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Khazaeinezhad, Reza,Kassani, Sahar Hosseinzadeh,Hwanseong Jeong,Kyung Jun Park,Byoung Yoon Kim,Dong-Il Yeom,Kyunghwan Oh IEEE 2015 IEEE photonics technology letters Vol.27 No.15
<P>We demonstrate all-fiber mode-locked laser based on a tapered optical fiber saturable absorber (SA) enclosed in tungsten disulfide (WS<SUB>2</SUB>) 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. WS<SUB>2</SUB> nanosheets were prepared via a liquid phase exfoliation method to form a uniform dispersion. Subsequently, the WS<SUB>2</SUB> 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.</P>
Passive Q-Switching of an All-Fiber Laser Using WS <sub>2</sub>-Deposited Optical Fiber Taper
Khazaeinezhad, Reza,Nazari, Tavakol,Hwanseong Jeong,Kyung Jun Park,Byoung Yoon Kim,Dong-Il Yeom,Kyunghwan Oh IEEE 2015 IEEE photonics journal Vol.7 No.5
<P>We demonstrate the passive Q-switching operation of an erbium-doped fiber laser using a few-layer WS<SUB>2</SUB>-based saturable absorber with evanescent field interaction. WS<SUB>2</SUB> flakes have been synthesized via a liquid-phase exfoliation method, and an optical fiber taper was selected as a light-material template that causes to achieve a long interaction length of the evanescent wave. Subsequently, the prepared solution was optically deposited around the 3-mm interaction length of the fabricated optical fiber taper with a 5-μm waist diameter. The proposed Q-switched fiber laser based on the WS<SUB>2</SUB>-deposited fiber taper saturable absorber could generate stable output pulses at 1565 nm, with a pulsewidth ranging from 1.3 to 3 μs and a repetition rate of 33-108 kHz.</P>
Khazaeinezhad, Reza,Hosseinzadeh Kassani, Sahar,Hwanseong Jeong,Dong-Il Yeom,Kyunghwan Oh IEEE 2015 Journal of lightwave technology Vol.33 No.17
<P>We investigated nonlinear optical characteristics of Tungsten disulfide (WS<SUB>2</SUB>) films and experimentally demonstrated their high potential for application as nonlinear saturable absorbers in passively mode-locked fiber lasers. Side polished fiber (SPF) was fabricated and WS<SUB>2</SUB> film was overlaid to provide an efficient evanescent field interaction. The WS<SUB>2</SUB> film was prepared using two methods: liquid phase exfoliation to form few-layer nano-sheets, and chemical vapor deposition (CVD) to grow uniform multilayer WS<SUB>2</SUB> on a SiO<SUB>2</SUB> substrate. Two SPF saturable absorbers were prepared by either spin coating WS<SUB>2</SUB> solution or lifting off the multilayer CVD WS<SUB>2</SUB> on SPF. An all-fiber ring cavity was built and the WS<SUB>2</SUB> film overlaid on SPF was employed as a mode locker along with Er-doped fiber as a gain medium. Using the spin-coated WS<SUB>2</SUB> SPF, stable soliton-like pulses were generated with a spectral width of 5.6 nm and 467 fs pulse duration. The fiber laser cavity containing CVD WS<SUB>2</SUB> SPF generated a transform-limited soliton pulse train with a spectral width of 8.23 nm and a pulse duration of 332 fs. Our study confirmed a high potential of WS<SUB>2</SUB> film as a novel 2-D nonlinear optical material for laser applications.</P>
Khazaeinezhad, Reza,Kassani, Sahar Hosseinzadeh,Hwanseong Jeong,Nazari, Tavakol,Dong-Il Yeom,Kyunghwan Oh IEEE 2015 IEEE photonics journal Vol.7 No.1
