# Polarization Sensitive Optical Coherence Tomography: A Review of Technology and Applications
## 2 Principles of Light Polarization and PS-OCT
### Technical Approaches to PS-OCT
#### 2.4.1 PS-OCT with a Single Circular Input State  
PS-OCT with a single circular input state relies on a polarization sensitive low coherence interferometer design devised by [PERSON] et al. in 1992 [13]. The basic scheme using a Michelson interferometer is shown in Figure 4b. Light from a low coherent light source is linearly polarized before being split up into a reference arm (top) and a sample arm (right). In the sample arm, the beam passes a QWP oriented at 45\({}^{\circ}\) which renders the original linear polarization into a circular polarization state and then illuminates the sample. Sample illumination by circular light offers sensitivity to any transverse orientation of birefringent media. If linearly polarized light were used for sample illumination, the sample's fast or slow birefringent axis could align with the interrogating linear polarization such that no birefringence would be observed. In the case of circular sample illumination shown in Figure 4b, a birefringent sample will in general produce an elliptical state. The reflected or backscattered light will transmit the QWP again and interfere with the reference beam at the beam splitter. Due to double passing a QWP oriented at 22.5\({}^{\circ}\) in the reference arm, the reference beam is a linearly polarized light beam oscillating at 45\({}^{\circ}\) which provides equal intensity components in the horizontal and vertical orientation, respectively. At the interferometer exit, the OCT light beam is split up into its horizontal (H) and vertical (V) component, which are detected by separate detection units. By the respective amplitudes \(A_{H,V}\) and the relative phase difference \(\Delta\Phi\), Jones vectors are detected for every image pixel. These Jones vectors enable the calculation of the sample's birefringent properties, namely of phase retardation \(\delta\)[13; 39] and fast birefringent axis orientation \(\vartheta\)[14] as well as sample reflectivity \(R\):  
\[R\propto A_{H}^{2}+A_{V}^{2} \tag{6}\]  
\[\delta=arctan\bigg{(}\frac{A_{V}}{A_{H}}\bigg{)} \tag{7}\]  
\[\vartheta=\frac{\pi-\Delta\Phi}{2} \tag{8}\]  
Since \(\delta=\Delta n\cdot L\) accumulates as a function of the light path \(L\) travelled in a birefringent material, phase retardation measurements are cumulative. However, the measurement of \(\delta\) is restricted to 0-90\({}^{\circ}\) due to the arctangent, which leads to cumulative retardation images with a banded structure caused by increasing and artificially decreasing \(\delta\) in strongly birefringent samples (cf. Figures 9 and 10). From the detected amplitudes \(A_{H,V}\) and the relative phase difference \(\Delta\Phi\), the Stokes vector elements can also be calculated for every image pixel [19]. These may then serve as the input for depolarization images, for instance based on DOPU [20].  
The beauty of the above scheme lies in its simplicity. Most implementations were done using free-space optics [14; 40; 41; 42; 43; 44], however fiber optic prototypes have also been reported based on polarization maintaining (PM) fiber optics [45; 46; 47; 48; 49; 50; 51; 52] and regular single mode fibers [53; 54; 55].