214 CHAPTER 5. LIGHTWAVE SYSTEMS
Most reflections in a fiber link originate at glass–air interfaces whose reflectivity
can be estimated by usingRf=(nf− 1 )^2 /(nf+ 1 )^2 , wherenfis the refractive index
of the fiber material. For silica fibersRf= 3 .6% (− 14 .4 dB) if we usenf= 1 .47.
This value increases to 5.3% for polished fiber ends since polishing can create a thin
surface layer with a refractive index of about 1.6. In the case of multiple reflections
occurring between two splices or connectors, the reflection feedback can increase con-
siderably because the two reflecting surfaces act as mirrors of a Fabry–Perot interfer-
ometer. When the resonance condition is satisfied, the reflectivity increases to 14%
for unpolished surfaces and to over 22% for polished surfaces. Clearly, a considerable
fraction of the signal transmitted can be reflected back unless precautions are taken to
reduce the optical feedback. A common technique for reducing reflection feedback is
to use index-matching oil or gel near glass–air interfaces. Sometimes the tip of the
fiber is curved or cut at an angle so that the reflected light deviates from the fiber axis.
Reflection feedback can be reduced to below 0.1% by such techniques.
Semiconductor lasers are extremely sensitive to optical feedback [133]; their oper-
ating characteristics can be affected by feedback as small as−80 dB [126]. The most
dramatic effect of feedback is on the laser linewidth, which can narrow or broaden by
several order of magnitude, depending on the exact location of the surface where feed-
back originates [122]. The reason behind such a sensitivity is related to the fact that the
phase of the reflected light can perturb the laser phase significantly even for relatively
weak feedback levels. Such feedback-induced phase changes are detrimental mainly
for coherent communication systems. The performance of direct-detection lightwave
systems is affected by intensity noise rather than phase noise.
Optical feedback can increase the intensity noise significantly. Several experiments
have shown a feedback-induced enhancement of the intensity noise occurring at fre-
quencies corresponding to multiples of the external-cavity mode spacing [123]–[125].
In fact, there are several mechanisms through which the relative intensity noise (RIN)
of a semiconductor laser can be enhanced by the external optical feedback. In a simple
model [127], the feedback-induced enhancement of the intensity noise is attributed to
the onset of multiple, closely spaced, external-cavity longitudinal modes whose spac-
ing is determined by the distance between the laser output facet and the glass–air inter-
face where feedback originates. The number and the amplitudes of the external-cavity
modes depend on the amount of feedback. In this model, the RIN enhancement is
due to intensity fluctuations of the feedback-generated side modes. Another source
of RIN enhancement has its origin in the feedback-induced chaos in semiconductor
lasers. Numerical simulations of the rate equations show that the RIN can be enhanced
by 20 dB or more when the feedback level exceeds a certain value [134]. Even though
the feedback-induced chaos is deterministic in nature, it manifests as an apparent RIN
increase.
Experimental measurements of the RIN and the BER in the presence of optical
feedback confirm that the feedback-induced RIN enhancement leads to a power penalty
in lightwave systems [137]–[140]. Figure 5.13 shows the results of the BER measure-
ments for a VCSEL operating at 958 nm. Such a laser operates in a single longitu-
dinal mode because of an ultrashort cavity length (∼ 1 μm) and exhibits a RIN near
−130 dB/Hz in the absence of reflection feedback. However, the RIN increases by as
much as 20 dB when the feedback exceeds the−30-dB level. The BER measurements