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Results of the first experimental demonstration of the recently proposed technique for improvement of the pump absorption in double-clad fibers by their simultaneous coiling and twisting are reported. The peak absorption (14 dB) of 3-m long hexagonal thulium-doped fiber was increased by 8 dB by its simultaneous coiling and twisting. Explanation of the effect is given by numerical modelling of the pump absorption in hexagonal and panda-type double-clad fibers. Improvement of fiber laser performance was also proved. The slope efficiency increased from 19.6% of the straight fiber to 23.9% of the coiled only fiber and 29.4% of the simultaneously coiled and twisted fiber.
© 2016 Optical Society of America
1. Introduction
The high-power operation of fiber lasers was enabled especially thank to the invention of cladding-pumping in a double-clad fiber structure. A number of cross sections with broken circular symmetry was proposed to enhance the pump absorption, for example D-shaped, hexagon, stadium, stress-element inclusion (e. g., panda-type polarization maintaining fibers), spiral cladding, etc [1–5]. Tight coiling also helps to increase the absorption of pump radiation in the inner cladding but only a few experimental studies were published in the open literature concerning the coiling optimization. The figure-eight- [6] and the kidney-shape [7] coiling were experimentally proved to increase the pump absorption. With regard to the numerical studies, they were almost exclusively limited to optimization of the inner cladding shape under the assumption of a straight fiber. To our knowledge the effect of coiling the fiber on the pump absorption has been theoretically studied for the first time in the seminar paper [8], where a rigorous beam-propagation numerical method for the simulation of the speckle pattern of the multimode pump transversal distribution was combined with a heuristic ray-optics approach for the description of the coiling. Recently, we developed a new numerical model respecting twist of the fiber. The fiber curvature is simulated by the modified refractive index profile and twist is simulated by its rotation around fiber longitudinal axis [9, 10]. More recently, we have proposed new method of enhancement of the pump absorption efficiency that consists in simultaneous coiling and twisting of the double-clad fiber [10, 11], see Fig. 1(b). The twisting may be achieved also by rotation of the preform during the fiber drawing; the twist is then preserved in the drawn fiber. Review of the initial numerical modelling results together with analysis of pump absorption in two-fiber bundle, so-called GTWave double-clad fiber structure, can be found in [12]. By using numerical simulations we have shown that the pump absorption efficiency for twisted and coiled fiber with hexagonal shape of the inner cladding cross section may be close to ideal (ergodic) limit of absorption for the straight fiber and in special cases even better than the ergodic limit.
Fig. 1 Coiling methods for improving pump absorption in double-clad rare-earth-doped fibers: kidney shape of the spool (a) and simultaneous twisting and coiling of the fiber on a round spool (b).
In this paper we present an experimental demonstration of the proposed method for enhancing the pump absorption by simultaneous coiling and twisting of the fiber. The particular case of thulium-doped double-clad fibers with hexagonal shape of the inner cladding is studied. Explanation of the improvement effect is given by numerical modelling of the pump field along the thulium-doped fibers of hexagonal and panda cross sections. In addition to the absorption measurements presented in a conference paper [13] we present characterization of the double-clad fiber with hexagonal shape of the inner cladding cross section in a Fabry-Perot laser cavity.
