Search

We had developed a unique porous thin films by a special coating method1. In this technique, two dielectric materials A and B having different refractive indices nA and nB ,where nA＞nB are simultaneously deposited in vacuum on a substrate such as fused silica or optical glasses. Then the coated surface is processed in ultra-pure water which preferentially dissolves the material B. These processes result in a porous thin film which has gradient refractive index and has the antireflection (AR) property over broad bandwidth. The porous coating obtained by this method cannot apply depositing a multilayered dielectric thin film. We have developed a novel method. The present technique, a dielectric material D and a plastic P are simultaneously deposited in vacuum on a heated-substrate such as fused silica, ceramic or optical glasses. Then the coated surface forms an adaptively mixed thin film ( AMTF ) with dielectric material and plastic. In this coating process, plastics partially evaporate due to the heated-substrate. The refractive index of the coated AMTF mainly decided by the mixing ratio of the dielectric material and plastic. In our samples the damage threshold was confirmed to be 115 J/cm2 at 10 ns and λ=1064 nm. The band width of AMTF with MgF2 and Teflon (AMTF: MgF2 ) was confirmed to cover from 200 to 8000 nm. This AMTF: MgF2 can be applicable not only to AR thin film, but to a high reflectance mirror and polarizer in various high intensity laser syetems. 1K.Yoshida, H.Yoshida, Y.Kato, and C.Yamanaka, Appl.Phy.Lett.47,911(1985)

We have fabricated a 410 x 468 mm size deformable mirror with 100 Bimorph piezoceramic actuators for the LFEX laser system at Osaka University. In the case of Bimorph-type deformable mirrors, the mirror surface had to be polished and coated after bonding the piezoceramic actuators to the rear side of the thin mirror substrate. This provides a good surface figure, but the coating temperature for the high-reflection mirror was strictly limited because of the thermal fragility of piezoceramic actuators. The mirror substrate with the actuators was polished, and an ion-assisted multilayer dielectric coating was produced at 60 degrees Celsius with our 80-inch coating chamber. The flatness of the mirror just after coating was 7 μm, and reduced by aging to 3.2 μm when the mirror was assembled. The surface figure of the assembled mirror with 20 piezostack bonded actuators is demonstrated and a laser-induced damage threshold tested with a witness sample is also reported.

We report the first demonstration of polycrystalline Nd-doped YAG ceramics with almost perfect pore-free structure and Nd-doped YAG single crystal by advanced ceramic processing. The laser conversion efficiency of pore-free polycrystalline Nd:YAG ceramics is extremely high and its optical quality is comparable to that of commercial high quality Nd:YAG single crystal. Moreover, we have succeeded also in fabrication of Nd:YAG single crystal, which enables laser oscillation, by solid-state reaction method. Laser oscillation efficiency was very low when the pores were remained inside single crystal, however the laser oscillation efficiency of pore-free Nd:YAG single crystal was slightly higher than that of polycrystalline Nd:YAG ceramics having grain boundaries. From this fact, it was found that the optical scattering inside the Nd:YAG ceramics occurs mainly at the residual pores and the scattering at the grain boundary is very little. In addition, we confirmed that high concentration Nd:YAG single crystal can be fabricated by sintering method. Applying the above single crystallization technology by sintering method, we have demonstrated the fabrication of layer-by-layer single crystal composite and micron size spherical single crystal.

Text: The laser-induced damage threshold ofpolished fused silica surfaces is much lower than the damage threshold ofits bulk. It is well known that contaminations ofpolished surface are one ofthe causes oflow threshold of laser-induced surface damage. Particularly, polishing contamination such as cerium dioxide (Ce02) compound used inoptical polishing process is embedded inside the surface layer, and cannot be removed by conventional cleaning.For the enhancement of surface damage resistance, various surface treatments have been applied to the removalof embedded polishing compound. In this paper, we propose a new method using selective chemical removalwith high-temperature sulfuric acid (H2S04). Sulfuric acid could dissolve only Ce02 from the fused silicasurface. The surface roughness offused silica treated H2504 was kept through the treatment process. At thewavelength of 355 nm, the surface damage threshold was drastically improved to the nearly same as bulkquality. However, the effect of our treatment was not observed at the wavelength of 1064 nm. The comparisonwith our previous results obtained from other surface treatments will be discussed.Keywords: fused silica, laser-induced surface damage, cerium dioxide (CeO2), selective chemical removal,sulfUric acid (H2S04)

