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We demonstrate the use of an all-fiberized, mode-locked 1.94 μm laser with a saturable absorption device based on a tungsten disulfide (WS2)-deposited side-polished fiber. The WS2 particles were prepared via liquid phase exfoliation (LPE) without centrifugation. A series of measurements including Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM) revealed that the prepared particles had thick nanostructures of more than 5 layers. The prepared saturable absorption device used the evanescent field interaction mechanism between the oscillating beam and WS2 particles and its modulation depth was measured to be ~10.9% at a wavelength of 1925 nm. Incorporating the WS2-based saturable absorption device into a thulium-holmium co-doped fiber ring cavity, stable mode-locked pulses with a temporal width of ~1.3 ps at a repetition rate of 34.8 MHz were readily obtained at a wavelength of 1941 nm. The results of this experiment confirm that WS2 can be used as an effective broadband saturable absorption material that is suitable to passively generate pulses at 2 μm wavelengths.
© 2015 Optical Society of America
1. Introduction
Mid-infrared fiber lasers are considered to be promising light sources that can be used for various applications in material processing, laser surgery, free-space communication, long-range light detection and ranging (LIDAR), and gas detection in air [1–6]. Fiber lasers have inherent advantages in that they provide alignment-free operation, excellent beam quality, and environmental stability and as a result are more suitable for industrial applications than their free-space optics-based solid state counterparts [7,8]. To date, many studies have been conducted regarding mid-infrared fiber lasers as both continuous wave (CW) [9,10] and pulsed lasers [11–13]. In terms of pulsed lasers, mode-locked pulses with temporal widths in the range of sub-picoseconds have received an increasing amount of attention for industrial use in fine material processing due to their applicability without thermal accumulation [14,15].
Sub-picosecond pulses can be passively produced from a laser cavity through the use of a saturable absorber (SA). III-V compound semiconductors [16–20], carbon nanotubes (CNTs) [21–26], and graphene [27–38] are well-known saturable absorption materials, and these can be readily used in SAs. The SAs can be incorporated into the laser cavity to readily obtain mode-locked or Q-switched pulses. Topological insulators (TIs) have recently been identified as saturable absorption materials and a series of studies on their use with pulsed lasers have thus been conducted at various wavelengths, including 1 μm [39–41], 1.5 μm [42–45], and 2 μm [46–48]. Furthermore, there is a new emerging two-dimensional (2D) material called “black phosphorus” which also shows the broadband saturable absorption [49].
Recently, molybdenum disulfide (MoS2), which is another kind of 2D material, has received an increasing amount of attention in the field of photonics due to its unique optoelectronic properties, including strong photoluminescence [50] and high nonlinear optical response [51]. Since MoS2 has an indirect bandgap of 1.29 eV in the bulk form and a direct bandgap of 1.8 eV in the monolayer form, MoS2 with a highly perfect lattice structure has been known to not provide any saturable absorption behavior at wavelengths of light larger than 1000 nm [50]. MoS2 is known to exhibit linear absorption and nonlinear saturable absorption at 1.55 μm and 2 μm due to defects and impurities. Note that Wang et al. theoretically demonstrated that the bandgap of the MoS2 can substantially be reduced down to ~0.08 eV by the suitable introduction of defects in [52]. It was also experimentally reported that the edge surface defects can induce sub-bandgap saturable absorption in MoS2 [53,54]. Furthermore, it was recently inferred that the same sort of sub-bandgap absorption could occur in other TMD materials such as tungsten disulfide (WS2), MoSe2, and MoTe2 [55]. Mode-locked and Q-switched fiber lasers implemented using MoS2-based SAs have thus been extensively investigated at wavelengths ranging from 1 μm to 2 μm [56–62], Also, very recently it was experimentally demonstrated that broadband saturable absorption of MoSe2 could be used to generate Q-switched fiber laser operating at 1060, 1566, and 1924 nm due to edge state-driven sub-bandgap saturable absorption in [63].
MoS2 belongs to transition metal dichalcogenides (TMDs) family that exhibits a layered lattice structure of the MX2 form [64,65]. Various materials, such as MoSe2, MoTe2, WS2, and WSe2, also belong to the TMD family [64,65]. Previous results obtained for MoS2-based SAs led to the investigation of the saturable absorption properties of WS2, mainly focusing in the 1.55 μm wavelength region [66–68]. In previous report [66], Mao et al. demonstrated that the absorption spectrum of WS2 reaches the 2 μm region, indicating that these materials are applicable as mid-infrared SAs. It is well known that monolayered WS2 is a direct bandgap material with a large exciton binding energy, while multilayered one is an indirect bandgap material. The exciton binding energy level is known to be 0.3 ~1 eV [69–71]. However, it is not clear yet whether or not the excitonic characteristics of WS2 can account for the saturable absorption effect at 1.55 μm and longer wavelengths [72]. It should be noticed that recent investigations on WS2 showed that WS2 in imperfect, finite, multilayered structures can also exhibit non-negligible linear and nonlinear absorption at 1.55 μm like MoS2 [66–68]. It was reported that boundary effects, defects, and impurities of WS2 could readily modify the bandgap structure in a manner similar to how the MoS2 bandgap is tuned.
