A multimodal light manipulator

Interferometers, devices that can modulate aspects of light, play the important role of modulating and switching light signals in fibre-optic communications networks and are frequently used for gas sensing and optical computing.

Now, applied physicists at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have invented a new type of interferometer that allows precise control of light’s frequency, intensity and mode in one compact package.

Called a cascaded-mode interferometer, it is a single waveguide on a silicon-on-insulator platform that can create multiple signal paths to control the amplitude and phase of light simultaneously, a process known as optical spectral shaping. By combining mechanisms to manipulate different aspects of light into a single waveguide, the device could be used in advanced nanophotonic sensors or on-chip quantum computing.

Published in Science Advances, the research was led by postdoctoral fellow Jinsheng Lu, who works in the lab of Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering. The devices were made at Harvard’s Center for Nanoscale Systems.

“Conceptually, this is a very big step forward compared to the state of the art for commercial high-speed modulators that are particularly used for communications,” Capasso said.

The most widely used such devices, known as Mach-Zehnder interferometers, work by splitting a beam of light down two paths to toggle its output. Despite their widespread use, Mach-Zehnder interferometers have their limitations — they are not very good at simultaneously controlling different aspects of light. Today, multiple interferometers are needed in succession to make up for these limitations, taking up space and restricting the amount of signal that can travel through.

The new cascaded-mode interferometer is a reimagining of a Mach-Zehnder device integrated into a single-chip waveguide. Rather than the traditional split beam, the new interferometer has a unique, nanoscale pattern of gratings etched into the waveguide that control the energy exchange between different modes of light.

This makes the new interferometer able to control the spectrum of light passing through by finely adjusting the intensity and characteristics of different colours. Light can move through in different patterns, or transverse modes. And the device allows for precise, sharp lines of colour, or light waves with distinct features.

In the paper, the team not only demonstrates the capabilities of their new interferometer but also lays out the theoretical framework for extending the physics of the device to many different modes of light.

Image credit: Capasso Lab/Harvard SEAS.