Relaxation dynamics measurements in bi-fiber lasers

Since the discovery of a wideband (220-300 nm) infrared photoluminescence in bismuth containing alumosilicate glass centered in a 1.1-1.25 mum wavelength band, researchers have focused on exploiting this luminescence in oxide silicate glasses of various compositions for laser purposes. Interest in this effect is mainly associated with the possibility to create Bi-doped fiber amplifiers in the second telecommunications window and for accomodating the future increase of data traffic in telecommunications optical fiber networks. A linear cavity laser containing a 12-m piece of alumosilicate fiber doped with 3times10^-18 cm^-3 of Bi was used to investigate the characteristics of Bi. The cavity was defined by a 5% output coupler and a 1/600 mm^-1 reflection grating operating in a Littrow configuration. The laser was pumped with a 1060-nm Yb fiber laser, supplying up to 1.7 W of pump power to the Bi-fiber. The relaxation oscillation frequency was determined from the repetition period of the small-amplitude oscillations, using and RF-analyzer and an oscilloscope. For lambda>1170 nm, the laser operates in a quasi-4-level scheme. In contrast, lambda<1170 nm, oscillations exhibit significant spectral variation due to thermal population of the terminal energy level at room temperature. This indicates a change in the type of laser transition from four- to three-level type when going to shorter wavelengths.