A Passively Q-Switched Holmium-Doped Fiber Laser with Graphene Oxide at 2058 nm

: 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 µ s as the pump power was adjusted from ~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 ~11.61 mW and 207.05 nJ, respectively, at a ~1072 mW pump power.


Introduction
Q-switched lasers operating at the~2-µm spectral region are attractive coherent light sources for numerous applications such as medicine, LiDAR, material processing, and gas sensing [1][2][3][4]. Compared with other types of lasers, fiber lasers have a range of benefits like low heat accumulation, environmental stability, alignment-free operation, and compactness [5]. Depending on the requirement of an external electrical signal source, Q-switched fiber lasers can be categorized into two groups: passive and active. Active Q-switching technique is commonly used in most of commercial products of pulsed fiber lasers with output pulse widths from nanoseconds to microseconds, due to its features of a high energy output and a controllable pulse repetition rate. Comparatively speaking, passive Q-switching technique is still of intensive research interest due to its future potential associated with a simple configuration that does not require an external modulator.
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 Bao 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 µm 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-µm wavelengths, whereas a Ho-doped fiber cavity does so at around 2 µm. Until now, most of the investigations in short pulse generation in the wavelength band from 1.9 to 2.1 µm have been focused on the use of a Tm-doped fiber cavity due to easy availability of semiconductor pumping sources at 0.79 and~1.56 µm [55][56][57][58]. Ho-doped silica fiber produces photon emission over a wavelength region of 2.05~2.17 µm [59] and is pumped by laser sources operating at~1.1 and 1.9 µm. One should note that semiconductor-based pump sources are not easily available at wavelengths of~1.1 and~1.9 µm [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, Chamorovskiy et al. demonstrated a Q-switched Ho-doped fiber laser incorporating a CNT-based SA [72], Sholokhov et al. reported a Q-switched Ho-doped fiber laser with a heavily Ho-doped fiber SA [74], and Wang et al. demonstrated the use of a heavily Ho-doped fiber SA for the realization of high-power Q-switched pulses from a Ho-doped fiber laser [75].
In this work, a Q-switched Ho-doped fiber laser using a low-cost SA based on GO is experimentally demonstrated. The SA, which has an all-fiber structure, was implemented through deposition of GO particles onto the flat polished surface of a side-polished fiber. In order to pump a Ho-doped fiber gain medium in the laser ring resonator, our built continuous-wave (CW) Tm-doped fiber laser at 1928 nm, which was followed by a power amplifier operating was used. By inserting the GO-based SA in the prepared Ho-doped fiber-based ring resonator, the pulsed output was found to be readily obtainable at a wavelength of 2058 nm due to passive Q-switching. The measured temporal width of the output pulses was 1.56 µs at a 56.12 kHz repetition rate. Both output pulse width and repetition were tunable by changing the pump power. It should be noticed that this study's constructed laser has an all-fiber ring configuration unlike the Q-switching-based Ho-doped fiber laser incorporating a SA based on CNT, demonstrated by Chamorovskiy et al. in which a Fabry-Pérot cavity was employed with a free-space coupled mirror-type SA [72]. The main goal of this work is to demonstrate the ultimate potential of graphene oxide (GO) as a base material for the implementation of our SA since it is a low-cost alternative to expensive high-purity graphene in terms of saturable absorption. Note that GO-based SAs have not been used for passive Q-switching of a fiber laser operating at the wavelength band beyond 2 µm, even if they have been well-investigated at wavelengths of 1, 1.5, and 1.9 µm.

