Contamination of optics observed in LFEX compression chamber was a critical problem for maintaining high damage
threshold and high optical performance for mirrors and gratings in the vacuum environments. We conducted a study for
understanding this problem, and we found important knowledge on the nature of contamination, namely materials of
contaminants, source of contamination, the invasion mechanism, and removal method of contamination. We also found
the samples for the damage test is easily contaminated in the storage environment. This means the optical coating
accumulate contaminations even in the air. So we tested in-situ damage test in a gas flowing chamber with controlled
contaminants. The degradation was time-dependent phenomena, and proportional to the vapor
contaminants. Several materials were tested, and even in water vapor, the damage threshold was decreased about 10%.
We also found out two methods for removing contaminations from the coatings. According to these studies, our
conclusion is special treatments should be used for knowing the intrinsic damage threshold of the coatings.
Photo-thermal deflection (PD) method is one of the typical pump-probe techniques for absorption evaluation. In this
study, the PD method was used for detection of a signature of laser-induced damage prior to critical destruction. A setup
for the PD technique was incorporated into the nanosecond damage testing system and the absorption of some optics was
measured in combination with N-on-1 damage test. The absorption decreased with pulse-laser irradiation at first. Then,
the absorption increased at some point. Finally, the laser damage was caused and the absorption decreased. The
relaxation times of the absorption were also evaluated to reveal the reason for the variation. The measured results
indicated that the decrease came from the disposal of initial contamination and the effect might be considered as laser
conditioning. On the other hand, the increase of the absorption might be attributed to generation of laser-induced defects
In high power laser systems, the laser-induced damage threshold (LIDT) in optical coating is very important parameters for obtaining high performances. Recently, LIDT was found to have strong temperature dependences in the bulk, surface of substrates, and in coatings. These temperature dependences of LIDT were carefully measured, and the damage formation model was constructed regarding to this temperature dependence. To explain this temperature dependence of LIDT, the temperature dependences of the initial electron generation and electron multiplication in the avalanche process were taken into account. On the other hand, LIDT in optical coating is very sensitive to organic contaminations accumulated in coating layers during storage and using condition. This paper also introduces the oil-contamination problem in LFEX laser system for First Ignition scheme in the laser fusion. We have analyzed contaminants and evaluated the effects of the contamination. We also developed new cleaning methods to remove contamination from the coating, and we have succeeded to prevent the degradation in LIDT for the duration of evacuation with Silica-gel in the chamber. The quantitative analysis of contamination on LIDT was made. We have investigated the characteristics of LIDT in dielectric coatings under the controlled contamination with several materials.
The temperature dependence of the laser-induced damage threshold on optical coatings was studied in detail for laser
pulses from 123 K to 473 K at different temperature using Nd:YAG laser (wavelength 1064 nm and pulse width 4 ns)
and Ti:Sapphire laser (wavelength 800 nm and pulse width 100 fs, 2 ps, and 200 ps). The six kinds of optical monolayer
coatings were prepared by electron beam evaporation and the coating materials were SiO2, Al2O3, HfO2, ZrO2, Ta2O5,
and MgF2. For pulses longer than a few picoseconds, the laser-induced damage threshold of single-layer coatings
increased with decreasing temperature. This temperature dependence was reversed for pulses shorter than a few
picoseconds. We describe the physics models to explain the observed scaling. The electron avalanche is essential to
explain the differences in the temperature dependence. In other words, the balance between linear process such as
electron avalanche etc. and nonlinear process such as multiphoton ionization etc. will be able to decide the tendency of
the temperature dependence. The proposed model also gives one of possibility for an extremely high LIDT optics.
In high power laser system, laser-induced damage threshold (LIDT) in optical coating is very important for obtaining
high performances. The dependence of LIDT on the pulse duration and the repetition rate are well known phenomena.
But recently, LIDT was found to have strong temperature dependences in the bulk, surface of substrates, and in coatings.
This temperature dependence of LIDT was carefully measured, and the damage formation model was constructed
regarding to this temperature dependence. This paper introduces LFEX laser system for First Ignition scheme in the
laser fusion. A large-scale pulse compression chamber was designed and constructed, and segmented grating system has
been employed for large-scale pulse compressor. This compressor provided good pulse compression performances, but
we observed a heavy oil-contamination of optics in this chamber. We have analyzed contaminants and evaluated the
effects of the contamination. We also developed new cleaning methods to remove contamination from the coating, and
the quantitative analysis of contamination on LIDT was made. We have investigated the characteristics of LIDT in
dielectric coatings under the controlled contamination. LIDT of coating drops to 1/2 in the saturated toluene vapor at
Studies for temperature dependences of laser-induced damage thresholds for optical devise is introduced in this paper.
Additionally, the temperature dependence of the laser-induced damage threshold of single-layer optical coatings as
resent progress was clarified using Nd:YAG and Ti:sapphire lasers. The wavelengths of the lasers were 1064 nm and
800 nm and the pulse widths were 4 ns, 200 ps, 2 ps, and 100 fs. For pulses longer than a few picoseconds, the laser-induced
damage threshold of coated substrates increased with decreasing temperature. This temperature dependence was
reversed for pulses shorter than a few picoseconds. A flowchart was presented including the several mechanisms for
laser damage mechanism. The differences in the temperature dependence are explained by the flowchart. As one of
results in theoretical analysis, the electron resistivity i. e. electron mobility is key point to elucidate the temperature
dependence of laser-induced damage threshold.
The new method to estimate the erosion threshold of various material surfaces with high power pulsed electron beam
was extended to the surface erosions with various pulsed laser lights. We are interested in the erosions of candidate
materials to be useful under very severe eovironmental conditions with various laser lights, including ArF and Nd:YAG
Laser-induced damage threshold (LIDT) in optical coating is very sensitive to organic contaminations accumulated in coating layers during storage and using condition. The sources of contamination are commonly exists, and optical coatings are easily contaminated regardless to the environment pressure, LIDT at ns region decreased largely by contamination, but LIDT at ps seems insensitive. In this study, we have investigated the influence of contamination of optical coating on LIDT and other optical properties. We examined several kinds of coating to clarify the sensitivity to the contamination. Degradations of LIDT were commonly observed in e-beam deposition, IAD and IBS. Some coatings changed spectral characteristics by contamination, and other coatings did not change. Some samples were contaminated as received condition, and some were very clean. Furthermore, we have investigated the characteristics of LIDT in dielectric coatings under the controlled contamination. LIDT of coating drops to 1/2 in the saturated toluene vapor at room temperature.
Temperature dependence of laser-induced damage thresholds were measured by Nd:YAG laser (1064-nm wavelength, 4-
ns pulse width) and Ti:Sapphire laser (800-nm wavelength, 100-fs, 2-ps, and 200-ps pulse widths) to elucidate the effects
of laser-induced damage mechanisms. As experimental samples, SiO2, MgF2, Al2O3, HfO2, ZrO2, and Ta2O5 were
prepared by electron evaporation. With longer pulses than few picoseconds, laser-induced damage thresholds were
increased with decreasing temperature. Temperature dependence was reversed for shorter pulses than a few picoseconds.
The effects of temperature at different pulse width to laser-induced damage mechanisms were considered with separated
processes. In the conclusions, a temperature effect to free-electron generations by photoionization and multi photon
ionization is negligible. However, the temperature affects to electron multiple (electron avalanche) and critical density.
Electron multiple decreased at low temperature and the laser-induced damage thresholds increased. On the other hand,
critical density decreased at low temperature and the laser damage thresholds decreased. Influence of electron avalanche
is much greater than the impact of critical density. Thus, the trend and the strength of the temperature dependence on
laser-induced damage threshold will be decided by electron avalanche.
Laser-induced damage thresholds for dielectric and metal single-layer coatings at different temperature conditions
(123-473 K) were measured by 1064-nm wavelength and 4-ns pulses to elucidate the effects of initial temperature to laser
damage mechanisms. SiO2, MgF2, gold, silver and copper single-layer coatings were prepared as experimental samples.
In the experimental results, temperature dependence of LIDTs for optical substrates and all dielectric single-layer
coatings indicated same trend as that for bulk silica glasses, which increased linearly with decreasing the temperature.
However, all metallic coatings had the inverse trend of the dependence for dielectric coatings. The effects of initial
temperature to laser damage mechanisms were considered with separated processes from the experimental results. In the
conclusions, free-electron generation and electron multiple caused difficultly at low temperature and the laser-induced
damage thresholds increased. On the other hand, plasma heating caused easily at low temperature and the laser-damage
Laser-induced damage thresholds of 2.2 picoseconds pulse for HR coatings prepared by Japanese optics makers were
measured. The damage thresholds were compared with that of 10 nanoseconds pulse measured at last year. In
picoseconds pulse, almost HR coatings had an average threshold of about 5 J/cm2. This meant that the damage thresholds
of picoseconds pulse were unconnected to a little defects and/or contaminants in the coatings.
A heavy oil-contamination was observed on the optical components in LFEX pulse compressor. This contamination
came from the wall of compression chamber, and the damage threshold of the mirror dropped to 1/2 or1/3 of the original
value. The same contamination was observed in different compression chambers in our institute. The contamination
materials were identified as Paraffin-oil and DBP (Di-n-butyl phthalate). Several cleaning schemes were tried, but no
significant improvement was obtained. Finally, we found well-baked silica gel placed in the vacuum chamber improved
the contamination very much. In a small vacuum chamber, the damage threshold increase by 3 times, and this result
indicated the contamination of damage test sample. We also tried to remove contamination with dipping optics in the
water-alcohol mixture, and we obtained almost the same improvements with the silica gel.
Laser-induced damage thresholds of HR and AR coatings prepared by Japanese optics makers were measured at
10-ns pulse and 1064-nm wavelength. The data would be an important database to improve the damage threshold for makers,
and to guide the designs of laser systems for optics users.
A linear increase of the laser-induced damage thresholds in silica glasses with decreasing the temperature was reported
in this conference at last year. Various nonlinear phenomena should be generated in silica glasses besides the damage in
high intensity. Temperature dependences of the nonlinear refractive indices and the SBS (stimulated Brillouin scattering)
thresholds in silica glasses at temperature 173 K to 473 K were measured with single-mode Q-switched Nd:YAG laser at
fundamental wavelength. As the result, the nonlinear refractive indices increased with decreasing temperature. Because
the change was not enough to explain the temperature dependence of laser-induced damage thresholds, the temperature
dependence of nonlinear refractive indices would be negligible on laser-induced damage thresholds. On the other hand,
the SBS thresholds also increased with decreasing temperature. This result means that acoustic phonons arise easily at
high temperature. Probably, the SBS phenomenon is one of reasons for temperature dependence of laser-induced damage
The laser-induced damage thresholds in silica glasses at different temperature conditions (123 K - 473 K) by Nd:YAG
laser fundamental (wavelength 1064 nm) and third harmonic (wavelength 355 nm) 4 ns of pulses were measured. In the
results, the damage thresholds increased at low temperature. At 1064 nm, the temperature dependence became strong by
the concentration of impurities. However, at 355 nm, the temperature dependences of almost sample were almost the
same for different concentration of impurities.