High hopes have been maintained for decades concerning optical logic, optical switching matrices (e.g. for communications), and optical computing. The missing link in realizing this promise is a practical circuit element that allows one light to be turned on or off purely by application of another light to the device - rather like voltage on the control gate of a field effect transistor. This missing link has now been developed through a novel application of the complex behavior exhibited by coupled lasers.

A multinational team of researchers led by the Vienna University of Technology and including members from Princeton, Yale, and ETH-Zurich has made a rather amazing discovery. Take a properly prepared pair of coupled lasers, each of which has its own optical pump source. Begin by turning on one of the optical pumps so that its laser begins to lase. Now, while keep the intensity of the first optical pump constant, slowly turn on the second optical pump.

Output modes of a pair of coupled lasers as a function of the optical pump intensity on the second laser. The first laser turns off, and later both lasers begin to lase

The application of optical pumping to the second laser simply turns off the first laser without turning on the second laser. A complex interaction between the two lasers has increased their combined sensitivity so that neither laser can lase. One plus one equals zero - in this intermediate state, there is no functioning laser. When still stronger optical pump intensity is delivered to the second laser, both lasers begin to lase. The result is a three-state device, for which you might label the states [00], [10], and [11]. The relationship between the input and output of this three-state device is sufficiently complex to allow the construction of a general-purpose programmable optical computer.

The switching effect and the existence of three stable states is not the result of interfering outputs canceling each other. Rather, it is the result of interactions between the two lasers which involve both interference and the gain of the laser cavities. A great deal more study is needed to uncover all the fruit of this fascinating new tree, but the effect could easily produce a field of digital photonics as deep and productive as that of digital electronics.

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