Single Photon Avalanche Diode Line Research Articles

Multipoint Raman Spectrometer Based on a Time-Resolved CMOS SPAD Sensor

The potential of in-line Raman spectrometers for process monitoring applications has been shown for many industrial processes, but in most cases, only one measurement point has been monitored by one spectrometer. In this letter, we describe and demonstrate a novel, time-resolved method for measuring Raman spectra and fluorescence lifetimes from multiple points using a single excitation source and a single spectrometer. This technique is based on a combination of a time-resolved CMOS SPAD (single-photon avalanche diode) line sensor and a fitting optical light guiding system. The line sensor is designed to make multiple individual measurements at intervals of tens of nanoseconds and the optical light guiding system, in turn, produces matching temporal differences for optical signals from different measurement points. Thereby, signals from different points are distinguished in the time domain. A combined Raman and fluorescence lifetime monitoring of two measurement points was demonstrated with an oil-ethanol emulsion sample.

3-D Raman Imaging Using Time-Resolving CMOS SPAD Line Sensor and 2-D Mapping

The capability of Raman imaging to produce 2D and 3D chemical presentations of samples has gained a lot of interest in different application fields. In this paper, we present a 3D chemical image reconstruction based on 2D scanning of a sample utilizing a time-resolved Raman spectrometer based on a CMOS single-photon avalanche diode (SPAD) line sensor. The 2D scanning data contains the lateral information (XY-plane) whereas the time-of-arrival data of the Raman photons measured by the sensor carries the axial information (i.e., depth information, Z-axis). The sensor is fabricated in 110-nm CMOS technology. It has 256-spectral channels and each channel has its own 7-bit on-chip time-to-digital converter (TDC) with an adjustable resolution from 25 ps to 65 ps. In addition to the 3D chemical reconstruction of the scanned sample, we have also shown the ability to retrieve depth profiling information of each scanned pixel such as the boundaries and middle points of any selected layer over the depth range of the scanned object by means of a single measurement for each scanned pixel. In addition, we have discussed the system components and the post-processing parameters that affect the depth profiling accuracy and the 3D reconstruction operation the most. Results showed that the instrument response function of the system and the time gate window width in a post-processing phase are playing the most important role in determining the axial (depth) accuracy. We believe that our system will enable a whole new class of Raman applications that will allow simultaneous 3D chemical geometric representation at the cm-level during Raman operations.

Time-correlated single photon Raman spectroscopy at megahertz repetition rates

Significant improvements in time-correlated single photon counting (TCSPC) Raman spectroscopy acquisition times can be achieved through exploitation of megahertz (MHz) laser repetition rates. We have developed a TCSPC Raman spectroscopy system based on a high peak power (>40W) pulsed laser, a high pulse repetition rate (40 MHz), a custom f/1.5 spectrometer, and a 512 spectral channel × 16 time bin single photon avalanche diode line sensor. We report millisecond Raman spectrum acquisition times, a peak Raman count rate of 104 kcps, and a linewidth aggregated count rate of 440 kcps with a diamond sample. This represents a three-order-of-magnitude increase in measured Raman count rate in comparison with a 104 kHz pulsed laser operating at 300 W and a four-order-of-magnitude increase over a 0.1 W pulsed laser operating at 40 MHz. A Raman-to-fluorescence ratio of 4.76 is achieved with a sesame oil sample at a 20 MHz repetition rate. Achieving high count rates and Raman-to-fluorescence ratios unlocks the potential of combined Raman/fluorescence lifetime spectroscopy for imaging and other short acquisition time applications.

