LSTM-assisted OFDR for DMGD characterization of a short few-mode fiber

We demonstrate an LSTM-assisted optical frequency domain reflectometry (OFDR) method for differential modal group delay (DMGD) characterization of short few-mode fibers. Sparse phase-domain resampling is first applied to compensate for laser sweep nonlinearity with a low sampling rate. Then, the resampled beat signal is extrapolated by an LSTM network to improve the resolution. In a 5-m single-mode fiber, the distance resolution is improved from 4.95 mm to 0.44 mm with a 20.74-GHz sweep bandwidth. The method resolves the overlapped LP₀₁ and LP₁₁ modal peaks in 1-m and 6-m two-mode fibers, yielding a differential modal group delay of 1.911 ps/m for the 1-m fiber, in good agreement with the reference values. A 2.2-m four-mode fiber is further tested to demonstrate the applicability of the method. It provides a route for modal characterization of short fibers under a limited sweep bandwidth.