This bench is used to characterize the thickness and index of refraction of transparent thin films. It can measure films whose thickness is in the range (100 nm – 10 µm) at wavelengths from 400 nm to 1.5 µm.


M-line measurement is based on coupling light to the modes of a planar waveguide by prism coupling. The effective indices of the guided modes are measured by analysing the reflected beam at the output facet of the prism. From the knowledge of the effective indices, the thin film characteristics such as thickness and refractive index can be computed. This principle of operation entails a few restrictions on the films that may be characterized using this method:

– The thin film as well as the underlying substrate must be transparent at the operating wavelength

– The thin film must act as a waveguide (its refractive index must be higher than the substrate’s refractive index. The film must also have a minimum thickness that allows having at least one guided mode.

Technical specifications:

The characteristics of the proposed m-line bench are the following:

– Measurement both in TE and TM polarizations

– Single wavelength measurement but several wavelengths are available, ranging from 400nm to 1.5µm.

– Accuracy on refractive index ~5.10-4 (depending on film characteristics)

– Relative accuracy on film thickness ~1% (depending on film characteristics)

Requirements on the samples:

– Dimensions of the sample should be at least 1cm x 1cm

Case study:

This equipment can be used to characterize the optical and physical properties of transparent thin films, for example to validate a layer deposition process.

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