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9.9 Optical Flow Measurement 245

profiles in a 55-µm-wide microchamber were measured. This technique permits the
high-resolution three-dimensional mapping and analysis of a wide range of velocity
profiles in confined spaces that measure a few micrometers in dimension. The
particle trajectories are mapped and it is assumed that the particles trace out the
flow lines.

9.9 Optical Flow Measurement

Although almost all optical flow sensors are not strictly MEMS-based, they are,
however, included in this chapter as they can be used in areas, which are important,
but for which MEMS cannot yet cater for. Fiber optic sensors have a number of
advantages over their electrical counterparts. They are safe around volatile chemi-
cals, are free from electromagnetic interference, and provide electrical isolation. In
some applications, fiber sensors show higher durability at elevated temperatures,
and they are corrosion resistant. For example, Eckert et al. [108] developed a
mechanooptical sensor to measure flow in metallic melts of about 350°C. Flow
rates between 1 and 14 cm/s in eutectic InGaSn melt could be measured. Borosilicate
glass can be used up to temperatures of 350°C and quartz glass up to 1,000°C [108].
The major disadvantage of optical measurement systems is their size. Lasers, optical
power meters, lenses, couplers, and mirrors are needed, making the system setup
rather expensive and not suitable for portable systems or for use in small, confined
spaces. Optical devices are not suitable for operation in unclean conditions for long
periods of time (e.g., on the engine block of a car) because dirt and condensation
lead to problems.

9.9.1 Fluid Velocity Measurement
A flow sensor using a silicon cantilever with a wave guide on its surface is described
by Chun et al. [109]. It uses a similar principle to the sensors based on drag force,
but here, the sensing is not detected by an implanted piezoresistor but rather opti-
cally. Light is transmitted across the wave guide and is used to detect the movement
of the cantilever. The intensity of the optical beam changes with the deformation of
the silicon cantilever due to fluid flow [Figure 9.36(a)]. An optical fiber is used for
the light input to the wave guide, and a second optical fiber is used to detect the light
intensity. The optical fibers are fixed to the silicon chip by V-grooves. Unfortu-
nately, neither minimum or maximum flow rate nor the sensitivity of the sensor is
given in the paper.
An optical fiber drag force flow sensor to measure the speed and direction of
fluid flow was published by Philip-Chandy et al. [110]. The flow sensor comprises a
fiber optic strain gauge, a cantilever element made of rubber, and a spherical drag
element. The fiber optic strain gauge was produced by inserting six grooves into a
multimode optical fiber of 1-mm diameter. As the fiber bends, the variation in the
angle of the grooves causes an intensity modulation of the light transmitted through
the fiber [Figure 9.36(b)]. The flow sensor has a repeatability of 0.3% and measures
wind velocity up to 30 m/s with a resolution of 1.4 m/s.