1. Field of the Invention
The present invention relates to an optical power level-controlling device and more particularly to an optical power level-controlling device of an optical disk device capable of recording data on a rewritable optical disk such as CD-RW.
2. Description of the Related Art
Recordable optical disk may generally be categorized into write-once read-many optical disks such as CD-R and rewritable optical disks such as CD-RW. FIG. 1 illustrates optical power level of a laser beam used for recording data on a CD-RW. In the following text, terms “power level” and “level” are understood to mean “optical power level”. As can be seen in FIG. 1, the power level is varied between two values, i.e., WRITE level and ERASE level, with BIAS level being the reference level. In such a case, the laser beam at WRITE level causes a change of state of the recording layer of the optical disk from crystalline to amorphous, thereby forming a record mark. On the other hand, the laser beam at ERASE level causes a change of state of the recording layer of the optical disk from amorphous to crystalline, thereby erasing the recorded mark.
As can be seen in FIG. 2, in order to reproduce information from a CD-RW, the laser beam is set at READ level which is at the same level as BIAS level and then a high-frequency signal of about 300 MHz is added thereon, so that the power level of the laser beam will be changed into a burst form (multi-mode). This is due to a return optical noise caused by a laser beam that is emitted from the laser diode, reflected on the optical disk and then returned to the laser diode. Also, oscillation of the diode can prevented from becoming unstable and being stopped. According to the above-mentioned multimode operation, oscillation of the laser diode can be stabilized.
Adding a high-frequency signal of about 300 MHz during a recording operation of a CD-RW as described above does not cause any problem during periods in which the laser beam is at BIAS (READ) level or at ERASE level. However, during periods at WRITE level, an excess current may flow in the laser diode due to the high-frequency signal. Such an excess current may shorten life duration of the laser diode. On a characteristic curve of output power level against input electric current of the laser diode shown in FIG. 5, “B” indicates BIAS level, “E” indicates ERASE level and “W” indicates WRITE level. It is to be noted that when the high-frequency signal is added, each of the points “B”, “E” and “W” will move along the horizontal axis.
Accordingly, in the related art, instead of adding a high-frequency signal during recording of CD-RW, the difference between ERASE level and BIAS level (labeled “DW” in FIG. 1) is indicated by a fixed value and the difference between ERASE level and WRITE level (labeled “DP” in FIG. 1) is also indicated by a fixed value. This may cause an unstable oscillation of the laser diode due to the return noise during a recording operation.