Akira Naka

In-line amplifier transmission distance determined by self-phase modulation and group-velocity dispersion

The propagation of an intensity-modulated signal in an optical fiber is numerically analyzed, taking self-phase modulation, group-velocity dispersion, and 2nd-order group-velocity dispersion into account. Transmission distances yielding a prescribed eye-opening penalty are shown to relate to three characteristic lengths: the dispersion length, the 2nd-order dispersion length, and the nonlinear length. These relations, with a slight modification for the signal power, are also found to be valid for in-line amplifier systems, in which each optical amplifier, though adding undesired amplified spontaneous emission, compensates the signal attenuation occurring in the preceding fiber. The resulting relations give us informative guidelines for designing such systems. >

Comparison between NRZ and RZ signal formats for in-line amplifier transmission in the zero-dispersion regime

Nonreturn-to-zero (NRZ) and return-to-zero (RZ) signal formats are experimentally and numerically compared for single-channel long-distance transmission in an in-line amplifier system with dispersion management providing average zero dispersion and local nonzero dispersion at an interval equal to the in-line amplifier spacing. Among a 20-ps RZ signal, a 40-ps RZ signal, and an NRZ signal transmitted in 10 Gb/s straight-line experiments, the last signal achieves the longest transmission distance of 6000 km while the others are limited to 4400 km. Numerical simulations explain these results well and show that, along with linear amplified spontaneous emission (ASE) accumulation, signal waveform distortion due to the combined effect of higher order group-velocity dispersion (GVD) and self-phase modulation (SPM) dominates the performance. Nonlinear optical noise enhancement is not obvious because of the fiber dispersion arrangement. Signals with large pulse widths are less affected by the combined effect, while small-width signals yield superior initial signal-to-noise ratio (SNR) as determined by optical noise. A detailed simulation indicates that a pulse width of about 60 ps is optimum for long distance transmission under the fiber dispersion arranged in this paper.

Transmission distance of in-line amplifier systems with group-velocity-dispersion compensation

Group-velocity dispersion (GVD) compensation in in-line amplifier systems is evaluated from the viewpoint of improving the transmission distance. The nonlinear Schrodinger equation, which simulates signal propagation in optical fibers, is numerically evaluated to clarify the optimum configuration for GVD compensation. It is shown that the optimum amount of GVD compensation is about 100% of the GVD experienced by the transmitted signal. The optimum compensation interval is found to be a function of the bit rate, signal power, and dispersion parameter. For dispersion parameter values ranging from about -0.1 ps/nm/km to -10 ps/nm/km, and an amplifier noise figure of about 6 dB, the optimum compensation configuration can eliminate the GVD from in-line amplifier systems, thus improving transmission distances to those limited by self-phase modulation and higher-order GVD. >

Inline amplifier transmission experiments over 4500 km at 2.5 Gb/s

An inline amplifier system was constructed with erbium-doped fiber amplifiers spaced at 100 km and 80 km intervals. The system transmits 2.5 Gb/s signals over 2500 km with continuous-phase frequency-shift-keying heterodyne detection and over 4500 km with intensity-modulation direct detection. With respect to amplifier output signal power levels, it is experimentally shown that there exists a dynamic range within which long-distance signal transmission can be achieved with only small receiver sensitivity degradation. The ranges upper and lower limits are determined by fiber nonlinearities and amplifier noise characteristics, respectively. >

Super high density multi-carrier transmission system by MIMO processing

We propose a super high density multi-carrier transmission system with MIMO processing by estimating the precise subcarrier spacing. We experimentally and numerically demonstrate that the proposed system enables the sub-carrier spacing to be reduced to half the baud rate.

An Analytical Signal and Noise Expression for Optical Preamplifier Receivers and its Application

Signal-to-noise ratio (SNR) theory based on beat noise consideration allows us to estimate the basic performance of optical amplifier systems [1].

Kerr Nonlinearity Limits on Transmission Distance and Data Rate of In-Line Optical Amplifier Systems

In-line optical amplifiers allow fiber transmission systems to have increased regenerative repeater spacing [1],[2]. Transmission distance and data rate are evaluated in terms of fiber loss, dispersion, and nonlinearity. Loss and dispersion limits, which are estimated through optical noise accumulation and waveform distortion, respectively, have already been discussed [3],[4]. Fiber nonlinear effects, however, have not received any extensive discussion except for a few limited cases [5]-[7]. This paper discusses the optical Kerr effect in in-line optical amplifier systems and derives two simple formulae to evaluate nonlinearity- limited system performance. Expected overall performance is also described by taking fiber loss, dispersion and nonlinear limits into account.

A noise-loading method for evaluating in-line amplifier systems

The authors propose a noise-loading method for evaluating inline optical amplifier systems. This method takes the linear accumulation of amplified spontaneous emission into account. Comparison against the measured performance of an experimental transmission system confirms that the method accurately simulates inline amplifier systems. The method can be used to evaluate transmitter-receiver pairs to be used in such systems.<<ETX>>

Characteristics Investigation of High-Speed Multi-Carrier Transmission Using MIMO-Based Crosstalk Compensation in Homodyne Detection Scheme

We experimentally investigate the characteristics of a multi-carrier optical transmission system with neighboring-crosstalk compensation using multiple-input multiple-output processing as an integrated part of a polarization-demultiplexing stage in a homodyne detection scheme for high-speed optical signals. We show that the crosstalk compensation yields adequate performance in the presence of high polarization mode dispersion and high-speed changes in the state of polarization. We also clarify the performance required of local oscillators in the receivers for adequate crosstalk compensation in this scheme. Furthermore, we show the applicability of the system equipped with delay compensation to long-haul transmission through 480-km WDM transmission experiments employing 256-Gb/s optical multi-carrier signals with the subcarrier spacing of 25 GHz, which corresponds to 78% of the signal baud rate.

Compatibility between nonlinear compensation and crosstalk compensation using MIMO processing in super-high-density multi-carrier transmission system

We experimentally demonstrate that nonlinear compensation and MIMO-based neighbouring-crosstalk compensation achieve a Q-factor improvement of 2-3 dB without mutual performance degradation in a broad range of fibre-launched power for two-sub-carrier 32-Gbaud signals with the sub-carrier spacing of 25 GHz.