Bidirectional coupling of semiconductor lasers (SLs) through optical injection is a well established method to generate chaotic signals which, through their dynamics, may give rise to several applications from sensing to monitoring and from communication to security. Recent works have shown the capability of joint behavior or complete synchrony of mutually coupled networks of SLs. In these works, the coupling architecture, the operational conditions and the properties of the active elements determine the types of dynamics of the emitted optical signals, through which the network can potentially be synchronized. In this experimental work, a network of mutually coupled semiconductor lasers has been synchronized through chaotic optical signals that spectrally extend over 10GHz. The synchronization among the lasers that participate in the coupled network is affected, besides the structural and operational conditions, by the signals' bandwidth that circulates optically. Here we show that the synchronization performance of the detected signals when monitoring the network nodes through optoelectronic conversion is in direct dependence on the signal bandwidth. Smaller signal bandwidth at the GHz range may result in synchronization with cross-correlation values over 0.97 in most of the SL nodes, rejecting higher frequencies that are not optimally synchronized. Another source of improving the synchronization of the network that has been recorded in this experimental setup is by harnessing the de-synchronization events that are almost always apparent, especially when emitted signals include power dropouts.
Semiconductor lasers (SL) have been proven to be a key device in the generation of ultrafast true random bit streams. Their potential to emit chaotic signals under conditions with desirable statistics, establish them as a low cost solution to cover various needs, from large volume key generation to real-time encrypted communications. Usually, only undemanding post-processing is needed to convert the acquired analog timeseries to digital sequences that pass all established tests of randomness. A novel architecture that can generate and exploit these true random sequences is through a fiber network in which the nodes are semiconductor lasers that are coupled and synchronized to central hub laser. In this work we show experimentally that laser nodes in such a star network topology can synchronize with each other through complex broadband signals that are the seed to true random bit sequences (TRBS) generated at several Gb/s. The potential for each node to access real-time generated and synchronized with the rest of the nodes random bit streams, through the fiber optic network, allows to implement an one-time-pad encryption protocol that mixes the synchronized true random bit sequence with real data at Gb/s rates. Forward-error correction methods are used to reduce the errors in the TRBS and the final error rate at the data decoding level. An appropriate selection in the sampling methodology and properties, as well as in the physical properties of the chaotic seed signal through which network locks in synchronization, allows an error free performance.
The potential of conventional semiconductor lasers to generate complex and chaotic dynamics at a bandwidth that extends up to tens of GHz turns them into useful components in applications oriented to sensing and security. Specifically, latest theoretical and experimental works have demonstrated the capability of mutually coupled semiconductor lasers to exhibit a joint behaviour under various conditions. In an uncoupled network consisting of N similar SLs - representing autonomous nodes in the network - each node emits an optical signal of various dynamics depending on its biasing conditions and internal properties. These nodes remain unsynchronized unless appropriate coupling and biasing conditions apply. A synchronized behaviour can be in principle observed in sub-groups of lasers or in the overall laser network. In the present work, experimental topologies that employ eight SLs, under diverse biasing and coupling conditions, are built and investigated. The deployed systems incorporate off-the-shelf fiber-optic communications components operating at the 1550nm spectral window. The role of emission wavelength detuning of each participating node in the network - at GHz level - is evaluated.
Two multi-semiconductor-laser (SL) topologies, based on mutually coupled semiconductor lasers - representing a startype and a mesh-type network - are evaluated in terms of their synchrony potential and their sensitivity towards critical SLs' intrinsic and operational parameters. The coupling topology, the coupling conditions and the values of key SL parameters determine the type of dynamics of the emitted optical signals. The number of nodes and the detuning in their fundamental properties have been assessed to be decisive in terms of efficiency and quality of synchronized outputs, as wells as for the overall dynamical map of the network. Our investigation mainly focuses on discrepancies in SL parameter values and their effect on the efficiency of synchronized dynamics. This type of investigation will provide preliminary guidelines on building experimentally large scale networks of coupled SLs under various coupling matrices that could support optical sensing or cryptographic applications.
A thorough study of an all-optical chaotic communication system, including experimental realization real-world testing
and performance characterization through bit-error-rate analysis, is presented. Pseudorandom data that are effectively
encrypted in the chaotic emitter and sent for transmission are recovered at the receiver with bit-error-rate (BER) values
as low as 10-7 for 1 Gb/s data rate. Different data code lengths and bit-rates at the Gb/s region have been tested. Optical
transmission using 100km fiber spools in an in-situ experiment and 120km in an installed optical network showed that
chaotic communication systems does not act as a considerably deteriorating factor in the final performance.
A detailed investigation of the decoding properties of different receiver configurations in an all-optical chaotic transmission system is presented for two data-encoding techniques and for various dispersion compensation maps. A semiconductor laser subjected to optical feedback generates the chaotic carrier while data is encoded either by Chaotic Modulation (CM) or Chaotic Shift Keying (CSK) methods. The complete transmission module consists of various dispersion management maps, in-line amplifiers and Gaussian optical filters. The receiver, employing a high facet reflectivity laser, is either forming a closed-loop configuration operating at the non-amplification regime or a strongly injected open-loop one. For the latter configuration the possibility of utilizing an anti-reflection (AR) coated laser is also investigated. System's performance is numerically tested by calculating the Q-factor of the eye diagram of the 1 Gb/s received data. The influence of the optical power launched into fibre or the transmission distance to the quality of the decoded message has been investigated. The closed-loop scheme had better performance relative to the open-loop, while CSK method and maps utilizing Dispersion Shifted Fibres are superior to CM and that employing Dispersion Compensating Fibres respectively. When an AR-coated laser is used in the open-loop receiver setup, improved decoding performance occurs.
The performance of an all-optical closed-loop chaotic communication system in a transmission link consisting of single mode fibers (SMF) applying two different dispersion management techniques is numerically studied. The first technique is implemented by the usage of dispersion compensating fibers (DCFs), while the second utilizes optical phase conjugators (OPCs). The latter is implemented by means of four wave mixing (FWM) in a dispersion shifted fiber (DSF), where the chaotic carrier corresponds to the signal wave and a high power continuous wave corresponds to the pump wave. Calculation of the recovered message Q-factor values obtained from the corresponding eye diagrams has been carried out applying chaotic modulation (CM) and chaos shift keying (CSK) encryption techniques for two repetition rates (2.4Gbps, 5Gbps). It is shown that the optical phase conjugation is an effective dispersion and non-linear effects compensation technique even if high-bit rate message encoding is applied. The superiority of a transmission system including OPCs to that utilizing (DCFs) is presented. The influence of key system parameters such as optical power, OPC spacing, pump power level, etc. to the transmission performance has been investigated. Acceptable system performance is presented for approximately 600Km at 2.4Gbps and 400Km at 5Gbps.