Time-Resolved Raman Spectrometer With High Fluorescence Rejection Based on a CMOS SPAD Line Sensor and a 573-nm Pulsed Laser

Abstract

A time-resolved Raman spectrometer is demonstrated based on a 256×8 CMOS SPAD line sensor and a 573 nm fiber-coupled diamond Raman laser delivering pulses with duration below 100 ps FWHM. The collected back scattered light from the sample is dispersed on the line sensor using a custom volume holographic grating having 1800 lines/mm. Efficient fluorescence rejection in the Raman measurements is achieved due to a combination of time gating on sub-100 ps-time scale and a 573 nm excitation wavelength. To demonstrate the performance of the spectrometer, fluorescent oil samples were measured. For organic sesame seed oil having a continuous wave mode fluorescence-to-Raman ratio of 10.5 and a fluorescence lifetime of 2.7 ns, a signal-to-distortion value of 76.2 was achieved. For roasted sesame seed oil having a continuous wave mode fluorescence-to-Raman ratio of 82 and a fluorescence lifetime of 2.2 ns, a signal-to-distortion value of 28.2 was achieved. In both cases, the fluorescence-to-Raman ratio was reduced by a factor of 24-25 owing to time gating. For organic oil, spectral distortion was dominated by dark counts while for the more fluorescent roasted oil, the main source of spectral distortion was timing skew of the sensor. With the presented post-processing techniques, the level of distortion could be reduced by 88-89 % for both samples. Compared to common 532 nm excitation, approximately 73 % lower fluorescence-to-Raman ratio was observed for 573 nm excitation when analyzing the organic sesame seed oil. Index Terms-Fluorescence rejection, Raman laser, Raman spectrometer, Raman spectroscopy, SPAD sensor, time-correlated single photon counting, time gating, timing skew I. INTRODUCTION AMAN spectroscopy is used in a wide range of fields including food and oil industries, mining industry, medical diagnostics, pharmacy, forensic science and archaeometry [1]

Time-correlated single photon Raman spectroscopy at megahertz repetition rates

“…Pulsed lasers used in combination with time-resolving photon detection systems can enhance Raman signal discrimination in the presence of fluorescence compared with continuous-wave (CW) techniques [1]. In recent years, time-gated [2][3][4] and time binning [5] single photon avalanche diode (SPAD) detectors have been used for time-correlated single photon counting (TCSPC) Raman spectroscopy (see the recent review by Kogler et al [6].) Time-gating SPADs depend on activating photon detection over a time window spanning the excitation laser pulse, before most of the fluorescence signal arrives.…”

“…The laser repetition rate in TCSPC sets an upper saturation limit on the Raman count rate since the number of signal photons per laser period in a given spectral channel is at most one or usually less. It is advantageous therefore to make the laser repetition rate as high as possible to maximize the detection rate [4]. Significant improvements in time-correlated single photon Raman spectroscopy acquisition times can be achieved through exploitation of high peak power and MHz laser repetition rates.…”

“…In Fig. 6(a), we present time-resolved spectra of sesame oil [4] (fluorescence). The time origin is set to the center of the first time bin, the bin width is set to 800 ps, and on-chip delay is used to position the signal peak at 2.4 ns.…”

Tracking Molecular Diffusion across Biomaterials’ Interfaces Using Stimulated Raman Scattering

“…To address these problems, methods of generating or collecting Raman spectra have been developed in which these undesirable scattering features are eliminated from the measured signals. Among these, time-resolved Raman spectroscopy and stimulated or coherent techniques such as the pump-probe-based stimulated Raman scattering (SRS) spectroscopy are most notable …”

High-Resolution Time-to-Digital Converters (Tdcs) with a Bidirectional Encoder

“…Time-to-digital converters (TDCs) are high-precision time sensors converting a time interval (TI) into a digital code. They are widely used in time-resolved applications, such as particle physics [1][2][3], positron emission tomography (PET) [4,5], random number generation [6,7], Raman spectroscopy [8,9], and light detection and ranging (LiDAR) [10,11].…”