the need for Optics Primer regeneration every so many kilometers. Longer spacing between amplifiers leads to degraded OSNR figures. In DWDM systems, the larger the number of channels, the higher the amplification requirement to maintain an acceptable OSNR. Higher speed also requires improved OSNR (for exam- ple, 10-Gbps systems require 6 dB more OSNR than 2.5-Gbps systems). Nonlinearities give rise to the following other problems that have to be dealt with appropriately: Stimulated Raman Scattering (SRS) Stimulated Brillouin Scattering (SBS) Four-wave mixing Self-phase modulation Cross-phase modulation 2.4 Transmission Approaches The two basic approaches to optical transmission-noncoherent and coher- ent-are described in Figure 2-25. DIRECT DETECTION Figure 2-25 Transmission Single-mode Fiber methods. Laser Receiver Photodetector COHERENT DETECTION Single-mode Fiber Laser Receiver Photodetector Coupler External Modulator Tunable Local Laser Chapter 2 2.4.1 Noncoherent Transmission Noncoherent methods involve on-off keying and the direct detection of a sig- nal; this is a variant of amplitude modulation (AM), which is specifically a unipolar digital scheme of amplitude shift keying (ASK). The amount of information transmitted is based on the source/reception clocking. Building on this basic method, several noncoherent optical channels can be operated over the low-loss/low-dispersion ranges of an optical fiber. This requires fil- ters to separate the optical channels. Such devices (filters) are called WDMs. Initially, such multiplexers were designed to support two channels in the 1.30-um wavelength region and three channels in the 1.50-um region. Com- mercial products now support several dozen channels and laboratory tests have demonstrated the possibility of using several hundred channels of 1 to 10 Gbps, utilizing narrow spectrum lasers and direct detection. Even this multiplexing, however, does not use the available fiber band- width to its maximum potential (100 channels at 10 Gbps would equate to 1 terabit per