We propose and demonstrate a MoS2-based passively Q-switched Er-doped fiber laser with a wide tuning range of 1519.6–1567.7 nm. The few-layer MoS2 nano-platelets are prepared by the liquid-phase exfoliation method, and are then made into polymer-composite film to construct the fiber-compatible MoS2 saturable absorber (SA). It is measured at 1560 nm wavelength, that such MoS2 SA has the modulation depth of ∼2% and the saturable optical intensity of ∼10 MW/cm2. By further inserting the filmy MoS2-SA into an Er-doped fiber laser, stable Q-switching operation with a 48.1 nm continuous tuning from S- to C-waveband is successfully achieved. The shortest pulse duration and the maximum pulse energy are 3.3 μs and 160 nJ, respectively. The repetition rate and the pulse duration under different operation conditions have been also characterized. To the best of our knowledge, it is the first demonstration of MoS2 Q-switched, widely-tunable fiber laser.
© 2014 Optical Society of America
Passively Q-switched fiber lasers (PQFLs)  have been widely applied in material processing, range findings, telecommunications and medicine. In particular, wide wavelength tunability is a key feature of PQFL, and is highly desirable in some specific applications such as WDM technology, spectroscopy and biomedical research. The key element for obtaining widely-tunable PQFL is a broadband saturable absorber (SA). At present, semiconductor saturable absorber mirrors (SESAMs)  are thought as one of the most mature and commercial SAs, but SESAMs are expensive for fabrication and have narrow operation bandwidth (typically few-tens nm ), which possibly limits the broadband tunable Q-switching operation. Therefore, the low-cost, broadband and high-performance SAs are in high demand. In the past decade, the considerable attention has been given to nanomaterial-based SAs such as single-wall carbon nanotubes (SWNTs) [4–7], graphene [8–18] and topological insulators (TIs) [19–25], mainly due to their advantages such as broadband operation, easy-fabrication and low-cost. However, SWNTs usually require the broad tube-diameter distribution for obtaining the broadband operation, leading to high additional loss. Graphene is an intrinsic ultra-broadband SA, but its modulation depth is often low (typically <1% per layer ). Therefore, researchers still have strong motivations and are making every effort to seek new ideal SAs.
In the recent years, ultrathin molybdenum disulfide (MoS2) as another new-type nano-material has been attracting great interest, due to its extraordinary electronic and optical properties . Since Wang et al. firstly reported the nonlinear saturable absorption property of MoS2 nanosheets in 2013 , very few works have carried out MoS2-based pulsed fiber lasers [30–33]. Zhang and Liu et al. presented the 1 and 1.5 μm mode-locked fiber lasers using few-layer MoS2 SAs [30, 31], respectively. Most recently, Woodward et al. obtained a 1068 nm Yb-doped fiber laser Q-switched by MoS2 SA . However, so far, no widely-tunable Q-switched fiber laser has been reported using MoS2 SA, and MoS2 broadband saturable absorption is not fully exploited.
In this paper, we demonstrate for the first time a widely-tunable, MoS2 passively Q-switched Er-doped fiber laser. The broadband absorption of few-layer MoS2 enables the tunable Q-switching with 48.1 nm continuous tuning range of 1519.6–1567.7 nm, limited only by our tunable filter. The pulse duration and pulse energy of the tunable Q-switching operation have been characterized as the maximum pulse energy of 160 nJ and the shortest pulse duration of 3.3 μs.
