Gupta Pradeep K, Singh Chandra P, Singh Amarjeet, Sharma Sunil K, Mukhopadhyay Pranab K, Bindra Kushvinder S
Appl Opt. 2016 Dec 10;55(35):9961-9967. doi: 10.1364/AO.55.009961.
We report, for what we believe is the first time, generation of stable chair-like pulses (a pulse shape with an initial long flat portion followed by a short high peak power portion resembling the shape of a chair) by mode locking of a Ytterbium (Yb)-doped fiber laser. Chair-like pulse shapes are achieved by implementing dual saturable absorbers, one based on a nonlinear optical loop mirror (NOLM) and the other based on nonlinear polarization rotation (NPR) inside the cavity. The transmission characteristics of the NOLM-NPR pair leading to the formation of chair-like pulses are numerically investigated. We also report the amplification characteristics of chair-like pulses in an external multistage Yb-doped fiber amplifier setup at different repetition rates of the pulse train. It was found that the chair-like pulses are suitable for amplification, and more than 10 W of average power at 460 kHz repetition rate have been obtained at total pump power of ∼20 W coupled to the power amplifier. At a lower repetition rate (115 kHz), ∼8 W of average power were obtained corresponding to ∼70 μJ of pulse energy with negligible contribution from amplified spontaneous emission or stimulated Raman scattering. We believe that such an oscillator-amplifier system could serve as an attractive tool for micromachining applications.
据我们所知，我们首次报道了通过锁模掺镱（Yb）光纤激光器产生稳定的椅状脉冲（一种脉冲形状，初始为长的平坦部分，随后是短的高峰值功率部分，类似于椅子的形状）。通过在腔内采用双饱和吸收体实现椅状脉冲形状，一个基于非线性光学环镜（NOLM），另一个基于非线性偏振旋转（NPR）。数值研究了导致形成椅状脉冲的NOLM - NPR对的传输特性。我们还报道了在外部多级掺镱光纤放大器装置中，不同脉冲序列重复率下椅状脉冲的放大特性。发现椅状脉冲适合放大，在耦合到功率放大器的总泵浦功率约为20 W时，在460 kHz重复率下获得了超过10 W的平均功率。在较低的重复率（115 kHz）下，获得了约8 W的平均功率，对应约70 μJ的脉冲能量，放大的自发辐射或受激拉曼散射的贡献可忽略不计。我们认为这样的振荡器 - 放大器系统可作为微加工应用的有吸引力的工具。
