Optical fiber and optical fiber device

Abstract

An optical fiber (10) comprising at least one first core (2) and at least one second core (12) wherein: the first core (2) comprises at least one active dopant (9); the first core (2) and the second core (12) are separated by a first cladding (3); the first cladding (3) has a first refractive index (5) which is less than the refractive index (4) of the first core (2) and less than the refractive index (14) of the second core (12); the optical fiber (10) is characterized in that: when the optical fiber (10) is configured in a straight line or with a uniform bend radius, a first optical mode (101) having a first propagation constant can propagate along the second core (12), but the first optical mode (101) is isolated from the active dopant (9); when the optical fiber (10) is disturbed (29), the first optical mode (101) can be coupled to a second optical mode (102) having a second propagation constant, and the second optical mode (102) overlaps the active dopant (9); the first optical mode (101) can be selectively coupled to the second optical mode (102) with one or more of the perturbations (29) by propagating pumping radiation along the second core (12) in the first optical mode (101), and using the active dopant (9) to absorb the pump radiation that has been selectively coupled to the second optical mode (102) to control at least one of a population inversion fraction of the active dopant (9) along the optical fiber (10), an optical gain characteristic of the optical fiber (10), and a thermal load on the optical fiber (10), the active dopant (9) is capable of amplifying, by stimulated emission, one or more signal patterns (31) guided by the first core (2).

Description

Technical Field

[0001] This invention relates to an optical fiber and an optical fiber device. The invention can be applied to increasing the pulse energy output by a fiber laser. The invention can be applied to lasers used for cleaning, cutting, and welding. Background Technology

[0002] Industrial fiber lasers have important applications in cleaning, marking, cutting, scribing, welding, laser sintering of metal powders (a process often referred to as additive manufacturing or 3D printing), and other industrial materials processing. These lasers are used in multiple industries, including consumer electronics, medical devices, automotive, and aerospace. These lasers can be pulsed or continuous wave.

[0003] High pulse energy requires optical fibers with high saturation energy. This necessitates a large core size and a low emission cross-section at the emission wavelength. Furthermore, ideally, the laser's gain peak should be as close as possible to the emission wavelength to avoid parasitic amplification of spontaneous emission and parasitic lasing.

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

[0005] When increasing the core size to achieve higher pulse energies, the absorptivity of the pump radiation in the core tends to increase. High pump absorptivity implies a high population inversion near the pump injection point, causing the laser's gain peak to shift towards shorter wavelengths with potentially larger emission cross-sections. This can lead to parasitic amplified spontaneous emission, parasitic lasing, and may also reduce the maximum output pulse energy of the laser. Furthermore, higher population inversion increases the tendency for photodarkening in the core, a known cause of laser performance degradation. A higher pump absorptivity can also increase the thermal load on the fiber near the pump injection point, potentially damaging the fiber. Typical coatings used on optical fibers have a finite temperature range; storage or operation above this range will degrade the coating. Even with measures to maximize heat dissipation (such as placing the fiber in a metal recess on a cooling plate), a high pump absorptivity may still exceed this temperature range.

[0006] For at least the reasons mentioned above, as the fiber core size increases, it is desirable to reduce the pump absorption rate per unit length of the fiber. Known methods exist to achieve this, but all have significant drawbacks.

[0007] The most direct method is to increase the cladding size to maintain the ratio of cladding area to core area. However, once the core size exceeds a certain value, it becomes impossible to simultaneously maintain a sufficiently small fiber diameter (to ensure the fiber is flexible enough to be packaged and coiled into a spool). A 2mm diameter glass fiber is closer to a glass rod than an optical fiber.

[0008] Pump absorption can also be reduced by decreasing the concentration of the active dopant. However, this does not change the fractional inversion of the active dopant, which is determined solely by the pump intensity and absorption cross-section. Since the fractional inversion determines the emission characteristics, this solution cannot achieve the key advantage of shifting the fiber gain peak towards the emission wavelength.

[0009] Pump absorption can be reduced by using different pump wavelengths with lower pump absorption cross-sections. However, this approach is generally not convenient. For example, shifting to shorter pump wavelengths increases quantum defect (the energy difference between pump photons and signal photons), reduces laser efficiency, and thus increases the heat generated by the laser at the same output power. Efficient pump sources may not exist at the desired wavelength, and even if they could be achieved, they would significantly increase complexity and cost (such as cascaded pumping schemes for pumping fiber lasers near 1 micrometer). Cost and power efficiency are critical parameters for industrial laser systems.

[0010] There is a need for optical fibers and optical fiber devices that can adjust the pump absorption rate through means other than fiber size, doping concentration or pump wavelength. Summary of the Invention

[0011] According to a non-limiting embodiment of the present invention, an optical fiber is provided, the optical fiber comprising at least one first core and at least one second core, wherein:

[0012] The first fiber core includes at least one active dopant;

[0013] • The first fiber core and the second fiber core are separated by the first cladding;

[0014] • The first cladding has a first refractive index, which is less than the refractive index of the first fiber core and less than the refractive index of the second fiber core;

[0015] The characteristics of optical fiber are:

[0016] • When an optical fiber is configured as a straight line or has a uniform bending radius, it has a first propagation constant. The first optical mode can propagate along the second fiber core, but the first optical mode is isolated from the active dopant;

[0017] When the optical fiber is disturbed, the first optical mode can be coupled to a mode with a second propagation constant. The second optical mode, and the second optical mode overlaps with the active dopant;

[0018] This enables control of at least one of the following: the population inversion fraction of the active dopant along the fiber, the optical gain characteristics of the fiber, and the thermal load on the fiber, by propagating pump radiation along the second fiber core in a first optical mode, selectively coupling the first optical mode to a second optical mode using one or more perturbations, and absorbing the pump radiation that has been selectively coupled to the second optical mode using an active dopant. The active dopant is capable of amplifying one or more signal modes guided by the first fiber core through stimulated emission.

[0019] Optical gain characteristics can be the wavelength dependence of optical gain, or the variation in the amplitude or wavelength distribution of amplified spontaneous emission.

[0020] Optical fibers can be used in amplifying optical devices, such as fiber lasers or optical amplifiers, where pump radiation is coupled to a second fiber core. The first cladding, along with perturbations, allows for the design control of the proportion of pump radiation overlapping with the active dopant in the first core as a function of the fiber length. This, in turn, enables the control of the laser's population inversion fraction, increases the pulse energy of emitted laser pulses, reduces amplified spontaneous emission from the laser, and decreases the thermal load on the fiber. It also provides the ability to tune the pump absorption coefficient as a function of the fiber distance, for example, reducing the absorption coefficient near the pump launch point where the pump intensity is highest, thereby better distributing the thermal load.

[0021] Disturbances may include spatial frequency components .

[0022] The disturbance may include at least one spatial frequency component that couples at least some of the first optical modes to each other.

[0023] Disturbances may include spatial frequencies other than those spatial frequencies that couple signal modes to each other.

[0024] The disturbance may include at least one of the following: a change in bending radius, a change in bending orientation, extrusion pressure, a 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.

[0025] The active dopant can be selected from the group consisting of ytterbium, erbium, neodymium, praseodymium, thulium, samarium, holmium, dysprosium, transition metals, or semiconductors. The active dopant can be erbium co-doped with ytterbium.

[0026] Due to the presence of the first cladding, the first optical mode propagating along the second fiber core does not overlap with the first fiber core. The first cladding acts as a barrier, allowing only the second optical mode (not the first optical mode) to overlap with the first fiber core. For light initially propagating in a certain first optical mode to overlap with the first fiber core, it must first be coupled to a certain second optical mode through a perturbation. The power ratio of inter-mode coupling per unit length is greater for stronger perturbations than for weaker perturbations. Examples of stronger perturbations include large changes in bending radius, large changes in bending orientation, and strong squeezing forces. The first cladding acts as a barrier layer restricting the coupling between the first and second optical modes unless the fiber is perturbed by perturbations including the following spatial frequency components:

[0027] .

[0028] Therefore, the coupling amount can be controlled by selecting one or more of the following: the intensity of the disturbance, the number of disturbances, and the distribution of the disturbances along the fiber.

[0029] It is important to note that increasing the pulse energy output of the laser requires increasing the diameter of the first fiber core. If the first cladding is absent, the size of the second fiber core also needs to be increased. When the required pulse energy exceeds 10 millijoules, the total diameter of the fiber rapidly exceeds 1 millimeter, making it difficult to encapsulate, cut, splice, and dissipate heat. The first cladding, which separates the first and second fiber cores, reduces the effective brightness of the pump radiation overlapping with the second fiber core. In other words, the same amount of pump radiation propagates along the smaller second fiber core, but depending on the intensity, quantity, and distribution of the disturbance, the proportion of pump radiation overlapping with the active dopants in the first fiber core can be reduced.

[0030] Light rays with an angle smaller than the minimum divergence angle to the fiber axis are confined within the second core (guided by the second core) and resist the first cladding; while some light rays with larger divergence angles can propagate along the second core, the first cladding, and the first core, and can overlap with and be absorbed by the active dopant. Therefore, in an fiber with a first cladding, the absorption rate of pump radiation can be lower than in an fiber without a first cladding. Including a first cladding allows the fiber diameter to be smaller than that of an equivalent fiber providing the same output power and pulse energy. This also allows the fiber to operate at lower temperatures, thereby improving the reliability of the fiber coating.

[0031] The highly divergent pump radiation overlapping with the first fiber core can be absorbed by the active dopant, thus reducing the amount of pump radiation overlapping with the active dopant in the first fiber core. The second fiber core can still guide a significant amount of pump radiation, but the first cladding prevents this pump radiation from overlapping with and being absorbed by the active dopant. Therefore, compared to optical fibers without a first cladding, the pumping effect of pump radiation on the active dopant can be suppressed.

[0032] To maintain sufficient absorption along the fiber length, a mechanism is needed to supplement the higher divergent pump radiation. This supplementation can be achieved by applying perturbations to the fiber at multiple locations along its length, which convert the lower divergent pump radiation guided by the second core into higher divergent pump radiation that can overlap with the active dopant. Perturbations can be achieved in various ways, including: changes in bending radius or bending orientation, applying compression or stress to the fiber (e.g., using a mechanical grating), changes in fiber diameter, long-period gratings, fiber rotation, changes in fiber shape (e.g., localized polishing of the second core using a carbon dioxide laser), or changes in the material composition of the second core along the fiber length.

[0033] The presence of the first cladding suppresses the overlap of pump radiation with the first core, and the design perturbation of the fiber controlslably increases this overlap. This combination allows for the design selection of the pump absorption rate along the fiber. The pump absorption rate can be more than two times lower than that without the first cladding, preferably more than three times lower, and more preferably more than five times lower. By designing the strength of the perturbation, the rate of pump radiation absorbed per unit length of the fiber can be independently controlled in different segments. Stronger perturbations (e.g., tighter bends, stronger compression, stronger long-period gratings) couple more pump radiation, thereby increasing the absorption rate of pump radiation in subsequent segments of the fiber.

[0034] The ability to control the amount of pump radiation coupled from the second core to the first core allows for the storage of large amounts of energy in the active dopant without generating excessive amplified spontaneous emission and parasitic lasing. This is because, by preventing the pump radiation from coupling out of the second core, the pump absorption rate along the fiber can be reduced, allowing for the use of higher intensity pump radiation and longer fiber lengths. As shown below, this invention enables nanosecond-level laser sources with pulse energies exceeding 100 millijoules. Such lasers are difficult to design without extremely large diameter fibers; and because fiber stiffness is highly dependent on diameter, extremely large diameter fibers require very large bending radii to prevent breakage, thus making them unsuitable for packaging in appropriately sized modules.

[0035] An optical fiber may include multiple first cores.

[0036] An optical fiber may include multiple second cores.

[0037] The second core can be non-circular.

[0038] The optical fiber may include a second cladding. The second cladding may surround a second core. The second core may have a second refractive index, which is less than a first refractive index.

[0039] The second cladding layer can be non-circular.

[0040] The first and second fiber cores may be surrounded by a second cladding.

[0041] 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 fiber using chemical, thermal, or mechanical methods. Advantageously, this allows the first core to be fused to the signal input fiber, and the second core to be fused to the pump fiber.

[0042] The optical fiber may include a pump fiber that is optically contacted along its length with a second core. The second core may surround the first core. The second core and the pump fiber may be surrounded by a second cladding.

[0043] The second cladding can be a polymer. A characteristic of optical fibers is that the pump fiber and the second core can be separated from each other.

[0044] At least one of the first fiber core and the second fiber core may be a toroidal fiber core.

[0045] The width of the first fiber core can be greater than 50 micrometers, preferably greater than 100 micrometers, and more preferably greater than 250 micrometers.

[0046] The width of the first cladding layer can be greater than 5 micrometers, preferably greater than 10 micrometers, and more preferably greater than 15 micrometers.

[0047] The width of the second fiber core can be greater than 50 micrometers, preferably greater than 100 micrometers, and more preferably greater than 150 micrometers.

[0048] The refractive index of the second fiber core may be greater than the first refractive index of the first cladding by more than 0.001, preferably more than 0.004, and more preferably more than 0.01.

[0049] The refractive index of the first fiber core can be greater than that of the second fiber core. The refractive index of the first fiber core can be more than 0.015 greater than that of the first cladding.

[0050] The active dopant can be uniformly distributed within the first fiber core. Alternatively, the concentration of the active dopant in the central region of the first fiber core can be higher than that in its edge regions. This improves the beam quality of stimulated laser radiation emitted from the optical fiber.

[0051] The present invention also provides an optical fiber spool comprising the optical fiber of the present invention. The optical fiber spool may include multiple perturbations.

[0052] Disturbances can include changes in the bending radius.

[0053] The disturbance may include a long-period grating.

[0054] The optical fiber can be placed inside the groove.

[0055] The width of the second core of the optical fiber can be less than 1 mm, preferably less than 0.9 mm, and more preferably less than 0.8 mm.

[0056] The fiber optic spool may be characterized by having a minimum bending radius of less than 20 mm, preferably less than 15 mm, and more preferably less than 10 mm.

[0057] The fiber optic spool may 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.

[0058] The present invention also provides an amplifying optical device for outputting a signal at the transmit signal wavelength, the amplifying optical device comprising the fiber spool of the present invention and at least one pump source. The pump source may be connected to a second fiber core to allow pump radiation from the pump source to be guided by the second fiber core and coupled to an active dopant via a perturbation. The output signal may include stimulated emission from the active dopant. Stimulated emission is preferably guided by a first fiber core. More than one pump source may be provided, which may emit at the same wavelength or at different wavelengths.

[0059] The optical fiber is characterized by having a temperature higher than that of the substrate. The intensity and distribution of the disturbance are such that when the optical fiber is pumped by a pump source, the temperature rise is less than 100°C, preferably less than 70°C, and more preferably less than 50°C.

[0060] The magnifying optical device may have a population inversion fraction of at least 70%, less than 10%, preferably less than 8%, and more preferably equal to 7% of the active dopant along the length of the optical fiber.