Characterization and Preparation of the GO-Based Saturable Absorber
The conventional, low-cost GO was obtained from a commercial company for this experimental demonstration. The measured Raman spectrum of the GO particles is illustrated in Figure 1a. The D band and G band were observed at 1348 and 1592 cm −1 , each. The D peak is known to be induced by the lattice disorder of graphene [76], while the G peak is caused by the phonon excitation at the Brillouin zone center [77]. The X-ray photoelectron spectroscopy (XPS) spectrum was also measured and the spectrum in the C 1s region of the GO particles is shown in Figure 1b. After curve fitting, it was found that the fitted C 1s peaks could be decomposed by three peaks at~284.8,~286.7, and~287.9 eV, which correspond to groups of C-C/C=C, C-O, and C=O, respectively. These peaks correspond to those of the previously reported values [78].
Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 10 based SAs have not been used for passive Q-switching of a fiber laser operating at the wavelength band beyond 2 μm, even if they have been well-investigated at wavelengths of 1, 1.5, and 1.9 μm.

Characterization and Preparation of the GO-Based Saturable Absorber
The conventional, low-cost GO was obtained from a commercial company for this experimental demonstration. The measured Raman spectrum of the GO particles is illustrated in Figure 1a. The D band and G band were observed at 1348 and 1592 cm −1 , each. The D peak is known to be induced by the lattice disorder of graphene [76], while the G peak is caused by the phonon excitation at the Brillouin zone center [77]. The X-ray photoelectron spectroscopy (XPS) spectrum was also measured and the spectrum in the C 1s region of the GO particles is shown in Figure 1b. After curve fitting, it was found that the fitted C 1s peaks could be decomposed by three peaks at ~284.8, ~286.7, and ~287.9 eV, which correspond to groups of C-C/C=C, C-O, and C=O, respectively. These peaks correspond to those of the previously reported values [78].  Figure 2a illustrates the linear optical absorption spectrum for the GO particles, which was measured with a spectrophotometer. The linear absorption spectrum indicates that the GO particles have a broad absorption band that include the 2.0~2.1 μm regime. A schematic diagram of the GO-deposited side-polished fiber is shown in Figure 2b. One side of a standard single-mode fiber (SMF) was finely polished to obtain a side-polished fiber, which enables us to induce evanescent field interaction with the deposited GO particles. The physical distance between the polished surface and core edge was ~5 μm. The polished length of the fiber was ~3 mm, and the length covered with GO was ~8 mm. The used GO has a single layer with a thickness of ~500 nm. Note that the used GO has no atomic layer structure. The insertion loss (IL) of the side-polished fiber was ~0.7 dB and its polarization-dependent loss (PDL) was ~0.04 dB. After the GO particles were deposited onto the polished surface, the IL was changed into ~2.8 dB, while the PDL became ~3.4 dB.  Figure 2a illustrates the linear optical absorption spectrum for the GO particles, which was measured with a spectrophotometer. The linear absorption spectrum indicates that the GO particles have a broad absorption band that include the 2.0~2.1 µm regime. A schematic diagram of the GO-deposited side-polished fiber is shown in Figure 2b. One side of a standard single-mode fiber (SMF) was finely polished to obtain a side-polished fiber, which enables us to induce evanescent field interaction with the deposited GO particles. The physical distance between the polished surface and core edge was~5 µm. The polished length of the fiber was~3 mm, and the length covered with GO was~8 mm. The used GO has a single layer with a thickness of~500 nm. Note that the used GO has no atomic layer structure. The insertion loss (IL) of the side-polished fiber was~0.7 dB and its polarization-dependent loss (PDL) was~0.04 dB. After the GO particles were deposited onto the polished surface, the IL was changed into~2.8 dB, while the PDL became~3.4 dB. Next, nonlinear transmission measurement was performed as a function of the pulse peak power for the fabricated GO-deposited SA, using an experimental se Figure 3a. The incident beam was coupled into the GO-based SA in the transverse e (TE) mode. For this measurement, we used a passively mode-locked fiber laser repetition rate of ~36.94 MHz and a temporal width of ~711 fs at 1902 nm. Since an ul laser source operating at a 2.0~2.1-μm wavelength region did not exist in our labo this particular measurement was carried out at 1.9 μm with our built mode-locke doped fiber laser. The nonlinear transmission curve of the GO-based SA, is illustra Figure 3b together with its corresponding fitting curve [79]: where T(I) and ΔT are the transmission and modulation depth, respectively. I and the input-pulse energy and saturation energy, respectively. Tns represents the nons ble loss. The modulation depth and saturation power were estimated at ~13.1% and W, respectively.

