PROCEEDINGS ARTICLE | April 29, 2009
Proc. SPIE. 7319, Next-Generation Spectroscopic Technologies II
KEYWORDS: Actuators, Optical filters, Mirrors, Fabry–Perot interferometers, Sensors, Calibration, Germanium, Silicon, Reflectivity, Transmittance
This paper reports work on the development of rugged micro-electromechanical systems (MEMS)-based
microspectrometers for real-time applications in agriculture. The devices are electrostatically actuated, first order Fabry-
Perot tuneable optical filters, hybridised with InGaAs photodiode detectors. Tuning range and resolution of the devices
are 1615 nm to 2425 nm and 52 nm (FWHM) at 2000 nm, respectively. To our knowledge, this tuning range is the
largest reported for a MEMS-based Fabry-Perot filter. Three-layer distributed Bragg reflectors are used for the Fabry-
Perot mirrors, and consist of e-beam evaporated layers of germanium - silicon monoxide - germanium. The moveable
mirror also includes two silicon nitride layers that act as the MEMS flexures, stress compensation layers, and as an
encapsulant for the mirror layers. The spectral resolution matches the theoretical expected for the mirror structures used
when the residual bowing of the mirror (~15 nm across a diameter of 70 μm) is included, and can be improved to ~10 nm
if five layer mirrors are used. The out of band rejection is approximately 20 dB. Experimental results show that the
throughput of the device is sufficient to allow transmittance, specular reflectance and diffuse reflectance spectra to be
measured. The primary outstanding issue is wavelength calibration, and is being addressed using a number of
approaches including incorporation of wavelength calibration standards in the hybrid structure and accurate, real-time
measurement of the separation of the two mirrors.