[0061] The first fiber core is characterized by having an absorption length at the pump wavelength. The first fiber core has a first core area, and the second fiber core has a second core area. The optical fiber is characterized by having a core area ratio equal to the sum of the first and second core areas divided by the first core area. The length of the optical fiber is at least twice as long as the product of the absorption length and the core area ratio, preferably at least three times, and more preferably at least five times.

[0062] The length of the optical fiber may be at least twice the length of the equivalent optical fiber, which does not have a first cladding and may optionally be used with an amplifying optical device while absorbing the same amount of pump radiation along its length; wherein, in the equivalent optical fiber, the first cladding of the optical fiber is replaced by a region having the same refractive index as the second core of the optical fiber, thereby enabling the second core of the equivalent optical fiber to act as a pump cladding surrounding the first core.

[0063] The characteristic of an amplifying optical device may lie in its pulse energy. The intensity and distribution of the disturbance can satisfy the following: when the optical fiber is pumped by a pump source, the pulse energy is higher than the pulse energy when the amplifying optical device uses an equivalent optical fiber.

[0064] The characteristics of an amplifying optical device can be found in its peak gain and peak gain wavelength. The intensity and distribution of the perturbation can satisfy the following: when the fiber is pumped by a pump source, the peak gain wavelength is longer than the peak gain wavelength when the amplifying optical device uses an equivalent fiber. When the amplifying optical device uses this fiber, the gain at this peak gain can be higher than the gain when it uses an equivalent fiber.

[0065] The magnifying optical device may include more than one pump source. One of the pump sources may emit pump radiation at a different pump wavelength than the other pump source.

[0066] The active dopant may include ytterbium ions, and the gain peak may be located at 1060 nanometers.

[0067] The active dopant may include erbium or erbium co-doped with ytterbium, and the gain peak wavelength may be in the range of 1555 nm to 1650 nm.

[0068] The active dopant may contain holmium, and the gain peak wavelength may be in the range of 1990 nm to 2150 nm. The amplifying optics may include more than one pump source. One pump source may emit pump radiation at 915 nm. Another pump source may emit pump radiation at 976 nm.

[0069] The active dopant (9) may include thulium, and the gain peak wavelength may be in the range of 1900 nm to 2100 nm. Advantageously, thulium ions can be pumped at 793 nm.

[0070] The difference between the signal wavelength and the peak gain wavelength can be less than 10 nanometers, preferably less than 5 nanometers, and more preferably less than 1 nanometer.

[0071] The magnifying optics can be configured such that pump radiation exceeding 1 kW, preferably exceeding 2 kW, and more preferably exceeding 3 kW is guided by a second fiber core.

[0072] The amplifying optical device may include a seed laser connected to the first fiber core to allow signal energy to be guided from the first fiber core and amplified along the optical fiber.

[0073] The seed laser can be a pulsed laser.

[0074] The seed laser can emit signal pulses with pulse widths between 100 picoseconds and 10 milliseconds, and the amplifying optics are configured to emit single pulse energies greater than 50 millijoules, preferably greater than 100 millijoules, and more preferably greater than 200 millijoules.

[0075] The amplifying optical device may include an optical feedback structure configured to promote light generation within the optical fiber to produce a laser.

[0076] The magnifying optical device may include at least one reflecting device configured to reflect light energy back to the first fiber core.

[0077] The magnifying optical device may include an optical switch connected to the first fiber core.

[0078] The present invention also provides a method for providing optical radiation, the method comprising:

[0079] • Provide an optical fiber comprising at least one first core and at least one second core, wherein the first core comprises 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, the first refractive index being less than the refractive index of the first core and less than the refractive index of the second core;

[0080] • Pump radiation with a first propagation constant The first optical mode propagates along the second fiber core;

[0081] • Select the intensity and distribution of at least one disturbance;

[0082] • The optical fiber is perturbed using at least one perturbation to couple pump radiation from the second fiber core to a second optical mode having a second propagation constant, and the second optical mode overlaps with the active dopant;

[0083] • Utilize active dopants to absorb pump radiation;

[0084] • Propagate one or more signal patterns along the first fiber core;

[0085] • Amplify signal modes via stimulated emission using active dopants;

[0086] • Output the amplified signal pattern in the form of optical radiation;

[0087] The method is characterized by:

[0088] • When the optical fiber is configured to be straight or configured with a uniform bending radius, the first optical mode can propagate along the second fiber core, but the first optical mode is isolated from the active dopant.

[0089] The method may include the following steps: controlling at least one of the population inversion fraction of the active dopant along the optical fiber, the optical gain characteristics of the optical fiber, and the thermal load on the optical fiber. This can be achieved by selecting the intensity and distribution of the perturbation.

[0090] The disturbance includes spatial frequency components. .

[0091] The disturbance includes at least one spatial frequency component that couples at least some of the first optical modes to each other.

[0092] The disturbance includes spatial frequencies other than those spatial frequencies that couple signal modes to each other.

[0093] The disturbance includes at least one of the following: a change in bending radius, a change in bending orientation, extrusion pressure, a 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.

[0094] The method may include the following steps: providing a signal mode by radiating a signal from a seed laser to a first fiber core.

[0095] The method may include the following steps: providing an optical feedback structure to facilitate light generation within an optical fiber, thereby generating a laser. The optical feedback structure may include at least one reflecting device configured to reflect optical energy back to a first fiber core. The reflecting device may include a fiber Bragg grating.

[0096] The method may include the following steps: providing an optical switch and connecting the optical switch to a first fiber core.

[0097] The method of the present invention may include one or more steps required for utilizing the above-described optional aspects of the optical fiber, optical fiber spool, or magnifying optical device of the present invention. Attached Figure Description

[0098] Embodiments of the present invention are described below by way of example and with reference to the accompanying drawings, wherein:

[0099] Figure 1 An optical fiber according to the present invention is shown;

[0100] Figure 2 The light is shown to be coupled out from the second fiber core, absorbed by the active dopant, and emitted as light transmitted by the first fiber core;

[0101] Figure 3 This demonstrates multiple optical modes guided by optical fibers;

[0102] Figure 4 An optical fiber with a non-circular second core is shown;

[0103] Figure 5 This illustrates an optical fiber with a non-circular second cladding.

[0104] Figure 6 An optical fiber with four second cores disposed in the second cladding is shown;

[0105] Figure 7 An optical fiber with four second cores disposed in the first cladding is shown;

[0106] Figure 8An optical fiber with four first cores and three second cores is shown, all of which are disposed in a first cladding.

[0107] Figure 9 An optical fiber is shown having three first cores disposed in a second core, each first core being surrounded by a first cladding;

[0108] Figure 10 An optical fiber is shown, wherein a second core is surrounded by a first cladding, and the first cladding is surrounded by the first core;

[0109] Figure 11 An optical fiber is shown, wherein a first cladding surrounds a first core, and wherein the first core and the first cladding can be separated from the second core by removing a second cladding and stretching.

[0110] Figure 12 An optical fiber is shown, wherein a first cladding surrounds a second core, and wherein the first core can be separated from the second core and the first cladding by removing the second cladding and stretching.

[0111] Figure 13 An optical fiber is shown, comprising a separate pump fiber, wherein the pump fiber can be separated from a second core by removing the second cladding and stretching.

[0112] Figure 14 An optical amplifier is shown, comprising an optical fiber according to the invention, the optical fiber being end-pumped, and the amplifier being integrated as a power amplifier into a master oscillator power amplifier;

[0113] Figure 15 The diagram shows a bundle of pump fibers and a signal fiber inside a capillary that forms part of the output combiner.

[0114] Figure 16 Showing a kind of compatible Figure 15 Fiber bundle fusion splicer fiber;

[0115] Figure 17 This illustrates a planar helical fiber spool configured with straight edges and rounded corners;

[0116] Figure 18 The optical fiber of the present invention is shown wound on a winding mold;

[0117] Figure 19 The optical fiber illustrating the present invention provides a perturbation via a long-period grating that couples pump radiation from a mode guided by a second core to other modes that overlap with the active dopant.

[0118] Figure 20 A laser is shown, the laser comprising: Figure 4 Fiber optic cables;

[0119] Figure 21 A laser is shown, the laser comprising: Figure 13 Fiber optic cables;

[0120] Figure 22 A master oscillator power amplifier is shown, the amplifier comprising: Figure 11 Fiber optic cables;

[0121] Figure 23 A master oscillator power amplifier is shown, which includes two amplifiers and Figure 14 Amplifier;

[0122] Figure 24 A Q-switched laser is shown, which includes Figure 14 Amplifier;

[0123] Figure 25 The curves showing the absorption and emission cross sections of ytterbium ions as a function of wavelength are shown in the ytterbium-doped optical fibers used in Examples 1-6.

[0124] Figure 26 and Figure 27 The curves showing the population inversion fraction as a function of fiber length are shown in Examples 1-6.

[0125] Figure 28 and Figure 29 The following are examples 1-6, showing the pump absorption rate as a function of fiber length.

[0126] Figure 30 The magnified spontaneous emission spectra are shown in Examples 1, 3, 4, 5, and 6;

[0127] Figure 31 The optical fiber used in Example 1 is shown;

[0128] Figure 32 An equivalent optical fiber that does not conform to the present invention is shown, which does not have a first cladding. Detailed Implementation

[0129] Figure 1 An optical fiber 10 is shown, the optical fiber comprising at least one first core 2 and at least one second core 12, wherein:

[0130] • The first fiber core 2 includes at least one active dopant 9;

[0131] • The first fiber core 2 and the second fiber core 12 are separated by the first cladding 3;

[0132] The first cladding layer 3 has a first refractive index 5, which is less than the refractive index 4 of the first core 2 and less than the refractive index 14 of the second core 12; and

[0133] The characteristics of fiber optic 10 are:

[0134] • When the optical fiber 10 is configured as a straight line or has a uniform bending radius, it has a first propagation constant. The first optical mode 101 can propagate along the second fiber core 12, but the first optical mode 101 is isolated from the active dopant 9; and

[0135] • When the optical fiber 10 is disturbed 29 (reference) Figure 2 As shown), the first optical mode 101 can be coupled to a second optical mode 102 having a second propagation constant, and the second optical mode 102 overlaps with the active dopant 9;

[0136] This allows control over at least one of the following: the population inversion fraction of the active dopant 9 along the optical fiber 10, the optical gain characteristics of the optical fiber 10, and the thermal load on the optical fiber 10, by propagating pump radiation along the second fiber core 12 in the first optical mode 101, selectively coupling the first optical mode 101 to the second optical mode 102 using one or more of the perturbations 29, and absorbing the pump radiation that has been selectively coupled to the second optical mode 102 by the active dopant 9. The active dopant 9 is capable of amplifying one or more signal modes 31 guided by the first fiber core 2 by stimulated emission.

[0137] The coupling from the first optical mode 101 to the second optical mode 102 can be direct or indirect. An example of indirect coupling is that perturbation 29 couples the first optical mode 101 to a skew mode that does not overlap with the active dopant 9. Additional coupling (achieved through one perturbation, another perturbation, or via an asymmetric structure designed in the fiber 10) can couple this skew mode to the second optical mode 102 that overlaps with the active dopant 9.

[0138] The disturbance 29 may include at least one of the following: a change in bending radius, a change in bending orientation, extrusion pressure, 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 10 along its length.

[0139] The active dopant 9 may be selected from the group consisting of ytterbium, erbium, neodymium, praseodymium, thulium, samarium, holmium, and dysprosium. Alternatively or additionally, the active dopant may be a transition metal or a semiconductor. More than one active dopant 9 may be selected. For example, the active dopant 9 may include erbium co-doped with ytterbium.

[0140] Optical gain characteristics can be the wavelength dependence of optical gain, or the variation in the amplitude or wavelength distribution of amplified spontaneous emission.

[0141] In rare-earth-doped fiber amplifiers, the higher and lower energy levels of the active dopant 9 comprise a large number of subtly different and strongly coupled levels, thus optical amplification can be achieved without reaching the conventional population inversion of greater than 50%. For longer wavelengths where emission is typically much stronger than absorption, gain can be achieved even at low excitation levels. Based on this, when modeling the fiber, the population inversion fraction N—the ratio of the number of ions or atoms in the active dopant 9 in excited states to those in lower energy states—is used. It is the population inversion fraction that allows optical amplification via stimulated emission; the higher the population inversion fraction, the higher the optical gain. A higher population inversion fraction also leads to a higher level of spontaneous emission when the active dopant 9 relaxes to lower energy states by emitting photons. The combination of high optical gain and high level of spontaneous emission results in a higher level of amplified spontaneous emission (ASE); this is generally undesirable because it reduces the amount of gain available for the signal, and ASE is typically an unwanted output signal.

[0142] The optical gain g in an optical amplifier is determined by the population inversion fraction and the emission and absorption cross sections (which are wavelength-dependent).

[0143]

[0144] in, Let N be the doping concentration of the active dopant, and N be the population inversion fraction of the active dopant. and These are the emission cross section and absorption cross section at the emission wavelength, respectively.

[0145] When energy is extracted from the amplifier, the population inversion fraction decreases, and therefore the gain decreases accordingly. (The saturation energy of the laser gain medium...) This refers to the pulse energy of the incident signal pulse, which causes the gain to drop to its initial value. .

[0146]

[0147] For high-multimode fibers, A represents the core area containing active dopant 9, and The energy of the photon is determined by the wavelength of the emitted light.

[0148] It is generally believed that the highest possible pulse energy that can be extracted from an optical amplifier (for extremely long pulses) is about 10 times the saturation energy; however, for pulses with a pulse width of 100-1000 nanoseconds, the actual limit is closer to 3 or 4 times the saturation energy.

[0149] For ytterbium-doped fiber, the emission cross section at 1030 nm Approximately three times larger than at 1065 nanometers; and at these two wavelengths, the absorption cross-section... All are much smaller than the launch cross-section This means that for a signal at 1065 nanometers, its saturation energy... This is approximately three times larger than a 1030 nm signal. Therefore, if maximizing pulse energy is important, it is desirable to operate using a 1065 nm signal.

[0150] The peak gain wavelength is the wavelength at which light achieves the highest gain when passing through an amplifier. From the formula for gain g, it can be deduced that the peak gain wavelength depends on both the cross-section and population inversion; therefore, the peak gain wavelength can be controlled by adjusting the pump absorption rate along the fiber. Population inversion varies along the length of the amplifier and also varies over time. Therefore, the peak gain wavelength typically refers to the wavelength corresponding to the time-averaged gain obtained by integrating along the entire length of the amplifier.

[0151] Amplified spontaneous emission (ASE) is a wide-bandwidth light generated by random spontaneous emission within an amplifier. Spontaneous emission achieves optical gain in the same way as deliberately injected light signals. Because the initial input is broadband and the gain varies with wavelength, the ASE spectrum at the amplifier output typically reaches its maximum at or near the amplifier's gain peak wavelength.

