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Industrial Sensors and Contr ol
Temperature-controller 331
AIGaAs-LED
Transmitter 1
Temperature
InGaAsP sensor
Transmitter 2
-LED
Output Receiver
Display Signal Optical fiber
Processor
Ge-APD
FIGURE 7.3 System confi guration of the optical-fi ber thermometer with GaAs
light absorber.

of λ is intensity-modulated by temperature. On the other hand, GaAs
s
is transparent for the light of λ , which is then utilized as a reference
r
light. After detection by a germanium avalanche photodiode (GeAPD),
the temperature-dependent signal λ is normalized by the reference
s
signal λ in a microprocessor.
r
The performance of the thermometer is summarized in Table 7.1.
An accuracy of better than ±2°C is obtained within a range of –20 to
+150°C. The principle of operation for this temperature sensor is
based on the temperature-dependent direct fluorescent emission from
phosphors.

7.3.2 Semiconductor Temperature Detector Using
Photoluminescence
The sensing element of this semiconductor photoluminescence sensor
is a double-heterostructure GaAs epitaxial layer surrounded by two
Al Ga As layers. When the GaAs absorbs the incoming exciting
x 1–x
light, the electron-hole pairs are generated in the GaAs layer. The
electron-hole pairs combine and reemit the photons with a wavelength
determined by temperature. As illustrated in Fig. 7.4, the luminescent
wavelength shifts monotonically toward longer wavelengths as the
temperature T increases. This is a result of the decrease in the energy
gap E with T. Therefore, analysis of the luminescent spectrum yields
g
the required temperature information. The double heterostructure of
the sensing element provides excellent quantum efficiency for the
luminescence because the generated electron-hole pairs are confined
between the two potential barriers (Fig. 7.5).
The system is configured as shown in Fig. 7.6. The sensing ele-
ment is attached to the end of the silica fiber (100-μm core diameter).
The excitation light from an LED, with a peak wavelength of about
750 nm, is coupled into the fiber and guided to a special GRIN lens
mounted to a block of glass. A first optical inference filter IF , located
1
between the GRIN lens and the glass block, reflects the excitation