Active waveguides for high-power lasers

Abstract

An active waveguide comprising an active rod and a passive rod having respective polymer claddings, the active rod and the passive rod being mechanically and optically coupled to each other so as to define a lateral pumping scheme. One or more elements are embedded in one or both of the active rod and the passive rod and have a refractive index at least 1*10 lower than the minimum of the respective refractive indices of the active rod and the passive rod. ‑3 The MM core of the active rod includes an inner concentric region and an outer concentric region, wherein the luminophore concentration in the outer region is more than 50% lower than the luminophore concentration in the inner region, and the radius of the inner region is at most 92% of the radius of the outer region. The unabsorbed pump light at the output of the active waveguide accounts for less than 1% of the delivered pump light, which, combined with the embedded elements and the refractive index of the selectively doped core region, results in a laser efficiency of at least 86%.

Description

Technical Field

[0001] The present invention relates to kW power fiber lasers and amplifiers that output signal light substantially in fundamental mode. In particular, the present disclosure relates to an active waveguide comprising active rods and pump rods defining a lateral pumping configuration, wherein at least one of the rods comprises one or more elements embedded in a silica cladding and configured to increase pump light absorption in a centrally doped MM core of the active rod. The disclosed laser source demonstrates a reduction in signal light in the cladding to about 2% and a reduction in unabsorbed pump light to less than 0.5%, which combined together result in a laser efficiency of at least 86% and a wall plug efficiency greater than 50% at a desired wavelength. Background Art

[0002] Due to increasing energy costs on the one hand and energy efficiency regulations on the other, the environmental performance of various laser-based systems (including energy efficiency factors) has attracted academic and industrial research and development. Considering industrial laser-based tools, methods for improving environmental performance include, among others, the following three main categories: correct process and machine tool selection, optimized machine tool design, and optimized process control. While the first and last categories are mainly controlled by the process planner or machine tool operator, the original equipment manufacturer has a major influence on the system design, which, within the scope of the present invention, is a fiber laser source.

[0003] Figure 1 A typical schematic diagram of a fiber laser configured with a resonant cavity is shown, the resonant cavity being defined between a high fiber Bragg grating (FBG) 5 and a low fiber Bragg grating (FBG) 6, which are written into the corresponding input signal passive fiber 3 and output signal passive fiber 8. Obviously, without the FBG shown, Figure 1 will represent a fiber amplifier. The essence of the following description applies equally to both oscillators and amplifiers.

[0004] Figure 1The fiber laser includes an active double-clad signal or active fiber 2 having an MM core doped with luminescent ions that provide amplification of signal light of a signal wavelength λs. The schematic diagram shown utilizes an end-pumping technique, wherein, after coupling the signal light and the pump light (where the wavelengths of the signal light and the pump light are different, λp≠λs) to the multiplexer 1, they are respectively injected into the doped core and inner cladding of the active fiber 2. The demultiplexer 9 extracts the amplified signal light λs from the unabsorbed pump light λp, and then the amplified signal light is coupled into the signal output passive fiber 8. Typically, the pump light guided in the pump light transmission fibers 4 and 7 is injected into the opposite ends of the cladding of the signal fiber 2, thereby causing it to propagate in two directions. However, not all of the injected pump light is absorbed.

[0005] The unabsorbed pump light makes Figure 1 The efficiency of the fiber laser is lower than its theoretical threshold. For example, it affects the generation of signal light and leads to unsatisfactory gain. Another reason is that the multiplexer / demultiplexer is damaged due to, for example, back reflection of the pump light from the joint between the opposite end of the corresponding active fiber 2 and the adjacent fiber. In addition, the unabsorbed pump light propagating in the opposite direction can damage the pump light source, FBG, and the device for guiding the amplified optical signal from the fiber laser source 10. The above are just some of the many adverse consequences of unabsorbed pump light.

