Active waveguides for high-power, high-efficiency ECO lasers.

Abstract

To provide an active waveguide that includes an active rod and a passive rod having respective polymer claddings mechanically and optically coupled to each other to define a side-pump mode.SOLUTION: One or more elements embedded in one or both of active and passive rods have a refractive index that is at least 1*10-3 lower than the lowest refractive index of the respective active and passive rods. A MM core of the active rod includes a concentric inner region and an outer region, luminescent material concentration in the outer region is less than 50% of the luminescent material concentration in the inner region, and a radius of the inner region is at most 92% of the radius of the outer region. Unabsorbed pump light at output of an active waveguide is less than 1% of delivered pump light, thereby contributing to laser efficiency of at least 86% so as to be combined with the refractive index of the embedded element and the selectively doped core region.SELECTED DRAWING: Figure 6A

Description

[Technical Field]

[0001] The present disclosure relates to a kW-power fiber laser and amplifier that outputs signal light substantially in the fundamental mode. Specifically, the present disclosure relates to an active waveguide including an active rod and a pump rod that define a side-pumped configuration, at least one of which includes one or more elements embedded in a silica cladding configured to enhance pump light absorption in the centrally doped multimode (MM) core of the active rod. The disclosed laser source exhibits a reduction of signal light in the cladding to approximately 2% and a reduction of unabsorbed pump light to less than 0.5%, which in combination contributes to a laser efficiency of at least 86% and a wall-plug efficiency of greater than 50% at the desired wavelength. [Background technology]

[0002] Due to the increasing cost of energy as well as regulations on energy efficiency, the environmental performance, including energy efficiency factors, of various laser-based systems has attracted the attention of academic and industrial research and development. Approaches to improving environmental performance, taking industrial laser-based tools into consideration, include three main categories: appropriate process and machine tool selection, optimized machine tool design, and optimized process control, among others. While the first and last categories are primarily controllable by the process planner or machine tool operator, it is the OEM supplier that has the dominant influence on system design, which is the fiber laser source within the scope of this invention.

[0003] Figure 1 shows a general schematic diagram of a fiber laser consisting of a resonant cavity defined between a high fiber Bragg grating (FBG) 5 written in an input signal passive fiber 3 and a low fiber Bragg grating 6 written in an output signal passive fiber 8. Without the FBG shown, Figure 1 would clearly represent a fiber amplifier. The essentials of the following discussion are equally applicable to both oscillators and amplifiers.

[0004] The fiber laser of Figure 1 includes an active double-clad signal fiber, i.e., active fiber 2, with an MM core doped with luminescent ions that amplify signal light at a signal wavelength λs. The illustrated schematic utilizes end-pumping technology, in which signal and pump light of wavelength λs ≠ λp are coupled into multiplexer 1 and then launched into the doped core and inner cladding of active fiber 2, respectively. The amplified signal light λs extracted from demultiplexer 9 is then coupled from the unabsorbed pump light λp into signal output passive fiber 8. Conventionally, pump light guided in pump light delivery fibers 4 and 7 is launched into both ends of the cladding of signal fiber 2, thereby propagating in both directions. However, not all of the launched pump light is absorbed.

[0005] The fiber laser of FIG. 1 is less efficient than theoretical threshold in the presence of unabsorbed pump light for several reasons. For example, the unabsorbed pump light affects the generation of signal light, resulting in insufficient gain. Another reason is damage to the multiplexer / demultiplexer due to back-reflection of the pump light from, for example, the splices between the opposing ends of each active fiber 2 and the adjacent fiber. Also, unabsorbed pump light propagating in both directions damages the pump light source, the FBG, and the means for directing the amplified optical signal from the fiber laser source 10. These are just a few of the many undesirable effects of unabsorbed pump light.

[0006] Figure 2 shows the power of unabsorbed pump light at wavelength λ propagating through the respective multiplexers 1 and 9 to the respective pump optical fibers 4 and 7 as a function of the total input pump power in the fiber laser source 10 of Figure 1. As can be seen, the fraction of unabsorbed pump light remains high, which becomes particularly problematic when the input pump power reaches high power levels. As a result, the environmental performance of the fiber laser source of Figure 1 should be improved.

