NIST details fiber geometry round-robins

NIST details fiber geometry round-robins

WILLIAM B. GARDNER

Tim Drapela and coworkers at the National Institute of Standards and Technology (NIST) in Boulder, CO, have published NIST Technical Note 1378, which presents detailed results of five interlaboratory optical measurement comparisons completed in 1994 and 1995. Such "round-robins" are frequently used for evaluating test methods and calibration procedures.

These five round-robins deal with fiber and connector geometrical parameters and were coordinated by NIST for the Telecommunications Industry Association (TIA) and the International Telecommunication Union (ITU). TIA Working Group FO-6.6.1 on Round-robins, chaired by Doug Franzen of NIST, provided guidance for most of this work.

The first round-robin, which concerned fiber geometry, showed the value of a calibration artifact and led to Standard Reference Material being provided by NIST in the United States and by the National Physical Laboratory in the U.K.

The second round-robin led to the creation of a similar artifact for fiber coating diameter, which is expected to be available from NIST in 1996 (see Lightwave, November 1995, page 14).

The third round-robin involved ceramic ferrules that position fibers within connectors. The inside diameter of the ferrule must be only slightly larger than the fiber diameter to ensure the snug fit needed for accurate fiber alignment.

Two methods of measuring the ferrule`s inside diameter were evaluated. The first method was a go/no-go measurement performed by inserting calibrated pin gauges or fibers into the ferrule. This method proved effective in establishing upper and lower bounds for the ferrule inside diameter. With available pin gauges, however, these bounds were usually 1 micron apart. A 1-micron measurement resolution is generally insufficient when working with singlemode fibers.

The submicron resolution obtained with video methods, however, did not lead to improved accuracy. In addition, video methods measure only the ferrule endface. Unlike pin gauges, they are unable to detect tapering along the length of the ferrule. The present TIA Test Method (in draft form) for measuring the inside diameter of the ferrule requires the use of pin gauges with diameters traceable to NIST and tolerances of +0.25 micron greater than the nominal, or 0 micron less than the nominal. This traceability should be improved with the availability in 1996 of a NIST pin gauge diameter Standard Reference Material.

Because pin gauges wear with normal use, their diameters must be verified periodically. Therefore, the fourth round-robin dealt with pin-gauge diameter measurements. Eight participants measured 18 pin gauges using both contact and noncontact methods. Although systematic errors of 0.5 micron occurred, the measurement spread for a single participant was typically 0.1 micron. These results suggest that improved calibration--perhaps with the NIST pin gauge Standard Reference Material--could reduce the systematic errors.

The fifth round-robin measured ferrule geometrical parameters, such as outside diameter, concentricity error, surface finish and roundness. Only the outside diameter carries a TIA specification: 2.499 millimeters ۪.5 micron. The measurement spread indicates that the systematic errors--one standard deviation of typically 0.3 micron for ferrule outside diameter--could be reduced to approximately 0.19 micron by improved calibration. The latter number probably represents insufficient measurement accuracy to support the industry tolerance target of ۪.5 micron.

NIST Technical Note 1378 is available from Tim Drapela at tel: (303) 497-5858 or e-mail: drapela@boulder.nist.gov. q

More in Optical Tech