Cutoff wavelength tests updated
WILLIAM B. GARDNER
The cutoff wavelength of a singlemode fiber establishes the lower end of the range over which the fiber operates in a true singlemode manner. Below the cutoff wavelength, singlemode fibers can perform as multimode fibers. Because the cutoff wavelength depends on the fiber`s length and bend characteristics, the values of these characteristics must be established when defining and measuring the cutoff wavelength using industry-standard test methods (see Lightwave, October 1995, page 20).
Until 1996, there were two options for defining, specifying, and measuring this cutoff wavelength:
Cable cutoff wavelength--Expose 1 meter of singlemode fiber at each end of a 22-meter-long cable. Keep the 20-meter cabled portion substantially straight. Form a 40-mm radius loop in each of the exposed fiber ends, thus simulating the conditions in splice cases. The acceptable maximum values are 1260 and 1270 nm when specifying the cable cutoff wavelength under International Telecommunication Union (itu), International Electrotechnical Commission (iec), and Telecommunications Industry Association (tia) test methods.
Fiber cutoff wavelength--In a 2-meter length of uncabled fiber, form one loop of 140-mm radius. Keep the remainder of the fiber substantially straight. Using the 1260- and 1270-nm cable cutoff wavelength maxima now requires mapping from the fiber cutoff values.
The itu, iec, and tia have all designated the cable cutoff wavelength as the preferred option because it comes closer to characterizing performance in the field.
Fiber jumpers used in equipment bays cannot be adapted to either of the above schemes. They are generally provided with a protective sheath--just as cables are--but they are usually much less than 22 meters long. Both itu Study Group 15 and Working Group 1 in the iec Subcommittee SC86A addressed the jumper problem during 1996 by adding a third cutoff wavelength definition and test configuration:
Jumper cable cutoff wavelength--Form one loop in a 2-meter-long jumper. Keep the remainder of the jumper essentially straight. Although 76 mm is the only suggested value for the loop`s radius, so far, this parameter has not yet been approved as the standard value. Similar to the cable cutoff wavelength, the maximum values of 1260 and 1270 nm are acceptable for the jumper cable cutoff wavelength.
Similar texts adopted by the itu and iec acknowledge that the fiber cutoff wavelength values generally exceed cable cutoff wavelength values. However, the previous 1280-nm maximum value for the fiber cutoff wavelength that appeared in the old text has been deleted. In fact, the 1260- and 1270-nm cable maximum values apply to both dispersion-unshifted and dispersion-shifted fibers (itu Recommendations G.652 and G.653, respectively).
itu`s Reference Test Method (rtm) for the fiber cutoff wavelength now includes the rtm for jumper cable cutoff wavelength as well. The curve-fitting algorithm for improving test precision has also been incorporated as an option. The tia is using this new text in consolidating its cable cutoff Test Method (fotp-170) with its fiber cutoff Test Method (fotp-80).
A component needed attention: Optical devices frequently include fiber pigtails less than 2 meters long. The new text states that any pigtail (or jumper) less than 2 meters must be made from fiber whose fiber cutoff wavelength does not exceed 1250 nm. This value was selected because shorter lengths result in less attenuation of higher-order modes that might be present. q
William B. Gardner represents Lucent Technologies, Norcross, GA, on sev eral fiber standards committees. He can be contacted at tel: (770) 798-2674; fax: (770) 798-4654; e-mail: wbgardner@ lucent.com.