2. Theoretical modelling
The theoretical model is based on finite-element and beam-propagation methods whilst the bending and twisting of the fiber are simulated by using conformal transformation of the refractive index [9–12]. Implementation of the methods is described in detail in [10]. The simulations were performed for two thulium-doped fibers. Firstly, we considered a fiber with a Tm3+ concentration NTm = 7000 mol ppm in the core of diameter of 12 µm, a numerical aperture (NA) of 0.12, and a pump wavelength of λ = 793 nm. The thulium absorption is modelled by means of complex refractive index ncore. Imaginary part of the refractive index is related to the absorption coefficient α through:
where absorption cross-section σaTm = 8.5 × 10−25 m2, stochiometric coefficient k = 1, density of silica ρ(SiO2) = 2203 kg/m3, and mass of silica molecule m(SiO2) = 9.98 × 10−26 kg. The double-clad fiber with a hexagonal inner cladding with a flat-to-flat distance of 130 µm and NA = 0.46 was assumed. A speckle pattern that mimics the pump field of the 105/125 µm multimode- pump-delivery fiber with NA = 0.22 is used for the input, see Fig. 2, z = 0 m. The pump field was synthesized by propagation of fundamental mode of the pump delivery fiber through sections of straight, bent and rotated pump delivery fiber in order to achieve mode mixing. The results of simulations for the hexagonal fiber are shown in Fig. 2 and 3(a). The coiled and twisted hexagonal fiber provides highly efficient pump absorption, close to the ergodic limit for the straight fiber that is given by the core/cladding area ratio, see Fig. 3(a). The advantageous effect of coiling and twisting for pump-absorption enhancement can be explained using Fig. 2. The effective area of the pump radiation is decreased by the bending; the speckle pattern is squeezed towards the outer part of the spool for z>0 m. If no twisting is applied to the fiber, the effective area of the pump field in the coiled fiber is smaller than in the straight fiber. But its outer shape orientation is not changed and the mode scrambling is rather limited. On the other hand, for simultaneously coiled and twisted fiber the rotating corners of the hexagon disrupt the pump field and leads to effective mode-scrambling and improved pump absorption. As a second fiber structure, we considered a double-clad panda-type fiber with two stress-rod inclusions in the inner cladding that enable maintaining of the polarization of the signal propagating in the core. The rod inclusions also serve as elements that broke the circular symmetry of the fiber and enhance pump absorption. The coiling and twisting helps to increase the pump absorption also in panda-fiber though the absorption is not as close to the ideal limit as it was the case for hexagonal fiber. Fiber parameters are summarized in Table 1, while the results of numerical simulations are shown in Fig. 3(b). The numerical model of coiled and twisted fiber does not account for geometrical deformation of the cross-section and stress-induced birefringence that may appear especially when fiber is twisted during its coiling on the spool [10]. Both effects would even more promote mode mixing and therefore the observed improvement in measured absorption. Since twisting of a cold (drawn) fiber undermines its long term reliability due to fatigue, in practical devices it would be more beneficial to use fiber that was spun during drawing so that the twist is preserved in the drawn fiber.Fig. 2 793 nm pump-field distribution at the input and three other longitudinal positions along hexagonal fiber coiled on 3-cm radius spool and twisted with rate 1°/mm. One particular corner of the rotating hexagon is labeled with a star.
Fig. 3 Numerical modelling of pump absorption efficiency along hexagonal fiber (a) and panda fiber (b) for various coiling conditions.
Table 1. Parameters of the fibers in the simulations and experiment
It should be noted that the pump absorption curves shown in Fig. 3 may depend on pump launch conditions. For the stadium-like double-clad fiber cross section we tested a different pump-speckle-pattern distribution and the so-called a flat-top pump beam distribution, where the pump field is evenly distributed across the inner cladding cross section [10]. The same trend in absorption was observed for different pump-field distributions. However, the detailed and systematic study of pump launch distribution is beyond the scope of this paper and it would require time consuming calculations. For example, the simulation of absorption in 3-m long panda fiber in Fig. 3(b) required 10 days by using computer with Intel i7 3930k processor.
3. Experiment
The experimental thulium-doped fiber preform was prepared by using the modified-chemical vapor deposition and the solution doping methods. The preform was ground on its sides into hexagonal shape of the cross section. The drawn fiber has 16 μm core diameter, NA = 0.18, cca 9600 mol ppm of Tm3+ concentration and 115 μm flat-to-flat distance of the inner cladding. The parameters of the actually drawn fiber differed from the designed parameters resulting in a higher cladding absorption than it was predicted. The spectral attenuation of a 3-m long fiber coiled on a spool of 6.5 cm diameter is shown in Fig. 5(a). Halogen lamp was used as a source of wideband radiation. The pump absorption reached almost 14 dB at peak absorption wavelength of 793 nm. The fiber was not twisted during the drawing and also special care was taken to prevent the casual twist during coiling of the fiber on the spool. When the fiber was coiled and simultaneously twisted with one 360° twist per turnaround, the peak absorption increased by 8 dB, confirming the trend predicted by the numerical model. The absorption of the multimode laser diode pump was also measured. The peak-wavelength of the pump was temperature tuned to maximum fiber absorption and the laser diode was operated at low currents above threshold to avoid saturation effects in thulium and to keep the narrowband operation of the laser diode. The measured pump absorption difference between the two coiling methods of 7 dB was measured. It is in good agreement with the measurement by using halogen lamp source presented in Fig. 5(a). It should be noted that the input pump field patterns are usually slightly different when halogen lamp and multimode pump laser diodes are used.
