In the solid state laser a solid crystalline material is used as the lasing medium, pumped usually by optical radiations. Solid state lasers should not be confused with semiconductor or diode lasers which are also 'solid state' but are almost always electrically pumped (in principle, optical pumping may be possible with some).

Ruby laser. As a working matter of ruby laser the crystal of oxide of aluminum Al₂O₃ (corundum) is used, into which at growing the oxide of chrome Сг₂О₃ was added in a kind of admixture. In a grating of Al₂O₃ - crystal the ion of Cr⁺³ substitutes an ion of Al⁺³. Hereupon two bars of absorption appear: one in green (G) part of spectrum, the other in blue (B) part of spectrum. In a red ruby crystal the concentration of ions Сг³⁺ is equal of 1 %. Near with green and blue bars of absorption there are two narrow energy levels of E₁ and E'₁. In transitions from these levels the light is emitted with a wave-lengths of 694,3 nm and 692,8 nm. The width of lines at a room temperature is equal approximately 0,4 nm. Possibility of the forced transitions for a line of 694,3 nm is more than for 692,8 nm. Therefore it is simpler to work with the radiation with a wave-length of 694,3 nm.

The diagram of energy levels of ruby is shown on fig.1,a. At the irradiation of ruby by the radiations of wide spectrum the blue and green parts are absorbed, and the red part is reflected. For the optical pumping of ruby laser a xenon lamp is used, that gives the splashes of light of high intensity. Pulse of current when passing through the lamp heats a gas to a few thousands of Kelvins. Radiation of pumping lamp is close to the radiation of absolute black body (ABB). As a result, the radiations are absorbed by Сг⁺, which pass onto the certain energy levels. But from these levels the ions quickly pass without emission on the power level of E₁ and E'₁. The levels of E₁ and E'₁ are metastable. Life-span at the level of E₁ is equal of 4,3 ms. In the process of pumping the excited atoms are accumulated  at levels E₁ and E'₁. The atoms form a considerable population inversion in relation to a level E_o.

The crystal of ruby is usually made as a round cylinder by the length L ~ 5 sm and diameter of d ~ 1 sm. The xenon lamp is placed in an elliptic cavity with a well reflecting internal surface. Thanking it the radiation is sent on a ruby. The radiation density is near to radiation density of source of pumping.

Laser on an yttrium-aluminum garnet (YAG). The laser consists of active environment set between two mirrors. Mirrors repeatedly reflect the radiations that go out from an active environment and strengthen them. Active environment of Nd:YAG - laser includes the atoms of neodymium, incorporated into an yttrium-aluminum garnet. On fig.1,b the simplified chart of power levels of Nd:YAG - laser is shown. The YAG - laser represents by itself a four level’s chart, which possess by large advantages, in particular, by low power of excitation. Efficiency of the YAG - laser is by 20 times higher, than that of the ruby laser. The YAG - laser also, as well as the ruby laser, is excited by a gas-discharge lamp. Energy of excitation is absorbed by atoms of Nd that are excited to the metastabe power level of E₂. Radiation with a wave-length of 1064 nm is generated as a result of difference of energies between two levels (E₁ and E₂). At the frequent passing through an active environment the radiation is amplified. This laser radiates also spectral lines of 1220 nm and 1338 nm that are corresponding to other excited levels but their intensity is considerably lower.

Nd:YAG - laser works in the modes of free generation, modulation of quality factor and mode-locking. Work of laser is possible also in the continuous mode.

Free generation mode of Nd:YAG - laser. After excitation of the YAG - crystal by the radiation of pulsed lamp a laser forms the row of the densely packed short single pulses. This mode is known as a mode of free generation. Duration and form of pulses are determined by the parameters of radiations of pumping lamp and by the parameters of optical resonator.

