Fluresence Multi layer Disk

The first generation of disc productions from Constellation 3D will be a family of 120 mm multi-layer FM-discs with capacity up to 140 GBytes and with read speed up to 1 GBytes/s. It's interesting to compare them with DVD that keep up to 17.4 GBytes. New discs will be capable for example to keep up to 20 hours of compressed movies in HDTV format.

Compact discs were a revolutionary product at its time and influenced many spheres of human activity. People started recording music of high quality which didn't get worse with the time as it 

happens to be on tape or vinyl. As soon as CDs appeared in computer industry they immediately became an undoubted helper both for users and for programmers. The latter were able to increase volume of their program products by adding video and audio elements etc. Later discs were used for digital video (VideoCD).

 

But technologies are progressing. Data are growing faster and faster. A usual CD is far not enough (640 MBytes). So, there appeared DVD technology. Of course we are happy with those 17 GBytes that can be kept on one DVD disc, but this is a limiting point. 

FMD-ROM: operating principle

In optical discs such as CD, DVD and MÎ the process of reading is implemented the following way. A beam of a semi-conducting laser gets on the surface of an informational layer and then reflects from aluminum (or any other metallic) layer and fixed with a detector-receiver. In FMD there is no reflected laser beam: when a laser beam reaches an informational layer the latter starts radiating. 

The principle of operation of FM-discs is based on a phenomenon of photochromism. Some years ago russian chemists discovered a stable organic material a "stable photochrome" which when acted upon by a laser beam obtains fluorescent properties.

The matter is that an informational element of FM-disc (photochrome) can change its physical properties (such as color and presence of fluorescence) under influence of a laser of a definite power and wavelength. Initially photochrome doesn't possess fluorescent properties. When switching on a laser a photochemical reaction starts what causes fluorescent properties to appear. When reading, this matter becomes excited again but with a laser of lower power. The fluorescence is caught up by a photo-receiver and is fixed as a value "1".

In usual optical discs (CD/DVD) with increasing number of informational layers a signal gets worse. It's explained by the fact that these technologies use a reflected signal, it means that there is necessity in mirror surfaces. That's why in DVD technology an external layer is made to be semitransparent in order to allow a laser to reach an internal one.

And a signal while passing an external layer leaves a part of its energy because of reflecting. Signals reflected from both layers interfere because of their coherence, it results in losses of useful signal. Increasing number of layers aggravates an effect of multiple interference between the layers what makes reading more complicated. The problem can be solved by improving detector-receivers, but it is possible only in laboratory. In case of fluorescent discs the quality of the signal gets worse much slower with increasing number of layers. Look at the graph below: According to FMD-ROM developers, even with a hundred layers a useful signal will be acceptable.

Here is a list of advantages of FMD/C:

1. Multilayer disc is transparent and homogeneous
2. Small loss of useful signal while passing through several layers
3. Fluorescence of a separate element easily passes through disc layers
4. Less sensitivity (than of CD/DVD) to different imperfections of reading devices. Fluorescent technology doesn't require special manufacturing conditions
5. Reflective fluorescent light from any layer is not coherent, it prevents a problem of multiple interferences
6. FMD-technology is compatible with CD and DVD formats supporting the same data distribution system on each layer.

Parameter	CD	DVD	FMD
Disc diameter, mm	120	120	130
Capacity, GBytes	0,64	17,4	50,8
Number of layers	1	2 (on each side)	12
Distance between layers , micron	-	40	25±5
Total width of informational layers, micron	0,11	2	275
Format	CD	DVD	Modified DVD
Distance between tracks, micron	1,6	0,74	0,8
Optical system: Wavelength, nm	780	635-650	532