Ocean Optics ARCoptix ANIR - Manual (Page 34)

A: How the ARCoptix ANIR Works
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Noise
The last aspect treated in this chapter is related to the effect of noise in Fourier transform
spectroscopy. The interferogram signal is always accompanied by statistical errors and we want to
know their effect on the spectrum. Several types of noise and their origins will be presented in this
section.
Signal-to-noise ratio
We define the Signal-to-Noise ratio (SNR) of a set of measurements as the ratio between the mean
value of the signals and their standard deviation.
Types of Noise
In spectroscopy, we usually distinguish between additive noise and multiplicative noise. The first one
is the noise added to the signal; the latter varies with the signal power. Previously, we have mentioned
the multiplex advantage of an interferometer over a dispersive spectrometer (Fellgett advantage). This
advantage occurs only for additive noise.
Additive Noise
Thermal noise (or Johnson noise) arises from thermal fluctuations of the current in the detector and is
independent of the signal level. In the infrared region, photon energies are smaller than these thermal
fluctuations and, therefore, detectors will mainly be limited by thermal noise.
In addition, we mention the dark-current noise, which is an additive noise as well. Photodetection
devices also generate dark-current noise, which is present even in the absence of light and results from
random electron-hole pairs generated thermally or by tunneling.
Multiplicative Noise
Photon noise (or quantum noise, shot noise) is associated to the electromagnetic radiation itself and is
a fundamental quantity. It refers to the fluctuations resulting from the random arrival of photons at the
detector. The standard deviation of these fluctuations is proportional to the square root of the number
of photons arriving. Therefore, the noise power is proportional to the square root of the signal power.
Photon noise is a multiplicative noise. In the near-infrared and visible region, very sensitive detectors
(e.g. photodiodes) are available, which allow to approach photon-noise-limited detection. In this case,
there is no multiplex advantage of the FTS over a dispersive instrument (note that the interferometer
still enjoys the luminosity or Etendue advantage).