Physical principles that lie in the basis of work of lasers can be used for creation of the high-effective photovoltaic modules of a new generation. In next articles the achievements in technologies of creation of laser environments and generation of laser radiations will be analyzed, and also possibility of the use of laser structures in the systems of the solar power engineering is considered.

Signs of classification of lasers. Lasers can be classified in accordance with different signs: mode of operations; type of active matter; method of pumping and cooling; functional purpose and spectral composition of radiation.

In accordance with the mode of operations the lasers are divided into the devices of continuous (quasi-continuous) and pulsed radiations.

In obedience with the type of active matter (environments) the lasers can be distinguished into the devices with solid state, gas and liquid environments.

According to the method of pumping the lasers can be classified onto the devices with optical, chemical, electronic, gas-discharge, x-ray and plasma pumping.

The generation of continuous radiation is possible for all active environments, if in their matter the condition of creation of permanent population inversion is executed. For the continuous mode the gases (gas mixtures) and semiconductor diode structures are the more promising. The factors limiting the use of lasers of continuous radiation are followings:

-  low power efficiency of transformation of energy of excitation into the laser radiation;

-  heating of laser environment;

-  degradation of components.

The pulsed mode of operations is possible both in solid state environment and in gaseous environment.

Lasers on the neutral atoms of gas mixture work in a visible range, in near infrared part and in middle infrared part of optical spectrum, from 600 nm to 3400 nm. The main spectral lines of radiation of gas environments are represented in tabl.1. The more known is a helium-neon (He-Ne) laser pumped by an electric current. Power of radiation that is attained with He-Ne – laser hesitates in scopes from a few milliwatts to 100 mW. Laser possess by high monochromatic and stable radiation when working in the one mode cycle. It is widely used in holography, systems of navigation and aiming, devices of scientific experiment, in particular, in those are used for measuring of distances. The radiation of laser is used also as a pilot ray in the infra-red optical-electronic systems, for adjusting of the optical-mechanical and optical-electronic systems.

Ionic gas lasers work in ultraviolet and visible ranges of light-spectrum, from 235 nm to 800 nm. The typical spectral lines of radiation of gas environments are represented in table 2. With the different gas mixtures it is possible to get more than 50 spectral lines of radiation in ultraviolet and visible ranges of light-spectrum. The most known samples are the krypton and argon lasers. An argon laser is widely used in holography, spectroscopy, lidar, systems of laser graphic and medicine.

Molecular gas lasers radiate in infra-red part of light-spectrum from 5500 nm to more than 78000 nm. The typical spectral lines of gas environments are shown in table 1. A carbon dioxide (CO₂) laser is the most powerful within all lasers of continuous radiation. In infra-red part of spectrum on a wave-length of 10600 nm about 10 kW power was attained. Unlike other molecular lasers the carbon dioxide laser is chemically proof at an electric gas discharge. A laser on carbon dioxide is widely used in technological processes and in lidar. The lasers on the steam of water (H₂O) and on the cyanide of hydrogen are also found the wide use.

To the pulsed lasers, the molecular and ionic gas structures and also solid state systems belong. The main spectral lines of radiationd of these lasers are shown in table 1.

Table 1.

Typical spectral lines of radiation of lasers

Active environment: wave-length, nm

  Gas on neutral atoms          helium: 593,9; 632,8; 1152,3; 3391,3

oxygen: 844,6; 2026,2; 2651,1

           Gas ionic                                 neon (IV): 235,8

argon (IV):  262,5

helium-cadmium: 325,0; 441,6

helium-selenium: 460,0; 480,0; 520,0

neon (II): 332,4

krypton (II): 467,2; 530,8; 568,2; 647,1

argon (II): 476,5; 488,0; 514,5

xenon (II): 526,2; 597,1

Gas molecular                      CO: 5600

CO2: 10600

H2O: 27974;  47693; 118650; 220340

HeN: 310887; 336558

IeN: 773500

Pulsed gas and solid state   nitrogen (molecular): 337,1; 540,1

copper (steam): 510,0; 578,0

frequency doubled  Nd: YAG: 532,0

mercury (ion): 619,9

gold (steam): 628,0

chrome (ruby): 694,3

chrome (alexandrite): 701,0...818,0

chrome (emerald): 750,0...759,0

neodium (YAG): 1064,0

neodium (glass): 1060,0

Lasers on ionic crystals or crystalline lasers have certain advantages comparatively with other lasers. Life-span of metastable power levels of ions of admixtures in crystals in the excited state is large (10^-3 s) in comparison with gas lasers (10^-6 s) and semiconductor injection lasers (10^-11 s). Therefore crystalline lasers can be used as accumulators of energy and to get pulses of extraordinary large power (to 10¹⁰ W). The active ions of such crystalline lasers are strongly diluted by the matter of basic crystal; therefore a crystal does not need the intensive cooling, as in semiconductor lasers.

The modes of operations of crystalline lasers can be stationary and unstationary. To the stationary modes operations the mode of continuous generation belongs, and also pulsed mode, at which duration of pulses of pumping does exceeds sufficiently the duration of process of generation. To unstationary mode the mode of monopulse belongs that appears by bringing of optical shutter into the resonator, which changes the quality factor of the resonator. 

Information about the active environments and operating temperature conditions of solid state lasers is presented in table 2. A temperature strongly affects the spectrum of generation. Relocation bias of spectrum of generation of ruby laser is equal of (1,2…1,5) nm at the change of temperatures in the interval of (30…350)K, and for a laser on CaF₂: Dy²⁺ - 0,2 nm at temperature of (20…80)K. For narrowing of spectrum of generation of laser the spectral selectors are used, but power of generation goes down in the case. In lasers with optical shutters a spectrum width is equal to (0,03…0,07) nm. By using the passive optical shutters it is possible to narrow the spectrum of generation to 0,002 nm (without selector) and to 10^-4 nm – at presence of selector.

