Passively mode-locked laser based on an ultra-large dispersion Yb-doped fiber

Summary form only given. We report on the first realization of a passively mode-locked oscillator featuring an ultra-large dispersion Yb-doped Bragg fiber. The gain medium is a double-clad Bragg fiber featuring a 20 μm Yb-doped core surrounded by a 120 μm inner cladding [1]. The 2 m long gain fiber is cladding-pumped with a fiber-coupled laser diode emitting at 976 nm. Passive mode locking is achieved by the combined actions of nonlinear polarization evolution and a SESAM. Mode-locking is initiated by optimizing the saturation criteria on the saturable absorber mirror using an adequate focusing lens. The emission wavelength of the oscillator is controlled using a passive spectral filter introduced inside the cavity. Two distinct regimes of operation are observed depending on the emission wavelength. For short wavelengths, the laser generates highly-chirped pulses with 3 ps duration and 4 nm spectral widths around 1035 nm. These pulses could be dechirped down to 500 fs duration [1]. When laser emission is tuned to longer wavelengths, long pulses with a narrow spectrum are obtained. Figure 1 summarizes the results obtained when laser emission is centered at 1040 nm. The laser generates long picoseconds pulses with a narrow spectrum of less than 70 pm which corresponds to the resolution of the optical spectrum analyzer (Fig. 1(a)). The FWHM of the recorded autocorrelation trace is 70 ps, corresponding to a pulse width of ~45 ps (assuming a Gaussian pulse shape). The beam profile measured at the laser output (Inset in Fig. 1(a)) corresponds to a supermode structure resulting from the coupling between the fundamental core mode and the LP04 mode of the first ring, as confirmed by calculation (inset in Fig. 1(b)). Dispersion measurements conducted on a passive Bragg fiber exhibiting the same index profile as the gain fiber used in the oscillator, show that the dispersion associated with the supermode structure is as high as -1500 ps/(nm.km) [2]. This corresponds to ~50 times the dispersion of silicate at ~1.04 μm (~ - 30 ps/nm/km).