Standing-Wave Spectrometer May Fill Instrumentation Gap

Hank Hogan

Researchers at International University Bremen and at Forschungszentrum Jülich, both in Germany, have demonstrated a standing-wave spectrometer that consists of a partially transmissive optoelectronic detector and a mirror. Compared with that of a traditional spectrometer, the design has fewer components, and the layout is linear, with the device and mirror aligned. As a result, it should be possible to pack standing-wave spectrometers into a dense array on a chip. Such devices could fill an instrumentation gap.

Digital cameras have low spectral but high spatial resolution. Grating spectrometers exhibit the opposite characteristics. Dietmar Knipp, an assistant professor of electrical engineering at the university, noted that the spectrometer could bridge the two.

“The system would be something like a multispectral camera,” he said. “The system should have a relatively high spectral resolution, combined with the capability of providing 2-D images.”

In constructing the standing-wave spectrometer, the researchers placed an optoelectronic detector in front of a mirror. The device consisted of a 30- to 40-nm-thick film of amorphous silicon deposited on a glass substrate by plasma-enhanced chemical vapor deposition and coated with transparent conductive oxide layers. The silicon was doped to form an NIP diode, with some carbon added. The researchers patterned and etched the film to make the device.

In operation, light passes through the device, bounces off the mirror and travels back out. This forms a standing wave with a wavelength half that of the incoming light. The intensity of light at the device depends on its distance from the mirror and varies from nothing (as a result of total destructive interference) to somewhat less than four times the incoming intensity (in the case of complete constructive interference).

Moving the mirror back and forth changes the photocurrent measured by the device, which the investigators demonstrated that they could use to differentiate lasers with wavelengths of 594 and 633 nm. The spectral resolution was a function of the mirror movement, with a scan of 4.7 µm needed to distinguish the two lasers.

In an experimental demonstration of the instrument, they used a piezo positioning system from PI (Physik Instrumente) GmbH & Co. KG to modulate the mirror and an adjustable current amplifier from Femto Messtechnik GmbH to amplify the photocurrent.

There are limitations to the approach. The standing-wave spectrometer works best at a particular wavelength. The researchers thus envision its use where an incoming signal is being correlated to a reference, rather than for absolute measurement.

Knipp nonetheless sees a number of applications for multispectral cameras built with the spectrometers, including food inspection and the detection of skin cancer.

Applied Physics Letters, Feb. 20, 2006, 083509.