The surface-damage resistance of fused silica was enhanced 2.8-fold by removing a subsurface damage. For the conventionally polished fused silica surface, μm-scale subsurface damage and a shallow (20 nm to 100 nm) structurally modified zone produced during grinding and polishing were formed on the top of surface. Several surface etching techniques and super-precise polishing process were used to remove subsurface damage from a fused silica surface. First the conventionally polished surfaces were chemically etched in a buffered HF solution to remove 300μm of surface material, and then super-precise polishing was performed to obtain an optical surface. After that, the polishing compound was removed by using ion-beam etching. The effect of subsurface damage on laser damage resistance was characterized by the measuring of the laser-induced damage threshold (LIDT) for the laser radiations of 1064 nm and 266 nm respectively. For the wavelength of 1064 nm, the effect of the removal of subsurface damage wasn't clearly seen, although the enhancement of surface-damage resistance by the ion-beam etching could be confirmed. However, in the case of 266 nm, enhanced LIDT of 28 J/cm2 was obtained from the subsurface damage removed surface. The surface LIDT increased by 2.8 times compared to that of conventionally polished fused silica surfaces.

Silica glasses can be used as optical material in various applications such as deep-ultraviolet lithography and nuclear fusion, because they have no internal absorption and extremely small defects. In these applications, laser-induced bulk damage is an important factor in practical use. The laser-induced damage threshold (LIDT) is expected to depend on the types and production conditions of silica glasses. In this paper, the LIDT of synthetic fused silica which contains 4~1220ppm of OH, 1.7x1018~2x1019 molecules/cm3 of H2, and 100~3700 ppm of fluorine were studied by irradiating the higher harmonics of Nd:YAG laser at 355 and 266 nm with pulse width of 4 ns. Current experimental results show that the improvement of the LIDT with impurity contained silica glass is possible.

High-power solid-state ultraviolet (UV) lasers by using a have been in high demand because of their convenient operation procedure. An effective technique for UV generation is cascaded sum-frequency generation pumped by the output of near-IR solids-state lasers. The performance of such solid-state UV lasers appears to depend on the ability and reliability of nonlinear optical (NLO) crystals that are employed for laser frequency conversion. Discovery of CsLiB 6 O 10 (CLBO) crystals have enabled the production of such practical high-power all solid-state UV lasers. In 2001, UV output power up to 23.0 W by fourth harmonic generation of Nd:YAG laser was achieved. It is fact that laser-induced damage of NLO crystal is a limiting factor on reliable operation of high-power solid-state UV lasers. Bulk laser-induced damage of NLO crystal is related to the crystal's quality. In this paper, we have investigated the relationship among the bulk laser-induced damage threshold (LIDT), dislocation density and absorption of laser light in CLBO crystals with various crystallinity. The bulk LIDT of CLBO increased with decreasing dislocation density. High-quality crystals with a higher LIDT (15 - 18 GW/cm 2 ) have a lower dislocation density of 6.6 x 10 3 /cm 2 than that of conventional CLBO (~15.0 x 10 3 /cm 2 ). The relationships between crystal quality and absorption of laser light will be presented.

A transparent low refractive index SiO2 film was laminated on a glass substrate with photochemical reaction by an Xe2* excimer lamp in the presence or NF3, 02 and silicon wafer at room temperature. The glass substrate and the silicon wafer were placed in the reaction chamber, which was filled with NF3 and O2 gases in the mixing ratio of 10:1 and under 330 Torr. The Xe2* excimer lamp was, then, irradiated for 20 minutes. The SiO2 film was spontaneously laminated on the glass substrate by repeating an adsorption of SiF4 and a photochemical oxidization with NO2, which was photo-dissociated from a mixed gas of NF3 and O2. The film thickness was 160 nm and the infrared spectrum was measured; the Si-O peaks were depicted at 600, 700, 1100[1/cm-1]. And Si-F peak was observed at 740[1/cm-1]. Then, the refractive index of the SiO2 film was 1.36. After annealing the film for one hour at 200 degree, the refractive index increased to 1.42.

