A ultrashort pulse Nd:YAG rod gain medium regenerative amplifier side-pumped by A laser diode array was studied. The SESAM mode locking fiber seed pulses with 10ps pulse duration 1nJ single pulse energy 50 MHz repetition-rate and the wavelength of 1064 nm, was amplified to 1 mJ at 1 kHz by our regenerative amplifier, corresponding to a peak power of 0.1 GW, with the maximum amplification about 3.3×106 . And when the repetition rate changed to 10 kHz, an average power of 6 W was obtained, corresponding to an amplification of 2×106 . The repetition-rate to period doubling of regenerative amplifier pulse was experimentally studied.
We demonstrate a short pulsed Yb-doped fiber laser system comprising two coherently combined fibers with 15ps pulse width and 20MHz repetition rate. The system delivers an average laser power of 6.6mW, of which the RMS error is only 0.13%. The stability is optimal for a coherent beam combining system with multiple combining stages, thanks to an active feedback control loop implemented in the system. The feedback control loop constantly checks the system output power, automatically maximizing and stabilizing it. Implementation of the active feedback control loop is simple, robust, and versatile.
We describe the design and experimental result of a LD pumped Nd:YAG laser with 12J energy at repetition rate of 10Hz. The temperature distribution was controlled to less than 2℃ on the surface by means of uniform pump and cooling. The ASE was calculated by energy storage code and fit well with the measurement results which was about 1.72 in average. The beam quality was controlled by means of mechanical design and adjustment and compensation by a home-made deforming mirror. The far field was measured to 3.23 TDL. The stability of energy and pointing were paid great attention and control by means of full absorption and high stability mechanical design. The energy stability was less than1%(RMS) and pointing stability was 73μrad(PtV), which made the laser very comfortable for use.
Spatial-spectral interference carries the spectral phase difference information between short pulses. We propose a new method of time delay retrieval via the slope of spatial-spectral interference fringe in the case of only time delay without high-order spectral phase difference between short pulses. The analytical expression is deduced based on the principle of spatial-spectral interference. The simulation results show that the slope of spatial-spectral interference fringe and the crossing angle between short pulses are both important for the calculation accuracy. This proposed method has advantages of no direction-of-time ambiguity, simple principle and calculation process, which are helpful for the measurement and control of the time delay between short pulses in coherent combination, plasma parameter diagnosis and so on.
KEYWORDS: Wavefronts, Adaptive optics, Digital signal processing, Control systems, Image processing, Adaptive control, Image transmission, Process control
Adaptive optics is a real-time compensation technique using high speed support system for wavefront errors caused by atmospheric turbulence. However, the randomness and instantaneity of atmospheric changing introduce great difficulties to the design of adaptive optical systems. A large number of complex real-time operations lead to large delay, which is an insurmountable problem. To solve this problem, hardware operation and parallel processing strategy are proposed, and a high-speed adaptive optical control system based on DSP is developed. The hardware counter is used to check the system. The results show that the system can complete a closed loop control in 7.1ms, and improve the controlling bandwidth of the adaptive optical system. Using this system, the wavefront measurement and closed loop experiment are carried out, and obtain the good results.
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 coherent amplification network (CAN) aims at developing a laser system based on the coherent combination of multiple laser beams, which are produced through a network of high beam quality optical fiber amplifiers. The scalability of the CAN laser facilitates the development of many novel applications, such as fiber-based acceleration, orbital debris removal and inertial confinement fusion energy. According to the requirements of CAN and the front end of high-power laser facilities, a millijoule polarized fiber laser system was studied in this paper. Using polarization maintaining Ytterbium-fiber laser system as the seed, and 10-μm core Yb-doped fiber amplifier as the first power amplifier and 40-μm core polarizing (PZ) photonic crystal fiber (PCF) as the second power amplifier, the all-fiber laser system outputs 1.06-mJ energy at 10 ns and diffraction limited mode quality. Using 85-μm rod-type PCF as the third power amplifiers, 2.5-mJ energy at 10-ns pulse width was obtained with better than 500:1 peak-to-foot pulse shaping ability and fundamental mode beam quality. The energy fluctuation of the system is 1.3% rms with 1-mJ output in one hour. When using phase-modulated pulse as the seed, the frequency modulation to amplitude modulation (FM-to-AM) conversion ratio of the system is better than 5%. This fiber laser system has the advantages of high beam quality, high beam shaping ability, good stability, small volume and free of maintenance, which can be used in many applications.
