Laser Auditory Effect


Laser communication using the photoacoustic effect


Researchers from the MIT Lincoln laboratory created a laser communication system based on the the photoacoustic effect which consists of the generation of pressure waves or sound waves in a material following light absorption. They determined that water vapor, which is found in all environments, absorbs light at the infrared wavelength of 1907 nm and as a result generates mechanical/pressure waves which are perceived as sound when they reach a person's ear. They used a laser of the above wavelength as carrier on which they modulated sound information or messages. In one technique, the message is heard by all persons crossing the beam and in another technique the message is delivered at a specific distance from the transmitter and is heard only by the person found at the specified location.






Laser communication using the laser-induced plasma effect

Initial efforts of generic sound (and light) generation


Researchers from the U.S. military’s Joint Non-Lethal Weapons Program (JNLWD) announced in March 2018 that they created a system that produced sound or light at a distance based on the laser-induced plasma effect. This could be used for deterrence if for instance the sound of a flashbang grenade used for disorientation was generated.
Their efforts consisted of the use of a combination of femtosecond and nanosecond laser excitations in the so-called "igniter-heater" scheme*. Specifically, a femtosecond laser removes electrons from air molecules, thereby generating plasma which corresponds to a field of "electrified gas" which is significantly responsive to electromagnetic manipulation. Then, a nanolaser manipulates the plasma field in order to produce sound (noise) or light.
The distance at which this effect can be mediated depends on the dimensions of the used optics system and is theoretically considered to be at the range of tens of kilometers. The Kerr effect, which refers to the change of the refractive index of a material in response to an electromagnetic field, makes it easier to focus at large distances.



Transcript of video (slightly modified): We have whole propagated weakly-focused laser pulses, one of which is a femtosecond 30 mJoule igniter pulse which creates a very dilute plasma. The plasma is subsequently densified by copropagating a nanosecond pulse with about 3 Joules of pulsed energy.


At this point only a nanosecond pulse is propagating. The femtosecond pulse is blocked. As you can see, although the nanosecond pulse has huge energy, it doesn't have any visible effect and does not produce any amount of plasma except for occasional sparks due to the dust in the lab.


You can see that if I insert different objects in the beam like this piece of paper, then immediately on the surface there is dielectric breakdown (electrical breakdown). This is all left-over dust triggering the breakdown.


Depending on the type of materials I put in the beam, dark or light and so forth, the amount of noise that the plasma produces is different, the amount of plasma is different.


This is a blackened foil. Of course immediately the amount of plasma and noise increases.


Now let me turn on the femtosecond pulse. The femtosecond pulse will trigger the plasma formation in the gas and the nanosecond pulse will enhance the plasma, and create another plasma in the air.


Now the igniter and heater pulses propagate together and the level of sound is (...) maybe at about a loud voice level. The plasma is created in the form of a string which expands maybe to a distance of 10 cm.


If I block the igniter femtosecond pulse, the effect disappears even though the Joule pulse is still propagating.


Now I am again opening again the igniter pulse and I see the stable formation of plasma in the air.





Laser communication using the laser-induced plasma effect

Recent implementations:

1) Sound modulation and mimicking human speech

2) Discomfort effect for deterrence

Researchers from the U.S. military’s Joint Non-Lethal Weapons Program (JNLWD) announced in July 2019 that following their creation of a system that produced sound or light at a distance based on the laser-induced plasma effect, they had implemented sound modulation for audio messages. This included experimentation with algorithms for human speech mimicking by adjusting low and high frequencies.
The researchers commented that it suffices to create plasma at a location of choice (target) for sound to be then induced at the location through modulation. Additionally, sequential sounds similar to those of flashbang grenades used for disorientation have been developed. Next steps include the extension of the effect range.
Another implementation uses skin heating. The high energy laser with a very short pulse rate removes electrons from air molecules thereby generating plasma. This is used to create microscopic pores in a person's skin even below clothing thereby activating nerve endings and generating discomfort or pain.