Optical characteristics of a new commercially available perfluorinated organic polymer have been measured to prepare a quarterwave AR coating for Nd3+:YAG crystal and other optical materials. Coatings are carried out by dip method from solution at room temperature, they have a refractive index of 1.364. The laser-induced damage threshold at 1064nm, 532nm, 355nm, and 266nm is approximately same values with Teflon AR 2400.

There has been great interest in high-repetition, high-power ultraviolet (UV) source for various applications in semiconductor processing, micro machining, and other fields. Discovery of CsLiB 6 O 10 (CLBO) crystals have enabled the production of such practical high-power all solid-state UV lasers. In 2001, UV output power up to 23.0 W by fourth harmonic generation of Nd:YAG laser was achieved. In general, one of the limiting factors for the development of high-power solid-state UV lasers is laser-induce damage of NLO crystal due to some kinds of defects inside the materials. Recently, we have succeeded to grow the high crystallinity CLBO with an enhanced bulk laser damage resistance. On these samples, an increase in the surface damage resistance could be expected. Measurement of the surface laser-induced damage threshold (LIDT) on CLBO crystals with various crystallinity was performed by using a 266 nm laser. For the crystal with high damage resistance (15-18 GW/cm 2 ), LIDT of as-polished surface was 1.3 times higher than that of crystal with conventional damage resistance (9-12 GW/cm 2 ). In addition, polishing compound embedded inside the crystal surface was removed by using an ion-beam etching process. We have observed 1.5 times improved surface LIDT by ion beam etching for both high damage resistance and conventional damage resistance samples. The relationships between vickers hardness and crystal quality will be presented.

Laser-induced damage of thin films coated on optical components used for UV and DUV lasers are serious problems. Gradient refractive index antireflection films are produced by the combination of evaporation and leaching process on fused silica substrate. It has a high laser-induced damage threshold of 7.4-8.7 J/cm 2 at 266 nm. The reflectance is about 0.35%.

High-power solid-state UV lasers are in high demand because of the convenient operation procedure. An effective technique for UV generation is cascaded sum-frequency generation pumped by the output of near-IR solid state lasers. The major challenge to such a UV laser seems to depend on the ability and reliability of the nonlinear optical (NLO) crystals that employed for frequency up conversion. The recent development of borate materials such as CsLiB 6 O 10 (CLBO) and (open square)-BaB 2 O 4 (BBO) have permitted an improved UV generation. In this work, we have investigated the bulk laser damage and the light scatter from bulk defects in CLBO and BBO crystals.

UV and DUV lasers are very important for many applications such as micro-machining, new material creation, medical care, and photo-lithography. Laser-induced damage of thin films coated on optical components used for UV and DUV lasers are serious problem. We have developed a new technique for producing anti- reflection (AR) coating on various substrate materials including fused silica glass and CaF2 crystal. In this technique, porous magnesium fluoride (MgF2) AR coating was developed. The reflectance of the porous AR coating on fused silica substrate was 0.3% at 500 nm.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The development of high power lasers requires highly damage- resistant optical coatings. Present multilayer dielectric coatings do not have sufficient laser-induced damage thresholds (LIDTs) to pulsed lasers, particularly in the short wavelength region. The LIDT strongly depends on the absorption coefficient of optical coatings and the absorbing contaminants on the optical substrate. The absorption of optical coatings can be minimized by optimizing the deposition condition. However, polishing compound embedded inside subsurface among the absorbing contaminants cannot be completely removed by standard optical cleaning techniques. In this paper, the significant improvement of LIDT of optical coatings on subsurface-damage removed fused silica glass due to ion etching is presented.

