TABLE OF CONTENTS The emphasis in this book has been on single mode laser emitters with narrow and stable linewidths. However, future high-capacity optical networks need to employ tunable sources and detectors to increase capacity and flexibility [31], and for this purpose trial systems have been built with 80 or more channels in a 30 nm window centred around 1515 nm [32] using erbium-doped fibre amplifiers. Tunable lasers that are capable of wide tuning ranges are complex, expensive and have variable tuning ranges because of sensitivity to manufacturing tolerances. Sensing and measuring instruments can already accept the expense of currently available devices, so that high-volume communication applications should promote the improvement and cost reduction of tunable lasers.
In Chapter 2 it was pointed out that the gain spectra typically covered a frequency range ? f ~k T/h, giving wavelength ranges to half-maximum gain of ??~ ? 2k T/hc, or in other words, to a first order, ?? ~50 nm centred on 1500 nm, reducing to ??~15 nm centred at 850 nm. These ranges can be extended to ~80 nm around 1550 nm central wavelength by pumping harder to increase the width of the gain spectrum and making use of low-loss material with adequate feedback. Figure 8.17 indicates schematically how quantum-well material, at high drive, can significantly increase the bandwidth for gain compared with that of conventional bulk material. At low gain levels when only the lowest level is...
