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.