding 3 separating the first core 2 and the second core 12.

[0217] The ytterbium concentration was reduced to decrease the pump absorption rate. The average pump absorption was 1.1 dB / m, and the maximum pump absorption was 1.2 dB / m. These values ​​were the same as in Example 1. The maximum thermal load at an average output of 2385 W was 70 W / m, which was also the same as in Example 1. The maximum temperature rise relative to the temperature of substrate 175 was 35°C.

[0218] It was also possible to obtain an output signal 145 having pulses with a pulse repetition frequency of 20 kHz, an average output of 2385 W, and a pulse energy of 91 mJ.

[0219] The gain peak wavelength 2317, corresponding to the gain peak 2318 of amplifier 140, was 1028 nm. There was an amplified spontaneous emission of 498 W, equivalent to 21% of the total output power. These values ​​are similar to those in Example 2, indicating that while reducing the concentration of active dopant 9 can improve thermal performance, it cannot shift the gain peak wavelength 2317 to a suitable wavelength or improve optical performance. This is because reducing the doping concentration reduces total absorption while not changing the reversal rate of the active ions, which determines the emission characteristics. Therefore, the ASE level does not change significantly compared to Example 2, and, similar to Example 2, shows a substantial decrease in laser performance. The accompanying high gain also poses a risk of laser damage.

[0220] Example 4. The fiber design and configuration of Example 4 were the same as in Example 1, except that the gentle bend of the optical fiber 310 at the output end 179 (pump inlet end) used in Example 1 was removed. The groove 173 had a curved section with a radius of curvature of 100 mm, and there were no sections with a larger radius of curvature at the output end 179.

[0221] This change had essentially negligible effects on the optical properties compared to Example 1, but it caused an increase in peak absorption at the output end 179, and therefore a slight increase in peak thermal load and maximum fiber temperature.

[0222] Example 5. The fiber design and configuration in Example 5 were the same as in Example 1, except that the pump radiation was introduced into the optical fiber 310 from a combiner fiber having a simple step-index profile. The pump radiation 209 overlapped with the core of the combiner fiber and was coupled to both the first core 2 and the second core 12 of the optical fiber 310 in proportion to their cross-sectional areas.

[0223] This change had only a slight effect on the optical properties compared to Example 1. The gain peak wavelength 2317 corresponding to the gain peak 2318 of amplifier 140 was 1060 nm. There was 13 W of amplified spontaneous emission for an average output power of 2.4 kW. These values ​​are similar to those in Example 1, indicating that a very short section of pump radiation 209, which is highly absorbed by direct incidence of the first core 2, does not significantly affect the spectral performance of the amplifier.

[0224] However, the very high gain in the short section near the pump injection point significantly affected the thermal characteristics. The maximum absorption was 6.1 dB / m, the corresponding maximum thermal load was 495 W / m, and the maximum temperature rise for substrate 175 was 245°C. Such high temperature rises are undesirable and can lead to reliability problems.

[0225] Example 6. The fiber design and configuration of Example 6 were the same as in Example 1, except that the optical fiber 310 was incorporated into a helical groove with no abrupt changes in the radius of curvature. Along the entire length of the 15m spool, the radius of curvature decreased very slowly and continuously. As a result, the maximum absorption occurring near the signal output end 179 (pump input end) of fiber 10 was similar to that of Examples 1, 3, and 4, but the absorption decreased rapidly in the rest of fiber 310 as a result of the absence of perturbation 29 due to the lack of change in the radius of curvature. Consequently, the average pump absorption was much lower than the desired value, and since most of the pump radiation 209 was not coupled from the first optical mode 101 guided by the second core 12 to the second optical mode 102 which overlaps with and is absorbed by the active dopant 9, it resulted in reduced efficiency and reduced output power.

[0226] [Table 1] [Table 2]

[0227] Figure 25 shows the absorption and emission cross-sections 251 and 252 for wavelength 250 of ytterbium ions in ytterbium-doped silica phosphate glass used in the example. Absorption and emission cross-section σ abs , σ em In Examples 1 to 6, this is a measure of the probability that a photon is absorbed or emitted from the active dopant 9, which is a ytterbium ion.

