A. Bogris

Chaos-based communications at high bit rates using commercial fibre-optic links

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.

Ultralow Noise, Broadband Phase-Sensitive Optical Amplifiers, and Their Applications

Frequency nondegenerate phase-sensitive amplifiers (PSAs) have the potential to realize broadband and noiseless amplification. However, the rigorous requirement of phase- and wavelength-locking of the input waves has significantly hampered their progress. In this paper, we review recent research on this type of optical amplifier. This work is based on a copier-PSA scheme, which consists of a parametric phase-insensitive copier followed by one or more PSAs. Broadband gain and noise performance of a fiber-based nondegenerate PSA has been characterized, both theoretically and experimentally. A record-low 1.1 dB noise figure was measured at >;26 dB gain, and a clear phase dependent gain was observed. Moreover, potential applications including phase noise squeezing and ultralow noise, multichannel and modulation-format-transparent linear amplification with up to 6 dB link noise figure advantage over conventional EDFA-amplified links have been experimentally demonstrated. The prospects and practical challenges of this intriguing amplification technology are also discussed.

Regenerative properties of a pump-modulated four-wave mixing scheme in dispersion-shifted fibers

A new scheme for all-optical signal reshaping is proposed. It is shown that proper adjustment of the operating conditions of a wavelength converter based on four-wave mixing (FWM) in dispersion-shifted fiber (DSF) results in significant extinction ratio enhancement and noise suppression of the converted signal. The key issue in the proposed idea is the application of the modulation bitstream on the input pump wave. Detailed numerical simulation shows that it is possible to achieve a wavelength-converted output with extinction ratio of 20 dB when the extinction ratio of the input signal is 10.5 dB, while at the same time, the improvement in the Q-factor is almost 6 (from 6.5 at the input, it becomes more than 12 at the output). The theoretical predictions have been experimentally confirmed by measuring the transfer function of the FWM in a 17-km-long DSF under different operating conditions. The obtained results are in a very good agreement with those predicted theoretically.

Investigation of a 2R all-optical regenerator based on four-wave mixing in a semiconductor optical amplifier

The properties of an all-optical 2R regenerator based on four-wave mixing (FWM) in a semiconductor optical amplifier (SOA) are investigated. The regeneration is based on the nonlinear FWM transfer function and a study of the systems static behavior reveals the operating conditions, under which the transfer function approaches most the ideal, step-like discrimination characteristic function. A fiber Bragg grating (FBG) is employed in order to overcome the SOAs speed limitations due to limited carrier dynamics. The simulations with dynamic input data by means of extinction ratio (ER) and Q-factor calculations, showed satisfactory regenerative behavior up to 40 Gb/s.

Coherent All-Optical Phase and Amplitude Regenerator of Binary Phase-Encoded Signals

The performance of future ultralong-haul communication systems exploiting phase-encoded signals is likely to be compromised by nonlinear phase noise generated during signal transmission. One potential way to mitigate against nonlinear phase noise is to use phase-sensitive amplifiers (PSAs) that have been demonstrated to help remove such phase noise as well as to provide simultaneous signal amplitude noise suppression when operated in saturation. Recently, we have shown that a PSA-based signal regenerator based on degenerate four-wave mixing could be implemented in a network-compatible manner in which only the (noisy) signal is present at the device input (black-box operation). However, this scheme was tested only with relatively high-frequency deterministic perturbations applied to the signal. Here, we address both theoretically and experimentally the important issue of how such a regenerator works with more realistic random broadband amplitude/phase noise distributions. Good regenerative performance is demonstrated and our study also illustrates an additional unique feature of PSA-based regenerators-namely error correction for differentially encoded signals when placed in front of a DPSK receiver. Furthermore, we present a simplified regenerator implementation providing highly stable operation and representing a significant further step toward a practical device.

Experimental evaluation of an open-loop all-optical chaotic communication system

An experimental investigation of the performance of an open-loop optical chaotic communication system has been carried out under different optical injection conditions. The most popular message encoding techniques have been considered, tested and compared at gigabit rates and for different levels of optical injection to the receiver, by bit-error-rate (BER) measurements. Different low-pass electrical filters have been implemented for the various message bit-rates tested, in order to employ the most appropriate one for message decoding. The best performance of all the examined encryption methods for messages of 1.5 Gb/s bit-rate emerged for the chaos modulation encryption method and was characterized by Q-factor values up to 5.1, after synchronizing in the strong injection regime, while the corresponding BER of the decoded message was as low as 7/spl middot/10/sup -5/. In order to confirm the secure encryption of the message, BER measurements are also performed for the chaotic carrier into which the message is encrypted.

Numerical investigation of fiber transmission of a chaotic encrypted message using dispersion compensation schemes

A detailed numerical investigation of the transmission properties of all-optical chaotic communication systems is presented for two data-encoding techniques and for various dispersion compensation maps. A semiconductor laser subjected to optical feedback generates the chaotic carrier, and the data is encoded on it by chaotic modulation (CM) or chaotic-shift-keying (CSK) methods. The complete transmission module consists of different types of fiber, inline amplifiers, and Gaussian optical filters. Different dispersion maps based on either Nonzero dispersion-shifted fibers (NZ-DSFs) or combinations of single-mode fibers (SMF) along with dispersion-compensating fibers (DCF) were considered. The systems performance is numerically tested by calculating the Q factor of the eye diagram of the received data for 1 and 2.4 Gb/s. The influence of the optical power launched into fiber and the transmission distance to the quality of the decoded message has been investigated. The CSK scheme appears to have better performance relative to the CM scheme, while dispersion maps utilizing NZ-DSFs are superior to that employing DCF. In all encoding methods and transmission maps, a decrease in the Q factor is observed when the repetition bit rate of the encoding message and the transmission distance increases.

Investigation of an all-optical wavelength converter with reshaping properties based on four-wave mixing in passive microring resonators

The properties of an all-optical wavelength converter with reshaping characteristics, based on four-wave mixing (FWM) in a passive GaAs-AlGaAs microring resonator side-coupled to a bus waveguide, are investigated in this paper. The reshaping properties are based on the nonlinear FWM transfer function as a result of the two-photon absorption enhanced by the resonance effect. A detailed study of the static and dynamic reshaping characteristics of the wavelength converter has been carried out. Its performance, evaluated by extinction ratio and Q-factor calculations, showed satisfactory regenerative properties up to 10 Gb/s.

Phase locking and carrier extraction schemes for phase sensitive amplification

We demonstrate two schemes which enable simultaneous phase locking, carrier extraction and frequency shifting for use in stand-alone phase sensitive amplifiers. The schemes provide phase locked pumps at appropriate frequency spacing for a number of non-degenerate PSA configurations.

Experimental evaluation of subcarrier modulation in chaotic optical communication systems

Data streams with bit rates of up to 1 Gbit/s have been applied though the subcarrier modulation technique in an experimentally deployed optical communication system based on chaos data encryption and have been recovered efficiently. By shifting the spectral components of the encrypted data in regions where the chaotic carrier is powerful, a more efficient encryption and final message recovery is recorded with respect to the baseband modulation technique, improving the up-to-date performance of analogous systems.