Microspectrometer based on a tunable optical filter of porous silicon
Sensors and Actuators A: Physical
Volume 92, Issues 1–3, 1 August 2001, Pages 52–59

We present a tunable interference filter for infrared and visible light that scans the desired part of the optical spectrum within milliseconds. A single pixel detector measures serially the intensity at selected wavelengths. This concept avoids expensive linear detectors as used for grating spectrometers. The tunable optical interference filter is fabricated by a new porous silicon batch technology using only two photolithography steps. The refractive index of this filter microplate is gradually modulated in depth to create a Bragg mirror or a Fabry-Perot bandpass filter for a transmission wavelength between 400 nm and 6 μm. Two thermal bimorph micro-actuators tilt the plate by up to 90°, changing the incidence angle of the beam to be analyzed. This tunes the wavelength transmitted to the detector by a factor of 1.25, e.g. from 4 to 5 μm.

The filter area can be chosen between 0.27 mm×0.70 mm and 2.50 mm×3.00 mm, its thickness is typically 30 μm. The spectral finesse Δλ/λ of 25 is sufficient for most diagnosis applications, e.g. detection of CO2 and CO in combustion processes by their IR absorption bands. Online colorimetry and color correction of desktop printers can be envisaged.