High Energy Laser Applications Research Articles

Post-process annealing of MgAl2O4 spinel optics for high energy laser windows

Magnesium aluminate (spinel) has been investigated as a low-absorption material for applications such as high-strength windows, armor, and high energy laser (HEL) apertures. To develop spinel for HEL applications, it is critical to lower the absorption of the lasing medium at the operating wavelength to reduce optical path distortion. In this work, post-processing of spinel via annealing is investigated to lower the absorption at 1.064 μm. Specifically, spinel optics are annealed in ambient air, oxygen, and nitrogen at temperatures of 1000–1400 °C and their transmission, absorption and photoluminescence properties are measured. This annealing process reduces the absorption by 73% from 463 ppm/cm to 126 ppm/cm and results in optics that have transmission within 1% of the Fresnel limit over the 1.0–2.0 μm wavelength range. The resulting optical characterizations imply a direct oxidation of Fe2+ to Fe3+ and the formation of VO1+ at distorted octahedral sites to mediate this reaction that results in lower optical absorption in the near-IR (NIR). Further, performance of equibiaxial flexure testing and corresponding Weibull analysis indicates that spinel manufactured using this method can outperform sapphire windows from a design strength perspective.

Enhanced laser-induced damage performance ofall-glass metasurfaces for energetic pulsed laserapplications.

To fabricate optical components with surface layers compatible with high-power laser applications that may operate as antireflective coatings, polarization rotators, or harness physical anisotropy for other uses, metasurfaces are becoming an appealing candidate. In this study, large-beam (1.05cm diameter) 351-nm laser-induced damage testing was performed on an all-glass metasurface structure composed of cone-like features with a subwavelength spacing of adjacent features. These structures were fabricated on untreated fused silica glass and damage tested, as were structures that were fabricated on fused silica glass that experienced a preliminary etching process to remove the surface Beilby layer that is characteristic of polished fused silica. The laser-induced damage onset for structures on untreated fused silica glass was 19.3J⋅c m -2, while the sample that saw an initial pretreatment etch exhibited an improved damage onset of 20.4J⋅c m -2, only 6% short of the reference pretreated glass damage onset of 21.7J⋅c m -2. For perspective, the National Ignition Facility operational average fluence at this wavelength and pulse length is about 10J/c m 2. At a fluence of 25.5J⋅c m -2, the reference (pretreated) fused silica initiated 5.2 damage sites per m m 2, while the antireflective metasurface sample with a preliminary etching process treatment initiated 9.8 damage sites per m m 2. These findings demonstrate that substrate-engraved metasurfaces are compatible with high energy and power laser applications, further broadening their application space.

Kinetics of Metastable Argon Optical Excitation and Gain in Ar/He Microplasmas.

The optically pumped rare-gas metastable laser is capable of high-intensity lasing on a broad range of near-infrared transitions for excited-state rare gas atoms (Ar*, Kr*, Ne*, Xe*) diluted in flowing He. The lasing action is generated by photoexcitation of the metastable atom to an upper state, followed by collisional energy transfer with He to a neighboring state and lasing back to the metastable state. The metastables are generated in a high-efficiency electric discharge at pressures of ∼0.4 to 1 atm. The diode-pumped rare-gas laser (DPRGL) is a chemically inert analogue to diode-pumped alkali laser (DPAL) systems, with similar optical and power scaling characteristics for high-energy laser applications. We used a continuous-wave linear microplasma array in Ar/He mixtures to produce Ar(1s5) (Paschen notation) metastables at number densities exceeding 1013 cm-3. The gain medium was optically pumped by both a narrow-line 1 W titanium-sapphire laser and a 30 W diode laser. Tunable diode laser absorption and gain spectroscopy determined Ar(1s5) number densities and small-signal gains up to ∼2.5 cm-1. Continuous-wave lasing was observed using the diode pump laser. The results were analyzed with a steady-state kinetics model relating the gain and the Ar(1s5) number density.

