LaserEllips™: Polarization Measurement of Phase Retardation Application Note

Most surfaces of optical components display isotropic polarizability. However, the non-normal incidence may break the symmetry, resulting in polarization-dependent reflections and absorptions. Special care is necessary when a beam transport system must deliver a well-defined polarization. As an example, consider the several 90 degrees reflections occurring in the beam guide of a laser cutting robot. To preserve the desired circular polarization, special designs of zero-phase reflectors must be employed. The verification of desired phase tolerance makes the development of polarization-measuring devices necessary.

Operation Principle

The overall design concept is shown in Figure 1. It consists of a laser source illuminating the sample under test, a rotating polarizer, and a detector.

The beam reflected from the sample is transmitted through a polarizer. The measured power ratio r_o = P_min/P_max and the power maximum occurring at a polarizer orientation a_o are recorded using the software. The phase retardance is related to the power ratio and polarizer orientation by the equations below in Figure 2. The phase retardance can be calculated from the measurement data using least square optimization.

The key specifications of laser ellipsometry are listed below. To the best of our knowledge, there have yet to be any commercial products in the market fulfilling similar laser optics measurement purposes:

We illustrate the determination of polarization angles using the following measurement data. The periodic trace measures a linearly polarized laser beam since the power goes down to zero. For example, when the time of the maximum power signal overlaps with the occurrence of the encoder signal peak in the bottom box, the orientation of the laser polarization electric field is known to coincide with the polarimeters’ orientation.

Applications

Within the visible regime, laser ellipsometry is commonly used for measurements of thickness and refractive index. The development of a phase retardance measurement system resulted in compact, accurate polarization instrumentation for near-infrared and mid-infrared laser beams. Applications with high-power industrial beams include the testing and adjustment of either the ideal linearity or circularity of the polarization. Further applications include component testing and in general, infrared ellipsometry.

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