[0228] Figures 26–30 show the spontaneous emission amplified light, inversion ratio, and pump absorption in Examples 1–6. The calculations correspond to the point in time when a seed pulse from the middle of a pulse chain with a pulse repetition frequency of 20 kHz is about to be incident on fibers 310 and 320. The results represent the steady-state performance of the laser, excluding transient effects that may occur, particularly near the beginning of the pulse chain.

[0229] Figure 26 shows the inversion ratios 261, 262, 263, and 266 for distance 260 along the optical fibers 310, 320 from the signal input end 171, as shown with reference to Figures 14 and 17, calculated for Examples 1, 2, 3, and 6, respectively. The inversion ratios 264 and 265 for Examples 4 and 5 are shown in Figure 27, respectively. These were similar to the inversion ratio 261 of Example 1, except near the signal output end 179, which is the end where the pump radiation 209 enters the optical fiber 310. The inversion ratio 266 of Example 6 increases towards the signal output end 179 and decreases to a steady state lower than the inversion ratio 261 for the rest of the fiber 310. The inversion ratios 262 and 263 corresponding to Examples 2 and 3 are significantly higher than the inversion ratio 261 of Example 1.

[0230] Figure 28 shows the pump absorptions 281, 282, 283, and 286 calculated for distance 260 for Examples 1, 2, 3, and 6, respectively. Pump absorptions 284 and 285 for Examples 4 and 5, respectively, are shown in Figure 29 and are similar to pump absorption 281 in Example 1. Pump absorption 282 in Example 2 is significantly higher than pump absorption 281. Pump absorption 283 was designed to be similar to pump absorption 281 by reducing the dopant concentration of active dopant 9. Pump absorption 286 in Example 6 (no perturbation) increases rapidly toward the signal output end 179 because the first cladding 3 isolated the pump radiation 209 propagating along the second core 12 from the active dopant 9.

[0231] Figure 30 shows the spontaneous emission amplification (ASE) spectra 301, 302, 303, and 306 emitted by the optical fiber, calculated for wavelength 250 for Examples 1, 2, 3, and 6, respectively. The ASE spectra 304 and 305 for Examples 4 and 5, respectively, overlap almost with the ASE spectrum 301 of Example 1 and are therefore not shown. The ASE spectra 302 and 303 of Examples 2 and 3 peak at approximately 1030 nm, while the ASE spectra 301, 304, 305, and 306 of Examples 1, 4, 5, and 6 peak at approximately 1060 nm.

[0232] Referring to Figures 26 and 27, the ASE spectra 301, 304, 305, and 306 in Figure 30 peak around 1065 nm for the desired signal wavelength 2316, because the inversion ratios 261, 264, 265, and 266 along at least the first half of the optical fiber 310 are smaller than the transition inversion ratio 267. For the ytterbium dopant, the transition inversion ratio 267 is less than 10%, preferably less than 8%, and more preferably 7%. This can be compared to the inversion ratios 262 and 263, which are higher than the transition inversion ratio 267 along most of the optical fiber 320, in which case the ASE peaks at 1030 nm.

[0233] Examples 1 to 3 demonstrate that the first cladding 3 provides a mechanism to reduce the coupling of pump radiation 209 from the first optical mode 101 propagating along the second core 2, and the coupling of the second optical mode 102 overlapping with the active dopant 9, without changing the external dimensions of the second core 12 or the concentration of the active dopant 9. This not only significantly reduces the thermal load on the fiber but also shifts the gain peak wavelength 2317 from approximately 1030 nm to approximately 1060 nm. This is suitable for operation at a signal wavelength 2316 where the emission cross-section 252 is small enough to allow sufficient energy storage for generating nanosecond pulses without generating excessive ASE and without the risk of the laser being destroyed by parasitic oscillations. In operation at 1065 nm, the reversal rate of the active dopant 9 is less than 10%, preferably less than 8%, and more preferably 7% over at least 70% of the length of the optical fiber 310.