[0152] Unlike pulsed signals, ASE will always be emitted whenever the amplifier is pumped. If the signal wavelength is far from the gain peak wavelength, the gain it receives will be lower than the ASE; this leads to an increase in the proportion of ASE in the amplifier's total output. To generate pulses with maximum efficiency (and minimize the light intensity between pulses), the signal preferably has a wavelength close to the gain peak wavelength. The difference between the signal wavelength and the gain peak wavelength can be less than 10 nanometers, preferably less than 5 nanometers, and more preferably less than 1 nanometer.

[0153] Figure 1The fiber 10 shown can be used in fiber lasers or optical amplifiers, where pump radiation is coupled to a second core 12. Note that to increase the pulse energy of the laser output, the diameter 6 of the first core 2 needs to be increased. If the first cladding 3 is not present, the size of the second core 12 also needs to be increased. When the required pulse energy exceeds 10 millijoules, the total diameter of the fiber rapidly exceeds 1 millimeter, making the fiber difficult to encapsulate, cut, splice, and dissipate heat. The first cladding 3, which separates the first core 2 from the second core 12, reduces the effective brightness of the pump radiation propagating along the second core 12. That is, the same amount of pump radiation propagates along a smaller second core 12, but depending on the perturbation 29, the proportion of pump radiation overlapping with the active dopant 9 can be reduced. Therefore, the absorption rate of the pump radiation can be lower. The inclusion of the first cladding 3 allows the fiber 10 to have a smaller diameter than the equivalent fiber providing the same output power and pulse energy. This also allows the fiber 10 to operate at lower temperatures, thereby improving the reliability of the fiber coating.

[0154] The ability to control the pump radiation coupled from the second fiber core 12 to the first fiber core 2 enables the storage of large amounts of energy in the active dopant 9 without generating excessive amplified spontaneous emission and parasitic lasing. As shown below, this invention enables nanosecond-level laser sources with pulse energies exceeding 100 millijoules. Such lasers are difficult to design without using extremely large diameter optical fibers; and because fiber stiffness is highly dependent on diameter, extremely large diameter fibers require very large bending radii to prevent fiber 10 breakage, thus making them unsuitable for encapsulation in appropriately sized modules.

[0155] The optical fiber 10 may include a second cladding 13 having a second refractive index 15, which is less than the refractive index 14 of the second core 12. The second refractive index 15 is preferably less 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 also be air or vacuum; in waveguide theory, both air and vacuum have a refractive index of 1 and can both be considered as cladding. Without the second cladding 13, optical modes propagating in the second core 12 may leak.

[0156] The first fiber core 2 may form part of a signal waveguide, which may include additional fiber cores and cladding. The first fiber core 2 may be positioned along or off-axis of the central axis of the fiber 10. The second fiber core 12 may form part of a pump waveguide, which may include additional fiber cores and cladding.

[0157] The optical fiber 10 may also include an optional third cladding 18 having a third refractive index 19. The third cladding 18 may be a glass or polymer coating. The third cladding 18 may also be air or vacuum; in waveguide theory, the refractive index of air and vacuum is 1, and both can be considered as cladding.

[0158] The first refractive index 5 can be greater than the second refractive index 15. The first refractive index 5 can be less than the second refractive index 15 and greater than the third refractive index 19.

[0159] The optical modes of multimode waveguides can be modeled using light rays. Figure 2 A first optical mode 101 is shown, represented by a ray 21 propagating along the second core 12 of the optical fiber 10. The ray 21 is characterized by the angle between it and the local longitudinal axis 20 of the optical fiber 10. (22). Ray 21 is a low-divergence ray that undergoes total internal reflection 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. As indicated by perturbation 29, the optical fiber 10 is perturbed. Perturbation 29 can be a change in the bending radius, an applied compressive force, or other forms of perturbation. Perturbation 29 can couple the first optical mode 101 to the second optical mode 102 represented by ray 23. Ray 23 is characterized by the angle between it and the local longitudinal axis 20 of the optical fiber 10. (24). Included angle (24) greater than the included angle (22), and its size is sufficient to prevent the light ray 23 from being totally internally reflected by the first cladding 3. Therefore, the second optical mode 102 represented by the light ray 23 is not guided by the second fiber core 12. The light ray 23 shown represents higher divergent pump radiation that passes through the first fiber core 2 and is absorbed by the active dopant 9. Subsequently, the spontaneous emission or stimulated emission generated by the active dopant 9 excites the signal mode 31 represented by the light ray 25, which is characterized by the angle between its angle and the local longitudinal axis 20 of the fiber 10. (26). The ray 25 undergoes total internal reflection at the boundary between the first fiber core 2 and the first cladding 3, and is guided by the first fiber core 2. The angle between the rays 25 propagating along the first fiber core 2 and the ray 25 is... (26) is less than the angle between the light rays 23 and the first fiber core 2. (twenty four).

[0160] Figure 3 Multiple modes that can propagate along fiber 10 are shown:

[0161] • “Signal modes” 31 are guided by the boundary between the first core 2 and the first cladding 3. The effective refractive indices 32 of the signal modes 31 are between the first refractive index 5 of the first cladding 3 and the refractive index 4 of the first core 2. As shown in the figure, there are effective index mode spacings 33 between each signal mode 31.

[0162] • The first optical mode 101, or "A modes," is guided by the boundary between the second core 12 and the first cladding 3 and the second cladding 13. The effective refractive index 35 of the first optical mode 101 is less than the refractive index 14 of the second core 12, and lies between the larger of the first refractive index 5 of the first cladding 3 and the second refractive index 15 of the second cladding 13. As shown in the figure, there is an effective refractive index mode spacing 36 between each of the first optical modes 101.

[0163] • “B modes” 37 are guided by the boundary between the second core 12 and the second cladding 13. The effective refractive index 38 of B modes 37 is less than the first refractive index 5 of the first cladding 3 and greater than the second refractive index 15 of the second cladding 13. As shown in the figure, each B mode 37 has an effective refractive index mode spacing 39 between the different modes. Some B modes 37 may be skew modes that do not overlap with the active dopant 9. Therefore, not all B modes 37 are… Figure 1 The first optical mode 101 in the middle.

[0164] • “C modes” 45 are guided by the boundary between the second cladding layer 13 and the third cladding layer 18. The effective refractive index 46 of the C modes 45 is less than the smaller of the first refractive index 5 of the first cladding layer 3 and the second refractive index 15 of the second cladding layer 13. As shown in the figure, there is an effective refractive index mode spacing 47 between each C mode 45. Some C modes 45 may be oblique modes that do not overlap with the active dopant 9.

[0165] The figure shows that the effective refractive index mode spacings of 32, 36, 39, and 47 within each mode group are uniform. However, in reality, the mode spacing within each mode group may vary to some extent.

[0166] The above description of modes means that when the effective refractive index of a mode is lower than that of the adjacent cladding, the fiber core cannot guide that mode. This phenomenon is called cut-off. When the effective refractive index is below the cut-off value, the mode cannot exist as a normal waveguide mode. However, below the cut-off value, wave propagation does not abruptly stop. Instead, many modes can transform themselves into leakage waves, which have relatively low loss and can propagate a considerable distance along the fiber core. For the purposes of this discussion, core-guided modes also include leakage modes or rays that are effectively guided by the fiber core when the fiber remains straight. When fiber 10 is perturbed in perturbation 29, these modes can couple with other optical modes in fiber 10 that overlap with the active dopant 9. "Effective guidance" means attenuation of less than 0.1 dB / m.

[0167] If the first refractive index 5 of the first cladding layer 3 is greater than the second refractive index 15 of the second cladding layer 13 ( Figure 3 (As shown in the diagram), the first optical mode 101 guided by the second fiber core 12 can be coupled to mode B 37 and / or mode C 45 via perturbation 29. Parts of mode B 37 and mode C 45 overlap with the active dopant 9 in the first fiber core 2, from which the active dopant 9 can absorb energy. The spontaneous or stimulated emission generated by the active dopant 9 can excite or amplify the signal mode 31 guided by the first fiber core 2.

[0168] When the disturbance has spatial frequency components Time (of which) Let be the wavelength of the light radiation, and and (the effective refractive indices of the coupled first and second modes, respectively), this perturbation can have a propagation constant The first mode is coupled to a mode with a propagation constant. The second mode. So, for example, if the perturbation is a sinusoidal deflection of the fiber with a period of 1 millimeter (mm), the spatial frequency will be 1000 cycles / m. At wavelength... At that time, the perturbation will couple the effective refractive index difference to 0.001 or... The mode. At each deflection point of the optical fiber, the coupled power, which is phase-consistent, will cumulatively add up along its length. Similarly, if the period of the sinusoidal deflection is 10 millimeters, then the spatial frequency is 100 cycles / meter. At wavelength... At that time, the disturbance will couple the effective refractive index difference to 0.0001 or... The pattern.

[0169] The bending stiffness of an optical fiber increases proportionally to the fourth power of its glass diameter. This means that in optical fibers with relatively large diameters, it is easier to couple modes with smaller effective refractive index differences than modes with larger differences, because the latter require perturbations with higher spatial frequencies to couple the modes. For example, for a 0.5 mm diameter fiber, achieving a sinusoidal deflection at a spatial frequency of 100 Hz is easier than achieving the same deflection at 1000 Hz. Mode coupling is extremely dependent on the fiber diameter. The optical fiber acts as a low-pass filter, filtering out high spatial frequency perturbations generated by bending forces.

[0170] like Figure 3As shown, the dependence of mode coupling on the effective refractive index mode spacing between different modes can be utilized. The mode spacing 36 between successive mode groups in the second core 12 is preferably smaller than the mode spacing 33 between successive mode groups in the first core 2. This is because it is desired that the perturbation 29 couples the light radiation of the first optical mode 101 propagating in the second core 12 to mode B 37. Part of mode B 37 overlaps with and is absorbed by the active dopant 9, which then amplifies the light radiation propagating in signal mode 31 in the first core 2 by stimulated emission. It is generally preferred that the perturbation 29 does not couple signal mode 31 to mode B 37, as this would result in loss and reduced efficiency. Therefore, it is preferable to satisfy at least one of the following conditions: (i) the spacing 33 is greater than the spacing 36; and / or (ii) the spatial frequency of the perturbation is selected to preferentially couple the first optical mode 101 to mode B 37 rather than the signal mode 31 to mode B 37.

[0171] The average spacing between the effective refractive indices of the continuous mode groups is approximately inversely proportional to the width or diameter of the fiber core. Therefore, by ensuring that the diameter 6 of the first fiber core 2 is smaller than the diameter 17 of the second fiber core 12, the mode spacing 33 between the continuous mode groups in the first fiber core 2 can be designed to be larger than the mode spacing 36 between the continuous mode groups in the second fiber core 12. Figure 1 The fiber 10 shown is an example of this. The difference in refractive index distribution between the first core 2 and the second core 12 can also be used to explain the difference in spacing that affects the effective refractive index of the mode.

[0172] The perturbation 29 can couple (or scatter) the first optical modes 101 to each other, or couple the first optical modes 101 to mode B 37, which overlaps with the active dopant 9. For coupling the first optical modes 101 to each other, the length of the perturbation 29 is preferably less than the adiabatic length, which is the minimum perturbation length required to produce negligible intermode coupling. For a typical step-index fiber with a core diameter of 660 micrometers, the adiabatic length is approximately 10 millimeters. Therefore, to achieve mode coupling between the first optical modes 101, the fiber needs to be perturbed by bending, applying stress, changing the diameter, changing the refractive index, and / or other perturbations with a longitudinal spatial frequency less than 10 millimeters. This can be achieved by mechanically bending the fiber.

[0173] The insulation length is roughly proportional to the core width. Therefore, for a typical first core 2 with a width of 170 micrometers, the insulation length is approximately 2.6 millimeters. In a core with a diameter of 660... In an optical fiber, generating a bend with a spatial frequency component of 2.6 mm is more difficult than generating a bend with a spatial frequency component of 10 mm. Therefore, the difficulty of coupling (or scattering) the first optical modes 101 propagating in the second core 12 to each other is much lower than the difficulty of coupling the signal modes 31 propagating in the first core 2 to each other.

[0174] Similarly, if the diameter of the second core is doubled from 17 to 1320... The thermal insulation length increases to approximately 20 mm, and the bending stiffness increases by 16 times. Such optical fibers will be difficult to encapsulate into spools unless the spool diameter is very large, or if the stress within the fiber can be released (e.g., by melting the glass with a flame or electric arc).

[0175] Therefore, by selecting its spatial frequency components The perturbation 29 can be designed to couple the first optical mode 101 to the second optical mode 102 to meet the following conditions:

[0176] .

[0177] However, this typically only couples the first optical mode 101, whose effective refractive index is very close to that of the second optical mode 102. Therefore, it is preferable that one or more perturbations 29 include at least one spatial frequency component that couples at least some of the first optical modes 101 to each other.

[0178] For lasers or amplifiers, the beam quality of the signal is often an important parameter. Signal mode 31, with the highest effective refractive index 32, provides higher beam quality. As discussed earlier, Figure 3 The mode spacing 33 of the intermediate signal mode 31 is greater than the mode spacing 36 of the first optical mode 101. Therefore, by designing the perturbation 29 to include spatial frequencies other than those that couple the signal modes 31 to each other, the beam quality of the signal modes 31 as they propagate along the fiber 10 can be preserved. This design parameter can be theoretically achieved as shown in Example 1, or experimentally.

[0179] Figure 2 The light ray 23 shown is a meridional ray passing through the first fiber core 2. Many optical modes include oblique rays that do not pass through the first fiber core 2. If the fiber core is off-center, oblique or meridional rays may also not pass through the first fiber core 2. If oblique rays cannot couple to the light passing through the first fiber core 2, they will not overlap with or be absorbed by the active dopant 9. Therefore, it is preferable that at least one of the first cladding 3, the second fiber core 12, and the second cladding 13 is non-circular to facilitate coupling between oblique rays and light overlapping with the active dopant 9.

[0180] The second fiber core 12 can be a regular polygon or an irregular polygon, such as... Figure 4The optical fiber 40 shown has an octagonal second core 12. This polygon can be a Lurau polygon. The optical fiber 40 may include an optional third cladding 18. The second core 12 may have a D-shaped cross-section with one plane, or it may have more than one plane. Alternatively or additionally, the second cladding 13 may be non-circular. The second cladding 13 may be a regular polygon or an irregular polygon, such as... Figure 5 The fiber 50 shown has an octagonal second cladding 13. This polygon can also be a Luró polygon. The second cladding 13 can have a D-shaped cross-section with one plane or more than one plane. Removing the circular symmetry of the second core 12 and / or the second cladding 13 can increase coupling between different first optical modes 101 (AA mode coupling) and between different B modes 37 (if present) (BB mode coupling) (e.g., in the case where the second core 12 is non-circularly symmetrical), and can also increase coupling between different C modes 45 (if present) (CC mode coupling) (e.g., in the case where the second cladding 13 is non-circularly symmetrical). Removing the circular symmetry can increase coupling with the active dopant 9, thereby improving the efficiency of amplifying optical devices (such as optical amplifiers or lasers) made from the fiber 10.

