A projection optical system, which is telecentric with respect to an object side and an image side, projects a pattern onto an image plane, which is substantially parallel with the pattern, at a substantial equi-magnification. It consists of two steps: the step of dry etching an aluminum interconnect stack by using an etch gas composed mainly of boron trichloridechlorinefluoroformnitrogen, and the step of removing etch remnants by using a vapor plasma. A second processor generates a second subset of the codeword symbols based on the original symbols. The optical receiver calculates the distance of the remote object with reference to delay information between transmitted and detected optical signals, with the delay information being determined by gated detection of optical signals at the photo-detector.