[0007] The excitation light absorption can be estimated from the following equation:

[0008]

number

[0009] where αcore is the core absorption, and Acore and Aclad are the core area and inner cladding area of ​​the double-clad (DC) active fiber 2, respectively. From the above, it can be seen that pump light absorption increases with core absorption, i.e., with increasing doping concentration and / or core / cladding ratio. However, both of the above options have limitations. In particular, photodarkening effects and background losses set upper levels for rare-earth ion concentrations. High background losses result in reduced efficiency of the fiber laser source. This is one of the reasons why typical slope efficiencies in the DC active fiber 2 of Figure 1 are below 70-80%, despite the theoretical limit exceeding 90%.

[0010] Another option for improving pump absorption, core area / cladding area scaling, can be achieved by increasing the core diameter with a concomitant reduction in the numerical aperture (NA). However, if the core is configured to support multiple modes, the core diameter cannot be increased indefinitely due to the excitation of multiple higher-order modes (HOMs). The excitation of HOMs results in an M of often less than 1.2, and in practice closer to 1.05. 2 The quality of the output signal light required to be in the fundamental mode of the modulus is degraded, and the fundamental mode (FM) has a substantially Gaussian-shaped intensity profile.

[0011] The pump light in the inner cladding of the active fiber 2 in Figure 1 propagates in a highly MM regime. These modes can be effectively grouped into two categories: "highly absorbing" and "poorly absorbing" modes. Modes in the high-absorption category have an axially symmetric field distribution, their intensity is maximized in the doped core of the active fiber 2, and they are well absorbed, thus contributing efficiently to the gain. Modes in the low-absorption category have less overlap with the doped core and therefore do not contribute particularly to pump absorption, but these helical modes carry a significant fraction of the pump power, making the laser source less efficient overall than if these modes were absorbed.

[0012] 3 shows a cross section of a typical refractive step index profile of DC fiber 2, where core 10 has the highest refractive index and is doped with rare earth elements such as ytterbium (Yb), erbium (Er), neodymium (Nd), thulium (Tm), holmium (Ho), and other known luminescent materials. DC fiber 2 further includes an inner cladding that receives pump light through a termination, and an outer protective cladding 12 that has the lowest refractive index to guide the pump light into the inner cladding.

[0013] The pump light of pump wavelength λ p launched into the inner cladding of rod 11 through both ends of fiber 2 consists of meridian and skew rays. The meridian rays (not shown) intersect core 10 and are efficiently absorbed. However, skew rays 13 propagate along the inner cladding in a helical trajectory that does not substantially intersect core 10 and therefore without significant absorption to further contribute to the unabsorbed pump light portion.

[0014] Many attempts have been made to correct the problems associated with skew or helical modes and improve energy conversion efficiency. Referring to FIG. 4A, the DC fiber 2 of FIG. 1 is shown with a radial asymmetry at the interface between the rod's inner and outer claddings. This approach assists in scattering the skew rays 13 so that some of them cross over and are absorbed into the core 10. FIG. 4B illustrates another approach in which multiple regions 14 are formed within the cladding of the active rod 11. The regions 14 have respective refractive indices different from that of the inner cladding 11, so that the skew rays 13 are radially scattered by these regions 14 and pass through the core 10, but at least some are absorbed by the core 10.

[0015] Both solutions shown in Figures 4A and 4B are somewhat effective. However, aligning all the fibers coupled together along a common optical axis, as required by end-pumping technology, is a difficult task and often results in unacceptable signal light losses and unreliable launch of pump light. Structures that serve to reduce optical losses simply require sophisticated and complex configurations that are economically unjustified and therefore do not improve the environmental performance of high-power fiber lasers and amplifiers.

[0016] Figure 5 illustrates another approach to improving pump absorption. The illustrated structure is based on a side-pumping technique in which the active fiber 2 and the pump light delivery fiber 15 are in optical (and mechanical) contact along their respective peripheries. The outer cladding 12 wrapped around both rods 11 and 15 is constructed with the lowest refractive index to prevent light decoupling from the inner cladding. It is easy to see that the configuration of Figure 5 has fewer problems associated with optical loss and / or the structural complexity of devices based on the end-pumping techniques of Figures 4A and 4B.

