Ultrafast Pulsed All-Fiber Laser Based on Tapered Fiber ... - IEEE Xplore

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 15, AUGUST 1, 2015

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Ultrafast Pulsed All-Fiber Laser Based on Tapered Fiber Enclosed by Few-Layer WS2 Nanosheets Reza Khazaeinezhad, Sahar Hosseinzadeh Kassani, Hwanseong Jeong, Kyung Jun Park, Byoung Yoon Kim, Member, IEEE, Dong-Il Yeom, and Kyunghwan Oh, Member, IEEE Abstract— We demonstrate all-fiber mode-locked laser based on a tapered optical fiber saturable absorber (SA) enclosed in tungsten disulfide (WS2 ) nanosheets. Tapered fibers were fabricated using the standard flame brushing method to an interaction length of 3 mm with waist diameters of 10 and 15 µm. WS2 nanosheets were prepared via a liquid phase exfoliation method to form a uniform dispersion. Subsequently, the WS2 nanosheets were optically deposited along the interaction length of the tapered fibers by evanescent field interactions. We built a ring laser including the fabricated mode-lockers. The SA with a 10-µm taper diameter delivers the pulses with a pulse duration of 369 fs and 3-dB spectral bandwidth of 7.5 nm; on the other hand, the output pulses using the mode-locker with 15-µm waist diameter were found to have 563-fs pulse duration and 5.2 nm of 3-dB bandwidth. It is shown that the smaller waist diameter of tapered fiber causes wider spectral bandwidth of the ultrafast pulses and narrower 3-dB bandwidth. Index Terms— Mode-locked fiber laser, tapered optical fiber, saturable absorber, nonlinear optical materials, Tungsten disulfide.

I. I NTRODUCTION N RECENT decades, researchers have devoted great effort into the development of ultrafast pulse lasers based on different materials and configurations. Ultra-short pulse lasers have extensive applications in various fields including optical communications, microstructure fabrication, spectroscopy and biomedical research [1]–[3]. Passive mode-locking is the well-known method to generate femtosecond pulses where saturable absorbers (SA), or mode-lockers are popularly used for environmentally stable and self-starting operation of the laser. Integrated fiber SAs can be designed to take

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Manuscript received February 16, 2015; revised March 30, 2015; accepted April 12, 2015. Date of publication April 24, 2015; date of current version July 7, 2015. This work was supported in part by the Institute of Physics and Applied Physics, Yonsei University, Seoul, Korea, in part by the ICT Research and Development Program through the Ministry of Science, ICT and Future Planning (MSIP)/Institute for Information and Communications Technology Promotion under Grant 2014-044-014-002, and in part by the Nano Material Technology Development Program through the National Research Foundation (NRF) within the MSIP under Grant NRF-2012M3A7B4049800. The work of H. Jeong and D.-I. Yeom was supported by NRF of Korea under Grant NRF-2013R1A1A2A10005230 and Grant 2011-0027920. (Corresponding authors: Dong-Il Yeom and Kyunghwan Oh.) R. Khazaeinezhad, S. H. Kassani, and K. Oh are with the Photonic Device Physics Laboratory, Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Korea (e-mail: [email protected]; [email protected]; [email protected]). H. Jeong and D.-I. Yeom are with the Department of Physics and Energy Systems Research, Ajou University, Suwon 443-749, Korea (e-mail: [email protected]; [email protected]). K. J. Park and B. Y. Kim are with the Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2015.2426178

