High-power Yb:YAG single-crystal fiber amplifiers for femtosecond lasers in cylindrical polarization

Abstract : We demonstrate a three-stage diode-pumped Yb:YAG single-crystal-fiber amplifier to generate femtosecond pulses at high average powers with linear or cylindrical (i.e., radial or azimuthal) polarization. At a repetition rate of 20 MHz, 750-fs pulses were obtained at an average power of 85 W in cylindrical polarization and at 100 W in linear polarization. The report includes investigations on the use of Yb:YAG single-crystal fibers with different length/doping ratio and the zero-phonon pumping at a wavelength of 969 nm in order to optimize the performance. In the last decade, laser beams with cylindrical polarization states, namely radial and azimuthal polarizations, have sparked increasing interest. Radially and azimu-thally polarized laser beams have unique properties that are advantageous for a wide range of applications, including laser material processing. The isotropic properties of the polarization state increase cutting speed and lead to higher aspect ratio holes with improved symmetry for drilling [1], and reduce spattering in welding applications [2]. Moreover, the doughnut-like intensity distribution of cylindrically polarized beams exhibit steeper slopes than Gaussian beams, which leads to higher processing quality with sharper edges and higher process efficiencies. Therefore, it becomes very interesting to implement the cylindrical polarization also in ultra-fast systems dedicated to micromachining. Cylindrically polarized laser beams can directly be generated within a laser cavity or by converting linearly polarized beams by means of an extra-cavity polarization converter [3–7]. In the latter case, easy-to-implement and robust segmented wave-plates can be used [8], which are particularly well adapted to high average powers and allows to decrease the constraints on the oscillator, which is especially important for femtosecond oscillators. The master oscillator power amplifier (MOPA) scheme therefore is a very convenient approach to combine cylindrical polarization, high average power, and ultrashort pulses. Yb-doped diode-pumped solid-state lasers (DPSSL) in MOPA configuration clearly dominate the field of high average power ultrafast lasers. Among the geometries used so far for high-power Yb-doped DPSSL as slabs [9,10], rods [11], and thin disks [12], the single-crystal fiber (SCF) technology was recently shown to have a high potential for the amplification of ultrashort pulses thanks to a very efficient thermal management and high optical efficiencies. This technology combined with the cubic crystal structure of Yb:YAG offers a cylindrical symmetry of the optical and thermo-mechanical properties. Yb:YAG SCFs are therefore well suited for the amplification of cylindrically polarized beams. CW beams with radial and azimuthal polarization have recently successfully been amplified up to a power of 100 W in an Yb:YAG SCF [13]. Here we now report on the use of an SCF to directly amplify femtosecond pulses in a passive MOPA to achieve high-energy and high average power pulses with radial and azimuthal polarizations without any stretching and recompression of the pulses. The experimental setup is schematically shown in Fig. 1. The seed laser oscillator provided 360-fs pulses at a repetition rate of 20 MHz with an average power of 1.5 W and a spectral width of 3.45 nm FWHM centered at a wavelength of 1031 nm. The amplifier consisted of three Yb:YAG SCF TARANIS modules. The implementation of the three separate SCF amplifier stages was an opportunity to investigate different solutions with respect to the pumping diode, gain media, and setup configuration in order to optimize the overall performance. The setup is composed of two Yb:YAG SCF preamplifiers operating in linear polarization to favor high gain amplification, and a last amplifier stage operating in radial/azimuthal polarization achieved by adding the polarization converter at the input.
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Optics Letters, Optical Society of America, 2015, 40 (11), pp.2517-2520. 〈10.1364/OL.40.002517〉
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Fabien Lesparre, Jean Thomas Gomes, Xavier Delen, Igor Martial, Julien Didierjean, et al.. High-power Yb:YAG single-crystal fiber amplifiers for femtosecond lasers in cylindrical polarization. Optics Letters, Optical Society of America, 2015, 40 (11), pp.2517-2520. 〈10.1364/OL.40.002517〉. 〈hal-01179174〉

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