Power Laser Systems Research Articles

Micro‐Shaping, Polishing, and Damage Repair of Fused Silica Surfaces Using Focused Infrared Laser Beams

Localized infrared (IR) laser heating of fused silica optics has proven highly effective in reducing or removing surface flaws, which tend to limit performance in high power laser systems. Here, we present both simulation and experimental results to examine the use of IR laser light to polish, anneal, and micro‐shape fused silica surfaces used in high power laser systems. We show how the resulting material response can be tuned by considering the temperature‐dependent optical constants of the material and choosing the appropriate laser parameter set. For example, non‐evaporative laser polishing of glass surfaces to heal crack networks is shown most effective when using mid‐IR lasers, which lead to laser energy coupling up to ≈1 mm in depth. In contrast, long‐wave IR light tuned to the Restrahlen frequency of the material is shown to evaporate material most efficiently with penetration depths of <1 μm. Through calibrated, time‐resolved thermal imaging we are able to monitor the laser polishing process, to control material response. The results of our studies can be applied beyond the practical application of damage mitigation in high energy pulsed laser systems to any which require laser‐smoothing and shaping of silica surfaces.

Dynamics of defects in Ce³⁺ doped silica affecting its performance as protective filter in ultraviolet high-power lasers.

We investigate defects forming in Ce³⁺-doped fused silica samples following exposure to nanosecond ultraviolet laser pulses and their relaxation as a function of time and exposure to low intensity light at different wavelengths. A subset of these defects are responsible for inducing absorption in the visible and near infrared spectral range, which is of critical importance for the use of this material as ultraviolet light absorbing filter in high power laser systems. The dependence of the induced absorption as a function of laser fluence and methods to most efficiently mitigate this effect are presented. Experiments simulating the operation of the material as a UV protection filter for high power laser systems were performed in order to determine limitations and practical operational conditions.

Laser-induced damage on ordered and amorphous sol-gel silica coatings

A sol-gel antireflective (AR) coating with an ordered structure has been studied recently for its promising application in high power laser system. However, it is not clear how good the coating is when compared with the traditional sol-gel ones. To address this issue, a comparative study of the laser-induced damage thresholds (LIDTs) of three sol-gel silica coatings was conducted with 1064 nm laser irradiation. Ordered mesoporous, amorphous porous and dense coatings were prepared with acid or base catalysts, and were compared in terms of the skeleton and pore structures, the absorptions, the LIDTs, the damage morphologies and so on. It was found that the former two coatings were comparably strong in resisting laser damage, and that high porosity was the most important factor affecting the LIDT. Besides, the ordered mesoporous coating with a fiber-like skeleton led to the minimum damage of the substrate in all the samples.

Energy coupling in short pulse laser solid interactions and its impact for space debris removal.

Significant advances have been made over the last decade to improve the performance, efficiency, and contrast of high peak and average power laser systems, driven by their use in a wide variety of fields, from the industrial to the scientific. As the contrast of the lasers has improved, interactions with contrasts of 1012 are now routinely undertaken. At such high contrasts, there is negligible preplasma formation and the ionized surface layer created by subpicosecond-duration pulses typically forms a highly reflective "plasma mirror" capable of reflecting between 70% and 90% of the incident energy. Although such interactions are of significant interest for applications such as harmonic source production and to enable the underlying physics to be studied, their low absorption can limit their usefulness for applications such as space debris removal.

Ultra-high contrast frontend for high peak power fs-lasers at 1030 nm.

We present the results from a new frontend within a double-chirped pulse amplification architecture (DCPA) utilizing crossed-polarized wave generation (XPW) for generating ultra-high contrast, 150 μJ-level, femtosecond seed pulses at 1030 nm. These pulses are used in the high energy class diode-pumped laser system Polaris at the Helmholtz Institute in Jena. Within this frontend, laser pulses from a 75 MHz oscillator-pulse train are extracted at a repetition rate of 1 Hz, temporally stretched, amplified and then recompressed reaching a pulse energy of 2 mJ, a bandwidth of 12 nm and 112 fs pulse duration at a center wavelength of 1030 nm. These pulses are temporally filtered via XPW in a holographic-cut BaF₂ crystal, resulting in 150 μJ pulse energy with an efficiency of 13 %. Due to this non-linear filtering, the relative intensity of the amplified spontaneous emission preceding the main pulse is suppressed to 2×10⁻¹³. This is, to the best of our knowledge, the lowest value achieved in a high peak power laser system operating at 1030 nm center wavelength.

