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Annual symposium on optical materials for high power lasers
Conference Report Annual Symposium on Optical Materials for High Power Lasers 27-29
October
1993, Bouldel; Colorado,
USA
The symposium on Optical Materials for High Power Lasers, commonly known as the Boulder Damage Conference, has been an annual event ever since it started in 1969. It has grown from being just a meeting sponsored by the ASTM to a fully grown major conference which draws its participants from all parts of the world and publishes its proceedings through the SPIE (Vol 2114). The 1993 conference was attended by 132 people all actively engaged in research, development and use of high power laser systems. The affilliation split was interesting showing that, this year, it was dominated by industrial personnel (59) rather than by university (35), government (29) or the military (9) even though the single largest contingent (13) came from the Lawrence Livermore National Laboratories. It was also noticeable that there was a larger than usual international presence (36). The conference took the form of four sessions per day plus 2 X 1 hour long poster sessions/refreshment breaks. Apart from a 25-year review of the laser damage symposium by Dr A. H. Guenther there were five invited papers, 32 read papers and 41 poster papers. The breakdown of the 78 papers presented indicates that 20 were on the subject of materials and measurements, 11 on surface effects, 34 on coatings and 13 on fundamental issues/theory. Although the symposium’s main aim is to encourage research into new and improved optical materials it is axiomatic that after 25 years the supply of new materials will be scarce. The only ‘new’ materials on the horizon, which everybody is looking to for high damage thresholds, is CVD diamond. However, there were only three papers mentioning this and only one of these gave any values for LIDT. Other materials under discussion were: l dye impregnated porous glass for ‘solid’dye laser rods, l Ti:A1203 laser host material, l gamma and ultra-violet induced colour centres in glass, alkali halides and magnesium oxide, l LIDT testing of 50 samples of NaCl crystals at five wavelengths. The measurement of laser-induced damage threshold has finally come of age with the promulgation of IS011254. This lays down a set procedure for making measurements based on the percentage damage/damage frequency versus increasing power or energy density technique rather than the older American technique of plotting damage or no damage versus energy density. The search goes on to discover methods of characterizing materials so that the probability of laser-induced damage at a certain flux can be adduced without damaging the material or component. Although this may never truly come to pass 0030-3992/94/05/0373-02 Optics & Laser Technology Vol 26 No 5 1994
there are several very powerful techniques that have been developed including the photothermal technique, atomic force microscopy and the use of liquid crystal layers to show up the onset of damage. It has long been realized that the top few micrometres of the surface of the bulk material are extremely susceptible to laser-induced damage and the search goes on for better polishing techniques. Methods discussed this year included a chemical etch followed by laser treatment for the removal of crystal grains at the surface of CVD diamond. Ion exchange treatment of surfaces to lower the VSWR, COZ laser polishing and thermal and laser conditioning of material and thin film surfaces were all discussed as having advantages in particular circumstances. Another subject that reared its head again (most subjects repeat in a cyclic fashion after a few years) was that of the effect of surface contamination on both the deposition of thin films and on the LIDT of bulk and coated optics. Although a mini symposium on coatings was planned to be a special feature of this year’s conference with two invited talks and several overviews of what had been published in the past the actual content was limited in that coatings are now very well understood and there are very few new coating materials under development. In the ultra-violet, visible and near in&+red the choice is now firmly for oxides and for the it&a-red fluorides. There is, however, a growing resistance to the use of thorium fluoride and this is commonly replaced by yttrium or yterbium fluoride. The only newer materials which threaten the dominance