## 2 Materials and Methods  
### Experimental Setup  
The polarization sensitive setup is related to one of our previous systems [15]. A scheme of the system is shown in Figure 1. The light from a superluminescent diode (cBLMD-D-840-HP-I, Superlumdiodes, Cork, Ireland) is sent to a fiber based non-polarizing beam splitter (FOBS-22P, OZ Optics LTD, Ottawa, Ontario, Canada) consisting of polarization maintaining fibers. Further, 80%of the light is directed to the reference arm that includes a dispersion compensation prism pair, a quarter wave plate oriented at 22.5\({}^{\circ}\) and a mirror. The light is reflected by the mirror and after double passing the quarter wave plate, the light has a linear polarization state that is oriented at 45\({}^{\circ}\) with respect to the incident polarization state. This ensures that equal reference power is provided to both polarization detection channels. Furthermore, 20% of the light is directed to the sample arm where the light exits the fiber and first traverses a variable collimator. With the variable collimator, the focus of the beam can be adjusted to fall within the central depth range of the cornea [12]. Then, the light traverses a quarter wave plate that is oriented at 45\({}^{\circ}\). After passing the wave plate, the light will be in a circular polarization state. The beam is then deflected by two galvanometer scanning mirrors (Thorlabs, Dachau, Germany) and traverses a dichroic mirror and the aspheric condenser lens (Thorlabs ACL5040U-B, Dachau, Germany). The dichroic mirror is used to couple light from the fixation target into the system and to monitor the position of the eye via a camera. A total power of 1 mW is sent to the eye which is well below the corresponding limits for safe exposure given by the laser safety standards [16]. Considering that the light is focused on the cornea, an angular extent of \(-49\times 10^{-3}\) rad on the retina was calculated that results in a safe retinal exposure of up to 19 mW.  
The light that is backscattered from the cornea interferes with the light returning from the reference arm at the non-polarizing fiber beam splitter and is split into the two orthogonal polarization states by the fiber based polarizing beam splitter (FOBS-12P, OZ Optics LTD, Ottawa, Ontario, Canada). The light in each polarization state is then spectrally dispersed and detected by two identical spectrometers (CS800-840/114-250-OC2K, Wasatch Photonics, Morrisville, NC, USA). The dispersion compensation prism pair was adjusted with a mirror at the sample location by maximizing the coherence signal (point spread function). The cameras can be operated at different speeds that resulted in A-scan rates of up to 100 kHz. However, in order to increase the sensitivity in the current study, an imaging speed of 50 kHz was used. By attenuating the beam that is incident on a mirror as the sample, a sensitivity of 97 dB for this A-scan rate was measured. The sensitivity drops to 83 dB at 2/3 of the axial measurement range. The measured axial resolution was 4.1\(\upmu\)m in air. This corresponds to 2.96 \(\upmu\)m in the corneal  
Figure 1: Scheme of the optical setup. SLD (superluminescent diode), NPBS (non-polarizing fiber based beam splitter), VC (variable collimator), QWP (quarter wave plate), GS (galvanometer scanner), DM (dichroic mirror), SL (spherical lens), BS (beam splitter), FT (fixation target), C (collimator), DC (dispersion compensation), M (mirror), PBS (polarizing beam splitter), SPEC (spectrometer), FG (frame grabber). All fibers are polarization maintaining.  
tissue (assuming a refractive index of 1.385 [17]) which is \(\sim\)2 times better than our previously reported instrument. The total imaging depth range was 2.9 mm and sufficiently large for imaging the entire cornea because of the used conical scanning pattern.