Ablation and mechanical properties of ZrC-reinforced PBI resin matrix composites

With the rapid development of laser technology, there is an increasing demand for laser protective materials. Thus, the mechanical and ablation properties of composite materials need to be further enhanced. Herein, solution impregnation and hot-press molding were used to develop composites containing various mass ratios of ZrC to polybenzimidazole (PBI) resin (thickness = 3 mm) for laser ablation applications. The results showed that adding short carbon fiber (SCF) to ZrC/PBI composites improves the ablation and mechanical properties. The ZrC/PBI/SCF composites were ablated using a high-energy continuous laser (7 kW cm−2), and the composites did not burn through. The composites did not peel off or split as the ablation process progressed. With the increase in the ZrC content in the composites, dense oxide layers are formed, enhancing the ablation properties of the composites. The ZrC/PBI/SCF composite (the ZrC/PBI mass ratio = 2:1) exhibited the lowest mass loss (2.24%), mass ablation rate (0.119 mg s−1) and linear ablation rate (0.032 mm s−1). This indicates that ZrC/PBI/SCF composites can be used as protective materials in high-energy continuous laser applications.

Improving the laser performance of ion beam sputtered dielectric thin films through the suppression of nanoscale defects by employing a xenon sputtering gas

Laser damage-prone precursors in high index materials such as hafnia are believed to be the primary limiter in the performance of dielectric multilayer films to advance ultra-high power and energy laser applications. Removing or suppressing these precursors is the key to fabricating laser damage resistant thin films for the enabling technologies. Early work has revealed that nanobubbles formed by entrapped argon (Ar) working gas in ion beam sputtering (IBS) produced hafnia films are primarily responsible for the onset of laser damage upon exposure to UV, ns-laser pulses. In this study, we demonstrate that the UV ns-laser damage onset of IBS produced hafnia films can be improved to 3.1 +/- 0.2 J/cm2 by substituting the conventional Ar working gas with xenon (Xe), a nearly 1 J/cm2 increase from that of the Ar produced hafnia films. In addition to the suppression of the overall point-defect density of the hafnia films, the reduction of the Xe entrapment eliminates the nanobubbles and the generation of plasmas that initiates the laser damage. The defect suppression and its correlation to the increase in laser damage threshold is revealed by the combined analysis of Rutherford backscattering spectroscopy, electron paramagnetic resonance spectroscopy, transmission electron microscopy, and laser damage testing. Monte Carlo simulations suggest a much smaller entrapment of Xe gas by comparison to Ar, which is attributed to the significant difference in the energy of the reflected neutrals (3X) which are likely to be implanted. These results provide an effective process route with a fundamental understanding for producing high laser damage resistant dielectric films for high power and high energy laser applications.

Uniform and efficient beam shaping for high-energy lasers.

Phase-only beam shaping with liquid crystal on silicon spatial light modulators (SLM) allows modulating the wavefront dynamically and generating arbitrary intensity patterns with high efficiency. Since this method cannot take control of all degrees of freedom, a speckle pattern appears and drastically impairs the outcome. There are several methods to overcome this issue including algorithms which directly control phase and amplitude, but they suffer from low efficiency. Methods using two SLMs yield excellent results but they are usually limited in the applicable energy due to damage to the SLM's backplane. We present a method which makes use of two SLMs and simultaneously gives way for high-energy laser applications. The algorithm and setup are designed to keep the fluence on the SLMs low by distributing the light over a large area. This provides stability against misalignment and facilitates experimental feasibility while keeping high efficiency.

