This device is an arrangement for detecting a signal for effecting a focus control of an optical head with respect to an optical information recording medium by making use of a reflected light beam from the optical information recording medium. On an optical path for a reflected light beam reflected...http://www.google.com.tw/patents/US5023439?utm_source=gb-gplus-share專利 US5023439 - Arrangement for detecting a signal for effecting a focus control of an optical head
