Study of methodology for gain-switched holmium-doped fiber laser combined with Q-switched Thulium-doped fiber laser

Abstract

Pulsed fiber laser operating at 2000 nm is of great interest in applications such as LIDAR system using very low atmospheric absorption bands, material processing for transparent plastics, etc. Thulium- and holmium-doped silicate glass fibers have been the best choice as a gain medium for the fiber lasers. In particular, a holmium-doped fiber laser (HDFL) has drawn much attention because it opened up the possibility to extend the operating wavelength beyond 2100 nm. The pump source of the HDFL has been typically a thulium-doped fiber laser (TDFL) due to the availability of high power at 1950 nm where the HDF has the maximum absorption. A gain-switched HDFL is the pulsed fiber laser built on the same architecture. The pump source, in this case, has been either a Q-switched TDFL pump by a continuous wave (CW) laser at 793 nm or another gain-switched TDFL pumped by a pulsed laser at 1550 nm. Such cascaded pumping scheme leads the laser configuration to be complicated having redundant optical components [1, 2]. We proposed a couple of methodologies for integrating the gain-switched HDFL and the Q-switched TDFL on a single laser cavity. The main idea here is to let the HDF, the saturable absorber of the Q-switched TDFL, be the gain at the same time in the cavity, thereby being a gain-switched HDFL. One of proposed methods to realize the idea appears in Fig. 1(a). The laser consists of a ring cavity with a linear section. The TDF and HDF are in the ring cavity, and a fiber Bragg grating (FBG, reflection at 1950 nm), a variable optical attenuator and a fiber mirror are in the linear section. In the Q-switched TDFL configuration, the HDF functions as a saturable absorber. The FBG determines the lasing wavelength of 1950 nm at the highest level of attenuation (no reflection from the fiber mirror), which is for case (i) in Fig. 1(b). Very stable output pulses are observed in this case. The lasing wavelength jumps up to the wavelength of 2008 nm at the middle level of attenuation, which corresponds to case (ii) in Fig. 2(b). This is because the effective thulium gain at 2008 nm gets higher than that at 1950 nm. In the meantime, output pulses become unstable due to the low modulation depth of the HDF at the wavelength. At the lowest level of attenuation, the laser operates at two different wavelengths as shown in case (iii) in Fig. 2(b): one is at 1987 nm and the other is at 2013 nm. The attenuator can control the distance of separation, which can be understood by considering the spectral emission and absorption profiles of TDF and HDF. The shorter wavelength (1987 nm) is expected to be from the Q-switched TDFL and the longer wavelength (2013 nm) the gain-switched HDFL. In this presentation, we will introduce two methods for integrating gain-switched holmium-doped fiber laser and Q-switched thulium-doped fiber laser and describe their experimental results.