Handbook for Sound Engineers

Transmission Techniques: Fiber Optics 469

equivalent to a low-pass RC filter yields an approxima-
tion of its bandwidth. Fig. 15-25 shows the equivalent
circuit model of a PIN photodetector.

15.7 Transmitter/Receiver Modules

In most cases, fiber optic engineers will not design their
own transmitters and receivers. They will use completed
transmitter-receiver modules. A transmitter module may
consist of the following elements:

1. Electronic interface analog/digital input.


2. Analog/digital converter.


3. Drive circuits (preamplifiers, etc.).


4. Optical monitoring circuit.


5. Temperature sensing and control for laser diodes.


6. LED or laser diode as light source.


7. FO connector or pigtail at output.

A receiver module may consist of the following
elements:

1. PIN or APD photodiode at the input.


2. Amplification circuits.


3. Signal processor A/D.


4. Analog/digital electrical signal at the output.

Usually, the FO engineer will use a matched pair of
transmitter and receiver modules as shown in Fig.
15-26. When considering transmitter/receiver modules
one must consider the following requirements:

1. Type of modulation.


2. Bandwidth.


3. Noise.


4. Dynamic range.


5. Electrical and optical interface.


6. Space and cost.

15.8 Transceivers and Repeaters

Transceivers and repeaters are two important compo-
nents in fiber optics. A transceiver is a transmitter and
receiver both in one package to allow transmission and

reception from either station. A repeater is receiver

driving a transmitter. The repeater is used to boost sig-

nals when the transmission distance is so great that the

signal will be too highly attenuated before it reaches the

receiver. The repeater accepts the signal, amplifies and

reshapes it, and sends it on its way by retransmitting the

rebuilt signal.

One advantage of digital transmission is that it uses

regenerative repeaters that not only amplify a signal but

reshape it to its original form as well. Any pulse distor-

tions from dispersion or other causes are removed.

Analog signals use nonregenerative repeaters that

amplify the signal, including any noise or distortion.

Analog signals cannot be easily reshaped because the

repeater does not know what the original signal looked

like. For a digital signal, it does know.

15.8.1 Demand on Gigabit Optical Transceivers

The industry is now experiencing that the world needs

more bandwidth for today’s high-definition technolo-

gies. Ethernet has become a standard using both copper

and fiber. Manufacturers must keep up with the demand

for higher bit rates. The audio/video industry is now

employing 1/2/4/10 Gigabit optical transceivers for

these high bandwidth applications. Also the need to

carry these audio/video signals at distances of 10 km or

greater has become a reality. Fiber optics can carry a

signal with higher bandwidth and greater distance than

their copper counterpart.

The price of copper has gone up tenfold due to the

world market consumption of copper, especially in the

China market. This has bought the price of fiber and

fiber optic transceivers down considerably from 2 years

ago. An example is the company 3Com, who manufac-

tures fiber optic transceivers. Most of these fiber optic

transceivers employ a SFP (small form factor plug-in)

duplex type LC connector. Table 15-5 gives 3Com

optical transceiver specification data. Fig. 15-27 is a

photo of the 3Com Optical Transceiver.

Figure 15-25. Equivalent circuit model of a PIN diode.

Id Rd Cd Figure 15-26. A short-wavelength lightwave data link.

Courtesy Agilent Technologies.