New optical switch could lead to ultra-fast all-optical signal processing

An artist’s illustration of an optical switch, dividing light pulses according to their energies. Credit: Y. Wang, N. Thu and S. Zhou

Caltech engineers developed a switch, one of the most fundamental components in computing, using optical rather than electronic components. The development could contribute to efforts to achieve ultra-fast all-optical signal processing and computation.

Optical devices have the ability to transmit signals much faster than electrical devices by using pulses of light rather than electrical signals. This is why modern devices often use optics to send data; for example, consider fiber optic cables that offer much faster Internet speeds than conventional Ethernet cables.

The field of optics has the potential to revolutionize computing by doing more, at faster speeds and with less power. However, one of the major limitations of optics-based systems today is that at some point they still need transistor-based electronics to process data efficiently.

Now, using the power of optical nonlinearity (more on that later), a team led by Alireza Marandi, assistant professor of electrical engineering and applied physics at Caltech, has created an all-optical switch. Such a switch could possibly allow data processing using photons. The research was published in the journal Nature Photonics July 28.

Switches are among the simplest components of a computer. A signal enters the switch, and depending on certain conditions, the switch allows the signal to move forward or stop it. This on/off property is the foundation of logic gates and binary computation, and it is what digital transistors were designed to accomplish. However, until this new work, performing the same function with light has proven difficult. Unlike electrons in transistors, which can strongly affect each other’s flux and thus cause “switching”, photons generally do not easily interact with each other.

Two things made the breakthrough possible: the material used by Marandi’s team and the way they used it. First, they chose a crystalline material known as lithium niobate, a combination of niobium, lithium and oxygen that does not exist in nature but has, over the past 50 years, s has proven essential in the field of optics. The material is inherently nonlinear: due to the particular way the atoms are arranged in the crystal, the optical signals it produces at the output 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 allowed Marandi and his team to create lithium niobate-based integrated photonic devices that can confine the light in a tiny space. The smaller the gap, the greater the intensity of light with the same power. As a result, light pulses carrying information through such an optical system could provide a stronger nonlinear response than would otherwise be possible.

Marandi and his colleagues also confined the light temporally. Essentially, they reduced the duration of the light pulses and used a specific design that would keep the pulses short as they propagated through the device, which gave each pulse a higher peak power.

The combined effect of these two tactics – spatio-temporal confinement of light – is to dramatically improve the strength of nonlinearity for a given pulse energy, which means photons now affect each other much more strongly.

The net result is the creation of a nonlinear splitter in which light pulses are routed to two different outputs based on their energies, allowing switching in less than 50 femtoseconds (one femtosecond is one quadrillionth of a second). By comparison, state-of-the-art electronic switches take tens of picoseconds (a picosecond is one trillionth of a second), a difference of several orders of magnitude.

The article is titled “Femtojoule femtosecond all-optical switching in lithium niobate nanophotonics”.

The hand of light holds the key to better optical control

More information:
Qiushi Guo et al, Femtojoule femtosecond all-optical switching in lithium niobate nanophotonics, Nature Photonics (2022). DOI: 10.1038/s41566-022-01044-5

Provided by California Institute of Technology

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