We report a new distributed fiber optic sensing technique using optical carrier based microwave interferometry. The concept has been demonstrated using different types of optical fibers including singlemode fiber, multimode fiber, single crystal sapphire fiber and polymer fiber. Using the microwave-photonic technique, many fiber interferometers with the same or different optical path differences were interrogated and their locations could be unambiguously determined. The distributed sensing capability was demonstrated using cascaded low-finesse Fabry-Perot interferometers fabricated by fs laser micromachining. Spatially continuous, fully distributed temperature and strain measurements were used as examples to demonstrate the capability of the proposed concept.
A multimode fiber (MMF) based cascaded intrinsic Fabry-Perot interferometers (IFPIs) system is presented and the distributed strain sensing has been experimentally demonstrated by using such system. The proposed 13 cascaded IFPIs have been formed by 14 cascaded reflectors that have been fabricated on a grade index MMF. Each reflector has been made by drawing a line on the center of the cross-section of the MMF through a femtosecond laser. The distance between any two adjacent reflectors is around 100 cm. The optical carrier based microwave interferometry (OCMI) technique has been used to interrogate the MMF based cascaded FPIs system by reading the optical interference information in the microwave domain. The location along with the shift of the interference fringe pattern for each FPI can be resolved though signal processing based on the microwave domain information. The multimode interference showed very little influence to the microwave domain signals. By using such system the strain of 10-4 for each FPI sensor and the spatial resolution of less than 5 cm for the system can be easily achieved.
This paper reports the stress-induced birefringence generated in an optical fiber using femtosecond laser (fs) irradiations and the fabrication of in-fiber waveplates and polarizers. Optical birefringence was created in a single-mode fiber by introducing a series of symmetric cuboid stress rods on both sides of the fiber core and along the fiber axis using a femtosecond laser. The stress-induced birefringence was estimated to be 2.4×10-4 at the optical wavelength of 1550 nm. By controlling the length of the stress rods, waveplates of the desired retardance can be fabricated. The stress-induced birefringence was further explored to fabricate in-fiber polarizers based on the polarization-dependent long-period fiber grating (LPFG) structure. For the in-fiber polarizer based on low order mode LPFG, a polarization extinction ratio of more than 25 dB was observed at the wavelength of 1527.8 nm. A high order mode LPFG based in-fiber polarizer, with a broad bandwidth of 100 nm near 1550 nm, was investigated as well. The in-fiber polarization devices with low insertion loss may be useful in optical communications and fiber optic sensing applications.
A U-shaped optical fiber inline microchannel was fabricated by femtosecond laser irradiation and subsequent selective
chemical wet etching. A high quality micro-cavity embedded inside the channel was obtained to construct a Fabry-Perot
interferometer (FPI). A fringe visibility of 20 dB in spectrum domain was achieved. High temperature survivability of
this micro device was also demonstrated. The proposed assembly-free optical fiber inline interferometer is attractive for
sensing applications in high-temperature harsh environments.
We report a method to fabricate SERS fiber probe by femtosecond laser micromachining on a microfiber tip. Multimode optical fibers are tapered to outer diameter of 4 to 20 μm. Femtosecond laser pulses are used to form nanostructures on the cleaved endface of the microfiber tip. This endface is then activated by silver sputter coating. To confine the excitation and reflection signal within the tapered core of the microfiber tip, the probe is side-coated with silver plating. High quality SERS signal of Rhodamine 6G molecules is detected through back excitation and collection from a lead in fiber of up to several meters long. The small size of the SERS micro-probe is promising for intra-cellular or even single cell detection, while the back excitation and collection setup make it suitable for remote sensing applications.
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