Is There a Light at the End of the Tunnel?

Testing a fiber-optic transmission system involves the same steps as evaluating any other transmission medium. Supply a signal, interface it to the medium at both the sending and the receiving sites, connect analysis equipment, and see what the transmission link does to the signal.

Three types of 1-mW signal sources are available to evaluate a fiber-optic system. The first uses a fixed-wavelength laser, the second a tunable laser, and the third an LED source. If the system is equipped with multimode optics, the typical band of interest is 850 nm. Wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) systems operate in the 1,310-nm band in one direction and the 1,550-nm band in the other.

The receiving site may use a spectrum analyzer to measure the entire band of interest, possibly 600 nm to 1,700 nm. The sensitivity should be -90 dBm to handle the worst-case scenario. The dynamic range typically is 60 dB. A lightwave multimeter with a sensitivity range of -70 dBm to -110 dBm gives a good determination of optical insertion loss for both single-mode and double-mode components.

Other instruments may include a polarization controller to maximize system accuracy for power-sensitive applications like polarization-dependent gain or loss, or a polarization analyzer with a graphic output display. Other accessories include switches, terminations, and optical manipulators.

A valuable piece of test equipment is the optical time domain reflectometer (OTDR). This instrument uses backscatter to determine the distance to a reflection interface surface by measuring the time it takes for a lightwave pulse to reflect from the imperfection. It displays the reflected waves on a time scale, locating the fault very precisely for an installation or repair team.

OTDRs are available for the 1,310-nm and 1,550-nm wavelength modes. Generally, they have a dynamic range of 40 dB or more to help examine long stretches of fiber. Fault resolution is better than 6″. .

Recognizing that users have the same general test requirements, some manufacturers provide packages for typical applications. For example, EXFO offers the IQ-200 Optical Test System for laboratory and factory test and the FTB-300 Universal Test System for use in installation and maintenance. Options for each configuration include the basic units needed to generate and analyze signals, but the in-house system has more computer power and the capability for more in-depth analysis to support development programs.

The optimum system can be upgraded as fiber-optic technology advances, and this accounts for the popularity of a modular test system architecture. For a look at progress in the fiber-optic test industry, here is information on several new or upgraded products.

Spectrum Analyzer

 

The MS9710C Optical Spectrum Analyzer can be used over the 600-nm to 1,750-nm range of wavelengths to measure WDM and DWDM systems. The wavelength accuracy is ±20 pm, with a linearity of ±20 pm at 1,550 nm. The typical dynamic range is 42 dB. Resolution of 50 pm is accurate to 3% at 0.2 nm with an optical input level of +23 dBm at 1,600 nm. The rugged unit weighs less than 35 lb. $37,000. Anritsu, (800) 267-4878.

Network Monitor

 

The Network Access Agent™ is an unattended, software-controlled performance monitor installed permanently in a broadband optical network and managed from a central location. It provides intuitive testing for each channel in the network and reports on demand to the central location. Call company for price. Digital Lightwave, (800) 548-9283.

Laboratory Test Assembly

The IQ-200 Optical Test System is composed of interchangeable modular instruments built around a standard PC and an expansion chassis. The selection of functions includes laser generation, power measurement, attenuation, polarization control, optical spectrum analysis, and performance analysis all under software control in a windows environment. Turnkey packages provide fiber assembly test, calibration, environmental test, passive component test, and optical amplifier test. An IEEE 488 interface is standard. Call company for price. EXFO, (800) 663-3936.


Analyzer Family

 

The 86140A family of optical spectrum analyzers measures closely spaced channels. The 86142A benchtop and 86145A portable units have a 40- dB dynamic range at 25-GHz offset from the peak of a channel. One sweep from 1,250 nm to 1,650 nm displays legacy traffic at 1,310 nm, pump power at 1,480 nm, all DWDM channels between 1,525 and 1,560 nm, and a monitor signal at 1,625 nm. 86142A: >$41,700; 86145A: >$37,500. Hewlett-Packard, (800) 452-4844.

Modular System Family

The 600 Series Modular System is a computer-controlled assembly with LED or laser sources, power meters, attenuators, and switches for detailed evaluation of a fiber link. The receivers are power meters with up to 15 readings/s. The dynamic range is +3 dBm to -80 dBm. Modules are available for several wavelengths in the 635-nm to 1,550-nm range. Call company for price. Rifocs.

Network AnalyzerThe Q7750 Optical Network Analyzer provides amplitude, chromatic dispersion, and group-delay measurements for fiber-optic systems. It collects and presents 600 data points in 4 s. With its group-delay measurement range of 25 ns and resolution of 0.1 ps, the Q7750 determines the overall chromatic dispersion of a system. The analyzer operates in the 1,530-nm to 1,600-nm wavelength range and has an optical frequency resolution of 50 MHz over a 40-dB dynamic range. $219,000. Tektronix, (800) 426-2200.


Analyzer Upgrade

An upgrade for the T-BERD 310 Communications Analyzer helps you evaluate and maintain DWDM fiber-optics systems at speeds to OC-48/48c with concatenation. The basic T-BERD measures return loss and optical power on 1,310 and 1,550 nm systems as well as total attenuation, output power, and stability of the optical transmitters. $39,995 . TTC, (800) 638-2049.

Fiber Verifiers

The FV40 Series of fiber verifiers for 850-nm multimode fiber-optic cable installations measures power loss in a system. The FV42 is a -20-dBm light source. The FV43 meter indicates power at the receiving end of the cable, with 10 LED bars showing the level in the -22-dBm to -40-dBm range. The FV43 combines these functions into one unit. Each portable unit operates from an internal 9-V battery. FV42 and 43: $195; FV41: $295. Wavetek Wandel Goltermann, (619) 279-2200.

Copyright 1999 Nelson Publishing Inc.

September 1999


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