{HEAD ~ A system} 1, wherein the system comprises a phase locked loop for measuring {TAIL ~ the amplitude} of a photoacoustic signal 
{ HEAD ~ A system } 1 , wherein the system comprises a phase locked loop for measuring { TAIL ~ the amplitude } of a photoacoustic signal
HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH