ACTIVE WAVEGUIDE FOR HIGH-POWER LASER

Abstract

An active waveguide including active and passive rods which have respective polymeric claddings mechanically and optically coupled to one another so as to define a side pumping scheme. One or a plurality of elements are embedded in one of or both active and passive rods and have a refractive index lower than the lowest of refractive indices of the respective active and passive rods at least 1*10.sup.−3. The MM core of the active rod includes inner and outer concentric regions with a concentration of light emitters in the outer region being lower than that of the inner region at more than 50% and, a radius of the inner region being at most 92% of that of the outer region. The unabsorbed pump light at the output of the active waveguide constitutes less than 1% of the delivered pump light which in combination with the refractive index of the embedded elements and selectively doped core regions contribute to laser efficiency of at least 86%.

Claims

1. An active waveguide including active and passive rods which are optically and mechanically coupled to one another in a side-pumping scheme, the passive rod delivers pump light to the active rod, and the active rod is provided with a multimode (MM) core configured to amplify a generated signal radiation, and wherein the improvement comprises: one or more elements embedded in the active rod or passive rod or both and having a refractive index lower than that of material of the rods, which surrounds the elements, at at least 1*10.sup.−3; the MM core of the active rod including inner and outer concentric regions with a concentration of light emitters in the outer region being lower than that of the inner region at more than 50%, and a radius of the inner region being at most 92% of that of the outer region, wherein the unabsorbed pump light at the output of the active waveguide constitutes less than 1% of the delivered pump light which in combination with the refractive index of the embedded element and selectively doped core regions contribute to laser efficiency of at least 86%.

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

3. The active waveguide of any of the above claims further comprising a plurality of the elements embedded in the cladding of the active rod.

4. The active waveguide of any of claims 1 or 2 further comprising a plurality of the elements embedded in the cladding of the passive rod.

5. The active waveguide of any of claims 1 and 2 further comprising a plurality of the elements embedded in the claddings of respective active and passive rods.

6. The active waveguide of any of the above claims, wherein the claddings of respective active and passive rods have central parts coupled together to define a coupling path for the pump light into the active rod, the central part of the passive rod having a diameter smaller than that of opposite ends thereof.

7. The active waveguide of any of the above claims, wherein the claddings of respective active and passive rods have central parts coupled together to define a coupling path for the pump light into the active rod, the central part of the active rod having a greater diameter than that of opposite ends thereof.

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

9. The active waveguide of any of the above claims, wherein the claddings of respective active and pump rods have central parts coupled together to define a coupling path for the pump light into the active rod, the central part of the pump rod having a diameter smaller than that of opposite ends thereof while the central part of the active rod being greater than that of opposite ends thereof.

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

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The above and other features and advantages of the disclosed structure are further discussed in the specific description accompanied with the following drawings, in which:

[0030] FIG. 1 is a standard schematic of fiber laser source of the known prior art.

[0031] FIG. 2 illustrates the dependence of the power of unabsorbed pump light from the input pump light power in the schematic of FIG. 1.

[0032] FIG. 3 illustrates a cross-section of the typical DC fiber of the known prior art.

[0033] FIGS. 4A and 4B are respective realizations of the DC fiber of FIG. 2 configured to improve absorption of pump light in the known prior art.

[0034] FIG. 5 illustrates a typical side pumping arrangement of the prior art.

[0035] FIG. 6A-6C illustrate respective modifications of the inventive active waveguide.

[0036] FIGS. 7A, 7B, 7C and 7D illustrate respective doping profiles of the active rod configured in accordance with the invention.

[0037] FIGS. 8A and 8B illustrate laser efficiency of inventive and known active waveguides at respective output powers of signal light and the percentage of unabsorbed pump light and signal light in the cladding of respective known and inventive active waveguides at a given signal wavelength.

SPECIFIC DESCRIPTION

[0038] Reference will now be made in detail to the disclosed system. Wherever possible, same or similar reference numerals 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 far from precise scale.

[0039] The disclosed structure is specifically configured to meet the heightened requirements for efficiency of MM fiber laser provided with a side-pumping arrangement and outputting kW-level signal light in substantially a fundamental mode. It distinguishes from the known prior art by a new combination of known elements which decreases the unabsorbed pump light below 0.5% and signal light in the cladding to about 2% thus increasing the laser efficiency to 86-90%.

