Optical transmitters use an electrical signal to modulate the power of a light source. The most common application is simple on-off modulation used to convert high-speed binary signals to light pulses. At the receiving end, detectors need only detect the presence or absence of light to accurately reproduce the original digital stream. Digital modulation is a conceptually simple and robust format, and relatively high noise levels can be tolerated. At the present state of the art, speeds in excess of 10 gigabits per second (Gb/s) are commercially deployed, whereas optical losses of 30 dB can be tolerated between transmitters and receivers.

Figure 12.13 shows theoretical bit error rate as a function of signal-to-noise ratio for a baseband, binary transmission system.12 Quite moderate S/N ratios (compared with those required for modulated analog NTSC signals) yield very good bit error rates. Baseband digital modulation is frequently employed in fiber-deep architecture (fiber-to-the-curb or fiber-to-the-home) due to the lower cost of the transmitters relative to those commonly used in HFC systems. Chapter 19 discusses fiber-deep architecture in detail.