of these are the nitrides which are notoriously hard to deposit but which have superior thermo-mechanical properties. The methods of deposition discussed included ion-beam and ion-assisted sputtering, electron-beam, dc magnetron sputtering, chemical vapour deposition, molecular beam epitaxy, Langmuir Blodgett films and the sol-gel process. It was emphasized several times that no one deposition technique could be used for the whole range of coating materials and that every coating material had perhaps two deposition techniques which could be used with equal success. High energy deposition was agreed to yield the best coating quality in most cases as the growth of nodules/ defects was to be discouraged. Two schools of use have also sprung up, the first being those who require a long lasting coating on a component which is thrown away once it is damaged. The other category of coating is that where the component is costly, the lifetime in terms of number of laser pulses is not incredibly high and where a soft, replaceable coating can be used. The former categorey covers most of the commercial and military laser systems while the latter is
@ 19194 Butterworth-Heinemann
Ltd 373
Conference that of the large fusion lasers where polymer coatings have been used to advantage. Although theory is a difficult subject to cover at a conference primarily devoted to experimental measurement, the use of the poster sessions to enable the whole of a mathematical treatment to be on view alongside an explanation of the rationale and conclusions makes a good way to allow experimentalists to appreciate what is being done without making the session wearisome. Two subjects came up in some detail this year, one being the calculation of the temperature distribution for both pulsed and cw beams throughout a material and the other being the significance of the spot size/number of data points in the damage frequency method. A further interesting paper was concerned with the calculation of the local electric field in an irradiated sample. As would be expected at a conference subtitled ‘laser damage symposium’ the majority of the experimental papers reported laser damage thresholds. Unfortunately, each wavelength, each pulse length and, to a certain extent, each laser focused spot size yields slightly different results and therefore the subjects of scaling laws, 1 on 1 and n on 1 damage as well as the pulse frequency dependance have to be discussed in detail. It is relevant to note that the early laser damage studies were made using near i&a-red lasers (usually 1.060 km Nd:glass and 1.064 pm Nd:YAG), that they then moved to longer wavelength lasers (10.6 pm) before moving back to the visible and ultra-violet. So much data has now been amassed that the wavelengths of interest vary along with the application. Early work centred on the military laser rangefinder and govermental fusion lasers. It was instructive to note that at this conference the military involvement had shrunk to very low proportions and even the fusion laser interest was waning. Even three years ago the conference would have been dominated by personnel from the US armed forces and laboratories (e.g. China Lake) and by personnel from Los Alamos and Lawrence Livermore. While the single largest contingent did come from the latter establishment the other two establishments were not well represented. Much of the drive for better optical components now comes from the manufacturers of laser systems, the applications and interests of the participants included: 0 diamond windows fo; the Gee-electron laser and dual band laser systems, 0 the development of solarization resistant laser windows and host materials, 0 materials for optical parametric oscillators, 0 non-linear materials, 0 diffractive optical components, 0 kineform lens systems for focusing lasers into optical fibres, 0 the development of high prf lasers for machining, and materials inspection, 0 lidar, spectrophotometry optical coatings for a wide range of interests. :: tentative date for the 1994 symposium on optical materials for high power lasers has been set as the week starting Monday 24 October. It was also announced that Applied Optics would be
Report
celebrating ‘25 years of laser damage’ with an issue featuring papers on the subject. The manuscript submission date is 30 April 1994 with a publication date of 1 November 1994. The guest editors will be Dr A. H. Guenther (Sandia National Laboratories) and T. W. Humpherys (US DOD). R. M.
