Abstract

The development of high energy short pulse sources at 2 µm wavelength range demonstrates a steady progress during recent years. Real world applications such as medical surgery and polymer micromachining [1] require compact, monolithically integrated all-fibered configurations. While table-top systems deliver outstanding performances on the order of hundreds of µJ and 2 GW peak power in 200 fs compressed pulses [2], the few µJ level is still a hurdle for monolithic all-fiber systems. The best results obtained in relatively compact systems are achieved using thulium-doped fiber master oscillator power amplifier (MOPA) systems operating at center wavelengths around 2 μm to avoid water vapor absorption in the air near the optimal 1.9 μm regime of thulium. Several microjoules of energy with sub-picosecond pulses were achieved with such configurations [3] [4]. In this communication, we present a 10 µJ-class ultrafast thulium-doped fiber MOPA system operating at 1940 nm. The chirped pulse amplification (CPA) approach was applied for the seed pulses directly generated at the operating wavelength of 1940 nm. Our main oscillator is working in the dissipative soliton (normal dispersion) regime [5]. This concept allows all-fibre integration [6] and offers several advantages such as i) high seed average power, ii) adjustable broad spectrum to fit dispersive components transmission band and iii) pre-stretched pulse. To make the total system compact, free-space elements must be avoided. This architecture requires the use of step-index large mode area (LMA) active fibres in the power amplifier stage instead of rod-type photonic crystal fibers. Therefore, the final stage in our MOPA scheme (amplifier N2 in Fig.1a) is based on highly doped thulium doped fibre featuring 25-micron core diameter. In order to boost the pulse energy above the µJ level, the nonlinear threshold, imposed by active fibre core size, is increased by stretching the seed pulse to 550 ps by means of a chirped fiber Bragg grating (CFBG) stretcher. The system also includes fiberized acousto-optical modulator for repetition rate control. Finally, the output pulse was compressed using chirped volume Bragg grating (CVBG). The linear amplification regime was preserved up to the maximum available pump power where output average power reached P_out = 2.4 W at a repetition rate of 100 kHz (Figure 1b, bottom x-axis, left y-axis). After compression with 80 % efficiency this corresponds to P_compressed = 1.9 W. The AC trace for maximum achieved performance is presented in Figure 1b (top x-axis, right y-axis). The careful evaluation of the energy concentrated in the central part of the pulse (~54%) led to a pulse energy E_p = 10.3 µJ and to a peak power of 5.1 MW in 2 ps pulses (slightly higher than the Fourier limit of 1.6 ps). Corresponding optical spectrum is presented in Figure 1c (black) together with that measured directly at the amplifier output (red).

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