Page 247 - Optical Communications Essentials

Performance Measures

Performance Measures 237

■ The rate at which the eye closes as the sampling time is varied (i.e., the slope
of the eye-pattern sides) determines the sensitivity of the system to timing
errors. The possibility of timing errors increases as the slope becomes more
horizontal.
■ Timing jitter (also referred to as eye jitter or phase distortion) in an optical
fiber system arises from noise in the receiver and pulse distortion in the op-
tical fiber. If the signal is sampled in the middle of the time interval (i.e., mid-
way between the times when the signal crosses the threshold level), then the
amount of distortion ∆T at the threshold level indicates the amount of jitter.
Timing jitter is thus given by
∆T
Timing jitter (percent) 100 percent (14.4)
T b
where T b is the bit interval.
■ Traditionally, the rise time is defined as the time interval between the point
where the rising edge of the signal reaches 10 percent of its final amplitude to
the time where it reaches 90 percent of its final amplitude. However, in meas-
uring optical signals, these points are often obscured by noise and jitter effects.
Thus, the more distinct values at the 20 percent and 80 percent threshold
points normally are measured. To convert from the 20 to 80 percent rise time
to a 10 to 90 percent rise time, one can use the approximate relationship

(14.5)
T 10–90 1.25T 20–80
■ Any nonlinearities of the channel transfer characteristics will create an asym-
metry in the eye pattern. If a purely random data stream is passed through a
purely linear system, all the eye openings will be identical and symmetric.

14.2. Optical Signal-to-Noise Ratio
An important point about the BER is that it is determined principally by the
optical signal-to-noise ratio, or OSNR. Therefore it is the OSNR that is meas-
ured when a WDM link is installed and when it is in operation. The OSNR does
not depend on factors such as the data format, pulse shape, or optical filter
bandwidth, but only on the average optical signal power P signal and the average
optical noise power P noise . The TIA/EIA-526-19 standard defines OSNR over a
given reference spectral bandwidth B ref (normally 0.1 nm) as (in decibels)

P signal Bm
OSNR(dB) 10log 10log (14.6)
P noise B ref
where B m is the noise-equivalent measurement bandwidth of the instrument
being used, for example, an optical spectrum analyzer. The noise may be from
sources such as the transmitter, crosstalk, or amplified spontaneous emission
(ASE) from an erbium-doped fiber amplifier (EDFA). OSNR is a metric that can

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