# Polarization Sensitive Optical Coherence Tomography: A Review of Technology and Applications
## 2 Principles of Light Polarization and PS-OCT
### Polarization Effects
\begin{array}{cc}p_{1}&0\\ 0&p_{2}\end{array} \right]\] (2) where \(p_{1}\) and \(p_{2}\) correspond to the respective signal attenuation \(p_{1,2}=\exp\left(-\mu_{a_{12}}L\right)\) with attenuation coefficients \(\mu_{a}\) and the length \(L\). Similar to the linear birefringence, the Jones matrix for a diattenuating element oriented at \(\vartheta\) can be computed by sandwiching Equation (2) by rotation matrices. An extreme case of a diattenuating element with \(p_{1}=1\) and \(p_{2}=0\) is a linear polarizer which only transmits light along axis 1. It should be noted that diattenuation in biological tissue is usually very weak and thus is often assumed negligible for PS-OCT [16; 17], although attempts have been made to quantify diattenuation using PS-OCT [15; 17; 18]. * Depolarization. Depolarization or polarization scrambling refers to a more or less random change of the incident polarization state at spatially adjacent sample locations. Using Stokes-Muller polarimetry, depolarization can be described by the DOP discussed in Section 2.1. However, owing to the coherent detection in OCT, DOP will always equal unity in any pixel of a PS-OCT dataset [9]. Therefore, in order to analyze polarization scrambling using PS-OCT, the randomization of polarization states among neighboring speckles is investigated [19; 20]. The Stokes vectors of adjacent speckles will be more or less parallel in polarization preserving or weakly birefringent media, while they will point in different directions in depolarizing media (Figure 3a). For the purpose of depolarization assessment, the average Stokes vector can be calculated within a small kernel including several speckles (Figure 3b). Typical kernel sizes for this calculation are on the order of \(\sim\)100 pixels spanning 2-3 times the axial resolution in depth and 2-3 times the transverse resolution laterally [20]. Note also that pixels with low reflectivity (i.e., less than several decibels above the noise floor) are usually excluded from the analysis. The length of the average normalized Stokes vector is referred to as the degree of polarization uniformity (DOPU) [20]: \[DOPU=\sqrt{\overline{Q}^{2}+\overline{U}^{2}+\overline{V}^{2}}/\overline{I}\] (3) where the overbar indicates the ensemble average and \(\overline{I}\) denotes the average Stokes vector length for normalization. As shown in Figure 3b, DOPU or the average Stokes vector length will be close to unity in polarization preserving tissue, while it will be lower in the case of polarization scrambling where the orientation of the Stokes vectors is more diverse. In DOPU images, DOPU values at every spatial coordinate are color-coded. Recently, advanced depolarization measureshave been developed including DOPU with noise floor normalization [21], spectral DOPU [22], the depolarization index independent of the incident polarization state [23], and the differential depolarization index providing a larger dynamic range for depolarization mapping [24]. Different mechanisms can cause polarization scrambling, and the actual cause of depolarization observed in DOPU images has not always been completely clarified. In biological tissues, depolarization can be caused by multiple scattering or by scattering at non-spherical particles such as melanin granules [25]. Since the strength of depolarization is proportional to the concentration of polarization scrambling scatterers, depolarization measures such as DOPU may for instance enable a quantitative assessment of the melanin concentration in ocular tissues [26]. In general, polarization effects may be subject to dispersion, that is, their strength depends on wavelength. Since OCT is based on broadband light covering a wide range of wavelengths, efforts have been made to mitigate effects such as polarization mode dispersion in PS-OCT systems based on optical fibers [18; 27; 28; 29; 30].