The grating tiling technology is one of the most effective means to increase the aperture of the gratings. The line-density error (LDE) between sub-gratings will degrade the performance of the tiling gratings, high accuracy measurement and compensation of the LDE are of significance to improve the output pulses characteristics of the tiled-grating compressor. In this paper, the influence of LDE on the output pulses of the tiled-grating compressor is quantitatively analyzed by means of numerical simulation, the output beams drift and output pulses broadening resulting from the LDE are presented. Based on the numerical results we propose a compensation method to reduce the degradations of the tiled grating compressor by applying angular tilt error and longitudinal piston error at the same time. Moreover, a monitoring system is setup to measure the LDE between sub-gratings accurately and the dispersion variation due to the LDE is also demonstrated based on spatial-spectral interference. In this way, we can realize high-accuracy measurement and compensation of the LDE, and this would provide an efficient way to guide the adjustment of the tiling gratings.
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.
The paper presents the technical design and progress on a special high-power laser facility, i.e. XG-III, which is being used for high-field physics research and fast ignition research. The laser facility outputs synchronized nanosecond, picosecond and femtosecond beams with three wavelengths, i.e. 527 nm, 1053 nm and 800 nm respectively, and multiple combinations of the beams can be used for physics experiments. The commissioning of the laser facility was completed by the end of 2013. The measurement results show that the main parameters of the three beams are equal to or greater than the designed ones.
In a high-energy chirped-pulse-amplified laser system, grating tiling technology provides an effective means to increase the aperture of the gratings and to scale the energy and irradiance of short-pulse lasers. The difficulties lie in controlling tiling errors accurately between the sub-gratings and keeping long time stability. In this paper, a two-pass full-tiled grating-compressor (TGC) with real- time control unit is developed for the first time. The far-field distributions of the 0th order and -1 st order diffracted beams of the two pairs of tiled gratings are monitored by the same CCD system, with the main laser chain being not disturbed. In this way, we realize online real-time control of tiling errors. Through a method of locking the far-field image to compensating the temporal drift, we can realize the automation of the assembly. The TGC has successfully applied in the multi-function XGIII laser facility, and focusing focal spot and output pulse width are obtained.
KEYWORDS: Sensors, High power lasers, Coherent beam combination, Signal detection, Research facilities, Numerical simulations, Beam splitters, Automatic control, Process control, Calibration
Array element tiling is one of the key technologies for the coherent beam combination in a high-power laser facility. In this paper, we proposed a method of the array element auto-tiling based on capacitive displacement sensor. The method was verified on a double-pass tiled-grating compressor in XG-III laser facility. The research showed that the method is an effective way to control the misalignment errors automatically, with high precision and long-term stability.
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 × 1016 W/cm2 is obtained, with an energy transfer up to 16.8%. An output pulse of petawatt power is theoretically demonstrated feasible.
Chirp pulse amplification (CPA) has been promoted as an effective way to explore the intensity frontier. High order dispersion induced by the stretcher and materials in the CPA system, which deteriorates both the pulse duration and temporal contrast, however, can not be absolutely compensated by the compressor. Placed at the Fourier plane of a 4f zero-dispersion stretcher consisting of a grating, the deformable mirror (DM) has been demonstrated as the modulator to compensate high order dispersion. Using the method of ray tracing, the relation between spectrum and position on DM has been obtained. It shows that the resolution of the deformable mirror can be controlled by adjusting the focal length and incident angle. We have simulated a typical Ti:sappire CPA system to revise the spectral phase by the DM. The result illustrates that if the spectral phase can be compensated, the temporal contrast will be improved by 2 order of magnitude.
Precise physical experiments place strict requirements on target illumination uniformity in Inertial Confinement Fusion. To obtain a smoother focal spot and suppress transverse SBS in large aperture optics, Multi-FM smoothing by spectral dispersion (SSD) was studied combined with continuous phase plate (CPP) and polarization smoothing (PS). New ways of PS are being developed to improve the laser irradiation uniformity and solve LPI problems in indirect-drive laser fusion. The near field and far field properties of beams using polarization smoothing were studied and compared, including birefringent wedge and polarization control array. As more parameters can be manipulated in a combined beam smoothing scheme, quad beam smoothing was also studies. Simulation results indicate through adjusting dispersion directions of one-dimensional (1-D) SSD beams in a quad, two-dimensional SSD can be obtained. Experiments have been done on SG-III laser facility using CPP and Multi-FM SSD. The research provides some theoretical and experimental basis for the application of CPP, SSD and PS on high-power laser facilities.
