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Abstract
An all-fiberized, mode-locked fiber laser that operates in the 1912 nm region with a saturable absorber (SA) that is based on a composite consisting of bulk-like, molybdenum diselenide (MoSe2) particles and polyvinyl alcohol (PVA), is experimentally demonstrated here. The MoSe2 particles were prepared using liquid phase exfoliation (LPE) without a centrifugation. The SA was implemented on a side-polished fiber platform and deposited with the MoSe2/PVA. The saturation power and modulation depth of the SA were measured as ~25.7 W and ~4.4%, respectively. Using the prepared SA, the stable soliton pulses with a temporal width of ~920 fs could be produced at 1912 nm from a thulium-holmium (Tm-Ho) co-doped fiber ring cavity. The mode-locked pulses were operated at a repetition rate of ~18.21 MHz, and a 3-dB bandwidth was measured as ~4.62 nm. The signal-to-noise ratio (SNR) was measured as ~65 dB at the fundamental frequency of ~18.21 MHz. To the best of the authors’ knowledge this is the first time demonstration of the use of a saturable absorber based on a transition metal dichalcogenide for femtosecond mode-locking of a 2-μm fiber laser.
© 2017 Optical Society of America
1. Introduction
Mid-infrared (IR) lasers have been used in various applications, such as laser spectroscopy [1], LIDAR [2], free-space optical communication [3], and laser surgery [4]. To date, most of the commercially available mid-IR lasers have been implemented using solid-state gain media [5] or semiconductors based on quantum cascade structures [6]. Recently, the use of fiber-laser technology in the mid-IR spectral region attracted a huge extent of technical attention since it allows for a number of advantages over the solid-state counterparts, such as an alignment-free operation, a sound beam quality, compactness and an environmental stability [7–10].
One important issue in the field of fiber lasers is the way that the ultrashort optical pulses are produced from the fiberized cavity. In particular, the use of mode-locking to generate femtosecond optical pulses has been a hot topic. To induce the mode-locking within a fiberized cavity, a saturable absorber (SA) has been implemented on a fiberized platform in a number of studies [16,31,39,67]. Until now, most of commercially available SAs have been implemented with the III-V compound semiconductors because of their proven reliability and performance [11–14]. However, a number of limitations hinder the semiconductor-based SAs such as a narrow operating bandwidth and the requirement of an expensive/complicated fabrication process. For the past decade, intensive investigations into alternative nonlinear optical materials for the implementation of SAs have been conducted, and the following range of novel materials have been identified as saturable absorption materials: carbon nanotubes (CNTs) [15–23], graphene [24–34], graphene oxide (GO) [35–39], graphite [40–42], topological insulators (TIs) [43–55], transition metal dichalcogenides (TMDs) [56–81], gold nanoparticles [82–87], black phosphorus (BP) [88–92], and filled skutterudites [93].
Among the previously mentioned saturable-absorption materials, TMDs are technically interesting materials that exhibit the layer-number-dependence of the bandgap properties, which have a huge potential regarding various optoelectronic applications [94]. TMDs form a material group with the formula MX2, where M is a transition metal (Mo, W, Nb and so on), and X is a chalcogen (S, Se, Te). The layered structure of the TMDs comprises X-M-X, wherein a layer of the transition-metal atom is sandwiched between two layers of chalcogen atoms [94]. The adjacent layers are weakly bonded together by the van der Waals force, and therefore the thin sheets can be easily exfoliated from the bulk. The typical TMDs are WS2, WSe2, MoS2, MoSe2, and WTe2. Since the first demonstration of the nonlinear saturable absorption property of MoS2 by Zhang et al. [56], the nonlinear saturable absorption properties of TMDs have been intensively investigated worldwide [56–81]. However, compared to the sulfide-based TMDs of MoS2, and WS2, less technical attention has been paid to selenide-based TMDs such as MoSe2 and WSe2.Very recently, however, many studies on the saturable absorption properties of MoSe2 and WSe2 have been reported [70–79]. Regarding the mode-locking demonstrations that are based on the selenide-based TMDs, only a limited number of works have been conducted in the wavelength regions of 1 μm to 1.5 μm [73–77]. Woodward et al. demonstrated the use of a MoSe2-based SA to generate Q-switched fiber laser operating at 1924 nm [71]. But, to the best of the authors’ knowledge, no report has been made on the use of an SA that is based on a selenide-based TMD for mode-locking in the 2 μm wavelength region. Furthermore, the successful use of a TMD-based saturable absorber for femtosecond mode-locking in the 2 μm region has not yet been reported, even if a couple of saturable absorbers based on sulfide-type TMDs have been demonstrated as mode-lockers operable for 2-μm fiber laser [63,65].
