Fig. 1. (a) Schematic diagram of the modular inverse spatially offset Raman spectroscopy (SORS); (b) Schematic diagram of the principle of forming a ring-shaped laser beam of the adjustable radius with the axicon

Fig. 2. (a) Pictures of sheep scapula with 3 and 4 mm thickness. The mixed Raman spectra obtained from the two-layer biological tissue model of sheep scapula thickness d=3 mm (b) and (c) d=4 mm under different spatial offsets. The Raman spectra of sheep scapula and paracetamol were measured separately and listed at the top and bottom for reference; (d) The relative Raman intensity ratio corresponding to paracetamol (1 326 cm^-1) and sheep scapula (960 cm^-1) as a function of the offset value

Fig. 3. (a) Schematic diagram of three-layer model; (b) The Raman spectra of the pig skin, silicone rubber, and paracetamol are shown for reference;(c) A set of SORS spectra measured with the three-layer model of silicone rubber of 3 mm (c) and 6 mm (d) under different spatial offset values

Fig. 4. (a) The relative Raman intensity ratio corresponding to silicone rubber (490 cm^-1) and pig skin (1 450 cm^-1) as a function of the offset value; (b) The relative Raman intensity ratio corresponding to paracetamol (1 326 cm^-1) and pig skin (1 450 cm^-1) as a function of the offset value. The thickness of silicone rubber is 3 and 6 mm