## 1 Introduction  
The interpretation of polarization changes at dental hard tissue with polarized light microscopy (PLM) represents a milestone in caries research that has provided significant contributions to the understanding of the histopathological nature of the caries process [1]. In 1958, it was shown that demineralization of enamel is accompanied by distinct changes of polarization properties [2]. Polarization-sensitive optical coherence tomography (PSOCT) obtains depth-resolved images of polarization properties noninvasively with high resolution (typically 5-20 \(\upmu\)m) and, as it is based on near-infrared light, without the hazards of ionizing radiation. In 2000, [PERSON] et al. presented the first PSOCT measurements of dental structures [3] and characterized the birefringence of the enamel, which is due to the anisotropy of hydroxyapatite crystals, organized in rodlike structures.  
The depolarizing effect of initial enamel lesions observed in PSOCT measurements was reported by [PERSON] et al. [4] and has been documented for natural and artificial lesions [5,6]. In the context of dental PSOCT applications, depolarization has usually been detected by evaluating co- and cross-polarization intensities, and previous work demonstrated the potential of the cross-polarization signal for quantifying enamel demineralization and remineralization [7]. In particular, calculating the integrated cross-polarization reflectivity of near-surface signals showed significant results [8,9]. However, both native enamel birefringence and demineralization-induced depolarization contribute to an increased cross-polarization signal [5], which can be especially detrimental to the assessment of hidden caries, i.e., sound birefringent enamel with underlying depolarizing caries. In recent studies, we concluded that the representation of the degree of polarization (DOP) as a PSOCT-based measure of depolarization seems to be the most appropriate representation for the detection of early carious lesions in comparison with other measures such as co- and cross-polarization intensities, total reflectivity, retardation, and optic axis orientation [10,11]. Furthermore, we improved this approach by noise-immune processing, as introduced by [PERSON] et al. [12], and demonstrated that the DOP also supports the assessment of suspect occlusal enamel lesions [13]. Hence, the feasibility of DOP-based caries detection has been demonstrated, but not yet its potential in the distinction of stages.  
Most in vitro studies for caries research involve extracted teeth, representing very heterogeneous specimens with differences in tooth morphology, caries progression stage, age, and dietary-dependent alterations that cause varying optical properties. The application of a controlled, standardized demineralization model at comparable specimens allows a more coherent investigation of consecutive stages of the caries process and related changes of optical properties [14]. As an appropriate dental substrate for these models, bovine enamel specimens with artificial carious lesions show a mineral distribution and structure that resembles lesions at human teeth, and that can be considered as a comparable alternative to human teeth in caries research [6,15,16].  
The presented study aims at assessing the stage of enamel demineralization by PSOCT-based DOP measurements. Although studies on cross-polarization OCT indicate a correlation due to the presumably same source of contrast, i.e., depolarization, the correlation of demineralization stages and the DOP has not yet been presented. Therefore, we analyze the DOP at bovine enamel specimens in an artificial demineralization model with consecutive stages of lesion progression and compare those findings with the lesion depth measured at thin cross sections with PLM.