# Polarization Sensitive Optical Coherence Tomography: A Review of Technology and Applications
## 1 Introduction  
Optical coherence tomography (OCT) is an imaging modality providing 2D and 3D images with micrometer scale resolution [1; 2; 3]. Often considered an optical analog of ultrasound imaging, OCT detects light backscattered from sample structures. However, since the speed of light is much greater than that of sound, subtle differences in time delays corresponding to optical path lengths from different scatter locations within the sample cannot easily be measured in a direct way. For instance, the time delay corresponding to an optical path length of 10 \(\upmu\)m is only on the order of \(\sim\)30 fs. In order to assess such short delay times, OCT employs the interference of low coherent light [4; 5]. Low-coherent light sources span a broad wavelength range, usually covering several tens of nanometers when used for OCT. Before the light interacts with the sample, it is split into two. One portion is directed onto the sample, while the other portion--the so-called reference beam--travels a defined path length before being recombined and interfered with the light beam scattered by the sample. Now, since low-coherent light consists of a continuum of wavelengths, the interference spectrum will be subject to modulations which depend on the path length difference between the sample beam and the reference beam. From the interference signal in the time or spectral domain, the axial position of scattering structures within the sample can be reconstructed [4; 5]. The respective depth profile (backscatter intensity vs. depth) is called an axial scan (A-scan) and forms the basic unit of OCT images. By scanning the beam laterally across the sample, two- and three-dimensional images can be assembled from the acquired A-scans. OCT is a rapid imaging method providing 3D data comprising up to several millions of axial scans within few seconds [6]. The axial resolution of OCT is usually in the order of a few micrometers and, despite the high imaging speeds, the interferometric approach enables detection sensitivities of reflected light signals as low as \(10^{-10}\) of the input. During the past 25 years since its invention, OCT experienced a multitude of technological advances leading to higher imaging speeds, improved resolution, and novel contrast mechanisms. One so-called functional extension of OCT is polarization sensitive (PS) OCT. PS-OCT adds polarization contrast to the technique. While the standard OCT is based solely on the intensity of light backscattered or reflected by the sample, PS-OCT also detects its polarization state. Since the polarization state can be measured for every pixel in a depth scan, PS-OCT can enhance the image contrast and also enables quantitative measurements of a sample's polarization properties. These properties are often linked to the micro- or even ultrastructure of the sample which themselves are below the optical resolution limit of OCT [7]. Hence, the detection of changes of polarization properties--be they due to disordered microstructure in pathological tissue or due to stress and strain in a technical sample--may provide access to quantities and markers that are of interest for a broad variety of applications. PS-OCT has been applied in many biomedical as well as non-medical fields. Figure 1 shows results of a search for scientific publications on PS-OCT using the free literature search engine PubMed ([[https://www.ncbi.nlm.nih.gov/pubmed/](https://www.ncbi.nlm.nih.gov/pubmed/)]([https://www.ncbi.nlm.nih.gov/pubmed/](https://www.ncbi.nlm.nih.gov/pubmed/))). In Figure 1a, the number of publications per year was plotted beginning with the first article on PS-OCT published in 1997 [8]. A rising trend of published documents per year can be observed, peaking with more than 30 publications per year for the last three years. The set of 360 publications was classified into seven medical fields and one non-medical field. The latter included articles primarily focusing on technological aspects or on measurements of technical (i.e., non-biological) samples.