Sanguk Park

Hybrid mode-locked Er-doped fiber femtosecond oscillator with 156 mW output power

We report on an Er-doped fiber oscillator that produces 146 fs pulses with 156 mW average power at a repetition rate of 49.9 MHz. The pulse energy reaches 3.13 nJ, surpassing the conventional power limit in the dispersion-managed soliton regime. Such high pulse power is obtained by devising a hybrid mode-locking scheme that combines saturable absorption with nonlinear polarization evolution. The oscillator also offers excellent temporal purity in the generated pulses with high power, providing a robust fiber-based frequency comb well suited for industrial uses.

Wireless Digital Packet Communication and Analog Image Communication Systems for Fire Fighting Robot

Frequent occurrences of a fire cause tremendous loss of human lives and their property. Recently, in order to cope with such catastrophic accidents, researches on fire-fighting robots are carried out in developed countries. Under the dangerous situations, it is sometimes impossible for fire-fighting men to access the firing place because of explosive materials, smoke, high temperature and so on. In such an environment, fire-fighting robots can be useful to extinguish the fire. It is usually very dangerous place where fire-fighting robots operate. Hence, these robots should be controlled by remote users who are for away from the firing place exploiting remote communication systems. This paper considers the communication systems between fire-fighting robots and remote users. The communication systems consist of two parts; digital packet communication systems and analog image communication systems. Digital packet communication systems transfer data packets in order to control fire-fighting robots and to check the state of the fire-fighting robots. Remote users watch the video around the fire-fighting robots by exploiting the analog image communication systems. In the future, the more prosperous the commercial communication network systems will be, the more evolved the communication systems for fire-fighting robots are.

Large-aperture ground glass surface profile measurement using coherence scanning interferometry

We present a coherence scanning interferometer configured to deal with rough glass surfaces exhibiting very low reflectance due to severe sub-surface light scattering. A compound light source is prepared by combining a superluminescent light-emitting diode with an ytterbium-doped fiber amplifier. The light source is attuned to offer a short temporal coherence length of 15 μm but with high spatial coherence to secure an adequate correlogram contrast by delivering strongly unbalanced optical power to the low reflectance target. In addition, the infrared spectral range of the light source is shifted close to the visible side at a 1,038 nm center wavelength, so a digital camera of multi-mega pixels available for industrial machine vision can be used to improve the correlogram contrast further with better lateral image resolutions. Experimental results obtained from a ground Zerodur mirror of 200 mm aperture size and 0.9 μm rms roughness are discussed to validate the proposed interferometer system.

Hybrid femtosecond fiber laser outcrossing Er-doped fiber and Yb-doped fiber

A hybridized scheme of a fiber femtosecond pulse laser was devised with the aim of grafting the frequency comb of an Er-doped fiber oscillator, stabilized around a 1.550 μm center wavelength, onto the 1.0 μm emission range of an Yb-doped fiber amplifier. Test results showed that the frequency comb is successfully transferred to a new 1.034 μm center wavelength with a spectral bandwidth of 21 nm, upholding an original frequency stability of 3.71 × 10–13 at 10 s averaging. This work demonstrates the feasibility of outcrossing different kinds of fibers to shift the spectral range of the frequency comb over a large operating span without loss of stability.

Low-repetition-rate fiber femtosecond laser oscillator

We demonstrate a ring type, mode-locked all-normal-dispersion ytterbium-doped fiber laser which have 9.8 MHz repetition rate through the combination of saturable absorber mirror and nonlinear polarization evolution (NPE) phenomena.

Wireless image communication system for fire-fighting robots

Recently, it has sometimes been impossible for fire-fighting personnel to access the site of a fire, even as the fire causes tremendous property damage and loss of human life, due to high temperatures or the presence of explosive materials. In such environments, fire-fighting robots can be useful for extinguishing a fire, and they should be controlled by remote operators. In order to help a remote operator who is located far away from the fire-fighting robot, wireless image communication systems have been investigated in this study. The developed system uses a High-Speed Downlink Packet Access (HSDPA) network to transmit images, showing the view from a fire-fighting robot. The fire-fighting robots and remote controllers are equipped with industrial computers, and image-transmitting and image-receiving programs function according to various protocols to allow communication between the robots and controllers

A New Channel Estimation Method Using Phase-Shifted Pilot Sequence for MIMO-OFDM Systems

In this paper, we propose a new MIMO-OFDM channel estimator with a phase-shifted pilot (PSP) sequence to enhance the performance and overcome the problems of conventional schemes. The proposed scheme overcomes conventional PSP estimators limitation of requiring a uniformly distributed pilot position and performs better by adopting a minimum mean square error criterion that cannot be applied to conventional schemes. The proposed PSP channel estimator can perfectly separate multiple channel impulse responses, even though all transmit antennas use the same pilot position. Furthermore, the simulation results show that the performance of the proposed scheme is close to lower bound of the mean square error.