2. Preparation and characteristics of few-layer MoS2
The few-layer MoS2 used in our experiment was prepared by the liquid-phase exfoliation method as follows. Initially, the bulk MoS2 was exfoliated in the dimethyl formamide (DMF) solution by 20-hour sonication to produce few-layer MoS2 suspension. Then, the few-layer MoS2 suspension was centrifuged for 30 min at 1000 rpm to remove the residual bulk MoS2. Different from one before sonication [Fig. 1(a)], the suspension after sonication became deeper color with good uniformity, indicating that the bulk MoS2 had been exfoliated and well dispersed in DMF. As shown in Figs. 1(b)–1(d), we characterized the exfoliated MoS2 by X-Ray diffractometer (XRD) and atomic force microscopy (AFM). Compared with the rhombohedral phase of MoS2 (JCPDs No. 06-0097), the measured XRD pattern [Fig. 1(b)] can be indexed to some characteristic peaks, implying that the exfoliated MoS2 nanosheets maintain the good crystalline property. Figures 1(c) and 1(d) give the AFM image and the height profile diagram of the exfoliated MoS2 nanosheets. The height profile [Fig. 1(d)] shows that the average thickness is around 2–3 nm. This indicates that the exfoliated MoS2 nanosheets are around 3–4 layers, because the single-layer thickness of MoS2 is 0.65 nm .
In order to allow easy-integration and flexibility of use, one could prefer that the MoS2 nanosheets are made into polymer-composite structure. Thus, we collected the as-prepared few-layer MoS2 suspension into phials and dispersed in the polyving akohol (PVA), which can be easily film-forming for further application. Subsequently, we investigated the optical absorption properties of the few-layer MoS2 nanosheets. Firstly, a PVA-MoS2 thin film was obtained by drying the as-prepared PVA-MoS2 solution on a glass substrate. Then, using an optical spectrometer (Perkinelmer Lambda 750) scanning from 300 to 2700 nm, we measured its transmission spectrum (i.e. linear-absorption spectrum). As shown in Fig. 2(a), the MoS2 sample has a flat absorption-curve from near-infrared to mid-infrared region (800–2700 nm), indicating the potential of the few-layer MoS2 as a broadband optical material. As seen in the inset of Fig. 2(b), a PVA-MoS2 film was transferred onto a fiber ferrule, and then connected with another fiber ferrule for constructing a fiber-compatible device. We measured the nonlinear absorption property of this filmy-MoS2 fiber device by a balanced twin-detector measurement system. The measurement system is similar to Ref.. The illumination light source used in this measurement is a home-made SWNTs-mode-locked fiber laser (center wavelength: 1560 nm, repetition rate: 22.15 MHz, pulse duration: ∼250 fs, output power after amplification: ∼12 mW). With the different optical intensity into the filmy-MoS2 sample by adjusting an electrical tunable attenuator, we recorded in real time the nonlinear transmission curve. As shown in Fig. 2(b), the typical saturable absorption was observed, and the experimental data can be well fitted by the nonlinear saturable-absorption formula . The modulation depth of the filmy-MoS2 fiber device can be estimated to be ∼2%, comparable to 1.3% of graphene , 0.94% of SWNTs  and 1.7% of TI:Bi2Te3 . The saturable optical intensity (Isat) of the filmy-MoS2 fiber device is ∼10 MW/cm2 at 1560 nm wavelength, also comparable to other nanomaterial-based SAs (e.g. SWNTs , graphene , TIs ). Therefore, we believe that using such filmy-MoS2 SA, Q-switching or mode-locking in fiber or solid-state lasers could be achieved.
Because of the MoS2 bulk bandgap ∼1.29 eV (corresponding to ∼1 μm) , one could concern why the few-layer MoS2 sample has the saturable absorption at 1.56 μm. When MoS2 is exfoliated from bulk to few-layer structure, the bandgaps of few-layer MoS2 become inhomogeneous because of the blending of the 1T (metallic-like) and 2H (semiconducting) phases . Similar to the zero bandgap of graphene and the surface state of TIs, the 1T phase of few-layer MoS2 could also exhibit the broadband saturable absorption characteristics under the help of the Pauli-blocking effect . Therefore, the saturable absorption of few-layer MoS2 at 1560 nm could be easily understand.
