Periodic nanoscale array structures are of great importance in various fields including photonic crystals, diffraction gratings, etc. In this study, a dielectric-film-based polarization modulation scheme on an orthogonal two-axis Lloyd’s mirrors interference system was proposed for patterning high-uniformity nanoscale two-dimensional (2D) grating over a large area. We established a beam reflection model of three media-layer structure of air-dielectric film (MgF2)-metal substrate (Al) and calculated the integrative amplitude reflection coefficients. We systematically analyzed the spatial polarization states of the interference beams and determined the optimal exposure conditions to automatically eliminate the additional interference at certain incident angles. We plotted the optimal period of fabricable 2D grating at different thickness of dielectric film MgF2, where a thickness of 66.3 nm was selected for experimental demonstration. Then, 2D gratings with various periods of 740 nm, 780 nm, 1000 nm, and 1250 nm were fabricated, which presented a high consistency with the simulation results and revealed the fabrication ability over a period range from 730 nm to 840 nm. This dielectric-film based polarization modulation mechanism enables to extend the fabricable 2D grating with a smaller pitch, which is corresponding to a larger area. The proposed dielectric-thin-film-based polarization modulation mechanism provides a promising approach for fabricating large-area, high-uniformity, 2D-crossed gratings with a high throughput.
The planer grating with large area plays an important role in the optical encoder for large stroke measurement. Compared with the conventional fabrication techniques such as interference lithography and mechanically ruling for large-area manufacture, splicing small-area grating units into a large integral mosaic grating is promising to reduce the technical difficulty and time-consumption. We proposed a novel grating splicing method to construct large-area mosaic grating applicable for optical encoder. The splicing apparatus is capable of providing four degree-of-freedom including one outof-plane straight motion (ΔZ) and three rotation motions (Δθx, Δθy, Δθz) for mechanically adjusting the grating units. The diffracted beams from the spliced gratings are utilized to guide the adjustment, i.e., zero-order diffracted beams for ΔZ, Δθx, Δθy and first-order diffracted beams for Δθz, respectively. The diffracted-beams orientations of the first grating were adopted as the basic reference, and the posture of the other gratings are continuously adjusted by matching the position of the diffracted beams. Five grating units with 20×20 mm2 area and 1 μm period were spliced together. Finally, a coordinate measuring machine was utilized to evaluate the flatness of the mosaic gratings, showing a good flatness as small as ~60 μm. It is demonstrated that our proposed splicing method is promising to achieve good flatness of mosaic grating potentially applied for large-stroke optical encoder.
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