The photodetectors are positioned such that {HEAD ~ the output signals} are in {TAIL ~ phase} quadrature 
are in
The photodetectors are positioned such that { HEAD ~ the output signals } are in { TAIL ~ phase } quadrature
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL OTH