R.S. Kaler

Comparison of pre-, post- and symmetrical-dispersion compensation schemes for 10 Gb/s NRZ links using standard and dispersion compensated fibers

In this paper, we investigate pre-, post- and symmetrical-dispersion compensation methods for 10 Gb/s non-return to zero (NRZ) links using standard and dispersion compensated fibers through computer simulations to optimize high data rate optical transmission. The influence of EDFA power and increase in length of each type of fiber has been studied to evaluate the performance of optical communication systems. The performance characteristics like bit error rate, eye diagrams and eye closure penalty at the output are studied by simulating different systems. The results of three compensation methods have been compared and it is found that the symmetrical compensation method is superior to pre- and post-compensation methods. On comparing pre- and post-compensation methods, it is found that the later is superior to the former. Further, it has also been observed that system needs proper matching between the EDFA power and length of the fiber for optimum performance.

Approximate and Exact Small Signal Analysis for Single-Mode Fiber Near Zero Dispersion Wavelength with Higher Order Dispersion

This article presents the comparison of approximate and exact small-signal theories for analyzing the influence of the higher-order dispersion terms on dispersive optical communication systems operating near zero dispersion wavelength for linear single-mode fiber. For the approximate theory, the generalized conversion matrix has been reported and gives the transfer function of intensity and phase from the fiber input to fiber output for a laser source including the influence of any higher-order dispersion term. In addition, expressions for the small-signal frequency response and the relative intensity noise (RIN) response of an ultrafast laser diode including noises are derived. However, it is observed that the approximation assumed for the second-order dispersion term for the approximate analysis is not valid. From the approximate theory, the exact generalized conversion matrix and exact expressions for small-signal frequency response and relative intensity noise (RIN) are obtained. We show that for the exact theory, the second-order dispersion term has no effect on intensity and frequency response even at large modulating frequencies and large propagation distances contrary to the approximate theory as reported by other authors. But we show that third-order dispersion term certainly has some minute impact on the frequency and RIN response for long distance links at high modulating frequencies.

Minimization of cross-gain saturation in wavelength division multiplexing by optimizing differential gain in semiconductor optical amplifiers

We successfully simulated the 10 × 40 Gbit/s soliton RZ-DPSK WDM signals over 1050 km with spectral efficiency approaching 0.4 bit/s/Hz using semiconductor optical amplifiers (SOAs) as in-line amplifier. The cross-gain saturation of SOA can be minimized by settling crosstalk at a lower level by decreasing the differential gain. This decrease in differential gain is in such a way that we get nil power penalty. The maximum transmission distance of 1050 km is possible with differential gain 210 atto cm2 of SOA. The impact of amplification factor, ASE noise power, crosstalk, quality factor and bit error rate for different differential gain has been investigated. It has been shown that with the increase in differential gain of SOA, the transmission distance goes on decreasing. At high value of differential gain 2.5 × 10−16 cm2 for the transmission distance 1050 km, all channels produce inter channel crosstalk with bit error rate greater than 10−6. But for lower differential gain 190 atto cm2, the quality of all channel increases at the cost of large power penalty. With slight increase in differential gain 200 atto cm2, the maximum transmission distance observed is 4550 km with quality of received signal more than 15 dB and having nil power penalty. We observed clear eye diagrams and optical power spectra for received signal with transmission distance 1050 km and 4550 km using soliton RZ-DPSK system. The bit error rate for all channels increase more than 10−10 with the increase in launched input power that is due to power saturation.

Power penalty analysis for realistic weight functions using differential time delay with higher-order dispersion

In this paper, the power penalty analysis for approximate and realistic weight functions has been presented for combating the pulse broadening effects of group-velocity dispersion in a fiber-optic communication link using differential time delay method including higher-order dispersion terms. The expressions for root mean square (RMS) phase deviation, optimum chirp factor and figure of merit have been evaluated for approximate and realistic systems. We show that the optimum value of chirp factor corresponds to dispersion compensation. The power penalty graphs for second-, third-, and fourth-order dispersion and their combinations have been presented for distance up to 300 km for this chirp factor for different weight functions. It is observed that the power penalty for realistic weight functions is less in comparison with the approximated weight function. It has also been shown that it is possible for a short pulse to propagate without significant broadening over the lengths many times longer than the usual dispersion length of fiber.  2002 Elsevier Science (USA). All rights reserved.

