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