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

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.