216 CHAPTER 5. LIGHTWAVE SYSTEMS
Figure 5.14: Feedback-induced power penalty as a function of MSR for several values ofNand
rI= 0 .01. Reflection feedback into the laser is assumed to generateNside modes of the same
amplitude.
value. Thus, reflection feedback can degrade system performance to the extent that
the system cannot achieve the desired BER despite an indefinite increase in the power
received. Such reflection-induced BER floors have been observed experimentally [125]
and indicate the severe impact of reflection noise on the performance of lightwave
systems. An example of the reflection-induced BER floor is seen in Fig. 5.13, where
the BER remains above 10−^9 for feedback levels in excess of−25 dB. Generally
speaking, most lightwave systems operate satisfactorily when the reflection feedback
is below−30 dB. In practice, the problem can be nearly eliminated by using an optical
isolator within the transmitter module.
Even when an isolator is used, reflection noise can be a problem for lightwave sys-
tems. In long-haul fiber links making use of optical amplifiers, fiber dispersion can
convert the phase noise to intensity noise, leading to performance degradation [130].
Similarly, two reflecting surfaces anywhere along the fiber link act as a Fabry–Perot
interferometer which can convert phase noise into intensity noise [128]. Such a con-
version can be understood by noting that multiple reflections inside a Fabry–Perot inter-
ferometer lead to a phase-dependent term in the transmitted intensity which fluctuates
in response to phase fluctuations. As a result, the RIN of the signal incident on the
receiver is higher than that occurring in the absence of reflection feedback. Most of
the RIN enhancement occurs over a narrow frequency band whose spectral width is
governed by the laser linewidth (∼100 MHz). Since the total noise is obtained by inte-
grating over the receiver bandwidth, it can affect system performance considerably at
bit rates larger than the laser linewidth. The power penalty can still be calculated by
using Eq. (5.4.15). A simple model that includes only two reflections between the re-
flecting interfaces shows thatreffis proportional to(R 1 R 2 )^1 /^2 , whereR 1 andR 2 are the