In this paper, we demonstrate the use of a WS2-based SA to generate mode-locked pulses from a fiber laser operating at a wavelength of 1.94 μm. To the best of the authors’ knowledge, there has been no report to date on the use of a WS2-based SA for pulsed laser operation in the 2 μm wavelength region. Therefore, we believe that it is necessary to investigate the potential of WS2 as a base material for SAs in the wavelengths, to elucidate the technological basis of such devices. The WS2 particles were prepared via liquid phase exfoliation (LPE), and a series of measurements were carried out, including Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). The particles that were prepared were found to have thick nanostructures with more than 5 layers. A side-polished SM2000 fiber with WS2 particles deposited on its flat side was prepared, and the experiments that were carried out showed that stable ~1.3-ps mode-locked pulses could be readily obtained from a thulium(Tm)-holmium(Ho) co-doped fiber-based ring cavity. The electrical signal-to-noise ratio (SNR) of the produced pulses was measured at ~72 dB.
The SA is a key device for the generation of pulsed outputs from a passively mode-locked laser cavity. We believe that research into emerging saturable absorption materials is a very important step for a better understanding of its potentials. Until now, widely used and reliable SAs in the practical application have been based on compound semiconductors. However, they require complicated and expensive fabrication facilities and their operating wavelength bandwidth is limited to several tens of nanometers. Recently, graphene has been regarded as an excellent alternative; however, it still requires complicated fabrication processes such as chemical vapor deposition to obtain a pure, single layered structure [27]. On the contrary, our used WS2 sample preparation method is very simple since perfect layer structures are not required. Note that defects and impurities are allowed in our prepared sample. This is the reason why a simple LPE method without centrifugation could be used in our experimental demonstration.
2. Preparation and characterization of nanostructured WS2 particles
Commercially available bulk WS2 powder (99%, Sigma-Aldrich) was used as a starting material, and the nanostructured WS2 solution was obtained by inserting bulk WS2 powder in a solvent consisting of distilled water and sonicating with an ultrasonicator for 24 hours. We later found that the saturable absorption performance of the prepared WS2 particles was independent of whether or not centrifugation was used, so an additional centrifugation step was not conducted to eliminate large agglomeration, unlike in [66]. Figure 1(a) shows a schematic of the ultrasonication process used to prepare the WS2 particles, and Fig. 1(b) shows a photograph of the WS2 solution that was prepared.
Fig. 1 (a) Schematic of the ultrasonication used to prepare the nanostructured WS2 particles in a solvent consisting of distilled water. (b) Photograph of the WS2 water solution that was prepared.
A small amount of WS2 in water was dropped on top of a slide glass and was dried for 5 hours. We subsequently characterized the material structures and the optical properties of the WS2 particles. Figure 2(a) shows an SEM image of the WS2 particles. The size of the WS2 particles was observed to be within the range from 100 to 400 nm. AFM measurements were also conducted to determine the thickness of the particles, and the thickness of the WS2 particles was found to be larger than 6 nm, as shown in Fig. 2(b).
Fig. 2 Measured (a) SEM image and (b) AFM image of the prepared WS2 solution, which was dropped on top of a slide glass and was dried for 5 hours.
We then used a spectrophotometer to measure the linear absorption of the WS2 particles. Figure 3(a) shows the absorbance measured from 400 nm to 2500 nm. The absorption band of the WS2 solution can be seen to cover a wide spectral bandwidth, including the short mid-infrared region. The dip around 630 nm for the absorbance can be attributed to the direct bandgap of the WS2 [66, 73].
Next we measured the Raman spectrum of the WS2 at 532 nm (InVia Raman Microscope, Renishaw). The Raman spectrum of the WS2 sample is shown in Fig. 3(b). Two major peaks can be seen for E12g at ~350 cm−1 and A1g at ~420 cm−1. E12g is the in-plane vibrational mode and A1g is the out-of-plane vibrational mode. The peak intensity ratio of E12g and A1g was of 1.64, and the peak intensity ratio indicates that the WS2 particles that were prepared had a thick-multilayered structure with more than 5 layers [74]. The results of this Raman measurement coincide with what we observed in the AFM measurements.
