Chair-like pulses in an all-normal dispersion Ytterbium-doped mode-locked fiber laser

• Published in: Applied optics (2004) 2016-12, Vol.55 (35), p.9961-9967
• Main Authors: Gupta, Pradeep K, Singh, Chandra P, Singh, Amarjeet, Sharma, Sunil K, Mukhopadhyay, Pranab K, Bindra, Kushvinder S
• Format: Article
• Language: English
• Subjects: Amplification; Fiber lasers; Nonlinearity; Pulse shape; Raman scattering; Repetition; Spontaneous emission; Ytterbium
• ISSN: 1559-128X; 2155-3165
• 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.