Optical Duobinary Signals Research Articles

320 Gbit/s WDM repeaterless transmissionusing fully encoded 40 Gbit/s optical duobinary channelswith dispersion tolerance of 380 ps/nm

320 Gbit/s WDM repeaterless transmission over 100 km dispersion-shifted fibre is demonstrated using fully encoded 40 Gbit/s optical duobinary channels without dispersion compensation. A record dispersion tolerance of 380 ps/nm is achieved using a 40 Gbit/s optical duobinary signal.

A 1580-nm band WDM transmission technology employing optical duobinary coding

This paper reports 1580-nm band wavelength division multiplexed (WDM) transmission employing optical duobinary coding over dispersion-shifted fibers. By using the 1580 nm band, the generation of four-wave mixing (FWM) over dispersion-shifted fibers (DSFs) can he suppressed. Optical duobinary coding is dispersion-tolerant because of its narrow bandwidth, and enables the use of the conventional binary intensity modulated direct detection (IM-DD) receiver. First, comparisons are made for WDM transmission performance in the 1580-nm band between conventional binary nonreturn-to-zero (NRZ) coding with and without postdispersion compensation, and optical duobinary coding by computer simulation is described. From the numerical simulations, it is found that the optical duobinary coding has superior transmission performance to the conventional binary coding without any dispersion compensation, and that the difference in the transmission performance between two coding methods is very small even if postdispersion compensation at the optical receiver is applied to the NRZ coding method. Second, transmission performance between the conventional binary NRZ and the optical duobinary signals without any dispersion compensation is compared with the straight-line experiment over 500-km dispersion-shifted fiber. The experimental results reveal that the transmission distance with optical duobinary coding is doubled in comparison with that of the conventional binary NRZ signals. Finally, 16-channel, 10-Gb/s optical duobinary WDM signals in the 1580-nm band are successfully transmitted over 640 km (80 km/spl times/8) of DSF without any dispersion compensation or management.

Differential precoder IC modules for 20-and 40-Gbit/s optical duobinary transmission systems

This paper reports on 20- and 40-Gbit/s differential precoder modules for optical duobinary transmission systems. These precoder modules overcome the speed limit of a conventional precoder by parallel processing. The proposed precoders handle two or four parallel signals before multiplexing with data rates of one-half or one-quarter the transmission bit rate, and the final preceded signal is obtained by multiplexing the precoder output bit by bit, production-level 0.2-/spl mu/m gate-length GaAs MESFET's were used to fabricate the precoders. The precoders are mounted in an RF package. They successfully performed 20- and 40-Gbit/s precoding for the first time, and the 20-Gbit/s precoder achieved a maximum precoding rate of 22 Gbit/s, which is 76% faster than that of the conventional circuit using the same MESFETs. The 40-Gbit/s precoder performs 40-Gbit/s precoding when combined with a 40-Gbit/s multiplexer unit. Twenty-Gbit/s optical duobinary transmitter and receiver circuits using the 20-Gbit/s precoder module successfully generate fully encoded optical duobinary signal at this rate for the first time. These circuits show a receiver sensitivity of -28.6 dBm for a bit error rate of 1/spl times/10/sup -9/.

160 Gbit/s WDM transmission experimentusing four 40 Gbit/s optical duobinary channels

Four-channel multiplexed 40 Gbit/s optical duobinary signals are successfully transmitted over a 100 km dispersion-shifted fibre for the first time. 40 Gbit/s based four-channel WDM transmission without individual channel dispersion compensation was achieved by virtue of the high dispersion tolerance of the optical duobinary signal.

