Hung Nguyen Tan

No guard-band wavelength translation of Nyquist OTDM-WDM signal for spectral defragmentation in an elastic add-drop node.

We demonstrate a seamless spectral defragmentation in an elastic all-optical add-drop node based on wavelength division multiplexing (WDM) channels of Nyquist optical time division multiplexing (OTDM) signal. A 172 Gbaud Nyquist OTDM signal occupying a 215 GHz range is elastically shifted adjacent to its neighboring channel, completely filling a variable spectral gap caused by the dropped channel. The frequency shift is done in a dual-stage polarization-diversity four wave mixing-based converter using polarization-maintaining highly nonlinear fiber. The spectrally defragmented signals are successfully transmitted over a 80 km fiber link with BER<10(-9).

Transmission and pass-drop operations of mixed baudrate Nyquist OTDM-WDM signals for all-optical elastic network

We propose the use of Nyquist OTDM-WDM signal for highly efficient, fully elastic all-optical networks. With the possibility of generation of ultra-coarse yet flexible granular channels, Nyquist OTDM-WDM can eliminate guard-bands in conventional WDM systems, and hence improves the spectral efficiency in network perspective. In this paper, transmission and pass-drop operations of mixed baudrate Nyquist OTDM-WDM channels from 43 Gbaud to dual-polarization 344 Gbaud are successfully demonstrated over 320 km fiber link with four FlexGrid-compatible WSS nodes. A stable clock recovery is also carried out for different baudrate Nyquist OTDMs by optical null-header insertion technique.

Counter-Dithering Pump Scheme for Cascaded Degenerate FWM Based Wavelength Converter

We demonstrate counter-dithering of pumps between two cascaded FWM processes for highly-efficient format-independent wavelength conversion. Up-to -1.2dB FWM conversion efficiency is obtained over 32-nm. 86-Gbps DP-QPSK are converted with OSNR penalty below 0.3dB at 10-3 BER.

Optimized WDM Transmission Impairment Mitigation by Multiple Phase Conjugations

Multiple phase conjugations within a transmission-optimized fiber link enable significant nonlinearity cancellation in the wavelength division multiplexed (WDM) systems. In particular, we demonstrate the fiber impairment mitigation of WDM 24×48 Gb/s dual-polarization quadrature phase shift keying (DP-QPSK) signals using 12 optical phase conjugations (OPC). The power excursion and dispersion-tailored transmission line in which the phase conjugations acted periodically, enabled an 8-dB higher nonlinearity threshold for the WDM 1.1-Tb/s DP-QPSK signals. With the nearly perfect nonlinearity cancellation, large input powers per span became available. Under these circumstances, we also present a first approach to the analysis of the impact of multiple phase conjugations in a distributed Raman amplified link. The analysis considers the performance of the experimentally measured OPC subsystem within an optimized transmission line limited only by an amplified spontaneous emission noise. This raw estimate gives insight into the linear noise performance limit of the employed phase conjugator when inserted multiple times in such a transmission system.

On the Cascadability of All-Optical Wavelength Converter for High-Order QAM Formats

With the evolution to ever more complex modulation formats for optical signals, all-optical wavelength converters offer great benefits to optical networks, thanks to the possibility of translating the wavelength of an optical signal regardless of its data rate or modulation format. To deliver its full advantages over conventional optical-electrical-optical conversion, an all-optical wavelength converter has to possess important features in terms of transparency, bandwidth, and cascadability for practical applications in real systems. In this paper, we review a recent development of an all-optical wavelength converter, which exhibits seamless conversion in the C-band with a low noise figure of 6.2 dB for dual-polarization (DP) phase-modulated signals. We then carry out an investigation by both experiment and calculation of the cascaded operation of the developed wavelength converter for high-order quadrature amplitude modulation (QAM) formats. A cascadability comparable to an erbium-doped fiber amplifier is achieved for DP-QPSK, DP-16QAM, and DP-64QAM signals, thanks to the exemplary characteristics of the wavelength converter.

Multi-tone counter dithering of terabit/s polarization multiplexed signals for enhanced FWM with a single pump

We demonstrate low-noise degenerate four-wave mixing (FWM) of a 24×100Gb/s DP-QPSK signal with a high 50% conversion-efficiency, enabled by counter-phase modulating it and a CW pump at two frequencies. An improved signal Q2-factor by an average of 5.3 dB is achieved.

