Abstract
A $16\times256$ element single-photon avalanche diode array with a 256-channel, 3-bit on-chip time-to-digital converter (TDC) has been developed for fluorescence-suppressed Raman spectroscopy. The circuit is fabricated in $0.35~\mu \text{m}$ high-voltage CMOS technology and it allows a measurement rate of 400 kframe/s. In order to be able to separate the Raman and fluorescence photons even in the presence of the unavoidable timing skew of the timing signals of the TDC, the time-of-arrival of every detected photon is recorded with high time resolution at each spectral point with respect to the emitted short and intensive laser pulse (~150 ps). The dynamic range of the TDC is set so that no Raman photon is lost due to the timing skew, and thus the complete time history of the detected photons is available at each spectral point. The resolution of the TDC was designed to be adjustable from 50 ps to 100 ps. The error caused by the timing skew and the residual variation in the resolution of the TDC along the spectral points is mitigated utilizing a calibration measurement from reference sample with known smooth fluorescence spectrum. As a proof of concept, the Raman spectrum of sesame seed oil, having a high fluorescence-to-Raman ratio and a short fluorescence lifetime of 1.9 ns, was successfully recorded.
Highlights
M EASURING the chemical environment and molecular composition of a medium is important in many applications, e.g. in the agricultural, food, oil and pharmaceuticals industries [1]–[4]
The time-gated 16 × 256 single-photon avalanche detectors (SPAD) line detector with a 256-channel, 3-bit to-digital converter (TDC) and having adjustable resolution between 52 ps and 104 ps was fabricated in a 0.35 μm High Voltage CMOS technology
The dark count rates (DCR) of the SPADs were measured in the time-gated operation mode and hot pixels were eliminated so that the best possible performance in the Raman measurement could be achieved
Summary
M EASURING the chemical environment and molecular composition of a medium is important in many applications, e.g. in the agricultural, food, oil and pharmaceuticals industries [1]–[4]. To make SPAD-based Raman spectroscopy more practicable for most of the potential in situ applications with highly fluorescent samples having nanosecond-scale fluorescence lifetimes, a shorter time gate width in the deep sub-ns range (∼100...200ps) is needed to achieve a high enough fluorescence rejection ratio. In addition to distorting the recorded spectrum, the timing skew deteriorates the signal-to-noise ratio of the measurement, especially if the sample has a high fluorescence background with a short, nanosecond-scale fluorescence lifetime [23]. The design of the chip follows the above design principles and the effectiveness of the suggested techniques are demonstrated using samples (olive oil and sesame seed oil) which are known to be difficult to measure with conventional continuous wave (CW) Raman spectroscopy due to the high fluorescence background and relatively short fluorescence time constant.
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