3. WS2-deposited side-polished fiber
The WS2 solution that was prepared was deposited on the flat surface of a side-polished fiber (SPF) that had been prepared by side-polishing an optical fiber, as shown in Fig. 4. Nonlinear saturable absorption occurs due to the mutual interaction of the WS2 particles deposited onto the flat side of a SPF with the evanescent field of the oscillating beam within our fiberized cavity. This scheme has been first demonstrated by Song et al. [23]. The SPF was prepared by polishing one side of the SM2000, which was fixed onto a V-grooved quartz block. The distance between the flat side and the top side of the fiber core was measured at ~7 μm. The estimated interaction length between the deposited WS2 particles and the oscillating beam was ~1.5 mm. It is not easy to control the deposited particle thickness and homogeneity in using the solution deposit process. We thus think that the deposited particles have a nonuniform distribution. The insertion loss and polarization dependent loss (PDL) of the prepared WS2-based SA at 1.94 μm were ~3 dB and ~2.2 dB, respectively. A PDL level of ~2.2 dB is believed to be low enough to eliminate the possibility of mode-locking through a nonlinear polarization rotation [37].
The nonlinear transmission of the WS2-based SA was also measured as a function of the incident peak power of the input pulses. Figure 5(a) shows the measurement setup for nonlinear transmission of the WS2-based SA. We used a mode-locked fiber laser that we built to generate 1925 nm, ~1.5-ps optical pulses at a repetition rate of 178 MHz. Figure 5(b) shows the transmission that was measured as a function of the incident peak power together with a fitting curve [43].
where is the transmission, is the modulation depth, is the input pulse energy, is the saturation energy, and is the nonsaturable loss. The estimated modulation depth was ~10.9%, and the saturation power was ~1.9 W. The nonlinear transmission curve in Fig. 5(b) was first measured at the transverse electric (TE) mode of the incident pulse beam. When we changed the input beam polarization state into the transverse magnetic (TM) mode, the modulation depth decreased to ~4%. We also measured its nonlinear transmission curve at 1560 nm to check its broadband saturable absorption. The estimated modulation depth and saturation power at 1560 nm were ~1.8% and ~3.2 W, respectively, as shown in Fig. 5(c). Note that the lower performance of the WS2-based SA at 1560 nm can be attributed to the fact that our WS2-based SA was optimized for the operation at 2 μm region.Fig. 5 (a) Measurement setup for nonlinear transmission of the WS2-based SA. Measured nonlinear transmission curves (b) at a wavelength of 1925 nm and (c) at 1560 nm.
Then, in order to measure the damage threshold of our prepared WS2-based SA, we launched a 1550 nm CW-amplified laser beam of maximum 1 W power into it, but no substantial optical damage of the device was observed within the optical power level. Note that we used a 1550 nm high power beam since a high power CW beam at a wavelength of 2 μm was not available in our laboratory. The damage threshold must be larger than 1 W. However, it was impossible to measure the precise value due to limited availability of a high power laser in our lab.
4. Laser setup and experimental output measurements
The WS2-deposited SPF was used as an all-fiberized SA for a mode-locked ring cavity fiber laser based on a 1-m length Tm-Ho co-doped fiber as the gain medium (TH512, CorActive), as shown in Fig. 6. A polarization-independent isolator was employed for unidirectional beam propagation, and a 10:90 coupler was used to extract the output pulses from the cavity. The WS2-deposited SPF was used after the isolator, and a polarization controller (PC) was also used to adjust the state of the polarization of the oscillating beam within the cavity. A 1550-nm semiconductor laser diode with a maximum power of 250 mW was used as a pump source, coupled into the laser cavity by using a 1550/2000 nm wavelength division multiplexing (WDM) coupler. The total cavity length was ~5.94 m, and the group velocity dispersion (GVD) of the fiberized cavity was estimated to be of −0.387 ps2. Note that this cavity GVD estimation neglected the GVD variation, which could possibly be caused by the WS2 deposition on the SPF.
Stable mode-locked pulses were readily obtained when the pump power was set at a maximum power of 250 mW while the PC was properly adjusted. The average output power was measured at ~0.6 mW and the obtainable maximum pulse energy was ~17.2 pJ at a maximum pump power of ~250 mW. Figure 7(a) shows optical spectrum measured for the output pulses obtained with an optical spectrum analyzer (AQ6375, Yokogawa). The center wavelength and 3-dB bandwidth were measured at 1941 nm and 5.6 nm, respectively. Kelly sidebands were clearly shown on the optical spectrum, indicating that the mode-locked pulses operated in the anomalous dispersion region [75]. Sub-sidebands also appeared in the optical spectrum around the Kelly sidebands as shown in Fig. 7(a). It is believed to be associated with vector soliton sidebands [76]. Figure 7(b) shows the oscilloscope traces captured with a 16 GHz real time oscilloscope (DSA71604C, Tektronix). A photodetector with a 28 ps rise/fall time was used to detect the output mode-locked pulses. Figure 7(b) shows stable mode-locked pulses, and the inset shows the evident single-pulse operation. The time interval of the successive pulses was ~28.7 ns, which corresponds to the fundamental repetition rate of 34.8 MHz.
