Wavefront sensor has been widely used for various research fields. Among these wavefront measuring techniques, Shack-Hartmann wavefront sensor has the merit of simple structure, real-time detection and wide waveband. Traditional Shack-Hartmann wavefront sensor is usually based on geometrical optics, which builds a micro-lens array in front of a CMOS sensor. Here amplitude-only photon-sieve array was proposed to replace the micro-lens array to reduce the cost and simultaneously to improve the measurement accuracy by super-resolution focusing of test wavefront. A simulation experiment was carried out through software GLAD and another optical experiment was implemented by Liquid crystal spatial light modulators (LC-SLM) to verify the effectiveness. As a kind of amplitude-only diffractive lens, largeaperture photon-sieve array can be easily fabricated by lithography, which has great potential to be applied for measurement of large-scale laser beam and optical element.
In this paper, a new mechanism of hot image formation introduced in frequency conversion process is introduced. Under the effect of second order nonlinear effect, defects can also cause downstream hot image, where the peak light intensity can reach several times of the average light intensity. The corresponding physical model was established, and the physical process and related laws of the upstream defect induced optical field enhancement under the second order nonlinear action were studied by theoretical analysis and numerical simulation. It is found that three wavelengths of laser will form corresponding hot images at different positions downstream of the nonlinear crystal. The effects of defect size and conversion efficiency on hot image strength are further studied. Finally it is found that the generation of hot image of triple frequency can be effectively inhibited by increasing the spacing between two crystals. This research can provide an important reference for beam quality control of final optical systems of high power laser deriver.
Radial-shearing digital holography has been proposed to reconstruct the wavefront of test object in a common path, which can take use of a bifocal diffractive lens to realize radial-shearing interference. For coherent light source, the recording plane is usually located near the focal point in order to reduce the crosstalk. As for incoherent light source, on the one hand, the above recording cannot take advantage of the space-bandwidth product of optical detector, and on the other, the optical path length is far greater than the coherent length of the light source, which results in failure of effective interferogram. A high contrast interferogram must meet the requirement of nearly equal intensity between the two interference beams. Based on this viewpoint, a better recording plane for radial-shearing holography is in the middle location between the two images. Compared to the recording location with equal beam diameter, the midpoint of the two images is a better choice to improve the image quality on condition that one focal length is much greater than the other. Otherwise, this difference will gradually decrease as one focus is close to the other. The experiment was carried out to verify the validity of our proposed method.
The crosstalk problem between the background light and the signal light of the diffractive optical element severely degrades the image quality and limits the practical application. In this paper, we proposed an off-axis imaging technology aimed at multifocal diffractive lenses and performed geometrical imaging analysis to investigate the imaging distribution. Off-axis imaging ensured a high signal-to-noise ratio by separating the signal and the noise spatially and provided a larger space for parallel synchronization processing in different image plane. Taking the bifocal Greek-ladder lens as an example, the experiment achieved zero-crosstalk multiplanar images and verified the law of diffractive imaging. Besides, the critical condition for zero-crosstalk imaging was also studied.
In high power laser system, the upstream flaw could induce light intensification in the downstream, thus damaging the optical component. In most of the research, the shape of the defect model is ideal, for example, Gaussian shape. However, the defect in the real system is non-ideal with different shapes. In this paper, the light intensification effect caused by defects with different shapes are compared by numerical simulation. Results show the shape dependence of downstream light intensification caused by flaws. When only the linear effect is considered, the change of defect shape could change the maximum light intensification factor and the downstream location for the maximum intensity. When the nonlinear effect is also considered, the light intensification effect will be more sensitive to the shape of defects. This research can provide some reference for the beam quality control and defect management in the high power laser systems.
High repetition rate slab amplifier (HRRSA) is extraordinarily indispensable for the future fusion power plant, ultra-short laser, laser weapon, and so on. Thermal controlling is the decisive factor for the repetition rate and the output energy of the slab amplifier. For larger clear aperture HRRSA, flash-lamp pumped slab amplifier based on neodymium phosphate glass (Nd:glass) is chosen with the liquid cooling. The liquid coolant circulates across the Nd:glass and takes off the thermal induced in the pumping process. A novel liquid coolant (Series A) whose refractive index is the same with Nd:glass is proposed to alleviate the wavefront distortion induced by thermal. The chemical stability of the liquid coolant under high energy flash-lamp irradiation with 200 shots and under the irradiation of a 1053nm laser with 19 hours and 37 hours are experimented. The results show that the chemical stability of the liquid coolant is stable under irradiation.