[0234] In Example 1, spontaneously emitted amplified light (ASE) accounted for only 2% of the total output power, while in Examples 2 and 3, where the first cladding 3 was not used, ASE accounted for 12% of the total output power. The absence of the first cladding 3 in Examples 2 and 3 degraded the laser performance by reducing the pulse energy and decreasing the contrast between pulse and inter-pulse power, leaving a very high risk of unstable parasitic laser oscillation at the ASE peak wavelength.

[0235] Examples 1 and 4 demonstrate the importance of the spool plate characteristics. Advantageously, the optical fiber 310 in Example 1 is positioned with a larger radius of curvature at the signal output end 179 (pump inlet end), resulting in reduced maximum absorption, maximum thermal load, and maximum temperature compared to Example 4, where all bent sections of groove 173 have the same radius of curvature. The impact on pulse energy and ASE power was negligible.

[0236] Examples 1 and 5 demonstrate the importance of the initial distribution of pump radiation 209. As in Example 5, when a large amount of pump radiation is incident on the first core 2, this radiation is partially confined within the first core 2 by the same mechanism that partially prevents pump radiation in the second core 12 from entering the first core 2. This resulted in a section of very high pump absorption at the signal output end 179. This high-absorption section continued until most of the pump radiation 209 confined within the first core 2 was absorbed by the active dopant 9. Because the high-absorption region was very short (approximately 20 cm), the overall gain characteristics of the amplifier 140 did not change significantly, and the impact on pulse energy and ASE power was negligible. However, higher pump absorption corresponds to higher local thermal loads and higher maximum temperatures, both of which are undesirable as they can cause damage to the optical coating and catastrophic damage to the optical fiber. Alternative fiber coatings, such as metal coatings and / or soldering to a heat sink, can be used. The fiber coating can also be completely removed in these high-temperature sections.

[0237] In Example 6, the radius of curvature of the optical fiber 310 changed only gradually, so there was no fiber perturbation that coupled the first optical mode 101 from the second core 12 to the second optical mode 102, which has a high divergence overlapping with the first core 2 and the active dopant 9. The initial absorption in Example 6 was a suitable value, slightly higher than the target value obtained in Example 1, but the overall absorption was lower than the value obtained in Example 1. This is because, as the pump radiation 209 propagated away from the signal output end 179, the pump rays with higher divergence were absorbed by the active dopant 9, while the pump rays with lower divergence continued to be guided by the second core 12 and therefore were not absorbed by the active dopant 9, resulting in a decrease in absorption along the length of the fiber 310. By the time the pump radiation 209 reached the signal input end 171, the pump absorption had decreased to <0.1 dB / m. The depletion of pump rays with higher divergence resulted in low total absorption (<<10 dB) for reasonable fiber lengths, leading to low efficiency and a large amount of wasted pump radiation that had to be safely disposed of. This is in contrast to Examples 1, 4, and 5, where perturbation 29 replenished pump radiation with high divergence that overlapped with the first core 2, allowing absorption to be maintained (or improved, if desired) along the entire length of fiber 310.

[0238] In the experiment, the seed laser 1410 in Figure 14 was a 20W TruPulse nanolaser manufactured by TRUMPF Laser UK Limited in Southampton, England. The TruPulse nanolaser has a single seed source 231 and first and second preamplifiers 232, 2313, as shown with reference to Figure 23, and is therefore equivalent to the seed laser modeled in Example 1. The TruPulse nanolaser can emit various pulse shapes with different pulse widths, powers, and pulse energies. The maximum pulse energy of the 20W TruPulse nanolaser is approximately 1 mJ. Referring to Figure 14, fiber 10 is fiber 310 in Figure 31, having the same design as modeled in Example 1, and was configured and mounted on substrate 175 as described with reference to Figure 17 and Example 1. Amplifier 140 was able to emit pulses of 100 mJ at a pulse repetition frequency of 20 kHz and an average power of 2 kW. The heat generated in the optical fiber 310 could be dissipated by the substrate 175, and the resulting temperature rise of the optical fiber 310 was within the acceptable limits necessary to ensure reliable operation over long periods of time.

