Continuous Phase Plate Research Articles

Laser irradiation uniformity for polar direct drive on ShenGuang III facility

Inertial confinement fusion utilizes sufficient laser beams to directly illuminate a spherical capsule, or convert the laser into thermal X-rays inside a high Z hohlraum to drive capsule implosion. The direct drive implosion is one of ways toward central ignition and similar to the indirect drive implosion, but has higher laser energy coupling efficiency and the potential for higher-gain implosion than indirect drive, and needs stringent laser condition. In order to develop and execute the direct drive experiment on the laser facility, which is configured initially for indirect drive, the polar direct drive has been proposed and validated on the Omega laser facility and the National Ignition Facility. The polar direct drive repoints some of the beams toward the polar and equator of the target, thus increasing the drive energy on the polar and equator of capsule and achieving the most uniform irradiation. The present article focuses on the laser irradiation uniformity of the target in polar direct drive on ShenGuangIII (SGIII) facility. Firstly, the laser beam configuration of the SGIII, the characteristics of laser spots, the laser beam repointing strategy and the principle of optimization are introduced. The 48 laser beams are distributed over four cones per hemisphere and the beam centerlines are repointed in polar direct drive. The continuous phase plates (CPPs) of the SGIII are designed to have unique shape to make the laser beam with a 250 m-radius circular section at the laser entrance hole in indirect drive, and thus the laser beams have ellipse cross sections with fixed major axis and different minor axes in different cones. Then, the irradiation uniformity of 540 m target is optimized by the three-dimensional (3D) view factor method on the assumption that the laser intensity distribution is super-Gaussian with three and five orders, and the energy deposition distributions are expressed as cos2 and cos . The irradiation nonuniformity of less than 5% on the polar direct drive capsule of 540 m in diameter is achieved. The pressure distribution of the hot spot at the neutron bang time with the optimized parameter is also simulated by Multi2D, and the results of 2D hydrodynamics simulation indicate that the hot spot under the assumption of cos distribution is more symmetric. Finally, the effects on irradiation uniformity of the beam-to-beam power imbalance, the repointing error and the target pointing error are estimated by the Monte Carlo method. According to the simulation results, the laser root mean square nonuniformity on the target will not become worse observably when the maximal beam-to-beam power imbalance is limited to a value of 5%, and the repointing error and the target pointing error are better than 7 m.

Analysis on the focal spot characteristics of random Gauss phase plate

Gaussian phase plate (GPP) is a special kind of continuous phase plate, which surface figure can be described as a Gaussian random process. In this paper we propose an analysis model to discuss the statistical characteristics of GPP’s focal spot. Using the gradient histogram instead of the diffract integral, the variance of the focus light intensity is analyzed. And the variance reflects the stability of the intensity envelope of the focal spot. Both the deducing process and the numerical simulation are presented. And the results show that the stability of intensity distribution in the focal spot is directly proportional to the correlation length of the phase plate.

Application of optical diffraction method in designing phase plates

Continuous phase plate (CPP), which has a function of beam shaping in laser systems, is one kind of important diffractive optics. Based on the Fourier transform of the Gerchberg–Saxton (G–S) algorithm for designing CPP, we proposed an optical diffraction method according to the real system conditions. A thin lens can complete the Fourier transform of the input signal and the inverse propagation of light can be implemented in a program. Using both of the two functions can realize the iteration process to calculate the near-field distribution of light and the far-field repeatedly, which is similar to the G–S algorithm. The results show that using the optical diffraction method can design a CPP for a complicated laser system, and make the CPP have abilities of beam shaping and phase compensation for the phase aberration of the system. The method can improve the adaptation of the phase plate in systems with phase aberrations.

Optical zooming based on focusing grating in direct drive ICF.

In direct drive ICF, optical zooming is an effective way to mitigate cross-beam energy transfer and increase the hydrodynamic efficiency, by reducing the spot size of the laser beams while target compressing. In this paper, a novel optical zooming scheme is proposed, which employs a focusing grating to focus the broadband laser pulse, changing the spot size on the target within single beamlet. Experimentally, a focusing grating with clean aperture of 40-mm × 40-mm placed after the collimated light successfully realized the peak-valley of defocusing wavefront distribution of 0.73 µm as the wavelength ranging from 1052.43 nm to 1053.23 nm. Extended to the full-sized focusing grating with laser beam of 360-mm × 360-mm, it is derived that the focal spot reduction reaches to 21.8% with the 3rd harmonic light ranging from 350.81 nm to 351.08 nm, decreasing from 375 µm to 294 µm with 300 µm shaping continuous phase plate.

Analysis and optimization of radial smoothing based on optical Kerr effect for irradiation improvement

In radial smoothing scheme, taking a super-Gaussian pulse train obtained by the pulse stacking scheme based on fibers and spatial shaping technology based on serrated-aperture apodizers as the pump laser, due to the hemispherical shape of the optical Kerr medium, the induced refraction index by the interaction of the optical Kerr medium and the pump laser is spherically distributed with periodical variation. Consequently, the transmission wavefront of the laser quads in the beamline is periodically modulated, resulting in the rapidly and periodic focal zooming in far field. This focal zooming smoothes the speckles on target plane in the radial direction in the sense of averaged over a finite time interval. The performance of the pump laser and the optical Kerr medium strongly affect the radial smoothing effect. In order to obtain better smoothing effect as that of smoothing by spectral dispersion, the propagation model of laser quads in the beamline with the radial smoothing scheme has been built up and further used to optimize the parameters of the pump laser and the optical Kerr medium. The beam smoothing effects of the joint use of continuous phase plate and polarization control plate with smoothing by spectral dispersion, as well as radial smoothing have been analyzed and compared in detail. Results indicate that, the delay time between each super-Gaussian pulse in the pump laser should be matched with the pulse width of each super-Gaussian pulse to achieve the best and stable radial smoothing effect, while the fluctuation of the peak intensity of each super-Gaussian pulse in the pump laser would degrade the radial smoothing effect. The selection of the optical Kerr medium directly determines its thickness and peak intensity of the pump laser to obtain the required wavefront modulation, which affects the feasibility of the radial smoothing scheme.

Direct patterning of vortex generators on a fiber tip using a focused ion beam.

The realization of spiral phase optical elements on the cleaved end of an optical fiber by focused ion beam milling is presented. A focused Ga+ ion beam with an acceleration voltage of 30keV is used to etch continuous spiral phase plates and fork gratings directly on the tip of the fiber. The phase characteristics of the output beam generated by the fabricated structures measured via an interference experiment confirmed the presence of phase singularity in the output beam. The devices are expected to be promising candidates for all-fiber beam shaping and optical trapping applications.

Research of beam conditioning technologies using continuous phase plate, Multi-FM smoothing by spectral dispersion and polarization smoothing

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 implemented on the SG-III laser facility. After using these techniques, the far field distribution of SG-Ⅲ laser facility can be adjusted, controlled and repeated accurately. The output spectrums of the cascade phase modulators used for Multi-FM SSD were stable and the FM-to-AM effect can be restrained. Experiments on SG-III laser facility indicate that 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 preamplifier and main amplifiers 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, 0.57 with CPP+PS and 0.84 with only CPP and wedged lens. Polarization smoothing using flat birefringent plate in the convergent beam of final optics assembly (FOA) was studied. The PS plates were manufactured and equipped on SG-III laser facility for LPI research. Combined beam smoothing and polarization manipulation were also studied to solve the LPI problem. Results indicate that through adjusting dispersion directions of SSD beams in a quad, two dimensional SSD can be obtained. Using polarization control plate (PCP), polarization on the near field and far field can be manipulated, providing new method to solve LPI problem in indirect drive laser fusion.

