WDM sources

Abstract

For several years tunable lasers and other WDM sources has been as one of the hottest components topics in photonics, but like other areas the downturn has hit hard. In this paper we will describe the latest developments in the area of optical sources for DWDM systems, including tunable lasers and multiwavelength sources, and we will look at the companies (still) competing in this area. We will start by introducing the basic technologies used for DWDM sources. After that we will give an overview of the source options, and discuss tuning methods and wavelength control issues. The source options under discussion will include monolithic tunable lasers, hybrid structures and external cavity lasers, wavelength selectable laser arrays, tunable VCSELs, 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 speed will be related to the expected areas of application. These areas include sparing, fixed wavelength transmitter replacement, and use in wavelength switched networks. TUNABLE LASER APPLICATIONS AND REQUIREMENTSThe ability to choose a particular wavelength of operation makes tunable lasers, or wavelength selectable laser arrays, with an uncommitted and adjustable wavelength attractive for applications such as sparing in DWDM optical networks. Instead of keeping a large number of spare transmitter cards (one for each wavelength used), a system operator only needs a limited number of spares. If tunable lasers become sufficiently cheap, one might even consider replacing fixed wavelength lasers by tunable lasers. Tunable lasers also support new network features based on the use of wavelength routing. One can use wavelength switching either as circuit switching or as packet switching. In addition, tunable lasers are also of interest in test and measurement applications as well as for various sensors.The key performance parameters are: • Power. For sparing and replacement applications the optical power level has to be comparable to that of present DFB lasers. •Tuning range. A wide tuning range reduces the number of lasers required to cover a given spectral range, but for widely tunable lasers there may be trade-off between tuning range and power level. •Tuning speed. For sparing applications tuning speed is not an issue, but for optical packet switching it is of prime importance. •Reliability. In addition to the usual requirements, wavelength stability has to be ensured.