281 research outputs found
Advanced photon counting techniques for long-range depth imaging
The Time-Correlated Single-Photon Counting (TCSPC) technique has emerged as a
candidate approach for Light Detection and Ranging (LiDAR) and active depth imaging
applications. The work of this Thesis concentrates on the development and
investigation of functional TCSPC-based long-range scanning time-of-flight (TOF)
depth imaging systems. Although these systems have several different configurations
and functions, all can facilitate depth profiling of remote targets at low light levels and
with good surface-to-surface depth resolution. Firstly, a Superconducting Nanowire
Single-Photon Detector (SNSPD) and an InGaAs/InP Single-Photon Avalanche Diode
(SPAD) module were employed for developing kilometre-range TOF depth imaging
systems at wavelengths of ~1550 nm. Secondly, a TOF depth imaging system at a
wavelength of 817 nm that incorporated a Complementary Metal-Oxide-Semiconductor
(CMOS) 32×32 Si-SPAD detector array was developed. This system was used with
structured illumination to examine the potential for covert, eye-safe and high-speed
depth imaging. In order to improve the light coupling efficiency onto the detectors, the
arrayed CMOS Si-SPAD detector chips were integrated with microlens arrays using
flip-chip bonding technology. This approach led to the improvement in the fill factor by
up to a factor of 15. Thirdly, a multispectral TCSPC-based full-waveform LiDAR
system was developed using a tunable broadband pulsed supercontinuum laser source
which can provide simultaneous multispectral illumination, at wavelengths of 531, 570,
670 and ~780 nm. The investigated multispectral reflectance data on a tree was used to
provide the determination of physiological parameters as a function of the tree depth
profile relating to biomass and foliage photosynthetic efficiency. Fourthly, depth
images were estimated using spatial correlation techniques in order to reduce the
aggregate number of photon required for depth reconstruction with low error. A depth
imaging system was characterised and re-configured to reduce the effects of scintillation
due to atmospheric turbulence. In addition, depth images were analysed in terms of
spatial and depth resolution
Spectrally and temporally resolved single photon counting in advanced biophotonics applications
Biomedicine requires highly sensitive and efficient light sensors to analyse light-tissue or light-sample interactions. Single-photon avalanche diode (SPAD) sensors implemented with complementary metal-oxide-semiconductor (CMOS) technology have a growing range of applications in this field. Single-photon detection coupled with integrated timing circuits enables us to timestamp each detected photon with high temporal resolution (down to picoseconds). Arrays of SPAD based pixels and CMOS technology offer massively parallel time-resolved single-photon counting for spectrally and temporally resolved analysis of various light phenomena.This thesis examines how time-resolved CMOS SPAD based line sensors with per pixel timing circuits can be utilized to advance biophotonic applications. The study focuses on improving the existing techniques of fluorescence and Raman spectroscopy, and demonstrates for the first time CMOS SPAD based detection in optical coherence tomography (OCT). A novel detection scheme is proposed combining low-coherence interferometry and time-resolved photon counting. In this approach the interferometric information is revealed from spectral intensity measurements, which is supplemented by time-stamping of the photons building up the spectra.Two CMOS SPAD line sensors (Ra-I and its improved version, Ra-II) were characterized and the effect of their parameters on the selected techniques was analysed. The thesis demonstrates the deployment of the Ra-I line sensor in time-resolved fluorescence spectroscopy with indications of the applicability in time-resolved Raman spectroscopy. The work includes integration of the sensor with surrounding electrical and optical systems, and the implementation of firmware and software for controlling the optical setup. As a result, a versatile platform is demonstrated capable of micro- and millisecond sampling of spectral fluorescence lifetime changes in a single transient of fast chemical reactions.OCT operating in the spectral domain traditionally uses CMOS photodiode and charge-coupled device (CCD) based detectors. The applicability of CMOS SPAD sensors is investigated for the first time with focus on the main limitations and related challenges. Finally, a new detection method is proposed relying on both the wave and particle nature of light, recording time-resolved interferometric spectra from a Michelson interferometer. This method offers an alternative approach to analyse luminous effects and improves techniques based on the light’s time of flight. As an example, a proof of concept study is presented for the removal of unwanted reflections from along the sample and the optical path in an OCT setup
