We experimentally investigated the impact of the mode filtering technique on the performances of pulse amplification in a fiber with a large core diameter. The technique was applied to a femtosecond pulse amplifier, and was based on a large area double-clad Yb-doped fiber. The mode filtering enabled selective excitation of the lowest transverse mode with minimal contamination of higher order modes. The output pulses with 110 fs duration, > 30 nJ pulse energy (> 3 W average power), and clean spatial/temporal profiles were successfully generated. Benefits of this technique are also discussed.

To obtain an ultra-short high-intensity pulse source, we investigated the amplification characteristics of two types of pulses (dissipative soliton and stretched pulses) produced by our Yb-doped fiber laser oscillator. Our results show that the dissipative soliton pulse can be amplified with less deterioration than the stretched pulse.

We review our recent work on Yb-doped and hybrid-structured solid photonic bandgap fibers (Yb-HS-SPBGFs) for linearly-polarized fiber lasers oscillating in the small gain wavelength range from 1160 nm to 1200 nm. The stack-and-draw or pit-in-jacket method is employed to fabricate two Yb-HS-SPBGFs. Both of the fiber shows optical filtering property for eliminating ASE in the large gain wavelength range from 1030 nm to 1130 nm and enough high birefringence for maintaining linear polarization, thanks to the photonic bandgap effect and the induced birefringence of the hybrid structure. The fiber attenuation of the Yb-HS-SPBGF fabricated by the pit-in-jacket method is much lower than that of the Yb-HS-SPBGF fabricated by stack-and-draw method. Linearly-polarized single stage fiber lasers using Yb-HS-SPBGFs are also demonstrated. Laser oscillation at 1180 nm is confirmed without parasitic lasing in the fiber lasers. High output power and high slope efficiency in linearly-polarized single-cavity fiber laser using the low-loss Yb-HS-SPGF fabricated by the pit-in-jacket method are achieved. Narrow linewidth, high polarization extinction ratio and high beam quality are also confirmed, which are required for high-efficient frequency-doubling. A compact and high-power yellow-orange frequency-doubling laser would be realized by using a linearly-polarized single-cavity fiber laser employing a low-loss Yb-HS-SPBGF.