# Polarization Sensitive Optical Coherence Tomography: A Review of Technology and Applications
## 2 Principles of Light Polarization and PS-OCT
### Brief Basics of OCT  
OCT is based on low coherence interferometry, i.e., the interference of broad band light [1; 4]. A multitude of different interferometer designs have been used for OCT. A sketch of a basic Michelson interferometer is shown in Figure 4a. Here, light from a low coherent light source such as a superluminescent diode or a broadband laser is split into one beam that is directed on the sample and another beam that serves as a reference. After the beam splitter, the beam in the sample arm is directed onto the sample, whereas the reference beam is reflected by a mirror. Light backscattered and reflected by the sample \(e_{S}\) and light reflected by the reference mirror \(e_{R}\) is recombined at the beam splitter. The sample and reference beam interfere and their interference signal is detected at the interferometer exit. The interference signal in the time domain can be described by  
\[I(z)=I_{R}+I_{S}+2\sqrt{I_{R}I_{S}}|\gamma(z-z_{0})|\cos[2k_{0}(z-z_{0})]. \tag{4}\]  
Here, \(I_{R}\sim\left|e_{R}\right|^{2}\) is proportional to the intensity of the reference beam and \(I_{S}\sim\left|e_{S}\right|^{2}\) is proportional to the intensity of light backscattered or reflected by the sample. The third term contains the interference information. Its first component \(\sqrt{I_{R}I_{S}}\) indicates that the strength of the interference signal will scale with both the sample and the reference amplitude. The high sensitivity of OCT is based on the fact that even weak light scatter signals \(I_{S}\) from the sample can be amplified by a strong  
Figure 4: Basic schemes for PS-OCT. (**a**) Basic scheme of OCT with a Michelson interferometer. (**b**) PS-OCT with a single circular input state; (**c**) PS-OCT based on probing with several input states; (**d**) PS-OCT based on a Mach–Zehnder interferometer with two input states generated by a polarization delay unit. See text for descriptions of different approaches. The grey arrows indicate the directions of light beams. Black pictograms indicate linear, circular, and elliptical polarization states. LS—light source, POL—polarizer, QWP—quarter wave plate, RM—reference mirror, L—lens, SAM—sample, PBS—polarizing beam splitter, DU—detection unit, POM—polarization modulator, PS1/PS2—polarization state 1/2, BS—beam splitter, \(\Delta z\)—path delay between polarization states, PDU—polarization delay unit. reference signal \(I_{R}\). The second component \(|\gamma(z-z_{0})|\) contains the complex degree of coherence \(\gamma(z)\) which is inversely related to the spectral bandwidth of the light source (via the Fourier transform of the spectral density). For broad bandwidth sources common in OCT, \(|\gamma(z)|\) will only be greater than zero for a shallow depth around every scattering sample interface along the propagation direction \(z\). Hence, the high resolution of OCT is contained in this term. The third component \(\cos[2k_{0}(z-z_{0})]\) describes a sinusoidal modulation of the interference signal along \(z\). Here \(k_{0}=2\pi/\lambda_{0}\) is the central wavenumber of the spectrum. In order to acquire depth scans in time domain OCT, the signal intensity \(I(z)\) is recorded at the interferometer exit while the reference mirror is axially translated in beam direction \(z\). Most modern OCT systems rely on frequency (or Fourier) domain detection of the interference signal [5,31]. In such Fourier domain OCT systems, the reference mirror position is fixed and the interference spectrum is acquired. This can be achieved by using a spectrometer at the interferometer exit which disperses the interference signal into its spectral intensity components. Alternatively, a broadband wavelength-swept light source can be used to rapidly tune the spectrum with a narrow instantaneous bandwidth. In this case, the interference spectrum is recorded as a function of time by a detector at the interferometer exit.