Diode-side-pumped joule-level square-rod Nd:glass amplifier with 1†Hz repetition rate and ultrahigh gain.

An effective diode-side-pumped joule-level square-rod Nd:glass multipass amplifier with 1 Hz repetition rate is developed. A Pockels cell is used to extend the amplification-pass to twelve-pass. The linear depolarization compensation method and adaptive optics based wavefront aberration correction method are used to minimize the thermal effects. Combined with the relay-imaging technology and beam-shaping method, good beam quality is obtained. Under the small-signal gain of 3.23 and the input energy of 50 µJ, the output energy of 1 J and the net gain of 2×104 are realized. The effective energy-extraction efficiency in the whole aperture of Nd:glass rod is 12%. The total wavefront aberration is 1.1 µm (peak valley). The Strehl Ratio is 0.62, and 90% of far-field energy is concentrated in 3 times the diffraction limit. The near-field modulation index of twelve-pass amplification at 1 J is 1.4.

Off-axis eight-pass neodymium glass laser amplifier with high efficiency and excellent energy stability.

A new relay-imaged off-axial eight-pass laser amplifier with several joules energy and 1 Hz repetition rate was demonstrated. The extraction efficiency and pulse-to-pulse energy stability were greatly improved. Under the single-pass small-signal gain of 3.6, a net gain of 900 and an effective extraction efficiency of 42.4% in the beam aperture were realized. Pulse-to-pulse energy stability of 0.83% (peak-valley) and 0.17% (root-mean-square) was achieved by the significant saturation of eight-pass amplification, which, to the best of our knowledge, is the best energy stability in several-joules-class amplifiers. The far-field quality was 2.52 times the diffraction limit, and the near-field modulation of the 90% beam aperture was 1.28. No parasitic oscillations or pencil beams were observed. Moreover, another key feature of the proposed amplifier was the ability to remarkably improve the pulse contrast with a unique design.

First Octahedral Spherical Hohlraum Energetics Experiment at the SGIII Laser Facility.

The first octahedral spherical hohlraum energetics experiment is accomplished at the SGIII laser facility. For the first time, the 32 laser beams are injected into the octahedral spherical hohlraum through six laser entrance holes. Two techniques are used to diagnose the radiation field of the octahedral spherical hohlraum in order to obtain comprehensive experimental data. The radiation flux streaming out of laser entrance holes is measured by six flat-response x-ray detectors (FXRDs) and four M-band x-ray detectors, which are placed at different locations of the SGIII target chamber. The radiation temperature is derived from the measured flux of FXRD by using the blackbody assumption. The peak radiation temperature inside hohlraum is determined by the shock wave technique. The experimental results show that the octahedral spherical hohlraum radiation temperature is in the range of 170-182 eV with drive laser energies of 71 kJ to 84 kJ. The radiation temperature inside the hohlraum determined by the shock wave technique is about 175 eV at 71 kJ. For the flat-top laser pulse of 3 ns, the conversion efficiency of gas-filled octahedral spherical hohlraum from laser into soft x rays is about 80% according to the two-dimensional numerical simulation.

Beam alignment based on two-dimensional power spectral density of a near-field image.

Beam alignment is crucial to high-power laser facilities and is used to adjust the laser beams quickly and accurately to meet stringent requirements of pointing and centering. In this paper, a novel alignment method is presented, which employs data processing of the two-dimensional power spectral density (2D-PSD) for a near-field image and resolves the beam pointing error relative to the spatial filter pinhole directly. Combining this with a near-field fiducial mark, the operation of beam alignment is achieved. It is experimentally demonstrated that this scheme realizes a far-field alignment precision of approximately 3% of the pinhole size. This scheme adopts only one near-field camera to construct the alignment system, which provides a simple, efficient, and low-cost way to align lasers.

Process-oriented adaptive optics control method in the multi-pass amplifiers.

In this talk, we propose and demonstrate the process-oriented adaptive optics (AO) wavefront control method, for optimizing the beam quality in the multi-pass amplifiers. Different from the conventional target-oriented wavefront control approach, the novel method divides the aberration correction process into several steps, to optimize the wavefront quality in time during the courses of the beam's transport and amplification. The experimental results show that the proposed method can effectively prevent the beam quality from worsening and ensure the successful reality of multi-pass amplification, so it has obvious advantages both in efficiency and accuracy over the traditional target-oriented method.

Aperture scalable, high-average power capable, hybrid-electrode Pockels cell.

We demonstrate a hybrid-electrode Pockels cell (HEPC) using a thin z-cut deuterated potassium dihydrogen phosphate (DKDP) crystal. The device is a reflective, longitudinally driven, longitudinally cooled Pockels cell, constructed with a plasma chamber providing the incident-side electrode and a silicon-substrate mirror serving the three purposes of a mirror, a back-side electrode, and a heat sink. The mirror and the DKDP are thermally coupled through a sub-millimeter, inert gas-filled gap. A time-multiplex pass-by driving method is proposed to favor low-voltage and fast-response operation. The experimental results support that the HEPC would be a competent device for high-energy and high-repetition-rate lasers.

Suppression of transverse stimulated Raman scattering with laser-induced damage array in a large-aperture potassium dihydrogen phosphate crystal.

A laser-induced damage array composed of numerous pinpoints is generated in a large-aperture KDP crystal to suppress the transverse stimulated Raman scattering (TSRS). The 36 cm × 8.5 mm × 7 mm damage array is used to block the propagation of the TSRS photons within the crystal and decrease the TSRS gain length. Then several series of experiments were conducted on a large-aperture laser system to test this method and experimental results show that the amplification of TSRS is significantly suppressed by the laser-induced damage array.