SD-OCT Base Unit
Chapter 3: Description
Rev E, November 7, 2018
Page 19
Polarization-Sensitive Optical Coherence Tomography
A standard OCT system is able to measure a depth scan of the sample using the phase and intensity of the
backscattered light from different positions in depth. Nevertheless, it does not account for polarization effects or
birefringence in the sample which might be interesting if the sample shows different birefringent properties
locally.
SD-PS-OCT is able to capture and visualize this property. The abbreviation SD-PS-OCT stands for spectral-
domain polarization-sensitive optical coherence tomography. This SD-OCT system uses a known and controlled
polarization state of the incident light which is changed by the birefringent properties of the sample. Maintaining
the polarization information from to sample to the spectrometer, the polarization state can be deconvolved into
the two polarization axes: s (‘senkrecht’, German for vertical) and p (‘parallel’). The s and p polarization
components are detected in two separate sensor units.
The controlled polarization state of the light is achieved by the broadband light source being linearly polarized
and guiding the light using polarization maintaining fibers. The magnitude of coherent interference between the
reference and the sample beam depends on the similarity between the polarization states of both beams. To
achieve a good sensitivity for both polarization axes, the polarization state of the reflected beam needs to be
rotated by 45° when interfering with the sample beam. To implement this, a quarter-wave plate (QWP) is used
in the reference arm. The polarization axis is rotated by 22.5° to the incident linearly polarized light which results
in elliptically polarized light. Being reflected at the retro reflector the phase of the polarized light is shifted by π.
Thus, traversing back through the QWP with an angle of 22.5° generates linearly polarized light which is rotated
by 45° to the incident linearly polarized light. This change in polarization is independent of the sample and only
affects the reference beam.
Figure 18 Schematic Diagram of an SD-PS-OCT System
In order to be sensitive to all polarization effects introduced by the sample and to be independent of the sample
orientation in plane, the sample is probed with circularly polarized light. To obtain circularly polarized light,
another QWP is placed in the sample arm which is turned by 45° to the incident, linearly polarized light. In case
the sample is a mirror, the circularity stays unchanged in the reflected light and after traversing through the
QWP a second time, linearly polarized light is obtained which is rotated by 90° compared to the incident, linearly
polarized light. If the sample changes the polarization state of the incident light, the backscattered light is
changed, e.g. to elliptically polarized light. After traversing through the QWP a second time, this ellipticity is
changed again but its composition stays the same so that the intensity measured in each sensor, for s- and for
p-polarized components, reflects the polarization and birefringence properties of the sample.
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