Yb3+ doped single-mode silica fibre laser system for high peak power applications

We present ytterbium doped silica single-mode fibre components for high power and high energy laser applications. We developed in-house the fibre laser with high efficiency of 65% according to the launched power, the threshold of 1.16W and the fibre length of 20 m. We also elaborated the fibre with 20 µm in diameter core suitable for amplifying the beam generated in oscillator. We implemented made in-house endcaps to prove the utility of the fibre towards high peak power applications. Full Text: PDF ReferencesStrategies Unlimited, The Worldwide Market for Lasers: Market Review and Forecast, 2020 DirectLink J. Zhu, P. Zhou, Y. Ma, X. Xu, and Z. Liu, "Power scaling analysis of tandem-pumped Yb-doped fiber lasers and amplifiers", Opt. Express 19, 18645 (2011) CrossRef IPG Photonics, Product information, accessed: October, 2020. DirectLink J.W. Dawson, M. J. Messerly, R. J. Beach, M. Y. Shverdin, E. A. Stappaerts, A. K. Sridharan, P. H. Pax, J. E. Heebner, C. W. Siders, and C. P. J. Barty, "Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power", Opt. Express 16, 13240 (2008) CrossRef W. Koechner, "Solid-State Laser Engineering", Springer Series in Optical Science, Berlin 1999 CrossRef A. V. Smith, and B. T. Do, "Bulk and surface laser damage of silica by picosecond and nanosecond pulses at 1064 nm", Appl. Opt. 47, 4812 (2008), CrossRef M. N. Zervas, C. Codemard, "High Power Fiber Lasers: A Review", IEEE J. Sel. Top. Quantum Electron. 20, 1, 2014 CrossRef D.J. Richardson, J. Nilsson, and W.A. Clarkson, "High power fiber lasers: current status and future perspectives [Invited]", J. Opt. Soc. Am. B, 27, 63, 2010, CrossRef M. Li, X. Chen, A. Liu, S. Gray, J. Wang, D. T. Walton; L. A. Zenteno, "Limit of Effective Area for Single-Mode Operation in Step-Index Large Mode Area Laser Fibers", J. Lightw. Technol., 27, 3010, 2009, CrossRef J. Limpert, S. Hofer, A. Liem, H. Zellmer, A. Tunnermann., S. Knoke, and H. Voelckel, "100-W average-power, high-energy nanosecond fiber amplifier", App.Phys.B 75, 477, 2002, CrossRef

Weak absorption and scattering losses from the visible to the near-infrared in single-crystal sapphire materials

Durable window materials with minimal optical loss are important for future high-energy laser (HEL) systems that will operate at or near the megawatt level. Sapphire is recognized as a promising HEL window candidate due to its outstanding optical transparency and mechanical properties. However, its weak scattering characteristics and absorption levels near the 1-μm wavelength region need to be lowered further for future HEL applications. In our study, the weak absorption and scattering of sapphire samples provided by three vendors were measured. Ultraviolet–visible spectroscopy measurements were made in the wavelength range 190 to 600 nm and several absorption bands due to vacancies and impurities were detected. The bulk absorption of several samples at wavelengths of 355, 532, and 1064 nm were measured using photothermal common-path interferometry and the absorption coefficient values obtained were in the range of 10 − 5 to 10 − 2 cm − 1 and increased as the wavelength decreased. An empirical weak absorption tail model was used to fit the measured data. In addition, scattering measurements on all samples were made at 405, 532, 633, 1064, and 1550 nm using an instrument developed to assess the bidirectional scatterance probability distribution function. The total integrated scatterance was in the range of 10 − 4 to 10 − 2 and increased for all samples as the wavelength decreased. Surface roughness was found to contribute insignificantly to the scattering loss, while bulk defects along with subsurface damage have major impact. A simple single-scatter model was developed and applied to the measured bulk scattering data. The model suggests that impurity particles, porosity, and other density variations exist with a range of sizes that contribute to scattering. Overall, the measurements indicate that both weak absorption and scattering losses are strongly related to defect structures such as lattice disorder and impurities that were introduced during crystal growth or postgrowth processing. Understanding these defects and their contributions to optical loss can lead to improved manufacturing and processing methods.

