Today’s semiconductor lasers radiate in a red and near infrared (IR) ranges of light-spectrum and have high energy effectiveness about 50%. The high levels of output power are attained, which allow replacing traditional lasers in a row of applications. The sizes of radiative area of diode laser are in a submillimetric range at output power in the tens and hundreds Watts. The decline of price on semiconductor lasers was instrumental in appearance of the inexpensive laser systems.

Spectral range of radiations of powerful GaAlAs – lasers is today limited by length-waves of 750…950 nm. The diode lasers on other geterostructures can generate the radiations in all visible range and in the IR - range of spectrum far to 30 µm. New diode lasers allow effectively converting electric energy into light energy. In future they can replace the splash lamps that are used for the optical pumping of solid state (Nd:YAG, Ho:YAG and Er:YAG) lasers and to make them compact and energy effective devices. In table 1 the spectral lines of radiations of semiconductor structures are presented.

Table 1.

Spectral lines of radiations of semiconductor lasers

Laser environment         Wave-length, nm

Zinc sulfide                                   330

Cadmium sulfide                           490

Zinc telluride                                 540

Gallium arsenide                           850

Indium arsenide                           3100

Indium antimonide                      5400

Construction of injection laser. As the active matter of laser the crystal of GaAs n-type (1) and p-type (3) is used. It has grey color and cubic form (fig.1).

        In a gallium arsenide laser the p-n - transition is formed by diffusion of acceptor admixtures (Zn, Cd, Mn, Na and Cu) into material doped by donor admixtures (Te and Se). The semiconductor of n - type is alloyed by tellurium to a concentration of (1...2)х10¹⁸ sm^-3. The depth of disposing of transition is equal to (2...100) μm. This transition lies approximately in the middle between edges, to which an electric voltage is applied.

        A crystal of GaAs is soldered to the molybdenum plate (5) that is covered by the layer of gold. On the surface of p - region the alloy of gold with silver is deposited. The applying of current and withdrawing of heat is provided through a clamp (4 and 5).

Two lateral edges of crystal are parallel between itself and form the mirrors of optical resonator (6 and 11). The length of resonator is about (0,01...2) µm. Due to the large index of refraction (n = 2,4...3,6) on the GaAs-air boundary a considerable coefficient of reflection is provided (20…40)%. The polished edges, creating an optical resonator, form the positive feed-back, necessary for the generation of radiations. If to decrease the coefficient of reflection by means of deposition of dielectric coverage on the surface of edges, a crystal will work as an active element or amplifier of the induced radiations.

Pumping of diode is executed by direct or alternating current. Duration of electrical waves is changed from a few microseconds to a few tenths of microseconds. Frequency of reiteration of pulses can be equal to hundreds of kilohertz. At the strength of current of 100 А and temperature of 770 К the sharp increase of intensity of radiations appears. At T = 4,2 К such increase flows at i = 6 A. Density of current is accordingly equal to 10000 A/sm² at T = 77 K and 600 A/sm² at T = 4,2 K.

Divergence of beam of radiations in vertical and horizontal planes is accordingly equal about 6° and 1°. These values are near to the diffraction limit for the thickness of p-n - transition about 20 μm, and for the width about 0,1 mm. The actual size of active region in vertical direction is equal approximately 1 µm, and the effective width of radiative region is equal near 10 µm. At the change of temperature from 4,2 K to 1250 K the value of threshold flux is multiplied by 25 times. At a room temperature the density of threshold current is equal of 10⁵  A/sm². The value of density of threshold current relies on the degree of alloying of semiconductor.

The maximal value of pulsed power is determined by the optical radiation firmness of radiative zone of semiconductor. Critical density of power of radiations is equal to (2...4) 10⁶ W/sm². At the temperature of crystal of 300 К the output power is equal approximately 60 W; frequency of generation - 1 kHz; duration of pulse - 100...200 ns. Frequency of generation of induced radiations can be increased to 100 kHz.

