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A Highly Cascadable Optically Bistable Device for Large Fan-out Optical Computing Applications

Nicholas C. Craft, Stanley Desmond Smith

Published Jan 1, 1987• 2 citations

Abstract

Smith et al have shown (1) that in order to ensure that the change in output from one bistable device is sufficient to switch the succeeding one, the technique know as ’hold and switch’ is desirable. This involves holding a device as close as possible to its switch point with one laser beam, and using the change in output from a previous similar device to induce switching, thus providing fully restoring logic. Due to the fact that the holding power cannot be made arbitrarily close to the switching power, and that a substantial "over switch" is required to avoid the effects of critical slowing down, there exists a fundamental limit upon the cascadability and fan out potential of conventional bistable devices. This is because their change in output power can only be less than or equal to their switching power. A better device would be one in which the change in output power could be significantly larger than the switching power. The twin cavity device (TCD) described here will be shown to possess this quality.

DOI: 10.1364/optcomp.1987.tuc3

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Smith et al have shown (1) that in order to ensure that the change in output from one bistable device is sufficient to switch the succeeding one, the technique know as ’hold and switch’ is desirable. This involves holding a device as close as possible to its switch point with one laser beam, and using the change in output from a previous similar device to induce switching, thus providing fully restoring logic. Due to the fact that the holding power cannot be made arbitrarily close to the switching power, and that a substantial "over switch" is required to avoid the effects of critical slowing down, there exists a fundamental limit upon the cascadability and fan out potential of conventional bistable devices. This is because their change in output power can only be less than or equal to their switching power. A better device would be one in which the change in output power could be significantly larger than the switching power. The twin cavity device (TCD) described here will be shown to possess this quality.

We describe two approaches to low-energy switching and optical bistability. In one approach, which is a more fundamental approach, we use the nonlinearity associated with bound excitons in CdS to demonstrate the lowest total switching energy (4 pJ) of any optical device exhibiting two stable output states for the same input intensity.1 Bound excitons were chosen because they are known to have a very large oscillator strength (f = 7), a fast radiative decay (tspont = 500 ps), and they do not diffuse in the crystal, thus permitting a sharp focusing geometry. Furthermore, they are a good approximation to quantum dots. In a different experiment we have operated a standard diode laser structure below threshold and have demonstrated an efficient optical switch that can be used in thresholding and decision-making applications. The device switches with <400 aJ (4 × 10−16 J or <3000 photons) of optical energy incident on the device.2 During the switching operation, only 10 pJ of electrical power is dissipated. The switch operates at room temperature, is compatible with optical communication systems, and has a gain of the order of 15 dB, leading to the important properties of cascadability and large fan-out. The operation of a high contrast (5:1) AND gate was demonstrated.

We report the demonstration of cascadability in bistable diode laser amplifiers operated below threshold. A schematic of the experimental set-up is shown in Figure 1. There it is shown that the optical input to the first bistable diode laser amplifier is modulated by a mechanical chopper at frequency Ω1. The output from the first bistable diode laser amplifier is then modulated at Ω2= 2Ω1using a second mechanical chopper. The light is thereafter used as the input to the second bistable device. When the input intensity to the second device is higher than a certain threshold, the output switches. These results are shown in Figure 2. For these experiments, each of the bistable diode laser amplifiers were carefully studied. Detailed bistability measurements at different bias currents and different cavity detuning's were performed. It was found that both the bistability loop and the switching power gets larger as the detuning increases. The size of the hysteresis loop also increased as the laser was operated closer to threshold. In a two beam experiment, it was found that the switching power in the data beam can be as small as 25 nW. Gains larger than 1000 have been demonstrated, indicating the large fan-out capability of these devices. The turn-on and turn-off times in these bistable diode laser amplifiers was measured to be faster than 1 ns and detector limited. These bistable diode laser amplifiers are the most efficient optical devices that have yet been demonstrated and will find applications in photonic switching networks.

We demonstrate an all-optical switch that operates at ultra-low-light levels and exhibits several features necessary for use in optical switching networks. An input switching beam, wavelength $\ensuremath{\lambda}$, with an energy density of ${10}^{\ensuremath{-}2}$ photons per optical cross section $[\ensuremath{\sigma}={\ensuremath{\lambda}}^{2}∕(2\ensuremath{\pi})]$ changes the orientation of a two-spot pattern generated via parametric instability in warm rubidium vapor. The instability is induced with less than

1\phantom{\rule{0.3em}{0ex}}\mathrm{mW}

of total pump power and generates several $\ensuremath{\mu}\mathrm{Ws}$ of output light. The switch is cascadable: the device output is capable of driving multiple inputs, and exhibits transistor-like signal-level restoration with both saturated and intermediate response regimes. Additionally, the system requires an input power proportional to the inverse of the response time, which suggests thermal dissipation does not necessarily limit the practicality of optical logic devices.