Antiresonant negative curvature hollow-core optical fibers (NC-HCFs) have proven to have great potential for the delivery of high-energy pulses and for continuous wave Mid-infrared fiber gas laser (FGL) systems. However, following the drawing fiber conditions on the designed fiber geometry, the resulting position of transmission bands of the NC-HCFs needs to be matched to both pumping and lasing wavelengths for the FGL with the particular filled gas. In this work, we investigated experimentally the possibilities of adjusting the transmission bands of a short length of the fabricated NC-HCFs to both UV the Mid-infrared directions that would further enhance their usage in FGL. The investigation was based on two steps. The first step involves fabricating a few fibers from a single preform with different inner geometry dimensions concerning the applied pressure into the preform’s capillaries. This would help to calibrate the correlation of adjusted fibers’ transmission bands to the desired wavelength with respect to the applied pressure while drawing. Increasing the pressure into the capillaries from 300 to 400 mm H2O while maintaining the pressure in the preform core at 30 mm H2O the bands were gradually blue-shifted by ~16.5 % to have about 330 nm shift at the first transmission band. This is attributed to the decrease in core diameter, capillaries gaps, and wall-thickness while increasing the capillaries diameter. The second step involves processing a short length of drawn fibers through local heating and tapering for adjusting their transmission bands' positions using a CO2 laser fusion splicer. Processing the whole length of a short fiber segment ~5 cm shows a complete redshift by ~12.2 % and a blueshift by ~10.2 % after heating and tapering respectively. The first transmission band of the processed fiber was shifted about 200 nm after heating and 122 nm after tapering. The parameters of processing were optimized; thus, no insertion losses were observed. Such results urge us to further develop the mechanism of processing fibers to process long lengths of fibers in the future.
Thulium-doped fiber lasers have been extensively investigated as the most promising source of efficient laser emission at wavelengths around 2 μm, i. e., in the eye-safer spectral region and in the atmospheric window as well. It allows for wide range of applications including medicine, defense, distance measurement or materials processing. To enhance pump absorption efficiency along the active double-clad fiber, good overlap of the pump light and doped fiber core should be achieved along the fiber length. The overlap can be increased by breaking the circular symmetry of the inner cladding by shaping its cross-section. Further mode-mixing and better pump absorption can be achieved by coiling and twisting of double-clad fibers. In this work we present experimental measurement of 792 nm pump cladding absorption of a series of double-clad thuliumdoped fibers with respect to their bend radius, the inner cladding cross-sectional shape and twist rate. With these fibers, we assembled a set of fiber lasers with different resonator setups and tested their performance. Twisting was introduced to fiber during drawing from an octagonal, CO2 laser-shaped or mechanically grinded preform so that the twist remained frozen in the drawn fiber. We have shown that the fiber twist significantly improves the pump absorption even in the case of straight or coiled fibers with large coil radii. We provide a preliminary comparison of two fiber laser resonators.
The high-power operation of fiber lasers was enabled mainly by the invention of cladding pumping within a double-clad fiber structure. Various cross-sectional shapes of double-clad fibers as well as unconventional coiling methods have been investigated both experimentally and theoretically in order to enhance the absorption of the multimode-pump. With enhanced pump absorption efficiency, the double-clad fiber of shorter length can be used in the fiber devices and in such a way the unwanted effects of background losses and nonlinear effects can be mitigated. In this paper we report on numerical modelling of optical pump absorption in double-clad octagonal active fiber of different fiber geometry and layouts. Namely we investigate the effect of the bending radii, twist rate of the fiber, doped core area (holmium is considered in this as a doping ion) and pump beam shape. The numerical model is based on FEM-BPM method. The optimized geometries and layouts shall finally result in a highly efficient laser of small footprint without the need of water cooling with great potential for application with low power consumption, tightly limited space and weight requirements. Optimized design will also minimize risk of damage of the fiber during operation of the fiber laser.
The high-power fiber lasers rely on the use of double-clad active fibers with noncircular symmetry of their inner-cladding cross-section. Therefore, the optical fiber preforms had to be shaped before fiber drawing. A new technique of preform-shaping by a CO2 laser is now available along with the conventional mechanical-based grinding. This innovative technique retains the advantages of enabling to produce complex inner-cladding shapes that not easily achievable by a conventional grinding technique. However, one of the drawbacks of the CO2 laser-based preform-shaping is weak of hydroxyl OH-groups reduction during the ablation process. The water is often penetrating into the preform surface via the oxyhydrogen flame during preform manufacturing. The thermophysical nature of the CO2 laser ablation process causes further diffusion of the OH-ions deeper towards the preform center during shaping. The diffused OH-groups in the glass material cause high attenuation at some wavelengths which are associated with the overtones of the fundamental OH absorption peaks. Unfortunately, some of these peaks lay rather close to the commonly used laser pumping wavelengths. This should be considered when designing a double-clad fiber laser as well as when selecting the preform-shaping technique. In this work, we will present a new method of mitigation of the water penetration into the optical fiber preform when a CO2 laser preform-shaping technique is applied. This method includes an optical fiber preform etching procedures prior to the preform laser shaping and to the fiber drawing. The acquired data helps also to predict the thickness of the layer that should be removed from the preform surface. The knowledge of the thickness of the optimal layers is of great benefit for the advanced estimation of the inner-cladding attenuation, an important parameter of double-clad fibers intended for high-power fiber lasers.
We experimentally investigated loss of multimode optical fibers (MMFs) drawn of thermally shaped optical fiber preforms (OFPs). Such preforms are typically used for fabrication of double clad active fibers. The investigation involved undoped shaped MMFs coated with a low refractive index polymer. The fibers were drawn of silica rod, prepared by collapsing a pure silica tube (Heraeus F300, OH content is 0.2 ppm) in the MCVD lathe. Background losses of undoped MMFs with inner cladding of various geometries shaped by CO2 laser were measured via cut-back method. Losses of the shaped MMFs were compared to the loss of the circular and of the mechanically shaped MMF. Constraints, drawbacks and advantages of shaping the fiber preform using the CO2 laser are discussed. Shaping OFPs with CO2 beam provides advantages of quick polishing, smooth surface, and freeform shape. Results show that mechanical polishing technique leads to significant OH content elimination, which is expressed as reduced absorption peaks at wavelengths of 0.945 μm and 1.24 μm, which correspond to the third and second overtone, respectively. The average perimeter length of fibers cross section governs absorption at polymer-glass interface.
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