Heterogeneous teams of robots, leveraging a balance between autonomy and human interaction, bring powerful capabilities to the problem of exploring dangerous, unstructured subterranean environments. Here we describe the solution developed by Team CSIRO Data61, consisting of CSIRO, Emesent and Georgia Tech, during the DARPA Subterranean Challenge. These presented systems were fielded in the Tunnel Circuit in August 2019, the Urban Circuit in February 2020, and in our own Cave event, conducted in September 2020. A unique capability of the fielded team is the homogeneous sensing of the platforms utilised, which is leveraged to obtain a decentralised multi-agent SLAM solution on each platform (both ground agents and UAVs) using peer-to-peer communications. This enabled a shift in focus from constructing a pervasive communications network to relying on multi-agent autonomy, motivated by experiences in early circuit events. These experiences also showed the surprising capability of rugged tracked platforms for challenging terrain, which in turn led to the heterogeneous team structure based on a BIA5 OzBot Titan ground robot and an Emesent Hovermap UAV, supplemented by smaller tracked or legged ground robots. The ground agents use a common CatPack perception module, which allowed reuse of the perception and autonomy stack across all ground agents with minimal adaptation.

Nicolas Hudson Fletcher Talbot Mark Cox Jason Williams Thomas Hines Alex Pitt Brett Wood Dennis Frousheger Katrina Lo Surdo Thomas Molnar Ryan Steindl Matt Wildie Inkyu Sa Navinda Kottege Kazys Stepanas Emili Hernandez Gavin Catt William Docherty Brendan Tidd Benjamin Tam Simon Murrell Mitchell Bessell Lauren Hanson Lachlan Tychsen-Smith Hajime Suzuki Leslie Overs Farid Kendoul Glenn Wagner Duncan Palmer Peter Milani Matthew O'Brien Shu Jiang Shengkang Chen Ronald C. Arkin

The DARPA Subterranean Challenge was designed for competitors to develop and deploy teams of autonomous robots to explore difficult unknown underground environments. Categorised in to human-made tunnels, underground urban infrastructure and natural caves, each of these subdomains had many challenging elements for robot perception, locomotion, navigation and autonomy. These included degraded wireless communication, poor visibility due to smoke, narrow passages and doorways, clutter, uneven ground, slippery and loose terrain, stairs, ledges, overhangs, dripping water, and dynamic obstacles that move to block paths among others. In the Final Event of this challenge held in September 2021, the course consisted of all three subdomains. The task was for the robot team to perform a scavenger hunt for a number of pre-defined artefacts within a limited time frame. Only one human supervisor was allowed to communicate with the robots once they were in the course. Points were scored when accurate detections and their locations were communicated back to the scoring server. A total of 8 teams competed in the finals held at the Mega Cavern in Louisville, KY, USA. This article describes the systems deployed by Team CSIRO Data61 that tied for the top score and won second place at the event.

Navinda Kottege Jason Williams Brendan Tidd Fletcher Talbot Ryan Steindl Mark Cox Dennis Frousheger Thomas Hines Alex Pitt Benjamin Tam Brett Wood Lauren Hanson Katrina Lo Surdo Thomas Molnar Matt Wildie Kazys Stepanas Gavin Catt Lachlan Tychsen-Smith Dean Penfold Leslie Overs Milad Ramezani Kasra Khosoussi Farid Kendoul Glenn Wagner Duncan Palmer Jack Manderson Corey Medek Matthew O'Brien Shengkang Chen Ronald C. Arkin

We demonstrate the fractional Talbot effect of nonpraxial accelerating beams, theoretically and numerically. It is based on the interference of nonparaxial accelerating solutions of the Helmholtz equation in two dimensions. The effect originates from the interfering lobes of a superposition of the solutions that accelerate along concentric semicircular trajectories with different radii. Talbot images form along certain central angles, which are referred to as the Talbot angles. The fractional nonparaxial Talbot effect is obtained by choosing the coefficients of beam components properly. A single nonparaxial accelerating beam possesses duality --- it can be viewed as a Talbot effect of itself with an infinite or zero Talbot angle. These results improve the understanding of nonparaxial accelerating beams and the Talbot effect among them.

