In magneto-optical devices as optical reproducing devices, for a magneto-optical disk of the magnetic super resolution type provided with a recording layer and an in-plane magnetized reproducing layer, a method has been proposed in which a light beam is irradiated to the magneto-optical disk from the reproducing layer side so as to reproduce a record mark smaller than a spot diameter of the light beam.
In the foregoing method, the magnetic property of the recording layer corresponding to a portion of the reproducing layer within an area irradiated by the light beam, in which the temperature rises above a predetermined level (hereinafter, the portion is referred to as an aperture), is copied onto the reproducing layer, and the magnetic property of the foregoing portion of the reproducing layer changes from in-plane magnetization to perpendicular magnetization. In this way, a record mark smaller than the spot diameter of the light beam can be reproduced.
In the foregoing method, although a driving current for generating the light beam is kept at a constant level, there are some cases where an optimal reproducing power of the light beam might vary depending on the changes in the ambient temperature during reproduction.
In this case, if the reproducing power of the light beam becomes much stronger, the aperture formed becomes too large. Consequently, the output of reproduction signals from tracks adjacent to the track being reproduced is increased, and the proportion of noise signals included in the reproduced data is increased, resulting in the increase in the probability of reading errors.
In addition, if the reproducing power of the light beam becomes much weaker, the aperture formed becomes smaller than the record mark, and the output of the reproduction signals from the track to be read is also reduced, also resulting in the increase in the probability of reading errors.
To cope with the foregoing problem, in a recording and reproducing device disclosed in Japanese Unexamined Patent Publication No. 8-63817/1996 (Tokukaihei 8-63817, published on Mar. 8, 1996), two types of reproducing power control marks having different lengths recorded on a magneto-optical disk, that is, a short mark and a long mark, are reproduced, and reproducing power is controlled in such a manner that a ratio between amplitudes of reproduction signals of these two marks gets close to a predetermined value. This arrangement allows the reproducing power of the light beam to be kept at an optimum value, reducing the probability of reading errors.
More specifically, the short mark is smaller than an aperture diameter. Therefore, by increasing the reproducing power and increasing the aperture diameter, the short mark occupies a smaller area in an aperture, resulting in a reduction in an amplitude of the short mark. However, when the reproducing power is increased, the quantity of light also increases, which acts in a direction to increase the amplitude of the short mark.
On the other hand, when reproducing the long mark, which is greater than the aperture diameter, the long mark always occupies 100% of the area of the aperture even if the aperture diameter changes depending on the reproducing power of the light beam. Therefore, it can be considered that a change in an amplitude of the long mark with respect to a change in the reproducing power corresponds to the amount of change in the quantity of light. Thus, a value obtained by dividing the amplitude of the short mark by the amplitude of the long mark, that is, a ratio between the amplitudes of the long and short marks obtained by normalizing the amplitude of the short mark by the amplitude of the long mark, becomes a value corresponding to the amount of change in the aperture diameter. Consequently, to keep the ratio between the amplitudes of the long and short marks constant means to keep the aperture diameter constant. As a result, by maintaining the ratio between the amplitudes of the long and short marks constant, it becomes possible to control the reproducing power so as to be always optimized with respect to an ambient temperature or a tilt.
Next, a method for optimizing the reproducing power of this kind will be specifically explained.
First, FIG. 5 shows a structure of an optical reproducing device, and FIG. 6 schematically shows a structure of a magneto-optical disk 220 reproduced by the optical reproducing device. A sector 300, which is one unit of a recording area in the magneto-optical disk 220, is structured so as to include an address area 301 for indicating the position of the sector 300, a reproducing power control area 302 for recording reproducing power control marks, and a data recording area 303 for recording digital information data.
The reproducing power control marks are constituted by a short mark pattern in which marks each having a length of 2T are provided at spaces each having a length of 2T as shown in FIG. 7(a), and a long mark pattern in which marks each having a length of 8T are provided at spaces each having a length of 8T as shown in FIG. 7(b).
More specifically, T represents a channel bit length, and the short mark pattern shown in FIG. 7(a) is structured such that a mark with a bit length of 2×T shown as a 2T mark and a space with a bit length of 2×T shown as a 2T space are repeated alternately. Besides, the long mark pattern shown in FIG. 7(b) is structured such that a mark with a bit length of 8×T shown as an 8T mark and a space with a bit length of 8×T shown as an 8T space are repeated alternately.
Therefore, for example, when the channel bit length is 1 bit, in binary notation, the short mark pattern specifically represents a pattern in which a bit arrangement of “1100” is repeated, and the long mark pattern specifically represents a pattern in which a bit arrangement of “1111111100000000” is repeated. These short mark pattern and long mark pattern are recorded in the reproducing power control area 302.
In the foregoing optical reproducing device, as shown in FIG. 5, when light emitted from a semicondoctor laser 202 reaches the address area 301 of the sector 300 on the magneto-optical disk 220, a target sector address is recognized by an address decoder (not shown). Next, the emitted light is directed to the reproducing power control area 302, then light reflected from a short mark pattern and light reflected from a long mark pattern, the patterns being recorded in the area, are converted to reprodution signals by a photo diode 203, then the reproduction signals are subjeted to A/D conversion by an A/D converter 205.
