{HEAD ~ The first and second diffractive structures} may be on a same surface or may be on {TAIL ~ opposite surface} of a same substrate 
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{ HEAD ~ The first and second diffractive structures } may be on a same surface or may be on { TAIL ~ opposite surface } of a same substrate
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH