means for regulating the intensity of the source of {TAIL ~ laser light} in response to the error signal, to thereby maintain the light intensity on {HEAD ~ the photo detector} substantially constant, 
means for regulating the intensity of the source of { TAIL ~ laser light } in response to the error signal , to thereby maintain the light intensity on { HEAD ~ the photo detector } substantially constant ,
OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH