{HEAD ~ A negative voltage} is applied to {TAIL ~ the anode} of the first photodiode and a positive voltage of the same magnitude is applied to the cathode of the second photodiode 
is applied to
{ HEAD ~ A negative voltage } is applied to { TAIL ~ the anode } of the first photodiode and a positive voltage of the same magnitude is applied to the cathode of the second photodiode
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH