Machine Design

Small and cheap spectrometers

Spectrometers detect substances present in ultralow concentrations.

A prototype spectrometer from Georgia Tech plugs into a laptop computer and can be used for environmental sensing. Ali Adibi is head of the project and an associate professor in the School of Electrical and Computer Engineering at Georgia Tech.

But the devices tend to be large and need highly complex and sensitive optics to work, which has limited their use.

Now a team of researchers from Georgia Tech and Duke University has found a way to significantly reduce the size and complexity of the devices. The breakthrough could usher in handheld spectrophotometers for detecting carbon monoxide, on-the-spot blood analysis, and other biomedical applications.

Conventional spectrometers contain a narrow slit, a lens, a grating to separate wavelengths, a second lens, and a detector to measure light power at different wavelengths. They work best using collimated light, though diffuse optical signals are needed for practical sensing applications. The problem is over-come in these spectrometers by blocking light in all but one direction using a slit and a lens, but at the expense of considerable power loss and lower efficiency.

The Georgia Tech device combines all these pieces into two parts: a volume hologram formed in an inexpensive polymer, and a detector. Volume holograms are recorded by the interference pattern of two light beams in a piece of photopolymer. The hologram handles both the alignment and focusing functions and needs no collimating hardware. Spectrophotometer efficiency and sensitivity scale with hologram complexity. Highly complex holograms are recorded much the same way as the less-complex versions so the spectrometer itself is not more complicated. The arrangement is lightweight and relatively insensitive to optical alignment.

So far, the team has built a prototype spectrometer comparable to those currently on the market but that would cost much less. Funding for the project comes from the National Institute on Alcohol Abuse and Alcoholism through the Integrated Alcohol Sensing and Data Analysis program.


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