Growing appeal of mini-OTDRs

The explosive growth of the Internet and other data-communications applications is spurring the use of fiber-optic cable in campus and premises-network environments. As a result, the makers of test equipment such as optical time-domain reflectometers (OTDRs) are attempting to meet the needs of what they see as a growing pool of new customers.

Manufacturers add features to make these portable testers even more user-friendly.

Michael Fahey

The explosive growth of the Internet and other data-communications applications is spurring the use of fiber-optic cable in campus and premises-network environments. As a result, the makers of test equipment such as optical time-domain reflectometers (OTDRs) are attempting to meet the needs of what they see as a growing pool of new customers.

For some time, the mini-OTDR has been the fastest-growing segment of this product category. The so-called mainframe version of the test instrument has lost ground to the smaller, more-portable battery-operated minis, which are easier to transport and handle on the job. The features and functions offered by different mini-OTDRs vary widely. Some versions challenge the capabilities of the more expensive and unwieldy full-sized models, while others are essentially optical fault locators with expanded capabilities such as the ability to store optical traces.

As fiber use proliferates, it appears that test-equipment makers want to round out their line of OTDRs. Noyes Fiber Systems (Laconia, NH) recently announced the M600 Mini-OTDR, which provides a wider set of features than its OFL 100 line. Meanwhile, according to industry sources, EXFO Electro-Optical Engineering Inc. (Vanier, QC, Canada) is working on a mini-OTDR that is less feature-rich than its ftb-250 mini-OTDR.

OTDR testing reveals end-to-end fiber length and distance to "reflective events" such as faults or splices, link loss, event return loss, and link return loss. The device measures these essential transmission characteristics of fiber-optic cable by sending a short light pulse down the fiber. Once in the fiber, the light scatters in all directions, and the OTDR measures the light coming back toward the source to provide a graphic display, or trace, of the backscattered light. The instrument interprets the trace by gauging the speed of the light pulse as it travels down the fiber and correlating what it sees in the backscattered light with an actual location in the fiber.

Identifying defects

An OTDR monitors two phenomena to gather the information needed to identify fiber defects. One is Fresnel reflection, which is caused by the difference between refractive indices of two media. For example, a Fresnel reflection could occur as a result of a reflective event, which could be a connector, mechanical splice, or fault in the optical fiber. If air separates two fibers, the power of the reflected light is typically 4% of the total power. This type of break appears as a pulse or spike in the backscattered signal of an OTDR`s display.

OTDRs also monitor Rayleigh backscattering. If a pulse travels through a fiber and hits an imperfection, the light will scatter. The level and slope of the backscattered area in the responsive curve can be measured to reveal the location of a fault or a potential break in the fiber.

In addition to decreasing size and weight while maintaining or adding important capabilities, manufacturers are focusing on ways to make mini-OTDRs more user-friendly. They are making the instruments easier to operate and simplifying the task of analyzing and interpreting the data collected.

This emphasis on ease of use and simplified interpretation and analysis of data generated by mini-OTDRs is an important dynamic in the design and marketing of mini-OTDRs aimed at all segments of the cabling installation and testing market. These improvements are particularly important for suppliers targeting the premises-network market, where the use of fiber has lagged behind its use in wide area networks.

"In the past, a company might have one fiber guru who handled fiber installation," says Ryan Irving, national sales manager at Noyes Fiber Systems. "As fiber proliferates, instead of one fiber expert, you may have 10 to 20 people installing fiber, and they want automatic features. They are not interested in fiddling and diddling with a lot of dials and settings."

Sean Pons, test-equipment support engineer at Siecor, agrees. The company`s OTDR Plus Multitester II, which features a one-button test, has been well-received, "especially in this market," he says, referring to the contractors who primarily focus on premises and campus networks. Like many high-end mini-OTDRs, Siecor`s OTDR Plus Multitester II offers a built-in power meter, visual fault locator, and singlemode laser source.

Sunjay Sudeora, product manager for optical products at Tektronix Inc. (Beaverton, OR), also touts simplicity of operation as a major selling point for his company`s mini-OTDR, the TFS3031 TekRanger2, which automatically monitors and changes the pulsewidth as it travels along the fiber. This feature eliminates the need for the technician to shoot multiple pulses to accommodate the need for varying pulse sizes. In fact, the single-button automated testing is being implemented by nearly all OTDR makers, says Stephane Duquet, OTDR product manager at EXFO. In addition to regulating pulse size, some mini-OTDRs let technicians set the instruments to determine if the cable being tested falls within an acceptable range.

