A pair of {TAIL ~ photodetector output signals} are positioned so as to be responsive to the interference pattern produced by the pair of {HEAD ~ laser beams} of the ring laser 
A pair of { TAIL ~ photodetector output signals } are positioned so as to be responsive to the interference pattern produced by the pair of { HEAD ~ laser beams } of the ring laser
OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH