Band 35, Heft 1

We have studied transmission properties of the previously proposed passive optical network (PON) using hybrid Raman amplifiers for upgrading the existing PON. We focus on the upgrading of 10G-EPON (also XG-PON) system, which uses optical burst signals for upstream transmission. This paper discusses the transmission properties of the hybrid Raman amplifiers for burst signals. In access networks, there are many splice points and connection points. These points may reflect signal waves and may interfere with themselves, which is called a multipath interference (MPI). We also study degradation due to MPI in the hybrid Raman amplification. With the obtained results, we evaluate required reflection coefficients of splices in the previously proposed PON using hybrid Raman amplifiers.

We have designed a four-channel wavelength demultiplexer based on photonic crystal ring resonators (PCRR), which can be used for photonic integrated circuits. Dropping efficiency and Q factor of single improved ring are 100% and 650, respectively. In order to achieve the structure of demultiplexer, three improved rings have been used, that every ring has an individual inner rod radius; it means that each ring has a varying resonant wavelength. By changing the radius of inner rods, various wavelengths can be chosen, therefore this device is tunable. The results of simulation using finite-difference time-domain (FDTD) method in our proposed structure reveals an average transmitted power higher than 90% for each output port. Channel spacing is about 7.5 nm and bandwidth for each individual channel is about 2.8 nm. The mean value of the crosstalk between output channels is about −27.11 dB. The area of the proposed structure is about 315 μm 2 .

Based on the simple series expression of fundamental mode in dispersion-shifted and dispersion-flattened fibers, we formulate analytical expressions for far-field pattern of such fibers and evaluate them. The concerned calculations require much less computations. Taking three typical trapezoidal fibers as example of dispersion-shifted fiber, we show that our results match excellently with the exact numerical results. As example of dispersion-flattened fiber, we choose two parabolic profile W and two step profile W fibers for our study and here also, our estimations agree excellently with the exact numerical results. The present formalism can also be used for predicting different propagation characteristics associated with the far-field. Further, one can also predict the fundamental modal field from the knowledge of far-field pattern. This simple but accurate formalism developed will benefit the system engineers working in the field of optical technology.

Bidirectional optical network based on direct detection optical code division multiple access (OCDMA) technique is implemented. In this architecture, in-band was used for up/downstream transmission data encoded by modified quadratic congruence (MQC) codes at channel spacing 12.5 GHz (i.e., 0.1 nm), and simultaneously transmitted since they are at different wavelengths. The results of the system with optimum parameters show low error rate transmission at 1 Gbit/s data rate and minimum receiver sensitivity −24 dBm which make the high performance is achievable. The results have indicated that bidirectional optical CDMA technique capable to support local area networks as an application for fiber to the home (FTTH).

Light-trail (LT) has been proposed as an attractive solution for optical networks to support emerging services such as video-on-demand, pseudo-wires, data-centers, etc. Due to the fact that light-trail (LT) is a shared-medium network, the media access control (MAC) protocol is essential to be implemented in the whole network. In this paper, we propose a novel MAC protocol called DAART (Delay-aware Adaptive Round Time) to improve the throughput in LT network. DAART MAC adopts the delay-aware mechanism to calculate the actual token holding time for every transmission node. More specifically, the token holding time can be evaluated from queue delay, transmission delay and slot-synchronization delay. To estimate the throughput performance of DAART MAC protocol, we give three different patterns of traffic and compare the throughput between DAART MAC and ART MAC. Simulation results show that DAART has more stable throughput performance than ART, which in return enhances the throughput of LT networks.

In this study a hybrid spectral amplitude coding for optical code division multiple access (i.e., called OCDMA) over wavelength division multiplexing (WDM) network is proposed to support large number of subscribers. The proposed system combines two different techniques, namely OCDMA and WDM. The modified double weight (MDW) code is used as a signature address for designing the system. This code has numerous advantages such as this code can accommodate more number of simultaneous active users under considerable standard bit error rate (e.g. ≤10 −9 ) and shorter code length. The proposed system can enhance the network capability to support high number of users. Therefore, this system can be considered as a promising solution for Metro Area Network (MAN).

In this paper, we have presented analysis of FSO communication system with various weather conditions using OPTISYSTEM TM simulator by a transmission length of 4 km at data rate of 2.50 Gbit/s and reported that as we move from clear weather to heavy fog, OSNR and Q factor as well as transmission range will decrease.