Effect of ion beam etching on surface damage resistance was investigated in CsLiB 6 O 10 (CLBO) crystal. In high-power UV operation, an as-polished CLBO surface was damaged due to absorption of the polishing compound embedded inside the crystal surface. In the as-polished surface of CLBO, polishing compound ZrO 2 (absorption edge is about 300 nm) was detected to a depth of 60 nm. We have removed polishing compound with ion beam etching without degrading the surface quality. The effects of polishing compound removal on surface damage were characterized for the surface laser-induced damage threshold (LIDT) at 355 nm (pulse width 0.85 ns) as a function of etching depth and surface lifetime for the generation of fourth-harmonic of ND:YAG laser (266 nm, 20 ns, 4 kHz). We found an improvement of the surface damage resistance. LIDT of etched surface increased up to 15 J/cm 2 as compared with that of the as-polished surface of 11 J/cm 2 . Etched CLBO surface also exhibits an improvement lifetime 4 times longer than that of as-polished surface.

The improved Sellmeier's equations and thermo-optic dispersion formula that reproduce well our experimentalresults for harmonic generation of C02 laser harmonics at 3.5303-5.2955 LL m are presented. These formulas are believed to be highly useful for predicting the temperature-tuned 90 ° phase-matched OPO in the mid-infrared. Keywords : potassium niobate, second-harmonic generation, sum-frequency generation, optical parametric oscillator,Sellmeier's equations, thermo-optic dispersion formula, C02 laser, Nd:YAG laser. 1. INTRODUCTION Although the linear and nonlinear optical properties of KNbO3 have been investigated in detail by Zysset et al',Biaggio et a12, Jundt et al3, Ghosh4, and Casix me5, until now five papers have been published on mid-infrared opticalparametric oscillation ( mid-rn OPO ) 610 in this material. In order to predict precisely the tuning curves for mid-JR OPO under 2 /2 m excitation along z ( =b ) axis, we have determined the refractive indices and thermo-optic constants fromthe phase-matching conditions and the temperature phase-matching bandwidths ( F'WFIM ) for type-i second-harmonic( SHG) and sum-frequency generation ( SFG ) of the harmonics of a TEA

The possibility has been investigated that the optical glasses could be used as a solid-state phase conjugator for high power lasers. The solid SBS (stimulated Brillouin scattering) materials as effective phase conjugator can be performed with stable operation for improving the beam quality of high- average-power lasers. The advantage using solid material as phase-conjugating media is its reliability, environmental harmlessness and easy handling. An optical fiber offers a low power SBS threshold and stable performance at input energies of a few mJ level. Using a bulk optical glass, flint glass of SF-6 showed a much lower SBS threshold at 1.06 micrometer wavelength than fused silica glass because of its high SBS gain coefficient (9 cm/GW). However, the operating energy range of flint glass is limited by its low damage threshold. For long pulse duration using a longer focusing lens, fused- silica glass had high SBS reflectivity over 90% without any damage occurrence at the incident energy of 600 mJ in 18 ns at a repetition rate of 10 Hz. It shows also high SBS reflectivity of approximately over 95% at a single shot operation with no damage observation at a pumping energy of 2.3 J in 18 ns. These properties of the fused-silica are almost comparable with those of highly performed gases or liquids. Damage-free operation using a fused-silica as a better phase conjugation material would lead to construct more compact laser-diode pumped all-solid-state laser system.

Multiple-pulse laser induced bulk damage of various types of silica glasses at wavelengths of 532 and 355 nm with laser pulse width of approximately 6 ns repetition rate of 10 Hz were measured. At 532 n, no apparent difference was observed in multiple pulse laser-induced bulk damage threshold (MLIDT) except for As fused quartz (FQ) produced by electrical melting of natural quartz powder whose MLIDT at a constant number of pluses is less than those of the other silicas. MLIDT at 355 nm were different among samples. The MLIDT of FQ disperse much more than that of synthetic fused silica. This could be derived from the distribution of the metallic impurity content in FQ.

Exfoliation of optical thin films at high humidity in the air is serious problem. Deterioration of three kind of optical thin films deposited on three types of substrate with different water-resistance at high humidity condition was studied. Thin films deposited on fused silica glass with highest water-resistance showed a best result for the exfoliation problem. Silica film seamed to be best for the surface observation at high humidity, but the laser -induced damage threshold was considerably lowered.

High-power all solid-state UV lasers are highly demanding for many applications because of their compactness and ease of operation. An effective technique for UV generation is cascaded sum-frequency generation pumped by the output of near-IR solid-state lasers. The performance of these laser systems is limited by the laser power handling capability of the nonlinear optical crystals that employed for frequency up-conversion.

