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In this paper, we demonstrated a passively mode-locked erbium-doped fiber (EDF) laser by incorporating a tungsten disulfide (WS2) film SA fabricated by pulsed laser deposition (PLD) method. The WS2 film was thickness-dependent, which had two different states: the bulk WS2 [faced to plasma plume] and tiny WS2 flakes [in the shadow of plasma plume]. This SA device demonstrated low insertion loss (IL) and high power tolerance ability. Interestingly, the SA device possessed different nonlinear absorption regimes related with the film states. By employing this new type of SA, we obtained stable fundamental mode-locking (FML) at pump power of 54 mW, and the generated soliton pulse had pulse duration of 675 fs and signal-to-noise ratio (SNR) of 65 dB. At the maximum pump power of 395 mW, we also obtained up to 1 GHz repetition rate of harmonic mode-locking (HML) with pulse duration of 452 fs and SNR of 48 dB. The experimental results show that WS2-PLD film can serve as a promising SA for ultrafast laser systems.
© 2015 Optical Society of America
1. Introduction
Mode-locked fiber lasers have widespread applications in optical communication, industrial material processing, optical sensing and biomedical diagnostics [1,2]. Compared to active mode-locking, passive mode locking has the advantages of compactness, simplicity and flexibility. Incorporating a real saturable absorber (SA) can achieve ultra-short pulses emission and enhance the environmental stability for a passive mode-locked laser. Therefore, new and high performance SA is always a hot topic in ultrafast laser research. Among the various types of SAs, the semiconductor saturable absorber mirrors (SESAMs) [3] are regarded as one mature SA with widely application in commercial laser system. But SESAMs require expensive equipment and complicated fabrication technique, and have limited bandwidth operation (typically few-tens nm [4]). Carbon-based SA, including of single-wall carbon nanotubes (SWCNT) and graphene, have been widely investigated for ultrashort pulse generation owe to their advantages such as ultrafast photo-response, easy fabrication and low cost. SWCNT [5–10] is an intrinsically selective-broadband SA depending on it’s tube-diameter, which usually leads to a larger non-saturable loss for obtaining a broadband operation. Unlike SWCNT, graphene [11–25] possesses unique zero bandgap and can behave as an excellent SA with ultra-broadband absorption property. But graphene also holds two main disadvantages, the weak modulation depth (typically ~1.3% per layer [11]) and the difficulty of creating an optical bandgap. Therefore, significant efforts have been focused on developing new SA beyond graphene from other layered crystal [26–29], such as topological insulators (TIs) [30–36], transition mental dichalcogenides (TMDs), including of molybdenum disulfide (MoS2) or tungsten disulfide (WS2), as well as their diselenide analogues [37,38].
In 2013, Wang et al. firstly revealed that the MoS2 nanosheets exhibited stronger SA than graphene based on the open-aperture Z-scan technique at 800nm [39], implying a huge potential in the development of nanophotonic devices, such as passive laser mode-locker, Q-switcher and optical limiter. Subsequently, Cui et al. [40] experimentally determined the excitation lifetimes of monolayer and bulk WSe2 to be 18 ps ± 1ps and 160 ps ± 10ps, respectively. This suggests that WSe2 can be developed as a new saturable absorber with two different relaxation time that can fit for specific applications. Zhang et al. [41] demonstrated the broadband saturable absorption properties of few-layer MoS2 from the visible band to the near-infrared band, and achieved stable picoseconds passively mode-locked ytterbium-doped fiber (YDF) laser by the few-layer MoS2. Several methods have been used to obtain the MoS2 SAs, such as solution processing method [42], CVD-grown method [43], evanescent-field-interaction method [44–47] and composite films composed of nanomaterial flakes in a polyvinyl alcohol (PVA) host [48,49]. In laser, these SAs are usually pasted on fiber ferrules, or embedded in photonic crystal fiber [19, 34], or deposited on microfiber or side polished fiber (SPF). The fiber ferrule-type SAs have inherently short nonlinear interaction length. The composite films SAs could maintain the thermal stability of SA materials, but they are vulnerable to destruction by high power operation. Moreover, it should be guaranteed that light must transmit through the SA materials. The microfiber-based or SPF-based SAs are attractive for high power tolerance and long interaction length. Compared with SPF, microfiber has a simple fabrication process by the mature fused-tapering method. Additionally, pulsed laser deposition (PLD) method is a well-known thin film fabrication technique. Therefore, a new type of SA would be created by deposition of TMD film on microfiber, which has combined advantages from the strong nonlinear optical response in material together with the sufficiently-long-range interaction length in fiber taper. However, the fabrication and characteristics of this type of SA, and their application in mode-locking, is yet lacking.
