1. Introduction

Passively Q-switched fiber lasers have attracted much interest in recent years owing to their wide range of applications in fields such as range finding, remote sensing, medicine and telecommunications [1, 2]. Generated by modulating the intracavity losses or Q factor of a laser oscillator, Q-switching has advantages of cost, efficient operation, and easy implementation, compared to mode-locking, which needs a careful design of the cavity parameters to achieve a balance of dispersion and nonlinearity. Semiconductor saturable absorber mirrors (SESAMs) [3, 4], transition metal-doped bulk crystals [5, 6] have been intensively investigated and demonstrated for passively Q-switched ytterbium-doped or erbium-doped fiber lasers as SAs. However, SESAMs and bulk crystal require complex fabrication, expensive price and have narrow operation bandwidth in typically few-tens nanometers. Therefore, the low-cost, broadband and high-performance SAs are in high demand. Since the discovery of carbon-based nanomaterials (e.g. CNTs, graphene) and topological insulator (TI), recent advances in nanomaterials open up new research on pulsed fiber lasers [7–15]. CNTs have been considered as excellent SAs for its easy fabrication and cheap cost, but their operation wavelength is determined by the nanotube diameters and chirality [7, 8]. Although the zero-gap material graphene possesses wavelength-independent saturable absorption, making it could be used as wideband SA, but it has a small absorption of 2.3% at 1550 nm [9]. Most recently, the topological insulator (TI) was extensively studied as a saturable absorber to obtain optical Q-switched pulse generation in fiber laser [13–15], whereas both the lasers have relatively long pulse duration of a few μs.

Researchers are still making considerable efforts to seek for new SAs which are expected to have the ideal characteristics of broadband-wavelength saturable absorption, low saturable optical intensity, large modulation depth, high damage threshold and low cost. Similar to the layered-structure of graphene, MoS₂ is a typical two dimensional material consisting of alternating hexagonal planes of Mo and S atoms bound by a weak van der Waals interaction, which allows formation of nano-sheets and its optical properties show interesting layer dependence. In contrast to graphene, which is a semi-metal with no bandgap by nature, bulk 2H-MoS₂ has an indirect bandgap of 1.29 eV, while monolayers offer an attractive semiconductor option due to a direct bandgap of 1.8 eV [16,17]. Recently, Wang et al. reported that MoS₂ dispersions performed faster saturable absorption response than the graphene dispersions at the same excitation condition [18]. The passively Q-switched with MoS₂ SA at wavelengths of 1.06 μm, 1.42 μm, 2.1 μm were firstly reported by Yu et al. in solid-state lasers, indicating that MoS₂ possesses broadband saturable absorption [19]. Depositing few-layered MoS₂ onto the end facet of fiber, Zhang et al. have shown the generation of mode-locked pulse of 800 ps in ytterbium-doped fiber laser [20]. Li et al. firstly used the chemical vapor deposition multilayer MoS₂ SA in erbium-doped fiber laser [21], the resultant output soliton pulses had central wavelength, pulse duration, and repetition rate of 1568.9 nm, 1.28 ps, and 8.288 MHz, respectively. Later on, Zhang et al. obtained mode-locked femtosecond pulse from an anomalous-dispersion erbium-doped fiber with proper cavity dispersion. The laser directly generated ?710 fs pulses centered at 1569.5 nm with a repetition rate of 12.09 MHz [22]. Up to date, MoS₂ passively Q-switched fiber laser has not been broadly reported, except that R. I. Woodward et al. using liquid phase exfoliation fabricated MoS₂ obtained 2.7 μs pulses in the ytterbium-doped fiber laser in CLEO 2014 [23], and Luo et al. reported a tunable erbium-doped fiber laser based on MoS₂ nano-platelets with pulse duration of 3.3 μs [24]. It is highly desirable to obtain Q-switched pulse generation based on MoS₂ SA and further explore its potential advantages in nanosecond-scale pulsed fiber laser.

In this contribution, we demonstrated stable Q-switched pulse generation in the erbium- doped fiber laser with the shortest pulse duration of 660 ns, and the repetition rate varying from 116 to 131 kHz. The high quality MoS₂ film deposited on the broadband reflectivity mirror with modulation depth up to 9% was obtained by pulsed laser deposition method. The Q-switched erbium-doped fiber laser operating in the 1.5 μm wavelength further demonstrated the broadband nonlinear saturable absorption potential of the MoS₂ for stable pulse generation.

