Light and general radiation laws. Coherence and incoherence. Emission, absorption and amplification of radiation. Units and physical constants

Fig. 2.4. The distribution of atoms in energy levels as a function of temperature.
Light amplification using a three-level system

Consider the three-level system of Fig. 2.5. Under the action of optical radiation with energy , atoms transfer from state E₁ to state E₃. From the state E₃, spontaneous transitions to E₂ and E₁ are possible. From the state E₂, in turn, spontaneous transitions to the state E₁ are possible. To obtain an inverted population between the levels E₁ and E₂, the state of E₂ should be more long-lived than E₃, i.e. the following conditions must be met:

A₂₁<31

A₂₁<32 (2.7)

where A₂₁, A₃₁ and A₃₂ are the transition probabilities between the corresponding energy levels.

Conditions (2.7) mean that the average lifetime of an atom in the state E₂ must be much greater than in the state E₃, i.e.

(2.8)
In practice , magnitudes of the order of 10^-6 s, while .

Fig. 2.5. The scheme of the three-level system.
The energy state E₂ satisfying these conditions is called metastable. If the radiation is sent to such a system with sufficient power with a frequency , the time elapsing n₂ will be greater than n₁.

In this medium, as a result of spontaneous transitions, quanta with frequency arise . When these quanta pass through an active medium with an inverted population, light is amplified. To effectively enhance light, we must increase the path of light in the active medium. For this purpose, the active medium is located between strictly parallel arranged flat or spherical mirrors. A system of such mirrors is called an optical resonator. Fotons, flying perpendicularly to the surface of mirrors and reflecting from them many times, will pass in the active medium a very large path, creating on its length more and more identical identical photons (positive feedback). In this case, the effect of quantum optical amplification will be especially strong for such a distance between the mirrors L, on which an integer number of half-waves with a wavelength corresponding to the laser transition is stacked . Obviously, for the case under consideration this condition is satisfied by the relation

(2.9)
where h is the Planck constant, c is the speed of light. And, finally, the last - it is necessary to provide the output of the laser radiation that emerged from the active medium, for which one of the mirrors is made partially transparent.

The role of the resonator is not only to increase the path length of light in the active medium. It also makes it possible to obtain a highly parallel and very close to monochromatic radiation.

At the present time, a great number of the most diverse quantum generators and amplifiers have been developed and successfully operated, differing from each other in design, type of active medium, power, spectral range and character (continuous or impulse) of radiation , the method of obtaining inverse population, the pumping method, the arrangement of a resonator, and the like. We will describe some of them in the following chapters.