Akio Sahara

Ultrahigh-speed long-distance TDM and WDM soliton transmission technologies

Recent progress on time-division multiplexed (TDM) and wavelength-division multiplexed (WDM) soliton transmission is described, in which dispersion management (DM) plays an important role in increasing the power margin and the dispersion tolerance. The characteristics of the DM soliton are compared with those of return-to-zero (RZ) and nonreturn-to-zero (NRZ) pulses. With a small dispersion swing, the system can still be described as an average soliton with a nonlinear Schrodinger equation (NLSE), whereas with a large dispersion swing, the soliton-like steady-state pulse becomes a chirped Gaussian pulse, in which the master equation is closer to a linear Schrodinger equation (LSE) with a parabolic potential well. An in-line modulation scheme up to 80 Gb/s per channel and its two-channel WDM transmission over 10000 km are described. A 640-Gb/s (40 Gb/s/spl times/16 channels) WDM soliton transmission over 1000 km is also reported with a DM single-mode fiber, without the use of in-line modulation. Finally, dark soliton transmission at 10 Gb/s over 1000 km is described as a different nonlinear pulse application.

Recent progress in soliton transmission technology

Recent progress on time-division multiplexed (TDM) and wavelength-division multiplexed (WDM) soliton transmission is described, in which dispersion management plays an important role in increasing the power margin and the dispersion tolerance. The characteristics of the dispersion-managed soliton are compared with those of return to zero and nonreturn to zero pulses. With a small dispersion swing, the system can still be described as an average soliton using the nonlinear Schrodinger equation, while with a large dispersion swing, the solitonlike steady-state pulse becomes a chirped Gaussian pulse, in which the governing equation is closer to a linear Schrodinger equation with a parabolic potential well. We describe an in-line modulation scheme for up to 80 Gbit/s per channel and its two channel WDM transmission over 10 000 km. Finally, we describe 640 Gbit/s (40 Gbit/sx16 channels) WDM soliton transmission over 1000 km with a dispersion-managed single-mode fiber. (c) 2000 American Institute of Physics.

Time-domain ABCD matrix formalism for laser mode-locking and optical pulse transmission

ABCD matrix formalism in the time domain has been newly developed on the basis of laser beam and resonator analyses which were developed under a Gaussian paraxial approximation. We derive matrix elements for amplitude and frequency modulations, group velocity dispersion, optical bandpass filter dispersion, and lumped self-phase modulation under a parabolic approximation. Applications to AM, FM, and stretched-pulse laser mode-locking are described by using these time-domain matrices. An application to pulse transmission in a dispersion-allocated system is also described.

40-Gb/s RZ transmission over transoceanic distance in a dispersion managed standard fiber using a new inline synchronous modulation method

We propose a modified soliton control method for a strongly dispersion managed line, which employs a highly nonlinear fiber in addition to conventional inline synchronous modulation. This method enables us to transmit a 10-Gb/s return-to-zero signal over 20000 km using a dispersion managed line composed of a standard (nondispersion shifted) fiber and dispersion compensation. Optimizing the length of the highly nonlinear fiber greatly increases the power margin of the transmitted optical power.

Optimum fiber dispersion for two-step dispersion-allocated optical soliton, RZ at zero GVD and NRZ systems

The optimum fiber dispersion conditions for two-step dispersion-allocated (D-A) optical soliton and nonreturn-to-zero (NRZ) systems are derived numerically. The transmission speed is 20 Gb/s, the amplifier spacing is SO km, and the two fiber segment lengths are 30 km and 50 km. The maximum transmission distance for a D-A soliton system can be obtained at an average dispersion of 0.1 ps/km/nm. The dispersion of the first fiber segment is 3.1 ps/km/nm. For an NRZ system, the maximum transmission distance can be obtained at zero average dispersion, in which the dispersion of the first segment is -2 ps/km/nm. The optimum average dispersion strongly depends on the dispersion allocation and on whether a soliton or an NRZ system is used.

Achievement of Subchannel Frequency Spacing Less Than Symbol Rate and Improvement of Dispersion Tolerance in Optical OFDM Transmission

We propose a novel optical orthogonal frequency-division multiplexing (OFDM) transmission, where the frequency spacing of subchannels is less than the symbol rate. We show experimentally that the novel OFDM transmission achieves an optical SNR (OSNR) sensitivity level and dispersion tolerance roughly equivalent to those of the conventional OFDM. We also show the influence of chromatic dispersion from the transmission channel on the OFDM signal. The chromatic dispersion causes a desynchronization of the symbol timing between subchannels and breaks the frequency orthogonality in OFDM. We confirm experimentally that it is possible to mitigate the signal degradation due to the chromatic dispersion by optimizing the symbol timing and orthogonally polarizing the subchannels to the neighboring subchannels.

Demonstration of optical burst data switching using photonic MPLS routers operated by GMPLS signaling

We demonstrate, for the first time, optical burst data switching using photonic MPLS routers controlled by GMPLS. Optical burst data can be transmitted at a bit rate of 10 Gb/s without burst data loss, and high network throughput is achieved.

40-Gb/s RZ transmission over a transoceanic distance in a dispersion managed standard fiber using a modified inline synchronous modulation method

This paper analyzes in detail numerically a 40-Gb/s return-to-zero (RZ) transmission system over a transoceanic distance in a strongly dispersion managed line composed of standard single-mode fiber (SMF) and dispersion compensation fiber (DCF). We derived a periodically steady-state pulse (a DM soliton) in a DM line. Since the pulse width of a steady-state pulse is too broad for a 40 Gb/s system, the conventional in-line synchronous modulation technique cannot greatly improve the transmission quality. However, we found that the modified inline synchronous modulation technique, which is reported as the black-box optical regenerator, can effectively extend the transmission distance even in such a strongly DM line. We discuss the mechanism of the modified synchronous modulation technique with respect to a steady-state pulse in a transmission line, and show that a 40-Gb/s RZ signal can be transmitted over 20 000 km.

Demonstration of colorless and directed/directionless ROADMs in router network

We demonstrate dynamic optical path operation of colorless ROADM and connection switching between routers in a router network. We also evaluate and demonstrate both colorless-directed ROADM and colorless-directionless ROADM in a router network.

Propagation of a solitonlike nonlinear pulse in average normal group-velocity dispersion and its unsuitability for high-speed, long-distance optical transmission

We detail the propagation characteristics of a solitonlike single nonlinear pulse in an average normal group-velocity dispersion (GVD) region under two-step dispersion management. We compare the pulse characteristics with numerical results obtained by the variational method. Even when the dispersion swing is large, a steady-state solitonlike solution can be obtained only when the average effective dispersion becomes anomalous. We describe a pulse–pulse interaction based on a pair of nonlinear pulses and show that there is a large pulse interaction in the average normal GVD region. We show with a Q-mapping technique that such a nonlinear pulse train is unsuitable for high-speed, long-distance optical communication because of this strong pulse interaction.