2. Preparation and characterization of MoS₂

There are several methods available for preparation of 2-D MoS₂ materials such as hydrothermal intercalation/exfoliation [20], micromechanical exfoliation [25], chemical vapor deposition [26], and pulsed laser deposition [19]. Pulsed laser deposition (PLD) is an efficient technique to control the imperfections in the produced MoS₂ sample. Modified by defects, the bandgap of MoS₂ atomically layers is reduced and the materials exhibits broadband absorption and more advantages in the passively Q-switched lasers. With MoS₂ powder as precursor material, the MoS₂ target was cold pressed into a 40 mm diameter pellet. A KrF exciter laser with pulse width of 20 ns and at the wavelength of 248 nm was used for radiating and ablating the target. The repetition rate of 5 Hz and energy of 600 mJ per pulse (corresponding to an energy density of 8.5J/cm²) of the laser were employed for the broadband high-reflectivity mirror substrate. The base pressure of the vacuum chamber system was about 8.9 × 10⁻⁵ Pa, and it raised to about 5 × 10⁻⁴ Pa during the deposition process. In order to enhance the uniformity of the film, both the target and the substrate were rotated, and the substrate temperature was fixed at 300°C.

Figure 1(a) shows a photograph of the two-dimensional fabricated MoS₂ sample. The MoS₂ film of 20 × 20 mm lying on the substrate is flat and has good adhesion. The MoS₂ area was characterized by atomic force microscopy (AFM). The AFM scan of 5 × 5 μm, as shown in Fig. 1(b). The measurement showed that the thickness was about 16 nm. Normally the thickness of single-layer was 0.65 nm and the MoS₂ layers bond via the Van der Waals’ interaction, so the samples we prepared were more than 20 layers.

 

Fig. 1 (a) Fabricated MoS₂ sample on the broadband all-reflectivity mirror. (b) Morphology of MoS₂ measured with AFM

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With Raman spectrometry, the Raman shifting is shown in Fig. 2(a). It is observed that the shifted peaks of layered MoS₂ samples appear at 408 cm⁻¹ and 382 cm⁻¹, corresponding to the in-plane E¹_2g vibration mode and out-plane A_1g vibration mode, respectively [27]. For the 1T MoS₂, the spectrum does not show the E¹_2g Raman mode. Thus it implied that the MoS₂ nano-sheets were 2H polytypic. Meanwhile, a frequency difference of ~26 cm⁻¹ was obtained for the spectrum, showing an average thickness of more than ~6 layers.

 

Fig. 2 (a) Raman spectrum of MoS₂. (b) MoS₂ saturable absorption characteristic at 1.5 μm

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In order to investigate its nonlinear saturable absorption property, power-dependent measurement system was constructed at 1554 nm. The pump source used was a home-made SESAM mode-locked femtosecond fiber laser with a pulse width of 400 fs, repetition rate of 20 MHz and maximum output power of 2 mW. 10% of the output power was monitoring the input power. The 90% residual signal was directly incident into the MoS₂ SA through a variable optical attenuator and then received by a power meter. The result of the power-dependent reflectivity is plotted in Fig. 2(b). We used the following formula to obtain the fitting curve shown in Fig. 2(b) [9]:

Where I is the intensity of the input optical pulse, α_s and α_ns are the saturable absorption and non-saturable absorption respectively. I_sat is the saturation intensity. The initial reflectivity was about 50.5% and the saturable reflectivity was about 59.5%, which indicated the modulation depth was as high as 9.0%. The saturable incident power was ~0.2 mW and the mode-field diameter about the fiber butted to the SA was ~10.4 μm. Then the corresponding saturation intensity was ~27 MW/cm².

MoS₂ saturable absorption behavior in the wavelength of 1550 nm (~0.8 eV) was lower than the bandgap energy of MoS₂ in both bulk and monolayer forming. In Wang’s former work [19], they have theoretically and experimentally demonstrated that in the process of fabricating MoS₂ by PLD method, with Mo or S atomic defects, the bandgaps would be changed. When R is located in the range of 1.89 to 2.09, few layered MoS₂ becomes a semiconductor and the bandgap is also decreased. By the introduction defects in a suitable range, the MoS₂ bandgap can be reduced from 1.08 (R = 1:2) to 0.08 eV (R = 1:2.09) corresponding to an maximum absorption wavelength of 15.4 μm (0.08 eV). We did not exclude that saturation of edge states, which arise from large edge to surface area ratios of few-layer MoS₂ flakes would be a factor attributed to the saturable absorption behavior at longer wavelengths [28].