Fig. 7 (a) Optical spectrum and (b) oscilloscope trace of the output pulses. Inset: Oscilloscope trace for a narrow span.
Next, we measured the temporal width of the output pulses using a second harmonic generation (SHG)-based autocorrelator. Figure 8(a) shows the pulse width that was measured together with a sech2() fitting curve. The measured pulse width was ~1.3 ps, and considering that the 3-dB optical bandwidth of the output pulses was ~5.6 nm, the estimated time-bandwidth product was expected to be ~0.579, which indicates that the output pulses were chirped. Such a high chirp level of the output pulses is believed to be due to the variation in GVD, which could be induced by WS2-deposition on the SPF, even if the net cavity GVD in terms of the optical fiber that was used is close to that of the laser in [48]. Further investigation is therefore required to confirm the reason for such a high output pulse chirp.
Fig. 8 (a) Autocorrelation of the output pulses. (b) Electrical spectrum of the output pulses. Inset: Electrical spectrum for a span of 1.5 GHz.
Finally, the stability of the output pulses was confirmed by measuring the electrical spectrum, as shown in Fig. 8(b). A strong signal peak with an electrical signal-to-noise ratio (SNR) of ~72 dB was obtained at a fundamental pulse repetition rate of 34.8 MHz. The inset in Fig. 8(b) shows the electrical spectrum measured with a wide span for 1.5 GHz. Strong beat signals were clearly observed, which indicates highly stable mode-locking.
5. Performance comparison of our laser with previously demonstrated Tm-doped fiber lasers using other types of saturable absorption materials
In order to figure out the pros and cons of our mode-locked 1.94-um fiber laser incorporating a WS2-based SA relative to the same sort of lasers using compound semiconductor- or graphene-based SAs, the output performance of our laser was compared to that of those lasers. Table 1 summarizes the results. The temporal width of the output pulses from our laser is almost comparable to those from the lasers using compound semiconductors or graphene. Even if the modulation depth of our WS2-based SA is approximately a half of those of compound semiconductors, its saturation fluence is much smaller than those of compound semiconductors-based ones. The modulation depth of our WS2-based SA is approximately 2.5 times larger than that of the graphene-based one in [32].
Table 1. Output performance of our mode-locked Tm-doped fiber laser incorporating a WS2-based SA in comparison with that of the Tm-doped fiber laser incorporating compound semiconductor- or graphene-based SAs
Next, the output performance of our laser was compared with that of a Tm-doped fiber laser incorporating a MoS2-based SA. Table 2 shows the performance comparison results. As shown in the table, similar modulation depths were obtained for both cases, even if they have different interaction mechanisms. The saturation intensity of our WS2-based SA is much lower than that of the MoS2-based one, whereas the non-saturable loss of our WS2-based SA is larger than that of MoS2-based one. The temporal widths of the output pulses are significantly different since Tian et al. produced dissipative soliton pulses [62], while our laser generated pure soliton pulses.
Table 2. Performance comparison at 2 μm wavelengths between our laser incorporating a WS2-based SA and a fiber laser using a MoS2-based one
6. Conclusion
We have experimentally demonstrated the use of a WS2-based, ultrafast mode-locked fiber laser operating in the 2 μm region. An SPF platform deposited with multilayered WS2 particles was shown to readily obtain stable mode-locking of a Tm-Ho co-doped fiber laser. One notable point of this demonstration was that centrifugation to eliminate large agglomeration was skipped during WS2 water solution preparation, unlike in [66], since we found that the saturable absorption performance of the WS2 particles was independent of whether or not such centrifugation was used.
In conclusion, the results of the experiment confirm that thick-nanostructured, multilayered WS2 can be used as an effective broadband saturable absorption material for passive pulse generation at 2 μm wavelengths.
Acknowledgments
This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2015R1A2A2A11000907), Republic of Korea. This work was also supported by the Industrial Strategic Technology Development Program (10039226, Development of actinic EUV mask inspection tool and multiple electron beam wafer inspection technology) funded by the Ministry of Trade, Industry & Energy, Republic of Korea.
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