TECHNICAL FIELD

The present disclosure relates to laser projection systems and more particularly to Scheimpflug LIDAR systems and methods.

BACKGROUND ART

A LIDAR or laser radar is an optical device for detection and ranging with applications in a very broad range of environments, from industrial combustion furnaces to ecosystem monitoring. In contrast to the now wide-spread topographical LIDAR systems which detect and range hard targets, atmospheric LIDARs have sufficient sensitivity to retrieve a continuous molecular echo from entirely clean air.

Atmospheric LIDARs have been around for several decades and they have been extensively applied to vertical profiles of aerosols in the troposphere. They are typically implemented in containers or trucks and comprise systems weighting several tons. Smaller commercial systems weighing several hundred kilograms have also been developed. The cost of such systems is approximately 1 Euro per gram of equipment. The conventional method used by such systems relies on Time-of-Flight (ToF) principles with expensive and bulky pulsed neodymium-doped yttrium aluminium garnet lasers of several hundred kilograms. The laser provide high peak powers (˜100 MW) but the system has poor resolution in time and space (approx. ˜1 minute and ˜50 m). Elastic aerosol LIDAR systems are exceedingly challenging to calibrate and yield very little specificity for the aerosols sensed unless they are expanded by N2and O2Raman channels. Raman channels only work during night time, are very noisy and require even longer averaging times of typically 10 minutes to produce a usable signal.

A highly specific atmospheric LIDAR method is the Differential Absorption LIDAR (DIAL). In this method, a pulsed tuneable laser targets specific molecular absorption lines and concentration profiles of a gas can be acquired. In practice, high peak powers (MW), short pulses (ns), narrow bands (<pm) and tunability contradict each other. Such DIAL systems typically require a small team of PhDs in laser physics to run. Some progress has been reported on lighter and smaller DIAL systems using micro-LIDAR, but still with time resolution in the order of 10 minutes. As a consequence, DIAL systems are immensely expensive and there are only a handful operational on a global basis. The low resolution, the cost and the bulkiness of DIAL systems prevent many practical applications such as industrial process optimization and mapping of greenhouse gas sources and fluxes.

There exists a need for a LIDAR method sufficiently sensitive enough to allow profiling, mapping and visualization of the exhausted O2-hole from a single breath, e.g., with a resolution of millimetres, and capable of operating within milliseconds. This would allow the capture of the exhaust plume as it flies by, and the determination of the exhaust temperature.

Known problems with existing atmospheric LIDAR systems include the limitations on the optical performance. For example, existing systems may suffer from non-linear and ambiguous range dependency, ghosting and optical artefacts, and poor collection efficiency. A solution is needed to address and overcome these limitations in order to advance the technology and make it practically competitive on the market.

SUMMARY

It is an objective of the disclosure to at least partly overcome one or more of the above-identified limitations of the prior art. One or more of these objectives, as well as further objectives that may appear from the description below, are at least partly achieved by means of a method for data processing, a computer readable medium, devices for data processing, and an optical apparatus according to the independent claims, embodiments thereof being defined by the dependent claims.

A first aspect of the disclosure provides a device for detecting a property of a gas comprising: a light source configured to emit a light along at least a first axis, a light detection arrangement comprising: a lens arrangement having a lens plane, a displaced image plane, and a front focal plane, and being configured to direct the light from the light source and scattered by the gas to a light sensor, the light sensor comprising at least one column of pixels aligned to an image plane and configured to output a sensor signal, wherein the first axis, the lens plane, and the image plane intersect such that a Scheimpflug condition is achieved, wherein the displaced image plane, the front focal plane of the lens arrangement, and a relationship between the light source and the light detection arrangement fulfil the Hinge rule intersection, and wherein the light source comprises an expander aperture, and wherein the expander aperture and lens arrangement are configured such that:a spot size of the emitted light along the first axis is matched to a pixel footprint of pixels configured to receive light from corresponding distances along the first axis, andan effective range resolution of at least one column of pixels or probe volume deteriorates linearly with respect to the range. The dimensions of the light source may be matched with the respective focal lengths of the pixels of the at least one column of pixels. The light source may be configured to emit a sheet of light along at least the first axis, wherein the thickness of light sheet, in combination with the focal length of the lens arrangement, is matched to the pixel width of the pixels of the at least one column of pixels. The light source may be one or more of; a narrowband single-mode source, a broad band multi-mode source, a high power multimode diode laser, a high power multimode fibre laser, a high power tapered amplifier seeded by a tuneable single mode diode laser, a high-power fibre amplifier seeded by a tuneable single mode diode laser, and a high power tuneable CO2laser. The light sensor may comprise a transmissive layer and wherein the lens arrangement being configured to direct modulated light scattered by the gas on to the transmissive layer at Brewster's angle such that P-polarized light is transmitted through the transmissive layer of light sensor. The light sensor may be configured to only detect P-polarized light and wherein the light source is configured to emit polarized light. The light source may comprise a polarization rotation controller comprising at least one of: a liquid crystal, a Pockels cell, and an arrangement comprising a super position of a plurality of emitters with orthogonal polarizations in combination with a polarization beam splitter. The lens arrangement may comprise at least one of: an imaging lens comprising one or more light refracting components, and a mirror lens comprising a catadioptric optical system. Preferably, a data processing device is employed to process the sensor signal to determine a property of the gas.

In another aspect of the disclosure, a method is provided for detecting a property of a gas comprising: emitting a light along at least a first axis, generating a sensor signal using a detection arrangement comprising: a lens arrangement having a lens plane, a displaced image plane, and a front focal plane, and being configured to direct the light from the light source and scattered by the gas to a light sensor, the light sensor comprising at least one column of pixels aligned to an image plane and configured to output a sensor signal, wherein the first axis, the lens plane, and the image plane intersect such that a Scheimpflug condition is achieved, and wherein the displaced image plane, the front focal plane of the lens arrangement, and a relationship between the light source and the light detection arrangement fulfil the Hinge rule intersection wherein the light source comprises an expander aperture, and wherein the expander aperture and lens arrangement are configured such that:a spot size of the emitted light along the first axis is matched to a pixel footprint of pixels configured to receive light from corresponding distances along the first axis, andan effective range resolution of at least one column of pixels or probe volume deteriorates linearly with respect to the range.

