Thermal analysis of a high-power cw solar-pumped laser under development as a magnesium energy cycle driver has
been conducted experimentally and analytically. The laser system is equipped with a Fresnel lens and a cone-shaped
secondary mirror chamber (SMC). The SMC realizes a hybrid-pumping scheme combining axial- and side-pumping
configurations to enhance solar light absorption to a rod-shaped laser medium. A non-uniform temperature profile was
obtained during experiments due to combination of volumetric heating and surface cooling, which leads to a nonuniform
variation of index of refraction in the laser medium. The thermal lensing and thermal stress-induced
birefringence are analyzed.
For EUV lithography the generation of clean and efficient light source and the high-power laser technology are key issues. Theoretical understanding with modeling and simulation of laser-produced EUV source based on detailed experimental database gives us the prediction of optimal plasma conditions and their suitable laser conditions for different target materials (tin, xenon and lithium). With keeping etendue limit the optimal plasma size is determined by an appropriate optical depth which can be controlled by the combination of laser wavelength and pulse width. The most promising candidate is tin (Sn) plasma heated by Nd:YAG laser with a pulse width of a few ns. Therefore the generation technology of clean Sn plasma is a current important subject to be resolved for practical use. For this purpose we have examined the feasibility of laser-driven rocket-like injection of extremely mass-limited Sn or SnO2 (punched-out target) with a speed exceeding 100m/s. Such a mass-limited low-density target is most preferable for substantial reduction of ion energy compared with usual bulk target. For high average power EUV generation we are developing a laser system which is CW laser diode pumped Nd:YAG ceramic laser (master oscillator and power amplifier system) operating at 5-10 kHz repetition rate. The design of practical laser for EUV source is being carried out based on the recent performance of >1 kW output power.
Properties of laser-produced tin (Sn) plasmas were experimentally investigated for application to the Extreme Ultra-Violet (EUV) lithography. Optical thickness of the Sn plasmas affects strongly to EUV energy, efficiency, and spectrum. Opacity structure of uniform Sn plasma was measured with a temporally resolved EUV spectrograph coupled with EUV backlighting technique. Dependence of the EUV conversion efficiency and spectra on Sn target thickness were studied, and the experimental results indicate that control of optical thickness of the Sn plasma is essential to obtain high EUV conversion efficiency and narrow spectrum. The optical thickness is able to be controlled by changing initial density of targets: EUV emission from low-density targets has narrow spectrum peaked at 13.5 nm. The narrowing is attributed to reduction of satellite emission and opacity broadening in the plasma. Furthermore, ion debris emitted from the Sn plasma were measured using a charge collector and a Thomson parabola ion analyzer. Measured ablation thickness of the Sn target is between 30 and 50 nm for the laser intensity of 1.0 x 1011 W/cm2 (1.064 μm of wavelength and 10 ns of pulse duration), and the required minimum thickness for sufficient EUV emission is found to be about 30 nm under the same condition. Thus almost all debris emitted from the 30 nm-thick mass-limited Sn targets are ions, which can be screened out by an electro-magnetic shield. It is found that not only the EUV generation but also ion debris are affected by the Sn target thickness.
It is very effective for mass-limited tin-foil targets to adapt for the EUV source. Tin-foil targets in account of formation, size, and thickness have been developed for debris mitigation. The amount of ions from targets is 40 % decreased tin-foil targets of 1μm or 5μm thickness than tin-bulk targets. The ion velocity is one order of magnitude less than bulk targets. The EUV emission spectra of tin-foil are more narrowing than bulk targets. The targets supply for high repetition rate of 10 kHz is applied for a novel method. It is called "Punch-out" method. The flight of graphite foil that it is a test targets was succeed to observe by using a gated ICCD camera. The target velocity is achieved to be about 120 m/s. This value can be applied for targets supply with high repetition rate of 10 kHz.
We propose an automobile driven by piston motion, which is driven by water-laser coupling. The automobile can load a solar-pumped fiber laser or can be driven by ground-based lasers. The vehicle is much useful for the use in other planet in which usual combustion engine cannot be used. The piston is in a closed system and then the water will not be exhausted into vacuum. In the preliminary experiment, we succeeded to drive the cylindrical piston of 0.2g (6mm in diameter) on top of water placed inside the acrylic pipe of 8 mm in inner diameter and the laser is incident from the bottom and focused onto the upper part of water by the lens (f=8mm) attached to the bottom edge.
Extreme ultraviolet (EUV) emission from laser produced tin plasma was investigated for 1064, 532 and 266 nm laser wavelengths. The EUV conversion with tin target tends to be high for shorter laser wavelength and is optimized at 4-5x1010 W/cm2 for 1064 and 532 nm. The EUV emission exhibits laser wavelength dependence in terms of angular distribution and structures of emission spectra. It is found that spectra for 532 nm and 266 nm showed spectral dips at around 13.5 nm and these dips are well replicated in computer simulations. Both the angular distribution together with the spectral dips may suggest existence of opaque plasmas surrounding the EUV emission region.
Extreme ultraviolet (EUV) emission from laser produced plasma attracts much attention as a next generation lithography
source. The characterization of EUV emission has been carried out using GEKKO XII laser system. The twelve beams
irradiated tin or tin-oxide coated spherical targets uniformly and dependence of EUV spectra on laser intensity were
obtained with a transmission grating spectrometer and two grazing incidence spectrometers. The EUV Conversion
Efficiency (CE, the ratio of EUV energy at the wavelength of 13.5 nm with 2 % bandwidth to incident laser energy) was
measured using an absolutely calibrated EUV calorimeter. Optimum laser intensities for the highest conversion were
found to be 0.5- 1x1011 W/cm2 with CE of 3 %. The spectroscopic data indicate that shorter wavelength emission
increases at higher laser intensities due to excessive heating beyond optimum temperatures (20- 40 eV). The CE was
almost independent on the initial coating thickness down to 25 nm.
Extremely ultraviolet (EUV) light at around 13.5 nm of wavelength is the most probable candidate of the light source for lithography for semiconductors of next generation. We have been studying about the EUV light source from laser-produced plasma. Detailed understanding of the EUV plasma is required for developments of modeling with simulation codes. Several parameters should be experimentally measured to develop the important issues in the simulation codes. We focused on density profile, properties of EUV emission, and opacity of the laser-produced plasmas. We present re-cent experimental results on these basic properties of the laser-produced EUV plasmas.
Extreme Ultra Violet (EUV) light source produced by laser irradiation emits not only the desired EUV light of
13 ~ 14 nm (about 90 eV) but also shorter x-rays. For example, emissions around 4 ~ 8 nm (about 150 ~ 300 eV)
and 1 ~ 2.5 nm (about 0.5 ~ 1.2 keV) are experimentally observed from Sn and/or SnO2 plasmas. These
emissions are correspond to the N-shell and M-shell transitions, respectively. From the view point of energy
balance and efficiency, these transitions should be suppressed. However, they may, to some extent, contribute
to provide the 5p and 4f levels with electrons which eventually emit the EUV light and enhance the intensity.
To know well about radiative properties and kinematic of the whole plasma, atomic population kinetics and
spectral synthesis codes have been developed. These codes can estimate the atomic population with nl-scheme
and spectral shapes of the EUV light. Radiation hydrodynamic simulation have been proceeding in this analysis.
Finally, the laser intensity dependence of the conversion efficiency calculated by these codes agrees with that of
the corresponding experimental results.
A new research project on extreme ultraviolet (EUV) source development has just been started at the Institute of Laser Engineering, Osaka University. The main task of this project is to find a scientific basis for generating efficient, high-quality, high power EUV plasma source for semiconductor industry. A set of experimental data is to be provided to develop a detailed atomic model included in computer code through experiments using GEKKO-XII high power laser and smaller but high-repetitive lasers. Optimum conditions for efficient EUV generation will be investigated by changing properties of lasers and targets. As the first step of the experiments, spherical solid tin and tin-oxide targets were illuminated uniformly with twelve beams from the GEKKO XII. It has been confirmed that maximum conversion efficiency into 13.5 nm EUV light is achieved at illumination intensity less than 2 x 1011 W/cm2. No significant difference is found between laser wavelengths of one μm and a half μm. Density structure of the laser-irradiated surface of a planar tin target has beem measured experimentally at 1012 W/cm2 to show formation of double ablation structure with density plateau by thermal radiation transport. An opacity experiment has just been initiated.
