Volume 33, Issue 4

We vary the refractive index of second core of dual concentric cores of a single mode Fiber Raman Amplifier (FRA), having a step profile. Based on well known scalar analysis, we study crucially the effects of this variation on the performance characteristics of FRA for the first time. For practically important frequency shift band of 20 cm −1 to 700 cm −1 , our interesting observation is that values of refractive indices in a certain higher range show better flattening of effective raman gain in comparison to those in lower range with single pump; side by side, larger negative coefficient of dispersion is achieved in the operating range of wavelength with increase of refractive indices keeping the outer core radius, core gap radius and refractive index of the first core fixed. These promising criteria should attract the attention of system designers engaged in fabrication of FRA.

A novel fiber Bragg grating (FBG)-based en/decoder for the two-dimensional (2-D) time-spreading and polarization multiplexer optical coding is proposed. Compared with other 2-D en/decoders, the proposed en/decoding for an optical code-division multiple-access (OCDMA) system uses a single phase-encoded FBG and coherent en/decoding. Furthermore, combined with reconstruction-equivalent-chirp technology, such en/decoders can be realized with a conventional simple fabrication setup. Experimental results of such en/decoders and the corresponding system test at a data rate of 5 Gbit/s demonstrate that this kind of 2-D FBG-based en/decoders could improve the performances of OCDMA systems.

Employing the simple series expression based on Chebyshev technique, we predict the fundamental modal field inside the core as well as cladding in case of dispersion-shifted trapezoidal and dispersion-flattened graded and step W fibers. This formalism also estimates cladding decay parameters ( W ) of the said fibers. The analysis involves use of a linear relationship of the ratio of first and zero order modified Bessel functions in the form with 1/ W over a wide and practical range of W values appropriate for guidance of only fundamental mode in such fibers. The present method prescribes analytical expressions for the concerned propagation characteristics and these are executable with little computations. Our results are found to match excellently with the available exact results.

Tm 3+ /Yb 3+ -codoped tellurite glass and fibers have been prepared, Judd–Ofelt parameters and spontaneous transition probabilities of Tm 3+ ions were calculated. Fluorescence of Tm 3+ /Yb 3+ -codoped tellurite glass and fibers were investigated under the excitation of a 980 nm diode laser. 480 nm, 650 nm, 1120 nm, 1220 nm and 1370 nm fluorescence were recorded both in glass and fibers fluorescence spectra. The mechanism of the fluorescence was presented upon analyzing the multi-photon pumping process and J–O calculation results.

We have studied Raman amplification in passive optical networks (PON). A use of a reflector, which reflects pump wave for Raman amplification, is proposed in this paper to mitigate pump power. The reflector also serves as a pump blocking filter for each subscriber. To clarify the effects of a reflector in PON, the wavelength dependence of Raman gain (or loss) and amplified spontaneous emission (ASE) has been investigated theoretically and experimentally. Raman amplification with a reflector is applied to both ordinary PON systems and wavelength division multiplex (WDM) PON systems as case studies. Finally, effects of Raman amplification with a reflector are discussed.

In the optical multicast network, node and link failures have much important influence on the network survivability. A node failure or link failure may lead to multiple destination nodes can not receive data. A multiple-quality segment protection approach based on pre-configured cycle (p-Cycle) was presented to improve the link utilization and reduce the delay of high priority multicast in the paper. Simulation results showed that the segment protection approach under p-Cycle could ensure the 100% full protection of node and link failures, and high-priority traffic has less propagation delay.

In this paper, the performance of a hybrid scheme of Subcarrier Multiplexing (SCM) technique in a Spectral Amplitude Coding (SAC) Optical Code Division Multiple Access (CDMA) transmissions system is evaluated. The hybrid system is design using new code family; we call it Dynamic Cyclic Shift (DCS) code. The DCS code design for SAC-OCDMA system to reduce the effect of Multi Access Interference due to it property of low cross-correlation ( λ C  ≤ 1) between code words. In contrast, the SCM scheme shows the ability to increase the data rate of SAC-OCDMA system. Consequently, the hybrid SCM/SAC-OCDMA system could be one promising solution to the high-capacity access network with low cost effective, good flexibility and enhanced security, which makes an attractive candidate for next-generation broadband access network.

