202 CHAPTER 5. LIGHTWAVE SYSTEMS
contrasted with the TAT–8 capacity of 8000 channels in 1988, which in turn should be
compared to the 48-channel capacity of TAT–1 in 1959.
5.4 Sources of Power Penalty
The sensitivity of the optical receiver in a realistic lightwave system is affected by
several physical phenomena which, in combination with fiber dispersion, degrade the
SNR at the decision circuit. Among the phenomena that degrade the receiver sensitivity
are modal noise, dispersion broadening and intersymbol interference, mode-partition
noise, frequency chirp, and reflection feedback. In this section we discuss how the
system performance is affected by fiber dispersion by considering the extent of power
penalty resulting from these phenomena.
5.4.1 Modal Noise
Modal noise is associated with multimode fibers and was studied extensively during the
1980s [59]–[72]. Its origin can be understood as follows. Interference among various
propagating modes in a multimode fiber creates aspeckle patternat the photodetector.
The nonuniform intensity distribution associated with the speckle pattern is harmless
in itself, as the receiver performance is governed by the total power integrated over
the detector area. However, if the speckle pattern fluctuates with time, it will lead to
fluctuations in the received power that would degrade the SNR. Such fluctuations are
referred to asmodal noise. They invariably occur in multimode fiber links because
of mechanical disturbances such as vibrations and microbends. In addition, splices
and connectors act as spatial filters. Any temporal changes in spatial filtering translate
into speckle fluctuations and enhancement of the modal noise. Modal noise is strongly
affected by the source spectral bandwidth∆νsince mode interference occurs only if
the coherence time (Tc≈ 1 /∆ν) is longer than the intermodal delay time∆Tgiven by
Eq. (2.1.5). For LED-based transmitters∆νis large enough (∆ν∼5 THz) that this
condition is not satisfied. Most lightwave systems that use multimode fibers also use
LEDs to avoid the modal-noise problem.
Modal noise becomes a serious problem when semiconductor lasers are used in
combination with multimode fibers. Attempts have been made to estimate the extent
of sensitivity degradation induced by modal noise [61]–[63] by calculating the BER
after adding modal noise to the other sources of receiver noise. Figure 5.6 shows the
power penalty at a BER of 10−^12 calculated for a 1.3-μm lightwave system operating at
140 Mb/s. The graded-index fiber has a 50-μm core diameter and supports 146 modes.
The power penalty depends on the mode-selective coupling loss occurring at splices
and connectors. It also depends on the longitudinal-mode spectrum of the semiconduc-
tor laser. As expected, power penalty decreases as the number of longitudinal modes
increases because of a reduction in the coherence time of the emitted light.
Modal noise can also occur in single-mode systems if short sections of fiber are
installed between two connectors or splices during repair or normal maintenance [63]–
[66]. A higher-order mode can be excited at the fiber discontinuity occurring at the
first splice and then converted back to the fundamental mode at the second connector