Xiao Ai

High-resolution random-modulation cw lidar

A high-resolution random-modulation continuous wave lidar for surface detection using a semiconductor laser diode is presented. The laser diode is intensity modulated with the pseudorandom binary sequence. Its enhanced resolution is achieved via interpolation and a novel front-end analog technique, lowering the requirement of the analog-to-digital converter sampling rate and the associated circuitry. Its mathematical model is presented, including the derivation of the signal-to-noise ratio and the distance standard deviation. Analytical and experimental results demonstrate its capability to achieve distance accuracy of less than 2 cm within 2.6 ms acquisition time, over distances ranging from 1 to 12 m. The laser diode emits 1.4 mW of optical power at a wavelength of 635 nm.

An efficient algorithm for pothole detection using stereo vision

In this paper, a stereo vision based pothole detection system is proposed. Using the disparity map generated from an efficient disparity calculation algorithm, potholes can be detected by their distance from the fitted quadratic road surface. The system produces the size, volume and position of the potholes which allows the pothole repair to be prioritised according to its severity. The quadratic road surface model allows for camera orientation variation, road drainage and up/down hill gradients. Experimental results show robust detection in various scenarios.

Pseudo-Random Single Photon Counting for space-borne atmospheric sensing applications

The ability to accurately observe the Earths carbon cycles from space gives scientists an important tool to analyze climate change. Current space-borne Integrated-Path Differential Absorption (IPDA) lidar concepts have the potential to meet this need. They are mainly based on the pulsed time-of-flight principle, in which two high energy pulses of different wavelengths interrogate the atmosphere for its transmission properties and are backscattered by the ground. In this paper, feasibility study results of a Pseudo-Random Single Photon Counting (PRSPC) IPDA lidar are reported. The proposed approach replaces the high energy pulsed source (e.g. a solid-state laser), with a semiconductor laser in CW operation with a similar average power of a few Watts, benefiting from better efficiency and reliability. The auto-correlation property of Pseudo-Random Binary Sequence (PRBS) and temporal shifting of the codes can be utilized to transmit both wavelengths simultaneously, avoiding the beam misalignment problem experienced by pulsed techniques. The envelope signal to noise ratio has been analyzed, and various system parameters have been selected. By restricting the telescopes field-of-view, the dominant noise source of ambient light can be suppressed, and in addition with a low noise single photon counting detector, a retrieval precision of 1.5 ppm over 50 km along-track averaging could be attained. We also describe preliminary experimental results involving a negative feedback Indium Gallium Arsenide (InGaAs) single photon avalanche photodiode and a low power Distributed Feedback laser diode modulated with PRBS driven acoustic optical modulator. The results demonstrate that higher detector saturation count rates will be needed for use in future spacebourne missions but measurement linearity and precision should meet the stringent requirements set out by future Earth-observing missions.

Obstacle detection using U-disparity on quadratic road surfaces

This paper addresses the problem of detecting obstacles that protruding from the road. Traditionally, the road surface has been considered flat, and camera orientation is fixed. However, both assumptions are not strictly true in urban scenarios. The proposed algorithm employs a time-of-flight (ToF) camera. It allows dynamic pitch/roll angles, height variations and represents the ground as a quadratic surface. The range information given by the camera is represented in both Euclidean and disparity domains, so that their domain characteristics support each other to achieve accurate and efficient detection results. Gradient filtering of the disparity image presents Euclidean planner patches, with which outliers can be minimised during road fittings. Obstacle points are subsequently detected by the connect component labelling algorithm. Experimental results show that the proposed method can effectively segment and detect multiple obstacles and presents their bounding boxes in complex scenarios.

U-V-Disparity based Obstacle Detection with 3D Camera and steerable filter

This paper presents an obstacle detection system which is robust to non-flat road surface and interference of illumination. A 3D camera is used to generate depth information without the need of camera calibration. The depth map is then transformed into U-V-disparity domain, where obstacles and ground surface are projected as lines. Hough Transform is employed to extract line features; it has been modified to fit the characteristic of the U-V-disparity in order to boost the speed and accuracy. In addition, steerable filters are applied to the u-v histogram before Hough Transform for noise reduction. By categorising extracted lines according to their position and posture, road surface and on-road obstacles can be detected. Finally, results obtained using both U and V disparity maps are combined to eliminate road side surface and post processing. Experiments show that the proposed system is Able to detect obstacles accurately under various environments.

