Polarization smoothing (PS) can effectively improve the uniformity of the focal spot by changing the polarization distribution of the beam. In this paper, a technical scheme for achieving multidimensional PS by cascading birefringent wedge (BW) has been proposed. A near-field and far-field diffraction transmission model with multiple PS and a continuous phase plate (CPP) was first constructed, and parameters such as the wedge angle and placement position of BW are optimized through theoretical analysis to increase the number of sub-focal spots on the target plane and change the distribution direction. Then one-dimensional (1-D) power spectral density (PSD) and root mean square (RMS) were used to quantify the difference in the focal spot uniformity in different directions, the results show that the cascaded BW not only ensures the smoothing effect of a single BW, but also has a smoothing performance in a specific frequency band at specific wedge angles.
Laser multi-focus separation technology is used as an efficient way to cut thick transparent materials and composites. In this paper, we propose a multi-focal design and experimental scheme based on refractive Fresnel lenses. Various refractive type multi-focal Fresnel lens designs were completed by combining theoretical calculations and Trace Pro software simulations, and the designed tri-focal optical system was experimentally verified by using N-BK7 glass, and the experimental errors were analyzed. The results show that the organic plastic (PMMA) processed multi-focal Fresnel lens can be applied to the high-power laser multi-focal optical system. The multi-focal optical system is highly stable and low cost, and has great application prospects.
High-power laser systems have high requirements on the size, shape, and uniformity of the focal spot. Both continuous phase plate (CPP) and lens array (LA) can be used for beam shaping and smoothing. However, their ability is limited and there are few studies on the cooperation of CPP and LA. In this paper, the joint design of the CPP and LA was proposed, we theoretically analyzed its principle and studied the method to achieve better shaping and smoothing performance. Then, the far-field focal spot properties of three beam smoothing methods of the LA, the CPP, the CPP combined with the LA were obtained and compared through numerical simulation. The result shows that the LA is mainly contribution to shaping and the CPP mainly reduces the high-contrast intensity modulation to a certain extent. The joint design of the CPP and the LA greatly improves the uniformity of the far-field focal spot, which is valuable for expanding the application scope of the CPP and the LA.
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.
Realizing the main amplification system without window glasses is important for high-power laser devices. Not only optical components and B-integration of the system are reduced, but also the output capability and beam quality of the system are improved. While the primary problem is how to maintain the cleanliness after the window glasses being removed. In response to this problem, this paper proposes a cleanness control scheme suitable for realizing the windowless operation of the main amplification system.Take one main amplification system of a high-power laser device for example, some work are conducted: integrated design of optical pipelines, introduction of air knives, and simulation analysis of airflow field and related parameters. The integrated design of the beam tubes makes the entire main amplification system to maintain a closed environment.The air knife which can generate a high-speed air curtain and block the exchange of airflow, divides the entire system into three parts: main amplifiers, beam tubes, other optical components. For each part, we use in-situ control to achieve cleanliness. To beam tubes and main amplifiers, theoretical analysis and verification of gas flow field characteristics for different process parameters are conducted. For other components, the effectiveness of air knife air curtain protection has also been experimentally studied and analyzed. Finally, it provides important guidance for realizing the remove of the window glasses of the main amplification system.
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.
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.
The influence of phase mismatching on output pulse temporal waveform in the process of second and third harmonic generation at various power densities with different detuning angles is studied based on nonlinear coupled-wave equations. The rules of different frequencies pulse width vary with the detuning angle have been analyzed quantitatively in potassium dihydrogen phosphate (KDP) crystals. The results of numerical simulation show that in high power laser frequency conversion system, the changes of fundamental pulse width are more sensitive to the detuning angle than second harmonic in the process of doubling, and the maximum variation of the full-width at half-maximum (FWHM) of fundamental at different detuning angles is about 19.0%. For tripling, the output pulse width changes of fundamental and second harmonic are sensitive to the detuning angle at high power density, and the maximum variation of their FWHM at different detuning angles are about 19.0% and 19.2%, respectively. However, the SH pulse width almost unchanged at low power density. The results of this study may provide a promising route to determine the best angle for phase matching in the angle adjusting process of experiment.
Experimental performance of one-dimensional (1D) smoothing by spectral dispersion (SSD) combined with distributed
phase plate (DPP) on the ninth beam of SG-II is presented. Without the application of SSD, normalized focal-spot
non-uniformity of an 85% energy concentration is about 60%. Then, spectral bandwidth of the 3-ns, 1053-nm laser pulse
is broadened to 0.3 nm (as 270GHz in 3ω) by a 3-GHz modulator and a 10-GHz modulator integrated in the front-end
system. Spectral dispersion of 236 μrad/Å is achieved by a Littrow-configuration, 1480-l/mm grating placed between the
Φ40mm faraday isolator and the third Φ40mm rod-amplifier. By using such SSD, normalized non-uniformity with the
same energy concentration is decreased to 16%. A scheme of spatial power spectral density (PSD) in different directions
is adopted to analyze the intensity distribution of the far-field irradiation. Based on the spatial PSD analysis, theoretical
predictions of spectral peak caused by SSD’s color cycles is in excellent agreement with the experimental result. With
double-frequency modulation, the amplitude of the spectral peak is reduced by ~10dB. The temporal waveform of the 1ω
laser is measured. Waveform distortion criterion defining the frequency modulation to amplitude modulation conversion
(FM-to-AM) is about 6% with 1ω laser energy of ~1.8kJ.
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