In this paper, we demonstrated a passively mode-locked EDF laser by incorporating a WS2 film SA. The WS2 film was deposited on microfiber by PLD method, which had two different states: the bulk WS2 [faced to plasma plume] and WS2 flakes. This SA device demonstrated low insertion loss (IL) and high damage threshold. Interestingly, the SA possessed two different nonlinear absorption features. By employing this new type of SA, we obtained stable fundamental mode-locking (FML) at pump power of 54 mW, and the generated soliton pulse had pulse duration of 675 fs and signal-to-noise ratio (SNR) of 65 dB. At the maximum pump power of 395 mW, we also obtained up to 1 GHz repetition rate of harmonic mode-locking (HML) with pulse duration of 452 fs and SNR of 48 dB. The experimental results show that WS2-PLD film can serve as a promising SA for ultrafast laser systems.
2. Preparation and characterization of WS2-based SA
The WS2 target, fabricated by the mat-world.com, had a diameter of 49 mm and a thickness of 3 mm. Figure 1 shows the WS2 target in plastic sack and one of the enlarged regions. It exhibited a typical morphology of bulk WS2. The purity of WS2 was larger than 99.5%. In elemental composition, the tungsten (W) had a mass percentage of 74.14% and the sulfur (S) had 25.86%. [Here, W has an atomic weight of 183.84 and S has an atomic weight of 32.065. For the 1:2 ratio of W to S atoms (e.g. S-W-S), the percentage can be easily deduced]. In PLD process, the target was placed into a vacuum chamber where the vacuum degree was set at 5 × 10−4 pa. A high energy Nd:YAG laser (SL II-10, Surelite) could emit 2 mJ/pulse laser beam, which was delivered into the chamber and focused on the target to inspire out of the plasma plume. When arrived to the microfiber, the WS2 element would grow on the side surface of microfiber. In experiment, the deposition time was 2 hours, and the deposition temperature was fixed at room temperature. To verify that the film was really deposited on microfiber, we executed a scanning electron microscope (SEM) on both the microfiber and the WS2 film morphology, as shown in Fig. 2. The waist region of microfiber, marked by the arrow in Fig. 2(a), had a diameter of ~18 μm and a length of 1 mm. At this scale, the guided light in taper would effectively penetrate into the film and be modulated along the whole waist region. Figure 2(b) shows the film morphology, in which the arrow marked out the direction faced to plasma plume. It is obviously that the film has different morphology. For the film faced to plasma plume [as in region I], it was compact and smooth. But in the shadow [as in region II], the film was floppy and like stacked tiny flakes. Figure 2(c) shows the film in region I, it has a thickness of ~1.1 μm and can be treated as bulk WS2. Figure 2(d) shows the film in the region II, it has a thickness of ~160 nm. This result illustrated that the film was indeed a thickness-depended SA, which was quite different from the previously reported SA with relatively uniform size and thickness by LPE method. Although the thickness of single piece of flakes could not be obtained under our condition, the tiny WS2 flakes might be treated as nanomaterials based on the following reason: 1) they were grown on the shadow side of microfiber and multilayer stacked very floppily [as in region II]. 2) More deeply into the shadow side, the film would become more thin and floppy.
Fig. 2 SEM characteristic of the microfiber based WS2 SA. (a) Microfiber waist region; (b) Film thickness at the side that faced to the plasma plume; (c) Film thickness at the opposite side. (d) Film morphology.
For convenient measuring the film characteristics, we also fabricated the WS2 film on quartz glass under the same PLD condition. The elemental composition and stoichiometry were studied by energy dispersive spectroscopy (EDS, Oxford 7582), as shown in Fig. 3.The mass ratio of W and S were 79.9% and 20.1% respectively. Compared with the bulk WS2 target, it was evident that there existed S defects in the PLD film because that the percentage of S was lower than the initial value of 25.86%.