Q-Switching of a 2058-nm Fiber Laser
The whole laser schematic diagram including a Ho-doped fiber ring resonator Tm-doped fiber-based high power pumping source is presented in Figure 4. Our las constructed with single-clad Ho-doped fiber rather than double clad one, since o geted laser did not require a high output power. We chose a 1900-nm Tm-dope laser as a pumping source for the Ho-doped fiber considering the fact that the pum sorption based on the transition of 5  Next, nonlinear transmission measurement was performed as a function of the input pulse peak power for the fabricated GO-deposited SA, using an experimental setup in Figure 3a. The incident beam was coupled into the GO-based SA in the transverse electric (TE) mode. For this measurement, we used a passively mode-locked fiber laser with a repetition rate of~36.94 MHz and a temporal width of~711 fs at 1902 nm. Since an ultrafast laser source operating at a 2.0~2.1-µm wavelength region did not exist in our laboratory, this particular measurement was carried out at 1.9 µm with our built mode-locked Tmdoped fiber laser. The nonlinear transmission curve of the GO-based SA, is illustrated in Figure 3b together with its corresponding fitting curve [79]: where T(I) and ∆T are the transmission and modulation depth, respectively. I and I sat are the input-pulse energy and saturation energy, respectively. T ns represents the nonsaturable loss. The modulation depth and saturation power were estimated at~13.1% and~12.5 W, respectively.  Next, nonlinear transmission measurement was performed as a function of the input pulse peak power for the fabricated GO-deposited SA, using an experimental setup in Figure 3a. The incident beam was coupled into the GO-based SA in the transverse electric (TE) mode. For this measurement, we used a passively mode-locked fiber laser with a repetition rate of ~36.94 MHz and a temporal width of ~711 fs at 1902 nm. Since an ultrafast laser source operating at a 2.0~2.1-μm wavelength region did not exist in our laboratory, this particular measurement was carried out at 1.9 μm with our built mode-locked Tmdoped fiber laser. The nonlinear transmission curve of the GO-based SA, is illustrated in Figure 3b together with its corresponding fitting curve [79]: where T(I) and ΔT are the transmission and modulation depth, respectively. I and Isat are the input-pulse energy and saturation energy, respectively. Tns represents the nonsaturable loss. The modulation depth and saturation power were estimated at ~13.1% and ~12.5 W, respectively.

