Memory needs of digital processing and computing has been
formed so far around the idiosyncrasy of electronic RAM. Between
types of electronic memories, static RAMs have been
the preferred choice for performance-sensitive applications, implementing
cache memories on the processor chips of HPCS.
Multiple single static RAM cells arranged in rows and columns
form what is widely known as a RAM bank structure. A typical
two dimensional (2D) arrangement of 4 4 static RAM
bank is shown in Fig. 1(a), where 16 separate single RAM cells
are independently controlled and each row can store a 4-bit
word. Shared among the RAM cells of a single row, the “Word”
signal grants simultaneous access for Read or Write operation
according to the logical value of the corresponding signal.
The most basic element in electronic RAM bank structures
for static memory design has been the 6-transistor (6T) RAM
cell [6], the layout of which is shown in Fig. 1(b). It consists
of two pass gates for access control and two cross-coupled inverters
with two possible states. Although it has been widely
used in electronic cache memories, its speed performance imposes
the major limit to the overall system processing speed.
To overcome the long foreseen “Memory Wall”, research
has shifted focus on developing all optical RAM alternatives
based on ultra-fast bistable latching and memory elements.
Optical Set-Reset (SR) Flip-Flop (FF) operation has already
Continuous Wave (CW) light [12] or even the polarization
bistability of a cylinder-shaped single mode Vertical Cavity
Surface Emitting Laser [13] have also been exploited.
Despite the numerous FF implementations, only a few true
all optical static RAM cells have been presented so far. The
first optical RAM cell was presented in [14] consisting of two
SOAs acting as Access Gates and two coupled SOA based
MZIs serving as a FF, as shown in Fig. 1(c), experimentally
demonstrating successful operation at 5 Gb/s along with a
performance analysis for reaching 40 Gb/s Read/Write speeds
[15]. The RAM cell presented in [16] also exploits four SOAs,
two of them serving as Access Gates, to achieve Read and
Write operation. Recently an integrated 4-bit optical RAM has
been demonstrated in [17] with ultra-low power consumption
and ultra-small footprint characteristics, exhibiting however a
fall time of 7 nsec. On the other hand, an 8 8 RAM bank has
been presented in [18] implementing however a dynamic RAM
configuration. To this point, we must mention that all optical
RAMimplementations reported so far employ a separate access
gate per RAM cell, mimicking in this way the electronic architectural
layout and not exploiting the full potential of optical
properties.