10.1. BASIC CONCEPTS 479
Figure 10.1: Schematic illustration of a coherent detection scheme.10.1 Basic Concepts
10.1.1 Local Oscillator.........................
The basic idea behind coherent detection consists of combining the optical signal co-
herently with a continuous-wave (CW) optical field before it falls on the photodetector
(see Fig. 10.1). The CW field is generated locally at the receiver using a narrow-
linewidth laser, called thelocal oscillator(LO), a term borrowed from the radio and
microwave literature. To see how the mixing of the received optical signal with the
LO output can improve the receiver performance, let us write the optical signal using
complex notation as
Es=Asexp[−i(ω 0 t+φs)], (10.1.1)
whereω 0 is the carrier frequency,Asis the amplitude, andφsis the phase. The optical
field associated with the local oscillator is given by a similar expression,
ELO=ALOexp[−i(ωLOt+φLO)], (10.1.2)whereALO,ωLO, andφLOrepresent the amplitude, frequency, and phase of the local
oscillator, respectively. The scalar notation is used for bothEsandELOafter assuming
that the two fields are identically polarized (polarization-mismatch issues are discussed
later in Section 10.5.3). Since a photodetector responds to the optical intensity, the
optical power incident at the photodetector is given byP=K|Es+ELO|^2 , whereKis a
constant of proportionality. Using Eqs. (10.1.1) and (10.1.2),
P(t)=Ps+PLO+ 2√
PsPLOcos(ωIFt+φs−φLO), (10.1.3)where
Ps=KA^2 s, PLO=KA^2 LO, ωIF=ω 0 −ωLO. (10.1.4)
The frequencyνIF≡ωIF/ 2 πis known as theintermediate frequency(IF). Whenω 0 =
ωLO, the optical signal is demodulated in two stages; its carrier frequency is first con-
verted to an intermediate frequencyνIF(typically 0.1–5 GHz) before the signal is de-
modulated to the baseband. It is not always necessary to use an intermediate frequency.