Reduction of Backward Scatterings at the Low-Coherence Kunwu Laser Facility.

We report the first experimental observation on the reduction of backward scatterings by an instantaneous broadband laser with 0.6% bandwidth in conditions of interest for inertial confinement fusion at the low-coherence Kunwu laser facility. The backscatter of stimulated Brillouin scattering (SBS) was robustly reduced by half at intensities of 1-5×10^{14} W/cm^{2} with the 0.53-µm broadband laser in comparison with the monochromatic laser. As SBS dominates energy loss of laser-plasma interactions, the reduction of that demonstrates the enhancement of laser-target coupling by the use of broadband laser. The mitigation of filamentation leads to the reduction of stimulated Raman backscattering at low intensities. In addition, the three-halves harmonic emission was reduced with the broadband laser as well.

Understanding and optimizing diffraction gratings by the blazing analysis of total internal reflection.

The diffraction grating is a classic and important optical element, and its design usually traverses the whole parameter space to search for an optimal solution, which is time consuming and inefficient. In order to specify the optimization direction of the grating to obtain clearer physical images and to improve the design efficiency, a new blazing model based on the total internal reflection (TIR) is proposed to analyze the diffraction behavior of the grating from a geometry perspective. The optical tunnel along the ridge direction can be used to understand and quantify the blaze of the grating. This TIR blazing model is demonstrated via three types of surface-relief grating with simple formulas, resulting in the solution space decreasing significantly. By utilization of the estimated upper limit of the diffraction efficiency and the range of the depth and slanted angle generated by the TIR blazing model, how the grating delivers the majority of the light energy to a required diffraction order is revealed. Binary and slanted gratings with >0.93 efficiency of T1 order have been obtained with high probability within the calculated parameter range, regardless of the duty cycle and polarization. The reason why a transmission sawtooth grating cannot blaze the most energy to a high order at normal incidence has been clarified, and the method of using the first or second TIR blaze has also been provided. Through this TIR blazing model, the grating design could be simplified, and accommodation to various application requirements could be optimized as well.

High-stability, high-beam-quality, and pulse-width-tunable 1319 nm laser system for VISAR applications in high-power laser facilities.

A high-energy, high-beam-quality, and pulse-width-tunable Nd:YAG laser system, pumped by vertical cavity surface emitting laser arrays and laser diodes, is demonstrated and applied to a velocity interferometry system for any reflector (VISAR) application in a high power laser facility. A multistage multipass amplification structure is used to fully extract the amplifier energy and obtain a high-energy pulse. The temporal waveform is compensated to provide a square waveform, with a flatness less than 8% (peak-to-peak value). The peak power is greater than 100 kW with a frequency-doubling efficiency of 25% for a 50 ns pulse width. The laser operates as a single shot with 1-5 Hz repetition frequency and 0.7% rms energy stability.

Phase-shifting radial-shearing digital holography with Greek-ladder zone plates.

Holography has been widely applied in fields of imaging and measurement, but the optical setup of traditional phase-shifting holography is complicated. Here, a kind of phase-shifting radial-shearing holography with Greek-ladder zone plates is presented, which largely simplifies the optical configuration for holographic recording and enhances the stability of system. A 1951 U.S. Air Force resolution test target is measured by experiment to verify the validity of our proposed method. Benefiting from amplitude-only diffractive lenses and high stability of the common path, the phase-shifting radial-shearing interference with Greek-ladder zone plates has great potential in biological imaging and x-ray holography for the next generation of synchrotron light sources.

Multiplanar imaging properties of Theon sieves.

Greek ladders with diffraction-limited array foci provide a probability to realize array imaging with equal intensity. Here, taking the ancient Theon sequence as an example, we design the optical structure and have measured the focusing properties by digital holography. Then, we verify the multiplanar imaging with different magnifications by experiment. The experimental results agree well with the theoretical analysis. In addition, bi-Fourier planes filtering technology is proposed to solve the problem of crosstalk between different imaging planes to further improve the imaging resolution. Therefore, we can freely design the focal length of the bifocal lens to achieve high-quality imaging at different resolutions. As a kind of amplitude-only diffractive lens, multifocal imaging provides a possibility of application in array biological imaging, ophthalmology, and an optical zoom system.

Generation of broadband laser by high-frequency bulk phase modulator with multipass configuration.