Optical Nyquist Filtering for Elastic OTDM Signals: Fundamentals and Demonstrations

Optical Nyquist filtering offers an efficient method to generate ultra-high speed optical time division multiplexing (OTDM) signals with bandwidth equal to the symbol rate without intersymbol interference. Since the Nyquist filtering can be simply implemented in a passive optical device such as a liquid crystal on silicon-based switching element, optical Nyquist filtering is potentially advantageous over digital filtering method in terms of energy efficiency and capacity. Especially in extremely coarse granular optical networks based on high baud rate Nyquist-OTDM signals, it would offer better spectral efficiency and elasticity through efficient add/drop multiplexing with small guard band and flexibility in bandwidth allocation. In this paper, we will discuss important roles of the optical Nyquist filtering as a platform to realize ultra-coarse granular elastic networks based on wavelength division multiplexing of Nyquist OTDM signals. While concepts of basic building blocks, such as elastic multi-Tbit/s transceiver and add/drop multiplexing, are presented, we will introduce proof-of-concept experimental demonstrations placing emphasis on the elasticity and the network aspect operations. We will also argue that spectral defragmentation is a critical network functionality at elastic add/drop nodes to improve the spectral utilization, and the realization of the spectral defragmentation by means of an all-optical wavelength converter is superior to the optical-electrical/electrical-optical approach thanks to the modulation format independent property of the employed optical parametric processes.

Low noise frequency comb carriers for 64-QAM via a Brillouin comb amplifier

Optical frequency comb lines with poor carrier to noise ratio (CNR) are significantly improved by Brillouin amplification using its extreme narrow bandwidth gain to suppress out of band noise, enabling higher quality signal modulation. Its application to spectral lines of narrow 10 GHz pitch and poor CNR is shown to suppress the otherwise strong phase distortion caused by poor CNR after encoding with 96 Gb/s DP-64-QAM signals and restore the bit error rate (BER) to below the limit for standard forward error correction (FEC). This is also achieved with the required frequency shifted optical pump for amplification obtained by seeding it from the comb itself, sparing the need for lasers and frequency locking. Simultaneous CNR improvement for 38 comb lines is also achieved with BER restored to below the FEC limit, enabled by a multi-line pump that is pre-dispersed to suppress its spectral distortion from the Kerr effect in the gain medium. Carrier performance at minimum BER shows minimal noise impact from the Brillouin amplifier itself. The results highlight the unique advantage of Brillouin gain for phase sensitive communications in transforming otherwise noisy spectral lines into useful high quality signal carriers.

Wavelength Translation of Dual-Polarization Phase-Modulated Nyquist OTDM at Terabit/s

Wavelength translation is one of the key functions to realize spectral defragmentation in an elastic optical add/drop network. Optically translating the wavelength of Terabit/s signals is challenging in the sense that it requires a signal processing scheme to be independent of the symbol rate and modulation format, wideband, and highly efficient simultaneously. In this paper, we present an all-optical wavelength translation for a single-carrier 1.376-Tb/s dual-polarization quadrature phase-shift keying (DP-QPSK) Nyquist optical time-division multiplexing (OTDM) signal by using an all-optical wavelength converter which fulfills all of the basic requirements. A seamless wavelength translation covering almost C-band is achieved with a boost in OSNR thanks to the improved conversion efficiency. The system performance is measured by a polarization-insensitive coherent receiver using linear sampling which implements simultaneously the demultiplexing of the time-domain tributaries, polarization demultiplexing, and carrier-phase recovery for the dual-polarization phase-modulated OTDM signal. The employed receiving scheme, therefore, resolves one of the most shortcomings of the conventional OTDM receiver, which is polarization sensitive due to the use of nonlinear sampling.

First demonstration of wavelength conversion of DP-64QAM signal using an improved counter-dithering pump scheme

Wavelength conversion of a 144-Gbit/s DP-64QAM signal is demonstrated for the first time. An improved counter-dithering pump scheme realizes high efficiency and negligible phase noise, resulting in only 1-dB Q-factor penalty at 36-dB output OSNR.