[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Build-Up of Bidirectional Ultrashort Pulse Laser Generation: From Q-Switched Instabilities to Mode-Locking

摘要： Development of new technologies in electronics has made it possible to observe real-time spectral evolution of ultrashort pulses via time-stretching techniques [1]. These experimental methods complement the theoretical completed work on the rich build-up dynamics of solitons in ultrafast lasers [2, 3]. In this work, we have obtained the formation dynamics of counter-propagating solitons in a bidirectional hybrid mode-locked erbium-doped fibre laser using the dispersive Fourier transform (DFT) technique [4]. The bidirectional ring Erbium-doped fibre laser, adopted from [5] is mode-locked via the co-action of single-walled carbon nanotube and non-linear polarization rotation. The laser generates near transform-limited 790 and 570 fs soliton pulses at 14.78 MHz repetition rate with ~1 mW output power. Real-time dynamics were evaluated using a pair of 50-GHz fast photodetector and a 33 GHz real-time digital storage oscilloscope. The DFT is realised in the dispersion compensated fibre (DCF) with overall dispersion of D = -1200 ps/nm. DFT resolution is determined by the total accumulated dispersion and limited by the fundamental Fourier limit to Δλ = 0.28 nm [4]. We have observed soliton formation in frequency domain for both clockwise (CW) and counter-clockwise (CCW) directions. The whole bidirectional pulses formation takes almost 62000 roundtrips from first modulation instabilities until stable ultrafast bidirectional regime is established. Fig. 1a shows real-time spectrum formation for both pulses from 60100 to 62900 roundtrips, when the stable mode-locking is established in both directions. The whole formation dynamics differ from the earlier demonstrated in unidirectional lasers in [2, 3]. The generation starts from characteristic modulation instabilities, which spectral bandwidth rapidly increase, transforming to Q-switched generation (Fig. 1b). Bound soliton formation and unstable mode-locking generation observed before stable mode-locking regime is occurred (Fig. 1e) from unstable Q-switched generation. We believe that obtained results would constitute a basis for the further investigations of counter-propagating soliton dynamics and interactions in bidirectional fibre laser cavities and the role of Q-switched instabilities in mode-locking regime formation.