Neodymium phosphate glass (Nd:glass) is the typical gain medium in the large clear aperture slab amplifiers, and provide more than 99% energy of a high power laser facility. Cladding structure is employed for decreasing the amplified spontaneous emission (ASE) and parasitic oscillation (PO) which are the key limits for the gain of large clear aperture slab amplifier. Polymer cladding is generally used since it was proposed.
Polymer is an organic glue which has risk to decompose and fracture under high flash-lamp irradiation while the irradiation is a common condition in the operation of a high power laser facility. Once the polymer glue falls off, the suppression of ASE and PO of cladding structure would be damaged. In addition, the impurities existing in the glue may form the counteractive scattering sources and reduce the suppression of the cladding structure. Moreover, the decomposer of the organic glue may affect the cleanliness of the slab cavity. Thus, an inorganic edge cladding structure based on hydroxide-catalysis bonding (HCB) was proposed which can match the requirements of the cladding structure and evade the organic glue in the high flash-lamp irradiation slab cavity. An antireflective film was used as the medial material whose refractive index was matched with the cladding structure. Bonded samples were prepared based on HCB and confirmatory experiment under high flash-lamp irradiation was finished.
The latest progress on high power laser facilities in NLHPLP was reported. Based on a high power laser prototype, damage behavior of 3ω optics was experimentally tested, and the key influencing factors contributed to laser-induced damage in optics were deeply analyzed. The latest experimental results of advanced precision measurement for optical quality applied in the high power laser facility were introduced. At last, based on the accumulated works of 3ω elements damage behavior status in our laboratory, beam expanding scheme was presented to increase the total maximum output 3ω energy properly and decrease the laser induced damage risking of ω optics simultaneously.
Optical components are often damaged by hot images in high power laser system, especially for the final optics assembly. There are several nonlinear optical elements and a focusing lens. So both the hot images in free space propagation and in the focusing system are theoretically and numerically studied. We find that the focusing lens moves the hot images towards the lens. Through Fresnel number, the connection of hot image position in free space propagation and in the focusing system is discussed. What’s more, the nonlinear effect of the focusing lens is also considered for the hot images formation because the lens is sometimes very thick. At last, the influence of the size and modulation depth of scatter on the hot images position and intensity are also given. The hot images analysis is essential for the final optics assembly design, which can shed some light on avoiding the optical damage.
A new high power laser facility with 8 beams and maximum output energy of one beam 5kJ/3.4ns/3ω has been performed and operated since 2015. Combined together the existing facilities have constructed a multifunction experimental platform including multi-pulse width of ns, ps and fs and active probing beam, which is an effective tool for Inertial Confinement Fusion (ICF) and High Energy Density (HED) researches. In addition another peculiar high power laser prototype pushes 1ω maximum output energy to 16kJ in 5ns and 17.5kJ in 20ns in flat-in-time pulse, this system is based on large aperture four-pass main amplifier architecture with 310mm×310mm output beam aperture. Meanwhile the near field and far field have good quality spanning large energy scope by use of a wide range of technologies, such as reasonable overall design technique, the integrated front end, cleanness class control, nonlinear laser propagation control, wave-front adaptive optics and precision measurement. Based on this excellent backup, 3ω damage research project is planning to be implemented. To realize the above aims, the beam expanding scheme in final transport spatial filter could be adopted considering tradeoff between the efficient utilization of 1ω output and 3ω damage threshold. Besides for deeply dissecting conversion process for beam characteristic influence of 1ω beam, WCI (Wave-front Code Image) instrument with refined structure would be used to measure optical field with simultaneous high precision amplitude and phase information, and what’s more WCI can measure the 1ω, 2ω and 3ω optical field in the same time at same position, so we can analyze the 3ω beam quality evolution systematically, and ultimately to improve the 3ω limited output.
In a word, we need pay attention to some aspects contents with emphasis for future huger laser facility development. The first is to focus the new technology application. The second is to solve the matching problem between 1ω beam and the 3ω beam. The last is to build the whole effective design in order to improve efficiency and cost performance.
