June 06, 2020

Researchers use picosecond lasers to eliminate fog

A group of a dozen years ago in Lyon, France, showed in the lab that when the light produced by the ultrafast laser pulses passed through the fog they did not attenuate as much as expected because when the filaments themselves were blocked , It draws back energy from the surrounding "photonic bath" to replenish the energy in the light.

Now, researchers at the University of Geneva in Switzerland and Trumpf Scientific Lasers are using light from picosecond lasers to create a path through the fog again in the lab, which may allow other types Of the laser beam unhindered through the fog and clouds. This "open access" path created by picosecond lasers in the fog or cloud has many potential applications, including free space communication, remote sensing, or changing the local weather even further.

Shock wave clear mist droplets

In the experimental setup, pulsed light with a duration of 1.3 ps, a wavelength of 1030 nm and an energy of 100 mJ was generated using a Thru-beam Dira disc laser which was slightly focused into a 40 cm long mist chamber. The repetition frequency of the laser varies from 100 to 1000 Hz with an average output power of 10 to 100 W.

The fog is blown onto the laser beam through the exhaust port, providing a total of about 50 cm long fog path for the light beam. The fog itself consists of droplets with an average size of 5 μm but its concentration is about 100 times higher than that of a typical outdoor fog.

Femtosecond lasers produce three to four filaments, which carry approximately 15% of the total beam energy (the remaining light provides a photonic bath). The beam is imaged onto a screen and then imaged through an optical filter CCD camera that completely blocks the shorter spectrum of continuum resulting from the nonlinear optical interactions in the air but only partially blocks the fundamental frequency .

The beam transmittance through the fog was 0.1% at 100 Hz repetition rate and 32% at 1000 Hz. At higher transmissivity, the fog is completely removed across the entire beam path, not just its own path. The researchers hypothesized that the energy deposition in the droplets in the filaments creates a depressurized (0.5 atm) channel, and the resulting shock waves in the air eject droplets from the filaments and the larger beam path.

Figure 1: Computer simulations show that at a repetition rate of 100 to 1000 Hz, a shock wave generated by a picosecond laser pulse clears water droplets of varying concentration in the cylinder over time. Note that the higher repetition rate allows no drop on the path.

Although the repetition rate is a pulse in a 100 Hz beam, droplets can be cleared to some extent from the beam path, but the time between pulses is long enough for the droplets in the beam to be replenished between pulses (see FIG. 1). However, a beam with a repetition rate of 1000 Hz will produce a steady state, which leaves no time for the droplets to re-enter the beam path between the pulses.

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