Gain-switched, Yb-doped, all-fiber laser with narrow bandwidth

Abstract

Summary form only given. Rare-earth doped fiber lasers have emerged as attractive alternatives to traditional lasers in material processing due to the ruggedness, high efficiencies, and excellent beam quality [1]. The development of pulsed fiber lasers is driven by the fact that specific applications require different peak powers, repetition rates, and pulse energies. Some of the methods of pulsing the lasers completely avoid free-space coupled components and they benefit greatly from the all-fiber robustness. Examples of all-fiber pulsed laser technologies are mode-locking by nonlinear polarization rotation and Q-switching by using custom-made saturable absorber fiber [2]. Gain-switching of a fiber laser is a less-investigated method that makes use of the inherent relaxation oscillations of the fiber laser by fast modulation of the pump [3,4]. The advantage is that only readily available components for a CW laser are required. Gain-switched fiber lasers can deliver high pulse energies and has applications in e.g. supercontinuum generation [4], where the increased peak power reduces the dependence on the zero dispersion wavelength [5].Here we report on generating narrow bandwidth pulses by a gain-switched Yb-doped fiber laser. The master oscillator (MO) seen in Fig. 1(a) is built from readily available components made for PM CW fiber lasers and the only modification is the pulsed pumping scheme [4]. The bandwidth and peak power of the output of the MO have been characterized. In Fig. 1(b) it can be seen that a high peak power of 700 W is obtainable (at the highest repetition rate), however the high gain at the time of pulse emission results in significantly broadened spectra. The peak power is reduced at lower repetition rate due to decay of excited Yb-ions in the long time (100's of μs) between pump pulses. In Fig. 1(d) the absorbed pump energy is reduced by a factor of two and the bandwidth increases now only slightly with repetition rate remaining below 0.1 nm. A side effect is that the peak power is reduced to 150 W. This narrow bandwidth gain-switched all-fiber MO can be used for second harmonic generation (SHG) in a periodically poled stoichiometric lithium tantalate crystal (ppSLT). Efficient SHG in ppSLT requires a narrow bandwidth (typically 0.1 nm), linear polarization, and high peak intensity. We amplify the pulses by a pulsed pumped amplifier (PA) into the kW-range and focus the light into the nonlinear crystal. A conversion efficiency of 37% was achieved with a maximum pulse energy of 84 μJ at 532 nm, as can be seen in the table in Fig.1(c).In conclusion, we have demonstrated that an all-fiber, narrow bandwidth, pulsed laser can be constructed from commercially available components by applying gain-switching.