Electronic and optical pumping. A laser with electronic excitation is other type of semiconductor laser. Electronic beam with energy in tens of kilowatts, and on occasion in hundreds of kilowatts fall onto a semiconductor plate athwart to the plane. The coherent radiations go out through edge, athwart to a direction of motion of electrons of excitant electronic beam. At interaction of electronic beam with a semiconductor in the last the electron-hole pairs appear. The electrons of basic beam excite the electrons of valency zone and transfer them into zone of conductivity. The excited electrons get energy that exceeds the width of the forbidden zone. During the collisions with the atoms of crystalline grate, they, in the turn, transfer the electrons from a valency zone into zone of conductivity. Thus, a process is developed avalanche-like. Electrons enter into the crystal on a depth, that almost in one hundred times exceeds the thickness of irradiative layer in lasers on p-n – transition. That creates possibility to receive radiations of very high power. The necessity of providing of the deep cooling of the crystal is the lack of this type of laser.

What is especially important, the population inversion can be got at excitation of semiconductor with the beam of photons (by optical pumping). For this purpose the luminescent crystals are used. Under action of photons, which energy exceeds the width of the forbidden zone, in a semiconductor the transition of electrons is achieved from a valency zone into zone of conductivity with formation of electron-hole pairs. Rationally to execute pumping in the narrow interval of frequencies, when energy of quantum only slightly exceeds the width of the forbidden zone. In this case the inversion of electrons and holes appears mainly between levels, imbedded into upper part of valency zone and into bottom of zone of conductivity. Ordinary sources are radiated in the wide range of light-spectrum, therefore often as a source of pumping the lasers are used, for example, solid state ruby laser or semiconductor laser with other wave-length (fig.2). Penetration of light into a semiconductor is achieved on a depth that exceeds the depth of penetration of electronic beam. This gives possibility to receive large powers; simultaneously the effectiveness of laser structure grows to 50%. 

Perspectives of the use of laser semiconductor technologies in the elements of solar power engineering are explained by the wide spectral range of wave-lengths of radiations that includes visible and near infrared regions of the spectrum (to 3 μm) and also by high power of radiations. Sun photo-electric modules that are used today in the solar power engineering may be substituted for the photovoltaic semiconductor laser structures, if to take into account their high reliability, simplicity of service, compactness and low price. Pulsed lasers also can be used in the future for reduction of cost of the sun modules.

From other side, laser diodes are well combined with solid state nonlinear elements. It allows changing the bar of absorption and achieving resonance absorption of light. In addition to the increase of efficiency of transformation, it is possible also to regulate the spectrums of absorptions and radiations of concentrating and photovoltaic elements (fig.2).

Reduction of sizes of laser can be attained also by the increase of quantum efficiency of active matter. For this purpose it is needed to multiply concentration of ions of Nd+++ in a crystal without worsening of his optical and mechanical properties.  Research of different materials showed perspective of some phosphate crystals. In a neodymium-lanthanum pentaphosphate (Nd_xLa_1-xP₅O₁₄ at x » 0,5) and in lithium-neodymium tetraphosphate (LiNdP₄O₁₂)  the concentration of active  centers (Nd++) can be by 30 times higher, than in Nd:YAG, where it is near 1%. In the phosphate lasers which are sometimes called lasers with a 100 percent excitation of active ions, the border of generation can be equal the tenth of miliwatts. Length of active region is diminished to hundreds of microns, and its transversal crossing – to tens of microns. At the correct choice of spectrum of radiations of pumping element the high (more than 50 %) use of optical energy is achieved, and general effectiveness is near 20%. Width of spectral line Dl remains the same as in Nd: YAG - lasers.

The application of optical radiations with a wave-length of 3 μm necessitates of the use of fluorine-zirconium wave-guides, transparent in this range of light-spectrum.

Written by Vasil Sidorov on August 17, 2010 in queltanews.com

Technopark QUELTA,

Nizhyn Laboratories of Scanning Devices

sidorovvasil@gmail.com