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Powers J. Introduction to fiber optic systems (2ed., MGH, 1997)(600dpi)(T)(355s)_EO_.djvu |
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Size 4.5Mb Date Jul 9, 2005 |
) and incorporates a narrow region of p type material between the intrinsic region and the
n+ region...
The dark current is the output of the device with no input illumination and is
primarily due to thermal generation of charge carriers in the depletion region and surface leakage
currents due to surface defects near the edges of the semiconductor...
This excess noise factor causes some unusual
effects in the application of this device, as we shall see in considering the signal-to-noise ratio...
When using an ADP detector, the amplification factor will make the shot noise the dominant
noise source...
A relatively small fraction of the
carriers are involved in this process, but, because the diffusion process is quite slow, it can
be a limiting effect on the device speed...
Analog Detector Analysis
Having considered how a simple detector (i.e., without any amplifier) responds to a digital sig-
signal, we now consider its performance for analog signals...
(Table 6.4 on the next page gives the ranges of typical FET parameters for three
FET types.)
The parameters I2, I3, and If are the Personick integrals [8, 18, 20]...
common-source gain-bandwidth product /j\ It can be shown that [8]
/r = 2ir{c9m+c ■ F-104)
We are usually interested in optimizing the receiver performance at high bit rates...
i i i
-^- 106 2345 107 2345 108 2345 109 234 1010
Bit rate (b/s)
Figure 6.20 Comparison of mean-square noise current for typical FET front-ends and a BJT front-end...
F.124)
)
qX \ M
6.6 Eye Pattern Analysis 201
For this same case, the optimum gain is [8, 19]
Mopt = 4=\/ т D>f^tal - * + 1 (for noise due to 4uik negligible), F.125)
у/к у qh&RQ
where к is the ionization ratio of the avalanche process, described earlier...
The jitter is defined as
AT
Timing jitter (in %) = —- x 100%,
Jь
F.126)
where Ть is the bit spacing in the data stream...
The channel capacity required will be equal to the baud rate.) There are
several variations of the RZ code...
In a link design, one
usually knows (or estimates) the data rate required to meet the objectives...
From these values we now want to compute the
drive current of the LED source required to produce each value of power...
For a step-index fiber with length L, the modal-
dispersion delay is given by
L(ni -n2) , v
A^modal = • G.30)
The modal delay time for a graded-index fiber is a more complicated expression...
Hence, the dynamic ranges of the signal and the noise in the link become important; too
strong a signal will saturate the amplifier and too weak a signal will be lost in the noise...
Among the
potential disadvantages of these amplifiers are that they are limited to operation near 1550 nm
(although they are moderately broadband in the vicinity of that wavelength), that they require
a non-trivial amount of pump power D0 to 50 mW) at 1480 nm, that they require lengths of
fiber that exceed several meters (i.e., shorter lengths do not give as much gain), and that they
add noise to the optical signal [14, 15]...
The powers shown are the total power out of each amplifier, the signal power out of each amplifier, and
the power in amplified stimulated emission at the output of each amplifier...
Cascaded Amplifiers: Other Effects
An additional effect of N cascaded amplifiers will be an accompanying decrease in the bandwidth
of the channel...
Other optical amplifiers include the Raman fiber amplifier that uses the Raman nonlinear ef-
effect discussed earlier in Chapter 3 on fiber properties...
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