New uses approved for the otdr
By William B. Gardner
The optical time-domain reflectometer (otdr) earned its status as an essential test set through its unique ability to locate faults and measure fiber splice loss. Other uses then emerged, such as the measurement of attenuation uniformity (see Lightwave, August 1993, page 26), return loss (see Lightwave, September 1994, page 21), and fiber length (see Lightwave, March 1997, page 62). Recently, some less obvious applications have developed and are gaining the approval and encouragement of standards bodies.
In August 1995, the Telecommunications Industry Association (tia) issued the Informative Test Method itm-6, "Characterization of Mode Field Diameter and Cutoff Wavelength of Single-Mode Optical Fiber by otdr." The fiber under test is spliced to a reference fiber whose mode field diameter and cut-off wavelength values are known. If you measure the change in the backscatter across the splice in both directions, the mode field diameter of the fiber under test can be determined. Repeating this procedure at a second wavelength provides enough information to calculate the fiber`s cut-off wavelength as well.
Efforts are now under way to elevate the otdr mode field diameter measurement to a standard test procedure. Tom Hanson of Corning Inc. in Corning, NY, has authored a test procedure that has undergone balloting in the tia.
A similar mode-field-diameter test procedure was introduced into Working Group 1 of the International Electrotechnical Commission`s Subcommittee 86A. Balloting closed in April 1997. Essentially the same text was introduced into the International Telecommunication Union`s (itu`s) Working Party 4/15 in April 1997. Some clarification was requested in the itu about two adjustable parameters used in the mode-field-diameter calculation. Data supplied by Corning in the United States and Optical Fibres in the United Kingdom shows a repeatability of the otdr measurement of mode field diameter of 0.02 micron, compared to 0.05 micron for the widely used variable aperture measurement in the far field.
The latest role for the otdr is in the measurement of chromatic-dispersion uniformity. As optical-power levels and unregenerated distances increase, four-wave mixing could impair system operation. Careful control of chromatic dispersion can be used to suppress four-wave mixing. Non-zero dispersion fibers can be used as a means of introducing a small, controlled amount of dispersion (see Lightwave, December 1996, page 62). There is some concern that unintentional variations in dispersion might cause the dispersion to actually drop to zero at some points in the fiber. The otdr`s ability to assess fiber uniformity makes it a natural candidate for investigating this question.
Nippon Telegraph & Telephone in Japan has submitted to itu Working Party 4/15 a test method for measuring a fiber`s chromatic-dispersion uniformity using the otdr. The otdr measures only the waveguide component of the dispersion; the material component must be determined separately from a knowledge of the fiber`s dopant concentration. The two components can then be added to calculate the chromatic dispersion. This proposal will be discussed further at the next itu Working Party 4/15 meeting in Geneva in February 1998.q