3-D Raman Imaging Using Time-Resolving CMOS SPAD Line Sensor and 2-D Mapping
The capability of Raman imaging to produce 2-D and 3-D chemical presentations of samples has gained a lot of interest in different application fields. In this article, we present a 3-D chemical image reconstruction based on 2-D scanning of a sample utilizing a time-resolved Raman spectrometer based on a complementary metal–oxide–semiconductor (CMOS) single-photon avalanche diode (SPAD) line sensor. The 2-D scanning data contain the lateral information (XY plane), whereas the time-of-arrival data of the Raman photons measured by the sensor carry the axial information (i.e., depth information, Z -axis). The sensor is fabricated in 110-nm CMOS technology. It has 256-spectral channels, and each channel has its own 7-bit ON-chip time-to-digital converter (TDC) with an adjustable resolution from 25 to 65 ps. In addition to the 3-D chemical reconstruction of the scanned sample, we have shown the ability to retrieve depth profiling information of each scanned pixel, such as the boundaries and middle points of any selected layer over the depth range of the scanned object by means of a single measurement for each scanned pixel. In addition, we have discussed the system components and the postprocessing parameters that affect the depth profiling accuracy and the 3-D reconstruction operation the most. Results showed that the instrument response function (IRF) of the system and the time gate window width in a postprocessing phase are playing the most important role in determining the axial (depth) accuracy. We believe that our system will enable a whole new class of Raman applications that will allow simultaneous 3-D chemical geometric representation at the centimeter level during Raman operations.