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Industrial Sensors and Contr ol

FIGURE 7.38 A gas detection system with 30 detection cells. 359

the distributed beams is transmitted through a 1-km length of fiber to
a corresponding gas detection cell. The receiving unit is constructed of
three optical switches, a rotating sector with four optical interference
filters, and three Ge photodiodes. Each optical switch can select any ten
returned beams by specifying the number of the cell. The peak trans-
mission wavelength of the optical filter incorporated in the sensor is
1.666 μm for CH , 1.690 μm for C H , 1.625 μm for C H , and 1.600 μm
4 3 8 2 2
for a reference beam. After conversion to electrical signals, the signal
amplitudes for the three gases are normalized by the reference ampli-
tude. Then, the concentration of each gas is obtained from a known
absorption-concentration calibration curve stored in a computer.
An intrinsic distributed optical-fiber gas sensor for detecting the
leakage of cryogenically stored gases such as CH , C H , and N has also
4 2 4 2
been developed. The sensor’s operation is based on the temperature-
dependent transmission loss of optical fiber—that is, the optical fiber
is specially designed so that the transmission loss increases with
decreasing temperature by choosing the appropriate core and clad-
ding materials. Below the critical temperature, in the region of –55°C,
most of the light has transferred to the cladding layer, and the light in
the core is cut off. By connecting this temperature-sensitive fiber
between a light source and a detector and monitoring the output light
level, the loss of light resulting from a cryogenic liquid in contact
with the fiber can be detected directly.

7.19 Using Avalanche Photodiodes to Improve
System Performance
Avalanche photodiode detectors (APD) are used in many diverse appli-
cations, such as laser range finders (Fig. 7.39), data communications,