SPAD-based time-of-flight technologies for near-infrared spectroscopy

Abstract

Near-infrared spectroscopy (NIRS) is a technique that applies light sources of multiple wavelengths within the object for non-invasive property detection. The current NIRS techniques include Continuous-Wave NIRS (CW-NIRS), Frequency-Domain NIRS (FD-NIRS), and Time-Domain NIRS (TD-NIRS). Both CW-NIRS and FD-NIRS apply constant or intensitymodulated photon-emitting light sources and photodiodes as sensors to detect the power of the light from the target in a certain time. Even though FD-NIRS uses a frequency sinusoidal function (MHz) for the modulation of the light source, which brings in more information than CW-NIRS, such as average intensity (DC), the amplitude of intensity (AC), and phase shift between the detected and emitted light, the sensitive nature of the frequency modulation and detection techniques employed makes the measurements susceptible to motion artefacts and environmental noise. These external factors can disrupt the phase and amplitude of the modulated signal, resulting in distortions that impact the accuracy of the reconstruction process and require careful consideration and mitigation strategies in the measurements.

TD-NIRS applies lasers as light sources and single-photon sensors for light detection, which not only inherits the merits of CW-NIRS on the intensity measurement, but also provides up to picosecond time resolution for the average photon path length calculation and the chance to extract the absorption and scattering coefficient of the target for absolute haemoglobin concentration qualification based on the nature of the histograms. The research on TD-NIRS has been over 20 years, however, there are only few instruments in the market due to the complexity of the system and the high price, which hinders its application for clinical practices and daily care, and blocks the feedback from the actual usage. In recent years, high-resolution and high-frequency laser drivers and sources, and single-photon detectors have become more accessible and cheaper, which contributes to further research and development of TD-NIRS systems.

This thesis targets to reduce the complexity and size of the TD-NIRS system while keeping the quality of spatial resolution, time resolution, and frame acquisition rate, utilising laser diodes, a Single Photon Avalanche Diode (SPAD) sensor, and a Field Programmable Gate Array (FPGA) based processing circuits. Different from previous publications, the Time-to-Digital Converter (TDC) is moved out of the SPAD and implemented in an FPGA for cost-saving and system flexibility. A novel method is applied to the TDC by encoding the states of the delay lines instead of the thermometer code used in the conventional TDCs to improve the linearity. The achieved raw Differential Non-Linearity (DNL) which is the DNL without compensation and calibration, together with the zero empty bins, to our knowledge, exceeds previously reported of the FPGA-based TDCs. The TDC is applied to the TD-NIRS system, with the achieved system Impulse Response Function (IRF) of 165 ps at the maximum repetition rate of 20 Mhz. The concept of different lasers with a sync phase shift of 2 ns to generate multi-IRFs in one histogram frame was successfully proved, providing a high-speed and real-time solution for achieving high spatial resolution.