Open Access
Add to BibSonomyAbstract
We have fabricated an Yb-doped passively Q-switching fiber laser based on WS2 saturable absorber. Both the operating wavelength and the repetition rate can be tuned in a wide range. The operating wavelength can be continuously tuned from 1027 nm to 1065 nm under the Q-switching state at a fixed pump power, while the repetition rate increases from 60.2 kHz to 97.0 kHz by varying pump power at a fixed wavelength of 1048.1 nm. The shortest pulse duration of 1.58 µs was observed. To the best of our knowledge, it’s the first demonstration of WS2-based passively Q-switching fiber laser with a wide tunable range at 1.0 μm band.
© 2015 Optical Society of America
1. Introduction
Since the first graphene saturable absorber (GSA) based passively mode-locked fiber laser was demonstrated by Bao et al. in 2009 [1], new types of nano materials based saturable absorber have drawn intense attention [2–5] due to their instinct advantages of wide operation bandwidth, high damage threshold and low fabrication expense compared with semiconductor saturable absorber mirrors (SESAMs) and single-walled carbon nanotubes (SWCNTs).
Among variable saturable absorbers, transition metal dichalcogenides (TMDs) stand out. In TMDs, a trilayer sheet is formed by two layers of chalcogen atoms sandwiching a layer of transition metal atoms, then these sheets are stacked together by weak van der Waals forces, allowing them easily exfoliated into mono- and few-layer nanosheets. In 2013, Wang et al. firstly demonstrated the saturable absorption of the molybdenum disulfide (MoS2) [6]. After that, MoS2-based passively mode-locked and Q-switching fiber lasers have been successively reported [7–9]. Moreover, mode-locked and Q-switching of widely-tunable fiber lasers have also been demonstrated [10–12].
Recently, tungsten disulfide (WS2), another outstanding delegate of TMDs, has been widely investigated as a new substitution for saturable absorber since Mao et al. firstly achieved WS2-based passively mode-locked fiber laser [5]. Few-layer tungsten disulfide has been already demonstrated saturable absorption at the band of 532 nm [13], 800 nm [14], 1550 nm [15–17] and 1940 nm [18]. However, few reports on the saturable absorption of WS2 at 1060 nm band have been published [19]. Most recently, our group has successfully achieved WS2-based passively mode-locked fiber laser at 1064 nm, but WS2-based Q-switching fiber laser at 1060 nm band, especially wavelength tunable Q-switching fiber laser has never been demonstrated.
In this letter, we report what we believe to be the first widely-tunable, WS2-based passively Q-switching Yb-doped fiber laser. The wavelength can be continuously tuned from 1027 nm to 1065 nm at Q-switching state. We believe the tuning range is mainly limited by our tunable filter rather than WS2 itself. Meanwhile, at a certain wavelength (e. g. 1048.1 nm), the repetition rate can monotonously increase from 60.2 kHz to 97.0 kHz by increasing the pump power, and the shortest pulse duration of 1.58 µs was observed. Our results show that WS2 is a promising saturable absorber for the wideband operation.
2. Preparation and characteristics of few-layer WS2
The WS2 nanoplates used in our experiment was synthesized by lithium-based chemical exfoliation [20] and then dispersed in deionized water with a concentration of 1.0 mg/mL.The scanning electron microscopy (SEM) image of the WS2 nanosheets is shown in Fig. 1(a). The flakes diameter falls in the range of 20-500 nm. We also characterized the Raman spectrum of the prepared WS2 using Ar+ laser at 514 nm, as depicted in Fig. 1(b). The characteristic bands at 350.8 and 420 cm−1 on the Raman spectrum can be clearly observed, corresponding to the in-plane (E2g) and out-of-plane (A1g) vibrational modes of WS2 [21]. Then 3 mL WS2 aqueous solution was mixed with 7 ml deionized water and 0.2 g polyvinyl alcohol (PVA) and ultrasonicated for ~8 h. After that, the WS2-PVA solution was stirred sufficiently and then dropped on a Petri dish for slow evaporating in an oven at 45 °C, resulting in a WS2-PVA composite film. The linear transmission spectrum of the WS2-PVA film was measured in Fig. 1(c). The dip near 632 nm in the transmission spectrum is a typical fingerprint of WS2 nanosheets due to the direct bandgap transition [22]. The absorption band extends from 400 nm to 2000 nm, indicating that WS2 is a promising broadband optical material. The insertion loss of the individual WS2-PVA film at 1064 nm is about 1.5 dB, and according to Fig. 1(d), the fabricated saturable absorber has a polarization independent loss.
