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We demonstrated a Q-switched fiber laser based on Tungsten Disulfide (WS2) saturable absorber. The WS2 nano-sheets were prepared by liquid phase exfoliation method and the saturable absorber was fabricated by spin-coating of few-layer WS2 nano-sheets on a side-polished fiber for pulsed operation of a fiber laser. By inserting the absorber into an Erbium-doped fiber laser cavity pumped by a 980 nm laser diode, a stable Q-switched laser operation was achieved with a tunable repetition rates from 82 kHz to 134 kHz depending on the applied pump power. The properties of the deposited WS2 film was examined using scanning electron microscopic (SEM) and atomic force microscope (AFM). Detailed optical properties of the laser output are also discussed.
© 2015 Optical Society of America
1. Introduction
Q-switched fiber lasers have gained intense research interests for their applications in medical laser treatments and laser materials processing [1–3]. Compared to actively Q-switched fiber lasers, passively Q-switched fiber lasers are of great interest due to their capability of compactness, and simplicity in design [4,5]. To achieve the saturable absorption in the passively Q-switched fiber laser, several kinds of saturable absorbers (SAs) have been proposed including semiconductor saturable absorber mirrors (SESAMs) [6], carbon nanotubes (CNTs) [7], and Dirac type material such as graphene and topological insulator [8,9]. However, in order to obtain ideal SAs with the characters of wavelength independent, high damage threshold and cost-effective manufacturing process, exploration of new nonlinear optical materials still remains as an on-going issue.
Recently, molybdenum disulfide (MoS2), a transition metal dichalcogenide, has revealed a high potential in electronic and optic applications [10,11]. In particular, it has been demonstrated that MoS2 possess saturable absorption behavior [12] and pulse shaping ability of few-layer MoS2 has been experimentally demonstrated [13–15]. Tungsten disulfide (WS2) is another layered transition metal dichalcogenide, analogous to MoS2. Each atomic layer of WS2 consists of a layer of tungsten sandwiched by two layers of sulphur. The indirect band gap of bulk WS2 is ~1.3 eV and the direct band gap of its monolayer form is up to 2.1 eV [16]. Very recently, monolayer WS2 showed an extraordinarily large second-order nonlinear susceptibility [17]. And WS2 monolayers have been used for ultra-short optical pulse characterization via second harmonic generation [18]. While MoS2 has received increasing attention for ultra-short pulse generation, little attention has been paid to WS2 nano-sheets and WS2-based Q-switched fiber laser has not been reported, to the best of author’s knowledge.
In this study, we prepared few-layer WS2 nano-sheets by the liquid phase exfoliation (LPE) of bulk layered materials [19]. By spin coating of the solution on a side polished fiber, WS2 saturable absorber was fabricated and its applicability for Q-switching in the laser cavity was experimentally investigated for the first time. The properties of the deposited WS2 film were examined using scanning electron microscopic (SEM) and atomic force microscope (AFM).The compact all-fiber laser based on WS2-SA emits stable Q-switched pulses with central wavelength of 1568.9 nm, pulse duration from 2.82µs to 0.71 µs, and repetition rates from 82 kHz to 134 kHz. The experimental results showed that the few-layer WS2 can be regarded as a promising broadband SA for ultra-fast laser systems.
2. Fabrication and characterization of the WS2-SA
Weak bonding between the WS2 layers makes it possible to separate WS2 bulk into single molecular layers by a liquid route. WS2 nano-sheets used in our experiment were prepared by lithium-based chemical exfoliation with large populations of few layers WS2 in ethanol [19]. The exfoliated WS2 nano-sheets are characterized by optical absorption spectroscopy (OAS) and transmission electron microscopy (TEM).
Figure 1(a) depicts the linear absorption spectrum of WS2 nano-sheets spin-coated on a sapphire substrate. Almost flat absorption distribution around near infrared wavelength confirms a broadband optical material. Figure 1(b) is a high contrast transmission electron microscopy (TEM) image of an exfoliated few-layer WS2 nano-sheets on a TEM grid. Results show that very thin sheets of WS2, with lateral sizes tens of nanometers can be obtained after exfoliation. We prepared a side-polished fiber by polishing one side of a conventional single mode fiber (SMF) fixed onto a V-grooved quartz block. The polishing was stopped when evanescent field appeared at the central region of the block surface due to removing a few microns of the fiber cladding. The insertion loss of the fabricated SPF was less than 1 dB and an index matching oil drop test showed a loss of ~25 dB which is typical value used in fabrication of SPF type coupler [20]. The corresponding interaction length of the SPF is estimated to be about 3 mm. The WS2 nano-sheets were subsequently deposited onto the surface of SPF using the spin coating method and showed ~1 dB polarization dependent loss. Figure 1(c) schematically depicts the deposited SPF and its cross-sectional view. Figure 1(d) shows SEM image of the WS2 nano-sheets deposited on the SPF showing that the SPF surface was completely covered with WS2 nano-sheets with a rough morphology as the nano-sheets are randomly stacked. Using atomic force microscopy the film thickness of WS2 nano-sheets spin coated on SPF was determine to be about 350 nm (see Fig. 1(e)). The inset of Fig. 1(e) indicates the clear step between spin coated and clean area of the deposited layer on the surface of SPF.
Fig. 1 (a) Linear optical absorption spectrum of prepared solution. (b) TEM images of the WS2 nano-sheets on the TEM grid. (c) Schematic image of fabricated SPF and cross section view. (d) SEM image of the spin coated solution on the SPF. (e) AFM measurement of spin coated WS2 nano-sheets on SPF (inset: edge of the deposited layer).