In this work, an SA that is based on a composite of bulk-like MoSe2 and polyvinyl alcohol (PVA) that can operate as a mode-locker in the 2 μm wavelength region is experimentally demonstrated. More specifically, a fiberized SA that is based on MoSe2/PVA, which is implemented on a side-polished fiber platform, is used to mode-lock a thulium-holmium (Tm-Ho)-codoped fiber ring cavity for the generation of femtosecond soliton pulses. The MoSe2 particles were prepared from bulk MoSe2 crystal using the liquid phase exfoliation (LPE) method, and a series of measurements including Raman spectroscopy, atomic force microscopy (AFM), and energy dispersive spectroscopy (EDS) were carried out to determine the material properties. Further, the attainment of soliton pulses from an all-fiberized cavity, with a temporal width of 920 fs at a wavelength of 1912 nm, was readily demonstrated using the MoSe2/PVA SA.
2. Fabrication and characterization of MoSe2/PVA-based saturable absorber
The MoSe2 bulk crystal (MoSe2, HQ graphene) was used as a starting material, and the MoSe2 particles were prepared using a bath-type ultra-sonification in 20 ml of distilled water for 12 hours. Figure 1(a) shows the measured AFM image of a sampled MoSe2 particle. The inset of the Fig. 1(a) shows the line profile of a prepared MoSe2 particle and the thickness of the MoSe2 particle was measured as ~52 nm, which is much thicker than the film thickness of the few-layer or nanosheet MoSe2 SAs in Refs [70–73] and [75–77], indicating that the MoSe2 particles of the present study are closer to the bulk status [74]. As shown in the scanning electron microscope (SEM) image of Fig. 1(b), the size of the MoSe2 particles varied from tens of nanometers to about 10 micrometers. Therefore, the quality of our prepared MoSe2/PVA composite was not as good as that of the few-layer MoSe2/PVA composite that was demonstrated in Ref [71].
Fig. 1 (a) Measured AFM image of a sampled MoSe2 particle. Inset: line profile. (b) Measured SEM Image of the prepared MoSe2 particles. (c) Measured EDS spectrum of the MoSe2 particles.
Then, the measured energy dispersive spectroscopy (EDS) of the MoSe2 particles is shown in Fig. 1(c). For the EDS measurement, a small amount of MoSe2 particle solution was dropped and dried on top of a slide glass. Therefore, the MoSe2 particles used for this measurement were 100% MoSe2. The spectrum shows two strong peaks corresponding to Mo and Se, respectively, and the atomic ratio between Mo and Se is around 1:2.
To form a thin film, the PVA was mixed into the prepared MoSe2 solution. The concentration of MoSe2 for our prepared MoSe2/PVA composite was ~0.2 mg·mL−1. The prepared MoSe2/PVA composite dropped on a slide glass was characterized using Raman spectroscopy and linear-absorption measurements. The measured Raman spectrum that was excited by a 532 nm laser is shown in Fig. 2(a). The out-of-plane vibration-mode A1g peak is observable at ~240 cm−1 [95,96]. As shown in this spectrum, the position of the A1g peak of the prepared MoSe2/PVA composite is similar to that of the bulk MoSe2 [74]. Figure 2(b) shows the linear optical absorption of the MoSe2/PVA composite. It is clearly observed that the broad absorption band of the prepared MoSe2/PVA film is wide and can cover the 2 μm regime. The inset of the Fig. 2(b) shows the linear optical absorption of the MoSe2/PVA composite film over a spectral range from 500 nm to 1000 nm. The two absorption peaks, located at ~710 nm (A) and ~800 nm (B), correspond with the two spin-orbit-split transitions at the K point of the Brillouin zone [97,98].
Fig. 2 Measured (a) Raman spectrum and (b) linear optical absorption spectrum of the MoSe2/PVA composite. Inset: linear optical absorption spectrum over a spectral range from 500 nm to 1000 nm.