[0239] The experimentally measured beam parameter product (BPP) of output signal 145 was 10 mm.mrad, which is larger than the BPP of Example 1. The BPP varied slightly depending on the spool format. This suggests that the primary cause of the increase in BPP of output signal 145 was not intentionally induced fiber bending (macrobending). Other causes of the increase in BPP may include microbending (random variation of the fiber) and increases in BPP caused by other optical components such as combiners. The results confirm that the absorption of pump radiation can be manipulated by controllingly bending fiber 10 without significantly degrading the signal BPP.

[0240] For a beam transmission cable 206 of appropriate length, its core size is preferably larger than the first core diameter 6 of the optical fiber 10 in order to avoid excessive nonlinearity for high peak power pulses. If this increase in core size is achieved not by simple splicing but by expansion using tapered coupling or free-space optics, it is possible to maintain a BPP similar to that of the optical fiber 10. Preferably, the taper is an adiabatic taper.

[0241] The lasers and amplifiers in Figures 14 and 20-24, and in Examples 1, 4, 5, and 6, are optical amplifiers for emitting an output signal 145 at a signal wavelength 2316. By appropriately selecting the intensity and distribution of the first cladding 3 and perturbation 29, it is possible to increase the available pulse energy, reduce the level of ASE between pulses, and design a gain peak wavelength 2317 at an optimal wavelength for the selection of the active dopant 9 and the glass material of the optical fiber 10.

[0242] For example, active dopant 9 may contain ytterbium ions, and the gain peak wavelength 2317 may be 1060 nm. Active dopant 9 may contain erbium, or ytterbium-doped erbium, and the gain peak wavelength 2317 may be in the range of 1555 nm to 1650 nm. Active dopant 9 may contain holmium, and the gain peak wavelength 2317 may be in the range of 1990 nm to 2150 nm. The optical amplifier may include multiple pumps. One pump can emit pump radiation at 915 nm. Another pump can emit pump radiation at 976 nm. Active dopant 9 may contain thulium, and the gain peak wavelength 2317 may be in the range of 1900 nm to 2100 nm. The thulium ions are pumped at a pump wavelength 2320 of 793 nm, enabling so-called 2-in-1 pumping, where two pump photons are associated with each photon emitted by stimulated emission.

[0243] The invention described with reference to the figures and examples can be used in a variety of ways, including cleaning, paint removal, rust removal, etc., all of which can be carried out by a method that includes the step of providing an optical fiber 10 according to the present invention and controlling the transition rate of pump radiation from the second core 12 to the first core 2 using a perturbation 29. In such a process, the required BPP is typically 20 or less.

[0244] The embodiments of the present invention described above with reference to the accompanying drawings are for illustrative purposes only, and modifications and additional components may be provided to improve performance. The individual components shown in the drawings are not limited to their use in those drawings, but may be used in other drawings and in all embodiments of the present invention. The present invention extends to the individual components mentioned and / or shown above, either alone or in any combination.

Claims

1. An optical fiber (10) having at least one first core (2) and at least one second core (12), The first core (2) has at least one active dopant (9) The first core (2) and the second core (12) are separated by the first cladding (3), The first cladding (3) has a first refractive index (5) which is smaller than the refractive index (4) of the first core (2) and smaller than the refractive index (14) of the second core (12). When the optical fiber (10) is composed of a straight or uniform radius of curvature, the first propagation constant β 1 The first optical mode (101) having can propagate along the second core (12), but is isolated from the active dopant (9), When the optical fiber (10) is subjected to a perturbation (29), the first optical mode (101) has a second propagation constant β 2 It has and can be coupled to a second optical mode (102) that overlaps with the active dopant (9), This allows the pump radiation to propagate along the second core (12) as the first optical mode (101), to be selectively coupled to the second optical mode (102) by one or more perturbations (29), and to be absorbed by the active dopant (9), which can amplify one or more signal modes (31) guided by the first core (2) by stimulated emission, thereby enabling control of at least one of the inversion rate of the active dopant (9) along the optical fiber (10), the optical gain characteristics of the optical fiber (10), and the thermal load of the optical fiber (10). An optical fiber characterized by the following features.

