Reversed Time

 With the increasing number of items of electronic equipment in civil and military equipment, checking the electromagnetic compatibility of the different systems is essential. For example, when a combat aircraft approaches its aircraft carrier, how can we be sure that the aircraft carrier's powerful radars will not interfere with the aircraft's electronic equipment, making the landing dangerous? In the civil field, cases have been reported of ABS braking systems that do not work near airports or airbags that go off on their own. Not forgetting lightning, which has direct impact on structures, and also indirect impact due to the strong electromagnetic fields that it induces. This is what motivates the activities of the Onera electromagnetic compatibility team.

Test chambers called "mode stirring reverberating chambers" are used to test the electromagnetic compatibility of different systems. They need very intense electromagnetic fields to be created in order to approach the critical conditions for the equipment. However, creating such fields is expensive. "Time reversal offers us a means of creating strong fields with generators of limited power", says Florent Christophe, Assistant Director of Onera's Electromagnetism and Radar Department in Toulouse. Time reversal can concentrate the energy emitted by a generator over a "long" period (in reality, several microseconds) into a hundred or thousand times shorter period. So the maximum power of the electromagnetic wave is a hundred to a thousand times higher. You can then do the same thing with a low cost generator yielding under one hundred watts as you can with a several kW amplifier costing tens of thousands of Euros.

Reverberating chamber for the Onera time reversal experiment

But how does this time reversal technique work? It was first perfected for acoustic waves by the Mathias Fink's team at the École supérieure de physique et chimie industrielles (Graduate School of Industrial Physics and Chemistry). Here it has been adapted for electromagnetic waves. Firstly, a short pulse wave is sent into a chamber and reflects many times off the walls. In the center of the chamber there is a device for measuring the wave. We observe that it is degraded because of the reflections off the walls: the frequencies are out of phase; the signal lasts longer and the power is lower. This signal is then recorded and reproduced "backwards", like in old films on video cassettes when the film runs backwards during rewinding. Then, this reversed signal is returned to the reverberating chamber and there we get almost the original short pulse with an energy concentrated over a very short time. "Generally, reverberation is a handicap, but here, on the contrary, we use it to concentrate the signal".

First part of the experiment: a short signal is sent and recovered, reverberated and degraded, at a chosen point (here at the foot of the antenna).

Second part: the preceding degraded signal is reversed, amplified and sent back in the same way: at the foot of the antenna we obtain a signal very close to the initial signal, amplified.
 

Cécile Michaut, scientific reporter.