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