White light lasers for remote sensing

摘要

The advent of non-linear photonic crystal fibres with engineered optical properties has enabled the production of compact high-brightness super-continuum sources with a spectral power density in excess of 1 mWm throughout the visible and near-infrared spectral regions. Such sources have intrinsically good beam quality and, when properly collimated, the various spectral components propagate in a co-linear fashion, thus retaining spectral fidelity along the beam path. These properties are ideal for an active hyper-spectral remote sensing system.rnWe report the construction and testing of a white light transceiver for measurement of the spectral reflectivity of remote targets. The transmission section of the transceiver comprises a commercial white light source with apochromatic optics to ensure simultaneous collimation at all wavelengths. The receiver section comprises a telescope coupled to a fibre-optic visible band spectrometer. A portion of the received light is directed onto a camera to facilitate accurate pointing of the system.rnThe transceiver has been used to measure the spectral reflection from both diffuse and retro-reflecting targets at an outdoor range. The spectral return from retro-reflective targets was successfully measured at ranges up to 1.2 km. For diffuse targets, the useful range was limited to a few hundred metres, beyond which the signal was dominated by ambient daylight. The propagation of the white light beam along the 1.2 km has been studied. The fidelity of measured spectra was affected by atmospheric turbulence which caused the beam to break up into a time-varying pattern of coloured regions. This effect imposed a lower limit on the integration time required to measure individual spectra, independent of the signal to noise ratio.