advantage of direct or indirect light interaction. The fiber ferrule geometry uses direct light interaction, while tapered fiber [4] and side-polished fiber (SPF) [5] use evanescent field interactions. Compared to those with fiber ferrules or the SPF, the tapered fiber SA has a long interaction length and higher damage threshold. New materials with unique and outstanding optical properties are always among the hottest research topics in optics and photonics. Up to now, semiconductor saturable absorber mirrors (SESAMs), carbon nanotubes (CNTs), Graphene, topological isolators (TIs) and other two dimensional (2D) materials have been widely studied due to their excellent optical properties and pulse shaping abilities [6]–[11]. SESAM has a challenging and costly fabrication process and CNT needs to be engineered to match their diameter-dependent band-gap and chiral effects. Recently, studies of layered structured materials such as transition metal dichalcogenide have received increasing attention due to their great diversity and notable physical and optical properties. 2D materials such as MoS2 and WS2 have layer-dependent band-gap properties. As a result, these 2D materials have attracted much attention for applications in various research areas, including optoelectronics and short pulse generation. A lot of researchers have reported high third order nonlinearity of MoS2 and application of this material in lasers [12], [13]. In contrast, there have not been many reports on WS2 , which is also one of the layered materials analogous to graphene and have a direct band-gap in the few layers form [14]–[16]. Tungsten disulfide is one of the 2D materials consisting of stacked S-W-S slabs (S: Sulfur atom and W: tungsten atom). In the layers, the sulfur and tungsten atoms are hexagonally bonded very strongly, while the bonds between layers are formed through weak van der Waals bonds. There are various methods to prepare few-layer nano-sheets of bulk-structured 2D materials such as mechanical exfoliation, liquid phase exfoliation and chemical vapor deposition (CVD) methods [17]. The mechanical exfoliation method is not efficient and the CVD method needs expensive equipment. So far, the liquid-phase exfoliation method is a practical and costeffective means to fabricate nano-sheets of 2D materials. In this letter, we reported a passively mode-locked Er-doped fiber laser based on a SA consisting of tapered fiber encircled by few-layer WS2 . The WS2 nano-sheets were exfoliated with liquid-phase exfoliation method and optically deposited around the tapered fiber. We built a laser cavity using a SA with a tapered fiber enclosed by WS2 nano-sheets where the pulse duration can be engineered by proper selection of tapered diameter. Using a 10 µm waist diameter tapered fiber SA,

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Fig. 1. Characterization of WS2 nano-sheets, (a) Raman spectrum, (b) Linear optical absorption spectrum, (c) TEM images of the flakes with 50 and 2 nm resolutions, (d) AFM measurements of the nano-sheets along with three height profiles.

optical pulses were generated with pulse duration of 369 fs and 3dB bandwidth of 7.5 nm. A 15 µm diameter SA generated 563 fs pulses with 3dB bandwidth of 5.2 nm. II. FABRICATION AND C HARACTERIZATION OF WS2 SA Tungsten disulfide (WS2 ) nano-sheets were synthesized through well-known liquid phase exfoliation (LPE) method due to the weak interlayer forces between the layered structures and the detailed LPE fabrication process has been fully explained in Ref [17]. Briefly, after dispersing bulk WS2 in solvent, ultrasound-assisted exfoliation is then used to exfoliate few layers and finally exfoliated nano-sheets are separated from thick flakes via ultracentrifugation. Then, the prepared few layer WS2 nano-sheets were uniformly dispersed in ethanol and the quality of the nano-sheets was characterized. In Fig. 1(a), Raman characterization shows that the typical 1 at 355.5 cm−1 Raman peaks of bulk WS2 (phonon modes: E 2g −1 and A1g at 420.5 cm , in-plan and out of plan motion of W and S atoms, respectively) are shifted and the frequency difference between these two modes are decreased due to the successful exfoliation [18]. In Fig. 1(b), the optical absorption spectrum of the as-deposited WS2 nano-sheet thin film was recorded via UV-VIS-NIR spectrophotometer. The WS2 showed an excitonic absorption peak ∼ 625 nm which arises from a direct band-gap transition at the K point [19]. Figure 1(c) shows an ultra-high resolution transmission electron microscope images which confirms the thin few layer nano-sheets have been formed with hexagonal rings shapes of tungsten and sulphur atoms in lattice structure. Furthermore, we measured the thickness of nano-sheets via an atomic force microscopy (AFM), to find 80% of the flakes had the thickness of 5-6 nm. This thickness correspond to 7∼9 layers according to prior report [19] (Fig. 1(d)). We used tapered fiber as a template for interaction between evanescent field and WS2 due to its simple fabrication process and long interaction length. The evanescent field interaction of light with WS2 substantially increased the optical damage threshold, in contrast to direct interaction of light with material which causes thermal damage and prevents operation in the high-power regime. The tapered fiber not only significantly

Fig. 2. (a) SEM images of the waist diameter of tapered fibers, (b) Schematic view of optical deposition setup (inset: the output power versus time during the deposition process), (c) Before and after deposition pictures of tapered fiber, (d) Nonlinear transmission measurements for 10 and 15 µm waist diameter of the deposited tapered fibers.

