A novel method for high-sensitive measurements of optical attenuation in metal-coated optical fibers in a wide range of wavelengths is demonstrated. Some part of the radiation transmitted through the waveguiding core of the metallized silica fiber is scattered and eventually absorbed in the carbon layer or the metal coating, thus heating it. Absorption in the silica core also contributes to the overall fiber heating. The method used for determination of optical attenuation coefficients of metallized fibers is based on the measurement of the change of temperature-dependent electrical resistance of metal coating induced by transmitted laser radiation. A number of single-mode and multimode metallized fibers with different geometry were investigated using several laser sources operating in visible and near infrared ranges. Experimentally obtained spectral dependence of optical losses of copper-coated fibers was analyzed. Possible explanations for optical attenuation mechanisms at different wavelengths and in different fibers were proposed. The obtained results can help to optimize various devices based on metal-coated fibers, such as laser radiation power fiber sensors or high-power laser sources.
In this paper we introduce a novel approach for the measurement of the intensity profile of high-power laser radiation, which does not require any preliminary attenuation. It is based on the application of the matrix made of multimode passive copper-coated optical fibers. It is well known, that laser radiation experiences Rayleigh and Mie scattering while transmitting along an optical fiber. Radiation scattered inside the fiber core is completely absorbed by the outer copper layer leading to its heating. The temperature change of the metal coating proportional to the transmitted optical power is determined directly by measuring its electrical resistance. A matrix sensor was assembled for measuring the transverse intensity distribution of the laser beams. It comprised nineteen 660 μm (core diameter 600 μm) multimode copper-coated optical fibers. Intensity profile measurements were carried out for the 67 W single-mode Yb-doped fiber laser and 72 W multimode laser diode sources. The laser radiation was directed into the polished end faces of the fiber matrix elements. Optical power transmitted through each fiber was proportional to the incident optical intensity at corresponding location of the fiber end face. The transverse intensity profile of the laser beam was evaluated by measuring the resistance change of the copper coating of each fiber. Preliminary the calibration of the resistance dependence on the incident optical power was separately conducted for all 19 fibers. An introduced technique can be also applied for the determination of the radiation beam quality factors such as M<sup>2</sup> and BPP.