Ruby laser

The ruby ​​laser in 1960 by Theodore Maiman at all developed the first laser. He is one of the solid-state lasers ( solid state laser ). The main emission line is at 694.3 nm

Construction

The active medium consists of ruby (chrome aluminum oxide), that is, from a single-crystal Al2O3 host crystal (sapphire / corundum ) doped with chromium ions. Since the optimum doping level is around 0.03-0.05 %, only specially manufactured rubies come into question (for natural rubies the chromium content is higher). The rubies are produced in rod shape and the ends very smooth polished. The roughness has to be less than half the laser wavelength, which due to the extraordinary hardness of rubies is a major problem.

Previously, the mirrors were formed directly on the crystal by vapor deposition of a silver layer on the ends of the rod. Today, the ends are usually provided with an anti -reflective coating, and the mirrors are mounted externally.

The crystal is optically pumped by means of xenon flash lamps as the light is absorbed particularly well. In very rare cases, continuous pumping takes place, for example by means of mercury vapor lamps. In the first set-ups a helical flash lamp was used was at the center of the ruby rod. In modern lasers, however, the light of one or more rod-shaped lamps also by an elliptical mirror is focused directly onto the laser crystal for a more effective excitation usually.

Operation

The ruby ​​laser is a three- level laser. Through optical pumping a majority of the electrons of the chromium ions in one of the energy bands is 4F1 or 4F2 raised. A small part of it immediately relaxes back by spontaneous fluorescence emission to the ground state 4A2.

The vast majority, however, goes into the metastable laser level 2E in a radiationless transition. There they remain relatively long. So it may come to the population inversion by staying more electrons at the level 2E than in the ground state 4A2. From the 2E level, the electrons relax by itself very slowly (in the range of about 3 milliseconds) with spontaneous emission of photons of wavelength 694.3 nm These few photons can now, however, give rise to stimulated emission in the excited medium, whereby the coherent laser radiation is generated and pass the excited electrons to the ground state.

Since the laser level 2E is a doublet level, the red laser line is actually a double line.

While most laser can be operated in both the pulse and in cw mode ( continuous wave), the ruby ​​laser is found almost exclusively in the pulsed version because its efficiency is by far the highest here. To operate it continuously, you have to keep the performance is very low because the rubies can heat up very strongly by the pump and the heat can be poorly dissipate.

Application

The ruby ​​laser has now largely lost its importance because its efficiency is comparatively low and the wavelength is accessible by other lasers.

In dermatology it is not used due to the high pulse energy and good absorption of the laser wavelength of melanin in the treatment of pigmented spots and for the removal of tattoos.

Literature and links

  • Sam 's Laser FAQ (English)
  • How a Laser Works ( English)
  • Laser beam source
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