Fig. 1 (a) The SEM image of WS2 nanoplates. (b) The Raman Spectrum of WS2 nanoplates. (c) The linear transmission spectrum of the WS2-PVA film. (d) The polarization dependent loss of the WS2-PVA film.
The saturable absorption of WS2-PVA film was measured by the power-dependent transmission technique with a homemade picosecond fiber laser centered at 1064 nm. From the nonlinear transmission curve in Fig. 2, we can conclude that the WS2-PVA film has a modulation depth of 3.87% and a saturation intensity of 6.2 MW/cm2.
3. Experimental setup
The schematic of the fiber laser is shown in Fig. 3. The laser cavity consists of ~70cm highly ytterbium-doped fiber (LIEKKI 1200-4/125), a polarization independent isolator (PI-ISO), a wavelength division multiplexer(WDM), a polarization controller (PC), and a 7:3 output coupler with 70% output. Our Q-switching fiber laser can operate without the PC, the PC placed inside the cavity is to make the fiber laser works at the optimum state all the time in case of the polarization state changing by the ambient effects. A manually adjustable polarization insensitive tunable filter with a tuning range of 38 nm (1027-1065 nm) and 1.3 nm bandwidth from OZ Optics is inserted into the cavity to select the oscillation wavelength. The WS2-PVA film was cut into small pieces and sandwiched by two FC/PC fiber connectors. The total length of the fiber cavity is about 10 m. The optical spectrum was measured by an optical spectrum analyzer (ANDO AQ6317B) and the oscilloscope traces were measured by a 10 GHz photodetector and a 4 GHz oscilloscope (LECTOY Waverunner 640Zi). The radio frequency (RF) spectrum was measured by a 2 GHz RF spectrum analyzer.
Fig. 3 The experimental setup of the Q-switching fiber laser. LD, laser diode, WDM, wavelength-division multiplexer, YDF, ytterbium-doped fiber, PI-ISO, polarization independent isolator, OC, output coupler, PC, polarization controller.
When the pump power increased to 99 mW, the stable Q-switching pulses appeared at a wavelength of 1048.1 nm (depend on the tunable filter). The temporal behavior of the pulse train and the single pulse shape at a pump power of 122 mW are shown in Figs. 4 (a) and 4(b). The full width at half maximum (FWHM) is 1.65 µs. The output optical spectrum is depicted in Fig. 4(c) which is centered at 1048.1 nm with 3 dB bandwidth of 0.067 nm. The corresponding RF spectrum with a 5-KHz span and a 1-Hz resolution bandwidth (RBW) in Fig. 4(d) shows the signal-to-background ratio (SBR) of the repetition rate is about 50 dB in a narrow linewidth, indicating the stable Q-switching. The repetition rate was measured of 81.50 KHz, The average output power was 2.36 mW, corresponding to a pulse energy of 28.8 nJ.
Fig. 4 The characteristics of the Q-switching operation at the pump power of 122 mW. (a) the output pulse train; (b) single pulse profile; (c) the output optical spectrum; (d) corresponding RF spectrum.
Figure 5 shows the repetition rate and the pulse duration as a function of the pump power. By increasing the pump power from 99 mW to 131 mW, the repetition rate monotonically increased from 60.17 kHz to 97.04 kHz with about 37 kHz tuning range. The minimum pulse duration appeared at the pump power of 122 mW with FWHM of 1.65 µs. This pump-dependent operation is the typical feature of the Q-switching laser.
Fig. 5 The repetition rate and pulse duration of the 1048.1 nm Q-switching operation versus the pump power.
By adjusting the tunable filter, the stable Q-switching operation is achieved with the wavelength continuous tuning across the whole tuning range of our tunable filter (1027- 1065 nm) [as shown below]. We believe the tuning range is mainly limited by our tunable filter rather than WS2 itself. With a broader tunable filter, we expect the Q-switching fiber laser can be tuned over an even wider spectral range. As shown in Figs. 6(a)-6(d), at a fixed pump power of 128 mW, the repetition rate varies at different wavelengths. In order to get deeper insight of this wavelength-dependent Q-switching operation, we investigated the repetition rate and the pulse duration as a function of the wavelength at the fixed pump power of 128 mW, as shown in Fig. 7(b). The repetition rate nearly monotonically decreased from 106.16 kHz to 65.28 kHz with the wavelength increasing from 1027nm to 1065nm. The pulse duration varied from 1.57 µs to 2.11 µs. As we know, the gain of the YDFA varies with different wavelengths. Also, some of the components (tunable filter, coupler, WDM, etc) in fiber laser have wavelength-dependent loss. The comprehensive effect of these factors may cause the pulse repetition rate, pulse duration and output intensity varying with the operating wavelength with no obvious trend.
