Microspectrometer based on a tunable optical filter of porous silicon

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 run and 6 gm. Two thermal bimorph micro-actuators tilt the plate by up to 90 degrees, 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 mum. The filter area can be chosen between 0.27 mm x 0.70 mm and 2.50 mm x 3.00 mm, its thickness is typically 30 pm. The spectral finesse Delta lambda/lambda, 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. (C) 2001 Elsevier Science B.V. All rights reserved.