480 CHAPTER 10. COHERENT LIGHTWAVE SYSTEMS
In fact, there are two different coherent detection techniques to choose from, depend-
ing on whether or notωIFequals zero. They are known ashomodyneandheterodyne
detection techniques.
10.1.2 Homodyne Detection
In this coherent-detection technique, the local-oscillator frequencyωLOis selected to
coincide with the signal-carrier frequencyω 0 so thatωIF=0. From Eq. (10.1.3), the
photocurrent (I=RP, whereRis the detector responsivity) is given by
I(t)=R(Ps+PLO)+ 2 R√
PsPLOcos(φs−φLO). (10.1.5)Typically,PLOPs, andPs+PLO≈PLO. The last term in Eq. (10.1.5) contains the
information transmitted and is used by the decision circuit. Consider the case in which
the local-oscillator phase is locked to the signal phase so thatφs=φLO. The homodyne
signal is then given by
Ip(t)= 2 R√
PsPLO. (10.1.6)The main advantage of homodyne detection is evident from Eq. (10.1.6) if we note that
the signal current in the direct-detection case is given byIdd(t)=RPs(t). Denoting the
average optical power byP ̄s, the average electrical power is increased by a factor of
4 PLO/P ̄swith the use of homodyne detection. SincePLOcan be made much larger than
P ̄s, the power enhancement can exceed 20 dB. Although shot noise is also enhanced,
it is shown later in this section that homodyne detection improves the signal-to-noise
ratio (SNR) by a large factor.
Another advantage of coherent detection is evident from Eq. (10.1.5). Because the
last term in this equation contains the signal phase explicitly, it is possible to trans-
mit information by modulating the phase or frequency of the optical carrier. Direct
detection does not allow phase or frequency modulation, as all information about the
signal phase is lost. The new modulation formats for coherent systems are discussed in
Section 10.2.
A disadvantage of homodyne detection also results from its phase sensitivity. Since
the last term in Eq. (10.1.5) contains the local-oscillator phaseφLOexplicitly, clearly
φLOshould be controlled. Ideally,φsandφLOshould stay constant except for the inten-
tional modulation ofφs. In practice, bothφsandφLOfluctuate with time in a random
manner. However, their differenceφs−φLOcan be forced to remain nearly constant
through an optical phase-locked loop. The implementation of such a loop is not sim-
ple and makes the design of optical homodyne receivers quite complicated. In addition,
matching of the transmitter and local-oscillator frequencies puts stringent requirements
on the two optical sources. These problems can be overcome by the use of heterodyne
detection, discussed next.
10.1.3 Heterodyne Detection
In the case of heterodyne detection the local-oscillator frequencyωLOis chosen to
differ form the signal-carrier frequencyω 0 such that the intermediate frequencyωIFis