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High-Speed Wide-Field Time-Correlated Single-Photon Counting Fluorescence Lifetime Imaging Microscopy
Fluorescence microscopy is a powerful imaging technique used in the biological sciences to identify labeled components of a sample with specificity. This is usually accomplished through labeling with fluorescent dyes, isolating these dyes by their spectral signatures with optical filters, and recording the intensity of the fluorescent response. Although these techniques are widely used, fluorescence intensity images can be negatively affected by a variety of factors that impact the fluorescence intensity. Fluorescence lifetime imaging microscopy (FLIM) is an imaging technique that is relatively immune to intensity fluctuations and also provides the unique ability to directly monitor the microenvironment surrounding a fluorophore. Despite the benefits associated with FLIM, the applications to which it is applied are fairly limited due to long image acquisition times and high cost of traditional hardware. Recent advances in complementary metal-oxide-semiconductor (CMOS) single-photon avalanche diodes (SPADs) have enabled the design of low-cost imaging arrays that are capable of recording lifetime images with acquisition times greater than one order of magnitude faster than existing systems. However, these SPAD arrays have yet to realize the full potential of the technology due to limitations in their ability to handle the vast amount of data generated during the commonly used time-correlated single-photon counting (TCSPC) lifetime imaging technique. This thesis presents the design, implementation, characterization, and demonstration of a high speed FLIM imaging system. The components of this design include a CMOS imager chip in a standard 0.13 μm technology containing a custom CMOS SPAD, a 64-by-64 array of these SPADs, pixel control circuitry, independent time-to-digital converters (TDCs), a FLIM specific datapath, and high bandwidth output buffers. In addition to the CMOS imaging array, a complete system was designed and implemented using a printed circuit board (PCB) for capturing data from the imager, creating histograms for the photon arrival data using field-programmable gate arrays, and transferring the data to a computer using a cabled PCIe interface. Finally, software is used to communicate between the imaging system and a computer.The dark count rate of the SPAD was measured to be only 231 Hz at room temperature while maintaining a photon detection probability of up to 30\%. TDCs included on the array have a 62.5 ps resolution and a 64 ns range, which is suitable for measuring the lifetime of most biological fluorophores. Additionally, the on-chip datapath was designed to handle continuous data transfers at rates capable of supporting TCSPC-based lifetime imaging at 100 frames per second. The system level implementation also provides sufficient data throughput for transferring up to 750 frames per second from the imaging system to a computer. The lifetime imaging system was characterized using standard techniques for evaluating SPAD performance and an electrical delay signal for measuring the TDC performance. This thesis concludes with a demonstration of TCSPC-FLIM imaging at 100 frames per second -- the fastest 64-by-64 TCSPC FLIM that has been demonstrated. This system overcomes some of the limitations of existing FLIM systems and has the potential to enable new application domains in dynamic FLIM imaging
Parallel reconfigurable single photon avalanche diode array for optical communications
There is a pressing need to develop alternative communications links due to a number of
physical phenomena, limiting the bandwidth and energy efficiency of wire-based systems or
economic factors such as cost, material-supply reliability and environmental costs. Networks
have moved to optical connections to reduce costs, energy use and to supply high data rates. A
primary concern is that current optical-detection devices require high optical power to achieve
fast data rates with high signal quality. The energy required therefore, quickly becomes a
problem.
In this thesis, advances in single-photon avalanche diodes (SPADs) are utilised to reduce the
amount of light needed and to reduce the overall energy budget. Current high performance
receivers often use exotic materials, many of which have severe environmental impact and have
cost, supply and political restrictions. These present a problem when it comes to integration;
hence silicon technology is used, allowing small, mass-producible, low power receivers.
