• Journal of the Optical Society of Korea
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• v.13 no.1
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• pp.28-32
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• 2009

In order to obtain high quality images of thin objects, we performed an experiment of proton radiography by using low energy protons generated from the interaction of an ultrashort ultraintense laser with solid targets. The protons were produced from a thin polyimide target irradiated by the laser pulse, and their maximum energy was estimated at up to 1.8 MeV. A CR-39 nuclear track detector was used as a proton radiography screen. The proton images were obtained by using an optical microscope and the spatial resolution was evaluated by a Modulation Transfer Function (MTF). We have achieved about $10\;{\mu}m$ spatial resolution of images. The obtained spatial resolution shows about $4{\sim}5$ times better value than the conventional X-ray radiography for inspection or non-destructive test (NDT) purpose.

• Proceedings of the Optical Society of Korea Conference
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• 2009.02a
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• pp.335-336
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• 2009

• Journal of Communications and Networks
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• v.15 no.5
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• pp.469-475
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• 2013

In this paper, we propose an efficient interference management technique which controls the number of resource blocks (or subcarriers) shared with other cells based on statistical interference levels among cells. The proposed technique tries to maximize average throughput of a femto cell user under a constraint on non-real time control of a femto cell network while guaranteeing a target throughput value of a macro cell user. In our proposed scheme, femto cells opportunistically use resource blocks allocated to other cells if the required average user throughput is not attained with the primarily allocated resource blocks. The proposed method is similar to the underlay approach in cognitive radio systems, but resource block sharing among cells is statistically controlled. For the statistical control, a femto cell sever constructs a table storing average mutual interference among cells and periodically updates the table. This statistical approach fully satisfies the constraint of non-real time control for femto cell networks. Our simulation results show that the proposed scheme achieves higher average femto user throughput than conventional frequency reuse schemes for time varying number of users.

• Journal of Communications and Networks
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• v.14 no.3
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• pp.252-256
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• 2012

We consider resource allocation in femto-cell networks to maximize the throughput while minimizing interference to macro-users nearby. This can be achieved by allocating spectrum resource in a cognitive radio way. The proposed resource allocation is performed in two steps; spectrum sensing and resource scheduling. The femto base station detects idle frequency assignments (FAs) free from the occupation by macro-users and then allocates sub-channels in an idle FA to femto-users, effectively managing the interference problem. Finally, the effectiveness of the proposed scheme is verified by computer simulations.

• Journal of the Optical Society of Korea
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• v.13 no.1
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• pp.53-59
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• 2009

A chirped-pulse amplification Ti:sapphire laser system has been designed using a 10-Hz 100-TW Ti:sapphire laser to generate 0.1-Hz 0.5-PW laser pulses and optimize their temporal qualities such as temporal contrast and pulse duration. A high-energy booster amplifier to be added is expected to produce an energy above 30 J through the parasitic lasing suppression and the efficient amplification. To improve the temporal contrast of the laser pulses, a high-contrast 1-kHz amplifier system is used as a front-end. A grating stretcher which makes the laser pulse have 1-ns duration is used to prevent optical damages due to high pulse energy during amplification. A grating compressor has been designed with group delay analysis to obtain the recompressed pulse duration close to the transform-limited pulse duration. The final laser pulses are expected to have energy above 20 J and duration below 40 fs.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2005.06a
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• pp.1179-1180
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• 2005

3D micro scaled shapes are fabricated with the method of direct writing and superposing grooving in ambient air using femto-second laser pulses and copper, aiming at establishing an industrially useful femto-second laser processing machine to be able to fabricate three dimensional micro-scale structures, especially micro scaled molds, and processing techniques. For the several advantages, there is no thermally influenced region around the area irradiated by the laser beam and surfaces irradiated laser beam are smooth and substances ablated to form are no attached on the surface of works and so on, the femto-second laser technology is anticipated for advanced micro/nano precision technology.

• Journal of the Optical Society of Korea
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• v.13 no.1
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• pp.8-14
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• 2009

In a laser-plasma wakefield accelerator, the ponderomotive force of an ultrashort high intensity laser pulse excites a longitudinal wave or plasma bubble in a way similar to the excitation of a wake wave behind a boat as it propagates on the water surface. Electric fields inside the plasma bubble can be several orders of magnitude higher than those available in conventional RF-based particle accelerator facilities which are limited by material breakdown. Therefore, if an electron bunch is properly phase-locked with the bubble's acceleration field, it can gain relativistic energies within an extremely short distance. Here, in the bubble regime we show the generation of stable and reproducible sub GeV, and GeV-class electron beams. Supported by three-dimensional particle-in-cell simulations, our experimental results show the highest acceleration gradients produced so far. Simulations suggested that the plasma bubble elongation should be minimized in order to achieve higher electron beam energies.

• Journal of the Optical Society of Korea
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• v.13 no.1
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• pp.15-21
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• 2009

High-order harmonic generation from a solid surface affected by the contrast of a laser pulse was studied using an oscillating mirror model. High-order harmonics generated from solid surfaces have unusual properties such as spectral redshift, and an intensity difference between even- and odd-order harmonics which is not reported for high-order harmonics generated by a gas medium. We confirmed that high-order harmonics from solid surfaces have selectivity of polarization as well as cut-off extension and the enhancement of conversion efficiency proportional to laser intensity. And the principle of operation and the characteristics of a plasma mirror system developed for achieving high contrast laser pulses to pursue the experimental realization of high-harmonic generation from solid surfaces are reported. Energy fluence on the plasma mirrors is tunable between $10\;J/cm^2$ and $1000\;J/cm^2$ and around 1000 shots are available before the plasma mirrors require replacement.

• Korean Journal of Optics and Photonics
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• v.17 no.2
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• pp.203-206
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• 2006

Femtosecond laser-induced nano-patterning of an Al surface coated on a slide glass is reported in this paper. It was found that the period of the laser-induced nano-patterning was much dependent on the incident laser power and pulse number. Through finely adjusting the laser power and pulse number, uniform nano-patterns could be formed on the Al surface. It is based on the interference of the incident laser beam with some form of a surface scatted electromagnetic wave. It was also found that an Al oxide layer played an important role in forming the nano-patterning on the Al surface.

• Journal of Advanced Navigation Technology
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• v.16 no.3
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• pp.471-479
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• 2012

In cellular femto systems, femto Base stations(f-BSs) can be installed unnecessarily and overcrowded in small areas. This will cause an interference problem and it can impact on the capacity, blocking probability, and coverage of femtocells in the shared channel systems. In this paper, we propose a load distribution scheme by using forced handover and probabilistic subchannel scheduling policy to resolve the problem. The proposed scheme operates in distributed manner though communication with neighboring f-BSs, and includes self-detection of overcrowded area and radio resource management based on measurements. We evaluate the performance of the proposed scheme in terms of average cell throughput and average throughput per users.