XG-III laser facility is a petawatt laser which has a unique feature of three synchronized pulses output for various pump-probe experiments. To realize the synchronization with zero timing jitter, we have designed and implemented a novel front-end system based on super-continuum injected femtosecond optical parametric amplification (fs:OPA). Critical parameters of fs:OPA were optimized for the best conversion efficiency. Experimental results verified that major design specifications such as pulse energy, central wavelength and spectral width were fully accomplished and a high pulse contrast ratio was also achieved by the fs:OPA process.
Multi-FM SSD and CPP was experimentally studied in high fluence and will be equipped on all the beams of SG-III laser facility. The output spectrum of the cascade phase modulators are stable and the residual amplitude modulation is small. FM-to-AM effect caused by free-space propagation after using smoothing by spectral dispersion is theoretically analyzed. Results indicate inserting a dispersion grating in places with larger beam aperture could alleviate the FM-to- AM effect, suggesting minimizing free-space propagation and adopting image relay. Experiments taken on SG-III laser facility indicate when the number of color cycles (Nc) adopts 1, imposing of SSD with 3.3 times diffraction limit (TDL) did not lead to pinhole closure in the spatial filters of the preamplifier and main amplifier with 30-TDL pinhole size. The nonuniformity of the focal spot using Multi-FM SSD and CPP drops to 0.26, comparing to 0.84 only using CPP. The experiments solve some key technical problems using SSD and CPP on SG-III laser facility, and provide a flexible platform for laser-plasma interaction experiments. Combined beam smoothing and polarization smoothing are also analyzed. Simulation results indicate through adjusting dispersion directions of one-dimensional SSD beams in a quad, two-dimensional SSD could be obtained. The near field and far field properties of beams using polarization smoothing were also studied, including birefringent wedge and polarization control plate (PCP). By using PCP, cylindrical vector beams could be obtained. New solutions will be provided to solve the LPI problem encountered in indirect drive laser fusion.
Applying the self-diffraction process to clean ultrashort laser pulses temporally is a recently developed effective way to temporal contrast enhancement. In this paper, we attempt to clean ultrashort laser pulses temporally by the self-diffraction process. Experiments were carried out to study the temporal contrast improvement in the front-end system of an ultraintense and ultrashort laser facility, i.e. the super intense laser for experiment on the extremes (SILEX-I). The results show that the maximum conversion efficiency of the first-order self-diffraction (SD1) pulse is 11%. The temporal contrast of the SD1 signal is improved by two orders of magnitude, i.e. to 103, for a 2.4-ns prepulse with initial contrast of ~10. For a 5.5 -ns prepulse with initial contrast of 2×103, the temporal contrast of the SD1 signal is improved by more than three orders of magnitude.
The paper presents the development of a sub-petawatt ultrashort laser facility, i.e. the upgraded super intense laser for
experiment on the extremes (SILEX-I). The facility is a multi-stage Ti:sapphire chirped pulse amplification (CPA) laser
system. Cross-polarized wave generation was used to improve the temporal contrast. An adaptive optical system was
utilized to correct wavefront aberrations and to improve focusability before each shot. After upgrading, the maximum
energy is 20.1 J, the recompressed pulse width is 26.8 fs and the peak power is up to 750 TW. The temporal contrast is
around 109. The on-target focal spot size (full width at half maximum (FWHM)) is Φ6.5 μm and the focused intensity is
greater than 4x1020 W/cm2.
To improve the performance of SG-III prototype facility (TIL-Technical Integration Line), final optics assembly (FOA) is re-designed. It contains that stray light and focusing ghosts are optimized, operational performance and environments are improved and the total thickness of optics is reduced. With the re-designed FOA, Some performance advantages are achieved. First, the optics damages are mitigated obviously, especially crystals and Focus lens; Second, stray light and focusing ghosts are controlled better that organic contamination sources inside FOA are eliminated; Third, maintenance and operation are more convenient for the atoms environment; Fourth, the focusable power on target is increased for lower B-integral.