Fast focal zooming scheme for direct drive fusion implemented by inserting KD2PO4 crystal

The highly required uniformity of target in direct-drive fusion is difficult to achieve and maintain during the entire laser fusion implosion. To mitigate the increasing nonuniformity, the fast focal zooming scheme implemented by inserting an electro-optic (EO) crystal in the front end of beamline has been proposed. Functioning as a phase plate, the specifically designed EO crystal may add the induced spherical wavefront to the laser beam and alter its focusing characteristics. However, in order to zoom out the focal spot by half, the required voltage for KD2PO4 (DKDP) with single pair of electrodes is relatively high. In order to decrease the voltage while maintaining the zooming performance, the DKDP cylinder with multi pairs of electrodes has been presented. The continuous phase plate (CPP) is designed according to both the injected beam and the output field. However, the conventional CPP is designed based on the assumption of an injected beam without wavefront distortion, which would zoom in the focal spot variation in the focal zooming scheme. In order to zoom out the focal spot, a redesigned CPP has been proposed by adding a spherical wavefront to the phase variation of the conventional CPP and further optimizing. On the basis, the focusing characteristics of laser beam during the fast focal zooming process have been analyzed. Results indicate that the focal spot size decreases with the increasing voltage on DKDP crystal, meanwhile the uniformity maintains high during the focal zooming process.

Vortex Electromagnetic Waves Generated by Using a Laddered Spiral Phase Plate and a Microstrip Antenna

ABSTRACTA laddered spiral phase plate excited by a microwave antenna to generate a vortex electromagnetic wave is studied. By replacing the continuous spiral phase plate with several planar metal plates, a ladder spiral phase plate that is easily built is designed. When the spiral phase plate is slightly tilted away from the xoy-plane to approximate the parabola plate, and it is further excited by a microstrip patch antenna, different orbital angular momentums are obtained. The presented scheme provides a convenient and simple approach to generate a vortex wave at microwave frequencies.

The light diffraction effect on stimulated Raman scattering

The stimulated Raman scattering is simulated with different size of incident light using code LAP3D. The reflectivity of Raman scattering is very sensitive to the size of single speckle of the incident light. Small reflectivity is obtained when width of speckle is small, which is explained by the effect of light diffraction. For inhomogeneous plasma, the reflectivity is larger with a positive density gradient than that with a negative density gradient, which is consistent with the theory result. The incident light smoothed by the continuous phase plate contains many speckles and has an intensity distribution. The fractional power above intensity profile is very important, because the high intensity power can excite the stimulated Raman scattering and reduce the threshold of average intensity.

A novel radial beam smoothing scheme based on optical Kerr effect

Laser-beam illumination uniformity is a key issue in inertial confinement fusion facilities. In order to fulfill the requirement of improving illumination uniformity, a radial smoothing (RS) scheme is proposed. For smoothing the focal-spot pattern on a short time scale compared with the hydrodynamic response time of the target, the optical Kerr effect with extremely response time is taken into consideration. The basic principle of RS based on optical Kerr effect is that by using the interaction between optical Kerr medium and periodic Gaussian pulses to modulate a periodic spherical phase, to modulate periodic sphericel phase added at the wavefront of laser transmission wave, change the focal-spot size of the laser beam in far field, and further induce the fast radial redistribution of the speckles inside the focal spot in far field, and further induce the fast radial redistribution of the speckles inside the focal spot in far field. This fast radial redistribution of the speckles smoothes the intensity modulation of the focal spot on the target and eventually achieves the beam smoothing in the radial direction. The application of RS in the beamline is detailed. The optical Kerr medium is inserted in the front-end of the bemline, before the laser beam is injected into the main amplifier. The periodic Gaussian pulse for pumping the optical Kerr medium is obtained by the pulse stacking system based on fibers. The pulse width of stacked Gaussian pulse and the time delay between Gaussian pulses are set to be on a picosecond time scale or subpicosecond time scale. The induced refractive index of the optical Kerr medium by the pump laser fits spherical distribution with periodic variation, and results in the radial distribution of the speckles in focal plane. By establishing the theoretical model of the radial beam smoothing scheme implemented with continuous phase plate (CPP), the focusing characteristics of laser beam with RS and CPP are discussed in detail. The influences of the selection of optical Kerr medium and the characteristics of the radial redistribution on the radial smoothing effect are simulated and analyzed. Results indicate that the RS based on optical Kerr effect could efficiently achieve the periodic radial redistribution of the speckles on focal plane, and therefore improves the illumination uniformity in the radial direction while eliminating the stripe pattern presented in far field by one-dimensional smoothing spectral dispersion (SSD). The smoothing performance of RS is different from that of the conventional SSD due to its radial smoothing direction. Moreover, the combined application of RS with continuous phase plate could achieve a better smoothing level with a shorter time. The utilization of radial smoothing scheme in high power laser system may significantly improve the laser-beam irradiation with little influence on the performance of the beamline.