2. The aforementioned perturbation (29) is the spatial frequency component Λ = (β 1 -β 2 The optical fiber according to claim 1, having ) / 2π.

3. The optical fiber according to claim 1 or 2, wherein the perturbation (29) has at least one spatial frequency component that couples at least several of the first optical modes (101).

4. The optical fiber according to any one of claims 1 to 3, wherein the perturbation (29) has a spatial frequency other than the spatial frequency that couples the signal modes (31) together.

5. The optical fiber according to any one of claims 1 to 4, wherein the perturbation (29) comprises at least one of a change in radius of curvature, a change in bending direction, compressive force, long-period grating, a change in the diameter of the optical fiber (10), a change in the cross-sectional shape of the optical fiber (10), rotation of the optical fiber (10), and a change in the material composition of the optical fiber (10) along the length of the optical fiber (10).

6. An optical fiber according to any one of claims 1 to 5, having a plurality of the first cores (2).

7. An optical fiber according to any one of claims 1 to 6, having a plurality of the second cores (12).

8. The optical fiber according to any one of claims 1 to 7, wherein the second core (12) is non-circular.

9. The second core (12) is surrounded by a second cladding (13), The optical fiber according to any one of claims 1 to 8, wherein the second cladding (13) has a second refractive index (15) that is smaller than the first refractive index (5).

10. The optical fiber according to claim 6, wherein the second cladding (13) is non-circular.

11. The optical fiber according to claim 9 or 10, wherein the first core (2) and the second core (12) are surrounded by the second cladding (13).

12. The second cladding (13) is a polymer, The optical fiber according to any one of claims 9 to 11, wherein the first core (2) and the second core (12) are separable from each other.

13. The pump fiber (131) is included in the second core (12) along its length and in optical contact with the second core (12), The second core (12) surrounds the first core (2), The optical fiber according to any one of claims 9 to 12, wherein the second core (12) and the pump fiber (131) are surrounded by the second cladding (13).

14. The second cladding (13) is a polymer, The optical fiber according to claim 13, characterized in that the pump fiber (131) and the second core (12) are separable from each other.

15. The optical fiber according to any one of claims 1 to 14, wherein at least one of the first core (2) and the second core (12) is a ring core.

16. The optical fiber according to any one of claims 1 to 15, wherein the width (6) of the first core (2) is greater than 50 μm, preferably greater than 100 μm, and more preferably greater than 250 μm.

17. The optical fiber according to any one of claims 1 to 16, wherein the width (7) of the first cladding (3) is greater than 5 μm, preferably greater than 10 μm, and more preferably greater than 15 μm.

18. The optical fiber according to any one of claims 1 to 17, wherein the width (16) of the second core (12) is greater than 50 μm, preferably greater than 100 μm, and more preferably greater than 150 μm.

19. An optical fiber spool (170) having an optical fiber (10) according to any one of claims 1 to 18, and including a plurality of perturbations (29).

20. The optical fiber spool (170) according to claim 19, wherein the optical fiber (10) is arranged in a groove (173).

21. The optical fiber spool (170) according to claim 19 or 20, wherein the second core (12) of the optical fiber (10) has a width (17) of less than 1 mm, preferably less than 0.9 mm, and more preferably less than 0.8 mm.

22. The optical fiber spool (170) according to any one of claims 19 to 21, characterized in that the optical fiber spool (115) has a minimum radius of curvature of less than 20 mm, preferably less than 15 mm, and more preferably less than 10 mm.