3. Experiments and discussions
The experimental setup of our proposed widely-tunable, passively Q-switched Er-doped fiber laser based on few-layer MoS2 SA is shown in Fig. 3. A section of 4.6 m Er-doped fiber (EDF, Nufern-EDFC-980-HP) with absorption loss of ∼3 dB/m at 980 nm acts as the gain medium. The EDF is pumped by a 974 nm laser diode (LD) through a 980/1550 nm wavelength-division-multiplexer (WMD). The as-fabricated filmy-MoS2 fiber device [Fig. 2(b)] was spliced into this cavity as a passive Q-switcher with the insertion loss of ∼48.5%. A fiber Fabry-Perot (FP) filter from Micron Optics, Inc., with a free spectra range (FSR) of 48.5 nm (centered at ∼1539 nm) is used to select the oscillation wavelength inside the fiber ring cavity. The transmission wavelength of the FP filter can be adjusted by applying different electrical voltages, and therefore the lasing wavelength can be readily tuned within its FSR. A 20/80 optical coupler (OC) is used to extract 20% lasing signal. A polarization-independent optical isolator is inserted in the cavity to ensure an unidirectional propagation. The laser output is simultaneously monitored by an optical spectrum analyzer (OSA, HP 70951B), and a high-speed photodetector together with a 200 MHz digital oscilloscope (Tektronix TDS2024) or radio-frequency (RF) spectrum analyzer (Gwinstek GSP-930).
Firstly, by purposely removing the filmy-MoS2 fiber device from the cavity, we excluded the possiblity of self-Q-switching of the laser at any tunable wavelength. Then, we spliced the filmy-MoS2 fiber device into the cavity, and stable Q-switching can easily occurred at a low pump intensity, manifesting that the filmy-MoS2 SA plays a key role in passive Q-switching. This system did not start the mode-locking operation which is possibly limited by 1) the large cavity loss of ∼9 dB, and 2) the narrow bandwidth of the tunable filter. The wavelength-tunable Q-switching operation was investigated specially at a fixed pump power of 46.1 mW. As shown in Fig. 4(a), by tuning the lasing wavelength, the stable Q-switched pulse trains on the oscilloscope screen were observed always. The center wavelength of Q-switching could be continuously tuned in the wide spectral range of 1519.6–1567.7 nm (across S and L bands, Fig. 4(b)). The 48.1 nm tuning range is comparable to the ∼30 nm one reported for graphene Q-switched fiber laser , and is much larger the 5 nm one for SWNT Q-switched fiber laser . Moreover, our tuning range is limited mainly by the FSR (48.5 nm) of the tunable FP filter (see Fig. 4(b)), not by the filmy-MoS2 SA. We believe that the Q-switched tuning-range could be further enlarged if using a larger-FSR tunable filter.
One can clearly see from Fig. 4(a) that at the same pump power of 46.1 mW, the pulse period (i.e. repetition rate) of Q-switching varied greatly with lasing wavelength. This implies that the Q-switching is wavelength-dependent. Therefore, we also investigated the effects of tunable lasing-wavelength on the output power, repetition rate and pulse duration of Q-switching operation. Figure 5(a) gives the average output power as a function of the tunable lasing wavelength at the fixed pump power of 46.1 mW. The power difference along the 48.1 nm tuning-range is less than 3-dB, originating from the gain difference of EDF at different wavelengths. Moreover, the higher output power is obtained around 1530–1560 nm, which is mainly attributed to the larger gain of EDF in the spectral region. As shown in Fig. 5(b), we also recorded the repetition rate and the pulse duration as a function of the lasing wavelength. As tuning the lasing wavelength from 1519.6 to 1567.7 nm, the repetition rate sharply changes between 10.6 kHz and 34.5 kHz, and the variation tendency is similar to that [Fig. 5(a)] of the output power. This is easily explained as follows. At a larger-gain wavelength of EDF (e.g. ∼1530 nm), the intracavity laser is stronger, and the bleaching of MoS2 SA is faster under a faster population inversion/depletion, leading to a larger repetition rate. It should be noted in Fig. 5(b) that the pulse duration could vary from ∼5 μs to ∼9 μs in the tuning range,
