Atmospheric monitoring with SLidar

Atmospheric monitoring with SLidar

A new approach to atmospheric monitoring measures laser backscatter range as a function of angle using the Scheimpflug principle, instead of the more common time of flight method known as Lidar, which means it can monitor a large volume of space all at once. Given the name SLidar, it was demonstrated to work during daylight to measure cloud height and particle emissions using a diode laser. Compared with Lidar, the lasers and optics required for SLidar are cheaper and less complex, which could lead to small, robust and affordable devices for monitoring the skies.
Beam of light in the sky above a city

Atmospheric monitoring with SLidar

LIDAR -- LIght Detection And Ranging1 -- is the optical equivalent of RADAR, which uses radio waves to track objects like aeroplanes in the sky. Light waves have vastly shorter wavelengths and therefore offer improved resolution, both in terms of the size of objects that can be tracked and the distance to them.

One drawback of Lidar is the equipment cost in remote sensing applications such as atmospheric studies. High-power pulsed lasers are typically required to receive adequate levels of backscattered radiation, and the transmission optics must also be capable of handling those high energy pulses.

A new approach by Swedish researchers uses a continuous wave (CW) laser diode, and instead of measuring the time of flight of a laser pulse, as with Lidar, instead measures range as a function of angle using the Scheimpflug principle, which describes the relationship between the plane of focus when an object plane and image plane are non-parallel. They call this method SLidar2.


Illustration of the Scheimpflug principle (Attribution: By Fil Hunter at English Wikipedia [Public domain], via Wikimedia Commons)
The Scheimpflug Principle3

CW laser diodes are capable of producing high powers at low cost, and the laser damage thresholds of the optical components can be significantly lower for apparatus using them. This helps minimize the overall cost of a working SLidar device and also opens up the opportunity to perform atmospheric studies during daylight hours. Lidar applications are typically restricted to night-time use because of the low backscattered signal strengths amidst the deleterious effects of background radiation from the sun.

The research successfully demonstrated SLidar's ability to measure cloud height and particle emissions into the atmosphere. They used a laser diode emitting at 808 nm, and the backscattered radiation was received by a Newtonian telescope located 100 m from the laser source. A tilted CCD array was used as the imaging sensor, in accordance with the Scheimpflug principle, allowing an entire volume of atmosphere to be monitored at once.

A proposal for future work is to introduce either a multi-band light source or a means of measuring the polarization of the reflected light, to discriminate particle size and type.

Mei, L., & Brydegaard, M. (2015). Atmospheric aerosol monitoring by an elastic Scheimpflug lidar system Optics Express, 23 (24) DOI: 10.1364/OE.23.0A1613

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