The bandgap of the bulk MoSe2 is 1.09 eV (1138 nm), while that of the monolayer MoSe2 is 1.58 eV (785 nm) [76,99]. The photon absorption from the bulk MoSe2 samples above the wavelength of 1138 nm is impossible in terms of the bandgap as a result; however, Fig. 2(b) clearly indicates the optical absorption of the prepared MoSe2/PVA sample in the 2 μm wavelength region (> 1138 nm). The existence of the long-wavelength absorption could be attributed to the sub-bandgap absorption, which is due to the defect-driven bandgap decrease [58] and the edge-state absorption [61,100,101]. In fact, the sub-bandgap absorption has been previously reported in TMD materials such as molybdenum disulfide (MoS2) [56–63,102–104] and tungsten disulfide (WS2) [64–70]. It is well known that perfect monolayer MoSe2 grown by chemical vapor deposition does not possess sub-bandgap structures [107]; however, it was reported that crystallographic defects could lead to sub-bandgap structures of MoSe2 [71]. The sub-bandgap absorption phenomenon, which was first reported for MoS2 by Roxlo et al. [101,108], was commonly observed in TMDs [58]. The electronic structures of TMDs are known to be modified due to broken symmetry and unsatisfied bondings between M and X atoms at the edges of atomic planes [100].
For the implementation of an all-fiberized SA that is based on the prepared MoSe2/PVA composite, a side-polished fiber platform was used in the experiment of the present study. The side-polished fiber was prepared through the polishing of one side of the SM2000 fiber, which was fixed onto the V groove of a quartz block. The distance between the flat side and the fiber core was measured as ~10 μm using a microscope. The beam propagation loss and the polarization dependent loss (PDL) of the prepared side-polished fiber were 1.2 dB and 0.05 dB, respectively. Without MoSe2/PVA composite deposition, it was impossible to induce mode-locking based on nonlinear polarization rotation even if the side-polished fiber was inserted in a laser cavity, since the PDL level was not high enough [65].
The MoSe2/PVA solution was deposited onto the flat side of the side-polished fiber using the solution-dropping method and dried at room temperature for 24 h. The solution dropping is a simple method that a small amount of solution is dropped and dried on top of a substrate. Using this method, it is straightforward to form a thin film; however, it is difficult to control the uniformity and thickness of the film. Furthermore, this method is not suitable for the precise control of the drop size and particle distribution. The thickness of the MoSe2/PVA film on the side-polished fiber was measured at ~5 μm with an alpha step profiler, and the length of the MoSe2/PVA composite film deposited on side-polished fiber was ~3 mm. Figure 3(a) shows the schematic diagram and side-view of the MoSe2/PVA composite-deposited side-polished fiber. After the deposition, the insertion loss and the PDL of the prepared MoSe2/PVA-based SA were measured as ~3.1 dB and ~7 dB, respectively. The polishing length of the prepared side-polished fiber was ~2.5 mm. In order to check the roughness of the prepared side-polished surface, an SEM measurement for the flat surface of the prepared side-polished fiber was performed, as shown in Fig. 3(b). The surface roughness was estimated to be less than 4 μm.
Fig. 3 (a) Schematic diagram and side-view of the proposed MoSe2/PVA composite side-polished fiber. (b) Measured SEM Image of the flat surface of the prepared side-polished fiber. (c) Nonlinear transmission curve of the side-polished fiber deposited with the MoSe2/PVA composite film. This measurement was performed for the TE- mode beam.
Next, the nonlinear transmission curve of the MoSe2/PVA-deposited side-polished fiber was measured as a function of the incident peak power through the use of a mode-locked fiber laser with a pulse width of ~1.5 ps at ~1903 nm. The modulation depth measurements were performed for both transverse electric (TE) and transverse magnetic (TM) mode beams; however, no saturable absorption was observed for the TM mode due to the high PDL [38]. Figure 3(c) shows the measured nonlinear transmission curve for the TE mode together with the fitting curve [105]. The saturation power and modulation depth were estimated to be ~25.7 W and ~4.4%, respectively.
3. Fiber laser setup and mode-locking performance measurement
The schematic for the passively mode-locked fiber laser of the present study is shown in Fig. 4. The gain medium is a 1-m-long Tm-Ho co-doped fiber (TH512, CorActive) with an absorption of ~13 dB/m at a wavelength of 1550 nm. A 1550-nm pump laser diode (PSL-450, Princeton Lightwave Inc.) with a maximum power of ~297 mW was used as the pump source that was entered into the gain medium through a 1550/2000 nm wavelength division multiplexer (WDM). For the unidirectional light propagation, a polarization-insensitive optical isolator was used after the gain medium. The mode-locked laser-output power was extracted from the ring cavity using the 10% port of a 90:10 coupler. A polarization controller (PC) was used to optimize the polarization state of the oscillating beam within the laser cavity. The MoSe2/PVA-deposited side-polished fiber was placed after the PC.
