On-chip optical filter processes wide range of light wavelengths

Silicon-based system offers smaller, cheaper alternative to other “broadband” filters; could improve a variety of photonic devices.

MIT researchers have designed an optical filter on a chip that can process optical signals from across an extremely wide spectrum of light at once, something never before available to integrated optics systems that process data using light. Image: E. Salih Magden
MIT researchers have designed an optical filter on a chip that can process optical signals from across an extremely wide spectrum of light at once, something never before available to integrated optics systems that process data using light. Image: E. Salih Magden

MIT scientists have recently developed an on-chip optical filter that can process optical signals from across an extremely wide spectrum of light at once. This is for the first time, scientists have achieved this to integrated optics systems that process data using light.

The filter works by matching the broadband coverage and precision performance of the bulky filters. It takes an extremely broad range of wavelengths within its bandwidth as input and efficiently separates it into two output signals, regardless of exactly how wide or at what wavelength the input is. That capability didn’t exist before in integrated optics.

This on-chip optical filter has architecture, similarly as dichroic filters. There are two sections of precisely sized and aligned silicon waveguides that coax different wavelengths into different outputs. These waveguides typically made of a “core” of high-index material — meaning light travels slowly through it — surrounded by a lower-index material.

At the point when light experiences the higher-and lower-record materials, it tends to skip toward the higher-index material. In this manner, in the waveguide light winds up caught in, and goes along, the center.

The MIT analysts utilize waveguides to unequivocally control the light contribution to the relating signal yields. One segment of the scientists’ filter contains a variety of three waveguides, while the other area contains one waveguide that is marginally more extensive than any of the three individual ones.

In a gadget utilizing a similar material for all waveguides, light tends to move along the largest waveguide. By tweaking the widths in the variety of three waveguides and holes between them, the specialists influence them to show up as a single wider extensive waveguide, however just to light with longer wavelengths. Wavelengths are estimated in nanometers, and altering these waveguide measurements makes a “cutoff,” which means the exact nanometer of the wavelength above which light will “see” the variety of three waveguides as a single one.

Emir Salih Magden, a former PhD student in MIT’s Department of Electrical Engineering and Computer Science (EECS) said, “That these long wavelengths are unable to distinguish these gaps, and see them as a single waveguide, is half of the puzzle. The other half is designing efficient transitions for routing light through these waveguides toward the outputs.”

The design also allows for a very sharp roll-off, measured by how precisely a filter splits an input near the cutoff. If the roll-off is gradual, some desired transmission signal goes into the undesired output. Sharper roll-off produces a cleaner signal filtered with minimal loss. In measurements, the researchers found their filters offer about 10 to 70 times sharper roll-offs than other broadband filters.

As a final component, the researchers provided guidelines for exact widths and gaps of the waveguides needed to achieve different cutoffs for different wavelengths. In that way, the filters are highly customizable to work at any wavelength range.

Magden said, “Once you choose what materials to use, you can determine the necessary waveguide dimensions and design a similar filter for your own platform.”

Paper co-authors along with Magden, who is now an assistant professor of electrical engineering at Koç University in Turkey, are: Nanxi Li, a Harvard University graduate student; and, from MIT, graduate student Manan Raval; former graduate student Christopher V. Poulton; former postdoc Alfonso Ruocco; postdoc associate Neetesh Singh; former research scientist Diedrik Vermeulen; Erich Ippen, the Elihu Thomson Professor in EECS and the Department of Physics; Leslie Kolodziejski, a professor in EECS; and Michael Watts, an associate professor in EECS.

The study is published in the journal Nature Communications.