For checking the Raman shift property of the as-prepared materials, the Raman spectra were measured by using a Raman spectrometer (LabRAM HR Evolution) with a laser at 514 nm. As previously reported in [50], our WS2 target had two optical phonon modes (E2g1 at 356.1 cm−1 and A1g at 420.18 cm−1) and two typical longitudinal acoustic modes (LA(M)) at 173.6 cm−1 and 349.5 cm−1 in Fig. 4(a) where the E2g1 is an in-plane optical mode and A1g corresponds to the out-of-plane vibrations along the c-axis direction of the S atoms. Figure 4(b) shows the measured Raman spectra of WS2 film on quartz glass together with that of bare quartz glass. Notably, these peaks were also observed for the film with locations of LA(M) at 174 cm−1, 2 LA(M) at 350 cm−1, E2g1 at 356 cm−1 and A1g at 420.7 cm−1, respectively.
Fig. 4 (a) Raman spectrum of bulk WS2, (b) Raman spectra of bare quartz glass and WS2 film on quartz glass.
To construct a practical SA device, the PLD film on microfiber should be packaged, as shown in the inset in Fig. 5(a).The linear transmission of the SA device was measured in the range from 1300 nm to 1600 nm by using an ASE source (Glight, 1250 nm ~1650 nm) and optical spectrum analyzer (OSA). It was at the level of 68% ± 2%, as shown in Fig. 5(a). The transmittance (T) at 1560 nm was 68.2%. For measuring of the saturable absorption, a home-made fs laser (central wavelength: 1562 nm, repetition rate: 22.5 MHz, pulse duration: 650 fs, Average power: 12 mW) was utilized as test source, a variable optical attenuator (VOA) was applied to continuously change the input optical intensity into the sample. A 50:50 optical coupler (OC) was used to split the laser into two arms with the one arm for power-dependent transmission measurement of SA device and another one arm for reference. A two-channel power meter with measuring range from 10 μW~10 mW was used to measure the power. The setup was inserted in Fig. 5(b). As increasing the optical intensity from 4 MW/cm2 to 500 MW/cm2 into the SA device, the optical transmittance was recorded. Again, the optical transmittance was also measured via decreasing optical intensity from 500 MW/cm2 down to 4 MW/cm2, as plotted in Fig. 5(b). Interestingly, the curve can be divided into different regimes according to the evolution trend. In regime I [optical intensity: < 98 MW/cm2], the modulation depth, saturable intensity was fitted to be ~1.2% and 25 MW/cm2 by the standard two-level saturable absorber model [59]. In regime II [optical intensity: 98 MW/cm2 ~365 MW/cm2], a relative high modulation depth of ~8% was measured at this wavelength, and the saturable intensity was about 200 MW/cm2. However in regime III, the transmittance became lower when optical intensity was beyond 365 MW/cm2. This phenomenon was attributed to optical limiter effect [60], which was mainly due to nonlinear scattering, two-photon absorption (TPA) effect, and other nonlinear absorption effects. SWCNT suspensions and SWCNT hosted in PVA film have been reported to have optical limiting effects [60, 61].
For the semiconductor-type SA, a precondition of saturable absorption is that the photos should have energy larger than the bandgap so as to excite the electrons from the valance band to the conduction band by absorbing the photons. One could concern: 1) Why this SA has the different saturable absorption properties relative to the optical intensity? 2) Why the SA has the saturable absorption around the 1.56 μm because the bulk WS2 is an indirect semiconductor with bandgap ~1.35 eV (corresponding to ~0.92 μm)?
