We propose a physically asymmetric thin film structure to support long-range surface plasmons (LRSPs), which consists
of a low index medium on a metal film on a dielectric layer (membrane) over air, as a suspended waveguide. An analytic
formulation is derived in 1D geometry yielding a transcendental equation that ensures symmetry of the transverse fields
of the LRSP within the metal film by properly configuring the thicknesses of the metal film and the membrane. The
theoretic results from the formulation agree with the transfer matrix calculations quantitatively for a candidate slab
waveguide consisting of an H<sub>2</sub>O-Au-SiO<sub>2</sub>-air structure. These results are promising for sensors that operate with an
aqueous solution that would otherwise require a low refractive index substrate to achieve LRSP guiding.
Intensity-referenced and temperature-independent high accuracy displacement measurement is demonstrated by using a
Gaussian-chirped tilted fiber Bragg grating (TFBG). A specially designed bending cantilever beam is used to provide a
displacement-induced Gaussian-strain-gradient and internal tilt angle modulation along the sensing TFBG. Compared
with a straight FBG, the peak intensity of the Bragg resonance in the TFBG shows an improved sensitivity to the
displacement but is immune from the spatially uniform temperature changes. A displacement resolution up to 1×10<sup>-3</sup> mm
and thermal fluctuation less than 2% full-scale for a temperature range of 0~80°C are experimentally achieved.
We propose and experimentally demonstrate the feasibility high refractive indices measurement (larger than the fiber
cladding index) by using individual radiation mode resonances of tilted fiber Bragg gratings. By tracking a single
cladding mode relative to the Bragg resonance, improved sensitivity for refractive index measurement has been achieved
and the temperature fluctuations could be eliminated. The maximum index sensitivity is 5×10<sup>-4</sup> with a detected power
resolution of 0.1dB.
Fiber Bragg gratings with strong resonance peaks for both Bragg and cladding modes are made in photonic crystal fiber
modified with a germanium doped core. Experimental results for strain, temperature and refractive index sensitivities of
fiber Bragg gratings are reported. We show the existence of an inner cladding mode that has the largest coupling from
the core mode and that is insensitive to surrounding index changes. The core mode, inner cladding mode and outer
cladding modes all have the same temperature sensitivity. By tracking the cladding mode resonances shifts relative to
the core mode, a temperature and index insensitive strain sensor can be made.
Weakly tilted fiber Bragg gratings with gratings planes tilted at small angles with respect to the fiber axis couple light to
backward going core mode and cladding modes. Their transmitted spectrum is characterized by narrow resonance dips
below the Bragg wavelength corresponding to the core mode coupling. The amplitude spectral evolutions of weakly
tilted fiber Bragg gratings in response to diverse physical perturbations such as temperature, mechanical strains, bending
and surrounding refractive index changes are presented. Different techniques allowing to efficiently correlate the spectral
evolution with the information to be measured are reported. We demonstrate that a selective monitoring of one cladding
mode shift with respect to the Bragg wavelength gives temperature-insensitive strain measurements whereas a global
monitoring of the cladding modes spectrum offers temperature-insensitive surrounding refractive index measurements.
We also point out the possibility of using this global monitoring for bending and transverse strain sensing purposes.
Finally, we present the effect of coating (the grating is covered by a polymer) on the sensitivity of weakly tilted fiber
Bragg grating to surrounding refractive index changes. For every application, the performances of weakly tilted fiber
Bragg gratings sensors are discussed.
In this paper, the hybrid modes are used to analyze the cladding mode resonances of TFBGs and compared with those
using the linear polarization modes. As the TFBG spectrum span is larger than 30nm, the material dispersion effect can
not be ignored and must be considered for accurately analyzing the cladding mode resonance peaks. Both analysis and
experimental results will be presented, and they are well matched. By accurately analyzing the higher order cladding
mode resonances with double or triple peaks, the tracking of higher order cladding mode resonances is facilitated, and
these modes have the largest differential strain and index sensitivities relative to the core modes.
