We report on the measurement of the longitudinal temperature distribution in a fiber amplifier fiber during high power operation. The measurement signal of an optical frequency domain reflectometer is coupled to an ytterbium doped amplifier fiber via a wavelength division multiplexer. The longitudinal temperature distribution was examined for different pump powers with a sub mm resolution. The results show even small temperature variations induced by slight changes of the environmental conditions along the fiber. The mode instability threshold of the fiber under investigation was determined to be 480W and temperatures could be measured overall the measured output power values.
We demonstrate a quasi-continuous wave laser amplifier with a peak output power of 6.8 kW pumped by an industrial thin-disk laser. A high slope efficiency of 84 % has been obtained within a duty cycle of 10 % at a signal wavelength of 1071 nm. For cw-pumping we measured a maximum output power of 985W. The amplifier fiber had a step index profile with a core diameter of 45μm and a pump core diameter of 120μm. A signal to ASE peak ratio of 48 dB could be determined.
In this contribution we demonstrate a single mode continuous wave laser amplifier with 146 W of power at a wavelength of 1009 nm. On one hand this experiments constitutes an extension of the wavelength range of high power fiber lasers, furthermore, emission wavelength well below 1030 nm find use for efficient high-brightness tandem pumping of high power fiber amplifiers. The wavelength and bandwidth of the seed oscillator are defined by a pair of fiber Bragg gratings. This seed is amplified in a two-stage Ytterbium-doped rod-type amplifier to 146 W with a high slope efficiency of 64 %, an excellent beam quality and an ASE-suppression as high as 63 dB.
With increasing output power of lasers, absorption in optical components grows larger and demands on heat withdrawal become challenging. We report on the fabrication of a Faraday isolator for high power fiber laser applications (P = 1 kW) at a wavelength of 1080 nm and operation at ambient conditions. We investigate direct bonding of Terbium Gallium Garnet to sapphire disks, to benefit from the good heat spreading properties (having a 6-fold higher thermal conductivity than TGG) at high transparency of the latter. Successful bonding was achieved by extensive cleaning of the plane and smooth surfaces prior to low pressure plasma activation. The surfaces to be bonded were then contacted in a vacuum environment at elevated temperature under axial load. Our measurements show that the bonded interface has no measurable influence on transmission properties and bonded samples are stable for laser output powers of at least 260 W. As compared to a single Terbium Gallium Garnet substrate, wavefront aberrations were significantly decreased by bonding sapphire disks to Terbium Gallium Garnet.
We investigate the influence of seed polarization on nonlinear effects in a high power fiber amplifier for different orientations of the linear seed polarization and for different ellipticities of the seed polarization (linear, elliptic, circular polarized). We show that it was possible to considerably reduce the power of the Raman scattered light. Maximum reduction to around 50% could be achieved by changing the seed polarization from linear to circular. Furthermore, we demonstrate that not only the threshold of nonlinear effects could be influenced by changing the orientation of the linear seed polarization as only parameter but even the limiting effect could be changed: For all orientations of the linear seed polarization Raman scattering was the dominant nonlinear effect except for linear polarization along the slow fiber axis of the slightly birefringent amplifier fiber, where also modulation instability was observed. From our results we estimate the importance of the polarization state as further parameter to increase the nonlinear threshold of high power fiber amplifier systems.