Man Seop Lee

Migration to the Next Generation Optical Access Networks Using Hybrid WDM/TDM-PON

Due to the rapid growth of Internet and multimedia services, broadband fiber access technologies, such as Passive Optical Networks (PONs), come to the forefront of the research field. Network service providers mostly deploy Time Division Multiplexed (TDM)-PON everywhere in the world. In order to mitigate the future demand, researchers have investigated some next-generation PON systems, such as Wavelength Division Multiplexed (WDM)-PON and hybrid WDM/TDM-PONs. In this report, we propose the architecture of a self-restored hybrid WDM/TDM-PON. Due to the restorable capacity of the architecture, the availability of the PON system increases. Furthermore, the proposed architecture is a cost effective solution compared to the existing restorable PON architectures . Finally, we analyzed the power budget and signal quality of the proposed architecture , which proves the applicability of the proposed architecture to the access networks.

Formation mechanism of self-organized nanogratings on a titanium surface using femtosecond laser pulses

Abstract. We demonstrate the formation of self-organized nanogratings on a titanium surface under the irradiation of a single-beam femtosecond laser. Self-formed, periodic nanogratings are printed on a titanium surface by varying the average pulse energy, pulse width, and number of laser pulses in each spot. The direction of the nanogratings is perpendicular to the direction of the laser polarization. The nanograting period shows obvious dependence on the average pulse energy, pulse width, and number of laser pulses. The period of the self-organized nanogratings shows an increasing trend with the increase of laser energy and pulse width, and a decreasing trend with an increase of number of applied laser pulses. We qualitatively explain the formation mechanism of the self-organized nanogratings and their dependence on various laser parameters.

Migration to the next generation passive optical network

Due to the rapid growth of Internet with new generation of services and applications, demand for faster and cheaper access network has been rising. To address the present and future demand, broadband fiber access technologies such as passive optical networks (PONs) are a potential solution. Mostly, time division multiplexed (TDM)-PON is deployed in all parts of the world. In order to mitigate the future demand, some next-generation PON systems have been investigated by the researchers. In this paper, we examine the current status of PONs and investigate the probable future PONs. We also explain the smooth migration process from the current status to the future technologies. Architecture of a self-restored tree-type hybrid wavelength division multiplexed/TDM-PON (WDM/TDM-PON) has been proposed, for migrating from TDM to WDM-PON. Due to the restorable capacity of the architecture, the availability of the system is increased. In addition, cost analysis of different PON architectures are performed and compared with the cost of the proposed architecture. It is found that, the proposed architecture provides more cost effective solution.

Micromachining of Grooves for Cutting Fused Silica Plates with Femtosecond Laser Pulses

Micromachining of grooves and their morphologies for cutting fused silica plates are reported in this paper. A femtosecond laser (pulse width: 172 fs, wavelength: 780 nm, and repetition rate: 1 kHz) is used to machine grooves. Ablations are investigated varying pulse energies, scanning speeds and number of consecutive passes.

Femtosecond laser based in-fiber long period grating fabrication for improved solution sensing

This work reports femtosecond laser based fabrication of long period fiber gratings (LPFG). Index modulation in the core of single mode fiber (SMF) is written employing femtosecond pulse filamentation technique. Highly repeatable filamentary voids written in line-by-line femtosecond laser inscription technique enables steady and noise free growth of LPFGs. The sharp transmission valley (with a narrow full width at half maximum of 5 nm) of long period grating offers better resolution for refractive index (RI) measurement of a solution. The LPFGs inscribed by femtosecond laser radiation show RI sensing sensitivity of 29.199 nm/RIU which is three times higher than the sensitivity of LPFGs written by UV radiation (sensitivity: 11.179 nm/RIU). The position of transmission dip of a grating can be tailored relatively easily simply by varying the period of index modulation.

