Fabrication of optical mosaic gratings

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.