A high resolution compact all-fiber spectrometer based on periodic refractive index modulation

The realization of a miniaturized spectrometer with high resolution is highly desired but is still a big challenge. Although all-fiber spectrometers based on speckle detection show their great potential for high resolution ones, their long fiber lengths set the greatest obstacle for the miniaturized design. Here, we demonstrate a compact all-fiber speckle spectrometer by using cascading coreless fibers and photonic crystal fibers. A unique cascaded structure readily excites more guided modes, in which the speckle patterns are formed by modal interferences. Using only a 10 cm-long fiber with 20-segment spliced elements, a resolution of 0.03 nm over a bandwidth from 1540 to 1560 nm is achieved. The spectral resolution is comparable to that of a 2 m multimode fiber spectrometer and approximately 20 times higher than that of the same length multimode fiber. Narrow linewidth and broadband spectra are individually reconstructed to demonstrate the excellent performance of the spectrometer. The proposed processing technique of the dispersive element is versatile, reproducible, and controllable, promising for different application scenarios.