Two kinds of tunable Thulium-doped fiber laser (TDFL) respectively using a Sagnac loop mirror and a novel tunable multimode interference (MMI) fiber filter are experimentally demonstrated. The TDFL with the Sagnac loop mirror made by a 145.5-cm polarization-maintaining fiber (PMF) can operate with stable dual-wavelength lasing or tunable single-wavelength lasing around 1860nm. Both stable dual-wavelength and tunable single-wavelength lasing are achieved by adjusting a polarization controller in the Sagnac loop mirror. The TDFL with a novel tunable MMI fiber filter formed by splicing a segment of a special no-core fiber that is an all silica fiber without fiber core to single mode fibers can achieve tuning range from 1813.52 nm to 1858.70 nm. The no-core fiber with a large diameter of 200 μm is gradually vertically covered by refractive index matching liquid, which leads to a wavelength tuning of the transmission peak of the MMI fiber filter. The relationship between the refractive index of the refractive index matching liquid and the peak wavelength shift of the MMI fiber filter is also discussed. Using the MMI fiber filter, a Thulium-doped fiber laser with a tuning range of 45.18 nm is demonstrated.
A Fabry-Perot interferometer sensor based on a fiber-tip bubble-structure micro-cavity is proposed, fabricated, and demonstrated for hydrostatic pressure sensing and transverse load sensing. A segment of a well-cleaved multimode fiber with a core diameter of 62.5μm is processed with chemical etching based on a solution of HF 40% and the bubblestructure micro-cavity is fabricated by using arc discharge at the end of the processed multimode fiber. The sensor can be considered as a two-beam Fabry-Perot interferometer with one beam from the silica-air interface on the left side of the bubble and the other from the air-silica interface on the right side of the bubble. The broadband light is injected into the fiber-tip bubble-structure micro-cavity by splicing the multimode fiber with the bubble-structure micro-cavity to a 3- dB optical coupler and the reflective spectrum of the bubble-structure micro-cavity is measured by an optical spectrum analyzer. Both hydrostatic pressure sensing with a sensitivity of ~0.1 nm/MPa and transverse load sensing with sensitivity of 3.64 nm/N are experimentally demonstrated based the proposed fiber-tip bubble-structure micro-cavity sensor. The proposed sensor is demonstrated with a relative low temperature sensitivity of about 2 pm/°C. Properties of the fiber-tip bubble-structure micro-cavitys with different sizes are investigated. The sensor has the advantages of lowcost, ease of fabrication and compact size, which make it a promising candidate for hydrostatic pressure sensing or transverse load sensing in harsh environments.