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