Ltd
Articles for inclusion
in future issues of Optics & Laser Technology include:
Quantitative Schlieren measurements of a high energy electromagnetic transducer acoustic shock field M. T. Carnell, D. C. Emmony Nd: YAG laser delivery system utilizing GRIN-rod lenses as focusing-collimating optics R. Falciai, A. Schena, A. Del Bianco, T. Pascucci A continuously variable attenuator A. E. Smith, N. R. Heckenberg
for the CO, laser
Effects of stray reflections on the performance of semiconductor laser amplifiers as in-line optical repeaters C. Y.J. Chu, H. Ghafouri-Shiraz Evanescent field fibre-optic chlorine A. Safaai-Jazi, J. V. Petersen
sensor
Research on optical multistage butterfly interconnection and optoelectronic logic operations De-Gui Sun, Na-Xin Wang, Li-Ming He, Mai Xu, GuoDong Liang, Jie Zheng
Optics
374
Wood
Cosolas
& Laser Technology Vol 26 No 5 1994
October
1993, Bouldel; Colorado,
USA
The symposium on Optical Materials for High Power Lasers, commonly known as the Boulder Damage Conference, has been an annual event ever since it started in 1969. It has grown from being just a meeting sponsored by the ASTM to a fully grown major conference which draws its participants from all parts of the world and publishes its proceedings through the SPIE (Vol 2114). The 1993 conference was attended by 132 people all actively engaged in research, development and use of high power laser systems. The affilliation split was interesting showing that, this year, it was dominated by industrial personnel (59) rather than by university (35), government (29) or the military (9) even though the single largest contingent (13) came from the Lawrence Livermore National Laboratories. It was also noticeable that there was a larger than usual international presence (36). The conference took the form of four sessions per day plus 2 X 1 hour long poster sessions/refreshment breaks. Apart from a 25-year review of the laser damage symposium by Dr A. H. Guenther there were five invited papers, 32 read papers and 41 poster papers. The breakdown of the 78 papers presented indicates that 20 were on the subject of materials and measurements, 11 on surface effects, 34 on coatings and 13 on fundamental issues/theory. Although the symposium’s main aim is to encourage research into new and improved optical materials it is axiomatic that after 25 years the supply of new materials will be scarce. The only ‘new’ materials on the horizon, which everybody is looking to for high damage thresholds, is CVD diamond. However, there were only three papers mentioning this and only one of these gave any values for LIDT. Other materials under discussion were: l dye impregnated porous glass for ‘solid’dye laser rods, l Ti:A1203 laser host material, l gamma and ultra-violet induced colour centres in glass, alkali halides and magnesium oxide, l LIDT testing of 50 samples of NaCl crystals at five wavelengths. The measurement of laser-induced damage threshold has finally come of age with the promulgation of IS011254. This lays down a set procedure for making measurements based on the percentage damage/damage frequency versus increasing power or energy density technique rather than the older American technique of plotting damage or no damage versus energy density. The search goes on to discover methods of characterizing materials so that the probability of laser-induced damage at a certain flux can be adduced without damaging the material or component. Although this may never truly come to pass 0030-3992/94/05/0373-02 Optics & Laser Technology Vol 26 No 5 1994
there are several very powerful techniques that have been developed including the photothermal technique, atomic force microscopy and the use of liquid crystal layers to show up the onset of damage. It has long been realized that the top few micrometres of the surface of the bulk material are extremely susceptible to laser-induced damage and the search goes on for better polishing techniques. Methods discussed this year included a chemical etch followed by laser treatment for the removal of crystal grains at the surface of CVD diamond. Ion exchange treatment of surfaces to lower the VSWR, COZ laser polishing and thermal and laser conditioning of material and thin film surfaces were all discussed as having advantages in particular circumstances. Another subject that reared its head again (most subjects repeat in a cyclic fashion after a few years) was that of the effect of surface contamination on both the deposition of thin films and on the LIDT of bulk and coated optics. Although a mini symposium on coatings was planned to be a special feature of this year’s conference with two invited talks and several overviews of what had been published in the past the actual content was limited in that coatings are now very well understood and there are very few new coating materials under development. In the ultra-violet, visible and near in&+red the choice is now firmly for oxides and for the it&a-red fluorides. There is, however, a growing resistance to the use of thorium fluoride and this is commonly replaced by yttrium or yterbium fluoride. The only newer materials which threaten the dominance of these are the nitrides which are notoriously hard to deposit but which have superior