A novel Mach-Zehnder interferometer (MZI) based on a pair of long period fiber gratings (LPGs) in an embedded-core hollow optical fiber (ECOF) are proposed and demonstrated experimentally. Two similar LPGs with peak attenuation of approximately 3dB are fabricated in an ECOF by using high-frequency CO2 laser to form an in-fiber MZI. The dependences of the resonant peak on the temperature and the axial strain were studied. Experimental results show that sensitivities of the temperature and the axial strain are 64.3 pm/°С and 0.4 pm/με, respectively.
In the present work, a compact all-fiber plasmonic focusing beam generator with single and multiple spots is proposed and demonstrated in a conventional multimode fiber. Here, the focusing beam generator is composed of air slit arrays perforated through the gold films deposited on the end facet of a multimode fiber. The array of nanoscale slits with varying widths is used to modulate phase distribution of the focused light. An all-fiber focusing beam generator provides many advantages, such as self-alignment, high flexibility, lower insert loss, and easy portability, which is of importance to realize optical trapping, micromanipulation, beam shaping, and fiber integrated devices.
A linear 5-core fiber was sandwiched in between two single mode fibers (SMF) to construct an all fiber Mach-Zehnder interferometer (MZI). The interferences between the fundamental supermodes, between the fundamental supermodes and the high order core modes, and between the fundamental supermodes and the low order cladding mode are investigated. The experimental results show both the interference between the core modes and the interference between the core modes and the cladding modes have approximately equal temperature sensitivity. The interference between the core modes is insensitive to the axial strain.
A novel twin-core fiber connector has been made by two side-polished fibers. By using side polishing technique, we present a connector based on the twin-core fiber (TCF) and two D-shaped single-core fibers. After simple alignment and splicing, all fiber miniaturizing connector can be obtained. Two cores can operate independently and are non-interfering. The coupling loss of this connector is low and the fabrication technologies are mature. The connector device could be used for sensors or particle trapping.
In this paper, we analyzed the mode field distributions of the seven-core fiber consisting of a central core and six symmetrically surrounding single mode cores in a common cladding by the finite element method (FEM). The excitation characteristics in the splicing process between the seven-core fibers were numerically investigated. The efficiency of the fundamental mode excited by the incident light was studied by calculating the excitation coefficients in two cases, in which a lateral offset and a rotation offset are introduced between the two fiber ends, respectively. The results show that the excitation coefficient of the fundamental mode decreases from 1.0 to 0.1816 when the lateral offset increases from 0 to 9 μm. Similarly, when the rotation offset increases gradually from 0º to 10º, the excitation coefficient of the fundamental mode in the six surrounding cores gradually and synchronously decreases from 1.0 to 0.1779, while that of the central core is always constant.
A novel Bragg fiber grating (FBG) in an embedded-core hollow optical fiber (ECHOF) has been proposed and experimentally demonstrated. The high-quality FBG fabricated with phase-mask technique by using 248 nm ultraviolet laser, has a resonant wavelength of ~943.1 nm and a dip of ~24.2 dB. Subsequently, the dependences of the resonant peak on the temperature and the axial strain were studied. Experimental results show that the temperature and axial stain sensitivity are 6.5 pm/°С and 1.1 pm/με, respectively. In addition, a 0.03 nm shift of the transmission dip can be obtained when the polarization state changes from X polarization to Y polarization.
The bending characteristics of the long period fiber grating (LPFG) in hollow eccentric optical fiber (HEOF) were investigated experimentally. The results indicate that the HEOF-LPFG is insensitive to the bending and the biggest sensitivity only is 1.3nm/m-1 in the range of 0~5m-1. More than that, the dependences of the resonant peak on the temperature and the axial strain were also studied, obtaining the response of 56.7 pm/°С and 0.3 pm/με, respectively. Obviously, the HEOF-LPFG is more sensitive to the temperature and immune to the curvature and the strain. The HEOF-LPFG can be employed to measure the single parameter and simplify the measurement equipment in the practical application.
A simple structure of all-solid microstructured fiber, which is composed of 8 linear arrays of high refractive index dielectric rods radially symmetry arranged around a low refractive index substrate, is proposed and numerically investigated. The calculated results show that the space between the adjacent band gaps is very small, minimum loss of 0.35 dB/km can be achieved. With increasing the distance between high refractive index rods and the refractive index difference between the high refractive index rod and the core, the band gap will move to the long wavelength. With decreasing the bending radius, the short wavelength edge of PBG is significantly affected and shifts to the long wavelength, while long wavelength edge of PBG suffers relatively little impact. By adjusting the layer number of high refractive index dielectric rods, we can effectively change the effective mode area, resulting in large mode area fiber.
