The low frequency and d.c. components of the photo detector output are compared with {HEAD ~ a reference voltage} to derive {TAIL ~ an error signal} 
The low frequency and d.c . components of the photo detector output are compared with { HEAD ~ a reference voltage } to derive { TAIL ~ an error signal }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL