CSEM Scientific and Technical Report 2008

TUGON – Micro-spectrometer Demonstrator for Visible, Near- and Mid-infrared
Spectroscopy
M. Tormen, R. Lockhart, P. Niedermann, T. Overstolz, A. Hoogerwerf, J.-M. Mayor, J. Pierer, C. Bosshard, R. P. Stanley
MEMS based microspectrometers are gaining more and more interest due to their huge potential, especially in the near- and mid-Infrared. A
compact MEMS-based spectrometer that can operate in the visible, near-infrared and mid-infrared wavelength ranges, has been conceived,
designed and demonstrated.
MEMS based micro-spectrometers have a huge potential due
to their flexibility as well as low cost, low mass, small volume
and power savings. Interest in this technology, especially in
the near-infrared and mid-Infrared, comes from many different
fields, such as industrial quality and surface control, chemical
analysis of soil and water, detection of chemical pollutants,
exhausted gas analysis, food quality control, process control
in pharmaceuticals and astronomy, just to name a few.
A compact MEMS-based spectrometer for near-infrared and
mid-infrared operation has been conceived, designed and
demonstrated [ 1] . The design is based on tunable MEMS
blazed gratings, developed by CSEM [2] , achieving state of the
art results in terms of spectral resolution, operational
wavelength range, light throughput, overall dimensions and
power consumption.
In the spectrometer, the MEMS grating is stretched like an
accordion. The change in the size of the grating changes
directly the period of the grating and hence the wavelength at
which the grating filters the input light.
Figure 1 shows a processed MEMS device which comprises
of the tunable optical grating and the electrostatic comb drives
which stretch the grating. The device has been fabricated
using standard MEMS manufacturing techniques in Comlab.
The complete die measures 6x6 mm, with a 1x1 mm grating.
The grating itself is formed from free-standing beams with a
12 µm period and a 50% duty-cycle. The beams are attached
to each other using leaf springs. So that it can be stretched in
its plane, the grating is free standing.
Figure 1: SEM image of a tunable MEMS blazed grating
The MEMS blazed gratings have been successfully tested at
600 nm, 800 nm, at 1500 nm and at 10 μm, demonstrating a
tuning range up to 8%.
The properties of the developed MEMS blazed grating are
such that a novel, simple and compact geometry of
monochromator could be adopted. Light is coupled into and
out of the MEMS grating using a pair of fibres and collimating
lenses; then a basic system of mirrors is used to re-direct the
light internally and makes it impinge onto the grating at the
blazed angle; finally a system of separated optical paths
improves stray-light properties. The monochromator can
achieve high spectral resolution with linewidths of 1 nm at
1500 nm and 40 nm at 10 μm.
Figure 2: The minature MEMS monochromator
Figure 2 shows the MEMS monochromator. Its dimensions
are 32x53x72 mm, including fiber connectors.
Finally, the miniature MEMS monochromator is integrated with
light input and ouput ports and a detector. External electronics
and software were developed to run the control of the
monochromator and for data acquisition. The spectrometer
has been demonstrated in preliminary tests with first spectral
measurements made at 1500 nm. The system can acquire at
a rate of about 1 kHz, providing a 1024 point spectrum in
about 1 sec.
The benefits derived from the use of the developed
technology are manifold: possibility to operate in the visible,
near-infrared and mid-infrared up to 10 µm wavelength; no
complex mechanics required to control multi-degree of
freedom rotations; high spectral resolution by operating at
high orders, as high as 0.6 nm at 600 nm, 1.2 nm at 1500 nm
or 40 nm at 10 μm; high throughput at all wavelengths, ideally
100%; the optical head dimensions are dominated by fiber
connector dimensions without compromising in optical
resolution; due to the electrostatic actuation, power
consumption is reduced.
[1] M. Tormen, et al., “MEMS tunable grating microspectrometer”,
International Conference for Space Optics (ICSO), (2008)
[2] M. Tormen, et al., “Deformable MEMS grating for wide tunability
and high operating speed”, SPIE, v 6114 (2006), 61140C
15