1.  Field of the Invention
This invention relates to a driving device for an ablation type write-once optical disc for illuminating a recording lightbeam for recording a recording pattern on the disc and a method for calibrating the recording light volume thereof. 
2.  Description of the Related Art
In a write-once type optical disc capable of recording/reproducing data by a user, a thin-film recording material not in need of development after recording is generally used, a laser light beam is converged onto this recording material for producing thermally irreversible changes for recording data thereon.  These changes include, for example, ablation (hole-making), phase change and film deformation. 
The driving device for an ablation write-once optical disc converges the laser light beam on the recording surface and forms a hole during marking for effecting recording, as shown in FIG.  1. 
The length of the hole produced in the recording surface becomes shorter than an optimum value the lower the recoding light volume, that is the smaller the laser power, and becomes longer the higher the laser power, as shown in FIG.  2.  The proportion of the length of the hole produced in the recording surface depends on heat diffusion as from the time the light emission pulse is turned off so that it is not dependent significantly on the length of the recording mark. 
In general, the ablated write-once optical disc is of a structure in which the disc surface can be contacted with outside air, so that sensitivity to the laser power tends to be changed with lapse of time.  Thus, with the driving device for an ablated type write-once optical disc, the recording light volume needs to be optimized prior to actual data recording for absorbing changes in disc sensitivity.  In general, in calibrating the recording light volume, the recording light volume is swept for recording and the optimum recording light volume is estimated from the amplitude of the playback signal. 
Meanwhile, the power margin for the ablated type write-once optical disc, that is the power margin capable of recording an optimum mark, is on the order of 30% and hence narrower than that for the magneto-optical disc which is on the order of 50%.  In addition, with pulse width modulation recording, that is mark length recording, even the groove is destroyed if an excessive power is used for recording to render the servo unstable. 
Therefore, in the recording device for the ablated type write-once optical disc, the recording light volume needs to be calibrated accurately without using an excessive power. 
However, in calibrating the recording light volume in the driving device for the ablated type write-once optical disc, the excessive power (over-power) cannot be detected, while the optimum recording light volume cannot be directly searched, so that sufficient precision cannot be achieved.