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