Development of a large depth of field collection optics for on-line LIBS application

It is common knowledge that LIBS dedicated optical systems usually operate over a quite broad spectral region, typically ranging from 200 nm to 1100 nm. It should also be noticed that in this spectral region, especially at the shortest wavelength, common lenses suffer from great chromatic aberration. Achromatic lenses do exist, but they are expensive and with a limited aperture: for these reasons, the use of mirror based optical systems has become a quite common practice. A typical configuration is the one made of two ellipsoidal mirrors that conjugate the plasma emitting region with the spectrometer slit, which is virtually immune from any kind of aberration. Although suitable for tabletop applications, due to its limited depth of field this system starts failing in application where the distance between the sample and the collection optics may vary from shot to shot, such as in measurements performed on raw minerals transported by conveyor belts. This problem has been tackled in the past by using optical systems with variable focus or by means of mechanical systems that keep the optics at a fixed distance from the sample. To overcome the difficulties connected to the use of feedback based instrumentation, we investigated the possibility of developing an innovative LIBS setup, with a large depth of field, where moving parts are not required. To evaluate the collection efficiency of different mirror based configurations we developed a specific ray tracing software. Main boundary conditions imposed to the optical system during simulations were the specifications of the collecting optical fiber (600μm core diameter, 0.22NA) and the distance between the first mirror and the sample (about 1m). According to these conditions, we found that the use of two spherical mirrors with the same curvature radius provide a collection optics free from spherical aberrations. However, due to the off axis configuration of the mirrors, a slight astigmatism still remains. Although this aberration could be compensated by using customized mirrors, an astigmatic system still provides a greater depth of field without a substantial decrease in the collection efficiency. Tests carried out with an experimental setup show that the collection efficiency of the innovative optical system is in good agreement with the simulations.