256 Modulation and Demodulation Photodetector Front-end amplifier Receive filter Clock/timing recovery Sampler Decision circuit Figure 4.5 Block diagram showing the various functions involved in a receiver. turn impose additional limits on channel capacity. Recent work to quantify the spec- tral efficiency, taking into account mostly cross-phase modulation [Sta99, MS00], shows that the achievable efficiencies are of the order of 3­5 b/s/Hz. Other nonlinear- ities such as four-wave mixing and Raman scattering may place further limitations. At the same time, we are seeing techniques to reduce the effects of these nonlinearities. Another way to increase the channel capacity is by reducing the noise level in the system. The noise figure in today's amplifiers is limited primarily by random spontaneous emission, and these are already close to theoretically achievable limits. Advances in quantum mechanics [Gla00] may ultimately succeed in reducing these