A highly efficient reflective 1×3 splitting grating with triangular structure operating in 1.064μm wavelength under normal incidence for TE polarization is designed. The schematic of the grating has four layers. The first layer with SiO2 is triangular structure. Rigorous coupled wave analysis (RCWA) and Simulated Annealing (SA) algorithm are adopted to design and analyze the properties. The theoretical efficiency is nearly about 99%. The bigger error tolerance is also analyzed by rigorous coupled wave analysis. These reflective gratings as splitters should be useful optical elements in the field of high-power laser as well as other reflective applications.
Edge extraction has found applications in various image processing fields, such as in pattern recognition. In this paper, a new method is proposed for edge extraction of three-dimensional objects in optical scanning holography (OSH). Isotropic and anisotropic edge extraction of 3D objects is simulated using spiral phase plates in OSH operating in an incoherent mode. We propose to use a delta function and a spiral phase plate as the pupil functions to realize isotropic and anisotropic edge extraction. Our computer simulations show the capability of extracting the edges of a given 3D object by spiral phase filtering in OSH.
A 3×3 high divergent 2D-grating with period of 3.842μm at wavelength of 850nm under normal incidence is designed and fabricated in this paper. This high divergent 2D-grating is designed by the vector theory. The Rigorous Coupled Wave Analysis (RCWA) in association with the simulated annealing (SA) is adopted to calculate and optimize this 2D-grating.The properties of this grating are also investigated by the RCWA. The diffraction angles are more than 10 degrees in the whole wavelength band, which are bigger than the traditional 2D-grating. In addition, the small period of grating increases the difficulties of fabrication. So we fabricate the 2D-gratings by direct laser writing (DLW) instead of traditional manufacturing method. Then the method of ICP etching is used to obtain the high divergent 2D-grating.
This paper will report our recent works on fabrication, evaluation, and applications of gratings. We are using the Dammann parallel laser writing facility for fabrication of gratings. High-efficiency reflective gratings and large-sized grating are fabricated. We have fabricated high-power reflective laser vortex grating with expectation of a new laser drilling effect for laser fusion in the future, which is just evaluated by our developed method. These gratings are essential elements for high-power laser systems and other high-demanding metrology applications.
The Direct Laser Writing (DLW) technique has become a well-established, flexible and multi-functional method of micro- and nano-technology. A DLW system, mainly containing blue light writing module and red light autofocus module, is established and efficiently applied for the fabrication of diffractive optical elements (DOEs). In the DLW system, the stability of the writing beam is always a concern. Although the autofocus module is employed to eliminate the influence of the drifting focus point resulting from ambient vibration, the inherent defocusing error still has a serious impact on the lithography accuracy of the DLW system. As the refractive index of the lithography objective lens with a high numerical aperture (NA, 0.9) for blue light (405nm) differs from that for the auto-focus red light (650nm), the focal planes of the two beams will not coincide. Furthermore, the two beams can’t be mounted seriously parallel to the axis of the objective lens in practice. The misalignment will impact the location of the focus point axially and laterally. The above defocusing error is determined experimentally, and then is pre-compensated, which improves the fabrication accuracy dramatically. The relationship between defocusing amounts and line widths of the stripes is obtained, which can be used for writing gratings with different line widths. A 100×100 mm sized fused-silica grating with a period of 2 μm is obtained with the DLW system, and some microscope images are presented to show the effectiveness of the error-eliminating methods.
Although there are several well -known methods such as RCWA, FMM, for analyzing the diffraction properties of
gratings, design of these optical elements with specified spectral properties is commonly a challenging problem. It is
relatively not easy for the researchers to design narrow line-with diffraction filters based on guided mode resonance
phenomenon with common diffraction algorithm.
Simulated Annealing (SA) method is evolutionary, robust technique that has been widely utilized to design optical
diffraction components. This method is inspired by the physical process of heating and controlled cooling of metal
material to increase the size of its crystals and reduce their defects. The most distinctive features of this method lie in its
powerful ability of convergence towards the global minimum in a reasonable computation time and the independence of
the initial parameter values.
In this paper, first, the physical basis of SA and its mathematical realization are introduced. Then, a Guided-Mode
Resonant Grating (GMRG) filters with single layer is designed by using SA algorithm. The central wavelength of
GMRG filter is locked at 532nm and its line-width is fixed at 1nm. The plane wave light radiates the grating from air
cover with normal incidence.
The optimized parameters are refractive indices and thicknesses of high and low material of grating, other parameters are
grating period and fill factor of the grating. It is shown from our calculation that an excellent reflection spectrum with
narrow line-width, high peak and low sideband can be obtained after optimizing the grating parameters. Next, a double
layered GMRG filter with line-width of 4nm, which is relatively easy fabrication in experiment, is designed at central
wavelength of 1064nm. The optimized parameters are grating period, groove depth, refractive index of waveguide layer
and fill factor respectively. The grating substrate and waveguide layer are Sio2 and Hfo2 respectively, the grating
structure is directly etched on the waveguide layer. The above grating values should be included in reasonable ranges in
consideration of grating fabrication in our experiment condition.
It is demonstrated from the calculations with the parameters obtained from SA optimization algorithm that the peak
diffraction efficiency is more than 99% at central wavelength 1064nm and the sideband reflection is depressed at the
level bellow 5% in a large wavelength range. Moreover, the parameters of a triple layer GMRG filter structure are also
provided with this powerful method. Meanwhile, the results found by SA method are compared with RCWA theory.