Page 112 - Optical Communications Essentials

Light Sources and Transmitters

102 Chapter Six

Figure 6.15. A typical industry standard 14-pin hermetically sealed butterfly package for housing a laser diode mod-
ule. (Photo courtesy of Bookham Technology; www.bookham.com.)

factor (SFF) package that has optical fiber connectors integrated into the package.
The term pigtail or flylead refers to a short length of optical fiber that is attached to
a device in the factory. This allows easy attachment of a connector for coupling of the
device to a transmission fiber. Chapter 8 has more details on fiber-to-fiber coupling.
Figure 6.15 shows a typical industry standard 14-pin hermetically sealed but-
terfly package for housing a laser diode module, such as that shown in Fig. 6.13.
The package contains the laser diode, an optical isolator, a thermoelectric cooler
(TEC), a power-monitor photodiode, and a wavelength-monitor photodiode. The
optical isolator prevents unwanted optical power reflections from entering the
laser diode and causing output instabilities. Although the 14-pin package has
become a de facto standard for optical fiber components such as laser diodes, there
is no standard designation as to the functions (such as bias voltage, monitoring
photodiode output, TEC control voltage, etc.) assigned to specific pin numbers.
Figure 6.16 shows a typical SFF package. In 1998 a number of vendors signed
a multisource agreement (MSA) to establish the SFF configuration as an inter-
nationally compatible package for fiber optic transmission modules. The MSA
standardized the package outline, the circuit board layout, and the pin function
definitions. The electrical and optical specifications are compatible with param-
eter values listed in standards such as the ITU-T G.957 SDH recommendation
and the IEEE 802.3z Gigabit Ethernet standard. The optical connector is not
specified, but it normally is an industry standard connector (see Chap. 8).

6.5. External Modulators
When direct modulation is used in a laser transmitter, the process of turning
the drive current on and off produces a widening of the laser linewidth.

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