Nonlinear-optical crystals are attractive materials for the high-power frequency converter with high damage threshold, phase matching characteristics, wide transparency range, and large effective nonlinear coefficients. Especially as to the power laser applications, its laser-induced damage threshold determines the limit of performance in the optical system. The threshold depends not only on the intrinsic material parameters but also on the laser beam parameters is use. We have investigated the bulk damage threshold of several crystals at a single-shot operation for frequency converter depending on the laser irradiation direction and its polarization. For KDP and CLBO crystals, the damage threshold in the direction of c-axis is about tow times higher than that in the a- or b-axis at 1.064 micrometers of wavelength. This result is consistent with the molecular bonding structure in different directions of the crystal. The relation between the bulk damage pattern and the crystal structure is also discussed.

Bulk laser induced damage in KDP crystal was measured using a single-shot 1-ns pulse Nd:YAG laser in a transverse and longitudinal single mode. It is found that the damage threshold of KDP single crystal depends on the laser irradiation direction, polarization direction and laser wavelength. The damage threshold in the direction of c-axis is about two times higher than that in the a(b)-axis at 0.532 and 1.064-micrometers wavelength. This result consistent with the experimental results that (001) direction is mechanically weaker than (100) direction with the result of mechanical strength tests.

Laser induced damage threshold (LIDT) of various types of vitreous silica at 1064, 532, 355 and 266 nm were investigated. At 1064 nm no difference of LIDT were observed for all samples. At 1064-355 nm, wavelength dependence of LIDT of synthetic fused silica (SFS) can be well described by a relation Ith equals 1.45 (lambda) 0.43 where Ith is LIDT in J/cm2 and (lambda) is wavelength in nm. At 266 nm, however, LIDTs were smaller than a half of the calculated value from above relation. This difference can be explained by the damage mechanism; at 266 nm, two-photon absorption-induced defects lowered the LIDT same as in the case of KrF-excimer-laser induced defects, whereas at longer wavelength two-photon processed is not occur. LIDTs of fused quartz (FQ) at 532 and 355 nm, and that of a SFS containing about 1000 ppm of Cl and no OH at 355 nm were a little lower than those of the other SFS. This may be related to the absorption of metallic impurities in FQ and dissolved Cl2 molecules in SFS. At 266 nm, on the other hand, LIDTs of FQs were lower than those of most SFSs.

Optical coatings deposited by conventional evaporation processes have a number of shortcomings including weak adhesion which depends upon the subsurface condition of substrate and the change of spectral characteristics in the relative humidity. Thin films deposited by reactive-rf- magnetron sputtering improve these problems. Laser-induced damage thresholds at 355nm, measured by 1-on-1 and R-on-1 methods, of substrate and single high index zirconia, hafnia and aluminum oxide, and low-index silicon dioxide coated by magnetron sputtering, are reported.

The laser-damage resistance of CLBO and fused silica surfaces was successfully improved after removing polishing compound by ion beam etching. The polishing compound embedded in the CLBO and fused silica surfaces was to a depth of less than 100 nm. We were able to remove polishing compound without degrading the surface condition when the applied ion beam voltage was less than 200 V. After surface etching, the effects of polishing compound removal on surface damage were characterized for the surface laser- induced damage threshold at 355 nm and surface lifetime at 266 nm as a function of etching depth. In the fused silica surface, we found improvement of the surface LIDT up to 15 J/cm2 as compared with that of the as-polished surface of 7.5 J/cm2. For the irradiation of a 266 nm high- intensity and high-repetition laser light, the exit surface lifetime of CLBO and fused silica could be more doubled compared with that of the as-polished surface.

Grinding is considered to be a rough machining process in the field of optics; a polishing process must follow the grinding process for getting optical-quality surfaces. An ultraprecision surface grinder with hydrostatic oil bearings and a glass-ceramic spindle of extremely low thermal expansion was developed to get smooth optical surfaces without any polishing process. Various optical materials such as NbF1, BK7, LHG08 fused silica, KTP, KDP and CLBO were ground into optical surfaces after empirically determining the conditions required to attain ductile-mode grinding. An extremely smooth surface less than 0.1 nm rms was obtained on BK7 glass by the ultraprecision grinding process. The laser-induced damage threshold was measured on variously finished LHG-8 laser glass at (lambda) equals 1.053 micrometers and 1-ns pulse width. The damage threshold was measured at 22.2 J/cm 2 on a ground surface with the polarization parallel to the grinding direction. This number is higher than that obtained by optical polish. The damage threshold of 293 J/cm 2 was also obtained on a ground LHG-8 glass surface at (lambda) equals 1.053 micrometers and 30-ns pulse width.

