Ytterbium doped and co-doped Q-switched fiber lasers utilizing passive saturable absorbers

Abstract

Q-switched 2 micron fiber laser is a new area of research. Q-switching using passive technique by means of saturable absorber is quite rare in the research work. A stable passive Q-switched lasers operating at 1 micron, 1.5 micron and 2 micron wavelength regions were demonstrated using Ytterbium doped, Erbium-Ytterbium co-doped and Thulium-Ytterbium co-doped fibers, respectively as the gain medium. Carbon nanotubes and graphene based saturable absorbers (SA) were explored as the passive Q-switcher in the proposed lasers. The Q-switched Ytterbium doped fiber laser (YDFL) operating at 1060.2 nm was realized using a multi-walled carbon nanotubes / PEO saturable absorber. The repetition rate of the laser were varied from 7.92 kHz to 24.27 kHz by varying the pump power from 53.42 mW to 65.72 mW. At the 59.55 mW pump power, the lowest pulse width and the highest pulse energy were obtained at 12.18 μs and 143.5 nJ, respectively. The YDFL was then used to demonstrate an all-fiber based MOPA system where the maximum pulse energy of 354.2 nJ was obtained at the maximum cladding pump power of 800 mW. The Qswitching of Erbium-Ytterbium fiber laser (EYFL) was demonstrated using a multilayer graphene film based saturable absorber. The proposed laser was operated at 1532.5 nm and self-started at pump threshold of 44 mW to produce Q-switching pulse with repetition rate of 12.33 kHz and pulse width of 9.36 μs. At the maximum pump power of 78 mW, the maximum pulse energy of 5.8 nJ and the shortest pulse duration of 2.68 μs were achieved. Multi-wavelength and Q-switched fiber lasers were also demonstrated based on the newly developed octagonal shape double-clad Thulium-Ytterbium fiber (TYF) operating at 2 micron wavelength region. By incorporating the home-made multi-wall carbon nanotubes saturable absorber (MWCNTs SA) in the ring cavity, a Q-switching pulse train operating at 1983.4 nm was successfully demonstrated. By varying the 905 nm multimode pump power from 1570 to 1606 mW, the pulse repetition rate increased from 27.4 to 37.8 kHz and the pulse width fluctuated from 3.8 μs to 4.9 μs. The maximum pulse energy of 10.6 nJ was obtained at pump power of 1570 mW. Besides showing good Q-switching performance, the proposed saturable absorbers are easy to fabricate and inexpensive.