Thermo-optic coefficient of B2O3 and GeO2 co-doped silica fibers

New measurements of the thermo-optic coefficients (TOCs) in ternary GeO2 and B2O3 co-doped silica core optical fibers are presented. Material additivity models are applied to the measurements to shed more light on the relative contributions by each constituent. Three of the ternary fibers studied are drawn at different temperatures, but from the same preform, providing insight into the influence of fabrication conditions. It is found that the TOC of the B2O3 constituent is somewhat less negative than previously reported and trends towards more negative values with increasing draw temperature. Two approaches are taken in determining the TOC of one of the fibers, including measuring the LP11 cutoff wavelength as a function of temperature. The latter suggests an attractive application for fiber cores with a TOC lower than the cladding: fibers whose V-number intrinsically decreases with increasing temperature. Such thermally-mode-reducing optical fibers could be of great consequence for high-energy laser applications.

A remote laser focusing system with spatial light modulator

This paper designs a remote laser focusing system (RLFS) based on spatial light modulator (SLM). In this system, the SLM serves as an active optical element that modulates the wavefront of the laser beam, causing variation in beam transmission. Theoretical analysis shows that a reflective SLM can compensate the residual aberrations at different operating distances, enabling the accurate focusing of targets without sacrificing the energy convergence. Based on optical design software, the geometrical parameters were optimized for the presented system through simulation. Finally, experimental demonstration and error analysis were carried out. The results show that the proposed RLFS-SLM is more stable and capable for controlling energy convergence than the traditional mechanical RLFS. Our system provides a desirable tool for high-energy laser applications.

Calcium silicate and fluorosilicate optical fibers for high energy laser applications

Passive calcium silicate and fluorosilicate glass optical fibers are fabricated using the molten core method, and their properties investigated. Reductions up to −11.5 dB in Brillouin gain coefficient, −2 dB in Raman gain coefficient, and −2.5 dB in thermo-optic coefficient, relative to silica, are measured. These results support the continued study of alkaline earth oxide silicates as host materials in high energy fiber-based applications since they offer less optical nonlinearity-limited properties for power-scaling than do current analogs. Other properties, such as refractive index and acoustic velocity, are also reported and discussed. Finally, modeling work is performed to study and identify the contributions of CaO and CaF2 glass materials on the fiber properties.

Rational Design of a LiNbO3-like Nonlinear Optical Crystal, Li2ZrTeO6, with High Laser-Damage Threshold and Wide Mid-IR Transparency Window.

With existing and emerging technologies urgently demanding the expansion of the laser wavelengths, high-performance nonlinear optical (NLO) crystals are becoming indispensable. Here, a prospective NLO crystal, Li2ZrTeO6, is rationally designed by the element substitution of Nb for Zr and Te from LiNbO3, which has been recognized as one of the most commercial NLO crystals. Li2ZrTeO6 with R3 symmetry inherits the structural merits of LiNbO3 (space group R3 c) and thus meets the requirements for NLO applications, including noncentrosymmetric crystal structure, moderate birefringence, and phase-matchability. Moreover, it can be exploited to achieve more outstanding optical damage resistant behavior (>1.3 GW cm-2), exceeding 22 times that of LiNbO3, which is more suitable for high-energy laser applications. Notably, this compound displays the widest IR absorption edge (7.4 μm) among all of the noncentrosymmetric tellurates reported so far. These excellent attributes suggest that Li2ZrTeO6 is a promising candidate for providing high NLO performance. The substitution of Nb for Zr and Te from LiNbO3 demonstrates a viable strategy toward the rational design of NLO crystals with anticipated properties.

Oxyfluoride Core Silica-Based Optical Fiber With Intrinsically Low Nonlinearities for High Energy Laser Applications

A few-moded silica-based optical fiber fabricated from core materials that possess intrinsically low optical nonlinearities is reported. Specifically, the 8-μm core, 125-μm cladding diameter silicate fiber was composed of a strontium aluminosilicate oxyfluoride core with a fused silica cladding and was fabricated using the molten core method. Relative to conventional optical fibers, reductions of ∼6.3 dB in Brillouin gain coefficient (gB ), ∼0.9 dB in Raman gain coefficient (gR), and ∼2.2 dB in thermo-optic coefficient were realized as was a “silica-like” nonlinear refractive index (n2) with a value of ∼3 × 10 –20 m2/W. The role of each core material constituent on parameters that drive optical nonlinearities is discussed to provide a materials solution route for low nonlinearity fiber systems. Materially addressing optical nonlinearities represent a simpler and more effective approach to mitigating power-scaling limits in high energy fiber laser systems compared to the geometric approaches employed using microstructured fibers.

