All-optical switch promises ultrafast signal processing

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By Rich Pell

Researchers at Caltech say they have developed a switch – one of the most fundamental components of computing – using optical, rather than electronic, components. Since optical devices have the capacity to transmit signals far faster than electrical devices by using pulses of light rather than electrical signals, the development could aid efforts to achieve ultrafast all-optical signal processing and computing.

One of the major limitations of optics-based systems at present, say the researchers, is that at a certain point, they still need to have electronics-based transistors to efficiently process the data. Now, using the power of optical nonlinearity, the researchers say they have created an all-optical switch that could eventually enable data processing using photons.

The on/off property of a switch in computing is the foundation of logic gates and binary computation, and is what transistors in digital circuits were designed to accomplish. However, until this new work, say the researchers, achieving the same function with light has proved difficult. Unlike electrons in transistors, which can strongly affect each other’s flow and thereby cause “switching,” photons usually do not easily interact with each other.

Two things made the breakthrough possible: the material the researchers used, and the way in which they used it. First, they chose a crystalline material known as lithium niobate, a combination of niobium, lithium, and oxygen that does not occur in nature but has, over the past 50 years, proven essential to the field of optics. The material is inherently nonlinear: Because of the special way the atoms are arranged in the crystal, the optical signals that it produces as outputs are not proportional to the input signals.

While lithium niobate crystals have been used in optics for decades, more recently, advances in nanofabrication techniques have enabled the researchers to create lithium niobate-based integrated photonic devices that allow for the confinement of light in a tiny space. The smaller the space, the greater the intensity of light with the same amount of power. As a result, the pulses of light carrying information through such an optical system could provide a stronger nonlinear response than would otherwise be possible.

The researchers also confined the light temporally. Essentially, they decreased the duration of light pulses, and used a specific design that would keep the pulses short as they propagate through the device, which resulted in each pulse having higher peak power.

The combined effect of these two tactics — the spatiotemporal confinement of light — is to substantially enhance the strength of nonlinearity for a given pulse energy, which means the photons now affect each other much more strongly, say the researchers. The net result is the creation of a nonlinear splitter in which the light pulses are routed to two different outputs based on their energies, which enables switching to occur in less than 50 femtoseconds (a femtosecond is a quadrillionth of a second).

By comparison, state-of-the-art electronic switches take tens of picoseconds (a picosecond is a trillionth of a second), a difference of many orders of magnitude. For more, see “Femtojoule femtosecond all-optical switching in lithium niobate nanophotonics.”


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