Optical Networks: A Practical Perspective - Part 23. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 190 Components Applied voltage b Figure A lithium niobate external modulator using a Mach-Zehnder interferometer MZI configuration a Device configuration b Theoretical switching response as a function of applied voltage V. V denotes the voltage required to achieve a n phase shift between the two arms. Note that the MZI has a periodic response. modulator uses a material such that under normal conditions its band gap is higher than the photon energy of the incident light signal. This allows the light signal to propagate through. Applying an electric field to the modulator results in shrinking the band gap of the material causing the incident photons to be absorbed by the material. This effect is called the Franz-Keldysh effect or the Stark effect. The response time of this effect is sufficiently fast to enable us to realize Gb s and 10 Gb s modulators. The chirp performance of EA modulators while much better than directly modulated lasers is not as good as that of lithium niobate MZI modulators. While ideally there is no chirp in an external modulator in practice some chirp is induced in EA modulators because of residual phase modulation effects. This chirp can be controlled precisely in lithium niobate modulators. Pump Sources for Raman Amplifiers One of the biggest challenges in realizing the Raman amplifiers that we discussed in Section is a practical high-power pump source at the right wavelength. Since Transmitters 191 High-reflectivity fiber Bragg gratings Low-reflectivity fiber Bragg grating Input pump HOOnm 1455 1366 1288 1218 1155 Output putnp 1155 1218 1288 1366 1455 1455 nm Figure A high-power pump laser obtained by cascading resonators after Gru95 . the Raman effect is only seen with very high powers in the fiber pump powers on the order of several watts are required to provide effective amplification. Several approaches have been proposed to realize high-power pump sources. One method is to combine a number of high-power .
