5.4. SOURCES OF POWER PENALTY 203
Figure 5.6: Modal-noise power penalty versus mode-selective loss. The parameterMis defined
as the total number of longitudinal modes whose power exceeds 10% of the peak power. (After
Ref. [61];©c1986 IEEE; reprinted with permission.)
or splice. Since a higher-order mode cannot propagate far from its excitation point, this
problem can be avoided by ensuring that the spacing between two connectors or splices
exceeds 2 m. Generally speaking, modal noise is not a problem for properly designed
and maintained single-mode fiber-optic communication systems.
With the development of the vertical-cavity surface-emitting laser (VCSEL), the
modal-noise issue has resurfaced in recent years [67]–[71]. The use of such lasers in
short-haul optical data links, making use of multimode fibers (even those made of plas-
tic), is of considerable interest because of the high bandwidth associated with VCSELs.
Indeed, rates of several gigabits per second have been demonstrated in laboratory ex-
periments with plastic-cladded multimode fibers [73]. However, VCSELs have a long
coherence length as they oscillate in a single longitudinal mode. In a 1994 experi-
ment the BER measurements showed an error floor at a level of 10−^7 even when the
mode-selective loss was only 1 dB [68]. This problem can be avoided to some extent
by using larger-diameter VCSELs which oscillate in several transverse modes and thus
have a shorter coherence length. Computer models are generally used to estimate the
power penalty for optical data links under realistic operating conditions [70]. Analytic
tools such as the saddle-point method can also provide a reasonable estimate of the
BER [71].