D. Miksys

Automated test station for laser-induced damage threshold measurements according to ISO 11254-2 standard

All laser components can withstand a limited intensity of optical radiation and the measurement of laser-induced damage thresholds (LIDT) is required. In the case of repetitive pulses the LIDT measurements should be performed according ISO 11254-2 standard. This procedure is time consuming and puts a high requirement on human resources. In order to speed up the LIDT measurements with a minimal human resource we developed the automated station for LIDT measurements according ISO-11254-2 standard. In this paper we overview the main parts of this automated station and present the results of LIDT measurements with repetitive pulses. In order to control the LIDT measurements, software based on LabView programming package was created. The LIDT software controls experimental sample positioning in X and Y directions, laser pulse energy attenuation and shutter. It also automates damage detection and performs statistical analysis. The program recognizes damage by detecting scattered light from damaged surface. The input of sample and laser beam technical parameters is required to start the measurements. The minimal distance between test sites on the sample surface is calculated automatically, and the surface area is divided in to a hexagonal matrix. The program also chooses the laser pulse train energy for each test site. The program also allows fast damage inspection by translating the sample under the Nomarski microscope. After completion of measurement and damage inspection, the program automatically generates the measurement report.

The microstructure and LIDT of Nb2O5 and Ta2O5 optical coatings

High power laser systems are one of the most rapidly growing areas in the development of laser technology. This also leads towards higher requirements for environmental stability of optical components and their resistance to laser radiation. There are some reports showing that porous dielectric coatings are more resistant to intense laser radiation, however they have smaller environmental stability than denser coatings, which are more sensitive to laser radiation. The influence of important technological parameters (deposition rate, substrate temperature, energy of ions) on optical and microstructural properties of high reflection dielectric coatings based on Nb2O5/SiO2, and Ta2O5/SiO2 in VIS spectral region is presented. Furthermore the LIDT measurements using repetitive nanosecond laser pulses of Nb2O5/SiO2 and Ta2O5/SiO2 high reflecting optical coatings based on ISO 11254-2 standard are presented.

Characterization of KDP crystals used in large aperture doublers and triplers

We report on laser-induced damage threshold (LIDT) and UV-laser excited defect formation measurements in large aperture KDP crystals developed as doublers and triplers for mega-Joule laser. Measurements of LIDT were performed according to the ISO 11254-2 standard for repetitive pulses with duration ~ 4 ns and repetition rate of 10 Hz. The results for different laser wavelengths (1064, 532 and 355 nm) and polarizations are presented. The largest LIDT was observed for 532 nm pulses and the 1064 nm wavelength had a strong dependence on laser polarization. The LIDT values at 532 nm and 355 nm also depended on the crystal cutting angle, which is different for doublers and triplers. A comparison of LIDT with earlier reported crystal absorptance at different wavelengths is also performed. The UV-laser induced defect formation was investigated by the means of pump-probe technique. The excitation was performed with a single pulse of ns Nd:YAG laser (355 or 266 nm wavelength) and probing with another Nd:YVO4 laser system (532 nm) operating at 1kHz. This gave us a temporal resolution of 1ms. The transient absorption of defect states relaxed non-exponentially and fully disappeared in ~10 s. A comparison is made between crystal grown by distinct growth methods and between different laser polarizations. An influence of laser conditioning on UV induced defect state formation is also revealed.

Comparison of optical resistance of ion assisted deposition and standard electron beam deposition methods for high reflectance dielectric coatings

The ion assisted thin film deposition (IAD) method has been used extensively for more than two decades, but questions about possibility of improving of the laser-induced damage threshold (LIDT) by this method compared with the conventional electron-beam evaporation (non-IAD) method are still not fully answered. A more complete understanding of different factors that can influence laser-induced damage threshold is necessary for continued development of multilayer dielectric coatings optimized for high-power laser applications. To clarify these factors we performed comparison of LIDT for IAD and non-IAD coatings in nanosecond and femtosecond pulse ranges. High reflectance mirrors at 800 nm and 532 nm were tested. Mirror coatings were made of ZrO2 and SiO2. Automated LIDT measurements were performed according to the requirements of current ISO 11254-2 standard. Two lasers were used for the measurements: Nd:YAG (λ = 532 nm, τ = 5 ns) and Ti:Sapphire (λ = 800 nm, τ = 130 fs). Measurements at 800 nm and 532 nm were performed at 1-kHz and 10 Hz pulse repetition rate respectively (S-on-1 test). The damage morphology of coatings was characterized by Nomarski microscopy and relation of LIDT with coating parameters was analyzed.

Laser-induced damage threshold measurements of high reflecting dielectric layers

A quest for higher laser powers is one of the main driving forces in development of laser technology. Unfortunately all laser components have some limit to the intensity of optical radiation that can be applied on them - the so-called laser-induced damage threshold (LIDT). To enable further power scaling of laser devices, novel highly resistant optical components have to be developed. Such components are laser crystals, mirrors, fibers and other components typically coated with periodic dielectric layers made using e-beam, sputtering or sol-gel technologies. The production materials and methods of all the mentioned optics are under constant development, which requires a reliable quality test to provide the feedback to the manufacturing process; one of such tests are the measurements of LIDT. LIDT measurement procedure using repetitive laser pulses, as described in ISO 11254-2 standard, is time- and human resource consuming, if performed without automation. We developed an automated station for the measurements of LIDT that greatly reduces the required human resources and allows fast data collection. In this presentation, we briefly describe the main components of this automated LIDT test station. Furthermore we present the comparison of the latest results obtained on LIDT measurements of ZrO2/SiO2, Nb2O5/SiO2, Ta2O5/SiO2 and TiO2/SiO2 periodic high reflecting dielectric layers performed using repetitive nanosecond laser pulses.

