# On Fabry P\'erot Etalon based Instruments. II. The Anisotropic (Birefringent) Case

## TL;DR

This paper analyzes the birefringent effects in uniaxial Fabry-Pérot etalons used in magnetographs, providing analytical models and evaluating their impact on polarization measurements in different optical configurations.

## Contribution

It presents analytical expressions for retardance and Mueller matrices of uniaxial etalons, and assesses their polarimetric behavior in collimated and telecentric setups.

## Key findings

- Retardance increases with oblique incidence and optical axis misalignment.
- Birefringence can produce artificial polarization signals.
- Polarimetric response depends on incident angle and optical configuration.

## Abstract

Crystalline etalons present several advantages with respect to other types of filtergraphs when employed in magnetographs. Specially that they can be tuned by only applying electric fields. However, anisotropic crystalline etalons can also introduce undesired birefringent effects that corrupt the polarization of the incoming light. In particular, uniaxial Fabry-P\'erots, such as LiNbO3 etalons, are birefringent when illuminated with an oblique beam. The farther the incidence from the normal, the larger the induced retardance between the two orthogonal polarization states. The application of high-voltages, as well as fabrication defects, can also change the direction of the optical axis of the crystal, introducing birefringence even at normal illumination. Here we obtain analytical expressions for the induced retardance and for the Mueller matrix of uniaxial etalons located in both collimated and telecentric configurations. We also evaluate the polarimetric behavior of Z-cut crystalline etalons with the incident angle, with the orientation of the optical axis, and with the f-number of the incident beam for the telecentric case. We study artificial signals produced in the output Stokes vector in the two configurations. Last, we discuss the polarimetric dependence of the imaging response of the etalon for both collimated and telecentric setups.