Multichannel Time-correlated Single-photon Counting Research Articles

A 16 Channel Spad Array for High-Throughput Tcspc Measurements of Single-Molecule FRET of Freely Diffusing Molecules

Single-molecule techniques probe the spectroscopic properties of biomolecules one molecule a time. These approaches allow detecting and quantifying subpopulations which would be masked in ensemble measurements. Single-molecule FRET (smFRET), which indirectly measures the distance between fluorophores, has been extensively used to characterize the conformation of biomolecules or molecular interactions in complex biomolecular assemblies among many applications. FRET efficiency is typically computed ratiometrically from photon counts detected in two spectral channels. However, this approach requires careful correction and calibrations to account for background and differences in quantum yield and detection efficiency between the two channels. Alternatively, time-correlated single-photon counting (TCSPC) measurements can provide the same result from the donor fluorescence lifetime only, which is much less sensitive to these experimental aspects.In both approaches, a fundamental limitation remains the long acquisition time needed to accumulate sufficient statistics from freely-diffusing molecules, severely limiting the throughput of these techniques and preventing the study of rapidly evolving systems. To circumvent these limitations, we have developed a multi-spot setup using sensitive SPAD arrays, achieving high-throughput data acquisition for FCS and smFRET. These systems had so far been limited to counting applications.In this work, we present the first proof-of-principle TCSPC multispot system employing a simple line-excitation, a new dedicated SPAD array with low jitter and integrated multichannel TCSPC electronics[1]. This system allows single-molecule detection on all 16 channels using a total excitation power of 100 mW. We report measurements on short doubly-labeled dsDNA molecules and demonstrate uniform performance across the 16 detection channels. These results are a first step toward extending current single-spot TCSPC spectroscopic techniques (ns-ALEX or PIE) to a multispot geometry for high-throughput time-resolved smFRET measurements.[1] Cuccato, A et al. IEEE Phot. J. 5-5, p6801514, 2013.

Eight-Channel Fully Adjustable Pulse Generator

This paper describes an eight-channel differential pulse generator based on a fast differential bipolar transistor output stage. It has been designed specifically to test new multichannel time-correlated single-photon counting (TCSPC) instruments, while maintaining a wide range of regulation possibilities in terms of frequency, amplitude, delay, and duty cycle. The input signal for each channel can be provided either by an on-board programmable frequency synthesizer or by an external reference shared by all channels. The chosen input is delivered to a delayer loop controlled by a field programmable gate array (FPGA), which provides the desired delay, minimizing the jitter contribution. The delayed signal is fed to an analog differential stage that provides the fast output, whose amplitude and offset are adjusted by a digital-to-analog converter, controlled by the FPGA. The experimental results on the instrument show an output slew rate of 1600 V/ $\mu \text{s}$ over the complete full-scale range, which is between −2 and 6 V. The feedback delayer regulates the delay between 0 ns and 30 $\mu \text{s}$ whereas the high time is adjustable from a minimum of 10 ns to a maximum of 30 $\mu \text{s}$ . Finally, the frequency range is from 2 kHz to 50 MHz. All these adjustable parameters are managed by a microcontroller that works as an interface between the FPGAs and dedicated PC software. The implemented structure allows the system to achieve a time resolution down to 6 ps across all channels, making this instrument perfect for testing high time resolution TCSPC systems.

8-channel acquisition system for time-correlated single-photon counting

Nowadays, an increasing number of applications require high-performance analytical instruments capable to detect the temporal trend of weak and fast light signals with picosecond time resolution. The Time-Correlated Single-Photon Counting (TCSPC) technique is currently one of the preferable solutions when such critical optical signals have to be analyzed and it is fully exploited in biomedical and chemical research fields, as well as in security and space applications. Recent progress in the field of single-photon detector arrays is pushing research towards the development of high performance multichannel TCSPC systems, opening the way to modern time-resolved multi-dimensional optical analysis. In this paper we describe a new 8-channel high-performance TCSPC acquisition system, designed to be compact and versatile, to be used in modern TCSPC measurement setups. We designed a novel integrated circuit including a multichannel Time-to-Amplitude Converter with variable full-scale range, a D∕A converter, and a parallel adder stage. The latter is used to adapt each converter output to the input dynamic range of a commercial 8-channel Analog-to-Digital Converter, while the integrated DAC implements the dithering technique with as small as possible area occupation. The use of this monolithic circuit made the design of a scalable system of very small dimensions (95 × 40 mm) and low power consumption (6 W) possible. Data acquired from the TCSPC measurement are digitally processed and stored inside an FPGA (Field-Programmable Gate Array), while a USB transceiver allows real-time transmission of up to eight TCSPC histograms to a remote PC. Eventually, the experimental results demonstrate that the acquisition system performs TCSPC measurements with high conversion rate (up to 5 MHz/channel), extremely low differential nonlinearity (<0.04 peak-to-peak of the time bin width), high time resolution (down to 20 ps Full-Width Half-Maximum), and very low crosstalk between channels.

A time-domain diffuse fluorescence and optical tomography system for breast tumor diagnosis

Aiming at enhancing the reliability of breast diffuse optical tomography,a combined time-domain diffuse fluorescence and optical tomography system is proposed based on the multi-channel time-correlated single-photon counting technique.Aligning 32 coaxial fibers around the tissue surface equally;the system scans objects in a parallel-beam mode analogous to X-ray CT so that the time-resolved projections at different incident positions can be obtained.By applying the relevant iteration reconstruction algorithm,promising images have been produced from measurements on different phantoms.The results indicate this system works reliably and is one of the ideal platforms for optical breast tumor diagnosis.

Multichannel time-correlated single photon counting: Spectroscopy and time-gated imaging using a resistive anode photomultiplier tube

A multichannel single photon counting detection system for steady state, time-resolved luminescence spectra, time-gated imaging, or time-gated Raman is described. This system, which has approximately 80 ps time resolution, is based on a microchannel plate photomultiplier with a position-sensitive anode (Mepsicron). This detector can operate at very low light levels with excitation wavelengths from 200 nm to the near infrared. For time-resolved luminescence applications, the system simultaneously collects 1024 decays of up to 1024 spectral channels. The measured time dispersion for spectral bandwidths greater than 100 nm are shown to be negligible.