thermo-mechanical properties. The methods of deposition discussed included ion-beam and ion-assisted sputtering, electron-beam, dc magnetron sputtering, chemical vapour deposition, molecular beam epitaxy, Langmuir Blodgett films and the sol-gel process. It was emphasized several times that no one deposition technique could be used for the whole range of coating materials and that every coating material had perhaps two deposition techniques which could be used with equal success. High energy deposition was agreed to yield the best coating quality in most cases as the growth of nodules/ defects was to be discouraged. Two schools of use have also sprung up, the first being those who require a long lasting coating on a component which is thrown away once it is damaged. The other category of coating is that where the component is costly, the lifetime in terms of number of laser pulses is not incredibly high and where a soft, replaceable coating can be used. The former categorey covers most of the commercial and military laser systems while the latter is
@ 19194 Butterworth-Heinemann
Ltd 373
Conference that of the large fusion lasers where polymer coatings have been used to advantage. Although theory is a difficult subject to cover at a conference primarily devoted to experimental measurement, the use of the poster sessions to enable the whole of a mathematical treatment to be on view alongside an explanation of the rationale and conclusions makes a good way to allow experimentalists to appreciate what is being done without making the session wearisome. Two subjects came up in some detail this year, one being the calculation of the temperature distribution for both pulsed and cw beams throughout a material and the other being the significance of the spot size/number of data points in the damage frequency method. A further interesting paper was concerned with the calculation of the local electric field in an irradiated sample. As would be expected at a conference subtitled ‘laser damage symposium’ the majority of the experimental papers reported laser damage thresholds. Unfortunately, each wavelength, each pulse length and, to a certain extent, each laser focused spot size yields slightly different results and therefore the subjects of scaling laws, 1 on 1 and n on 1 damage as well as the pulse frequency dependance have to be discussed in detail. It is relevant to note that the early laser damage studies were made using near i&a-red lasers (usually 1.060 km Nd:glass and 1.064 pm Nd:YAG), that they then moved to longer wavelength lasers (10.6 pm) before moving back to the visible and ultra-violet. So much data has now been amassed that the wavelengths of interest vary along with the application. Early work centred on the military laser rangefinder and govermental fusion lasers. It was instructive to note that at this conference the military involvement had shrunk to very low proportions and even the fusion laser interest was waning. Even three years ago the conference would have been dominated by personnel from the US armed forces and laboratories (e.g. China Lake) and by personnel from Los Alamos and Lawrence Livermore. While the single largest contingent did come from the latter establishment the other two establishments were not well represented. Much of the drive for better optical components now comes from the manufacturers of laser systems, the applications and interests of the participants included: 0 diamond windows fo; the Gee-electron laser and dual band laser systems, 0 the development of solarization resistant laser windows and host materials, 0 materials for optical parametric oscillators, 0 non-linear materials, 0 diffractive optical components, 0 kineform lens systems for focusing lasers into optical fibres, 0 the development of high prf lasers for machining, and materials inspection, 0 lidar, spectrophotometry optical coatings for a wide range of interests. :: tentative date for the 1994 symposium on optical materials for high power lasers has been set as the week starting Monday 24 October. It was also announced that Applied Optics would be
Report
celebrating ‘25 years of laser damage’ with an issue featuring papers on the subject. The manuscript submission date is 30 April 1994 with a publication date of 1 November 1994. The guest editors will be Dr A. H. Guenther (Sandia National Laboratories) and T. W. Humpherys (US DOD). R. M.
Ltd
Articles for inclusion
in future issues of Optics & Laser Technology include:
Quantitative Schlieren measurements of a high energy electromagnetic transducer acoustic shock field M. T. Carnell, D. C. Emmony Nd: YAG laser delivery system utilizing GRIN-rod lenses as focusing-collimating optics R. Falciai, A. Schena, A. Del Bianco, T. Pascucci A continuously variable attenuator A. E. Smith, N. R. Heckenberg
for the CO, laser
Effects of stray reflections on the performance of semiconductor laser amplifiers as in-line optical repeaters C. Y.J. Chu, H. Ghafouri-Shiraz Evanescent field fibre-optic chlorine A. Safaai-Jazi, J. V. Petersen
sensor
Research on optical multistage butterfly interconnection and optoelectronic logic operations De-Gui Sun, Na-Xin Wang, Li-Ming He, Mai Xu, GuoDong Liang, Jie Zheng
Optics
374
Wood
Cosolas
& Laser Technology Vol 26 No 5 1994