Miniature spectrometer targets lab-on-a-chip

07 Nov 2006

Researchers in the US unveil details of a tiny diffractive imaging spectrometer.

Combining integrated optical and free-space components is the key to designing a new generation of miniature imaging spectrometers, say researchers at Wayne State University, US. Such devices will be crucial for lab-on-a-chip systems where the area dedicated to optical spectroscopy could be as little as several square millimetres. (Applied Optics 45 7811)

"We can squeeze the optical part of our spectrometer (not including the image detector) into a volume of just 2x2x2 mm," Ivan Avrutsky from the Department of Electrical and Computer Engineering told optics.org. "Our device can work with dozens of independent optical inputs. This is like having around 35 independent optical spectrometers in that tiny volume, each of which can cover the entire visible range with a spectral resolution of 2 nm. Such resolving power is suitable for absorption and fluorescence spectroscopy."

Avrutsky's spectrometer consists of multiple input channels, a planar waveguide expansion section, a diffractive optical element (DOE), an aberration correction prism and an image sensor. The key component is the DOE as it combines the function of input collimating optics, a diffraction grating and output focusing optics. The DOE is a chirped grating made up of a set of curved grooves.

According to Avrutsky, manufacturing the DOE was a challenge because it contains features as small as 100 nm. "Direct writing by electron-beam lithography and focused ion-beam milling is prohibitively expensive and can only be used at the research stage," he said. "The most promising technology for manufacturing the DOE is nanoimprint lithography."

To test their design, the researchers produced a lens-based prototype where a uniform waveguide grating with a microlens mimicked the DOE. All optical elements within the prototype measured less than 1 cm in all dimensions. The end result was a miniature spectrometer covering the spectral range from 450 to 650 nm with a spectral resolution of 0.5 nm. The group presented details of a DOE-based prototype recently at Optics East, which was held in Boston, US, in October.

Avrutsky and colleagues are now working to improve the spectral resolution and throughput of their device. The team is also hoping to commercialise its idea and suggests that any interested parties should contact Wayne State's Technology Transfer Office.