Spontaneously generated noise photons

The ASE process is initiated by the spontaneously generated noise photons (SPs) with intensity... 

Comparing Eq. (110) with Eq. (66), we find that both equations have extra terms (the e or e terms) which make the solutions oscillatory, but the physical reason for oscillations is different in both cases. In Eq. (66) different from zero e comes from the nonzero initial value of the second-harmonic mode intensity, while in Eq. (110) the nonzero value of e comes from the quantum noise. We can interpret this fact in the following way. It is the spontaneous emission of photons, or vacuum fluctuations of the second harmonic mode, that contribute to the nonzero value of the initial intensity of the second harmonic mode and lead to the periodic evolution. This means that the very small quantum fluctuations can cause macroscopic effects, such as quantum-noise-induced macroscopic revivals [38], in the nonlinear process of second-harmonic generation. 

For the purpose of this consideration, the dynamic range is the ability of the detector to count photons at both the low and high fluxes with the same effectiveness, and by the background noise we mean the events similar to those generated by the absorbed photons, but occurring randomly and spontaneously in the detector without photons entering the detector. 

As we have already seen, quantum noise changes the character of the evolution of the field in the second-harmonic generation by making it periodic. But periodic behavior is also seen for classical solution if we assume that there is a small classical signal of the second harmonic mode when the evolution starts. One can thus say that the quantum noise, or spontaneously emitted photons, play a role of the classical signal that makes the evolution periodic. 

Spontaneous emission The random generation of a photon due to the spontaneous recombination of an electron-hole pair. Spontaneous emission is the fundamental noise-generating process in an optical amplifier. 

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