Pseudo-Random Single Photon Counting for Time-Resolved Optical Measurements

Abstract

In time-resolved optical instrumentations, an ultra-short light pulse is used to illuminate the subject of interest, while the time-dependent transmittance, reflectance and fluorescence in response to the illumination are measured. By taking advantage of the high information content of the time-dependent measurements, researchers can uncover the structure and dynamics of the sample under investigation. Currently, time-correlated single photon counting (TCSPC) is the most widely used method for acquiring the temporal profile of the response to an ultra-short light pulse. Despite its various striking benefits, TCSPC has a problem of limited photon count rate which usually results in low data acquisition speed. In addition, a typical TCSPC system would require a pulsed laser, which is of high cost and renders the system bulky. In this thesis, a new time-resolved optical measurement method termed as pseudo-random single photon counting (PRSPC) was developed to provide a valuable alternative approach of conducting time-resolved optical measurements. The new method combines the spread spectrum time-resolved (SSTR) method with single photon counting. A pseudo-random bit sequence is used to modulate a continuous wave laser diode, while single photon counting is used to build up the optical signal in response to the modulated excitation. Periodic cross-correlation is performed to obtain the temporal profile of the subject of interest. Compared with conventional TCSPC, PRSPC enjoys many advantages such as low cost and high count rate without compromising much on the sensitivity and time-resolution. The PRSPC system reported in this thesis can reach a temporal resolution of 130 ps and a photon count rate as high as 3 Mcps (counts per second). In addition, considering that the PRSPC system uses a continuous wave laser instead of a pulsed laser, it has high potential to be easily integrated into a portable device. To explore the potential application of the PRSPC method, the PRSPC system was integrated into a time-resolved diffuse optical imaging (DOI) experimental system for phantom studies. The lipofundin phantom based experiments demonstrated that the PRSPC system is capable of fast acquisition of temporal profile of diffuse photons and has high potential in time-domain DOI systems. The PRSPC system was also integrated into a time-resolved spectroscopic system for human blood glucose testing studies. By analyzing the temporal profiles of the photons diffused through human finger, the connection between the human blood glucose level and the blood transparency in the near infrared was explored. The preliminary results from this study may shed light on the topic of non-invasive human blood sugar monitoring and further the development of truly non-invasive blood sugar testing devices.