Fiber-optic communication systems form the high-capacity transport infrastructure that enables global broadband data services and advanced Internet applications. The desire for higher per-fiber transport capacities and, at the same time, the drive for lower costs per end-to-end transmitted information bit has led to optically routed networks with high spectral efficiencies. Among other enabling technologies, advanced optical modulation formats have become key to the design of modern wavelength division multiplexed (WDM) fiber systems. In this paper, we review optical modulation formats in the broader context of optically routed WDM networks. We discuss the generation and detection of multigigabit/s intensity- and phase-modulated formats, and highlight their resilience to key impairments found in optical networking, such as optical amplifier noise, multipath interference, chromatic dispersion, polarization-mode dispersion, WDM crosstalk, concatenated optical filtering, and fiber nonlinearity

Advanced Optical Modulation Formats

Progress in optical networking has stimulated interest in optical performance monitoring (OPM), particularly regarding signal quality measures such as optical signal-to-noise ratio (SNR), Q-factor, and dispersion. These advanced monitoring methods have the potential to extend fault management and quality-of-service (QoS) monitoring into the optical domain. This paper reviews OPM applications and techniques, while examining the role of OPM as an enabling technology for advances in high-speed and optically switched networks.

Optical performance monitoring

After over two decades of exploration, tunable diode lasers are beginning to find significant applications, driven largely by the huge demand for bandwidth that is guiding many developments in the optical fiber communication business today. In the paper, some of the history and key developments that have led to the technologies available today are reviewed from the perspective of the author. After discussion of some of the early work, the focus shifts to widely tunable diode lasers, which would appear to be key enablers for future dense wavelength-division multiplexing and optical switching and networking systems. The distinguishing characteristics of the current technological alternatives are summarized.

Monolithic tunable diode lasers

A communications network has a plurality of nodes interconnected by an optical transmission medium. The transmission medium is capable of a carrying a plurality of wavelengths organized into bands. A filter at each node for drops a band associated therewith and passively forwards other bands through the transmission medium. A device is provided at each node for adding a band to the transmission medium. Communication can be established directly between a pair of nodes in the network sharing a common band without the active intervention of any intervening node. This allows the network to be protocol independent. Also, the low losses incurred by the passive filters permit relatively long path lengths without optical amplification.

WDM optical network with passive pass-through at each node

We discuss optical multi-band orthogonal frequency division multiplexing (OFDM) and show that by using multiple parallel OFDM bands, the required bandwidth of the digital-to-analogue/ analogue-to-digital converters and the required cyclic prefix can significantly be reduced. With the help of four OFDM bands and polarization division multiplexing (PDM) we report continuously detectable transmission of 10 times121.9-Gb/s (112.6-Gb/s without OFDM overhead) at 50-GHz channel spacing over 1,000-km standard single mode fiber (SSMF) without any inline dispersion compensation. In this experiment 8 QAM subcarrier modulation is used which confines the spectrum of the 121.9 Gb/s PDM-OFDM signal within a 22.8 GHz optical bandwidth. Moreover, we propose a digital signal processing method to reduce the matching requirements for the wideband transmitter IQ mixer structures required for PDM-OFDM.

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121.9-Gb/s PDM-OFDM Transmission With 2-b/s/Hz Spectral Efficiency Over 1000 km of SSMF

First all-optical 100 Gbit/s wavelength conversion employing cross-phase modulation is demonstrated with a recently introduced completely integrated and packaged delayed-interference configuration.

100 Gbit/s All-Optical Wavelength Conversion with an Integrated SOA Delayed-Interference Configuration

Clock recovery from a 160 Gbit/s optical time-division-multiplexed data stream is experimentally demonstrated using an electroabsorption modulator-based phase-locked loop. The recovered clock signal exhibits excellent stability. With an RMS timing jitter of <230 fs, a dynamic range of 25 dB, and a locking range of 16 MHz.

160 Gbit/s clock recovery using electroabsorption modulator-based phase-locked loop

In this paper intrachannel nonlinearities in a return-to-zero differential-phase-shift-keyed (RZ-DPSK) 40-Gb/s 32/spl times/100-km system are compensated in the absence of signal power symmetry using a single LiNbO/sub 3/ conjugator and results in two decades of improvement in bit-error rate (BER). The reduction of transmission impairments enables the reach of the system to be extended from 5200 to 6400 km with a measured BER =5/spl times/10/sup -4/. This paper also presents a first-order perturbation analysis that describes the effects of optical-phase conjugation and dispersion mapping on the optical field of pulses.

Compensation of intra-channel nonlinearities in 40 Gb/s pseudo-linear systems using optical phase conjugation

We present a novel InP differential quadrature phase-shift-keying modulator design comprised of two electroabsorption modulators in a three-arm interferometer. The modulator is extremely compact, measuring only 1500 mum times 250 mum. We demonstrate it at 21.4 Gb/s and investigate preliminary performance at 107 Gb/s.

Compact High-Speed InP DQPSK Modulator

We demonstrate 100-Gb/s high spectral efficiency long-haul optical networking. Ten co-polarized 107-Gb/s electronically multiplexed/demultiplexed NRZ-DQPSK channels at 1.0 b/s/Hz spectral efficiency are sent over 1200 km, with an optical routing node every 200 km.

G. Raybon

Papers

100 Gbit/s All-Optical Wavelength Conversion with an Integrated SOA Delayed-Interference Configuration

160 Gbit/s clock recovery using electroabsorption modulator-based phase-locked loop

Compensation of intra-channel nonlinearities in 40 Gb/s pseudo-linear systems using optical phase conjugation

Compact High-Speed InP DQPSK Modulator

10 x 107-Gb/s NRZ-DQPSK Transmission at 1.0 b/s/Hz over 12 x 100 km Including 6 Optical Routing Nodes