Wilfried Uhring

Streak camera in standard (Bi)CMOS (bipolar complementary metal-oxide-semiconductor) technology

The conventional streak camera (CSC) is an optoelectronic instrument that captures the spatial distribution as a function of time of an ultra high-speed luminous phenomenon with picosecond temporal resolution and a typical spatial resolution of several tens of micrometers. This paper presents two tubeless streak camera architectures called MISC (matrix integrated streak camera) and VISC (vector integrated streak camera), which replicate the functionality of a CSC on a single CMOS chip. The MISC structure consists of a lens, which spreads the photon flux on the surface of a specific pixel array-based (Bi)CMOS sensor. The VISC architecture is based on a sensor featuring a single column of photodetectors, where each element is coupled to a front-end and a multi-sampling and storage unit. In this case the optical objective used in front of the sensor focuses the luminous event on the several tens of micrometers wide photosensitive column. For both architectures, the spatial resolution is linked to the size of the photodetector and the temporal resolution is determined by the bandwidths of the photodetectors and the signal conditioning electronics. The capture of a 6 ns full width at half maximum 532 nm laser pulse is reported for two generations of MISC and a first generation of VISC.

200 ps FWHM and 100 MHz Repetition Rate Ultrafast Gated Camera for Optical Medical Functional Imaging

The paper describes the realization of a complete optical imaging device to clinical applications like brain functional imaging by time-resolved, spectroscopic diffuse optical tomography. The entire instrument is assembled in a unique setup that includes a light source, an ultrafast time-gated intensified camera and all the electronic control units. The light source is composed of four near infrared laser diodes driven by a nanosecond electrical pulse generator working in a sequential mode at a repetition rate of 100 MHz. The resulting light pulses, at four wavelengths, are less than 80 ps FWHM. They are injected in a four-furcated optical fiber ended with a frontal light distributor to obtain a uniform illumination spot directed towards the head of the patient. Photons back-scattered by the subject are detected by the intensified CCD camera; there are resolved according to their time of flight inside the head. The very core of the intensified camera system is the image intensifier tube and its associated electrical pulse generator. The ultrafast generator produces 50 V pulses, at a repetition rate of 100 MHz and a width corresponding to the 200 ps requested gate. The photocathode and the Micro-Channel-Plate of the intensifier have been specially designed to enhance the electromagnetic wave propagation and reduce the power loss and heat that are prejudicial to the quality of the image. The whole instrumentation system is controlled by an FPGA based module. The timing of the light pulses and the photocathode gating is precisely adjustable with a step of 9 ps. All the acquisition parameters are configurable via software through an USB plug and the image data are transferred to a PC via an Ethernet link. The compactness of the device makes it a perfect device for bedside clinical applications.

A time-gated near-infrared spectroscopic imaging device for clinical applications.

A time-resolved, spectroscopic, diffuse optical tomography device was assembled for clinical applications like brain functional imaging. The entire instrument lies in a unique setup that includes a light source, an ultrafast time-gated intensified camera and all the electronic control units. The light source is composed of four near infrared laser diodes driven by a nanosecond electrical pulse generator working in a sequential mode at a repetition rate of 100 MHz. The light pulses are less than 80 ps FWHM. They are injected in a four-furcated optical fiber ended with a frontal light distributor to obtain a uniform illumination spot directed towards the head of the patient. Photons back-scattered by the subject are detected by the intensified CCD camera. There are resolved according to their time of flight inside the head. The photocathode is powered by an ultrafast generator producing 50 V pulses, at 100 MHz and a width corresponding to a 200 ps FWHM gate. The intensifier has been specially designed for this application. The whole instrument is controlled by an FPGA based module. All the acquisition parameters are configurable via software through an USB plug and the image data are transferred to a PC via an Ethernet link. The compactness of the device makes it a perfect device for bedside clinical applications. The instrument will be described and characterized. Preliminary data recorded on test samples will be presented.

Streak-mode optical sensor in standard BiCMOS technology

A streak-mode optical sensor suitable for the accurate observation of nanosecond luminous phenomena is presented. The imager is fabricated in a standard 0.35 µm SiGe BiCMOS technology. It reaches a total acquisition rate of 512 GS/s and a temporal resolution of 490 ps at λ = 400 nm. The sensor has been successfully employed to acquire the fluorescence profile of a stilbene sample, demonstrating its capability to record nanosecond-order transients with high temporal resolution.