User interface

Along with ease of operation, the quality of the OTDR`s user interface is an important selling point. "The widespread use of laptop computers has raised expectations among OTDR customers when it comes to resolution, storage space, and processing power of mini-OTDRs," says Steve Wolszczak, OTDR product manager for Wavetek Wandel Goltermann (Indianapolis, IN). He says mini-OTDRs equipped with user-configurable data points make it easier for technicians to analyze the fiber trace. Wolszczak adds that the more data points the better, noting that his company`s mts 5100 Mini OTDR can provide up to 62,000 data points.

The addition of color to OTDR displays has made it easier for users to interpret OTDR traces. For example, a technician can more easily compare multiple OTDR traces if each is a different color. "If you are in a telecommunications closet looking at 16 traces, it is really helpful if they are different colors," say Siecor`s Pons.

Modular design has become an important attribute of mini-OTDRs, enabling installers to test singlemode and multimode fiber. In addition to providing separate modules suitable for singlemode and multimode fiber, "quad" systems enable technicians to test singlemode and multimode fiber operating at different wavelengths. "We hear customers saying, `We`re doing a lot of multimode testing, but we think we will be doing more singlemode work in the next year or so,` " says Pons.

A modular design enables contractors to purchase a mini-OTDR that satisfies their current needs while providing an upgrade path to meet their future requirements. The ability to spread the cost of equipment by starting with the configuration most essential to the customer`s immediate need is an important benefit, since mini-OTDRs are fairly pricey, ranging from $7000 to $18,000, depending on the instrument`s features. This benefit is particularly appealing to cabling contractors, since their budgets for capital expenses are usually much smaller than those of telephone companies and cable-TV systems operators. Moreover, since the telephone companies and large multisystem cable-TV operators purchase many units, they are able to leverage their buying power to obtain discounts from the OTDR suppliers.

"In the contracting business, you usually like to wait for a large job so you can justify the expense of new equipment," says Bill McClendon, an engineer and project estimator for Adkins Cabling, an Arizona-based cabling contractor. In addition to doing installation work for telephone companies, Adkins Cabling`s list of customers includes corporations like Raytheon. McClendon says many of its customers require Adkins to provide OTDR documentation of its fiber installations. And even if it isn`t a customer requirement, he says, "we use it for our own benefit." According to McClendon, the documentation provided by an OTDR avoids any finger-pointing that may result if post-installation problems arise.

Crestcom Engineering (Bowling Green, KY) installs a lot of optical fiber, including large local-area-network (LAN) systems for manufacturing facilities. But according to Emmett Secrest, general manager, very few customers demand OTDR testing. "Most of the fiber work we do requires loss measurement with a loss meter, rather than actual OTDR measurement," says Secrest. When an OTDR measurement is required, Crestcom Engineering rents an instrument. Nearly all of the OTDR suppliers have mini-OTDRs and full-sized OTDRs available for rent.

Premises-backbone testing

Bruce Riddle, an operations manager for Lockheed Martin in Washington, DC, says his company`s customers frequently require OTDR testing and documentation. Lockheed Martin also uses an OTDR as a quality-control tool for checking on work performed by subcontractors. "Say there were 96 fibers installed, we might take traces on 20 of them," says Riddle. While an OTDR is useful for testing long fiber runs providing connection between buildings on campus LANs, Riddle says he does not recommend use for shorter fiber backbones used in LANs confined to a single building unless the customer requires OTDR documentation as part of the network-commissioning process. "Let`s say you have a five-story building with the computer room on the first floor and stacked communications closets directly above one another. If the ceilings are 20 feet high, the maximum length of the fiber run is 200 feet," he says.

Chuck Lohrmann, a master instructor for cabling installation and maintenance training programs offered by BICSI (Tampa, FL) and a principal in Compass Telecommunications Consulting, a Ft. Worth, TX-based provider of training and consulting services, essentially agrees with Riddle regarding OTDR testing for intrabuilding LAN backbones. But Lohrmann offers one caveat regarding LANs with collapsed backbones: In LANs with this topology, the horizontal cabling is spliced to the vertical backbone in a telecommunications closet. Lohrmann says the OTDR is beneficial for testing collapsed backbones because it provides a picture of the network that indicates the locations of the splices and their loss measurement.

The primary reason an OTDR loses its effectiveness as fiber links become shorter is because of the "dead zone," which is a result of an OTDR`s need to recover after a reflective event. This problem is generally most acute when the light pulse is first launched from the OTDR. The reflection from connector on the OTDR front panel is stronger than the reflection returning from the fiber and thus creates a blind spot, or dead zone, in which the OTDR is unable to detect an event. Another problem stems from OTDR resolution; the instrument cannot see distinct features in the cable if the pulse is going through them simultaneously, as is often the case in LANs.

Vendors have worked hard to limit the length of dead zones but they are an inevitable result of the OTDR`s indirect method of acquiring information by monitoring reflections that travel through the fiber and return to the test instrument.

Michael Fahey is a freelance writer in Waltham, MA.

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