Page 110 - Optical Communications Essentials

Light Sources and Transmitters

100 Chapter Six

using multimode fibers. An example would be OC-3/STM-1 links between nearby
buildings, which operate at 155 Mbps. For local-area network use, an LED trans-
mitter often is contained in the same physical package as the corresponding
receiver. This configuration allows a simple, low-cost, and convenient way to
interface a printed-circuit board (PCB) to an optical fiber.
A typical LED transmitter will contain a 1310-nm InGaAsP surface emitting
LED and driver circuitry for the source. Such a transmitter uses a single 3.3-V
power supply and operates over a standard temperature range of 0 to 70°C, or
optionally (for a more expensive device) over a 40 to 85°C range.

6.4.2. Laser transmitters
Since the wavelength of a laser drifts by approximately 0.1nm/°C and because the
output efficiency changes with temperature, a standard method of stabilizing
these parameters is to use a miniature thermoelectric cooler. This device uses a
temperature sensor and an electronic controller to maintain the laser at a constant
temperature. In addition there normally is a photodiode in the package that mon-
itors the optical power level emitted by the laser, since this level can change
with factors such as age or bias voltage. The module also may contain a second
photodiode for precisely monitoring the peak output wavelength. Figure 6.13
shows a typical structure for the laser, thermoelectric cooler, and monitoring pho-
todiode.

Figure 6.13. Construction of a laser transmitter that has a thermoelectric cooler and a monitoring pho-
todiode.

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