Optical-loss test sets--mainstay for fiber testing

An optical-loss test set is the main test equipment needed for optical fiber installed in premises and campus environments. The test set consists of two separate devices, a light source and an optical power meter. These devices may be purchased separately, or as a common unit. The power meter alone is sufficient to perform continuity checks of fiber-optic cables, as well as transmitter and receiver power-level tests. Both light source and power meter are needed for an end-to-end performance test

May 1st, 1995
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Arlyn S. Powell, Jr.

An optical-loss test set is the main test equipment needed for optical fiber installed in premises and campus environments. The test set consists of two separate devices, a light source and an optical power meter. These devices may be purchased separately, or as a common unit. The power meter alone is sufficient to perform continuity checks of fiber-optic cables, as well as transmitter and receiver power-level tests. Both light source and power meter are needed for an end-to-end performance test.

The first question to consider in thinking about this test equipment is: What is your application? In telephony applications, for example, devices must operate over great distances; and in cable TV, the capability to measure high power levels is required because of the high optical power transmitted along the fiber. Most cabling installers, however, will work on fiber-backbone and fiber-to-the-desk cable runs, which call for far less dynamic range and resolution than more expensive benchtop, cable-TV and telephone company models.

"Field installers will also be looking for rugged design, small size and light weight," says Larry Johnson, president of The Light Brigade Inc. (Kent, WA), a company specializing in fiber-optic products, services and training. "An AC/DC option is useful," he adds, "but battery power is essential for field work. Another question to ask is: How long does one battery charge last? If it is less than seven or eight hours, you may have to stop in the middle of a shift to recharge or replace batteries."

A crucial issue with light sources is that their output must be stabilized to prevent drifting. If the power level of the source drifts up and down, so does the measurement at the power meter, creating inaccurate readings. Johnson says, "An easy way to check a light source for drifting is to hook it directly to the power meter and turn it on, watching the power meter for fluctuations in power level." Some companies suggest you let the light source warm up and stabilize for five to 10 minutes before using it. But this may not be practical if you must frequently turn the device on and off in the field.

Stabilization, according to Norman Elsasser, optical test equipment product manager for 3M`s Telecom Systems Division (Austin, TX), will not usually present a problem in premises applications. "The technology of power meters and light sources is no longer black-box," he says. "Most operate with feedback loops. They monitor what`s going out and try to maintain it at a certain level. If power rises or falls, current is increased or decreased accordingly."

Most premises networks use multimode fiber, and multimode, in turn, requires a light-emitting diode for a light source. "LEDs are less susceptible to warm-up problems," Elsasser says. "They come up very quickly and are very stable." Stability within 0.1 decibel should be adequate for premises work.

Also, keep in mind that drifting may occur with temperature change. If you work in a cold climate and install in campus environments, what will happen when you go from indoors in one building, walk across a snowy campus and enter a second building? Test your device to see if power level is affected by such changes.

Wandel & Goltermann Inc. (Morrisville, NC) tests and specifies its optical-loss test sets not only for temperature change, but also for humidity caused by condensation when moving from warm air to cold, according to business unit manager Wolfgang Schmid. The company`s in-house testing laboratory checks isolation from radio-frequency and electromagnetic interference as well.

Stability of measurement in the power meter is as important as the stability of power output of the source. Check how long the manufacturer warrants the calibration of the power meter. According to Johnson, "Although the manufacturer may warrant its device for a longer period, power meters should be calibrated annually against standards set by the National Institute of Standards and Technology. This calibration can be done by returning the device to the manufacturer or by sending it to a company that performs such calibrations at a reasonable rate. Make sure that devices are stickered when calibrated, and that a calibration certificate is retained in case a customer asks for such documentation."

Connectors and alignment

Most contractors work at a number of different job sites each year, and may encounter several different types of optical connectors. "Although the industry is standardizing on SC and ST connectors," says Johnson, "many other types exist, and each must be connected to the power meter and light source to do end-to-end testing." This is not so much a problem at the power-meter end, because the photodetector in this device is larger than the optical fiber being connected to it. However, fiber alignment can be critical at the light-source end because it is necessary to maintain a fixed and stable power level through the fiber during testing.

The normal procedure is to keep a fixed connector on the light source and then attach this connector to the fiber under test using a connector adapter or a hybrid jumper (see "Adapt your dissimilar connectors," April 1995, page 10). If, for example, your power meter has an ST connector and the cable being tested has SC connectors, you need an ST-to-SC adapter. Hybrid jumpers and adapters cover all common connector types and combinations.

"When using hybrid jumpers," Johnson adds, "make sure they have high-quality connectors on each end and that the fiber endfaces are properly polished. If jumpers have been stretched, tied in knots, stepped on or otherwise abused, they may not function properly." Also, the fiber used in the jumper must have the same diameter and numerical aperture as the fiber under test; otherwise, the jumper may cause excessive loss.