increases the damage threshold but also raises the light interaction length without increasing the fabrication process complexity. In order to make the tapered fiber, the widely known flame brushing method was used [20]. By heating the tapered region and pulling the fiber at both sides simultaneously, the fiber diameter D at the transition region is exponentially decreased and the taper waist region remains as same diameter at the end of transition region, following the equation D=D0 exp(-pulling length/taper waist length) [20]. In standard single mode fiber (i.e. weakly guiding fiber), as fiber cladding diameter is decreased less than 30 µm, light in the core mode is no longer confined to the core and can propagate via silica-air guiding. Due to the claddingair total internal reflection, this evanescent field outside of the fiber can interact with the deposited material around the interaction length. In this experiment, the interaction length of tapered fiber was 3 mm and the waist diameters were 10 and 15 µm. The insertion losses (IL) of the prepared SAs at a wavelength of 1.5 µm before WS2 deposition were measured to be approximately 1 dB. Figure 2(a) shows scanning electron microscopy (SEM) images of the interaction length of the fabricated tapered fibers. Subsequently, the mode-locker was assembled via optical deposition of WS2 nano-sheets around the interaction region of the fabricated tapered fiber. Figure 2(b) schematically shows the simple deposition setup using a laser source (1.5 µm) and in-situ monitoring of the deposition via an optical power meter. The inset of Fig. 2(b) shows the output power versus time during the deposition process of the tapered fiber. Using a micropipette, ∼2 µL of WS2 solution was drop-cast on the interaction region of the tapered fiber and it can be clearly

KHAZAEINEZHAD et al.: ULTRAFAST PULSED ALL-FIBER LASER BASED ON TAPERED FIBER

observed that after a while the deposition of nano-sheets causes a sudden decrease of optical transmission. After the deposition process, we did not observe any polarizationdependent loss of the prepared samples due to the uniform deposition of nano-sheets around the interaction length of tapered fiber and the IL of the tapered fibers at 1.5 µm after WS2 deposition were again measured to be ∼6 and 5 dB for 10 and 15 µm waist diameter samples, respectively. We confirmed the deposition of WS2 nano-sheets around the tapered fiber by capturing images of the tapered fiber before and after deposition using a red light source as shown in Fig. 2(c). The after-deposition picture clearly demonstrates increased scattering due to the deposited nano-sheets. Afterwards, we investigated the intensity-dependent transmission behavior of the fabricated and deposited tapered fibers using a lab-built Er-doped fiber laser. The laser source was working at a central wavelength of 1560 nm and pulse duration of 400 fs. The measuring method was carried out similar to our previous work [21], [22]. A variable optical attenuator along with an optical power meter was programmed to read the input and output power simultaneously. Figure 2(d) plots the measured transmission as a function of input laser power at 1560 nm and the data were fitted with a simple two-level saturable absorber model. The transmission of both WS2 tapered fibers intensified as the input power increased. The 10 µm waist diameter tapered fiber showed more than 0.6% modulation depth (MD), however the transmission was not fully saturated because of limited light source power. The MD of the tapered fiber with 15 µm waist diameter was about 0.5%. From the measurement results, it can be clearly seen that the MD of the deposited tapered fiber with 10 µm waist diameter is potentially larger than that of 15 µm. The transmission was measured via two methods, to confirm that the nonlinear results were caused primarily by WS2 nano-sheets. The input power of the laser source was decreased from high to low and increased the opposite way from low to high; however, the transmittance curves overlapped satisfactorily, proving that the mode-locker device has high and robust performance. The results evidently demonstrated the saturable absorption properties of WS2 . III. M ODE -L OCKED F IBER L ASER Figure 3 schematically depicts the all-fiber laser setup including the mode-locker device. A 1-m highly-doped Erbium-doped fiber was used as the gain medium which was pumped by a 980 nm laser diode (LD) using a wavelength division multiplexer (WDM). An isolator was inserted at the end of the amplifier section to maintain unidirectional laser operation and a polarization controller (PC) was used to optimize the mode-locking conditions. Light was then extracted from the cavity using a 10/90 coupler with 10% output ratio. Our laser cavity has anomalous intra-cavity dispersion, with overall negative group velocity dispersion, in order to facilitate soliton-like pulse shaping. Figures 4(a) and 4(d) illustrate the typical output spectra of mode-locked pulses at 10 and 15 µm waist diameter of fabricated SAs, respectively. The evident Kelly sidebands of the spectra indicated that the lasers were operating in the

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Fig. 3. Passively mode-locked fiber laser cavity schematic, (LD: laser diode, WDM: wavelength division multiplexer, SA: saturable absorber, PC: polarization controller, EDF: Erbium-doped fiber).

