Page 106 - MEMS Mechanical Sensors

5.4 Optical Techniques 95

photodetector. A simple arrangement is for the microsensor to move in response to
the measurand and for this movement to be arranged to block the path of the light
beam incident on a photodetector. Figure 5.7 illustrates a simple transmissive
arrangement, although reflected light is also used in some arrangements.
The optical source is shown as a light emitting diode (LED) since a coherent
source is not required for intensity-based sensors. Alternative optical sources could
be a laser, the output of an optical fiber, or simply an incandescent lamp.
The major difficulty with intensity-based systems is variations in intensity
caused by factors not related to the measurand. For example, the output of an opti-
cal source can vary with time and temperature. For this reason intensity-based sen-
sors often incorporate some form of reference measurement of the optical source
intensity and a ratio taken between the optical intensity before and after modulation
by the microsensor. This problem often negates the simplicity of intensity-based
sensors. Variations in the sensitivity of the optical detector can also cause difficulties
and complications.
A qualitative estimate of the resolution of intensity-based sensors can be
obtained by estimation of the optical beam size. The minimum beam size is of the
order of the wavelength of the optical source, so this gives an indication of the dis-
placement required to give a 100% modulation of intensity.

5.4.2 Phase
As photodetectors do not respond directly to phase variation, it is necessary to con-
vert a variation in phase to an intensity variation for measurement at the photodi-
ode. This is usually achieved by using an interferometer to combine one or more
optical beams that have interacted with the microsensor with one or more optical
beams that are unaffected by the microsensor. A coherent source such as a laser
diode is therefore typically used in phase-based optical sensing. The interaction with
the microsensor has the effect of altering the optical path length of that optical beam
and hence its phase. This can simply be achieved by reflecting the optical beam off
the microsensor and the microsensor moving in response to the measurand so as to
vary the optical path length.
A major advantage of phase-based systems is that subwavelength phase varia-
tions can be resolved, which equates to submicron displacement of the microsensor.
Difficulties can be caused by the fact that the output of the interferometer is peri-
odic; therefore, care has to be taken to establish the start point and the position rela-
tive to that. This can lead to complexity in the reference electrodes and errors in
initializing the system.

Emitted light Transmitted light

Optical fiber

LED
Photodiode
Moving microstructure

Figure 5.7 An example of a simple intensity modulation sensing system.