Laser-induced damage threshold (LIDT) of various types of vitreous silica at 1064, 532, 355 and 266 nm was investigated. At 1064 nm no difference of LIDT was observed for all samples. At 1064-355 nm, wavelength-dependence of LIDT of synthetic fused silica (SFS) can be well described by a relation Ith equals 1.45 (lambda) 0.43, where Ith is LIDT in J/cm2 and (lambda) is wavelength in nm. At 266 nm, however, LIDTs were smaller than a half of the calculated value from above relation. This difference can be explained by the damage mechanism; at 266 nm, two-photon absorption-induced defects lowered the LIDT same as a KrF-excimer-laser induced defects, whereas at longer wavelengths two photon process does not occur. LIDTs of fused quartz (FQ) at 532 and 355 nm, and that of a SFS containing about 1000 ppm of Cl and no OH at 355 nm were a little lower than that of the other SFSs. This may be related to the absorption of metallic impurities contained in FQ and dissolved Cl2 molecule in SFS.

This paper investigates ion etching process to the surface of CsLiB 6 O 10 (CLBO) crystal. Laser-induced surface damage was reduced and surface durability of CLBO crystal was improved by removing the subsurface embedded polishing compound. There was no surface degradation as a result of the ion etching. The effects of ion etching on surface damage were measured by a 1-on-1 test at a laser wavelength of 266 nm. Durability of the CLBO crystal was tested by the approximately 7 W fourth harmonic generated by a Nd:YAG laser. The durability of the ion etched surface was improved more than 10 times as compared with the as-polished surface.

Optical coatings deposited by conventional evaporation processes have a number of shortcomings including weak adhesion which depends upon the subsurface condition of substrate, and the change of spectral characteristics and film stress in the relative humidity. Thin films coated by reactive-rf-magnetron sputtering improve these problems. In this paper, optical properties of thin films deposited by reactive-rf-magnetron sputtering are presented.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The design study of the laser fusion power plant KOYO has been conducted as a joint program of universities, national laboratories, and industries in Japan and also with international collaborations. In the design of KOYO, the gain scaling of direct drive implosion with 0.35 micrometers wavelength laser light is used. A driver of diode pumped solid state laser generates 4 MJ/pulse with 12 Hz and the output pulses are switched to deliver the laser energy success five to four chambers, which operate with 3 Hz. The chamber wall is protected with thick liquid metal which flows down in a SiC woven tube. Following to the conceptual design study, the critical key issues which may affect the technical and economical and economical feasibility of the commercial power plant KOYO have been examined. Research and development of some key technologies have been performed. As the results of the studies on KOYO, it is concluded that thee technical and economical feasibility of laser fusion reactor is well in our scope to reach.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A dielectric barrier discharge type Xe excimer lamp has been used for etching of some organic materials which can be widely used for opt-electric applications. Using polymethylmetacrylate (PMMA) and polyimidic (PI) resin as samples, they were irradiated at an intensity of 12 mW/cm2. In both cases, etch rates could be varied by changing the species and pressures of gases in the chamber. The maximum etch rates for PMMA and PI were approximately 7 and 10 nm/min, respectively. The etched surface was found to be very smooth compared to the surfaces etched by the ArF excimer laser. Because the excimer lamp produces incoherent radiation, uniform irradiation over a large area without speckling or interference fringe has been obtained. Furthermore, because the peak power of the lamp is significantly lower than that of excimer lasers, effective quantum effects and photochemical effects without thermal effects have been observed. We applied this technique for etching single crystalline organic opt-electric materials such as 1-arginin phosphate monohydrate and 1-histidine tetrafluoroborate.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The development of high power lasers requires highly damage-resistant optical coatings. Present multilayer dielectric coatings, do not have sufficient laser-induced damage thresholds (LIDTs) to pulsed lasers, particularly in the short wavelength region. LIDT strongly depends on the absorption coefficient of optical coatings and the impurities on the optical substrate. The absorption coefficient of optical coatings can be minimized by optimizing the deposition conditions. However, the impurities on the optical substrate cannot be completely removed by standard optical cleaning techniques. In this paper, the significant improvement of LIDT of optical coatings on subsurface-damage removed fused silica glass due to ion beam etching is presented.

