An innovative approach for security-enhanced optical stealth transmission in a synchronous digital hierarchy network is proposed and experimentally investigated. The security enhancement is achieved through a signal modulation format, so-called polarization-modulated-code-shift-keying, which is implemented with two superstructured fiber Bragg gratings-based optical-code-division multiple-access encoders and a polarization modulator. The proposed modulation format can provide a constant energy level for both bits 0’s and 1’s, which avoids secure vulnerability of single-stealth-user with on-off-keying modulation before coupling into the host channel and after the cascade of filters. Moreover, a self-made cost-effective gain-switched distributed feedback laser with relatively narrow spectrum is first employed as a stealth optical source, which greatly reduces the system cost and complexity. The stealth signal is recovered and detected asynchronously. The experimental results show high secure performance and robustness against eavesdropping, while keeping a bit error rate below forward error correction limit.
KEYWORDS: Electro optics, Modulators, Computer programming, Code division multiplexing, Electrodes, Receivers, Picosecond phenomena, Signal attenuation, Transmittance, Signal to noise ratio
An electro-optical time gating technique, which is based on an electrical return-to-zero (RZ) pulse driven Mach-Zehnder modulator (MZM) for eliminating multiple access interference (MAI) in optical code-division multiple access (OCDMA) networks is proposed. This technique is successfully simulated in an eight-user two-dimensional wavelength-hopping time-spreading system, as well as in a three-user temporal phase encoding system. Results show that in both systems the MAI noise is efficiently removed and the average received power penalty improved. Both achieve error-free transmissions at a bit rate of 2.5 Gb/s. In addition, we also individually discuss effects of parameters in two systems, such as the extinction ratio of the MZM, the duty cycle of the driven RZ pulse, and the time misalignment between the driven pulse and the decoded autocorrelation peak, on the output bit error rate performance. Our work shows that employing a common MZM as a thresholder provides another probability and an interesting cost-effective choice for a smart size, low energy, and less complex thresholding technique for integrated detection in OCDMA networks.
A principle-of-concept demonstration about the effect of filter parameters on the output performance of a highly nonlinear fiber-based all optical thresholding technique (OTT) is proposed. The center wavelength, bandwidth, and rejection parameters of an optical bandpass filter (OBPF) used in fiber-based OTT are discussed. Carefully adjusting each parameter of the OBPF allows a steeper transforming characteristic of optical thresholding, and a fourth-order polynomial power transfer function is achieved, which is the steepest thresholding curve ever reported. To the best of our knowledge, this is the first demonstration of fiber-based OTT focusing on OBPF parameters, which makes the thresholder more flexible and allows customization of thresholding performance to meet requirements in various systems.
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