The converted signals are respectively inputted to a short mark amplitude detection circuit 221 and a long mark amplitude detection circuit 222. Then, an amplitude value of a short mark in the short mark pattern and an amplitude value of a long mark in the long mark pattern are obtained respectively. Incidentally, the A/D conversion is carried out at timings by clocks extracted from the reproduction signals by a reproducing clock extracting circuit 204 constituted by a PLL (Phase Locked Loop).
The amplitude value of the short mark, that is, the 2T mark, in the short mark pattern and the amplitude value of the long mark, that is, the 8T mark, in the long mark pattern obtained in such a manner are respectively inputted to a division circuit 210. The division circuit 210 outputs an amplitude ratio calculated byAmplitude ratio=amplitude value of the 2T mark +amplitude value of the 8T mark.
The amplitude ratio and a target amplitude ratio are compared by a differential amplifier 211, and a laser power control circuit 212 outputs a driving current for the semiconductor laser 202 in such a manner that feedback is applied in a direction to reduce the difference between the two ratios.
After laser light is controlled so as to provide an optimum reproducing power in such a manner, the emitted light is directed to the data recording area 303, and a read-out reproduction signal is inputted to a binarization processing circuit 213, then information data reproduced with a low error rate is outputted. When the emitted light reaches the next sector 300, the same processing is repeated, and the reproducing power is set again to a new optimum level.
In this manner, by providing the area for recording the reproducing power control marks in each sector 300, dispersedly on the whole, and detecting the quantity of the reproduction signal for reproducing power control at each sector 300, the reproducing power can respond at a short time interval, and follow fluctuations in an optimum reproducing power caused in a short time.
However, the foregoing conventional optical reproducing device has a disadvantage that, since the reproducing power control area 302 should be provided in each sector 300, the area for recording information data is reduced by the amount of the reproducing power control area 302, resulting in a decrease in the utilization ratio of an optical recording medium.
Consequently, another method can be considered in which the reproducing power control area 302 is not provided in each sector 300, but instead, the 2T mark and the 8T mark are detected from a bit arrangement pattern of information data recorded in the data recording area 303, and amplitude values are obtained from reproduction signals corresponding to the marks. However, this method has the following problem.
First, as shown in FIG. 8(a), a reproduction waveform of the short mark pattern constituted by only the 2T mark and the 2T space, shown as “2T2T2T2T”, is expressed in a curve shown in FIG. 8(a). On the other hand, as shown in FIG. 8(b), a reproduction waveform of a mark pattern “3T2T2T3T” in which a 3T mark and a 3T space are arranged before the 2T space and after the 2T mark, respectively, is expressed in a curve shown in FIG. 8(b). As apparent from these waveforms, amplitude values of the 3T mark and the 3T space become greater than those of the 2T mark and the 2T space.
Besides, as shown in FIG. 8(c) , a reproduction waveform of a mark pattern “4T2T2T4T” in which a 4T mark and a 4T space are arranged before the 2T space and after the 2T mark, respectively, is expressed in a curve shown in FIG. 8(c). As apparent from the waveform, amplitude values of the 4T mark and the 4T space become further greater than those of the 3T mark and the 3T space.
Likewise, as shown in FIG. 8(d), a reproduction waveform of a mark pattern “5T2T2T5T” in which a 5T mark and a 5T space are arranged before the 2T space and after the 2T mark, respectively, is expressed in a curve shown in FIG. 8(d). As apparent from the waveform, amplitude values of the 5T mark and the 5T space become further greater than those of the 4T mark and the 4T space.
Here, since the 2T mark is smaller than the aperture diameter of the light beam, its reproduction waveform suffers waveform interference from a mark or a space before and after the 2T mark. The degree of the waveform interference differs depending on the length of the mark or the space before and after the 2T mark.
An actual result confirmed by the measurement using reproduction signals read out from the magneto-optical disk 220 is shown in FIG. 9. The horizontal axis represents the respective mark patterns shown in FIGS. 8(a) through 8(d), and the vertical axis represents the measurement results of the amplitude values of the 2T marks in the reproduction signals of the respective mark patterns.
It can be confirmed also from this measurement result that the amplitude value of the 2T mark varies depending on the length of a space before and after the 2T mark. That is, as the length of the space before and after the 2T mark increases as 3T, 4T, the amplitude value of the 2T mark becomes greater, influenced by the space of a longer length.
In this manner, since the amplitude value of the 2T mark varies depending on the length of the space before and after the 2T mark, individual amplitude values of the 2T marks have considerable variations. As a result, in order to obtain an amplitude value of the 2T mark from a reproduction waveform of the 2T marks included in the bit arrangement pattern of the information data, it is clearly preferable to average the individual 2T mark amplitude values obtained by detecting a plurality of the 2T marks.
For example, when the reproducing power is controlled for each sector 300, the 2T marks are detected from all the bit arrangement patterns of the information data included in each sector 300, and the obtained 2T mark amplitude values are averaged.
In this case, however, since occurence probability of the space of each length before and after the 2T mark differs depending on each sector 300, the 2T mark amplitude value obtained by averaging also has considerable variations depending on each sector 300, resulting in causing a great error in the reproducing power controlled based on the 2T mark amplitude value.