Fundamental Gaussian mode content measurements on active large core CCC fibers

Summary form only given. Applications in gravitational wave detectors [1] require high power lasers with approximately 200 W of single-frequency output in linearly polarized TEM₀₀-mode. Single-frequency signals have been amplified to more than 500 W in master oscillator power amplifier (MOPA) configurations, employing photonic crystal fibers (PCF) [2] as well as chirally coupled core (CCC) fibers [3]. For a PCF amplifier system, a fundamental Gaussian mode power of 203 W was demonstrated in linear polarization [4]. In this contribution we report for the first time measurements of the overlap of high power CCC fiber amplifiers output beam with the TEM₀₀-mode of a scanning non-confocal ring cavity [5].The seed signal for our experiments had a linewidth of about 1 kHz at 1064 nm and a maximum power level of about 11 W. Nominal core and modefield diameters of the tested active fibers were 37 μm and about 30 μm, respectively. The fiber was available in standard polymer-coated double-clad geometry (413 μm) and with an airclad (260 μm inner and 440 μm outer cladding). However, due to the non-optimum cladding shape (round clad) of the polymer-coated fibers in this batch approximately 50 % of the coupled pump light remained unabsorbed. Therefore, here we concentrate on results obtained with air-clad fibers. TEM00 mode content (%)Fig. 1(a) shows the output power versus absorbed pump power for 2.2 m of an air-clad version of this fiber, as well as the corresponding fractional TEM00-mode content. This fiber had a nominal absorption of 9 dB/m and was mounted on a cold-plate with a coiling diameter of 30 cm. With increasing pump power, the fundamental mode content first increased from 95.6 % at 19 W of amplifier output to 97 % at output powers levels between 49 W and 78 W. Concurrently, the PER increased from 17.8 dB to more than 20 dB. At an amplifier output of 103 W, about 98 W of power could be extracted in the lin- arly polarized TEM00-mode. The corresponding modescan is shown in Fig. 1(b). Increasing the pump power led to a decreased beam quality and a higher sensitivity of the modal content to the seed alignment. Power scaling was limited by the onset of modal instabilities [6] to 230 W at the amplifier output, at which point the highest TEM00-mode power reached 186 W, corresponding to a fractional fundamental mode content of 80.7 %.