[0017] However, problems still persist with regard to unabsorbed pump light. For example, while the pump power used in the side-pumping technique can be made very large, the cladding area Aclad in Equation 1 also increases, since it is the sum of the cladding areas of the respective fibers 2 and 15. As with the end-pumping technique, not all of the pump light at wavelength λp in FIG. 5 is coupled into the cladding 11 of the active fiber 2 and contributes to increasing the output power of the unabsorbed pump light. Furthermore, some of the pump radiation coupled into the active rod 11 contains helical modes 13 that only partially overlap with the core 10 and are therefore not fully absorbed. The unabsorbed pump light can be significant, thus preventing the illustrated fiber laser from operating at the desired high level of efficiency. Thus, while the configuration of FIG. 5 is more efficient than that of FIGS. 4A-4B, it still benefits from more efficient pump light energy focusing onto the energy of the signal light at wavelength λs.

[0018] Pump light absorption is the primary, but not the only, factor contributing to the overall inefficiency of the illustrated fiber laser. As previously mentioned, in most cases, high quality signal light at wavelength λs, i.e., light having a substantially single transverse mode (SM), is important. For example, the core 10 of the active fiber 2 of Figures 4 and 5 is SM if the V parameter at wavelength λs is less than 2.405 based on the equation:

[0019]

number

[0020] where r is the core radius, ncore is the refractive index of the core 10, and nclad is the refractive index of the cladding 11.

[0021] Despite numerous innovations to minimize excitation of HOMs in MM cores designed to operate substantially in the fundamental mode (FM), their total suppression is hardly feasible. Furthermore, a wide variety of laser applications require high-power single-mode lasers with output powers ranging from about 1 kW.

[0022] Higher power requirements require larger core diameters. As a rule, active fibers are typically wound in fiber blocks FB, which require low bending losses. Low bending losses can be achieved if the numerical aperture Δn=ncore-nclad is large. For example, for a typical core radius of 10 μm, Δn=2*10 3 For a silica fiber with nclad = 1.4495 at a signal wavelength λs = 1070 nm, the V parameter is 4.47. A fiber with such a large V parameter is an MM, where HOM is amplified at the signal wavelength λs. Because all modes in the MM core compete for the same pump energy, the efficiency of the pump energy to generate and amplify FM is reduced.

[0023] One known technique for minimizing HOM excitation in MM fiber involves doping only the center of the MM core 10 of Figures 4 and 5. Another technique involves fiber geometry. Specifically, bottlenecked fibers are widely used to reduce HOM amplification.

[0024] Based on the foregoing, what is needed is a fiber laser or amplifier having an active waveguide that includes a signal fiber having an MM core doped with a light emitting material, and a pump fiber configured to side-pump the signal fiber, wherein the disclosed fiber laser / amplifier operates very close to the maximum theoretical level of efficiency of about 90%. Summary of the Invention [Means for solving the problem]

[0025] The disclosed active waveguide for fiber lasers and / or amplifiers fulfills this need. This inventive configuration incorporates all of the above-disclosed features and other features known from end-pumped configurations and incorporates side-pumping technology. Unlike the disclosed device, no fiber laser device known to applicants uses side-pumping technology that operates with a laser efficiency of at least 86% and a wall-plug efficiency greater than 50%.

[0026] According to one aspect of the disclosed fiber laser device, the active waveguide includes an active rod having an MM core doped with a phosphor and a pump light delivery rod, configured in this way to represent a side-pumped configuration in which the pump rod delivers MM pump light at wavelength λp and the active rod amplifies the generated signal light at signal wavelength λs, outputting the amplified signal light substantially in the fundamental mode.

[0027] One feature of the disclosed waveguide is that at least one or both of these rods are constructed using at least one element embedded in a silica cladding, the refractive index of which is at least 1*10 smaller than the refractive index of the delivery rod. -3 The helical modes of the MM pump light, which are efficiently reflected by these elements, propagate along the inner cladding of the DC fiber with no or minimal overlap with the MM core, with increasing overlap. As a result, the absorption of the pump light in the MM core is increased compared to known prior art.