[0181] Figure 6 An optical fiber 60 is shown having four second cores 12. These four second cores 12 are located within the same second cladding 13. A first core 2 is surrounded by a first cladding 3, which in turn is surrounded by a second cladding 13. A second refractive index 15 of the second cladding 13 is shown to be greater than a 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. Having more than one second core 12 helps to retain the brightness of the pump radiation along the optical fiber 60. A second optical mode 102 is guided by the boundary between the second cladding 13 and the third cladding 18.

[0182] Figure 7 An optical fiber 70 is shown having four second cores 12 within a first cladding 3. The first cores 12 are surrounded by a cladding 71, which in turn is surrounded by the first cladding 3. The refractive index 72 of the cladding 71 is higher than the first refractive index 5 of the first cladding 3. Having more than one second core 12 helps retain the brightness of the pump radiation along the fiber 70. A second optical mode 102 is guided by the boundary between the first cladding 3 and the third cladding 18.

[0183] Figure 8 An optical fiber 80 is shown, having four first cores 2 and three second cores 12, each surrounded by a first cladding 3. A second cladding 13 surrounds the first cladding 3. A first refractive index 5 is greater than a second refractive index 15. A second optical mode 102 is guided by the boundary between the first cladding 3 and the second cladding 13.

[0184] Figure 9 An optical fiber 90 is shown having three first cores 2, each of which is surrounded by a first cladding 3. These three first claddings 3 are surrounded by second cores 12. The second cores 12 are surrounded by second claddings 13. The first refractive index 5 of the first cladding 3 is greater than the second refractive index 15 of the second cladding 13. A second optical mode 102 is guided by the boundary between the second cores 12 and the second claddings 13.

[0185] Figure 10 An optical fiber 100 is shown, wherein a first core 2 is a ring core surrounding a second core 12. The first core 2 and the second core 12 are separated by a first cladding 3. A second optical mode 102 is guided by the boundary between the first core 2 and the second cladding 13.

[0186] Figure 11 An optical fiber 110 is shown, wherein a first cladding 3 and a second core 12 are surrounded by a second cladding 13. The second core 12 is preferably in optical contact with the first cladding 3 along its length. The second core 12 is shown as circular, but alternatively may be non-circular. The cross-sectional area of ​​the second core 12 may be larger than that of the first core 2, such that the effective refractive index spacing 36 of the first optical mode 101 can be smaller than a reference spacing. Figure 3 The effective refractive index spacing 32 of the signal mode 31 is shown. A larger cross-sectional area also enables higher optical power from the first core 2. The second cladding 13 can be a polymer. By removing the second cladding 13, the first core 2 and the second core 12 can be separated from each other. This allows one end of the second core 12 to be separated from the first core 2 and connected to a pump source.

[0187] Figure 12 An optical fiber 120 is shown, which is connected to... Figure 11 The fiber 110 is similar, except that the first cladding 3 is arranged around the second core 12.

[0188] Figure 13 An optical fiber 130 is shown, comprising a pump fiber 131 preferably in optical contact with the second core 12 along its length. The pump fiber 131 and a 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 with a cross-sectional area larger than that of the second core 12. The pump fiber 131 shown is circular, but alternatively it may be non-circular. By removing the second cladding 13, the pump fiber 131 and the second core 12 can be separated from each other. This allows one end of the pump fiber 131 to be separated from the second core 12 and connected to a pump source.

[0189] The light radiation injected into the pump fiber 131 can propagate along the pump fiber 131 in mode 133, where the effective refractive index 134 of mode 133 is greater than the first refractive index 5 of the first cladding 3. Mode 133 can be coupled along its length to a first optical mode 101 of the second core 12. When the fiber 130 is disturbed, the first optical mode 101 guided by the second core 12 can couple to a second optical mode 102 in the fiber 130 that overlaps with the active dopant 9. The effective refractive index 135 of the second optical mode 102 is less than the first refractive index 5.

[0190] refer to Figure 1 and Figure 4-13 The first cladding 3 of the fibers 10, 40, 50, 60, 70, 80, 90, 100, 110, 120, and 130 shown acts as a barrier layer, preventing optical power coupling between the first core 2 and the second core 12. As shown, the first core 2 can be located centrally or offset from the fiber's central axis. When used in amplifying optical devices such as lasers or amplifiers, fibers 10, 40, 50, 60, 70, 80, 90, 100, 110, 120, and 130 can be compared to existing double-clad fibers that do not have the barrier layer provided by the first cladding 3. The absorption length of the double-clad fiber used in such devices is approximately equal to the absorption length of the pump radiation 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 then divided by the cross-sectional area of ​​the first core 2. By incorporating the first cladding 3, pump radiation coupling from the second core 12 guiding pump radiation to the first core 2 guiding the signal can be significantly reduced. Therefore, the length of the optical fiber 10 can be much longer, typically at least twice as long as the product of the absorption length and the area of ​​the second core 12 divided by the area of ​​the first core 2, preferably at least three times longer, and more preferably at least five times longer.

[0191] refer to Figure 2 The perturbation 29 shown couples the first optical mode 101 guided by the second core 12 to a second optical mode 102 overlapping with the active dopant 9. Pump radiation propagating along the second core 12 in 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 through stimulated emission.

[0192] The effective refractive index 135 of the second optical mode 102 is less 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, the refractive index 4 is preferably higher than the refractive index 14 of the second core 12, but it may also be lower than or equal to the refractive index 14.

[0193] Figure 1 and Figure 4-13 In the optical fibers 10, 40, 50, 60, 70, 80, 90, 100, 110, 120 and 130 shown, the diameter or width 6 of the first fiber core 2 can be greater than 50 micrometers, preferably greater than 100 micrometers, and more preferably greater than 250 micrometers.

[0194] Figure 1 and Figure 4-13 In the optical fibers 10, 40, 50, 60, 70, 80, 90, 100, 110, 120, and 130 shown, the width 7 between the first fiber core 2 and the second fiber core 12 (reference) Figure 1 The width 7 (as shown) can be greater than 5 micrometers, preferably greater than 10 micrometers, and more preferably greater than 15 micrometers. A width 7 of 10 micrometers can more effectively isolate the modes propagating along the second core 12 from other modes overlapping with the active dopant 9 compared to a width 7 of 5 micrometers. For a 1-meter-long straight fiber, the proportion of optical power propagating along its second core 12 that is coupled to other modes overlapping with the active dopant 9 is preferably no more than 5%, and more preferably no more than 1%. Preferably, compared to an equivalent fiber without the first cladding 3, the first cladding 3 reduces the overlap of optical radiation injected into the second core 12 to at least 1 / 2, preferably at least 1 / 10.

[0195] The preferred width 16 of the second core 12 can be the minimum width that allows sufficient pump light coupling to achieve the required injection power while primarily injecting light into the first optical mode 101. The width 16 of the second core 12 can be greater than 50 micrometers, preferably greater than 100 micrometers, and more preferably greater than 150 micrometers. In other cases, a smaller second core 12 may be required.

[0196] 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 more preferably more than 0.01.

[0197] The refractive index 4 of the first fiber core 2 can be greater than the refractive index 14 of the second fiber core 12. The refractive index 4 of the first fiber core 2 can be more than 0.015 greater than the first refractive index 5 of the first cladding 3.

[0198] The active dopant 9 can be uniformly distributed in the first fiber core 2. Alternatively, the concentration of the active dopant 9 at the center of the first fiber core 2 can be higher than that at its edge region, in order to improve the beam quality of stimulated laser radiation emitted from the first fiber core 2 compared to the beam quality of stimulated laser radiation emitted from an optical fiber in which the active dopant 9 is uniformly distributed in the first fiber core 2.

[0199] The second core 12 and / or the second cladding 13 may be circular or non-circular.

[0200] Fiber optic cables 80, 90, 110, 120, and 130 may include references. Figure 1 The third cladding layer 18 is shown. The third cladding layer 18 may be circular or non-circular.

[0201] Figure 1 and Figure 4-13 The optical fibers 10, 40, 50, 60, 70, 80, 90, 100, 110, 120, and 130 shown are depicted as 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.

[0202] Optical fibers 10, 40, 50, 60, 70, 80, 90, 100, 110, 120 and 130 may include silica, silicate glass, phosphate glass, or may be soft glass (such as fluoride glass or a member of the chalcogenide glass family).

[0203] Figure 14 An optical amplifier 140 is shown, comprising an optical fiber 10 and at least one pump source 141 for providing pump radiation 209 having at least one pump wavelength 2320. The pump source 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 a first core 2 to an output fiber 144, which outputs an output signal 145. An optional combiner fiber 146 connects the optical fiber 10 to the combiner 143 via a fusion 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 comprise fused silica. The output signal 145 may be collimated by a collimating lens 1416 and focused onto a material 1418 to be processed by a focusing lens 1417.

[0204] Optical amplifier 140 amplifies the input signal 1411 emitted from seed laser 1410. This input signal 1411 is input to input end 171 of optical fiber 10, amplified by optical fiber 10, and output from output end 179 of optical fiber 10. The input signal 1411 can be coupled to the first core 2 of optical fiber 10 via input fiber 149 and fusion splice 148. Seed laser 1410 can be a fiber laser, disk laser, solid-state laser, slab laser, or semiconductor laser. Seed laser 1410 can be a continuous-wave laser, Q-switched laser, master oscillator power amplifier laser, or mode-locked laser. Controller 1412 can be configured to control pump source 141, preferably synchronized with seed laser 1410. Pump source 141 can be turned on before seed laser 1410 emits input signal 1411, so as to activate active dopant 9 before input signal 1411 reaches optical fiber 10.

[0205] Output combiner 143 may include Figure 15 The fiber bundle 150 is shown. Six pump fibers 142 and one center output fiber 144 are bundled together within a capillary tube 151. The capillary tube 151 may be made of silica, may include silica, or may be fluorine-doped silica. The capillary tube 151 may also be other types of glass (such as soft glass). As shown, the capillary tube 151 may have a single hole or multiple holes. The fiber bundle 150 may be coupled to a combiner fiber 146 or directly coupled to fiber 10. Coupling is preferably performed by fusion splicing, but it can also be achieved by coupling devices such as butt splicing or imaging optics. The capillary tube 151 can be contracted by heating and applying tension to fix the pump fibers 142, and the fiber bundle can be modified to adjust the dimensions of the pump fibers 142 and the center output fiber 144 to properly match the dimensions of the fibers to be fused (which may be the combiner fiber 146 or fiber 10).

[0206] The output fiber 144 preferably includes a core 152 surrounded by a cladding 153, wherein the refractive index 158 of the core 152 is higher than the refractive index 159 of the cladding 153. The core 152 may be made of silicon dioxide or include silicon dioxide, and the cladding 153 may be fluorine-doped silicon dioxide. The inclusion of the cladding 153 isolates the core 152 from pump radiation 209 propagating along the pump fiber 142 within the output combiner 143.

[0207] Alternatively or additionally, the pump fiber 142 may include a core 155 surrounded by an optional cladding 156, wherein the refractive index 1510 of the core 155 is higher than the refractive index 1511 of the cladding 156. The core 155 may be made of or include silicon dioxide, and the cladding 156 may be fluorine-doped silicon dioxide. The inclusion of the cladding 156 isolates the core 152 of the output fiber 144 from the pump radiation 209 propagating along the pump fiber 142. The cladding 156 also isolates 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.

[0208] The combiner fiber 146 can be used for Figure 16 The bundler fiber 160 shown includes a central core 161 surrounded by a ring core 162. The central core 161 is preferably surrounded by an inner cladding 163, the refractive index 167 of which is less than the refractive index 165 of the central core 161 and the refractive index 166 of the ring core 162. This selection allows pump radiation 209 coupled from the pump fiber 142 to the ring core 162 to be isolated from the central core 161, thereby coupling only to the second core 12 of the fiber 10 and not to the first core 2 of the fiber 10. The width 1611 of the inner cladding 163 may be equal to or greater than... Figure 1The width 7 of the first cladding 3 is shown. The annular core 162 may be surrounded by an outer cladding 164, the refractive index 168 of which is less than the refractive index 166 of the annular core 162. The outer cladding 164 may be glass or a polymer coating. The central core 161 may be made of silicon dioxide or include silicon dioxide, and the inner cladding 163 may be fluorine-doped silicon dioxide.

[0209] Preferably, the pump power 209 injected into the pump fiber 142 is primarily coupled to the second core 12 of the fiber 10. The higher the proportion of the pump power 209 coupled to the second core 12 relative to the total pump power coupled to the fiber 10, the less pump power is absorbed by the active dopant 9 in the first core 2 before the perturbation 29. This is highly advantageous because the absorbed pump power generates heat, which, if causing an undesirable temperature rise in the fiber 10, needs to be removed. To maximize this proportion:

[0210] · Figure 15 The diameter 157 of the core 155 of the pump fiber 142 shown is preferably equal to or less than 157. Figure 16 The width of the annular fiber core 162 shown is 169;

[0211] The width 169 of the annular fiber core 162 is preferably equal to or less than 169. Figure 1 The width of the second fiber core 12 shown is 16;

[0212] • The centerline of each fiber core is preferably aligned with its outer diameter to prevent light radiation from coupling to the cladding region surrounding the fiber core;

[0213] Each fiber core receiving pump radiation 209 preferably has a numerical aperture equal to or greater than the numerical aperture of the pump radiation 209 emitted from its preceding fiber core. The numerical aperture of the pump radiation 209 can be controlled by the injection conditions into the optical fiber. Alternatively or additionally, it can also be controlled by selecting the numerical aperture of the fiber core relative to its cladding.

[0214] Preferably, the signal radiation emitted from the first core 2 of optical fiber 10 is coupled as efficiently as possible to the center core 161 (if present) of the combiner fiber 160 and the core 152 of the output fiber 144. To maximize this efficiency:

[0215] · Figure 1 The diameter 6 of the first fiber core 2 shown is preferably equal to or less than 6. Figure 16 The diameter 1610 of the central core 161 (if present) of the combiner fiber 160 shown;

[0216] The diameter 1610 of the central core 161 of the combiner fiber 160 is preferably equal to or less than that of the fiber itself. Figure 15 The diameter of the core 152 of the output optical fiber 144 shown is 154.

[0217] • The centerline and outer diameter of each fiber core should be aligned to prevent signal radiation from coupling to the cladding region around the fiber core;

[0218] Each fiber core receiving signal radiation has a numerical aperture equal to or greater than the numerical aperture of the signal radiation emitted from its preceding fiber core. The numerical aperture of the signal radiation can be controlled by the injection conditions into the corresponding fiber. Alternatively, or additionally, it can also be controlled by selecting the numerical aperture of the corresponding fiber core relative to its cladding.