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

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 → 5 I 7 allows for better quantum efficiency than that of 5 I 8 → 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~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) × 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~4.5 dB/m. The core and inner cladding diameter were 10 and 130 µm, respectively. The 1928-nm pumping beam, which had a maximum power of~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.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 10 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 ~1928 nm. The output of the seed oscillator was coupled into a preamplifier  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 ~660 mW. Subsequently, the CW lasing mode was converted into the Q-switching mode as soon as the pump power arrived at ~759 mW. The measured optical spectrum of the Q-switched pulses at a pump power of ~759 mW is shown in Figure 5a. The peak wavelength was ~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 laser configuration. Figure 5b   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~660 mW. Subsequently, the CW lasing mode was converted into the Q-switching mode as soon as the pump power arrived at~759 mW. The measured optical spectrum of the Q-switched pulses at a pump power of~759 mW is shown in Figure 5a. The peak wavelength was~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 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. This sort of pulse temporal behavior depending on pump power is a typical phenomenon, which is generally reported in passively Q-switched fiber lasers using an SA [16,24,35].
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. This sort of pulse temporal behavior depending on pump power is a typical phenomenon, which is generally reported in passively Q-switched fiber lasers using an SA [16,24,35]. Subsequently, the average optical power, pulse energy, and repetition rate of the output pulses were monitored while changing the pump power. The results are illustrated in Figure 6. The pulse width decreased from 2.01 to 1.56 μs as the pump power was adjusted from ~759 to 1072 mW, while the repetition rate changed from 45.56 to 56.12 kHz, as shown in Figure 6a. The pump power-dependent pulse width change could be attributed to the pump-induced gain compression effect [16,35,83]. The pulse energy as well as the average output power was observed to increase as the pump power was enlarged, as shown in Figure 6b. The maximum values of pulse energy and average output power were ~11.62 mW and 207.05 nJ, respectively, each at the maximum pump power of ~1072 mW. Finally, a performance comparison between previously demonstrated Q-switched Ho-doped fiber lasers with SAs and our laser is presented in the Table 1. It is obvious from the table that our laser exhibited better performance than the laser using a CNT-based SA in terms of pulse energy. From a perspective of pulse energy scalability, the Ho-doped fiber SA is the best among the three due to its high damage threshold. Apparently, nanomaterial-based SAs have damage threshold issues, which are critical to obtaining high energy pulses directly from a laser cavity.  Subsequently, the average optical power, pulse energy, and repetition rate of the output pulses were monitored while changing the pump power. The results are illustrated in Figure 6. The pulse width decreased from 2.01 to 1.56 µs as the pump power was adjusted from~759 to 1072 mW, while the repetition rate changed from 45.56 to 56.12 kHz, as shown in Figure 6a. The pump power-dependent pulse width change could be attributed to the pump-induced gain compression effect [16,35,83]. The pulse energy as well as the average output power was observed to increase as the pump power was enlarged, as shown in Figure 6b. The maximum values of pulse energy and average output power werẽ 11.62 mW and 207.05 nJ, respectively, each at the maximum pump power of~1072 mW. Finally, a performance comparison between previously demonstrated Q-switched Hodoped fiber lasers with SAs and our laser is presented in the Table 1. It is obvious from the table that our laser exhibited better performance than the laser using a CNT-based SA in terms of pulse energy. From a perspective of pulse energy scalability, the Ho-doped fiber SA is the best among the three due to its high damage threshold. Apparently, nanomaterialbased SAs have damage threshold issues, which are critical to obtaining high energy pulses directly from a laser cavity.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 10 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. This sort of pulse temporal behavior depending on pump power is a typical phenomenon, which is generally reported in passively Q-switched fiber lasers using an SA [16,24,35]. Subsequently, the average optical power, pulse energy, and repetition rate of the output pulses were monitored while changing the pump power. The results are illustrated in Figure 6. The pulse width decreased from 2.01 to 1.56 μs as the pump power was adjusted from ~759 to 1072 mW, while the repetition rate changed from 45.56 to 56.12 kHz, as shown in Figure 6a. The pump power-dependent pulse width change could be attributed to the pump-induced gain compression effect [16,35,83]. The pulse energy as well as the average output power was observed to increase as the pump power was enlarged, as shown in Figure 6b. The maximum values of pulse energy and average output power were ~11.62 mW and 207.05 nJ, respectively, each at the maximum pump power of ~1072 mW. Finally, a performance comparison between previously demonstrated Q-switched Ho-doped fiber lasers with SAs and our laser is presented in the Table 1. It is obvious from the table that our laser exhibited better performance than the laser using a CNT-based SA in terms of pulse energy. From a perspective of pulse energy scalability, the Ho-doped fiber SA is the best among the three due to its high damage threshold. Apparently, nanomaterial-based SAs have damage threshold issues, which are critical to obtaining high energy pulses directly from a laser cavity.

Conclusions
A passively Q-switched 2058 nm laser with an all-fiber configuration incorporating a GO-based SA has been experimentally demonstrated in this study. The all-fiber SA was fabricated with GO particles deposited onto the polished side surface of a standard optical fiber. It was discovered that stable microsecond pulses could be generated at 2058 nm when the prepared SA was inserted into a ring resonator incorporating a Ho-doped fiber gain medium. The minimum pulse width was~1.56 µs at a~1072 mW pump power.
These experimental results are believed to be a useful part of a database of pulsed Ho-doped fiber lasers, which have not been extensively investigated until now.