Freeform corrector for laser with large aperture YAG:ND3+ active element

The results of transmitted wavefront distortion correction are presented for a YAG:ND 3+ active element with a diameter 45 mm. Halftone mask and proximity printing were used for fabrication of the freeform corrector. Experimental results show a three-fold decrease of the wavefront distortion. Because the corrector presented a high damage threshold, it can be used with high power laser systems.

Crystal growth and application of large size YCOB crystal for high power laser

Yttrium calcium oxyborate YCa4O(BO3)3 (YCOB) is a novel non-linear optical crystal possessing good thermal, mechanical and nonlinear optical properties. It is regards the important candidate frequency conversion material for the high-average power laser system. In this work, we described our effort to achieve the successful growth of large size YCOB single crystals, and the crystal quality of large size YCOB crystal grown by the Bridgman method. The OPCPA application of YCOB element was also introduced simply. The results confirmed that Bridgman technology can be used for the growth of large size YCOB crystal as an alternative to Czochralski method.

On the entropy of radiation reaction

The inexorable development of ever more powerful laser systems has re-ignited interest in electromagnetic radiation reaction and its significance for the collective behaviour of charged matter interacting with intense electromagnetic fields. The classical radiation reaction force on a point electron is non-conservative, and this has led some authors to question the validity of methods used to model ultra-intense laser-matter interactions including radiation reaction. We explain why such concern is unwarranted.

Total Internal Reflection-Based High Efficiency Grating Design for a Metal-Free Polarizing Filter Applications Using Hybrid Optimization Procedure

This paper presents a fast and rigorous design method for grating-based metal-free polarizing filter applications using two-step hybrid optimization techniques. Grating structures utilizing the total internal reflection in a lamellar configuration were used to achieve metal-free solution, which is a key technology in the chirped pulse amplification for high power laser system. Here two polarizing filters were designed: polarization sensitive and polarization insensitive. Those polarization performances were characterized by the rigorous coupled-wave analysis (RCWA), and the design parameters of grating structures, pitch, depth, and filling factor were optimized by two-step hybrid optimization procedure because the diffraction characteristics of grating-based polarizing filters are highly sensitive to small changes in design parameters. The Taguchi method is incorporated into selection process in the genetic algorithm, which indicates that the Taguchi method optimizes the design parameters in a coarse manner, and then, coarsely optimized parameters are finely optimized using the genetic algorithm. Therefore the proposed method could solve global numerical optimization problems with continuous variables. The proposed two-step hybrid optimization algorithm could effectively optimize the grating structures for the purpose of polarization filter applications, and the optimized grating structures could selectively filter the incident light up to 99.8% as to TE or TM waves.

Thin-disk athermal laser system

We present a theoretical scheme for Yb3+-doped high power radiation-balanced (athermal) thin-disk laser system. This high power laser system consists of several thin disks operating athermally. Athermal amplification is achieved by the balance between heat generated during the amplification process and laser cooling with anti-Stokes fluorescence in the system of active Yb3+ ions doped in a KGW host. For athermal operation the pump power has to be distributed properly in the system of several thin disks.

Extreme Light Infrastructure – Nuclear Physics (ELI-NP) European Research Centre

Extreme Light Infrastructure (ELI) is one of the 48 infrastructures of the European Strategic Forum for Research Infrastructure (ESFRI) roadmap and represents a major step forward in quest for producing extreme electromagnetic fields. Extreme Light Infrastructure – Nuclear Physics (ELI-NP) is one of the three pillars of the ELI initiative, which aims to use extreme electromagnetic fields for nuclear physics research. A high power laser system and a very brilliant gamma beam are the two main research equipment at the core of ELI-NP. Their targeted operational parameters are described. The related experimental areas are also presented, together with the main directions of the proposed scientific programme.

Propagation characteristics of small-bandwidth pulsed beams with smoothing by spectral dispersion in high power laser system

In this paper, we numerically study the propagation characteristics through spatial filter and free space of small-bandwidth pulsed beams with smoothing by spectral dispersion (SSD) in high power solid-state laser system. The numerical simulation with time-, space-and spectrum-resolution is achieved by using laser propagation and amplification software developed by Research Center of the Laser Fusion. The graphs of corresponding time-, space-and spectrum-distribution are presented, and the propagation characteristics between the SSD and non-SSD beam are compared. Besides, the effects of some SSD parameters on the diffraction propagation are further discussed. The results can help to choose the proper beam divergence of SSD which matches with the size of the spatial filter hole, to design the appropriate modulation frequency and the color circle number and so on. Moreover, it could guide the design of the light path, especially the design of free space propagation of small-diameter laser.