Yiqi Zhang Hua Zhong Milivoj R. Belić Changbiao Li Zhaoyang Zhang Feng Wen Yanpeng Zhang Min Xiao

The temporal Talbot effect refers to the periodic self-imaging of pulse trains in optical fibers. The connection between the linear and nonlinear temporal Talbot effect is still not fully understood. To address this challenge, we use Soliton Radiation Beat Analysis and numerically investigate the evolution of a phase-modulated continuous-wave laser input in a passive single-mode fiber. We identify three input-power-dependent regimes and their Talbot carpets: the quasi-linear regime for low input powers, the intermediate one, and separated Talbot solitons for higher powers. We show that the intermediate regime hosts soliton crystals rather than rogue waves, as reported in the literature. The Talbot-solitons beating can be used for pulse repetition-rate multiplication in the nonlinear regime. We also show two types of solitons involved: some encoded in the whole frequency comb and the individual solitons carried only by particular comb lines.

Marina Zajnulina Michael Böhm

We demonstrate that under certain conditions, fractional Talbot revivals can occur in heterostructures of composite metamaterials, such as multilayer positive and negative index media, metallodielectric stacks, and one-dimensional dielectric photonic crystals. Most importantly, without using the paraxial approximation we obtain Talbot images for the feature sizes of transverse patterns smaller than the illumination wavelength. A general expression for the Talbot distance in such structures is derived, and the conditions favorable for observing Talbot effects in layered heterostructures is discussed.

Simin Feng Klaus Halterman Pamela L. Overfelt

We report the first observation of the periodical properties for Talbot effect with {\pi} phase jump. Analytical expressions are derived from simplified modal method to analyze the novelty phenomenon of the Talbot effect with {\pi} phase jump, which can deepen our understanding of physical diffraction process. Importantly, the physical reason of {\pi} phase jump can be attributed to that the two even grating modes make the left derivative and right derivative of real part of the E1 opposite in sign, which results in the physical information of first order diffractive wave hidden in the near field Talbot effect image. We expect that this theoretical work will be helpful for the tremendous potential applications of the Talbot effect.

Shubin Li

When light is incident upon a diffraction grating, images of the grating appear at periodic intervals behind the grating. This phenomenon and the associated self-imaging distance were named after Talbot who first observed them in the nineteenth century. A century later, this effect held new surprises with the discovery of sub-images at regular fractional distances of the Talbot length. In this paper, we show that water waves enable one to observe the Talbot effect in a classroom experiment. Quantitative measurements, of for example the Talbot distances, can be performed with an easy to use digital Schlieren method.

Alexandra Bakman Shmuel Fishman Mathias Fink Emmanuel Fort Sander Wildeman

The Talbot effect, also referred to as self-imaging or lensless imaging, was originally discovered in the 1830's by Henry Fox Talbot. Over the years, various investigators have found different aspects of this phenomenon, and a theory of the Talbot effect capable of explaining the various observations based on the classical theory of diffraction has emerged. Unfortunately, many of the standard Optics textbooks do not discuss the Talbot effect. The goal of the present paper is to bring to the reader's attention the essential features as well as an elementary explanation of this wonderful phenomenon.

Masud Mansuripur

The temporal Talbot effect refers to the periodic revivals of a pulse train propagating in a dispersive medium, and is a temporal analog of the spatial Talbot effect with group-velocity dispersion in time replacing diffraction in space. Because of typically large temporal Talbot lengths, this effect has been observed to date in only single-mode fibers, rather than with freely propagating fields in bulk dispersive media. Here we demonstrate for the first time the temporal Talbot effect in free space by employing dispersive space-time wave packets, whose spatio-temporal structure induces group-velocity dispersion of controllable magnitude and sign in free space.

Layton A. Hall Sergey A. Ponomarenko Ayman F. Abouraddy

Conventional projection Talbot lithography usually employs opaque (amplitude) or transparent (phase) masks for creating a periodic array of Fresnel diffraction fringes in the photosensitive substrate. For particular mask design the longitudinal periodicity of Talbot carpet can be avoided producing quasi uniform striped pattern (Talbot stripes). We propose a novel hybrid amplitude-phase mask which is engineered for obtaining extremely smooth Talbot stripes and simultaneously high lateral optical contrast and extreme spatial resolution better than a third of laser wavelength. By means of the numerical simulations, we demonstrate the robustness of produced striped diffraction patterns against mask design deviation and light incidence angle variations. The reproducibility of the Talbot stripes is reported also for 1D and 2D metal-dielectric projection masks.

Yu. E. Geints I. V. Minin O. V. Minin