Page 111 - Optical Communications Essentials

Light Sources and Transmitters

Light Sources and Transmitters 101

6.4.3. Transmitter controllers
Figure 6.14 illustrates various functions that a commercially available optical
transmitter controller might perform with respect to maintaining laser stability
and operating an external modulator (See, for example, www.pinephotonics.
com). The external modulator can be in a separate package, or it can be inte-
grated within the laser package (see Sec. 6.5). The transmitter control functions
include the following:
The temperature control function uses a temperature sensor and a thermo-
electric cooler to set the laser diode temperature at a particular value (say,
25°C) and to maintain it to within a 0.02°C stability.
The wavelength controller can stabilize the laser output wavelength to a few
picometers (say, 5pm 0.005nm).
The laser bias control works in conjunction with an optical power monitoring
photodiode to continuously adjust the drive current for the laser.
The transmitter output power controller works in conjunction with a variable
optical attenuator (VOA) to maintain a constant optical output level from the
external modulator.
The alarm processor provides alerts for abnormal operation conditions.
The transmission control signal processor enables both low-speed and high-
speed interfaces for data exchange with a motherboard microprocessor.
A typical size for such a device is 50 50 8mm (2.0 2.0 0.3in), and
it normally resides on a PCB along with the laser package, a modulator driver, the
external modulator if it is a separate package, and some peripheral electronics.

6.4.4. Transmitter packages
Two popular transmitter package configurations that can be mounted on a PCB are
the butterfly package with fiber pigtails and the industry-standardized small form

Figure 6.14. Functions that an optical transmitter controller might perform with
respect to maintaining laser stability and operating an external modulator.

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