Sony Corp. presented its achievements on data reading/writing with the use of "Micro-Reflector recording," a type of holographic recording at the "ISOM'06" event. Compared to the previous systems, the new scheme can reduce the cost of device and medium, so that the holographic recording may be more readily available for use in consumer devices.
In the "Micro-Reflector recording" demonstrated by Sony, a laser light emitted from a blue violet semiconductor laser diode is split into two so that one of them irradiates the front side of a medium as a reference light while the other is emitted to the backside as a recording light. By precisely aligning focal points of the two lights with a servo technology, a minute interference fringe corresponding to a 1 bit recording mark is formed. When a laser light (reproduction light) is emitted on the front side of the medium having interference fringes, the recording light is reproduced. This light advances from the fringes to the medium front side as if the fringes reflect the reproduction light. This is why the system is called "Micro-Reflector recording".
The medium comprises a 0.3 mm thick photopolymer sandwiched by 0.6 mm glass substrates. Since the depth of the interference fringes can be controlled by changing the depth of the focal points of the laser lights, multi-layer recording with 10-20 layers can be provided. The recorded interference fringes are small enough to prevent the photopolymer from expanding/contracting due to temperature change. This eliminates the need of adjusting the wavelength of the laser in accordance with the temperature. Therefore, a commercial blue violet semiconductor laser diode is expected to be used as is. Moreover, the setup can easily achieve a reduction in component cost because it requires no spatial light modulators, CMOS sensors or other parts.
Sony demonstrated read/write operations by using an optical system with a wavelength of 405 nm and a numerical aperture (NA) of 0.5. The system involves 17 PP modulation as with the Blu-ray Disc. This time, the company only recorded data on a single layer. The recording density is as low as 1.25 GB in terms of a 12 cm-disk capacity. The company says that it aims to achieve multilayer recording by increasing the numerical aperture and assess the system's potential.