3. Experimental setup and results

Taking advantage of the saturable absorption of the fabricated MoS₂ in fiber laser, a linear cavity configuration was constructed as shown in Fig. 3. The cavity was constituted with 1.5 m erbium-doped fiber with core absorption coefficient of 7dB/m at 980 nm as gain medium. The 974 nm laser diode pump light of 503 mW maximum power was launched into the gain fiber by a wavelength division multiplexer. The MoS₂ saturable absorber mirror was located after the gain fiber, and output with 30% portion from the cavity was extracted through a narrow-band fiber bragg grating with its central wavelength of 1549.815 nm and high reflectivity of 70%. The total length of the cavity is about 2.8 m. An optical spectrum analyzer (Yokogawa, AQ6370), a 1 GHz real-time oscilloscope(Agilent,DSO7104B) with a 1.8 GHz photo-detector(Thorlabs, DET01CFC) were employed to monitor the laser output.

 

Fig. 3 Experimental setup of passively Q-switched erbium-doped fiber laser. WDM: wavelength division multiplexer; EDF: erbium-doped fiber; FBG: fiber bragg grating

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In the experiment, the erbium-doped fiber laser started the continuous-wave operation at the pump power of 35 mW. As increasing the power, stable Q-switched laser pulse train occurred at 371 mW incident pump power. When the pump power was increased, the repetition rate increased at the same time,which is the typical feature of passive Q-switching. The shortest pulse width of 660 ns was achieved for 470 mW incident pump power (Fig. 4(a) and 4(b)). Figure 5 summarizes the typical spectrum of the Q-switched operation at this power. The output optical spectrum centered at 1549.83 nm with a 3dB spectral bandwidth at the level of 0.02 nm, corresponding to the reflection wavelength of the FBG.

 

Fig. 4 (a) The pulse profile with the pump power of 470 mW. (b) The Q-switched pulse train with pump power of 470 mW.

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Fig. 5 The optical spectrum of the Q-switched pulse with pump power of 470mW

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When the pump power was over 470 mW, the Q-switched pulses became unstable, as usually observed in some passively Q-switched fiber lasers reported previously [7,10,14]. We think one possible reason for the unstable Q-switched is the over-saturation of the MoS₂ at high incident intensity. That means, two-photon absorption (TPA) process in the few-layer MoS₂ has been excited under the higher optical intensity. Thus, the absorption coefficient increased as continually increasing of pump power and Q-switched operation could not be maintained. Increasing the pump power further to 503 mW, the Q-switching stopped. However, after the pump power was decreased from 450 mW, the stable Q-switched operation observed again at the pump power of < 470 mW. This phenomenon indicated that the sample was not damaged in the relative high power. In the lasers, there was no thermal damage observed in the MoS₂ sample and the output power was stable during laser operation up to 470 mW. Limited by the maximum output power of the pump ~503 mW, we could not measure the exact damage threshold.

Figure 6 shows the pulse widths and repetition rates versus different incident pump powers. The pulse widths decreased from 660 ns to 760 ns while the pump power increasing from 371 mW to 470 mW, and the repetition rates of the laser can be widely turned from 116 kHz to 131 kHz since pulse generation relies on saturation. The average output power was also found to linearly increasing with the rise of incident pump power. At pump power of 470 mW, the maximum output power was 20 mW, so the maximum single pulse energy was about 152 nJ. To verify whether the Q-switched operation is purely contributed by the saturable absorption of the MoS₂, the MoS₂-based gold mirror was purposely replaced with an ordinary gold mirror. In this case, no Q-switching was observed, despite that the pump power was increased from zero to the maximum available power in a full range.

 

Fig. 6 Pulse widths and repetition rates versus incident pump powers

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4. Conclusions

In summary, we have presented a passively Q-switched erbium-doped all-fiber laser based on MoS₂ SA. Fabricated by pulsed laser deposition technique, the MoS₂ SA yielded modulation depth as high as 9% with 1.5 μm femtosecond laser excitation. Employing this device into an erbium-doped fiber laser, we have achieved stable Q-switched nanosecond pulses generation. Through fine increasing the pump power, the repetition rate could be changed from 116 kHz to 131 kHz, and pulse duration from 760 ns narrow to 660 ns. The Q-switching operation at the 1.5 μm waveband further demonstrates the high-performance broadband saturable absorption of the MoS₂ SA in fiber lasers.