an irradiation beam photo detector for producing an amount of irradiation beam detection signal according to the amount of {HEAD ~ a laser beam} irradiated from said {TAIL ~ laser beam source} onto an optical disk; and 
irradiated from
an irradiation beam photo detector for producing an amount of irradiation beam detection signal according to the amount of { HEAD ~ a laser beam } irradiated from said { TAIL ~ laser beam source } onto an optical disk ; and
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH