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.
Perhaps a quick review of a few real-world examples of proven, high-performance optical spectrum analyzers may help in understanding how these instruments work and how they can best be used in different applications. Optical communications systems, for example, operate in different modes, such as with single-mode (SM) and multimode (MM) optical fibers, and some users may wonder if it is possible to use a single analyzer for both SM and MM measurements to save test equipment costs? One of the more versatile optical spectrum analyzers, the Anritsu MS9740A optical spectrum analyzer, covers the broad wavelength range of 600 to 1750 nm and can work in SM and MM modes of operation for measurements on both types of optical cables. The analyzer can be used for measurements of modulated and pulsed light signals. With optical sensitivity of -90 dBm, it features a wide dynamic range across the wide wavelength range is a relative lightweight equipment package that is designed for portability.
The Keysight 8164B lightwave measurement system is a mainframe with high-resolution display screen that can be equipped with single- and dual-channel optical power meters and single or dual-wavelength fixed or tunable optical sources at wavelengths of interest. With a compact size of 3.5 × 8.4 × 15.0 (88 × 213 × 380 mm) and 9.3 lbs (4.2 kg), the Keysight 8163B mainframe is also designed for portability and ready transport to different job sites. The mainframe is designed for use with the company’s 8153A Series plug-in modules and 8163A/B Series modules, which consist of precision sources, return-loss modules, and various other optical measurement functions which can be quickly inter-changed as needed.
The Yokogawa AQ6370 Series of optical spectrum analyzers includes several models with different spans of wavelengths, so that a model can be selected for a specific wavelength of interest. For example, the Yokogawa AQ6370D operates from 600 to 1700 nm, the AQ6373B from 350 to 1200 nm, and Yokogawa model AQ6375 from 1200 to 2400 nm. With signal power sensitivity to -90 dBm, these analyzers feature dynamic optical power measurement ranges approaching 80 dB. Each analyzer incorporates a precision calibration source and automatic calibration function to achieve high wavelength accuracy of ±0.1 nm across the full wavelength range and better for selected portions of the wavelength coverage.
The test signals that represent, for example, carriers in optical communications systems can take on many different forms, including pulsed signals and various types of modulation, and generation of optical test signals is a critical part of any optical test and measurement system, whether it is performed as an integral part of an optical spectrum analyzer or by a separate, stand-alone optical signal source. In the future, this blog series will take a closer look at optical test signals and sources for them, stand-alone optical sources which can be used for producing multiple modes and channels within a single box.
These instruments are just a few examples of some of the measurement tools available for optical testing; more information on these and more instruments can be found on the Axiom Test Equipment website. To learn more, please visit Axiom’s website at www.axiomtest.com, contacting Axiom Test Equipment’s sales department at firstname.lastname@example.org or by calling an Axiom account manager at 760-806-6600.