Fig. 4. Mode-locked lasers characteristics: (a) and (d) Optical spectrum of 10 and 15 µm waist diameter tapered fiber SA (inset: Pulse train), (d) and (e) Pulse trains of the 10 and 15 µm SA, (c) and (f) RF spectra of fundamental frequency and wide span rang of the 10 and 15 µm tapered fiber SA. TABLE I D ETAILED C HARACTERISTICS OF THE M ODE -L OCKING O PERATION OF F IBER L ASERS

soliton regime. The inset of these figures show the output time trace of the pulses by the oscilloscope after an opticalto-electrical conversion using a fast photodiode. The measured autocorrelation trace of the output pulses of the 10 and 15 µm waist diameter SAs were presented in the Figs. 4(b) and 4(e), which were fitted with a sech2 pulse profile. Figures 4(c) and 4(f) show the radio frequency (RF) spectra of the output lasers for 10 and 15 µm SAs, respectively. High signal to noise ratios are observed for the fundamental peak, which indicates great stability in the mode-locking regime. In the inset of Figs. 4(c) and 4(f), wideband RF spectrum of the pulses are shown. Table I summarizes the modelocking performance of fiber lasers based on 10 and 15 µm waist diameter of the tapered fiber SAs. Once stable output pulses were achieved by introducing an adjustment to the PC, no further PC disturbance was needed and we could decrease the pump power to the mode-locking

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threshold while maintaining ultrafast pulses. The output pulses of the fiber lasers illustrate high stability, which is proved by the RF spectra, and the measured repetition rates were in good agreement with the design parameters of the cavities length. In addition, to verify the effects of WS2 on mode-locking, the tapered fiber deposited inside the laser cavity was replaced by a non-deposited tapered fiber, resulting in no pulsed operation observed. This confirmed that the above mode-locked was induced by WS2 . The previous mode-locking reports based on WS2 [15], employed ferrule type SA which is vulnerable to optical and mechanical damage and side-polished fiber which has complicated fabrication process compare with tapered fiber SA. We optically deposited WS2 nano-sheets around the tapered fiber which is much simpler than pulse laser deposition (PLD) on microfiber [16], so optical deposition is more efficient method to customize the performance and specifications of the SA. Up to date, there was no report on spectral dependent fiber laser based on the waist diameter of tapered fiber SA. Although, there are several factors in laser operation for different pulse properties, there is a possibility that the smaller waist diameter of SA exhibits wider optical spectrum which could have more intense evanescent field interaction compare with larger waist diameter and caused to stronger interaction of light with WS2 . We found that the smaller waist diameter of SA exhibits wider optical spectrum due to stronger interaction of light with WS2 . While the waist diameter increases from 10 to 15 µm, the 3 dB spectral bandwidth varies from 7.5 to 5.2 nm, and the pulse duration is increased from 369 to 563 fs. With the fixed interaction length and upon increasing the waist diameter, the 3dB spectral bandwidth decreased, while the pulse duration increased. Furthermore, the central wavelength of the output pulses shifts toward longer wavelengths. This suggests a simple way to modify and tune the spectral pulse width. The ultrafast pulses with tunable spectral width have widely application such as bio-imaging. The laser showed high stability which we could consider it for mass production by compact packaging the tapered fiber SA using fiber optic protection micro-sleeves to full protection of the as-deposited tapered fiber. Although the exact mechanism of 2D materials operation below the bandgap is still unclear, defect-states and edge-states of saturable absorber have been suggested by Ref. [23] and Ref [13], respectively. This letter suggests that WS2 has a potential application in ultrafast photonics such as ultra-short pulse generation and optical switching.

bandwidths. Our results demonstrated that the tapered fiber SA embedded in WS2 nano-sheets is a promising device for practical ultrafast pulses generation. R EFERENCES

IV. C ONCLUSION We have prepared few-layer WS2 nano-sheets using liquid phase exfoliation method. Tapered fibers have been fabricated via the flame brushing method with 10 and 15 µm waist diameters and an interaction length of 3 mm. Subsequently, the prepared WS2 nano-sheets solution was optically deposited around the tapered fiber. Upon employing the tapered fiber SAs, we achieved stable fundamental mode-locking at wavelength of 1.5 µm with pulse duration of femtoseconds. By using a different waist diameter of tapered fiber, we obtained femtosecond soliton pulses with various spectral

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