We presented a novel scheme to improve the stability of the orbital angular momentum (OAM) modes transmission by adding a dip at the edge of the annular high-index region of the air-core fiber. The simulation indicated a larger effective index difference of the vector modes that composed OAM modes in the same order, promising a stable transmission of the OAM modes. The intensity of the modes was concentrated better in this scheme decreasing the crosstalk between adjacent fibers. The propagation properties of the OAM modes in bent fiber were investigated.
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.
The laser pulse should be shaped to satisfy the ICF physical requirement and the profile should be flattened to increase the extraction efficiency of the disk amplifiers and to ensure system safety in ICF laser facility. The spatial-temporal distribution of the laser pulse is affected by the gain saturation, uniformity gain profile of the amplifiers, and the frequency conversion process. The pulse spatial-temporal distribution can’t be described by simply analytic expression, so new iteration algorithms are needed. We propose new inversion method and iteration algorithms in this paper. All of these algorithms have been integrated in SG99 software and the validity has been demonstrated. The result could guide the design of the ICF laser facility in the future.
Physical model was established to describe the pulse superposition in multi-pass amplification process when the pulse reflected from the cavity mirror and the front and the end of the pulse encountered. Theoretical analysis indicates that pulse superposition will consume more inversion population than that consumed without superposition. The standing wave field will be formed when the front and the end of the pulse is coherent overlapped. The inversion population density is spatial hole-burning by the standing wave field. The pulse gain and pulse are affected by superposition. Based on this physical model, three conditions, without superposition, coherent superposition and incoherent superposition were compared. This study will give instructions for high power solid laser design.
KEYWORDS: Phase shift keying, Modulation, Near field optics, High power lasers, Near field, Solid state lasers, Wavefronts, Laser systems engineering, Laser optics, Spatial frequencies
The Power Spectral Density (PSD) is the specification of mid-spatial-frequency phase error in high power solid-state laser facilities. Its evolution during propagation and connection with laser performance have not been fully discussed due to the complexity of nonlinear propagation. We present in this paper the analytic forms of PSD of output phase and intensity modulation, which could be obtained from the input phase PSD introduced by imperfect optics. Moreover, we connect the PSD with the important laser performance parameters: the relative intensity of wings on the focal spot and the contrast ratio of the near-field.
optical propagation simulation by SG99 code and invert algorithm has been made for two typical laser architecture,
namely the National Ignition Facility (model A) and SG-III laser facility (model B) based on measured 400mm aperture
Nd:glass slab gain distribution data on ITB system. When the gain nonuniformity is about 5%, 7%, and 9% respectively
within 395x395mm2 aperture and output beam aperture is 360x360mm2, and output energy is about 16kJ/5ns(square)
with B-integral limited, 1ω(1053nm) nearfield modulation is about 1.10, 1.15, and 1.30 respectively for model A (11+7
slab configuration), and 1.07, 1.08, and 1.17 respectively for model B (9+9 slab configuration) without spatial gain
compensation. With the above three gain nonuniformity and slab configuration unchanged, to achieve flat-in-top output
near field, the compensation depth of the input near field is about 1.5:1, 2.0:1, and 6.0:1 respectively for model A, and
1.3:1, 1.4:1, and 3.5:1 respectively for model B. Compared with model A (the beam aperture unchanged in multi-pass
amplification), the influence of slab gain nonuniformity on model B (beam aperture changed) is smaller. All the above
simulation results deserve further experiment study in the future.
The temporal contrast is an important factor affecting the application of ultra-intense and ultra-short lasers. In this paper, we develop a double chirped-pulse-amplification (CPA) front-end system with an intermediate nonlinear temporal pulse filter to improve the temporal contrast at a sub-petawatt Ti:sapphire laser facility, i.e. the super intense laser for experiment on the extremes (SILEX-I). The temporal pulse filter employs cross-polarized wave (XPW) generation to suppress the amplified spontaneous emission (ASE). The design output energy is 320 mJ for the front-end system. The experimental results show that the output energy of the double CPA system is 360 mJ. The ASE pedestal is suppressed significantly and the temporal contrast is improved by around three orders of magnitude.
Temporal contrast is an important factor affecting the application of ultraintense and ultrashort laser systems. In this
paper, we employ cross-polarized wave (XPW) generation to improve the temporal contrast for ultraintense and
ultrashort pulses in a 300 TW Ti:Sapphire laser facility, i.e. the super intense laser for experiment on the extremes
(SILEX-I). We designed a double chirped-pulse amplification (CPA) system with an intermediate nonlinear temporal
pulse filter based on XPW generation and the estimated output energy is more than 300 mJ for the new front-end system.
The experimental results show that the output energy of the double CPA system is greater than 370 mJ. The amplified
spontaneous emission (ASE) pedestal is suppressed significantly and the temporal contrast is improved by more than two
orders of magnitude.
Laser-induced damage is a key lifetime limiter for optics in large laser facilities. After tested on a
large-aperture high-power laser facility, a damaged fused silica component is disassembled and
conditioned to receive damage test on a small-aperture laser. The damage threshold and growth
behavior show the corners on the component are less damage resistant. The acid etch on corner has not
effectively increased the damage threshold but lowered the damage growth coefficient. A
statistic-based model is presented to extrapolate the threshold data in small-aperture test to predict the
damage threshold under functional conditions.
High power laser facility for ICF will routinely operate at high fluence level. The damage on the large-area FOA optics
is a key lifetime limiter. The optics should be checked after each laser shot for damage initiation and growth. On-line
monitoring equipments are installed for this purpose. Damage pictures of a fused silica component are successfully taken
and the luminance of the pictures could reflect the deterioration of the operational environment. Damage initiation and
growth behaviors at 351nm high-fluence laser were observed. Damage density and damage growth are exponential with
the shot number and some conclusions could be drawn. These results bring forward demands for future monitoring
equipments and more experiments to establish a lifetime model.
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