Folded Spectrometer Offers Range, Resolution and Speed

Richard Gaughan

Spectrometer design usually involves compromises among acquisition time, spectral resolution and wavelength range. A spectrometer that was developed at Fudan University in Shanghai, China, using a two-dimensional back-thinned silicon CCD has demonstrated a resolution of 0.07 nm over a spectral range from 200 to 1000 nm with an acquisition time of 100 ms.

At the heart of the system is a grating consisting of 10 subgratings that produces 10 separate spectral regions. Each of these 1200-groove-per-millimeter subgratings is aligned with a different angle of incidence, making 10 horizontal wavelength channels on a 1340 × 1300-pixel CCD, a VersArray:1300B, from Princeton Instruments Inc. of Trenton, N.J. Each of the 1340 × 1300-pixel spectral regions covers 80 nm, ranging from a 200- to 280-nm window to a 920- to 1000-nm window. By matching the blaze angle to the center wavelength, the first-order dispersion for each wavelength subregion is identical, creating uniform resolution across the detector.

The 10-element grating at the heart of the spectrometer folds the optical path to image a wide spectrum at high resolution onto a two-dimensional detector. Because there are no moving parts, the acquisition also can be fast. Courtesy of Fudan University.

The unique grating alignment effectively folds a 268-mm dispersion path, creating 10 separate 26.8-mm dispersion regions without any moving parts. And because there are no mechanically moving elements, the minimum acquisition time is limited by the data transfer rate from the CCD to the data storage device.

Although the unique 10-element grating was manually aligned with setscrews in the prototype, Liang Yao Chen of the university’s department of optical science and engineering said that the well-defined incidence angles should allow the grating to be manufactured separately.

With the prototype performing so well, the researchers hope to collaborate with an industrial partner to commercialize the technology. Potential applications include real-time spectral analysis during thin-film deposition.

“We hope people working in similar fields around the world can use this concept and method in many applications,” Chen said.