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State of the art optical systems become more complex. There are more lenses required in the optical design
and optical coatings have more layers. These complex designs are prone to induce more thermal stress into the optical
system which causes birefringence. In addition, there is a certain degree of freedom required to meet optical specifications
during the assembly process. The mechanical fixation of these degrees of freedom can also lead to mechanical stress in
the optical system and therefore to birefringence. To be able to distinguish those two types of stress a method to image
the birefringence in the optical system is required. In the proposed setup light is polarized by a circular polarization filter
and then is transmitted through a rotatable linear retarder and the tested optical system. The light then is reflected on the
same path by a mirror. After the light passes the circular polarization filter on the way back, the intensity is recorded.
When the rotatable retarder is rotated, the recorded intensity is modulated depending on the birefringence of the tested
optical system. This modulation can be analyzed in Fourier domain and the linear retardance angle between the slow and
the fast axis as well as the angle of the fast axis can be calculated. The retardance distribution over the pupil of the optical
system then can be analyzed using Zernike decomposition. From the Zernike decomposition, the origin of the birefringence
can be identified. Since it is required to quantify small amounts of retardance well below 10nm, the birefringence of the
measurement system must be characterized before the measurement and considered in the calculation of the resulting
birefringence. Temperature change of the measurement system still can produce measurement artifacts in the calculated
result, which must also be compensated for.
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