Yongjoo Rhee

Simple method for the temporal characterization of amplified spontaneous emission in femtosecond terawatt Ti:sapphire lasers

We have analyzed the temporal characteristics of amplified spontaneous emission (ASE) in femtosecond terawatt Ti:sapphire lasers by using a simple method based on fast photodiodes. Instead of measuring ASE directly with fast photodiodes, we created a narrow gap in the spectrum of seed pulses and, after amplification, detected the pure ASE signal through the gap by using a fast photodiode covered with a bandpass filter with high transmission at the gap. Because the detected ASE signal was completely separated from amplified main pulses, preceding and even trailing ASEs could be characterized quantitatively in a single-shot measurement. We believe that our method is a good alternative or a complement to conventional methods for ASE measurements.

High-resolution spectroscopy of Sm I performed with an extended-cavity violet diode laser

We performed high-resolution spectroscopy of Sm I in the near-UV transitions by using an extended-cavity violet diode laser. By adopting traditional Doppler-free saturated-absorption spectroscopy, we made accurate measurements of the isotope shifts for Sm I. For an atomic excitation laser we used an extended-cavity violet diode laser that was operated in the wavelength range 398–400 nm. As for the experimental results, Doppler-free spectra of the 399.002- and 399.102-nm transition lines of Sm were obtained and the isotope shifts between the even-mass isotopes were measured for the first time to our knowledge. Additionally, we used the King-plot method to check the consistency of the measured isotope shifts and to analyze them as mass shifts and field shifts. For a more quantitative analysis, we performed an ab initio calculation by using the relativistic multiconfiguration Dirac–Fock method.

Silver-mirror-based multipass preamplifier for a broadband terawatt Ti:sapphire laser.

We have developed a silver-mirror-based multipass preamplifier for a broadband amplification in a terawatt Ti:sapphire laser. With the extremely broad bandwidth of the silver mirrors, a very broad amplified spectrum can be generated at an amplified energy of 4 mJ; the amplified spectral width is 65 nm at half maximum and 160 nm at -25 dB without any spectral shaping technique. Such a broad amplification can be explained well by the simulation that includes gain narrowing and gain saturation. Even after a further amplification to an energy of 600 mJ, the amplified spectrum is broad enough to support an approximately 20 fs transform-limited pulse duration.

Optical Parametric Chirped - pulse Amplification of Femtosecond Ti : sapphire Laser Pulses by Using a BBO Crystal

We have characterized the optical parametric chirped-pulse amplification of femtosecond Ti:sapphire laser pulses by using a BBO crystal. It is numerically verified that a high gain and a broad gain bandwidth can be obtained with a 532-nm pump laser. The dependence of the gain profile of OPA on phase matching angles, pump intensity, and crystal length is numerically investigated. Experimental results shows that the temporal fluctuation of a pump laser causes the modulation of an amplified spectrum in OPCPA.

Sequence effect of optical elements in a femtosecond Ti:sapphire laser oscillator

We have numerically and experimentally verified that the sequence of optical elements in a femtosecond Ti:sapphire laser affects the laser’s characteristics. Even with the same net dispersion in a cavity, the output spectrum showed a significant change in width and shape when the sequence of dispersive elements was changed in a Kerr-lens mode-locked Ti:sapphire laser with fused-silica prisms for dispersion compensation. It was also shown that the output coupler’s position can affect the dynamics of pulse formation by the combined action of output coupling and self-phase modulation. We explain such phenomena based on pulse formation in a laser cavity with discrete distribution of dispersion and self-phase modulation.

Development and Characterization of a 1-kHz Self-Seeding-Type Ti:Sapphire Laser Oscillator

We have developed and characterized a 1-kHz self-seeding-type pulsed Ti:sapphire laser oscillator pumped by a frequency-doubled Nd:YAG laser. It is a dual-cavity laser composed of a grazing-incidence cavity and a feedback mirror. We found that the narrow-linewidth lasing can be achieved even though the grazing-incidence cavity for seeding does not lase separately without the feedback mirror. The maximum average output power is 1 W (1 mJ/pulse) at a 9-W (9 mJ/pulse) pumping, and the linewidth is less than 380 MHz.

Self-Induced Transparency Phenomena of Single-Mode Pulse Laser Propagating in a Thick Ytterbium Vapor Column

The variations in pulse shape of a single-mode pulse laser propagating through ytterbium (Yb) atomic medium were investigated experimentally. The laser pulse shape was changed as a function of pulse area and it could be explained well using the area theorem. When the pulse area was close to 2π, the pulse shape was transformed to a hyperbolic secant function after transmission. The transmission curve of the propagating laser vs. the pulse area demonstrated self-induced transparency phenomena well. However, the transmission was markedly reduced when the atomic density was increased above ~4×1010 atoms/cm3. Several causes; such as collision-induced polarization relaxation, Doppler broadening, laser incoherence, and power broadening, which could induce reduced transmission, were examined. It was the incoherence of the laser that caused reduced transmission because the laser line width was broader than the Fourier-transform limit, and the absorbed energy dissipated through spontaneous emission.

Neutron yield dependence on deuterium ion energy distribution from deuterated methane clusters with an intense femtosecond laser

Neutron generation experiment with deuterated methane (CD4) clusters exposed to a femtosecond laser beam (620 mJ, 28 fs) was done. The relation of ion energy distribution to cluster size was studied. By analyzing TOF ion signals, neutron yield was estimated to be 2 × 105 per laser shot.

Fast neutron emission from a deuterated polystyrene solid target irradiated by a high-intensity laser pulse

Fast neutrons were generated from a deuterated polystyrene (C8D8) solid-target irradiated by a high-intensity laser. A TOF neutron signal is analyzed in detail. A neutron yield of 1206 n/pulse is estimated from a CR-39 neutron detection, and a maximum deuterium ion energy of 76 keV is measured from a Faraday cup signal. The employed laser has a 27-fs pulse width and a 260-mJ pulse energy.

Neutron generation induced by laser fusion by using a 10TW femtosecond Ti:Sapphire laser

A 10 TW Ti:sapphire laser is used to generate DD fusion neutrons and the characteristics of plasma has been investigated. Neutrons are generated from deuterium clusters which are formed from deuterium gas jet when cooled down to liquid nitrogen temperature.