A hollow optical fiber (HOF) has an unique modal distribution of a central evanescent field due to its structure.
The HOF consists of a central air hole, a Ge-doped ring core placed at the inmost layer, and silica cladding,
which induces the weak evanescent field at the central hole. By the structure with geometric symmetry, it is
possible to inject a refractive fluid into the hole and to modify the modal distribution. When a refractive index of
the fluid is same with or higher than the core's, guiding of light becomes dominant at the center and the ringshaped
field turns into a LP01 mode. During the process, optical force is induced and the net momentum of the
fluid is changed. The direction of optical force is opposite to that of light propagation, and the fluid come to be
dragged along the central channel in the HOF. In order to further investigate the phenomenon, we have changed
the refractive index of the fluid and measured resultant optical force. The direction and strength of the optical
force was dependent on the refractive index of the central fluid, which shows ample potential of the HOF as a
refractive index sensor.
In this paper, we experimentally demonstrate the potential of quasi-distributed high temperature sensor based on fiber
Bragg grating (FBG) utilizing high thermal conductive sheath, which can be a cost-effective alternative for conventional
distributed temperature sensors based on Raman, Brillouin, and Rayleigh scattering. A unique Fire Sensing Cable (FSC)
used in this experiment is constructed from a 304 stainless steel sheath with 16 optical fibers imbedded in a conductive
fluid. One of the fibers contains FBGs for temperature sensing. Total of seventy seven FBGs were serially inscribed with
the spacing of six meter over the total length of 468 meter. FSC was heated by various hot zones formed by IR furnace
and nitrogen heat nozzle, as the shifts of FBGs were monitored. Although FBGs were 6 meter apart each other, high
thermal conductivity of the stainless steal sheath made it possible to check temperature change in the region between
gratings. These preliminary results clearly show a high potential of FBGs combined with FSC in applications of quasi-distributed
fire sensing cables and monitoring systems.
We propose and experimentally demonstrate an effective method to reduce far-field speckle noise in multimode fiber
with a short cylindrical piezoelectric transducer (PZT) vibrating in radial direction. In this study, the fiber was coiled as
tightly as possible around the mandrel of the PZT and periodic stretching effect was caused by the radial oscillations of
the actuator. This technique can be adapted at a high modulation frequency, so the speckle patterns can be time-averaged.
The output of the optical fiber was intensively observed by a CCD camera. By counting all the pixels
corresponding to relative intensity graded 256 levels in selected area and by calculating the mean value and standard
deviation of the intensity, we can measure the speckle contrast and vibration effect in quantitative measurands. It was
clearly observed that the characteristics of the speckle pattern in vibration-ON-state were signinficantly reduced than that
of vibration-OFF-state by comparing the proposed measurands as well as direct CCD images. We expect that the
proposed speckle reduction technology would find viable applications in realization of fiber laser, laser marking, optical
trapping and projection display systems.
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