Simultaneous measurement of temperature and salinity based on a hole-assisted dual-core fiber.

An optical fiber sensor based on a hole-assisted dual-core fiber (HADCF) has been proposed and experimentally demonstrated for dual-parameter measurements. The dual-mode interferometer created uses the LP01 mode and LP11 mode in the suspended core of a specialist optical fiber, combined with a directional coupler formed by using the suspended core and the center core in a 16 mm long HADCF. Using this, the simultaneous measurement of salinity (due to the presence of NaCl) and temperature has been achieved through monitoring the interference dip and resonance dip. The sensitivities of the measurement of salinity and temperature are 190.7 pm/‰ and -188.2 pm/°C, respectively. The sensor developed has the advantages of simplicity of fabrication, a high level of integration and the potential for measurement of dual parameters, supporting its potential applications in marine environment measurements.

All-optical vector magnetic field sensor based on a side-polished two-core fiber Michelson interferometer.

A magnetic field sensor based on a side-polished two-core fiber (SPTCF)-based Michelson interferometer (MI) has been developed and demonstrated. The magnetic field sensor is composed of a standard single mode fiber (SMF) and a section of tapered TCF. By side-polishing a segment of the TCF, the effective index of the exposed core can be made sensitive to the environmental refractive index (RI). To evaluate its performance, a magnetic fluid is used to cover the polished region with a magnetic field sensitive material, where the sensor then measures the magnetic field intensity by sensing the RI change of the magnetic fluid through the evanescent field in the polished core. The SPTCF MI device developed allows for vector magnetic field sensing because of its asymmetric structure, with its highest directional sensitivity being 55.2 pm/degree. Experimental results obtained show that when the magnetic field is parallel to the side-polished plane, a sensitivity of 1.262 nm/mT can be achieved, operating over the magnetic flux density region of 0-5 mT and over a temperature range of 20∼85 °C, where the device is minimally affected by temperature changes. The sensor is well suited to a variety of potential applications given its low cost, strong anti-interference ability, simple structure and high stability.

Characterization of a fast response fiber-optic pH sensor and illustration in a biological application.

Optical, and especially fiber-optic techniques for the sensing of pH have become very attractive and considerable research progress in this field has been made over recent years. The determination of the value of pH across a broad range of applications today, important for areas of study such as life sciences, environmental monitoring, manufacturing industry and widely in biological research is now accessible from such optical sensors. The need for such technology arises because familiar, commercial sensors are often limited in terms of their response time and the presence of drift, all of which emphasize the value of newer and rapidly developing technologies such as fiber-optic sensors, to address these wider applications. As a result, a new compact sensor design has been developed, designed around a specially-formed fiber-optic tip, coated with a pH-sensitive dye, and importantly covalently linked to a hydrogel matrix to provide high stability. The sensor developed was designed to have a very fast response time (to 90% of saturation, Δt90) of <5 s and a sensing uncertainty of ∼±0.04 pH units. Given the covalently bonded nature of the dye, the problem of leaching of the indicator dye is reduced, creating a probe which has been shown to be very stable over many days of use. Illustrating this through extended continuous use, over ∼12 h at pH 7, this stability was confirmed showing a drift of <0.05 pH h-1. In order to give an illustration of the value of the probe in an important biological application, the monitoring of pH levels between pH 7 to pH 8 in an AMES' medium, a substance which is important to maintain the metabolism of retinal cells is shown and the results as well as temperature stability of the probe discussed.

Characteristics of silicon nanowire solar cells with a crescent nanohole.

In recent years, newly emerging photovoltaic (PV) devices based on silicon nanowire solar cells (SiNW-SCs) have attracted considerable research attention. This is due to their efficient light-trapping capability and large carrier transportation and collection with compact size. However, there is a strong desire to find effective strategies to provide high and wideband optical absorption. In this paper, a modified circular nanowire (NW) with a nanocrescent hole is newly introduced and analyzed for solar cell applications. The crescent hole can strongly improve the light absorption through the NW due to the excitation of numbers of modes that can be coupled with the incident light. The material index, volume, and position of the nanohole are studied to significantly increase the optical absorption efficiency and hence the power conversion efficiency (PCE). The absorption performance can be further preserved by using a silicon substrate due to the coupling between the supported modes by the NW, and that of the substrate. The optical and electrical characteristics of the suggested design are investigated using finite difference time domain and finite element methods via Lumerical software packages. The reported asymmetric design offers higher optical and electrical efficiencies compared to the conventional NW counterpart. The proposed NW offers a short circuit current density (Jsc) of 33.85 (34.35) mA/cm2 and power conversion efficiency (PCE) of 16.78 (17.05) % with an enhancement of 16.3 (16.8) % and 17.3 (18.4) % for transverse magnetic (TM) and transverse electric (TE) polarizations, respectively, compared to the conventional cylindrical counterpart.