A Fully Characterizable Asynchronous Multiphase Delay Generator

A fully characterizable, 128-stage asynchronous Multiphase Delay Generator (MDG) integrated in standard 0.35 μm CMOS technology is presented. The circuit consists of a mirror Voltage-Controlled Delay Line (VCDL), driven by a Delay-Locked Loop (DLL), and an analog memory block. The master DLL ensures the stability over temperature and the absolute precision of the delay, whereas the mirror VCDL allows an asynchronous operation of the MDG with respect to the DLL reference clock. The memory block carries out a precise stage-to-stage delay and jitter characterization by analog sampling the state of the mirror VCDL upon an external request. Two versions of the circuit differing by their VCDL layout configurations were processed in order to compare their absolute time accuracy and jitter performance. In the first variant the two delay lines were laid out in an interlaced arrangement, whereas in the second, the mirror VCDL was positioned under the master VCDL. A maximal temporal dynamic range of 125 ps-1 ns was achieved. The single-stage delay variation with temperature was less than 1% over the 10-60°C range considered. The mean RMS jitter level per stage remained below 3% of the elementary delay over the entire dynamic range of the MDG for both circuit versions.

High-dynamic-range microscope imaging based on exposure bracketing in full-field optical coherence tomography

By applying the proposed high-dynamic-range (HDR) technique based on exposure bracketing, we demonstrate a meaningful reduction in the spatial noise in image frames acquired with a CCD camera so as to improve the fringe contrast in full-field optical coherence tomography (FF-OCT). This new signal processing method thus allows improved probing within transparent or semitransparent samples. The proposed method is demonstrated on 3 μm thick transparent polymer films of Mylar, which, due to their transparency, produce low contrast fringe patterns in white-light interference microscopy. High-resolution tomographic analysis is performed using the technique. After performing appropriate signal processing, resulting XZ sections are observed. Submicrometer-sized defects can be lost in the noise that is present in the CCD images. With the proposed method, we show that by increasing the signal-to-noise ratio of the images, submicrometer-sized defect structures can thus be detected.

A 64 single photon avalanche diode array in 0.18 µm CMOS standard technology with versatile quenching circuit for quick prototyping

Several works have demonstrated the successfully integration of Single-photon avalanche photodiodes (SPADs) operating in Geiger mode in a standard CMOS circuit for the last 10 years. These devices offer an exceptional temporal resolution as well as a very good optical sensitivity. Nevertheless, it is difficult to predict the expected performances of such a device. Indeed, for a similar structure of SPAD, some parameter values can differ by two orders of magnitude from a technology to another. We proposed here a procedure to identify in just one or two runs the optimal structure of SPAD available for a given technology. A circuit with an array of 64 SPAD has been realized in the Tower-Jazz 0.18 μm CMOS image sensor process. It encompasses an array of 8 different structures of SPAD reproduced in 8 diameters in the range from 5 μm up to 40 μm. According to the SPAD structures, efficient shallow trench insulator and/or P-Well guard ring are used for preventing edge breakdown. Low dark count rate of about 100 Hz are expected thanks to the use of buried n-well layer and a high resistivity substrate. Each photodiode is embedded in a pixel which includes a versatile quenching circuitry and an analog output of its cathode voltage. The quenching system is configurable in four operation modes; the SPAD is disabled, the quenching is completely passive, the reset of the photodiode is active and the quenching is fully active. The architecture of the array makes possible the characterization of every single photodiode individually. The parameters to be measured for a SPAD are the breakdown avalanche voltage, the dark count rate, the dead time, the timing jitter, the photon detection probability and the after-pulsing rate.

Integrated streak camera in standard (Bi)CMOS technology

The conventional streak camera (CSC) is an optoelectronic instrument which captures the spatial distribution versus time of a ultra high-speed luminous phenomena with a picosecond temporal resolution and a typical spatial resolution of 60 μm. This paper presents two Integrated Streak Camera (ISC) architectures called MISC (M for Matrix) and VISC (V for Vector) which replicate the functionality of a streak camera on a single CMOS chip. The MISC structure consists of a pixel array, where the column depth together with the sampling rate determine the observation window. For proper operation, the image of the slit has to be spread uniformly over the rows of the imager. The VISC architecture is based on a single column of photosensors, where each element is coupled to a front-end and a multi-sampling and storage unit. The observation window is determined by the sampling rate and the depth of the memory frame. The measurement of a 6 ns FWHM 532 nm light pulse laser is reported for both ISCs. For the two architectures, the spatial resolution is linked to the size and the number of the photodetectors.

A single photon avalanche detector in a 180 nm standard CMOS technology

We present the performance characteristics of a Single Photon Avalanche Detector fabricated in a 180 nm standard CMOS image sensor technology. The SPAD implemented in 8 different diameters between 5 μm and 40 μm shows a DCR below 10 kHz at 15°C with a low afterpulsing probability (0.2% at 300 mV), a good photodetection efficiency (20%) and a very good time resolution (80 ps at 450 nm).

Optical implementation of the filtered backprojection algorithm

Filtered backprojection (FBP) is the basic operation of image reconstruction algorithms in tomography. It is widely used but very time-consuming. We propose a new implementation, based on an optoelectronic architecture, providing a speedup of about two orders of magnitude over a classical digital implementation. The realization of the optical core, based on a rotated Dove prism, requires careful attention in order to ensure good image quality. This aspect has been studied in simulation with a suitable model of the architecture, and in practice with an experimental setup. Results are very encouraging.