A novel arrangement scheme of laser quads for spherical hohlraum in laser indirect-driven facility

In traditional laser quads arrangement schemes for spherical hohlraum in indirect-driven laser facilities, the laser quads to bring about the laser entrance hole (LEH) to close when they are incident at a large angle (55), while the complicated cross and overlap of laser quads inside the spherical hohlraum may be generated when they are incident at a small angle (35). To overcome these problems, a novel laser quads arrangement scheme for spherical hohlraum is proposed. The laser quads injected into the single LEH are divided into two cones (the incident angle of the inner cone is 35, and that of the outer cone is 55). Furthermore, the contrast and the fractional power above the intensity have been proposed to evaluate the irradiation uniformity of single laser quad, while the dispersion degree and the duty ratio are proposed to evaluate the distribution uniformity of all laser quads on the spherical hohlraum wall. Based on the beam smoothing scheme implemented by the combination of one-dimensional smoothing by spectral dispersion, the continuous phase plate and polarization control plate, the propagation model of laser quads in the spherical hohlraum has been built up, and further used to analyze the irradiation uniformity of single laser quad and all the laser quads on the spherical hohlraum wall. On this basis, the irradiation characteristics on the LEHs and the spherical hohlraum wall, and the propagation characteristic of laser quads in the novel and traditional laser quads arrangement schemes have been analyzed and compared. Results indicate that, compared with the traditional arrangement scheme of laser quads, the novel laser quads arrangement scheme has following advantages: The irradiation uniformity on the spherical hohlraum wall of single laser quad and all laser quads remains unchanged. Not only the LEH closure problem can be alleviated, but also the complicated cross and overlap of laser quads inside the spherical hohlraum in the traditional scheme could be avoided. The novel scheme may provide useful reference for the design of spherical hohlraum structure in laser indirect-driven facilities due to its obvious advantages over the traditional scheme.

小特征尺寸连续位相板离子束修形的误差分析

小特征尺寸连续位相板离子束修形的误差分析

基于相位调制的单次曝光波前测量在高功率激光驱动器中的应用

Coherent modulation imaging (CMI) based algorithm has been employed for on- shot laser beam diagnostics in high power laser facility and off- line measurement of large- scale optic element. A single- shot intensity measurement is sufficient for wave-front reconstruction after the random phase plate has been accurately pre- characterized, and the iteration process can be finished in 30 seconds with the acceleration of graphics processing unit (GPU) device. Furthermore, a compact structure is also available after combining CCD and phase plate together, which can be placed almost anywhere of high power laser facilities. Series of experiments have been finished for focal-spot prediction, measurement of large-scale continuous phase plate, and on-shot wave-front reconstruction of seed pulse in our high power laser facility with amplifiers turned off. The flexibility of CMI for focal- spot prediction, on- shot wave- front diagnostic and off- line measurement of optic elements has been demonstrated for its characteristic. It has significant influence on the development of high power laser facilities as a new method for wave-front measurement and diagnostics.

Statistic characteristics of random Gaussian phase plate

We present a special kind of continuous phase plate (CPP), which probability density function of the phase variations can be defined as a random Gaussian function, called Gaussian phase plate (GPP). Using the statistic theory the 2nd-order and 4th-order statistical properties of GPP is calculated in detail. The approximate analytic functions of the covariance of GPP are deduced. Both the numerical and analytic results show that the uniformity of the far-field distribution if GPP related to its correlation length. With the reducing of GPP's correlation length, the variance of far-field distribution reduces.

