Optical communications systems are highly regarded
for their generous bandwidths and capabilities of transferring massive amounts of data
during extremely short transmission times. Of course, those systems rely on durable,
high-performance components connected by single-mode (SM) and multi-mode (MM) cables,
which must be checked and maintained at regular intervals. That can be done using fiber-optic
test equipment designed specifically for the task: optical spectrum analyzers and optical
waveform generators.
This blog will focus on selecting an optical spectrum analyzer, while a future blog
will offer advice on understanding the specifications for selecting an optimum precision
optical test source. An optical spectrum analyzer is essentially a receiver that can
detect and displaying signal waveforms within a designated wavelength range. Matching
an optical analyzer to a measurement requires some basic understanding of the key performance
parameters of a modern optical spectrum analyzer, since different optical wavelengths
and bandwidths are used in high-speed optical communications systems and other optical
applications. The optical analyzer of choice should meet or exceed the span of optical
wavelengths required by system to be tested or a device under test (DUT), such as 600
to 1700 nm (which is also designated as 0.6 to 1.7 µm) which is suitable for testing
most optical communications networks and components.
Key Optical Analyzer Specs
As with a spectrum analyzer designed for measuring electromagnetic (EM) energy, an
optical spectrum analyzer can be evaluated in terms of many key performance parameters,
including its dynamic range, which is the difference between the largest and the lowest-level
optical signals that it can detect and display with an acceptable amount of accuracy.
The lower end of the dynamic range is set by an analyzer's optical sensitivity, which
is the lowest-level signal that it can detect and display. The higher end is set by the
maximum optical level that an analyzer can read without excessive distortion. The dynamic
range can be boosted at the higher end using optical attenuators, either integrated as
part of the design of the analyzer for automatic use or added to the output of a DUT
as an external attenuator (operating over the wavelength to be measured) prior to the
input port of the optical spectrum analyzer, to reduce test signals to acceptable levels
for an analyzer.
As with RF/microwave spectrum analyzers, the cost of an optical spectrum analyzer
will increase with its capabilities and performance, with the need for high optical accuracy
over a wide dynamic range, for example, bearing with it a relatively high price tag and
being dictated by the measurement requirements of a particular application or set of
applications, such as characterizing the output levels of optical components based on
laser diodes (LDs) or light-emitting-diode (LED) semiconductor devices or the precision
of distributed feedback laser diode (DFB-LD) components for optical communications systems
or equipment.
For a given dynamic range, for example, 60 or 70 dB, an optical spectrum analyzer
will be characterized in terms of its wavelength resolution, or how small a portion of
a wavelength it can clearly display, such as within ±1 nm or finer, at ±0.1 nm, and the
amplitude flatness of the displayed waveform, such as ±1 dB or finer, at ±0.1 dB. Its
optical response will also be characterized in terms of its linearity, with a typical
response provided within an amplitude window, such as ±0.1 or ±0.5 dB, which is expected
to be the worst-case variations in amplitude for all optical measurements ...
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