Handbook for Sound Engineers

470 Chapter 15

15.9 The Fiber Optic Link

A basic fiber optic link, as shown in Fig. 15-28, consists
of an optical transmitter and receiver connected by a
length of fiber optic cable in a point-to-point link. The
optical transmitter converts an electrical signal voltage
into optical power, which is launched into the fiber by
either na LED or laser diode.
At the receiving point, either a PIN or APD photo-
diode captures the lightwave pulses for conversion back
into electrical current.

It is the fiber optic system designer’s job to deter-

mine the most cost- and signal-efficient means to

convey this optical power, knowing the trade-offs and

limits of various components. He or she must also

design the physical layout of the system.

15.9.1 Fiber Optic Link Design Considerations

Fiber optic link design involves power budget analysis

and rise time analysis. The power budget calculates

total system losses to ensure that the detector receives

sufficient power from the source to maintain the

required system SNR or bit-error-rate (BER). Rise time

analysis ensures that the link meets the bandwidth

requirements of the application.

BER is the ratio of correctly transmitted bits to

incorrectly transmitted bits. A typical ratio for digital

systems is 10–9, which means that one wrong bit is

received for every one billion bits transmitted. The BER

in a digital system often replaces the SNR in an analog

system and is a measure of system quality.

15.9.2 Passive Optical Interconnections

In addition to the fiber, the interconnection system

includes the means for connecting the fiber to active

devices or to other fibers and hardware for efficiently

packaging the system to a particular application. The

three most important interconnects are FO connectors,

splices, and couplers.

Interconnect losses fall into two categories—

intrinsic and extrinsic.

• Intrinsic or fiber-related factors are those caused by
  variations in the fiber itself, such as NA (numerical
  aperture) mismatch, cladding mismatches, concen-
  tricity, and ellipticity, see Fig. 15-29.


• Extrinsic or connector-related factors are those
  contributed by the connector itself, Fig. 15-30. The

Table 15-5. 3Com Optical Transceiver—Part No.
3CSFP92

1.25 Gb Gigabit Ethernet/1.063G Fiber Channel

Application: This 100% 3Com compliant 1000 BASE LX SFP

Transceiver is hot-swappable and designed to plug directly into

your SFP/GBIC interface slot in your router and switch for Ether-

net and Fiber Channel network interface applications.

Reach 10 km (32, 820 ft)

Fiber Type SMF (Single Mode Fiber)

Fiber Optic Connector LC

Center Wavelength O 1310 nm

Min TX Power 9.5 dBm

Max Input Power 3 dBm

RX Sensitivity 20 dBm

Max Input Power 3 dBm

Link Budget 10.50 dB

Dimensions MSA SFP Standard

Height: 0.33 inches (8.5 mm)

Width: 0.52 inches (13.4 mm)

Depth: 2.18 inches (55.5 mm)

Power 3.3V

Operating Temperature 0°C–70°C

Standards: IEEE 802.3 2003; ANSI X3.297-1997

Compliance IEC-60825; FDA 21; CFR 1040.10,

CFR 1040.11

Warranty 1 Year Full Replacement

Figure 15-27. 3Com Optical Transceiver. Courtesy of
3Com Corporation.

Figure 15-28. Basic fiber optic system.

Transmitter

Receiver

Analog or

digital

interface

Signal

output

Light source

Light detector

Source to fiber

connection

Fiber to detector

connection

Fiber optical cable