Multiple pulse laser-induced damage of antireflection coated lithium triborate

An investigation of laser-induced damage thresholds (LIDT) of antireflection coated lithium triborates used for second and third harmonic generation were performed at 1064, 532, and 355 nm wavelengths for ~4 ns pulses. Two types of coatings were tested: an anti-reflection coatings with dual peak at 1064 nm and 532 nm and anti-reflection coatings with three peaks at 1064 nm, 532 nm, and 355 nm, respectively. Ion-beam sputtering and magnetron sputtering technologies where used for coatings deposition. Automated LIDT measurements were performed according to the requirements of current ISO 11254-2 standard. The obtained LIDT were in range of 3-15 J/cm2.

Effect of deposition method and substrate surface quality on laser-induced damage threshold for repetitive 13-ns and 130-fs pulses

A comparison of laser induced damage thresholds (LIDT) of ion assisted deposition (IAD) and standard electron beam deposition dielectric coatings on BK7 glass with different surface roughness was performed. Five types of high reflectance mirrors at 800 nm and two types of high reflectance mirrors at 1064 nm were tested. Mirror coatings were made of ZrO2 and SiO2. Automated LIDT measurements were performed according to the requirements of current ISO 11254-2 standard. Two lasers were used for the measurements: Nd:YAG (l = 1064 nm, t = 13 ns) and Ti:Sapphire (l = 800 nm, t = 130 fs ). All measurements were performed at 1-kHz pulse repetition rate (S-on-1 test). A fixed spot size was used for each laser. For 1064 nm it was ~ 70 um and for 800 nm ~ 500 um. The damage morphology and structure of coatings were characterized by an atomic force microscopy (AFM), Nomarski microscopy and X-ray diffraction (XRD).

Laser-induced damage thresholds of starched PMMA waveplates

Polymethyl methacrylate (PMMA) is a versatile polymeric material that is well suited for fabrication of many commercial optical components: lenses, fibers, windows, phase waveplates and others. Our focus is achromatic zero-order waveplates made of anisotropic PMMA which can be used to modify the state of polarization of electromagnetic radiation. Such waveplates have a broad range of application in devices where polarized radiation is used. For example, when tunable lasers are used or when spectropolarimetric measurements are performed, one needs an achromatic waveplate providing a specific retardation in a wide wavelength range. Herewith anisotropic properties of PMMA subjected to one-axis stretching are analyzed and the technology for manufacturing such achromatic and super-achromatic, one-axis-stretched PMMA waveplates is described. This technology excludes any mechanical processing of waveplate component surfaces. Technical characteristics of achromatic and super-achromatic waveplates manufactured of PMMA including results of laser-induced damage threshold (LIDT) measurements are discussed below.

Laser-induced damage thresholds of ion beam sputtered and magnetron sputtered AR-coatings on lithium borate

Lithium triborate, LiB3O5 (LBO) is a popular nonlinear optical crystal typically used for frequency conversion. For power scaling of laser radiation in transparency range of LBO crystals with maintaining low reflection losses is very important to reach high optical resistance of anti-reflective dielectric coatings. The measurements of laser-induced damage thresholds (LIDT) of AR coated LBO used for second and third harmonic generation of Nd:YAG lasers were performed at 1064, 532, and 355 nm wavelengths for ~4 ns pulses. Two types of coatings where tested; a dual peak anti-reflection at 1064 nm and 532 nm, three peak anti-reflection at 1064 nm, 532 nm, and 355 nm. Ion beam sputtering and magnetron sputtering technologies were used for coatings deposition. Automated LIDT measurements were performed according to the requirements of current ISO 11254-2 standard. The obtained LIDT were in range of 5-21 J/cm2.

Influence of ion-assisted deposition on laser-induced damage threshold and microstructure of optical coatings

High density, improved adhesion and environmental stability are the main features of dielectric optical coatings produced using ion-assisted deposition (IAD) technology. However, investigations of resistance of IAD coatings to intensive laser radiation show controversial results. A series of experiments were done to examine the influence of ion gun operation on the transmittance of fused silica substrates. It was shown that operation of ion source introduced extinction in UV spectral range. Optical properties of single hafnia layers and multilayer dielectric mirrors deposited using conventional e-beam evaporation and different modes of IAD were investigated. Microstructural analysis using X-ray diffraction (XRD) measurements and AFM scanning of coated areas was carried out. Single hafnia layers deposited using high energy ion assistance had more amorphous structure with smaller crystallites of monoclinic phase. High reflection UV mirrors deposited using high energy ion assistance had slightly higher mean refractive indices of hafnia, higher extinction than conventional e-beam deposition, but demonstrated slightly higher laser induced damage threshold (LIDT) values measured at 355 nm. Deposition using the lowest energy ions produced the most porous coatings with the best LIDT of 7.7 J/cm2.