Compact all-fiber plasmonic Airy-like beam generator is demonstrated. A single slit and a 1D groove array were fabricated by focused ion beam (FIB) milling on the end facet of a single mode optical fiber. The single slit excites the surface plasmonic polaritons (SPPs), which are decoupled into free space by the groove array. The phase of decoupling SPPs is adjusted by the grooves position. Experimental generation of the single Airy-like beam has good consistency with theoretical predictions. The transverse acceleration and nondiffraction properties are observed. The presented plasmonic Airy-like beam generator is of importance to realize all-fiber optical trapping, beam shaping, and fiber integrated devices.
We propose and experimentally demonstrate a novel long period fiber grating (LPFG) in an embedded-core hollow optical fiber (ECHOF). Without the structural deformation of the air hole and the fiber core, the high-quality LPFG can be fabricated within a few scanning cycles by a high-frequency CO2 laser with a low energy density of 0.896J/mm2. The ECHOF LPFG reveals a high temperature sensitivity of 50.2 pm/°C and a low strain sensitivity of 0.4 pm/με. Due to the good performance and easy fabrication, the ECHOF LPFG will be important to develop novel in-fiber devices.
A technique to enhance the response of Brillouin distributed sensors is proposed and experimentally validated. The method consists in creating a multi-frequency pump pulse interacting with a multi-frequency continuous-wave probe. The power of each pulse at a distinct frequency is lower than the threshold for nonlinear effects, while the sensor response remains given by the total power of all pulses. Distinct frequency pulses are delayed to avoid temporal overlapping and cross-interaction; this requires to smartly reconstruct the traces before photo-detection. The method is validated in a 50 km-long sensor using 3 frequencies, demonstrating a signal-to-noise ratio enhancement of 4.8 dB.
A focused CO2 laser beam has been previously used to successfully fabricate both symmetric and asymmetric long period fiber gratings which have been used for a variety of sensing applications. However fabrication by a CO2 laser beam demands a time consuming laser scanning process which increases the difficulty and cost of fabrication. In this paper a fibre sensor based on a fibre heterostructure with a simple configuration consisting of a series of periodical tapers in a photonic crystal fibre (PCF) sandwiched between two singlemode fibres is proposed and investigated experimentally. The tapers are periodically fabricated along the PCF section using a CO2 laser beam. The proposed fibre heterostructure can be used for strain sensing by measuring the wavelength blueshift of the multimode interference dip of the transmission spectrum as a function of strain. An average stain sensitivity of -68.4 pm/μ ε has been experimentally achieved over a microstrain range from 0 to 100 μ ε. Assuming in practice that the sensor is interrrogated with a ratiometric power measurement system, then the strain resolution is estimated to be better than 1.18×10-2 microstrain. The mechanisms for refractive index modulation periodically tapered PCF under tensile strain measurements are complex but may be regarded as a combination of stress-relaxation and refractive index perturbations over the length of the tapered PCF induced by strain and by tapering. The proposed fibre strain sensor has the advantage of low temperature sensitivity (average 8.4 pm/°C) and an experimental demonstration of this reduced sensitivity is also presented. The proposed strain sensor benefits from simplicity of fabrication and achieves a competitive sensitivity compared with other existing fibre-optic sensors.
A CO2-induced LPG with an over-coupled resonant dip is fabricated. The bending characteristics of proposed LPG are experimentally studied in detail. The results show that the spacing between the two resonant wavelengths has a periodic behavior along circular directions. The spacing between the two resonant wavelengths changes nearly linearly against the curvature under certain bending directions. The bend sensitivities under two bending directions are 7.19nm/m and 3.13nm/m, respectively. In addition, the over-coupled dip splits into two dips when increases curvature under a special bending direction, which may be attributed to that new cladding mode meet coupling condition is excited.
