Edward Fisher

A Reconfigurable Single-Photon-Counting Integrating Receiver for Optical Communications

A reconfigurable Single-Photon Avalanche Diode integrating receiver in standard 130 nm CMOS is presented for optical links with an array readout bandwidth of 100 MHz. A maximum count rate of 58 G photon/s is observed, with a dynamic range of ≈ 79 dB, a sensitivity of ≈ - 31.7 dBm at 100 MHz and a BER of ≈ 1 ×10-9. The sensor core draws 89 mW at the maximum count rate and obtains a peak SNR of ≈157 dB. 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 32 × 32 SPAD array, achieves a minimum dead time of 5.9 ns, and a median dark count rate of 2.5 kHz/SPAD. The internal gain of SPADs and spatio-temporal summation removes the need for analogue amplification. High noise devices can be weighted or removed to optimize the SNR. The power requirements, transient response and received data are explored and limiting factors similar to those of photodiode receivers are observed.

Progress towards non-intrusive optical measurement of gas turbine exhaust species distributions

We report on the development of three systems intended to provide fast, non-intrusive measurement of cross-sectional distributions of pollutant species within gas turbine exhaust flows, during ground-based testing. This research is motivated by the need for measurement systems to support the introduction of technologies for reducing the environmental impact of civil aviation. Tomographic techniques will allow estimation of the distributions of CO2, unburnt hydrocarbons (UHC), and soot, without obstruction of the exhaust, bypass or entrained flows, from measurements made in a plane immediately aft of the engine.

A reconfigurable 14-bit 60GPhoton/s Single-Photon receiver for visible light communications

A reconfigurable Single-Photon Avalanche Diode integration mode receiver in 130nm CMOS is presented for optical links with an array readout bandwidth of 100MHz. The all-digital 32×32 SPAD array achieves a minimum dead time of 5.9ns, and a median dark count rate of 2.5kHz per SPAD. Pulse shortening increases the dynamic range by preventing pulse overlap. An in-pixel feedback loop allows synchronous or self-clocked asynchronous time division multiplexing. The internal gain of SPADs and spatio-temporal summation removes the need for analogue amplification. A maximum count rate of 58GHz is observed, with SNR of 79dB, a sensitivity of -31.7dBm at 100MHz and a BER of 10-9. The sensor core draws 89mW at maximum count rate.

Implementation of non-intrusive jet exhaust species distribution measurements within a test facility

We report on the installation and commissioning of two systems for the measurement of cross-sectional distributions of pollutant species in jet exhaust, within the engine ground test facility at INTA, Madrid. These systems use optical tomography techniques to estimate the cross-sectional distributions of CO2 and soot immediately behind the engine. The systems are designed to accommodate the largest civil aviation engines currently in service, without obstruction of the exhaust or bypass flows and with negligible effect upon the entrained flow behavior. We describe the physical construction and installation status of each system. In the case of the CO2 system, we examine the challenges of achieving the structural rigidity necessary for adequate suppression of pointing error within 126 laser-based transmittance measurements, each utilizing a 7 m overall path length. We describe methods developed for efficient implementation of co-planarity and 4-degree-of-freedom alignment of individual paths within this beam array. We also present laboratory performance data for three alternative optical designs that differ in their approach to the management of pointing error and turbulence-induced beam wander and spread. The FLITES soot monitoring capability is based on laser induced incandescence (LII) and uses a short-pulse fiber laser and two CCD cameras, in an autoprojection arrangement. We describe the measurement geometry currently being implemented in the test cell and discuss optical design issues, including once again the effect of the plume itself.

An FPGA-based lock-in detection system to enable Chemical Species Tomography using TDLAS

This paper presents the design, implementation and test of a compact, low-cost and fully digital signal recovery system for tunable diode laser absorption spectroscopy (TDLAS) in narrow line-width gas sensing applications. An FPGA-based digital lock-in amplifier (DLIA), in conjunction with TDLAS using the wavelength modulation spectroscopy (WMS) technique, is utilized to demodulate and extract first (1f) and second (2f) harmonic signals for a narrow CO2 feature in the spectrum region of 1997.2nm. The spectrum in this wavelength region shows suitably weak water absorption, enabling CO2 detection with high resolution. Gas-cell experiments were carried out using the DLIA and a conventional rack-mounted commercial lock-in amplifier. The comparison between the two systems shows good agreement, validating the feasibility of this approach and demonstrating the prospect for extension to a massively multichannel system to implement Chemical Species Tomography.

