Femtosecond direct laser writing has recently shown great potential for the fabrication of complex integrated devices in the cladding of optical fibers. Such devices have the advantage of requiring no bulk optical components and no breaks in the fiber path, thus reducing the need for complicated alignment, eliminating contamination, and increasing stability. This technology has already found applications using combinations of Bragg gratings, interferometers, and couplers for the fabrication of optical filters, sensors, and power monitors. The femtosecond laser writing method produces a local modification of refractive index through non-linear absorption of the ultrafast laser pulses inside the dielectric material of both the core and cladding of the fiber. However, fiber geometries that incorporate air or hollow structures, such as photonic crystal fibers (PCFs), still present a challenge since the index modification regions created by the writing process cannot be generated in the hollow regions of the fiber. In this work, the femtosecond laser method is used together with a pre-modification method that consists of partially collapsing the hollow holes using an electrical arc discharge. The partial collapse of the photonic band gap structure provides a path for femtosecond laser written waveguides to couple light from the core to the edge of the fiber for in-line power monitoring. This novel approach is expected to have applications in other specialty fibers such as suspended core fibers and can open the way for the integration of complex devices and facilitate miniaturization of optical circuits to take advantage of the particular characteristics of the PCFs.
The initial results from continuing long-term monitoring of a 67 km of an aerial fiber optic cable installed on a 500 kV power line cable (total fiber length of 134km) using BOTDA are presented. The effects of thunderstorms and rime ice on the cable were identified by monitoring strain on OPGW fibers. Variations of strain between day and night on the OPGW cable were observed and can potentially be exploited.
We present 100-μm crack detection and 100-km distributed strain and temperature sensing by fiber-optic distributed
strain and temperature sensor (DSTS) based on coherent interaction of probe-pump. The DSTS products have been
employed to detect cracks on ceramic by measuring the strain distributions along the surface of the ceramic and a
continuing long-term field monitoring of local temperature and stress changes in a 70-km buried fiber-optic cable.
Distributed Brillouin scattering sensor system was employed to measure the hoop strain in an internally pressurized steel
pipe with wall thinning due to erosion. The difference of Brillouin frequency shift from regions that have different
degree of wall thickness lost can be observed on strain distribution. The strain from thin wall region is higher than that
from the thick wall region. Therefore, the inner wall thinning can be discriminated from the corresponding strain
measurements, which shows that fiber optic sensor technology based on distributed Brillouin scattering offers the great
potential as a "nervous system" for infrastructure elements that allow high performance, cost effective health and
damage assessment systems to be achieved.
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