Mr. Xiang Gao Profile

Xiang Gao

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Proceedings Article | 20 November 2024 Paper

Moiré pattern-based fringe locking and alignment in fabrication of optical mosaic gratings

Zijian Zhong, Xiang Gao, Jingwen Li

Proceedings Volume 13241, 132410U (2024) https://doi.org/10.1117/12.3036338

KEYWORDS: Optical gratings, Diffraction gratings, Optical alignment, Fabrication, Holography, Lithography, Reflection gratings, Control systems, Phase shifts

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Large diffraction gratings have been widely applied in fields from industrial measurement to modern scientific facilities. The scale and geometrical accuracy of a grating directly affect its operating performance. Optical mosaic grating fabrication, which produces a large diffraction grating by multi-exposing different areas of a substrate, has shown great advantages in terms of simplicity and productivity over other methods. However, groove shape defects resulted from disturbance of interference fringes and mosaic seams resulted from misalignment of exposure patterns compromise the quality of the produced gratings and impede this method from expanding to larger scales. As a result, active fringe locking and alignment control must be performed during and between each exposing process.

This paper proposes a fringe locking system consisting of Moiré pattern monitoring via a CCD camera and closed-loop piezoelectric-driven mirror control. To ensure the geometric consistency of the fabricated gratings and the effectiveness of the fringe locking system, an alignment method based on morphological operations and Fast Fourier Transform (FFT) analysis of the image is introduced. Experiments show the proposed system operates at 250 Hz, with significant suppression of low-frequency disturbance components below 100 Hz. The Root-Mean-Square (RMS) value of phase drifts remains below 0.02 fringe periods during the 60-second exposure, laying the foundation for fabricating large optical mosaic gratings.

Proceedings Article | 8 November 2024 Paper

Fabrication of optical mosaic gratings

Xiang Gao, Zijian Zhong, Tianshi Lu, Jingwen Li, Xinghui Li

Proceedings Volume 13234, 1323409 (2024) https://doi.org/10.1117/12.3036399

KEYWORDS: Diffraction gratings, Optical gratings, Mirrors, Fabrication, Diffraction, Wavefronts, Wavefront errors, Holography, Microscopes, Ferroelectric materials

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Large-scale diffraction gratings, ranging from sub-meter to meter in size, play a vital role in spectrometers and various measurement technologies. The primary method for fabricating these gratings is holographic exposure, which requires large-aperture laser beams and low-aberration wavefronts, limiting the size of gratings that can be fabricated in a single exposure. To expand the grating size, we employ a mosaic exposure method, where different areas of the substrate are sequentially exposed by repositioning the beams. This requires compensation for mosaic errors, including phase, period, and tilt errors. Mosaic errors are monitored by observing the interference fringes formed by the -1st and 0th-order diffraction waves of a reference grating. By analyzing the variations in the reference fringes caused by these errors, we correct them through adjustments to the phase, incidence angle, and azimuth angle of the laser beams. In our experiments, we achieved a 2×1 mosaic grating area of (9+9) mm × 9 mm. Fizeau interferometer measurements showed that the -1st-order diffraction wavefront error had a PV value of 0.1595λ and an RMS value of 0.0259λ.

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