An novel solution for secure optical communication system based on dynamic strong dispersion control is proposed. Firstly, the theoretical principle of dynamic strong dispersion control and the system structure, especially the composition of encryption part and decryption part, are presented. Secondly, the feasibility and performance of the secure system are verified by numerical analyzing in a 40 Gb/s simulation system. Lastly, the huge application potential of this system in various fields is pointed out.

We present a study of the influence of dispersion induced phase noise for CO-OFDM systems using FFT multiplexing/IFFT demultiplexing techniques (software based). The software based system provides a method for a rigorous evaluation of the phase noise variance caused by Common Phase Error (CPE) and Inter-Carrier Interference (ICI) including – for the first time to our knowledge – in explicit form the effect of equalization enhanced phase noise (EEPN). This, in turns, leads to an analytic BER specification. Numerical results focus on a CO-OFDM system with 10–25 GS/s QPSK channel modulation. A worst case constellation configuration is identified for the phase noise influence and the resulting BER is compared to the BER of a conventional single channel QPSK system with the same capacity as the CO-OFDM implementation. Results are evaluated as a function of transmission distance. For both types of systems, the phase noise variance increases significantly with increasing transmission distance. For a total capacity of 400 (1000) Gbit/s, the transmission distance to have the BER < 10 −2 for the worst case CO-OFDM design is less than 800 and 460 km, respectively, whereas for a single channel QPSK system it is less than 1400 and 560 km.

The focus of this paper is to investigate a 40 Gbit/s DWDM system over the optical span of 100 km in the presence of Four Wave Mixing (FWM) under the impact of equal channel spacing at varied values of laser line-width. In this proposed work, the performance of DWDM system is carried out by evaluating received electrical power, Q-factor and average eye opening at varied values of equal channel spacing of 0.26 nm and 0.24 nm at laser line-widths of 100 kHz, 10 MHz and 100 MHz under the impact of fiber dispersion.

The joint distributed fast polarization scramblers (D-FPSs) and forward error correction (FEC) is one of methods used to mitigate polarization mode dispersion (PMD). The distribution pattern of FPSs, which indicates how the FPSs are deployed along the fiber link, is a significant factor for this PMD mitigation scheme. The performance of the bit error rate (BER) for the case with or without FPSs is investigated by simulations. Five different distribution patterns of FPSs along the fiber link are designed in this paper, and the performance for each pattern is depicted by a novel representation that is known as the “Ring Chart.” The results show that the property of PMD mitigation varies with different distribution patterns of FPSs.

In recent years, Optical Code Division Multiple Access (OCDMA) system had received great attentions due to its potential to support applications with various data rates and Quality of Service requirements in physical layer. Since the main part of any fiber-based transmission system is the optical fiber itself, OCDMA also suffers from fiber impairments especially dispersion. In this paper, a review of fiber dispersion effects on system performance of different coding techniques is presented, focusing on single mode fiber. The different coding techniques include direct-sequence, spectral amplitude encoding and wavelength hopping/time spreading. In addition, the basic dispersion compensation approaches, which have been proposed in the literature, for these particular coding schemes are also discussed.

We report what, as per our knowledge, is the first theoretical investigation of the excitation efficiency in presence of possible transverse and angular misalignments in case of single-mode circular core triangular refractive index profile fiber excitation by laser diode via hemispherical microlens on the fiber tip. The present analysis takes into consideration limited aperture allowable by the hemispherical lens. Using ABCD matrix for refraction of paraxial rays by a hemispherical microlens, we prescribe analytical expressions for the coupling efficiencies in presence of the said mismatches. The evaluation of the concerned efficiencies and associated losses will require little computations. The results found will be extremely useful in the field of optimum launch optics involving such coupler.

In this work, an effort has been attempted for the comprehensive theoretical analysis to evaluate the impact of higher order dispersion (HOD) coefficients on non-linear crosstalk in SCM–WDM communication systems. The results are obtained to compute the inter-channel crosstalk due to XPM fiber nonlinearity under the individual influence of 2OD-, 3OD-, 4OD- and 5OD-coefficient at different modulation frequencies. The results are reported at varied values of optical span and input channel power, as these both parameters enhance the amount of crosstalk due to fiber nonlinearity in the SCM-WDM systems. This work is also carried out by taking into consideration of the different values of modulation index and walk-off parameter.