Wavelength calibration proves challenging
WILLIAM B. GARDNER
Zero-dispersion wavelength is a key measurement parameter in determining the chromatic dispersion of optical fiber. New lightwave developments, such as dense wavelength-division multiplexing (dwdm) and dispersion management of long unrepeatered systems, are causing some fiber purchasers to demand subnanometer accuracy in the measurement of zero-dispersion wavelength (see Lightwave, November 1995, page 40).
Wavelength resolution and the repeat ability of commercially available test sets are generally adequate for this task, but the problem of absolute wavelength calibration is a challenge for both test-set designers and standards bodies. The measurement problem exists not only for dispersion test sets, but also for instruments such as optical spectrum analyzers, which are used to measure the spectral properties of light sources and fibers.
Wavelength calibration strategies are presently being explored in Telecommunications Industry Association Working Group FO-6.6.1 and International Telecommunication Union Working Party 4/15. Also, International Electrotechnical Commission TC86 Working Group 4, Calibration of Fiber Optic Test Equipment, chaired by Grant Watkins of Corning, has instituted Subworking Group 3, Calibration of Chromatic Dispersion, chaired by Arthur Barlow of EG&G in the United Kingdom, and Subworking Group 5, Optical Spectrum Analyzers, chaired by S.-I. Furukawa of Nippon Telegraph & Telephone in Japan.
While the work in these standards bodies progresses, several options are or will soon be available for those interested in seeking subnanometer accuracy in wavelength calibration. The National Physical Laboratory (NPL) in the United Kingdom provides a wavelength measurement service that uses various absorption and emission characteristics of gases. This service enables the calibration of instruments at discrete wavelengths between 1200 and 1600 nm. To obtain continuous calibration over this range, the NPL uses an air-spaced cavity etalon "artifact" in a reflection mode. These calibrations are fully traceable to the national optical standards held at NPL. Additional information can be obtained by calling NPL`s David Humphreys at 44-181-943-6389 in the United Kingdom.
EG&G Fiber Optics in the United Kingdom sells a similar (temperature-controlled) artifact (Model CD301B) for use in calibrating its CD300 chromatic-dispersion measurement system and PMD4 and CD331 polarization mode dispersion measurement systems. This calibration method results in a ۪.2-nm accuracy in the measurement of zero-dispersion wavelength and can also be used for calibrating optical-spectrum analyzers.
In the United States, the National Institute of Standards and Technology (nist) is expected to offer a wavelength reference absorption cell for sale. This cell will contain acetylene gas, which has numerous absorption lines of accurately determined wavelengths between 1510 and 1540 nm.
The acetylene absorption cell has been assigned the number 2519 in nist`s Standard Reference Material (SRM) Program. Another useful SRM is number 2524, a fiber approximately 10 km long, whose zero- dispersion wavelength is certified by nist to approximately ۪.08 nm. For more information, call the SRM Program Office in Gaithersburg, MD, at (301) 975-6776.
Another candidate for a future cell is hydrogen cyanide, which has absorption lines near 1550 nm. For those seeking subnanometer accuracy, note that wavelengths at about 1550 nm measure approximately 0.4 nm lower in air than they do in a vac uum. This difference can be accounted for in calibration, but the amount of the difference is generally not negligible. q
Gardner photo and bio
"New lightwave developments are causing some fiber purchasers to demand subnanometer accuracy."
William B. Gardner represents Lucent Technologies, Norcross, GA, on several fiber standards committees. He received a B.S. from the University of Alabama and a Ph.D. from Johns Hopkins University, both in physics.