The m-line spectroscopy allows a reliable measurement of thickness and refractive index of films as well as to pinpoint the light propagation modes into the guide. This technique is mainly based on the prism coupling technique (figure 1 and figure 2). A conventional Metricon mod. 2010 apparatus shown in figure 3 is used. The excitation sources (two He-Ne lasers with 632.8 and 543.5 nm lines plus an external tunable argon laser with emission at 457, 477, 488, 496 and 514 nm) permit to measure the refractive index at several wavelength. For the optical characterization in the NIR region a second Metricon apparatus with the 632.8, 1330 and 1540 nm lines and with losses measurement option is used (figure 4). Both TE and TM modes can be measured inserting a polarisation rotator (l/2) on the beams.
Figure 1: M-line scheme
Figure 2: Detail of the m-line prism, sample holder
and rotation turret