Fluorescence suppression in Raman spectroscopy using a time-gated CMOS SPAD

Abstract

A Raman spectrometer technique is described that aims at suppressing the fluorescence background typical of Raman spectra. The sample is excited with a high power (65W), short (300ps) laser pulse and the time position of each of the Raman scattered photons with respect to the excitation is measured with a CMOS SPAD detector and an accurate time-to-digital converter at each spectral point. It is shown by means of measurements performed on an olive oil sample that the fluorescence background can be greatly suppressed if the sample response is recorded only for photons coinciding with the laser pulse. A further correction in the residual fluorescence baseline can be achieved using the measured fluorescence tails at each of the spectral points.

Raman spectroscopy: techniques and applications in the life sciences

“…This is achieved using a pulsed laser synchronized with time-gated detection, usually by means of an intensity-activated Kerr gate [174]. Emerging fast detectors also allow time-gated detection [175]. Time-gated Raman spectroscopy has been used to detect Raman spectra from various samples including bone [176] and breast tissues [177].…”

Fluorescence lifetime imaging (FLIM): Basic concepts and some recent developments

“…Line sensors allow high fill-factor by allowing pulse processing electronics to be placed below the detectors. Advanced realizations of these line sensors are beginning to emerge for time-resolved Raman spectroscopy (Blacksberg et al, 2011;Kostamovaara et al, 2013;Maruyama et al, 2014;Nissinen et al, 2011).…”

A 4 × 128 SPAD array with a 78-ps 512-channel TDC for time-gated pulsed Raman spectroscopy

“…Some portion of the fluorescence photons will always be generated at the same time as the Raman photons, and thus the fluorescence cannot be totally eliminated just by counting the photons during the laser pulse. This residual fluorescence can be minimized, however, by measuring the level of fluorescence background after the laser pulse and then subtracting a scaled version of this from the total result [8]. In order to do that, a 3-bit TDC has been used here to expand the dynamic range for the measurement of the time position of the recorded fluorescence photons, so that the fluorescence level can be estimated from the photons collected within the last four bins, for example.…”

“…Nowadays, SPADs can also be manufactured using CMOS technologies, and thus the time gating can be realized more easily than in the structures presented in the above references. Sub-ns time gating was achieved with the structures presented in [7,8], and these structures were used effectively in time-gated Raman spectroscopy. CMOS technologies also enable one to construct 2-D detector arrays, which opens up a possibility for manufacturing time-gated Raman spectroscopy devices for use in out-oflaboratory applications.…”

“…This 3-bit flash TDC covers a time measurement range of approximately 3.5 ns but has an expanding resolution, allowing high-resolution (78 ps) measurement of the times of arrival of the Raman photons at the beginning of the range (during the laser pulse), and more relaxed resolution in the measurement of fluorescence photons at the end of the range (after the laser pulse). This makes it possible to estimate the residual fluorescence level of the measurement result simultaneously and to deduct this residual level from the Raman result [8]. This structure optimizes the time resolution needed to within a certain range and reduces the complexity of the circuit, as explained in detail below.…”