Fabrication of optical mosaic gratings with phase and attitude adjustments employing latent fringes and a red-wavelength dual-beam interferometer

We present a method to make optical mosaic gratings that uses the exposure beams and the latent grating created by the previous exposure to adjust the lateral position and readjust the attitude of the substrate for the current exposure. As thus, it is a direct method without using any auxiliary reference grating(s) and it avoids the asynchronous drifts between otherwise independent exposure and alignment optical sub-systems. In addition, the method uses a red laser wavelength in the plane-mirror interferometers for the multi-dimensional attitude adjustment, so the adjustment can be done at leisure. The mosaic procedure is described step by step, and the principles to minimize substrate alignment errors are explained in detail. Experimentally we made several mosaics of (50 + 30) x 50 mm(2) final grating area. The typical peak-valley and root-mean-square values of the measured -1st-order diffraction wavefront errors are 0.036 lambda and 0.006 lambda, respectively.