23. The optical fiber spool (170) according to any one of claims 19 to 22, characterized in that the optical fiber spool (115) has a cross-sectional width (172) of less than 500 mm, preferably less than 400 mm, and more preferably less than 300 mm.

24. An optical amplifier for emitting an output signal (145) at a signal wavelength (2316), A fiber optic spool (170) according to any one of claims 19 to 23, It has at least one pump (141), An optical amplifier in which the pump (141) is connected to the second core (12), so that the pump radiation from the pump (141) is guided by the second core (12) and coupled to the active dopant (9) by the perturbation (29).

25. The optical amplifier according to claim 24, wherein the optical fiber (10) is characterized by a temperature rise relative to the temperature of the substrate (175), and the intensity and arrangement of the perturbation (29) are such that when the optical fiber (10) is pumped by the pump (141), the temperature rise is less than 100°C, preferably less than 70°C, and more preferably less than 50°C.

26. The optical amplifier according to claim 24 or 25, wherein the inversion rate of the active dopant (9) along at least 70% of the length of the optical fiber (10) is less than 10%, preferably less than 8%, and more preferably equal to 7%.

27. The optical amplifier according to any one of claims 24 to 26, wherein the first core (2) is characterized by an absorption length at a pump wavelength (2320), the first core has a first core area, the second core has a second core area, the optical fiber (10) is characterized by a ratio of core areas equal to the sum of the first core area and the second core area divided by the first core area, and the length of the optical fiber is at least 2 times, preferably at least 3 times, and more preferably at least 5 times, the product of the absorption length and the ratio of core areas.

28. The optical fiber (10) has no first cladding (3) and has a fiber length at least twice that of an equivalent optical fiber, which is used selectively with the optical amplifier while absorbing the same amount of pump radiation along its length, wherein the first cladding (3) of the optical fiber (10) is replaced by a region in the equivalent optical fiber having the same refractive index (14) as the second core (12) of the optical fiber (10), so that the second core (12) of the equivalent fiber can function as a pump cladding surrounding the first core (2), according to any one of claims 24 to 27.

29. The optical amplifier according to claim 28, characterized by pulse energy, wherein the intensity and arrangement of the perturbation (29) are such that the pulse energy is higher when the optical fiber (10) is pumped by a pump source than when the optical amplifier uses an equivalent optical fiber.

30. The optical amplifier according to claim 28 or 29, characterized by a gain peak (2321) and a gain peak wavelength (2317), wherein the intensity and arrangement of the perturbation (29) are such that when the optical fiber (10) is pumped by a pump source, the gain peak wavelength is longer than when the optical amplifier uses an equivalent optical fiber.

31. The optical amplifier according to claim 30, wherein when the optical amplifier uses the optical fiber (10), the gain peak (2321) has a higher gain than when the optical amplifier uses an equivalent optical fiber.

32. The optical amplification device according to any one of claims 24 to 31, comprising a plurality of pumps (141), wherein one of the pumps (141) emits pump radiation at a different pump wavelength than the other pumps (141).

33. The optical amplification apparatus according to any one of claims 24 to 32, wherein the active dopant (9) has a ytterbium ion and has a gain peak of 1060 nm.

34. The optical amplification apparatus according to any one of claims 24 to 32, wherein the active dopant (9) has erbium and the gain peak wavelength is in the range of 1555 nm to 1650 nm.

35. The optical amplification apparatus according to any one of claims 24 to 32, wherein the active dopant (9) has holmium and the gain peak wavelength is in the range of 1990 nm to 2150 nm.

36. The optical amplification device according to any one of claims 24 to 32, wherein the active dopant (9) has thulium and the gain peak wavelength is in the range of 1900 nm to 2100 nm.

37. The optical amplification device according to any one of claims 30 to 36, wherein the difference between the signal wavelength (2316) and the gain peak wavelength (2317) is less than 10 nm, preferably less than 5 nm, and more preferably less than 1 nm.

