Abstract

We have carried out experiments of mode propagation and identification in a nominally single-mode fiber having 1.2 µm cutoff wavelength. We operated at a low V-value of 6.2 by launching mode-locked pulses from a dye laser operating at λ = 615 nm. These pulses were measured to have FWHM of 97 ps being detector-oscilloscope limited. Other measurements furnished FWHM of 165 fs. The pulses had a repetition rate of 130 MHz. The modes we identified at the output of 1-km long fiber were LP₀₁, LP₁₁, LP₁₂, LP₀₂, LP₂₁ and LP₃₁. These modes were identified by looking at the output intensity distributions and moving the focused spot size laterally at the input end of the fiber. This enabled each mode to be launched preferentially and hence be identified. We calculated the temporal modal dispersion, between the fundamental LP₀₁ and each higher-order mode, using Gloge’s weakly guiding theory, and observed good correlation with the measured values. The broadened output pulses are explained in terms of material dispersion, which is 280 ps/nm/km at the launching wavelength. By introducing a V-groove mode mixer at the input end, we could eliminate all the higher-order modes and be left with only the LP₀₁ mode.

© 1985 Optical Society of America