We report the design of 12-LP-mode (21-spatial-mode) fiber with a low differential mode group delay (DMGD), a large effective area, and a low bending loss that is adapted to a mode-division-multiplexing system over the C+L band. Based on the designed fiber parameters, we characterize the few-mode fiber (FMF) with the DMGD, an effective area, and a bending loss. Over the C+L band, the maximum DMGD is 0.106 ps/m, and the effective area is in the range of 150∼485 μm 2. The bending loss of the designed FMF reduces to zero when the bending radius is greater than 9 mm, and the DMGD is below 0.0413 ps/m at the wavelength of 1550 nm.
Using principal modes (PMs) can avoid the inter-mode crosstalk in fiber-optic communication and reduce the complexity of digital signal processing. In this paper, a new detection method for PMs based on spatially and spectrally resolved imaging (S 2) is used to recover their intensity distributions and inter-mode dispersion parameters. This method collects the optical interference information on a two-dimensional plane at different frequencies. The collected data can be used to characterize the principal modes, including their patterns and differential mode group delays. Due to the frequency invariance of PMs even in mode coupling state, which is studied carefully, this proposed method is shown to be robust. Analyses based on a four-mode fiber show that the distributed fluctuation of the coupling coefficients break the frequency invariance of PMs. We experimentally measure the mode characteristics of a four-mode fiber with S 2 method. The results show that the mode group delays of degenerate modes can be separated in mode coupling state, which is exactly appropriate for the modified measurement method.
We propose to use multiple lasers to write long-period gratings into few-mode fibers for significant reduction of the mean DMD by 280 times from 1700 to 6.2 ns at 1500 km.
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