KEYWORDS: Modulation, Radio optics, Hybrid fiber radio, Single mode fibers, Linear filtering, Signal detection, Single sideband modulation, Dispersion, Radio over Fiber, Signal generators
A full-duplex radio over fiber system with optimum optical carrier to sideband ratio (OCSR) of 0 dB and wavelength reuse for uplink are proposed. At the central office, single-sideband modulation with OCSR larger than 0 dB is realized based on optical injection. At the base station, a notch filter with the notch depth corresponding to the OCSR of the transmitted signal is used to achieve the optimum OCSR of 0 dB. The reflected weakly modulated carrier is reused as the carrier for upload signals. A simulation is carried out to verify the proposed system. Results show that the download 60-GHz-RF signal is almost not affected by the fiber chromatic dispersion, and the power penalty after transmission over a 50-km single-mode fiber is negligible in uplink and is only 1.27 dB in downlink.
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.
Realization of a wideband tunable optoelectronic oscillator based on a chirped Mach–Zehnder modulator (MZM) and a chirped fiber Bragg grating is proposed and demonstrated. By simply adjusting the direct-current bias of the chirped MZM, the frequency of the oscillating signal is tuned. A theoretical model is established, then verified by an experiment. A high-purity microwave signal with a tunable frequency from 5.8 to 11.8 GHz is generated. The single-sideband phase noise of the generated signal is −112.6 dBc/Hz at a frequency offset of 10 kHz.
A wideband tunable filter with a complex coefficient is proposed and demonstrated. The output of a polarization modulator is filtered by the optical bandpass filter, an optical single sideband modulation will be generated. When it passes through a polarization controller and a polarizer, a complex coefficient will be achieved by tuning the angle of the polarization controller before the polarizer, and the real coefficients are controlled by the optical power of the lasers. As the real coefficients and complex coefficient can be tuned, a two-taps incoherent photonic microwave filter with a tunable range over the entire full free spectral range is realized by adjusting the polarization angle from −90 deg to 90 deg while the shape of filter remains unchanged.
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.
An all-optical realization of a two-tap complex-coefficient microwave photonic filter, working under incoherent mode, is proposed and demonstrated. The complex coefficient is realized by simply adjusting the bias voltage applied to the dual-electrode Mach–Zehnder modulator. The real coefficient is controlled by adjusting the power of the two taps. As a result, the frequency response of the filter can be continuously tuned, over a full free spectral range, without changing the shape of the filter.
Optical en/decoder is one of the key components in Optical Code Division Multiple Access (OCDMA) systems.
Therefore, the improvement of performance of en/decoder is very necessary for improving the whole performance of
OCDMA system. Among all kinds of factors which affecting the performance of en/decoder, the compatibility between
optical source and en/decoder is proved to be an inevitable item. In this paper, as to equivalent phase shift super-structured
FBG (EPS-SSFBG), an advanced en/decoding technology, both optimum pulse width and proper central
wavelength deviation between optical source and EPS-SSFBG based en/decoder are first investigated, which lead to a
novel method of optimizing the compatibility between optical source and en/decoder. The simulation results show that
there exists an optimum pulse width, not the narrowest one, fit for EPS-SSFBG based en/decoder, what's more, it is
proved advisable that proper central wavelength deviation would be beneficial when pulse width is relatively wide.
Simulation and experimental results prove the feasibility and effectivity of the proposed method.
Combined DLP algorithm and sampled theory, super-narrow FBG filter for arbitrary shaping spectrum can be
successfully synthesized, and the process of design and fabrication using one uniform mask are demonstrated. A novel
method for a single stage illumination to fabricate dc-free apodized sampled FBG is proposed based on the chirp of
sample period. Above design and fabrication method are testified by simulation and experiment results, and a super-narrowband
triangle shaping FBG filter with 25pm 3dB bandwidth and side lobes suppression beyond 20dB is
fabricated.
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