One of the main issues with reading back multi-layer discs is that the reproduction signal gets weaker the more layers the disc has. To overcome this issue, Hitachi, Ltd. has developed an optical technology, which when used with an optical disc device, can amplify the reproduction signal by 10 times. As an example, this technology can work for the efficient read-back of Blu-ray discs of 4 to 8 layers.
The technology involves Homodyne detection, which is widely used in optical communications to amplify the reproduction signal. It works by splitting the laser light source in two beam paths such as with a half mirror, modulating one beam to use as the signal beam and keeping the other as a reference beam. Both arrive at the photo detector, where they are multiplexed and intefered with each other. This results in a reproduction signal much stronger than that of just using a signal beam only, which provides a signal to noise ratio of around 10 times better. To use the technology with optical disc devices, it involves the addition of three modifications to the optical system.
Further details, including more in-depth technical information, can be read here on this Nikkei Business Publications article.