[0006] Figure 2 shows the power of the unabsorbed pump light of wavelength λp, which is propagated into the corresponding pump light fibers 4 and 7 through the corresponding multiplexers 1 and 9. Figure 1 As can be seen, the fraction of unabsorbed pump light is still high, which is particularly troublesome when the input pump power reaches high power levels. Therefore, it should be improved Figure 1 Environmental performance of fiber laser sources.

[0007] The pump light absorption can be estimated by the following expression:

[0008]

[0009] in is the core absorptivity, and Acore and Aclad are the corresponding areas of the core and inner cladding of the double-clad (DC) active fiber 2, respectively. As can be seen from the above, the pump light absorptivity increases with the core absorptivity (i.e., with increasing doping concentration) and / or with the core / cladding ratio. However, both of the above options have limitations. In particular, the photo-darkening effect and background losses determine the upper limit of the rare earth ion concentration. Large background losses will lead to low efficiency of the fiber laser source. This explains why, although the theoretical limit exceeds 90%, Figure 1 This is one of the reasons why the typical slope efficiency of DC active fibers is less than 70-80%.

[0010] Another option to achieve pump absorption enhancement is to scale the core / cladding area by increasing the core diameter while reducing 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 degrades the quality of the output signal light, which usually requires the output signal light to be in the fundamental mode with M 2 The factor is below 1.2 and in practice close to 1.05, with the fundamental mode (FM) having an intensity curve of essentially Gaussian shape.

[0011] exist Figure 1 The pump light in the inner cladding of the active fiber 2 propagates in a highly MM mechanism. Effectively, these modes can be grouped into two categories: "well-absorbed" modes and "weakly-absorbed" modes. The modes of the first category have an axially symmetric field distribution that has a maximum intensity at the doped core of the active fiber 2 and is therefore well absorbed, making an efficient contribution to the gain. The modes of the other category have a poor overlap with the doped core and therefore do not contribute significantly to the pump absorption rate, but these helical modes carry a large portion of the pump power, which makes the overall efficiency of the laser source not as high as when these modes are absorbed.

[0012] Figure 3 A cross section of a typical refractive index step curve of a DC fiber 2 having a core 10 having the highest refractive index and doped with rare earth elements, such as ytterbium (Yb), erbium (Er), neodymium (Nd), thulium (Tm), holmium (Ho) and other known luminophores is shown. The DC fiber 2 also includes an inner cladding that receives pump light through its end and an outer protective cladding 12 having the lowest refractive index to waveguide the pump light in the inner cladding.

[0013] The pump light of the pump wavelength λp injected into the inner cladding of the rod 11 through the opposite end of the optical fiber 2 includes meridian and skew rays. The meridian (not shown) passes through the core 10 and is effectively absorbed. However, the skew ray 13 propagates along the inner cladding in a spiral trajectory, does not actually pass through the core 10, and therefore has no meaningful absorption, which further results in a portion of the pump light that is not absorbed.

[0014] Many attempts have been made to correct the problem of skew or helical modes and improve the energy conversion efficiency. Figure 4A , showing Figure 1 The DC fiber 2 of the embodiment of the present invention has a radially asymmetric interface between the inner and outer claddings of the rod. This method helps to scatter some of the deflected rays 13 so that they pass through the core 10 and are absorbed in the core 10. Figure 4B A different approach is shown, in which a plurality of regions 14 are formed in the cladding of the active rod 11. The regions 14 have respective reflection coefficients that differ from the reflection coefficient of the inner cladding 11, so that these regions 14 scatter deflected rays 13 in radial direction through the core 10, wherein at least some of the deflected rays 13 are absorbed in the core 10.

[0015] Respectively Figure 4A and Figure 4B Both solutions shown in are somewhat effective. However, aligning all the fibers to be spliced ​​together along the same optical axis, as required by end-pumping techniques, is a difficult task, often accompanied by unacceptable losses of signal light and unreliable emission of pump light. Structures that can be used to reduce optical losses require delicate and complex configurations that are simply not economically justifiable, thus preventing improvements in the environmental performance of high-power fiber lasers and amplifiers.

