Samuel Burri

Architecture and applications of a high resolution gated SPAD image sensor

We present the architecture and three applications of the largest resolution image sensor based on single-photon avalanche diodes (SPADs) published to date. The sensor, fabricated in a high-voltage CMOS process, has a resolution of 512 × 128 pixels and a pitch of 24 μm. The fill-factor of 5% can be increased to 30% with the use of microlenses. For precise control of the exposure and for time-resolved imaging, we use fast global gating signals to define exposure windows as small as 4 ns. The uniformity of the gate edges location is ∼140 ps (FWHM) over the whole array, while in-pixel digital counting enables frame rates as high as 156 kfps. Currently, our camera is used as a highly sensitive sensor with high temporal resolution, for applications ranging from fluorescence lifetime measurements to fluorescence correlation spectroscopy and generation of true random numbers.

Towards a High-Speed Quantum Random Number Generator

Randomness is of fundamental importance in various fields, such as cryptography, numerical simulations, or the gaming industry. Quantum physics, which is fundamentally probabilistic, is the best option for a physical random number generator. In this article, we will present the work carried out in various projects in the context of the development of a commercial and certified high speed random number generator.

Nonuniformity Analysis of a 65-kpixel CMOS SPAD Imager

While CMOS single-photon avalanche diode (SPAD) technology has steadily advanced, improving noise, timing resolution, and sensitivity, spatial resolution has been increasing as well. The increase in the number of pixels has made a comprehensive analysis of nonuniformity and its effects meaningful, allowing a more accurate comparison of SPAD imagers with other high-end scientific imagers, such as electron multiplying charge-coupled device and scientific CMOS. A comprehensive nonuniformity analysis was conducted on a 512 × 128 pixel gated SPAD imager, where dark noise, afterpulsing, crosstalk, signal response, and shot noise were measured. This analysis has led to a variety of postprocessing algorithms to improve the linearity of the response as for example required by ground state depletion microscopy-based superresolution microscopy and other techniques. We derived a new correction formula for the count rate applicable to 1-b SPAD imagers, and we measured a significant improvement of photon detection efficiency using microlenses. These techniques were used to validate the suitability of the imager in fluorescence microscopy examples.

Virtual ways: efficient coherence for architecturally visible storage in automatic instruction set extensions

Customizable processors augmented with application-specific Instruction Set Extensions (ISEs) have begun to gain traction in recent years. The most effective ISEs include Architecturally Visible Storage (AVS), compiler-controlled memories accessible exclusively to the ISEs. Unfortunately, the usage of AVS memories creates a coherence problem with the data cache. A multiprocessor coherence protocol can solve the problem, however, this is an expensive solution when applied in a uniprocessor context. Instead, we can solve the problem by modifying the cache controller so that the AVS memories function as extra ways of the cache with respect to coherence, but are not generally accessible as extra ways for use under normal software execution. This solution, which we call Virtual Ways is less costly than a hardware coherence protocol, and eliminate coherence messages from the system bus, which improves energy consumption. Moreover, eliminating these messages makes Virtual Ways significantly more robust to performance degradation when there is a significant disparity in clock frequency between the processor and main memory.

LinoSPAD: a time-resolved 256×1 CMOS SPAD line sensor system featuring 64 FPGA-based TDC channels running at up to 8.5 giga-events per second

LinoSPAD is a reconfigurable camera sensor with a 256×1 CMOS SPAD (single-photon avalanche diode) pixel array connected to a low cost Xilinx Spartan 6 FPGA. The LinoSPAD sensor’s line of pixels has a pitch of 24 μm and 40% fill factor. The FPGA implements an array of 64 TDCs and histogram engines capable of processing up to 8.5 giga-photons per second. The LinoSPAD sensor measures 1.68 mm×6.8 mm and each pixel has a direct digital output to connect to the FPGA. The chip is bonded on a carrier PCB to connect to the FPGA motherboard. 64 carry chain based TDCs sampled at 400 MHz can generate a timestamp every 7.5 ns with a mean time resolution below 25 ps per code. The 64 histogram engines provide time-of-arrival histograms covering up to 50 ns. An alternative mode allows the readout of 28 bit timestamps which have a range of up to 4.5 ms. Since the FPGA TDCs have considerable non-linearity we implemented a correction module capable of increasing histogram linearity at real-time. The TDC array is interfaced to a computer using a super-speed USB3 link to transfer over 150k histograms per second for the 12.5 ns reference period used in our characterization. After characterization and subsequent programming of the post-processing we measure an instrument response histogram shorter than 100 ps FWHM using a strong laser pulse with 50 ps FWHM. A timing resolution that when combined with the high fill factor makes the sensor well suited for a wide variety of applications from fluorescence lifetime microscopy over Raman spectroscopy to 3D time-of-flight.

