5.4. SOURCES OF POWER PENALTY 215
Figure 5.13: Experimentally measured BER at 500 Mb/s for a VCSEL under optical feedback.
The BER is measured at several feedback levels. (After Ref. [139];©c1993 IEEE; reprinted with
permission.)
at a bit rate of 500 Mb/s show a power penalty of 0.8 dB at a BER of 10−^9 for−30-dB
feedback, and the penalty increases rapidly at higher feedback levels [139].
The power penalty can be calculated by following the analysis of Section 4.6.2 and
is given by
δref=−10 log 10 ( 1 −r^2 effQ^2 ), (5.4.15)
wherereffis the effective intensity noise over the receiver bandwidth∆fand is obtained
from
r^2 eff=1
2 π∫∞−∞RIN(ω)dω= 2 (RIN)∆f. (5.4.16)In the case of feedback-induced external-cavity modes,reffcan be calculated by
using a simple model and is found to be [127]
r^2 eff≈r^2 I+N/(MSR)^2 , (5.4.17)whererIis the relative noise level in the absence of reflection feedback,Nis the number
of external-cavity modes, and MSR is the factor by which the external-cavity modes
remain suppressed. Figure 5.14 shows the reflection-noise power penalty as a function
of MSR for several values ofNby choosingrI= 0 .01. The penalty is negligible in the
absence of feedback (N=0). However, it increases with an increase inNand a decrease
in MSR. In fact, the penalty becomes infinite when MSR is reduced below a critical