Today's sun technologies of receipt of thermal and, especially, of electric energy ran into a large problem that will brake their subsequent introduction. This obstacle is low energy effectiveness of transformation of energy of sun radiation into electric energy and the high cost of elements and systems of the solar power engineering. One of possible ways of overcoming of these difficulties – integration of laser technologies into the structure of concentrating elements and photo-electric transformers.

Quantum-mechanical model of laser. Principle of work of laser is based on the quantum nature of radiations and absorptions of light, more precisely on the mechanism of dynamic equilibrium between the radiation and material bodies. In a state of thermodynamic equilibrium the cavities of body and radiations in a cavity are found. The radiations appears by the aggregate of quanta with the energy ε = ђω. Quanta can be absorbed by atoms, which pass to a higher power level with energy E₁ = E₀ + ђω, where E₀ – the initial power level of atom. At transition of the atom from a power level E₁ onto a power level E₀ a quantum with energy E₁ - E₀ = ђω is emitted. On the fig. 2,a these levels are marked by indexes 0 and 1 (lower level and upper level).

In obedience to principle of equilibrium an exchange by quanta is to be counterbalanced for every frequency (every mode) of radiations separately. In accordance with the law of conservation of energy the transitions from a lower level onto upper level are possible only with absorption of quantum of energy, or under act of radiations, falling onto the atom. Such transitions are called forced transitions. Transitions from a top level onto lower level can be as forced (under act of radiations falling onto atom), so spontaneous, or independent of radiations, falling on an atom.

Let use the following conventional signs: ν₁₀^(c) - frequency of spontaneous transitions of atoms from a top level onto lower level; ν₁₀^(b) -  forced transitions of atoms from a top level onto lower level;  ν₀₁^(b) - forced transitions of atoms from a lower level on upper level. The condition of dynamic equilibrium of the system will be written as follow: ν₁₀^(c) + ν₁₀^(b) = ν01 ^(b).         

If to bring the system of atoms into the unequilibrium state and thus to violate strongly the Boltzmann’s distribution, it is possible easily to attain the change of concentration of atoms on the different levels.  We attain a condition N₁ ~ N₀ or N₁ > N₀. In first case α ~ 0 and a beam will travel through an environment without absorption. In other case of α > 0 a beam will be amplified, in other words, an environment operate as an amplifiers of light flux.

The diagram of dependence of absorption from density of flux of energy S is shown on fig. 2,b. When S → ∞ an absorption fully disappears and insulation comes. Creation of the population inversion of power levels can be attained by means of action of independent light onto the atoms. The simplest way of achievement of population inversion is provided in the three-level`s systems. On fig. 2,c  a distribution of the population of the system is represented, that is found in the equilibrium state. At action onto the system by the radiations of large power with population frequency ω_p = (Е₂ – Е₀)/ђ the value Е₀ and Е₂ are practically equal. Time of life of atoms at the level of E₂ is very small, and they pass to the level E₁ spontaneously. Atoms will be accumulated at the level E₁, as a result the population inversion is created between levels E₁ and E₀ (fig. 2,d).

Fill a space between plates M1, M2 of Fabry-Perot interferometer by an active environment LM (fig. 3). This system forms an active optical resonator R. One of mirrors M1 has maximally possible coefficient of reflection, and the second mirror M2 is semitransparent. In an active environment at successive reflections from mirrors a light flux is increased. The light in certain proportion go out through a mirror M2, forming the radiations, that are called the laser radiations.

The losses of energy on mirrors are taken into account by the coefficients of reflection of ρ₁ and ρ₂. For one cycle the two reflections of light are observed, and accordingly, the decrease of flux is proportional to ρ₁ρ₂. For one cycle the light passes in an active environment a way equal to 2L. Therefore strengthening of flux for a cycle is proportional to exp(α2L), where α – value of amplification factor for a cycle. The complete strengthening of density of flux of energy for one cycle is described by a formula S = S₀ρ₁ρ₂e^2αL or S = S₀e^2αL-2f, where S₀ - density of flux of energy at the beginning of cycle 2f = - ln(ρ₁ρ₂).

The condition of threshold of generation can be described as α₀L = f, where α₀ - amplification factor in absence of light flux.

The condition of stationary generation can be described as αL = f, where α - amplification factor in presence of light flux.

The losses of energy during the period of oscillations are characterized by quality factor – ratio of the energy W, accumulated in the system, to the losses of energy for one oscillation ΔW: Q = W/ΔW = 2L//(fλ) = m/f, where v – velocity of radiations in an environment, m = 2L/λ – number of standing waves in a resonator; σ – area of cross section of laser beam; ω – volume density of energy; λ – wave-length of radiations; T – period of laser  radiations.

To achieve the generation, the strengthening on a half of wave-length segment should be equal or exceeds the value of 1/Q. The smaller is the quality factor, the higher is threshold of generation. From other side, the greater are losses, the smaller is quality factor. It means that the radiations of laser are concentrated in the narrow parallel cone of rays; the angle of convergence of rays is limited by diffraction.

Population inversion is achieved by pumping. Depending on the time and nature of action, the pumping can be continuous or pulse-like. If pumping is executed by pulses, the radiations of laser are pulse-like. At continuous pumping the radiations of laser are continuous. But pulsed mode of radiations is also possible in this case. It is possible to get a few pulses of radiation with one pumping impulse. For the increase of power of radiations it is necessary to increase the amount of atoms that take part in amplifying of light flux in a resonator. The other method of increasing of power of laser is the decreasing of duration of pulses.

Written by Vasil Sidorov on August 06, 2010 in queltanews.com

Technopark QUELTA,

Nizhyn Laboratories of Scanning Devices

sidorovvasil@gmail.com

References

1.       Rebrin Y.K., Sidorov V.I. // Optical Deflectors. Kiev: Tékhnika, 1988. 136 pp.

2.       Rebrin Y.K., Sidorov V.I. Optical mechanical and holographic deflectors // Results in science and technology. Radio engineering. Vol. 45. - Moscow: VINITI, 1992. - 252 pp.

3.     Rebrin Y.K., Sidorov V.I. Holographic devices of control of an optical ray. – Kiev:  KHMAES, 1986. - 124 pp.