Laser characteristics of the Tm-doped fiber under different coiling conditions were also measured. The laser setup is shown in Fig. 4. The Fabry-Perot laser cavity was formed by high-reflectivity fiber-Bragg-grating (FBG) and perpendicularly cleaved fiber end of 107 cm long Tm-doped fiber. Note that the fiber length shorter than expected optimum device length of the straight fiber case was selected in order to demonstrate the beneficial effect of new coling method on fiber laser performance. Shorter device length is important in many fiber lasers as it mitigates harmful influence of background losses and nonlinear effects. The FBG was inscribed into a single mode fiber with Al2O3/GeO2 doped-core by deep-UV (266 nm) femtosecond laser system and by using Talbot-interferometer setup to illuminate the fiber and create the grating [14]. The FBG reflectivity was >99%, centered at 1951.1 nm, and FWHM was 2.1 nm. The core diameter and NA were 8.9 μm and 0.12, respectively. The improvement of fiber laser performance by coiling and twisting of the fiber is apparent from the laser output vs. input pump power characteristics shown in Fig. 5(b). The slope efficiency increased from 19.6% of the straight fiber to 23.9% of the coiled only fiber and 29.4% of the simultaneously coiled and twisted fiber. The relatively low slope efficiencies were mainly attributed to the fiber core background losses that were about 3.25 dB/m at 2000 nm. Great care was taken to distinguish between the three cases. The straight fiber was laid directly from the spool of the drawn fiber; then it was coiled without twist and only then it was coiled with simultaneous twist of the fiber. Such a procedure was adopted because fibers may preserve some temporal twist due to shape memory of the acrylate used.
Fig. 5 (a) Difference of the pump absorption between the simultaneously coiled and twisted fiber and coiled only fiber for 3 m long hexagonal fiber. (b) Fiber laser characteristics for different coiling condition of 107 cm long hexagonal fiber.
4. Conclusions
Results of the first experimental demonstration of the recently proposed new technique for improvement of the pump absorption in double-clad fibers by their simultaneous coiling and twisting were reported. The 14 dB peak absorption of 3-m long thulium-doped fiber sample with hexagonal inner cladding cross section was increased to 21.9 dB by adding the fiber twisting to the coiling process. The absorption was improved mainly thanks to the enhanced mode mixing provided by the fiber twisting and also thanks to the squeezing of the effective area of the pump radiation due to fiber bending. To our knowledge, the importance of the squeezing effect for the pump absorption in double-clad fibers was recognized only by the rigorous method that takes into account fiber bending for modeling of the pump absorption. The effective mode mixing by rotating the cross section with broken circular symmetry plays the key role in the pump absorption improvement. The rigorous numerical modeling opens a new way to design double-clad fibers and to optimize the pump absorption efficiency. With optimized pump absorption efficiency, the double-clad fiber of shorter length can be used in the fiber lasers and amplifiers. In such a way the harmful influence of background losses and nonlinear effects can be minimized. We have experimentally proved the beneficial effect of fiber twisting on fiber laser performance. The slope efficiency increased from 19.6% of the straight fiber to 23.9 and 29.4% of the coiled only and simultaneously coiled and twisted fiber, respectively. It means that the slope efficiency was improved by more than 50% thanks to simultaneous coiling and twisting of the fiber.
Acknowledgments
This work was supported by the Technology Agency of the Czech Republic, project No. TH01010997, the Czech Ministry of Education, Youth and Sports, project No. LD15122 and by the European Action COST MP1401 “Advanced Fibre Laser and Coherent Source as tools for Society, Manufacturing and Lifescience”.
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