Cladding standard improves fiberoptic measurements
Matt Young
A new standard of optical fiber cladding diameter developed by the National Institute of Standards and Technology (NIST; Boulder, CO) has led most major US fiber manufacturers to recalibrate their fiber drawing towers. The standard is important to companies that manufacture or install connectors or connector parts in that accurate diameter measurements permit low-loss connections to be made with minimal effort.
Represented by members of the Telecommunications Industry Association, the fiber industry approached NIST several years ago and asked for help in calibrating its gray-scale method of measuring fiber diameter. A gray-scale system is a video microscope used in conjunction with computer-based acquisition and image-processing software. These systems measure the diameter and lack of circularity of fibers and the decentering between the core and cladding. The TIA representatives asked for an artifact standard that consisted of a fiber end whose diameter was known with an uncertainty less than 0.1 µm.
Improved measurement techniques were required to achieve such accuracy. NIST`s Optoelectronics Division, collaborating with the Precision Engineering Division, responded by developing a contact micrometer that has an overall uncertainty less than 50 nm (see Fig. 1). The micrometer uses an air bearing for nearly frictionless motion and a commercial heterodyne interferometer for accurate position measurement. The measured fiber diameter had to be corrected for the compression of the fiber by the jaws of the micrometer.
Because the fiber is thin and flexible, the usual method of calculating the compression failed, so NIST scientists developed a graphical method instead: they plotted the measured diameter as a function of the 2/3-power of the force applied to the fiber and extrapolated to zero force. They used the 2/3-power because of a theoretical analysis of the compression of two crossed cylinders subjected to a known force.
NIST scientists also developed two other instruments, a scanning confocal microscope and an interference microscope, to verify the accuracy of the micrometer. Both used a heterodyne interferometer to measure position. The scanning confocal microscope is an instrument that uses a microscope objective to focus a laser beam onto the specimen. The beam that reflects off the specimen is focused though a beam splitter onto a pinhole, and a detector collects the light that passes through the pinhole. The specimen is scanned laterally across the focal point of the microscope objective, and the reflected light is digitized by a computer. NIST`s instrument scanned across a cleaved fiber end face and determined the diameter from measurements of several chords.
The interference microscope is a computer-controlled video microscope that uses a relatively broadband, incoherent source and an interference objective. The fiber is laid on its side, contacted to an optical flat that lies on the microscope stage, and examined near the cleaved end. The microscope is focused onto the surface of the fiber and scanned vertically, that is, perpendicularly to the axis of the fiber. As the surface comes into focus, the computer records a burst of interference fringes because of the relatively short coherence length of the source; the brightest of these fringes locates the surface of the fiber. The surface of the flat is located similarly, and the fiber diameter is taken to be the difference between the two measurements. The three instruments agreed with one another within approximately 25 nm.
Proof of the pudding
The benefit of the new standard was recently demonstrated through an international intercomparison in which laboratories, including NIST, reported their measurements of fiber cladding diameter. Early round robins revealed both variations of fiber diameter from specimen to specimen and significant disagreement among participants. In the most recent round robin, however, participants measured the same fiber ends, so specimen-to-specimen variation was eliminated. Figure 2 plots the average, high, and low measurements by participants in that round robin.
Participants are identified by geographic region: North America, Europe, and the Pacific. All the participants in a given region measured the same seven fiber ends. Those who had calibrated their instruments against the NIST standard agreed with one another, on average, by less than 0.15 µm (۪.075 µm), or about half the earlier disagreement. Participants who had calibrated their instruments against other national standards laboratories likewise agreed with NIST and with the NIST-calibrated laboratories within approximately ۪.1 µm. Participants who had not calibrated their instruments against a standards laboratory sometimes disagreed with one another by more than ten times that value.
NIST has sold more than 40 of the new standards, mostly to fiber and instrument manufacturers, and is also preparing standards for the outside diameters of connector ferrules and for the diameters of the steel wires, or pin gages, used to assess the inside diameters of ferrules. In addition, NIST scientists are beginning work on a glass rod that will be used as a standard of coating diameter. The coatings are the polymers that protect the fiber. They have a nominal diameter of 245 µm, and that diameter must be held within certain limits in order to make ribbon cables accurately. Actual coatings are considered unsuitable for use as standards, however, because they are polymers and their dimensions may change with time or exposure to index-matching fluids. Hence, NIST is working with a fiber manufacturer to develop a glass rod standard that has the right diameter and index of refraction.
MATT YOUNG is a Physicist in the Optoelectronics Division of the National Institute of Standards and Technology. For further information on any of these standards, please contact National Institute of Standards and Technology, Office of Standard Reference Materials, Room B311, Chemistry Building, Gaithersburg, MD 20899; tel.: (301) 975-6776. Refer to SRM 2520, Optical Fiber Diameter Standard; SRM 2522, Pin Gage Standard for Optical Fiber Ferrules; or SRM 2523, Diameter Standard for Optical Fiber Ferrules.
FIGURE 1. A carefully cleaved and calibrated fiber end is retracted within the aluminum housing of the NIST Standard Reference Material 2520, Optical Fiber Diameter Standard. The black-anodized storage container holds both the housing and approximately 2 m of fiber.
FIGURE 2. Round-robin measurements of several fiber ends expressed as offsets from NIST micrometer measurements show the mean, high, and low values of each of 25 participants. Filled circles represent calibration to the NIST standard; filled squares, calibration to another national standards laboratory; and open circles, uncalibrated or problematic measurements. (Figure courtesy of NIST/ Timothy Drapela)
