Optical wireless communication systems usually adopt on-off keying (OOK), binary pulse position modulation (BPPM), and binary phase-shift keying (BPSK). In this work, the scheme of systems based on a turbo code of OOK, BPPM, and BPSK is proposed, and the three different iterated maximum a posteriori (MAP) algorithms of turbo code are deduced. To use the turbo code based on different modulated formats over optical wireless channels we deduce the formula of an atmospheric channel signal-to-noise ratio versus the atmospheric visibility. Simulations show that the turbo code based on BPSK is well suited for optical wireless communication systems.
The wavelength conversion based on cascaded second-order nonlinearity by using pulsed pump waves is theoretically analyzed in a periodically poled lithium niobate waveguide. By numerical simulation, the evolution of pulses and the walk-off between pulses is discussed, and a distorted waveform of converted pulse is illustrated. A tunable and single-to-dual-channel wavelength converter is realized by pumping with picosecond pulses at a repetition rate of 40 GHz, and the waveform distortion of a converted pulse is demonstrated in our experiments.
Atmospheric optical wireless communication is a fading channel because of the effect of atmospheric attenuation. We introduce a novel turbo code named the two-fold turbo code scheme to provide high performance for optical wireless communication. To use the two-fold turbo code over optical wireless channels, first deduce the formula of atmospheric channel signal-noise-ratio versus atmospheric visibility, then analyze the coding and decoding principle of the two-fold turbo code. Simulations show that the proposed two-fold turbo code significantly improves the performance of the system compared to the common turbo code.
Optical wireless communication (OWC) is beginning to emerge in the telecommunications market as a strategy to meet last-mile demand owing to its unique combination of features. Turbo codes have an impressive near Shannon-limit error correcting performance. Twofold turbo codes have been recently introduced as the least complex member of the multifold turbo code family. In this paper, at first, we present the mathematical model of signal and optical wireless channel with fading and bit error rate model with scintillation, then we provide a new turbo code method to use in OWC system, we can obtain a better BER curse of OWC system with twofold turbo code than with common turbo code.
In this paper, a comprehensive broad-band model of tunable wavelength converter based on four-wave mixing (FWM) in semiconductor fiber ring laser (SFRL) is presented. Critical factors, eg., the material gain profile, the longitudinal variation of the optical field, the carrier density and the broad-band spontaneous noise emission are considered in the model. By numerical simulation, the effects of the input signal power, injection current, the coupling of the output couplers and the lasing wavelength on the performances of the wavelength converter, such as the conversion efficiency and the SBR, are investigated. Simulation results are in agreement with the experimental results in literatures.
The longitudinal spatial hole burning (LSHB) in semiconductor optical amplifiers (SOA) is investigated using an improved wideband numerical model. The main new feature of the model is that it takes into account the current self-distribution effect, which induced by the axial variations of the separation between quise-Fermi levels in active region. The current self-distribution effect leads to the nonuniform current injection, and it tends to smoothen the carrier density distribution over the active region and reduces the strength of LSHB. It is found that the internal series resistance of SOA, which comes from mental-semiconductor Ohmic contacts, heterointerface and semiconductor bulk resist, significantly influences the strength of current self-distribution effect. The assumption of current injected uniformly gives rise to an overestimation of the strength of LSHB in SOA. The simulation results also show that the series resistance influences the gain and noise figure of SOA greatly. It is proposed that reducing the series resistance can enhance the small signal gain and reduced the noise figure of SOA efficiently.
The cascadability of Semiconductor optical amplifier (SOA) gates and the size limitations for several kinds of switch architectures based on SOA's are studied theoretically. The analysis shows that the sizes of matrix-vector switches are severely limited owing to the splitting losses, waveguide losses and coupling losses. However for distributed gain matrix-vector switch and Benes switch, the accumulation of amplified spontaneous emission (ASE) noise and non-ideal extinction ratio also greatly influence the maximal sizes of switches. The calculation results also reveal that the gain optimum strategy for switches based on SOA's, which derived from steady state analysis, will not be optimum when non-ideal extinction ratio and dynamical gain saturation is considered
To realize practical wavelength division multiplexing (WDM) system, a high-performance AWG with an all-optical polarization state controller (AOPSC) is introduced. This device is polarization-independence based on asymmetric planar waveguide in the silicon-oxynitride material system with a high-refractive-index difference n =0.7 % and large a/b (here, a is the width and b is the thickness of the core layer waveguide). Only TE0 mode can exist in the device, the TM0 mode is cutoff. Finally, an eight-channel AWG using 100GHz channel spacing (the center wavelength is 1552.52nm) is designed. In order to get more realistic results, the BPM method was used to simulate the light propagation in the AWG. The simulation was calculated for both TE and TM. Using BPM method, and we found only TE0 can propagate through the device. No polarization dependence was measured in the wavelength response.
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