Kangin Lee

Reducing temperature dependence of the output energy of a quasi-continuous wave diode-pumped Nd:YAG laser

It is demonstrated by numerical modeling that spectrally dispersed compound pumping diodes and low-loss pumping chamber reduced the temperature dependence of the output energy of quasi-continuous wave diode-pumped Nd:YAG lasers considerably. Several compound diodes with different spectral profiles were tested for pumping. The laser energy was calculated as a function of diode temperature from -30°C to 60°C. When a compound diode with a flat-top spectrum was used for pumping, the mean laser energy was 83% of the maximum energy of a Nd:YAG laser pumped by a diode with a narrow bandwidth. In addition, a compound diode with three emission lines was tested for pumping. When the wavelength gap between the adjacent emission lines of the pumping diode was in the range of 3-10 nm, the mean energy of the Nd:YAG laser became similar to that of a Nd:YAG laser pumped by a diode with a flat-top spectrum.

Laser Beam Shaping Using Hollow Optical Fiber and Its Application in Laser Induced Thermal Printing

A Gaussian beam of single mode fiber laser was changed into a ring-shaped pattern after it was transmitted through the hollow optical fiber. The ring-shaped beam was focused on a plane by an f-. lens and it was scanned by a Galvano-mirror. The spatial profile of laser energy incident on a plane had two peaks at both sides of the scanned linear track. The profile was compared with the result obtained when the Gaussian beam was dithered transversely by an acousto-optic modulator. It is found that hollow optical fiber beam shaper can replace acousto-optic beam dithering device which is employed in a laser induced thermal printing system.

Pump laser effect on temporal jittering of pulses from passively Q-switched Nd:YVO4 laser

Temporal jittering has been problem in industrial application of passively Q-switched laser. To address this, we tried two methods. The first is preventing pump laser absorption in the Cr:YAG, which is used for saturable absorber. The saturation of the Cr:YAG should be affected by intracavity photons generated by stimulated emission in Nd:YVO4 crystal for reliable Q-switching operation. To prevent pump absorption, we applied 808 nm mirror coating on the Cr:YAG crystal. This coating also enhanced efficiency of pump laser utilization because the reflected beam was reabsorbed in the Nd:YVO4 crystal. As the second method, we applied pulsed current to the laser diode. The Q-switched laser is generated only after the pulse diode laser beam was absorbed in the Nd:YVO4 crystal. The pulsewidth of current pulse was carefully adjusted for Q-switched output pulse per each pump pulse. Air circulation is reduced by contacting crystals together and applying output coupler coating on the Cr:YAG crystal surface.

Study of optimal laser beam profile for laser induced thermal printing of organic film

Thin organic film is coated on the laser absorbing layer. The organic material coated heat absorbing layer is closely contacted with the substrate. When the laser is incident on the heat absorbing layer, the absorbing layer is expanded and the organic film is transferred to the substrate. When the laser has Gaussian spatial profile, the profile of printed organic material becomes non-uniform. We tried several laser beam profile shaping methods to get a clean and sharp printing. The dependences of eliminated organic material profile from donor plate on the laser power, beam shaping method, and the layer thickness are also investigated.

Optimization of reflector design for diode-pumped Nd:YAG laser with single-side pumping geometry

We evaluated characteristics of various shapes of reflector to get uniform and efficient pumping of Nd:YAG rod even with single-side pumping geometry using commercial software ZEMAX and LASCAD. The combined application of the software enabled us to evaluate numerically the laser performance and absorbed pump diode profile. We tried various shape of reflector such as semi-circular, square, elliptical reflectors, and cusp-shape, to get uniform pumping. In the calculation, water flows inside the cooling tube which surrounds the Nd:YAG rod. For further improvement a planarconcave window was placed in front of the diode to enlarge pump beam divergence angle. Through several trial and error, we found optimal pumping chamber configuration for single-side pumping, which gives reasonably uniform and efficient pumping of Nd:YAG rod. The output power and pulsewidth of Q-switched laser output were optimized by varying the laser cavity length and the output coupler reflectivity.

Stabilization of Output Pulses from a Passively Q-switched Nd:YVO 4 Laser Pumped by a Continuous-wave Laser Diode

결정과 접촉시켜 공진기를 구성하였다. Cr:YAG의 한 면에 여기 다이오드 레이저를 반사시키는 코팅을 입혀 공진기의 광변환 효율을 높임과 동시에 , 여기빔에 의한 포화흡수체의 표백현상을 방지하여 안정된 출력이 나오게 하였다. 레이저 이득 매질 및 포화흡수체의 온도는 열전 냉각기와 냉각수를 사용하여 안정화 시켰다. 온도안정화를 시킨 상태에서 발진되는 펄스의 첨두 출력 요동 (peak to peak)은 4%였다. 다이오드 레이저의 출력이 1 W일 때, 출력 펄스의 반복률은 평균 9 KHz 였으며, 최소 펄스폭은 7.11 ns, 최고 출력은 16.27 mW였다.