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Generation of Two-Dimensional Optical Reference Signals Based on Parametric Minimum Cross Entropy

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4 Author(s)
Yi-Sheng Su ; Dept. of Comput. Sci. & Inf. Eng., Chang Jung Christian Univ., Tainan, Taiwan ; Tsung-Cheng Wu ; Chung-Hsuan Wang ; Min-Kuan Chang

In this letter, a novel design of two-dimensional (2-D) zero reference codes (ZRCs) based on parametric minimum cross entropy (PMCE) is proposed to generate 2-D optical zero reference signals (ZRSs) with low second maximum for grating alignment systems. An optical ZRS is necessary to obtain an absolute measurement in grating alignment systems. A method to acquire an optical ZRS is by means of illuminating two identical superimposed ZRCs. Because the movement between the two ZRCs is in two-axis, 2-D ZRCs are required. However, they are hard to design due to the high computational complexity, especially for large codes. The proposed PMCE method not only reduces the second maximum of 2-D optical ZRSs, but also decreases the computational complexity. Simulation results indicate that there are 8.33% ~ 22.22% reductions in the second maximum of several 2-D optical ZRSs, as compared with those of the recently proposed cross-entropy (CE) method. The PMCE method proves to be a powerful tool for the design of 2-D ZRCs.

Published in:

Photonics Technology Letters, IEEE  (Volume:23 ,  Issue: 12 )