Sahar Hosseinzadeh Kassani

Mode-locking of Er-doped fiber laser using a multilayer MoS2 thin film as a saturable absorber in both anomalous and normal dispersion regimes.

Application of a multilayer Molybdenum Disulfide (MoS2) thin film as a saturable absorber was experimentally demonstrated by realizing a stable and robust passive mode-locked fiber laser via the evanescent field interaction between the light and the film. The MoS2 film was grown by chemical vapor deposition, and was then transferred to a side polished fiber by a lift-off method. Intensity-dependent optical transmission through the MoS2 thin film on side polished fiber was experimentally observed showing efficient saturable absorption characteristics. Using erbium doped fiber as an optical gain medium, we built an all-fiber ring cavity, where the MoS2 film on the side polished fiber was inserted as a saturable absorber. Stable dissipative soliton pulse trains were successfully generated in the normal dispersion regime with a spectral bandwidth of 23.2 nm and the pulse width of 4.98 ps. By adjusting the total dispersion in the cavity, we also obtained soliton pulses with a width of 637 fs in the anomalous dispersion regime near the lasing wavelength λ = 1.55 μm. Detailed and systematic experimental comparisons were made for stable mode locking of an all-fiber laser cavity in both the normal and anomalous regimes.

All-fiber Er-doped Q-Switched laser based on Tungsten Disulfide saturable absorber

We demonstrated a Q-switched fiber laser based on Tungsten Disulfide (WS2) saturable absorber. The WS2 nano-sheets were prepared by liquid phase exfoliation method and the saturable absorber was fabricated by spin-coating of few-layer WS2 nano-sheets on a side-polished fiber for pulsed operation of a fiber laser. By inserting the absorber into an Erbium-doped fiber laser cavity pumped by a 980 nm laser diode, a stable Q-switched laser operation was achieved with a tunable repetition rates from 82 kHz to 134 kHz depending on the applied pump power. The properties of the deposited WS2 film was examined using scanning electron microscopic (SEM) and atomic force microscope (AFM). Detailed optical properties of the laser output are also discussed.

Flexible transition metal dichalcogenide nanosheets for band-selective photodetection.

The photocurrent conversions of transition metal dichalcogenide nanosheets are unprecedentedly impressive, making them great candidates for visible range photodetectors. Here we demonstrate a method for fabricating micron-thick, flexible films consisting of a variety of highly separated transition metal dichalcogenide nanosheets for excellent band-selective photodetection. Our method is based on the non-destructive modification of transition metal dichalcogenide sheets with amine-terminated polymers. The universal interaction between amine and transition metal resulted in scalable, stable and high concentration dispersions of a single to a few layers of numerous transition metal dichalcogenides. Our MoSe2 and MoS2 composites are highly photoconductive even at bending radii as low as 200 μm on illumination of near infrared and visible light, respectively. More interestingly, simple solution mixing of MoSe2 and MoS2 gives rise to blended composite films in which the photodetection properties were controllable. The MoS2/MoSe2 (5:5) film showed broad range photodetection suitable for both visible and near infrared spectra.

Ultrafast Pulsed All-Fiber Laser Based on Tapered Fiber Enclosed by Few-Layer WS 2 Nanosheets

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.

Suspended Ring-Core Photonic Crystal Fiber Gas Sensor With High Sensitivity and Fast Response

We demonstrate the design and fabrication of an in-line evanescent field gas sensor based on a novel suspended ring-core photonic crystal fiber (PCF) to further overcome limitations in sensing capability. The suspended ring-core PCF is made by the stack-and-draw technique with a large central capillary of ~60 μm and adapted small capillaries in a larger cladding tube. Sensing capabilities of two variations of the proposed suspended ring-core PCFs were investigated experimentally by detecting absorption lines of acetylene gas. The sensitivity of the two PCF structures was also numerically analyzed using a full-vectorial finite-element method. The dynamic response of the sensors was quantified in detail. The results demonstrated significant improvement in both sensitivity and response time of the suspended ring-core PCF as this special PCF can provide larger gas diffusion and higher overlap between the sensing species and the guided light than either a Ge-doped ring defect PCF or a conventional index-guided PCF.