Simulation Results for DWDM Systems with Ultra-High Capacity

We present stimulation results for DWDM systems with an ultra-high capacity up to 1.28 Tbit/s and spectral efficiency approaching 0.4 bit/s/Hz. The impact of signal-to-noise ratio (SNR) on parameters such as channel spacing, length of fiber, dispersion, and number of channels has been investigated and the results obtained have been explained on the basis of fiber nonlinear effects. It has been shown that with an increase in channel spacing, the SNR increases to the maximum optimum value and then decreases to a steady value. With an increase in number of channels, the SNR decreases for small wavelength spacing. For large wavelength spacing, it becomes independent of the number of channels. Keeping channel spacing constant, the SNR decreases with an increase in the length of the fiber. The SNR also improves with a small increase in dispersion of the fiber. Further, it is observed that, with increase in length over dispersion-shifted fiber, the received power decreases and the bit error rate increases.

Wide-Band Optical Wavelength Converter Based on Four-Wave Mixing Using Optimized Semiconductor Optical Amplifier

We have simulated 50 nm up and down wavelength conversion for a non-return to zero differential phase shift keying (NRZ-DPSK) signal using four-wave mixing in an optimized semiconductor optical amplifier (SOA) at 10 Gb/s for the first time. For this we optimized the SOA parameters to achieve sufficient quality and enhancement in four-wave mixing effect. This can be done in such a manner that the SOA never saturates and produces maximum four-wave mixing signals with minimum gain fluctuations. The quality of the converted signal is best before the saturation of SOA. Finally, we have numerically simulated cascaded wavelength converters up to 1300 km transmission distance.

Large signal analysis of FM-AM conversion in dispersive optical fibers for PCM systems including second order dispersion

By using large signal analysis for dispersive optical fiber, the FM-AM conversion with respect to binary intensity modulated PCM systems including second order dispersion term is discussed. The modified expression for power penalty has been derived and its impact on laser linewidth and bit rate has been investigated. For power penalty less than 0.5 dB, the plots between bit rate and transmission distance are plotted. It is seen that the transmission distance increases with decrease in linewidth over significant bit rates. The transmission distance with first order dispersion term for 300 MHz linewidth is approximately 800km. With proper first order dispersion compensation, i.e., with second order dispersion only, the transmission distance can be enhanced to 10 8 km for this linewidth. The linewidth requirements for systems with different bit rates and transmission distances are also calculated and discussed. Further, it is seen that by including the second-order dispersion term, the bit rate and transmission distance decreases. For higher linewidths, this decrease in bit rate and transmission distance is very less and vice versa. For 300 MHz linewidth, the decrease in transmission distance is just 30 km, and for 30 MHz linewidth, the decrease is approximately 600 km over significant bit rates.

On the performance analysis of wireless receiver using generalized-gamma fading model

In this paper, we provide a unified analysis for wireless system over generalized fading channels that is modeled by the two parameter generalized gamma model. This model is versatile enough to represent short-term fading such as Weibull, Nakagami-m, or Rayleigh as well as shadowing. The performance measures such as the amount of fading, average bit error rate, and signal outage are considered for analysis. With the aid of moment generating function (MGF) approach and Padé approximation (PA) technique, outage probability and average bit error rate have been evaluated for a variety of modulation formats. We first use the PA technique to find a simple way to evaluate compact rational expressions for the MGF of output signal-to-noise ratio, unlike previously derived intricate expressions in terms of Fox’s H and MeijerG functions. Using these rational expressions, we evaluate the performance of wireless receivers under a range of representative channel fading conditions. Our results are validated through computer simulations, which shows perfect match.

A novel optical burst switching architecture for high speed networks

A novel optical burst switching (OBS) high speed network architecture has been proposed. To verify its feasibility and evaluate its performance, just-enough-time (JET) signaling has been considered as a high performance protocol. In the proposed architecture, to avoid burst losses, firstly, a short-prior-confirmation-packet (SPCP) is sent over the control channel that simulates the events that the actual packet will experience. Once SPCP detects a drop at any of the intermediate nodes, the actual packet is not sent but the process repeats. In order to increase network utilization, cost effectiveness and to overcome some limitations of conventional OBS, inherent codes (e.g., orthogonal optical codes (OOC)), which are codified only in intensity, has been used. Through simulations, it shows that a decrease in burst loss probability, cost effectiveness and a gain in processing time are obtained when optical label processing is used as compared with electronic processing.

Performance analysis of an integrated scheme in optical burst switching high-speed networks

A new integrated scheme based on resource-reservation and adaptive network flow routing to alleviate contention in optical burst switching networks is proposed. The objective of the proposed scheme is to reduce the overall burst loss in the network and at the same time to avoid the packet out-of-sequence arrival problem. Simulations are carried out to assess the feasibility of the proposed scheme. Its performance is compared with that of contention resolution schemes based on conventional routing. Through extensive simulations, it is shown that the proposed scheme not only provides significantly better burst loss performance than the basic equal proportion and hop-length based traffic routing algorithms, but also is void of any packet re-orderings.