A novel surface plasmon resonance (SPR) sensor based on the multihole optical fiber with TiO2 layer is proposed and numerically characterized. The finite element method(FEM) is used to analyze the characteristics of the SPR sensor. The effects of the pitch between air holes, the thicknesses of the gold film and titanium dioxide layer on the sensor characteristics are investigated, and the sensitivity of proposed sensor is also given. The results indicate that when the thickness of gold film increases, the resonance wavelength shifts to longer wavelength, and the resonance peak broadens. The spectral tuning of the plasmon resonance can be more efficiently realized by changing the TiO2 layer thickness. The maximal refractive-index resolution of the proposed sensor for aqueous analytes is 5×10-5.
We propose a novel embedded-core hollow optical fiber composed of a central circular air hole and a semi-elliptical core
embedded in an annular cladding. Both the phase birefringence and group birefringence are investigated based on the
finite element method (FEM). The embedded-core hollow optical fiber has polarization-preserving properties. The
birefringence magnitude of the proposed fibers is the same order as that of the side-hole optical fiber. The theoretical
results reveal that the birefringence of embedded-core hollow optical fibers can reach the order of 10-4. The group
birefringence of the fiber is obtained by using the wavelength scanning technique and can reach 4.7×10-5. The
measurement results are basically consistent with the theoretical simulations.
A Michelson interferometer based on near-surface-core fibers (NSCF) was demonstrated and applied to measure the
refractive index (RI) of NaCl solution. The reference arm and the sensing arm of the interferometer are constituted by a
single mode fiber and a chemical-etching near-surface-core fiber, respectively. Two different samples were tested. The
wavelength shift of the interference spectrum with the variation of the environmental refractive index was investigated.
The results show that the proposed sensor has a measurement resolution up to 120nm/RIU in the range of 1.333-1.3684.
Furthermore, the resolution can be increased by adjusting the parameters of the NSCF reasonably.
We study local field energy density enhancement in planar metamaterials at normal incidence based on the finite element
method. The microwave metamaterials composed of asymmetric resonators with/without quartz substrates are utilized to
investigate the resonant response to incident electromagnetic waves. The trapped-mode resonant feature results from the
excitation of an antisymmetric current mode due to the broken symmetry between two resonators and the quality factor
and local field energy density enhancement strongly depend on the asymmetry. The proposed metamaterial on glass
substrate shows the high possible quality factor of about 1000 and energy density enhance factor of up to 150000. To
reduce losses of metamaterials further, freestanding metallic structure is considered being treated as perfect electric
conductor and real-loss metal respectively. Real metallic metamaterial provides a very sharp trapped-mode resonance
having the quality factor of up to 1500.
The mode field characteristics of eccentric optical fibers are investigated theoretically. The eccentric optical fiber is
converted to be a concentric three-layered optical fiber through the conformal representation and the corresponding field
distribution is given by solving eigenvalue equation of a general three-layered optical fiber waveguide. The method
should be proved a simple solution for optimizing the configurations of special optical fibers.
A optical fiber with hole-assisted structure is proposed to construct the surface plasmon resonance (SPR) refractive index
sensor. The finite element method is utilized to analyze characteristics of the surface plasmon resonance sensor. The
effects of the thickness of metal films, fiber core size, and refractive index of liquid on the resonance wavelength are
investigated. The sensitivity of sensor is also given. The result shows that the resonance wavelength is sensitive to the
thickness of metal film and refractive index of liquid, while the resonance wavelength doesn't change basically when the
fiber core size. The proposed surface plasmon resonance sensor exhibits high sensitivity up to 10-4.
In the present paper, a novel ultrahigh birefringent index-guided triangular-arrayed photonic crystal fiber with two small
elliptical air holes in the core is proposed, which guides light by total internal reflection (TIR). The birefringence of
PCFs is investigated using full-vector finite-element method. The impacts of the geometrical parameters and position of
the elliptical air holes on the properties of birefringence are discussed. Our suggested structures show that the
birefringence can be as high as 1.5x10-2 at 1.55μm wavelength.
The modes in annular core fibers are presented based on the vector finite element method and the beam propagation
method. The biconical tapered coupling characteristics between the conventional single mode fiber and the annular core
fiber are investigated. The effects of parameters on the coupling characteristic are discussed, including the biconical
taper shape and the relative deviation of the single core fiber to the annular core fiber in the splicing process.
