The laser beam and the optical axis

The laser beam and the optical axis of the measuring system shall be coaxial. Select the diameter (crosssection)of the optical system such that it accommodates the entire cross section of the laser beam andso that clipping or diffraction loss is smaller than 10 % of the intended measurement uncertainty.Arrange an optical axis so that it is coaxial with the laser beam to be measured. Suitable opticalalignment devices are available for this purpose (e.g. aligning lasers or steering mirrors). Mount theattenuators or beam forming optics such that the optical axis runs through the geometrical centres.Care should be exercised to avoid systematic errors.NOTE 1 Reflections, external ambient light, thermal radiation and air currents are all potential sources of errors.After the initial preparation is completed, make an evaluation to determine if the entire laser beamreaches the detector surface. For this determination, apertures of different diameters can be introducedinto the beam path in front of each optical component. Reduce the aperture size until the output signalhas been reduced by 5 %. This aperture should have a diameter at least 20 % smaller than the apertureof the optical component. For divergent beams, the aperture should be placed immediately in front ofthe detector to ensure total beam capture.NOTE 2 Remove these apertures before performing the power (energy) measurements described in Clause 7.