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.
In the research of inertial confinement fusion, laser plasma interaction (LPI) is becoming a key problem that affects ignition. Here, multi-frequency modulation (Multi-FM) smoothing by spectral dispersion (SSD), continuous phase plate (CPP) and polarization smoothing (PS) were experimentally studied and equipped on SG-III laser facility. After using these technologies, the focal spots of SG-III laser facility can be adjusted, controlled and repeated accurately. Experiments on SG-III laser facility indicate when the number of color cycles adopts 1, imposing SSD with 3.3 times diffraction limit (TDL) did not lead to pinhole closure in the spatial filters of the preamplifier and the main amplifier with 30-TDL pinhole size. The nonuniformity of the focal spots using Multi-FM SSD, CPP and PS drops to 0.18, comparing to 0.26 with CPP+SSD, and 0.84 with CPP and wedged lens. Polarization smoothing using flat birefringent plate in the convergent beam of final optics assembly (FOA) was studied.
Backward Raman amplification (BRA) in plasma has been demonstrated an effective way to produce high power laser pulses. However, most experiments of BRA are carried out around the pump wavelength of 800 nm. In recent years, the 1053 nm pump pulse becomes more and more essential as the development of the chirped pulse amplification (CPA) around this wavelength. Here we design an experiment of BRA with a 1053 nm, 20 ps pump pulse and a 1200 nm, 50 fs seed pulse based on the facility of XG III. The simulation results obtained by a 1-d particle-in-cell (PIC) code show that the amplified peak seed intensity of ∼ 5 × 10<sup>16</sup> W/cm<sup>2</sup> is obtained, with an energy transfer up to 16.8%. An output pulse of petawatt power is theoretically demonstrated feasible.
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.
SG-III laser facility is now the largest under-construction laser driver for
inertial confinement fusion (ICF) research in China, whose 48 beams will deliver 180kJ/3ns/3ω energy to target in one shot. Till the summer of 2014, 4 bundle of lasers
have finished their engineering installation and testing, and the A1 laser testing is
undergoing. A round of physics experiment is planned in Oct. 2014 with 5 bundle of
lasers, which means the facility must be prepared for a near-full-capability operation
before the last quarter of 2014. This paper will briefly introduce the latest progress of
the engineering and research progress of SG-III laser facility.
For better performance of laser coupling in inertial confinement fusion (ICF), beam shaping of the focus spot is
required. Among all the beam smoothing methods, the multi frequency modulation smoothing by spectral dispersion
(MultiFM-SSD) proposed by LLE has the advantages of the faster smoothing and better operability. Strong
frequency modulation to amplitude modulation conversion(FM-to-AM) will take place because of the complex
spectrum imposed by the multi frequency modulators applied in the Multi FM-SSD method. The FM-to-AM effect is
studied with numerical simulation including the polarization mode dispersion and group velocity dispersion. The
results reveal that the modulation frequencies and bandwidths of multi modulators will influence the contrast degree
of the FM-to-AM effect. The compensation of the FM-to-AM with arbitrary waveform generator (AWG) is also
numerically simulated. The FM-to-AM effect is effectively suppressed, i.e. the non-uniformity of the pulse decreases
substantially, by applying multiple intensity and phase compensation (the compensation function is obtained via G-S
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 395x395mm<sup>2</sup> aperture and output beam aperture is 360x360mm<sup>2</sup>, 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 under-construction SG-III laser facility is a huge high power solid laser driver, which contains 48 beams and is
designed to deliver 180kJ energy at 3ns pulse duration. The testing ending up at September 2012 validated that the first
bundle lasers of SG-III facility had achieved all the designed requirements. And shortly later in December 2012, the first
round of running-in physics experiment provided a preliminary X-ray diagnostic result. In the testing experiment,
detailed analysis of the laser energy, the temporal characteristics, the spatial distribution and the focusing performance
was made by using the Beam Integrated Diagnostic System. The 25kJ 3ω energy produced by the first bundle lasers
created the new domestic record in China. These great progresses in the laser performance and the physics experiment
have already demonstrated that the facility is in excellent accordance with the designs, which establish a solid foundation
for completing all the construction goals.
The injection lens of the high power solid laser facility built soon was designed as double
lens. The tolerance of the double lens was analyzed, and the optical performance of which was
detected in experiment.