TDLAS using FPGA-based lock-in detection for multi-channel Chemical Species Tomography

In this paper, a large scale, multi-channel optical Chemical Species Tomography (CST) system is presented to aid research into aero jet-engine performance. Through high-speed, non-intrusive measurement and spatio-temporal imaging of gas concentration dynamics; fuel efficiency, emissions, combustion diagnostics and novel engine designs, can be tested to achieve greener aviation. A 126-channel tunable diode laser absorption spectroscopy (TDLAS) system is proposed, utilizing wavelength modulation spectroscopy (WMS) for robust noise performance in harsh industrial environments. Narrow CO2 absorption spectra, in the region of 1997.2nm, can be obtained from first (1f) and second (2f) harmonics of WMS signals by an FPGA-based digital lock-in amplifier (DLIA). Comparative single-channel gas-cell experiments, using the DLIA and a commercial rack-mounted system, demonstrate the clear prospect of all-digital, scalable, multi-projection CST of jet-engine exhaust-plumes. Further, the low cost and compact nature of FPGAs indicate the feasibility of reduced image reconstruction distortion, through massively multi-channel diagnostic tomography instrumentation.

An experimentally verified model for estimating the distance resolution capability of direct time of flight 3D optical imaging systems

This report introduces a new statistical model for time-resolved photon detection in a generic single-photon-sensitive sensor array. The model is validated by comparing modelled data with experimental data collected on a single-photon avalanche diode sensor array. Data produced by the model are used alongside corresponding experimental data to calculate, for the first time, the effective distance resolution of a pulsed direct time of flight 3D optical imaging system over a range of conditions using four peak-detection algorithms. The relative performance of the algorithms is compared. The model can be used to improve the system design process and inform selection of the optimal peak-detection algorithm.

Interleaving and Error Concealment to Mitigate the Impact of Packet Loss in Resource-Constrained TDLAS/WMS Data Acquisition

Tomographic imaging of pollutant gas emissions from aeroengines is attractive for the development of engines and fuels. A 126-beam tomographic setup has previously been proposed utilizing tunable diode laser absorption spectroscopy aiming for fast spatially resolved measurement of CO2 concentration. The custom data acquisition system uses a distributed architecture with at-site digital lock-in amplification, but remains resource constrained. A calibrated model is fitted to quadrature the first and second harmonic data, however, packet loss in ethernet and/or wireless networks can cause nondeterministic errors in the curve fitting and increased errors in recovered gas concentrations. Packet loss in this case, is a product of the available protocol, the high-vibration and high-noise industrial testing environment, the high network utilization expected, and the interrupt behavior of the embedded microprocessors. In this paper, the structure of the data acquisition system and the curve fitting approach are briefly discussed. Packet loss is then performed numerically to demonstrate the introduction of errors, as this cannot be swept experimentally without introducing other factors and increasing additive noise. An interleaving and error concealment mitigation approach is reported, that reduces this error, and can be applied to other resource-constrained remote acquisition systems such as Internet of Things applications. This approach is evaluated over parameters including extent of packet loss, interleaving ratio, and number of wavelength samples per packet. Viewing packet loss as a measurement SNR modifier, interleaving is shown to recover some SNR, but is ultimately limited. Processing of the received data using error concealment prior to spectrographic fitting is shown to increase tolerance.

Constrained models for optical absorption tomography

We consider the inverse problem of concentration imaging in optical absorption tomography with limited data sets. The measurement setup involves simultaneous acquisition of near-infrared wavelength-modulated spectroscopic measurements from a small number of pencil beams equally distributed among six projection angles surrounding the plume. We develop an approach for image reconstruction that involves constraining the value of the image to the conventional concentration bounds and a projection into low-dimensional subspaces to reduce the degrees of freedom in the inverse problem. Effectively, by reparameterizing the forward model, we impose, simultaneously, spatial smoothness and a choice among three types of inequality constraints, namely, positivity, boundedness, and logarithmic boundedness in a simple way that yields an unconstrained optimization problem in a new set of surrogate parameters. Testing this numerical scheme with simulated and experimental phantom data indicates that the combination of affine inequality constraints and subspace projection leads to images that are qualitatively and quantitatively superior to unconstrained regularized reconstructions. This improvement is more profound in targeting concentration profiles of small spatial variation. We present images and convergence graphs from solving these inverse problems using Gauss-Newtons algorithm to demonstrate the performance and convergence of our method.