By William B. Gardner
As the optical power, density of wavelength-division multiplexing, and distance between regenerators all increase, nonlinear optical effects tend to grow in importance. One parameter that helps in characterizing a fiber's tendency toward nonlinearity is the effective area Aeff (see Lightwave, February 1998, p. 64). Since the effective area is a geometrical concept that is somewhat related to the mode field, existing test methods for the mode-field diameter can be modified to become Aeff test methods. Three of these methods (near-field, direct far-field, and variable aperture in the far-field) appear in the Telecommunications Industry Association (TIA) Fiber Optic Test Procedure FOTP-132, "Measure ment of the effective area of singlemode optical fiber."
Key nonlinear effects such as self-phase modulation, cross-phase modulation, and four-wave mixing are proportional to the ratio n2/Aeff, where n2 is the nonlinear refractive index. In this case, it is the ratio n2/Aeff, called the "nonlinear coefficient," that is the real measure of a fiber's tendency toward nonlinearity.
Since the nonlinear coefficient is not the relatively simple geometrical parameter that Aeff was, sufficient optical power must be used to generate actual nonlinearities when measuring it. Kokusai Denshin Denwa (KDD-Japan) has proposed a method in the International Telecommunication Union (ITU) that uses transform-limited optical pulses. An optical spectrum analyzer measures the output pulses, which are distorted by self-phase modulation. Measurement of the nonlinear phase shift in the output pulses enables the fiber's nonlinear coefficient to be determined.
TIA recently balloted the continuous-wave (CW) test method FOTP-202, "CW dual-frequency method for measuring the nonlinear coefficient of singlemode fibers," and submitted the method to ITU Study Group 15 and the International Electrotechnical Commission's (IEC) SC 86A. In FOTP-202, signals from two CW lasers propagate in the test fiber. Self-phase modulation in the fiber acts on the resulting beat envelope to create sidebands in the frequency domain.
The ratio of the intensities of the fundamental frequencies to the first sidebands is measured with an optical spectrum analyzer. From this ratio, the nonlinear phase shift is determined, which in turn allows the nonlinear coefficient to be calculated.At its June 1999 meeting, TIA's Working Group FO-6.6.1, chaired by Tim Drapela of the National Institute of Standards and Technology, organized a nonlinear coefficient round robin using both the CW and pulsed methods. Preliminary results were reported at last January's meeting. Although no systematic difference between the two methods is evident in the data from the first five reporting participants, some participants using the CW method measured differences as large as 30%, suggesting that more work is needed to understand the reason for these discrepancies. Definitive conclusions await the completion of the study in early 2000.