[0028] According to a further feature of the disclosed active waveguide, the MM core of the active fiber is composed of an inner region and an outer region, the radius of the inner region being no greater than 92% of the radius of the outer region. The concentration of the luminescent material in the inner region is at least 50% higher than that in the outer core region. This feature can result in substantially less HOM amplification at the signal wavelength than known side-pumping schemes.

[0029] The disclosed waveguide incorporating both of the above-discussed features addresses the problem of poor optical efficiency of SM light generation at kW-level powers above 87%, allowing fiber lasers / amplifiers based on the disclosed active waveguides to operate with overall wall-plug efficiencies above 50%.

[0030] The disclosed waveguide further comprises an outer cladding that surrounds both rods and ensures their optical and mechanical contact. The refractive index of the outer cladding is smaller than that of the rods, which may have respective refractive indices substantially the same as or different from that of the active rod, which is larger than that of the delivery rod. Finally, the outer cladding is surrounded by a protective sleeve made of a material with a refractive index higher than that of the outer cladding.

[0031] In one modification of the disclosed active waveguide, one or more elements are inserted into the active rod. In another embodiment, both the active rod and the delivery rod are provided with respective elements. In yet another embodiment, only the delivery rod contains an element that reflects the radial mode of the pump light towards the MM core of the active rod.

[0032] The above and other features and advantages of the disclosed structure are further discussed in the following detailed description accompanied by the drawings. [Brief explanation of the drawings]

[0033] [Figure 1] 1 is a standard schematic diagram of a known prior art fiber laser source; [Figure 2] FIG. 2 is a diagram showing the dependence of the power of the non-absorbed pump light on the input pump light power in the schematic diagram of FIG. 1. [Figure 3] FIG. 1 shows a cross section of a typical DC fiber of the known prior art. [Figure 4A] 3 is a diagram of an implementation of the DC fiber of FIG. 2 configured to improve absorption of pump light in the known prior art. [Figure 4B]3 is a diagram of an implementation of the DC fiber of FIG. 2 configured to improve absorption of pump light in the known prior art. [Figure 5] FIG. 1 illustrates a typical side-pumped arrangement of the prior art. [Figure 6A] FIG. 1 shows a modification of the inventive active waveguide. [Figure 6B] FIG. 1 shows a modification of the inventive active waveguide. [Figure 6C] FIG. 1 shows a modification of the inventive active waveguide. [Figure 7A] FIG. 10 shows the doping profile of an active rod constructed in accordance with the present invention. [Figure 7B] FIG. 10 shows the doping profile of an active rod constructed in accordance with the present invention. [Figure 7C] FIG. 10 shows the doping profile of an active rod constructed in accordance with the present invention. [Figure 7D] FIG. 10 shows the doping profile of an active rod constructed in accordance with the present invention. [Figure 8A] 1 shows the laser efficiency of the inventive active waveguide and the laser efficiency of the known active waveguide at each output power of the signal light. [Figure 8B] 1A and 1B show the percentage of non-absorbing pump and signal light at a given signal wavelength in the cladding of a known active waveguide and the cladding of an inventive active waveguide, respectively. DETAILED DESCRIPTION OF THE INVENTION

[0034] Reference will now be made in detail to the disclosed system. Wherever possible, the same or similar reference numbers are used in the drawings and the description to refer to the same or like parts or steps. The drawings are in simplified form and are not to scale.

[0035] The disclosed structure is specifically configured to meet increasing efficiency requirements for MM fiber lasers with side-pumping configurations and kW-level signal output in a substantially fundamental mode. The novel combination of known elements that distinguishes the disclosed structure from known prior art reduces unabsorbed pump light to less than 0.5% and signal light in the cladding to approximately 2%, thus improving laser efficiency to 86-90%.

[0036] Improved laser efficiency invariably reduces power consumption, reduces environmental impact, and improves safety for maintenance personnel, to name just a few benefits. Therefore, it is not uncommon for improvements to demonstrate small quantitative improvements to radically change the marketability of the improvement. What may be considered minor in terms of the improvement's characteristics outside the target industry may be considered pioneering by ordinary and less-than-ordinary workers in this particular industry.