Stable mode-locked pulses were readily obtained when the pump power was set as ~274 mW, while the PC was properly adjusted. Since our prepared MoSe2/PVA-based SA had a high polarization dependent loss, it is believed that both nonlinear polarization rotation (NPR) and nonlinear saturable absorption contributed to the mode locking of our laser [109]. The average output power was measured as ~4.3 mW. Figure 5(a) shows the measured optical spectrum of the mode-locked pulses for which an optical-spectrum analyzer was used. The center wavelength and the 3 dB bandwidth were measured as ~1912.6 nm and ~4.62 nm, respectively. Figure 5(b) shows the measured oscilloscope trace of the output pulses with a 16 GHz real-time oscilloscope, while the inset of Fig. 5(b) shows a magnified view of a single output pulse. The temporal period of the output pulses is ~54.9 ns, which corresponds with the fundamental frequency of ~18.21 MHz.
Fig. 5 Measured (a) optical spectrum and (b) oscilloscope trace of the output pulses. Inset: oscilloscope trace for a narrow span.
Next, an autocorrelation measurement was performed using a two-photon absorption-based autocorrelator. Figure 6(a) shows the measured autocorrelation trace of the output pulses with a sech2() fitting curve. The measured pulse width is ~920 fs. Considering the 3-dB bandwidth of ~4.62 nm, the expected estimated time-bandwidth product is ~0.348, which is slightly higher than the 0.315 of the transform-limited sech2() pulses. This means that the output pulses are slightly chirped.
Fig. 6 Measured (a) autocorrelation trace of the output pulses and (b) electrical spectrum of the output pulses. Inset: electrical spectrum for a span of 1 GHz.
Lastly, the electrical spectrum was measured to check the phase noise of the output pulses, as shown in Fig. 6(b). A strong signal peak with an electrical signal-to-noise ratio (SNR) of ~65 dB was clearly observed at the fundamental frequency of ~18.21 MHz. The inset in Fig. 6(b) shows the electrical spectrum that was measured with a 1 GHz span. Strong harmonic signals were clearly observed, thereby strongly indicating that the output pulses are stable mode-locked pulses.
The pump power range for the fundamental mode-locking was from 207 to 274 mW. Above the pump power of 274 mW, stable mode locking was broken due to the multiple-soliton generation and the soliton energy quantization effect [110–112]. During our experiment no harmonic mode-locking phenomenon was observed.
Table 1 summarizes the output performance of the mode-locked, Tm-Ho co-doped fiber laser in comparison with that of the mode-locked, Tm-doped fiber lasers for which other saturable absorption materials were incorporated. As shown in the table, the modulation depth of the Bi2Te3-based SA is high (~20.6%) and comparable with the semiconductor-based SAs. The authors’ fabricated MoSe2/PVA-based SA exhibited a modulation depth (~4.4%) that is similar to the SAs that are based on graphene, gold nanorods, and BP. As reported in Ref [106], a modulation depth of 4% is high enough to induce mode-locking within a fiberized laser cavity with a proper anomalous dispersion. In terms of the temporal width of the output pulses, the fiber laser for which a CNT-based SA was used, showed the smallest output-pulse width of 450 fs [23]. The proposed mode-locked fiber laser produced output pulses with a temporal width that is twice as large as that of the output pulses of the CNT-based fiber laser, even if the output-pulse width is at the subpicosecond level.
Table 1. Performance comparison between the present work and the previous mode-locked Tm-doped fiber lasers for which other saturable absorption materials were incorporated
4. Conclusion
In conclusion, a MoSe2/PVA-based SA that can be used for the generation of stable mode-locked pulses in the 1912 nm region has been experimentally demonstrated here. The SA was fabricated using a side-polished fiber platform that was deposited with a MoSe2/PVA composite. Using the prepared MoSe2/PVA SA, it was possible to obtain stable ultrafast pulses with a ~920 fs temporal width from a Tm-Ho co-doped fiber laser.
This experimental demonstration provides meaningful evidence of the use of a bulk-like MoSe2/PVA composite film as an effective SA for ultrafast laser operations in the mid-IR region. Also, it has been confirmed that high-crystalline-quality, atomic-layered MoSe2 films are not essential for mode-locked laser operations. Further theoretical and experimental investigations need to be conducted, however, to determine the benefits and problems of the use of bulk and nano-structured MoSe2 as saturable absorption materials in terms of the mode-locked laser applications.
Funding
National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2015R1A2A2A11000907), Republic of Korea; MSIP (Ministry of Science, ICT and Future Planning), Korea, under the ITRC (Information Technology Research Center) support program (IITP-2017-2015-0-00385), supervised by the IITP (Institute for Information and Communications Technology Promotion).
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