Two-dimensional MEMS arrays pave way for mobile spectrometers

A digital micromirror device serving as the spatial light modulator can overcome shortcomings of traditional spectroscopy architectures.

In the field of near-infrared (NIR) spectroscopy, a system that combines portability with the accuracy and functionality of high-performance laboratory systems would significantly enhance real-time analysis.

The development of small handheld spectrometers, powered by a battery, could lead to more efficient monitoring of industrial processes or assessments of food ripeness in the field.

spec memes yoleSpectroscopy is used to evaluate the chemical makeup of items in the lab and the field. Courtesy of Texas Instruments.

Most dispersive IR spectroscopic measurements begin in the same manner. The light to be analyzed passes through a small slit that, combined with a grating, controls the resolution of the instrument. The diffraction grating is designed specifically to reflect light of different wavelengths at known angles. This spatial separation of wavelengths allows other systems to measure the light intensity on a wavelength-by-wavelength basis.

Traditional architectures for spectroscopic measurements differ mainly in how the dispersed light is measured. Two very common methods are 1) a single element (or point) detector combined with physical scanning of the dispersed light, and 2) imaging the dispersed light onto an array of detectors.

Approach using MEMS technology

Many limitations of traditional spectroscopy methods can be overcome using a new approach based on optical microelectromechanical systems (MEMS) array technology with a single-point detector. A solid-state optical MEMS array replaces the traditional motorized grating in a single-point detector-based system with a simple, spatial wavelength filter. This approach can utilize the performance advantages of a single-point detector while eliminating the issues of a finely controlled motorized system. In recent years, such systems have been produced in which the scanning grating is eliminated and the MEMS device filters each specific wavelength into a single point detector. This method has demonstrated high performance while resulting in a more compact and robust spectrometer.

meter memes yoleA spectrometer is used to analyze pharmaceuticals. Courtesy of Texas Instruments.

The use of an optical MEMS array has several advantages over a linear array detector architecture. First, a larger, single element detector can be used to increase light collection and greatly reduce system cost and complexity, especially for infrared systems. Also, by eliminating the array detector, the pixel-to-pixel noise is eliminated, which can be a significant improvement in the signal-to-noise ratio (SNR) performance. The increase in SNR allows more accurate measurements to be obtained in less time.

In a spectroscopy system using MEMS technology, the diffraction grating and focusing elements perform the same functions as before, but now light from the focusing element is imaged onto the MEMS array. To select a wavelength for analysis, a specific band of spectral response is activated so as to direct light to the single point detector element for collection and measurement.

The advantages are realized if the MEMS device is highly reliable and can produce a filter response that is predictable and constant over time and temperature.