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We experimentally demonstrated compact orange-light passively Q-switched praseodymium (Pr3+)-doped all-fiber lasers utilizing two-dimensional (2D) transition-metal dichalcogenides (TMDs, WS2 and MoS2) as saturable absorbers (SAs). The filmy TMDs SAs are fabricated by evaporating the composite of TMDs nanosheets and polyvinyl alcohol (PVA), and then inserted into an orange-light Pr3+-doped fiber laser cavity, which is designed by coating dielectric mirrors on fiber ends. Stable passively Q-switched orange lasers at 604 nm are successfully achieved. The passive Q-switching generated the short pulse duration of 435 ns and 602 ns, as well as the tunable repetition rate in the range of 67.3 ~132.2 kHz and 50.8 ~118.4 kHz, respectively. This is, to the best of our knowledge, the shortest-wavelength operation of 2D-material-based pulsed fiber lasers.
© 2016 Optical Society of America
1. Introduction
Due to the inherent advantages of compactness and flexibility and low-cost, pulsed fiber lasers have been widely used in industrial and scientific applications [1, 2]. Motivated by a more convenient and cost-effective way for generating compact pulsed fiber lasers, passive Q-switching or mode-locking based on saturable absorber (SA) is more attractive than active one by an acousto-optic [3] or electro-optic modulator [4]. In recent years, two-dimensional (2D) layered materials, including graphene [5–9], topological insulators (TIs) [10–13], transition-metal dichalcogenides (TMDs) [14–18], and black phosphorus [19, 20], have been recognized as the ideal SAs and extensively applied to the near-infrared fiber lasers for Q-switching/mode-locking operation. However, in the short (visible/ultraviolet) and long (mid-infrared) wavelengths, 2D-material-based SAs for pulsed fiber lasers have not been fully exploited until now.
Visible pulsed fiber lasers are highly demanded for biomedical diagnosis, underwater detection, and indoor optical communication. At present, only a few works have reported the 2D materials SAs as Q-switchers for pulsed operation in red fiber lasers [21–23]. One would concern if 2D-material-based SAs are available to shorter-wavelength operation (e.g. at orange, yellow, blue or ultraviolet). So far, there exists no reports about compact orange-light pulsed all-fiber laser operation modulated by 2D-material-based SAs. Although Fujimoto et al. have observed pulsed laser phenomena by inserting graphene thin film into an orange fiber laser [24], most of the passive Q-switching features were not observed. Definitely, the layered TMDs are considered as the type of existing 2D semiconductors with a direct bandgap at the visible frequency regions [25]. Besides, our latest report on compact red-light pulsed fiber lasers with the TMDs-based SAs, further confirms their advantages in the visible spectral region [23]. Here, we report the first demonstration of orange-light passively Q-switched all-fiber-based laser that produces pulses at 604 nm using a few-layer WS2 or MoS2 SA. The pulse repetition rate of the few-layer WS2-SA-based Q-switching orange all-fiber laser can be continuously tuned from 67.3 to 132.2 kHz, and the few-layer MoS2-SA-based one is between 50.8 kHz and 118.4 kHz. The narrowest pulse duration of 435 ns is derived from the few-layer WS2-SA-based Q-switched all-fiber laser.
2. Experiments
2.1 TMDs characterization
As reviewed in our latest work [23], the saturable absorption properties of the few-layer TMDs at red and green wavelengths used in our experiments have been characterized by the Z-scan measurements. The measured modulation depth can be of ~7% (635 nm) and ~14% (530 nm), respectively, showing that TMDs-based SAs are available for passive Q-switching or mode-locking in visible spectral range. Nevertheless, one could have the concern about the fundamental mechanism of saturable absorption of the layered WS2 and MoS2 at visible wavelength. Owing to WS2 and MoS2 have wide direct bandgaps of ~2.0 eV (WS2) and ~1.9 eV (MoS2) [25, 26], respectively, their corresponding resonant wavelengths of ~620 nm and ~650 nm are just located in the visible spectral region [27]. Under the visible-light illumination, electron-hole pairs from the layered TMDs can be in resonance, which is similar to carbon nanotubes [28]. This resonant process could make Pauli blocking very powerful and facilitate the enhanced visible-wavelength saturable absorption for nonlinear and ultrafast photonic applications. Hence, we believe that the few-layer TMDs could also have a strong saturable absorption at the orange wavelength.
