Page 113 - Optical Communications Essentials

Light Sources and Transmitters

Light Sources and Transmitters 103

Figure 6.16. A typical industry-standardized small-form-factor
package. (Photo courtesy of Agilent Technologies; www.
agilent.com.)

This phenomenon is referred to as chirp and makes directly modulated lasers
undesirable for operation at data rates greater than 2.5Gbps (see Sec. 15.5). As
shown in Fig. 6.17, for these applications it is preferable to use an external mod-
ulator either that is integrated physically in the same package with the light
source or that can be a separate device. The two main device types are the elec-
trooptical modulator and the electroabsorption modulator.
The electrooptical (EO) modulator typically is made of lithium niobate (LbNiO 3 ).
In an EO modulator the light beam is split in half and then sent through two
separate paths, as shown in Fig. 6.18. A high-speed electric signal then changes
the phase of the light signal in one of the paths. This is done in such a manner
that when the two halves of the signal meet again at the device output, they will
recombine either constructively or destructively. The constructive recombina-
tion produces a bright signal and corresponds to a 1 pulse. On the other hand,
destructive recombination results in the two signal halves canceling each other,
so there is no light at the output. This of course corresponds to a 0 pulse. LbNiO 3
modulators are separately packaged devices and can be up to 12cm (about 5in)
long.
The electroabsorption modulator (EAM) typically is constructed from
indium phosphide (InP). It operates by having an electric signal change the

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