Characteristics of optical duobinary signals in terabit/s capacity, high-spectral efficiency WDM systems

Optical duobinary signals have been applied to dense wavelength division multiplexing (WDM) systems with high-spectral efficiency to fully utilize a limited gain bandwidth of about 35 nm (4.4 THz) for an erbium-doped fiber amplifier (EDFA). These signals are one type of partial response signals and have narrower bandwidth than conventional intensity modulation (IM) signals. Thus, the use of these signals should make possible to attain ultradense WDM systems. In this paper, characteristics of optical duobinary and IM signals in ultradense WDM systems are compared through experimental evaluations at 20 Gb/s. High-spectral efficiency of 0.6 b/s/Hz was reached in this demonstration.

Duobinary transmitter with low intersymbol interference

A duobinary transmitter employing two-level electrical drive signals for the modulator, exhibits negligible penalties for transmitting 2/sup 7/-1 as well as 2/sup 31/-1 pseudorandom bit sequences (PRBS's). Previous implementations of duobinary transmitters have relied on driving the modulator with three-level electrical signals, which are very susceptible to distortion in saturated amplifiers. The logic operation necessary for generating an optical duobinary signal is carried out in the dual-drive Mach-Zehnder modulator.

Expansion of tolerable dispersion rangein a 40 Gbit/s optical transmission systemusing an optical duobinary signal

The authors demonstrate the high dispersion tolerance of a 40 Gbit/s optical duobinary signal. The tolerable dispersion range of the signal was 175 ps/nm. This dispersion range is more than twice that obtained using a binary non-return-to-zero intensity modulated signal.

Key technologies for terabit/second WDM systems with high spectral efficiency of over 1 bit/s/Hz

To fully utilize a limited gain bandwidth of about 35 nm (4.4 THz) in an erbium-doped fiber amplifier, an increase in signal spectral efficiency is required. In this paper, we investigate the key technologies to achieve terabit/second wavelength-division multiplexing (WDM) systems with over 1 bit/s/Hz spectral efficiency. Optical duobinary signals, which have narrower optical spectra than conventional intensity modulation signals, were applied to such dense WDM systems. The measured minimum channel spacing for 20-Gbit/s optical duobinary signals was 32 GHz and a spectral efficiency of over 0.6 bit/s/Hz was reached. By using polarization interleave multiplexing, spectral efficiency was expected to reach 1.2 bit/s/Hz in an ideal case with no polarization dependencies along the transmission lines. In such ultradense WDM systems with narrower channel spacing, stabilizing the wavelengths of laser diodes is an important issue for achieving stable operation over long periods. To do this, we developed a simple and flexible wavelength stabilization system which uses a multiwavelength meter. The wavelengths for 116 channels with 35-GHz spacing were stabilized within /spl plusmn/150 MHz. The stabilization system is applicable to ultradense WDM signals with a spectral efficiency of over 1 bit/s/Hz by employing wavelength interleave multiplexing and an optical selector switch. On the basis of these investigations, we demonstrated a 2.6-Tbit/s (20 Gbit/s/spl times/132 channels) WDM transmission by using optical duobinary signals. In addition, 1.28-Tbit/s (20 Gbit/s/spl times/64 channels) WDM transmission with a high spectral efficiency of 1 bit/s/Hz was achieved by using polarization interleave multiplexing.

Dispersion-tolerant optical transmission system using duobinary transmitter and binary receiver

We propose a dispersion-tolerant optical duobinary transmission system which uses a binary intensity modulation direct detection (IM-DD) receiver. The proposed system also relaxes the fiber input power limitation due to stimulated Brillouin scattering (SBS). Tolerance to chromatic dispersion in the proposed system is estimated by a simulation. A 10 Gb/s optical duobinary signal can be transmitted over 200 km of standard single-mode fiber (SMF) with 1 dB dispersion penalty. The advantages of the proposed system are confirmed experimentally. The optical duobinary signal was successfully received by the binary IM-DD receiver at the same sensitivity as a binary IM signal. Power penalty due to chromatic dispersion over a 164-km standard SMF was about 1.8 dB at 10 Gb/s. The SBS threshold of the 10 Gb/s optical duobinary modulated light was more than +20 dBm.