Simple and precise characterization of differential modal group delay arising in few-mode fiber

In this Letter, we propose a simple and high-precision differential modal group delay (DMGD) characterization method for few-mode fibers (FMF) by using the frequency-modulated continuous wave. Since the detected signals are located at the low-frequency range, our DMGD characterization method waives the use of expensive equipment, such as vector network or optical spectrum analyzers. Due to the high linearity of the used Mach-Zehnder modulator, our DMGD measurement is free from the complex auxiliary interferometer, leading to an improvement of characterization precision. Meanwhile, we propose a novel spectrum recovery algorithm to overcome the shortcoming that the traditional fast Fourier transform (FFT) method is incapable to deal with spectrum features arising in a periodic signal. Therefore, the characterization precision is no longer limited by the FFT length. When a commercial 23299.8 m two-mode fiber is used in the experiment, the DMGD measurement of LP₁₁ mode relative to LP₀₁ mode has a high precision of ±0.007ps/m over the C-band. Our proposed method shows the potential for characterizing the wavelength-dependent DMGD of FMF with more than two LP modes.