[0219] Figure 1 The signal waveguide shown, including the first fiber core 2 and the first cladding 3, can be multimode. The input fiber 149 can be a single-mode fiber or a multimode fiber operating in fundamental mode. The fusion splice 148 may include a variable diameter section. A cladding mode stripper (not shown) may be included in the fusion splice 148, the input fiber 149, or the seed laser 1410. This structure reduces the problem of damage to the seed laser 1410 caused by amplification of light reflected from the workpiece by the amplifier 140, because the input fiber 149 and / or the cladding mode stripper act as a mode filter, reducing the back-reflected radiation energy reaching the seed laser 1410. It should be noted that the variable diameter section may increase the divergence of light propagating along the fiber core. Therefore, care should be taken to avoid excessive coupling of light radiation from the fiber core due to the variable diameter, which could introduce undesirable thermal loads on the fiber 10 during operation.

[0220] Figure 14 The optical amplifier 140 shown employs reverse pumping, meaning pump radiation 209 is injected into the output terminal 179, which is the end of the optical fiber 10 that transmits the output signal 145. Reverse pumping typically offers the advantage of increased output power and pulse energy. The optical amplifier 140 may also employ forward pumping (i.e., pump radiation 209 is injected into the input terminal 171 of the optical fiber 10), or a combination of reverse and forward pumping. The second fiber core 12 can guide pump radiation 209 exceeding 1 kW, preferably exceeding 2 kW, and more preferably exceeding 3 kW.

[0221] Figure 14 The optical fiber 10 shown can be configured as follows: Figure 17 The spindle is shown in the form of 170. An optical fiber 10 is inserted into a helical groove 173 of a substrate 175. The groove 173 may include sections with a constant or varying radius of curvature. The bending transition length of the optical fiber 10 can be controlled by making the width 174 of the groove 173 greater than the width or diameter 1710 of the optical fiber 10.

[0222] The substrate 175 can serve as a heat sink, allowing for air cooling via a fan or water cooling via channels within the substrate 175. The groove 173 has a width 174, which can be designed to force the optical fiber 10 to follow a desired trajectory to optimize mode coupling. The greater the difference between the width 174 of the groove 173 and the width 1710 of the optical fiber 10, the less constrained the optical fiber 10 is to follow the curvature of the groove 173. When the optical fiber 10 is placed within the groove 173, it relaxes to a shape of minimum stress. This allows an optical fiber located within a wider groove to experience a change in its own radius of curvature over a longer distance when encountering a change in the groove's radius of curvature; that is, it creates a longer effective transition length or a perturbation 29 with a lower spatial frequency. The width 174 can be 25 to 150 micrometers larger than the width 1710. Other widths of 174 are also possible.

[0223] The optimal radius of curvature and transition region can be determined experimentally, i.e., by changing the perturbation 29 of fiber 10, adjusting the difference between the width 1710 of fiber 10 and the width 174 of groove 173, and measuring the cross-coupling between different mode groups. Alternatively, optimization can also be achieved through theoretical calculations as described in Example 1.

[0224] The spool 170 can be in the form of a rounded regular polygon or a rounded irregular polygon, where the radius of curvature of its side 176 is greater than the radius of curvature at corner 177. Side 176 can be a straight side. For example, it can be a rounded polygon with 3 to 12 sides and arc-shaped corners. This rounded polygon can be a rounded rectangle, a rounded square, a hyperellipse, or a square-round shape. To simplify manufacturing, Figure 17 The spool 170 shown is a rounded square with straight sides 176 and rounded corners 177. Each rounded corner 177 has the same length and radius and is staggered to form a helical structure. At the transition section 178 between each side 176 and each corner 177 of the fiber 10, pump radiation can be coupled from the second core 12 to the mode or light absorbed by the active dopant 9. By providing multiple transition sections 178, pump radiation 209 can be coupled in a controllable manner along the length of the fiber 10.

[0225] Figure 3 The mode coupling of the first optical mode 101 to mode B 37 is caused by the transition section 178 between different curvature radii in the fiber 10. The sharper the transition, the higher the spatial frequency component of the disturbance 29. The sharpness of the transition is related to the rate of curvature change of the fiber 10 along the length of the transition section, which is limited by the glass diameter 8 of the fiber 10 and therefore by the fiber stiffness. Figure 1 The glass diameter 8 in the diagram represents the diameter of the second cladding 13. If the third cladding 18 is glass and non-polymer, then the glass diameter 8 represents the diameter or width of the third cladding 18. Similarly, if the second cladding 13 is polymer, then the glass diameter 8 represents the diameter or width 17 of the second core 12.

[0226] refer to Figure 17 Preferably, the curvature radii of fiber 10 at angle 177 and transition section 178 are selected to achieve Figure 3 Coupling between the first optical mode 101 and mode B 37 is avoided, while coupling between signal modes 31 and mode B 37 is prevented. This objective is achievable because... Figure 1 and Figure 3 The effective mode spacing 33 of the signal mode 31 in the fiber 10 is greater than the effective mode spacing 36 of the first optical mode 101. The perturbation of the coupled signal mode 31 has a higher spatial frequency (sharper transition region) than the perturbation of the coupled first optical mode 101. Therefore, by avoiding these higher spatial frequencies, the first optical modes 101 can be coupled to each other and to mode B 37, while minimizing the coupling of signal mode 31 to other signal modes 31 and to mode B 37. Thus, at each transition segment 178, the power in the first optical mode 101 can be scattered from the second core 12 into the mode overlapping with the active dopant 9.

[0227] Alternatively, the curvature radii of fiber 10 at angle 177 and transition section 178 can be selected to prevent coupling between signal modes 31, as such coupling may reduce [signal quality / performance]. Figure 14 The beam quality of the output signal 145. This may be advantageous for certain dicing and micro-welding applications. Alternatively, the radius of curvature of the fiber 10 at angle 177 and transition section 178 may be selected to cause coupling between signal modes 37, generating a multimode output signal 145 with relatively poor beam quality. This may be advantageous for certain welding or cleaning applications. Preferably, the mode coupling between signal modes 31 is less than the mode coupling between the first optical modes 101.

[0228] The optical amplification process (absorption and stimulated emission) generates heat. The output end 179 of the spool 170 (i.e., the end of the optical fiber 10 that transmits the output signal 145) preferably has no abrupt transitions between fiber segments with different bending radii, thus resulting in lower mode coupling compared to the rounded corner 177 and transition section 178. The optical fiber 10 is characterized by a temperature higher than that of the substrate 175, which preferably acts as a heat sink. The intensity and distribution of the disturbance 29 preferably ensure that when the optical fiber 10 is pumped by the pump source 141, the temperature rise is less than 100°C, preferably less than 70°C, and more preferably less than 50°C.

[0229] The radius of curvature at each corner 177 of the spool 170 and / or the transition segment 178 may be the same. Alternatively, the radius of curvature at at least one corner of the spool 170 may be different from the other corners. The latter may be advantageous for achieving coupling between different mode groups.

[0230] The radius of curvature of fiber 10 at its output end 179 (i.e., the fiber end connected to output fiber 144) can be greater than its radius of curvature at its input end 171 (i.e., the fiber end connected to seed laser 1410). This may be beneficial in regions with higher signal power, reducing coupling between the first optical mode 101 and the second optical mode 102, as well as subsequent absorption by the active dopant 9. Alternatively or additionally, it can increase coupling in regions with lower signal power.

[0231] In the spool 170 shown in the figure, the optical fiber 10 is constrained by a helical groove 173. Alternatively or additionally, the optical fiber 10 may be constrained by adhesive and / or bosses, pins or studs to achieve the desired bending transition.

[0232] Figure 18 A spool 180 is shown, wherein an optical fiber 10 is wound on a winding die 181 having straight edges and rounded corners. The shape of the winding die 181 is preferably a rounded polygon. The shape of the spool 180 may be similar to that of a reference spool. Figure 17 The various spool shapes are similar in design.

[0233] The optical fiber 10 used in spool 170 or spool 180 has a glass diameter 8 that is less than 1 mm, preferably less than 0.9 mm, and more preferably less than 0.8 mm.

[0234] The spool 170 or spool 180 may be characterized in that the minimum bending radius at the angle 177 and the transition section 178 is less than 20 mm, preferably less than 15 mm, and more preferably less than 10 mm.

[0235] The spool 170 or spool 180 may be characterized in that its cross-sectional width 172 is less than 500 mm, preferably less than 400 mm, and more preferably less than 300 mm. Spools made of optical fibers excluding the first cladding 3 and having equivalent optical properties to spool 170 typically require the use of optical fibers with a glass diameter 8 larger.

[0236] The optical fiber 10 in spool 170 or spool 180 can be respectively in Figure 4-13 The fiber optic cables shown are 40, 50, 60, 70, 80, 90, 100, 110, 120, or 130.

[0237] Figure 19 The perturbation 29 is shown in the form of a long-period grating 190, which is written into the reference. Figure 10In the second core 12 of the optical fiber 100, a long-period grating 190 has regions 191 whose refractive index changes upon ultraviolet radiation. The spatial frequency of region 191 is equal to 1 / pitch 192, chosen to couple the mode guided by the second core 12 to other modes in the optical fiber 100 that overlap with the active dopant 9 in the first core 2. The modes guided by the second core 12 couple or scatter to each other through the long-period grating 190, and to other modes in the optical fiber 100. Pitch 192 is preferably equal to the pump wavelength. Divide by the effective mode spacing 36. This structure is suitable for optical fibers such as fiber 100: in such fibers, the effective mode spacing 33 between the signal modes 31 guided by the first core 2 is smaller than the effective mode spacing 36 between the first optical modes 101 guided by the second core 12. This is because, in such fibers, if a structure such as... Figure 17 The bending and bending transition methods shown make it more difficult to couple the first optical modes 101 to each other without coupling the signal modes 31 to each other.

[0238] Long-period gratings can also be used in optical fibers 10, 40, 50, 60, 70, 80, 90, 110, 120 and 130 to couple a first optical mode 101 guided by a second core 12 to a second optical mode 102 that overlaps with the active dopant 9.

[0239] Other forms of long-period gratings may also be used, including those based on long-period gratings formed by applying periodic bending or pressure to the fiber 100.

[0240] Figure 20 An end-pumped fiber laser 200 is shown, comprising an optical fiber 40, an optional combiner fiber 160 located between the optical fiber 40 and a corresponding output combiner 143, an optional input fiber 204, and an optional output fiber 144. Reflectors 201 and 202 are located at both ends of the optical fiber 40 to form a laser cavity 203. Reflectors 201 and 202 are preferably fiber Bragg gratings, which can be incorporated into the combiner fiber 160. Alternatively, fiber Bragg gratings can also be incorporated into the optical fiber 40, or into the input fiber 204 and the output fiber 144.

[0241] Pump source 141 via pump fiber 142, reference Figure 14-16 The described output combiner 143 and optional combiner fiber 160 are coupled to the second core 12 of fiber 40. (Reference) Figure 16 The optional combiner fiber 160 preferably has a ring core 162, the shape and size of which are the same as the second core 12 of the fiber 40. Alternatively, the ring core 162 may be circular and smaller than the second core 12 to reduce splicing requirements.

[0242] Pump source 141 may include a laser diode, a laser diode module including multiple laser diodes, or a laser diode strip. Pump source 141 may include one or more fiber lasers. Pump source 141 may include multiple pump sources 207, the outputs of which are combined by pump combiner 208. Pump source 207 may be a single laser diode, a laser diode module each including multiple laser diodes, a laser diode strip, or a fiber laser. Pump combiner 208 may be a fused fiber pump combiner. Multiple pump sources 207 may emit pump radiation 209 having the same pump wavelength 2320. Alternatively, different pump sources 207 may emit pump radiation 209 with different pump wavelengths 2320. If the pump absorption rate of the active dopant 9 is different at different wavelengths, the ability to select different pump wavelengths 2320 may be advantageous.

[0243] Optical fiber 40 is perturbed by one or more perturbation pairs 29 to couple pump radiation 209 emitted by pump source 141 from second fiber core 12 to active dopant 9. The configuration of optical fiber 40 can be compared with reference to... Figure 17-19 The optical fiber 10 is the same as or similar to the optical fiber 40. The optical fiber 40 can be replaced with the following respectively (refer to...). Figure 1 and Figure 5-10 The optical fiber is 10, 50, 60, 70, 80, 90 or 100.

[0244] The fiber laser 200 can be connected to an optional beam transmission fiber 206, which can be terminated by an end cap 1415. A collimating lens 1416 and a focusing lens 1417 can also be provided to focus the output signal 145 onto the material to be processed 1418.

[0245] Figure 21 A side-pumped laser 210 is shown, which includes a reference Figure 13 The optical fiber 130 has had its second cladding 13 removed from both ends, and the pump fiber 131 and the second core 12 have been separated from each other. Pump radiation 209 from at least one pump source 141 is coupled into the pump fiber 131 via optical fiber 142. The pump radiation 209 is coupled into the second core 12 and then coupled into the active dopant 9 within the first core 2 via perturbation 29. Reflectors 201 and 202 may be dichroic mirrors or gratings configured to promote light generation within the optical fiber 130 by reflecting optical energy back into the first core 2. Reflectors 201 and 202 are preferably fiber Bragg gratings. The generated output signal 145 is output via an optional beam transmission fiber 206. Alternatively or additionally, the laser 210 may also be end-pumped, for example by setting a reference... Figure 14 The output combiner 143 couples the pump radiation 209 into the second fiber core 12.

[0246] Figure 22 A side-pumped laser 220 is shown, which is in the form of a master oscillator power amplifier, compared with a reference... Figure 14 The laser described is similar. The side-pumped laser 220 includes a reference... Figure 11 The optical fiber 110. A second core 12 is connected to at least one pump source 141. The first core 2 is connected to a seed laser 1410 via an optical fiber 221. The input signal 1411 emitted by the seed laser 1410 is amplified by the active dopant 9 within the first core 2, and the generated output signal 145 is output through a beam transmission optical fiber 206. The optical fiber 110 can be replaced with a reference fiber. Figure 11 and Figure 13 The optical fiber 110 or 130. If optical fiber 130 is used instead of optical fiber 110, then the second core 12 of optical fiber 130 can be a reference. Figure 14 The end-face pumping method described above can also be a side-face pumping method.

[0247] The fiber lasers 200, 210, and 220 shown in the figure all employ a combination of reverse and forward pumping. If fiber lasers 200, 210, and 220 were to use only reverse pumping, higher pulse energies might be achievable. Only one amplification stage is shown in the figure, but lasers can include multiple amplification stages.

[0248] Figure 23 Showing will Figure 14 The amplifier 140 is integrated as a power amplifier into the master oscillator power amplifier (MOPA) 230. The MOPA 230 includes at least one seed source 231 for providing seed radiation 2315, and a first preamplifier 232 for amplifying the seed radiation 2315. A second preamplifier 2313 may also be provided.

[0249] Optical isolators 233 can be provided 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. These optical isolators 233 protect the seed source 231 and the first and second preamplifiers 232 and 2313 from the reverse transmission radiation 2310 emitted by the amplifier 140.