Analysis of a random modulation single photon counting differential absorption lidar system for space-borne atmospheric CO 2 sensing

The ability to observe the Earths carbon cycles from space provides scientists an important tool to analyze climate change. Current proposed systems are mainly based on pulsed integrated path differential absorption lidar, in which two high energy pulses at different wavelengths interrogate the atmosphere sequentially for its transmission properties and are back-scattered by the ground. In this work an alternative approach based on random modulation single photon counting is proposed and analyzed; this system can take advantage of a less power demanding semiconductor laser in intensity modulated continuous wave operation, benefiting from a better efficiency, reliability and radiation hardness. Our approach is validated via numerical simulations considering current technological readiness, demonstrating its potential to obtain a 1.5 ppm retrieval precision for 50 km averaging with 2.5 W average power in a space-borne scenario. A major limiting factor is the ambient shot noise, if ultra-narrow band filtering technology could be applied, 0.5 ppm retrieval precision would be attainable.

High-brightness all semiconductor laser at 1.57 μm for space-borne lidar measurements of atmospheric carbon dioxide: device design and analysis of requirements

The availability of suitable laser sources is one of the main challenges in future space missions for accurate measurement of atmospheric CO2. The main objective of the European project BRITESPACE is to demonstrate the feasibility of an all-semiconductor laser source to be used as a space-borne laser transmitter in an Integrated Path Differential Absorption (IPDA) lidar system. We present here the proposed transmitter and system architectures, the initial device design and the results of the simulations performed in order to estimate the source requirements in terms of power, beam quality, and spectral properties to achieve the required measurement accuracy. The laser transmitter is based on two InGaAsP/InP monolithic Master Oscillator Power Amplifiers (MOPAs), providing the ON and OFF wavelengths close to the selected absorption line around 1.57 μm. Each MOPA consists of a frequency stabilized Distributed Feedback (DFB) master oscillator, a modulator section, and a tapered semiconductor amplifier optimized to maximize the optical output power. The design of the space-compliant laser module includes the beam forming optics and the thermoelectric coolers. The proposed system replaces the conventional pulsed source with a modulated continuous wave source using the Random Modulation-Continuous Wave (RM-CW) approach, allowing the designed semiconductor MOPA to be applicable in such applications. The system requirements for obtaining a CO2 retrieval accuracy of 1 ppmv and a spatial resolution of less than 10 meters have been defined. Envelope estimated of the returns indicate that the average power needed is of a few watts and that the main noise source is the ambient noise.

Local stereo disparity estimation with novel cost aggregation for sub-pixel accuracy improvement in automotive applications

This paper presents a local disparity calculation algorithm on calibrated stereo images based on cost aggregation. Unlike most of the existing cost aggregation methods which are mainly based on the grouping of colour similarities, the proposed algorithm is grouped by local cost similarities. The proposed algorithm also applies a bilateral filter to enhance the normalised cost volume and, then, uses the winner-take-all technique to select the correspondence candidates. Finally, a quadratic polynomial interpolation is performed using the candidates and their neighbourhood values to achieve sub-pixel disparity resolution. The experimental results indicate that the proposed algorithm is able to provide dense disparity maps with sub-pixel resolution and achieves better accuracy compared to two similar stereo matching algorithms.

Obstacle detection with 3D camera using U-V-Disparity

Obstacle detection has been one of the most critical features for reliable driving scene analysis. This paper presents an approach for an automatic obstacle detection system. The proposed system makes use of depth information generated by a 3D camera mounted on the front of a moving vehicle. Obstacles projected as line features in the V-U-Disparity map can be extracted to detect the road surface and obstacles. A Steerable Filter is employed at an early stage to dramatically lower the noise. Furthermore, a modified Hough Transform is placed to extract the straight line feature from the depth map with improved accuracy. The system is robust in dealing with fault detection caused by roadside features which is a commonly shared problem in many other obstacle detection approaches.

Atmospheric CO

We propose an integrated path differential absorption (IPDA) lidar system based on a hybrid master oscillator power amplifier (MOPA) and single photon counting detection for column-averaged measurements of atmospheric CO2. The random modulated continuous wave (RM-CW) approach has been selected as the best suited to the average output power obtained from hybrid and monolithically integrated MOPAs. A compact RM-CW IPDA lidar instrument has been designed and fabricated. High-sensitivity and low-noise single photon counting has been used for the receiver. Colocated 2-km horizontal trial path experiments with a pulsed system and in situ measurements were performed for comparison. The RM-CW IPDA lidar instrument shows a relative accuracy of the order of about ±10% or ±40 parts per million CO2 concentration in absolute terms. The measurements qualitatively demonstrate the feasibility of CO2 IPDA measurements with an RM-CW system.