This work presents optimization analysis of the sensitivity to variations of the external refractive index (RI) of long-period fiber grating (LPFG) coated with a nano-overlay of diamond-like carbon (DLC) material. Through numerical simulations, we have shown that both the dual-resonance and mode transition phenomena can be simultaneously exploited to substantially increase the sensitivity to variations of the external RI. The tuning of the DLC layer thickness to displace the dual-resonance band into a more suitable region of the spectrum is also reported. To perform this analysis, we implemented a novel pseudo-heuristic simulation model based on a 4-layer step-index fiber layer model and coupled mode theory. The dispersion dependence on the DLC overlay thickness was modeled from experimental data. LPFG parameters were fitted to an experimental transmission spectrum. The simulation model and the obtain results provides guidance for the fabrication of the device.
The paper presents a modification of the refractive-index (RI) response of a photonic crystal fiber (PCF) based
intermodal interferometer using a thin plasma-deposited silicon nitride (SiNx) overlay with a high refractive index. We
show that the film overlay can effectively change the distribution of the cladding modes and thus tune the RI sensitivity
of the interferometer. Thanks to the nano-coating we were able to increase RI sensitivity eightfold in the range required
for biosensors (nD~1.33). Due to the extreme hardness of SiNx films and their excellent adhesion to the fiber surface, we
believe that after the deposition the device can still maintain its advantages, i.e. lack of degradation over time or with
temperature.
An analytical model for long-period grating manufactured using an electric-arc micromachining method is developed.
This model takes into account the resulting modulation of the refractive index and the diameter in both the core and the
cladding of the fiber structure. Based on the local-mode theory, the model introduces coupling coefficients and detuning
parameters that vary longitudinally. Comparisons are made with the standard model for long-period grating
manufactured using the UV micromachining method.
This paper presents a photonic crystal fiber (PCF) refractive index sensor. The sensor structure is quite simple. It is
composed of three segments of optical fibers spliced together. The multimode fibers with core diameter of 50 μm are
used for light input and output. The middle fiber is a short segment of PCF, ESM-12-01. Although it has some
advantages such as being able to operate in single mode for a large number of light wavelengths and has great
temperature stability, it also has a common drawback of the PCFs, that is, the tiny holes will collapse when they are
spliced. This paper makes use of this drawback to facilitate the generation of the surface plasmon resonance. The spliced
region of a PCF actually becomes a thin silica rod that is no longer a PCF or a traditional optical fiber. For this reason
once the light travels into this region it diverts in all possible directions. Thus, the splice acts as a mode converter that
converts the core modes of the multimode fiber into a set of the modes spreading into the PCF cladding. Among those
modes some are suitable for SPR excitation. The width and the depth of the output spectrum dip depend on the length of
the sensing part and the thickness and uniformity of the gold coating, and hence these parameters affect the properties of
the sensor. The developed sensor is compact in size, simple to fabricate, promising in performance, and has a potential
for practical applications.
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