We report on a wavelength-tunable filter based on multimode interference (MMI) effects. A typical MMI filter consists of a multimode fiber (MMF) spliced between two single-mode fibers (SMF). The peak wavelength response of the filter exhibits a linear dependence when the length of the MMF is modified. Therefore a capillary tube filled with refractive-index-matching liquid is used to effectively increase the length of the MMF, and thus wavelength tuning is achieved. Using this filter a ring-based tunable erbium-doped fiber laser is demonstrated with a tunability of 30 nm, covering the full C-band.

A widely tunable erbium-doped all-fiber laser has been demonstrated. The tunable mechanism is based on a novel tunable filter using multimode interference effects (MMI). The tunable MMI filter was applied to fabricate a tunable erbium-doped fiber laser via a standard ring cavity. A tuning range of 60 nm was obtained, ranging from 1549 nm to 1609 nm, with a signal to noise ratio of 40 dB. The tunable MMI filter mechanism is very simple and inexpensive, but also quite efficient as a wavelength tunable filter.

In this letter, a torsion sensing arrangement based on a three beam path Mach–Zehnder interferometer (MZI) is presented. The MZI with three beam paths was fabricated by fusion splicing a piece of double ytterbium‐doped double‐cladding photonic crystal fiber (Yb‐d‐DPCF) between two segments of single mode fiber (SMF). Moreover, in our sensing arrangement the Yb‐d‐DPCF segment is acting both as the MZI as the gain medium. Here, a spectral MZI fringe shifting is induced by applying torsion over the Yb‐d‐DPCF. By this setup, a torsion sensitivity of 0.001 nm/° was achieved. Finally, it is shown that the sensing arrangement is compact and robust. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1857–1860, 2015

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