Optical Networks: A Practical Perspective - Part 26. 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. | 220 Components The advantage of CGM is that it is conceptually simple. However there are several drawbacks. The achievable extinction ratio is small less than 10 since the gain does not really drop to zero when there is an input 1 bit. The input signal power must be high around 0 dBm so that the amplifier is saturated enough to produce a good variation in gain. This high-powered signal must be eliminated at the amplifier output by suitable filtering unless the signal and probe are counterpropagating. Moreover as the carrier density within the SOA varies it changes the refractive index as well which in turn affects the phase of the probe and creates a large amount of pulse distortion. Interferometric Techniques The same phase-change effect that creates pulse distortion in CGM can be used to effect wavelength conversion. As the carrier density in the amplifier varies with the input signal it produces a change in the refractive index which in turn modulates the phase of the probe hence we use the term cross-phase modulation for this approach. This phase modulation can be converted into intensity modulation by using an interferometer such as a Mach-Zehnder interferometer MZI see Section . Figure shows one possible configuration of a wavelength converter using cross-phase modulation. Both arms of the MZI have exactly the same length with each arm incorporating an SOA. The signal is sent in at one end A and the probe at the other end B . If no signal is present then the probe signal comes out unmodulated. The couplers in the MZI are designed with an asymmetric coupling ratio y . When the signal is present it induces a phase change in each amplifier. The phase change induced by each amplifier on the probe is different because different amounts of signal power are present in the two amplifiers. The MZI translates this relative phase difference between its two arms on the probe into an intensity-modulated signal at the output. This approach has a few .