Femtosecond Soliton Pulse Generation Using Evanescent Field Interaction Through Tungsten Disulfide (WS 2 ) Film

We investigated nonlinear optical characteristics of Tungsten disulfide (WS2) 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 WS2 film was overlaid to provide an efficient evanescent field interaction. The WS2 film was prepared using two methods: liquid phase exfoliation to form few-layer nano-sheets, and chemical vapor deposition (CVD) to grow uniform multilayer WS2 on a SiO2 substrate. Two SPF saturable absorbers were prepared by either spin coating WS2 solution or lifting off the multilayer CVD WS2 on SPF. An all-fiber ring cavity was built and the WS2 film overlaid on SPF was employed as a mode locker along with Er-doped fiber as a gain medium. Using the spin-coated WS2 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 WS2 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 WS2 film as a novel 2-D nonlinear optical material for laser applications.

Fast response in-line gas sensor using C-type fiber and Ge-doped ring defect photonic crystal fiber

An in-line chemical gas sensor was proposed and experimentally demonstrated using a new C-type fiber and a Ge-doped ring defect photonic crystal fiber (PCF). The C-type fiber segment served as a compact gas inlet/outlet directly spliced to PCF, which overcame previous limitations in packaging and dynamic responses. C-type fiber was prepared by optimizing drawing process for a silica tube with an open slot. Splicing conditions for SMF/C-type fiber and PCF/C-type fiber were experimentally established to provide an all-fiber sensor unit. To enhance the sensitivity and light coupling efficiency we used a special PCF with Ge-doped ring defect to further enhance the sensitivity and gas flow rate. Sensing capability of the proposed sensor was investigated experimentally by detecting acetylene absorption lines.

Mode-Locked All-Fiber Lasers at Both Anomalous and Normal Dispersion Regimes Based on Spin-Coated

We demonstrate an all-fiberized mode-locked laser by employing Molybdenum Disulfide (MoS2) spin-coated onto a side-polished fiber (SPF) as an in-line saturable absorber. Single crystal MoS2 has been exfoliated via the Li intercalation method and then dispersed with large population of few-layer MoS2 nano-sheets in ethanol. Subsequently, a saturable absorber has been prepared using a relatively simple method: spin-coating the uniform MoS2 solution on the fabricated SPF without using any polymer or toxic procedure. Power-dependent transmission property of the prepared MoS2 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 MoS2 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 MoS2 saturable absorbers and applications for pulsed laser operation.

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A new extraordinary application of deoxyribonucleic acid (DNA) thin-solid-film was experimentally explored in the field of ultrafast nonlinear photonics. Optical transmission was investigated in both linear and nonlinear regimes for two types of DNA thin-solid-films made from DNA in aqueous solution and DNA-cetyltrimethylammonium chloride (CTMA) in an organic solvent. Z-scan measurements revealed a high third-order nonlinearity with n2 exceeding 10−9 at a wavelength of 1570 nm, for a nonlinarity about five orders of magnitude larger than that of silica. We also demonstrated ultrafast saturable absorption (SA) with a modulation depth of 0.43%. DNA thin solid films were successfully deposited on a side-polished optical fiber, providing an efficient evanescent wave interaction. We built an organic-inorganic hybrid all-fiber ring laser using DNA film as an ultrafast SA and using Erbium-doped fiber as an efficient optical gain medium. Stable transform-limited femtosecond soliton pulses were generated with full width half maxima of 417 fs for DNA and 323 fs for DNA-CTMA thin-solid-film SAs. The average output power was 4.20 mW for DNA and 5.46 mW for DNA-CTMA. Detailed conditions for DNA solid film preparation, dispersion control in the laser cavity and subsequent characteristics of soliton pulses are discussed, to confirm unique nonlinear optical applications of DNA thin-solid-film.

Nano-Sheets on a Side-Polished Fiber

We investigated nonlinear characteristics and applications of WS 2 for mode-locked fiber laser. The saturable absorber was prepared by transferring the synthesized CVD WS 2 onto a fabricated side-polished fiber. WS 2 showed promising potential for ultrafast-pulse generation.