Ekawit Tipsuwannakul

Approaching Nyquist Limit in WDM Systems by Low-Complexity Receiver-Side Duobinary Shaping

A novel low-complexity coherent receiver solution is presented to improve spectral efficiency in wavelength-division multiplexing (WDM) systems. It is based on the receiver-side partial-response equalization and maximum-likelihood sequence detection (MLSD) in prefiltered WDM systems. The partial-response equalization shapes the channel into an intermediate state with a known partial response which is finally recovered by MLSD without the need of channel estimation. In this scheme, the severe intersymbol interference induced by the prefiltering can be shared between the partial-response equalization and MLSD. Therefore, a tradeoff can be made between complexity and performance. The feasibility of receiver-side partial-response shaping relaxes the efforts and requirements on the transmitter-side prefiltering, which permits the mature WDM components to implement prefiltering. In addition, the partial-response equalization or shaping structure is also improved based on our prior art, which further simplifies the overall scheme. For near-baudrate-spacing optically prefiltered WDM systems, duobinary response is experimentally proved as a good intermediate response to shape. Due to the short memory of the duobinary response, the complexity of the receiver based on duobinary shaping has been reduced to a low level. As a whole, the proposed scheme provides a good alternative to Nyquist-WDM at comparable spectral efficiencies. With the proposed receiver-side duobinary shaping technique, three sets of experiments have been carried out to verify the improved duobinary shaping scheme and meanwhile demonstrate the main features, including 5 ×112-Gb/s polarization-multiplexed quadrature phase-shift keying (PM-QPSK) WDM transmission over a 25-GHz grid, single-channel 40-Gbaud PM-QPSK experiment, and 30-GHz-spaced 3 × 224-Gb/s PM 16-ary quadrature amplitude modulation transmission.

Format Conversion of Optical Multilevel Signals Using FWM-Based Optical Phase Erasure

An optical format conversion scheme of multilevel phase-shift keying (M-ary PSK) signals using the four-wave mixing based optical phase erasure is discussed in this paper. The conversion from differential quadrature phase-shift keying (DQPSK) to differential phase shift keying at 10 and 160 Gbaud, and the conversion from differential eight-ary phase-shift keying to DQPSK at 40 Gbaud have been experimentally demonstrated. The proposed scheme could be used as a phase erasure, in which a redundant tributary of the M-ary PSK signal is removed, or as an “updater” where the unwanted phase pattern is replaced with the new pattern.

Cancellation of Nonlinear Phase Distortion in Self-Homodyne Coherent Systems

Using the self-homodyne detection scheme, it is possible to compensate for the nonlinear phase distortion that, if left uncompensated, would distort the received constellation diagram. The suggested compensation method is evaluated using numerical simulations and validated in an experiment.

Phase-sensitive amplified DWDM DQPSK signals using free-running Lasers with 6-dB link SNR improvement over EDFA-based systems

For the first time, we experimentally demonstrate a modulation-format independent, WDM-compatible link concept with up to 6-dB SNR advantage over conventional EDFA amplified links, by using a frequency-non-degenerate copier and phase-sensitive fiber amplifier based design.

Low-complexity duobinary signaling and detection for sensitivity improvement in Nyquist-WDM coherent system

With complexity-reduced digital signal processing, duobinary signaling and detection are proposed for Nyquist-WDM coherent systems. Best sensitivity is experimentally demonstrated i n 25GHz-grid 5×112-Gb/s PM-QPSK WDM systems.

Transmission of 240 Gb/s PM-RZ-D8PSK over 320 km in 10 Gb/s NRZ-OOK WDM system

We present the first demonstration of 40 Gbaud PM-RZ-D8PSK and its successful transmission in a 100 GHz-spaced 10 Gb/s NRZ-OOK WDM system over a 320-km transmission link along with the studies of WDM nonlinear crosstalk.

Influence of fiber-Bragg grating-induced group-delay ripple in high-speed transmission systems

The implementation of a chirped fiber-Bragg grating (FBG) for dispersion compensation in high-speed (up to 120 Gbit/s) transmission systems with differential and coherent detection is, for the first time, experimentally investigated. For systems with differential detection, we examine the influence of group-delay ripple (GDR) in 40 GBd 2-, 4-, and 8-ary differential phase shift keying (DPSK) systems. Furthermore, we conduct a nonlinear-tolerance comparison between the systems implementing dispersion-compensating fibers and FBG modules, using a 5×80 Gbit/s 100-GHz-spaced wavelength division multiplexing 4-ary DPSK signal. The results show that the FBG-based system provides a 2 dB higher optimal launch power, which leads to more than 3 dB optical signal-to-noise ratio (OSNR) improvement at the receiver. For systems with coherent detection, we evaluate the influence of GDR in a 112 Gbit/s dual-polarization quadrature phase shift keying system with respect to signal wavelength. In addition, we demonstrate that, at the optimal launch power, the 112 Gbit/s systems implementing FBG modules and that using electronic dispersion compensation provide similar performance after 840 km transmission despite the fact that the FBG-based system delivers lower OSNR at the receiver. Lastly, we quantify the GDR mitigation capability of a digital linear equalizer in the 112 Gbit/s coherent systems with respect to the equalizer tap number (Ntap). The results indicate that at least Ntap = 9 is required to confine Q-factor variation within 1 dB.

Format Conversion From 120-Gb/s RZ-D8PSK to 80-Gb/s RZ-DQPSK Through FWM-Based Optical Phase Erasure

An optical format conversion scheme from 120-Gb/s return-to-zero differential 8-ary phase-shift keying (RZ-D8PSK) to 80-Gb/s RZ differential quadrature phase-shift keying (RZ-DQPSK) is demonstrated by optically removing one tributary through optical phase erasure. It is achieved through four-wave mixing in a highly nonlinear fiber. An 8-dB sensitivity improvement is obtained for the converted DQPSK signal relative to that of the input D8PSK signal. This is potentially useful to enable the interconnection among neighboring networks where different formats are deployed.

Optical injection-locking-based pump recovery for phase-sensitively amplified links

An injection-locking-based pump recovery system for phase-sensitively amplified links is proposed and studied experimentally. Measurements with 10 Gbaud DQPSK signals show penalty-free recovery of 0.8 GHz FWHM bandwidth pump with 63 dB overall amplification.

Performance Comparisons of DP-16QAM and Duobinary-Shaped DP-QPSK for Optical Systems With 4.1 Bit/s/Hz Spectral Efficiency

This paper, for the first time, experimentally presents direct comparisons of two 25 GHz spaced wavelength division multiplexing (WDM) systems using conventional dual-polarization (DP) 16-ary quadrature amplitude modulation and duobinary-shaped DP-quadrature phase shift keying (QPSK) modulation. Both systems operate at the same bit rate per channel of 112 Gbit/s, yielding a spectral efficiency of 4.1 bit/s/Hz. The comparisons are conducted for three different cases, i.e., the back-to-back sensitivity, the nonlinear tolerance over a 640-km standard single-mode fiber link, and the phase-noise tolerance (by means of simulations). The results show that the duobinary-shaped DP-QPSK system not only provides a 3.4 dB superior back-to-back sensitivity, but also exhibits a 3 dB higher tolerance against nonlinear impairments after 640 km transmission with three WDM channels. In addition, the numerical results indicate that both investigated systems provide similar tolerances to the laser phase noise given that the block length used in the carrier phase estimation is optimized.