JW2A.74.pdf
CLEO:2015 © OSA 2015
Passively Mode-locked Fiber Laser based on CVD WS2 1
Reza Khazaeizhad,1 Sahar Hosseinzadeh Kassani,1 Hwanseong Jeong,2 Dong-Il Yeom,2,* and Kyunghwan Oh1,** Photonic Device Physics Laboratory, Institute of Physics and Applied Physics, Yonsei University, Seoul 120–749, South Korea 2 Department of Physics & Energy Systems Research, Ajou University, Suwon 443-749, South Korea *
[email protected] and **
[email protected]
Abstract: We investigated nonlinear characteristics and applications of WS2 for mode-locked fiber laser. The saturable absorber was prepared by transferring the synthesized CVD WS2 onto a fabricated side-polished fiber. WS2 showed promising potential for ultrafast-pulse generation. OCIS codes: (190.4400) Nonlinear optics, materials; (140.7090) Ultrafast lasers.
1. Introduction Passively mode-locked fiber laser based on nonlinear materials is a unique method for pulse generation. Application of carbon nano-tubes and two dimensional (2D) materials such as Graphene have been investigated intensively which have some limitations such as using polymer in saturable absorber for preserving the nonlinearity and CNTs absorption depends on its bundle size [1,2]. In contrast, intensity dependent transmission behavior of Tungsten disulfide (WS2), one of the 2D materials with unique band-gap structure [3], has not been reported. Application of WS2 as a nonlinear material for generating ultrafast pulses in a laser cavity has not been attempted yet. We investigated nonlinear optical characteristics of WS2 thin film and experimentally demonstrated its potential for application as a saturable absorber in passively mode-locked fiber lasers. The WS2 thin film was prepared using chemical vapor deposition (CVD) to grow uniform multilayer WS2 on a SiO2 substrate [3]. Side polished fiber (SPF) was fabricated and WS2 film was transferred via lift-off method to provide an efficient evanescent field interaction. An all-fiber ring cavity was built with an Er-doped fiber as a gain medium and the prepared WS2 thin film on SPF was employed as a mode locker. Subsequently, stable soliton-like pulses were generated with a spectral width of 8.23 nm and pulse duration of 332 fs. Moreover, there was no need for using any kind of composite or polymer matrix for pulse generation or preserving the nonlinear behavior for this material. Our study confirmed potential of WS2 film as a novel 2D nonlinear optical material and demonstrated its pulse shaping ability in an all-fiber ring cavity for obtaining femtosecond pulses, for the first time. 2. Saturable absorber fabrication process Fiber ferrule-type Saturable absorber (SA) has a short interaction length which restricts the pulse shaping ability and it is vulnerable to mechanical and optical damage [4]. We used side polished fiber (SPF) to overcome these limitation which provide a longer interaction length and a higher damage threshold. SPF was fabricated by polishing a single mode fiber (SMF 28) buried in a V-grooved quartz block, as shown in Fig. 1 (a). SPF provides efficient evanescent field interaction between the deposited materials on the surface and propagating light in the core. Before deposition, insertion loss of the SPF was ~0.1 dB. The WS2 thin film was synthesized via CVD processes to grow multilayer WS2 [3]. Subsequently, the WS2 film was spin casted with a thin layer of Polymethyl methacrylate (PMMA) and the WS2/PMMA layer was etched using a 30% potassium hydroxide (KOH) solution; see Fig. 1(b). Then, the lifted-off layer was washed in DI water to remove any residue and the WS2/PMMA film transferred onto the clean surface of the prepared SPF. Finally, the sample was immersed into the acetone to remove the PMMA and simultaneously the output power of the prepared sample was monitored via a power meter during the PMMA removal process as shown in the Fig. 1(c). After the PMMA removing process, insertion loss of the prepared sample was increased to ~7.5 dB.
Fig. 1 (a) Schematic cross section of SPF, (b) Lifting procedure of the CVD WS2 in KoH, (c) Monitored output power of the SPF while PMMA was removed, (inset: Removal process of PMMA with acetone,), (d). Raman spectrum of the CVD WS2 film on SPF.
