We investigate how to scale a free-space wavelength blocker to implement a spectral phase scrambling system to secure high data rate optical transmissions within a wavelength division multiplexing optical network. This technique belongs to the optical code-division multiple access family, and its implementation can be carried out using a free-space pulse shaping system based, for instance, on a wavelength blocker or spectral equalizer architectures. After a brief recall of the encryption principle, a model of the optical system is given, and we discuss the impact of phase distribution and the scrambling mask used to encrypt the data on penalty sources. We emphasize the importance of correctly choosing certain geometrical parameters, such as the beam waist in the spectral plane and the mask fill factor. The tolerance of this solution with respect to mask positioning errors is then investigated, and we finally discuss how such a solution could be implemented using existing devices, such as a wavelength blocker, for security applications without inducing additional system penalties.
This paper proposes a non-invasive optical scrambling technique to secure optical transmissions at high data rates
(>10Gb/s). The proposed method belongs to the optical code-division multiple access (OCDMA) technique, using
spectral phase encoding, based on overlapping of adjacent scrambled/spread pulses to encrypt transmitted data. In our
system, data confidentiality is directly related to scrambled/spread pulse interference, avoiding direct detection by a
power detector, in contrast to network access application (OCDMA), where this overlapping should be avoided. Our goal
is to secure data transmission without impacting the physical layer, by guaranteeing the optical transparency of the
encryption technique with respect to conventional transmission equipments. Therefore, we simulated the system penalty
as a function of the transmission distance for a bit error rate (BER) target of 10-9 to estimate the impact of the linear and
non-linear transmission effects on our encryption technique. We consider a point-to-point span for mono-channel and
multi-channel setups where self-phase modulation (SPM) and cross-phase modulation (XPM) become significant. In the
last section, we discuss the resilience of our encryption technique to some realistic attack scenarios. The eavesdropper
can use the linear optical sampling (LOS) technique, which with coherence conditions on the waveform under test,
permits to extract the amplitude and the phase of each spectral compound, enabling, to determinate the phase filter used
to encrypt. Determining the necessary time to crack the mask allows us to establish the mask refreshment to guarantee
data confidentiality.
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