Q-switched Nd:YAG - laser. The laser in this mode radiates considerably shorter pulses (fig.2,a), than in the mode of free generation. Typical duration of pulses is about 5...20 ns. The change of population density is achieved on power level of E₂, filled by electrons by means of optical excitation. Modulation of quality factor is executed by an optical shutter – passive, when certain solutions of dyes become transparent at absorption of intensive radiations of active environment, or active, in the case when an optical element with the help of electric signal is transformed from the opaque state into transparent one. The rapid freeing of power level of E₂ and forming of short (10^-8 s) pulse of radiations of high optical density (100 W/sm²) passes at the opened optical shutter.^.

Mode-locked Nd:YAG - laser. Principle of work of the laser is described in a section Synchronization of modes. The YAG – laser, working in this mode, radiates the row of very short pulses with duration from 7 ns to 20 ns. Every cycle proceeds about 30…80 ns with an interval of 5…10 ns. Duration of series relies on the amount of peaks. The greater is amount of longitudinal modes, the shorter is duration and higher is power of every separate pulse. On fig.2,b four longitudinal modes are represented. As a result the splashes of high intensity with a permanent interval T between them are formed. Amplitude of the splashes is amplified to the maximal value, and then at the end of series falls down. Energy of the splashes, creating one series, thanking small duration of them is by 100 times higher, comparatively with energy of pulse in the mode of modulation of quality factor. 

On the power levels near 70 mJ and sometimes at lower levels of energies there is a clear tendency to self-focusing of radiations of a mode-blocked laser. A similar effect is not observed for Q-switched lasers even at high energies of radiations.

Frequancy doubled Nd: YAG - laser. The first frequency doubled Nd:YAG - laser consisted of the following parts (fig.3,a): gilded elliptic reflector (2); two tungsten quartz halogen lamps (5) with a power of 3000W each; crystal of yttrium-aluminum garnet (3) activated by neodymium; two mirrors (1) with dielectric coverage; crystal of barium-sodium niobate (4) in a cuvette with thermo-stabilization.

Basic wave-length of Nd:YAG - laser is equal of 1064 nm. At establishment of nonlinear crystal between Nd: YAG – crystal and output mirror of resonator the radiations of the second harmonic are achieved as a green ray with a wave-length of 532 nm. The output of radiations is executed through the front mirror of resonator, which has dielectric coverage that maximally reflects the radiations with a wave-length of 1064 nm and maximally transmits the radiations with a wave-length of 532 nm.

Source of excitation that forms the population inversion in Nd:YAG - crystal represents by itself two tungsten filaments lamps bounded in parallel. The voltage that is given on lamps is changed from 120 V to the maximal value. This way the power of laser is regulated. The source of feed works from a network of 220 V at frequency of 50 Hz. In last years the frequency doubled Nd:YAG – laser was considerably modernized. As a frequency doubling element the crystal of Potassium Titanium Oxide Phosphate (KTP) is used, which maintains enormous optical powers (fig.3,b). Potassium Titanium Oxide Phosphate is an efficient nonlinear optical crystal, working in the visible and infrared spectral regions with relatively low cost.

Chart of process of formation of the second and forth harmonics at passing of laser radiations through a KTP-crystal is shown on fig.4. At passing of radiations through a KTP-crystal the doubling of frequency of Nd: YAG – laser occurs and accordingly, half abbreviation of wave-length. During the repeated passing through a KTP-crystal the UV - radiations are formed with a wave-length of 266 nm.

Lines of spectrum of radiations, that are formed by combining of dye laser with Nd:YAG – laser, are shown on fig.5. In this case the frequency of  Nd:YAG – laser is quadrupled and used for excitation of dye laser. Such lasers allow getting the generation in the wide range of ultraviolet light-spectrum.

Actual lasing mediums. The heart of any laser is the actual lasing medium which in the case of a solid state laser, is a solid. Most of these environments consist of a crystalline host doped with the active lasing atoms or ions. Less common is an amorphous (glass) host. Recent developments in ceramic fabrication techniques promise to greatly expand the availability of low cost high quality lasing materials in much larger sizes than possible with crystals. The most common by far to date have been:

-            Ruby - Chromium doped aluminum oxide (Cr:Al₂O₃). Synthetic ruby is chemically similar to gem stone ruby but of higher purity and quality. It is pinkish in appearance with a lasing wavelength is 694.3 nm. The beam is very deep red at the border of IR.

