A long uniform taper applied to an all-fiber Tm3+ doped double-clad fiber laser

A long uniform taper fabricated on a large mode area Tm3+-doped double-clad fiber laser, which was clad-pumped by a laser diode (LD), was found to be an effective wavelength filter while improving the output beam quality and narrowing the line-width significantly. The long uniform taper was fabricated directly on the multi-mode Tm3+-doped fiber by heating and stretching method, and located several centimeters before the output fiber end. By slightly bending the taper section, the output laser spectrum was left with only one peak with a line-width less than 0.5 nm, compared to the multi-peak spectrum with a 5 nm line-width before tapering, indicating that the multi-mode fiber could produce quasi-single wavelength output with a long uniform taper. The beam quality factor M2 declined from 6.6 to 2.6 compared. Only a slight decrease in slope efficiency, from 19.2 to 17.5%, was observed. The main output wavelength had a blue shift of 8 nm.