Robert Lamb began his career with the UK MoD 1986 developing high power solid state lasers using phase conjugation with stimulated Brillouin scattering and multi-wavelength lasers with stimulated Raman scattering. He later began the first work on the use of time correlated single photon counting for long range lidar, rangefinding, target identification and ultra-low light level sensing. He is now the Chief Technologist for Electro-optics at Leonardo with responsibility for future technology investment and applied research in lasers and photonics.
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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.
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.
However, as protection measures and signal processing techniques advance, one can anticipate that the requirement for sophisticated laser emission in the MWIR becomes more refined. In particular, broadband emission covering a wider, continuous, spectral region will prove harder to counter than that from a few discrete wavelengths. A supercontinuum has been suggested as a possible mechanism for broadband emission. In most investigations into supercontinuum generation, the emphasis has been on producing a wide, flat spectrum covering several hundred nanometres in the visible, near and short wave infrared for stand-off spectroscopic sensing of chemical agents, atmospheric sensing or hyperspectral sensing. These supercontinua are characterised by a spectral bandwidth to pump wavelength ratio of, δλ/λp<1 for a pump wavelength λp in the visible or near infrared. In most applications, the simultaneous generation of a wide spectrum is not required; instead a tuned output suffices. This has the added benefit of improving the efficiency of the laser sensor system since wavelengths which are not required, are not generated. The problem is to understand how a limited continuum might be generated. In the context of DIRCM, the spectral requirement is to produce a controlled spectral emission which matches the 3-5μm atmospheric transmission window.
In this paper, a theoretical calculation is presented which shows that a continuous spectrum spanning a few hundred nanometres in the mid infrared (δλ/λp~0.2) can be generated in a simple pump geometry from a mode-locked, ultra-short pulse train using self phase modulation (SPM). Spectral broadening centered on the CO2 absorption band at 4.26μm can be excited to produce all wavelengths for emission in band IV DIRCM. The parameters which affect the spectral output such as pulse power, interaction length, pulse duration and pulse shape are considered for the case where the pump geometry is a collimated beam propagating through a mid infrared glass characterised by a non-linear refractive index n2. The prospects for developing a suitable pump laser are also discussed, in particular, the possibility of using a modelocked QCL.
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