-            Nd:YAG - Neodymium doped Yttrium Aluminum Garnet (Y₃Al₅O₁₂). Synthetic crystal. It is purplish/lavender in appearance and lases at 1,064 nm. This is near-IR but totally invisible and NOT eye-safe. Nd:YAG has been by far the most common solid state laser material - much more so than ruby - partly because its lasing threshold is much lower so more output energy is available for a given input.

-            Nd:YVO₄ (Neodymium-doped Yttrium orthoVanadate) - This crystal, often just called "vanadate", is the material of choice in low to medium power (up to a few watts) diode pumped solid state (DPSS) lasers operating at 1,064 and 1,340 nm (or doubled to 532 and 670 nm) due to its large stimulated emission cross-section, low lasing threshold, and polarized output.

-            Nd:GdVO₄ (Neodymium-doped Gadolinium orthoVanadate) - This is a relatively new material with absorption and emission wavelegnths similar to Nd:YVO₄ but with thermal properties similar to Nd:YAG.

-            Nd:Gd_0.64Y_0.36VO₄ - Even newer material which in addition to the benefits of both Nd:YVO₄ and Nd:GdVO₄, has superior energy (upper level) storage, can be pumped more efficiently in a quasi three level mode at 879 nm (in addition to 808 nm), and may be manufactured by the much simpler floating zone method rather than the Czochralski crystal pulling technique.

-            Nd:Glass - Instead of a crystal, an amorphous glass is used as the substrate. Its use for the NOVA and NIF lasers at Lawrence Livermore National Labs. Its lasing characteristics are similar to Nd:YAG but since glass has a lower thermal conductivity, cooling becomes a problem for high power applications. The main reason Nd:Glass is used for these lasers is that it is the lasing medium that can be manufactured in sizes larger than a foot in diameter (as required to keep the fluence - energy density - within acceptable limits to avoid damage to the optics in these multi-kilojoules pulsed lasers). Not to mention that it is also less expensive - which may or may not matter with these projects. :) So, where thousands of large diameter optics are needed and a dozen shots a day is adequate performance as with the lasers at LLNL, Nd:Glass is the way to go. Someone was recently selling various large Nd:Glass rods and slabs that must have come from one of these sources!

-            Erbium doped - Er:YAG (2,940 nm) and Er:Glass (1,540 nm).

-            Holmium doped - Ho:YAG, Ho:YLF, and Ho:Glass (2,000 to 2,100 nm).

-            Thulium doped - Tm:YAG, Tm:LuAG, Tm,Ho:YLF (2,000 to 2,030 nm).

-            Ytterbium doped - Yb:KGW (1,025 to 1,045 nm).

-            Alexandrite (655 to 815 nm).

-            Ti:Sapphire (840 to 1,100 nm).

The efficiency can vary from well under 1% for flash-lamp and arc lamp pumped lasers to 25% or more for those pumped with laser diode.

A diode pumped Nd:YAG may have a 40% efficiency, and the pump diodes themselves have about a 45% efficiency, resulting in a net 18% of efficiency. At increased pumping powers, thermal issues may cause the efficiency to decrease after a certain point. This decrease is power dependent, as well as resonator and pump assembly design dependent.

Conclusion: quantum-electronic transformations that flow in solid state laser structures are determined by properties of admixtures and spectral characteristics of pumping radiations. Only the certain narrow range of spectrum creates terms for the effective generation of laser radiations. Efficiency of absorption of pumping radiations with a next transformation into the laser radiations can be equal about 50%.

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

Technopark QUELTA,

Nizhyn Laboratories of Scanning Devices

sidorovvasil@gmail.com