Time domain chromatic dispersion measurements in single mode optical fibers

A time domain chromatic dispersion measurement system for single-mode optical fibers is presented for easy measurements in both the laboratory and the field. This technique uses a relative group delay of a band of light at slightly different wavelengths from a single pulsed laser diode to determine chromatic dispersion and the zero chromatic dispersion (ZCD) wavelength of the fiber. Time domain dispersion measurements find the group delay directly, eliminating the need for a vector voltmeter and the added phase calibration and group delay calculations needed in most frequency (phase) domain techniques. With the use of a monochromator as a bandpass filter, a number of group delay data points can be taken throughout the spectral width of a single laser diode. Since the group delay data are relative measurements, access is needed only to the output of a length of fiber. This makes this time domain technique especially well suited for field use. Using the modified Sellmeier equation, only three group delay measurements need to be taken to find the zero chromatic dispersion (ZCD) wavelength and a highly accurate approximation to the chromatic dispersion curve. The modified Sellmeier equation coefficients are found by a simple BASIC program, eliminating the need for curve-fitting and numerical differentiation routines. The desired chromatic dispersion curve and ZCD wavelength are found by straightforward differentiation.