The few-layer WS2 used in our experiment was prepared by the liquid-phase exfoliation method as follows. Initially, the purchased WS2 (325 mesh power, alfa aesar) were added into the N-2-methyl pyrrolidone (NMP) solution and sonicated for 20 hours to produce the few-layer WS2 suspension. Different from one before sonication [see Fig. 1(a)], the suspension after sonication became deeper color with good uniformity, indicating that the bulk WS2 had been exfoliated and well dispersed in NMP. Purchased bulk WS2 was characterized by X-ray diffraction (XRD) in Fig. 1(b). All the labeled peaks can be readily indexed to rhombohedral WS2 (JCPDS no. 08-0237). The XRD pattern of few-layer WS2 [see Fig. 1(b)] showed a high [002] orientation and some characteristic peaks disappeared compared to bulk WS2, which indicates that bulk WS2 had been successfully exfoliated as we expected. As shown in Fig. 1(c), Raman spectroscopy, a powerful nondestructive characterization tool, was also used to estimate the thickness of the few-layer WS2 sample. The two characteristic peaks at 351 and 418 cm−1 are assigned to E12g and A1g modes of the bulk WS2. Compared with the bulk WS2, few-layer WS2 sample shows an obvious red-shift of peak. Furthermore, the thickness of the as-prepared few-layer WS2 was characterized by atomic force microscopy (AFM), as shown in Fig. 1(d). The average thickness from the height profile diagram [see Fig. 1(d) inset] was measured to be ~2-3 nm. This indicates that the WS2 nanosheets are around 3-4 layers, because the single-layer thickness of WS2 is about 0.7 nm [29].
Fig. 1 (a) Optical photos of the WS2 before and after sonication. (b) The XRD of bulk WS2 and few-layer WS2. (c) Raman spectrum of bulk WS2 and few-layer WS2. (d) AFM image of few-layer WS2 and the height profile diagram (inset) of the few-layer WS2.
Preparation of few-layer MoS2 is almost the same as few-layer WS2 except the organic solvent is dimethyl formamide (DMF). After sonication, the color of the suspension became deeper with good uniformity [see Fig. 2(a)], indicating that the bulk MoS2 had been exfoliated and well dispersed in DMF. XRD [see Fig. 2(b)] showed the rhombohedral structure of bulk MoS2 (JCPDS no. 06-0097) and bulk MoS2 had been successfully exfoliated as we expected. Raman spectra [see Fig. 2(c)] showed the blue-shift of E12g peak and the red-shift of A1g peak for few-layer MoS2 compared with bulk MoS2, indicating that the MoS2 was successfully exfoliated with a thickness of 1-4 layers [30]. AFM [see Fig. 2(d)] showed average thickness from the height profile diagram [see Fig. 2(d) inset] was ~2-3 nm, indicating that the MoS2 nanosheets are around 3-4 layers as the single-layer thickness of MoS2 is 0.65 nm [31]. Herein, we dispersed the as-prepared few-layer TMDs suspension into the polyvinyl alcohol (PVA), which is helpful for film-forming to be practically used.
Fig. 2 (a) Optical photos of the MoS2 before and after sonication. (b) The XRD of bulk MoS2 and few-layer MoS2. (c) Raman spectrum of bulk MoS2 and few-layer MoS2. (d) AFM image of few-layer MoS2 and the height profile diagram (inset) of the few-layer MoS2.
