Optical Spectrum Analyzers Shed Light On Fiber-Optic Cables

Communications networks often depend on pulses of light for high-speed links. Unlike wireless systems sending modulated electromagnetic (EM) signals through the air or conductive metal cables, fiber-optic systems transfer modulated optical signals through miles of glass cables. For the best results, these fiber-optic cables should be measured before installation and tested as part of a regular maintenance schedule. Numerous test equipment provides insights into fiber-optic cable quality, but one of the handiest and most versatile for the task is the optical spectrum analyzer (OSA).

Fiber-optic cables consist of glass or plastic cores surrounded by cladding which reflects light back into the core to help direct optical energy along the path formed by the cable. The cladding is usually surrounded by an outer protective coating. Compared to copper cables for electricity, fiber-optic cables provide much wider bandwidths, immunity to interference, and work over longer distances with less loss. Optical signals are well controlled within specific wavelength regions and cables can transfer light signals one wavelength at a time, in single-mode (SM) or transverse mode form, or working with optical signals at many simultaneous wavelengths, in the form of multimode (MM) fiber-optic cables.

As part of a fiber-optic communications network, optical cables work with repeaters and accessory components, such as amplifiers, switches, and connectors, to transfer light pulses with minimal loss and distortion. An OSA provides the functions and capabilities to measure key fiber-optic-cable characteristics quickly, accurately, and repeatably. An OSA measures and displays the distribution of light across a single wavelength or wavelength range. It can be used to explore how environmental effects, such as temperature, impact fiber-optic cable performance within its range, and how physical effects, such as bending or looping of cable, can affect performance.

Fiber-optic technology supports telecommunications as well as remote sensor monitoring in critical situations such as in hospitals and military applications. Optical cables provide high-speed communications over great distances, using SM or MM cables. MM cables accommodate many simultaneous signals using wavelength division multiplexing (WDM) to boost the amount of information that can be transferred across a single cable. Cables are used (and tested) within different wavelength bands, such as O-band (1260 to 1360 nm), C-band (1530 to 1565 nm), and L-band (1565 to 1625 nm). Shorter wavelengths, such as 350 to 1200 nm, are also used and often the cables must be manufactured to maintain the polarization of the light pulses for these applications.

Shining Light on OSAs

Evaluating the performance of fiber-optic cables requires a unit such as an OSA with the wavelength range to cover the bands and wavelengths used within any applications of interest. Commercial OSAs pack a great deal of measurement power into benchtop/rack-mount enclosures that can be operated from front-panel controls or a USB- or Ethernet-connected computer. In general, when specifying an OSA, it must provide the measurement functions and performance over the wavelength range required for a desired set of applications. The quality of the unit’s performance can be determined by the accuracy and resolution of the wavelength readings, wavelength linearity, the dynamic range or minimum and maximum power levels which the analyzer can measure, noise levels, and measurement speed, especially when testing multimode fiber-optic cables with WDM signals. In addition to generous measurement power, a practical OSA should be easy to use with a clear and easy-to-follow user interface (UI), such as provided on rack-mount benchtop units from Yokogawa and Anritsu.

The Yokogawa AQ6370D OSA, operates over the wide optical wavelength range from 600 to 1700 nm and is well suited for telecommunications and general-purpose optical testing. It can perform 0-nm measurements as well as across wavelength spans from 0.5 to 1100 nm wide. Full-span measurement accuracy is ±0.1 nm, reaching ±0.02 nm for wavelengths from 1520 to 1580 nm or 1580 to 1620 nm and ±0.04 nm for wavelengths from 1450 to 1520 nm. The OSA’s wavelength linearity is ±0.01 nm from 1520 to 1580 nm and ±0.02 nm from 1450 to 1520 nm or 1580 to 1620 nm. The wavelength measurement resolution can be set at 0.02, 0.05, 0.1, 0.2, 0.5, 1, or 2 nm with wide dynamic range at all settings. For a wavelength resolution of 0.02 nm, the wavelength accuracy is ±0.02 nm across a 55-dB dynamic range. For wavelength resolution of 0.05 nm, the benchtop OSA delivers ±1.0-nm wavelength accuracy across a 73-dB dynamic range.

This benchtop OSA safely handles input power levels as high as +25 dBm and has 0.001-nm minimum sampling resolution. It features many automated functions to simplify measurements, including WDM tests on multimode cables. These measurements show peak wavelength, peak optical level, and optical signal-to-noise ratio (OSNR) for as many as 1024 simultaneous WDM channels. Automatic test functions simplify characterization of active and passive components such as optical amplifiers and WDM filters, respectively. The OSA’s large front-panel display can be used, for example, to show optical signal power versus wavelength before and after amplification.

For additional wavelengths, Yokogawa’s AQ6373B OSA covers 350 to 1200 nm while the Yokogawa AQ6375E OSA scans 1200 to 2400 nm in the short-wavelength-infrared (SWIR) region. Both analyzers offer wavelength accuracy as good as ±0.05 nm (depending upon wavelength range) with wide power measurement ranges: -80 to +20 dBm in the model AQ6373B and -70 to +20 dBm in the model AQ6375E. The wavelength resolution can be set from 0.01 to 10 nm in the model AQ6373B and from 0.05 to 2 nm in the model AQ7375E. Additional Yokogawa OSAs reach higher in wavelength coverage, including the middle-wavelength-infrared (MWIR) Yokogawa AQ6376 with range of 1500 to 3400 nm and the Yokogawa AQ6377 OSA with range of 1900 to 5500 nm. Both units provide ±0.5 nm wavelength accuracy.

Over these different wavelength spans, the Yokogawa OSAs share a free-space optical input port configuration that simplifies instrument connections to DUTs. It offers high coupling efficiency and repeatability for SM or MM cables as well as active and passive optical components. The input port uses free space rather than interconnecting cables to attach a DUT—there is no wear on physical mounting surfaces for fiber-optic cables with core diameters as large as 800 μm. The test port accepts both PC and APC connectors.

Another leading supplier of OSAs, Anritsu, offers its model MS9740B OSA for evaluating SM and MM optical cables at wavelengths from 600 to 1750 nm. Users can set wavelength resolution at 0.07, 0.1, 0.2., 0.5, and 1.0 nm to display optical power versus wavelength for a dynamic range of 57 to 62 dB. The wavelength accuracy is ±300 pm from 600 to 1750 nm, and as good as ±50 pm from 1530 to 1570 nm. As with the OSAs from Yokogawa, the MS9740B is designed with powerful internal measurement software and can perform numerous automatic measurements by itself or connected to a personal computer (PC).

More information on any of these OSAs is available on the Axiom Test Equipment website at www.axiomtest.com. By following the guidelines for specifying an OSA, the best unit for a specific measurement application can be found, with the capabilities needed for ensuring that fiber-optic cables and other system components provide the performance needed for light-speed communications under all operating conditions.