Abstract
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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) 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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 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.
Highlights
In recent years, the demand for communication capacity has been increasing rapidly [1]
We propose a cost-effective, agile and feasible method for characterizing principal modes (PMs) based on spatially and spectrally resolved imaging (S2) [7], which only needs a single frequency scan to obtain the patterns of all PMs and their corresponding differential mode group delays (DMGDs)
We theoretically prove that the detection results of S2 method are the interferences between PMs
Summary
The demand for communication capacity has been increasing rapidly [1]. One way to solve this problem is to design weak mode coupling fiber [3] Another effective way is to use principal modes (PMs), a set of unique orthogonal modes with first-order frequency invariance [4], which has great potential for applications in MDM. The method currently used to resolve and regenerate PMs is mainly based on swept wavelength interferometry (SWI) [5], in which a rapidly tunable laser, a pair of (de)multiplexers and a highspeed real-time oscilloscope are required to measure the transmission matrix of a fiber. Measuring a N×N transmission matrix with SWI, where N is the total number of modes in a FMF, requires N2 frequency sweep or extra time-domain multiplexing [6], with increased equipment and time.
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