Fig. 6 The pulse trains of Q-switching operation at different wavelengths with a fixed pump power of 128 mW. (a) 1027 nm, (b) 1040 nm, (c) 1055 nm, and (d) 1065 nm.
Fig. 7 (a) The widely-tunable spectra of the Q-switching operation at the fixed pump power of 128 mW. (b) The repetition rate and duration versus the wavelength at the fixed pump power of 128 mW.
In order to verify whether the Q-switching was resulted from the WS2, we purposely remove the WS2-PVA film. No matter how to tune the PC or vary the pump power, no Q-switching was observed. So we can conclude that the Q-switching operation is purely induced by WS2 rather than other components.
4. Conclusion
In summary, we reported a widely-tunable, WS2-based passively Q-switching Yb-doped fiber laser. Attributed to the wide-band saturable absorption of WS2, the Q-switching fiber laser can be continuously tuned from 1027 nm to 1065 nm with 38 nm of tunability. The repetition rate increases from 60.2 kHz to 97.0 kHz by increasing pump power and the shortest pulse duration obtained is 1.65 µs at the fixed wavelength of 1048.1 nm. Our results not only prove the potential of WS2 as a saturable absorber, but also provide a simple and cost-effective source for metrology, environmental sensing, and bio-medicine.
Acknowledgments
The authors would thank the support of the National Natural Science Foundation of China (Nos. 11374285 and U1330104).
References and links
1. Q. L. Bao, H. Zhang, Y. Wang, Z. H. Ni, Y. L. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic-layer Graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]
2. Z. Sun, T. Hasan, F. Torrisi, D. Popa, G. Privitera, F. Wang, F. Bonaccorso, D. M. Basko, and A. C. Ferrari, “Graphene mode-locked ultrafast laser,” ACS Nano 4(2), 803–810 (2010). [CrossRef] [PubMed]
3. C. J. Zhao, H. Zhang, X. Qi, Y. Chen, Z. T. Wang, S. C. Wen, and D. Y. Tang, “Ultra-short pulse generation by a topological insulator based saturable absorber,” Appl. Phys. Lett. 101(21), 211106 (2012). [CrossRef]
4. S. Wang, H. Yu, H. Zhang, A. Wang, M. Zhao, Y. Chen, L. Mei, and J. Wang, “Broadband few-layer MoS2 saturable absorbers,” Adv. Mater. 26(21), 3538–3544 (2014). [CrossRef] [PubMed]
5. D. Mao, Y. Wang, C. Ma, L. Han, B. Jiang, X. Gan, S. Hua, W. Zhang, T. Mei, and J. Zhao, “WS2 mode-locked ultrafast fiber laser,” Sci. Rep. 5, 7965 (2015). [CrossRef] [PubMed]
6. K. Wang, J. Wang, J. Fan, M. Lotya, A. O’Neill, D. Fox, Y. Feng, X. Zhang, B. Jiang, Q. Zhao, H. Zhang, J. N. Coleman, L. Zhang, and W. J. Blau, “Ultrafast saturable absorption of two-dimensional MoS2 nanosheets,” ACS Nano 7(10), 9260–9267 (2013). [CrossRef] [PubMed]
7. H. Zhang, S. B. Lu, J. Zheng, J. Du, S. C. Wen, D. Y. Tang, and K. P. Loh, “Molybdenum disulfide (MoS₂) as a broadband saturable absorber for ultra-fast photonics,” Opt. Express 22(6), 7249–7260 (2014). [CrossRef] [PubMed]
8. H. Liu, A. P. Luo, F. Z. Wang, R. Tang, M. Liu, Z. C. Luo, W. C. Xu, C. J. Zhao, and H. Zhang, “Femtosecond pulse erbium-doped fiber laser by a few-layer MoS2 saturable absorber,” Opt. Lett. 39(15), 4591–4594 (2014). [CrossRef] [PubMed]
9. R. I. Woodward, E. J. Kelleher, T. Runcorn, S. V. Popov, F. Torrisi, R. T. Howe, and T. Hasan, “Q-switched fiber laser with MoS2 saturable absorber,” in CLEO (OSA, 2014), paper SM3H.6.
