Laser-induced Damage Threshold Measurements Research Articles

Bulk growth and physical properties of diguanidinium phosphate monohydrate (G2HP) as a multi-functional crystal

Bulk single crystals of diguanidinium phosphate monohydrate (G2HP) with dimensions up to 55 × 28 × 27 mm3 have been grown by solution growth methods. Thermal properties including specific heat, thermal conductivity and thermal expansion were investigated as a function of temperature. The specific heat was measured to be 1.339 to 1.614 J g−1 K−1 over the temperature range of 293 to 347 K. The thermal conductivities are descending as the temperature increases. The average linear thermal expansion coefficients along a and c axes from 298 to 338 K are α11 = 1.878 × 10−5 K−1 and α33 = 1.930 × 10−5 K−1, respectively. The laser-induced damage threshold measurements show that the G2HP crystal possesses an excellent resistance to laser radiation with a high threshold up to 5.29 GW cm−2. The optical behaviors, including the transmission spectrum and reflective indexes, were investigated to study its linear and nonlinear optical properties. The results reveal that the crystal is a phase matchable nonlinear optical material in the visible and UV regions. A complete set of dielectric, elastic and piezoelectric constants of G2HP have also been measured, and the results display that the crystal also possesses fine piezoelectric properties with d14 = 10.00. Furthermore, the structure–property relationships are discussed on the basis of the crystal structure combined with theoretical calculations. As a multi-functional crystal, G2HP is a particularly promising candidate for a variety of potential applications in many fields.

Fluence uncertainties of laser damage threshold measurements

The laser-induced damage threshold (LIDT) determines how optical components are used in a high power laser system, and its uncertainty is key for making comparisons between different LIDT measurements. In this work, we present an analysis of fluence uncertainties of LIDT measurements. The corresponding mathematical expressions for the fluence uncertainties have been established to deal with the statistical data obtained from a one-on-one damage experiment. The results show that the uncertainties are mainly caused by the measurement and calculation of the laser fluence and the damage probability at each fluence level, and the linear fit in a plot of laser fluence versus damage probability.

Design and realization of advanced multi-index systems

Practically all thin film systems for normal incidence can be realized using only two-layer materials. But for oblique incidence, polarization effects occur, designs may become complex, and polarization control is difficult or impossible to achieve. Here multi-index or gradient designs offer additional degrees of freedom, and can simplify or even enable challenging designs. Such gradient thin film stacks can be designed ab initio without any start or index profile approximations using a new design software developed by Carl Zeiss. With this software, a rugate omnidirectional AR coating was calculated and transferred to three different multi-index systems. All three examples were realized using ion beam sputter technology, and characterized at Laser Zentrum Hannover. Here we present comparative measurements of the optical performance together with femtosecond laser-induced damage threshold measurements.

Measuring the effective pulse duration of nanosecond and femtosecond laser pulses for laser-induced damage experiments

We report on our approach to measure the quantity named effective pulse duration as defined in the ISO 21254-1:2011 standard, which deals with laser-induced damage (LID) threshold measurements. The approach is applied to measure pulses from two laser sources: an injection-seeded electro-optically Q-switched Nd:YAG nanosecond system with 10-Hz pulse repetition frequency, and a fully integrated Ti:sapphire laser with 150–400 fs and 2-kHz pulse repetition frequency. For comparison, the full-width-half-maximum (FWHM) of the same pulses is also measured. The analysis and description of the measurement process, the experimental results, and the corresponding uncertainties are presented. A smaller combined uncertainty is obtained for the effective pulse duration than for the FWHM-defined pulse duration for each time scale involved in experiments. This suggests that the effective pulse duration is the appropriate parameter to characterize the pulse duration in LID experiments.

Glass Direct Bonding for Optical Applications

The direct wafer bonding technology is applied to join glass substrates for optical devices in high power laser applications. Uncoated as well as coated fused silica substrates were bonded to each other by hydrophilic direct bonding and -for comparison- sodium silicate-solution bonding. Both technologies are expected to generate materials-adapted Si-O-Si bonds at uncoated interfaces. Optical transmission and reflection in the spectral range of 200 nm to 1200 nm were measured and reveal superior transmission for the direct bonding technology in the ultra-violet range. Even in the near infrared at 980 nm, better performance with direct bonding as compared to silicate-solution bonding is evidenced by laser induced damage threshold measurements. For all coated samples, a distinct reduction in bonding strength relative to uncoated ones is observed in 3-point bending tests.

