Fumio Wani

High-efficiency operation of chemical oxygen-iodine laser using nitrogen as buffer gas

High-efficiency operation of supersonic chemical oxygen-iodine laser (COIL) with an advanced jet-type singlet oxygen generator using nitrogen as buffer gas was demonstrated. Laser output was remarkably increased when buffer gas was cooled with liquid nitrogen. The effects of buffer gas temperature on the characteristics of the oxygen-iodine laser medium was discussed. A net chemical efficiency of 23.4% was obtained at 405 W when the chlorine molar flow rate was 19 mmol/s.

Parametric study of a twisted aerosol-type singlet oxygen generator

A twisted aerosol singlet oxygen generator (TA-SOG), which is a new high-pressure SOG for the supersonic chemical oxygen-iodine laser (COIL), was developed. Its operational characteristics were compared to those of a conventional liquid-jet-type singlet oxygen generator. It was shown that TA-SOG is operated at an internal gas velocity of 85 m/s, which is three times higher than that of the jet-type SOG. Cl/sub 2/ utilization of 70% and singlet oxygen O/sub 2/ (/sup 1//spl Delta/) yield of 85% were obtained under this condition. The results of our two-dimensional model calculation suggested that there must be some enhancement of a specific surface area in the reaction zone. The mechanism of this effect is discussed.

Wave optics simulation of diode pumped alkali laser (DPAL)

A numerical simulation code for a diode pumped alkali laser (DPAL) was developed. The code employs the Fresnel- Kirchhoff diffraction integral for both laser mode and pump light propagations. A three-dimensional rate equation set was developed to determine the local gain. The spectral divergence of the pump beam was represented by a series of monochromatic beams with different wavelengths. The calculated results showed an excellent agreements with relevant experimental results. It was found that the main channel of the pump power drain is the spontaneous emission from the upper level of the lasing transition.

Output power enhancement by the injection of dissociated iodine in supersonic chemical oxygen-iodine laser

Iodine molecule was dissociated prior to injection in supersonic Chemical Oxygen-Iodine Laser (COIL). In some cases, output power enhancement was observed. However, the output power was decreased at the optimum titration (iodine/oxygen ratio). A quasi-two dimensional simulation was employed to analyze the effect ofiodine pre-dissociation. It was revealed that the high iodine atom concentration at the plenum is responsible for the power reduction. The method to improve the output power by pre-dissociation is discussed.

HIGH-PRESSURE, HIGH-EFFICIENCY OPERATION OF A CHEMICAL OXYGEN-IODINE LASER

High-pressure subsonic mode operation of a chemical oxygen-iodine laser (COIL) was demonstrated. Singlet delta oxygen generated by a liquid-jet type singlet oxygen generator (SOG) was directly utilized in the laser cavity, without supersonic expansion. The operating pressure in the laser cavity was 0.80 kPa (6.0 Torr). Cooled nitrogen gas was added to the singlet oxygen flow to enhance the output power. An output power of 448 W was obtained for a chlorine input rate of 19.7 mmol/s. This is equivalent to a chemical efficiency of 25.0%, and it is comparable to the highest reported chemical efficiency of COIL using nitrogen as the buffer gas. The obtained value of specific energy of 3.5 J/liter, is more than fivefold higher than that for our supersonic COIL device.

Output power characteristics of diode-pumped cesium vapor laser

We examine the output power as a function of the cesium vapor density in a diode-pumped cesium vapor laser. Since the pump light bandwidth of our apparatus is considerably wider than the absorption bandwidth, a fair amount of the pump power is unabsorbed. An optical-to-optical conversion efficiency of 70% is observed with respect to the absorbed pump power. Beyond a certain point, the output power starts to reduce despite linear increase in absorption power along with increase in the Cs number density. We perform a numerical simulation to study the observed phenomena, and it is found that spontaneous emission from the upper laser level is the main channel of the pump power loss.

INDUSTRIAL COIL IN JAPAN

First, a historical overview of the development of a Chemical Oxygen-Iodine Laser(COIL) for industrial use in Kawasaki will be given. For industrial use, both high power and long term stable oscillation is required. Besides, the reduction of operational costs is very important. Our recent research about these problems will be discussed.

Experimental study of the diode pumped alkali laser (DPAL)

A small-scale cesium diode-pumped alkali laser (DPAL) apparatus has been developed for fundamental researches. A commercial laser diode with volume Bragg grating outcoupler is used to pump the gain cell longitudinally. Both windows of the gain cell are set at Brewster’s angle for minimum loss and maximum durability. Output coupling coefficient is continuously variable from 13% to 85% by the slanted quartz plate outcoupler inserted in the optical resonator. Small signal gain is measured with a laser diode probe at various gain cell temperatures. A 6.5 W continuouswave output with 56% optical-to-optical conversion efficiency (based on the absorbed power) has been achieved. A numerical simulation code is developed and its calculation results are in good agreement with the experiments.

Experimental investigation and numerical simulation of exciplex pumped alkali lasers

An investigation of exciplex pumped alkali laser (XPAL) has been conducted. A 4-cm gain cell containing Cs as an active medium is pumped by a tunable Ti: sapphire pulse laser. Laser oscillation is observed in both five-level and four-level systems. Small signal gain and saturation intensity of the active medium is measured by both probe laser amplification and oscillation experiment with varied output coupling. A one-dimensional numerical simulation is developed and the result of calculations is compared with the experiments. The pulse shape and oscillation threshed agree reasonably while calculated optical-optical conversion efficiency is underestimated.

High-power COIL and Nd:YAG laser welding

We have constructed a laser welding system, which enabled high-power laser welding by combining three laser beams of 1 µm wavelength. Its wavelength enables optical silica fibers transmission and the flexible system. The heart of this system consists of a 4 kW and a 6 kW Nd:YAG lasers and a 10 kW class Chemical Oxygen-Iodine Laser (COIL) beams of 6 kW Nd:YAG laser and COIL are combined in a coaxial beam and its maximum average power is 19 kW. The third laser beam, 4 kW Nd:YAG laser beam, is added obliquely from the same side of workpiece or oppositely from the reverse one. The effects of various welding parameters were investigated, such as the laser power, pulse modulation, and so on. As a result of the welding test with the 6 kW Nd:YAG laser, it was clarified that the pulse wave (PW) has good efficiency of deeper penetration at low welding speed. When the combined beam with CW COIL and PW Nd:YAG laser was used, 20 mm penetration on the stainless steel could be achieved at a welding speed of 1 m/min. By adding the third laser beam, the both side welding on 30mm thickness plate could be achieved.