Omar Manzardo, University of Neuchâtel
Fourier transform spectroscopy is poised to take the next step forward. Thanks to silicon micromachining techniques, high-resolution spectrometers will soon move out of the lab and into commercial applications. Instruments based on microelectromechanical systems (MEMS) technology, as illustrated by a lamellar grating interferometer developed at the University of Neuchâtel's Institute of Microtechnology in Switzerland, will be produced in large quantities and at low cost.
Among the spectroscopic methods that are used in industry and in research labs, Fourier transform spectroscopy is probably the most powerful, enabling the investigation of weak sources with high resolution. Their superiority over dispersive-grating instruments arises from their throughput and from their ability for multiplexing, which enable them to collect large amounts of energy at high resolution. The throughput advantage means that an extended source at the entrance aperture does not reduce the resolution significantly. The multiplexing advantage allows the interferometer to receive information about the entire spectral range at any moment of the measurement, increasing the signal-to-noise ratio as a function of the number of spectral elements.
At present, a variety of Fourier spectrometers are commercially available, but the high resolution of these instruments necessitates a high degree of mechanical precision, which results in devices with large footprints and high costs. As a result, most compact spectrometers today use a dispersive grating. Miniature instruments that exploit the advantages of Fourier spectroscopy have been rare.
Because of MEMS technology, however, this is changing.
Choice of configuration
Miniature stationary Fourier spectrometers generally adopt a Twyman-Green, Sagnac or Wollaston configuration, all of which are robust and have no moving parts. Nevertheless, they require a photodiode array, and therefore offer no significant advantages over the grating-based compact spectrometers.
A microfabricated time-scanning configuration benefits from the advantages of Fourier transform spectroscopy because it employs a unique photodiode and its resolution is not restricted by a limited number of pixels in the photodiode array. Yet when device features are scaled down to micron size, limitations arise from the difficulties attending the integration of the micro-optical elements. For instance, in the case of the Michelson configuration, which is most commonly used for time-scanning Fourier spectroscopy, the insertion of a beamsplitter in a miniaturized instrument becomes complex.
This type of spectroscopy also can be performed using a lamellar grating interferometer, which avoids the need for an integrated beamsplitter and thus profits from the convenience of silicon microfabrication technology. This device employs a binary grating with a variable depth that operates in the zero order of the diffraction pattern. The grating is composed of a series of fixed and mobile mirrors. The depth of the grating -- i.e., the distance between the fixed and the mobile mirrors -- determines the optical path difference.
The modulation of the zero order is recorded in function of the optical path difference, as in the Michelson interferometer. Therefore, the basic equation of Fourier transform spectroscopy applies to the lamellar grating interferometer. The spectrum is the Fourier transform of the recorded intensity. In general, these types of spectrometers are used for wavelengths longer than 100 µm; for shorter wavelengths, the tolerances are too tight for most machine shops. Silicon micromachining is the ideal technology to overcome this.
Our team at the university has produced such a lamellar grating instrument. In this interferometer, an electrostatic comb drive actuator, microfabricated by the deep reactive-ion etching of a silicon-on-insulator wafer, controls the motion of the series of mirrors. The height of the mirrors is 75 µm, the number of grating periods is 32, the dimension of the grating period is 100 µm, and the total dimension of the MEMS device is 7 × 4 mm.
The spectrometer can measure wavelengths from 380 to 1700 nm with good resolution and accuracy. To demonstrate its potential, we have investigated the complex spectral structures of a low-pressure xenon arc lamp with a resolution of 0.8 nm at a wavelength of 400 nm and of 10 nm at 1540 nm. The position accuracy of the emission peaks was better than 0.5 nm, and the maximum optical path difference was 240 µm.
We plan to extend the path difference to 1.2 mm, which would enable a resolution of 8 cm–1. Such resolving power would enable numerous applications in the near-infrared and infrared.