118 CHAPTER 3. OPTICAL TRANSMITTERS
Figure 3.24: Measured linewidth as a function of emitted power for several 1.55-μm DFB lasers.
Active layer is 100 nm thick for the bulk laser and 10 nm thick for MQW lasers. (After Ref. [86];
©c1991 IEEE; reprinted with permission.)
3.6 Transmitter Design
So far this chapter has focused on the properties of optical sources. Although an optical
source is a major component of optical transmitters, it is not the only component. Other
components include a modulator for converting electrical data into optical form (if
direct modulation is not used) and an electrical driving circuit for supplying current to
the optical source. An external modulator is often used in practice at bit rates of 10 Gb/s
or more for avoiding the chirp that is invariably imposed on the directly modulated
signal. This section covers the design of optical transmitters with emphasis on the
packaging issues [95]–[105].
3.6.1 Source–Fiber Coupling
The design objective for any transmitter is to couple as much light as possible into the
optical fiber. In practice, the coupling efficiency depends on the type of optical source
(LED versus laser) as well as on the type of fiber (multimode versus single mode). The
coupling can be very inefficient when light from an LED is coupled into a single-mode
fiber. As discussed briefly in Section 3.2.1, the coupling efficiency for an LED changes
with the numerical aperture, and can become<1% in the case of single-mode fibers.
In contrast, the coupling efficiency for edge-emitting lasers is typically 40–50% and
can exceed 80% for VCSELs because of their circular spot size. A small piece of fiber
(known as a pigtail) is included with the transmitter so that the coupling efficiency can