强激光系统光束质量的非线性退化及补偿研究

强激光系统光束质量的非线性退化及补偿研究

高功率激光系统中近场滤波设计与理论模拟

高功率激光系统中近场滤波设计与理论模拟

Hartmann wavefront sensor based on multielement amplitude masks with apodized apertures

A theoretical and experimental study of a Hartmann sensor intended for measurements of light beam wavefronts and implemented on the basis of multielement amplitude masks with apodized apertures is presented. We perform a comparative analysis of the optical performance of the amplitude masks with hard-edge circular apertures and half-tone masks (HTMs) with apodized apertures forming smooth fast decaying optical responses on a photosensor. A technique for fabricating chromium HTMs is developed, which is based on laser thermochemical method of recording. This technique includes two main stages: first – the exposure of a chromium film by a focused laser beam, whose power is varied in a smooth and nonlinear manner versus the required density of HTM’s structure, and second – the developing of the film in a selective etcher. Specimens of HTMs consisting of 64×64 Gaussian apertures were fabricated and applied in a modified Hartmann sensor to measure the wavefront of powerful laser systems.

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.

High temporal contrast frontend with optical parametric amplifier for high powerful femtosecond laser

Abstract We demonstrate a new method for generating reliable ultrashort 800 nm laser pulse with high temporal contrast. The experimental setup is based on the combination of hollow core fiber, cross-polarized wave generation and optical parametric amplifier. Laser pulses with pulse duration of 10 11 at ~10 ps before the main pulse. High temporal contrast, broad spectrum and the potential high energy of this system make it a reliable frontend for a high powerful femtosecond laser system.

A compact Laboratory Transmission X-ray Microscope for the water window

In the water window (2.2 – 4.4 nm) the attenuation of radiation in water is significantly smaller than in organic material. Therefore, intact biological specimen (e.g. cells) can be investigated in their natural environment. In order to make this technique accessible to users in a laboratory environment a Full-Field Laboratory Transmission X-ray Microscope (L-TXM) has been developed. The L-TXM is operated with a nitrogen laser plasma source employing an InnoSlab high power laser system for plasma generation. For microscopy the Lyα emission of highly ionized nitrogen at 2.48 nm is used. A laser plasma brightness of 5 × 1011 photons/(s × sr × μm2 in line at 2.48 nm) at a laser power of 70 W is demonstrated. In combination with a state-of-the-art Cr/V multilayer condenser mirror the sample is illuminated with 106 photons/(μm2 × s). Using objective zone plates 35 – 40 nm lines can be resolved with exposure times < 60 s. The exposure time can be further reduced to 20 s by the use of new multilayer condenser optics and operating the laser at its full power of 130 W. These exposure times enable cryo tomography in a laboratory environment.

Polarization-independent electro-optic modulator based on PMNT electrically-controlled birefringence effect and Sagnac interferometer

Abstract A novel polarization-independent electro-optic modulator is proposed and investigated by using the electrically-controlled birefringence effect of transparent lead magnesium niobate titanate (PMNT) electro-optic ceramic and a Sagnac interferometer structure. The PMNT electro-optic ceramic is used as a phase retarder for two counter-propagating waves in which their polarization directions are adjusted to parallel and vertical to the electrical-field vector respectively. Then the output light intensity from the Sagnac interferometer will be modulated completely with the driven electrical signal and it is independent with the polarization direction of incident light. The extinction ratio is more than 21 dB. This modulated device will be very appropriate as a Q-switch or optical attenuator in the high power laser system.

Basic Study on Important Parameters of Spatial Filter Used in High Power Laser System

The focal length of the lens in the spatial filter should simultaneously satisfy the requirement of vacuum and the minimum spherical aberration. In order to filter the high frequency components, the divergence angle of the filter must be less than a certain value according to the system requirement. So the aperture of the filter hole used in the high power laser system is about 20 times of the diffraction limitation. At the same time, the scope of the location of the filter hole in the optical axis is reasonable as long as the location within the focal depth. In addition, the model was built by means of the optical design software to simulate validation. The simulation results show that the designed spatial filter according to the method presented in the paper can effectively filter out the high frequency components and stray light in the system.