# Polarization Sensitive Optical Coherence Tomography: A Review of Technology and Applications
## 2 Principles of Light Polarization and PS-OCT
### Recent Advances in PS-OCT Technology  
The development of PS-OCT has greatly advanced since [PERSON] and coworkers first presented birefringence-sensitive ranging [13]. Not only have PS-OCT devices become faster and the detection schemes become more sophisticated, as briefly described in the previous section, but also the analysis of PS-OCT images has improved a lot. The first PS-OCT prototypes provided axial scan rates on the order of several hertz [8; 13; 14]. Later rapid reference scanning schemes [79; 80] and advanced beam scanning approaches such as transverse-scanning PS-OCT [40] sped up the technique to several frames (B-scans) per second. The advent of Fourier domain OCT (or: frequency domain OCT), which computes A-scan signals by a Fourier transform of the interference spectrum, provided a huge increase in detection sensitivity and the possibility to scan even faster since no more mechanical reference mirror movement was required to perform depth scanning [5; 81; 82; 83]. First high-speed PS-OCT systems with spectrometer-based detection provided scan rates of several tens of A-scans per second [41; 68]. These spectral domain (SD) PS-OCT prototypes employed two spectrometer cameras, one for each orthogonal polarization channel. In order to reduce system complexity, cost, and alignment efforts, SD PS-OCT approaches based on single camera detection were developed [42; 49; 84; 85; 86; 87; 88; 89]. Fourier domain OCT can also be performed by using a frequency-swept laser and a high-speed detector, such that interference spectra are acquired as a function of time rather than in parallel with a spectrometer [90; 91; 92]. This variant of OCT is usually called swept-source (SS) OCT and sometimes also referred to as optical frequency domain imaging (OFDI) or time-encoded frequency domain OCT.  
Providing the same sensitivity and speed advantages as spectrometer-based Fourier domain OCT, SS-OCT was soon expanded by polarization sensitivity [47; 71; 75; 93; 94; 95; 96; 97]. In particular the advent of commercial laser technology providing longer imaging ranges led to the development of PS-OCT at ultrahigh imaging speeds of 100,000 axial scans per second [18; 54; 73; 74; 98]. Even higher imaging speeds were achieved by experimental swept lasers operating at several hundred kilohertz [50; 99; 100; 101; 102]. While PS-OCT technology has greatly advanced, there are still some limitations to the technique. Being an optical method, its applicability is limited to imaging of superficial locations in tissues and other objects. Further, PS-OCT has been used for qualitative imaging mostly; the exploitation of quantitative measurements however bears great potential for diagnostics and other applications, as will be demonstrated in the next sections. PS-OCT was also combined with other functional OCT extensions such as Doppler OCT or OCT angiography [30; 68; 103; 104; 105; 106]. Such combinations may not only improve the contrast for vascular tissue components but also provide additional, complementary insight into disease patterns [30; 103; 105; 106]. In order to perform PS-OCT beneath the body surface, endoscopic and needle-based PS-OCT was developed [28; 29; 107; 108; 109; 110; 111; 112]. To further increase the contrast and image range, PS-OCT has been combined with other technologies such as ultrasound and fluorescence imaging [113; 114]. In parallel to the impressive evolution of PS-OCT hardware, PS-OCT image processing also underwent massive improvements. Real-time display of PS-OCT data was enabled by parallel computing [115]. Computational methods were devised for removing polarization artifacts in order to produce clearer PS-OCT images [21; 27; 28; 29; 57; 116; 117]. As PS-OCT is an interferometric technique based on coherent light, images are subject to speckling which sometimes obscures structural details. The size of speckles can be kept small by using broadband light sources and optics providing high transverse resolution [46; 118].