A reconfigurable SPAD-based integrating receiver in standard 130nm imaging CMOS is presented
for links with a readout bandwidth of 100MHz. A maximum count rate of 58G photon/s
is observed, with a dynamic range of ≈ 79dB, a sensitivity of ≈ −31.7dBm at 100MHz and
a BER of ≈ 1x10−9. We investigate the properties of the receiver for optical communications
in the visible spectrum, using its added functionality and reconfigurability to experimentally
explore non-ideal influences. The all-digital 32x32 SPAD array, achieves a minimum dead
time of 5.9ns, and a median dark count rate (DCR) of 2.5kHz per SPAD. High noise devices
can be weighted or removed to optimise the SNR. The power requirements, transient response
and received data are explored and limiting factors similar to those of photodiode receivers are
observed.
The thesis concludes that data can be captured well with such a device but more electrical
energy is needed at the receiver due to its fundamental operation. Overall, optical power can
be reduced, allowing significant savings in either transmitter power or the transmission length,
along with the advantages of an integrated digital chip
Emerging semiconductor nanostructure materials for single-photon avalanche diodes
Detecting of light at the single photon level has a far-reaching impact that enables a broad range of applications. In sensing, advances in single-photon detection enable low light applications such as night-time operation, rapid satellite communication, and long-range three-dimensional imaging. In biomedical engineering, advancing single-photon detection technologies positively impacts patient care through important applications like singlet oxygen detection for dose monitoring in cancer treatment. In industry, impacts are made on state-of-the-art technologies like quantum communication which relies on the efficient detection of light at the fundamental limit. While the high impact of single-photon detection technologies is clear, the potential for improvement and challenges faced by prominent single-photon detection technologies remains. Superconducting single-photon detectors push the bounds of performance, but their high cost and lack of portability limits their prospect for far reaching applicability. Single-photon avalanche diodes (SPADs) are a promising alternative which can be made portable, absent of the need for cryogenic cooling, but they generally lack the performance of superconducting detectors. The materials in SPAD designs dictate operation, and conventional materials implemented being defined according to intrinsic material properties, limits SPAD performance. However, new classes of advanced materials are being realized which exhibit modified electromagnetic properties from the engineered arrangement of subwavelength structural units and low-dimensional properties. Such materials include metamaterials and low-dimensional materials, and they have been shown to enhance optoelectrical properties that are critical to avalanche photodiodes, like rapid photo response, enhanced absorption, and reduced dark current. In this work, the application of such advanced materials in SPADs is explored. Tapered nanowires and nanowire arrays are optimized for enhanced absorption and shown experimentally at room temperature to demonstrate high speed near-unity absorptance response at the single-photon level. In the metamaterial and nanowire devices, the gain and timing jitter are shown to be significantly improved over conventional bulk-based designs. Furthermore, the modelling of metamaterials in a SPAD device design and its operation with external single-photon detection circuitry is studied. The analysis is further shown to extend down to single nanowire devices which offers an elegant approach for integrated photonic circuits
Probabilistic modeling for single-photon lidar
Lidar is an increasingly prevalent technology for depth sensing, with applications including scientific measurement and autonomous navigation systems. While conventional systems require hundreds or thousands of photon detections per pixel to form accurate depth and reflectivity images, recent results for single-photon lidar (SPL) systems using single-photon avalanche diode (SPAD) detectors have shown accurate images formed from as little as one photon detection per pixel, even when half of those detections are due to uninformative ambient light. The keys to such photon-efficient image formation are two-fold: (i) a precise model of the probability distribution of photon detection times, and (ii) prior beliefs about the structure of natural scenes. Reducing the number of photons needed for accurate image formation enables faster, farther, and safer acquisition. Still, such photon-efficient systems are often limited to laboratory conditions more favorable than the real-world settings in which they would be deployed.