Recently, researches on Orthogonal Frequency Division Multiplexing (OFDM) based Free Space Optics (FSO) communication system gain interest since it combines the advantages of both techniques by mitigating the multipath propagation effects to improve the transmission data rate. The performance of such systems depends on base band modulation used, number of subcarriers, turbulence modeling, nature of laser beam and so on. In this paper, a performance comparison is carried out considering three different levels (weak, medium and strong) of atmospheric turbulence with log-normal and Gamma- Gamma scintillation model for plane, spherical and Gaussian beams. The Gamma-Gamma distribution analysis shows that for all kinds of laser beams, Bit Error Rate (BER) is high for weak turbulence which decreases further for medium and strong. The only difference is that the BER variation for Gamma-Gamma distribution with Gaussian beam is more prominent than for plane beam. The decrease in BER is clearly observed at higher SNR value. This kind of improvement in BER is observed with log-normal PDF at low SNR with weak turbulence. Thus we can say that weak atmospheric turbulence can be modeled by log-normal PDF and medium to strong condition can be modeled by Gamma-Gamma PDF.

We present a comparative study of the influence of dispersion induced phase noise for n-level PSK systems. From the analysis, we conclude that the phase noise influence for classical homodyne/heterodyne PSK systems is entirely determined by the modulation complexity (expressed in terms of constellation diagram) and the analogue demodulation format. On the other hand, the use of digital signal processing (DSP) in homodyne/intradyne systems renders a fiber length dependence originating from the generation of equalization enhanced phase noise. For future high capacity systems, high constellations must be used in order to lower the symbol rate to practically manageable speeds, and this fact puts severe requirements to the signal and local oscillator (LO) linewidths. Our results for the bit-error-rate (BER) floor caused by the phase noise influence in the case of QPSK, 16PSK and 64PSK systems outline tolerance limitations for the LO performance: 5 MHz linewidth (at 3-dB level) for 100 Gbit/s QPSK; 1 MHz for 400 Gbit/s QPSK; 0.1 MHz for 400 Gbit/s 16PSK and 1 Tbit/s 64PSK systems. This defines design constrains for the phase noise impact in distributed-feed-back (DFB) or distributed-Bragg-reflector (DBR) semiconductor lasers, that would allow moving the system capacity from 100 Gbit/s system capacity to 400 Gbit/s in 3 years (1 Tbit/s in 5 years). It is imperative at the same time to increase the analogue to digital conversion (ADC) speed such that the single quadrature symbol rate goes from today's 25 GS/s to 100 GS/s (using two samples per symbol).

This paper presents Long Reach Wavelength Division Multiplexing Passive Optical Network (WDMPON) system capable of delivering downstream 10 Gbit/s data and upstream 10 Gbit/s data on a single wavelength. The optical source for downstream data and upstream data is CW laser at central office and reflective semiconductor optical amplifier (RSOA) at each optical network unit. The use of dispersion management for both up and downstream transmission removes the error floor owing to the chromatic dispersion induced excess intensity noise. It has also been experimentally confirmed that both pre and post-dispersion compensating schemes are effective ways to suppress the intensity noise. Error free bidirectional transmission over 35 km is demonstrated by using Erbium-doped fiber amplifiers at the RN and at central office. Optical power budgets 14.565 dB and 29.171 dB are achieved for downstream performance and upstream performance respectively. This paper demonstrates an effective mitigation of Rayleigh backscattered noise for upstream data and downstream data and also demonstrates that receiver sensitivity is improved for upstream and downstream data. Bit error rate were measured to demonstrate the proposed scheme. In this paper long reach and large data service aspects of a WDM-PON is presented.

A highly reliable wavelength division multiplexing passive optical network delivering up/downstream and broadcast signals simultaneously is proposed in this paper. In this scheme, subcarrier multiplexing is introduced at the optical line terminal. The optical carriers are used for the transmission of point-to-point and broadcast signals. The combination of intensity modulation and phase modulation is employed to improve modulation efficiency. Pure double-sideband subcarriers function as the light source for upstream modulation. The design of OLT and RN can provide protection for downstream and broadcast signal when fiber failure occurs. A simulation study is performed to demonstrate this transmission system at the end of this paper.

Employing the prescribed ABCD matrix for refraction of paraxial rays by hemispherical lens, we derive analytical expression of efficiency of coupling of a laser diode to single-mode circular core dispersion-shifted as well as dispersion-flattened fiber via hemispherical microlens on the fiber tip. For the sake of simplicity, we assume Gaussian field distribution for both the source and the fiber. In our analysis, we also take care of limited aperture allowed by the hemispherical lens. The lens refracted spot size of the source, which is taken to be Gaussian, is matched with Petermann II spot size of the fiber for maximum coupling efficiency, since deviation from Gaussian nature of field for such fiber as per the recent design has to be taken care of. We carry out our study for two different wavelengths namely 1.3 μm and 1.5 μm. Our formalism is simple but accurate and executable easily requiring little computations. The results obtained by such user friendly method should benefit the designer of such coupling device in the field of optimum launch optics.