Multiphoton microscopy (MPM) is a nonlinear high-resolution technique that can be applied to collagen studies such as collagen type I denaturation during caries process. The MPM lies in its simultaneous image creation based on the fluorescence signals produced by a multiphoton excitation (2-photon excited fluorescence [2PEF]) and a coherent nonlinear phenomenon, second harmonic generation (SHG). This study aims to investigate the structural modifications during the dentin caries process by monitoring the variations of the SHG and the SHG/2PEF ratio recorded by nonlinear optical spectroscopy (NLOS). The International Caries Detection and Assessment System (ICDAS) was used as a clinical reference in vitro. To this end, teeth for each ICDAS score were selected and longitudinal sections through the caries lesion were performed. SHG and 2PEF images and spectra were recorded for each ICDAS score (0 to 6). 2PEF and SHG images highlighted the microarchitecture and optical properties of the collagen denaturation in dentin during the caries process. The SHG images of carious dentin demonstrated a destruction of the collagen matrix. The variation of intensity of the 2PEF, SHG and the ratio SHG/2PEF allowed the merge of ICDAS scores: ICDAS 0/1; ICDAS 2/3; ICDAS 4/5/6. The new groups tend to be similar to the International Caries Classification and Management System (ICCMS). The 2PEF and SHG intensities and the ratio SHG/2PEF are highly altered during the caries process. The ratio SHG/2PEF is a relevant parameter to monitor the caries evolution and it defined three stages: initial lesion (ICDAS 0/1), moderate lesion (ICDAS 2/3) and extensive lesion (ICDAS 4/5/6). The development of a clinical device based on the nonlinear optics will bring a valuable diagnostic tool for caries detection in an everyday practice.