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 at the MIT 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," says 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 today 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.