Fiber Optic Data Links
Typical Fiber Optic Datalink
Fiber optic transmission systems all use
data links that work similar to the
diagram shown
above. Each fiber link consists of a transmitter on one end of a fiber
and a
receiver on the other end. Most systems operate by transmitting in one
direction on one fiber and in the reverse direction on another fiber
for full duplex operation. It's possible to transmit both directions on
one fiber but it requires couplers to do so and fiber is less expensive
than couplers. A FTTH passive optical network (PON) is one of the only
systems using bidirectional transmission over a single fiber because
its network architecture is based around couplers already.
Fiber Optic Transceiver
Most
systems use a "transceiver" which includes both transmission and
receiver in a single module. The transmitter takes an electrical
input and converts it to an optical output from a laser diode
or LED. The light from the transmitter is coupled into the fiber
with a connector and is transmitted through the fiber optic cable
plant. The light from the end of the fiber is coupled to a receiver
where a detector
converts the light into an electrical signal which is then conditioned
properly for use by the receiving equipment.
More on fiber optic transceivers and their components
Analog or Digital?
Analog
signals are continuously variable signals where the information in the
signal is contained in the amplitude of the signal over time. Digital
signals are sampled at regular time intervals and the amplitude
converted to digital bytes so the information is a digital number.
Analog signals are the natural form of most data, but are subject to
degradation by noise in the transmission system. As an analog signal is
attenuated in a cable, the signal to noise ratio becomes worse so the
quality of the signal degrades. Digital signals can be transmitted long
distances without degradation as the signal is less sensitive to noise.
Fiber optic datalinks can be either analog or digital in nature, although most are digital. Both have
some common critical parameters and some major differences. For
both, the optical loss margin or power budget is most important. This is determined
by connecting the link up with an adjustable attenuator in the
cable plant and varying the loss between transmitter and receiver until one can generate the curve
shown above. Analog datalinks will be tested for signal to noise
ratio to determine link margin, while digital links use bit error
rate as a measure of performance. Both links require testing over
the full bandwidth specified for operation, but most data links
are now specified for a specific network application, like AM
CATV or RGB color monitors for analog links and SONET, Ethernet or Fibre Channel for digital links.
Datalink Performance
Just as with copper
wire or radio transmission, the performance of the fiber optic
data link can be determined by how well the reconverted electrical
signal out of the receiver matches the input to the transmitter.
The ability of any fiber optic system to
transmit data ultimately
depends on the optical power at the receiver as shown above, which
shows the data link bit error rate as a function of optical power
at the receiver. (BER is the inverse of signal-to-noise ratio, e.g.
high BER means poor signal to noise ratio.) Either too little or
too much power will cause
high bit error rates. Too much power, and the receiver amplifier
saturates, too little and noise becomes a problem as it interferes with
the signal. This receiver
power depends on two basic factors: how much power is launched
into the fiber by the transmitter and how much is lost by attenuation
in the optical fiber cable plant that connects the transmitter and
receiver.
The optical power budget of the link is determined by two factors,
the sensitivity of the receiver, which is determined in the bit
error rate curve above and the output power of the transmitter
into the fiber. The minimum power level that produces an acceptable
bit error rate determines the sensitivity the receiver. The power
from the transmitter coupled into the optical fiber determines
the transmitted power. The difference between these two power
levels determines the loss margin (power budget) of the link.
High speed
links like gigabit or 10gigabit Ethernet LANs on multimode fiber have
derating factors for the bandwidth of fiber caused by dispersion spreading out the data pulse. Older 62.5/125 OM1 fiber
will generally operate only on shorter links while links on 50/125 OM3
laser-optimized fiber will go the longest distance. Even long distance
singlemode fiber links may have limitations caused by chromatic or
polarization-mode dispersion.
If the link is
designed to operate at differing bit rates, it is necessary to
generate the performance curve for each bit-rate. Since the total
power in the signal is a function of pulse width and pulse width
will vary with bit-rate (higher bit-rates means shorter pulses),
the receiver sensitivity will degrade at higher bit-rates.
Every manufacturer of datalinks components and systems specifies
their link for receiver sensitivity (perhaps a minimum power required)
and minimum power coupled into the fiber from the source. Typical
values for these parameters are shown in the table below. In order for a manufacturer or system designer to
test them properly, it is necessary to know the test conditions.
For data link components, that includes input data frequency or bitrate
and duty cycle, power supply voltages and the type of fiber coupled
to the source. For systems, it will be the diagnostic software
needed by the system.
Typical Fiber optic link/system performance parameters
| Link type |
Source/Fiber Type |
Wave-
length (nm)
|
Transmit Power (dBm) |
Receiver Sen- sitivity (dBm) |
Margin (dB) |
| Telecom |
laser/SM |
1300/1550 |
+3 to -6 |
-30 to -45 |
30 to 40 |
|
DWDM |
1550 |
+20 to 0 |
-30 to -45 |
40 to 50 |
| Datacom |
LED/VCSEL |
850 |
-3 to -15 |
-15 to -30 |
3 to 25 |
|
LED/laser |
1300 |
-0 to -20 |
-15 to -30 |
10 to 25 |
| CATV(AM) |
laser/SM |
1300/1550 |
+10 to 0 |
0 to -10 |
10 to 20 |
Within the world of datacommunications links and networks, there are
many vendor-specific fiber optic systems, but there are also a
number of industry standard networks such as Ethernet which have fiber optic standards. These networks have agreed
upon specifications common to all manufacturers' products to insure
interoperability. This page in FOA Tech Topics shows a summary of specifications for many of these systems.
Summary
Datalinks must have proper receiver power, neither too little
nor too much, for proper operation.
The link margin can be measured with a power meter and variable
attenuator.
Test Your Comprehension
After you study this page and "More on fiber optic transceivers and their components" you should test your comprehension here.
Table of Contents: The FOA Reference Guide To Fiber Optics
