Harvard SEAS Develops Interferometer for Exact Light Control

Applied physicists at Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a new type of interferometer that offers precise control of light frequency, intensity, and mode in a compact form.

Image Credit: sspopov/Shutterstock.com

Interferometers, which modify the properties of light, are used in fiber-optic communication networks, gas sensing, and optical computing applications.

The new interferometer, a cascaded-mode device, uses a single waveguide on a silicon-on-insulator platform. It can generate multiple signal channels simultaneously to adjust the amplitude and phase of light, a process called optical spectrum shaping. This technology combines multiple light manipulation techniques into a single waveguide, making it suitable for advanced nanophotonic sensors or on-chip quantum computing.

The study was led by postdoctoral scholar Jinsheng Lu in Federico Capasso’s lab. The work received funding from the Air Force Office of Scientific Research (award No. FA9550-23-1-0699), and the devices were created at Harvard's Center for Nanoscale Systems, supported by the National Science Foundation (award number ECCS-2025158).

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.

Federico Capasso, Robert L. Wallace Professor of Applied Physics, John A. Paulson School of Engineering and Applied Sciences, Harvard University

The most common type of interferometer, the Mach-Zehnder interferometer, splits a light beam into two channels to toggle its output. While widely used, these devices have limitations, including the inability to control multiple properties of light simultaneously. As a result, multiple interferometers are often needed in succession to compensate for these limitations, taking up space and reducing signal throughput.

The newly developed cascaded-mode interferometer improves on the Mach-Zehnder design by integrating it into a single chip waveguide. Instead of splitting the beam, this device uses a nanoscale pattern of gratings etched into the waveguide to manage the energy exchange between different light modes.

This design allows the new interferometer to precisely control the light spectrum by adjusting the intensity and properties of different wavelengths. It enables the creation of distinct lines of color or light waves with specific characteristics. The study demonstrates the interferometer’s capabilities and provides the theoretical foundation for extending its functionality to handle more modes of light.

Journal Reference:

Lu, J. et. al. (2025) Cascaded-mode interferometers: Spectral shape and linewidth engineering. Science Advances. doi.org/10.1126/sciadv.adt4154