Huaiting Jia

Comparison of efficient third-harmonic generation between phase modulated broadband and narrowband lasers

Based on the existing design in our prototype laser facility, the third-harmonic generation of the phase modulated broadband of 0.3 nm and narrowband lasers are theoretically analyzed and experimentally studied in this paper. The output third-harmonic generation (THG) laser characteristics such as the conversion efficiency, the spectrum distribution and the intensity modulation are compared in detail. It is found that the THG conversion efficiencies for the narrowband and such broadband lasers are fundamentally identical without spectrum narrowing. In addition, the interrelationship between THG conversion efficiency and incident fundamental light intensity for both lasers is experimentally demonstrated and the experimental results agree with the theoretical analysis. The broadband THG spectrum with ideal repetition and without loss of spectrum is obtained in the experiment.

Efficient fourth harmonic generation of Nd: glass lasers in ADP and DKDP crystals

Experimental research on non-critical phase-matching fourth harmonic generation with ADP and DKDP crystals are reported. The characteristics of 2ω-to-4ω efficiency as function of incident angle, crystal temperature and input 2ω intensity have been investigated in detail, and the 2ω-to-4ω conversion efficiency has been demonstrated up to 84.1% with ADP crystal and 85.1% with DKDP crystal, respectively. Nevertheless, the spatial non-uniformity of rapid grown DKDP crystal has to be improved while the temperature control uniformity should be upgraded for ADP crystal to realize large aperture high efficiency fourth harmonic generation.

Small-scale self-focusing in a tapered optical beam

The analysis of Bespalov and Talanov concerning small scale self-focusing or filamentation effects for plane waves is generalized to a tapered beam case in this paper. A model of nonlinear phase and amplitude perturbations different from Siegmans is suggested. The previous conclusion that self-focusing is very ineffectual for strongly tapered, both divergent and convergent, beams in optical Kerr media as compared to small-scale self-focusing in plane waves is found to be inappropriate. Moreover, the different conditions of phase matching in cases of cylindrical and spherical waves are explicated, which could throw some light on the issue.

Demonstration of broadband frequency tripling based on cascaded partially deuterated KDP crystals

The cascaded KDP crystal scheme with different deuteration levels for enhancing the bandwidth of third-harmonic generation (THG) is experimentally demonstrated in this paper. The deuteration level of a partially deuterated KDP crystal is used as a degree of freedom to alter the phase-matching (PM) wavelength. The experimental results show that this scheme can increase the bandwidth acceptance in broadband frequency tripling. In addition, the spacing between the two cascaded crystals has significant influence on the THG spectrum as well as THG conversion efficiency, and a relatively ideal THG spectrum can be obtained by the optimized parameters. Meanwhile, nonlinear-coupled wave equations are used to theoretically analyze the experimental results, and the experimental results are consistent with the numerical simulation.

A multi-beam alignment method at target area for ICF laser facility based on invariant sub-area configuration of the alignment sensor

A multi-beam alignment method is proposed to reduce the total time for aligning at the target area all the laser beams of an ICF laser facility. A number of sub-areas with invariant size and position are extracted from the image acquired by the alignment sensor. An alignment route is comprised of a certain part of those sub-areas, and several alignment routes can cover all the sub-areas. The invariant layout of the sub-areas and the alignment routes is called an invariant sub-area configuration of the alignment sensor. The focused spots of the alignment beams are adjusted in a specific sequence along the alignment routes, and finally reach the desired position on the alignment sensor. The adjustment of all the spots inside each sub-areas is carried out concurrently, and the adjustment along one route for a spot moving from one sub-area into the next sub-area is carried out consecutively. The estimated total time for aligning all the laser beams at target area shows that the proposed multi-beam alignment method has a much higher efficiency.

Dark spot downstream from nonlinear hot image

The dark spot effect downstream from the nonlinear hot image is accounted for in this paper. The conditions for the formation of dark spot are carefully discussed. The explanation is based on analytical analysis, and the results are verified by numerical simulations. The dependence of the location of the dark spot on the nonlinear phase delay seems to suggest a probable method for measuring the nonlinear refractive coefficient of materials.

Efficient Frequency Tripling of Phase Modulated Laser Based on Two KDP Mixers in Tandem Array

By using the cascaded KDP crystals, the third harmonic generation (THG) of temporal phase modulated broadband laser is experimentally demonstrated in this paper. The spectral characteristic of the generated third harmonic waves is investigated in detail. The experimental results show that the cascaded crystals method is able to satisfy the acceptance bandwidth of broadband laser beam. In addition, the influence on the THG spectrum and THG conversion efficiency of the air spacing between the two cascaded crystals is experimentally studied and it is remarkable on THG spectrum but insignificant for THG conversion efficiency. The relatively ideal THG spectrum is obtained by the optimal parameter of air spacing and the stability of THG spectrum is very good in our experiment.

Frequency conversion system design and development for the large laser facility

The design of the Large Laser Facility incorporates a type-I-type-II third harmonic generator to convert the 1.053μm fundamental wavelength of the laser amplifier to a wavelength of 0.351μm for target irradiation. To understand the design of frequency conversion system, we have carried out a series of theory researches and experiments, including parameters optimization of the third harmonic converter, experiments on high-intensity third harmonic generation (THG), researches on Transverse Stimulated Raman Scattering (TSRS) in the converting crystal and on broad-band third harmonic generation.

Numerical analysis of the polarization smoothing in a convergent beam

The polarization smoothing (PS) of the focal spot on target is a key technology for inertial confinement fusion (ICF) laser. A mathematical model is presented to analyze the polarization smoothing in a convergent beam. The relation between the separations (both transverse and longitudinal) of focal spots and the parameters of the crystal are established. Via numerical simulation, the three-dimensional distributions of the far-field with and without PS are demonstrated. The relation between the property of the focal spot and the crystal’s thickness and tilt angle are obtained. Best smoothing can be achieved with the optimized thickness and tilt angle of the PS crystal.

Online tuning technique of frequency conversion crystals of high power solid-state laser facility at low 1ω drive irradiance

Advanced an online low 1ω drive irradiance tuning technique of frequency conversion crystals of high power solid-state laser facility, which can acquire the best match angle of frequency conversion crystals through online low 1ω drive irradiance tuning curve test, and achieve fast and high precision angle correction to assure the frequency conversion crystals to achieve the highest energy conversion efficiency in shot experiments. Analyzed the possibility of online low 1ω drive irradiance tuning technique of frequency conversion crystals, researched the technical scheme of online low 1ω drive irradiance tuning of frequency conversion crystals, and applied this technique on SG facility, which achieved 60%~70% frequency conversion efficiency in high energy shots.