# A very special light

The word LASER is the acronym for Light Amplification by Stimulated Emission of Radiation.

A laser is a very coherent light source. The high degree of spatial coherence allows the beam to be highly collimated and also very good focusable.

Further more the spectral longitudinal coherence is very high. All photons do have very similar energies and are therefore monochromatic.

These properties make laser light very suitable to form the standing light wave where the molecules are diffracted.

Laserlight can be highly polarized. In the KDTLI we use polarization optics to adjust the light power reaching the molecules and to separate the departing and returning laser beams.

Extra: Polarisation optics

Many lasers have a cross sectional intensity profile that can be described by a Gaussian function. In the interferometer we need a beam focussed to only $$20 \times 1000 \, \mathrm{\mu m}^2$$. Accordingly, we use a cylindrical lens to obtain a homogeneous field distribution over the whole molecular beam.

Extra: Gaussian beams