This paper is focused on the research of SSD and CPP carried out on TIL. A bulk phase modulator with 9.2-GHz modulation frequency is adopted in SSD. The output spectrum of the phase modulator is stable and the residual amplitude modulation is small. FM-to-AM effect caused by free-space propagation after using smoothing by spectral dispersion is theoretically and experimentally studied. Results indicate inserting a dispersion grating in places with larger beam aperture alleviates the FM-to-AM effect, suggesting minimizing free-space propagation and adopting image relay. Experiments indicate when the number of color cycles (Nc) adopts 1, imposing of SSD with 4.26 times diffraction limit (TDL) did not lead to pinhole closure in the spatial filters of the preamplifier with 20 TDL and main amplifier with 26 TDL. Experimental results also indicate SSD didn’t influence the load capacity of the laser facility. The contrast of the 440-μm diameter focal spot with 95% energy included using SSD and CPP drops to 0.47, comparing to 1.71 not using SSD and CPP. When the pulse width of the third harmonic wave is 1 ns and the energy is 1115 J, no damage is found in CPP and other final optics. The experiments solves some key technical problems using SSD and CPP on high-power laser facilities, and provides a flexible platform for the laser-plasma interaction experiments.
With the methods of time-division multiplexing in Frontend and angle detuning in FOA, each beam pulse on SG-III
prototype facility is controlled independently and so the systematic variations of power imbalance are eliminated
entirely.
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.
Plasma Pockels cell (PPC), which can use a thin crystal to perform the uniform electro-optical effect, is ideal component
as average-power optical switch with large aperture. In this paper, by reformative design and employing a capacity to
share the gas discharge voltage, the DKDP PPC driven by one pulse is realized. As gas breakdown delay time is stable,
and discharge plasma is uniformly filled the full aperture, it meets the demand of plasma electrode for the repetition-rate
PPC with DKDP crystal. A rep-rate plasma Pockels cell (PPC) with Φ30mm aperture has been fabricated. It is optimized
with the limited space of repetition rate diode pumped laser. The specification of the PPC is: static transition of 97.2%,
switching efficiency of 99.8%, the switch rising time of 8.6ns. In the LD pumped Yb:YAG plate laser system, the PPC
can steadily work on 10Hz repetition rate performed as Q-switch. The key problems in PPC are analyzed for
repetition-rate application, and thermo-optical effects are simulated by means of numerical modeling when average
power laser is loaded on the electro-optical crystal. Furthermore, the principium design of rep-rate PPC with
longitudinally conductive cooled structure is described in this paper. It will efficiently abate the thermo-optical effects
under repetition rate application.
KEYWORDS: Tolerancing, Lens design, High power lasers, Solids, Optical design, Optical amplifiers, Optical filters, Telescopes, Near field optics, Electronic filtering
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.
KEYWORDS: Optical amplifiers, High power lasers, Fiber amplifiers, Prototyping, Solid state lasers, Mirrors, Laser development, Near field, Integrated optics, Fusion energy
We are currently developing a large aperture neodymium-glass based high-power solid state laser, Shenguang-III
(SG-III), which will be used to provide extreme conditions for high-energy-density physical experiments in China. As a
baseline design, SG-III will be composed of 48 beams arranged in 6 bundles with each beam aperture of 40cm×40cm. A
prototype of SG-III (TIL-Technical Integration experimental Line) was developed from 2000, and completed in 2007.
TIL is composed of 8 beams (four in vertical and two in horizontal), with each square aperture of 30cm×30cm. After
frequency tripling, TIL has delivered about 10kJ in 0.351 μm at 1 ns pulsewidth. As an operational laser facility, TIL has
a beam divergence of 70 μrad (focus length of 2.2m, i.e., 30DL) and pointing accuracy of 30 μm (RMS), and meets the
requirements of physical experiments.
In this paper, a new focusing method adopting an axicon for the demand of the plasma measurements in inertial confinement fusion (ICF) drivers is presented. In order to improve the performance of this element, annular-aperture and Super-Guassian apodization are introduced to remove the on-axis oscillations. Meanwhile, the lateral width is optimized through choosing appropriate radius ratio of the inner ring to outer ring of the element. Furthermore, the feasibility is conformed by numerical evaluation of Fresnel diffraction integral .The results obtained are accordant with our designed intention. At last, as an example and for specific application, we designed an axicon, which has almost unchanged axial intensity, a focal depth more than 3mm, beam size smaller than 100μm and the maximal relative intensity of side lobe less than 2%. The performance of this element satisfies the requirements of plasma measurements in ICF drivers.
