3-D Raman Imaging Using Time-Resolving CMOS SPAD Line Sensor and 2-D Mapping

The capability of Raman imaging to produce 2D and 3D chemical presentations of samples has gained a lot of interest in different application fields. In this paper, we present a 3D chemical image reconstruction based on 2D scanning of a sample utilizing a time-resolved Raman spectrometer based on a CMOS single-photon avalanche diode (SPAD) line sensor. The 2D scanning data contains the lateral information (XY-plane) whereas the time-of-arrival data of the Raman photons measured by the sensor carries 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 ps to 65 ps. In addition to the 3D chemical reconstruction of the scanned sample, we have also 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 post-processing parameters that affect the depth profiling accuracy and the 3D reconstruction operation the most. Results showed that the instrument response function of the system and the time gate window width in a post-processing 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 3D chemical geometric representation at the cm-level during Raman operations.