• Korean Journal of Optics and Photonics
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• v.10 no.2
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• pp.146-151
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• 1999

We demonstrated a harmonic mode locking scheme that used regeneratie phase modulation to get a high and stable repetition rate in a figure-of-eight fiber soliton laser. From the detected beat spectra of the laser output, a sinusoidal clock freguency tone of 400 MHz, the 96th harmonics of the fundamental mode locking frequency, was extracted with a high Q filter and was used to drive the phase modulator, resulting in stable output of soliton pulse train synchronized with the modulation signal. Generated soliton pulses had FWHM pulsewidth of 930 fs and 3.1 nm linewidth, yielding pulsewidth-bandwidth product of 0.359 that was close to the transform limit. As the modulation frequency always followed the beat frequency of laser modes, stable harmonic mode locking was achieved without the adjustment of the cavity length, which has been commonly required in actively mode-locked lasers.

• Korean Journal of Optics and Photonics
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• v.13 no.4
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• pp.347-355
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• 2002

We have constructed a high efficiency and broad tunable cw Ti:sapphire laser with a four-mirror symmetric Z-type laser cavity to increase the laser usability. From theoretical analyses and experimental data for a symmetric Z-type laser cavity containing a Kerr medium, the cavity mode size and the Kerr-lens mode-locking (KLM) strength for KLM lasers can be confirmed as function of the position in the cavity, the intracavity laser power, and the stability parameter. As a result, the slope efficiency and the maximum average output power of cw Ti:sapphire laser at 5 W pumping power of Ar-ion laser were 31.3% and 1420 ㎽ respectively. The tunablility was ranged from 730 ㎚ to 908 ㎚ with average output power above 700 ㎽. We obtained the KLM operation easily by self-aperturing effect in the Kerr medium and the slope efficiency and the maximum average output power of KLM Ti:sapphire laser was 16% and 550 ㎽ respectively. The spectral bandwidth was 33 ㎚ at the center wavelength of 807 ㎚ and the pulse width was 27 fs with a repetition rate of 82 ㎒.

• Journal of the Optical Society of Korea
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• v.7 no.3
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• pp.135-138
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• 2003

A chirped-pulse amplification femtosecond Ti:sapphire laser operating at 1 KHz has been developed. The laser system consisted of a long-cavity femtosecond oscillator, a four-pass grating pulse stretcher, two multi-pass amplifiers and a double-pass grating pulse compressor. Thermal lensing at the amplifiers was reduced by cooling Ti:sapphire crystals using Peltier coolers. Gain narrowing and residual phase errors were compensated for by the use of an acousto-optic pulse shaper. The final laser output had an energy per pulse of 2.0 mJ and a pulse duration of 19.5 fs, reaching 0.1 TW at 1 KHz.

• Journal of the Optical Society of Korea
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• v.13 no.1
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• pp.86-91
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• 2009

The electrons injected into a laser wakefield undergo betatron oscillation and give rise to the emission of intense X-ray radiation. To investigate the generation conditions of the X-rays, the relativistic motion of an electron injected in an off-axis position has been simulated with wakefield profiles which are pre-calculated with a two-dimensional particle-in-cell code. The wakefield with a plasma density of $1.78{\times}10^{18}\;cm^{-3}$ is generated by the laser with an intensity of $1.37{\times}10^{18}\;W/cm^2$ and a pulse width of 30 fs. From the calculation of the single particle motion, the characteristics of the betatron radiation are investigated in the time domain. As the transverse injection position increases, the power and the duration time of the radiation increase, but the width of each pulse decreases.

• Korean Journal of Optics and Photonics
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• v.16 no.6
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• pp.490-493
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• 2005

We have experimentally studied supercontinuum generation in a photonic crystal fiber as a function of pump wavelength and intensity with 100 fs pulsewidth. A supercontinuum over 750 nm spectral width with amplitude variation less than 10 dB has been achieved. It was generated by coupling femtosecond pulses from a mode-locked Ti:Sapphire laser into a 2 m long photonic crystal fiber. Adjusting the parameters of the pump source, it was also possible to control different spectral features of the supercontinuum radiation.

