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