Tatsumi Goto

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...

Electron Density Measurements in UV-Preionized XeCl and CO2 Laser Gas Mixtures

A Langmuir probe technique has been used to measure electron densities and temperatures in UV-preionized XeCl excimer and CO2 laser gas mixtures in a laser tube. For this experiment, only pin electrodes (preionization sparks) were operated with no discharge between the main electrodes. The measured electron densities were about 108 cm-3 in both the excimer and CO2 laser gases, compared with 1010 cm-3 in pure He gas. The electron density was found to increase due to the proximity of the main electrodes. The coefficients of absorption for excimer and CO2 laser gas were obtained from the characteristics of the electron densities vs the distance from the UV source. Based on the absorption coefficient for XeCl, 0.9 cm-1 atm-1, we propose pin-electrode arrangements for spatially uniform preionization.

Square and uniform laser output device : theory and applications

We have developed a device whereby a laser beam is converted to a square and uniform output intensity distribution. This device consists of a square pillar. The laser beam enters the square pillar at a specified incident angle, and uniform output intensity distribution is obtained at the end of the pillar. We describe the theory and the characteristics and uses of this device. Furthermore, we developed the ruby and argon laser systems in which this device is incorporated as the handpiece. It is mainly for clinical use, but also for industrial purposes such as soldering and marking.

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...

Longitudinally excited N2 lasers without high-voltage switches

We have developed novel excitation circuits without high-voltage switches for two longitudinally excited N(2) lasers (wavelength: 337 nm). One uses a single tube without a trigger and the other uses a tandem tube with a trigger. In both systems, the discharge tube acts as a switch. In the single-tube system, the laser output energy was 125.8 microJ and the efficiency was 0.16% at 18 Torr (2.4 kPa) when a slow-rising voltage pulse of -28 kV was applied (rise time: 21.3 micros). In the tandem-tube system, the laser output energy was 259.4 microJ and the efficiency was 0.11% at 18 Torr when a slow-rising voltage pulse of -48 kV was applied (rise time: 27 micros).

Longitudinally Excited N2 Laser Pumped by Lamplike Discharge

Nitrogen laser oscillation is observed in a lamplike discharge within a longitudinal excitation tube. The laser consists of a 30-cm-long Pyrex glass tube with an inner diameter of 2.5 mm and a step-up transformer directly coupled to the discharge tube without a high-voltage switch. The excitation is produced by wall-coupled discharge. The laser output energy is 1.7 µJ at 12 Torr (1.6 kPa) and 40 Hz when applying a slow-rising voltage pulse of 37 kV (rise time of 572 ns).

Power enhancement for argon II narrow tube lasers by a transverse magnetic field

Appreciable enhancement in output and power efficiency for compact Ar II lasers (488.0 nm) is obtained by applying a transverse magnetic flux density at approximately 700 G. The enhancement factor amounts to as large as 3 in laser output and the efficiency becomes more than twice as much as without magnetic flux. A sharp rise in output occurs above the critical magnetic flux density, 450 G in the present work, where the cyclotron radius of electrons is nearly equal to one‐half of the tube radius.

5 kHz high repetition rate and high power XeCl excimer laser

A 5 kHz high repetition rate excimer laser has been developed. An average laser power of 0.56 kW at 5 kHz operation, which remains stable for 1 s, is obtained. This duty time is limited only by overheating of the switching thyratrons. The 5 kHz operation is attained by some improvements to the gas conditions, the UV‐preionization scheme, and the gas flow speed in the discharge region. The gas is circulated at an effective flow velocity of 137 m/s by a newly developed two‐stage axial blower. The gas conditions are optimized experimentally to achieve 5 kHz operation. To obtain uniform preionization, the locations of pin electrodes for the preionization are determined on the basis of numerical results.

Effects of H2 and CO Additives on the O2 Accumulation in the TEA CO2 Laser

A change in gas mixture in a sealed-off TEA CO2 laser discharge was analyzed gas-chromatographically. It has been found that the addition of a small amount of H2 and CO will reduce the O2 formation in the mixture. In particular, when a mixture of N2:CO2:CO:H2:He=12:12:6:1.6:68.4% was used, a sealed-operation lifetime exceeding 5 × 105 shots without any oxygen formation was obtained. However, it has also been found that CO2 was decomposed by discharge and formed O2 was missing. No nitrogen oxide formation was recognized, implying that the missing oxygen was dissipated to some other reactions. Furthermore, notable differences in the change in the electrode surface, with addition of CO or H2 to the mixture, were recognized by visual observation. Hence, we proposed electrode surface reactions for the reduction in the O2 formation in the mixture.

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.