Hitachi, Ltd. has developed an optical technology for use with an optical disc device to amplify the reproduction signal output from a photodetector by ten times. In general, an optical disc with multiple recording layers has a problem in that the reproduction signal tends to get weaker. It is expected, however, that this bottleneck can be solved by the latest technology. The company claims that it is possible to efficiently detect the reproduction signal of a Blu-ray Disc with a capacity of 100-200 GB on a single surface which is obtained by the provision of four to eight recording layers, for example.
The company conducted a principle experiment and confirmed that the reproduction signal from a photodetector can be increased by ten times compared to the case without using the latest optical technology. Details are unveiled at the ODS 2007 optical disc international conference being held from May 20 to 23 in Portland, Oregon.
Homodyne detection, widely used in optical communications, is utilized to amplify the reproduction signal. In the homodyne detection, the light beam output from the laser light source is split into two light paths by means of a half mirror or other devices. One of the split light beams is used as the signal light after being modulated and the other is used as the reference light without modulation. Then, the signal and reference lights are interfered with each other on the detector to obtain an electric signal. With the use of this method, even if the intensity of the signal light is low, the amplitude of the coherent light obtained by multiplexing the two lights becomes greater than that of the original signal light, when the intensity of the reference light is high enough. Compared to the case where only the signal light is used, the reproduction signal is less likely to be blocked by electric noises generated by the photodetector and electric circuits since the intensity of light that can be input to the detector is higher. In the experiment, Hitachi succeeded in boosting the S/N ratio of the electric signal output from the photodetector by ten times.
Hitachi added three modifications to the optical system in order to make the homodyne detection applicable to the optical disc devices. First, the reference light can be generated by just adding several pieces of optical parts to the existing optical system of the optical head. Second, polarization differential detection is employed. Third, variation in signal gain caused by the vibration of the recording surface of optical disc is reduced.
In general, an optical system of an optical head in an optical disc device uses a polarization beam splitter to totally reflect the light beam emitted from a laser light source, thereby guiding the light beam to the surface of an optical disc. Then, the system detects the light reflected from the disc surface as the signal light. In order to generate the reference light, a half-wave plate and other parts are effectively used so that a part of the light beam emitted from the laser unit penetrates the splitter. The penetrated light beam is then subjected to the total reflection on a mirror so that it is guided back into the splitter for use as the reference light.
More specifically, the latest development was achieved by providing a half-wave plate that can rotate the light phase by 180 degrees between the laser unit and the splitter and a quarter-wave plate in front of the reflection mirror. The half-wave plate is used to change the gradient of laser polarization so that a part of the light beam from the laser unit can penetrate the splitter.
In addition, polarization differential detection, which is used in the existing magnetic optical disc devices, is employed to detect the coherent light generated by the signal and reference lights.
The variation in signal gain generated by the vibration of the recording surface of an optical disc is reduced by providing an optical system that can compensate for the difference in length of optical paths of the reference and signal lights. At the time of playback, if a difference occurs in the length of optical paths of the reference and signal lights due to the vibration on the surface of a rotating optical disc, phase difference is generated between the two lights, causing a variation in the reproduction signal gain.
Another optical system using the polarization differential detection is added to compensate for the optical path length difference. The difference is compensated for by calculating the signals detected by the two polarization differential detectors.
In the principle experiment, a He-Ne laser unit is used as the light source and a reflective object is used in place of the optical disc. The intensity ratio between the reference and signal lights was set to 100:1. Although the intensities of the two lights are yet to be unveiled, Hitachi says that "the intensity of the reference light is lower than that of the laser beam output used when writing on a multilayered optical medium."
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