This thesis focuses on expanding the photon detection time models to address challenging imaging scenarios and the effects of non-ideal acquisition equipment. The processing derived from these enhanced models, sometimes modified jointly with the acquisition hardware, surpasses the performance of state-of-the-art photon counting systems.
We first address the problem of high levels of ambient light, which causes traditional depth and reflectivity estimators to fail. We achieve robustness to strong ambient light through a rigorously derived window-based censoring method that separates signal and background light detections. Spatial correlations both within and between depth and reflectivity images are encoded in superpixel constructions, which fill in holes caused by the censoring. Accurate depth and reflectivity images can then be formed with an average of 2 signal photons and 50 background photons per pixel, outperforming methods previously demonstrated at a signal-to-background ratio of 1.
We next approach the problem of coarse temporal resolution for photon detection time measurements, which limits the precision of depth estimates. To achieve sub-bin depth precision, we propose a subtractively-dithered lidar implementation, which uses changing synchronization delays to shift the time-quantization bin edges. We examine the generic noise model resulting from dithering Gaussian-distributed signals and introduce a generalized Gaussian approximation to the noise distribution and simple order statistics-based depth estimators that take advantage of this model. Additional analysis of the generalized Gaussian approximation yields rules of thumb for determining when and how to apply dither to quantized measurements. We implement a dithered SPL system and propose a modification for non-Gaussian pulse shapes that outperforms the Gaussian assumption in practical experiments. The resulting dithered-lidar architecture could be used to design SPAD array detectors that can form precise depth estimates despite relaxed temporal quantization constraints.
Finally, SPAD dead time effects have been considered a major limitation for fast data acquisition in SPL, since a commonly adopted approach for dead time mitigation is to operate in the low-flux regime where dead time effects can be ignored. We show that the empirical distribution of detection times converges to the stationary distribution of a Markov chain and demonstrate improvements in depth estimation and histogram correction using our Markov chain model. An example simulation shows that correctly compensating for dead times in a high-flux measurement can yield a 20-times speed up of data acquisition. The resulting accuracy at high photon flux could enable real-time applications such as autonomous navigation
Stellar Intensity Interferometry: Prospects for sub-milliarcsecond optical imaging
Using kilometric arrays of air Cherenkov telescopes, intensity interferometry
may increase the spatial resolution in optical astronomy by an order of
magnitude, enabling images of rapidly rotating stars with structures in their
circumstellar disks and winds, or mapping out patterns of nonradial pulsations
across stellar surfaces. Intensity interferometry (pioneered by Hanbury Brown
and Twiss) connects telescopes only electronically, and is practically
insensitive to atmospheric turbulence and optical imperfections, permitting
observations over long baselines and through large airmasses, also at short
optical wavelengths. The required large telescopes with very fast detectors are
becoming available as arrays of air Cherenkov telescopes, distributed over a
few square km. Digital signal handling enables very many baselines to be
synthesized, while stars are tracked with electronic time delays, thus
synthesizing an optical interferometer in software. Simulated observations
indicate limiting magnitudes around m(v)=8, reaching resolutions ~30
microarcsec in the violet. The signal-to-noise ratio favors high-temperature
sources and emission-line structures, and is independent of the optical
passband, be it a single spectral line or the broad spectral continuum.
Intensity interferometry provides the modulus (but not phase) of any spatial
frequency component of the source image; for this reason image reconstruction
requires phase retrieval techniques, feasible if sufficient coverage of the
interferometric (u,v)-plane is available. Experiments are in progress; test
telescopes have been erected, and trials in connecting large Cherenkov
telescopes have been carried out. This paper reviews this interferometric
method in view of the new possibilities offered by arrays of air Cherenkov
telescopes, and outlines observational programs that should become realistic
already in the rather near future.Comment: New Astronomy Reviews, in press; 101 pages, 11 figures, 185
reference
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