To answer these, the comparison of our results with the former results reported was listed. Compared the result of our WS2 film with that of WS2 nanosheet in PVA host and on SPF that was reported in [62], the existing of different regimes was remarkable in our case, further revealing of its inherently thickness-dependent SA property. Herein, the ~1.2% of modulation depth in regime I was comparable to the 1.8%, 2.96% or 0.95% of few-layer WS2 nanosheets hosted in PVA film or on SPF [62,63], respectively. Because the WS2 and MoS2 possess similar lattice structures and photonic properties, the ~1.2% of modulation depth was comparable to ~2.82% of microfiber-based MoS2 SA [45], 2% of MoS2-PVA film on fiber ferrule [48], 0.3~2.5% of MoS2-CVD film on SPF [46]. Apart from the layered TMDs, other real SAs have been demonstrated with a small modulation depth, e.g. 1.3% of graphene [11], 0.94% of SWCNTs [51], 1.7% of topological insulator (TI) [52]. Therefore, it was believed that such WS2-PLD film SA can readily provide the nonlinear saturable absorption for mode-locking operation. Additionally, the nonsaturable loss in regime I was around 30%, evidently lower than the 57.34% of microfiber-based MoS2 SA [45] and 69.5~79% of SPF-based MoS2-CVD film SA [46]. The saturable intensity in this regime was measured to be ~25 MW/cm2, which was larger than 0.34 MW/cm2 of few-layer MoS2-CVD film on fiber ferrule [43], but comparable to the 34 MW/cm2 of few-layer MoS2 PVA film in [47]. Based on the above comparison, it indicates that the tiny WS2 flakes undertake the role of saturable absorption in regime I. The edge-state or surface absorption [54–58] (i.e. the coexistence of both semiconducting and metallic phases) would attribute to the saturable absorption at 1.56μm. However in regime II, the transition of saturable absorption curve was remarkable. The ~8% of modulation depth was a relatively large value in the reported results. And the ~200 MW/cm2 of saturable intensity was comparable to the value of multilayer MoS2-CVD film SA on SPF [45]. The increasing of modulation depth and saturable intensity indicated that the thickness of WS2 film was also increased which provided higher optical modulation ability and required larger optical intensity to excite, like the performance of monolayer and multilayer graphene. Consequently, this difference of the saturable intensity in regime I and II must arise from the inherent material property (related with lattice structure, e.g. few-layer, multilayer or bulk) under the same measuring condition. Although the PLD film faced to plasma plume can be treated as bulk state, we tend to believe that the saturable absorption behavior was depended on the defect-induced bandgap decreasing effect [53] as evidenced in Fig. 3. It is the first demonstration of WS2 SA with different satruable absorption regimes. In laser mode-locking, the pulse can be readily established in regime I with low modulation depth and saturable intensity, which might be helpful to pave the pulse operation into the regime II.
The power tolerance ability is an important performance for SA devices. Figure 6 shows the ability that was measured by incident laser power of 1 W at 1550 nm. For our SA, it is notable that SA has a high power tolerance. The slope efficiency of output power vs. incident power was about 69.3%, close to the linear transmittance of 68.2% at 1560 nm. At highest incident power of 1.02 W, the output power was 0.71 W. The slope curve still maintained as a straight line. It is believed the power tolerance of our SA device would higher than that of MoS2-nanosheet deposited on microfiber in [44]. However, an optimization on the diameter of microfiber and the PLD film quality of our SA would further enhance this power tolerance ability.
3. Experimental setup and results
Figure 7 shows the schematic of mode-locked fiber laser with our WS2 SA device. The pump source was a laser diode (LD) with emission centered at 974.5 nm. A piece of 2.4 m EDF was used as the laser gain medium with absorption coefficient of 25 dB/m@980 nm (IsoGainTM I-25, Fibercore). The pump was delivered into EDF via a 980/1550 fused wavelength division multiplexer (WDM) coupler. A polarization dependent isolator (ISO), placed after the EDF, was used to ensure unidirectional operation and eliminate undesired feedback from the output end facet. A fused fiber optical coupler (OC) was used to extract 30% energy from the cavity. A polarization controller (PC), consisting of three spools of SMF-28 fiber, was placed in the ring cavity after the ISO. The WS2 SA was inserted between the PC and the WDM coupler. Apart from the gain fiber, all the fiber devices in cavity were made by SMF-28 fiber. The total cavity length was 10.57 m. The laser performance was observed using an optical spectrum analyzer (Yokogawa, AQ6370B), 1 GHz digital oscilloscope (Tektronix, DPO7104C), 3 GHz RF spectrum analyzer (Agilent, N9320A) coupled with a 15 GHz photodetector (EOT, ET-3500FEXT), and an optical autocorrelator (APE, PulseCheck).