Developing a bright 17 keV x-ray source for probing high-energy-density states of matter at high spatial resolution

A set of experiments were performed on the National Ignition Facility (NIF) to develop and optimize a bright, 17 keV x-ray backlighter probe using laser-irradiated Nb foils. High-resolution one-dimensional imaging was achieved using a 15 μm wide slit in a Ta substrate to aperture the Nb Heα x-rays onto an open-aperture, time integrated camera. To optimize the x-ray source for imaging applications, the effect of laser pulse shape and spatial profile on the target was investigated. Two laser pulse shapes were used—a “prepulse” shape that included a 3 ns, low-intensity laser foot preceding the high-energy 2 ns square main laser drive, and a pulse without the laser foot. The laser spatial profile was varied by the use of continuous phase plates (CPPs) on a pair of shots compared to beams at best focus, without CPPs. A comprehensive set of common diagnostics allowed for a direct comparison of imaging resolution, total x-ray conversion efficiency, and x-ray spectrum between shots. The use of CPPs was seen to reduce the high-energy tail of the x-ray spectrum, whereas the laser pulse shape had little effect on the high-energy tail. The measured imaging resolution was comparably high for all combinations of laser parameters, but a higher x-ray flux was achieved without phase plates. This increased flux was the result of smaller laser spot sizes, which allowed us to arrange the laser focal spots from multiple beams and produce an x-ray source which was more localized behind the slit aperture. Our experiments are a first demonstration of point-projection geometry imaging at NIF at the energies (>10 keV) necessary for imaging denser, higher-Z targets than have previously been investigated.

Measurement of the complex transmittance of large optical elements with modulation coherent imaging

Accurate measurements of the transmittance of large optical elements are essential to improve the energy density in inertial confinement fusion (ICF). The required complex transmittance of such optical elements used in ICF is obtained by computing the phase difference between the illuminating and transmitting fields using modulation coherent imaging (MCI). A phase plate designed as a modulator has a known transmission function in this technique. It presents a simple and quick method to measure the transmittance of large optical elements with irregular surface profiles by retrieving it from only two diffraction patterns recorded by a CCD camera. The complex transmittance of a continuous phase plate (CPP) with a large aperture used in ICF is measured experimentally, and the results are found to agree with the designed value.

Production of a broadband femtosecond optical vortex by use of a continuous spiral phase plate

In our work, a high-quality broadband femtosecond optical vortex is obtained by use of a continuous spiral phase plate (SPP) to modulate an ultrashort femtosecond (fs) laser with a broadband spectrum. The experimental results demonstrate that the continuous SPP is of good quality and that it can be used to efficiently produce a high-power fs optical vortex.

Wavefront and light intensity characteristics of continous phase plated fabricated by magnetorheological finishing

This paper analyzes characteristics of wavefront and light intensity of large-aperture continuous phase plates(CPPs) fabricated by magnetorheological finishing(MRF). Three groups of CPPs fabricated by different processing parameters such as scanning intervals and scanning offsets are compared, and the influence of mid-frequency error introduced by MRF on CPPs wavefront and light intensity are analyzed as well. Total analysis indicates that the performance of CPP is poor when scanning interval is 2 mm and scanning offsets are 0.1-0.3 mm; The iteration fabrication efficiency is higher when scanning interval is 2 mm and scanning offsets are 0.4-0.5 mm compared with that when scanning interval is 1 mm and scanning offsets are 0.1-0.3 mm, and the mid-frequency error is improved as well. Further analysis indicates that mid-frequency error introduced by MRF has a greater impact on CPPs wavefront gradient, near-field and side lobe.

Using the gradient histogram to analyze the continuous phase plate

The geometrical optical method has been used to discuss the far-field distribution characteristics of a continuous phase plate. The gradient histogram of the plate’s surface has been calculated. It has been proved that the gradient histogram can be used to show the angular spectrum of a phase plate. The gradient histogram can simplify the analysis process of the angular spectrum and describe the focal spot morphology more intuitively.