Characterization of femtosecond laser filament-fringes in titanium

In this paper, we characterize the femtosecond laser filament-fringes in titanium. In order to fabricate regular arrays of filaments, we place either a pinhole or a beam shaper in the optical path of the femtosecond laser beam that originates linear diffraction of the laser beam. Soda-lime glass is used as Kerr medium to produce the filaments. As a consequence, the intensity distribution of the laser beam is modulated and fringe type of filament distributions is evident. The suitable control over the size of the diaphragms (pinhole or beam shaper) leads us to adjust the shape, orientation, and number of filaments in each irradiated spots in titanium sample. By properly adjusting the diameter of a pinhole that was placed in the optical path, we are successful in forming a single filament in titanium. By using these single filaments, we fabricated high aspect ratio periodic holes in the titanium surface by moving the translation stage in both horizontal and vertical directions. The period of the holes in the horizontal direction is controlled by varying the scanning speed, whereas the period in the vertical direction is controlled by varying the vertical scanning step. We strongly believe that, filamentation technology described in this paper will have applications in forming a variety of micro/nano-structures in various materials.

Fabrication of micro/nano-fluidic channels by single-beam direct femtosecond laser writing

In this paper, we report on the formation of micro/nano-fluidic channels inside fused silica glass using single-beam femtosecond laser. The micro/nano-fluidic channels are fabricated by controlling the irradiation conditions of the femtosecond laser pulses, especially, pulse energy and scanning speed. We examine the production of this kind of channels both in air and water. In both cases, laser beam is focused inside the glass bar and shined horizontally with very low scanning speeds. In case of water, the glass sample is placed inside distilled water, which way is expected to reduce the surface roughness of the channels. The quality of the channels fabricated under different environment is compared as well. We further investigate the influence of various laser parameters on the production of channels. We also evaluate the fluid flowing ability of the fabricated micro/nano-fluidic channels of various diameters, fabricated under different environment and irradiation conditions.

Characterization of femtosecond laser filamentation in soda-lime glass

This paper demonstrates the characterization of femtosecond laser filamentation in soda-lime glass. We achieved femtosecond pulse filamentation of variable height and width by focusing the femtosecond laser beam in different layers inside the sample glass. The dielectric medium immediately before the target position increases the effective numerical aperture of the objective lens that causes the elongation of focal length of the laser beam in a tiny focal spot with high power density. As a result, high aspect ratio voids are engraved inside the glass samples. Furthermore, we investigate the characteristics of the fabricated voids and their dependence on the irradiation conditions of the laser beam and associated optical setup. The height and width of the voids are controlled by varying the laser power, numerical aperture & magnification of the objective lens, and width of the dielectric medium before laser focusing position. Using high power femtosecond laser pulse filaments, we fabricated very long voids inside the glass sample, which undergoes cleaving process with uniform mechanical pressure for smooth cutting of the glass substrate.

Nanostructure covered micro holes fabrication by femtosecond laser on fused silica glass surface

We report on the fabrication of nanostructure covered micro holes on the surface of fused silica glass by irradiation with single-beam direct femtosecond laser pulses. Nano-bumps or nano-pillars were formed on the side wall near the surface as well as near the bottom of the micro holes with a diameter down to 100nm. On the top of the nano-bumps, grouped and spherical shaped nanostructures down to 10 nm were observed. Nano-gratings with an average period of 175 nm-200 nm were found at the bottom of the holes, which is less than λi/2η period (η is the refractive index of the material).

Far rear surface focusing of femtosecond laser pulses for dicing display glass plates

A potential method for precise and fast dicing of display glass plates is proposed in this study. This technique facilitates the micromachining of cavities in both front and rear surfaces for a single pass of laser beam. The influences of focusing depth, input pulse energy, and scanning speed of the laser beam are investigated to study the morphology of the front and rear surface cavities. A commercial femtosecond laser with pulse duration of 172 fs, center wavelength of 780 nm, and repetition rate of 1 kHz is used for introducing the cavities.