Multifunction interferometry using the electron mobility visibility and mean free path relationship.

A conventional Michelson interferometer is modified and used to form the various types of interferometers. The basic system consists of a conventional Michelson interferometer with silicon-graphene-gold embedded between layers on the ports. When light from the monochromatic source is input into the system via the input port (silicon waveguide), the change in optical path difference (OPD) of light traveling in the stacked layers introduces the change in the optical phase, which affects to the electron mean free path within the gold layer, induces the change in the overall electron mobility can be seen by the interferometer output visibility. Further plasmonic waves are introduced on the graphene thin film and the electron mobility occurred within the gold layer, in which the light-electron energy conversion in terms of the electron mobility can be observed, the gold layer length is 100 nm. The measurement resolution in terms of the OPD of ∼ 50 nm is achieved. In applications, the outputs of the drop port device of the modified Michelson interferometer can be arranged by the different detectors, where the polarized light outputs, the photon outputs, the electron spin outputs can be obtained by the interference fringe visibility, mobility visibility and the spin up-down splitting output energies. The modified Michelson interferometer theory and the detection schemes are given in details.

Rigorous analysis of acoustic modes in low and high index contrast silica fibers.

The vector acoustic modes in both well-established and emerging designs of optical waveguides have been studied through use of a computer code which has been developed based on the finite element method (FEM). Dispersion curves and the displacement vectors for the transverse and longitudinal acoustic modes and the modal hybridness have been determined and these are shown for both low and high index contrast silica (SiO2) acoustic waveguides. Stimulated Brillouin scattering (SBS) frequencies are also reported for the subwavelength size SiO2 optical waveguides.

Rigorous analysis of the transverse acoustic modes in optical waveguides by exploiting their structural symmetry.

A full-vectorial finite-element-based approach has been developed to find accurate modal solutions of acoustic modes in Ge-doped planar silica waveguides. The structural symmetry is exploited, and Aitken's extrapolation is also used to improve the accuracy of the solution. The spatial dependences of the dominant and nondominant displacement vectors are shown for the fundamental and higher-order transverse modes. The modal hybridness and modal birefringence between the two fundamental transverse modes are also presented.

Rigorous characterization of acoustic-optical interactions in silicon slot waveguides by full-vectorial finite element method.

For the first time detailed interactions between optical and acoustic modes in a silicon slot waveguide are presented. A new computer code has been developed by using a full-vectorial formulation to study the acoustic modes in optical waveguides. The results have shown that the acoustic modes in an optical slot waveguide are not purely longitudinal or transverse but fully hybrid in nature. The model allows the effects of Stimulated Brillouin Scattering and the associated frequency shift due to the interaction of these hybrid acoustic modes with the fully hybrid optical mode also to be presented.

Sensitive detection of CO2 implementing tunable thulium-doped all-fiber laser.

In this paper a compact, yet sensitive gas detection system based on a modulated, tunable thulium-doped fiber laser in the 2 µm wavelength region is reported. The laser operating wavelength range centered at a wavelength of 1.995 µm has been selected to access the R(50) transition (ν1+2ν2+ν3) of CO2 based on its line strength and to achieve isolation from interfering high-temperature water absorption features. The laser linewidth and tuning range are optimized accordingly. The modulation of the fiber laser, achieved through pump source modulation and a locking detection mechanism, has been utilized to stabilize the laser system and therefore to create a compact gas sensor with high sensitivity. The absorption spectrum, as well as the line strength and the concentration level of CO2, have been monitored through absorption spectroscopy techniques. The measured minimum detectable concentration of CO2 obtained using the system shows that it is quite capable of detecting trace gas at the ppm (parts in 10(6)) level. The stable laser performance achieved in the sensor system illustrates its potential for the development of practical, compact, yet sensitive fiber-laser-based gas sensor systems.

Design and performance evaluation of polyvinyl alcohol/polyimide coated optical fibre grating-based humidity sensors.

Optical fibre grating based sensors make a significant contribution to relative humidity measurement for various applications and a number of different designs and sensor configurations have been produced and discussed in the literature. Recent developments have included using sensitive polymers and indeed other materials that experience a variation of their physical and optical properties as a result of the absorption of moisture and which thus creates a modification of the characteristic spectral features of the gratings used. The strain and the effective refractive index of the gratings are affected by the change induced by the polymer and these effects were calibrated against the relative humidity variations that initially induce variations in the polymer layer. In this work, an evaluation is made of the performance characteristics of two different moisture sensitive polymers, i.e., polyimide and polyvinyl alcohol, as the basis of such sensors, while at the same time using different types of fibre gratings i.e., fibre Bragg gratings and long period gratings. Their performance is discussed in light of the operation of similar types of polymer-coated and other grating based sensors reported in the literature.

