handbook of modern sensors

14.5 Cooled Detectors 423
(A)
(B)
Fig. 14.15. Examples of photoresistor applications: (A) light switch and (B) beacon light.
(Courtesy of Hamamatsu Photonics K.K.)
14.5 Cooled Detectors
For the measurement of objects emanating photons in the range of 2 eV or higher,
quantum detectors having room temperature are generally used. For the smaller energies
(longer wavelengths) narrower-band-gap semiconductors are required. However,
even if a quantum detector has a sufficiently small energy band gap, at room temperatures
its own intrinsic noise is much higher than a photoconductive signal. In other
words, the detector will sense its own thermal radiation. The noise level is temperature
dependent; therefore, when detecting long-wavelength photons, the signal-to-noise
ratio may become so small that accurate measurement becomes impossible. This is
the reason why, for the operation in the mid- and far-infrared spectral ranges, a detector
not only should have a sufficiently narrow energy gap, but its temperature has to
be lowered to the level where intrinsic noise is reduced to an acceptable level. Figure
14.16 shows typical spectral responses of some detectors with recommended operating
temperatures. The operating principle of a cryogenically cooled detector is about
the same as that of a photoresistor, except that it operates at far longer wavelengths
at much lower temperatures. Thus, the sensor design becomes quite different. Depending
on the required sensitivity and operating wavelength, the following crystals
are typically used for this type of sensor: lead sulfide (PbS), indium arsenide (InAs),
germanium (G), lead selenide (PbSe), and mercury–cadmium–telluride (HgCdTe).
Cooling shifts the responses to longer wavelengths and increases sensitivity. However,
the response speeds of PbS and PbSe become slower with cooling. Methods
of cooling include Dewar cooling using dry ice, or liquid nitrogen, liquid helium
(Fig. 14.17), or thermoelectric coolers operating on the Peltier effect (see Section
3.9 of Chapter 3). As an example, Table 14.2 lists typical specifications for an MCT
photoconductive detector. MCT stands for the mercury-cadmium-telluride type of a
sensitive element.
Applications of the cryogenically cooled quantum detectors include the measurements
of optical power over a broad spectral range, thermal temperature measurement
and thermal imaging, detection of water content and gas analysis.
Figure 14.18 depicts gas absorption spectra for various molecules. Water strongly
absorbs at 1.1, 1.4, 1.9, and 2.7 µm. Thus, to determine the moisture content, for
example, in coal, the monochromatic light is projected on the test and reference
samples. The reflected light is detected and the ratio is calculated for the absorption