4
APPLICATION SUPPORT
Optical Power Budget
and Link Penalties
The worst-case Optical Power
Budget (OPB) in dB for a fiber-
optic link is determined by the
difference between the minimum
transmitter output optical power
(dBm avg) and the lowest
receiver sensitivity (dBm avg).
This OPB provides the necessary
optical signal range to establish a
working fiber-optic link. The OPB
is allocated for the fiber-optic
cable length and the corre-
sponding link penalties. For
proper link performance, all
penalties that affect the link
performance must be accounted
for within the link optical power
budget. The Gigabit Ethernet
IEEE 802.3z standard identifies,
and has modeled, the
contributions of these OPB
penalties to establish the link
length requirements for 62.5/125
µm
and 50/125
µm
multimode fiber
usage. In addition, single-mode
fiber with standard 1300 nm
Fabry-Perot lasers have been
modeled and specified. Refer to
the IEEE 802.3z standard and its
supplemental documents that
develop the model, empirical
results and final specifications.
10 km Link Support
As well as complying with the LX
5 km standard, the HFCT-53D5
specification provides additional
margin allowing for a 10 km
Gigabit Ethernet link on single
mode fiber. This is accomplished
by limiting the spectral width and
center wavelength range of the
transmitter while increasing the
output optical power and
improving sensitivity. All other
LX cable plant recommendations
should be followed.
Data Line
Interconnections
Agilent Technologies’ HFBR/
HFCT-53D5 fiber-optic
transceiver is designed to directly
couple to +5 V PECL signals. The
transmitter inputs are internally
dc-coupled to the laser driver
circuit from the transmitter input
pins (pins 7, 8). There is no
internal, capacitively-coupled 50
Ohm termination resistance
within the transmitter input
section. The transmitter driver
circuit for the laser light source
is a dc-coupled circuit. This
circuit regulates the output
optical power. The regulated light
output will maintain a constant
output optical power provided
the data pattern is reasonably
balanced in duty factor. If the
data duty factor has long, con-
tinuous state times (low or high
data duty factor), then the output
optical power will gradually
change its average output optical
power level to its pre-set value.
As for the receiver section, it is
internally ac-coupled between the
pre-amplifier and the post-
amplifier stages. The actual Data
and Data-bar outputs of the post-
amplifier are dc-coupled to their
respective output pins (pins 2, 3).
Signal Detect is a single-ended,
+5 V PECL output signal that is
dc-coupled to pin 4 of the
module. Signal Detect should not
be ac-coupled externally to the
follow-on circuits because of its
infrequent state changes.
Caution should be taken to
account for the proper intercon-
nection between the supporting
Physical Layer integrated circuits
and this HFBR/HFCT-53D5
transceiver. Figure 3 illustrates a
recommended interface circuit
for interconnecting to a +5 Vdc
PECL fiber-optic transceiver.
Some fiber-optic transceiver sup-
pliers’ modules include internal
capacitors, with or without 50 Ohm
termination, to couple their Data
and Data-bar lines to the I/O pins
of their module. When designing
to use these type of transceivers
along with Agilent transceivers, it
is important that the interface
circuit can accommodate either
internal or external capacitive
coupling with 50 Ohm termina-
tion components for proper
operation of both transceiver
designs. The internal dc-coupled
design of the
HFBR/HFCT-53D5 I/O
connections was done to provide
the designer with the most
flexibility for interfacing to
various types of circuits.
Eye Safety Circuit
For an optical transmitter device
to be eye-safe in the event of a
single fault failure, the transmit-
ter must either maintain normal,
eye-safe operation or be disabled.
In the HFBR-53D5 there are
three key elements to the laser
driver safety circuitry: a monitor
diode, a window detector circuit,
and direct control of the laser
bias. The window detection
circuit monitors the average
optical power using the monitor
diode. If a fault occurs such that
the transmitter DC regulation
circuit cannot maintain the preset
bias conditions for the laser
emitter within
±
20%, the
transmitter will automatically be
disabled. Once this has occurred,
only an electrical power reset will
allow an attempted turn-on of the
transmitter.
The HFCT-53D5 utilizes an
integral fiber stub along with a
current limiting circuit to
guarantee eye-safety. It is
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