2.2 Experimental setup
The layout of the proposed TMDs-based passively Q-switched orange-light Pr3+-doped all-fiber laser is shown in Fig. 3. Both the schematic in Fig. 3(a) and the photograph in Fig. 3(b) included. It has a linear laser cavity with a total cavity length of about 2.5 m. We employed a 2 W GaN LD emitting central wavelength around 444 nm as pump source. A piece of 91.5 cm Pr3+-doped ZBLAN fiber (core/cladding: 6/125 µm, NA of 0.15, Pr3+ concentration of 1000 ppm) was manipulated as the gain medium. Each end facet of the Pr:ZBLAN fiber was mounted in the ferrule, perpendicularly polished, and both the ends were separately butt-coupled to the single-mode-fiber (SMF, 1060-XP). The pump laser can be coupled into the SMF with a maximum coupling efficiency of ~31.1%. Fiber pigtail mirrors (FPMs, M1 and M2) were obtained by directly coating multi-layer dielectric films onto the SMF 1060-XP fiber ends using a plasma sputter deposition system [23]. Figure 3(c) and Fig. 3(d) individually show transmission spectrum of FPMs M1 and M2. The input mirror M1 has a high transmittance of ~78% at the pump wavelength of ~444 nm and high reflectivity of ~97.4% at the lasing wavelength of 604 nm, and the output mirror M2 transmits ~7.2% orange light at 604 nm. Consequently, all-fiber-based compact laser resonator for orange-light oscillation could readily consist of the two FPMs M1 and M2. Here, we also give the appearance of the FPM in Fig. 3(e). Certainly, it is also a key step to compatibly insert the as-prepared few-layer TMDs into laser cavity so as to attain compact and stable visible-wavelength passive Q-switching. A piece of TMD film was sandwiched between the 1060-XP fiber ends to construct the fiber-compatible TMD-based SA and Fig. 3(f) reveals a diagram of the fabricated fiber-compatible TMD SA.
Fig. 3 (a) Schematic illustration and (b) experimental picture of compact TMDs-based passively Q-switched orange-light Pr3+-doped all-fiber laser. Transmission spectrum of the FPMs: (c) input mirror M1 and (d) output mirror M2. (e) An image of the FPM, and (f) a photograph of the fiber-end coated by a filmy TMD.
A polarization controller (PC) was utilized to optimize the Q-switching operation. The output pulses were detected by a photodetector (PD) (Thorlabs, DET10A) together with a 200 MHz digital storage oscilloscope (Tektronix TDS2024), and also measured radio-frequency (RF) output spectrum using a spectrum analyzer (Gwinstek GSP-930). The output optical spectrum was monitored by an optical spectrum analyzer (Ocean Optics, HR4000 or Advantest Q8384).
3. Results and discussion
Firstly, we characterized the output performance of the orange-light all-fiber laser without TMDs SAs. Figure 4(a) shows the output characteristic of orange Pr3+-doped all-fiber laser as a function of the incident pump power. A threshold of ~150 mW is recorded and the maximum output power is ~7 mW, where the slope efficiency with respect to the incident pump power is approximately 2.1%. At the incident pump power of 304.6 mW, the output optical spectrum with the central wavelength of 606 nm is plotted in Fig. 4(b). Indeed, it is an orange lasing that always operates at CW regime and no pulse train is observed, no matter how to change the incident pump powers or PC states.
Fig. 4 CW orange-light Pr3+-doped all-fiber laser. (a) the output power as a function of the incident pump power, (b) optical spectrum at the incident pump power Pp of 304.6 mW.
3.1 WS2-SA-based passively Q-switched 604 nm all-fiber laser
A WS2-based SA with the insertion loss of ~2.0 dB was embedded in the laser cavity, the orange-light started to oscillate at the incident pump power of 235.4 mW and the threshold with the WS2 SA was higher than the one (150 mW) without the WS2 SA. With further increasing the incident pump power to 245.8 mW, stable Q-switching operation was launched. As displayed in Fig. 5, we recorded the typical pulse trains under different incident pump powers. The pulse repetition rate of the orange Q-switched laser can be continuously tuned form 67.3 to 127.9 kHz by varying the incident pump power from 256.1 to 312.9 mW. These phenomena are typical characteristics of passive Q-switching, and thus confirming that the WS2-based SA can be applied as the orange wavelength Q-switcher.
Fig. 5 Passive Q-switching pulse trains of orange-light Pr3+-doped all-fiber laser based on WS2 SA under different incident pump powers Pp. (a) Pp = 256.1 mW, (b) Pp = 276.7 mW, (c) Pp = 292.2 mW, (d) Pp = 312.9 mW.
