Patent attributes
In a particular type of high-density optical recording systems, a solid immersion lens (SIL) is used in the objective system to focus a radiation beam onto an information layer of an optical record carrier. The distance between the exit surface of the SIL and the entrance surface of the record carrier is typically 25 nm to allow evanescent coupling of the radiation from the SIL to the optical record carrier for a system using a blue laser as radiation source. Such a system is also called a near-field system, deriving its name from the near field formed by the evanescent wave at the exit face of the SIL. A suitable gap signal, representing the width of the gap, is used to control the width of the gap during operation of the system. Tolerances in the optical elements and opto-mechanical components of the optical system of the optical recording system can lead to an offset in the position of the focus point of the objective system. The offset can be larger than the typically used gap width for such a recording system. This can lead to direct contact between the SIL and the record carrier, which may result in damaging one or both of them. The invention discloses a method and implementation for a focus initialization correcting the focus offset of the optical system used to read out or record the optical record carrier such that a reliable and robust gap signal can be derived for gap width control.
