Interferometers calibrate gauge blocks over optical fiber

If one were to generate a list of least known but most used measuring instruments, gauge blocks would certainly be included and might be listed near the top. They are "among the most commonly used standards for maintaining traceability in dimensional metrology," according to an online description by Britain's National Physical Laboratory (Middlesex, England).¹ Yet even at this year's annual meeting of the SPIE (Aug. 3–8; San Diego, CA), in which two sessions were devoted to calibrating gauge blocks, many if not a majority of general meeting attendees had probably never heard of a gauge block, according to Ted Doiron, a physicist in the precision metrology division of the National Institute of Standards and Technology (NIST; Gaithersburg, MD) who chaired one of the sessions.

According to one online history, the idea for a relatively small set of block-shaped, physical length standards that could be combined as necessary to meet industrial requirements was conceived a little more than a century ago to help armaments factories make interchangeable parts for rifles.² Currently, "gauge blocks ranging in sizes from 0.1 to 20 in. are required to support industrial processes in the United States," according to the NIST/SEMATECH e-Handbook of Statistical Methods.³

Optical interferometry with accuracies on the order of 10^–8 m provides the most sensitive method of calibrating gauge blocks. Mechanical calibration methods, which are faster, less expensive, and simpler to perform than interferometry, however, provide less calibration sensitivity. That accuracy is sufficient for most industrial users, who generally rely on mechanical calibration. Many research environments, as well as highly precise industrial applications, however, do need the extra order of magnitude of accuracy and would appreciate obtaining it without the cost and risk of transporting their sensitive and expensive gauge blocks to a calibration laboratory. Consequently, Akiko Hirai and colleagues at the National Metrology Institute of Japan (NMIJ: Ibaraki, Japan) are developing a method for delivering calibration-laboratory accuracy to gauge-block calibrators at remote locations using optical fibers.⁴

Tandem interferometers

The technique is based on using tandem low-coherence interferometers, with one interferometer actually at a calibration-laboratory location and the other at a remote location along with the gauge block to be calibrated (see figure). The interferometers are joined by a single-mode optical fiber. The coherence length of the interferometer's light source is much shorter than the gauge-block dimensions (the differences in optical path length that each interferometer will obtain). Because of the short coherence length, neither interferometer will produce interference fringes at the output individually, but the interferometers in combination will produce fringes (if the path-length difference is less than the coherence length).

An experimental setup for remote calibration of gauge blocks relies on tandem interferometers, one at a calibration laboratory and one at the gauge-block location, connected by single-mode optical fiber. Calibration is achieved when a photodetector senses matching of path-length differences between the two interferometers.Click here to enlarge image

The Japanese researchers used a 3-km length of single-mode optical fiber to connect two interferometers—the first illuminated with a superluminescent diode and measuring path-length differences based on national standards, and the second measuring the dimensions of an actual gauge block. The optical fiber carried light from the first interferometer to the second; when the optical path-length differences matched, an interference fringe was detected by a photodetector at the output of the second interferometer. Using this experimental model, the team calibrated a 50-mm-length gauge block with a standard deviation of 0.06 µm, according to Hirai, who presented the results at the SPIE annual meeting.

The remote interferometric calibration system is being developed in Japan as part of a five-year project, begun in 2001, focused upon improving calibration services provided to industry, Hirai said. The NMIJ team is currently working on reducing the measurement uncertainty obtained with their experimental model by improving the optical system as well as the environmental stability at the measurement site. They expect to use currently deployed dark fiber in providing actual services to industry.