[0037] The disclosed configuration is a good example of how intrinsically known elements incorporated into a novel structure bring this structure to the state of the art. The disclosed MM fiber laser / amplifier with a substantially single fundamental mode (FM) signal light output is based on a side-pumped technology in which an active pump rod and a passive pump rod are arranged in a side-by-side configuration. At least one of the active and passive pump rods is equipped with an element having a refractive index lower than that of the surrounding cladding to enhance pump mode conversion and absorption. However, the use of elements known from end-pumped systems is not obvious in a side-pumped configuration. The improvement of mode conversion in active fibers with asymmetric cores is well known to those skilled in the art of fiber laser technology. In a side-pumped configuration of the disclosed type, the MM core is positioned asymmetrically. Therefore, to the best of applicant's knowledge and belief, no attempt has been reported to insert any additional means into the active rod for mode conversion enhancement in a side-pumped configuration. As for the passive rod in the disclosed configuration, again, to applicant's knowledge, it is unknown for sufficient reasons. Typically, an active waveguide, including an active rod and a pump rod coupled side-by-side, is wound around a fiber block. It is believed that the pump mode in the wound fiber is distorted, resulting in poor pump mode absorption. However, typically, at the output of the fiber block housing, the unabsorbed pump light in a kW-level side-pumped fiber laser / amplifier is only a small fraction of the pump light delivered to the active rod. This amount of unabsorbed pump light is generally acceptable, and further improvement may have some negative impact on the overall efficiency of the laser. In contrast, the disclosed structure is configured to improve laser efficiency.

[0038] With the above in mind, the following description discloses an inventive configuration that significantly improves laser efficiency. Figure 6A shows the active waveguide 25 of the schematic diagram of Figure 1 wound generally in a fiber block FB. The active waveguide 25 shown represents a side-pumped configuration that includes an active fiber rod 11 having an MM core 35 doped with any of known light emitters or combinations thereof. For example, the light emitter may be an ion of ytterbium (Yb) that produces signal light at a wavelength λs of, for example, 1070 nm.

[0039] The active waveguide 25 further includes a passive rod 15, which has a refractive index at most equal to that of the active rod 11 and delivers MM pump light at a pump wavelength λp, e.g., 976 nm. An outer cladding 12, which has a refractive index lower than that of the rods 11 and 15, maintains the active rod 11 and passive rod 15 in mechanical and optical contact along the adjacent perimeters of their respective rods. The coupled perimeters of the active waveguide define a coupling range over the length over which the pump light continues to intersect the interface between the rods to be absorbed in the MM core 35. As discussed above, not all of the pump light is coupled into the active rod 11, and even the coupled pump light has a helical mode 13 that does not fully overlap the center of the MM core 35. Not all of the energy of the pump light is converted into signal light energy, thus affecting the laser efficiency and the output power of the signal light.

[0040] According to one aspect of the inventive concept, one or more elements 19 are inserted into a matrix material, such as silica, of the cladding 45 of the active rod 11. The elements 19 have a refractive index lower than that of the cladding 45 and are configured to redirect the helical modes 13 of the excitation light into the core 35, improving the absorption of these modes. To prevent any undesired loading of the core 35, the elements 19 are made of silica doped with fluoride (F) and possibly boron (B) ions, thereby reducing the refractive index n e is the refractive index n of the cladding 45 c11 At least 1*10 -3The latter limit is important for effective mode conversion to improve laser efficiency. In contrast, prior art techniques have shown that this difference between these factors is 1*10 -3 should not be exceeded, or the polarization properties of light guided in the core will be adversely affected. Furthermore, the disclosed active waveguides can be constructed with polarization-maintaining rods if desired.

[0041] A further feature that improves the environmental performance of the disclosed waveguide includes partially doping the MM core 35 of the active rod 11 with a phosphor. Different regions of the core 35 may be doped to a certain degree depending on the desired transverse mode. In light of this disclosure regarding the fundamental transverse mode, it is the relatively small central portion 17 that has a higher rare earth ion concentration than that of the outer core region 16. The outer core region 16 is either completely undoped with phosphor or has a phosphor concentration that is 50% or less of that of the central core region 17. Such selective doping reduces the use of pump energy to amplify HOMs propagating in the immediate vicinity of the core 35. Geometrically, the radius of the central core region 17 is at most 92% of the radius of the outer core region 16. With the MM core parameters disclosed above, more pump energy is available for FM generation and amplification.

