handbook of modern sensors

10.8 Optoelectronic Sensors 353
Fig. 10.13. Schematic of an optoelectronic pressure sensor operating on the interference phenomenon.
(Adapted from Ref. [12].)
resolution, and high accuracy. Especially promising are the optoelectronic sensors operating
with the light interference phenomena [11]. Such sensors use a Fabry–Perot
(FP) principle of measuring small displacements (covered in more detail in Section
7.5 of Chapter 7). A simplified circuit of one such a sensor is shown in Fig. 10.13.
The sensor consists of the following essential components: a passive optical pressure
chip with a membrane etched in silicon, a light-emitting diode (LED), and a
detector chip [12]. A pressure chip is similar to a capacitive pressure sensor as described
earlier, except that a capacitor is replaced by an optical cavity forming a
Fabry–Perot interferometer [13] measuring the deflection of the diaphragm. A backetched
single-crystal diaphragm on a silicon chip is covered with a thin metallic
layer, and a glass plate is covered with a metallic layer on its backside. The glass
is separated from the silicon chip by two spacers at a distance w. Two metallic layers
form a variable-gap FP interferometer with a pressure-sensitive movable mirror
(on the membrane) and a plane-parallel, stationary, fixed half-transparent mirror (on
the glass). A detector chip contains three p-n-junction photodiodes. Two of them are
covered with integrated optical FP filters of slightly different thicknesses. The filters
are formed as first surface silicon mirrors, coated with a layer of SiO 2 and thin metal
(Al) mirrors on their surfaces. An operating principle of the sensor is based on the
measurement of a wavelength modulation of the reflected and transmitted light depending
on the width of the FP cavity. The reflection and transmission from the cavity
is almost a periodic function in the inverse wavelength, 1/λ, of the light with a period
equal to 1/2w. Because w is a linear function of the applied pressure, the reflected
light is wavelength modulated.
The detector chip works as a demodulator and generates electrical signals representing
the applied pressure. It performs an optical comparison of the sensing cavity
of the pressure sensor with a virtual cavity formed by the height difference between
two FP filters. If both cavities are the same, the detector generates the maximum pho-