Laser propulsion has many advantages over other conventional methods of producing thrust in space applications. For example, laser energy can be delivered to a remote objects such as space debris which otherwise is impossible to make thrust on its surfaces to remove from the orbits. However, essential advantage of laser propulsion lies in the fact that the characteristics of laser propulsion can be controlled over wide range of parameters by changing laser irradiation conditions. This advantage is based on the capability of controlling specific energy carried by propellant. The specific energy is a key parameter of thrust performance since it determines the propellant temperature or expanding velocity and thus propulsion efficiency. A number of researches so far conducted have treated laser plasma interactions created on solid surfaces with laser parameters such as wavelength, pulse width, intensity, as well as ambient gas pressure. The present study will give a new insight to laser plasma interactions and/or new mechanism of laser thrust generation. Laser energy is deposited inside solid target and, as an initial condition, confined by solid material. Since the confinement time is an order of milli-second, both shock waves and thermal conduction can tale part in the energy transfer process and therefore, give more controllable parameters over the thrust characteristics. In this manner, specific energy carried by target material or propellant can be controlled by changing the depth of energy deposition region. This will give a new dimension of controlling laser plasma characteristics for laser propulsion. In this paper, experimental results and physical insights will be presented as to propelled mass and velocity dependence on laser energy and temporal behavior of impulse generation, as well as enhancement of impact generation over the conventional ablation scheme.
We present a technique of remote non-destructive inspection of concrete structures and location of inner defects. The technique is based on detection of ultrasonic vibration in concrete with the use of laser interferometer. The interferometer uses principles of dynamic holography via two-wave mixing in photorefractive crystal and homodyne detection. The use of the dynamic hologram of the inspected surface excludes possibility of wave front mismatch between interfering signals, one of which (probing signal) has speckle structure due to the roughness of the inspected surface. Bismuth Silicon Oxide (BSO) photorefractive crystal is used as a nonlinear medium for the recording of the dynamic hologram. CW YAG:Nd laser radiation of 532 nm wavelength is used for detection. Results of detection are verified by piezoelectric sensor. Vibration in concrete is initiated by hammering or laser impact. For completely remote operation, vibration should be initiated by laser impact. In purpose of reliable detection, we analyze both shape of propagating ultrasonic wave and its spectrum. Spectrum of low frequency (1-10 kHz) vibration provides information of the presence or absence of inner defects. Analysis of propagating ultrasonic wave of higher frequency (10-100 kHz) allows to locate the defect more precisely.
In order to increase the plasma production efficiency in the laser triggered lightening experiment, it is proposed that a hyper-velocity micro-particle flow produced by a chemical explosion shall be used along the laser beam path. A preliminary experiment to verify the proposal has successfully been done in the laboratory using 100 J 80 nsec CO2 laser pulse.
The dynamic holography is considered as the most perspective technique of correction for dynamic aberrations of large- aperture optical elements when operating in near IR range. The use of fluorocarbon Flourinert FC-72, characterized by Brillouin nonlinearity, for recording the dynamic hologram has been demonstrated experimentally. Resonant enhancement of the hologram's diffraction efficiency resulting in 10 times growth has been shown. Diffraction efficiency over 50% has been obtained. Different behavior of diffraction efficiency in dependence on intensity of recording beams and length of nonlinear medium has been shown. Capability of correction of laser beam distortions as well as dynamic behavior of recorded hologram have been tested. The random phase distortions bringing about 20 times increase of the laser beam divergence have been corrected resulting in near-diffraction quality of a point object image. It has been shown that lifetime of the hologram does not exceed 10-9 sec. Perspective of the use of Brillouin enhanced dynamic holography for nonlinear compensation of optical aberrations has been presented.
Generation of thrust by laser propulsion has gained reality and much attention due to the recent development of high average-power lasers and demonstrations of sizeable object launching. Generating thrust requires a large amount of energy or high average power, but the question is how it is provided. This study deals with effectiveness of highly repetitive ultrashort laser pulses on generating high momentum coupling coefficient, Cm in vacuum condition. Two laser parameters, pulse width and repetition rate, have been studied in terms of the enhancement of Cm. It was found that with pico- and femtosecond pulse ablation, higher Cm is generated compared with longer pulse duration although it is on the same scaling as longer pulse duration. With high repetition rate pulses (80 MHz), more than one order of magnitude enhancements in Cm have been observed compared with single pulse interaction.
This paper describes the widows for laser transport in the atmosphere. The limitations caused mainly by Rayleigh scattering, stimulated Raman scattering, thermal blooming and so on are discussed here. From these limitations, windows for laser space applications are found out. The widows have enough area in laser parameters for several applications of laser in the space.
