Total Detector Noise

In this consideration we neglect all other noise mechanisms, e.g., noise caused by nonuniformity of impurity distribution within detector ( pattern noise), all avalanche-related processes, burst ( popcorn ) noise, etc. [65, 66]. 

Since all of the considered noise mechanisms are independent, the squared values of all of them are summed, giving the square of total detector noise. 

Since photonic detectors most often operate at mid- and high frequencies, the most prominent noise component is g-r noise. Long [7] neglected 1//component to give an expression for noise current valid both for photovoltaic (PV) and photo-conductive (PC) detectors 

Since Johnson noise is much lower than g-r noise in the operating frequency range of photonic detectors, from the point of view of noise optimization it is most important to minimize generation-recombination processes. 

We consider further the determination of local values of generation-recombination noise for the case when carrier concentration within detector is position-dependent, i.e., when g-r rate and photoelectric gain are spatially inhomogeneous. We use such spatial distribution to determine total noise current (g-r plus thermal) through the whole detector. For the sake of simphcity, we assume that the gradient exists only along one direction, parallel to the y-axis. We consider a photocon-ductive device. 

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FTIR instrumentation is mature. A typical routine mid-IR spectrometer has KBr optics, best resolution of around 1cm-1, and a room temperature DTGS detector. Noise levels below 0.1 % T peak-to-peak can be achieved in a few seconds. The sample compartment will accommodate a variety of sampling accessories such as those for ATR (attenuated total reflection) and diffuse reflection. At present, IR spectra can be obtained with fast and very fast FTIR interferometers with microscopes, in reflection and microreflection, in diffusion, at very low or very high temperatures, in dilute solutions, etc. Hyphenated IR techniques such as PyFTIR, TG-FTIR, GC-FTIR, HPLC-FTIR and SEC-FTIR (Chapter 7) can simplify many problems and streamline the selection process by doing multiple analyses with one sampling. Solvent absorbance limits flow-through IR spectroscopy cells so as to make them impractical for polymer analysis. Advanced FTIR... 

It is evidently insufficient to consider only the response of a detector when analysing its usefulness for a particular application. It is generally necessary to analyse both intrinsic and extrinsic noise signals and compare them with the response. The result of this comparison can be expressed in many different ways. One of the most useful is the noise-equivalent power nep which is the power of an rms signal input (in watts) required to give a response equal to the total rms noise voltage AVN. Then ... 

The detector will also contribute to the noise in the measured CARS resonant signal. Depending on the type of device used in these experiments to digihze the dispersed CARS signal, the detector itself will have a noise component to contribute to the measured signal. The contribution of detector noise to the total noise in a CARS measurement is discussed by Snelling et al. 

The detection of molecules in a molecular beam by a bolometer is based on the bolometer s response to the total beam energy, including the center of mass translational energy (Zen, 1988). The bolometer consists of a liquid-helium-cooled thermocouple whose electrical response varies rapidly with the energy of the bolometer. The low temperature is necessary in order to reduce the heat capacity of the thermocouple, thereby increasing its sensitivity, as well as to minimize the thermal detector noise. 

As indicated by this equation, the multiplex gain at the frequency a- offered by Fourier transform spectroscopy in the presence of photon shot noise is a function of the spectral brightness at that frequency and of the overall structure in the total spectrum. This is completely different from the situation presented earlier where detector noise was assumed to dominate. 

Total noise Fig. 1 Different types of detector noise. 

The multiplex advantage is important enough so that nearly all infrared spectrometers are now of the Fourier transform type Fourier transform instruments are much less common for the ultraviolet, visible, and near-infrared regions, however, because signal-to-noise limitations for spectral measurements with these types of radiation are seldom a result of detector noise but instead are due to shot noise and flicker noise associated with the source. In contrast to detector noise, the magnitudes of both shot and flicker noise increase as the radiant power of the signal increases. Furthermore, the total noise for all of the resolution elements in a Fourier transform measurement tends to be averaeed... 

The radiation detector is located some distance from the readout. A shielded coaxial cable transmits the detector output to the amplifier. The output signal of the detector may be as low as 0.01 volts. A total gain of 1000 is needed to increase this signal to 10 volts, which is a usable output pulse voltage. There is always a pickup of noise in the long cable run this noise can amount to 0.001 volts. 

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