Abstract

The generation of high-energy pulses from Ytterbium-doped fiber lasers has been under intense investigation for the past few years. To date, the shortest and most energetic pulses are generated in a single-clad fiber laser using polarization additive pulse mode-locking. Self-similar femtosecond lasers with pulse energies above 14 nJ have been recently demonstrated [1]. The peak-power of pulses in mode-locked single-clad fiber laser has been limited because of overdriving of saturable absorber and nonlinearity accumulated into small fiber size core etc. The other approach to decrease the nonlinearity and, hence, generate higher pulse energies of ultra-short pulses from fiber laser is to use multimode (MM) fiber as a gain medium. Fermami et al. [2] have demonstrated the stable operation of passively modelocking MM fiber laser obtained under certain conditions. However, due to non-ideal excitation of fundamental mode in the MM fiber, small satellite pulses occur at the output of the fiber indicating poor quality of output pulses. In this contribution, we report the first experimental demonstration of passively mode-locked Yb-doped single-mode large- mode-area air-clad photonic crystal fiber laser. The fiber laser is mounted in a sigma cavity configuration. The ytterbium-doped core is fonned by seven missing air holes and has a diameter of 30 μm. The fiber consists of a hexagonal lattice of air holes with a diameter d of 0.8 μm and a hole-to-hole spacing A of approximately 9.23 μm, therefore d/Λ = 0.09. The fundamental mode-field-diameter ~25 pm (NA = 0.03). The inner cladding has a diameter of 135 μm (NA = 0.062). The pump light absorption of this structure is ~13 dB/m at 976 nm The length of the fiber inside the cavity is about 1.3 m. The fiber ends are polished at an angle of 8° to eliminate parasite reflection into the fiber or sub-cavity effects. The fiber laser is pmnped from just one side by a fiber coupled diode laser operating at 976 mn. The fiber loop is connected to a grating pair and semiconductor saturable absorber mirror with polarization-dependent optical isolator. The polarizing isolator provides unidirectional operation into the loop segment. The reflected polarization of the polarization beam splitter of the isolator serves as the output coupler. The output pulses have linearly polarized. The M²-value of the output laser beam is characterized to be 1.2, meaning a nearly diffraction-limited beam quality. In the linear segment of the cavity, grating pair with 600 lines/mm and AR coated semiconductor saturable absorber based on a multi layer of GaAs/AlAs Bragg mirror and low temperature molecular beam epitaxy grown InGaAs quantum wells in front of the mirror are inserted. The saturable absorber has a high modulation depth with a recovery time in the picosecond range. Passively mode locking is achieved through the optimizing of saturation criteria on the saturable absorber and by using the advantage of non-linear polarization rotation adjusting the orientation of the intra-cavity polarization controllers based wave-plates to optimize output pulse emission. In our experiment, the total second-order cavity dispersion is -0.048 ps² at 1035 mn. The laser operates thus in the anomalous dispersion regime. A stable self-starting pulse train at a 53.33 MHz repetition rate is obtained. The typical optical spectrum obtained is shown in Fig. la. The central wavelength is 1035 mn with optical spectrum bandwidth of ~ 7.5 mn. The autocorrelation trace obtained is presented in Fig. lb. The autocorrelation width is 760 fs, corresponding to a 493 fs pulse duration (assuming a seclr pulse shape), which indicates that the time-bandwidth product is 3.3 times higher than the theoretical limit. It is well known in soliton-like pulse fiber lasers that the pulse width undergoes small changes along the cavity round trip even for anomalous total cavity dispersion. In conclusion, we reported, for the first time to our knowledge, the generation of ultra-short pulses in an Ytterbium-doped large-mode-area air-clad photonic crystal fiber laser operating in the soliton-like regime. The fiber laser directly generates sub-500 fs pulse duration. In the single pulse regime, the laser delivers 880 mW of average power corresponding to pulse energy of more than 16.5 nJ with diffraction-limited quality. Thus, the pulse energy is considerably higher than in conventional single-mode fiber oscillators operating in the anomalous dispersion regime which typically generate few 100 pj before pulse break-up occurs. Scaling pj pulse energy appears feasible by employing single-mode LMA fiber with diameter larger than 50 pm and operating in the wave-breaking-free regime. Fig. 1. (a) Typical output spectrum of the laser operated in the soliton regime (b) Autocorrelation trace of the output.

© 2007 IEEE