a defect detection circuit configured to compare {HEAD ~ sad amount} of {TAIL ~ irradiation beam detection signal} with a reference voltage and maintain a substantially constant output voltage to said drive circuit during a first period of time, 
of
a defect detection circuit configured to compare { HEAD ~ sad amount } of { TAIL ~ irradiation beam detection signal } with a reference voltage and maintain a substantially constant output voltage to said drive circuit during a first period of time ,
OTH OTH OTH OTH OTH OTH OTH 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 OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH