Researchers present a new optical filter on a chip that can process optical signals from across a wide spectra. The filter can match the broadband coverage and precision performance of bulky dichroic filters, but can be manufactured using traditional silicon-chip fabrication methods. The filter is based on spectrally selective waveguides, which combine the broadband response of adiabatic transitions with the sharp spectral responses typically observed in interferometric filters. Researchers at Massachusetts Institute of Technology (MIT) said that the new filter can take a very broad range of wavelengths within its bandwidth as input and efficiently separates the input into two output signals, regardless of its width or wavelength. 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. Courtesy of E. Salih Magden. To build the filter, researchers created two sections of silicon waveguides and sized the waveguides precisely. One section of the filter contained an array of three waveguides. The other section contained a single waveguide. By tweaking the widths in the three-waveguide array and the gaps between the waveguides, researchers made the array appear as a single, wider waveguide to certain (longer) wavelengths. Adjusting the metrics of the waveguides created a cutoff point, measured in nanometers, at which a wavelength of light would “see” the three-waveguide array as a single waveguide. To longer wavelengths, the three-waveguide array appeared wider than the single waveguide located in the other section of the filter. To demonstrate, researchers created a single waveguide measuring 318 nm, and three separate waveguides measuring 250 nm each, with gaps of 100 nm in between. These sizes corresponded to a cutoff in the IR region of around 1540 nm. When a light beam entered the filter, wavelengths measuring less than 1540 nm could detect one wide waveguide on one side and three narrower waveguides on the other. These wavelengths moved along the wider, single waveguide. Wavelengths longer than 1540 nm could not detect the spaces between the three-waveguide array. Instead, they detected one large waveguide with no gaps, and were thus directed to move along the three-waveguide array. “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,” researcher Emir Salih Magden said. The design further allows for a sharp roll-off to produce a cleaner signal filtered with minimal loss. In measurements, the researchers found their filters offered about 10 to 70× sharper roll-offs than other broadband filters. As a final component, the team developed guidelines for the exact widths of the waveguides and the gaps in between that are needed to achieve different cutoffs for different wavelengths. The guidelines make the filters customizable so they can 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. Researchers believe that many of their broadband filters could be implemented in one system to flexibly process signals from across the entire optical spectrum. This could open the way for sharper optical combs, with more precisely spaced pulses and less signal noise. One promising new application for optical combs is powering optical clocks for GPS satellites and for detecting gravitational waves. Other applications include high-precision spectroscopy, enabled by stable optical combs combining different portions of the optical spectrum into one beam, to study the optical signatures of atoms, ions, and other particles. In these applications and others, it will be useful to have filters that cover broad, and vastly different, portions of the optical spectrum on one device. The research was published in Nature Communications (doi: 10.1038/s41467-018-05287-1).