In order to meet the increasing capacity requirements, network operators are extending their optical infrastructure closer to the end-user while making more efficient use of the resources. In this context, long reach passive optical networks (LR-PONs) are attracting increasing attention.Coherent LR-PONs based on high speed digital signal processors represent a high potential alternative because, alongside with the inherent mixing gain and the possibility of amplitude and phase diversity formats, they pave the way to compensate linear impairments in a more efficient way than in traditional direct detection systems. The performance of coherent LR-PONs is then limited by the combined effect of noise and nonlinear distortion. The noise is particularly critical in single channel systems where, in addition to the the elevated fibre loss, the splitting losses should be considered. In such systems, Kerr induced self-phase modulation emerges as the main limitation to the maximum capacity. In this work, we propose a novel clustering algorithm, denominated histogram based clustering (HBC), that employs the spatial density of the points of a 2D histogram to identify the borders of high density areas to classify nonlinearly distorted noisy constellations. Simulation results reveal that for a 100 km long LR-PON with a 1:64 splitting ratio, at optimum power levels, HBC presents a Q-factor 0.57 dB higher than maximum likelihood and 0.21 dB higher than k-means. In terms of nonlinear tolerance, at a BER of 2×10 ${}^{-3}$ , our method achieves a gain of ∼2.5 dB and ∼1.25 dB over maximum likelihood and k-means, respectively. Numerical results also show that the proposed method can operate over blocks as small as 2500 symbols.

We propose a novel digital signal processing (DSP) technique that encrypts multiple baseband signals by dividing them into several spectral slices, which are then shuffled. Afterwards, the shuffled signals modulate optical carriers and are propagated through transparent optical communication systems. Simulation results reveal that the reach of shuffled signals is ∼90% of that of unshuffled ones. We also evaluate a scenario where the shuffled signals are transmitted through multiple routes to increase security and investigate the robustness of the technique against brute force and chosen plaintext attacks.

We investigate the application of a new all-optical cryptography technique to wavelength division multiplexing (WDM)-compatible differential phase-shift keying (DPSK) signals. The technique uses current optical band-pass optical filters with narrow bandwidths to divide a given input WDM-compatible signal into several spectral slices, which are submitted to two cryptographic key stages. The first one impresses amplitude encoding, whereas the second imposes delay variations to each spectral slice. The technique performance is assessed by evaluating the encrypted and decrypted signals bit error rates. Computer simulations suggest that encrypted DPSK signals may be properly recovered even after being propagated through typical metropolitan area network distances.

In this work, we propose a comprehensive technique to enhance the security of superchannels (SCs). The spectral samples of the signals within a superchannel are shuffled and further encrypted in amplitude and phase, according to cryptographic keys. The strategy may be applied to SC signals with different modulation formats and bit rates. Moreover, the concept of SC is generalized, allowing for channels to be propagated through different optical routes. As a result, eavesdroppers need to tap multiple optical fibers to recover the content of a single signal. This simultaneously increases security and enhances the cost of spying. We investigate scenarios where dual polarization (DP) quadrature phase-shift keying (QPSK) and 16 quadrature amplitude modulation (16-QAM) signals are encrypted and propagated through transparent optical networks. Results reveal that no meaningful propagation penalty is introduced by signal encryption. From a security perspective, the technique is considerably more robust against brute force attacks than the Advanced Encryption Standard (AES), which is widely deployed in data encryption. A strategy to provide the properties of diffusion and confusion to the proposed encryption technique is discussed.