On-Chip Optical Filter Processes Wide Range of Light Wavelengths

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Optical filter implemented in a system to process signals from across the entire optical spectrum

Researchers at MIT have designed an optical filter on a chip that can process optical signals from across a wide spectrum of light at once. The technology may offer accuracy and flexibility for designing optical communication and sensor systems by studying photons and other particles. These optical filters are used to separate one light source into two separate outputs where one reflects unwanted wavelengths and the other transmit desired wavelengths.

These filters can be produced in large quantities inexpensively and cover a narrow band of the spectrum. Therefore, many filters must be combined to selectively filter larger portions of the spectrum. It takes a broad range of wavelengths within its bandwidth as input and separates it into two output signals, regardless of how wide or at what wavelength the input is. Instruments that require infrared radiation will use optical filters to remove any visible light and get cleaner infrared signals.

Dictating the flow of light

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

Waveguides have rectangular cross-sections which are made up of a core of high-index material through which light travels, surrounded by a low-index material. When light encounters the high and low-index materials, it tends to bounce toward the high-index material. Thus, the light got trapped in the waveguide and travels along the core.

The researchers use waveguides to 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 the individual ones.

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.

Sharper tools

These optical filters can be implemented within one system to process signals from across the entire optical spectrum including splitting and combining signals from multiple inputs into multiple outputs. For the applications, it is helpful to have filters that cover different portions of the optical spectrum on one device.

Once the device is implemented with sharp optical and radio-frequency signals, you can get more accurate positioning and navigation and better receptor quality. With spectroscopy, you can get access to phenomena you couldn’t measure before. The new device could be useful for sharper signals in fibre-to-the-home installations which connect optical fibre from a central point directly to homes and buildings.

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