Optical Filter on a Chip

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. (Source: E. S. 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. The tech­nology may offer greater precision and flexi­bility for designing optical communi­cation and sensor systems, studying photons and other particles through ultrafast techniques, and in other appli­cations.

Optical filters are used to separate one light source into two separate outputs: one reflects unwanted wave­lengths 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. 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,” says Emir Salih Magden, a former PhD student in MIT’s Depart­ment of Electrical Engi­neering and Computer Science (EECS).

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. These wave­guides have rect­angular cross-sections typi­cally made of a core of high-index material 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. 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.

Now, the researchers created a single waveguide measuring 318 nano­meters, and three separate wave­guides measuring 250 nano­meters each with gaps of 100 nano­meters in between. This corres­ponded to a cutoff of around 1,540 nano­meters, which is in the infrared region. When a light beam entered the filter, wavelengths measuring less than 1,540 nano­meters 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 1,540 nanometers, 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 says. 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. “Once you choose what materials to use, you can determine the necessary waveguide dimensions and design a similar filter for your own platform,” Magden says. Many of these broadband filters can be imple­mented within one system to flexibly process signals from across the entire optical spectrum, including splitting and combing signals from multiple inputs into multiple outputs.

This could pave the way for sharper optical combs, resulting in thousands of indi­vidual lines of radio-frequency signals that resemble teeth of a comb. Broadband optical filters are critical in combi­ning different parts of the comb, which reduces unwanted signal noise and produces very fine comb teeth at exact wave­lengths. Because the speed of light is known and constant, the teeth of the comb can be used like a ruler to measure light emitted or reflected by objects for various purposes. A promising new appli­cation for the combs is powering optical clocks for GPS satel­lites that could poten­tially pinpoint a cellphone user’s location down to the centi­meter or even help better detect 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’s helpful to have filters that cover broad, and vastly different, portions of the optical spectrum on one device. “Once we have really precise clocks with sharp optical and radio-frequency signals, you can get more accurate posi­tioning and navi­gation, better receptor quality, and, with spectro­scopy, get access to phenomena you couldn’t measure before,” Magden says.

The new device could be useful, for instance, for sharper signals in fiber-to-the-home instal­lations, which connect optical fiber from a central point directly to homes and buildings, says Wim Bogaerts, a professor of silicon photonics at Ghent Uni­versity. “I like the concept, because it should be very flexible in terms of design,” he says. “It looks like an interes­ting combi­nation of dis­persion engi­neering and an adiabatic coupler to make separation filter for high and low wave­lengths.” (Source: MIT)

Reference: E. S. Magden et al.: Transmissive silicon photonic dichroic filters with spectrally selective waveguides, Nat. Commun. 9, 3009 (2018); DOI: 10.1038/s41467-018-05287-1

Link: Research Laboratory of Electronics, Massachusetts Institute of Technology MIT, Cambridge, USA

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