Photon-Counting Arrays for Time-Resolved Imaging

The paper presents a camera comprising 512 × 128 pixels capable of single-photon detection and gating with a maximum frame rate of 156 kfps. The photon capture is performed through a gated single-photon avalanche diode that generates a digital pulse upon photon detection and through a digital one-bit counter. Gray levels are obtained through multiple counting and accumulation, while time-resolved imaging is achieved through a 4-ns gating window controlled with subnanosecond accuracy by a field-programmable gate array. The sensor, which is equipped with microlenses to enhance its effective fill factor, was electro-optically characterized in terms of sensitivity and uniformity. Several examples of capture of fast events are shown to demonstrate the suitability of the approach.

SPADs for quantum random number generators and beyond

Single-Photon Avalanche Diodes (SPADs) are solid-state photo-detectors capable of detecting single photons by exploiting the avalanche effect that occurs in the breakdown of a p-n junction biased above breakdown voltage. By this effect, a SPAD translates an incoming photon to a macroscopic current pulse. These devices are currently used for building medical devices characterized by a very high time resolution. An appealing application of SPAD is to use them as a basic block for building the entropy source of true random number generators. In this paper we focus on such application, and we explore the design challenges behind the realization of a quantum random number generator based on a massively parallel array of SPADs. The matrix under investigation comprises 512×128 independent cells that convert photons onto a raw bit-stream, which, as ensured by the properties of quantum physics, is characterized by a very high level of randomness. The sequences are read out in a 128-bit parallel bus, concatenated, and pipelined onto a de-biasing filter. Subsequently, we fabricated the proposed chip using a standard CMOS process. Our results, achieved on the manufactured device and coupling two matrices, show that our architecture can reach up to 5 Gbit/s while consuming 25pJ/bit, thus demonstrating scalability and performance for any random number generators based on SPADs.

A 65k pixel, 150k frames-per-second camera with global gating and micro-lenses suitable for fluorescence lifetime imaging

This paper presents our work on a 65k pixel single-photon avalanche diode (SPAD) based imaging sensor realized in a 0.35μm standard CMOS process. At a resolution of 512 by 128 pixels the sensor is read out in 6.4μs to deliver over 150k monochrome frames per second. The individual pixel has a size of 24μm2 and contains the SPAD with a 12T quenching and gating circuitry along with a memory element. The gating signals are distributed across the chip through a balanced tree to minimize the signal skew between the pixels. The array of pixels is row-addressable and data is sent out of the chip on 128 lines in parallel at a frequency of 80MHz. The system is controlled by an FPGA which generates the gating and readout signals and can be used for arbitrary real-time computation on the frames from the sensor. The communication protocol between the camera and a conventional PC is USB2. The active area of the chip is 5% and can be significantly improved with the application of a micro-lens array. A micro-lens array, for use with collimated light, has been designed and its performance is reviewed in the paper. Among other high-speed phenomena the gating circuitry capable of generating illumination periods shorter than 5ns can be used for Fluorescence Lifetime Imaging (FLIM). In order to measure the lifetime of fluorophores excited by a picosecond laser, the sensor’s illumination period is synchronized with the excitation laser pulses. A histogram of the photon arrival times relative to the excitation is then constructed by counting the photons arriving during the sensitive time for several positions of the illumination window. The histogram for each pixel is transferred afterwards to a computer where software routines extract the lifetime at each location with an accuracy better than 100ps. We show results for fluorescence lifetime measurements using different fluorophores with lifetimes ranging from 150ps to 5ns.

Imaging free and bound NADH towards cancer tissue detection using FLIM system based on SPAD array

Single photon detectors allow us to work with the weakest signals such as auto-fluorescent biological sources. In combination with time gated operation mode, an array of detectors can be used as Fluorescence Lifetime Imaging system with extremely high sensitivity.

Fluorescence lifetime imaging using a single photon avalanche diode array sensor (Conference Presentation)

Single photon detectors allows us work with the weakest fluorescence signals. Single photon arrays, combined with ps-controlled gating allow us to create image maps of fluorescence lifetimes, which can be used for in-vivo discrimination of tissue activity. Here we present fluorescence lifetime imaging using the ‘SwissSPAD’ sensor, a 512-by-128-pixel array of gated single photon detectors, fabricated in a standard high-voltage 0.35 μm CMOS process. We present a protocol for spatially resolved lifetime measurements where the lifetime can be retrieved for each pixel. We demonstrate the system by imaging patterns of Fluorescein and Rhodamine B on test slides, as well as measuring mixed samples to retrieve both components of the decay lifetime. The single photon sensitivity of the sensor creates a valuable instrument to perform live cell or live animal (in vivo) measurements of the weak autofluorescent signals, for example distinguishing unlabelled free and bound NADH. Our ultimate goal is to create a real time fluorescence lifetime imaging system, possibly integrated into augmented reality goggles, which could allow immediate discrimination of in vivo tissues.