Moreover, the output optical spectrum at the incident pump power of 273.6 mW is given in Fig. 6(a). The lasing peak wavelength locates at 604 nm. As plotted in Fig. 6(b), the signal-to-noise ratio (SNR) of fundamental radio-frequency (RF) peak ƒ0 at 79.2 kHz is as high as 41 dB, manifesting the good stability of the passive Q-switching. Also, the inset in the Fig. 6(b) illustrates the output pulse trains with the time interval of about 12.62 µs between two adjacent pulses, which is in agreement with the repetition-rate of 79.2 kHz. In addition, Fig. 6(c) presents the pulse-repetition-rate and the pulse duration as a function of the incident pump power. When gradually increased the incident pump power from 256.1 to 318.1 mW, the repetition rate can be almost linearly tuned from 67.3 to 132.2 kHz. And the pulse duration decreases from 1101 to 435 ns. We also exhibit the single pulse profile of the Q-switching pulse in Fig. 6(d), the full width at half maximum (FWHM) is 435 ns, which is the shortest pulse duration of our orange-light Q-switched all-fiber laser.
Fig. 6 Typical Q-switched characteristics of WS2-SA-based orange-light Pr3+-doped all-fiber laser. (a) the optical spectrum, (b) RF output spectrum (inset is the typical oscilloscope trace), (c) the repetition rate and the pulse duration versus the incident pump power, (d) the minimum single pulse envelope.
The maximum average output power of the 604 nm passively Q-switched all-fiber laser is measured to be 0.7 mW, and the maximum pulse energy is calculated to be 6.4 nJ. The lower output power and pulse energy might originate from the uncompetitive coupled efficiency and the unoptimized Pr:ZBLAN fiber length [32].
3.2 MoS2-SA-based passively Q-switched 604 nm all-fiber laser
A piece of MoS2-based SA with the insertion loss of ~2.6 dB was also incorporated into the orange-light CW all-fiber laser to achieve passive Q-switching operation. The measured CW laser threshold is 272.5 mW, and stable Q-switching is initiated as soon as the incident pump power reaching 278.8 mW. As shown in Fig. 7(a), we exhibit the oscilloscope traces under the incident pump powers of 284.1 mW, 304.6 mW and 335.6 mW. Also, the monotonously increased pulse-repetition-rate explains the typical feature of passive Q-switching. In Fig. 7(b), the output optical spectrum of 604 nm passively Q-switched all-fiber laser at the incident pump power of 335.6 mW is exemplified. Moreover, the SNR of fundamental RF peak at 108.2 kHz is ~39 dB, as illustrated in Fig. 7(c). Figure 7(d) clearly proves the relationship of the pulse-repetition-rate and the pulse duration against the incident pump power. By gradually increasing the incident pump power from 278.8 to 345.9 mW, the repetition rate can be tunable in the range of 50.8 ~118.4 kHz, and the pulse duration become narrower from 1955 to 602 ns. Likewise, the measured maximum average output power is 0.6 mW and the calculated maximum pulse energy is 5.5 nJ. A progressive work is needed to arrive at improving the capabilities of the orange-wavelength pulsed all-fiber lasers, such as the higher pulse energy and the narrower pulse width, which could be realized by optimizing the parameters of the all-fiber laser configuration (e.g. cavity arrangements and modulation depth of MoS2 SA).
Fig. 7 Experimental results of passive Q-switching orange-light Pr3+-doped all-fiber laser using the MoS2-based SA. (a) typical pulse trains, (b) output optical spectrum and (c) RF output spectrum, (d) repetition rate and pulse duration as a function of the incident pump power.
4. Conclusion
In summary, compact orange-light passively Q-switched all-fiber lasers enabled by TMDs-based SAs at 604 nm have been successfully demonstrated in this work. The pulse repetition rate of the WS2-SA-based Q-switched orange-light all-fiber laser is continuously tuned from 67.3 to 132.2 kHz, and the MoS2-SA-based one is from 50.8 to 118.4 kHz. And the pulse duration can be as short as ns-level. Such result reveals that the layered TMDs are available SAs for generating shorter laser pulses at the orange wavelength.
Acknowledgments
This work was sponsored by National Science Foundation of China (NSFC) (61275050); Specialized Research Fund for the Doctoral Program of Higher Education (20120121110034).
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