[0016] Figure 5 Another method for increasing pump absorption is shown. The structure shown is based on a lateral pumping technique, in which the active fiber 2 and the pump light delivery fiber 15 are optically (and mechanically) in contact along their respective peripheries. The outer cladding 12 wrapped around both the rod 11 and the rod 15 is configured with a minimum refractive index, thereby preventing light from being decoupled from the inner cladding. It can be easily seen that Figure 5 Configuration based on Figure 4A and Figure 4B The end-pumping technique has problems associated with optical losses and / or structural complexity of the device.

[0017] However, the problem of unabsorbed pump light still exists. For example, although the pump power used in the side pumping technique can be very high, the cladding area Aclad of Expression 1 also increases because it is the sum of the claddings of the corresponding fiber 2 and fiber 15. Similar to the end pumping technique, it is not Figure 5All of the pump light at wavelength λp is coupled into the cladding 11 of the active fiber 2, which helps to increase the output power of the unabsorbed pump light. In addition, part of the pump radiation coupled into the active rod 11 includes helical modes 13, which only partially overlap with the core 10 and are therefore not fully absorbed. The unabsorbed pump light can be substantial, thus preventing the illustrated fiber laser from operating at the desired high efficiency. Therefore, despite Figure 5 Configuration ratio Figure 4A-4B The configuration is more efficient, but can still benefit from more efficient focusing of the pump light energy into signal light energy at wavelength λs.

[0018] Although pump light absorption is the main factor contributing to the overall inefficiency of the fiber laser shown, it is by far not the only factor. As mentioned above, high-quality signal light (i.e., light with essentially a single transverse mode (SM)) at wavelength λs is critical. For example, if the parameter V at wavelength λs is less than 2.405 based on the following equation, then Figures 4 and Figure 5 The core 10 of the active optical fiber 2 in is SM.

[0019]

[0020] Where r is the core radius, ncore and nclad are the refractive indices of the core 10 and the cladding 11 .

[0021] Despite many innovations to minimize HOM excitations in MM fiber cores (designed to operate essentially in fundamental mode (FM)), their complete suppression is almost not feasible. However, high-power single-mode laser radiation in the power range starting from about 1 kW is still needed for a wide variety of laser applications.

[0022] High power requirements require larger core diameters. Typically, active fibers are usually wound in fiber blocks FB that require low bending losses. The latter can be provided if the numerical aperture Δn = ncore-nclad is higher. For example, at a typical core radius of 10 μm, Δn = 2*10 3 , nclad = 1.4495, and the parameter V is 4.47 in a silica fiber with a signal wavelength λs = 1070 nm. A fiber with such a high parameter V is a MM, where the HOM at the signal wavelength λs is amplified. Since all modes in the MM core compete for the same pump energy, the efficiency of the pump energy used to generate and amplify the FM is reduced.

[0023] One of the known techniques for minimizing HOM excitation in MM fibers consists in doping only Figure 5The central region of the MM core 10. Yet another technique is related to fiber geometry. Specifically, bottleneck-shaped fibers have been widely used to reduce the amplification of HOMs.

[0024] Based on the foregoing, there is a need for a fiber laser or amplifier having an active waveguide comprising a signal fiber having an MM core doped with a luminophore and a pump fiber arranged to laterally pump the signal fiber, wherein the disclosed fiber laser / amplifier operates at approximately 90% of the maximum theoretical efficiency level. Summary of the invention

[0025] The disclosed active waveguide for a fiber laser and / or amplifier meets this need. The configuration of the present invention includes all of the above disclosed content and other features known from end-pumping devices and combined in the side-pumping technology. Unlike the disclosed device, no fiber laser device with side-pumping technology known to the applicant 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 luminophore and a pump light delivery rod. The rods arranged in this way represent a side-pumping configuration, wherein the pump rod delivers MM pump light of wavelength λp, and the active rod amplifies the generated signal light of wavelength λs, which is output substantially in the fundamental mode.

