In recent years, tremendous progress has been made in the development of broadband erbium-doped fiber amplifiers (EDFAs), which form the backbone of high-capacity lighwave communication systems. Initially, the use of gain equalization filters increased the bandwidth of amplifiers by a factor of three (relative to first-generation amplifiers deployed in the field). Subsequently, the introduction of a two-band architecture, which includes amplifier sections for the C-band and the L-band, resulted in a further doubling of the bandwidth. In addition, this amplifier provides high output power and low noise figure to support the ever-increasing capacity demand on lightwave systems. Commercial systems with up to 80 wavelength division multiplexing (WDM) channels having a total capacity of up to 400 Gb/s are currently available, and terabit systems have been demonstrated in the laboratory. The recently discovered phenomenon of fast power transients in chains of EDFAs constituting an optical network has been shown to impair the performance of propagating channels in the event of channel failure or network reconfiguration. Several schemes to control the gain of EDFAs have been devised to mitigate the degradation caused by the fast power transient effect. Practical broadband amplifiers incorporating these and other control schemes, such as internal attenuation to control gain tilt, will enable future terabit and higher capacity networks.

Optical fiber amplifiers for WDM optical networks

The 25 years since the founding of the Journal of Lightwave Technology have seen more than three orders of magnitude increase in the capacity of optical transmission systems. This dramatic increase was brought about by the deployment of WDM and advances in high-speed transmission technologies.

High-Capacity Optical Transmission Systems

Optical networking, as embodied by dense wavelength division multiplexing (DWDM) and optical amplification, has revolutionized long-distance transport and has resulted in capacity expansion, cost reduction, and operations simplification. Extension to more wavelengths, emergence of reconfigurable wavelength add/drop and other optical network elements (NEs), as well as cost reduction of optical amplifiers and laser devices, are pushing the revolution into the local and metropolitan networks. Traffic generated in any format, such as SONET/SDH, asynchronous transfer mode (ATM), and plesiochronous digital hierarchy (PDH), with bit rates ranging from 45 Mb/s to 10 Gb/s, can be economically transported and routed optically without resorting to time division multiplexing (TDM). In this paper, we describe the latest advances in optical NEs and applications in the public and private networks. In addition, we discuss how optical networking creates flexibility in sharing existing fiber inventory between service providers. We present an example of how optical networking is capable of meeting the emerging demand for high-capacity clear-channel end-user services, a demand that TDM systems cannot meet. Finally, we discuss future directions and optical layer restoration as a prime target of network evolution.

http://ieeexplore.ieee.org/iel7/6731002/6767974/06767978.pdf

Optical networking

We review the use of optical frequency combs in wavelength-division multiplexed (WDM) fiber optic communication systems. In particular, we focus on the unique possibilities that are opened up by the stability of the comb-line spacing and the phase coherence between the lines. We give an overview of different techniques for the generation of optical frequency combs and review their use in WDM systems. We discuss the benefits of the stable line spacing of frequency combs for creating densely-packed optical superchannels with high spectral efficiency. Additionally, we discuss practical considerations when implementing frequency-comb-based transmitters. Furthermore, we describe several techniques for comb-based superchannel receivers that enables the phase coherence between the lines to be used to simplify or increase the performance of the digital carrier recovery. The first set of receiver techniques is based on comb-regeneration from optical pilot tones, enabling low-overhead self-homodyne detection. The second set of techniques takes advantage of the phase coherence by sharing phase information between the channels through joint digital signal processing (DSP) schemes. This enables a lower DSP complexity or a higher phase-noise tolerance.

https://research.chalmers.se/publication/503191/file/503191_Fulltext.pdf

Frequency Comb-Based WDM Transmission Systems Enabling Joint Signal Processing

The noise spectral densities resulting from detection of preamplified signals using an optical filter are discussed. The authors also carried out a simplified analysis of preamplified receiver sensitivities with ASK and FSK signals. The analysis shows that total optical bandwidths of less than 50 times the data rate result in (10/sup -9/ BER) sensitivities close to the limit set by the amplifier noise figure. The development of low-noise high-gain amplifiers in conjunction with narrowband tunable optical filters enable high-sensitivity FDM systems to be constructed, without the complication of a local oscillator or polarization control/diversity networks which are required for heterodyne coherent detection systems. >

Sensitivity of optically preamplified receivers with optical filtering

A description is given of high-bit-rate fiber-optic noncoherent frequency-shift keying (FSK) transmission field experiments using Er/sup 3+/-doped optical amplifiers. Transmission distances of 70 km for a four-channel 6.8-Gb/s capacity experiment and 177 km for single-channel 1.7-Gb/s transmissions were demonstrated in a typical field environment. Measurements of receiver sensitivity dispersion penalty, interchannel crosstalk, and long-term bit-error-rate performance are presented. Results show negligible degradations due to >3000-ps/nm dispersion and interchannel crosstalk, in addition to stable long-term performance. This performance demonstrates the applicability of noncoherent FSK and fiber amplifier technologies. >

C.M. Miller

Papers

A high capacity noncoherent FSK lightwave field experiment using Er/sup 3+/-doped fiber optical amplifiers