Transparent YAG ceramics with nearly the same optical characteristics as those of a single crystal were fabricated by a solid-state reaction method using high-purity powders. The average grain size and relative density of the 1.1 at % Nd:YAG ceramics obtained were about 50 micrometers and 99.98% respectively. An oscillation experiment was performed on a cw laser by the diode laser excitation system using the fabricated ceramics. The experimental results indicated an oscillation threshold and a slope efficiency of 309 mW and 28% respectively. These values were equivalent or superior to those of 0.9 at % Nd:YAG single crystal fabricated by the Czochralski (CZ) method.

Glass laser is the best one to radiate high-average power used for material processing and X- ray production. However, the thermal conductivity of laser glass is very low compared with other laser crystals. We report newly developed glass slab laser operating at 8 Hz without laser glass breakage, the geometrical shape of the glass slab is different from conventional ones. Our glass slab is composed of small square glasses. Slab laser (10 t mm X 40 w mm X 125 l mm) at pumping power of 1500 W emitted the average output power of 46 W.

Antireflective thin film coated by Teflon TM AF 2400 amorphous fluoropolymer to improve the pumping efficiency of disk amplifier used for laser fusion is presented. Improvement of pumping efficiency of about 8% can be attained by Teflon TM AF 2400 coating to the outer surface of Xe flashlamps and the shield glasses set up between Xe flashlamps and disk glasses. Flashlamp irradiation test to the Teflon thin film coated on flashlamp wall was carried out. Flashlamp-induced damage for 100 irradiations at about 15.4 J/cm 2 was not observed.

We have developed a coating chamber (diameter of 1600 mm (phi )) including ion guns for removal of subsurface which can deposit on the substrate of the maximum diameter of 800 mm (phi ). The damage threshold of AR coating on subsurface removed fused silica glass of 800 mm (phi ) showed 7 J/cm 2 at 355 nm, pulsewidth of 0.3 ns. (Abstract only)

The laser damage thresholds and other properties of 352 nm fluoride AR coatings for KDP substrate were investigated. The HoF 3 (or LaF 3 )/AlF 3 AR coating deposited by IAD process had a high threshold of 24 J/cm 2 (10 ns) when the laser conditioning procedure was used. These fluoride ARs were durable for wiping and also did not degrade during more than one year under room atmosphere.

We have developed a super-lightweight mirror usable for laser fusion for the first time. The porous fused silica with density of 0.2 g/cm3 was sandwiched between two fused silica plates. The porous silica and the fused silica plates were contacted under high temperature of about 1500 - 1600 degree(s)C. The weight and optical flatness of the mirror (3500 mm(phi ) X 36 mmt) were about 1.7 kg and (lambda) /8, respectively. A larger diameter mirror of 530 mm(phi ) is being fabricated.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

High-power glass laser systems for laser fusion use many BK-i glasses and the laser-induced damage of this glass is a serious problem. Since the BK-i glass is generally melted in platinum (Pt) crucibles the laser damage due to metallic Pt particles occurs inside the glass. The Pt particles cause fractures when irradiated at a fluence higher than 3 J/cm with 1064 nm 15 ns laser pulse. The size of the fracture increases with irradiation fluence and the damaged glass has to be replaced. In special cases BK-i glass is melted in ceramic crucibles but there is a problem of cristobalite production inside the BK-i glass. The laser damage threshold of cristobalite is about 4. 5-5. 5 JIcm with 1064 nni 15 ns laser pulse. We have found average damaged particles in BK-i glasses melted in Pt and ceramic crucibles to be 200-400 and 10 particles per liter respectively. At present laser damage in BK-i glass is being improved by changing a composition ratio of melting materials.