Ln3GaS6 (Ln = Dy, Y): new infrared nonlinear optical materials with high laser induced damage thresholds

Two new ternary rare earth chalcogenides, Dy3GaS6 (1) and Y3GaS6 (2), are reported here. They both crystallize in the orthorhombic space group Cmc21 (no. 36). Both are synthesized in pure phase and show phase-matchable second harmonic generation (SHG) of about 0.2 and 0.5 times, respectively for 1 and 2, as strong as that of KTiOPO4 (KTP) based on the powder SHG measurement at the wavelength of 1910 nm. They possess high powder laser induced damage thresholds (LIDTs), respectively, about 14 and 18 times that of AgGaS2 (AGS) based on the powder LIDT measurements under 1064 nm laser irradiation. They both exhibit wide transparency in the IR region (2.5–25 μm). It is believed that the title compounds are new candidates for nonlinear optical (NLO) materials in the IR region. To gain further insights into the NLO and LIDT properties of 1 and 2, the calculations of second-order NLO susceptibility and lattice energy density (LED) were also performed to explain their SHG efficiencies and high LIDTs.

Two phases of Ga2S3: promising infrared second-order nonlinear optical materials with very high laser induced damage thresholds

Two phases of Ga2S3 with different space groups Cc and F3m were synthesized in pure phase by a facile boron–sulfur–metallic oxide reaction. They both have a good transparency in the wavelength range of 0.44–25 μm and exhibit comparatively large second-harmonic generation (SHG) effects of about 0.7 and 0.5 times that of commercial KTiOPO4 (KTP), for the monoclinic and cubic Ga2S3 respectively. The monoclinic Ga2S3 is phase-matchable at the wavelength of 1910 nm while the cubic phase is non-phase-matchable. In order to study their powder laser induced damage threshold (LIDT) properties, a single pulse powder LIDT measurement method was proposed and it was found that they have very high powder LIDTs of about 30 and 100 times that of AgGaS2 (AGS), respectively for the monoclinic and cubic phase, under a single pulse 1064 nm laser radiation with a pulse width τp of 8 ns. To gain further insights into the nonlinear optical (NLO) and LIDT properties of the monoclinic and cubic Ga2S3, calculations of second-order NLO susceptibility and lattice energy density (LED) were also performed to explain their SHG efficiencies and high LIDTs.

Directly diode-pumped high-energy Ho:YAG oscillator

We report on the high-energy laser operation of an Ho:YAG oscillator resonantly pumped by a GaSb-based laser diode stack at 1.9 μm. The output energy was extracted from a compact plano-concave acousto-optically Q-switched resonator optimized for low repetition rates. Operating at 100 Hz, pulse energies exceeding 30 mJ at a wavelength of 2.09 μm were obtained. The corresponding pulse duration at the highest pump power was 100 ns, leading to a maximum peak power above 300 kW. Different pulse repetition rates and output coupling transmissions of the Ho:YAG resonator were studied. In addition, intracavity laser-induced damage threshold measurements are discussed.

Study on the feasibility of double stack high reflector coating at 355 nm

The feasibility of the idea of double stack HR coating was discussed in this paper both in theory and experiment. The theoretical simulation was made by employing optical coating design software. The analysis results showed that the design of double stack HR coating was feasible, which made the HR coating have ascendancy not only at reflectance but also at laser damage resistance. Then, the LaF 3/MgF 2 HR coating, the HfO 2/SiO 2 HR coating and the double stack HR coating were prepared for comparison, respectively. Transmittance spectra, surface morphologies and damage morphologies of these coatings were measured. Measurement of laser-induced damage threshold (LIDT) of S polarized light of the samples was performed at 355 nm, 45° incidence. The measurement results showed that the LIDT value of the LaF 3/MgF 2 HR coating with 30 layers was very high, but the reflectance was low. When the layer number was increased up to 36, lots of cracks appeared on the surface of the LaF 3/MgF 2 HR coating, with the LIDT badly declining. It was thought that the residual stress resulted in the cracks and the decline of the LIDT. The spectra measurement showed the double stack HR coatings could provide higher reflectance and wider reflection band than LaF 3/MgF 2 HR coating with less layer pairs. Any crack was also not found on the surface of the double stack coatings. Meanwhile, the double stack HR coatings possessed greater laser damage resistances than the HfO 2/SiO 2 HR coating. The damage morphologies showed that the damage of the double stack coating was even milder than that of the HfO 2/SiO 2 coating. Therefore, the double stack design was effective to gain high reflectance and great UV laser radiation resistance simultaneously.

Growth, crystal structure and characterization of a nonlinear optical crystal: Deuterated l-arginine trifluoroacetate

In order to reduce the absorption of l-arginine trifluoroacetate (LATF) in near-infrared region, deuterated LATF (DLATF) crystals have been synthesized and grown successfully. The grown crystals were characterized by the single crystal and powder X-ray diffraction methods, Fourier transform infrared and Raman spectra. DLATF belongs to monoclinic space system, space group P2 1, with unit cell parameters: a = 10.547(6), b = 5.696(3), c = 10.825(6) Å, β = 106.747(10)°, V = 622.8(6) Å 3 and Z = 2. Thermogravimetric (TG) and differential thermal analysis (DTA) studies were carried out to characterize its thermal behaviors. The specific heat has been investigated in the temperature range of 293–453 K. Optical transmission spectrum and second harmonic generation were investigated to study its optical properties. The laser-induced damage threshold measurement revealed that the crystal has a higher damage threshold than that of LATF.

