Ultralow-Power All-Optical RAM Chip Based on Nanocavities
Kengo Nozaki, Akihiko Shinya, Shinji Matsuo*, Yasumasa Suzaki*, Toru Segawa*,
Tomonari Sato*, Ryo Takahashi*, and Masaya Notomi
Optical Science Laboratory, *NTT Photonics Laboratories
An optical random-access memory (o-RAM) will enable us to develop various high-speed logic elements with a small power consumption , and is expected to play a key role in future optical routers in which high-bit-rate optical packets are processed without E-O/O-E conversion. A PhC nanocavity is a possible candidate for an o-RAM because of its ultrasmall size, ultralow power consumption, and large-scale integrability on a chip.
We achieved a novel buried heterostructure (BH) with which to form a PhC nanocavity, as shown in Fig. 1(a), where an ultra-compact InGaAsP is buried in an InP-PhC waveguide . This structure allowed us to strongly confine both photons and carriers, enabling a bistable behavior with very small optical power, as shown in Fig. 1(b). Thus memory state switching between the "on" and "off" states can be successfully obtained with a combination of CW bias light and optical pulse input. Operation power was only 30 nW, which is several orders of magnitude lower than of the power required by previously reported o-RAMs.
To demonstrate the feasibility for integration, we fabricated a four-bit memory array integrated in the same chip and demonstrated a four-bit RAM operation as shown in Fig. 1(c). The 40-Gb/s data ("1101" or "1010") were first spatially demultiplexed by an all-optical serialto-parallel converter, and then stored in four different cavities. Finally, the stored four-bit data were successfully read by injecting read pulses after a buffering time of 500 ns. There has been no previous report of integrated nanophotonics and our work is the first demonstration of "Integrated o-RAM chip".
This work was supported by the National Institute of Information and Communications Technology (NICT).
 M. Notomi et al., IET Circuits Devices & Systems 5 (2011) 84.
 K. Nozaki et al., Nature Photon. 6 (2012) 248.
Fig. 1. (a) Optical RAM element based on photonic crystal nanocavity having compact buried heterostructure. (b)
Optical bistability on optical input/output characteristic. (c) Integrated optical RAM operation for 40-Gb/s,
4-bit signal train by parallely-integrated nanocavities on a chip.
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