Beam smoothing

Mask projection supports the economic fabrication of large-area surface structures, thus facilitating its application in industrial environment. However, mask projection sets stringent requirements on the spatial homogeneity of the applied illuminating beam. Unfortunately, an inherent property of short pulse lasers is their poor beam homogeneity, caused by the high degree of coherence of these sources. At the same time, this coherent property prevents the use of well known beam smoothing methods, usually applied for standard laser sources. Therefore, new beam smoothing techniques are required, which are applicable to femtosecond lasers sources.

We introduced two methods for the spatial smoothing of femtosecond laser beams. One is active on each individual laser pulse; the other is employed for multiple pulse machining.

Two photon absorption

Nonlinear absorption can be used for spatial beam smoothing. Placing e.g. a two photon absorber in a laser beam, locations in the beam profile with high intensities (hot spots) will be attenuated more than less intense parts. Propagating the beam through an appropriate two photon absorber, a considerable increase of the spatial homogeneity can be achieved.

Figure 1: Beam smoothing arrangement by combining two photon absorption and variable beam displacement.

Variable beam displacement

Most laser micromachining applications require multiple shot irradiation of the sample surface. If the laser beam profile on the mask is displaced from shot to shot during irradiation, an averaging over the spatial profile occurs in the image plane, i.e. on the sample surface. Thus the smoothness of the accumulating irradiation profile on the sample surface increases with increasing number of shots. Such a beam displacement can be experimentally realized by placing a tilted plano-parallel plate in the beam, and rotating it around an axis along the beam propagation direction.

A combination of the two techniques results in a superior beam smoothness.