<P>We demonstrate an all-fiberized mode-locked laser by employing Molybdenum Disulfide (MoS<SUB>2</SUB>) spin-coated onto a side-polished fiber (SPF) as an in-line saturable absorber. Single crystal MoS<SUB>2</SUB> has been exfoliated via the Li intercalation method and then dispersed with large population of few-layer MoS<SUB>2</SUB> nano-sheets in ethanol. Subsequently, a saturable absorber has been prepared using a relatively simple method: spin-coating the uniform MoS<SUB>2</SUB> solution on the fabricated SPF without using any polymer or toxic procedure. Power-dependent transmission property of the prepared MoS<SUB>2</SUB> nano-sheets on the SPF was experimentally analyzed, providing the feasibility to apply it as an efficient in-line saturable absorption behavior. By deploying the MoS<SUB>2</SUB> SPF into a fiber laser cavity with finely tuned laser pump power and cavity dispersion, mode-locked pulses were obtained at anomalous and normal dispersion of the laser cavity. The self-started soliton pulses at the anomalous dispersion regime were generated with a spectral bandwidth of 9.96 nm at 1584 nm and a pulse duration of 521 fs. Moreover, by managing the dispersion to the normal regime, stable pulses at net-normal intra-cavity dispersion with 11.9 ps pulse duration were obtained with a spectral bandwidth of 18.2 nm at 1570 nm. The results prove the effectiveness of developing MoS<SUB>2</SUB> saturable absorbers and applications for pulsed laser operation.</P>
Nazari, Tavakol,Khazaeinezhad, Reza,Kassani, Sahar Hosseinzadeh,Jung, Woohyun,Shin, Inchul,Kang, Dooeui,Oh, Kyunghwan Optical Society of America 2014 Optics express Vol.22 No.22
<P>Enhanced optical transmission (EOT) and its polarization extinction ratio (PER) of a sub-wavelength polygonal aperture surrounded by polygonal grooves are investigated numerically by finite difference time domain (FDTD) method. Effects of light polarization on EOT were analyzed and compared for four types of geometrical structures: triangle aperture surrounded by triangle grooves, square aperture surrounded by square grooves, rhombus aperture surrounded by rhombus grooves, and pentagon aperture surrounded by pentagon grooves. The effects of relative angles between the symmetry axes of polygons and the light polarization were thoroughly analyzed. Among these plasmonic polygonal bull's eye structures, the rhombus showed a maximum EOT several times larger than others. In contrast to the prior normal incident condition, we further analyzed the impacts of the incident angle and we found a wideband tunability of EOT wavelengths.</P>
Nazari, Tavakol,Khazaeinezhad, Reza,Jung, Woohyun,Joo, Boram,Kong, Byung-Joo,Oh, Kyunghwan Optical Society of America 2015 Optics express Vol.23 No.14
<P>Dual resonant bands in UV and the visible range were simultaneously observed in the enhanced optical transmission (EOT) through star-shaped plasmonic structures. EOTs through four types of polygonal bull's eyes with a star aperture surrounded by the concentric star grooves were analyzed and compared for 3, 4, 5, and 6 corners, using finite difference time domain (FDTD) method. In contrast to plasmonic resonances in the visible range, the UV-band resonance intensity was found to scale with the number of corners, which is related with higher order multipole interactions. Spectral positions and relative intensities of the dual resonances were analyzed parametrically to find optimal conditions to maximize EOT in UV-visible dual bands.</P>
Enhancement of Surface Wettability by Intra-Helium Plasma for Liquid Core Fiber Lens
Kassani, Sahar Hosseinzadeh,Khazaeinezhad, Reza,Nazari, Tavakol,Wonho Choe,Kyunghwan Oh IEEE 2014 IEEE photonics technology letters Vol.26 No.20
<P>We generated an atmospheric pressure helium plasma inside a silica hollow optical fiber (HOF) and the surface wettability of the inside wall was flexibly controlled by the plasma intensity. By filling the glycerol into the HOF, we experimentally investigated the surface characteristics by contact angle measurements. Liquid-fiber optic lenses with various numerical apertures were proposed and the light intensity patterns through liquid core lens to the air were investigated numerically.</P>