38. The optical amplification device according to any one of claims 24 to 37, wherein the pump radiation exceeding 1 kW, preferably exceeding 2 kW, and more preferably exceeding 3 kW is guided by the second core (12).

39. An optical amplifier according to any one of claims 24 to 38, comprising a seed source (231) connected to the first core (2) to enable signal energy to be guided by the first core (2) and amplified along the optical fiber (10).

40. The optical amplification apparatus according to claim 39, wherein the seed source (231) is a pulsed laser.

41. The optical amplifier according to claim 40, wherein the seed source (231) is capable of emitting signal pulses (2319) having a pulse width of 100 ps to 10 ms, and the optical amplifier is configured to emit individual pulse energies greater than 50 mJ, preferably greater than 100 mJ, and more preferably greater than 200 mJ.

42. The optical amplification apparatus according to any one of claims 24 to 41, further comprising an optical feedback arrangement configured to promote light generation within the optical fiber (10) to generate a laser.

43. The optical amplification device according to claim 42, further comprising at least one reflecting device configured to reflect light energy back to the first core (2).

44. The optical amplification device according to claim 42 or 43, further comprising an optical switch (2314) connected to the first core (2).

45. A method for providing light emission, The present invention provides an optical fiber (10) having at least one first core (2) and at least one second core (12), wherein the first core (2) has at least one active dopant (9), the first core (2) and the second core (12) are separated by a first cladding (3), and the first cladding (3) has a first refractive index (5) that is less than the refractive index (4) of the first core (2) and less than the refractive index (14) of the second core (12), First propagation constant β 1 Pump radiation is propagated along the second core (12) in the form of a first optical mode (101) having, Select the intensity and arrangement of at least one perturbation (29), The optical fiber (10) is perturbed by at least one of the perturbations (29), and the pump radiation from the second core (12) is propagated by the second propagation constant β 2 It has and is coupled to a second optical mode (102) that overlaps with the active dopant (9), The activated dopant (9) absorbs the pump radiation, One or more signal modes (31) are propagated along the first core (2), The signal mode (31) is amplified by the activated dopant (9) through stimulated release. This includes outputting the amplified signal mode (31) in the form of light emission, If the optical fiber (10) is composed of a straight or uniform radius of curvature, the first optical mode (101) can propagate along the second core (12) but is isolated from the active dopant (9). A method characterized by the following features.

46. The method according to claim 45, comprising the step of controlling at least one of the inversion rate of the active dopant along the optical fiber, the optical gain characteristics of the optical fiber, and the thermal load of the optical fiber.

47. The aforementioned perturbation (29) is the spatial frequency component Λ = (β 1 -β 2 The method according to claim 45 or 46, wherein the property is ) / 2π.

48. The method according to any one of claims 45 to 47, wherein the perturbation (29) has at least one spatial frequency component that couples together at least several of the first optical modes (101).

49. The method according to any one of claims 45 to 48, wherein the perturbation (29) has a spatial frequency other than the spatial frequency that couples the signal mode (31) together.

50. The method according to any one of claims 45 to 49, wherein the perturbation (29) comprises at least one of a change in radius of curvature, a change in bending direction, compressive force, long-period grating, a change in the diameter of the optical fiber (10), a change in the cross-sectional shape of the optical fiber (10), rotation of the optical fiber (10), and a change in the material composition of the optical fiber (10) along the length of the optical fiber (10).

51. The method according to any one of claims 45 to 50, comprising the step of providing the signal mode (31) by coupling the signal emission from a seed laser to the first core (2).

52. The method according to any one of claims 45 to 51, comprising the step of providing an optical feedback arrangement for promoting light generation within the optical fiber (10) to generate a laser.

53. The method according to claim 52, wherein the optical feedback arrangement has at least one reflector configured to reflect light energy back to the first core (2).

54. The method according to claim 53, wherein the reflector has an optical fiber Bregg grating.

55. The method according to any one of claims 45 to 54, comprising the step of providing an optical switch (2314) and connecting the optical switch to the first core (2).

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