Page 66 - Optical Switching And Networking Handbook

03_200023_CH02/Batesx 1/17/01 8:17 AM Page 51

Basic Fiberoptics Technologies 51

When transmitting light on a multimode fiber, capacity is
quoted as an analog bandwidth measure.

On single-mode fiber, a modulating signal broadens the spectral
width (bandwidth) of the carrier signal.

In wavelength-division multiplexing systems, the modulating sig-
nal also broadens the spectral width. The bandwidth determination
is not straightforward. A squarewave signal has a fundamental fre-
quency of half the bit rate. A 1-Gbps digital signal has a fundamen-
tal frequency of 500 MHz operating as a simple sine wave. A
squarewave contains many higher-frequency harmonics at several
times the basic frequency. From there, the types of modulation tech-
niques used in the telecommunications arena can be analyzed.

Amplitude Modulation (AM) Lasers traditionally have been
very difficult to modulate with standard amplitude-modulation tech-
niques because of the nonlinear response typical of lasers.The major
use of AM is in CATV and hybrid fiber and coax (HFC) distribution
systems. An analog signal is prepared the same as for the coaxial
cable (CATV). Instead of putting it straight onto the cable, it is used
to modulate a laser. At the receiver, the signal is amplified electron-
ically and placed onto a section of coaxial cable.

Frequency-Shift Keying (FSK) or Frequency Modulation (FM)
It is difficult to modulate the frequency of a laser, and this is one of
the reasons that FM optical systems are not yet in general use. Some
lasers are being introduced that will use an FSK technique. For this
to work, the laser spectral line width has to be considerably nar-
rower than the bandwidth of the signal. FSK promises much higher
data rates than the pulse systems currently in use.

Phase-Shift Keying (PSK) or Phase Modulation (PM) The
phase of a laser’s output signal cannot be controlled directly. There-
fore, a laser will not produce phase-modulated light. However, a sig-
nal can be modulated in phase by placing a modulation device in the
light path between the laser and the fiber. PM has similar advan-
tages to FSK.