To further clarify the tunable Q-switched characteristics, we then focus on a fixed lasing wavelength of 1551.2 nm by setting the tunable FP filter. The Q-switching threshold for this wavelength is as low as 18.9 mW. Figure 6 summarizes the characteristics of the 1551.2 nm Q-switched pulses emitted from the fiber laser at the pump power of 46.1 mW. Figure 6(a) gives the optical spectrum of Q-switched operation with the center wavelength of 1551.2 nm and the 3-dB linewidth of 0.28 nm. Figure 6(b) shows the typical Q-switched pulse train with a pulse period of 59.6 μs (corresponding to 16.78 kHz repetition rate), and the emitted pulses have a nearly-uniform intensity without modulation. As seen from the inset of Fig. 6(b), the single pulse envelope has a symmetric Guassian-like intensity profile with a full width at half maximum of 5.7 μs. To evaluate the stability of the passive Q-switching, we measured the RF output spectra [Figs. 6(c) and 6(d)] of the Q-switched pulses with the resolution bandwidth (RBW) of 10 Hz. The RF signal-to-noise ratio (SNR) is over 50 dB, which is better or comparable to those reported fiber lasers Q-switched by graphene  and TIs . Furthermore, the broadband RF spectrum in Fig. 6(d) is regular and no spectral modulation. These indicate that the Q-switched laser is stable and could be suitable for practical applications.
As increasing the pump power, we also investigated the evolutions of repetition rate, pulse duration, average output power and pulse energy of the Q-switching at the fixed lasing wavelength of 1551.2 nm. As shown in Fig. 7(a), we recorded the pulse repetition rate and the pulse duration as a function of the pump power. As increasing the pump power from 18.9 to 227.1 mW, the repetition rate monotonously increases from 8.77 to 43.47 kHz, and meanwhile the pulse duration was narrowed from 26.7 to 3.3 μs. The pulse duration could be further narrowed by shortening the cavity length and improving the modulation depth of the MoS2 SA . As shown in Fig. 7(b), we also measured the average output power, and correspondingly calculated the single pulse energy. The average output power linearly increased, and the maximum output power was 5.91 mW at the pump power of 227.1 mW. One can see from Fig. 7(b) that the pulse energy fast grew in the initial stage, but after the pump power over 175 mW, the pulse energy became to saturate obviously. The maximum pulse energy was 160.0 nJ, comparable to those Q-switched fiber lasers using graphene [11,15,18,39], SWNTs  and TIs [22,25]. The higher pulse energy could be enabled by high-gain fiber (e.g. double-clad fiber ) and further optimizing the cavity parameters (e.g. SA’s performance, cavity loss). To understand the pulse-energy saturation, when further increasing the pump power over 230 mW, we noticed in our experiment that the Q-switched pulse trains became very unstable with strong amplitude fluctuation and even the Q-switching disappeared finally. The possible reason is the over-bleaching of MoS2 SA at the high pumping strength.
In summary, we have fabricated a filmy PVA-MoS2 SA with the advantages of easy-integration and flexibility of use, and measured its nonlinear saturable absorption using a balanced twin-detector measurement system. Further employing this MoS2 SA, we successfully demonstrated a widely-tunable, passively Q-switched Er-doped fiber laser across S+C bands. Stable Q-switching has been achieved in the 48.1 nm continuous-tuning range of 1519.6–1567.7 nm. The maximum pulse energy and the shortest pulse duration were obtained to be 160 nJ and 3.3 μs, respectively. Our results verified that the MoS2 SA possesses the advantage for broadband tunable pulsed laser, and such widely-tunable Q-switched laser could provide a simple and cost-effective solution to the applications of metrology, environmental sensing and bio-medicine.
This work is supported partially by the National Natural Science Foundation of China (NSFC) (No. 61177044, 661275050, 61475129 and 61107038).
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