[0027] A feature of the disclosed waveguide is that the at least one rod or both rods are provided with at least one element embedded in the silica cladding. The refractive index of the element is at least 1*10 lower than the refractive index of the transmission rod. -3 These elements effectively reflect the helical mode MM pump light that propagates along the inner cladding of the DC fiber with no or minimal overlap with the MM core, thereby increasing the overlap and, therefore, increasing the absorption of the pump light in the MM core compared to the known prior art.

[0028] According to another feature of the disclosed active waveguide, the MM core of the active fiber is configured with an inner region and an outer region, wherein the radius of the inner region does not exceed 92% of the radius of the outer region. The concentration of the luminophore in the inner region is at least 50% higher than the concentration of the luminophore in the outer core region. This feature allows a substantial reduction in the amplification of the HOM at the signal wavelength compared to known side pumping schemes.

[0029] The disclosed waveguide combining the above two features solves the problem of insufficient optical efficiency (above 87%) for kW-class power SM light generation, which allows the disclosed active waveguide-based fiber laser / amplifier to operate with an overall wall-plug efficiency of over 50%.

[0030] The disclosed waveguide further comprises an outer cladding surrounding the two rods and ensuring their optical and mechanical contact. The refractive index of the outer cladding is configured to be smaller than the refractive index of the rods, the two rods respectively having substantially the same or different refractive indexes, wherein the refractive index of the active rod is greater than the refractive index of the transmission rod. Finally, the outer cladding is surrounded by a protective sheath made of a material having a higher refractive index than the refractive index of the outer cladding.

[0031] In one variant of the disclosed active waveguide, one or more elements are inserted into the active rod. In another embodiment, both the active rod and the transmission rod are provided with corresponding elements. In another embodiment, only the transmission rod comprises elements that reflect radial mode pump light towards the MM core of the active rod. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The above and other features and advantages of the disclosed structure are further discussed in the detailed description in conjunction with the following drawings, in which:

[0033] Figure 1 is a standard schematic diagram of a fiber laser source known in the prior art.

[0034] Figure 2 Shown in Figure 1 Schematic diagram of the dependence of the power of the unabsorbed pump light on the input pump light power.

[0035] Figure 3 A cross section of a typical DC optical fiber known in the prior art is shown.

[0036] Figure 4A and Figure 4B is configured to improve the absorption of pump light in the known prior art Figure 2 The corresponding implementation method of DC optical fiber.

[0037] Figure 5 A typical lateral pumping arrangement of the prior art is shown.

[0038] FIG. 6A to FIG. 6C A corresponding modification of the active waveguide according to the invention is shown.

[0039] Fig. 7A , Figure 7B , Figure 7C and Fig.7D The corresponding doping profile of an active rod configured according to the invention is shown.

[0040] Fig. 8A and Figure 8B The laser efficiencies of the active waveguide of the present invention and the known active waveguide at the corresponding output powers of the signal light, and the percentages of the unabsorbed pump light and signal light in the claddings of the corresponding known active waveguides and the active waveguide of the present invention at a given signal wavelength are shown. DETAILED DESCRIPTION

[0041] Reference is now made in detail to the disclosed system. Wherever possible, the same or similar reference numerals are used in the drawings and description to represent the same or similar components or steps. The drawings are in simplified form and are not at all drawn to exact scale.

[0042] The disclosed structure is specifically configured to meet the high level requirements for the efficiency of MM fiber lasers, which are provided with lateral pumping means and output kW-level signal light substantially in the fundamental mode. It differs from the known prior art by a new combination of known elements, which reduces the unabsorbed pump light to less than 0.5% and the signal light in the cladding to about 2%, thereby increasing the laser efficiency to 86-90%.

