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Backward lasing – a breakthrough for remote sensing

This video shows backward and forward lasing signals from oxygen atoms in a flame recorded in real time. The forward lasing signal is stronger, but of limited diagnostic value.

Ever since it was invented in the 1960s, the laser has found new areas of use, and is today very useful in research, industry and society. In most laser applications, for example material processing, the laser can be placed very close to the area to be processed. However, there are many important application areas where this is problematic. For example, if you want to use the laser to study an area in the atmosphere, it would have to be placed behind the measurement volume so that the laser beam can penetrate through it and reach a detector on the ground. Although this is possible in principle, the concept is associated with significant limitations, technical challenges, and high costs. In this research we develop a new measurement concept based on backward lasing, that is a laser-like beam is generated in the opposite direction to a pump laser beam targeting the region of interest.

Today's atmospheric research is based on optical techniques where a laser placed on the ground emits a beam aimed at the measurement area, whereupon a detector, also placed on the ground, registers the laser light back-scattered off molecules and particles in the atmosphere, so-called light detection and ranging (LIDAR), or laser-radar technology. The major limitation with the laser-radar technology is that the laser light when it hits an air particle is essentially scattered equally in all directions, which means that an extremely small fraction of the scattered laser light reaches the detector on the ground. In addition, this proportion decreases quadratically with the distance to the measurement point. This is detrimental for the detection sensitivity and thus limits the measurement possibilities considerably.

Within this research project, we try to overcome this fundamental limitation. This is done by exploiting the fact that it is possible to generate laser action, or lasing as we call it, in air, or other gases, without using any mirrors surrounding the gas as normally needed for a laser. We have demonstrated the backward-lasing technique for remote sensing in various flames located at a distance of several meters from the pump laser and detector. Using wavelength-tunable ultrashort femtosecond pulses in the deep-UV regime (200-300 nm), we can create strong backward lasing from hydrogen (H) and oxygen (O) atoms. Detecting the backward-lasing signal time-resolved using an ultrafast detector (streak camera), we have demonstrated that we can detect these atoms with a spatial resolution on the order of 1 mm. This new measurement concept could open up unique possibilities both for industrial and security applications, for example measurements inside industrial facilities with limited optical access (that is only one window is available), detecting gas leaks from pipelines, or stand-off detection of trace elements associated with explosives or other dangerous substances.

Joakim Bood -