Unidirectional growth, linear and nonlinear optical, dielectric and mechanical properties of organic adduct of l-tartaric acid nicotinamide

An attempt has been made to grow l-tartaric acid nicotinamide (LTN); a complex of tartaric acid, by employing a modified unidirectional method. The crystalline structure and quality are investigated by single crystal XRD and rocking curve studies. The linear and nonlinear optical properties are studied by UV–vis-NIR spectral analysis, SHG test, phase matching and laser induced damage threshold measurement. For comparison, parallel growth of the crystal was carried out by conventional method and the properties of the LTN samples grown by the conventional and unidirectional methods are investigated. The mechanical, photoconductivity and dielectric behavior of LTN crystals are also investigated.

Graded index broadband antireflection coating prepared by glancing angle deposition for a high-power laser system

This paper reports that SiO2 is selected to fabricate broadband antireflection (AR) coatings on fused silica substrate by using glancing angle deposition and physical vapour deposition. Through accurate control of the graded index of the SiO2 layer, transmittance of the graded broadband AR coating can achieve an average value of 98% across a spectral range of 300–1850 nm. Moreover, a laser-induced damage threshold measurement of the fabricated AR coating is performed by using a one-on-one protocol according to ISOH254–1, resulting in an average damage threshold of 17.2 J/cm2.

Laser damage thresholds of optical coatings

Since the very beginning of laser technology, Laser Induced Damage Thresholds (LIDT) of optical components were always an obstacle for the application of laser systems operating at high power levels. Also, further progresses in the development of new high power laser concepts are often directly limited by the availability of advanced optical components with high quality and LIDT-values. Nowadays, in the course of the development of optical materials with excellent quality and power handling capability, the problem of laser induced damage has shifted from the bulk to the surface of the optical component. The optical surface is objected to various production steps and environmental influences, which modify its structure and composition. Especially, the thin film coating, which is deposited on the optical surface to adapt its reflectance and transmittance to the application, contributes predominantly to the reduction of the LIDT-values. As a consequence, the measurement and optimization of the power handling capability of thin films is considered as one of the primary research areas in modern optics technology and is supported by an extensive scientific community.In the present paper, a brief review will be given on selected fundamental damage mechanisms in thin films considering different operation conditions of modern laser systems. Also, the current standards for the measurement of LIDT will be described, and examples illustrating some practical aspects of high power optical coatings will be presented. Finally, recent trends in laser technology will be discussed in respect to research in laser induced damage.

Influence of plasma treatment on laser-induced damage characters of HfO_2 thin films at 355nm

HfO(2) thin films were deposited by e-beam evaporation, and were post-treated with plasma under different flow rate ratios of argon to oxygen. By measuring the surface defect density, weak absorption, laser-induced damage threshold (LIDT) and damage morphology, the influence of the flow rate ratio of argon to oxygen on the laser-induced damage characters of HfO(2) thin films were analyzed. The experimental results show that plasma treatment is effective in reducing the surface defect density of thin films. Compared with the as-grown sample, the absorption reduction is obvious after plasma treatment when argon and oxygen flow rate ratio is 5:25, but the absorption increases gradually with the continued increase of argon and oxygen flow rate ratio. LIDT measurements in 1-on-1 mode demonstrate that plasma treatment is not effective in improving LIDT of the samples at 355 nm. Damage morphologies reveal that the LIDT is dominated by nanoscale absorbing defects in subsurface layers, which agrees well with our numerical simulation result based on a spherical absorber model.

Measurement of femtosecond laser-induced damage and ablation thresholds in dielectrics

The paper is focused on the importance of accurate determination of surface damage/ablation threshold of a dielectric material irradiated with femtosecond laser pulses. We show that different damage characterization techniques and data treatment procedures from a single experiment provide complementary physical results characterizing laser–matter interaction. We thus compare and discuss two regression techniques, well adapted to the measurement of laser ablation threshold, and a statistical approach giving the laser damage threshold and further information concerning the deterministic character of femtosecond damage. These two measurements are crucial for laser micromachining processes and high peak-power laser technology in general.

