3.6. TRANSMITTER DESIGN 121
Figure 3.26: Driving circuit for a laser transmitter with feedback control to keep the average
optical power constant. A photodiode monitors the output power and provides the control signal.
(After Ref. [95];©c1988 Academic Press; reprinted with permission.)
respect to each other. Light propagating in one direction passes through the second
polarizer because of the Faraday rotation. By contrast, light propagating in the opposite
direction is blocked by the first polarizer. Desirable characteristics of optical isolators
are low insertion loss, high isolation (>30 dB), compact size, and a wide spectral
bandwidth of operation. A very compact isolator can be designed if the lens in Fig.
3.25(b) is replaced by a YIG sphere so that it serves a dual purpose [111]. As light
from a semiconductor laser is already polarized, a signal polarizer placed between the
YIG sphere and the fiber can reduce the feedback by more than 30 dB.
3.6.2 Driving Circuitry
The purpose of driving circuitry is to provide electrical power to the optical source and
to modulate the light output in accordance with the signal that is to be transmitted.
Driving circuits are relatively simple for LED transmitters but become increasingly
complicated for high-bit-rate optical transmitters employing semiconductor lasers as
an optical source [95]. As discussed in Section 3.5.2, semiconductor lasers are biased
near threshold and then modulated through an electrical time-dependent signal. Thus
the driving circuit is designed to supply a constant bias current as well as modulated
electrical signal. Furthermore, a servo loop is often used to keep the average optical
power constant.
Figure 3.26 shows a simple driving circuit that controls the average optical power
through a feedback mechanism. A photodiode monitors the laser output and generates
the control signal that is used to adjust the laser bias level. The rear facet of the laser
is generally used for the monitoring purpose (see Fig. 3.25). In some transmitters a
front-end tap is used to divert a small fraction of the output power to the detector.
The bias-level control is essential, since the laser threshold is sensitive to the operating