research activities in 2007 - CSEM

TUGON – Compact MEMS-based Spectrometers for Infra-Red Spectroscopy
M. Tormen, R. Lockhart, J-M, Mayor, R. P. Stanley
Deformable MEMS diffraction gratings have great promise as tuning elements for external cavity lasers and for compact spectrometers. The
challenge is to make high efficiency tunable MEMS gratings and incorporate them into practical devices. CSEM has successfully designed,
fabricated and tested MEMS gratings. Their spectral response has been tested and the potential to design ultra-compact spectrometers based
around this technology has been shown.
In Optical MEMS, the family of diffractive MEMS is interesting
for a wide range of applications because they can be
compact, fast and their narrow spectra response can be used
in spectrometers and for tunable lasers [1] . Commercially
available diffractive MEMS are used in displays, in
spectroscopy and optical telecommunications [2, 3] .
Optical grating
Comb drives
Figure 1: Overview of a tunable MEMS grating. The white dotted
region denotes the optical grating which is actuated by four sets of
electrostatic comb drives.
CSEM has been developing a tunable MEMS grating
technology, and this report demonstrates how it could be
incorporated into a compact spectrometer. 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
tuning of the grating. This method of tuning is completely
different from normal spectrometers where the grating is
rotated. The advantage of MEMS grating is that the complex
mechanics for controlling the rotation of the grating in the
standard configuration is replaced by simple electrostatic
comb drives which stretch the MEMS grating.
Figure 1 shows a processed MEMS device [4] which comprises
the tunable optical grating and the electrostatic comb drives
which stretch the grating. The device has been fabricated
using standard MEMS manufacturing techniques. The
complete die measures 6 x 3 mm, with a 1 mm x 1 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.
One of the challenges is to make a MEMS grating that has a
high diffraction efficiency. A grating with a square profile is the
easiest structure to manufacture but has only about 40%
diffraction efficiency in the first order. In contrast, blazed
gratings can have efficiencies close to unity. In order to
achieve this, the gratings have been blazed using anisotropic
KOH etching. This technique is widely used to make
V-grooves. It yields smooth angled surfaces while maintaining
the mechanical properties of the grating beams. Extending
this technology to a MEMS device has been a challenge. The
MEMS grating shown in Figure 1 is actually a blazed grating.
Figure 2 : A minature monochromator in the lab. The light is coupled
into and out of the grating (bottom centre) using a pair of fibres and
collimating lenses. For scale the MEMS chip is 12 x 6 mm.
The spectral response of the MEMS grating was measured
using an optical spectrum analyser and a collimated white
light source in a configuration shown in Figure 2. The optical
system is extremely compact. The resulting spectra are shown
for different drive voltages in Figure 3.
A tuning range of 3% and subnanometer linewidths have been
achieved. Potentially 10% is achievable with the improved
mechanical design. The spectral response can be better
appreciated in Figure 4 taken for a 1.5 mm long fixed grating
made with the same MEMS process technology. A 25 db
rejection has been achieved experimentally.
Efficiency (Normalized)
1
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5 mm
1505 1510 1515 1520
Wavelength (nm)
1525 1530 1535
Figure 3: The spectral response of the MEMS grating shown in
Figure 1 for several different drive voltages ranging from 0 to
55 volts.
A unique property of these MEMS gratings is that the
efficiency of the gratings is high at all wavelengths. Although,
the efficiency is strongly angular dependence, as the angle is
never varied, in contrast to standard scanning
monochromator, it remains constant. This means that the
same device can be used for a wide range of spectral regions
from the UV to the mid-IR. This versatility is very promising for
0V
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55V
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