1.56 µm sub-microjoule femtosecond pulse delivery through low-loss microstructured revolver hollow-core fiber

We report for the first time, to the best of our knowledge, on ~1 MW peak power femtosecond pulse delivery through ≈10 m-long air-filled microstructured revolver hollow-core fiber (RHCF) in the telecom spectral band near 1.56 µm wavelength. We have developed a high-power all-fiber master oscillator power amplifier source based on the novel large-mode area erbium-doped double-clad fiber with 980 nm multi-mode diode pumping that emits up to 530 nJ pulses shorter than 400 fs with 1.42 W maximum average power. These pulses have been further launched into low-loss (<30 dB km−1) RHCF with eight non-touched cylindrical capillaries-based cladding and 61 µm core size with more than 80% efficiency. Owing to low dispersion and nonlinearity of the RHCF developed, the output pulse characteristics (spectral and temporal) are close to the input ones for low and moderate pulse energies. However, we have observed significant nonlinear spectral filtering together with pulse shortening (down to 353 fs) at the maximum output average power of 0.94 W. We believe that the system developed may be highly promising for high-precision material processing and other high-energy and high-power laser applications.

Characterisation of adhesive-free bonded crystalline Yb:YAG for high energy laser applications

We report on the application of the adhesive-free bonding (AFB) technique to form Yb-doped crystalline yttrium aluminum garnet (Yb:YAG) gain media slabs. We performed experiments to characterise mechanical strength, optical quality and laser-induced damage threshold (LIDT) of bonded substrates. We demonstrate that mechanical properties of bonded samples are similar to those of monolithic substrates. We show that the presence of a bonding interface does not introduce unwanted wavefront deformations and does not increase the probability of laser-induced damage onset. Results indicate that the AFB technique constitutes a viable alternative for producing large aperture gain media slabs required for high-energy laser systems.

Growth, defect structure, and THz application of stoichiometric lithium niobate

Owing to the extraordinary richness of its physical properties, congruent lithium niobate has attracted multidecade-long interest both for fundamental science and applications. The combination of ferro-, pyro-, and piezoelectric properties with large electro-optic, acousto-optic, and photoelastic coefficients as well as the strong photorefractive and photovoltaic effects offers a great potential for applications in modern optics. To provide powerful optical components in high energy laser applications, tailoring of key material parameters, especially stoichiometry, is required. This paper reviews the state of the art of growing large stoichiometric LiNbO3 (sLN) crystals, in particular, the defect engineering of pure and doped sLN with emphasis on optical damage resistant (ODR) dopants (e.g., Mg, Zn, In, Sc, Hf, Zr, Sn). The discussion is focused on crystals grown by the high temperature top seeded solution growth (HTTSSG) technique using alkali oxide fluxing agents. Based on high-temperature phase equilibria studies of the Li2O–Nb2O5–X2O ternary systems (X = Na, K, Rb, Cs), the impact of alkali homologue additives on the stoichiometry of the lithium niobate phase will be analyzed, together with a summary of the ultraviolet, infrared, and far-infrared absorption spectroscopic methods developed to characterize the composition of the crystals. It will be shown that using HTTSSG from K2O containing flux, crystals closest to the stoichiometric composition can be grown characterized by a UV-edge position of at about 302 nm and a single narrow hydroxyl band in the IR with a linewidth of less than 3 cm−1 at 300 K. The threshold concentrations for ODR dopants depend on crystal stoichiometry and the valence of the dopants; Raman spectra, hydroxyl vibration spectra, and Z-scan measurements prove to be useful to distinguish crystals below and above the photorefractive threshold. Crystals just above the threshold are preferred for most nonlinear optical applications apart holography and have the additional advantage to minimize the absorption even in the far-infrared (THz) range. The review also provides a discussion on the progress made in the characterization of non-stoichiometry related intrinsic and extrinsic defect structures in doped LN crystals, with emphasis on ODR-ion-doped and/or closely stoichiometric systems, based on both spectroscopic measurements and theoretical modelling, including the results of first principles quantum mechanical calculations on hydroxyl defects. It will also be shown that new perspective applications, e.g., the generation of high energy THz pulses with energies on the tens-of-mJ scale, are feasible with ODR-doped sLN crystals if optimal conditions, including the contact grating technique, are applied.

