4. Fiber Optic Link Design 105
where the RIN value is specified in dB/Hz, BW is the receiver bandwidth,
M
r
is the receiver modulation index, and the exponent g is a constant
varying between 0 and 1 which relates the magnitude of RIN noise to
the optical power level. The maximum RIN noise penalty in a link can
usually be kept to below 0.5 dB.
4.3.4 JITTER
Although it is not strictly an optical phenomena, another important area
in link design deals with the effects of timing jitter on the optical signal.
In a typical optical link, a clock is extracted from the incoming data
signal which is used to retime and reshape the received digital pulse; the
received pulse is then compared with a threshold to determine if a digital
“1” or “0” was transmitted. So far, we have discussed BER testing with
the implicit assumption that the measurement was made in the center
of the received data bit; to achieve this, a clock transition at the center of
the bit is required. When the clock is generated from a receiver timing
recovery circuit, it will have some variation in time and the exact location
of the clock edge will be uncertain. Even if the clock is positioned at the
center of the bit, its position may drift over time. There will be a region of
the bit interval, or eye, in the time domain where the BER is acceptable;
this region is defined as the eyewidth [1–3]. Eyewidth measurements
are an important parameter for evaluation of fiber optic links; they are
intimately related to the BER, as well as the acceptable clock drift, pulse
width distortion, and optical power. At low optical power levels, the
receiver SNR is reduced; increased noise causes amplitude variations in
the received signal. These amplitude variations are translated into time
domain variations in the receiver decision circuitry, which narrows the
eyewidth. At the other extreme, an optical receiver may become saturated
at high optical power, reducing the eyewidth and making the system more
sensitive to timing jitter. This behavior results in the typical “bathtub”
curve shown in Figure 4.2; for this measurement, the clock is delayed
from one end of the bit cell to the other, with the BER calculated at
each position. Near the ends of the cell, a large number of errors occur;
towards the center of the cell, the BER decreases to its true value. The
eye opening may be defined as the portion of the eye for which the BER
remains constant; pulse width distortion occurs near the edges of the eye,
which denotes the limits of the valid clock timing. Uncertainty in the data
pulse arrival times causes errors to occur by closing the eye window and
causing the eye pattern to be sampled away from the center. This is one of

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