Depth Analysis of Semi-Transparent Media by a Time-Correlated CMOS SPAD Line Sensor-Based Depth-Resolving Raman Spectrometer

Raman spectroscopy has proved to have potential in deep surface analytical applications. We present here, to the best of our knowledge, the first time depth analysis of semi-transparent media by a depth-resolving Raman spectrometer based on an adjustable time-correlated CMOS SPAD (single-photon avalanche diode) line sensor that can measure the depth of target samples embedded in a centimeter-scale semi-transparent medium simultaneously with a normal Raman depth profiling operation and suppress the fluorescence background by means of adjustable picosecond time gating. The variability of the depth derivation was measured to be ± 0.43 cm at depths ranging from 2 to 9 cm. In addition, the advantages of the adjustable picosecond time gating in terms of depth derivation and fluorescence background suppression performance were shown by comparing gate widths ranging from 100 ps to 13 ns. We believe that the technology concerned could pave the way for a new kind of compact, practical depth-resolving Raman spectrometer for deep subsurface analytical applications.

PH sensing through a single optical fibre using SERS and CMOS SPAD line arrays.

Full exploitation of fibre Raman probes has been limited by the obstruction of weak Raman signals by background fluorescence of the sample and the intrinsic Raman signal of the delivery fibre. Here we utilised functionalised gold nanoshells (NS) to take advantage of the surface-enhanced Raman spectroscopy (SERS) effect to enhance the pH responsive spectrum of 4-mercaptobenzoic acid (MBA). However, the fibre background is still dominant. Using the photon arrival time-resolving capability of a CMOS single-photon avalanche diode (SPAD) based line sensor, we recover the SERS spectrum without a fibre background in a 10 s measurement. In this manner, pH sensing through a multimode fibre at a low excitation power that is safe for future in vivo applications, with short acquisition times (10 or 60 s), is demonstrated. A measurement precision of ± 0.07 pH units is thus achieved.

A <inline-formula> <tex-math notation="LaTeX">$2\times (4)\times 128$ </tex-math></inline-formula> Multitime-Gated SPAD Line Detector for Pulsed Raman Spectroscopy

A time-gated 2 × (4) × 128 single photon avalanche diode line detector for pulsed laser Raman spectroscopy has been developed and fabricated in a 0.35-μm high-voltage CMOS technology. The sample is illuminated with short laser pulses (~100 ps) at a rate of ~50 kHz and four time gates synchronized with these pulses and having selectable widths within the subnanoseconds range are used to measure the Raman photons and fluorescence background simultaneously. The fluorescence background measurement is used to suppress the residual fluorescence level to improve the quality of the Raman spectrum. The variation in the width of the time window was measured to be approximately ±17.5 ps along the spectral axis when set externally to a nominal value of 100 ps. Measurements with a reference sample demonstrate the effect of nonhomogeneities in the time gates on the quality of the recorded Raman spectrum and the residual fluorescence correction.

256 × 2 SPAD line sensor for time resolved fluorescence spectroscopy.

We present a CMOS chip 256 × 2 single photon avalanche diode (SPAD) line sensor, 23.78 µm pitch, 43.7% fill factor, custom designed for time resolved emission spectroscopy (TRES). Integrating time-to-digital converters (TDCs) implement on-chip mono-exponential fluorescence lifetime pre-calculation allowing timing of 65k photons/pixel at 200 Hz line rate at 40 ps resolution using centre-of-mass method (CMM). Per pixel time-correlated single-photon counting (TCSPC) histograms can also be generated with 320 ps bin resolution. We characterize performance in terms of dark count rate, instrument response function and lifetime uniformity for a set of fluorophores with lifetimes ranging from 4 ns to 6 ns. Lastly, we present fluorescence lifetime spectra of multicolor microspheres and skin autofluorescence acquired using a custom built spectrometer. In TCSPC mode, time-resolved spectra are acquired within 5 minutes whilst in CMM mode spectral lifetime signatures are acquired within 2 ms for fluorophore in cuvette and 200 ms for skin autofluorescence. We demonstrate CMOS line sensors to be a versatile tool for time-resolved fluorescence spectroscopy by providing parallelized and flexible spectral detection of fluorescence decay.