Koichi Yasuoka

Development of key components and technologies for a high repetition rate and high-power excimer laser

Key components and technologies have been developed for an ultrahigh repetition rate XeCl excimer laser of 5 kHz to be used for industrial applications. A compact axial blower having a revolution rate of up to 10 000 min−1 and a maximum pressure of 16.2 kPa in air was made with a canned magnetic coupler to circulate a laser gas at a flow velocity of over 150 m/s in a discharge region. Materials constituting a laser chamber were tested to prevent discharge instability by gas contamination to enable long time operation. The dominant cause of the instability was investigated by a simple simulation. For a preionization, a novel sealed-off x-ray tube was developed to compare the suitability in a high repetition rate operation with that of conventional UV preionization. The gas due to the shock and acoustic waves generated by discharge pulses was measured to design the damper, which enabled the suppression of the gas turbulence by around a tenth. To simplify cumbersome laser maintenance, a new power supply prov...

Design concept and performance considerations for fast high power semiconductor switching for high repetition rate and high power excimer laser

A semiconductor switching power supply has been developed, in which a novel structure semiconductor device, metal-oxide-semiconductor assisted gate-triggered thyristor (MAGT) was incorporated with a single stage magnetic pulse compression circuit (MPC). The MAGT was specially designed to directly replace thyratrons in a power supply for a high repetition rate laser. Compared with conventional high power semiconductor switching devices, it was designed to enable a fast switching, retaining a high blocking voltage and to extremely reduce the transient turn-on power losses, enduring a higher peak current. A maximum peak current density of 32 kA/cm2 and a current density risetime rate di/dt of 142 kA/(cm2×μs) were obtained at the chip area with an applied anode voltage of 1.5 kV. A MAGT switching unit connecting 32 MAGTs in series was capable of switching on more than 25 kV–300 A at a repetition rate of 5 kHz, which, coupled with the MPC, was equivalent to the capability of a high power thyratron. A high repe...

Long-pulse high-repetition-rate transversely excited CO2 laser for material processing

Using a TE-CO2 laser, we could obtain a long-pulsed laser beam of low initial spike by controlling the discharge current by a pulse forming network and optimizing the gas composition, discharge length to resonator length ratio, and output mirror reflectivity. The maximum laser output was 1.1 J; the initial spike energy, 100 kW; the tail output, 56 kW; and the 16 (mu) sec (FWHM). The maximum repetition rate was 500 Hz. A new type of circuit with small pre-ionization current made it possible to operate the laser at a high repetition rate so as to prolong the laser life. When a 5-inch lens was used, the laser power density at the focal point was 1*108 W/cm2, making it possible to use the laser with an unusually high energy density without causing the breakdown of air insulation. In fact, we succeeded in fine- cutting a 0.5 mm thick alumina ceramic with the laser. It was found that unlike other working methods, the newly developed laser does not cause cracks in ceramic work pieces.

Discharge-pumped excimer laser with high-speed and low-temperature gas flow

A high-speed wind tunnel, made by using a Ludwieg tube, has been successfully developed for a highly repetitive discharge-pumped excimer laser. This apparatus allows the gas flow of velocity approximately 204 m/s, pressure approximately 293 kPa, temperature approximately 254 K, and duration time approximately 48 ms. The rate constant for the recombination process of Xe+ + Cl- + Ne yields XeClX + Ne is found to increase to a maximum of 4.2 X 10-6 cm3/s at 180 K in a gas pressure of 294.2 kPa. The kinetic simulation of XeCl excimer laser using such a rate constant indicates the enhancement of the laser output in the lower gas temperatures.

Newly developed excitation circuit for kHz pulsed lasers

Pre-charging of peaking capacitors has been tested on a high-repetition-rate transversely excited atmospheric CO2 laser. The peaking capacitors, connected to the discharge electrodes, were divided into two blocks in series. One part was pre-charged by a DC voltage source and another part was pulse-charged. An efficiency of over 6% was obtained by only 7 kV pulse voltage, and was higher than that obtained in a charge-transfer-type circuit. A high repetition rate operation up to 1 kHz was demonstrated.

Breakdown characteristics in atmosphere by TEA-CO2 laser

The breakdown characteristics of a short-distance discharge gap in an atmosphere by TEA- CO2 laser have been studied to control the lightning artificially. It is efficient to enhance the probability of electrical breakdown induced if the focal point is set behind the discharge gap axis or near the negative high-voltage electrode. The length of the optical-breakdown plasma channel is elongated by using the micro-particles diffused in an atmosphere. Using 3 micrometers -diam. aluminum particles, the optical-breakdown threshold is lowered to 15 MW/cm2 compared to 0.5 GW/cm2 in the absence of the micro-particles.

Solid state direct-drive circuit for pumping gas lasers

A solid-state excitation circuit is demonstrated without any compression circuits. The new circuit contains the dc pre-charging capacitors and pulsed charging capacitors. The applied voltage between the electrode is the sum of the pre-charging voltage and the pulsed charging voltage. Therefore, the switching voltage is lowered when increasing the pre-charging voltage. In TEA-CO2 laser device, the stable discharge is excited by using only one thyristor device. In this circuit, the breakdown voltage is lower and output efficiency is higher than that obtained in the ordinary charge-transfer circuit. The high-repetition rate operation is tested up to 1 kpps. This circuit is also available for the excitation of KrF excimer laser.

Characteristics And Applications Of New Electrode System For High Power CO[sub]2[/sub] LASER

We have investigated the characteristics of a new electrode system developed for fast transvers-flow CO2 laser. The bar type electrode system makes it possible to reduce the number of electrodes and to increase the input discharge energy as compared with the conventional pin type electrode system. The oscillation mode at low laser output power is dramatically improved by adjusting the electrode arrangement.