Hayato Kondo

Interface properties of InAs quantum dots produced by antimony surfactant-mediated growth: Etching of segregated antimony and its impact on the photoluminescence and lasing characteristics

We present a method that improves the emission efficiency of InAs quantum dots (QDs) fabricated by antimony surfactant-mediated metal organic chemical vapor deposition. This process consists of removing the excess segregated antimony from the surface of InAs/Sb:GaAs QDs by applying a high arsenic pressure before capping. In such a way, one benefits from the advantages of InAs/Sb:GaAs QDs (high density, low coalescence) without the formation of antimony-induced nonradiative defects. Finally, we show that this better QD interface quality results in a strong decrease of the threshold current densities of InAs/Sb:GaAs QD lasers in the 1.3 μm band.

Positioning method for an AUV using a profiling sonar and passive acoustic landmarks for close-range observation of seafloors

Real-time accurate positioning is a key technology for autonomous underwater vehicles to perform close-range seafloor observations such as photo mosaicing. This paper proposes a real-time positioning method that realizes robust and drift-free positioning in a local area based on a passive acoustic landmarks set in the environment and a profiling sonar (profiler) mounted on the vehicle. The method stochastically updates the vehicles position based on all the sensory data available. Although the position of landmarks is generally unknown, this paper assumes the position is known without error for simplification. As an observation model of the profiler we propose a radial-angular representation with a non-Gaussian distribution based on experimental data. The performance of the proposed method is verified through tank experiments using the AUV Tri-Dog 1. The effects of the number of landmarks and observation model on the positioning accuracy is discussed. Through comparison with ground truth and off-line estimations based on actual data obtained during the experiments, the positioning accuracy of the proposed method remains within 5 centimeters at all times during the 2 hour duration of the experiment with a traveled distance of 600 meters, with two landmarks at a distance of 10 meters.

Navigation of autonomous underwater vehicles based on artificial underwater landmarks

Near underwater structures or the surface of shallow water, acoustic sensing based navigation of autonomous underwater vehicles (AUVs) suffers from inaccurate positioning which is caused by acoustic sensing such as multi-pass and noisy data. In the paper a new underwater navigation method for AUVs is proposed based on artificial underwater landmarks which are recognized by a vision system implemented in the robot. An underwater image processing strategy and an underwater vision environment analysis method are introduced to improve reliability of the underwater vision system. Experiments were carried out to demonstrate the efficiency of the proposed navigation method of AUVs.

Thruster fault-tolerant control of a hovering AUV with four horizontal and two vertical thrusters

This paper deals with the motion control of a hovering autonomous underwater vehicle (AUV) with four horizontal and two vertical thrusters, when one or more thrusters are completely malfunctioned. Three thruster fault cases are considered: one horizontal thruster is faulty; two horizontal thrusters are faulty; and one vertical thruster is faulty. Through a series of simulations and an experiment, it is validated that the AUV can track a planned path in a 3-D space with minimally three thrusters (two are horizontal ones); however, some cases require the changes of the vehicle’s preferred direction of motion and its movement manner. Additionally, when the number of active horizontal thrusters is less than the required degree-of-freedom, a continuous state feedback control law does not exist due to the non-holonomic constraint. This paper highlights that the hovering AUV can overcome the non-holonomic constraint if using the feature that their translational and rotational motions can be controlled independently. Graphical Abstract

Real time path-planning of an AUV based on characteristics of passive acoustic landmarks for visual mapping of shallow vent fields

Although underwater vent fields are of great scientific interest, accurate visual mapping is difficult because of the presence of bubble plumes that degrade the accuracy of conventional acoustic positioning systems such as long base line (LBL) and super short base line (SSBL). The authors had proposed a visual mapping method of shallow vent fields with an autonomous underwater vehicle (AUV) equipped with a profiling sonar, where positioning is based on vertical rod-shaped acoustic reflectors and bubble plumes. Although performance was verified through a series of experiments, there remain two challenges as follows. Firstly, observation is terminated if the vehicle was surrounded by bubble plumes, since the vehicle tries to avoid collision with not only artificial reflectors but also collision-safe bubble plumes. Secondly, the observation area drifts by disturbance since the waypoints are defined relative to the vehicles position after descending close to seafloor. This paper proposes a real-time path planning method of an AUV as a part of the proposed observation method. The path of the vehicle is defined based on the types of landmarks as well as the geometric relationship between the vehicle and the landmarks. The vehicle can distinguish landmark types using a sheet laser and a camera. The proposed method was implemented on the AUV Tri-Dog 1 and a series of experiments were carried out in order to verify its performance.

