Kyunghwan Oh

Novel hollow optical fibers and their applications in photonic devices for optical communications

Novel photonic devices based on a new type of waveguide, hollow optical fibers (HOF), are described. Utilizing unique three layered structure of HOF, the central air hole, germanosilicate ring core, and silica cladding along with its adiabatic mode transformation capability we demonstrated versatile applications in short-haul, long-haul optical communications, and tunable wavelength selective devices. Detailed design parameters, fabrication arts of the fibers, and operation principles of the devices are discussed.

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.

Electric and Photovoltaic Behavior of a Few‐Layer α‐MoTe2/MoS2 Dichalcogenide Heterojunction

Mo-based van der Waals heterojunction p-n diodes with p-type α-MoTe2 and n-type MoS2 are fabricated on glass, and demonstrate excellent static and dynamic device performances at a low voltage of 5 V, with an ON/OFF current ratio higher than 10(3) , ideality factors of 1.06, dynamic rectification at a high frequency of 1 kHz, high photoresponsivity of 322 mA W(-1) , and an external quantum efficiency of 85% under blue-light illumination.

Compact three segmented multimode fibre modal interferometer for high sensitivity refractive-index measurement

We experimentally demonstrate a new compact surrounding refractive-index sensor using a MMF-CSF-MMF (MCM) structure. The evanescent waves in the coreless silica fibre (CSF) region can directly interact with the surrounding medium. Due to its distinctive resonant spectral feature, the MCM sensor showed a high sensitivity of 4.37 × 10−4 for a refractive index range of 1.30 to 1.44, suggesting that the proposed device may be suitable for a wide range of biomedical and chemical sensor applications.

Ultracompact in-line broadband Mach-Zehnder interferometer using a composite leaky hollow-optical-fiber waveguide.

We report on an ultracompact fiber optic Mach-Zehnder interferometer (MZI) using a novel concatenated structure, single-mode-fiber (SMF)-hollow optical fiber (HOF)-SMF. An optimized air-hole size in the HOF provided two unique optical paths; the leaky waveguide modes through the central air hole and the radiation channel into the silica cladding, which enabled a compact MZI configuration with enhanced optical performances, such as high visibility, high temperature sensitivity, and wide dynamic range.

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.

Continuous-wave oscillation of thulium-sensitized holmium-doped silica fiber laser.

We have observed oscillation in a Tm(3+) and Ho(3+) codoped silica fiber from the (5)I(7) ? (5)I(8) transition of Ho(3+) in the 2-microm region, by pumping the fiber laser into a Tm(3+) absorption between 800 and 830 nm. By a change in the cavity length, the laser was tunable between 2.037 and 2.096 microm. When the laser was pumped with a Ti:sapphire laser at 820 nm, an absorbed threshold power extracted power of 214mW, a slope efficiency of 4.2%,and maximum extracted power of 12.5 mW were measured.

Elliptical defected core photonic crystal fiber with high birefringence and negative flattened dispersion

We propose a novel design of photonic crystal fiber (PCF) using an elliptical air hole in the core as a defected core in order to enhance the performance of modal birefringence and to control the properties of chromatic dispersion at the same time. From the simulation results, it is shown that the proposed fiber has high birefringence up to the order of 10(-2), negative flattened chromatic dispersion in a broad range of wavelengths, and low confinement loss less than that of the single mode fiber. The outstanding advantage of the proposed PCF is that high birefringence, negative flattened dispersion, and low confinement loss can be achieved just by adding a small sized elliptical air hole in the core to the elliptical air hole PCF, especially at the same time.

Electrically controllable long-period liquid crystal fiber gratings

An electrically controllable fiber-optic filter for broad-band rejection has been demonstrated by periodically poling a liquid-crystalline core in a hollow-core fiber by means of an external long-period-combed electrode. The periodically poled liquid-crystalline core in a period of 483 /spl mu/m couples the fundamental core mode to the leaky cladding modes. A maximum loss dip of approximately 15-nm bandwidth and 6 dB band rejection has been obtained for a nonpolarized light with a combed 250-V external voltage modulation. The experimental result matches well with a numerical expectation, which has been analyzed by the discretized coupled-mode theory for an anisotropic perturbation.