|
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.
|