[0250] An optical isolator 233 can also be installed at the output of amplifier 140 to attenuate light radiation reflected or emitted from the workpiece.

[0251] Seed source 231 may include a Fabry-Perot semiconductor laser.

[0252] Seed source 231 may include a superradiative diode.

[0253] Seed source 231 is capable of emitting signal pulses 2319 with pulse widths between 100 picoseconds and 10 milliseconds, and the amplifying optics can be configured to emit single pulse energies greater than 50 millijoules, preferably greater than 100 millijoules, and more preferably greater than 200 millijoules. It is believed that for pulses with pulse widths between 100 picoseconds and 1000 nanoseconds, achievable pulse energies can reach and exceed 1 joule.

[0254] Reflector 2311 can be configured to reflect a portion of seed radiation 2315 back to seed source 231. This helps prevent the generation of random pulses in MOPA 230, which could cause catastrophic damage to MOPA. Reflector 2311 can be a fiber Bragg grating.

[0255] Seed radiation 2315 may include light pulses 2319 characterized by a signal wavelength 2316. Amplifier 140 may have an optical gain 2318 that varies with wavelength, and the optical gain has a gain peak 2321 at a gain peak wavelength 2317. Seed source 231 may be selected such that the difference between signal wavelength 2316 and gain peak wavelength 2317 is within 10 nanometers, preferably within 2 nanometers, and more preferably within 1 nanometer.

[0256] MOPA 230 may include a depolarizer 2312 between seed source 231 and first preamplifier 232. This depolarizer 2312 may be a Lyot depolarizer. A Lyot depolarizer may include two polarization-maintaining fiber segments fused together, one segment being twice the length of the other. In practice, it has been found that when using a single seed source 231, the depolarizer can disrupt the polarization state of the seed radiation before it enters the first preamplifier 232, thereby increasing the peak power of the output signal 145 before nonlinear effects such as stimulated Brillouin scattering have an adverse effect. When using two or more seed sources 231, the polarization state can be disrupted by making the input polarization states of the signals from the two seed sources 231 at an angle to each other when they enter the coupler 234. This angle may be 45 degrees or 90 degrees.

[0257] MOPA 230 may include at least two seed sources 231. The outputs of these seed sources 231 can be combined via coupler 234. The seed sources 231 may have the same signal wavelength 2316. Coupler 234 may be a polarization combiner. The seed sources 231 may have different signal wavelengths 2316. Coupler 234 may be a wavelength division multiplexer. Coupler 234 is preferably a fused fiber coupler. Alternatively or additionally, the optical fiber 10 may include a plurality of first fiber cores 2, and the output of each seed source 231 may be coupled to a different first fiber core 2. Reference Figure 8 and Figure 9This type of optical fiber has already been described. A structure having more than one first fiber core 2 may be advantageous for certain welding applications that require imaging more than one laser spot onto a workpiece.

[0258] MOPA 230 may include an optical switch 2314. This optical switch 2314 may be an acousto-optic modulator. The optical switch 2314 can be used to reduce amplified spontaneous emission (ASE) emitted from the first preamplifier 232 between pulses 2319, thereby reducing ASE amplified by amplifier 140. The optical switch 2314 can also be used to adjust the shape of the pulses 2319 if the pulse shape output from the seed source 231 is not satisfactory. This may be advantageous if pulses with a slow rise edge are required, and such pulses are difficult to generate by semiconductor laser diodes.

[0259] The optical switch 2314 can also be placed after the second preamplifier 2313 or after the amplifier 140. However, the power rating of the optical switch 2314 may need to be higher, which would increase the cost.

[0260] MOPA 230 may include a visible laser diode 236 and a coupler 237, wherein the coupler 237 is configured to combine visible light radiation 238 emitted by the visible laser diode 236 with light radiation 239 emitted by the optical amplifier 140. The coupler 237 is preferably a wavelength division multiplexer. The wavelength division multiplexer may be a fused fiber coupler or a dichroic mirror.

[0261] The device may include a controller 1412 for controlling a seed source 231, a first preamplifier 232, a second preamplifier 2313 (if configured), an optical switch 2314 (if configured), and an amplifier 140. When amplifying the optical pulse 2319, it is preferable to activate the first preamplifier 232 and the second preamplifier 2313 before activating the amplifier 140 (i.e., by power supply from a pump source) to avoid damaging the amplifier 140. For example, the controller 1412 can activate the amplifier 140 before the first pulse 2319 arrives to ensure that the first pulse is amplified in the same way as subsequent pulses.

[0262] Figure 24 Showing will Figure 14 The amplifier 140 is integrated as a power amplifier into the Q-switched laser 240. The Q-switched laser 240 includes a reference... Figure 20 and Figure 21The aforementioned reflector 201, amplifier 140, Q-switch 241, and output coupler 242. The Q-switch 241 can be a free-space optical Q-switch. The light radiation emitted by the amplifier 140 can be coupled to the output coupler 242 through the collimating lens 244 and the Q-switch 241. Preferably, an end cap 1415 is provided at the end of the output fiber 144 to prevent light radiation from damaging the end face of the output fiber 144. The controller 249 synchronously controls the pumping of the fiber 10 and the activation of the Q-switch 241 to promote light generation within the fiber 10. The generated output signal 145 can be focused by the output lens 245 and output through the beam transmission fiber 206. The beam transmission fiber 206 can be a solid glass fiber, or an fiber with a microstructured core or a hollow core. The Q-switch 241 can be an acousto-optic modulator, an electro-optic switch, or, for example, a mechanical Q-switch including a rotating mirror or prism. Note that the amplifier 140 can be replaced with other amplifying optical structures (including reference optics). Figure 20-23 (The amplifier shown), and a reference can be used. Figure 1 and Figure 4-13 Any one of the optical fibers shown: 10, 40, 50, 60, 70, 80, 90, 100, 110, 120, and 130.

[0263] In the following examples, for various types of fiber optic 10, for Figure 23 The MOPA 230 shown is modeled. The MOPA 230 includes a seed source 231 and... Figure 14 Amplifier 140. The optical fiber used in Examples 1 and 3-6 is... Figure 31 Fiber 310, which is connected to Figure 4 The optical fiber 40 is similar, but does not include the third cladding 18. The second cladding 13 is a polymer.

[0264] The optical fiber used in Example 2 is an equivalent fiber 320, which is equivalent to fiber 310 but does not include the first cladding 3. The first core 2 of both fibers is identically designed, having the same refractive index 4, the same core diameter 6, and the same concentration of active dopant 9. The second core 12 of the equivalent fiber 320 extends to and optically contacts the outer circumference of the first core 2. The refractive index 14 of the second core 12 of fiber 310 is the same as that of the equivalent fiber 320. As will be explained below, when used in amplifying optical devices such as amplifiers or lasers, the fiber 310 of the present invention can be at least twice the length of the equivalent fiber while absorbing the same amount of pump radiation along its length. By selecting the intensity and distribution of the perturbation 29 on fiber 310, higher pulse energy, a longer gain peak wavelength 2317, and a higher gain at the gain peak 2321 at the signal wavelength 2316 can be obtained when using fiber 310 compared to using the equivalent fiber 320 in amplifying optical devices.

[0265] Limited to Figure 17 The bending of the optical fiber within groove 173 was modeled using the Coser rod theory, a method for modeling the bending and other deformations of slender rods. This was used to create a graph showing the relationship between the radius of curvature of optical fiber 10 and the distance along its length.

[0266] Pump absorption is modeled using the beam propagation method (BPM), which simulates the propagation of a light field within a specific refractive index structure. The radius of curvature, determined by the Cossella model, is represented by applying a gradient of the refractive index value along a cross-sectional direction of the fiber model. The absorption characteristics of the active dopant 9 on the pump radiation 209 are represented by the imaginary refractive index component (calculated based on the selected dopant concentration and the absorption cross-section of the selected dopant).

[0267] The signal gain and amplified spontaneous emission characteristics of fiber 10 are determined using a rate equation model, where pump absorption is calibrated using results from a beam propagation absorption model. In addition to the amplified spontaneous emission generated in power amplifier 140, the model also considers… Figure 23 The amplified spontaneous emissions generated in the first and second preamplifiers 232 and 2313 can be input to and amplified by amplifier 140. The amount of amplified spontaneous emissions input depends on the characteristics of preamplifiers 232 and 2313, as well as the (intentional or unintentional) wavelength filtering effect in optical isolator 233, and therefore will vary depending on the specific details of the laser system.

[0268] The thermal load of fiber 10 was calculated. The thermal load is caused by pump absorption and quantum defect. The quantum defect arises from the energy difference between the pump photon and the signal photon. A two-dimensional thermal model (representing the cross-section of the fiber and substrate 175) was used to calculate the highest temperature at the location of the highest thermal load along the fiber. 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 filler, and aluminum for substrate 175 were used. The maximum temperature rise values ​​given in the following examples refer to the point of highest temperature rise in the polyacrylate. Deterioration of the fiber coating due to heat is a known problem when fiber lasers operate at high optical power (and therefore high fiber temperatures).

[0269] BPM is also used to simulate signal perturbations in fiber 10. The gain is applied in the form of the imaginary refractive index component in the actively doped region of fiber 10 to simulate amplification. Studies have found that for a given gain and perturbation, the beam quality of the output signal 239 emitted by amplifier 140 is only slightly affected by the beam quality of the input signal 1411 input to amplifier 140.

[0270] The input signal 1411 is a multimode signal, and its field diameter matches the diameter 6 of the first core 2 of the fiber 10. The beam quality of the input signal 1411 is characterized by a beam parameter product (BPP) of 5.0. For a spool 170 with a groove width 174 of 1 mm, the beam parameter product (BPP) of the output signal 239 is approximately 5.5. In contrast, the BPP is 6.1 when the groove width 174 is 0.9 mm, and 5.0 when the groove width 174 is 1.3 mm. In comparison, the pump absorptivity is 0.5 dB / m in the spool with a groove width 174 of 1.3 mm and 1.6 dB / m in the spool with a groove width 174 of 0.9 mm, indicating that the pump absorptivity can be effectively controlled while maintaining good beam quality of the output signal 239.

[0271] Modeling based on Figure 14 The apparatus shown employs different designs and configurations of fiber optic 10. In each case, the average power of the input signal 1411 input to amplifier 140 is 20 watts.

[0272] Example 1 is a preferred embodiment, which uses an optical fiber 310 with a first cladding 3; the first cladding 3 isolates the active dopant 9 from the pump radiation 209 that is injected only into the second core 12. The active dopant 9 is a ytterbium ion. The optical fiber 310 is as follows... Figure 17 The illustrated perturbation uses a helical rounded square spool configuration, but with a very gentle perturbation at the output end 179 (the end where pump radiation 209 is injected) to minimize the coupling of pump radiation 209 with active dopant 9, thereby minimizing the thermal load on fiber 310. In Example 2, the role of the first cladding 3 and perturbation 29 is demonstrated by having the exact same configuration (fiber design, perturbation 29, and physical layout) but excluding the first cladding 3. Example 2 has extremely high absorption, necessitating a shorter fiber length. Example 3 uses the same fiber as Example 2, but with a reduced concentration of active dopant 9. Example 4 uses the exact same fiber as Example 1, but with a stronger perturbation 29 at the output end 179. Example 5 also uses the exact same fiber as Example 1, but at the output end 179, pump radiation is simultaneously injected into both the first core 2 and the second core 12. Finally, Example 6 reproduces the scheme of Example 1, but with a smaller change in the bending radius to reduce the coupling of pump radiation 209 from the second core 12.

[0273] Example 1

[0274] refer to Figure 23 Fiber 10 is Figure 31 The optical fiber 310 has a circular first core 2 with a diameter 6 of 170 micrometers. The active dopant 9 is ytterbium, and the dopant ion concentration is... The first core 2 is co-doped with phosphorus and aluminum in a certain proportion to avoid photodarkening. The second core 12 is an octagonal core with a diameter 17 of 660 micrometers. The first cladding 3 comprises fluorine-doped silicon dioxide to reduce its refractive index and has a thickness 7 of 10 micrometers. The second cladding 13 is a polymer coating with a diameter of 850 micrometers. A third cladding 18 is not provided.

[0275] The first refractive index 5 of the first cladding 3 is 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.

[0276] The second refractive index 15 of the second cladding 13 is 0.07 lower than the refractive index 14 of the second core 12.

[0277] A spool 170 is formed by inserting an optical fiber 10 into a spiral groove 173 in a substrate 175. The groove 173 employs... Figure 17 The square structure shown has rounded corners and a width of 174 mm. At corner 177 of the spindle 170, the minimum radius of curvature of the groove 173 is 100 mm, and along edge 176, the maximum radius of curvature of the groove 173 exceeds 10 mm.

[0278] The pump wavelength 2320 is 960 nm, and the signal wavelength 2316 is 1065 nm.

[0279] Optical amplifier 140 as reference Figure 14 The ground shown is pumped in reverse. Pump radiation 209 from the reference Figure 16 The combiner fiber 160 is introduced into fiber 310, and the pump radiation 209 is confined within the annular core 162. No pump radiation 209 is coupled to the central core 161. This results in all pump radiation 209 being injected into the second core 12 of fiber 310. The input signal 1411 is introduced to the other end of fiber 310 through a step-index fiber, the core size of which ensures that all input signals 1411 are injected into the first core 2 of fiber 310.

[0280] By increasing the pump power, an output signal 145 can be obtained, which includes pulses with a repetition frequency of 20 kHz, an average output power of 2458 watts, and a pulse energy of 121 millijoules. By increasing the diameter 6 of the first fiber core, pulse energies exceeding 200 millijoules can be achieved.

[0281] The average pump absorption rate along fiber 310 is 1.0 dB / m, the maximum pump absorption rate is 1.4 dB / m, and the maximum heat load is 91 W / m at an average output power of 2.4 kW.

[0282] The maximum temperature above the substrate temperature of 175°C is increased to 45°C.

[0283] The peak wavelength 2317 corresponding to the peak gain of amplifier 140 (gain 2318) is 1060 nm. The wavelength at which the amplified spontaneous emission spectrum reaches its peak is also 1060 nm. At an average output power of 2458 watts, the amplified spontaneous emission power is 41 watts, which is slightly less than 2% of the total output power.

[0284] Example 2

[0285] The fiber optic design and configuration in Example 2 are the same as in Example 1, except that fiber 10 is replaced with a reference fiber. Figure 32 The equivalent fiber 320 described above does not have a first cladding 3 used to separate the first fiber core 2 and the second fiber core 12. The first fiber core 2 has the same dimensions and design. The second fiber core 12 has the same... Figure 4 The second core 12 has the same octagonal shape as shown, but its inner diameter is equal to the outer diameter of the first core 2. Therefore, the second core 12 acts as the pump cladding of the prior art double-clad fiber. Example 2 is not part of the present invention, and is provided to illustrate the advantages of the first cladding 3 and the perturbation 29.