Rigorous modal analysis of silicon strip nanoscale waveguides.

A full-vectorial H-field Finite Element Method has been used for the rigorous modal analysis of silicon strip waveguides. The spatial variation of the full-vectorial H and E-fields are also discussed in details and further, the Poynting vector is also presented. The modal area, hybridness, single mode operation and birefringence are also described for such silicon strip waveguides.

Characterization of silicon nanowire by use of full-vectorial finite element method.

We have carried out a rigorous H-field-based full-vectorial modal analysis and used it to characterize, more accurately, the abrupt dielectric discontinuity of a high index contrast optical waveguide. The full-vectorial H and E fields and the Poynting vector profiles are described in detail. It has been shown through this work that the mode profile of a circular silicon nanowire is not circular and also contains a strong axial field component. The single-mode operation, vector field profiles, modal hybridness, modal ellipticity, and group velocity dispersion of this silicon nanowire are also presented.

Fiber Bragg gratings with enhanced thermal stability by residual stress relaxation.

Fiber Bragg gratings with greatly enhanced thermal stability have been fabricated by the use of femtosecond laser pulse irradiation on optical fibers with relaxed residual stress, through using high temperature annealing treatment. The grating reflectivity and resonant wavelength can be maintained for periods up to 20 hours using isothermal measurements and temperatures up to 1200 degrees C. No hysteresis was observed in the wavelength response when the gratings were annealed and the temperature cycled repeatedly between room temperature and 1200 degrees C.

Study of spectral and annealing properties of fiber Bragg gratings written in H2-free and H2- loaded fibers by use of femtosecond laser pulses.

The spectral and annealing properties of a series of fiber Bragg gratings (FBGs) written in both H(2)-loaded and H(2)-free fibers by use of 800nm femtosecond laser pulse irradiation and created through a phase mask, have been investigated. It is found that type II FBGs inscribed in H(2)- loaded fibers exhibit superior spectral quality when compared with those written in H(2)-free fibers. Isochronal annealing tests shows that type II FBGs written in H(2)-free fibers have the highest thermal stability, followed (in order of stability) by H(2)-loaded type II, H(2)-free type I and then H(2)-loaded type I FBGs. The thermal stability of the H(2)-loaded type II FBGs can effectively be increased by using a high temperature pre-annealing treatment. After the treatment, type II FBGs written into both H(2)-free and H(2)-loaded fibers can sustain long-term annealing (for more than 12 hours) at temperatures of more than 1000 masculineC while their high reflectivities can still be maintained. This demonstrates the real potential of the FBGs developed and investigated in this work to be used as the ideal sensing elements for a series of high temperature measurement applications.

Golden spiral photonic crystal fiber: polarization and dispersion properties.

A golden spiral photonic crystal fiber (GS-PCF) design is presented in which air holes are arranged in a spiral pattern governed by the golden ratio, where the design has been inspired by the optimal arrangement of seeds found in nature. The birefringence and polarization properties of this fiber are analyzed using a vectorial finite-element method. The fiber that is investigated shows a large modal birefringence peak value of 0.016 at an operating wavelength of 1.55 microm and exhibits highly tuneable dispersion with multiple zero dispersion wavelengths and also large normal dispersion. The GS-PCF design has identical circular air holes that potentially simplify fabrication. In light of its properties, the GS-PCF could have application as a highly birefringent fiber and in nonlinear optics, and moreover the 2D chiral nature of the pattern could yield exotic properties.

Numerical analysis of bent waveguides: bending loss, transmission loss, mode coupling, and polarization coupling.

A rigorous, full-vectorial and computationally efficient finite-element-based modal solution, together with junction analysis and beam propagation approaches have been used to study bending loss, transition loss, mode coupling, and polarization coupling in bent optical waveguides. The waveguide offset and their widths have been optimized to reduce the transition loss and the mode beating.

Characterization of surface-plasmon modes in metal-clad optical waveguides.

Finite-element analysis, based on the vector H-field formulation and incorporating the perturbation technique, is used to calculate the complex propagation characteristics of metal-coated dielectric waveguides. The propagation and attenuation characteristics of the surface-plasmon modes at the metal/dielectric interfaces are presented. The effects on the optical properties of metal-clad optical fibers with infinite and finite cladding thickness and the formation of the supermodes due to the coupling between the surface-plasmon modes in the presence of different surrounding materials are also investigated.