Robert A. Lamb

Single-photon three-dimensional imaging at up to 10 kilometers range

Depth and intensity profiling of targets at a range of up to 10 km is demonstrated using time-of-flight time-correlated single-photon counting technique. The system comprised a pulsed laser source at 1550 nm wavelength, a monostatic scanning transceiver and a single-element InGaAs/InP single-photon avalanche diode (SPAD) detector. High-resolution three-dimensional images of various targets acquired over ranges between 800 metres and 10.5 km demonstrate long-range depth and intensity profiling, feature extraction and the potential for target recognition. Using a total variation restoration optimization algorithm, the acquisition time necessary for each pixel could be reduced by at least a factor of ten compared to a pixel-wise image processing approach. Kilometer range depth profiles are reconstructed with average signal returns of less than one photon per pixel.

Towards industrial ultrafast laser microwelding: SiO 2 and BK7 to aluminum alloy

We report systematic analysis and comparison of ps-laser microwelding of industry relevant Al6082 parts to SiO2 and BK7. Parameter mapping of pulse energy and focal depth on the weld strength is presented. The welding process was found to be strongly dependent on the focal plane but has a large tolerance to variation in pulse energy. Accelerated lifetime tests by thermal cycling from -50° to +90°C are presented. Welds in Al6082-BK7 parts survive over the full temperature range where the ratio of thermal expansion coefficients is 3.4:1. Welds in Al6082-SiO2 parts (ratio 47.1:1) survive only a limited temperature range.

Analysis of three-dimensional scenes using photon-starved data in cluttered target scenarios (Conference Presentation)

This paper investigates a new computational method for reconstruction and analysis of complex 3D scenes. In the presence of targets, Lidar waveforms usually consist of a series of peaks, whose positions and amplitudes depend on the distances of the targets and on their reflectivities, respectively. Inferring the number of surfaces or peaks, as well as their geometric and colorimetric properties becomes extremely difficult when the number of detected photons is low (e.g., short acquisition time) and the ambient illumination is high. In this work, we adopt a Bayesian approach to account for the intrinsic spatial organization of natural scenes and regularise the 3D reconstruction problem. The proposed model is combined with an efficient Markov chain Monte Carlo (MCMC) method to reconstruct the 3D scene, while providing measures of uncertainty (e.g., about target range and reflectivity) which can be used for subsequent decision making processes, such as object detection and recognition. Despite being an MCMC method, the proposed approach presents a competitive computational cost when compared to state-of-the-art optimization-based reconstruction methods, while being more robust to the lack of detected photons (empty or non-observed pixels). Moreover, it includes a multi-scale strategy which allows a quick recovery of coarse approximations of the 3D structures, while is often sufficient for object detection/recognition. We assess the performance of our approach via extensive experiments conducted with real, long-range (hundreds of meters) single-photon Lidar data. The results clearly demonstrate its benefits to infer complex scene content from extremely sparse photon counts.

A computational approach to hyperspectral imaging for long-range target identification

For long range targeting, the limited focal length and aperture size associated with compact imaging sensors for airborne operation limit both the spatial resolution and the image brightness. This presents a serious challenge to the identification and tracking of targets. Algorithms that derive target shape and track movement through a scene require a resolved image and use pixel contrast to discriminate the target image from the background. This is of limited use when practical deployment demands the use of compact imaging systems with necessarily limited spatial resolution. To address this we consider a 2D mosaic filters sampling scheme to acquire an incomplete multispectral data cube on a single frame readout from a focal plane array. Specifically, the sparse data cube contains 4 x 4 spatial cells and 16 wavebands with each waveband sampled once per cell; this corresponds to a 1/16 undersampling of the data cube. Complete multispectral images are then computed using compressed sensing protocols. Results obtained using hyperspectral datasets from AVIRIS and Stanford University (SCIEN) are presented to demonstrate image reconstruction using 16 wavebands in the visible and near infrared. The function of the mosaic filter is mimicked by sampling the full dataset according to the design of a theoretical mosaic filter. This allows us to investigate different sampling strategies and, in particular, make a direct comparison between random and regular sampling. Our results show that the reconstruction error is strongly dependent on both the colour content and the sampling strategy in the test images, and that very good reconstruction can be achieved approaching the spatial resolution of the original image. Our results can be applied to both the MWIR and LWIR where the lower spectral resolution means that a smaller number of wavebands is likely to be sufficient for identification and tracking. The concept can also be extended to polarimetric imaging with a suitable polarimetric filter mask to provide a dual-mode polarimetric-multispectral imaging capability. This paper presents an overview of the technical approach and the general conclusions.

Challenges for the future of imaging: what's next and where can we draw inspiration from? (Conference Presentation)

Recent advances in computation, optical projection, and non-traditional imaging sensor designs are making it possible to overcome classical optical challenges which have stood for centuries: breaking resolution “limits” in real world scenarios and capturing indirect images of obscured objects.

Picosecond laser welding of optical to metal components

We report on practical, industrially relevant, welding of optical components to themselves and aluminum alloy components. Weld formation is achieved through the tight focusing of a 5.9ps, 400kHz Trumpf laser operating at 1030nm. By selecting suitable surface preparation, clamping and laser parameters, the plasma can be confined, even with comparatively rough surfaces, by exploiting the melt properties of the glass. The short interaction time allows for a permanent weld to form between the two materials with heating limited to a region ~300 µm across. Practical application of these weld structures is typically limited due to the induced stress within the glass and, critically, the issues surrounding post-weld thermal expansion. We report on the measured strength of the weld, with a particular emphasis on laser parameters and surface preparation.

Picosecond laser bonding of highly dissimilar materials

We report on recent progress in developing an industrially relevant, robust technique to bond dissimilar materials through ultra-fast microwelding. This technique is based on the use of a 5.9ps, 400kHz Trumpf laser operating at 1030nm. Tight focusing of the laser radiation at, or around, the interface between two materials allows for simultaneous absorption in both. This absorption rapidly, and locally, heats the material forming plasma from both materials. With suitable surface preparation this plasma can be confined to the interface region where it mixes, cools and forms a weld between the two materials. The use of ps pulses results in a short interaction time. This enables a bond to form whilst limiting the heat affected zone (HAZ) to a region of only a few hundred micrometres across. This small scale allows for the bonding of materials with highly dissimilar thermal properties, and in particular coefficients of thermal expansion e.g. glass-metal bonding. We report on our results for a range of material combinations including, Al-Bk7, Al-SiO2 and Nd:YAG-AlSi. Emphasis will be laid on the technical requirements for bonding including the required surface preparation of the two materials and on the laser parameters required. The quality of the resultant bonds are characterized through shear force measurements (where strengths equal to and exceeding equivalent adhesives will be presented). The lifetime of the welds is also discussed, paying particular attention to the results of thermal cycling tests.