Experimental measurement of effective refractive index difference for few mode polarization maintaining fibers using S~2 method

摘要

Polarization maintaining fibers (PMFs) can keep linear polarization state against external perturbations by inducing a high effective refractive index difference (Δneff) along one polarization axis. For few mode polarization maintaining fibers (FM-PMFs), Δneff is applicable between both orthogonal linear polarization modes (e.g. LP_(01x) and LP_(01y)) and orthogonal degenerated modes (e.g. LP_(11a) and LP_(11b)), which can enable advanced functionalities in multiple-input multiple- output-free spatial division multiplexing systems and optical fiber sensing systems. Therefore, the measurement of Δneff for polarization modes and degenerated modes is very important for determining the quality of a FM-PMF. However, measurement of the Δneff for FM-PMFs can be complicated due to the requirement for generating and demultiplexing of the higher order modes (HOMs). In this paper, we propose to measure the Δneff of FM-PMFs using Spatially and Spectrally resolved imaging (S~2) method for the first time. The presented method is simply by employing a tunable laser and an IR CCD camera, can avoid any mode converter or mode multiplexer/demultiplexer, featuring a rapid testing speed. A proof-of-concept experiment is carried out to measure FM-PMFs with a length of 1.1m and 5m. The Δneff between the orthogonal polarization modes (i.e. LP_(11ax-11ay), LP_(11bx-11by), LP_(21ax-21ay), and LP_(21bx-21by)) are characterized as 7.05×10~(-4), 6.91×10~(-4), 1.02×10~(-3) and 1.04×10~(-3) respectively. The Δneff of the orthogonal degenerated modes (i.e. LP_(11ax-11bx), LP_(11ay-11by), LP_(21ax-21bx) and LP_(21ay-21by)) are also characterized to be 1.39×10~(-4), 1.24×10~(-4), 5.61×10~(-5) and 6.53×10~(-5) respectively.