• Laser Solutions
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• v.9 no.3
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• pp.15-21
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• 2006

A Polypropylene (PP) film was ablated using a femtosecond laser with a center wavelength of 785 nm, a pulse width of 184 fs and a repetition rate of 1 kHz. Increments of both pulse energy and the shot number of pulses lead to co-occurrence of photochemical and thermal effect, demonstrated by the spatial expansion of rim on the surface of PP. The shapes of the laser-ablated PP films were imaged by a scanning electron microscope (SEM) and measured a 3D optical measurement system (NanoFocus). And, the oxygen transmission rate (ORT) of periodically laser-ablated PP film were characterized by oxygen permeability tester for modified atmosphere packaging (MAP) of fresh fruit and vegetable. Our results demonstrate that femtosecond pulsed laser is efficient tools for breathable packaging films in modifying the flow of air and gas into and out of a fresh produce container, where the micropatterns are specifically tailored in size, location and number which are easily controlled by laser pulse energy and pulse patterning system.

• Laser Solutions
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• v.15 no.3
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• pp.1-6
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• 2012

The friction force of patterned needle in elastomer have been investigated to verify the application for bio and plastic industry. The micro pattern on the needle surface were prepared by 266 nm, 20 ns laser and 800 nm, 220 fs laser, which were able to generate the different surface roughness. The friction force was measured by the load cell of 10 N capacity. As the results, the friction force of no patterned needle is almost constant during the needle penetrates the silicone rubber sample. However, the needle having asperities shows the variation of the friction force. The higher the surface roughness is, the smaller the friction force is until the surface roughness is very high. In our experiment conditions, the reduction of the friction force by 20 % compared to no pattern needle was achieved with straight and $50{\mu}m$ discrete line generated by 266 nm, 20 ns laser.

• Laser Solutions
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• v.14 no.3
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• pp.17-20
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• 2011

We fabricated a multi-layer diffraction grating inside fused silica glass by using a femtosecond laser direct writing method. The femtosecond laser with a wavelength of 515 nm, a pulse width of 250 fs, a repetition rate of 100 kHz, and an average output power of 6 W was used. Two layer diffraction grating with a grating period of $6{\mu}m$ was successfully fabricated with the layer gap of 0.5, 1, 2, 3, and $5{\mu}m$, respectively. Also, we investigated the diffraction pattern by illuminating a He-Ne laser beam. Finally, we demonstrated the diffraction grating with a grating period of $3{\mu}m$ by adjusting the gap of each layer with a grating period of $6{\mu}m$. Femtosecond laser direct writing technology of multi-layer has a potential to fabricate the diffraction grating with a grating period of below $1.5{\mu}m$.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.9
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• pp.1270-1275
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• 2004

Standard positive photoresist techniques were adapted to generate nano-scale patterns of gold substrate using self-assembled monolayers (SAMs) and femtosecond laser. SAMs formed by the adsorption of alkanethiols onto gold substrate are employed as very thin photoresists, Alkanethiolates formed by the adsorption of alkanethiols are oxidized on exposure to UV light in the presence of air to alkylsulfonates. Specifically, it is known that deep UV light of wavelength less than 200nm is necessary for oxidation to occur. In this study, ultrafast laser of wavelength 800nm and pulse width 200fs is applied for photolithography. Results show that ultrafast laser of visible range wavelength can replace deep UV laser source for photo patterning using thin organic films. Femtosecond laser coupled near-field scanning optical microscopy facilitates not only the patterning of surface chemical structure, but also the creation of three-dimensional nano-scale structures by combination with suitable etching methods.

• Journal of the Optical Society of Korea
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• v.20 no.1
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• pp.135-139
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• 2016

I propose a vector passive harmonic mode-locked fiber laser based on topological insulator Bi2Se3 interacting with a fiber taper with a diameter of 7 μm. The particles of topological insulator are deposited uniformly onto the fiber taper with light pressure effect. By incorporating the fabricated saturable absorber into an Er-doped fiber laser cavity, stable mode-locked fiber is obtained. Due to the intense evanescent field of the fiber taper, strong confinement of light enhances the nonlinearity of the laser cavity, and passive harmonic mode-locking is performed. I observe a maximum harmonic mode-locking of 356th, corresponding to a frequency of 3.57 GHz. The pulse duration is 824 fs, and the full width at half maximum of the spectrum is about 8.2 nm. The polarization dependent loss of the saturable absorber is ~ 2.5 dB in the wavelength range of the C band. As the cavity contains no other polarization dependent device, the mode-locked laser is functioning in the vector state. The harmonic order vs pump power is investigated. To the best of our knowledge, this report is the highest frequency mode-locked fiber laser based on Bi2Se3. Experimental results indicate that the topological insulator Bi2Se3 functioning with a thin fiber taper is effective for vector harmonic mode-locking.