JW2A.74.pdf
CLEO:2015 © OSA 2015
Since each material has its own phonon modes, Raman measurements was performed to determine the quality of the WS2 thin film. Raman spectrum of the bulk WS2 has E2g1 and A1g phonon modes at 355.5 cmE1 and 420.5 cmE1, respectively [5]. Figure 1(d) shows the Raman spectrum of the prepared CVD WS2 on SPF. The phonon mode values are slightly different from those of bulk WS2, as the A1g mode is red shifted and the E2g1 mode is blue shifted, which is in good agreement with the reported multilayer WS2 Raman spectrum [5]. 3. Intensity-dependent behavior and mode-locked fiber laser The nonlinear transmission characteristic of the prepared WS2 sample was investigated using a femtosecond Erbium fiber laser, with the same setup as Ref [6]. We measured the optical power-dependent transmission and experimental results were fitted with the two-level saturable absorber model, as shown in Fig. 2(a). The transmission of the sample was increased as the input power intensified. However, due to limited light source power, transmissions are not fully saturated. We confirmed the nonlinear transmission by repeating the measurements as well as by decreasing the input power from high to low and the fitted curves were satisfactory overlapped. Figure 2(b) schematically depicts the setup of the ring laser. One meter highly Er-doped fiber was used as a gain medium, and pumped by a 980 nm laser diode through a wavelength division multiplexer (WDM). A polarization controller (PC) was inserted to optimize the mode-locking conditions. A polarization independent isolator was used to maintain unidirectional laser operation. Light was then extracted from the cavity via a 10/90 tap coupler and the total length of the cavity was ~6.65 m. The fiber ring laser has an intra-cavity anomalous dispersion of -0.124 ps2. The fiber laser was operating in the soliton regime with Kelly sidebands in the optical spectrum, which are a typical feature of solution pulses, see Fig. 2(c). The laser operated at the center wavelength of G = 1565 nm with 8.23 nm spectral bandwidth. Figure 2(d) shoes the time trace of the oscilloscope with the pulse repetition rate of 31.11 MHz. The femtosecond pulses were measured using an intensity autocorrelator to have pulse duration of 332 fs which was fitted by a Sech2-shaped pulse as shown in Fig. 2(e). Finally, Fig. 2(f) represents the RF spectrum of the generated pulse train. The fundamental peak has the background noise of 81.6 dB and the inset shows the wide span RF spectrum clarifies this stable operation of the soliton mode locking without exhibiting Q-switching instability.
Fig. 2 (a) Intensity-dependent transmission data and its corresponding fitting, (b) Schematic of the fiber ring laser, (c) Optical spectrum of laser output, (d) Pulse train, (e) Intensity autocorrelator trace, (f) RF spectrum (inset: wide band RF spectra).
We replaced the deposited SPF in the ring cavity with a deposition-free SPF to verify the contribution of WS2 in mode locking and thus no pulses were generated, even by increasing the pump power or adjusting the PC. Finally, it can be concluded that WS2 is a promising materials for pulse generation with no need of additional polymer or composites. To the best of our knowledge, this is the first demonstration of mode-locked all-fiber lasers based on WS2 saturable absorbers. 4. References [1] T. Hasan, Z. Sun, F. Wang, F. Bonaccorso, P. H. Tan, A. G. Rozhin, and A. C. Ferrari, “Nanotube–Polymer Composites for Ultrafast Photonics,” Adv. Mater., 21, 3874-3899 (2009). [2] F. Bonaccorso, Z. Sun, T. Hasan, and A. C. Ferrari, “Graphene photonics and optoelectronics,” Nature Photon., 4, 611-622 (2010). [3] C. Cong, J. Shang, X. Wu, B. Cao, N. Peimyoo, C. Qiu, L. Sun and T. Yu, “Synthesis and optical properties of large-scale single-crystalline two-dimensional semiconductor WS2 monolayer from chemical vapor deposition,” arXiv:1312.1418 (2013). [4] R. Khazaeinezhad, S. H. Kassani, T. Nazari, H. Jeong, J. Kim, K. Choi, J. Lee, J. H. Kim, H. Cheong, D.-I. Yeom, and K. Oh, “Saturable optical absorption in MoS2 nano-sheet optically deposited on the optical fiber facet,” Opt. Commun., 335, 224-230 (2015). [5] A. Berkdemir, H. Gutiérrez, A. Botello-Méndez, N. Perea-López, A. Elías, C.-I. Chia, B. Wang, V. H. Crespi, F. López-Urías, J. Charlier, H. Terrones, and M. Terrones, “Identification of individual and few layers of WS2 using Raman Spectroscopy,” Sci. Rep., 3, 1755 (2013). [6] R. Khazaeizhad, S. H. Kassani, H. Jeong, D.-I. Yeom, and K. Oh, "Mode-locking of Er-doped fiber laser using a multilayer MoS2 thin film as a saturable absorber in both anomalous and normal dispersion regimes," Opt. Express 22, 23732-23742 (2014).
