General description

The goal of this experiment is to provide an insight into the operation of a solid-state laser that is excited by a diode laser and doubles the frequency of the laser light within the resonator due to its material properties. A diode laser made of GaAlAs is used for the optical excitation of the crystal. A diode laser is a semiconductor element in which the laser transition between the valence and conduction band takes place.

The population inversion required for laser operation is achieved by a current flow via the pn junction. The mirrors of the laser resonator are formed by the ends of the semiconductor element itself, as good reflectivity is achieved due to the high refractive index. By attaching narrow contact strips to the semiconductor, the current flow is limited to the area bounded by this strip. Lasers with a single strip generate outputs in the range of a few milliwatts. Higher output powers can be achieved with diode laser arrays or broad-angle lasers (BAL). Due to the phase coupling between the stripes, a diode laser array does not emit a Gaussian beam, but a complex interference pattern of the individual emitters. In broad-strip lasers, higher transverse modes are usually excited.

Therefore, the beam of a diode laser array or BAL is not spatially coherent and has poor imaging properties. In addition, the relatively large spectral width of 2 nm for a laser leads to a low spectral power density. These are the main reasons for not using a diode laser array in direct applications, but instead using it to excite a solid-state laser.

Compared to other pump sources, however, the spectral width is sufficiently narrow for selective absorption in the crystal. By varying the temperature, the wavelength of the diode laser can also be tuned to a suitable absorption band of the laser-active material. The entire power of the pump radiation is thus almost completely absorbed by the laser-active ions of the crystal. This significantly reduces the thermal load on the crystal compared to excitation with noble gas lamps.

The combination of a diode laser (BAL) as the pump source and a neodymium YAB solid-state laser achieves high efficiencies for visible emission. The areas of application range from ophthalmology or distance measurements to material processing, whereby precision work in the sub-mm range is particularly noteworthy.

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