Pockels Coefficients in Multicomponent Oxide Glasses

In this work, a corrected model for the additivity of the Pockels coefficients in multicomponent oxide glasses is derived. Given some material deformation, changes in density and electronic polarizability are known to be the dominant factors in the photoelastic effect. A trend of Pockels coefficients decreasing with increasing bond ionicity is postulated. This is discussed and shown to be due to the balancing of the relative changes in density and in polarization based on the bond strength and directionality. Alkali silicate data taken from the literature, shown to oppose the observed trends, is analyzed and compared to alkaline earth silicate data. The opposing trend is shown to be due to the bonding of the modifier ions. The alkali ions, associated with a single nonbridging oxygen (NBO), yield an increased relative density change and a decreased relative change in polarization. The alkaline earth ions, being associated with two NBOs, as expected experience significant density and polarizability changes that are more spatially distributed. Greater insights into the structural and chemical origins of the Pockels coefficient in multicomponent oxide glasses will enable the development of new optical fibers for high energy laser applications where photoelasticity‐influenced parasitic effects such as stimulated Brillouin scattering are mitigated.

Suitability of polycarbonate safety glasses for UV laser eye protection

Commonly used impact resistant polycarbonate (PC) safety glasses were evaluated for suitability of use in a high power, ultra-violet (UV) laser lab in place of laser eyewear. Product bulletins for the glasses tested all specified 99.9% or greater UV absorbance for their PC materials. Safety glasses from various manufacturers were exposed to 1,501 pulses of UV light (248 nm) from a 0.6 J krypton fluoride (KrF) excimer laser over a 30 s period. Radiant energy incident on the eye wear was reduced to a nominal 200 mJ through attenuating filters. Surface damage to lens coatings was rapid. Calculations for this laser system indicated that the safety glasses tested had a minimum optical density (OD) of 2.6 (2.58–3.40). At this wavelength the safety glasses would protect from an intra-beam (direct) exposure up to pulse energies of 800 μJ at the lowest OD rating. They would not be protective for an intra-beam exposure to the system as employed for these tests (minimum OD of 5 required). The damage threshold for surface coating destruction was 100 mJ/cm2. Damage only occurred at radiant energies above which the lenses could provide sufficient UV attenuation. That is to say ocular damage could result before damage to the coating became evident. As damage was allowed to continue the PC material began to blister, char and distort. At this point the damage resulted in small increases in optical densities. In general, PC safety glasses would not be suitable for high energy laser applications (all Class 4 lasers and many 3B lasers) whenever intra-beam viewing was possible. Safety glasses may be suitable for diffuse viewing (indirect) situations when it can be determined that the OD provided is sufficient to attenuate the scattered energy.

Low absorption magnesium aluminate spinel windows for high energy laser applications

Abstract

Large-energy, narrow-bandwidth laser pulse at 1645 nm in a diode-pumped Er:YAG solid-state laser passively Q-switched by a monolayer graphene saturable absorber

Nonlinear transmission parameters of monolayer graphene at 1645 nm were obtained. Based on the monolayer graphene saturable absorber, a 1532 nm LD pumped 1645 nm passively Q-switched Er:YAG laser was demonstrated. Under the pump power of 20.8 W, a 1645 nm Q-switched pulse with FWHM of 0.13 nm (without the use of etalon) and energy of 13.5 μJ per pulse can be obtained. To the best of our knowledge, this is the highest pulse energy for graphene-based passively Q-switched Er:YAG laseroperating at 1645 nm, suggesting the potentials of graphene materials for high-energy solid-state laser applications.