[0040] The higher efficiency of a laser always translates into the lower consumption of power, have a less devastating environmental impact, and provide the increased safety of the maintenance personnel, to name just a few advantages. Thus it is not unusual when the product, showing improvement of a fraction of percent may radically change the marketability of the improved product. What could be considered trivial in terms of enhanced product characteristics outside the targeted industry may be viewed as pioneering by ordinary and not so ordinary skill workers in this particular industry.

[0041] The disclosed configuration is a good example of how known in principle elements incorporated in a new structure render this structure to be on a technological cutting edge. The disclosed MM fiber laser/amplifier with signal light output in substantially a single, fundamental mode (FM) is based on a side-pumping technique in which active and passive pump rods are arranged in a side-to-side arrangement. At least one of the active and pump rods is provided with elements having a refractive index lower than that of the surrounding cladding to increase mixing and absorption of pump modes. However, the utilization of the elements, well known from the end pumping schemes, in a side-pumping arrangement is not obvious. It is well known by one of ordinary skill in the fiber laser arts that mode mixing in active fibers with an asymmetric core is improved. In the side-pumping arrangement of the disclosed type the MM core is asymmetrically located. That is why to the best of Applicants' knowledge and belief the attempts of inserting any additional means into active rods for the enhancement of mode mixing in the side pumping arrangement have not been reported. As to the passive rod in the disclosed structure, again to the best of Applicants' knowledge have not known and for a good reason. Typically the active waveguide including side by side coupled active and pump rods are coiled in the fiber blocks. The applicants tend to believe that pump modes in the coiled fiber deform which worsens the absorption of pump modes. However, typically at the output of fiber block housing a kW-level side-pumped fiber laser/amplifier unabsorbed pump light constitutes very few percent of the pump light delivered to the active rod. This amount of unabsorbed pump light is generally acceptable and further improvement may somewhat negatively affect the overall efficiency of the laser. In contrast, the disclosed structure is configured to increase the laser efficiency.

[0042] With the above in mind, the following description discloses the inventive configuration drastically improving the laser efficiency. FIG. 6A shows an active waveguide 25 of the schematics of FIG. 1 is typically coiled in fiber block FB. The illustrated active waveguide 25 is representative of a side-pumping arrangement including active fiber rod 11 with a MM core 35 which is doped with any of the known light emitters or a combination thereof. For example, the light emitter may be the ions of ytterbium (Yb) generating signal light at, for instance, 1070 nm wavelength λs.

[0043] The active waveguide 25 further includes passive rod 15 delivering MM pump light at pump wavelength λp, for example 976 nm and having an index of refraction at most equal to that of active rod 11. The outer cladding 12 with the index of refraction lower than that of rods 11 and 15 keeps active and passive rods 11, 15 in mechanical and optical contact along the adjoined peripheries of respective rods. The coupled peripheries of the active waveguide define a coupling stretch over the length of which pump light keeps crossing the interface between the rods so as to be absorbed in the MM core 35. As discussed above, not all pump light is coupled into active rod 11, and even the coupled pump light has spiral modes 13 not adequately overlapping the central region of MM core 35. Hence not all the energy of the pump light is converted into that of the signal light affecting thus laser efficiency and output power of signal light.

[0044] In accordance with one aspect of the inventive concept, one or more elements 19 are inserted into the host material of cladding 45, such as silica, of active rod 11. Having the refractive index lower than that of cladding 45, elements 19 are configured to redirect spiral modes 13 of pump light towards core 35 and improve the absorption of these modes. To prevent any undesirable load on core 35, elements 19 are made of silica doped with ions of fluoride (F) and possibly boron (B) which lower the index of refraction ne of elements 19 below index n.sub.c11 of cladding 45 at least 1*10.sup.−3. The latter limitation is critical for effective mode mixing leading to the increased laser efficiency. In contrast, the prior art teaches that this difference between these coefficients should not exceed 1*10.sup.−3, because otherwise the polarization properties of the core guided light may be affected. Yet, the disclosed active waveguide, if necessary, can be configured with polarization-maintenance rods.

[0045] A further feature providing the improved environmental performance of the disclosed waveguide includes partial doping of MM core 35 of active rod 11 with light emitters. Depending on the desired transverse mode different regions of core 35 may be more or less doped. In light of the present disclosure which is concerned with a fundamental transverse mode, it is a relatively small central region 17 which has a higher concentration of ions of rare earth elements than that of an outer core region 16. The latter may not be doped with light emitters at all or have their concentration lower than that of central core region 17 at 50% or less. Such a selective doping reduces the use of pump energy for amplification of HOM propagating close to the periphery of core 35. Geometrically, central core region 17 has a radius of at most 92% of that of outer core region 16. With the above disclosed parameters of the MM core, more pump energy goes on the generation and amplification of the FM.