A kind of diffractive optical elements (DOE) with star-ring topological structure is proposed and their focusing and imaging properties are studied in detail. The so-called star-ring topological structure denotes that a large number of pinholes distributed in many specific zone orbits. In two dimensional plane, this structure can be constructed by two constrains, one is a mapping function, which yields total potential zone orbits, corresponding to the optical path difference (OPD); the other is a switching sequence based on the given encoded seed elements and recursion relation to operate the valid zone orbits. The focusing and imaging properties of DOE with star-ring topological structure are only determined by the aperiodic sequence, and not relevant to the concrete geometry structure. In this way, we can not only complete the traditional symmetrical DOE, such as circular Dammam grating, Fresnel zone plates, photon sieves, and their derivatives, but also construct asymmetrical elements with anisotropic diffraction pattern. Similarly, free-form surface or three dimensional DOE with star-ring topological structure can be constructed by the same method proposed. In consequence of smaller size, lighter weight, more flexible design, these elements may allow for some new applications in micro and nanphotonics.
The non-collinear phase-matching in Potassium Dideuterium Phosphate (DKDP) crystal is analyzed in detail with signal pulse of center wavelength at 808 nm and pump pulse of wavelength at 526.5 nm. By numerical analysis, parametric bandwidths for various DKDP crystals of different deuteration level are presented. In particularly for DKDP crystals of 95% deuteration level, the optimal non-collinear angles, phase-matching angles, parametric bandwidths, walk-off angles, acceptance angles, efficiency coefficients, gain and gain bandwidths are provided based on the parameter concepts. Optical parametric chirped pulse amplifier based on DKDP crystal is designed and the output characteristics are simulated by OPA coupled wave equations for further discuss. It is concluded that DKDP crystals higher than 90% deuteration level can be utilized in ultra-short high power laser systems with compressed pulses broader than 30fs. The disadvantage is that the acceptance angles are small, increasing the difficulty of engineering regulation.
KEYWORDS: Modulation, Optical components, Standards development, High power lasers, Near field, Laser systems engineering, Phase modulation, Near field optics, Diffraction, National Ignition Facility
International standard ISO 10110-7 sets a strict limit on the size and quantity for surface defects of an optical element. For high-power laser, sub-beams caused by defects with different distributions interfere with each other in the transmission process, causing beam quality complex changes. So it is necessary to make a clear limitation on relative position of defects, thereby giving the standard a more comprehensive supplement. Based on the diffraction theory, the changes of beam modulation are studied. The influence of scratch depth on the distribution of near field beam modulation is also taken into account. Results demonstrate that when two parallel scratches are on the same or different surfaces of an element, they produce more severe modulation than single scratch, and the maximum modulation can be increased to 1.5 times. Meanwhile more strict requirements for scratch depth are put forward. The results could provide reference for the determination of defects specifications for large-diameter optical elements in high-power laser systems.
Zone plates and photon sieves can be used to focus soft X-rays and hard X-rays. Relative to the parallel plane wave incidence and focusing on the optical axis, we here present two different models to describe the other kinds of focusing properties. The former, the scaled zone plates or photon sieves are appropriate for the titled plane wave to image, which can alter the propagation direction. The latter, the eccentric elliptical zone plates or photon sieves are appropriate for the point-to-point off-axis focusing. Based on the above-mentioned models, the different algorithms are discussed in detail under the condition of different numerical apertures. Furthermore, the correctness of our model has been verified through the commercial software VirtualLAB. The obtained results can be used for the analysis, design, and simulation of different zone plates and photon sieves, meanwhile the non-coaxial characteristics can increase the flexibility of the optical system.
KEYWORDS: Modulation, Wavefronts, Phase shift keying, Near field, Near field optics, Contamination, High power lasers, Optical components, Particles, Diffraction
In high-power laser facilities for inertial confinement fusion, there are many large-diameter optical elements, which
inevitably have some flaws on the surface. The beam is modulated by these flaws after diffraction transmission, thereby
reducing the beam quality of the system. The optical field with high modulation may cause self-focusing in fused silica
and thus damage the optical components, which seriously affects the load capacity of the device. Therefore, for
high-power laser systems, near field beam quality also has a high demand. In this paper, the effect of flaws and nonideal
wavefront of sinusoidal modulation on the near-field quality at specific position behind the focusing lens is analyzed for
final target system of high power laser device based on the Huygens-Fresnel diffraction theory. Firstly the beam
modulation of ideal wavefront disturbed by flaws is investigated. And the modulation by phase type flaw is more serious
than amplitude type. Secondly, near field beam modulation of nonideal wavefront of sinusoidal phase disturbed by flaws
is analyzed. Results demonstrate that under some specific conditions of sinusoidal phase, modulation degree is reduced
and the beam quality is improved by the nonideal wavefront compared to the ideal wavefront. The results could give
some references to the improvement of near field beam quality and mitigation of risk of optical damage caused by
self-focusing.
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