Multiscale analysis of the laser-induced damage threshold in optical coatings

We have investigated the influence of laser beam size on laser-induced damage threshold (LIDT) in the case of single- and multiple-shot irradiation. The study was performed on hafnia thin films deposited with various technologies (evaporation, sputtering, with or without ion assistance). LIDT measurements were carried out at 1064 nm and 12 ns with a spot size ranging from a few tens to a few hundreds of micrometers, in 1-on-1 and R-on-1 modes. These measurements were compared with simulations obtained with the statistical theory of laser-induced damage caused by initiating inclusions. We show how to obtain information on the initiating defect properties and the related physical damage mechanisms with a multiscale study. Under certain conditions, it is possible with this method to discriminate different defects, estimate their densities, and follow the evolution of the defects under multiple irradiation. The different metrology implications of our approach, particularly for obtaining a functional LIDT of optical components are discussed.

Nanosecond laser-induced surface damage of optical multimode fibers and their preforms

High-power optical multimode fibers are essential components for materials processing and surgery and can limit the performance of expensive systems due to breakdown at the end faces. The aim of this paper is the determination of laser-induced damage thresholds (LIDT) of fibers (FiberTech) and preforms (Heraeus Suprasil F300). Preforms served as models. They were heated up to maximum temperatures of 1100, 1300 and 1500°C and cooled down to room temperature at rates of 10 K min−1 (oven) and ∼105 K min−1 (quenched in air) to freeze in various structural states simulating different conditions similar to a drawing process during the production of fibers. Single- and multi-pulse LIDT measurements were done in accordance with the relevant ISO standards. Nd:YAG laser pulses with durations of 15 ns (1064 nm wavelength) and 8.5 ns (532 nm) at a repetition rate of 10 Hz were used. For the preforms, LIDT values (1-on-1) ranged from 220 to 350 J/cm2 (1064 nm) and from 80 to 110 J/cm2 (532 nm), respectively. A multi-pulse impact changed the thresholds to lower values. The LIDT (1064 nm wavelength) of the preforms can be regarded as a lower limit for those of the fibers.

Influence of Technological Parameters on Nanosecond Laser-Induced Surface Damage of Optical Multimode Fibers

High-power optical multimode fibers are essential components for materials processing and surgery and can limit the reliability of expensive systems due to breakdown at the end faces. The breakdown threshold of fibers is determined by intrinsic materials properties and parameters of the technology applied. The aim of this paper is the identification of technological parameters that are crucial for the fiber quality. Fibers were drawn from preforms of Heraeus SWU with core material F300 and a low amount of OH-. Both, the cladding (fluorine doped SiO2) to core diameter ratio (CCDR) and the drawing speed were varied. CCDR values between 1.05 and 1.4 were used. Afterwards, the laser-induced damage thresholds (LIDT) of the fibers were determined. For comparison, also samples from preforms, which underwent different thermal treatments above the transition temperature, were tested with respect to their damage resistivity. Single and multi pulse LIDT measurements were done in accordance with the relevant ISO standards. Nd:YAG laser pulses with durations of 15 ns (1064 nm wavelength) and 8.5 ns (532 nm) at a repetition rate of 10 Hz were utilized. For the fibers, LIDT values (1-on-1, 1064 nm and 532 nm) increased with growing CCDR and with decreasing drawing velocities.

Laser-induced retinal damage threshold measurements with wavefront correction

An adaptive optics (AO) system was incorporated into a laser retinal exposure setup in order to correct for refractive error and higher-order aberrations of the nonhuman primate (NHP) eye during an in vivo retinal ED(50) measurement. Using this system, the ED(50) for a 100-ms, 532-nm small spot size exposure was measured to be 1.05 mJ total intraocular energy (TIE), a reduction of 22% from the value measured without aberration correction. The ED(50) for a 3.5-ns, 532-nm exposure was measured to be 0.51 microJ TIE, the lowest ED(50) reported for a ns-duration exposure. This is a reduction of 37% from the value measured without aberration correction and is a factor of only 2.6 higher than the maximum permissible exposure (MPE) for a 3.5-ns, visible wavelength small spot size exposure. The trend of in vitro measurements using retinal explants suggests that the in vivo ED(50) for small spot-size exposures could potentially be one order of magnitude smaller than the previously reported in vivo ED(50). Distortion of the incident laser beam by ocular aberrations cannot fully explain the discrepancy between the in vivo measurements with no aberration correction and the in vitro results.

SOCRATE: an optical bench dedicated to the understanding and improvement of a laser conditioning process

We present an automatic excimer laser bench (SOCRATE) allowing for the treatment of optical components by laser conditioning. This apparatus, developed at the Commissariat a l'Energie Atomique-Le Ripault, has been designed to add to this conditioning process an in situ, accurate laser-induced damage threshold (LIDT) measurement and different nondestructive optical techniques for the characterization of the component during treatment. Through different examples, we demonstrate the importance of these characterizations to improve the understanding of the laser conditioning. The role of an in situ adapted metrology associated in real time with a laser conditioning bench offers new opportunities to analyze laser-induced damage mechanisms and subsequently to increase the LIDT of optical components.