Abstract
We present a new method for measuring the group dispersion of the fundamental mode of a holey fiber over a wide wavelength range by white-light interferometry employing a low-resolution spectrometer. The method utilizes an unbalanced Mach-Zehnder interferometer with a fiber under test placed in one arm and the other arm with adjustable path length. A series of spectral signals are recorded to measure the equalization wavelength as a function of the path length, or equivalently the group dispersion. We reveal that some of the spectral signals are due to the fundamental mode supported by the fiber and some are due to light guided by the outer cladding of the fiber. Knowing the group dispersion of the cladding made of pure silica, we measure the wavelength dependence of the group effective index of the fundamental mode of the holey fiber. Furthermore, using a full-vector finite element method, we model the group dispersion and demonstrate good agreement between experiment and theory.
Highlights
The group dispersion, that is, the wavelength dependence of the group index, belongs to one of the fundamental dispersion characteristics of optical fibers
The technique utilizes an unbalanced Mach-Zehnder interferometer with a fiber under test placed in one arm while the other arm has adjustable path length to record a series of spectral signals and to measure the equalization wavelength as a function of the path length
We revealed that there is an apparent path length discrimination between the spectral signals associated with the fundamental mode supported by the fiber and light guided by the outer cladding of the fiber
Summary
The group dispersion, that is, the wavelength dependence of the group index, belongs to one of the fundamental dispersion characteristics of optical fibers. The chromatic dispersion, which can be obtained by differentiating the measured relative group index, is a significant characteristic that affects the bandwidth of a high speed optical transmission system through pulse broadening and nonlinear optical distortion. Chromatic dispersion of long length optical fibers is determined by two widely used methods [1]: the time-of-flight method which measures relative temporal delays for pulses at different wavelengths, and the modulation phase shift technique which measures the phase delay of a modulated signal as a function of wavelength. The spectral distribution of the phase delay over the full bandwidth of the white-light source is obtained in a single measurement by a Fourier transform of the cross-correlation interferogram [4]. The dispersion characteristics of the fiber sample under study can be obtained by differentiating the measured phase delay
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