In order to characterize the nonlinear transmission behavior of the fabricated WS2 SA, we used the transmission measurement set-up as shown in Fig. 2(a).The input source was a mode-locked fiber laser which operates at a central wavelength of 1563 nm with pulse duration of 800 fs, and repetition rate of 41 MHz. The laser output was tuned by a variable optical attenuator (VOA) and directed to the SA through a polarization controller (PC).
Fig. 2 (a) Experimental set-up of nonlinear transmission measurement. (b) The nonlinear transmission results for WS2 SA.
We first inserted a side polished fiber with no WS2 deposition into the nonlinear transmission setup and the output transmission was flat. Then, the WS2 SPF was inserted and Fig. 2(b) shows the result of nonlinear transmission which was fitted according to the two-level saturable absorber model [21]. The intensity dependent transmission can be clearly observed as the transmission increased by intensifying the input power. However, the WS2 SA could not be fully saturated due to limitations of the power of the light source currently used.
3. Fiber laser experiment and discussion
Figure 3 depicts the experimental setup of the proposed Q-switched fiber laser. A 980 nm laser diode (LD) was used for pumping the highly Er-doped fiber (EDF) with a length of 1 m. A polarization insensitive hybrid component was utilized which included a 980/1550 nm wavelength division multiplexing (WDM), a directional coupler with 10% output coupling ratio, and a polarization optical isolator to force the unidirectional operation of the ring laser. A polarization controller (PC) was inserted to adjust the polarization state of the ring laser. The total cavity length was:6.45 m, with a net dispersion of:-0.052 ps2. The fabricated WS2-SA was spliced in the cavity as a Q-switcher and the laser output was measured by an optical spectrum analyzer (Yokogawa AQ6370 B), a 7 GHz RF spectrum analyzer (Agilent technologies N9000A), and digital oscilloscope (Tektronix TDS 784D).
Figure 4 shows the typical Q-switched pulse trains under the different pump power without changing the PCs orientations. Q-switched pulse occurred at the threshold of 186 mw. At every specific repetition rate and pump power, the Q-switching pulse was stable and no significant pulse jitter was observed on the oscilloscope. The stable pulse train with the different repetition rates can be tuned from 82 kHz to 134 kHz by varying the pump power from 207.4 mW to 657.8 mW.
Fig. 4 Typical oscilloscope traces of the Q-switched pulse trains under different pump powers: (a) 207.4 mW, (b) 309.7 mW, (c) 454.7 mW 69.4, and (d) 657.8 mW.
Figure 5(a) shows a typical spectrum of the Q-switched ring laser at the pump power of 454.7 mW. The center wavelength of the Q-switched spectrum is 1567.8 nm. Furthermore, the broadband RF spectrum in the inset of Fig. 5(a) confirms the stability of Q-switching operation. At the same pump power, the pulse duration (FWHM) of 0.92 µs was obtained which was fitted by a Sech2-shaped pulse, as shown in Fig. 5(b).
Fig. 5 (a) Output optical spectrum of the Q-switched fiber laser. (Inset: broadband RF output spectrum) (b) Single pule profile. (c) The pulse duration and repetition rate as a function of pump power. (d) The output power as a function of pump power.
In order to further investigate the characteristics of the output Q-switched pulses, we fixed the orientations of the PCs and only changed the pump power. Figure 5(c) shows the dependence of the pulse-repetition rate (PRR) and pulse duration on the pump power. Initially, the pulse repetition rate was 82 kHz and the corresponding pulse duration was about 2.82 µs. We observed that the repetition rate increased and the pulse duration reduced with increasing pump power which is in consistent with the inherent characteristic of the Q-switched ring fiber lasers [2]. We measured a maximum repetition rate of 134 kHz which corresponded to minimum pulse duration of 0.71 µs.
In Fig. 5(d), the average output power and pulse energy of the Q-switched laser were measured as a function of the input pump power. Q-switching threshold was ~186 mW and as the pump was further increased to 207 mW, a stable and self-starting Q-switching pulse was observed and maintained up to the pump power of 657 mW without changing the polarization controller state. As the SPF type of SA provides a higher damage threshold and a long interaction length, no damage was observed even at the maximum power of the laser diode. The maximum average output power was 2.5 mW is corresponding to pulse energy of 19 nJ. These experimental results confirmed that the SA nature of WS2 thin film enabled highly stable Q-switching operation of the fiber laser by evanescent wave interaction. The performance of Q-switched pulses is expected to be further improved by optimization of the saturable absorber and the laser cavity design.
5. Conclusion
In summary, a stable all-fiber passively Q-switched ring laser is demonstrated near 1.5 µm based on the WS2-SA. WS2 nano-sheets used in our experiment were prepared by lithium-based chemical exfoliation. The WS2 nano-sheets were subsequently deposited onto the surface of SPF using spin coating method. The fabricated WS2-SA was spliced in the laser cavity as a Q-switcher. Q-swithched pulse occurred at the threshold of 186 mw and by adjusting the pump power level, the Q-switched laser has a range of pulse repetition frequency (PRF) from 82 to 134 kHz with a minimal pulse width is 0.71 µs. Our experimental results clearly demonstrated that the WS2 can be considered as a suitable candidate for fiber laser application.
Acknowledgments
This work was supported in part by Institute of Physics and Applied Physics, Yonsei University, in part by ICT R&D Program of MSIP/IITP (2014-044-014-002), in part by Nano Material Technology Development Program through NRF funded by the MSIP (NRF-2012M3A7B4049800). H. Jeong and D.-I. Yeom have been supported by NRF of Korea (NRF-2013R1A1A2A10005230).
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