It is also a good idea to test jumpers before using them: Take two jumpers and connect them with a sleeve. Then connect one end to the light source, the other to the power meter and take a loss reading. It may be useful to record the loss from the jumper pair for future reference. Inspect and clean jumpers before beginning each job, because contamination will degrade performance.

But what if you are testing 200 fibers in a day? Is it necessary to clean the jumpers before each test? Schmid says Wandel & Goltermann has thought through this problem: "Our experience is that sometimes the inner surface of a fiber-to-fiber connector gets dirty and scratched, so that subsequent measurements might give false results. Our solution is to create an air gap between the two fibers, to ensure there is no physical contact. The air gap protects the connector surface from damage and contamination."

Feature and price tradeoffs

The technology behind optical-loss test sets is not complicated; therefore, manufacturers distinguish themselves from each other by the extra features they add to their devices. Some of these features may be unnecessary, depending on your application; others can be quite useful. However, added features usually equate with higher cost.

"One useful feature some manufacturers include in their light sources is adjustable output," says Johnson. "This lets you adjust the output to a constant, such as -20 dBm, the standard for the typical LED. Then you can loop a jumper back from light source to power meter and detect any drift, canceling it out from time to time."

A similar capability of the power meter is the zeroing function. With the push of a button, you can set a baseline in decibels referenced to a milliwatt, so measured losses are read out in -dB. The baseline dBm measurement is stored in memory.

Since most local area networks today use multimode fiber, installers working in the premises and campus markets are going to test at both 850 and 1300 nanometers, the two wavelengths used for multimode fiber. It is less expensive to start with one wavelength--say, 850 nm--and then add 1300 nm as the applications using this wavelength come online, but is this the best long-term investment?

"The cost difference between a single-wavelength and dual-wavelength power meter is negligible," says Johnson, "typically less than $100, because the same photodetector can be used and tuned to two frequencies vs. a single frequency. Light sources, however, require two lasers or two LEDs to emit at different wavelengths, so the cost differential is substantial."

Right now, token ring and other current LAN applications run at 850 nm; but fiber distributed data interface, asynchronous transfer mode, Escon and other high-speed applications are slated for 1300 nm. "To go back and buy a second light source at this frequency will cost $600 or $700, rather than the extra $400 it may cost to invest in a dual-wavelength device now," says Johnson.

On every installer`s wish list should be a light source that changes wavelengths with a switch, so you don`t have to uncouple a fiber and plug it into a different connector on the device. Such a switch makes results repeatable and lowers the measurement variability resulting from disconnecting and reconnecting fibers to the tester.

Schmid says his company makes such a light source. "It incorporates two laser diodes," he adds, "but it has only one output. It switches between the two laser diodes using a toggle on the box." Used in conjunction with the company`s power meter, installers can do what Schmid calls twin-testing. "The transmitter switches back and forth between two wavelengths, and the power meter follows the switching, so you can measure both wavelengths without having to communicate with the other end."

Johnson adds that it might be smart to buy a triple-wavelength power meter if you anticipate doing singlemode testing. Singlemode fiber carries 1300-nm and 1550-nm wavelengths, so the power meter would be calibrated to 850, 1300 and 1550 nm. The extra cost for the third frequency would be negligible for the power-meter end.

Money-saving approaches

Some manufacturers offer savings to installers with modular devices, so you can add wavelengths to your base unit. Another approach has led to a recent product introduction of a plug-in module for a copper-based LAN tester that functions as an optical power meter.

Nortech Fibronic (Quebec, Canada) has introduced a power meter that attaches to a digital multimeter. Says vice president of marketing Jean-Noel Berube, "The object is to offer the power meter at a rock-bottom price. It is convenient to use, offers acceptable performance and is small, so installers and users can put it in their toolboxes."

He points out that most installers already have multimeters, and Nortech`s device attaches to most common models. He adds, "You can utilize all the features of the multimeter with our unit." For example, the measurements of the module read out on the dial of the multimeter to which it is attached. If the multimeter has absolute--as well as relative--power measurement capability, the power meter can provide readings in dBm as well as dB. "You don`t have to calculate anything," Berube says.

For the small contractor on a tight budget, testing copper and fiber with a single device may be appealing, but there is a down side to consider. Johnson explains: "The testing standard for fiber is well-established and has been in place for some time, but the test parameters for copper networks are still in flux. So combining the two functions in one unit could be dangerous. What happens, for instance, if the fiber tester remains viable while the copper tester becomes outmoded?"

Berube counters that the performance standard for field installation of copper networks, including field test procedures and specifications, is soon to be released by the Telecommunications Industry Association (Arlington, VA) as Technical System Bulletin 67. He adds, "The previous standard, Annex E of TIA-568, has held up well over the last year as TSB-67 has been drafted."

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Power meters, light sources, adapters and battery equipment by Wandel & Goltermann are housed in a rugged carrying case. The optical-loss test sets themselves are also padded with foam insulation to prevent damage if they are dropped during normal use.

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