A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

We review the current status of single-photon-source and single-photon-detector technologies operating at wavelengths from the ultraviolet to the infrared. We discuss applications of these technologies to quantum communication, a field currently driving much of the development of single-photon sources and detectors.

/pdf/invited-review-article-single-photon-sources-and-detectors-5bsn8auo37.pdf

Invited review article: Single-photon sources and detectors

Localized fluid flow measurements with an He-Ne laser spectrometer

XIAOTAO PAN,∗ CAILI AW,∗ YANAN DU,† HANRY YU†,‡ 7 and THORSTEN WOHLAND,∗,§ ∗Department of Chemistry, National University of Singapore, 9 3 Science Drive 3, Singapore 117543, Singapore †Institute of Bioengineering and Nanotechnology, A*STAR, 31 Biopolis Way, 11 The Nanos #04-01, Singapore 138669, Singapore ‡Department of Chemistry, National University of Singapore, 13 3 Science Drive 3, Singapore 117597, Singapore §chmwt@nus.edu.sg 15

Fluorescence correlation spectroscopy

The design and characterization of an imaging system is presented for depth information capture of arbitrary three-dimensional (3-D) objects. The core of the system is an array of 32 /spl times/ 32 rangefinding pixels that independently measure the time-of-flight of a ray of light as it is reflected back from the objects in a scene. A single cone of pulsed laser light illuminates the scene, thus no complex mechanical scanning or expensive optical equipment are needed. Millimetric depth accuracies can be reached thanks to the rangefinder's optical detectors that enable picosecond time discrimination. The detectors, based on a single photon avalanche diode operating in Geiger mode, utilize avalanche multiplication to enhance light detection. On-pixel high-speed electrical amplification can therefore be eliminated, thus greatly simplifying the array and potentially reducing its power dissipation. Optical power requirements on the light source can also be significantly relaxed, due to the array's sensitivity to single photon events. A number of standard performance measurements, conducted on the imager, are discussed in the paper. The 3-D imaging system was also tested on real 3-D subjects, including human facial models, demonstrating the suitability of the approach.

/pdf/design-and-characterization-of-a-cmos-3-d-image-sensor-based-17cj2iye1o.pdf

Design and characterization of a CMOS 3-D image sensor based on single photon avalanche diodes

A two-dimensional (2-D) array (4 by 8) of single-photon avalanche diodes integrated in an industrial complementary metal-oxide-semiconductor (CMOS) process is presented. Each pixel combines a photodiode biased above its breakdown voltage in the so-called Geiger mode, a quenching resistor, and a simple comparator. The pitch between the pixels is 75 /spl mu/m and the diameter of each pixel is 6.4 /spl mu/m. The full integration allows reducing the number of charge carriers in a Geiger pulse. The electroluminescence responsible for optical crosstalks between pixels is then reduced leading to a negligible optical crosstalk probability. Thanks to the cleanness of the fabrication process, no afterpulsing effects are noticed. At room temperature, most of the pixels exhibit a dark-count rate of about 50 Hz. The detection probability is almost identical for all 32 pixels of the array with relative variation in the range of a few percents. This letter demonstrates the feasibility of an array of single-photon detectors sensitive in the visible part of the spectrum. Besides low production costs and compactness, an undeniable benefit lies in the potential to easily modify the design to fit a specific application. Furthermore, the CMOS integration opens the way to on-chip data processing.

First fully integrated 2-D array of single-photon detectors in standard CMOS technology

This paper describes the design and the performance of a new high-speed laser Doppler imaging system for monitoring blood flow over an area of tissue. The new imager delivers high-resolution flow images (256×256 pixels) every 2 to 10 seconds, depending on the number of points in the acquired time-domain signal (32–512 points). This new imaging modality utilizes a digital integrating CMOS image sensor to detect Doppler signals in a plurality of points over the area illuminated by a divergent laser beam of a uniform intensity profile. The integrating property of the detector improves the signal-to-noise ratio of the measurements, which results in high-quality flow images. We made a series of measurements in vitro to test the performance of the system in terms of bandwidth, SNR, etc. Subsequently we give some examples of flow-related images measured on human skin, thus demonstrating the performance of the imager in vivo. The perspectives for future implementations of the imager for clinical and physiological applications are discussed.

High-speed laser Doppler perfusion imaging using an integrating CMOS image sensor

A system for full-field laser Doppler blood flow imaging has been developed and tested on biomedical samples. The new imaging system allows 2D flow maps or monitoring flux signals to be obtained from a plurality of measured points simultaneously by using a 2D array of photodetectors. The detection part of the system is based on an intelligent CMOS camera with a built-in digital signal processor and memory. The imaging time of the system is as much as to 4 times faster than for the conventional scanning laser Doppler imager. The performance of the system was evaluated in vitro and in vivo. The first measurement results with this new system on human skin are reported.

Full-field laser Doppler perfusion imaging and monitoring with an intelligent CMOS camera

We present parallel single molecule detection (SMD) and fluorescence correlation spectroscopy (FCS) experiments with a fully integrated complementary metal oxide semiconductor (CMOS) single-photon 2x2 detector array. Multifocal excitation is achieved with a diffractive optical element (DOE). Special emphasis is placed on parallelization of the total system. The performance of the novel single-photon CMOS detector is investigated and compared to a state-of-the-art single-photon detecting module [having an actively quenched avalanche photodiode (APD)] by measurements on free diffusing molecules at different concentrations. Despite the order of magnitude lower detection efficiency of the CMOS detector compared to the state-of-the-art single-photon detecting module, we achieve single molecule sensitivity and reliably determine molecule concentrations. In addition, the CMOS detector performance for the determination of the fraction of slowly diffusing molecules in a primer solution (two-component analysis) is demonstrated. The potential of this new technique for high-throughput confocal-detection-based systems is discussed.

https://www.spiedigitallibrary.org/journals/journal-of-biomedical-optics/volume-9/issue-5/0000/Parallel-single-molecule-detection-with-a-fully-integrated-single-photon/10.1117/1.1781668.pdf

Parallel single molecule detection with a fully integrated single-photon 2x2 CMOS detector array.

An instrument for high-speed laser perfusion imaging comprises a laser source, a detector, a signal-processing unit, data memory, and a screen to display the results. A section of the sample surface is illuminated with laser light; the reemitted light from the irradiated surface is collected by the focusing optics on a 2D array of integrating photo detectors. The elements of the 2D array can be accessed individually or in a pre-defined selection of pixels at high speed. This 2D array of random-pixel-access integrating photo detectors (for example an integrating CMOS image senor) is utilized to measure the intensity variations at each individual pixel. The average amplitude and the mean frequency of the measured signal contain information about concentration and speed of moving blood cells. For real-time imaging, the exposure time is used as a parameter to measure relative perfusion changes. These data are stored in a memory and processed with a signal-processing unit. The instrument delivers 2D flow maps of the illuminated sample section. In parallel a conventional image of the sample can be obtained with the same 2D array of photo detectors allowing a simple overlay between a conventional image and processed flow maps. The instrument enables objective high-speed tissue perfusion imaging and real-time perfusion monitoring.

Alexandre Serov

Papers

First fully integrated 2-D array of single-photon detectors in standard CMOS technology

High-speed laser Doppler perfusion imaging using an integrating CMOS image sensor

Full-field laser Doppler perfusion imaging and monitoring with an intelligent CMOS camera

Parallel single molecule detection with a fully integrated single-photon 2x2 CMOS detector array.

Instrument and method for high-speed perfusion imaging