# A Passively Q-Switched Holmium-Doped Fiber Laser with Graphene Oxide at 2058 nm
###### Abstract  
This study reports a Q-switching-based, 2058-nm holmium (Ho) fiber laser incorporating a saturable absorber (SA) based on graphene oxide (GO). The SA was prepared with a side-polished fiber, while GO particles were deposited onto the fiber-polished surface to realize an all-fiber SA. A continuous-wave thulium-doped all-fiber laser, which was configured with a master-oscillator power-amplifier (MOPA) structure, was constructed as a pumping source. By inserting the fabricated SA into an all-fiber ring resonator based on 1-m length of Ho-doped fiber, Q-switched pulses could readily be obtained at a wavelength of 2058 nm. The pulse width was observed to vary from 2.01 to 1.56 \(\upmu\)s as the pump power was adjusted from \(\sim\)759 to 1072 mW, while the repetition rate was tunable from 45.56 to 56.12 kHz. The maximum values of average optical power and pulse energy were measured as \(\sim\)11.61 mW and 207.05 mJ, respectively, at a \(\sim\)1072 mW pump power. holmium-doped fiber; fiber laser; graphene oxide; saturable absorber; Q-switching +
Footnote †: journal: Applied  
Among the aforementioned nonlinear absorption materials, graphene has been regarded as highly promising for high-performance devices in the field of photonics owing to its distinct and outstanding characteristics, such as its no energy bandgap structure, high carrier mobility, fast relaxation, large optical third-order nonlinearity, and optical transparency over a wide spectral band region [43; 44]. Therefore, graphene has proven to be useful in many photonic applications such as four wave-mixing [45], optical modulators [46], polarizers [47], and SAs [48; 49; 50; 51; 52]. In particular, the use of graphene for the realization of SAs has received significant attention due to its inherent advantages of fast recovery time, large operating bandwidth, and high optical damage threshold. Since [PERSON] et al. first reported that graphene could be used as an effective SA material in 2009 [12], the graphene-based SAs have been intensively investigated for their application to passively mode-locked or Q-switched lasers at various wavelengths [48; 49; 50; 51; 52]. Apart from graphene, GO and reduced GO, which are low-cost variations from graphene have also been investigated in terms of saturable absorption [13; 14; 53; 54]. In general, fiber lasers operating in the wavelength band from 1.9 to 2.1 \(\upmu\)m can be realized using thulium (Tm)-doped fiber, thulium-holmium (Tm-Ho) codoped fiber, or holmium (Ho)-doped fiber as a gain medium. It should be noticed that a Tm-doped fiber cavity generates a pulsed laser output at around 1.9-\(\upmu\)m wavelengths, whereas a Ho-doped fiber cavity does so at around 2 \(\upmu\)m. Until now, most of the investigations in short pulse generation in the wavelength band from 1.9 to 2.1 \(\upmu\)m have been focused on the use of a Tm-doped fiber cavity due to easy availability of semiconductor pumping sources at \(\sim\)0.79 and \(\sim\)1.56 \(\upmu\)m [55; 56; 57; 58]. Ho-doped silica fiber produces photon emission over a wavelength region of 2.05-2.17 \(\upmu\)m [59] and is pumped by laser sources operating at \(\sim\)1.1 and \(\sim\)1.9 \(\upmu\)m. One should note that semiconductor-based pump sources are not easily available at wavelengths of \(\sim\)1.1 and \(\sim\)1.9 \(\upmu\)m [60; 61; 62]. Until now, there have been quite a few reports on Ho-doped fiber-based pulsed lasers, and they were based on various pulse formation techniques such as gain-switching [63; 64; 65], nonlinear polarization rotation (NPR) [66], and saturable absorption [67]. It should be noted that the previous works on Ho-doped fiber-based pulsed lasers mostly focused on mode-locked pulse generation with various nonlinear saturable absorption materials, such as CNTs [67; 68], graphene [69; 70], and BP [71]; however, only a few investigations have been reported with respect to Q-switched pulse generation from a Ho-doped fiber cavity with an SA [72; 73; 74; 75]. More specifically, [PERSON] et al.