5 November 2010 Fabrication of optical mosaic gratings by consecutive holographic exposures employing a latent-fringe based alignment technique
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Abstract
Large-size diffraction gratings are essential for pulse compressors in chirped-pulse-amplified high-power laser systems, spectroscopic telescopes, etc. Fabricating large gratings requires large-aperture laser beams with collimated and aberration-free wavefronts. As an alternative a method of making monolithic gratings by optical mosaic has been proposed, which makes multiple-exposures in different areas of a substrate to enlarge the grating size. Between exposures the position and attitude of substrate must be adjusted to minimize the overall wavefront errors, with very tight accuracy requirements (~ dozens of nanometers and tenths of micro-radians, respectively). We fully utilize the latent fringes (exposed but undeveloped fringes in photoresist) as the core reference object and the exposure beams as the adjustment beams to adjust and lock the position and attitude between consecutive exposures. This approach greatly simplifies the alignment system and eliminates many system errors. However, the diffraction efficiency of a latent grating is extremely weak (~10-5) and excessive exposure of the latent fringes during the position and attitude adjustment should be avoided. We overcome these difficulties by carefully blocking stray lights and using a high-sensitivity CCD to monitor the interference fringes of the –1st- and 0th-order latent-fringe diffracted wavefronts. Experimentally we have made 2 x 2 mosaics of (60+28) x (53+30) mm2 grating area. Typical peak-valley and rootmean- square values of the measured –1st-order diffraction wavefront errors are 0.06 λ and 0.01 λ, respectively. The mosaic conditions, detailed alignment steps, and experimental results showing position and attitude controllability will be presented. Important issues of extending the present work to large-size (possibly sub-meter) fabrication will also be discussed.
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Lei Shi, Lijiang Zeng, "Fabrication of optical mosaic gratings by consecutive holographic exposures employing a latent-fringe based alignment technique", Proc. SPIE 7848, Holography, Diffractive Optics, and Applications IV, 78480S (5 November 2010); doi: 10.1117/12.869727; https://doi.org/10.1117/12.869727
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