[0043] To name just a few advantages, the higher efficiency of lasers always translates into less power consumption, has less destructive environmental impact, and provides enhanced safety to maintenance personnel. Therefore, it is not uncommon for a product to show an improvement of a few percent that can revolutionize the marketability of the improved product. What is considered insignificant in terms of enhancing a product's characteristics outside of the target industry may be considered groundbreaking by a person of ordinary skill in that particular industry and not by a person of ordinary skill in general.

[0044] The disclosed configuration is a good example of how known elements of the principles involved in the new structure can bring the structure to the forefront of the technology. The disclosed MM fiber laser / amplifier is based on the side pumping technology, in which an active pump rod and a passive pump rod are arranged side by side, and the MM fiber laser / amplifier has a signal light output substantially in a single fundamental mode (FM). At least one of the active rod and the pump rod is provided with an element having a refractive index less than the refractive index of the surrounding cladding to improve the mixing and absorption of the pump mode. Although the use of elements is well known in end pumping schemes, the use of elements in side pumping devices is not obvious. It is well known to a person skilled in the art in the field of fiber lasers that mode mixing in active fibers with asymmetric cores is improved. In side pumping devices of the disclosed type, the MM core is arranged asymmetrically. This is why, to the best of the applicant's knowledge and belief, no attempts to insert any additional device into the active rod to enhance mode mixing in side pumping devices have been reported. As for the passive rods in the disclosed structure, they are still unknown to the best of the applicant's knowledge and for good reasons. Typically, an active waveguide comprising active rods and pump rods coupled side by side is coiled in a fiber block. Applicants tend to believe that the pump mode in the coiled fiber is deformed, which deteriorates the absorption of the pump mode. However, typically at the output of a fiber block equipped with a kW-class side-pumped fiber laser / amplifier, the unabsorbed pump light accounts for only a very small percentage of the pump light delivered to the active rods. The amount of unabsorbed pump light is usually acceptable, and other improvements can negatively impact the overall efficiency of the laser to some extent. In contrast, the disclosed structure is configured to improve laser efficiency.

[0045] Based on the above considerations, the following description discloses an inventive configuration that significantly improves laser efficiency. Fig. 6A The optical fiber block FB is usually coiled in Figure 1 Active waveguide 25 in the schematic diagram. The active waveguide 25 shown represents a lateral pumping device, which includes an active fiber rod 11 with an MM core 35 doped with any known luminophore or combination thereof. For example, the luminophore may be ytterbium ions (Yb) that generate a signal light of wavelength λs of, for example, 1070 nm.

[0046] The active waveguide 25 also includes a passive rod 15 that transmits MM pump light at a pump wavelength λp (e.g., 976 nm) and has a refractive index at most equal to that of the active rod 11. An outer cladding 12 having a refractive index lower than that of the rods 11 and 15 maintains the active rod 11 and the passive rod 15 in mechanical and optical contact along the adjacent peripheries of the respective rods. The coupling periphery of the active waveguide defines a coupling stretch over the length of which the pump light remains to pass through the interface between the rods to be absorbed by the MM core 35. As described above, not all 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 with the central region of the MM core 35. Therefore, not all the energy of the pump light is converted into the energy of the signal light, thereby affecting the laser efficiency and the output power of the signal light.

[0047] According to one aspect of the inventive concept, one or more elements 19 are inserted into the bulk material (e.g., silica) of the cladding 45 of the active rod 11. The elements 19, which have a refractive index lower than that of the cladding 45, are configured to redirect the helical modes 13 of the pump light toward the core 35 and improve the absorptivity of these modes. To prevent any undesired loading on the core 35, the elements 19 are made of silica doped with fluorine ions (F) and possibly boron ions (B), which increases the refractive index n of the elements 19. e Reduced to a refractive index n smaller than that of the cladding 45 c11 At least 1*10 -3 The latter limit is crucial for efficient mode mixing, leading to increased laser efficiency. In contrast, the prior art teaches that the difference between these coefficients should not exceed 1*10 -3 , because otherwise the polarization properties of the light guided by the core will be affected. However, if necessary, the disclosed active waveguide can be configured with a polarization maintaining rod.