We are now constructing a technical integration experiment line (TIL) at CAEP, which is the prototype facility of Shenguang III laser fusion driver. Currently, many important results have been obtained on the first integrated beam line, which established a sound foundation for Shenguang III engineering design.
KEYWORDS: Optical amplifiers, Near field, High power lasers, Solid state lasers, Adaptive optics, Beam propagation method, Wave propagation, Diffraction, Laser systems engineering, Laser beam propagation
The characteristics of linear propagation and amplification of pulse in the high-power solid-state laser system were analyzed. The decomposition of linear propagation of the different parts in this system was also made. And the controlling means for beam quantity were put forward. At the same time, the measured near field and far field of the beam in TIL (Technical integrated experiment line, the prototype of SGIII (the Laser facility for ICF in China) were discussed, which proved these means were valid. These results of the theoretical analysis and experiment research become the general idea for investigating the problem of linear propagation in this system.
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 new method has been developed to fabricate refractive microlens by etching ammonium dichromate gelatin (ADG) with enzyme solution. Unlike previous methods which are used to fabricate refractive microlens with photoresist, the process of fabricating microlens by etching ADG with enzyme solution doesn't require the use of expensive equipment, and it isn't sophisticated and time consuming. The light exposes ADG through a high contrast binary mask, then the exposed parts of ADS generate cross- linking reaction. Usually, the relief achieved by water developing is very shallow (<1um) when nonpre-harden gelatin is used, so we compound a certain concentration enzyme solution, and because of surface tension, ADG turns to spherical structure after developing. The optimum technique parameters of this process are presented. Results are presented for experiments and evaluated by profile meter and interference microscope.
The fabrication of refractive microlens by introducing the melting process with coding gray-tone mask is reported. The applied mask is obtained by the coding method, nonlinear effects in intensity distribution through the gray-tone mask have been taken into account to correct the mask design. A continuous relief is formed in photoresist after exposing, and then the excellent surface shape microlens can be gained by melting. The technical parameters of this process are also presented. Results are presented for experiments and evaluated by profile meter and scanning electronic microscope. The fabrication technology of refractive microlens by this process is simple and it indicates the enormous potential to extend the fabrication range ofrefractive microlens.
In this paper, a new method has been proposed to realize fine OPC with gray-tone coding mask instead of gray-tone mask. The relationships of the gray-tone coding mask and gray-tone mask have been discussed, and OPC simulation results are given with the gray-tone coding mask.
A new method is proposed to design gray-tone masks for fabrication of surface relief microstructures. Unlike previous methods which modulate the light intensity by changing the cell size or cell pitch only, the method relays on adjusting both the shape and position of a cell which gives an extra freedom to control the design accuracy. Using the new method a gray-tone mask has been designed to produce a hemispherical shape relief structure. Based on the theory of partial coherent light and the resist development model, the intensity distribution through the gray-tone mask and exposure of photoresist have been simulated. Nonlinear effects in aerial image and resist development have been taken into account to correct the mask design. The accuracy of the gray-tone mask design has been confirmed by simulation of 3D resist profiles.
KEYWORDS: Diffraction, Optical components, Binary data, Optical filters, Error analysis, Refractive index, Chemical elements, Energy efficiency, High power lasers, Laser systems engineering
This paper aims at the request of dividing harmonic waves in the high power laser system used to perform Inertial Confinement Fusion. Dividing harmonic waves is realized by introducing binary optical element. Based on scalar diffraction theory, the distribution of its diffraction field was calculated and the fabrication parameters were also optimized. The element is fabricated with RIE. We also measured the relief structure and diffraction efficiency of each harmonic wave and analyze the errors.
In this paper, the term, fractal phase structure is presented. The recurrence formulas of Fourier spectral function of the Cantor relief fractal phase structure are derived out. A series of power spectral curves of the state that several generations coexist are produced by computer. The spectral characteristics fully shows the self-similarity property of the phase fractals. The Cantor relief fractal structure is successfully made by enzyme etching. Its power spectrum is recorded with CCD camera.
Deep etched diffractive optical elements could acquire some significant properties, for example, deep etched Fresnel lens could be used as fast lens, deep etched blazed grating has high dispersive index. So it is a new subject of binary optics. In this paper, a new kind of element, deep etched multi-phase level fractional Talbot grating is presented. Compared with common fractional Talbot grating, it has the property of high fill factor.
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