A new technique is presented for obtaining a large broadband nanosecond-laser pulse. This technique is based on multipass phase modulation of a single-frequency nanosecond-laser pulse from the integrated front-end source, and it is able to shape the temporal profile of the pulse arbitrarily, making this approach attractive for high-energy-density physical experiments in current laser fusion facilities. Two kinds of cavity configuration for multipass modulation are proposed, and the performances of both of them are discussed theoretically in detail for the first time to our knowledge. Simulation results show that the bandwidth of the generated laser pulse by this approach can achieve more than 100 nm in principle if adjustment accuracy of the time interval between contiguous passes is controlled within 0.1% of a microwave period. In our preliminary experiment, a 2 ns laser pulse with 1.35-nm bandwidth in 1053 nm is produced via this technique, which agrees well with the theoretical result. Owing to an all-solid-state structure, the energy of the pulse achieves 25 µJ. In the future, with energy compensation and spectrum filtering, this technique is expected to generate a nanosecond-laser pulse of 3 nm or above bandwidth with energy of about 100 µJ.

Performance of target irradiation in a high-power laser with a continuous phase plate and spectral dispersion.

We report on the performance of target irradiation at the SG-II high-power laser facility with a continuous phase plate (CPP) and the technique of smoothing by spectral dispersion (SSD). Simulative and experimental results are presented, where the irradiation uniformity and energy concentration of the target spots are analyzed. The results show that the designed CPP can focus the spot energy into the desired region and shape a profile with steep edge and flat top, but the actual performance of the fabricated CPP needs some improvements. It is also proved that the CPP is insensitive to the long-scale wavefront distortion in the incident beam. The one-dimensional SSD configuration evidently works in smoothing the fine-scale intensity modulation inside the target spot.

The generation of a fourth-harmonic probe and its application in Nomarski interferometry at Shengguang-II.

In this work, a design for the generation of a 4ω (263-nm) probe converted from a 1ω (1053-nm) laser is presented. The design is based on a beta-barium borate and potassium dihydrogen phosphate two-step frequency-conversion process. A suitable configuration for Nomarski interferometry based on the 4ω probe is proposed, for measuring the electron density of laser-produced plasmas. The signal-to-noise ratio of the output 4ω probe to 1ω and 2ω light after frequency quadrupling and harmonic separation is 103 with a 0.5 GW/cm2 1ω input but decreases to ∼102 at intensities below 0.1 GW/cm2. Additional noise suppression by a factor of 104 is achieved using filters before the interferometer recording camera. The spatial resolution of the diagnostic can reach 5.2 µm for a 10% modulation transfer function. An experiment validating the probe diagnostic system is conducted at the Shengguang-II laser facility. A clear interferogram of an aluminum plasma is obtained with 0.1 GW/cm2 input, suggesting a maximal electron density of about 2.5 × 1020 cm-3 as retrieved through an inverse-Abel transform. The design proposed in this paper is appropriate for a small laser device or a large laser facility that lacks a separate diagnostic beam, and it is an inexpensive solution as it requires small-aperture 1ω input at a relatively low intensity. All the key parameters necessary to implement the design are provided in detail, making it straightforward to reproduce or transplant the system for specific uses.

Design and calibration of hot-electron spectrometer array for angle-resolved measurement.

A hot-electron spectrometer array with two-dimensional distribution has been designed with a wide-angle range and high-energy resolution to measure the spatially resolved electron spectra for high-power-laser plasma interaction experiments. It consisted of 19 identical electron spectrometers set in three directions with an interval of 10°. Each electron spectrometer was designed with a uniform magnetic field to detect electrons in the range from 20 to 500 keV. The spectrometers were calibrated using electrons from an accelerator. In an experiment, the spatially resolved electron energy spectra, which approximately had a Maxwell distribution, were obtained from an aluminum foil target irradiated by a 0.53-µm laser pulse.

Two-dimensional performance of uniform irradiation with the use of an edge-softened lens array and spectral dispersion.

A lens array composed of edge-softened elements is used to improve on-target irradiation uniformity in the Shenguang II Laser Facility, with which a Fresnel pattern of suppressed diffraction peaks is obtained. Additional uniformity can be reached by reducing short-wavelength interference speckles inside the pattern when the technique of smoothing by spectral dispersion is also used. Two-dimensional performance of irradiation is simulated and the results indicate that a pattern of steeper edges and a flat top can be achieved with this joint technique.