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