4.3. RECEIVER DESIGN 149
Figure 4.11: Diagram of a digital optical receiver showing various components. Vertical dashed
lines group receiver components into three sections.
owing. The performance of a MSM photodetector can be further improved by using
a graded superlattice structure. Such devices exhibit a low dark-current density, a re-
sponsivity of about 0.6 A/W at 1.3μm, and a rise time of about 16 ps [50]. In 1998,
a 1.55-μm MSM photodetector exhibited a bandwidth of 78 GHz [51]. By 2001, the
use of a traveling-wave configuration increased the bandwidth beyond 500 GHz for a
GaAs-based device [52]. The planar structure of MSM photodetectors is also suitable
for monolithic integration, an issue covered in the next section.
4.3 Receiver Design
The design of an optical receiver depends on the modulation format used by the trans-
mitter. Since most lightwave systems employ the binary intensity modulation, we focus
in this chapter on digital optical receivers. Figure 4.11 shows a block diagram of such
a receiver. Its components can be arranged into three groups—the front end, the linear
channel, and the decision circuit.
4.3.1 Front End
The front end of a receiver consists of a photodiode followed by a preamplifier. The
optical signal is coupled onto the photodiode by using a coupling scheme similar to that
used for optical transmitters (see Section 3.4.1); butt coupling is often used in practice.
The photodiode converts the optical bit stream into an electrical time-varying signal.
The role of the preamplifier is to amplify the electrical signal for further processing.
The design of the front end requires a trade-off between speed and sensitivity. Since
the input voltage to the preamplifier can be increased by using a large load resistorRL,
a high-impedance front end is often used [see Fig. 4.12(a)]. Furthermore, as discussed
in Section 4.4, a largeRLreduces the thermal noise and improves the receiver sensi-
tivity. The main drawback of high-impedance front end is its low bandwidth given by
∆f=( 2 πRLCT)−^1 , whereRsRLis assumed in Eq. (4.2.2) andCT=Cp+CAis the
total capacitance, which includes the contributions from the photodiode (Cp) and the
transistor used for amplification (CA). The receiver bandwidth is limited by its slowest