10. Y. Huang, Z. Luo, Y. Li, M. Zhong, B. Xu, K. Che, H. Xu, Z. Cai, J. Peng, and J. Weng, “Widely-tunable, passively Q-switched erbium-doped fiber laser with few-layer MoS2 saturable absorber,” Opt. Express 22(21), 25258–25266 (2014). [CrossRef] [PubMed]
11. R. I. Woodward, E. J. R. Kelleher, R. C. T. Howe, G. Hu, F. Torrisi, T. Hasan, S. V. Popov, and J. R. Taylor, “Tunable Q-switched fiber laser based on saturable edge-state absorption in few-layer molybdenum disulfide (MoS₂),” Opt. Express 22(25), 31113–31122 (2014). [CrossRef] [PubMed]
12. M. Zhang, R. C. T. Howe, R. I. Woodward, E. J. R. Kelleher, F. Torrisi, G. H. Hu, S. V. Popov, J. R. Taylor, and T. Hasan, “Solution processed MoS2-PVA composite for sub-bandgap mode-locking of a wideband tunable ultrafast er:fiber laser,” Nano Res. 8(5), 1522–1534 (2015). [CrossRef]
13. K. G. Zhou, M. Zhao, M. J. Chang, Q. Wang, X. Z. Wu, Y. Song, and H. L. Zhang, “Size-dependent nonlinear optical properties of atomically thin transition metal dichalcogenide nanosheets,” Small 11(6), 694–701 (2015). [CrossRef] [PubMed]
14. X. Zheng, Y. Zhang, R. Chen, X. Cheng, Z. Xu, and T. Jiang, “Z-scan measurement of the nonlinear refractive index of monolayer WS2,” Opt. Express 23(12), 15616–15623 (2015). [CrossRef] [PubMed]
15. K. Wu, X. Zhang, J. Wang, X. Li, and J. Chen, “WS2 as a saturable absorber for ultrafast photonic applications of mode-locked and Q-switched lasers,” Opt. Express 23(9), 11453–11461 (2015). [CrossRef] [PubMed]
16. S. H. Kassani, R. Khazaeinezhad, H. Jeong, T. Nazari, D.-I. Yeom, and K. Oh, “All-fiber Er-doped Q-Switched laser based on Tungsten Disulfide saturable absorber,” Opt. Express 5(2), 373–379 (2015). [CrossRef]
17. P. G. Yan, A. J. Liu, Y. S. Chen, H. Chen, S. C. Ruan, C. Y. Guo, S. F. Chen, I. L. Li, H. P. Yang, J. G. Hu, and G. Z. Cao, “Microfiber-based WS2-film saturable absorber for ultra-fast photonics,” Opt. Mater. Express 5(3), 479–489 (2015).
18. M. Jung, J. Lee, J. Park, J. Koo, Y. M. Jhon, and J. H. Lee, “Mode-locked, 1.94-μm, all-fiberized laser using WS2-based evanescent field interaction,” Opt. Express 23(15), 19996–20006 (2015). [CrossRef] [PubMed]
19. M. Zhang, G. Hu, G. Hu, R. C. T. Howe, L. Chen, Z. Zheng, and T. Hasan, “Yb- and Er-doped fiber laser Q-switched with an optically uniform, broadband WS2 saturable absorber,” arXiv:1507.03188 (2015).
20. D. Yang and R. F. Frindt, “Li-intercalation and exfoliation of WS2,” J. Phys. Chem. Solids 57(6-8), 1113–1116 (1996). [CrossRef]
21. N. Perea-López, A. L. Elías, A. Berkdemir, A. Castro-Beltran, H. R. Gutiérrez, S. M. Feng, R. T. Lv, T. Hayashi, F. López-Urías, S. Ghosh, B. Muchharla, S. K. Talapatra, H. Terrones, and M. Terrones, “Photosensor device based on few-layered WS2 films,” Adv. Funct. Mater. 23(44), 5511–5517 (2013). [CrossRef]
22. J. N. Coleman, M. Lotya, A. O’Neill, S. D. Bergin, P. J. King, U. Khan, K. Young, A. Gaucher, S. De, R. J. Smith, I. V. Shvets, S. K. Arora, G. Stanton, H. Y. Kim, K. Lee, G. T. Kim, G. S. Duesberg, T. Hallam, J. J. Boland, J. J. Wang, J. F. Donegan, J. C. Grunlan, G. Moriarty, A. Shmeliov, R. J. Nicholls, J. M. Perkins, E. M. Grieveson, K. Theuwissen, D. W. McComb, P. D. Nellist, and V. Nicolosi, “Two-dimensional nanosheets produced by liquid exfoliation of layered materials,” Science 331(6017), 568–571 (2011). [CrossRef] [PubMed]