[0046] FIG. 6B illustrates another embodiment of active waveguide 25 based on the inventive concept. Similar to FIG. 6A, waveguide 25 is realized as the side pumping arrangement including active and passive rods 11 and 15, respectively. In contrast to the embodiment of FIG. 6A, this embodiment features element or elements 19 inserted in passive rod 15. The insertion of elements 19 in any of rods 11 and 15 is done by preliminary drilling the desired number of channels in the rod which later receive respective elements 19. The elements 19 are each configured with refractive index ne lower than index nc15 of rod 15 at least 1*10.sup.3. The pump light and particularly skew rays are directed towards active rod 11 in such a manner that the overlap between coupled into rod 11 spiral pump modes 13 and MM core 35 is increased. The MM core of waveguide 25 is configured similarly to that of FIG. 6A.

[0047] FIG. 6C illustrates yet another realization of the inventive concept including a combination of the inventive features of FIGS. 6A and 6B. Particularly, active and passive rods 11, 15 respectively each are provided with elements 19 disclosed above. The MM core 35 has two or more annular regions as discussed above in regard to respective FIGS. 6A and 6B.

[0048] The active waveguide of FIGS. 6A-6C may be provided with a third cladding 18 (shown in FIGS. 6 B and C) which serves as a shield from external mechanical loads. However, third cladding 18, shielding cladding 12 from physical damages, may have a refractive index greater than the claddings of respective active and passive rods.

[0049] In summary, the laser efficiency of the schematics of FIG. 1 including the side pumping arrangement with the inventive active waveguide is increased at least to 86% for the following structural particularities: [0050] Elements 19 increasing pump light absorption; and [0051] Selectively doped MM core of active rod 11 decreasing amplified HOMs at the signal wavelength.

[0052] In addition to the main structural innovation in the side pumping arrangement of the disclosed active waveguide, a few additional features are incorporate in any of the above disclosed embodiments and contribute to unprecedentedly high efficiency for side-pumped fiber lasers/amplifiers. The shape of active rod 11 may have a bottleneck shaped cross section along the optical axis of this rod with one or both ends each having a diameter smaller than that of central part. The passive rod 15 may be configured with a central part smaller than that of one or opposite ends. The bottleneck-shaped rods 11 and 15 may be incorporated in schematics of FIGS. 6A-6C together or either of them may be paired with the other uniformly shaped rod.

[0053] FIGS. 7A-7D illustrate respective configurations of the refractive step index profile of the active rod and dopant profile provided in its MM core. FIGS. 7A and 7D show a uniformly formed doped central core region 17 of the core and the undoped outer core region 16. FIG. 7B illustrates the dopant concentration of the central core region to be substantially greater than that of outer core region 16. FIG. 7C illustrates a frustoconically shaped dopant profile narrowing towards the center of the core from the interface between the core and cladding.

[0054] Numerous experiments with the above-disclosed active waveguide have been and continue to be conducted amounting to a substantial date. The advantages of elements 19 are clearly seen in FIGS. 8A and 8B. The unabsorbed pump power at the output of fiber block FB of FIG. 1 is sharply reduced from 21 W at 1200 W total input pump power with the prior art active rod to about 3.5 W in the inventive structure.

[0055] Referring to FIG. 8A, black curve 50 represents the maximum laser efficiency of 87.2% in the inventive structure compared to about 81% on a black curve 52 representing the configuration of the known prior art at the output power of FM signal light at a 1070 nm signal wavelength of 900 W and 977 nm pump wavelength.

[0056] The data shown in FIG. 8A is a direct result of the structural innovation of the inventive active waveguide including the reduced unabsorbed pump light and signal light in the cladding as shown in FIG. 8B. Only about 1.5% of signal light is detected in the cladding, as indicated by curve 56 (FIG. 8B). 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, the unabsorbed pump light at the output of the fiber block FB in the inventive structure is between 0.1-0.3% as shown by curve 58 of FIG. 8B, whereas the prior art device has about 2% and higher of the unabsorbed pump light at the maximum laser efficiency as illustrated by curve 60.

[0057] Accordingly, it is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.