The mode-locking state appeared at pump power of ~54 mW at a proper polarization state. Figure 8(a) shows the typical spectrum of mode-locked pulses. The generated optical soliton was centered at 1558.5 nm with a cw component coexisting in spectrum [52], indicating that the modulation depth was a small value. The radio frequency (RF) spectrum of the laser was shown Fig. 8(b). The fundamental repetition frequency was 19.58 MHz, which was consistent with the cavity length. The electrical signal-to-noise ratio (SNR) was 67 dB measured with a 1 kHz resolution bandwidth (RBW). Figure 8(c) shows the oscilloscope trace of the mode-locked pulse train. The pulse interval was ~51.07 ns. The autocorrelation trace had a full width at half maximum width (TFWHM) of 1.04 ps as shown in Fig. 8(d). Consequently, the pulse duration τ was 675 fs if a sech2 pulse profile was assumed. At this pump level, the average output power was 0.625 mW. If the cw component was removed, the actual mode-locking component was a lower value. Therefore, it can be deduced that the saturable absorption was provided in regime I. The fundamental mode locking operation was maintained below a pump of 67 mW.
Fig. 8 FML performance. (a) Spectrum; (b) RF spectrum; Inset: fundamental repetition frequency measured with a 1 kHz RBW; (c) Recorded pulse train; (d) Measured pulse duration.
To verify whether the SA contributed to the passive mode-locking, we replaced it in the ring laser by the same type of microfiber without WS2 film. In this case, we carefully adjusted PC orientation and changed pump power in this range, the mode-locking state could not be observed again. This result demonstrated that the mode-locking behavior was caused not by the nonlinear polarization rotation (NPR) effect under the same condition, which testified that the microfiber-based WS2-film SA was indeed contributing to the mode-locking operation.
At maximum pump power of 395 mW, the output spectrum was centered at 1559.7 nm with a 3-dB bandwidth of 6.6 nm, as can be seen from Fig. 9(a).The RF spectrum was measured in full rang with a SNR of 48 dB, as shown in Fig. 9(b). It can be seen that HML was achieved at repetition frequency of 1.04 GHz, corresponding to 53th harmonic of fundamental repetition frequency. Figure 9(c) illustrated the corresponding autocorrelation trace, which was measured to be 452 fs if sech2 shape was assumed. Thus, the time-bandwidth product (TBP) was 0.367, indicating that the output pulse was slightly chirped. The average output power was measured to be 11.3 mW.
Fig. 9 HML performance at the repetition frequency of 1.04 GHz. (a) Spectrum; (b) RF spectrum in full range with a 10 kHz RBW; (c) Measured pulse duration.
4. Conclusion
In conclusion, we firstly fabricated and characterized a microfiber-based WS2 film SA device based on the PLD method. Our experimental results revealed that the SA was thickness-dependent with different states: the bulk WS2 [faced to plasma plume] and tiny WS2 flakes [in the shadow of plasma plume]. This SA device demonstrated low insertion loss (IL) and high power tolerance. An intriguing phenomenon was that the SA device possessed different nonlinear absorption regimes related with the film states. Employing this SA device, we achieved a stable FML EDF laser at pump power of 54 mW, and the generated soliton pulse had pulse duration of 675 fs and a SNR of 65 dB. At the maximum pump power of 395 mW, we also obtained up to1.04 GHz repetition rate HML operation with pulse duration of 452 fs and SNR of 48 dB. We believed that a further optimization of the SA (such as taper waist, the thickness of WS2 film) together with the laser cavity design was essentially required for the substantial enhancement the mode-locking performance. Our experimental result indicates that the TMD e.g. WS2 and MoS2 can be effectively deposited on microfiber by the PLD method, which can serve as high performance SAs in passive mode-locked laser, nonlinear optical material for laser photonics devices, optical switcher and so on.
Acknowledgments
Supported by the NSFC (61275144&61308049), the natural science fund of Guangdong province (S2013010012235), the improvement and development project of Shenzhen key Lab (ZDSY20120612094924467), the science and technology project of Shenzhen City (JCYJ20120613172042264, JCYJ20130329142040731) and the science and technology project of Shenzhen City (JCYJ20140418091413568, JCYJ20120613112423982).
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