Kunihiko Hara

AlGaAs/GaAs pnpn differential optical switch operable with 400 fJ optical input energy

Differential optical switching operation with an optical input of as low as 400 fJ (0.07 fJ/μm2) was realized by using parallel‐connected AlGaAs/GaAs pnpn optical switches. An improvement in the sensitivity of more than two orders of magnitude has been attained by introduction of a new operating concept. The electrical transient time was 10 ns, almost independent of optical input levels.

AlGaAs-GaAs pnpn differential optical switch

A differential optical switch based on parallel-connected AlGaAs-GaAs pnpn structures is presented. Continuous input light makes it possible to discriminate between two optical inputs with a power difference of as low as 0.1 nW. The minimum operating energy with pulse inputs was 400 fJ. With the introduction of a storage operation of pnpn structures, the optical S-R flip-flop operation is also demonstrated with a 7.2-pJ optical input energy. The expansibility of the 2-D array is attractive in optical parallel processing systems, especially in optical neural networks. >

Optical flip-flop based on parallel-connected AlGaAs/GaAs pnpn structures.

An optical flip-flop using parallel-connected pnpn (thyristor) structures is demonstrated for the first time to our knowledge. We utilize the differential optical switching technique for inverting the state of the flip-flop and the slow dissipation of carriers stored in the gate layers of pnpn structures for retaining the state. The flip-flop operation with 7.2 pJ of optical input energy was attained by using an AlGaAs/GaAs device. The expansibility to monolithic two-dimensional arrays, the high on-off contrast, and the high optical gain of the flip-flop are useful in the design and fabrication of optical parallel processing systems.

Artificial retina chip with a 256 x 256 array of n-MOS variable sensitivity photodetector cells

The artificial retina chip, consisting of an array of variable sensitivity photo-detector cells (VSPD), attains versatile focal plane image processing using vector-matrix multiplication. We designed an n-MOS VSPD with a pn photodiode and a differential amplifier, which realizes programmable positive and negative sensitivity. The pixels of the fabricated 256 multiplied by 256 pixel artificial retina chip are 35 multiplied by 26 micrometers squared with a 25% aperture ratio in a 2 micrometer n-MOS process. The photosensitivity is 0.23 (mu) A/lx for 1 ms accumulation time. We demonstrate image capture in video mode and edge extraction mode.

Variable-Sensitivity Photodetector of pn-np Structure for Optical Neural Networks

A new structure of the variable-sensitivity photodetector (VSPD) is proposed. It is based on an interdigital pn-np structure. Analog and bipolar photosensitivities varied by the control voltage are described, and the photocurrent is compared with those of the metal-semiconductor-metal (MSM)-type VSPD. Introduction of the pn-np structure is effective for obtaining high photodetection efficiency, and also for modifying the variable-sensitivity characteristics. These advantages make it attractive for optical implementation of neural networks.

Realization of a Graded-Threshold Function Using an AlGaAs/GaAs pnpn Optical Switch

A graded-threshold, function using an AlGaAs/GaAs pnpn optical switch is proposed and demonstrated, which is very useful to optical neural networks.

Highly sensitive p-n-p-n differential optical switch

A novel operating technique has been developed for a differential optical switch based on a set of parallel-connected pnpn structures. The differential function at O.2nW input power and 400fJ input energy was realized with A1GaAs/GaAs pnpn devices. Its application to an optical S-R flip-flop was also demonstrated by introducing a storage operation of pnpn structures.