Optical recording/reproducing systems are designed to irradiate a laser beam to a recording medium, such as a CD, a DVD, or a next generation DVD (Blu-ray DISC or HD DVD). This writes, into the recording medium, data to be written as a recorded signal by a state change in a recording layer of the recording medium due to the heat of the irradiated laser beam. The optical recording/reproducing systems are also designed to reproduce data corresponding to a beam reflected from a plurality of recorded marks (also referred to as ‘recorded pits’) constituting the recorded signal. Such optical recording/reproducing systems have rapidly become common as data recording/reproducing systems.
In such a data recording/reproducing system, an acceleration of a linear velocity of the laser beam from 1× to 2×, . . . , 32× allows a rate or time of reproduction and/or recording to be reduced. The linear velocity represents a velocity of a laser beam travelling on a medium during recording and/or reproducing.
In such a data recording/reproducing system designed set forth above, a single-mode laser with a comparatively low operating current is used as a light source; this single-mode laser has a single longitudinal mode. A laser light outputted from a single-mode laser has very high coherency. For this reason, for reproducing data, it is required to maintain, at a high level, a ratio of a laser beam to noise, that is, CNR (Carrier to Noise Ratio); this noise may cause power fluctuations in a laser light outputted from the single-mode laser.
The noise that fluctuates the power of a laser beam includes external feedback noise and laser noise. The external feedback noise is due to interference with optical feedback from a recording medium and/or optical components. The laser noise is due to the fluctuations in temperature As described above, data writing (data recording) into a recording medium is carried out by a state change in a recording layer of the recording medium due to the heat of an irradiated laser beam. For this reason, there is a limit to the power of the irradiated laser beam during reproduction from the standpoint of the prevention of deterioration of the recording layer
In this respect, Patent Documents 1 and 2 change an optical coupling efficiency, which is a ratio of the quantity of part of a laser beam focused on a recording medium to the total of the laser beam to be irradiated from an optical source, according to its mode (recording mode and/or reproducing mode), the kind of the recording medium and/or its recording layer (single layer and/or multiple layer). This can maintain the CNR at a higher level while reducing the power of the irradiated laser beam.
As another method for reducing the external feedback noise, as disclosed in Patent Document 3, a high-frequency current of the order of hundreds of megahertz is superimposed on a drive current (direct current) for a laser beam outputted from a single-mode laser so that the outputted laser beam flashes (on and of). This changes the longitudinal mode of the laser beam to a multimode. This method will be referred to as “high-frequency superimposing method” hereinafter.
Patent Document 1: Japanese Patent Laid-Open No. 2002-260272
Patent Document 2: Japanese Patent Laid-Open No. 2003-196880
Patent Document 3: Japanese Patent Laid-Open No. 2005-346823