Crystalline lasers become excited by the method of pumping. The spectrum of absorption by an active environment allows using only small part of energy emitted by large spectrum sources. As a result the coefficient of transformation of power of pumping into output power goes down. Except for the losses of energy of pumping sources it is necessary to take into account also other types of losses. For example, with the use of elliptic or other types of reflector it is possible to concentrate on a crystal only 50% of power emitted by the pumping lamp.

Maximal power of laser, working in the continuous mode is determined by it coefficient of transformation and also by energy density of pumping lamp. Maximal power got for Nd: YAG - laser pumped by the voltaic arc is equal of 200 W.

Table 2.

Parameters of active environments of crystalline lasers

Basis – admixtures:     Wave-lengths, μm

A1203- 0,05% Cr3+       0,6943; 0,6929

A1203- 0, 05% Cr3+      0,7009; 0,7041; 0,7670

MgF2- 1% Ni2+              1,7500; 1,8030

ZnF2 - 1% Co2+             2, 6113

CaW04 - 1% Nd3+         0,9145; 1,3392

CaF2 - 1% Nd3+             1,0460

Y3Al5012 - Nd3+            1,0648

LaF3 - 1% Nd3+             1,0633

LaF-3 - 1% Pr3+             0,5985

CaW04- 0,5% Pr3+        1, 0468

Y203- 5% Eu3+              0, 6113

CaF2 - Ho3+                    0,5512

CaW04- 0,5% Ho3+       2,0460

Y3Al5012 - Ho3+            2,0975

CaW04- 1% Er3+           1,6120

Ca(Nb03) _2- Er3+            1,6100

Y3A15012 - Er3+           1,6602

CaW04 - Tu3+                1,9110

Y3Al5O12 - Tu3+           2,0132

CaF2- 0,05% U3+          2, 6130

Y3Al5O12 - Yb3+           1,0296

SrF2 - U3+                      2,4070

CaF2- 0,01% Sm2+        0,7083

CaF2- 0,01% Dy2+        2,3588

CaF2- 0,01% Tu2+         1,1160

One of the more often used source of radiation is a laser on the crystal of yttrium-aluminum garnet, activated by neodymium (Nd: YAG) and excited by tungsten lamps. Middle power of his radiations is about 100 W on a wave-length of 1064 nm. Frequency of reiteration of pulses is more than 20000 Hz on fundamental and doubled modes of radiation. Pulsed Nd: YAG – lasers work in the modes of free generation, Q-switching and mode–locking (synchronization of modes). They generate short pulses with duration of 10…30 ns. Such properties of solid state pulsed lasers, as forming of holes by a focused beam, the origins of shock acoustic and hydrodynamic wave do him by the mean of photodestruction. 

On fig.1 the positions of spectral lines of laser radiation of emerald (750,0…759,0 nm), alexandrite (701,0...818,0 nm), steam of gold (628,0 nm), steam of copper (578,0 nm), doubled frequency Nd:YAG (532,0 nm), nitrogen (337,0 nm) in visible part of electromagnetic spectrum are shown.

The high level of frequencies of reiteration of pulses (100 Hz) is got for a ruby laser. Their pumping is achieved by the lamps on the steam of mercury, located in an elliptic reflector. A pulsed lamp and ruby crystal often have a total axis. For a wave-length of 694,3 nm ruby laser in the quasi-continuous mode provides power equal more than 2…3 W.

A pulsed molecular nitric laser radiates energy in the mode of quasi-continuous series of pulses, each has duration of 10 ns and pick power of 100 kW. Middle power (a few kilowatts) is achieved in the UV- range on a wave-length of 337,1 nm.

The last achievements in technologies of creation of laser environments and generation of laser radiation allowed getting artificial laser sources, possessing by those important properties of radiations as high monochromaticity, coherence, polarization and high orientation in space. On the base of lasers the unique laser systems are created that are used in air and marine navigation, sounding of atmosphere and remote sensing of earthly surface from space apparatus, in the systems of formation of plasma for initiation of processes of thermonuclear synthesis, in the technological processes of cutting and welding of materials, systems of measuring of distances, spectroscopy, communication networks, computer technique and medicine. Laser rays are also the unique instrument of research of microcosm.

It is necessary to confess, a laser ray today in most cases is used as a high-cost optical instrument, executing the functions of electric or gas heater, mechanical drill, the mechanical or gas saw for cutting of materials. Efficiency of the use is illusive, and the necessity – doubtful. Therefore a search of effective ways of the use of possibilities of laser technologies proceeds.

 The increases of attention to the solar systems of energy in a recent year in connection with reduction of organic fuels and with the threat of global rise in a temperature witnessed imperfection of existent photo-electric sun technologies of receipt of electric current and impossibility to compete effectively on a cost with other technologies for providing by a heat and electric energy of necessities of population and industry. And only on occasion, for example, on the space satellites the use of the sun photo-electric modules is economically justified.

In a recent year the publications of results of researches of possibilities of spatial and spectral correction of radiation appeared. Part from them is directed onto creation of luminescent sun concentrators, other part is devoted to creation of absorbing and reemitting structures, which are intended for expansion of effective spectrum of sun radiations which are transformed later into an electric current. These selective fundamental and applied researches give birth to the ideas of combination of these approaches and to use of principles of generation of laser radiations for construction of principally new high effective coherent concentrating photovoltaic solar modules. As active environments they can use solid state structures, liquid solutions and gas mixtures.

Written by Vasil Sidorov on August 14, 2010 in queltanews.com

Technopark QUELTA,

Nizhyn Laboratories of Scanning Devices

sidorovvasil@gmail.com