[0042] Figure 6B shows another embodiment of an active waveguide 25 in accordance with the inventive concept. Similar to Figure 6A, the waveguide 25 is realized as a side-pumped arrangement including an active rod 11 and a passive rod 15. In contrast to the embodiment of Figure 6A, this embodiment features one or more elements 19 inserted into the passive rod 15. The insertion of elements 19 in any of the rods 11 and 15 is accomplished by pre-drilling the desired number of channels in the rod, which then receive the respective elements 19. The elements 19 are each oriented in a direction parallel to the refractive index n of the rod 15. c15 At least 1*10 3 Low refractive index n eThe pump light, and in particular the skew rays, are guided towards the active rod 11 in such a way as to increase the overlap between the helical pump mode 13 coupled into the rod 11 and the MM core 35. The MM core of the waveguide 25 has a configuration similar to that of Figure 6A.

[0043] Figure 6C shows another realization of the inventive concept, which includes a combination of the inventive features of Figures 6A and 6B. Specifically, each of the active rod 11 and passive rod 15 includes the above-disclosed element 19. The MM core 35 has two or more annular regions as discussed above in connection with Figures 6A and 6B, respectively.

[0044] The active waveguides of Figures 6A-6C may be provided with a third cladding 18 (shown in Figures 6B and 6C) that acts as a shield from external mechanical loads. However, the third cladding 18, the shielding cladding 12 from physical damage, may have a refractive index greater than that of the cladding of the respective active rod and the cladding of the passive rod.

[0045] In summary, the laser efficiency of the schematic diagram of Figure 1, which includes the side-pumped configuration with the inventive active waveguide, is Element 19 increases excitation light absorption; The selectively doped MM core of the active rod 11 reduces the amplified HOM at the signal wavelength; Due to the uniqueness of the structure, it will be improved to at least 86%.

[0046] In addition to the major structural innovation in the disclosed active waveguide side-pumped configuration, several additional features can be incorporated into any of the above-disclosed embodiments to contribute to the unprecedented high efficiency of side-pumped fiber lasers / amplifiers. The shape of the active rod 11 can have a bottleneck-shaped cross section along its optical axis, with one or both ends each having a smaller diameter than the central portion. The passive rod 15 can be configured with a central portion having a smaller radius than either or both ends. The bottleneck-shaped rods 11 and 15 can be incorporated together in the schematic diagrams of Figures 6A-6C, or either can be paired with another uniformly shaped rod.

[0047] Figures 7A-7D show the respective configurations of the refractive step-index profile of the active rod and the dopant profile imparted to its MM core. Figures 7A and 7D show the core's uniformly formed and doped central core region 17 and undoped outer core region 16. Figure 7B shows the central core region with a substantially higher dopant concentration than the outer core region 16. Figure 7C shows a truncated conical dopant profile that narrows from the interface between the core and cladding toward the center of the core.

[0048] Much experimentation has been carried out on the active waveguide disclosed above for quite some time and will continue. The advantages of element 19 are clearly seen in Figures 8A and 8B. The unabsorbed pump power at the output of the fiber block FB of Figure 1 is drastically reduced from 21 W using the prior art active rod to about 3.5 W in the inventive structure at a total input pump power of 1200 W.

[0049] Referring to FIG. 8A, at an FM signal light output power of 900 W at a signal wavelength of 1070 nm and a pump wavelength of 977 nm, black curve 50 represents a maximum laser efficiency of 87.2% for the inventive structure, compared to a laser efficiency of about 81% on black curve 52 representing the known prior art configuration.

[0050] The data shown in Figure 8A is a direct result of the inventive active waveguide's structural innovation, which includes reduced unabsorbed pump and signal light in the cladding, as shown in Figure 8B. As indicated by curve 56, only about 1.5% of the signal light is detected in the cladding (Figure 8B). In contrast, prior art structures operate with at least 6% unwanted signal light in the cladding, as seen in curve 54. Similarly, as shown by curve 58 in Figure 8B, the inventive structure has 0.1-0.3% unabsorbed pump light at the output of the fiber block FB, while prior art devices have approximately 2% or more unabsorbed pump light at maximum laser efficiency, as shown by curve 60.

