Dispersive Fourier transform using few-mode fibers for real-time and high-speed spectroscopy

Dispersive Fourier Transform (DFT) is a powerful technique for real-time and high-speed spectroscopy. In DFT, the spectral information of an optical pulse is mapped into time using group velocity dispersion (GVD) in the dispersive fibers with an ultrafast real-time spectral acquisition rate (>10 MHz). Typically, multi-mode fiber (MMF) is not recommended for performing DFT because the modal dispersion, which occurs simultaneously with GVD, introduces the ambiguity in the wavelength-to-time mapping during DFT. Nevertheless, we here demonstrate that a clear wavelength-to-time mapping in DFT can be achieved by using the few-mode fibers (FMFs) which, instead of having hundreds of propagation modes, support only a few modes. FMF-based DFT becomes appealing when it operates at the shorter wavelengths e.g. 1-μm range--a favorable spectral window for biomedical diagnostics, where low-cost single mode fibers (SMFs) and high-performance dispersion-engineered fibers are not readily available for DFT. By employing the telecommunication SMFs (e.g. SMF28), which are in effect FMFs in the 1-μm range as their cut-off wavelength is ~1260 nm, we observe that a 3nm wide spectrum can be clearly mapped into time with a GVD as high as -72ps/nm and a loss of 5 dB/km at a spectral acquisition rate of 20 MHz. Moreover, its larger core size than the high-cost 1-μm SMFs renders FMFs to exhibit less nonlinearity, especially high-power amplification is implemented during DFT to enhance the detection sensitivity without compromising the speed. Hence, FMF-based DFT represents a cost-effective approach to realize high-speed DFT-based spectroscopy particularly in the biomedical diagnostics spectral window.