Abstract

The lasers supporting dissipative-soliton (DS) regime produce the highest femtosecond pulse energy thus demonstrating superior performance over dispersion-managed (DM) soliton lasers [1, 2]. DS lasers have all-normal dispersion cavity [1] or at least the normal dispersion segments in the cavity are dominant [2]. This requirement becomes increasingly difficult to fulfill for longer wavelengths due to limited availability of fibers with normal dispersion. If at shorter wavelengths all-normal dispersion laser supporting dissipative solitons could be constructed from conventional fibers, above 1.2 μm fiber or bulk compensators producing normal dispersion should be employed thus inducing the dispersion map. Promising solution has been proposed recently which extends the dispersion-managed soliton theory to dissipative DM (DDM) solitons [3]. It was shown that DDM solitons could be generated at higher strengths of dispersion map as compared with DM strategy, leading to stable pulses with an increased energy. This finding is particularly important for construction of long-wavelength dissipative soliton systems. In this paper, a DDM soliton mode-locked 2 μm thulium-holmium doped fiber laser that operates in a stable single-pulse regime and demonstrates an increase in the pulse energy as compared with DM net anomalous-dispersion cavity. The mode-locked fiber laser with linear cavity is shown schematically in Fig. 1a. The gain was provided by 1.2 m-long Tm-Ho fiber. The cavity is terminated by 130 nm/cm chirp fiber Bragg grating (CFBG) and a semiconductor saturable absorber mirror (SESAM). By changing the length of passive fiber, the net cavity dispersion could be either normal or anomalous in a range from 0.47 ps² to −0.32 ps². The average output power was limited to 50 mW at pump power of 470 mW for normal-dispersion cavity. Fig. 1b shows autocorrelations of 11.7 ps pulse corresponding to the normal-dispersion of 0.24 ps² and 1.54 ps pulse for anomalous GVD of −0.18 ps². The evolution of pulse spectrum with the cavity dispersion is shown in Fig. 1c. As the normal GVD decreases from 0.47 ps² to 0.02 ps², the spectrum bandwidth increases from 3 to 10 nm which shows the promising potential for external pulse compression. The spectrum steep spectral edges for normal GVD are a signature of dissipative solitons [2, 3]. With anomalous GVD, the pulse spectral bandwidth increases from 6 nm to 7 nm when dispersion decreases from −0.32 ps² to −0.18 ps².

© 2011 Optical Society of America