New optical filter technology may yield greater precision and flexibility in a bevy of applications, including designing optical communication and sensor systems and studying photons and other particles through ultrafast techniques.
A team from the Massachusetts Institute of Technology (MIT) has created a new optical filter on a chip that is able to process optical signals from across a wide spectrum of light at once, combining the positive features of the two most commonly used types of filters.
“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,” Emir Salih Magden, a former PhD student in MIT’s Department of Electrical Engineering and Computer Science (EECS) and first author on the paper, said in a statement. “That capability didn’t exist before in integrated optics.”
Scientists use optical filters to separate one light source into two separate outputs—one that reflects unwanted wavelengths and another that transmits desired wavelengths.
Existing optical filters— such as discrete broadband filters called dichroic filters—process wide portions of the light spectrum. However, they are often large and expensive and could require several layers of optical coatings that reflect specific wavelengths.
Integrated filters, while able to be produced in large quantities inexpensively, often only cover an extremely narrow band of the spectrum and must be combined to efficiently and selectively filter larger portions of the spectrum.
The researchers developed new chip architecture that mimics dichroic filters by creating two sections of precisely sized and aligned silicon waveguides that coax different wavelengths into different outputs. One section of the filter contains an array of three waveguides that are 250 nanometers each with gaps of 100 nanometers in between and the other section contains just one waveguide that is 318 nanometers.
Light tends to travel along the widest waveguides in devices that use the same material for all of the waveguides. However, in the new device the researchers made the three waveguides and the gaps between them appear as a single-wide waveguide, but only to light with longer wavelengths.
“That these long wavelengths are unable to distinguish these gaps, and see them as a single waveguide, is half of the puzzle,” Magden said. “The other half is designing efficient transitions for routing light through these waveguides toward the outputs.”
The researchers found that the filters offer about 10 to 70 times sharper roll-offs—a measurement of how precisely a filter splits an input near the cutoff—than other broadband filters.
The team also provided guidelines for exact widths and gaps of the waveguides that are needed to achieve different cutoffs for different wavelengths that enable the filters to be highly customizable 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.
The study was published in Nature Communications.