Page 339 - Optical Communications Essentials

Test and Measurement

Test and Measurement 329

vendors offer such light sources that generate a true single-mode laser line for
every selected wavelength point. Typically the source is an external-cavity semi-
conductor laser. A movable diffraction grating may be used as a tunable filter
for wavelength selection. Depending on the source and grating combination, an
instrument may be tunable over (for example) the 1280- to 1330-nm, the 1370-
to 1495-nm, or the 1460- to 1640-nm band. Wavelength scans, with an output
power that is flat across the scanned spectral band, can be done automatically.
The minimum output power of such an instrument usually is 10dBm, and the
absolute wavelength accuracy is typically 0.01nm ( 10pm).
A broadband incoherent light source with a high output power coupled into a
single-mode fiber is desirable to evaluate passive DWDM components. Such an
instrument can be realized by using the amplified spontaneous emission (ASE)
of an erbium-doped fiber amplifier (EDFA). The power spectral density of the
output is up to 100 times (20dB) greater than that of edge emitting LEDs and
up to 100,000 times (50dB) greater than white-light tungsten lamp sources.
The instrument can be specified to have a total output power of greater than
3.5mW (5.5dBm) over a 50-nm range with a spectral density of 13dBm/nm
( 50µW/nm). The relatively high-power spectral density allows test person-
nel to characterize devices with medium or high insertion loss. Peak wave-
lengths might be 1200, 1310, 1430, 1550, or 1650nm.

19.5. Optical Spectrum Analyzer
The widespread implementation of WDM systems calls for making optical spec-
trum analyses to characterize the spectral behavior of various telecommunica-
tion network elements. One widely used instrument for doing this is an optical
spectrum analyzer (OSA), which measures optical power as a function of wave-
length. The most common implementation uses a diffraction-grating-based
optical filter, which yields wavelength resolutions to less than 0.1nm. Higher
wavelength accuracy ( 0.001nm) is achieved with wavelength meters based on
Michelson interferometry.
Figure 19.5 illustrates the operation of a grating-based optical spectrum ana-
lyzer. Light emerging from a fiber is collimated by a lens and is directed onto a
diffraction grating that can be rotated. The exit slit selects or filters the spec-
trum of the light from the grating. Thus, it determines the spectral resolution
of the OSA. The term resolution bandwidth describes the width of this
optical filter. Typical OSAs have selectable filters ranging from 10nm down to
0.1nm. The optical filter characteristics determine the dynamic range, which
is the ability of the OSA to simultaneously view large and small signals in
the same sweep. The bandwidth of the amplifier is a major factor affecting the
sensitivity and sweep time of the OSA. The photodiode is usually an InGaAs
device.
The OSA normally sweeps across a spectral band, making measurements at
discretely spaced wavelength points. This spacing depends on the bandwidth
resolution capability of the instrument and is known as the trace-point spacing.

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