[0051] Therefore, while the present invention has been described in conjunction with its detailed description, it should be understood that the foregoing description is intended to be illustrative and not to limit the scope of the invention as defined by the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims. [Explanation of symbols]

[0052] 1 Multiplexer 2. Active Fiber 3 Passive Fiber 4. Pump light delivery fiber 5 High-performance fiber Bragg gratings 6 Low Fiber Bragg Grating 7. Pump light delivery fiber 8 Passive Fiber 9 Demultiplexer 10 cores 11 Active Rod 12 Protective Cladding 13 Skew Rays 14 areas 15. Pump light delivery fiber, passive rod 16 outer core region 17 Central Core Region 18 Third Clad 19 elements 25 Active Waveguide 35MM Core 45 Clad

Claims

1. 1. An active waveguide comprising an active rod and a passive rod optically and mechanically coupled to each other in a side-pumped manner, the passive rod having a refractive index lower than that of the active rod, the passive rod delivering excitation light to the active rod, the active rod having a multimode (MM) core configured to amplify generated signal radiation, The refractive index of one or more elements embedded in one or both of the active rod or the passive rod is at least 1*10 lower than the refractive index of the material of the active rod or the passive rod that surrounds the element. －３ Low, the MM core of the active rod has concentric inner and outer regions, the phosphor concentration of the outer region being less than 50% of the phosphor concentration of the inner region; the phosphor concentration in the outer region continuously decreases from the boundary between the outer region and the inner region toward the outside, An active waveguide including an inner region and an outer region, wherein the radius of the inner region is at most 92% of the radius of the outer region, and wherein unabsorbed pump light at the output of the active waveguide is less than 1% of the delivered pump light, which, in combination with the refractive index of the embedded element and the selectively doped core region, contributes to a laser efficiency of at least 86%.

2. 2. The active waveguide of claim 1, further comprising at least one outer cladding surrounding each of the active and passive rods to maintain mechanical and optical contact between the active and passive rods, the one outer cladding having a refractive index lower than the lowest refractive index of the claddings of the respective active and passive rods.

3. 3. An active waveguide according to claim 1 or 2, further comprising a plurality of said elements embedded in the cladding of said active rod.

4. 3. An active waveguide according to claim 1 or 2, further comprising a plurality of said elements embedded in the cladding of said passive rod.

5. 3. An active waveguide according to claim 1 or 2, further comprising a plurality of said elements embedded in the cladding of each active and passive rod.

6. 6. An active waveguide according to claim 1, wherein the claddings of each active rod and passive rod have a longitudinal central portion along the optical axis that is coupled to each other to define a coupling path for the excitation light to the active rod, and the longitudinal central portion along the optical axis of the passive rod has a diameter smaller than the diameters of both ends of the passive rod in the longitudinal direction along the optical axis.

7. the claddings of each active rod and passive rod have longitudinal central portions along an optical axis that are joined to each other to define a coupling path for the pump light into the active rod; 7. An active waveguide according to claim 1, wherein the central portion of the active rod in the longitudinal direction along the optical axis has a diameter larger than the diameters of both ends of the active rod in the longitudinal direction along the optical axis.

8. 3. The active waveguide of claim 2 further comprising a protective cladding surrounding said one outer cladding.

9. the claddings of each active rod and passive rod have longitudinal central portions along an optical axis that are joined to each other to define a coupling path for the pump light into the active rod; 9. An active waveguide according to claim 1, wherein the central portion of the passive rod in the longitudinal direction along the optical axis has a diameter smaller than the diameters of both ends of the passive rod in the longitudinal direction along the optical axis, while the central portion of the active rod in the longitudinal direction along the optical axis has a diameter larger than the diameters of both ends of the active rod in the longitudinal direction along the optical axis.

10. The active waveguide of claim 1 , wherein the outer region of the MM core is free of light emitters.

Images