Chaotic light receiver for secure free space optics
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Researchers in Paris and Milan have developed a new type of secure receiver for free space optics (FSO).
A system of optical antennas integrated into a programmable optical processor (POP photonic chip allows the receiver to adapt in real time, maintaining the integrity of the signal even in harsh atmospheric conditions.
The work by researchers at Télécom Paris and the Politecnico di Milano paves the way for the use of chaos-based encryption for secure, high-speed FSO links in hostile environments.
Chaos-based communication encodes a key into a light signal which appears so unpredictable and complex that it is almost impossible to decipher. However, these signals face disruption by atmospheric issues that distort the transmissions and compromise the security.
The micro-antennas in the photonic chip capture light from multiple points and self-calibrates in real time to rebuild these fragments into a secure and reliable chaotic signal, even in heavy rain, wind or the presence of pollutants.
The integrated photonic processor was fabricated via a multi-project wafer run by a commercial silicon photonics platform at AMF in Singapore and combines a 2D array of 16 grating couplers and a POP. The grating couplers sample the incoming optical beam, and the POP combines them coherently. These are arranged in an outer ring with a 180 µm radius and and inner ring of 60 µm of 8 and 7 grating couplers, and a central one. The POP is a 16 × 1 self-configuring binary mesh with 15 balanced Mach-Zehnder interferometers (MZIs). Each one has two thermal shifters, and one of the outputs of each MZI is connected to an integrated photodiode (PD). The POP self-configures by exploiting local control loops for each MZI, needing neither prior calibration of the MZIs, nor prior knowledge of the turbulent optical system. The POP is connected through wire bonding to a custom electronic board (above) that reads the signal from the photodiodes and implements the required control loops that compensate for the relative amplitude and phase differences between the two input signals, maximizing the output power. The control finds the right power to be applied to each actuator by minimizing the received power at each diode, coupling into a single-mode fibre to send the signal on.
“Chaos is a robust system, but can only be used in cryptosystems if its inherent nature is fully preserved. Atmospheric turbulence degrades the optical signal and apparently destroys the properties of chaos, making it hard to maintain secure and reliable communications. With our approach, we’re not just mitigating the effects of turbulence, we’re completely restoring the chaos of light in all its intrinsic complexity,” said Sara Zaminga, of LTCI Télécom Paris, Institut Polytechnique de Paris.
“In remote areas or emergency zones, places where traditional networks fail, a chaos-based, turbulence-resistant system could provide a secure connection when it’s most needed,” said Francesco Morichetti, head of the Photonic Devices Lab at the Politecnico di Milano.
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