Fuquan Li

Suppression of FM-to-AM conversion in third-harmonic generation at the retracing point of a crystal.

FM-to-AM conversion can cause many negative effects (e.g., reducing of margin against damage to the optics) on the performance of a third-harmonic conversion system. In this Letter, the FM-to-AM conversion effect in third-harmonic generation is investigated both at and away from the retracing point of Type II potassium dihydrogen phosphate crystal. Obtained results indicate that the FM-to-AM conversion can be suppressed effectively when the crystal works at the retracing point.

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.

Growth and highly efficient third harmonic generation of ammonium dihydrogen phosphate crystals

The growth of ammonium dihydrogen phosphate (ADP) crystals and their optical properties were studied. ADP crystals were first obtained using a potassium dihydrogen phosphate (KDP) seed crystal. Then single ADP crystal was obtained by using ADP plate seed and tetragonal pyramid seed, respectively. The optical properties including transmittance, refractive index, nonlinear optical coefficient and laser-induced damage threshold were also reported and suggested that ADP crystals are a promising alternative to KDP crystals in inertial confinement fusion. The third harmonic generation efficiency was investigated systematically, and all of the samples achieved efficiencies higher than 50%.

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.

Laser-induced damage of a large-aperture potassium dihydrogen phosphate crystal due to transverse stimulated Raman scattering

A unique ?double-lung? laser-induced damage pattern of a large-aperture potassium dihydrogen phosphate (KDP) crystal is observed during a damage experiment carried out on a large-aperture laser system, which has not been predicted by damage tests on small-aperture laser systems. Investigations show that ?double-lung? damage results from transverse stimulated Raman scattering (SRS) and the non-uniform fluence and intensity distribution of transverse SRS leads to the ?double-lung? damage pattern.

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.

Bidirectional amplifying architecture with twin pulses for laser fusion facilities

A novel method to amplify laser pulses in inertial confinement fusion (ICF) facilities is presented, in which twin pulses are employed to propagate along the amplifier in opposite directions. The resulting bidirectional propagation provides a substantially higher extraction efficiency of the amplifier than the conventional multipass architecture for an equal B limit, while keeping the laser fluence at a low level that is beneficial to system reliability and stability. It is shown that an extraction efficiency of ~61% is achievable under a laser fluence of 10 J cm−2 (1ω, one pulse) in an 18-slab amplifier with the bidirectional amplifying design (slab thickness: 4 cm, small signal gain coefficient: 0.0525 cm−1), while the extraction efficiency is only ~40% under a fluence of 13 J cm−2 (1ω) for the conventional multipass architecture.

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.

Coherent addition of gratings for chirped-pulse-amplified lasers based on near-field and far-field measurements

The development of phased-array grating compressor is a crucial issue for high-energy, ultra-short pulse petawatt-class lasers. Almost all systems have adopted a tiled-grating approach to meet the size requirements for the compression gratings. We present a computer-control test system utilizing near-field interference and far-field focusing capable of monitoring and fast correcting tiled errors of the grating compressor. In this system, the tilt/tip errors between the two gratings are determined by the Fourier transform (FT) of the individual interference fringe, and the piston errors are determined by the ratio of the two primary peaks formed in the far-field pattern as a function of the piston difference. Monochromatic grating phasing is achieved experimentally and pulse compression is demonstrated with a tiled grating system.

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.