Navigation of the AUV for Investigation of Underwater Structures

There is a great demand for autonomous underwater vehicles (AUVs) to investigate artificial underwater structures such as piles and caissons in harbours, and risers and jackets of deep-sea oilfields. This paper proposes an autonomous investigation method of underwater structures using AUVs that is implemented by initially detecting the target objects, localizing them, then approaching them by taking video images while closely tracing their shape. A laser ranging system and a navigation method based on the relative position with respect to the target objects are introduced to realize this behaviour.

Underwater structure observation by the AUV with laser pointing device

To observe underwater structures such as artificial piles in harbors by taking visual images, the autonomous underwater vehicles (AUVs) have advantages that they have no problem related to the umbilical cable which may get entangled with the target object and surrounding obstacles. It is usually difficult to measure the precise configuration of such objects by using acoustic measurement systems because of poor resolution and diffused reflection of sound. This paper proposes sensing and navigation methods to observe such objects based on images by a CCD camera with laser pointers, on the assumption that the visibility is enough for taking their visual images. The navigation system cannot provide accurate enough position. Thus, AUVs change over their navigation basis from navigation system to relative position to the target so as to trace the shape of that. The proposed method was implemented in the testbed AUV Tri-Dog 1 and proved experimentally through tank tests. The vehicle accomplished the mission that autonomously detects 3 piles of 1 meter in diameter that are modeled on actual piles in a harbor, and turns around them. Based on the proposed system, AUVs will be able to carry out surveys of the target object without any help from outside.

On fault-tolerant control of a hovering AUV with four horizontal and two vertical thrusters

This paper discusses fault-tolerant control of a hovering AUV with four horizontal and two vertical thrusters. Three thruster fault cases are considered: the first one is when any one of the four horizontal thrusters is faulty; the second is when any two of the four horizontal thrusters are faulty; and the third is when any two of the four horizontal thrusters and any one of the two vertical thrusters are faulty. Through a series of simulations, it is shown that the AUV can track a planned path on a horizontal plane with minimally three thrusters (two are horizontal ones); however, some cases require sideways motion, cruising AUV-like motion, and oblique motion. Additionally, we briefly introduce our AUV simulator that can run hardware-in-the-loop simulation.

Ground state lasing at 1.30 µm from InAs/GaAs quantum dot lasers grown by metal–organic chemical vapor deposition

We investigated the effects of post-growth annealing on the photoluminescence (PL) characteristics of InAs/GaAs quantum dots (QDs) grown by metal-organic chemical vapor deposition (MOCVD). The onset temperature at which both the peak linewidth and the PL intensity degraded and the blueshift of the ground state emission wavelength occurred was found to depend on both the QD density and the In composition of the capping layer. This behavior is particularly important in view of QD integration in photonic devices. From the knowledge of the dependences of the PL characteristics after annealing on the QD and capping growth conditions, ground state lasing at 1.30 microm could be demonstrated from InAs/GaAs QDs grown by MOCVD. Finally, we compared the laser characteristics of InAs/GaAs QDs with those of InAs/Sb:GaAs QDs, grown according to the antimony-mediated growth technique, and showed that InAs/Sb:GaAs QDs are more appropriate for laser fabrication at 1.3 microm by MOCVD.

Motion estimation and mapping by autonomous underwater vehicles in sea environments

This paper describes a framework for motion estimation and mapping of unstructured underwater environment using multiple onboard sensors. The fundamental goal of this work is to estimate the current pose of the vehicle and build a map of its surroundings. This goal is framed with the context of improving autonomous underwater vehicle navigation for undersea explorations. The paper considers Inertial Navigation System (INS), Sector Scan Sonar and a monocular CCD camera for the purpose of mapping and motion estimation. It discusses a robust feature tracking technique from image sequences and thereby estimating the ego-motion. It also discusses about the trajectory recovery from the sonar and fusion of INS and sonar. Results are shown for the real world data set collected by an autonomous underwater vehicle (AUV) in an unstructured underwater environment.