Signal fluctuation limit

The analysis of the signal and background limits which follows, while detailed, is not exhaustive. Since thermal detectors respond to radiant power rather than photon arrival rate, the signal and background fluctuation limits derived below do not apply to them. Rather, it is necessary to repeat the derivations in terms of power rather than photon rate. An analysis of the background fluctuation limit for thermal detectors is found elsewhere [2.159]. 

Consider a photodiode viewing a monochromatic source of wavelength A. The short circuit signal current ig is given by [see (2.18)] 

Equation (2.65) implies that if the signal-to-noise power ratio is unity, then the minimum detectable power is just the energy of a photon multiplied by twice the measurement bandwidth and divided by the quantum efficiency. For any real 

In a practical sense, a value must be set for the probability of detecting a photon in a given observation interval. Let it be that if a source is emitting N photons per observation interval which reach the detector, the probability of one or more being detected in that interval must be at least 0.99. This is equivalent to saying that there is a one percent probability that no photons will be detected. Thus 

This result can be expressed in terms of a minimum detectable power. Since the energy per photon is hc/A, the minimum detectable power required in order that there be at least a 99% probability of detecting a photon in an observation time Tq is 

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Table 2.6. Signal fluctuation limit for 500° K black body source, 1 Hz bandwidth, and unit quantum efficiency in terms of long wavelength limit 2o...

Fig. 2.19. Minimum detectable monochromatic power as a function of wavelength for composite of signal fluctuation limit (SFL) and background fluctuation limit (BFL) for two detector areas and electrical bandwidths. Background temperature is 290 K and field ofview is 2x steradians. Detector long wavelength limit is assumed equal to source wavelength...

According to the Table, our ideal detector would have signal-fluctuation limited operation it would react momentary when illuminated by IR radiation its response outside the desired spectral range would be zero it could not only operate at room temperature, but also on elevated and lower temperatures, and the probability of its failure would be neghgible it would be robust and insensitive to the changes of operating conditions it would emit neither any harmful radiation nor... 

An important conclusion to be drawn from the present text is that a combination of equilibrium and nonequilibrium methods has potentials not only to reach the BLIP limit, but also to surpass it, nearing the contemporary infrared devices to the signal fluctuation limited mode of operation. There are no fundamental limits that would hinder novel generations of infrared photonic detectors to overcome the need for cooling. Of all types of photodetectors quoted in Part 1, the photonic devices remain the closest to the imaginary ideal device for detection of infrared radiation. 

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