Shiva Kumar

Design concept for optical fibers with enhanced SBS threshold

We propose a criterion to predict the relative value of the stimulated Brillouin scattering (SBS) threshold in single-mode optical fibers with different refractive index profiles. We confirm our results by several representative measurements. We show that with the proper profile design one can achieve more than 3 dB increase in the SBS threshold compared to the standard single-mode optical fiber.

Quasi-soliton propagation in dispersion-managed optical fibers

Combination of programmed chirp and dispersion profiles produced a solitonlike nonlinear pulse with a stationary structure (quasi-soliton). The quasi-soliton has more attractive properties than those of a soliton because of its reduced interaction and smaller peak power than a soliton and allows ~100-Gbit / s transmission over a 125-dispersion distance with reasonable power.

All-Optical Multihop Free-Space Optical Communication Systems

All-optical relaying techniques are proposed to improve the error performance and overall distance coverage of free-space optical (FSO) communication systems. An all-optical amplify-and-forward (OAF) relaying technique is presented where the received optical field is amplified at each relay. A novel channel model is developed including field distributions and weak turbulence. Simulation results indicate that OAF significantly enhances the BER performance, but is severely degraded by background light. In order to remove the impact of background noise, an optical regenerate-and-forward (ORF) relaying technique is also presented. At a bit rate of 10 Gbps, using two equally-spaced OAF relays under a turbulence-free atmospheric condition increases the total communicating distance by 0.9 km over direct transmission at a BER of 10-5, while using two ORF relays provides an additional gain in range of 1.9 km. In general, replacing OAF relays by ORF relays extends the total communicating distance at a cost of implementation complexity.

REDUCTION OF COLLISION-INDUCED TIME JITTERS IN DISPERSION-MANAGED SOLITON TRANSMISSION SYSTEMS

Nonadiabatic effects caused by lumped amplifiers in fiber soliton transmission systems are reduced by the use of dispersion-decreasing fibers between amplifiers. As a practical application, we consider a stepwise dispersion-decreasing fiber with M steps and show that the increase in M is almost equivalent to the reduction of amplifier spacing, Z(a), to Z(a)/M in reducing collision-induced jitters in soliton-based wavelength-division multiplexing systems.

Intrachannel nonlinear penalties in dispersion-managed transmission systems

We have developed a model for intra-channel impairments in dispersion-managed systems and validated it using numerical simulations. We have shown that intrachannel impairments can be reduced using optimal precompensation. We have studied the intrachannel penalty for different modulation formats, and results show that the optimum precompensation is independent of modulation format. The intrachannel penalty decreases as the spectrum of the modulation format broadens.

Chirped nonlinear pulse propagation in a dispersion-compensated system

We study nonlinear pulse propagation in an optical transmission system with dispersion compensation. A chirped nonlinear pulse can propagate in such a system, but eventually it decays into dispersive waves in a way similar to the tunneling effect in quantum mechanics. The pulse consists of a quadratic potential that is due to chirp in addition to the usual self-trapping potential and is responsible for the power enhancement and the decay.

Demonstration of negative dispersion fibers for DWDM metropolitan area networks

We present a detailed experimental and theoretical study, showing that a novel nonzero dispersion-shifted fiber with negative dispersion enhances the capabilities of metropolitan area optical systems, while at the same time, reducing the system cost by eliminating the need of dispersion compensation. The performance of this dispersion-optimized fiber was studied using different types of optical transmitters for both 1310- and 1550-nm wavelength windows and for both 2.5-and 10-Gb/s bit rates. It is shown that this new fiber extends the nonregenerated distance up to 300 km when directly modulated distributed feedback (DFB) laser transmitters at 2.5 Gb/s are used. The negative dispersion characteristics of the fiber also enhance the transmission performance in metropolitan area networks with transmitters that use electroabsorption (EA) modulator integrated distributed feedback (DFB) lasers, which are biased for positive chirp. In the case of 10 Gb/s, externally modulated signals (using either EA-DFBs or external modulated lasers using Mach-Zehnder modulators), we predict that the maximum reach that can be accomplished without dispersion compensation is more than 200 km for both 100- and 200-GHz channel spacing. To our knowledge, this is the first demonstration of the capabilities of a nonzero dispersion-shifted fiber with negative dispersion for metropolitan applications.

Intrachannel four-wave mixing in dispersion managed RZ systems

We model the interaction of three consecutive pulses in a dispersion managed transmission system, and obtain an analytical expression for the energy exchange. The results show that interaction can be suppressed using unequally spaced pulses, which has been verified by the computer simulations. The central pulse pumps energy to the adjacent pulses if the phase shift is zero, and the energy exchange process is reversed if the central pulse has a phase shift of /spl pi//2.

Effect of dispersion on nonlinear phase noise in optical transmission systems.

An analytic expression for the variance of nonlinear phase noise that uses a first-order perturbation technique is obtained. The results show that for highly dispersive transmission systems, amplified spontaneous emission-induced phase noise due to self-phase modulation becomes much smaller than that for the systems with no dispersion.

Gordon–Haus effect in dispersion-managed soliton systems

We derive an expression for the timing jitter of a soliton owing to amplifier noise in dispersion-managed fiber systems. We show that the timing jitter is a function of the pulse width as well as of the chirp at the amplifier location and that it can be minimized by a proper choice of the amplifier position relative to the dispersion map.