This paper will discuss the laser conditions for producing efficient thrust on debris. In order to obtain high thrust for given laser energies, thermal energies dissipated into laser ablation and debris bulk have to be minimized. It was suggested that this minimization can be achieved by using ultrashort pulse interactions where laser pulse duration is too short for thermal conduction to take place deep into debris. The results of momentum coupling coefficients measured with ps and ns pulse duration will be presented.
Generation of a long laser-plasma channel capable of triggering and guiding an electrical discharge is a crucial issue for laser-triggering protection system. We make a long plasma channel to increase the probability of triggered lightning by laser. To produce a long laser plasma channel, we propose da new technique called hybrid plasma channel method which combines weakly and strongly ionized plasma channels to maximize laser-energy efficiency of discharge guiding. We investigate the characteristics of the hybrid plasma channels to maximize laser-energy efficiency of discharge guiding. We investigate the characteristics of the hybrid plasma channel method through several laboratory experiments. The weakly ionized channel was generated by UV laser pulses in air. As the number density of electrons in weakly ionized channel is proportional to 1.1 power of laser intensity, nitrogen and oxygen molecules can not attributed to the source of ionized plasma. It is suggested that dissociation process of impurities in air whose density is 1011 - 1012 cm-3 plays an important role in plasma formation and leader triggering effect. The 50 percent flashover voltage using the hybrid plasma channel method is lower than that without the weakly ionized plasma channel. It was also found that higher repetition rate of the plasma generation on lowers the V50 furthermore.
Proc. SPIE. 3760, High-Resolution Wavefront Control: Methods, Devices, and Applications
KEYWORDS: Phase conjugation, Laser systems engineering, Laser energy, Four wave mixing, High power lasers, Atmospheric propagation, Laser scattering, Energy efficiency, Pulsed laser operation, Signal detection
Space debris removal from the earth orbit is getting substantial attention since the International Space Station (ISS) construction started in the orbit. Because of its size and length of operation period, it is considered that the ISS has high probability of being hit by debris. There is a certain range of debris size where its detection and protection are very difficult. This paper describes use of space borne laser system to protect space assets against debris. It is proposed that the system utilizes phase conjugation phenomena (Brillouin Enhanced Four Wave Mixing or BEFWM in short) to focus high power laser radiation to debris with very high accuracy. Basic study has been conducted to investigate energy efficiency, phase conjugation fidelity and phase conjugation dynamics.
Laboratory and field experiments for laser triggered lightning were performed in order to induce so called triggered lightning, in which an electric leader initiates from tall objects on the ground to thunderclouds, using a plasma channel produced by CO2 lasers and Nd lasers. Significant milestone was achieved in technological developments and verification of a scientific feasibility of the techniques such as determination of laser trigger timing which lead to laser triggered natural lightning. In the laboratory experiments technologies to produced long plasma channels and characteristics of discharge process induced by plasma channels were extensively investigated. In the field experiments developed are an automatic laser trigger system using an RF burst emission by a preliminary breakdown, methods to measure lightning path, thundercloud movement and field strength, and to make a continuous plasma channel at the top of the lightning tower which acts effectively for leader initiation.
A series of field experiments for laser triggered lightning have been carried out targeting the winter thunderstorms in Fukui, Japan. A combination of a 2 kJ CO2 laser system, a 600 J Nd-glass laser and a 4th harmonics 100 mJ YAG laser was used to produce a plasma channel which are effective to trigger and guide an electrical leader from the lightning tower. A new scheme of autotriggering laser system by preliminary breakdown (PB) has been developed. The PB triggering system demonstrated its capability of irradiating the laser at the right timing for triggering the leader. On February 11th 1997, in the field experiment, the electric leader was initiated by the laser plasma using the PB triggering system for the first time. A number of diagnostics including UHF interferometers, a tower current monitor, a capacitive antennas verified the initiation of leader triggered by the laser plasma both in time and space.
A series of laboratory experiments has been conducted to investigate the initiating and guiding effects of laser plasma channel on electrical discharge. It was confirmed that the plasma channels have strong guiding effects and reduce the required electrical field strength for electrical discharges to occur. A field experimental site targeting natural lightning is being prepared to develop the thunder storm monitoring system and to test the laser and optical systems against various weather conditions. The results from the laboratory experiments and laser transmission in snowy conditions as well as attempt of initiating electrical leader will be discussed.