[0048] Another feature that provides improved environmental performance of the disclosed waveguide includes partially doping the MM core 35 of the active rod 11 with a luminophore. Depending on the desired transverse mode, different regions of the core 35 can be doped more or less. In view of the present disclosure related to the fundamental transverse mode, the relatively small central region 17 has a higher concentration of rare earth ions than the outer core region 16. The latter may not be doped with luminophore at all, or its concentration is less than 50% or less of the concentration of the central core region 17. This selective doping reduces the use of pump energy for amplifying HOMs propagating near the periphery 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 parameters of the MM core disclosed above, more pump energy will be used for the generation and amplification of FM.

[0049] Figure 6BAnother embodiment of an active waveguide 25 based on the concept of the present invention is shown. Fig. 6A , the waveguide 25 is implemented as a lateral pumping device including an active rod 11 and a passive rod 15, respectively. Fig. 6A In contrast to the embodiment of the present invention, this embodiment is characterized by the insertion of one or more elements 19 in the passive rod 15. The insertion of the elements 19 in either the rod 11 or the rod 15 is accomplished by pre-drilling the desired number of channels in the rod, which then receive the corresponding elements 19. The elements 19 are each configured with a refractive index nc15 that is at least 1*10 less than that of the rod 15. 3 The pump light (especially the deflected ray) is guided to the active rod 11 in such a way that the overlap between the helical pump mode 13 coupled into the rod 11 and the MM core 35 is increased. Fig. 6A Configure similarly.

[0050] Figure 6C Shown include Fig. 6A and Figure 6B Another implementation of the inventive concept of the combination of the inventive features of the invention. In particular, the active rod 11 and the passive rod 15 are each provided with the element 19 disclosed above. As described above, respectively Fig. 6A and Figure 6B , the MM core 35 has two or more annular regions.

[0051] FIG. 6A to FIG. 6C The active waveguide may be provided with a third cladding layer 18 (eg Figure 6B and Figure 6C As shown), the third cladding 18 serves as a shield against external mechanical loads. However, the refractive index of the third cladding 18, which shields the cladding 12 from physical damage, can be greater than the refractive index of the cladding of the corresponding active rod and passive rod.

[0052] In summary, due to the following structural features, the lateral pumping device including the active waveguide of the present invention is Figure 1 Schematic diagram of laser efficiency increased to at least 86%:

[0053] An element 19 that improves pump light absorption; and

[0054] The selectively doped MM core of the active rod 11 reduces the amplified HOM at the signal wavelength;

[0055] In addition to the main structural innovations in the disclosed lateral pumping arrangement of active waveguides, some additional features are incorporated in any of the above disclosed embodiments and contribute to the unprecedented high efficiency of the lateral pumped fiber laser / amplifier. The active rod 11 may be shaped to have a bottleneck cross-section along the optical axis of the rod, wherein the diameter of one or both ends of the rod is smaller than the diameter of the central portion. The passive rod 15 may be configured with a central portion that is smaller than one or opposite ends. The bottleneck rod 11 and rod 15 may be incorporated together. Figure 6A-6C , or one of them can be paired with another uniformly shaped rod.

[0056] 7A to 7D The refractive index step profile of the active rod and the corresponding configuration of the dopant profile arranged in its MM core are shown. Fig. 7A and Fig.7D A central core region 17 formed by uniform doping of the core and an undoped outer core region 16 are shown. Figure 7B It is shown that the dopant concentration of the central core region is substantially greater than the dopant concentration of the outer core region 16 . Figure 7C A frustoconical dopant profile is shown that narrows from the interface between the core and the cladding toward the center of the core.

[0057] Extensive experiments have been conducted on the active waveguide disclosed above and are still ongoing, consuming considerable time. Fig. 8A and Figure 8B The advantage of element 19 is clearly seen in FIG. In the structure of the present invention, Figure 1 The unabsorbed pump power at the output of the fiber block FB is dramatically reduced from 21 W in the prior art active rod to about 3.5 W at a total input pump power of 1200 W.

