New Measurement Technique to Fill Critical Need for Fiber Optic Cable Industry

In consumer-driven industries, it is critically important to provide customers what they expect when they make their purchases. When you pay for a gallon of gas, you expect to get one gallon. When you go out to buy a pound of blueberries, you expect to receive a pound of blueberries. But, what if you're in the market for 7 kilometers of fiber optic cable? How would you know you're getting exactly what you've paid for? How would the supplier even know?

Some 19 million miles of optical fiber were installed in the U.S. last year. In this massive industry, Corning Cable Systems is among the world's largest suppliers. Measuring the length of cable for distribution is no small task because the actual length can vary significantly depending on the temperature of the cable and the tension applied during measurement. After looking for private industry solutions, Corning Cable Systems approached NIST to help tackle their unique measurement challenges, and researchers in PML's Semiconductor and Dimensional Metrology Division are at work calibrating a new set of length standards.

Corning and other manufacturers in this industry need to ensure that there are no discrepancies between what the customer wants and pays for and what is provided, explains Daniel Sawyer, leader of the Advanced Dimensional Measurement System for Manufacturing Project. "Corning Cable Systems needs accurate standards. These length standards will be used to accurately measure the lengths of the hundreds of thousands of kilometers of cable they sell each year. The standards will be used to reduce their scrap rate, which reduces cost and ultimately satisfies their customer needs by providing exactly what their customers requested." 

Adam Solodow, a Corning Cables Senior Quality Engineer, agrees: "Bottom line....this enables us to satisfy our customers and provide a competitive advantage."

"Corning could not go anywhere else for this type of measurement with the accuracy needed," Sawyer explains. NIST's unique dimensional measurement facility is 80 meters in length and has the precision temperature control needed to accurately measure the fiber optical length standards. "We can measure 60 meter displacements with uncertainties on the order of 0.3 micrometers per meter, which is very, very accurate," Sawyer states. "They needed the Nation's laboratory to respond. It is critically important and has high economic impact. 

"Corning wants very, very long lengths measured. One of the lengths is 7 kilometers. We had to develop some new instrumentation and new procedures to be able to measure those lengths quickly with the required accuracy."

The key word here is "quickly." NIST's facility already had the capability to make extremely accurate measurements of long cable lengths, but the measurement process—positioning an interferometer on a bench by hand across 60 m sections of cable—was time-consuming and extremely expensive. The interferometer is highly accurate, but it takes approximately 10 minutes to measure one 60 m length of cable.

The industry need dictates a much faster measurement with a slight sacrifice in accuracy. Sawyer and PML colleague Chris Blackburn (pictured left) have been able to achieve the desired speed using a laser tracker with an absolute distance meter (ADM). Their team also developed new hardware including a new target station which uses a camera and digital display instead of a conventional microscope. This eliminates the need for a technician to look through an eyepiece during each measurement.

Using this faster technique, the laser tracker is placed in the center of a 60 meter length of cable. Sequentially, the laser tracker shoots a laser beam in two different directions, reflecting off the target station at each end of the 60 meter length of cable. (The station is aligned to a fiducial mark on the cable.) The ADM within the tracker works on the principle of time-of-flight; it receives the reflected light from each target and calculates the distance based on the amount of time it takes for the light to return. "The tracker simultaneously measures the vertical and the horizontal angular directions of the laser beam," Sawyer describes. "If you know the distance and the vertical and horizontal angles from a known location, you know the coordinate." A 60 meter calibrated distance is set up and measured before each tracker measurement to establish traceability to the SI for each measurement performed.

The 60 meter distance is calibrated using a linear interferometer, which is traceable to NIST standards. Calibrating the length with a target uncertainty of approximately 1 micrometer per meter (the diameter of a human hair over the length of a football field) is not an easy task. "This is only possible because we have very accurate measurements of the temperature profile over the 60 meter measurement path. We need this information along with measurements of the relative humidity and barometric pressure to make the refractive index correction necessary to realize the unit of length so accurately," Sawyer explains.

The ADM measurements are accurate to about one micrometer per meter. The measurement takes place in seconds without the technicians needing to move from one end of the 60 meter bench to the other. After the measurements are taken, the technicians slide the cable another 60 m down the bench and repeat. "Three minutes per segment is our target," Sawyer states, an improvement better than three-fold with a measurement that is traceable to the older, slower interferometric measurement.

While the early results are very promising, there is still work to be done before these new standards can be rolled out to industry. "We've done some measurements," Sawyer explains, "but it's not quite ready for industry yet. The target stations are fairly heavy and hard to move around. We're looking into designing a lifting mechanism or mounting fixtures to make the process faster." 

Sawyer is encouraged by the work completed thus far: "We've assembled the hardware, we've got a preliminary design working, and we've actually done some comparison measurements. We now believe that we have a system that's going to work. We're going to calibrate these long lengths, and Corning Cable Systems will use these standards to help establish traceability of all their measurements back to the SI unit of length."

Written and photographed by Erik Secula

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