We present an abruptly tapered twin-core fiber optical tweezers, which is fabricated by fusing and drawing the twin-core
fiber (TCF). The two beams guided by the TCF, and a larger converge angle between the two beams are made due to the
abrupt tapered shape. The two beams converged at the micro-lensed tip, then forming a fast divergent optical field. The
microscopic particle trapping performance of this special designed tapered TCF tip is investigated. The distribution of
the optical field emerging from the tapered fiber tip is simulated based on the beam propagation method (BPM). By
using this two-beam combined technique, a strong enough gradient forces well is obtained for microscopic particles
trapping in three-dimensional. The abruptly tapered TCF optical tweezers is rigid and easy to handle, especially useful
for build-up a multi-tweezers system for trapping and manipulating micro-scale particles.
By using spherical designed three-core fiber, a micro structured light pattern generator for sensing of 3-D object shapes
has been demonstrated. The square or hexagon grid interferometric fringe pattern formed by the fiber optic
interferometric grid generator is projected on an object's surface. The deformed grid pattern containing information of
the object's surface topography is captured by a CCD camera and is analyzed using a 2-D Fourier transforming
profilometry. The use of fiber optic grid interferogram technique greatly simplifies the holographic interferometry system
and the carrier grid interferogram can be conveniently generated without the use of excessive auxiliary components or
sophisticated experimental devices, and moreover, the three-core fiber can be used in very narrow places due to its small
size. Finally, the square or hexagon grid interferometric fringe pattern provides a data fusion ability, which could further
improve the accuracy of the 3-D shape sensing results.
In the present paper, the two-dimension photonic crystal (PC) polarization coupler is mainly studied. The PC
waveguide coupler with a single row of dielectric rods in the interaction region is composed of square-arrayed dielectric
pillars in air. The photonic band gap maps are calculated by using plane wave expansion (PWE) method and the effects
of the geometrical parameters of the scatterers between two coupling channels on the property of the PC couplers are
analyzed by using the finite-difference time-domain (FDTD) method. The simulation results show that the coupling
property is sensitive to the shape variation of scatterers. The shape variation of scatterers between the two coupling
channels will reduce coupling length greatly.
The light transmission characteristics in the total reflection photonic crystal fiber used in the liquid fiber sensors are
investigated by using the finite element method. The double annular structure is analyzed in photonic crystal fibers,
which was injected by the liquid. On the basis of providing theoretical model, the mode field distributions of PCF with
different parameters are given, and then the influences of parameters on the mode area and energy distribution are
discussed. The results indicate that when PCF is injected by the liquid, the light field experiences changing from
diffusion to concentration. For the concave liquid surface, the energy distribution centralizes gradually with the increase
of the inner ring radius, and the inherent property of liquid also influences the light transmission characteristic in PCF.
Two-dimension photonic crystals coupler is studied in present paper. The photonic band gap structure and band gap map
are calculated on the basis of plane wave expansion method. The finite-difference time-domain (FDTD) method is
applied for analyzing transmission and coupling characteristic of photonic crystals coupler with the square lattice. The
effects of coupling length and waveguide-channel space on coupling ratio are discussed. Further, the field distribution is
given in this paper. The results indicate that photonic crystal can accomplish coupler function. The performance of the
coupler is more sensitive to coupling length than waveguide-channel space. The phenomenon between coupler branches
will disappear gradually when the waveguide-channel space increases. In terms of appropriate coupling length and
waveguide-channel space, the coupler with any coupling ratio can be achieved.
Symmetric air waveguide and antisymmetric air waveguide made with left-handed material (LHM) cladding are
constructed. Guided modes in the two kinds of air waveguides are studied on condition that the permittivity and
permeability of LHM are simultaneously negative. When absolute value of refraction of cladding is less than that of air,
fast modes and slow modes are possible to exist at the same time; when absolute value of refraction of cladding is more
than that of air, only slow modes could exist.
Fiber optical Moire interferometer is a new technology in configuration or deformation measurement. It is a potential powerful microstructure mapping technique, which is important in fine processing, product monitoring, NDT, etc. Comparing with the traditional measurement methods, the optical Moire interferometer has the advantage as flexible, scale-controllable, etc.
Some configurations of the fiber optic Moire interferometer are described in this paper. The ends of several polarization maintaining fibers set together under special design. They work as point optical sources to produce fringe patterns. The appearance and the trend of the fringes indicate the information of the shape or deformation of an object.
Both the simplest design, only two fibers are used as the illumination source, and the complex designs, three or four fibers are put together under different distribution, are discussed in this paper. The more fibers are introduced, the more complex is the fringe pattern with more information. A series of numerical simulations have been done to compare with the result of the experiments.