<title>What is new in WDM sources?</title>

Abstract

This paper will describe some of the latest developments in the area of optical sources for WDM systems, including tunable lasers and multiwavelength sources. We will consider both DWDM and CWDM. We will start by looking at the source requirements, such as wavelength switching time, for various applications. After that we will give an overview of the latest developments in source options and technologies. The source options under discussion will include external cavity lasers, wavelength selectable laser arrays, tunable VCSELs, monolithic tunable lasers, and non-semiconductor alternatives. Numerous examples will be shown, and the characteristics and performance of the various devices will be discussed. The key performance parameters, such as tuning range, power and switching time will be related to the expected areas of application. These areas include sparing, fixed wavelength transmitter replacement, and use in wavelength switched networks. We will pay particular attention to packaging and modules, and discuss the emergence of pluggable lasers. Finally we will consider the prospects for uncooled operation. TUNABLE LASER APPLICATIONS AND REQUIREMENTSThe ability to choose a particular wavelength of operation makes tunable lasers, or wavelength selectable laser arrays, with an adjustable or uncommitted wavelength, attractive for different applications in WDM systems. It is convenient to classify the application areas in terms of the required wavelength switching time. Below we discuss possible application areas in order of increasing demand on the switching speed. We will use the term "tunable" as a generic term, i.e. including wavelength selectable sources.An obvious application for tunable lasers is sparing. Rather than keeping spare transmitter modules, or even whole linecards, for a number of separate specific wavelengths, it is far more cost efficient to keep just a few spares which incorporate tunable lasers. The laser wavelength is then set when the transmitter is needed. Sparing was for a long time seen as the first major market segment for tunable lasers. However, the emergence of hot-pluggable transmitters has made sparing cheaper, and consequently reduced the premium which the tunable laser manufacturers can charge for transmitters for this application. It is of course necessary for tunable lasers used for sparing to have properties (like for example output power) comparable to those of fixed wavelength lasers. A related application is provisioning of circuits in response to traffic demand. Use of tunable transmitters means that new circuits can be set up at any desired wavelength without the need for a stock of fixed wavelength transmitters.The wavelength flexibility paves the way for constructing reconfigurable networks, and to use tunable lasers for restoration and protection. For these applications sub-second switching times are required.Ultimately tunable lasers can be used in packet switched networks. These can take the form of burst mode switching where data is transmitted on a selected wavelengt...