New multisoliton complex in Bi-doped fiber laser operated at 1450 nm

Summary form only given. The soliton - soliton interactions in mode-locked fiber lasers catch the attention of optical scientific society because it abounds in multifarious effects. As a sequence of these interactions (direct or indirect) the solitons constitute multi-soliton complexes with distinctive features. Bound solitons, bunch of solitons, and soliton rains have been widely covered in the literature, where were described the mechanisms of their formations and possible transition from one state to another [1, 2].We demonstrate a new type of multisoliton complex on the basis of Bi-doped frequency-shifted feedback fiber laser operated at 1450 nm. The fiber laser has the linear cavity configuration shown in Fig. 1a. The total cavity length is 11.3 m of fiber and 0.2 m of free space with corresponding fundamental repetition rate of 9 MHz. The laser is pumped by using a semiconductor disk laser at 1320 nm with a maximum pump power of 1.6 W. The total double-pass cavity dispersion amounts to 0.34 ps/nm. The new multisoliton complex features both a background of chaotic bunch dynamics and a pulse group with a constant interpulse spacing presented simultaneously in the cavity (Fig. 1 b,c). As it can been seen from the Fig. 1b the autocorrelation trace contains both a large noisy pedestal and the group of 5 bound pulses with 15.4 ps spacing. The optical spectrum reveals deep modulation with up to 30 dB amplitude and 0.47 nm (68 GHz) period (Fig. 1 c). The pulse duration was 1.89 ps. The spectrum width was equal to 1.2 nm corresponding to 0.33 of time-bandwidth product. The acoustooptic frequency shifter starts and maintains mode-locking and, due to strong polarization dependence of its crystal, it functions simultaneously as a polarization comb filter with a periodicity corresponding to the crystal properties. Bi-doped fiber provides sufficient gain for generation of plenty of solitons, at the same time it introduces slow relaxation absorption in the cavity due to its bi-exponential relaxation dynamics (<; 1 μs), which stimulates the soliton grouping [3]. The experimental study results are confirmed by theoretical investigation revealed all mechanism behind this type of soliton dynamics (Fig. 1d).