Page 109 - Optical Communications Essentials

Light Sources and Transmitters

Light Sources and Transmitters 99

Figure 6.12. Basic architecture of a vertical cavity surface
emitting laser (VCSEL).

lasing cavity. Figure 6.12 shows a general VCSEL structure. The layers of mir-
rors selectively reflect a narrow range of wavelengths, with the bottom stack
having a 99.9 percent reflectivity. The top has a 99.0 percent reflectivity which
allows the light to exit from the top. A circular metal contact on the bottom cre-
ates circular, low-divergent output light beams for easy and highly efficient cou-
pling into optical fibers. Because of manufacturing difficulties, VCSELs
were developed first at 850nm and used for applications such as Gigabit and
10-Gigabit Ethernet in a LAN environment. However, now VCSELs are avail-
able through the L-band region.

6.3.5. Pump lasers
In addition to the light sources used for data transmission, laser diodes are
needed for supplying external energy to optical amplifiers. The optical output
power of these pump lasers ranges from 200 to 500mW depending on the spe-
cific application. The lasers emit at wavelengths ranging from 1350 to 1520nm
for optical amplification in the S-, C-, and L-bands. Chapter 11 addresses opti-
cal amplifiers and their associated pump lasers in greater detail.

6.4. Optical Transmitters
As mentioned earlier, generally a light source is part of a transmitter package.
This package provides the following: a mounting block for the light source, a
holder for attaching a light-coupling fiber, a means for maintaining the tempera-
ture at a fixed value, and various control electronics. In some cases the transmit-
ter package also contains an external modulator for very high-speed applications.

6.4.1. LED transmitters
The low power output (typically 16dBm coupled into a 62.5-µm fiber) and slow
response time characteristics of an LED compared to a laser diode limit its use
to short-distance, low-speed (up to 200 Mbps over a few kilometers) applications

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