Mueller matrix maps of dichroic filters reveal polarization aberrations

TL;DR

This study measures the polarization aberrations of a dichroic filter using Mueller matrix imaging, revealing how polarization properties vary with incidence angle and wavelength, which is crucial for high-precision optical systems.

Contribution

It provides the first detailed polarization characterization of a specific dichroic filter at different angles, informing optical system design for exoplanet imaging.

Findings

At normal incidence, no polarization aberrations were observed.

At 45° incidence, the dichroic acts as a λ/4 waveplate at 630 nm.

Measured retardance was 2.9 radians with 0.95 diattenuation.

Abstract

Dichroic filters are used by instrument designers to split a field of view into different optical paths for simultaneous measurement of different spectral bands. Quantifying the polarization aberrations of a dichroic is relevant for predicting the incident polarization states downstream, which could affect the performance of diffraction limited systems. One important application is the fore-optics of exoplanet imaging coronagraphs. In this work, the polarization properties of the Edmund #69-205 650 nm roll-off dichroic are measured using a rotating retarder Mueller matrix imaging polarimeter. The polarization properties of this commercial dichroic are compared at normal and 45$^{\circ}$ angle of incidence. The normal incidence measurements verify the instrument calibration since no polarization aberrations were observed. Transmission measurements at 680 nm and 45$^{\circ}$ yield a 2.9 rad magnitude of retardance and 0.95 diattenuation. Effectively, at 630 nm the dichroic is a $\lambda$/4 waveplate with a horizontal fast-axis.