High-power solid-state laser programs at China Academy of Engineering Physics have made great progresses in recent years. A three-stage Ti:sapphire laser system, SILEX-I, was completed early in 2004 which could deliver 26-fs pulses at 5TW, 30TW, and 300TW to the corresponding target chambers for diverse applications. SILEX-I has been working very stably since its completion for experiments, demonstrating that it is the most powerful femtosecond Ti:sapphire laser for exploring strong-field phenomena in the world. The SG-III Nd:glass laser facility has been under conceptual design to meet the requirements from laser fusion applications. The SG-III facility is planned to have sixty-four beamlines divided into eight bundles with an output energy more than 100kJ at 0.35μm for 3- to 5-ns pulses. The eight-beamline TIL (Technical Integration Line), the prototype of the SG-III laser facility, has been installed in the new laboratory in Mianyang. The commissioning experiments have been conducted and one of the eight beams has produced 1-ns pulses of 3.0kJ and 1.2kJ at 1.053μm and 0.35μm, respectively. All the eight beamlines will be activated by the end of 2005 and completed in 2006 for operation. Meanwhile, the eight-beam SG-II laser in Shanghai Institute of Optics and Fine Mechanics has been operated for the experiments since 2001 and an additional beam, built in 2004, has been used for plasma backlighting experiments.
A Ti:sapphire laser system referred to as SILEX-I with the chirped pulse amplification technology has been built at CAEP which consists of three stages operating at 5TW, 30TW, and 300TW, each having a compressor and target chamber to meet different needs from diverse applications. The first and the second stages work at 10Hz, while the third at single shot. Pulse durations of 26fs have been obtained by installing an acousto-optic programmable dispersive filter (AOPDF) before the stretcher to compensate for the spectral gain narrowing in the regen. By taking a number of advanced measures for spatial beam control, such as spatial beam-shaping, relay-imaged propagation, precise alignment of compressor gratings and OAP, near-diffraction limited focal spots (FWHM) have been obtained. Focused intensities
are measured in the range of (1-5) x 10<sup>20</sup>W/cm<sup>2</sup> with an f/2.2 OAP. The laser system will be able to operate at 500TW and even higher soon. The SILEX-I has been operated for experiments since its completion early in 2004, covering electron and proton acceleration, hot electron production, transport and deposition, neutron production, x-ray radiation, femtosecond laser pulse propagation in air, warm matter, and other strong-field studies. The laser system has shown an excellent stability and reliability and has been the most powerful femtosecond Ti:sapphire laser facility to operate for experiments in recent years.
A peak power of 286-TW Ti:sapphire laser facility referred to as SILEX-I was
successfully built at China Academy of Engineering Physics, for a pulse duration of 30 fs in
a three-stage Ti:sapphire amplifier chain based on chirped-pulse amplification. The beam
have a wavefront distortion of 0.63μm PV and 0.09μm RMS, and the focal spot with an
f/2.2 OAP is 5.7μm, to our knowledge, this is the best far field obtained for high-power
ultra-short pulse laser systems with no deformable mirror wavefront correction. The
peak focused intensity of ~1021W /cm2 were expected.
In this paper, the physical models of the code SG99, which is used to simulate the pulse behavior in high power laser system, are presented in details. The experimental results are also presented to show that SG99 is capable of simulating pulse propagation well and yields reasonable results. In the last, some results in design of TIL(Technical Integrated Experiment Line), the prototype of ShenGuangIII, are also introduced.
A multihundred-terawatt Ti:sapphire laser facility was built at China Academy of Engineering Physics which could deliver femtosecond pulses at three power levels of 5TW, 30TW and hundreds TW to targets. Near-diffraction-limited focal spots were measured and it was found for the first time that alignment errors of grating groove parallelisms in compressors could be the major mechanism for producing elongated far fields. Pulse durations of 35fs were obtained with a Fastlite-produced AOPDF for spectral compensation.
A ray tracing software has been developed to allocate the stability requirements of a fusion laser facility. Using the developed software and by establishing a mathematical model of the layout of the fusion laser facility TIL, the task of analyzing the relations between the stability of any individual optical component and the position of the beam foci on the target has been fulfilled. Then, by adding random perturbation to the coordinate parameters of all optical components of the facility and calculating the possibility of the foci to locate in the sphere with radius of 30 μm (the target-shooting requirement of TIL), the stability requirements of the components of the facility has been acquired.