Optical Transmitters

Types of Modulation. Figure 7.1 illustrates two alternative ways to generate a modulated optical signal. In Fig. 7.1 (a), the laser is turned on and off by modulating its current; this method is known as direct modulation. In Fig. 7.l(b), the laser is on at all times, a so-called continuous wave (CW) laser, and the light beam is modulated with a kind of optoelectronic shutter, a so-called modulator; this method is known as external modulation. Direct modulation has the advantages of simplicity, compactness, and cost effectiveness, whereas external modulation can produce higher-quality optical pulses, permitting extended reach and higher bit rates. Direct as well as external modulation can be used to produce non-return-to-zero ( N U ) or return-to-zero (RZ) modulated optical signals. However, to produce very high-speed RZ-modulated signals, a cascade of two optical modulators, known as a tandem modulator, frequently is used. In this arrangement, the first optical modulator modulates the light from the CW laser with an NRZ signal and the second modulator, which is driven by a clock signal, coniverts the NRZ signal to an RZ signal in the optical domain.