On-chip optical filter processes wide range of wavelengths

Optical filters are used to separate one light source into two separate outputs: one reflects unwanted wavelengths — or colors — and the other transmits desired wavelengths. Instruments that require infrared radiation, for instance, will use optical filters to remove any visible light and get cleaner infrared signals.

Existing optical filters, however, have tradeoffs and disadvantages. Discrete (off-chip) 'broadband' filters, called dichroic filters, process wide portions of the light spectrum but are large, can be expensive, and require many layers of optical coatings that reflect certain wavelengths.

Integrated filters can be produced in large quantities inexpensively, but they typically cover a very narrow band of the spectrum, so many must be combined to efficiently and selectively filter larger portions of the spectrum.

Researchers from MIT’s Research Laboratory of Electronics have designed the first on-chip filter that, essentially, matches the broadband coverage and precision performance of the bulky filters but can be manufactured using traditional silicon-chip fabrication methods.

“This new filter 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,” said Emir Salih Magden, a former PhD student in MIT’s Department of Electrical Engineering and Computer Science (EECS) and first author on a paper describing the filters published in Nature Communications.

The MIT researchers designed a novel chip architecture that mimics dichroic filters in many ways. They created two sections of precisely sized and aligned (down to the nanometer) silicon waveguides that coax different wavelengths into different outputs.

Waveguides have rectangular cross-sections typically made of a 'core' of high-index material — meaning light travels slowly through it — surrounded by a lower-index material. When light encounters the higher- and lower-index materials, it tends to bounce toward the higher-index material. Thus, in the waveguide light becomes trapped in, and travels along, the core.

The MIT researchers use waveguides to precisely guide the light input to the corresponding signal outputs. One section of the researchers’ filter contains an array of three waveguides, while the other section contains one waveguide that’s slightly wider than any of the three individual ones.

In a device using the same material for all waveguides, light tends to travel along the widest waveguide. By tweaking the widths in the array of three waveguides and gaps between them, the researchers make them appear as a single wider waveguide, but only to light with longer wavelengths.

Wavelengths are measured in nanometers, and adjusting these waveguide metrics creates a 'cutoff,' meaning the precise nanometer of wavelength above which light will “see” the array of three waveguides as a single one.

In the paper, for instance, the researchers created a single waveguide measuring 318nm, and three separate waveguides measuring 250nm each with gaps of 100nm in between. This corresponded to a cutoff of around 1,540nm, which is in the infrared region.

When a light beam entered the filter, wavelengths measuring less than 1,540nm could detect one wide waveguide on one side and three narrower waveguides on the other. Those wavelengths move along the wider waveguide.

Wavelengths longer than 1,540nm, however, can’t detect spaces between three separate waveguides. Instead, they detect a massive waveguide wider than the single waveguide, so move toward the three waveguides.

“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,” Magden said.

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 customisable to work at any wavelength range.

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

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Image credit: MIT.