152 CHAPTER 4. OPTICAL RECEIVERS
Figure 4.13: Ideal and degraded eye patterns for the NRZ format.signal is maximum. At the same time,hout(t)=0 fort=m/B, wheremis an integer.
Sincet=m/Bcorresponds to the decision instant of the neighboring bits, the voltage
pulse of Eq. (4.3.6) does not interfere with the neighboring bits.
The linear-channel transfer functionHT(ω)that will result in output pulse shapes
of the form (4.3.6) is obtained from Eq. (4.3.4) and is given by
HT(f)=Hout(f)/Hp(f). (4.3.7)For an ideal bit stream in the nonreturn-to-zero (NRZ) format (rectangular input pulses
of durationTB= 1 /B),Hp(f)=Bsin(πf/B)/πf, andHT(f)becomes
HT(f)=(πf/ 2 B)cot(πf/ 2 B). (4.3.8)Equation (4.3.8) determines the frequency response of the linear channel that would
produce output pulse shape given by Eq. (4.3.6) under ideal conditions. In practice, the
input pulse shape is far from being rectangular. The output pulse shape also deviates
from Eq. (4.3.6), and some ISI invariably occurs.
4.3.3 Decision Circuit
The data-recovery section of optical receivers consists of a decision circuit and a clock-
recovery circuit. The purpose of the latter is to isolate a spectral component atf=
Bfrom the received signal. This component provides information about the bit slot
(TB= 1 /B)to the decision circuit and helps to synchronize the decision process. In
the case of RZ (return-to-zero) format, a spectral component atf=Bis present in
the received signal; a narrow-bandpass filter such as a surface-acoustic-wave filter can
isolate this component easily. Clock recovery is more difficult in the case of NRZ
format because the signal received lacks a spectral component atf=B. A commonly
used technique generates such a component by squaring and rectifying the spectral
component atf=B/2 that can be obtained by passing the received signal through a
high-pass filter.
The decision circuit compares the output from the linear channel to a threshold
level, at sampling times determined by the clock-recovery circuit, and decides whether
the signal corresponds to bit 1 or bit 0. The best sampling time corresponds to the
situation in which the signal level difference between 1 and 0 bits is maximum. It