Jacques Arnaud

Optical and Quantum Electronics, 34, 393-410, 2002

ABSTRACT

Lasers essentially consist of single-mode optical cavities containing two-level atoms with a supply of energy called the pump and a sink of energy, perhaps an optical detector. The latter converts the light energy into a sequence of electrical pulses corresponding to photo-detection events. It was predicted in 1984 on the basis of Quantum Optics and verified experimentally shortly thereafter that when the pump is non-fluctuating the emitted light does not fluctuate much. Precisely, this means that the variance of the number of photo-detection events observed over a sufficiently long period of time is much smaller than the average number of events.

Light having that property is said to be “sub-Poissonian”. The theory presented rests on the concept introduced by Einstein around 1905, asserting that matter may exchange energy with a wave at angular frequency ω only by multiples of ~ ω. The optical field energy may only vary by integral  multiples of ~ ωas a result of matter quantization and conservation of energy. A number of important results relating to isolated optical cavities containing two-level atoms are first established on the basis of the laws of Statistical Mechanics. Next, the laser system with a pump and an absorber of radiation is treated. The expression of the photo-current spectral density found in that manner coincides with the Quantum Optics result.

The concepts employed in this paper are intuitive and the algebra is elementary. The paper supplements a previous tutorial paper (Arnaud, 1995) in establishing a connection between the theory of laser noise and Statistical Mechanics.

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