I. Introdution
The -doped fluoride glass fibers have many applications, such as C-band fiber amplifiers [1], green upconversion fiber lasers [2] and high-power 3- fiber lasers [3]. In the case of C-band fiber amplifiers, low-noise operation of fluoride-based -doped fiber amplifier with -level pumping was reported by one of the present authors [1]. A brief review on commercial products of low power green upconversion laser for biomedical applications may be found elsewhere [4]. High-power fiber laser was demonstrated in -doped double-cladding fluoride glass fibers [3]. High efficiency upconversion processes always exist in all these devices due to energy-match and low phonon-energy of the glass host. The effects of upconversion, a key issue for device performance, on these devices are different. For a C-band fiber amplifier, upconversion is completely detrimental considered by almost all the researchers, while upconversion is preferable for a green laser. In the case of high-power 3- fiber lasers, it is more complicated that the terminal level can be depopulated by introducing into the fiber core and the operation of multiwavelength cascaded lasing [5]–[7]. The optimization of these devices can be realized by suppressing or enhancing upconversion by special designs. Therefore, study of those processes related to upconversion in -doped fluoride fibers is very necessary and important for exploring new applications of -doped fluoride fiber and for fully understanding the devices mentioned above. For this purpose, we have investigated free-running properties of -doped fluoride fiber excited at 974 nm, demonstrated high efficient cascaded two-wavelength 853-nm ( transition) and 1533-nm ( transition) lasing from -doped fluoride fiber pumped at 974 nm, and further investigated its effects on green upconversion lasers and C-band fiber amplifiers.