Spiral phase plate (SPP) for generating vortex hollow beams has high efficiency in various applications. However, it is difficult to obtain an ideal spiral phase plate because of its continuous-varying helical phase and discontinued phase step. This paper describes the demonstration of continuous spiral phase plate using filter methods. The numerical simulations indicate that different filter method including spatial domain filter, frequency domain filter has unique impact on surface topography of SPP and optical vortex characteristics. The experimental results reveal that the spatial Gaussian filter method for smoothing SPP is suitable for Computer Controlled Optical Surfacing (CCOS) technique and obtains good optical properties.
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.
In the final optics assembly of ICF driver, Diffractive Optical Elements (DOEs) are applied to achieve some important functions. Due to the advantage of easy integration, DOEs that achieve different functions can be combined into one element, and thus the system structure and performance of ICF driver can be optimized. In this paper, we present a new means to fabricate color separation grating (CSG) and beam sampling grating (BSG) on two surfaces of one fused-silica substrate with two-surface exposure method. The mask for CSG can be fabricated with common optical method since the period of CSG is large; whereas the mask for BSG cannot be made with normal micro-fabrication system whose resolution cannot meet the requirement of fabrication precision of slowly changed BSG fringes (from 2μm to 4μm). Therefore, we use e-beam direct writing method to fabricate mask for BSG. The alignment of CSG and BSG is designed in accordance with the requirement of actual system. Thus the functions of harmonic wave separation and beam sampling are realized through one silica plate. The satisfying experimental results are illustrated.
Proc. SPIE. 5636, Holography, Diffractive Optics, and Applications II
KEYWORDS: Diffractive optical elements, Modulation, High power lasers, Error analysis, Near field, Near field diffraction, Optical damage, Laser systems engineering, Diffraction gratings, Amplitude modulation
Color Separating Grating (CSG) is one of the important Diffractive Optical Elements (DOE) used in the final optical system of high power laser system. Its periodic step-phase structure can separate the third harmonic frequency from base- and second-harmonic waves in the far field. But the structure abbreviations of CSG, caused by the fabrication process, generate great modulations to the laser beam, which may lead to severe optical damages to CSG itself and the system. In this paper, a comprehensive error model is built, in which each structural parameter of CSG is expressed as a variable, and the structural parameter error induced by fabrication process is expressed as minute disturb of relative variable. With this error model, the near field diffraction pattern of CSG is calculated based on Fresnel diffractive theory. Through simulation and analysis, we obtain the amplitude modulation of different harmonic waves in near field, and also the relationship between the amplitude modulation and the fabrication error. The results show that beam modulations are mainly caused by the stair depth error and the slope error. This study provides a convenient way to estimate the possibility of optical damages induced by CSG.