Pyroelectric bolometer detector

The modulated beam is directed through either the sample or reference side of the sample compartment and is finally focused on the detector. For most mid-infrared work, a triglycine sulfate (TGS) pyroelectric bolometer is used as the detector because of its very high frequency response (> 1 MHz). 

An alternate approach involves the use of spatially uniform detectors. Blevin and Brown (26) developed a gold-film bolometer with a broadband response that was uniform to within 2% over an area of 9 x 4 mm for a 0.25-mm beam spot. The detector used a NaCl window and had a minimum detectable power of 4 nW for a bandwidth of 1 Hz. Hanssen and Snail (49) measured the spatial uniformity of a windowless, 14 x 14-mm gold-black coated pyroelectric (LiTaOj) detector at 10.6 /im. For a 0.25-mm beam spot, the response was uniform to within +1.7% over 13 mm, excluding one narrow scratched region where a 10-15% deviation was observed. The minimum detectable power was 1 /iW (standard uncertainty) for a bandwidth of 1 Hz (50). 

If the interferometer mirror speed is such that the optical velocity is 0.316 cm s (HeNe laser frequency of 5 kHz), 4000-cm radiation is modulated at 1.25 kHz (see Eq. 2.11). Thus, the response time of a detector for FT-IR spectrometry must be less than 1 ms. Although several cryogenically cooled detectors have response times this low, the only mid-infrared detectors that have an appropriate combination of high speed, reasonably good sensitivity, low cost, good linearity, and operation at or near room temperature are the pyroelectric bolometers. 

Detector photomultiplier tube thermal, pyroelectric, bolometers... 

Thermocouples, bolometers and pyroelectric and semiconductor detectors are also used. The first three are basically resistance thermometers. A semiconductor detector counts photons falling on it by measuring the change in conductivity due to electrons being excited from fhe valence band info fhe conduction band. 

Some radiation detectors, i.e., photoemissive detectors (vacuum phototubes or photomultipliers) or semiconductor detectors (photodiodes or phototransistors) directly produce an electrical signal by quantum effects. Their output is strongly dependent on the wavelength of the detected radiation. Thermal detectors, i.e., thermocouples and thermopiles, bolometers, pyroelectric detectors, or pneumatic and photoacoustic detectors record a temperature increase through radiation and convert this into an electrical signal. This is proportional to the flux of the absorbed radiant power, independent of the wavelength. 

The type of detector used in an FT-IR spectrometer is highly dependent upon the bandwidth (i.e. the spectral frequencies), the modulation rate of the interferometer, and the intensity of the radiant flux. Several types of detectors are used in the infrared regions photoconductive, photovoltaic, bolometers, pyroelectric and Golay cells. A detailed discussion of detectors may be found elsewhere.12 In general, the photovoltaic and photoconductive detectors can be used in the near- and mid-infrared regions as rapid response, high sensitivity detectors. Usually the bandwidths are limited and will not cover the total ran passed by the beamsplitter. Examples of such detectors are given in Table I. As can be seen from the... 

Other detectors that are useful in the near- and mid-infrared regions are bolometers and pyroelectric detectors. Both these detectors have very large bandwidths and can operate at room temperature however, they have long response times compared to the photodetectors and they have low D s. Pyroelectric detectors are useful in the far-infrared region with rapid-scanning spectrometers whereas Golay cell detectors are often used with slow scanning far-infrared interferometers. These cells are modulated at or below 20 Hz. 

In addition to the various types of bolometers and the pyroelectric effect, several other thermal effects have been exploited as radiation detectors. They are described below. 

Newton used a liquid in glass-thermometer to study heat radiation. Rumford and Leslie used a difierential gas thermometer. Herschel reverted to the liquid thermometer, but this was soon replaced by the thermopile (Melloni [3.4]). Some time later (Langley [3.5]) the first bolometers were used. More recently the use of the gas thermometer, in the shape of the Golay [3.6] and Luft cells has been reintroduced and is now widely used in spectrometers. Another type of thermal detector now widely used is that utilizing the pyroelectric effect. In addition to these, several other detection processes have been suggested, including thermal expansion and changed dielectric properties with temperature. 

In detectors employing an electrical readout mechanism, electrical noise fluctuations (Johnson noise plus possibly other noise sources such as low frequency contact noise in some cases) must be considered. There will be a Johnson noise source associated with the output impedance of the detector. In thermopiles and bolometers the output impedance is predominantly resistive so that the calculation of the Johnson noise is straightforward. The output impedance o f the pyroelectric detector is predominantly capacitative. In this case the resistive component associated with the dielectric loss factor of the... 

SBN pyroelectric detector (Liu and Maciolek [3.27]) to Plessey production ceramic pyroelectric detector it Thin film bolometer (Bessonneau [3.15])... 

Heterodyne detectors in the microwave and millimeter regions (hv< kT) include square-law mixers such as the crystal diode detector [7.93], the InSb photoconductive detector [7.94-96], the Golay cell [7.95], the pyroelectric detector [7.95], the metal-oxide-metal diode, and the bolometer [7.87]. The latter three types of detectors have also been used successfully in the middle infrared (at 10.6 pm) [7.97-100]. For this type of detector Johnson noise generally predominates, and the input SNR is given by [7.100]... 

Detectors for IR radiation fall into two classes thermal detectors and photon-sensitive detectors. Thermal detectors include thermocouples, bolometers, thermistors, and pyroelectric devices. Thermal detectors tend to be slower in response than photon-sensitive saniconduc-tors. The most common types of detectors used in dispersive IR spectroscopy were bolometers, thermocouples, and thermistors, but faster detectors are required for FTIR. FTIR relies on pyroelectric and photon-sensitive semiconducting detectors. Table 4.5 summarizes the wavenumber ranges covered by commonly used detectors. 

While the sensitivity of good pyroelectric detectors is comparable to that of Golay cells or high-sensitivity bolometers, they are more robust and therefore less delicate to handle. They also have a much better time resolution down into the nanosecond range [4.105]. 

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