384 CHAPTER 8. MULTICHANNEL SYSTEMS
the receiver and can be written as
CNR=
(mRP ̄)^2 / 2
σs^2 +σT^2 +σI^2 +σIMD^2, (8.5.2)
wheremis the modulation index,Ris the detector responsivity,P ̄is the average re-
ceived optical power, andσs,σT,σI, andσIMDare the RMS values of the noise currents
associated with the shot noise, thermal noise, intensity noise, and IMD, respectively.
The expressions forσs^2 andσT^2 are given in Section 4.4.1. The RMS valueσIof the
intensity noise can be obtained from Eq. (4.6.6) in Section 4.6.2. If we assume that the
relative intensity noise (RIN) of the laser is nearly uniform within the receiver band-
width,
σI^2 =(RIN)(RP ̄)^2 ( 2 ∆f). (8.5.3)
The RMS value ofσIMDdepends on the CSO and CTB distortion values.
The CNR requirements of SCM systems depend on the modulation format. In
the case of AM-VSB format, the CNR should typically exceed 50 dB for satisfactory
performance. Such large values can be realized only by increasing the received optical
powerP ̄to a relatively large value (> 0 .1 mW). This requirement has two effects.
First, the power budget of AM-analog SCM systems is extremely limited unless the
transmitter power is increased above 10 mW. Second, the intensity-noise contribution
to the receiver noise dominates the system performance asσI^2 increases quadratically
withP ̄. In fact, the CNR becomes independent of the received optical power whenσI
dominates. From Eqs. (8.5.2) and (8.5.3) the limited value of CNR is given by
CNR≈
m^2
4 (RIN)∆f. (8.5.4)
As an example, the RIN of the transmitter laser should be below−150 dB/Hz to realize
a CNR of 50 dB ifm= 0 .1 and∆f=50 MHz are used as the representative values.
Larger values of RIN can be tolerated only by increasing the modulation indexmor
by decreasing the receiver bandwidth. Indeed, DFB lasers with low values of the RIN
were developed during the 1990s for CATV applications. In general, the DFB laser is
biased high above threshold to provide a bias powerPbin excess of 5 mW because its
RIN decreases asPb−^3. High values of the bias power also permit an increase in the
modulation indexm.
The intensity noise can become a problem even when the transmitter laser is se-
lected with a low RIN value to provide a large CNR in accordance with Eq. (8.5.4).
The reason is that the RIN can be enhanced during signal transmission inside optical
fibers. One such mechanism is related to multiple reflections between two reflecting
surfaces along the fiber link. As discussed in Section 5.4.5, the two reflective sur-
faces act as an FP interferometer which converts the laser-frequency noise into intensity
noise. The reflection-induced RIN depends on both the laser linewidth and the spacing
between reflecting surfaces. It can be avoided by using fiber components (splices and
connectors) with negligible parasitic reflections (<−40 dB) and by using lasers with
a narrow linewidth (<1 MHz). Another mechanism for the RIN enhancement is pro-
vided by the dispersive fiber itself. Because of GVD, different frequency components