[0058] Reference Fig. 8A , black curve 50 represents the maximum laser efficiency in the structure of the present invention is 87.2%, compared with about 81% on black curve 52, which represents the configuration of the known prior art at an output power of 900W of FM signal light at a signal wavelength of 1070nm and a pump wavelength of 977nm.

[0059] Fig. 8A The data shown are a direct result of the structural innovations of the active waveguide of the present invention, including reduced unabsorbed pump and signal light in the cladding, such as Figure 8B As shown in curve 56 ( Figure 8B), only about 1.5% of the signal light is detected in the cladding. In contrast, the prior art structure operates with at least 6% of the unwanted signal light in the cladding, which can be seen on curve 54. Similarly, in the structure of the present invention, the unabsorbed pump light at the output of the fiber block FB is between 0.1% and 0.3%, as shown in FIG. Figure 8B As shown in curve 58 of FIG. 5A , the prior art device has about 2% or more of unabsorbed pump light at maximum laser efficiency, as shown in curve 60 .

[0060] Therefore, it should be understood that although the invention has been described in conjunction with the detailed description of the invention, the above description is intended to illustrate rather than limit the scope of the invention, and the scope of the invention is defined by the appended claims. Other aspects, advantages and modifications are within the scope of the appended claims.

Claims

1. An active waveguide comprising an active rod and a passive rod, the active rod and the passive rod being optically and mechanically coupled to each other in a lateral pumping scheme, the passive rod delivering pump light to the active rod, and the active rod being provided with a multimode MM core, the MM core being configured to amplify the generated signal radiation, and wherein, Improvements include: One or more elements are embedded in the active rods or the passive rods or in both the active rods and have a refractive index at least 1*10 lower than the material of the rods surrounding the elements. -3 The refractive index of , thereby reflecting the helical mode MM pump light to the MM fiber core; The MM core of the active rod comprises: concentric inner and outer regions, wherein the concentration of the luminophore in the outer region is more than 50% lower than the concentration of the luminophore in the inner region, and The radius of the inner region is at most 92% of the radius of the outer region, wherein the unabsorbed pump light at the output of the active waveguide accounts for less than 1% of the delivered pump light, and the less than 1% unabsorbed pump light combined with the refractive index of the embedded elements and the selectively doped fiber core region makes the laser efficiency at least 86%.

2. The active waveguide according to claim 1, further comprising at least one outer cladding surrounding the respective active rods and passive rods so as to maintain the passive rods in mechanical and optical contact with the active rods, wherein The at least one outer cladding has a refractive index lower than the lowest refractive index of the claddings of the corresponding active and passive rods.

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

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

5. The active waveguide according to claim 1 or 2, further comprising a plurality of elements embedded in the cladding of the respective active rods and passive rods.

6. The active waveguide according to claim 1, wherein: The claddings of the respective active and passive rods have central portions coupled together to define a coupling path for coupling pump light into the active rods, the central portion of the passive rods having a diameter smaller than a diameter of opposite ends of the passive rods.

7. The active waveguide according to claim 1, wherein: The claddings of the respective active and passive rods have central portions coupled together to define a coupling path for coupling pump light into the active rods, the central portion of the active rods having a diameter greater than a diameter of opposite ends of the active rods.

8. The active waveguide of claim 2, further comprising a protective cladding surrounding the at least one outer cladding.

9. The active waveguide according to claim 1, wherein: The claddings of the respective active and passive rods have central portions coupled together to define a coupling path for coupling pump light into the active rod, the central portion of the passive rod having a diameter smaller than a diameter of an opposite end of the passive rod and the central portion of the active rod having a diameter larger than a diameter of an opposite end of the active rod.

10. The active waveguide according to claim 1, wherein: The outer region of the MM core is free of luminophores.

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