A new Surface Plasmon Resonance (SPR) sensor design is proposed and fabricated based on an optical fiber with a photo-written Tilted Fiber Bragg Grating (TFBG) and a thin gold deposited layer. The TFBG allows the transfer of light from the core mode into a multitude of cladding modes, each wavelength corresponding to a different incidence angle. The most pronounced SPR effect was obtained for a gold thickness of 20 nm, however every tested thickness showed SPR at a certain level. To characterize the uniformity of gold films, the coated fiber were imaged using Atomic Force Microscopy (AFM) and showed a high level of graininess, as expected from such thin layers. Scanning Electron Microscope (SEM) images were used to characterize the quality of the gold coating before and after experiments. Despite the high non-uniformity and graininess of gold coating, the angular spread of SPR is as narrow as expected from theory. The sensitivity obtained reaches 454 nm per refractive index unit.
Fiber Bragg gratings (FBG) are one of several fiber optic sensor technologies currently being used in structural
health monitoring systems. When the effective refractive index of a fiber Bragg grating is changed by external
environmental variations (e.g. temperature, pH), the wavelength at which incident light experiences a maximum
reflection from the grating will correspondingly shift. To detect small environmental variations that occur during certain
chemical processes, one can enhance the sensitivity using either side-polished or tilted fiber Bragg gratings. Enhanced
sensitivity in each case is achieved by polishing the fiber on one side or writing the grating at some tilt angle. Side
polished FBG sensors having a 1542 nm Bragg wavelength and cladding thickness values from 1-3 &mgr;m provide a
maximum refractive index sensitivity of 7×10-4. Tilted FBG sensors having a 1566 nm Bragg wavelength and written
with a 4° degree tilt angle provide a maximum refractive index sensitivity of 5×10<sup>-5</sup>. Experiments on the tilted gratings
were done using 50, 80, 125 &mgr;m diameter fibers immersed in solutions in the index range 1.31-1.44. Since tilted FBGs
have enhanced sensitivity and the advantage of maintaining their full mechanical strength, they show greater promise as
reliable sensors for structural health monitoring applications.
In this paper, we present the differential strain sensitivity characteristics of core and cladding modes in weakly TFBG. Both experiment and analysis results are presented, and they are well matched. The results show that there are three different strain sensitivity regions for cladding mode resonances: the short wavelength region, the ghost mode region and the nearly linear sensitivity change region between them. By monitoring the cladding modes with different strain sensitivities, and noting that the different cladding modes have similar temperature sensitivities, weakly TFBG are attractive candidates for more accurate temperature-independent strain sensors.
Fiber Bragg gratings with grating planes tilted at small angles relative to the fiber axis couple light both to backward propagating core modes and cladding modes. The resonant wavelengths for these mode couplings depend differentially on external perturbations. Using the core mode back reflection resonance as a reference wavelength, the relative shift of the cladding mode resonances can be used to selectively measure perturbations affecting the region outside the cladding independently of temperature. We have measured a relative wavelength shift lower than 0.4 pm/degree in conventional single mode fiber while the sensitivity to external changes in refractive index can be larger than 300 pm per % index change. Experimental results on the bending selective sensitivity (relative to uniform axial strain) are also reported.
This paper presents photo-induced signal taps for power monitors in planar lightwave circuits. Both periodic perturbation and non-periodic perturbation structures are discussed. The numerical models based on the volume current method are used to simulate the interaction of guided light with photo-induced periodic and non-periodic perturbations. The results are compared with those from a commercially available FDTD simulation tool. The optimized designs for
the minimized polarization dependent loss taps are presented. Contrary to the power monitors currently used in planar lightwave circuits, which are based on directional coupler and deep etch technology, we propose the signal taps through photosensitive processes using laser light. Our approach is more cost efficient, flexible and reliable.