We present the measurement of distributed mode coupling along tapered single-mode fibers using optical frequency-domain reflectometry. The energy re-distribution through Rayleigh backscatter amplitude as a function of distance with a 13 μm resolution over the whole taper region is measured. The wavelength shifts between the fundamental mode and high order modes of the taper are theoretically derived and experimentally verified by using autocorrelation data processing. An accurate measurement of refractive index differences in both gentle and abrupt tapers is achieved with a spatial resolution of ~2 cm. The measured results and theoretical values match well in gentle taper. It is found that the physical length of mode coupling of abrupt taper is longer than its geometric length, while both lengths are the same in gentle taper. The measurement is rapid, repeatable and nondestructive.
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