Page 464 - Introduction to Information Optics

8.4. Bit-Pattern Optical Storage

Laser Light

Objective
Lens

Partial^
Transparent
Layers

Fig. 8.2. Multilayer optical disk.

having up to six layers for a read-only disk and up to four layers for a WORM
disk is also available [33]. There reportedly appear to be no fundamental
hurdles for read-only disks with 10 to 20 layers. Several formatted thin-disk
substrates are physically stacked with spacers into a single multilayer optical
disk. To change from one layer to another, the optical pickup refocuses the
objective lens to the new layer. The layers are separated by at least 100 ^m so
that other data surfaces are well out of focus. With the layers about 100 jum
apart, the laser beam could spread over 10,000 times more area on a layer
adjacent to the layer in focus, depending on the NA of the focusing lens.
Each disk layer must be partially transparent so that the laser can penetrate
all the layers in a stack. Each surface, however, must also be reflective enough
so the data can be read. On average, the surface would reflect only 5% of the
incident light. For comparison, a read-only optical disk reflects about 95% of
the incident light. The electronic part of the system should be modified to
amplify the output signal of the photodetector to a level compatible with
standard optical drives. At normal optical disk storage densities, a 10-layer
disk would store 6.5 gigabytes of information (a single layer stores 650
megabytes). A multilayer optical disk is schematically shown in Fig. 8.2.

8.4.4. PHOTON-GATING 3-D OPTICAL STORAGE

The objective of 3-D storage is to arrange and store a string of bits in a 3-D
structure. A book is a good analogy for 3-D storage. The 1-D bit pattern is
first arranged in a 2-D format called page memory. The stack of page memories
forms a 3-D storage medium. While we may freely open a book to select a page
to read, we cannot mechanically open a 3-D optical storage medium to select
a certain page or layer. A first approach to 3-D storage—a multilayer optical
disk —was discussed in the previous subsection. Depending on the separation