[0286] The absence of the first cladding 3 results in pump radiation 209 coupling to the active dopant 9 at a higher fiber length rate. The average pump absorptivity is 4.2 dB / m, and the maximum pump absorptivity is 4.6 dB / m. To maintain a total pump absorptivity of the equivalent fiber 320 similar to that achieved by fiber 310 in Example 1, the length of the equivalent fiber 320 is shortened from 15 meters to 3.9 meters.

[0287] The output signal 145 is achieved, consisting of pulses with a repetition frequency of 20 kHz, an average output power of 2371 W, and a pulse energy of 94 mJ. The peak gain wavelength 2317 is 1028 nm. At an average output power of 2.4 kW, the amplified spontaneous emission power is 460 W, equivalent to 19% of the total power. It is evident that the absence of the first cladding 3 leads to a higher pump absorption rate, resulting in pulse energy loss and a significant amount of amplified spontaneous emission (ASE), which degrades performance and poses a risk of parasitic lasing near the peak gain wavelength 2317, potentially damaging the laser.

[0288] The high pump absorption rate also results in an extremely high peak heat load of 276 W / m. The maximum temperature increase of the equivalent fiber 320 above the substrate 175 is 137°C.

[0289] Example 3

[0290] The fiber design and configuration in Example 3 are the same as in Example 2, except that the doping concentration of active dopant 9 (ytterbium ions) is lower, at 1.944 × 10⁻⁶. Example 3 is not part of the present invention because the equivalent fiber 320 does not have a first cladding 3 for separating the first core 2 from the second core 12.

[0291] The ytterbium concentration was reduced to decrease the pump absorption rate. The average pump absorption rate was 1.1 dB / m, and the maximum pump absorption rate was 1.2 dB / m. These values ​​are similar to those in Example 1. At an average output power of 2385 watts, the maximum heat load was 70 watts / m, also similar to Example 1. The maximum temperature increase above the substrate temperature of 175°C was 35°C.

[0292] It is also possible to achieve the following output signal 145: the output signal includes a pulse with a pulse repetition frequency of 20 kHz, an average output power of 2385 watts, and a pulse energy of 91 millijoules.

[0293] The peak gain wavelength 2317 corresponding to the peak gain 2318 of amplifier 140 is 1028 nm. The amplified spontaneous emission power is 498 W, equivalent to 21% of the total output power. These values ​​are similar to those in Example 2, indicating that reducing the concentration of active dopant 9 improves thermal performance but does not shift the peak gain wavelength 2317 to a suitable wavelength, nor does it improve optical performance. This is because, while reducing the doping concentration decreases the overall absorption, it does not change the population inversion fraction of the active ions; and it is the population inversion fraction that determines the emission characteristics. Therefore, the ASE level does not change significantly compared to Example 2, and is the same as in Example 2, indicating a significant degradation in laser performance. The associated high gain also poses a risk of laser damage.

[0294] Example 4

[0295] The fiber design and configuration in Example 4 are the same as in Example 1, except that the gentle bend of fiber 310 at the output end 179 (pump injection end) in Example 1 is omitted. The radius of curvature of the bending section of groove 173 is 100 mm, and there is no section with a larger radius of curvature at the output end 179.

[0296] Compared to Example 1, this change has a negligible effect on optical properties, but it leads to an increase in the peak absorptivity at the output 179, which in turn results in a moderate increase in peak thermal load and the maximum temperature of the fiber.

[0297] Example 5

[0298] The fiber design and configuration of Example 5 are the same as those of Example 1, except that the pump radiation is introduced into fiber 310 from a combiner fiber with a simple step refractive index distribution. The pump radiation 209 overlaps with the core of the combiner fiber and is coupled to the first core 2 and the second core 12 of fiber 310, with the coupling ratio being approximately proportional to its cross-sectional area.

[0299] Compared to Example 1, this change has a negligible impact on optical characteristics. The peak gain wavelength 2317 corresponding to the peak gain 2318 of amplifier 140 is 1060 nm. At an average output power of 2.4 kW, the amplified spontaneous emission power is 13 W. These values ​​are similar to those of Example 1, indicating that the extremely short segment with high absorptivity formed by the direct injection of pump radiation 209 into the first fiber core 2 has no significant impact on the amplifier's spectral performance.

[0300] However, the extremely high gain in the short fiber segment near the pump injection end significantly impacts the thermal characteristics. The maximum absorptivity is 6.1 dB / m, corresponding to a maximum heat load of 495 W / m, and a maximum temperature rise of 245°C above the substrate 175°C. Such a high temperature rise is undesirable and could lead to reliability issues.

[0301] Example 6

[0302] The fiber design and configuration of Example 6 are the same as in Example 1, except that fiber 310 is mounted in a helical groove with no abrupt change in bending radius. Over the entire 15-meter spool length, the bending radius decreases only very slowly and continuously. As a result, although the maximum absorption rate near the signal output end 179 (pump input end) of fiber 10 is similar to that in Examples 1, 3, and 4, the absorption rate drops rapidly in the rest of fiber 310 due to the lack of perturbation 29 caused by the unchanged bending radius. This results in a much lower average pump absorption rate than expected, leading to low efficiency and low output power because a large portion of the pump radiation 209 is 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.

[0303] Example 1 2 3 4 5 6 parameter First core diameter ( ) 170 170 170 170 170 170 Ytterbium doping concentration ( ) 7.775 7.775 1.944 7.775 7.775 7.775 First cladding thickness ( ) 10 none none 10 10 10 The refractive index difference between the first cladding and the second core -0.004 - - -0.004 -0.004 -0.004 Second core diameter ( ) 660 660 660 660 660 660 Second core shape octagon octagon octagon octagon octagon octagon Second cladding diameter ( ) 850 850 850 850 850 850 Second fiber diameter ( ) None none none none none none Minimum radius of curvature of the groove (mm) 100 100 100 100 100 85 Maximum radius of curvature of the groove (mm) >10000 >10000 >10000 >10000 >10000 115 Radius of curvature (mm) of the first curved section of the groove. 170 170 170 100 170 115 Groove width (mm) 1 1 1 1 1 1 Active fiber length (m) 15 3.9 15 15 15 15 Pump wavelength (nm) 960 960 960 960 960 960 Pump power (W) 3150 3150 3150 3150 3150 3150 Pump Injection Ring Ring Ring Ring round Ring Signal wavelength (nm) 1065 1065 1065 1065 1065 1065

[0304] Table 1: Examples 1-6: Design Parameters

[0305] Example 1 2 3 4 5 6 Average output power (W) 2458 2371 2385 2471 2507 1450 Pulse energy (mJ) 121 94 91 122 123 74 Average pump absorption rate (dB / m) 1.0 4.2 1.1 1.0 1.0 0.3 Maximum pump absorption rate (dB / m) 1.4 4.6 1.2 1.6 6.1 1.6 Maximum heat load (W / m) at 2 kW output 91 276 70 99 495 111 Higher than the maximum temperature rise of the radiator (°C) 45 137 35 49 245 55 Peak gain wavelength (nm) 1060 1028 1028 1060 1060 1060 ASE emission (W) at 2 kW output 41 460 498 40 42 10

[0306] Table 2: Examples 1-6: Implementation Performance

[0307] Figure 25 The diagram shows the absorption cross section 251 and emission cross section 252 of ytterbium ions as a function of wavelength 250 in the ytterbium-doped silicon phosphide glass used in the example. Absorption cross section With launch cross section It is an indicator that measures the probability of a photon being absorbed by or emitted from the active dopant 9; in Examples 1-6, the active dopant 9 is a ytterbium ion.

[0308] Figure 26-30The amplified spontaneous emission, fractional population inversion, and pump absorption rates for Examples 1-6 are shown. The calculations were performed at the moment a seed pulse from the middle of a pulse sequence with a pulse repetition frequency of 20 kHz is about to enter fibers 310 and 320. These results represent the steady-state performance of the laser and have intentionally excluded transient effects that may occur near the beginning of the pulse sequence.

[0309] Figure 26 The figures show the particle population inversion fractions 261, 262, 263, and 266 calculated for Examples 1, 2, 3, and 6, respectively, and their distances from the reference along fiber optic cables 310 and 320. Figure 14 and Figure 17 The relationship between the signal input terminal 171 and the distance 260 is shown. The particle number inversion fractions 264 and 265 of Examples 4 and 5 are shown respectively. Figure 27 These population inversion fractions are similar to the population inversion fraction 261 of Example 1, except near the signal output end 179, the end where pump radiation 209 is injected into the fiber 310. The population inversion fraction 266 of Example 6 is higher near the signal output end 179, then decreases to a steady state lower than the population inversion fraction 261 for the rest of the fiber 310. The population inversion fractions 262 and 263 corresponding to Examples 2 and 3 are significantly higher than the population inversion fraction 261 of Example 1.

[0310] Figure 28 The diagram shows the relationship between pump absorption rates 281, 282, 283, and 286 calculated for Examples 1, 2, 3, and 6, respectively, and a distance 260. The pump absorption rates 284 and 285 for Examples 4 and 5 are shown in the diagram. Figure 29 The pump absorptivity 282 in Example 2 is significantly higher than that in Example 281. The pump absorptivity 283 is designed to be similar to that in Example 281 by reducing the doping concentration of the active dopant 9. In Example 6 (unperturbed), the pump absorptivity 286 increases rapidly towards the signal output terminal 179 because the first cladding 3 isolates the pump radiation 209 propagating along the second fiber core 12 from the active dopant 9.

[0311] Figure 30 The diagram shows the relationship between amplified spontaneous emission (ASE) spectra 301, 302, 303, and 306, calculated for Examples 1, 2, 3, and 6, and wavelength 250 nm. The ASE spectra 304 and 305 of Examples 4 and 5 largely overlap 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.

[0312] refer to Figure 26 and Figure 27 , Figure 30 The ASE spectra 301, 304, 305, and 306 in the optical fiber reach peaks near the desired 1065 nm signal wavelength 2316 because, along at least the first half of fiber 310, the population inversion fractions 261, 264, 265, and 266 are all smaller than the transition population inversion fraction 267. For ytterbium dopants, the transition population inversion fraction 267 is less than 10%, preferably less than 8%, and more preferably 7%. This can be compared to population inversion fractions 262 and 263—along most of the length of fiber 320, population inversion fractions 262 and 263 exceed the transition population inversion fraction 267, and in these regions, ASE reaches its peak at 1030 nm.

[0313] Examples 1-3 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 12 to the second optical mode 102 overlapping with the active dopant 9; and this mechanism does not require changes to 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 advantageous for operation at the signal wavelength 2316, at which the emission cross section 252 is sufficiently low to achieve energy storage sufficient to generate nanosecond pulses without excessive ASE or the risk of parasitic lasing damaging the laser. For operation at 1065 nm, the population inversion fraction of the active dopant 9 is less than 10%, preferably less than 8%, and more preferably equal to 7% for at least 70% of the length of the fiber 310.

[0314] In Example 1, amplified spontaneous emission (ASE) accounts for only 2% of the total output power; while in Examples 2 and 3, which do not employ the first cladding 3, ASE accounts for 12% of the total output power. The absence of the first cladding 3 in Examples 2 and 3 reduces laser performance by decreasing pulse energy and the contrast between pulse power and inter-pulse power; and exposes the laser to a very high risk of unstable parasitic lasing at the peak wavelength of ASE.

[0315] Examples 1 and 4 illustrate the importance of the bobbin plate characteristics. Advantageously, in Example 1, the fiber 310 is arranged with a larger radius of curvature at the signal output end 179 (pump injection end); this reduces the maximum absorptivity, maximum heat load, and highest temperature compared to Example 4 (where all curved sections of the groove 173 have the same radius of curvature). The impact on pulse energy and ASE power is negligible.

[0316] Examples 1 and 5 illustrate the importance of the initial distribution of pump radiation 209. If a large amount of pump radiation is injected into the first fiber core 2, as shown in Example 5, this radiation will be partially confined within the first fiber core 2—using the same mechanism as partially preventing pump radiation from the second fiber core 12 from entering the first fiber core 2. This results in a section with extremely high pump absorption at the signal output 179. This high-absorption section persists until most of the pump radiation 209 confined within the first fiber core 2 is absorbed by the active dopant 9. Since the high-absorption region is very short (approximately 20 cm), the overall gain characteristics of the amplifier 140 are not significantly altered, and the effect on pulse energy and ASE power is negligible. However, the higher pump absorption corresponds to higher local thermal load and higher maximum temperature, both of which are undesirable as they can lead to damage to the optical coating and catastrophic damage to the fiber. Alternative fiber coatings (such as metallic coatings) can be used and / or soldered to the heat sink. Alternatively, the fiber coating can be completely removed in these higher-temperature sections.

[0317] In Example 6, the bending radius of fiber 310 changes only slowly, so there is no disturbance in the fiber that could couple the first optical mode 101 from the second core 12 to a second optical mode 102 with higher divergence, which overlaps with the first core 2 and the active dopant 9. Although the initial absorptivity of Example 6 has a suitable value (slightly higher than the target value achieved in Example 1), its total absorptivity is lower than that achieved in Example 1. This is because as the pump radiation 209 propagates outward from the signal output 179, the higher divergence pump light is absorbed by the active dopant 9, while the lower divergence pump light is still guided by the second core 12 and is therefore not absorbed by the active dopant 9; thus, the absorptivity decreases along the length of fiber 310. By the time the pump radiation 209 reaches the signal input 171, the pump absorptivity has decreased to less than 0.1 dB / m. The depletion of the highly divergent pump light results in a low overall absorption rate (well less than 10 dB) for any reasonable fiber length; this leads to inefficiency and generates a significant amount of wasted pump radiation that must be safely handled. This can be compared to Examples 1, 4, and 5, in which perturbation 29 supplements the higher divergent pump radiation overlapping with the first fiber core 2, allowing the absorption rate to be maintained (and even enhanced as needed) along the entire length of fiber 310.

[0318] In one experiment, Figure 14 The seed laser 1410 is a 20-watt TruPulse nanolaser manufactured by TRUMPF Laser UK Limited in Southampton, UK. This TruPulse nanolaser features a seed source 231 and a reference... Figure 23The first and second preamplifiers 232 and 2313 shown are therefore equivalent to the seed laser modeled in Example 1. This TruPulse nanolaser can emit various pulse shapes with different pulse widths, powers, and energies. The maximum pulse energy of the 20-watt TruPulse nanolaser is approximately 1 millijoule. (Reference) Figure 14 Fiber 10 is Figure 31 The fiber optic cable 310 has the same design as modeled in Example 1, and is in accordance with the reference... Figure 17 It is configured and mounted on substrate 175 as described in Example 1. Amplifier 140 is capable of emitting 100 millijoules of pulses with a pulse repetition frequency of 20 kHz and an average output power of 2 kW. The heat generated in optical fiber 310 can be removed by substrate 175, and the temperature rise generated by optical fiber 310 is within an acceptable range required to ensure long-term operational reliability.

