# A Passively Q-Switched Holmium-Doped Fiber Laser with Graphene Oxide at 2058 nm
## 3 Q-Switching of a 2058-nm Fiber Laser  
The whole laser schematic diagram including a Ho-doped fiber ring resonator and a Tm-doped fiber-based high power pumping source is presented in Figure 4. Our laser was constructed with single-clad Ho-doped fiber rather than double clad one, since our targeted laser did not require a high output power. We chose a 1900-nm Tm-doped fiber laser as a pumping source for the Ho-doped fiber considering the fact that the pump absorption based on the transition of \({}^{5}I_{8}\to{}^{5}I_{7}\) allows for better quantum efficiency than  
Figure 3: (**a**) Measurement setup for nonlinear transmission curve. (**b**) Measured nonlinear transmission curve of the prepared GO-based saturable absorber (SA). (PC: polarization controller, VOA: variable optical attenuator). Figure 2: (**a**) Linear optical absorption spectrum of the GO particles. (**b**) Schematic diagram of GO-deposited side-polished fiber. that of \({}^{5}I_{8}\to ^{5}I_{6}\)[80]. Taking into account that our prepared SA had a transmission type configuration, our laser cavity was constructed with a simple ring structure. The all-fiber continuous-wave (CW) thulium-doped master oscillator and power amplifier (MOPA) source consists of a CW seed oscillator, a Tm-Ho-codoped fiber prestage amplifier, and a Tm-doped fiber main-amplifier. The output beam from the seed oscillator was first amplified with a preamplifier stage and then further amplified with a main power amplifier stage. The output wavelength of the CW seed oscillator was \(\sim\)1928 nm. The output of the seed oscillator was coupled into a preamplifier stage via a 1550/2000-nm wavelength division multiplexer (WDM). For the prestage amplifier, 1-m length of Tm-Ho-codoped fiber an absorption of 13 dB/m at 1550 nm was used. A commercially available 1550 nm erbium-doped fiber amplifier (EDFA) was used as a pumping source for the prestage amplifier. The main stage amplifier was comprised a 793 nm laser diode (LD), a \((2+1)\times 1\) combiner, and 3-m length of double-clad (DC) Tm-doped fiber. The cladding absorption of the DC-Tm-doped fiber at 793 nm was \(\sim\)4.5 dB/m. The core and inner cladding diameter were 10 and 130 \(\upmu\)m, respectively. The 1928-nm pumping beam, which had a maximum power of \(\sim\)1.1 W was launched into the Ho-doped fiber ring resonator through a 1930/2080-nm WDM. A 1-m-long Ho-doped fiber (Nufern SM-HDF-10/130) was used as a gain medium for the cavity. To ensure unidirectional light propagation, an isolator was used. A 90:10 output coupler was utilized to extract the laser output, and the output pulses were obtained from the 10% port through a 90:10 coupler. In order to induce lasing from the all-fiber resonator, the pump power was enlarged. The CW lasing was observed to start when the pump power reached \(\sim\)660 mW. Subsequently, the CW lasing mode was converted into the Q-switching mode as soon as the pump power arrived at \(\sim\)759 mW. The measured optical spectrum of the Q-switched pulses at a pump power of \(\sim\)759 mW is shown in Figure 5a. The peak wavelength was \(\sim\)2058 nm and interestingly, multiple peaks were observed in the optical spectrum, implying that gain competition among many longitudinal modes occurred, such longitudinal mode instabilities in fiber laser devices were reviewed in [81; 82]. This intermode gain competition can be associated with the fact that no mode-selective filtering component was used in the  
Figure 4: Experimental setup of the Q-switched, holmium-doped fiber laser. (THDF: thulium-holmium co-doped fiber, DC TDF: double-clad thulium-doped fiber, HDF: holmium-doped fiber, PC: polarization controller, WDM: wavelength division multiplexer). laser configuration. Figure 5b illustrated the output oscilloscope traces at various pump power levels. It is clearly evident that stable Q-switching operation were maintained over a wide range of pump powers above the threshold of ~759 mW. The temporal period of the output pulses was observed to decrease when the pump beam power became larger.