Claims:1. A free-space laser communication system comprising of novel authentication scheme.
2. The authentication scheme comprises transmission of higher order Bessel beam of known order and detection of that order at the receiving end to ensure the origin of the data.
3. The detection scheme at the receiving end is comprised of
a. An interferometric set-up to produce interference between a part of the received beam with a locally generated plane wave.
b. A positive lens to generate the Fourier transform of the interference pattern.
c. A CCD camera to acquire the final pattern and operating software to match that fingerprint pattern with the stored one.
4. The other part of the received beam goes through photo-detector in order to decode the sent digitized message.
5. The final output generator is comprised of one AND gate that multiply the decoded digital message with the authentication scheme output to give the final message. , Description:In free space optical communications system a communication signal in the form of a narrow laser beam is sent from the transmitter to a receiver along a straight line optical path through the atmosphere. In case of a fiber based communications system the optical fiber waveguide provides a reasonably unobstructed path for the transmission signal to the receiver platform whereas in free space communications system the signal carrying visible laser beam propagates through the atmosphere and hence the various atmospheric conditions affects the quality of the received communication signal. In worst case, the propagating signal can be completely absorbed or scattered and hence no signal is being received in the receiver end of the free space communications signal. Besides, even if a signal is being received by the receiver end of the free space communications system it is never possible to conclude whether the received signal is part of the original encoded beam or stray light. The efficiency of optical encoding through Orbital Angular Momentum (OAM) in free-space laser communication has been well demonstrated in many literatures till
date1-5 , which describes its efficiency to add stability of the system against these adverse atmospheric conditions. Apart from using light intensity, frequency and polarization, encoding through optical orbital momentum of photon is gaining its own
importance. Many authors proposed different methods for data communication using laser beam with optical vortex, while encoding the beam using its topological charge number. This method does not only increase the data capacity of free-space communication, but also adds stability to the system owing to its self-healing ability.
However till date, to our knowledge, the final decoding has been done through photodetectors only depending upon the intensity of the beam. But even with the narrowfrequency photo-detectors (PD), it is not possible to infer that the light detected by the PDs is a part of the original encoded beam or stray light. Here we propose a method to ensure the origin our detected photon using its fingerprint singularity structure imposed on it at the time of its encoding.