[0319] The beam parameter product (BPP) of the output signal 145 measured in the experiment was 10 mm·mradians, which is greater than the BPP in Example 1. The BPP changes very little with the spool configuration. This indicates that the main source of the increase in the BPP of the output signal 145 is not intentionally induced fiber bending (macro-bending). Other possible sources of BPP increase exist, such as micro-bending (random variations in the fiber) and BPP increases caused by other optical elements such as the combiner. These results confirm that the absorptivity of the pump radiation can be controlled by controllably bending the fiber 10 without significantly reducing the signal's BPP.

[0320] For a reasonably long beam transmission optical cable 206, to avoid excessive nonlinearity in high-peak power pulses, its core size is preferably larger than the first core diameter 6 of the optical fiber 10. If this increase in core size is achieved through variable-diameter coupling or amplification using free-space optical elements, rather than through simple fusion splicing, a BPP similar to that in the optical fiber 10 can be maintained. This variable-diameter section is preferably an adiabatic variable-diameter section.

[0321] Figure 14 and Figure 20-24 The laser and amplifier in the examples, as well as the devices in Examples 1, 4, 5, and 6, are all amplifying optical devices for transmitting the output signal 145 at the signal wavelength 2316. By appropriately selecting the intensity and distribution of the first cladding 3 and the perturbation 29, it is possible to increase the available pulse energy, reduce the inter-pulse ASE level, and design the gain peak wavelength 2317 to be the optimal wavelength suitable for the selected active dopant 9 and the glass material of the fiber 10.

[0322] For example, the active dopant 9 may include ytterbium ions, and the gain peak wavelength 2317 may be 1060 nm. The active dopant 9 may include erbium or erbium co-doped with ytterbium, and the gain peak wavelength 2317 may be in the range of 1555 nm to 1650 nm. The active dopant 9 may include holmium, and the gain peak wavelength 2317 may be in the range of 1990 nm to 2150 nm. The amplifying optics may include more than one pump source. One pump source may emit pump radiation at 915 nm. Another pump source may emit pump radiation at 976 nm. The active dopant 9 may include thulium, and the gain peak wavelength 2317 may be in the range of 1900 nm to 2100 nm. Thulium ions may be pumped by a pump wavelength 2320 at 793 nm, a pumping method that enables so-called "two-in-one pumping," where each stimulated emission photon corresponds to two pump photons.

[0323] The invention described with reference to the accompanying drawings and examples can be applied in a variety of ways, including cleaning, paint removal, and rust removal; all of these applications can be achieved by a method comprising the steps of providing an optical fiber 10 according to the invention, and controlling the rate at which pump radiation is transferred from a second core 12 to a first core 2 using a perturbation 29. For such processes, a typical required BPP may be 20 or less.

[0324] It should be understood that the embodiments of the invention described above with reference to the accompanying drawings are given by way of example only, and modifications and additional components may be provided to improve performance. The various components shown in the drawings are not limited to use in the drawings shown therein, but may also be used in structures shown in other drawings and in all aspects of the invention. The invention also covers the various components mentioned and / or shown above, whether used alone or in any combination.

Claims

1. An optical fiber (10) comprising at least one first core (2) and at least one second core (12), wherein: The first fiber core (2) includes at least one active dopant (9); • The first fiber core (2) and the second fiber core (12) are separated by the first cladding (3); • The first cladding layer (3) has a first refractive index (5), which is less than the refractive index (4) of the first core (2) and less than the refractive index (14) of the second core (12). The optical fiber (10) is characterized in that: • When the optical fiber (10) is configured as a straight line or configured with a uniform bending radius, it has a first propagation constant. The first optical mode (101) can propagate along the second fiber core (12), but the first optical mode (101) is isolated from the active dopant (9); and When the optical fiber (10) is disturbed (29), the first optical mode (101) can be coupled to a mode with a second propagation constant. The second optical mode (102) overlaps with the active dopant (9); Thus, it is possible to control at least one of the following by propagating pump radiation along the second fiber core (12) in the first optical mode (101), selectively coupling the first optical mode (101) to the second optical mode (102) using one or more of the perturbations (29), and absorbing the pump radiation that has been selectively coupled to the second optical mode (102) by the active dopant (9), wherein the active dopant (9) is capable of amplifying one or more signal modes (31) guided by the first fiber core (2) by stimulated emission.

2. The optical fiber as described in claim 1, characterized in that, The disturbance (29) includes spatial frequency components. .

3. The optical fiber as described in claim 1 or 2, characterized in that, The disturbance (29) includes at least one spatial frequency component that couples at least some of the first optical modes (101) to each other.

4. The optical fiber as claimed in any one of the preceding claims, characterized in that, The disturbance (29) includes spatial frequencies other than those spatial frequencies that couple the signal modes (31) to each other.

5. The optical fiber as described in any one of the preceding claims, characterized in that, The disturbance (29) includes at least one of the following: a change in bending radius, a change in bending orientation, a squeezing 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 (10) along its length.

6. The optical fiber as claimed in any one of the preceding claims, characterized in that, The optical fiber includes a plurality of the first fiber cores (2).

7. The optical fiber as claimed in any one of the preceding claims, characterized in that, The optical fiber includes a plurality of second fiber cores (12).

8. The optical fiber as described in any one of the preceding claims, characterized in that, The second fiber core (12) is non-circular.

9. The optical fiber as claimed in any one of the preceding claims, characterized in that, It includes a second cladding (12) surrounding the second core (12), wherein the second cladding (12) has a second refractive index (15) which is less than the first refractive index (5).

10. The optical fiber as described in claim 6, characterized in that, The second cladding layer (13) is non-circular.

11. The optical fiber as described in claim 9 or 10, characterized in that, The first fiber core (2) and the second fiber core (12) are surrounded by the second cladding (13).

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

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

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

15. The optical fiber as claimed in any one of the preceding claims, characterized in that, At least one of the first fiber core (2) and the second fiber core (12) is a toroidal fiber core.

16. The optical fiber as claimed in any one of the preceding claims, characterized in that, The width (6) of the first fiber core (2) is greater than 50 micrometers, preferably greater than 100 micrometers, and more preferably greater than 250 micrometers.

17. The optical fiber as claimed in any of the preceding claims, characterized in that, The width (7) of the first cladding layer (3) is greater than 5 micrometers, preferably greater than 10 micrometers, and more preferably greater than 15 micrometers.

18. The optical fiber as claimed in any one of the preceding claims, characterized in that, The width (16) of the second fiber core (12) is greater than 50 micrometers, preferably greater than 100 micrometers, and more preferably greater than 150 micrometers.

19. An optical fiber spool (170) comprising an optical fiber (10) as described in any of the preceding claims and including a plurality of the aforementioned perturbations (29).

20. The optical fiber spool (170) as described in claim 19, characterized in that, The optical fiber (10) is disposed in the groove (173).

21. The optical fiber spool (170) as described in claim 19 or 20, characterized in that, The width (17) of the second core (12) of the optical fiber (10) is less than 1 mm, preferably less than 0.9 mm, and more preferably less than 0.8 mm.

22. The fiber optic spool (170) as claimed in any one of claims 19 to 21, wherein the fiber optic spool (115) is characterized in that: the minimum bending radius is less than 20 mm, preferably less than 15 mm, and more preferably less than 10 mm.

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

24. An amplifying optical device for an output signal (145) at a transmission signal wavelength (2316), the amplifying optical device comprising an optical fiber spool (170) as claimed in any one of claims 19 to 23 and at least one pump source (141), wherein the pump source (141) is connected to a second fiber core (12) to allow pump radiation from the pump source (141) to be guided by the second fiber core (12) and coupled to the active dopant (9) by the perturbation (29).

25. The amplifying optical device of claim 24, wherein the optical fiber (10) is characterized by a temperature rise above that of the substrate (175), and wherein the intensity and distribution of the disturbance (29) are such that when the optical fiber (10) is pumped by the pump source (141), the temperature rise is less than 100°C, preferably less than 70°C, and more preferably less than 50°C.

26. The magnifying optical device as described in claim 24 or 25, characterized in that, For at least 70% of the length of the optical fiber (10), the population inversion fraction of the active dopant (9) is less than 10%, preferably less than 8%, and more preferably equal to 7%.

27. The amplifying optical device 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 having a first core area, the second core having a second core area, and the optical fiber (10) is characterized by a core area ratio 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 twice as long as the product of the absorption length and the core area ratio, preferably at least three times as long, and more preferably at least five times as long.

28. The magnifying optical device as claimed in any one of claims 24 to 27, characterized in that, The optical fiber (10) has a fiber length at least twice that of the equivalent fiber length, the equivalent fiber having no first cladding (3) and optionally being used with the amplifying optical device, while absorbing the same amount of pump radiation along its length, wherein in the equivalent fiber, the first cladding (3) of the optical fiber (10) is replaced by a region having the same refractive index (14) as the second core (12) of the optical fiber (10), thereby enabling the second core (12) of the equivalent fiber to act as a pump cladding surrounding the first core (2).

29. The magnifying optical device as claimed in claim 28, characterized in that, The amplifying optical device has pulse energy, and the intensity and distribution of the disturbance (29) therein are such that when the optical fiber (10) is pumped by the pump source, the pulse energy is higher than the pulse energy when the amplifying optical device uses the equivalent optical fiber.

30. The magnifying optical device as claimed in claim 28 or claim 29, characterized in that, The amplifying optical device has a gain peak (2321) and a gain peak wavelength (2317), wherein the intensity and distribution of the disturbance (29) are such that when the optical fiber (10) is pumped by the pump source, the gain peak wavelength is at a longer wavelength than when the amplifying optical device uses the equivalent optical fiber.

31. The magnifying optical device as described in claim 30, characterized in that, When the amplifying optical device uses the optical fiber (10), the gain of the gain peak (2321) is higher than the gain of the amplifying optical device when using the equivalent optical fiber.

32. The magnifying optical device as described in any one of claims 24 to 31, characterized in that, It includes more than one pump source (141), and the pump wavelength of the pump radiation emitted by one of the pump sources (141) is different from the pump wavelength of the pump radiation emitted by the other of the pump sources (141).

33. The magnifying optical device as described in any one of claims 24 to 32, characterized in that, The active dopant (9) includes ytterbium ions, and the gain peak is located at 1060 nm.

34. The magnifying optical device as described in any one of claims 24 to 32, characterized in that, The active dopant (9) includes erbium, and the gain peak wavelength is in the range of 1555 nm to 1650 nm.

35. The magnifying optical device as described in any one of claims 24 to 32, characterized in that, The active dopant (9) includes holmium, and the gain peak wavelength is in the range of 1990 nm to 2150 nm.

36. The magnifying optical device as claimed in any one of claims 24 to 32, characterized in that, The active dopant (9) includes thulium, and the gain peak wavelength is in the range of 1900 nm to 2100 nm.

37. The magnifying optical device as described in any one of claims 30 to 36, characterized in that, The difference between the signal wavelength (2316) and the gain peak wavelength (2317) is less than 10 nanometers, preferably less than 5 nanometers, and more preferably less than 1 nanometer.

38. The magnifying optical device as claimed in any one of claims 24 to 37, characterized in that, The magnifying optics are configured such that pump radiation exceeding 1 kW, preferably exceeding 2 kW, and more preferably exceeding 3 kW is guided by the second fiber core (12).

39. The magnifying optical device as claimed in any one of claims 24 to 38, characterized in that, Includes a seed source (231) connected to the first fiber core (2) to allow signal energy to be guided by the first fiber core (2) and amplified along the optical fiber (10).

40. The magnifying optical device as claimed in claim 39, characterized in that, The seed source (231) is a pulsed laser.

41. The magnifying optical device as claimed in claim 40, characterized in that, The seed source (231) is capable of emitting signal pulses (2319) with a pulse width between 100 picoseconds and 10 milliseconds, and the amplifying optical device is configured to emit a single pulse energy greater than 50 millijoules, preferably greater than 100 millijoules, and more preferably greater than 200 millijoules.

42. The magnifying optical device according to any one of claims 24 to 41, characterized in that, It further includes an optical feedback structure configured to promote light generation within the optical fiber (10) to produce a laser.

43. The magnifying optical device as described in claim 42, characterized in that, It further includes at least one reflective device configured to reflect light energy back to the first fiber core (2).

44. The magnifying optical device as claimed in claim 42 or claim 43, characterized in that, It further includes an optical switch (2314) connected to the first fiber core (2).

45. A method for providing optical radiation, the method comprising: • Provide an optical fiber (10) comprising at least one first core (2) and at least one second core (12), wherein the first core (2) comprises 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) which is less than the refractive index (4) of the first core (2) and less than the refractive index (14) of the second core (12); • Pump radiation with a first propagation constant The first optical mode (101) propagates along the second fiber core (12); • Select the intensity and distribution of at least one perturbation (29); • The optical fiber (10) is perturbed using at least one perturbation (29) to couple the pump radiation from the second core (12) to a fiber having a second propagation constant. And the second optical mode (102) overlaps with the active dopant (9); • The pump radiation is absorbed by the active dopant (9); • Propagate one or more signal modes (31) along the first fiber core (2); • The signal mode (31) is amplified by stimulated emission using the active dopant (9); and • The amplified signal pattern (31) is output in the form of said optical radiation; The method is characterized by: • When the optical fiber (10) is configured as a straight line or configured with a uniform bending radius, the first optical mode (101) can propagate along the second fiber core (12) but is isolated from the active dopant (9).

46. ​​The method as described in claim 45, characterized in that, Further steps include: Control at least one of the following: the population inversion fraction of the active dopant along the optical fiber, the optical gain characteristics of the optical fiber, and the thermal load on the optical fiber.

47. The method as described in claim 45 or 46, characterized in that, The disturbance (29) includes spatial frequency components. .

48. The method according to any one of claims 45 to 47, characterized in that, The disturbance (29) includes at least one spatial frequency component that couples at least some of the first optical modes (101) to each other.

49. The method according to any one of claims 45 to 48, characterized in that, The disturbance (29) includes spatial frequencies other than those spatial frequencies that couple the signal modes (31) to each other.

50. The method according to any one of claims 45 to 49, characterized in that, The disturbance (29) includes at least one of the following: a change in bending radius, a change in bending orientation, a squeezing 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 (10) along its length.

51. The method according to any one of claims 45 to 50, characterized in that, Further steps include: The signal mode (31) is provided by coupling the signal radiation from the seed laser to the first fiber core (2).

52. The method according to any one of claims 45 to 51, characterized in that, Further steps include: An optical feedback structure is provided to facilitate light generation within the optical fiber (10) to produce a laser.

53. The method as described in claim 52, characterized in that, The optical feedback structure includes at least one reflective device configured to reflect light energy back to the first fiber core (2).

54. The method as described in claim 53, characterized in that, The reflective device includes a fiber Bragg grating.

55. The method according to any one of claims 45 to 54, characterized in that, Further steps include: Provide an optical switch (2314) and connect the optical switch to the first fiber core (2).

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